Josep M. Casanovas

Comparative study in the rat of the actions of different types of stress on the release of 5-HT in raphe nuclei and forebrain areas.

The effects of several stress procedures on the release of 5-HT in the dorsal and median raphe nuclei (DRN and MRN, respectively) and in forebrain structures of the rat brain innervated by both nuclei have been studied using intracerebral microdialysis. Handling for 30 sec, a saline injection and forced swimming for 5 min elevated significantly the 5-HT output in the MRN. The 5-HT output in the DRN was also enhanced by a saline injection. With regard to the forebrain structure examined, handling and forced swimming increased dialysate 5-HT in the amygdala. The injection of saline induced a slight, but significant, elevation of 5-HT in the medial prefrontal cortex. In contrast, the outflow of 5-HT was significantly reduced in the ventral hippocampus and medial prefrontal cortex following forced swimming and this effect persisted well beyond the cessation of the swim session. These results indicate that the efflux of 5-HT in the MRN appears to respond to different forms of stress, whereas that in the DRN only increases after the injection of saline. The release of 5-HT in the forebrain structures is also dependent on the type of stress procedure and the region studied.

Postsynaptic 5-HT1A receptors control 5-HT release in the rat medial prefrontal cortex

5-HTt1A receptor agonists reduce the neuronal release of 5-hydroxytryptamine (5-HT) by activation of raphe 5-HT1A autoreceptors. Using in vivo microdialysis in unanesthetized rats, we show that the local application of the selective 5-HT1A receptor agonist 8-OH-DPAT decreased the 5-HT output to approximately 50% of controls in medial prefrontal cortex (mPFC) but not in dorsal hippocampus. The decrease in 5-HT output was counteracted by the concurrent application of the selective 5-HT1A receptor antagonist WAY-100635. This agent also reversed the decrease in 5-HT output elicited by the novel 5-HT1A receptor agonist BAY x 3702 (30 microM) in mPFC and dorsal raphe nucleus. These results indicate that postsynaptic 5-HT1A receptors in mPFC also participate in the control of serotonergic activity.

The effect of the selective 5-HT1A agonists alnespirone (S-20499) and 8-OH-DPAT on extracellular 5-hydroxytryptamine in different regions of rat brain.

1 We have examined the effects of the systemic administration of the selective 5‐HT1A agonist alnespirone (S‐20499) on in vivo 5‐hydroxytryptamine (5‐HT) release in the dorsal raphe nucleus, the median raphe nucleus and four forebrain areas innervated differentially by both (dorsal striatum, frontal cortex, ventral hippocampus and dorsal hippocampus). 2 Alnespirone (0.1–3 mg kg−1, s.c.) dose‐dependently reduced extracellular 5‐HT in the six areas examined. In forebrain, the maximal reductions occurred in striatum and frontal cortex (maximal reduction to 23 and 29% of baseline, respectively). Those in dorsal and ventral hippocampus were more moderate (to ca 65% of baseline). In contrast, the decrease in 5‐HT elicited in the median raphe nucleus was more marked than that in the dorsal raphe nucleus (to ca 30 and 60% of baseline, respectively). The selective 5‐HT1A antagonist WAY‐100635 (0.5 mg kg−1, s.c.) prevented the decrease in 5‐HT induced by alnespirone (0.3 mg kg−1, s.c.) in frontal cortex. 3 8‐OH‐DPAT (0.025, 0.1 and 0.3 mg kg−1, s.c.) also reduced extracellular 5‐HT in a regionally‐selective manner (e.g., to 32% of baseline in striatum and to 69% in dorsal hippocampus at 0.1 mg kg−1, s.c.). In midbrain, 8‐OH‐DPAT reduced the dialysate 5‐HT slightly more in the median than in the dorsal raphe nucleus at all doses examined. 4 Doses of both compounds close to their respective ED50 values (0.3 mg kg−1 alnespirone, 0.025 mg kg−1 8‐OH‐DPAT) reduced 5‐HT to a comparable extent in all regions examined. However, the reductions attained at higher doses were more pronounced for 8‐OH‐DPAT. 5 These data show that the reduction of 5‐HT release elicited by alnespirone and 8‐OH‐DPAT is more important in forebrain areas innervated by 5‐hydroxytryptaminergic neurones of the dorsal raphe nucleus. This regional selectivity seems unlikely to be accounted for by differences in the sensitivity of 5‐HT1A autoreceptors controlling 5‐HT release, given the dissimilar effects of these two 5‐HT1A agonists in regions rich in cell bodies and nerve terminals. This suggests the presence of complex mechanisms of control of 5‐HT release by 5‐HT1A receptors.

Differential Effects of Ipsapirone on 5-Hydroxytryptamine Release in the Dorsal and Median Raphe Neuronal Pathways

Abstract: Serotonergic neurons of the dorsal and median raphe nuclei are morphologically dissimilar. Recent results challenge previous evidence indicating a greater inhibition of dorsal raphe neurons after 5‐hydroxytryptamine1A (5‐HT1A) autoreceptor activation. As both nuclei innervate different forebrain territories, this issue is critical to understanding the changes in brain function induced by anxiolytic and antidepressant drugs. Using microdialysis, we examined the modifications of 5‐HT release induced by the selective 5‐HT1A agonist ipsapirone in both neuronal pathways. Maximal and minimal basal 5‐HT values (in the presence of 1 µM citalopram) were 45.0 ± 4.8 fmol/fraction in the median raphe nucleus and 8.4 ± 0.4 fmol/fraction in the dorsal hippocampus. Ipsapirone (0.3, 3, and 10 mg/kg s.c.) reduced dose‐dependently 5‐HT in the two raphe nuclei and four forebrain areas. Maximal reductions (to ∼25% of predrug values) were observed in cortex and striatum and in median raphe nucleus. The effects were more moderate in dorsal and ventral hippocampus (to 66 and 50% of baseline, respectively). These results are consistent with a higher sensitivity of dorsal raphe neurons to 5‐HT1A autoreceptor activation. Yet the differential reduction of 5‐HT release in the median raphe nucleus and hippocampus suggests the presence of complex mechanisms of control of 5‐HT release in these neurons.

Control of the Serotonergic System by the Medial Prefrontal Cortex: Potential Role in the Etiology of PTSD and Depressive Disorders

The prefrontal cortex is involved in an array of higher brain functions that are altered in psychiatric disorders. Serotonergic neurons of the midbrain raphe nuclei innervate the prefrontal cortex and are the cellular target for drugs used to treat mood disorders such as the selective serotonin (5-HT) reuptake inhibitors.Anatomical evidence supports the existence of projections from the media prefrontal cortex (mPFC) to the dorsal raphe nucleus (DR). We report on a functional control on the activity of DR 5-HT neurons by projection neurons in the mPFC. The stimulation of the mPFC elicits two types of responses in DR 5-HT neurons, orthodromic excitations and inhibitions. Excitations are mediated by AMPA/KA and NMDA receptors, whereas inhibitions are mediated by GABAA and 5-HT1A receptors. The activation of a subgroup of 5-HT neurons increases 5-HT release that subsequently activates 5-HT1A autoreceptors on other 5-HT neurons. GABAA-mediated inhibitions involve GABAergic elements in the DR or adjacent areas.Pyramidal neurons of the mPFC co-express postsynaptic 5-HT1A (inhibitory) and 5-HT2A (excitatory) receptors. Consistent with the above observations, the selective activation of both receptors in mPFC reduced and increased, respectively, the firing activity of DR 5-HT neurons and the 5-HT release in mPFC. Overall, these data indicate that the activity of the 5-HT system is strongly controlled by the mPFC. Thus, the abnormal prefrontal function in post-traumatic stress disorder and depressive patients may induce a disregulation of 5-HT neurons projecting to other brain areas that can underlie the existing symptomatology in these psychiatric disorders.

Differential regulation of somatodendritic serotonin 5-HT1A receptors by 2-week treatments with the selective agonists alnespirone (S-20499) and 8-hydroxy-2-(Di-n-propylamino)tetralin: microdialysis and autoradiographic studies in rat brain

Abstract : Single treatment with the serotonin (5‐hydroxytryptamine) 5‐HT1A receptor agonists 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin (8‐OH‐DPAT) and alnespirone (S‐20499) reduces the extracellular 5‐HT concentration (5‐HText) in the rat midbrain and forebrain. Given the therapeutic potential of selective 5‐HT1A agonists in the treatment of affective disorders, we have examined the changes in 5‐HT1A receptors induced by 2‐week minipump administration of alnespirone (0.3 and 3 mg/kg/day) and 8‐OH‐DPAT (0.1 and 0.3 mg/kg/day). The treatment with alnespirone did not modify baseline 5‐HText but significantly attenuated the ability of 0.3 mg/kg s.c. alnespirone to reduce 5‐HText in the dorsal raphe nucleus (DRN) and frontal cortex. In contrast, the ability of 8‐OH‐DPAT (0.025 and 0.1 mg/kg s.c.) to reduce 5‐HText in both areas was unchanged by 8‐OH‐DPAT pretreatment. Autoradiographic analysis revealed a significant reduction of [3H]8‐OH‐DPAT and [3H]WAY‐100635 {3H‐labeled N‐[2‐[4‐(2‐methoxyphenyl)‐1‐piperazinyl]ethyl]‐N‐(2‐pyridyl)cyclohexanecarboxamide · 3HCl} binding to somatodendritic 5‐HT1A receptors (but not to postsynaptic 5‐HT1A receptors) of rats pretreated with alnespirone but not with 8‐OH‐DPAT. In situ hybridization analysis revealed no change of the density of the mRNA encoding the 5‐HT1A receptors in the DRN after either treatment. These data indicate that continuous treatment for 2 weeks with alnespirone, but not with 8‐OH‐DPAT, causes a functional desensitization of somatodendritic 5‐HT1A receptors controlling 5‐HT release in the DRN and frontal cortex.

Two actions are better than one : avoiding self-inhibition of serotonergic neurones enhances the effects of serotonin uptake inhibitors

The serotonin (5-HT)-increasing action of 5-HT uptake or monoamine oxidase inhibitors is limited by a negative feedback at somatodendritic level. The excess 5-HT produced by these antidepressant drugs in the interstitial space of the midbrain raphe activates somatodendritic 5-HT1A autoreceptors, thereby attenuating terminal 5-HT release. This effect is maximal in forebrain areas innervated by the dorsal raphe nucleus and can be prevented by the administration of non-selective [(-)pindolol, (-)tertatolol] and selective (WAY-100635) 5-HT1A antagonists. In keeping with these observations, the combined administration of selective serotonin reputake inhibitors (SSRIs) and 5-HT1A antagonists increase the cortical and striatal extracellular 5-HT concentration more than the former alone. Also, concurrent inhibition of the 5-HT and noradrenaline transporters with 20 mg/kg imipramine increases cortical extracellular 5-HT concentration more than SSRI doses which maximally block the 5-HT transporter. Moreover, the effects of fluoxetine on frontal cortex 5-HT are potentiated by a dose of desipramine that does not modify extracellular 5-HT by itself. Given the relevance of increased serotonergic transmission in the treatment of depression, these experimental data indicate that dual-action antidepressant treatments may be more effective than those which selectively inhibit the 5-HT transporter.

Strategies to Optimize the Antidepressant Action of Selective Serotonin Reuptake Inhibitors

It is beyond doubt that major depression can be treated by selectively manipulating the function of different brain neurotransmitters (1,2). Yet, of the many brain neuronal systems, the serotonin (5-hydroxytryptamine, 5-HT) system is the most common neurobiological target for such treatments. Tricyclic antidepressants (TCAs) act on 5-HT and noradrenergic (NE) neurons by inhibiting, with different potencies, transmitter reuptake (3,4), and monoamine (MAO) oxidase inhibitors (MAOIs) increase 5-HT and NE transmission by preventing their metabolism. Yet, it was not until the advent of the selective serotonin reuptake inhibitors (SSRIs) that the antidepressant potential of 5-HT transporter blockade was fully appreciated (5,6). The clinically useful SSRIs are chemically dissimilar, but share the property of selectively inhibiting the 5-HT reuptake process (Fig. 1) Unlike TCAs, the SSRIs display little affinity for aminergic receptors (7) and therefore lack the severe side effects associated with the use of the former agents. This results in both an improved quality of life for the patients and greater treatment compliance, which is compromised in some instances by the use of TCAs. It is generally recognized that the antidepressant efficacy of SSRIs is comparable to that of TCAs, although several studies have shown that the latter are more effective in severely depressed inpatients (5,6,8,9).

Control of serotonergic neurons in the dorsal raphe nucleus by the lateral hypothalamus.

Anatomical evidence indicates the presence of projections from the lateral hypothalamus to serotonergic (5-hydroxytryptamine, 5-HT) neurons of the dorsal raphe nucleus (DR). Using dual probe microdialysis and extracellular recordings in the DR, we show that the application of GABAergic agents in the lateral hypothalamus modulates the activity of 5-HT neurons in the DR. GABA and bicuculline or baclofen, applied in the lateral hypothalamus significantly reduced and increased, respectively, the 5-HT output in the DR. Likewise, the intrahypothalamic application of GABA and bicuculline reduced (14/20 neurons) and increased (8/12 neurons), respectively, the firing rate of 5-HT neurons in the DR. A smaller percentage of neurons, however, were excited by GABA (3/20) and inhibited by bicuculline (1/12). Application of tetrodotoxin in the lateral hypothalamus suppressed the local 5-HT output and reduced that in the DR. The 5-HT output in the DR increased transiently soon after darkness. The hypothalamic application of GABA attenuated and that of bicuculline potentiated this spontaneous change with an efficacy similar to that seen in light conditions. These results indicate that the lateral hypothalamus is involved in the control of 5-HT activity in the DR, possibly through excitatory (major) and inhibitory (minor) inputs.

Is the potent 5-HT1A receptor agonist alnespirone (S-20499), affecting dopaminergic systems in the rat brain?

The effects of the new methoxy-chroman 5-HT1A receptor agonist, alnespirone (S-20499), on the dopamine systems in the rat brain were assessed in vivo by means of electrophysiological and neurochemical techniques. Cumulative doses of alnespirone (0.032-4.1 mg kg(-1), i.v.) did not modify the spontaneous firing rate of dopamine neurons in the substantia nigra as well as in the ventral tegmental area. The local application of alnespirone (0.1-10 microM) by reverse microdialysis into the dorsal striatum did not affect the dopamine output but induced a moderate, although dose-independent, increase of 5-HT (5-hydroxytryptamine, serotonin) concentrations in the dialysate. As expected of a 5-HT1A receptor agonist, intraperitoneal (i.p.) administration of alnespirone at 2-32 mg kg(-1) markedly decreased 5-HT turnover in the striatum. Parallel measurements of dopamine turnover showed that alnespirone exerted no effect except at the highest dose (32 mg kg(-1), i.p.) for which a significant increase was observed. Interestingly, both alnespirone-induced reduction in 5-HT turnover and increase in dopamine turnover could be prevented by pretreatment with the selective 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexa ne carboxamide). Altogether, these data indicate that alnespirone does not exert any direct influence on central dopamine systems. The enhanced dopamine turnover due to alnespirone at high dose appeared to result from 5-HT1A receptor stimulation, further supporting the idea that this receptor type may play a key role in 5-HT-dopamine interactions in brain.