Interaction of the antidepressant mirtazapine with alpha2-adrenoceptors modulating the release of 5-HT in different rat brain regions in vivo.

Mirtazapine (MIR) is a novel antidepressant, reported to raise extracellular noradrenaline (NA) through blockade of alpha2-autoreceptors and serotonin (5-HT) output via (1) indirect activation of facilitatory alpha1-adrenoceptors on the cell bodies of ascending 5-HT neurones and (2) blockade of presynaptic release-modulating alpha2-heteroreceptors on 5-HT terminals in the forebrain. To further assess the effect of MIR on NA/5-HT system interplay, including putative regional differences in the effects of the drug on 5-HT release in rat forebrain, we used in vivo microdialysis in anaesthetised rats. Probes were implanted in the dorsal hippocampus (DH) and frontal cortex (FCx), representing median and dorsal raphe 5-HT projection areas, respectively. In the DH, MIR (10 mg/kg s.c.) completely blocked the 5-HT release-suppressing action of the selective alpha2-adrenoceptor agonist clonidine (0.1 mg/kg s.c.), but had no effect per se on the 5-HT output. Neither drug significantly changed the 5-HT levels in the FCx. MIR perfused locally (10 microM via reverse-dialysis) also failed to significantly elevate 5-HT output, and did not affect the clonidine response in either brain area. Thus, the data confirm the basic alpha2-adrenoceptor-blocking properties of MIR, but are only partly concordant with previous studies reporting an increase of 5-HT output after MIR alone. Moreover, we find no elevation in 5-HT by the reference alpha2-adrenoceptor antagonist idazoxan (0.3-1.0 mg/kg s.c.). The discrepancies encountered, and the potential ability of alpha2-adrenoceptor antagonists in general to raise the output of 5-HT, are discussed with particular reference to methodological and other factors that may influence the experimental outcome (e.g., brain regional aspects, different alpha2-adrenoceptor subtypes, potential differences in adrenoceptor tone under varying experimental conditions).

Serotonin autoreceptor function and antidepressant drug action.

This article briefly summarizes, within the context of a brief review of the relevant literature, the outcome of our recent rat microdialysis studies on (1) the relative importance of serotonin (5-HT)1A versus 5-HT1B autoreceptors in the mechanism of action of 5-HT reuptake blocking agents, including putative regional differences in this regard, and (2) autoreceptor responsiveness following chronic SSRI administration. First, our data are consistent with the primacy of 5-HT1A autoreceptors in restraining the elevation of 5-HT levels induced by SSRIs, whereas nerve terminal 5-HT1B autoreceptors appear to have an accessory role in this regard. Second, there is an important interplay between cell body and nerve terminal 5-HT autoreceptors, and recent findings suggest that this interplay may potentially be exploited to obtain regionally preferential effects on 5-HT neurotransmission in the central nervous system, even upon systemic drug administration. In particular, emerging data suggest that somatodendritic 5-HT1A autoreceptor- and nerve terminal 5-HT1B autoreceptor-mediated feedback may be relatively more important in the control of 5-HT output in dorsal raphe-frontal cortex and median raphe-dorsal hippocampus systems, respectively. Third, 5-HT autoreceptors evidently retain the capability to limit the 5-HT transmission-promoting effect of SSRIs after chronic treatment. Thus, although the responsiveness of these sites is probably somewhat reduced, residual autoreceptor capacity still remains an effective restraint on large increases in extracellular 5-HT, even after prolonged treatment. If a further increase in extracellular 5-HT is crucial to the remission of depression in patients responding only partially to prolonged administration of antidepressants, then sustained adjunctive treatment with autoreceptor-blocking drugs may consequently prove useful in the long term.

The role of 5-HT1A autoreceptors and alpha1-adrenoceptors in the modulation of 5-HT release--III. Clozapine and the novel putative antipsychotic S 16924.

Clozapine and the novel putative, antipsychotic S 16924 ((1-(benzodioxane-5-yl)-3-[3-(4-fluorophenacyl)pyrrolidine]-1-o xapropane HCl) share significant affinity for alpha1-adrenoceptors and 5-HT1A autoreceptors in vitro and display an 'atypical' behavioural profile in in vivo models used for detecting potential neuroleptic effects. In the present study, in vivo microdialysis was used to examine the effect of clozapine and S 16924 on 5-HT overflow in the rat ventral hippocampus, and to assess the relative role of putative alpha1-adrenoceptor antagonist and 5-HT1A autoreceptor agonist properties of the drugs in this regard. S 16924 (0.1-3 mg/kg, s.c.) reduced dialysate 5-HT in a dose- and time-dependent fashion by maximally approximately 70% from baseline 40-60 min after injection. Clozapine (0.1-10 mg/kg, s.c.) reduced 5-HT overflow in the same manner, with a maximum effect of approximately 60% from baseline, obtained after 60-80 min. The 5-HT decrease elicited by S 16924 (1.0 mg/kg, s.c.) was significantly, though only partially, antagonized by pretreatment with the selective 5-HT1A receptor antagonist WAY 100635 (0.3 mg/kg, s.c.). The selective alpha1-adrenoceptor agonist cirazoline (0.02 mg/kg, i.p.) alone did not significantly attenuate the effect of S 16924 (1.0 mg/kg, s.c.) on 5-HT overflow. Combined treatment with both WAY 100635 and cirazoline, however, totally reversed the 5-HT-suppressing effect of S 16924 (1.0 mg/kg, s.c.). By comparison, when given separately, neither WAY 100635 (0.3 mg/kg, s.c.) nor cirazoline (0.02 mg/kg, i.p.) antagonized the clozapine (0.3 mg/kg, s.c.)-induced decrease of 5-HT in ventral hippocampus dialysates. In the presence of both WAY 100635 and cirazoline, the response to this dose of clozapine was however significantly, though modestly, attenuated. In contrast, the WAY 100635/cirazoline combination failed to antagonise the 5-HT decrease resulting from a higher dose (3.0 mg/kg, s.c.) of clozapine. We conclude that both alpha1-adrenoceptor antagonist and 5-HT1A receptor agonist properties of clozapine and S 16924 contribute to the 5-HT release-reducing action of these drugs. Whereas these factors apparently explain the effect of S 16924 fully, additional mechanism(s) appear to be involved in the case of clozapine. With regard to the interplay between alpha1-adrenoceptor and 5-HT1A (auto)receptor mechanisms in the control of 5-HT release in the rat forebrain, the present data suggest that an excitation mediated by the former is outweighed by the simultaneous activation of the latter-inhibitory-receptors.

WAY100635-induced augmentation of the 5-HT-elevating action of citalopram: relative importance of the dose of the 5-HT1A (auto)receptor blocker versus that of the 5-HT reuptake inhibitor.

The elevation of extracellular 5-HT after systemic administration of 5-HT reuptake inhibiting drugs is strongly potentiated by agents capable of blocking 5-HT1A autoreceptors in the midbrain raphe. The present in vivo microdialysis study was aimed at assessing the relative importance of 5-HT reuptake inhibition versus 5-HT1A autoreceptor blockade in this interaction. Citalopram (0.5 or 5.0 mg/kg s.c.) dose-dependently increased dialysate 5-HT in the rat ventral hippocampus, maximally doubling the initial baseline values within 60 min after injection. The selective 5-HT1A receptor blocker, WAY100635 (0.01-0.3 mg/kg s.c.), further augmented, in a dose-dependent manner, the high-dose citalopram response (to approximately 4-5 x the pre-citalopram baseline). For comparison, the effect of low-dose (0.5 mg/kg s.c.) citalopram was mildly, but not significantly, potentiated by WAY100635 (0.3 mg/kg). WAY100635 given alone does not alter 5-HT under these conditions. The data confirm previous findings that 5-HT1A autoreceptor blockade enhances the citalopram-induced increase of extracellular 5-HT in the forebrain. To the extent the extracellular levels of 5-HT is a valid index, through 5-HT reuptake blockade appears to be the primary prerequisite for this interaction to occur. New drugs and/or treatment regimes based on the SSRI/5-HT1A autoreceptor blocker combination concept should, therefore, emphasize the former property.

Raphe 5-HT1A autoreceptors, but not postsynaptic 5-HT1A receptors or beta-adrenoceptors, restrain the citalopram-induced increase in extracellular 5-hydroxytryptamine in vivo.

In vivo microdialysis in rat ventral hippocampus was used (i) to verify the importance of 5-HT1A autoreceptors in the raphe as targets for drugs that enhance the citalopram-induced elevation of forebrain 5-hydroxytryptamine (5-HT), and (ii) to further examine the specificity of (-)-penbutolol in this regard. The selective 5-HT1A receptor antagonist WAY100635 (s.c., or intra-raphe) or the mixed 5-HT1A/1B/beta-adrenoceptor antagonist (-)-penbutolol (s.c.), potentiated the citalopram-induced 5-HT rise, whereas local "reverse' dialysis of WAY100635 into the ventral hippocampus did not. Furthermore, the (-)-penbutolol-induced augmentation proved stereoselective and not mediated by beta-adrenoceptors (no effect of s.c. (+)-penbutolol, or beta 1- and beta 2-adrenoceptor blockers (betaxolol, ICI118.551)). These data provide direct evidence that increased stimulation of 5-HT1A autoreceptors in the midbrain raphe impedes the effect of citalopram on forebrain extracellular 5-HT, whereas neither postsynaptic 5-HT1A receptors nor beta-adrenoceptors appear to be involved.

Studies on the role of 5-HT1A autoreceptors and alpha 1-adrenoceptors in the inhibition of 5-HT release--I. BMY7378 and prazosin.

UNLABELLED: The present study utilized in vivo microdialysis to investigate the importance of 5-HT1A autoreceptors and alpha 1-adrenoceptors in the decreased 5-HT release obtained following administration of the mixed 5-HT1A autoreceptor partial agonist/alpha 1-adrenoceptor antagonist BMY7378, the selective 5-HT1A receptor agonist 8-OH-DPAT and the alpha 1-adrenoceptor antagonist prazosin. BMY7378 (0.25 mg/kg, s.c.), 8-OH-DPAT (0.025 mg/kg, s.c.) and prazosin (0.1-1.0 mg/kg, s.c.) all suppressed ventral hippocampal 5-HT efflux. The BMY7378- and 8-OH-DPAT-induced inhibition of 5-HT release were reversed by a 40 min pre-treatment with either (+/-)pindolol (8 mg/kg, s.c.) or WAY-100635 (0.3 mg/kg, s.c.), to block 5-HT1A autoreceptors. Neitehr of these antagonists altered the prazosin-induced (0.3 mg/kg, s.c.) 5-HT disease. THE RESULTS: (i) confirm that both an alpha 1-adrenoceptor antagonist (prazosin) and 5-HT1A autoreceptor stimulants (BMY7378 and 8-OH-DPAT) may reduce cerebral 5-HT release; (ii) support that the BMY7378-induced decrease in 5-HT release results from 5-HT1A autoreceptor agonism, rather than alpha 1-adrenoceptor blockade; and (iii) argue against "physiological" antagonism (i.e. via blockade of beta-adrenoceptors, 5-HT1B receptors or some other mechanism) as an explanation for the reversal by pindolol of 5-HT1A autoreceptor agonist-induced suppression of 5-HT release. These data support the usefulness of pindolol, as well as the more specific compound WAY-100635, to block 5-HT1A autoreceptors.