5-HT3 receptors which modulate [3H]5-HT release in the guinea pig hypothalamus are not autoreceptors

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.

The pharmacology of the neurochemical transmission in the midbrain raphe nuclei of the rat

Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain, loaded with [(3)H]serotonin ([(3)H]5-HT), superfused and the release of [(3)H]5-HT was determined at rest and in response to electrical stimulation. Compartmental analysis of [(3)H]5-HT taken up by raphe tissue indicated various pools where the neurotransmitter release may originate from these stores differed both in size and rate constant. 5-HT release originates not only from vesicles but also from cytoplasmic stores via a transporter-dependent exchange process establishing synaptic and non-synaptic neurochemical transmission in the serotonergic somatodendritic area. Manipulation of 5-HT transporter function modulates extracellular 5-HT concentrations in the raphe nuclei: of the SSRIs, fluoxetine was found 5-HT releaser, whereas citalopram did not exhibit this effect. Serotonergic projection neurons in the raphe nuclei possess inhibitory 5-HT(1A) and 5-HT(1B/1D) receptors and facilitatory 5-HT(3) receptors, which regulate 5-HT release in an opposing fashion. This observation indicates that somatodendritic 5-HT release in the raphe nuclei is under the control of several 5-HT homoreceptors. 5-HT(7) receptors located on glutamatergic axon terminals indirectly inhibit 5-HT release by reducing glutamatergic facilitation of serotonergic projection neurons. An opposite regulation of glutamatergic axon terminals was also found by involvement of the inhibitory 5-HT(7) and the stimulatory 5-HT(2) receptors as these receptors inhibit and stimulate glutamate release in raphe slice preparation, respectively, Furthermore, postsynaptic 5-HT(1B/1D) heteroreceptors interact with release of GABA in inhibitory fashion in raphe GABAergic interneurons. Serotonergic projection neurons also possess glutamate and GABA heteroreceptors; NMDA and AMPA receptors release 5-HT, whereas both GABAA and GABAB receptors inhibit somatodendritic 5-HT release. Evidence was found for reciprocal interactions between serotonergic and glutamatergic as well as serotonergic and GABAergic innervations in the raphe nuclei. Serotonergic neurons in the raphe nuclei also receive noradrenergic innervation arising from the locus coeruleus and alpha-1 and alpha-2 adrenoceptors inhibited [(3)H]5-HT release in our experimental conditions. The close relation between 5-HT transporter and release-mediating 5-HT autoreceptors was also shown by addition of L-deprenyl, a drug possessing inhibition of type B monoamine oxidase and 5-HT reuptake. L-Deprenyl selectively desensitizes 5-HT(1B) but not 5-HT(1A) receptors and these effects are not related to inhibition of 5-HT metabolism but rather to inhibition of 5-HT transporter.

The auspicious role of the 5-HT3 receptor in depression: a probable neuronal target?

The serotonergic mechanisms have been successfully utilized by the majority of antidepressant drug discovery programmes, while the search for newer targets remains persistent. The present review focused on the serotonin type-3 receptor, the only ion channel subtype in the serotonin family. Behavioural, neurochemical, electrophysiological and molecular analyses, including the results from our laboratory, provided substantial evidence that rationalizes the correlation between serotonin type-3 receptor modulation and rodent depressive-like behaviour. Nevertheless, the reports on polymorphism of serotonin type-3 receptor genes and data from clinical studies (on serotonin type-3 receptor antagonists) were insufficient to corroborate the involvement of this receptor in the neurobiology of depression. The preclinical and clinical studies that have contradicted the antidepressant-like effects of serotonin type-3 receptor antagonists and the reasons underlying such disagreement were discussed. Finally, this critical review commended the serotonin type-3 receptor as a candidate neuronal antidepressant drug target.

Important messages in the 'post': recent discoveries in 5-HT neurone feedback control

The neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) mediates important brain functions and contributes to the pathophysiology and successful drug treatment of many common psychiatric disorders, especially depression. It is established that a key mechanism involved in the control of 5-HT neurones is feedback inhibition by presynaptic 5-HT autoreceptors, which are located on 5-HT cell bodies and nerve terminals. However, recent experiments have discovered an unexpected complexity of 5-HT neurone control, specifically in the form of postsynaptic 5-HT feedback mechanisms. These mechanisms have the physiological effects of 5-HT autoreceptors but use additional 5-HT receptor subtypes and operate through neural inputs to 5-HT neurones. A postsynaptic feedback system that excites 5-HT neurones has also been reported. This article discusses current knowledge of the pharmacology and physiology of these new found 5-HT feedback mechanisms and considers their possible contribution to depression pathophysiology and utility as a resource of novel antidepressant drug strategies.

5-HT3A receptor subunits in the rat medial nucleus of the solitary tract: subcellular distribution and relation to the serotonin transporter

The 5-hydroxytryptamine 3 (5HT3) receptor is a serotonin-gated ion channel implicated in reflex regulation of autonomic functions within the nucleus of the solitary tract (NTS). To determine the relevant sites for 5-HT3 receptor mediated transmission in this region, we used electron microscopic immunocytochemistry to examine the subcellular distribution of the 5HT3 receptor subunit A (5HT3A) in relation to the serotonin transporter (SERT) in the intermediate medial NTS (mNTS) of rat brain. The 5HT3A immunolabeling was detected in many axonal as well as somatodendritic and glial profiles. The axonal profiles included small axons and axon terminals in which the 5HT3A immunoreactivity was localized to membranes of synaptic vesicles and extrasynaptic plasma membranes. In dendrites and glia, the 5HT3A immunoreactivity was located on the plasma membranes or in association with membranous cytoplasmic organelles. The dendritic plasmalemmal 5HT3A labeling was prominent within and near excitatory-type synapses from terminals including those that resemble vagal afferents. The 5HT3A-labeled glial processes apposed 5HT3A-immunoreactive axonal and dendritic profiles, some of which also contained SERT. Terminals containing 5-HT3A and/or SERT were among those providing synaptic input to 5HT3A-labeled dendrites. Thus, 5HT3A has a subcellular distribution consistent with the involvement of 5-HT3 receptors in modulation of both presynaptic release and postsynaptic responses of mNTS neurons, some of which are serotonergic. The results further suggest that the neuronal as well as glial 5HT3 receptors can be activated by release of serotonin from presynaptic terminals or by diffusion facilitated by SERT distribution at a distant from the synapse.

Central 5-HT(3) receptors and water intake in rats

In the present paper, we studied in rats the effect of third ventricle administration of m-chlorophenylbiguanide hydrochloride (1-(3-chlorophenyl)biguanide (m-CPBG), a selective 5-HT(3) agonist, on water intake induced by three different physiological stimuli: water deprivation, acute salt load and hypovolemia. Central acute m-CPBG injections in the doses of 80 and 160 nmol significantly reduced water intake elicited by an acute salt load. Third ventricle injections of m-CPBG in the dose of 160 nmol significantly inhibited water intake in hypovolemic animals, whereas third ventricle injections of m-CPBG in a higher dose (320 nmol) were necessary to decrease water intake in water-deprived rats. Pretreatment with 1-methyl-N-[8-methyl-8-azabicyclo(3.2.1)-oct-3-yl]-1H-indazole-3-carboxamide (LY-278,584), a selective 5-HT(3) antagonist, abolished the inhibitory effect on water intake seen after central administration of m-CPBG in all groups studied. The central administration of m-CPBG was also able to inhibit water intake induced by pharmacological activation of central cholinergic and angiotensinergic pathways. Third ventricle injections of m-CPBG in the highest dose employed in this study (320 nmol) were unable to modify food intake in food-deprived rats. An aversion test has shown that acute third ventricle injections of m-CPBG do not induce illness-like effects that could explain the water intake inhibition here observed. Also, central administration of m-CPBG did not modify the intake of a "dessert" meal consisting of diluted condensed milk. It is concluded that central 5-HT(3) receptor activation exerts a specific inhibitory effect on water intake.

Feedback stimulation of somatodendritic serotonin release: a 5-HT3 receptor-mediated effect in the raphe nuclei of the rat

Slices from rat midbrain containing the raphe nuclei and from hippocampus were prepared, loaded with [3H]5-HT and superfused and the resting and the electrically stimulated [3H]5-HT release was measured. The 5-HT3 receptor agonist 2-methyl-5-HT (1 to 10 micromol/l) increased the resting tritium outflow in superfused raphe nuclei slices, EC50 5.3 micromol/l. The 2-methyl-5-HT-induced increase of tritium outflow was an external Ca2+-independent process and was not altered by reserpine pretreatment but it was reversed by addition of the 5-HT uptake inhibitor fluoxetine (1 micromol/l). The 5-HT3 receptor antagonists ondansetron and GYKI-46 903 (1 micromol/l) did not antagonize the stimulatory effect of 2-methyl-5-HT on resting tritium outflow. 2-Methyl-5-HT in lower concentration increased the electrically induced tritium overflow from raphe nuclei slices (EC50 0.56 micromol/l) and also from hippocampal slices preloaded with [3H]5-HT. These effects were reversed by 1 micromol/l of ondansetron and GYKI-46903. The 5-HT3 receptor antagonists (1 micromol/l) were without effects on depolarization-evoked [3H]5-HT release at 2 Hz stimulation, when 10 Hz stimulation was used, ondansetron and GYKI-46 903 reduced the tritium overflow from raphe nuclei slices. These data indicate that 5-HT3 receptors positively alter depolarization-induced somatodendritic 5-HT release in the raphe nuclei. They also show that 2-methyl-5-HT is able to evoke 5-HT release not only from vesicles but also from cytoplasmic stores via a transporter-dependent exchange process.

Modulation of fast synaptic transmission by presynaptic ligand-gated cation channels

There is now considerable evidence demonstrating that ligand-gated cation channels (i.e., P2X, nicotinic, kainate, NMDA, AMPA and 5-HT(3) receptors), in addition to mediating fast excitatory neurotransmission, may be located presynaptically on nerve terminals in the peripheral and central nervous systems where they function to modulate neurotransmitter release. This modulation can be facilitation, inhibition or both. In this article, we first outline the multiple mechanisms by which activation of presynaptic ligand-gated cation channels can modulate spontaneous and evoked neurotransmitter release, before reviewing in detail published electrophysiological studies of presynaptic P2X, nicotinic, kainate, NMDA, AMPA and 5-HT(3) receptors.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Presynaptic ionotropic receptors and the control of transmitter release

The quantity of neurotransmitter released into the synaptic cleft, the reliability with which it is released, and the response of the postsynaptic cell to that transmitter all contribute to the strength of a synaptic connection. The presynaptic nerve terminal is a major regulatory site for activity-dependent changes in synaptic function. Ionotropic receptors for the inhibitory amino acid GABA, expressed on the presynaptic terminals of crustacean motor axons and vertebrate sensory neurons, were the first well-defined mechanism for the heterosynaptic transmitter-mediated regulation of transmitter release. Recently, presynaptic ionotropic receptors for a large range of transmitters have been found to be widespread throughout the central and peripheral nervous systems. In this review, we first consider some general theoretical issues regarding whether and how presynaptic ionotropic receptors are important regulators of presynaptic function. We consider the criteria that should be met to identify a presynaptic ionotropic receptor and its regulatory function and review several examples of presynaptic receptors that meet at least some of those criteria. We summarize the classic studies of presynaptic inhibition mediated by GABA-gated Cl channels and then focus on presynaptic nicotinic ACh receptors and presynaptic glutamate receptors. Finally, we briefly discuss evidence for other types of presynaptic ionotropic receptors.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

5-HT-moduline, a 5-HT(1B/1D) receptor endogenous modulator, interacts with dopamine release measured in vivo by microdialysis

5-Hydroxytryptamine-moduline (5-HT-moduline) is an endogenous tetrapeptide (Leu-Ser-Ala-Leu) recently isolated and characterized from mammalian brain. This compound interacts with 5-HT1B receptors as a non-competitive, high-affinity antagonist and has the properties of an allosteric modulator. 5-HT-moduline could play an important role in the regulation of serotonergic transmission and also, through heteroreceptors, dopaminergic transmission. The aim of this work was to examine the potential ability of 5-HT-moduline to modify the basal extracellular concentration of dopamine and its metabolites (3-methoxytyramine, dihydroxyphenylacetic acid and homovanillic acid), in the rat striatum and to determine its potential interaction with the stimulating activity of a specific 5-HT1B receptor agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo [3,2-b] pyrid-5-one (CP-93,129), on the release of dopamine. The technique is based on in vivo microdialysis using probes implanted in the striatum of the conscious rat. Results showed that the perfusion of 5-HT-moduline directly into this structure (1.25 mM) increased the striatal level of dopamine by two-fold (104% of the absolute basal release values, P = 0.0015) and that of 3-methoxytyramine by 3-fold (293%, P = 0.0001) without any change in the terminal metabolite concentrations. The intrastriatal administration of CP-93,129 induced a statistically significant, dose-dependent increase of dopamine levels (P < 0.0001). Coperfusion of 5-HT-moduline did not significantly alter the effect of CP-93,129 at 0.1 and 0.5 mM, but appeared to have an additive effect on the lowest dose (P = 0.0406). The results obtained show that 5-HT-moduline directly administered into the striatum increases the release of dopamine in this area. Presumably, this effect results from the desensitization of 5-HT1B receptors located on dopamine terminals. However, the fact that a 5-HT1B receptor agonist (CP-93,129) also increased the release of dopamine in the striatum and that 5-HT-moduline exhibited a slight additive effect with that of a low concentration of CP-93,129 suggests that the two substances interact with different mechanisms.

5-HT3 serotonin hetero-receptors inhibit [3H]acethylcholine release in rat cortical synaptosomes

The present study was designed to verify the presynaptic localization of 5-HT3 serotonin receptors on cholinergic, serotonergic and dopaminergic nerve endings in rat brain regions where they have been shown to modulate the release of these neurotransmitters. We measured the effect of 5-HT3 agonists on [3H] neurotransmitter release from superfused synaptosomes as a functional assay of the presence of 5-HT3 serotonin receptors. m-Cl-phenylbyguanide (m-Cl-PBG, 1 microM) inhibited by 18% depolarization-evoked [3H]acethylcholine (ACh) release from cortical synaptosomes, and this effect was blocked by a potent and selective 5-HT3 antagonist based on the arylpiperazine skeleton (VC 135, 0.03 microM). Ondansetron (0.1 microM) per se had an inhibitory effect as well, thus making it difficult to evaluate its interaction with m-Cl-PBG. Up to 10 microM, m-Cl-PBG did not affect [3H]dopamine release in striatum, nucleus accumbens and frontal cortex. A similar, although not significant inhibition (16%) of [3H]ACh release, was obtained with 2-methylserotonin (10 microM), which, at this concentration, did not modify either basal or depolarization-induced release of [3H]serotonin in hippocampus or [3H]dopamine in striatum. IN conclusion, our data suggest that 5-HT3 hetero-receptors are located on cortical nerve endings where they directly inhibit acethylcholine release, but they do not seem to be located on serotonergic and dopaminergic nerve endings in the brain regions studied, probably having an indirect effect on these neurotransmitters release in rat brain.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Functional characterization of 5-HT1D autoreceptors on the modulation of 5-HT release in guinea-pig mesencephalic raphe, hippocampus and frontal cortex

1. The aims of the present study were (i) to characterize further the pharmacology of 5-HT1D autoreceptors modulating 5-HT release in guinea-pig mesencephalic raphe, hippocampus and frontal cortex; (ii) to determine whether 5-HT1D receptors in the mesencephalic raphe are located on 5-HT neurones; (iii) to determine whether 5-HT1D autoreceptors are coupled to G proteins; and (iv) to assess their sensitivity following long-term 5-HT reuptake blockade and inhibition of type-A monoamine oxidase. 2. In mesencephalic raphe, hippocampus and frontal cortex slices, the 5-HT1D/1B receptor agonist, sumatriptan and the 5-HT1 receptor agonist, 5-methoxytryptamine (5-MeOT) but not the 5-HT1B receptor agonist, CP93129, inhibited electrically the evoked release of [3H]-5-HT in a concentration-dependent manner. This effect was antagonized by the 5-HT1D/1B receptor antagonist GR127935 in the three structures, but not by the 5-HT1A receptor antagonist, (+)-WAY100635 in mesencephalic raphe slices. These results confirm the presence of functional 5-HT1D autoreceptors controlling 5-HT release within the mesencephalic raphe as well as in terminal regions. 3. The inhibitory effect of sumatriptan on K(+)-evoked release of [3H]-5-HT was not reduced by the addition of the Na+ channel blocker, tetrodotoxin to the superfusion medium, suggesting that these 5-HT1D receptors in the mesencephalic raphe are located on 5-HT neurones and may be considered autoreceptors. 4. The in vitro treatment with the alkylating agent N-ethylmaleimide (NEM) was used to determine whether these 5-HT1D autoreceptors are coupled to G proteins. The inhibitory effect of sumatriptan on electrically evoked release of [3H]-5-HT was attenuated in NEM-pretreated slices from mesencephalic raphe, hippocampus and frontal cortex, indicating that the 5-HT1D autoreceptors activated by sumatriptan are coupled to G proteins in these three structures. Taken together with our previous results, this suggests that, in addition to the 5-HT1D autoreceptor activated by sumatriptan, another subtype of 5-HT autoreceptor is activated by 5-MeOT in the hippocampus. 5. Following a 3-week treatment with the selective 5-HT reuptake inhibitor, paroxetine (10 mg kg-1 day-1) and a 48 h washout period, the electrically evoked release of [3H]-5-HT was enhanced in mesencephalic raphe, hippocampus and frontal cortex slices. There was an attenuation of the capacity of sumatriptan to inhibit the evoked release of [3H]-5-HT from mesencephalic raphe slices but not from frontal cortex and hippocampus slices. Only in the latter structure was the suppressant effect of 5-MeOT attenuated. After a 3-week treatment with the reversible type-A monoamine oxidase inhibitor, befloxatone (0.75 mg kg-1 day-1) and 48 h washout period, the effectiveness of sumatriptan and 5-MeOT on the evoked release of [3H]-5-HT was unaltered in the same brain structures. 6. The enhancement of [3H]-5-HT release by long-term paroxetine treatment is possibly due to a desensitization of 5-HT1D autoreceptors activated by sumatriptan in mesencephalic raphe and by terminal 5-HT autoreceptors activated by 5-MeOT in hippocampus. In the case of the frontal cortex, it appears that 5-MeOT and sumatriptan may act on the same 5-HT1D autoreceptor which is not desensitized either after paroxetine or befloxatone treatment, as previously reported.

Functional interaction between serotonin and other neuronal systems: focus on in vivo microdialysis studies

In this review, the functional interactions between serotonin (5-HT) and other neuronal systems are discussed with the focus on microdialysis studies in the mammalian brain (mainly rats). 5-HT release is negatively regulated not only by somatodendritic 5-HT1A and terminal 5-HT1B (5-HT1D) autoreceptors but also by alpha 2-adrenergic and mu-opioid heteroreceptors that are located on serotonergic nerve terminals. 5-HT by itself is involved in the inhibitory effects of noradrenaline release and the facilitatory regulation of dopamine release via multiple 5-HT receptors. Acetylcholine release appears to be regulated by inhibitory 5-HT1B heteroreceptors located on cholinergic nerve terminals. Long-term treatment with 5-HT-uptake inhibitors and noradrenaline-uptake inhibitor produces desensitization of 5-HT1A autoreceptors and alpha 2-heteroreceptors, respectively, which may be related therapeutically to the delayed onset of the effects of antidepressants. Some microdialysis studies have predicted that the combination of a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist might produce much greater availability of 5-HT in the synaptic cleft in terms of much faster induction of subsensitivity of 5-HT1A autoreceptors. Clinical trials based on this hypothesis have revealed that combination therapy with a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist ameliorated the therapeutic efficacy in depressive patients. Taken together, neurochemical approaches using microdialysis can contribute not only to clarification of the physiological role of the serotonergic neuronal systems but also might be a powerful pharmacological approach for the development of therapeutic strategies.

Pre- and post-synaptic effects of the 5-HT3 agonist 2-methyl-5-HT on the 5-HT system in the rat brain

Microiontophoretic applications of 5-HT and of the 5-HT3 agonist 2-methyl-5-HT produced a current-dependent suppression of firing activity of both hippocampal (CA1 and CA3) and cortical neurons in anesthetized rats. Concomitant microiontophoretic applications of the 5-HT3 antagonists BRL 46470A and S-zacopride, as well as their intravenous injection, did not antagonize the inhibitory effect of 5-HT and 2-methyl-5-HT. In contrast, the 5-HT1A antagonist BMY 7378, applied by microiontophoresis or administered intravenously, significantly reduced the inhibitory action of 5-HT and 2-methyl-5-HT. The firing activity of dorsal raphe 5-HT neurons was also reduced by 5-HT, 2-methyl-5-HT and the 5-HT1A agonist 8-OH-DPAT applied by microiontophoresis. While BRL 46470A (0.1 and 1 mg/kg, i.v.) did not antagonize the inhibitory effect of the three 5-HT agonists on 5-HT neuronal firing activity, only that of 8-OH-DPAT was attenuated by the 5-HT1A antagonist (+) WAY 100135. R-zacopride significantly reduced the duration of suppression of firing activity of CA3 pyramidal neurons induced by the electrical stimulation of the ascending 5-HT pathway, and this reducing effect was prevented by the three 5-HT3/5-HT4 antagonists renzapride, S-zacopride and tropisetron, but not by BRL 46470A. Finally, in in vitro superfusion experiments, both BRL 46470A and S-zacopride antagonized the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [3H]-5-HT in both rat frontal cortex and hippocampus slices. These findings suggest that, in vivo, the suppressant effect of 2-methyl-5-HT on the firing activity of dorsal hippocampus pyramidal, somatosensory cortical, and dorsal raphe 5-HT neurons is not mediated by 5-HT3 receptors, but rather by 5-HT1A receptors. The attenuating effect of R-zacopride on the effectiveness of the stimulation of the ascending 5-HT pathway is not mediated by 5-HT3 receptors. In contrast, in vitro, the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [3H]5-HT in both frontal cortex and hippocampus slices is mediated by 5-HT3 receptors.

Modulation of 5-HT release in the guinea-pig brain following long-term administration of antidepressant drugs

1. The aims of the present study were to determine whether long-term 5-hydroxytryptamine (5-HT) reuptake blockade and inhibition of type-A monoamine oxidase (MAO-A) lead to an enhancement of the electrically evoked release of tritum from guinea-pig brain slices preloaded with [3H]-5-HT, and to assess the sensitivity of the terminal 5-HT1D autoreceptor, the alpha 2-adrenoceptor also located on 5-HT terminals, and the 5-HT3 receptor that modulates 5-HT release following these two types of antidepressant treatments. 2. The electrically evoked release of tritium was significantly enhanced following a 21-day treatment with the 5-HT reuptake blocker, paroxetine and the reversible MAO-A inhibitor, befloxatone, in preloaded slices of the hypothalamus, hippocampus and frontal cortex 48 h after removal of the osmotic minipumps used to deliver the drugs. 3. The inhibitory effect of the terminal 5-HT autoreceptor agonist, 5-methoxytryptamine, on the evoked release of tritium was attenuated in slices of the hypothalamus, hippocampus, but not frontal cortex, following the paroxetine treatment. In the befloxatone group, the effectiveness of 5-methoxytryptamine was unaltered in the same brain structures. 4. The sensitivity of the alpha 2-adrenoceptor on 5-HT terminals, assessed using UK 14.304, was attenuated in hypothalamus, hippocampus, but not frontal cortex slices prepared from befloxatone-treated guinea-pigs and preloaded with [3H]-5-HT. The paroxetine treatment did not alter the sensitivity of this alpha 2-adrenoceptor in the hypothalamus. 5. The sensitivity of the alpha 2-adrenoceptor on noradrenaline terminals, also assessed using UK 14.304, was not altered in hippocampus and hypothalamus slices preloaded with [3H]-noradrenaline following the long-term befloxatone treatment. 6. In frontal cortex slices, [3H]-5-HT uptake was no longer significantly attenuated after a 21-day treatment with paroxetine, whereas it was still markedly inhibited in hypothalamus slices. The enhancing effect of paroxetine on the evoked release of [3H]-5-HT in the superfusion medium was no longer evident in frontal cortex slices of the paroxetine group. These data indicate that long-term 5-HT reuptake blockade desensitized the 5-HT transporter in the frontal cortex. 7. The capacity of the 5-HT3 receptor agonist, 2 methyl-5-HT, to enhance the electrically evoked release of tritium was not altered in hypothalamus, hippocampus, and frontal cortex slices prepared from befloxatone-treated guinea-pigs, but was significantly attenuated in the paroxetine group also treated for 21 days. Following a 2-day paroxetine treatment, the enhancing effect of 2-methyl-5-HT on tritium release was unaltered in frontal cortex slices.