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Psychoneuroendocrinology, Vol. 23, No. 8, pp. 905 – 926, 1998

© 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0306-4530/98 $ - see front matter

PII: S0306-4530(98)00060-2

ROLE OF THE VOMERONASAL INPUT IN MATERNAL BEHAVIOR

M. Cruz R. Del Cerro

Department of Psychobiology, Psychology School, U.N.E.D., Ciudad Universitaria s/n, 28040-Madrid, Spain

SUMMARY

This article reviews the role of the vomeronasal system in the induction of parental behavior in female and male rats, using, primarily, the sensitization model. The following questions are addressed: (1) Is the vomeronasal system sexually dimorphic? (2) Do the sex differences found in the VNS underlie those seen in behavior? (3) Do mechanisms, other than the classical ‘organizational’

effects of perinatal gonadal steroids, play a role in the organization of behavioral phenotypes in parental behavior? and (4) Does vomeronasal input play a role in the formation of the mother – in- fant bond in humans? The first question has been answered throughout the 1980’s in various studies of the organizational actions of postnatal exposure to gonadal steroids. The second aim has been addressed in a functional approach by lesion and neural activation studies. The experiments wich lead us to consider the hypothesis that nonsteroidal factors in development, and specifically GABA, could account for the expression of parental care are reviewed. Finally, research relevant to the existence of a vomeronasal organ in humans and a possible pheromonal input in the formation of mother – infant bonds in humans is reviewed. © 1998 Elsevier Science Ltd. All rights reserved.

Keywords — Maternal behavior; Vomeronasal system; Sexual dimorphism; GABAA– BDZ – Cl re- ceptor complex; Bed nucleus of the accessory olfactory tract; Diazepam; Picrotoxin.

INTRODUCTION

The most unquestioned form of love is that reflected in the interactions between a mammalian mother and her infant. This review addresses the question, is a ‘neurobiology of love’, from the perspective of chemosensory (vomeronasal and main olfactory) stimula- tion which may play a major role in the expression of maternal care. Specifically reviewed is the role of vomeronasal input in parental behavior. Evidence is reviewed indicating that:

(1) the vomeronasal system (VNS) is a sexually dimorphic neural network; (2) sex differences in reproductive behavior are based upon this VNS dimorphism; (3) mechanisms other than classical ‘organizational’ effects of perinatal gonadal steroids are involved in the organization of sex differences in parental behavior phenotypes; and, finally, (4) evidence for the hypothesis that VNS sensory input plays a role in the formation of the mother – in- fant bond in humans.

Address correspondence and reprint requests to: M. Cruz R. Del Cerro, Department of Psychobiol- ogy, Psychology School, U.N.E.D., Ciudad Universitaria s/n; 28040-Madrid; Spain (Tel: + 34 91 3986291; Fax: + 34 91 3986287; E-mail: delcerro@cu.uned.es).

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M. Cruz R. Del Cerro 906

Maternal behavior is a ‘motivated’ behavior which requires appropriate integration between external stimuli and internal states. Once maternal behavior is established, this motivated behavior can be elicited by a wide variety of sensory cues. The specific cues that elicit maternal responses vary among species. In most mammals, chemosensory communi- cation is a major form of sensory communication by which conspecifics mutually influence each other’s physiology and behavior (Wysocki et al., 1982). In rodents, the sense of smell plays a significant role in the expression and maintenance of social and, especially, parental behavior (Fleming and Rosenblatt, 1974a,b; Fleming et al., 1979; Marques, 1979;

Numan, 1994; Saito, 1986; Saito and Moltz, 1986; Saito et al., 1988).

Maternal behavior or parental care of pups can be considered as a form of ‘motivated love’. In order to measure this form of love, an operational definition is required to enable a consistent and systematic analysis of such a complex behavior pattern. Following repeated exposure to young pups, both male and female rats begin to exhibit high levels of parental behavior, which resemble the behaviors shown in females at the time of normal birth. This ‘sensitization’ (Noirot, 1972) or ‘induction’ (Rosenblatt, 1967) model provides a tool for examining one operational definition of parental behavior. This model also allows us to test our concept of the behavioral significance of morphological sex differ- ences in the VNS. An advantage of the sensitization paradigm is that it enables the analysis of maternal behavior in reproductively naive male rats as well as in virgin female rats. We and others (Numan, 1994) have studied sensitization-induced maternal behavior in these rats in relation to morphological sex differences in the VNS. Based upon research in the author’s laboratory, the vomeronasal system (VNS) plays a critical role in the early attachment between mother and infant in the rat, as elucidated using this ‘sensitization’

model (Del Cerro et al., 1991; Izquierdo et al., 1992).

In mammals, a close social relationship exists between a mother and her young. In the young this bond begins at birth and includes the odors of its mother, siblings and the nesting area. The mother, in turn, learns to recognize the odors of her/their infant(s). The chemosensory cues are varied; those that are of low molecular weight and volatile are processed by the main olfactory system (MOS), whereas those that are of high molecular weight and are non-volatile and, even low volatility, are processed by the VNS (Wysocki, 1979), which is also known as the ‘accessory olfactory system’ (AOS). The high molecular weight, non-volatile, substances mediate social interactions and are termed ‘pheromones’.

Most insects, amphibians, reptiles and mammals, including non-human primates (Tak- agi, 1989) and humans (Monti-Bloch et al., 1996), possess a well defined dual olfactory system that contains separate neural pathways regulating distinct olfactory functions (Dulka, 1993) and which includes secondary and tertiary projections (Segovia and Guilla- mo´n, 1993). The present review focuses on the role of the VNS in the control of parental behavior, and presents a tentative concept of vomeronasal (pheromonal) interaction in human maternal-filial bonding.

THE VOMERONASAL SYSTEM IS A SEXUALLY DIMORPHIC NEURAL NETWORK

The vomeronasal chemosensory system plays an important role in the physiology and behavior of mammals (Wysocki et al., 1985). The primary sensory cells, located in the vomeronasal organ (VNO), communicate — via the accessory olfactory bulb (AOB) with

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regions of the central nervous system (CNS) that have been implicated in the regulation of reproductive physiology and behavior (Halpern, 1987; Kevetter and Winans, 1981;

Lehamn and Winans, 1982; Scalia and Winans, 1975). Ablation studies have revealed that the VNS is involved in the attraction exerted by the female hamster’s vaginal secretions on the male (Powers et al. 1979), in the modulation of female cyclicity in the rat (Johns et al.

1978), in socially induced pregnancy block (Bellringer et al. 1980), in the modulation of hormone levels (Coquelin et al. 1984; Pfeiffer and Johnston 1994), in aggression (Novikov, 1993), in vasopressinergic transmission (Bluthe´ and Dantzer, 1993), in courtship and in mating (Clancy et al. 1984). Also, an important body of data refers to the role of main and accessory olfactory input, in maternal behavior (Numan, 1994).

At the beginning of the 1980’s Segovia and Guillamo´n (1982) showed that in rats the VNO is a sexually dimorphic chemosensory structure, and that VNO development is influenced by gonadal hormones present shortly after birth. The existence of steroid receptors (Pfaff and Keiner, 1973; Stumpf and Sar, 1982) and morphological sexual dimorphism in some VNS structures led them to hypothesize that the VNS might be a sexually dimorphic network (Fig. 1), and that the chemosensory stimuli conveyed by a sexually dimorphic VNO are most likely relevant to reproductive behavior, sexual and maternal behavior (Segovia and Guillamo´n, 1986). Morphological sexual dimorphism has been described in structures, also related to reproductive behavior, that receive input from, or are reciprocally connected with hypothalamic structures such as the medial preoptic area (MPA) (Bleier et al., 1982; Do¨rner and Staudt, 1968; Gorski et al., 1978; Raisman

Fig. 1. Anatomical and functional (motivational) approach showing two sexually dimorphic net- works (VNS – SNBS) that control sex differences in reproductive behaviors. I, II and III: prymary, sencondary and tertiary vomeronasal projections. SD: sex differences; AOB: accessory olfactory bulb; ARC: arcuate nucleus; BAOT: bed nucleus of the accessory olfactory tract; BST: bed nucleus of stria terminalis; C3: posteromedial cortical amygdaloid nucleus; LC: locus coeruleus; Me: medial amygdaloid nucleus; MPA-AH: medial preoptic area-anterior hypothalamus; PMV: ventral premam- millary nucleus of the hypothalamus, PVN: paraventricular nucleus of the hypothalamus; SNBS:

spinal nucleus of bulbocavernosus system; SO: supraoptic nucleus; VMH: ventromedial hypothala- mic nucleus; VNO: vomeronasal organ; lo: lateral olfactory tract; st: stria terminalis; and vn:

vomeronasal nerve (modified from Segovia and Guillamo´n, 1993).

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M. Cruz R. Del Cerro 908

and Field, 1971), the ventral region of the premammillary nucleus (PMV) (Do¨rner, 1976), the ventromedial hypothalamic nucleus (VMH) (Do¨rner and Staudt, 1969; Matsumoto and Arai, 1983) and the medial amygdaloid nucleus (Me) (Mizukami et al., 1983; Staudt and Do¨rner, 1976). The sexual dimorphism in the VNS, which depends on the organiza- tional actions of the gonadal steroids during the immediate postnatal period, has been demonstrated via the existence of sex differences in the VNO (Segovia and Guillamo´n, 1982) in the AOB (Caminero et al., 1991; Segovia et al., 1984, 1986; Valencia et al., 1986), the bed nucleus of the stria terminalis (BST) (Del Abril et al., 1987, 1990) and the bed nucleus of the accessory olfactory tract (BAOT) (Collado et al., 1990). These results indicate that gonadal steroids induce sexual dimorphism in the volume and/or number of neurons in all VNS structures during the perinatal period (Guillamo´n and Segovia, 1996;

Segovia and Guillamo´n, 1993, 1996). It is proposed here that the mammalian VNS is a useful model for understanding interactions between sexual dimorphism in the nervous system and the sexual dimorphism normally observed in parental behavior.

FUNCTIONAL IMPLICATIONS OF MORPHOLOGICAL SEXUAL DIMORPHISM OF THE VNS

Parental behavior has been defined by Numan (1994) as any behavior of a member of a species that increases the likelihood that the immature individual will survive to maturity. Maternal behavior fits into the larger category of parental behavior. Maternal behavior forms a complex, but stereotyped, behavioral pattern that includes nest building, grooming of the pups, licking their anogenital area, retrieving the litter to the nest site and nursing or crouching over them. All these behavioral parameters, plus others derived measures (Del Cerro et al., 1991), can be recorded via a computer program, the Maternal Behavior Recorder (MBR) (Claro et al., 1994). This program provides behavioral indices, sequential analysis of maternal patterns and rapid statistical processing, since all direct data can be directly imported to a statistical package.

Maternal behavior can be elicited in virgin female and male rats of many laboratory strains (Numan, 1994). Virgin female rats require several days exposure to young pups to display maternal behavior (Rosenblatt, 1967). This process of eliciting maternal behavior has been called ‘induction’ (Rosenblatt, 1967) or ‘sensitization’ (Noirot, 1972). Even though it is possible to induce maternal behavior in inexperienced males, males require, in comparision to females, longer exposure to pups, and infanticide frequently is observed (Menella and Moltz, 1988). Thus, there is sexual dimorphism in rats in the expression of maternal care. This observation is correlated with the fact that the VNS is a multisynaptic pathway whose structures are sexually dimorphic and have receptors for gonadal steroids.

The most frequent pattern of sexual dimorphism in VNS structures is that in which males contain a larger number of neurons than females (Segovia and Guillamo´n, 1993).

The total number of neurons and their relative proportions in different regions of the nervous system appear to be related to their function. For example in canaries the nuclei responsible for song contain a greater number of neurons in males than in females, and this morphological difference is associated with song production (Nottebohm et al., 1986).

The size of several VNS structures also correlates with parental behavior in rodents. In female rats, a significant increment is observed in dendritic bundling of supraoptic (SON) neurons after the induction of maternal behavior (Hatton et al., 1992). Gubernick and Alberts (1987, 1989) found in a biparental species of mouse (Peromyscus californicus), that

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Vomeronasal Control of Maternal Behavior 909

the MPA/brain size ratio was significantly larger in naive males than in virgin females.

When females of this species become mothers they do not show changes in MPA size;

however, fathers show a decrease in their MPA/brain size ratio compared with naive males.

Once the hypothesis regarding the sexual dimorphism of the complete VNS neural network was supported (Segovia and Guillamo´n, 1993), through the morphological studies cited above, the next step was to investigate the functional significance of the quantitative sex differences in the VNS. Furthermore, it has been shown that some VNS structures present sexual dimorphism in neurotransmitter substances that can in- hibit or facilitate reproductive behavior (De Vries, 1990). We started this functional approach by studying a comparatively unknown nucleus, the BAOT, wich recives input from the AOB and from the Amygdala (Fig. 1).

Different authors have suggested that virgin female rats find pups ‘aversive’ during the early phases of the sensitization process (Fleming and Rosenblatt, 1974b; Mayer and Rosenblatt, 1980). In support of this, nonmaternal females made anosmic by olfactory bulbectomy (eliminating main and accessory olfactory sensory input) did not avoid pups and became maternal within 2 days (Fleming and Rosenblatt, 1974b).

Both the MOS and VNS are involved in mediating the effects of these hypothesized aversive odors (Fleming et al., 1979). It is known that lesions in structures that are considered to be components of the VNS can facilitate or disrupt parental behavior.

The active avoidance period is markedly diminished in both males and females by VNO deafferentation (Fleming et al. 1979; Mayer et al., 1979) and VNO removal decreases infanticide behavior in male rats (Menella and Moltz, 1988). Facilitation of maternal behavior in virgin female rats also can be seen following lesion of different vomeronasal structures or tracts (Numan et al., 1993). In contrast, lesions of MPA, which receives vomeronasal input, disrupt maternal behavior (Numan and Callahan, 1980; Numan et al., 1977) and prevent the facilitation observed after amygdaloid le- sions (Fleming et al., 1983).

The BAOT is a cell group of the forebrain that is a component of the VNS. It is associated with the accessory olfactory tract (Broadwell, 1975; De Olmos et al., 1985;

Scalia and Winans, 1975), located rostrally, ventral to the nucleus of the lateral olfac- tory tract and its caudal portion is located ventrolaterally and adjacent to the anterior medial amygdala (Kevetter and Winans, 1981; Krettek and Price, 1978; Scalia and Winans, 1975). Histologically, the BAOT is composed of a small group of medium-size cells that have a high affinity for cresyl violet stain; the BAOT has been described as free of acetylcholinesterase and cholecystokinin activity (De Olmos et al., 1985). Embry- ologically, this nucleus has two phases of neurogenesis in the rat, the largest surge (74%) occurs around the 12th and 13th embryonic days (E), a later surge (17%) occurs about E15. The BAOT is a major target for fibers projecting from the AOB (De Olmos et al., 1978) and is reciprocally connected with the AOB (Davis et al., 1978; Scalia and Winans, 1975) and the posteromedial cortical amygdaloid nucleus (C3) (Kevetter and Winans, 1981; Ottersen, 1982). Moreover, BAOT receives afferents from the medial amygdaloid group (Me) (Kevetter and Winans, 1981) and is one of the main cellular groups that relays vomeronasal input to the MPA (De Olmos et al., 1985). After bilateral electrolytic lesions of the BAOT, we found shorter latencies for retrieving pups and becoming maternal (sensitized animals) compared with sham and control groups

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M. Cruz R. Del Cerro 910

Table I. Latency in days (medians9IQR) for the display of maternal behaviour in bilaterally BAOT lesioneed, sham and control virgin female rats

Groups Behavior patterns

Controls (15) Sham (16) Bilateral lesions (10) 8 (4.2–12) 5.5 (1–10.2) 10 (6–10)

Retrieval latencies

12 (9–12) 12 (88–12) 6 (3.2–12)* Sensitization latencies

31

Percentage of group 33 70

Days for retrieval and sensitization latencies are medians9(IQR) and were analyzed with the Mann–Whitney U-test, two- tailed.Differences are significant with respect to the control group (* pB.04;pB.05) and with respect to the sham group (pB.03).

(Tables I and II) (Del Cerro et al., 1991; Izquierdo et al., 1992), suggesting that the BAOT normally inhibits parental behavior in rats.

Since naive male rats require longer latencies than virgin female rats to become parental (Mayer et al., 1979) and males cannibalize more pups than females (Menella and Moltz, 1988), the results observed in BAOT lesioned males suggest that the inhibitory role of the BAOT in the expression of parental care is greater in male than in virgin female rats. This functional difference could be related to the sex differences found in BAOT volume and number of neurons (Collado et al., 1990), and supports the hypothesis (Segovia and Guillamo´n, 1993) that the larger number of neurons, consistently observed in male rat VNS structures has a functional role in the inhibition of parental behavior. We further hipothesized that this sexual dimorphic pattern might have two functional consequences: (1) to allow for a tonic inhibition of the expression of lordosis and maternal care; and (2) to enable the males to exhibit masculine sexual behavior (the role of the BAOT in sexual behavior is not reviewed here).

The functional meaning in the male of ‘extra’ or ‘supernumerary’ neurons is ex-

Table II. Latency in days to the onset of paternal behavior in Baot lesioned, sham, and control male rats

Groups

Behavior patterns Control Sham Bilateral lesions

(n = 9) (n = 10) (n = 7)

12 (8.7–12) 12 (12–12)

Retrieval latencies 2 (0–12)*

3 (0–12)‡§

12 (12–12) Sensitization latencies 12 (12–12)

0 10 71

Percentage of group

Data for retrieval and sensitization latencies are medians9interquartile range (IQR) and were analyzed with the Mann–Whitney U-test.

*†‡§Significant differences with respect to the control group (* pB.02;pB.001) and with respect to the sham group (pB.03;

§pB.01).

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Vomeronasal Control of Maternal Behavior 911

Fig. 2. Neurofunctional hypothesis on the significance of quantitative sex differences in the VNS and their relationships with sexually differentiated reproductive behaviors (modified from Segovia and Guillamo´n (1993)).

pressed by the behavioral sex differences in which the BAOT might be involved (Fig. 2).

Our studies had clearly shown, for the first time, the involvement of the BAOT in parental behavior. Based on this result we continued to investigate the hypothesis that the absence of maternal behavior in inexperienced rats is normally due to vomeronasal sensory input which via the BAOT tonically inhibits the MPA (Izquierdo et al., 1992). Pursuing this aim and also the effect of viscerosensory stimulation on 2-deoxyglucose (2-DG) levels in brain during the display of maternal care, we (Del Cerro et al., 1995b) compared regional brain levels of 2-DG (measured as relative optical density that was ascertained autoradiograph- ically with computerized densitometry) under different hormonal and stimulus experimen- tal conditions (Table III).

The effect of viscerosensory stimulation on 2-DG levels in brain was assessed by comparing a group of rats showing maternal behavior as a result of normal parturition with a group showing maternal behavior induced by hysterectomy on day 16 of pregnancy.

Since the hormonal milieu under both of these conditions is similar at least for 16 days (Rosenblatt et al., 1985), the most salient difference was the presence of the viscerosensory

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stimulation of the birth canal in the group that gave birth. The effect of chemosensory, but not viscerosensory or hormonal stimulation on levels of 2-DG in AOB, Me, MPA and BAOT also was assessed with a group of virgin females showing maternal behavior induced by sensitization. A fourth group consisted of virgin diestrous females not showing maternal behavior and lacking all the above forms of stimulation. We found an increase in MPA 2-DG levels in the parturient rats, suggesting that mechanostimulation of the birth canal during parturition stimulates afferent terminals in MPA. The absence of a significant increase in 2-DG levels in the MPA in the sensitized and hysterectomized groups suggests that presynaptic activity in MPA is not increased by these treatments;

however, it does not rule out the possibility that postsynaptic activity in cell bodies in the MPA is increased by these treatments, for instance in response to a reduction in tonic inhibitory input.

The relative decrease in 2-DG levels (Fig. 3) observed in sensitized females in certain regions of the VNS (AOB, Me, BAOT) can be interpreted as the result of a reduction of neural activity afferent to these vomeronasal structures. On the basis of ortho- and anti-dromic stimulation studies, there is evidence that 2-DG labeling in the CNS is due predominantly to local activity at synaptic terminals, regardless of whether they are excitatory or inhibitory, and only minimally to local postsynaptic cell discharge (Nudo and Masterton, 1986). Whereas increases in regional CNS activity measured by the 2-DG method are the predominant response to stimulation of afferent brain regions or nerves (Abram and Kostreva, 1986; Porro et al., 1991), local decreases in 2-DG levels in brain have been reported in response to deafferentation or specific pharmacological agents (Brett and Pratt, 1991; Chalmers and McCulloch, 1991; Dietrich et al., 1985;

Margulies and Hammer, 1991). A decrease in 2-DG levels in response to sensory stimulation is rare, but has been reported in the olfactory system (Jourdan, 1982).

Our findings of a significant decrease in 2-DG levels in the AOB, Me and BAOT of the sensitization group during the performance of maternal behavior support our proposal of the existence of a tonic vomeronasal inhibitory process starting in projections from the VNO bipolar neurons to the AOB and terminating in BAOT neurons. This findings suggest that: (a) the VNS exerts inhibitory control of maternal behavior; and (b) this control is exerted upon the MPA, because the MPA receives vomeronasal input from several VNS structures, especially the BAOT (De Olmos et al., 1985).

ROLE OF THE GABAA RECEPTOR IN THE DEVELOPMENT OF SEX DIFFERENCES IN VOMERONASAL STRUCTURES AND IN PARENTAL

BEHAVIOR

General assumptions regarding the sexual differentiation of brain and behavior have been supported predominantly by two proposals: (a) the concept of ‘organization’ based on the hypothesis that gonadal hormones differentiate or produce long-term changes in neural tissue and reproductive behavior during an early period of development, whereas these hormones have an ‘activating’ function on comparable systems during adulthood (Phoenix et al., 1959); and (b) the ‘cascade’ hypothesis which suggests that testos- terone and estradiol regulate a series of chained events, whose consequence is sexual

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M.CruzR.DelCerro913 Table III. Schematic description of the stimuli to which females were exposed (Del Cerro et al., 1995b)

Groups Experimental conditions

Hormonal state of gestation Hormonal state of parturition Birth canal distension Presence of pups Maternal behavior

P + + + + +

H + − − + +

S − − − + +

C − − − − −

P: parturient group; H: hysterectomized group; S: sensitized group; C: control group.

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Fig. 3. Regional 2-DG levels (group means9SEM of the relative autoradiograph density ratio between the specified brain region and the corpus callosum of the same brain sections) in the brain of female rats showing maternal behavior during maternal parturition (Par), maternally sensitized animals (Sen), hysterectomized on day 16 (Hys) and control group (Con) non-pregnant and not showing maternal behavior. Brain regions: MPA (medial preoptic area); AOB (accessory olfactory bulb); Me (medial amygdaloid nucleus) and BAOT (bed nucleus of the accessory olfactory tract).

Within each brain region, groups bearing different letters differ significantly from each other (Tukey’s Protected and test, two-tailed); upper case letters that are different from each other indicate pB.05. For example in the Me, the 2-DG level in the Sen group is significantly lower than in the control group at pB.01, and than the Hys group at pB.05 (modified from Del Cerro et al.

(1995a,b)).

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Vomeronasal Control of Maternal Behavior 915

dimorphism in the nervous system and reflected in the number of synapses and neurons, neurite growth and, finally, the volume of especific structures or nuclei (Toran-Allerand, 1984). Both proposals imply common basic mechanisms by which hormonal responsive- ness is related to the presence of receptor sites, which is turn mediates actions of steroid hormones at the level of gene expression (McEwen, 1988). These hormone receptors are proteins with a steroid binding domain and a DNA binding domain, which regulate genomic activity through binding to specific DNA sequences called ‘enhancers’ (Ya- mamoto et al., 1985).

As summarized here, morphological studies carried out on the vomeronasal sexual differentiation have established for the first time in mammals a sexually dimorphic multisynaptic pathway, the VNS (Segovia and Guillamo´n, 1993). This VNS pathway conforms, in general, to the proposals described above. However, genomic actions of gonadal steroids do not explain all brain sex differences, since monoaminergic neurons (Reisert and Pilgrim, 1991) and hypothalamic GABAergic neurons (Lieb et al., 1994), in culture can show sexual differentiation in the absence of exogenous gonadal steroids. A role for neurotransmitters in sexual differentiation was suggested by Do¨rner (1976) and confirmed in different studies showing that agonists and antagonists of several neurotrans- mitter systems can alter brain sexual differentiation during the critical periods of develop- ment (Do¨hler et al., 1991). Following this idea, and based on our previous works on the morphological effects of perinatal Diazepam (DZ) administration in VNS structures (Pe´rez-Laso et al., 1994; Segovia et al., 1991) we investigated the hypothesis that early posnatal administration of DZ, a benzodiazepine (BDZ), might facilitate maternal behav- ior in adult virgin female rats (Del Cerro et al., 1995a).

This hypothesis was based on the following rationale: (1) The ontogenetic period for BDZ central receptors extend from prenatal days 13 – 14, in which only 35% of adult levels are present, to postnatal day 18, in which adult levels are reached (Braestrup and Nielsen, 1978); (2) This ontogenetic period for BDZ central receptors overlaps the perinatal period of brain sexual differentiation (Segovia et al., 1991); (3) Perinatal administration of DZ feminizes vomeronasal structures in the male rat, such as the AOB, which participates in the control of maternal behavior (Segovia et al., 1991); (4) GABA is the inhibitory transmitter released from the granule cells in the olfactory bulb (Green et al., 1962; Nicoll, 1971); (5) The perinatal increase of the chloride (Cl) flux by the administration of BDZ mimics the morphological effects of neonatal male gonadectomy in the AOB (Segovia et al., 1991; Segovia and Guillamo´n, 1996); (6) The GABAA– BDZ – Cl ion channel receptor complex is involved in the sexual differentiation of VNS nuclei (Pe´rez-Laso et al., 1994; Segovia et al., 1991).

Furthemore, there is evidence implicating GABAergic system in the display of maternal behavior in the adult female rat; the behavioral profile recurring naturally in the lactating rat is blocked by the administration of BDZ antagonists, while BDZ elicits this behavior in nonmaternal rats (Hansen et al., 1985). In addition, pregnancy produces a decrement in the density of GABAA receptors and an increase in their affinity, probably due to the elevated levels of placental and adrenal endogenous steroids that occur during pregnancy (Majewska et al. 1989). These pharmacodynamic characteristics of the GABAA receptors may be related to the so called ‘postpartum blues’. Analogous changes in the density and affinity of GABAA receptors are obtained as a result of earlier (prenatal) treatment with DZ (Kellog, 1992).

Taken together these observations suggested the hypothesis thas early postnatal DZ administration would facilitate the expression of maternal care in adult virgin female rats.

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M. Cruz R. Del Cerro 916

Fig. 4. Cumulative percentages of animals showing retrieval throughout the 12 days of maternal induction test.

Daily DZ administration to rat pups from the day of birth (P0) to day 16 of life (P16) did in fact result in a statistically significant increase of the percentage of virgin female rats that became maternal (Fig. 4).

The higher maternal respoaensiveness of the DZ group versus control females was expressed as the number of females achieving retrieval of the complete litter (Fig. 4). This facilitatory effect seems to indicate that the DZ animals, when they were exposed to olfactory, auditory and tactile cues provided by the pups, more quickly overcame their

‘aversion to these stimuli’ (Guillamo´n et al., 1990).

Our results confirm and extend previous studies (Laviola et al., 1990, 1991; Majewska et al., 1989) suggesting that the GABAA– BDZ – Cl receptor complex is involved in the development of maternal behavior in female rats. A possible explanation of these facilitatory effects of postnatal DZ administration can be offered by observed changes in the density and affinity of brain GABAA receptors that occur after earlier (prenatal) treatments with DZ (Kellog, 1992). P0 – P16 DZ postnatal administration of DZ (day 0 – 16) to the female rat may have caused an increase in affinity and a decrement in the density of brain GABAA receptors and hence, an ‘imprinting’ of the functional status of these receptors in a similar way to those found postpaturition (Majewska et al., 1989).

Addressing the postulated ‘non-genomic’ effects of altered GABAergic transmission during critical periods on brain and behavior sexual differentiation, we have recently investigated (Segovia et al., 1996), the role of the GABAAreceptor in the development of sex differences in the AOB and in parental behavior. The agonist DZ and the antagonist picrotoxin (PTX) were administered from P0 (day of birth) to P16 (postnatal day) to male and female rats. We found that DZ was able to induce in adult males a female-like number of AOB mitral cells (Fig. 5) and maternal behavior (Fig. 6) while PTX masculinized the AOB and disrupted adult maternal behavior in females.

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We measured the levels of estradiol, testosterone and progesterone in plasma at P18 and P120, after the induction test was ended, in order to examine any hormonal 1modulatory effects caused by experimental treatments. Plasma levels of these hormones were normal in each sex (Table IV).

Brain and behavioral sexual dimorphism were changed by the administration of DZ and PTX, but hormonal patterns were not. These results suggest that the alterations observed in sexually demorphic behavior in adult animals may be due to early postnatal changes, such as those affected by the permeability of neuronal membranes to the Cl ion. These changes in the Cl permeability may have altered the number of mitral cells in the AOB and could have caused an ‘imprinting’ in the physiology of the GABAA receptor.

Since it has been shown that GABA accelerates excitotoxic cell death induced by excitatory amino acids, while blockers of the GABAA-gated Cl ion channel protect against excitotoxicity (Erdo¨ et al., 1991), we may hypothesize that an analogous process was associated to the reversal of sex differences in the AOB mitral cells observed after postnatal DZ and PTX treatments.

Furthermore, it has been reported that a single administration of DZ causes a signifi- cant increase of 5-alpha-reductase activity in the diencephalon of the male rat without altering testosterone levels (Kaneyuky et al., 1979). We suggest that a possible participa- tion of 5-alpha-reduced steroids of nongonadal origin should be taken into account in explaining the results we obtained concerning the organi- zation of sex differences in the AOB and maternal behavior (Segovia et al., 1996), it is possible that this mechanism also plays a role in the sexual differentiation of the brain and behavior. Through our studies we have established a new animal model that may contribute to our understanding the early postnatal differentiation of the brain, as well as processes involved when genetic sex and hormonal sex do not match brain sex and behavioral sexual orientation in adulthood.

Fig. 5. Bar diagrams showing sex differences in the number of AOB mitral cells and the effects of DZ and PTX treatments. ANOVA: F(3,16) = 12.24, pB0.0002; C-M: control males; C-F: control females; DZ-M; diazepam males and PTX-F: picrotoxin females. Data shown mean 9s.d.

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Fig. 6. Sex differences and effects of neonatal diazepam and picrotoxin treatments on induced maternal behavior. Data show median9interquartile interval. (A) Percentage of animals becoming maternal (x2= (3) = 91.65, pB.0000). (B) Nest building quality (x2= (3) = 30.93, pB.0000). (C) Cumulative percentage of animals achieving pup retrieval (x2= (3) = 60, pB.0000). (D) Time spent in nursing behavior (x2= (3) = 6.7, pB.08). C-M: control males; C-F: control females; DZ-M:

diazepam males, and PTX-F: picrotoxin females.

DOES VOMERONASAL SENSORY INPUT PLAY A ROLE IN THE FORMATION OF THE MOTHER – INFANT BOND IN HUMANS?

Although the VNS has been studied and described in a variety of nonhuman vertebrates, its existence in adult humans has only recently been established (Mora´n et al., 1995). It was

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previously assumed that the VNO was absent in adult humans except in rare cases, in which it is vestigial and had no function (Meredith, 1983). However, there are indications that the human VNO is involved in pheromone detection (Mora´n et al., 1995) on the basis of clinical studies in adults (Garcı´a-Velasco and Mondrago´n, 1991) and more consistently, electrophys- iological (Monti-Bloch and Grosser, 1991), electromicroscopic (Mora´n et al., 1991) and immunohistochemical studies (Mora´n et al., 1992; Takagi, 1989). The term ‘vomeropherins’

has been substituted for ‘pheromone’ to refer to substances that stimulate the VNO in humans (Monti-Bloch et al., 1994; Stensaas et al., 1991).

The reported frequency of occurrence of the VNO in adult humans ranges from 39% of 100 patients (Johnson et al., 1985) to 100% of 200 patients (Mora´n et al., 1991), this apparent discrepancy may be accounted for by differences in methodology. In one study, visual inspection was used and in the second microscopic observations of the characteristic vomeronasal pits were reported. There is evidence that in adult humans the VNO: (a) may exist; (b) has a unique electron-microscopic ultrastructure; (c) contains cells with several immunohistochemical markers for neurons; and (d) displays distinct chemoreceptive responses to specific chemical stimulants.

However, the presence of AOB has not been demonstrated clearly in adult humans. That is, there is no clear experimental evidence of a neuroanatomical connections between the human VNO and the brain. Nevertheless, specific autonomic changes (galvanic skin response and body temperature) occur during VNO stimulation by vomeropherins, suggesting the possibility that the VNO is functionally connected to the brain. The short latencies of these physiological responses indicate that the transmission of information from the VNO to the brain is mediated by a neuronal, rather than a blood-borne mechanism. Recent studies suggest that although the human AOB may disappear as a dictinct anatomical structure in the second trimester of gestation, the cells of the AOB may undergo a physical displacement, rather than neuronal degeneration, and persist into adult life (Mora´n et al., 1995).

Monti-Bloch et al. (1994) have reported sex differences in the specific responses of VNO and VNO-mediated effects of different groups of vomeropherins. However no sexual

Table IV. Sex differences in plasma gonadal hormone levels and the effect of diazepam and picrotoxin treatments

Hormone C-M C-F DZ-M PTX-F

E (pg ml−1) 11.294.35 (15) 25.5695.57 (12)e,f 10.9796.45 (10) 25.9095.36 (8)c,d P (ng ml−1) 9.9195.52 (15) 35.53911.06 (10)e,f 10.1994.30 (10) 44.1298.27 (10)e,f

0.3790.23 (10)a,b 2.1891.43 (10)

0.1990.13 (12)c,d 2.9191.90 (15)

T (ng ml−1)

Number of determinations in parentheses. ANOVA: F(3, 41) = 3.10, pB.000; F(3, 41)=0.40, pB.000 and F(3, 43)=8.76, pB.001, for E2, P and T, respectively.

With respect to the additional measures of plasma levels at P18: (number of determinations for each group) control males: E2 (10), P (13); control females: E2 (9), T (9), P (13) DZ-males: E2 (10), T (10), P (13); PTX-females: E2 (9), T (9), P (13). ANOVA:

F(3, 34) = 6.04, pB.002. Values for T: 1.0790.64 versus 0.3690.30 ng ml−1 for control males and females, respectively (pB.05). Interestingly, the levels of this hormone were similar for the control and DZ-males (0.9890.71), but different from those of PTX-females (0.2590.15; pB.01). Data show mean9SD.

aSignificant differences with respect to control males; pB.05.

bSignificant differences with respect to DZ-males; pB.05.

cSignificant differences with respect to control males; pB.01.

dSignificant differences with respect to DZ-males; pB.01.

eSignificant differences with respect to control males; pB.001.

fSignificant differences with respect to DZ-males; pB.001.

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dimorphism has been observed for olfactory responses. These reports coincide with the crucial role that the VNS plays in the detection of pheromones in animals (Wysocki, 1979).

The role of VNS in the onset of MB in rats has been reviewed above. By contrast with rodents, human mothers are highly responsive to visual stimuli from their infants.

However, the influence of olfactory cues is less obvious. At present there is no evidence for vomeronasal involvement in the early attachment between mother and baby. The largest amount of data relevant to maternal behavior in humans is related to the correlation between psychological and physiological influences in the expression of maternal care in the mothers. Psychobiological processes involved in the control of normal or deviant mothering are complex since background and situational factors seem to mask the role of biological factors in early mothering (Harkness and Super, 1995). Although the role of the vomeronasal system in human mothering has not been studied, there are relevant studies showing the sensory, hormonal and neural factors that regulate parental behavior in human beings (Benoist and Porter, 1991; Corter and Fleming, 1995).

There is substantial evidence in humans for the motivating effect of infant cries (Bell and Ainsworth, 1972; Boukydis, 1985) and a wide-ranging role of vision in mother – infant interactions (Klaus and Kennell, 1983; Messer and Vietz, 1984). There is a popular view that during the interval from pregnancy to the first days postpartum, women undergo dramatic changes in emotional lability, and specifically in odor sensitivity. New mothers can recognize their own infants on the basis of olfactory cues; for instance, they can discriminate their own infant’s soiled t-shirt from those of same-age infants, requiring very little interaction with their baby to do so (Corter and Fleming, 1995). Odor recognition ability may be related to early postpartum experience and circulating hormones. For example, mothers who correctly identified their own infant’s odors had experienced earlier and longer contact with their babies after birth, spent more time in close contact with their infants during interactions, and reported more positive maternal feelings and attitudes, in constrast to mothers who were unable to identify their infants odors (Corter and Fleming, 1995).

CONCLUSIONS

Induced structural changes in the VNS are related to modifications in sex-specific parental behavioral patterns in rats.

In this review the following experimental findings were discussed: (1) the VNS is a multisynaptic, sexually dimorphic neural network; (2)VNS sexual dimorphism is regulated firstly by the action of the perinatal gonadal steroids and secondly by membrane mechanisms affected by the GABAA– BDZ – Cl receptor complex; (3) the BAOT is a vomeronasal structure directly involved in the neural control of induced parental behavior;

(4) sexual dimorphism in parental behavior which can be reversed firstly by reversals in the morphological sexual patterns of the VNS structures, and secondly by induced changes in Cl ion channel on the GABAA receptor complex.

Although the process of brain and behavioral sexual differentiation is complex and limitations exist for animal models, findings from animals provide a neurobiological framework for studying the mechanisms through which external and internal variables can affect a well defined neural system, the VNS. Studies of the VNS and sexual dimorphism in the system provide a better understanding of the neurobiology of parental behavior, and suggest mechanisms through which olfactory processes may influence parental behavior, which is in turn crucial for species survival.

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Even in rodents, it is not yet clear whether vomeronasal cues are essential to the establishment of the mother – infant bond. The factors responsible for maternal responsive- ness are probably redundant in both rodents and humans. There is much to be learned regarding the role of olfaction in human maternal/parental behavior, and regarding the psychobiological factors responsible for these behaviors. Understanding these relationships may provide important insights into the neurobiology of what humans call ‘love’.

Acknowledgements: The works presented in this review have been supported by DGICYT grants PB93-291-CO3- 03, PB96-0107-C03-02, BE90/273 and NIH grant cP41 RR01638. I am grateful to Drs Segovia and Guillamo´n for their relevant contributions to the research programs which have given rise to this work. I also thank Drs Komisaruk and Rosenblatt (Rutgers Univ. USA) for their suggestions and editorial help and to L. Carrillo, L.

Troca, G. Moreno and R. Ferrado for the technical assitance. My thanks are particularly due to C.G. Malo de Molina and A. Marcos for their editorial and graphical support respectively.

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