TIPIFICACIÓN DEL DERECHO CONSTITUCIONAL A LA

As expected, administration of pCPA on 2 consecutive days reduced the 5-HT tissue content. The reduction was in the range of previously reported values: i.e. -78% and -86% in the frontal cortex and the hypothalamus, respectively (for comparison refer to the Introduction, section 7.1). Moreover, the reduction was equivalent to those which have been reported to reduce 5-HT concentration in dialysates and to antagonise the effects of CNS agents that normally increase 5-HT dialysate levels (Pozzi et al. 1999; O’Connell et al. 1991).

In a microdialysis study, Pozzi et al. (1999) showed that administration of pCPA (300 mg/kg, 72 h before microdialysis perfusion) significantly reduced tissue 5-HT

Chapter 7: Modulation of the noradrenergic response to iocai d-amphetamine and sibutramine by the serotonergic system

content by 90 % and 5-HT dialysate by 50 %, in the prefrontal cortex. This reduction prevented the increase in 5-HT concentration in the dialysates normally produced by the 5-HT reuptake inhibitors, fluoxetine and citalopram. Consequently, the pCPA treatment used in the present study was predicted to prevent, or at least reduce, any increase in 5-HT efflux following administration of c?-amphetamine and BTS 54 354.

Administration of pCPA also reduced the noradrenaline tissue content, especially in the hypothalamus (content reduced by 23 %). However, there was no overall reduction in noradrenaline dialvsate concentration. The reduction in noradrenaline tissue content was unexpected since pCPA is regarded as a specific agent when used at the dose used in the present study. However, others, using higher doses of pCPA in order to obtain maximum depletion in 5-HT tissue content, have already reported that this 5-HT synthesis inhibitor can reduce noradrenaline tissue content, also. Reader et a l (1986) observed a reduction of 23.5 % of noradrenaline content in the neocortex, following injection of pCPA 400 mg/kg, 48 h and 24 h before the measurement.

The decrease in noradrenaline tissue content following pCPA treatment could be explained by an increase in noradrenaline release leading to the reduction of the noradrenaline stores. There could be various reasons for this. One is that 5-HT exerts a tonic inhibition of the noradrenergic system. Consequently, the abolition of negative modulation of noradrenaline release following depletion of 5-HT would increase noradrenaline release. Supporting this hypothesis, McRae-Degueurce et al. (1985) reported that lesions of different serotonergic nuclei induced a rapid increase in tyrosine hydroxylase activity (the noradrenaline biosynthetic enzyme) in the locus coeruleus, suggesting a tonic inhibitory influence on the noradrenergic system in this brain area. Also, data from Reader et al. (1986) indicated that their pCPA treatment (400 mg/kg, 48 h and 24 h before the experiment) increased the spontaneous activity of neurones in the locus coeruleus. They also observed an increase in MOPEG tissue content (+862%), one of the major metabolites of noradrenaline, which supports the hypothesis that 5-HT exerts a tonic inhibitory influence on the noradrenergic system.

Another possibility could be that a deficit in central 5-HT transmission, due to 5-HT depletion, leads to an increase in noradrenaline transmission in order to compensate for over lapping functions. Although there are few reports of parallel changes of noradrenaline and 5-HT efflux in physiological mechanisms, there is some evidence that noradrenaline and 5-HT act in concert in the modulation of sleep. In a microdialysis study in cats, Shouse et a l (2000) reported the parallel and progressive decrease of noradrenaline and 5-HT concentration in dialysates, from waking to slow-wave-sleep (SWS). While collective increases in noradrenaline and 5-HT efflux appear to promote tonic electroencephalographic (EEG) arousal during waking, reduced noradrenaline and 5-HT efflux contribute to lower tonic activity during SWS.

7.4.2/ Effects of pCPA on the noradrenergic resp onse to CNS agents

Although the noradrenaline tissue content of animals treated with pCPA was reduced, in the hypothalamus at least, there was no difference in the noradrenaline concentration in dialysates of pCPA treated and control animals. This observation suggests that the pool of noradrenaline was not sufficiently diminished to affect the extracellular concentration of noradrenaline. Also, if release rate was increased (a possibility discussed in section 7.4.1), reuptake obviously increased to keep pace with release. Supporting the present results, Mateo et al. (2000), who administered 400 mg/kg pCPA, 24 h before the experiment, did not observe any difference in basal noradrenaline efflux in control and pCPA treated animals, in the locus coeruleus and the cingulate cortex, although the 5-HT tissue content was decreased by 75%.

The pCPA treatment had no effect on the increase in noradrenaline caused by administration of the a2-antagonist, atipamezole, suggesting that 5-HT has no influence

on the noradrenaline increase caused by atipamezole. This could be because atipamezole does not affect 5-HT efflux. It is noteworthy that the effects of a2-antagonists on 5-HT

efflux in vivo remain unclear (Bengtsson et al. 2000). Although a2-antagonists are

expected to block presynaptic a2-heteroceptors situated on serotonergic nerve endings

Chapter 7: Modulation of the noradrenergic response to iocai 6-amphetamine and sibutramine by the serotonergic system

mianserin (de Boer et al 1996), and RX821002 (Gobert et a l 1998), failed to increase 5-HT efflux.

In contrast, control and pCPA treated rats responded differently to the infusion of the reuptake inhibitor, BTS 54 354. In fact, in the latter group, the noradrenaline response to BTS 54 354 was abolished in the frontal cortex. Attenuation of the noradrenaline response to CNS agents that increase 5-HT efflux, after treatment with pCPA, have been reported before. In a dual-probe microdialysis study, Mateo et al. (2000) showed that treatment with pCPA (400 mg/kg, 24 h before the experiment) abolished the increase in noradrenaline efflux seen in the locus coeruleus after local administration of the selective 5-HT reuptake inhibitor, citalopram. It also abolished the simultaneous decrease in noradrenaline efflux seen in the cingulate cortex. The authors concluded that the effects of citalopram on the noradrenergic system depended on endogenous 5-HT.

Although BTS 54 354 is not only a 5-HT reuptake inhibitor but also a potent noradrenaline reuptake inhibitor, the finding that the noradrenaline response to BTS 54 354 is abolished by pCPA treatment suggests that the effects of this drug in the frontal cortex depend on endogenous 5-HT. When BTS 54 354 is infused into this brain area, the serotonergic system seems to have an excitatory effect on the noradrenergic system. Since the increase in noradrenaline and 5-HT efflux following administration of BTS 54 354 is due to extracellular accumulation of impulse-evoked release of neurotransmitters, the present findings suggest that the facilitatory effects of 5-HT on the noradrenergic system might be mediated through receptor activation. The receptors involved in this mechanism could be located at the level of the terminals, the cell bodies or on polysynaptic loops (refer to Figure 1.2). A number of studies of the regulation of noradrenaline release by 5-HT support the hypothesis of a receptor-mediated modulation of the noradrenergic system by 5-HT. Likely candidates are the 5-HTia receptor, (Gobert

et al. 1998; Suzuki et al. 1995), the 5-HT2a receptor (Gobert and Millan 1999) and the

5-HT3 receptor, all of which exert a facilitatory control on the noradrenergic system (refer

to Figure 1.1).

In contrast, in the hypothalamus, there was no difference in the noradrenaline response to the reuptake inhibitor, BTS 54 354, in pCPA treated and control rats. These

results indicate that, in the hypothalamus, the effects of BTS 54 354 do not depend on endogenous 5-HT. Therefore, unlike the modulation of noradrenaline efflux by

0(2-adrenoceptors which does not differ in the frontal cortex and the hypothalamus (as

discussed in Chapter 5), the modulation of noradrenaline efflux by 5-HT, following local administration of BTS 54 354, depends on the brain region.

One obvious reason for these observations could be that there are regional differences in the interactions between the two neurotransmitter systems. There is some evidence supporting the hypothesis that the modulation of the serotonergic system by noradrenaline depends on the brain area. Various studies have investigated the tonic excitatory influence of ai-adrenoceptors situated on serotonergic cell bodies. Blockade of oci-adrenoceptors by prazosin suppresses DRN firing and markedly reduces 5-HT dialysate in the striatum (Rouquier et al. 1994; Lejeune et al. 1994). The presence of tti-adrenoceptors in the MRN also was suggested by the decrease in 5-HT efflux, after administration of prazosin, observed in the hippocampus, a brain area preferentially innervated by the MRN (Rouquier et al. 1994; Hjorth et al. 1995). However, the degree of noradrenergic influence on the MRN and the DRN seems to differ since prazosin decreased 5-HT levels to a smaller extent in the striatum than in the hippocampus (Rouquier et al. 1994).

Another possibility would be that the local effects of BTS 54 354 on the serotonergic system differ in the frontal cortex and the hypothalamus and so would affect the modulation of noradrenaline efflux by 5-HT, also. Although there is some evidence that local sibutramine or BTS 54 505 (the other active metabolite of sibutramine) increase 5-HT efflux in the hypothalamus, the effects of local sibutramine and its metabolites have not been investigated in the frontal cortex as yet. Therefore, regional differences in the effects of local BTS 54 354 on 5-HT efflux cannot be excluded.

p C P A treated and control animals also responded differently to the infusion of ^-amphetamine. In control rats, the noradrenaline response to 10 pM d-amphetamine declined after 2 h of perfusion in both the frontal cortex and the hypothalamus (Figure 7.2). When animals received pCPA, this attenuation was abolished in both brain regions. These results indicate that 5-HT efflux, increased by d-amphetamine, imposes a