receptors. One of the explanations for this interest is due to the discovery of 8-O H -D P A T (Hjorth eta!., 1982), a selective agonist, and more recently
W A Y -100135 and W A Y-100635 (Fletcher e ta /., 1993; Fletcher ef a/., 1996), the latter of which is still the best selective high affinity antagonist. There are also a host of other substances which have been reported to have a potent affinity at the 5-H T ia receptor, including agonists buspirone (Dourish eta!., 1986) and 5-C T (see Bradley ef a/., 1986), antagonists methiothepin (Fozard eta!., 1987), and metergoline (Fozard eta!., 1987) and the p-adrenceptor antagonist pindolol
(Middlemiss eta!., 1977, see Hoyer, 1991). However, these compounds are not particularly selective, for example pindolol has been shown to have affinity for 5-HTib receptors, and methiothepin is known to interact with a-adrenoceptors and histamine receptors. Indeed even 8-O H -DPA T has been shown to act as a partial agonist at 5-HTy receptors (see Barnes & Sharp, 1999). Another
ligand affinity dependent on central location, functional model and species used (see Zifa & Pillion, 1992).
The use of autoradiography has provided a great deal of information as to the pattern of 5 -H Tia receptor location throughout the CNS (Pazos & Palacios, 1985; Hoyer et al., 1986). Inter species differences in this receptor subtype are few although the organisation of laminar regions in the cortex and hippocampus of rat has been shown to be different to that found in humans (Burnet et a!.,
1995). Levels of the 5-H T ia receptor are high in dorsal and median raphe nuclei, together with limbic brain areas such as the hippocampus, central nucleus of the amygdala and cortical regions. Binding sites have also been found throughout the brainstem including two vagal motor nuclei, the DVN and
NA (Pazos & Palacios, 1985), and the NTS (Dashwood eta!., 1988). However, areas including the basal ganglia and cerebellum contain very few binding sites. Both presynaptic and postsynaptic location of this receptor occurs. At forebrain levels postsynaptically located receptors have been identified whilst at the level of the raphe nuclei presynaptic location on the soma and dendrites of
serotonergic neurones has been demonstrated by a number of methods (Miquel
et a!., 1991, Miquel et a!., 1992). These presynaptic receptors are thought to function as somatodendritic autoreceptors, with selective agonists attenuating the ongoing activity of serotonergic neurones in dorsal raphe, whilst antagonism of the receptor reverses this attenuation (Sprouse & Aghajanian, 1986).
Another functional characteristic of 5-HTia receptors is their tonic activation in conscious cats, demonstrated by the ability of the antagonist W A Y 100635 to increase serotonergic cell firing (Fornal et a!., 1996). Further detail on the functioning of the 5-H Tia receptor can be found in a recent review (see Barnes & Sharp, 1999), including the role that these receptors may play in the
modulation of other neurotransmitters such as acetylcholine and noradrenaline. Other topics included are the behavioural responses to 5 -H Tia receptor
activation, however, one important aspect missing from this review is the
important role that this and other 5-HT receptor subtypes play in the modulation of cardiovascular control (see Ramage, 2001 ; section 1.6).
5 -H Tir an d 5-HTin re ce p to rs
The 5-H T1B/1D receptor classification has undergone a significant change over the last five years. The originally defined 5-HTid receptor, characterised in bovine tissue 15 years ago (Heuring & Peroutka, 1987), is now known to be a species variant of the rat 5-HTib receptor (Hartig et al., 1996). The similar central distribution of these two receptor subtypes suggested a close homology and indeed more recently the amino acid sequence has been characterised and reveals they are 93% identical overall (see Hoyer et al., 1994). There was also the identification of a further receptor subtype in higher species with similar pharmacology, which was given the classification 5-HTiDa, with the previous subtype classified as 5-HTiop, due to the similarities with rat 5-HTib.
Subsequently, the 5-HTiDa receptor was isolated in rat and the recent reassessment of the nomenclature began from there (Hamblin eta!., 1992). This now stands as the original 5-H Tid receptor being reclassified as 5 -H Tib, however, this must be prefixed with the species subtype, i.e. r5-HTiB for rodent and h5-HTiB for human. Furthermore, the 5-HTiDa receptor found in both rat and human has been reclassified as the 5-H Tid receptor. This has led to a certain degree of confusion in the use of selective pharmacological tools in research involved with these subtypes, possibly requiring a reappraisal of some of the relevant published work.
Receptor Ligands at 5 -H T w and 5 -H T w receptors
One of the recent advances in research into the individual characteristics of function of 5-HTib and 5-HTio receptors is the availability of selective ligands. One group of selective agonists at these binding sites is the triptans, including sumatriptan, zolmitriptan and others. Sumatriptan has high affinities specifically at 5-H T1B and 5-HTid and 5 -H T if receptors (pK ’s of 8.0, 8.3 and 7.6
respectively, see Barnes & Sharp, 1999, value given for h5-HTiB, data for r5-HTiB - pKj 6.3, see Hoyer eta!., 1994) and to a lesser extent 5-H Tia receptors (pKj 6.1, see Hoyer et al., 1994) and 5-HTy receptors (pKi 6.2, see Hoyer ef a/., 1994). One of the common attributes of 5-HTib and 5-HTid receptor ligands is their high selectivity for the 5-HTia receptor as well,
however, a more recently developed agonist for the 5-HTid receptor, MK 464, does have pronounced selectivity over the 5-H T ia receptor (Street et al., 1995).
In addition, C P-93,129 is a potent 5 -H Tib receptor agonist, which also has high selectivity over other 5-HT receptor subtypes (Koe et al., 1992). Whilst the compound G R 127935, reported to be an antagonist with high affinity for both 5- HTib and 5 -H Tid receptor binding sites (Skingle eta!., 1993), has been
demonstrated to have partial agonist activity at the 5 -H Tib receptor (Walsh at al., 1995). It has also been shown to have agonist properties at the 5 -H Tid
receptor (Starkey & Skingle, 1994; Pauwels & Golpaert, 1995). One ligand whose actions have been confused in light of the reclassification of 5 -H Tib and
5 -H Tid receptors, is G R55562. This compound is reported to act as an
antagonist at 5-HTiDp (or according to the new classification h5-HTiB) receptors (Walsh at al., 1995).
In addition, a selective 5 -H Tib receptor antagonist (SB-224289) has recently been developed which has been shown to be unable to induce 5-H T release in the frontal cortex (Roberts atal., 1997). Whilst another new compound, the 5-HTid receptor antagonist BRL-15572, is reported to have a 60-fold higher affinity for the 5 -H Tid versus the 5 -H Tib receptor. Thus, pharmacological tools are slowly beginning to emerge to differentiate these two similar receptor
subtypes in research at least. Finally, ketanserin, a 5-HT2A receptor antagonist, is able to differentiate between 5 -H Tid and 5 -H Tib receptors, having 15-30 fold selectivity for the 5 -H Tid over the 5 -H Tib receptor (Kaumann at al., 1994; Pauwels & Golpaert, 1996).
5 -H T w receptors
The highest concentrations of central 5-HTib receptor distribution are found in basal ganglia, striatum and frontal cortex (Pazos & Palacios, 1985). However, there are many other regions known to contain an appreciable density of this receptor, including the NTS (Manaker & Verderame, 1990) and raphe nuclei (Doucet atal., 1995). As described for 5 -H Tia receptors there is a reduction in 5 -H Tib mRNA in raphe nuclei after a 5-H T neuronal lesion (Doucet ef a/., 1995). Evidence for the presence of a 5-HTib autoreceptor has been demonstrated by strong correlations between the affinity of 5-H T receptor agonists at the 5 -H Tib binding site and the potency with which these agonists inhibit 5-H T release (see Middlemiss & Hutson, 1990). Additionally, in vivo, the selective 5-HTie receptor
agonist CP-93,129 has also evoked a marked reduction in 5-H T release (Hjorth & Tao, 1991). Furthermore, there is preliminary evidence for a small population of 5 -H Tib somatodendritic autoreceptors in the raphe nuclei (Davidson and Stamford, 1995). However, the ability of 5 -H Tib receptors to act as
autoreceptors and the tonic level of their activation may well vary according to brain region (see Barnes & Sharp, 1999).
There is growing evidence that the 5-HTib receptor also acts as a
heteroreceptor in some brain regions (Bruinvels etal., 1993; Bruinvels e t a i ,
1994; see Barnes & Sharp, 1999). One electrophysiological study
demonstrated a 5 -H Tib receptor-mediated reduction in glutamate release in locus coeruleus (Bobker & Williams, 1989), whilst heteroreceptors have also been implicated in the 5-HT-induced attenuation of G A B Abreceptor-mediated IPSPs in dopamine neurones in rat midbrain in vitro (Johnson e ta l., 1992). One other microdialysis study also reported an augmentation of acetylcholine release in rat frontal cortex by endogenous 5-HT mediated by 5-HTib receptors (Consolo etal., 1996).
Interest in the 5-HTib receptor is high at the moment after the discovery of the antimigraine properties of the ‘triptans’ (e.g. sumatriptan). There are currently thought to be two components of the therapeutic effect of these compounds; by activation of 5-HTib receptors on cerebral arteries resulting in vasoconstriction, or by 5 -H Tid/if receptor mediated presynpatic inhibition of trigeminovascular inflammatory responses, reducing nociception and neurogenic inflammation (see Villalon et al., 2002). The latter of which is supported by the ability of 5-HTib selective ligands to attenuate durai plasma protein extravasation evoked by trigeminal ganglion stimulation in mice, a response absent in 5 -H Tib receptor knockout mice (Yu etal., 1996). However, the success of these compounds in the treatment of migraine is unlikely to be solely due to 5-HTib receptor
activation, but probably includes the activation of other 5-HTi receptor subtypes e.g. the 5 -H T if receptor which has been implicated due to the affinity of the triptans for this receptor subtype. 5-HTib receptors are also thought to have an
important role in a number of other physiological and pathological states, such as locomotion and hypothermia (see Barnes & Sharp, 1999).