WAY-100635 and GR127935: effects on 5-hydroxytryptamine-containing neurones

CRISPR/Cas9-mediated in vivo gene editing reveals that neuronal 5-HT1A receptors in the dorsal raphe nucleus contribute to body temperature regulation in mice

Serotonin (5-HT) in the central nervous system regulates a variety of biological functions, from the basic homeostatic control to higher brain functions, by acting on fourteen known receptor subtypes. However, it is still usually unclear which receptor subtype is responsible for a specific function due to the lack of highly selective ligands for most of these receptors. Although 5-HT receptor knockout mice are useful, the brain-wide distribution of various receptors makes it difficult to dissect receptor functions in specific and brain regions and cell types. Recent advances in CRISPR/Cas9-mediated in vivo genome editing technology may overcome this problem. In this study, we constructed a viral vector expressing a single guide (sg)RNA targeting Htr1a (sgHtr1a) and Cre recombinase under the control of a neuron-specific promoter. Injection of the viral vector into the dorsal raphe nucleus (DRN) of Cre-dependent Cas9 knock-in mice induced Cre-dependent Cas9 expression mainly in DRN serotonin and GABA neurons. Mismatch cleavage assay and Sanger sequencing showed insertion or deletion formation at the target site. 5-HT_1A receptor agonist-induced hypothermia was attenuated and antidepressant effect of a selective serotonin reuptake inhibitor (SSRI) was enhanced by microinjection of the viral vector expressing sgHtr1a into the DRN of Cre-dependent Cas9 knock-in mice. These results suggest that this in vivo CRISPR/Cas9-mediated 5-HT receptor gene knockout strategy provides a reliable and low-cost method for elucidating 5-HT receptor functions in specific cell types and brain regions. Further, we demonstrate that the neuronal 5-HT_1A receptor in the DRN regulates body temperature and antidepressant effect of SSRI.

5-HT1A Receptor-Mediated Autoinhibition and the Control of Serotonergic Cell Firing

The idea that serotonergic synaptic transmission plays an essential role in the control of mood and the pharmacotherapy of anxiety and depression is one of the cornerstones of modern biological psychiatry. As a result, there is intense interest in understanding the mechanisms controlling the activity of serotonin-synthesizing (serotonergic) neurons. One of the oldest and most durable ideas emerging from this work is that serotonergic neurons are capable of autonomously regulating their own basal firing rate. Serotonergic neurons express on their surface 5-HT1A receptors (autoreceptors) that, when activated, induce the opening of potassium channels that hyperpolarize and thereby inhibit cell firing. Activity-dependent release of serotonin within serotonergic nuclei is thought to activate these autoreceptors, thus completing an autoinhibitory feedback loop. This concept, which was originally proposed in the 1970s, has proven to be enormously fruitful and has guided the interpretation of a broad range of clinical and preclinical work. Yet, remarkably, electrophysiological studies seeking to directly demonstrate this phenomenon, especially in in vitro brain slices, have produced mixed results. Here, we critically review this work with a focus on electrophysiological studies, which directly assess neuronal activity. We also highlight recent work suggesting that 5-HT1A receptor-mediated autoinhibition may play other roles in the control of firing besides acting as a feedback regulator for the pacemaker-like firing rate of serotonergic neurons.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

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.

Effects of serotonin on the intrinsic membrane properties of layer II medial entorhinal cortex neurons

Although serotonin (5-HT) is an important neuromodulator in the superficial layers of the medial entorhinal cortex (mEC), there is some disagreement concerning its influences upon the membrane properties of neurons within this region. We performed whole cell recordings of mEC Layer II projection neurons in rat brain slices in order to characterize the intrinsic influences of 5-HT. In current clamp, 5-HT evoked a biphasic response consisting of a moderately short latency and large amplitude hyperpolarization followed by a slowly developing, long lasting, and small amplitude depolarization. Correspondingly, in voltage clamp, 5-HT evoked a robust outward followed by a smaller inward shift of holding current. The outward current evoked by 5-HT showed a consistent current/voltage (I/V) relationship across cells with inward rectification, and demonstrating a reversal potential that was systematically dependent upon the extracellular concentration of K(+), suggesting that it was predominantly carried by potassium ions. However, the inward current showed a less consistent I/V relationship across different cells, suggesting multiple independent ionic mechanisms. The outward current was mediated through activation of 5-HT(1A) receptors via a G-protein dependent mechanism while inward currents were evoked in a 5-HT(1A)-independent fashion. A significant proportion of the inward current was blocked by the I(h) inhibitor ZD7288 and appeared to be due to 5-HT modulation of I(h) as 5-HT shifted the activation curve of I(h) in a depolarizing fashion. Serotonin is thus likely to influence, in a composite fashion, the information processing of Layer II neurons in the mEC and thus, the passage of neocortical information via the perforant pathway to the hippocampus.

SB-699551-A (3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4'-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride), a novel 5-ht5A receptor-selective antagonist, enhances 5-HT neuronal function: Evidence for an autoreceptor role for the 5-ht5A receptor in guinea pig brain

This study utilised the selective 5-ht(5A) receptor antagonist, SB-699551-A (3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4'-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride), to investigate 5-ht5A receptor function in guinea pig brain. SB-699551-A competitively antagonised 5-HT-stimulated [35S]GTPgammaS binding to membranes from human embryonic kidney (HEK293) cells transiently expressing the guinea pig 5-ht5A receptor (pA2 8.1+/-0.1) and displayed 100-fold selectivity versus the serotonin transporter and those 5-HT receptor subtypes (5-HT(1A/B/D), 5-HT2A/C and 5-HT7) reported to modulate central 5-HT neurotransmission in the guinea pig. In guinea pig dorsal raphe slices, SB-699551-A (1 microM) did not alter neuronal firing per se but attenuated the 5-CT-induced depression in serotonergic neuronal firing in a subpopulation of cells insensitive to the 5-HT1A receptor-selective antagonist WAY-100635 (100 nM). In contrast, SB-699551-A (100 or 300 nM) failed to affect both electrically-evoked 5-HT release and 5-CT-induced inhibition of evoked release measured using fast cyclic voltammetry in vitro. SB-699551-A (0.3, 1 and 3 mg/kg s.c.) did not modulate extracellular levels of 5-HT in the guinea pig frontal cortex in vivo. However, when administered in combination with WAY-100635 (0.3 mg/kg s.c.), SB-699551-A (0.3, 1 or 3 mg/kg s.c.) produced a significant increase in extracellular 5-HT levels. These studies provide evidence for an autoreceptor role for the 5-ht5A receptor in guinea pig brain.

8-Piperazinyl-2,3-dihydropyrrolo[3,2-g]isoquinolines: potent, selective, orally bioavailable 5-HT1 receptor ligands

The novel 8-piperazinyl-2,3-dihydropyrroloisoquinoline template was synthesized in nine steps. The template was N-substituted to give a series of compounds showing binding to human cloned 5-HT1A, 5-HT1B and 5-HT1D receptors with pKi's greater than 9 and selectivities up to 1000-fold against other serotonin, dopamine and adrenergic receptors. Several compounds were shown to possess weak partial agonist activity in cloned receptors, which translated to antagonism in in vitro studies.

Differential autoinhibition of 5-hydroxytryptamine neurons by 5-hydroxytryptamine in the dorsal raphe nucleus

5-Hydroxytryptamine neurons in the dorsal raphe nucleus are under autoinhibitory control by endogenous 5-hydroxytryptamine. Tonic activation of 5-hydroxytryptamine 1A autoreceptors was demonstrated in awake animals, but was inconsistently observed in anaesthetized animals and slice preparations, leading to questioning of its physiological significance. We re-evaluated autoinhibition in single-unit recordings from deeply seated 5-hydroxytryptamine neurons in slices in which endogenous 5-hydroxytryptamine bioavailability was restored by supplementing its precursor L-tryptophan. In these conditions, the application of the neutral 5-hydroxytryptamine 1A receptor antagonist WAY-100635 markedly increased 5-hydroxytryptamine neuron firing. Responses to WAY-100635 in single experiments ranged from a lack of effect to a several-fold increase in firing rate, suggesting that 5-hydroxytryptamine neurons in the dorsal raphe nucleus represent a heterogeneous population regarding their susceptibility to autoinhibition by endogenous 5-hydroxytryptamine.

Differential effects of the 5-hydroxytryptamine (5-HT)1A receptor inverse agonists Rec 27/0224 and Rec 27/0074 on electrophysiological responses to 5-HT1A receptor activation in rat dorsal raphe nucleus and hippocampus in vitro

The pharmacological properties of cyclohexanecarboxylic acid, {2-[4-(2-bromo-5-methoxybenzyl)piperazin-1-yl]ethyl}-(2-trifluoromethoxyphenyl)amide (Rec 27/0224), and cyclohexanecarboxylic acid, (2-methoxy-phenyl)-{2-[4-(2-methoxyphenyl)-piperazin-1-yl]ethyl}amide (Rec 27/0074), were characterized using radioligand displacement and guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS) binding assays, as well as electrophysiological experiments, in rat hippocampal and dorsal raphe nucleus (DRN) slices. Both compounds showed a high affinity (Ki, approximately 1 nM) and selectivity (>70-fold) at human 5-hydroxytryptamine (5-HT)1A receptors versus other 5-HT receptors. In [35S]GTPgammaS binding assays on HeLa cells stably expressing human 5-HT1A receptors, Rec 27/0224 and Rec 27/0074 inhibited basal [35S]GTPgammaS binding by 44.8 +/- 1.7% (pEC50 = 8.58) and 25 +/- 2.5% (pEC50 = 8.86), respectively. In intracellularly recorded CA1 pyramidal cells, 5-HT1A (hetero)receptor-mediated hyperpolarization, elicited by 100 nM 5-carboxamidoytryptamine (5-CT), was partially antagonized by Rec 27/0224 (approximately 50%; IC50 = 18.0 nM) and Rec 27/0074 (74%; IC50 = 0.8 nM). In extracellularly recorded DRN serotonergic neurons, Rec 27/0224 and Rec 27/0074 fully antagonized the inhibition of firing caused by the activation of 5-HT1A (auto)receptors by 30 nM 5-CT with an IC50 of 34.9 nM and 16.5 nM, respectively. The antagonism had a slow time course, reaching a steady state within 60 min. Both compounds also antagonized the citalopram-elicited, endogenous 5-HT-mediated inhibition of cell firing. In conclusion, Rec 27/0224 and Rec 27/0074 exhibited inverse agonism in [35S]GTPgammaS binding assays and differential antagonistic properties on 5-HT1A receptor-mediated responses in the hippocampus but not in the DRN. Whether this differential effect is causally related to inverse agonist activity is unclear. The qualitatively different nature of the antagonism in the hippocampus versus the DRN clearly distinguishes the compounds from neutral antagonists, such as N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-2-pyridinylcyclo-hexanecarboxamide (WAY-100635).

The role of sigma receptors in depression

Behavioral models used to test potential antidepressants have shown that ligands that bind to sigma receptors possess "antidepressant-like" properties. The focus of this review is to discuss the literature concerning sigma receptors and their ligands, with respect to their antidepressants properties. In addition to the behavioral data, we discuss electrophysiological and biochemical models demonstrating sigma receptors' ability to modulate important factors in the pathophysiology of depression and/or the mechanisms of action of antidepressants such as the serotonergic neurotransmission in the dorsal raphe nucleus (DRN) and the glutamatergic transmission in the hippocampus. We also discuss the significance of these two systems in the mechanism of action of antidepressants. Sigma ligands have potential as antidepressant medications with a fast onset of action as they produce a rapid modulation of the serotonergic system in the DRN and the glutamatergic transmission in the hippocampus. As these effects of sigma ligands may produce antidepressant properties by completely novel mechanisms of action, they may provide an alternative to the antidepressants currently available and may prove to be beneficial for treatment-resistant depressed patients.

Somatodendritic autoreceptor regulation of serotonergic neurons: dependence on L-tryptophan and tryptophan hydroxylase-activating kinases

The somatodendritic 5-HT(1A) autoreceptor has been considered a major determinant of the output of the serotonin (5-HT) neuronal system. However, recent studies in brain slices from the dorsal raphe nucleus have questioned the relevance of 5-HT autoinhibition under physiological conditions. In the present study, we found that the difficulty in demonstrating 5-HT tonic autoinhibition in slice results from in vitro conditions that are unfavorable for sustaining 5-HT synthesis. Robust, tonic 5-HT(1A) autoinhibition can be restored by reinstating in vivo 5-HT synthesizing conditions with the initial 5-HT precursor l-tryptophan and the tryptophan hydroxylase co-factor tetrahydrobiopterin (BH(4)). The presence of tonic autoinhibition under these conditions was revealed by the disinhibitory effect of a low concentration of the 5-HT(1A) antagonist WAY 100635. Neurons showing an autoinhibitory response to L-tryptophan were confirmed immunohistochemically to be serotonergic. Once conditions for tonic autoinhibition had been established in raphe slice, we were able to show that 5-HT autoinhibition is critically regulated by the tryptophan hydroxylase-activating kinases calcium/calmodulin protein kinase II (CaMKII) and protein kinase A (PKA). In addition, at physiological concentrations of L-tryptophan, there was an augmentation of 5-HT(1A) receptor-mediated autoinhibition when the firing of 5-HT cells activated with increasing concentrations of the alpha(1) adrenoceptor agonist phenylephrine. Increased calcium influx at higher firing rates, by activating tryptophan hydroxylase via CaMKII and PKA, can work together with tryptophan to enhance negative feedback control of the output of the serotonergic system.

Electrophysiological evidence for the tonic activation of 5-HT(1A) autoreceptors in the rat dorsal raphe nucleus

Serotonin (5-hydroxytryptamine, 5-HT) and norepinephine (NE) neurons have reciprocal connections. These may thus interfere with anticipated effects of selective pharmacological agents targeting these neurons. The main goal of the present study was to assess whether the somatodendritic 5-HT(1A) autoreceptor is tonically activated by endogenous 5-HT in anesthetised rats, using in vivo extracellular unitary recordings. In rats with their NE neurons lesioned using 6-hydroxydopamine (6-OHDA) and in controls administered the NE reuptake inhibitor desipramine to suppress NE neuronal firing, the alpha2-adrenoceptor agonist clonidine no longer inhibited 5-HT neuron firing, therefore indicating the important modulation of the firing activity of 5-HT neurons by NE neurons. In control rats, the administration of the potent and selective 5-HT(1A) receptor antagonist WAY 100,635 ((N-[2-[4(2-methoxyphenyl)-1-piperazinyl]ethy]-N-(2-pyridinyl)cyclohexanecarboxamide trihydroxychloride) (100 microg/kg, i.v.) did not modify the spontaneous firing activity of 5-HT neurons, but in NE-lesioned rats using either 6-OHDA or DSP-4, WAY 100,635 produced a mean firing increase of 80 and 69%, respectively. When desipramine and D-amphetamine were used in control rats to prevent alterations in the availability of NE in the dorsal raphe, again WAY 100,635 produced a significant disinhibition of the firing of 5-HT neurons (83 and 53%, respectively). These data support the notion that the NE system tonically activates the firing activity of 5-HT neurons. When the fluctuations of the function of NE neurons normally produced by WAY 100,635 were prevented, a tonic activation of 5-HT(1A) autoreceptors by endogenous 5-HT was unmasked.

Interleukin-1beta enhances non-rapid eye movement sleep when microinjected into the dorsal raphe nucleus and inhibits serotonergic neurons in vitro

Interleukin-1 (IL-1) and IL-1 receptors are constitutively expressed in normal brain. IL-1 increases non-rapid eye movements (NREM) sleep in several animal species, an effect mediated in part by interactions with the serotonergic system. The site(s) in brain at which interactions between IL-1 and the serotonergic system increase NREM sleep remain to be identified. The dorsal raphe (DRN) is the origin of the major ascending serotonergic pathways to the forebrain, and it contains IL-1 receptors. This study examined the hypothesis that IL-1 increases NREM sleep by acting at the level of the DRN. IL-1beta (0.25 and 0.5 ng) was microinjected into the DRN of freely behaving rats and subsequent effects on sleep-wake activity were determined. IL-1beta 0.5 ng increased NREM sleep during the first 2 h post-injection from 33.5 +/- 3.7% after vehicle microinjection to 42.9 +/- 3.0% of recording time. To determine the effects of IL-1beta on electrophysiological properties of DRN serotonergic neurons, intracellular recordings were performed in a guinea-pig brain stem slice preparation. In 26 of 32 physiologically and pharmacologically identified serotonergic neurons, IL-1beta superfusion (25 ng/mL) decreased spontaneous firing rates by 50%, from 1.6 +/- 0.2 Hz (before IL-1beta superfusion) to 0.8 +/- 0.2 Hz. This effect was reversible upon washout. These results show that IL-1beta increases NREM sleep when administered directly into the DRN. Serotonin enhances wakefulness and these novel data also suggest that IL-1beta-induced enhancement of NREM sleep could be due in part to the inhibition of DRN serotonergic neurons.

Suppression of the micturition reflex in urethane-anesthetized rats by intracerebroventricular injection of WAY100635, a 5-HT(1A) receptor antagonist

The influence of supraspinal 5-HT(1A) receptors on reflex bladder activity was evaluated in anesthetized rats by studying the effects of intracerebroventricular (i.c.v.) administration of WAY100635 (1-100 microg), a selective 5-HT(1A) receptor antagonist. The drug dose-dependently decreased the frequency and/or amplitude of isovolumetric reflex bladder contractions. Low doses (1-10 microg) increased the interval between contractions but only slightly reduced the amplitude of the contractions. However, 100 microg of WAY100635 elicited an initial complete block of bladder reflexes followed by a recovery period lasting 10-15 min during which the frequency of reflex contractions was normal but the amplitude was markedly suppressed by 70-80%. Mesulergine (0.1 mg/kg, i.v.), a 5-HT(2C) antagonist, which transiently eliminated bladder activity in some rats (five of 11), blocked the inhibitory effect of WAY100635 (10 or 100 microg, i.c.v.) in only two of six rats. Our data coupled with the results of previous studies suggest that spinal and supraspinal 5-HT(1A) receptors are involved in multiple inhibitory mechanisms controlling the spinobulbospinal micturition reflex pathway. The regulation of the frequency of bladder reflexes is presumably mediated by a suppression of afferent input to the micturition switching circuitry in the pons, whereas the regulation of bladder contraction amplitude may be related to an inhibition of the output from the pons to the parasympathetic nucleus in the spinal cord.

Rate-independent inhibition of 5-HT release by 5-HT in the somadendritic regions of raphe neurons

The somadendritic regions of raphe neurons respond to exogenous 5-hydroxytryptamine (5-HT) with an inhibition of spontaneous rate and a consequent reduction in local transmitter release, providing evidence for the operation of negative feedback regulation of spontaneous rate. Experiments were done to determine if a release process for 5-HT might also operate in the somadendritic regions that is independent of negative feedback and rate regulation. Slices of rabbit brain containing medullary or midbrain raphe nuclei, were stimulated in vitro at predetermined frequencies and the efflux of 3H-transmitter determined. The stimulation-induced pattern of transmitter release was independent of frequency, pointing to the absence of feedback. Further, exogenous 5-HT (1 x 10(-6)M) depressed the release of 3H-transmitter, but the inhibition, monitored over a range of frequencies, did not reflect competition with endogenous 5-HT for receptor sites. The antagonist methiothepin (3 x 10(-6)M) attenuated the inhibitions by 5-HT but did not by itself potentiate transmitter release, as expected if feedback inhibition were operative. Labeled transmitter release was antagonized by pretreatment with fluoxetine prior to 3H-HT incubation, and was severely curtailed in a calcium deficient medium, confirming that a neuronally relevant pool of transmitter was involved. It is concluded that serotonergic somadendritic sites contain inhibitory receptors for 5-HT release that operate independently of rate regulation and feedback. These findings could explain how other transmitters, and 5-HT itself (through dendritic release of transmitter), could exert synaptic effects on serotonergic and other neurons without being promptly countermanded by a somadendritic feedback-induced rate correction.

Ligands for the investigation of 5-HT autoreceptor function

The existence of multiple 5-HT autoreceptors in the central nervous system is now firmly established and they have been pharmacologically identified as belonging to the 5-HT(1A), 5-HT(1B), and 5-HT(1D) receptor subtypes. In addition, 5-HT(1F), 5-HT(5A), and 5-HT(7) receptors remain as potential candidates for additional autoreceptors. The emergence of selective ligands, such as SB-224289 (5-HT(1B) receptor antagonist), BRL 15572 (5-HT(1D) receptor antagonist), GR 127935 (a mixed 5-HT(1B/1D) receptor antagonist), LY 334370 (5-HT(1F) receptor agonist), and SB-269970 (5-HT(7) receptor antagonist), has aided the characterisation of 5-HT autoreceptors and has highlighted the complexity of mechanisms which modulate the release of 5-HT.

SB-272183, a selective 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptor antagonist in native tissue

A novel compound, SB-272183 (5-Chloro-2, 3-dihydro-6-[4-methylpiperazin-1-yl]-1[4-pyridin-4-yl]napth-1-ylaminocarbonyl]-1H-indole), has been shown to have high affinity for human 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors with pK(i) values of 8.0, 8.1 and 8.7 respectively and is at least 30 fold selective over a range of other receptors. [(35)S]-GTPgammaS binding studies showed that SB-272183 acts as a partial agonist at human recombinant 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors with intrinsic activities of 0.4, 0.4 and 0.8 respectively, compared to 5-HT. SB-272183 inhibited 5-HT-induced stimulation of [(35)S]-GTPgammaS binding at human 5-HT(1A) and 5-HT(1B) receptors to give pA(2) values of 8.2 and 8.5 respectively. However, from [(35)S]-GTPgammaS autoradiographic studies in rat and human dorsal raphe nucleus, SB-272183 did not display intrinsic activity up to 10 microM but did block 5-HT-induced stimulation of [(35)S]-GTPgammaS binding. From electrophysiological studies in rat raphe slices in vitro, SB-272183 did not effect cell firing rate up to 1 microM but was able to attenuate (+)8-OH-DPAT-induced inhibition of cell firing to give an apparent pK(b) of 7.1. SB-272183 potentiated electrically-stimulated [(3)H]-5-HT release from rat and guinea-pig cortical slices at 100 and 1000 nM, similar to results previously obtained with the 5-HT(1B) and 5-HT(1D) receptor antagonist, GR127935. Fast cyclic voltammetry studies in rat dorsal raphe nucleus showed that SB-272183 could block sumatriptan-induced inhibition of 5-HT efflux, with an apparent pK(b) of 7.2, but did not effect basal efflux up to 1 microM. These studies show that, in vitro, SB-272183 acts as an antagonist at native tissue 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors.

Electrophysiological evidence for postsynaptic 5-HT(1A) receptor control of dorsal raphe 5-HT neurones

Postsynaptic 5-hydroxytryptamine(1A) (5-HT(1A)) receptors have been proposed to participate in the control of dorsal raphe 5-HT neurone activity. To further investigate this hypothesis we performed single-unit extracellular recordings in anaesthetized rats. Pertussis toxin (2 microg/4 microl/day; 2 days, 24-72 h before the experiment) was applied close to the dorsal raphe nucleus to uncouple somatodendritic 5-HT(1A) autoreceptors from their effector system. After this treatment the spontaneous firing rate was higher (approximately +60% P<0.005) than in the vehicle-pretreated group. In addition, intravenous administration of 8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT) inhibited 5 out of 11 cells of the pertussis toxin-pretreated group (ED(50)=1.65+/-0.94 microg/kg), whereas in the vehicle-pretreated group, all tested cells were inhibited (ED(50)=1.87+/-0.39 microg/kg). Local administration of 8-OH-DPAT did not affect cells (n=12) in pertussis toxin-pretreated rats, even at doses much higher than those needed to completely inhibit 5-HT cells in vehicle-pretreated rats (ED(50)=3.34+/-0.62 fmol). These results confirm the involvement of distal postsynaptic 5-HT(1A) receptors in the control of 5-HT neurone activity in the dorsal raphe nucleus. However, this control does not appear to be exerted on all 5-HT neurones, but rather on a subpopulation of them.

Effects of WAY100635, a selective 5-HT1A-receptor antagonist on the micturition-reflex pathway in the rat

5-Hydroxytryptamine (5-HT) receptors in the central nervous system have been implicated in the control of micturition. The present study was undertaken to evaluate the effects of a selective 5-HT1A-receptor antagonist [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY100635)] on the micturition-reflex pathway in urethane-anesthetized female Wistar rats. Rhythmic isovolumetric bladder contractions evoked by bladder distension were abolished by 0.3- to 3-mg/kg iv or 30- to 100-microg intrathecal (it) administration of WAY100635 in a dose-dependent manner for periods of 3-15 min. Intrathecal injection of WAY100635 was effective only if injected at the L6-S1 spinal cord level, but not at the thoracic or cervical cord levels. WAY100635 (30-100 microg it) significantly reduced the amplitude of bladder contractions evoked by electrical stimulation of the pontine micturition center. However, the field potentials in the rostral pons evoked by electrical stimulation of pelvic nerve were not affected by intrathecal or intravenous injection of WAY100635. These results suggest that 5-HT1A receptors at the L6-S1 level of the spinal cord have an important role in the tonic control of the descending limb of the micturition-reflex pathway in the rat.

5-HT1A and 5-HT2 receptors differentially regulate the excitability of 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro

We have used intracellular recording techniques to examine the effects of 5-hydroxytryptamine (5-HT, serotonin) on 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro. Bath-applied 5-HT (30-300 microM) had two opposing effects on the membrane excitability of these cells, reflecting the activation of distinct 5-HT receptor subtypes. As demonstrated previously in the rat, 5-HT evoked a hyperpolarization and inhibition of 5-HT neurones, which appeared to involve the activation of an inwardly rectifying K(+) conductance. This hyperpolarizing response was blocked by the 5-HT(1A) receptor-selective antagonist WAY-100635 (30-100 nM). In the presence of WAY-100635, 5-HT induced a previously unreported depolarizing, excitatory response of these cells, which was often associated with an increase in the apparent input resistance of the neurone, likely due to the suppression of a K(+) conductance. Like the hyperpolarizing response to 5-HT, this depolarization could be recorded in the presence of the Na(+) channel blocker tetrodotoxin. In addition, the response was not significantly attenuated by the alpha(1)-adrenoceptor antagonist prazosin (500 nM), indicating that it is not due to the release of noradrenaline, or to the direct activation of alpha(1)-adrenoceptors by 5-HT. The 5-HT(3) receptor antagonist granisetron (1 microM) and the 5-HT(4) receptor antagonist SB 204070 (100 nM) failed to reduce the depolarizing response to 5-HT; however, ketanserin (100 nM), mesulergine (100 nM) and lysergic acid diethylamide (1 microM) significantly reduced or abolished the depolarization, indicating that this effect of 5-HT is mediated by 5-HT(2) receptors.

5-HT(1A) receptor mutant mice exhibit enhanced tonic, stress-induced and fluoxetine-induced serotonergic neurotransmission

Mutant mice that lack serotonin(1A) receptors exhibit enhanced anxiety-related behaviors, a phenotype that is hypothesized to result from impaired autoinhibitory control of midbrain serotonergic neuronal firing. Here we examined the impact of serotonin(1A) receptor deletion on forebrain serotonin neurotransmission using in vivo microdialysis in the frontal cortex and ventral hippocampus of serotonin(1A) receptor mutant and wild-type mice. Baseline dialysate serotonin levels were significantly elevated in mutant animals as compared with wild-types both in frontal cortex (mutant = 0.44 +/- 0.05 n M; wild-type = 0.28 +/- 0.03 n M) and hippocampus (mutant = 0.46 +/- 0.07 n M; wild-type = 0.27 +/- 0.04 n M). A stressor known to elicit enhanced anxiety-like behaviors in serotonin(1A) receptor mutants increased dialysate 5-HT levels in the frontal cortex of mutant mice by 144% while producing no alteration in cortical 5-HT in wild-type mice. There was no phenotypic difference in the effect of this stressor on serotonin levels in the hippocampus. Fluoxetine produced significantly greater increases in dialysate 5-HT content in serotonin(1A) receptor mutants as compared with wild-types, with two- and three-fold greater responses being observed in the hippocampus and frontal cortex, respectively. This phenotypic effect was mimicked in wild-types by pretreatment with the serotonin(1A) antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (p-MPPI). These results indicate that deletion of central serotonin(1A) receptors results in a tonic disinhibition of central serotonin neurotransmission, with a greater dysregulation of serotonin release in the frontal cortex than ventral hippocampus under conditions of stress or increased interstitial serotonin levels.

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.

Corticosteroid effects on serotonin responses in granule cells of the rat dentate gyrus

Granule cells in the rat dentate gyrus contain mineralocorticoid and glucocorticoid receptors to which the adrenal hormone corticosterone binds with differential affinity. These cells also express various receptor-subtypes for serotonin (5-HT), including the 5-HT1A receptor which mediates a membrane hyperpolarization accompanied by a decrease in membrane resistance. Earlier studies have shown that removal of corticosterone by adrenalectomy, particularly in the dentate gyrus, results in enhanced expression of the 5-HT1A receptor mRNA and increased 5-HT1A receptor binding capacity. This was normalized by activation of mineralocorticoid receptors or concurrent activation of both receptor types. In the present, intracellular recording study in vitro, we examined if the altered levels of 5-HT1A receptor mRNA and protein are associated with changes in the response to 5-HT. We found that the hyperpolarization and resistance decrease induced in granule cells by a submaximal (10 microM) dose of 5-HT were unaltered 2-4 days after adrenalectomy, indicating a dissociation between corticosteroid actions on 5-HT1A receptor mRNA/protein levels and functional responses to 5-HT. Subsequent occupation of mineralocorticoid receptors in vitro significantly suppressed the 5-HT induced change in resistance, 1-4 h after steroid application. Compared to this, concurrent activation of glucocorticoid receptors led to large responses to 5-HT. This modulation by steroids was not observed with a higher dose of 5-HT (30 microM). The data suggest that with moderate amounts of 5-HT, corticosteroids affect the information flow through the dentate gyrus such that excitatory transmission is promoted with predominant mineralocorticoid receptor activation and attenuated with additional glucocorticoid receptor occupation.

Reciprocal innervation between serotonergic and GABAergic neurons in raphe nuclei of the rat

Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain in order to study serotonergic-GABAergic interaction. The slices were loaded with either [3H] serotonin or [3H]GABA, superfused and the electrically induced efflux of radioactivity was determined. The GABA(A) receptor agonist muscimol (3 to 30 microM) and the GABA(B) receptor agonist baclofen (30 and 100 microM) inhibited [3H]serotonin and [3H]GABA release. These effects of muscimol were reversed by the GABA(A) antagonists bicuculline (100 microM). The GABA(B) antagonist phaclofen (100 microM) also antagonized the baclofen-induced inhibition of [3H]serotonin and [3H]GABA release. Phaclofen by itself increased [3H]serotonin release but it did not alter [3H]GABA overflow. Muscimol (10 microM) and baclofen (100 microM) also inhibited [3H]serotonin release after depletion of GABAergic neurons by isoniazid pretreatment. These findings indicate the presence of postsynaptic GABA(A) and GABA(B) receptors located on serotonergic neurons. The 5-HT1A receptor agonist 8-OH-DPAT (0.01 to 1 microM) and the 5-HT1B receptor agonist CGS-12066A (0.01 to 1 microM) inhibited the electrically stimulated [3H]serotonin and [3H]GABA release. The 5-HT1A antagonist WAY-100135 (1 microM) was without effect on [3H]serotonin and [3H]GABA efflux by itself but it reversed the 8-OH-DPAT-induced transmitter release inhibition. During KCl (22 mM)-induced depolarization, tetrodotoxin (1 microM) did not alter the inhibitory effect of CGS-12066A (1 microM) on [3H]GABA release, it did blocked, however, the ability of 8-OH-DPAT (1 microM) to reduce [3H]GABA efflux. After depletion of raphe serotonin neurons by p-chlorophenylalanine pretreatment, CGS-12066A (1 microM) still inhibited [3H]GABA release whereas in serotonin-depleted slices, 8-OH-DPAT (1 microM) was without effect on the release. We conclude that reciprocal influence exists between serotonergic projection neurons and the GABAergic interneurons or afferents in the raphe nuclei and these interactions may be mediated by 5-HT1A/B and GABA(A/B) receptors. Both synaptic and non-synaptic neurotransmission may be operative in the 5-HTergic-GABAergic reciprocal interaction which may serve as a local tuning in the neural connection between cerebral cortex and midbrain raphe nuclei.

Control of dorsal raphé 5-HT function by multiple 5-HT(1) autoreceptors: parallel purposes or pointless plurality?

The serotonergic cells of the dorsal raphé nucleus innervate much of the forebrain and are thought to be involved in the mechanism of action of antidepressants. Dysfunction of these cells might be involved in the neural mechanisms underlying depression and suicide. The traffic in pathways emanating from the dorsal raphé nucleus is controlled by 5-HT(1) autoreceptors. Until recently it was thought that the autoreceptors in the dorsal raphé nucleus were solely of the 5-HT(1A) subtype. In this article, we discuss evidence that the situation is more complex and that multiple 5-HT(1) subtypes govern different aspects of 5-HT function in the dorsal raphé nucleus presenting new therapeutic opportunities.

The question of feedback at the somadendritic region and antidepressant drug action

Presynaptic receptor theory has been expanded to encompass the regulation of the firing rate of serotonergic neurons through negative feedback mediated by the somadendritic release of transmitter. This has encouraged hypotheses as to the mechanisms of action of several classes of antidepressants and anxiolytics. One conspicuous example is the attribution of the clinical efficacy of 5-HT uptake inhibitors, such as fluoxetine and paroxetine, to desensitization of somadendritic 5-HT autoreceptors. An examination of the available evidence, mainly observations made with agonists, antagonists, monoamine oxidase inhibitors and uptake blockers, taken along with the theoretical expectations for a negative feedback loop, and the operational characteristics of inactivation pathways, indicates that negative feedback does not function at somadendritic sites to set firing rate or transmitter density, and suggests that the process may not function at all physiologically. The attribution of the effectiveness of neuroactive drugs to desensitization of raphe 5-HT inhibitory receptors, or to other interactions with feedback, is highly speculative and unlikely.

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.

Actions of 5-HT on human neocortical neurones in vitro

Using intracellular recordings, we have studied the action of 5-hydroxytryptamine (5-HT) on slices of human temporal, occipital and frontal cortex maintained in vitro. The recordings were usually made 1.2 to 1.5 mm down from the pial surface, in or around layer III. The action of 5-HT (30-50 microM) was studied on 21 cells (from 12 individuals) which had electrophysiological characteristics of glutamatergic pyramidal neurones. 5-HT depolarised the majority (11) of these cells with a median response of 5 mV. It produced a hyperpolarising response in five neurones (median=-4 mV) and a combined hyperpolarising/depolarising response in two others. No response was detected in three cells. The depolarising response was probably mediated by reducing a resting potassium conductance. Ketanserin (0.1 and 1.0 microM) and spiperone (1 microM) reduced the response indicating that it was likely mediated by 5-HT2A receptors. The hyperpolarising response was associated with the opening of ion channels and was blocked by the selective 5-HT1A receptor antagonist WAY-100635 (100 nM). 5-HT inhibited spontaneous synaptic potentials. This effect was reduced by ketanserin (1 microM) but not by WAY-100635 (100 nM). It is concluded that human neocortical neurones in vitro can be depolarised via 5-HT2A receptors and hyperpolarised via 5-HT1A receptors.

5-HT 1B/D receptor antagonists

1. 5-Hydroxytryptamine-1B (5-HT 1B, formerly designated 5-HT 1D beta) and 5-hydroxy-tryptamine-1D (5-HT 1D, formerly designated 5-HT 1D alpha) receptors are distinct molecular entities that mediate serotonergic neurotransmission. Both are G-protein-coupled receptors without introns in their coding region, negatively coupled to adenylate cyclase; their precise function in human beings remains to be defined. In brain, they are highly enriched in the globus pallidus and the substantia nigra. 2. Presynaptic 5-HT 1B/D receptors take part in the control of the release not only of 5-HT itself, but also of other neurotransmitters-for example, acetylcholine, glutamate, dopamine, noradrenaline and gamma-aminobutyric acid. Selective blockade of central 5-HT 1B/D autoreceptors should facilitate 5-HT neurotransmission and may offer a novel approach to antidepressant therapy. Other 5-HT 1B/D receptors are located postsynaptically; those receptors may be supersensitive in the pathophysiology of obsessive-compulsive disorder and may be a potential target for its treatment. 3. Few if any ligands show selectivity for 5-HT 1B or 5-HT 1D receptors or both. Most pharmacological studies have been performed with nonselective antagonists-for example, metergoline, I-naphthylpiperazine, methiothepin, ketanserin and ritanserin. Recently, a novel series of benzanilides have been reported as the first examples of selective 5-HT 1B/D receptor antagonists. GR 127935, a representative compound of this series, displays mixed agonist-antagonist properties both in vitro and in vivo. It induces upon systemic administration in the guinea pig either an opposite (decrease) effect or a small increase (65%, 5 mg/kg) in the concentration of cortical extracellular 5-HT compared with fluoxetine (218%, 10 mg/kg). The importance of blockade of 5-HT 1B/D receptors in the raphé region, their possible interaction with 5-HT 1A receptors, and consequent inhibition of 5-HT release in terminal 5-HT 1B/D receptor-containing regions are discussed. 4. To find out whether the available so-called 5-HT 1B/D receptor antagonists are indeed antagonists and not partial agonists, efficacy was measured at recombinant human 5-HT 1B and 5-HT 1D receptor sites by using a [35S]-GTP gamma S binding assay to membrane preparations of stably transfected rat C6-glial cell lines. Metergoline and the selective 5-HT 1B/D receptor ligands GR 127935 as well as GR 125743 showed significant intrinsic activity (43% to 69%) at the 5-HT 1D receptor subtype, whereas the nonselective ligand 1-naphthylpiperazine yielded less (15% to 19%) intrinsic activity at both receptor subtypes. In contrast, the nonselective ligands methiothepin, ketanserin and ritanserin are inverse agonists because they displayed negative efficacy (-14% to -28%). Differential blockade of 5-HT 1B/D receptors by neutral antagonists and inverse agonists is discussed in relation to the 5-HT tone on 5-HT 1B/D receptors. 5. It can concluded that 5-HT 1B/D receptor ligands modulate 5-HT neurotransmission through a terminal 5-HT 1B/D receptor. Future work should be directed toward the identification of selective 5-HT 1B and 5-HT 1D receptor ligands that display either neutral antagonist or inverse agonist properties to evaluate the therapeutic potential of 5-HT 1B/D receptor blockade.

EMD 68843, a serotonin reuptake inhibitor with selective presynaptic 5-HT1A receptor agonistic properties

The 5-HT1A receptor agonist 8-hydroxy-(di-n-propylamino)tetralin (8-OH-DPAT; 0.55 mg/kg s.c.) and the 5-HT (5-hydroxytryptamine, serotonin) reuptake inhibitor/5-HT1A receptor ligand 5-[4-[4-(5-cyano-3-indolyl)-butyl]-1-piperazinyl]-benzofuran-2-carbox ami de (EMD 68843; 55 mg/kg p.o.) inhibited ultrasonic vocalization in rats, an effect which was antagonized by the 5-HT1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridyl)- cyclohexancarboxamide (WAY 100635; 1.0 mg/kg s.c.). 8-OH-DPAT decreased body temperature in rats, an effect which was also antagonized by WAY 100635, whereas EMD 68843 neither affected body temperature by itself nor interacted with 8-OH-DPAT or WAY 100635. The selective 5-HT reuptake inhibitor fluoxetine (100 mg/kg p.o.) had no effect on ultrasonic vocalization or body temperature. Therefore EMD 68843 is suggested to be a 5-HT1A receptor agonist selective for presynaptic 5-HT1A receptors.

Modulation of the firing activity of noradrenergic neurones in the rat locus coeruleus by the 5-hydroxtryptamine system

1. The aim of the present study was to investigate the putative modulation of locus coeruleus (LC) noradrenergic (NA) neurones by the 5-hydroxytryptaminergic (5-HT) system by use of in vivo extracellular unitary recordings and microiontophoresis in anaesthetized rats. To this end, the potent and selective 5-HT1A receptor antagonist WAY 100635 (N-[2-[4(2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydroxychloride) was used. 2. In the dorsal hippocampus, both local (by microiontophoresis, 20 nA) and systemic (100 micrograms kg-1, i.v.) administration of WAY 100635 antagonized the suppressant effect of microiontophorectically-applied 5-HT on the firing activity of CA3 pyramidal neurones, indicating its antagonistic effect on postsynaptic 5-HT1A receptors. 3. WAY 100635 and 5-HT failed to modify the spontaneous firing activity of LC NA neurones when applied by microiontophoresis. However, the intravenous injection of WAY 100635 (100 micrograms kg-1) readily suppressed the spontaneous firing activity of LC NA neurones. 4. The lesion of 5-HT neurones with the neurotoxin 5,7-dihydroxytryptamine increased the spontaneous firing activity of LC NA neurones and abolished the suppressant effect of WAY 100635 on the firing activity of LC NA neurones. 5. In order to determine the nature of the 5-HT receptor subtypes mediating the suppressant effect of WAY 100635 on NA neurone firing activity, several 5-HT receptor antagonists were used. The selective 5-HT3 receptor antagonist BRL 46470A (10 and 100 micrograms kg-1, i.v.), the 5-HT1D receptor antagonist GR 127935 (100 micrograms kg-1, i.v.) and the 5-HT1A/1B receptor antagonist (-)-pindolol (15 mg kg-1, i.p.) did not prevent the suppressant effect of WAY 100635 on the firing activity of LC NA neurones. However, the suppressant effect of WAY 100635 was prevented by the non-selective 5-HT receptor antagonists spiperone (1 mg kg-1, i.v.) and metergoline (1 mg kg-1, i.v.), by the 5-HT2 receptor antagonist ritanserin (500 micrograms kg-1, i.v.). It was also prevented by the 5-HT1A receptor/alpha 1D-adrenoceptor antagonist BMY 7378 (1 mg kg-1, i.v.) and by the alpha 1-adrenoceptor antagonist prazosin (100 micrograms kg-1, i.v.). 6. These data support the notion that the 5-HT system tonically modulates NA neurotransmission since the lesion of 5-HT neurones enhanced the LC NA neurones firing activity and the suppressant effect of WAY 100635 on the firing activity of NA neurones was abolished by this lesion. However, the location of the 5-HT1A receptors involved in this complex circuitry remains to be elucidated. It is concluded that the suppressant effect of WAY 100635 on the firing activity of LC NA neurones is due to an enhancement of the function of 5-HT neurones via a presynaptic 5-HT1A receptor. In contrast, the postsynaptic 5-HT receptor mediating this effect of WAY 100635 on NA neurones appears to be of the 5-HT2A subtype.

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.

Multiple 5-HT receptors in the guinea-pig superior cervical ganglion

1. We have studied the pharmacology of the depolarization by 5-hydroxytryptamine (5-HT) of the guinea-pig isolated superior cervical ganglion (SCG) using the grease-gap technique. We studied the effects of selective and non-selective antagonists on the responses to 5-HT and other 5-HT receptor agonists. 2. We have extended the pharmacology of the 5-HT3 receptor in this preparation by studying the effects of granisetron, BRL 46470 and mianserin on the concentration-response curve (CRC) to 2-methyl-5-HT. As with other 5-HT3 receptor antagonists, these compounds exhibited a lower affinity for guinea-pig 5-HT3 receptors than for rat 5-HT3 receptors. 3. We have confirmed that low concentrations of 5-HT (< or = 1 microM) mediate ketanserin-sensitive responses and higher concentrations of 5-HT also recruit 5-HT3 receptors. The responses to low concentrations of 5-HT were antagonized by low concentrations of ketanserin, spiperone, mianserin, DOI and LSD indicating probably mediation by 5-HT2A receptors. At high concentrations, the hallucinogen, DOI, but not LSD, evoked a ketanserin-sensitive depolarization. 4. Although mianserin could bind to the 5-HT2A receptors in this preparation, we could not demonstrate a down-regulation of depolarizations evoked by these receptors after a 10 day oral treatment with mianserin (10 mg kg-1, daily). 5. 5-Carboxamidotryptamine (5-CT) evoked a prolonged depolarization. Although high concentrations of 5-CT (> or = microM) appeared to activate 5-HT2A receptors, lower concentrations of 5-CT evoked a response with a distinct pharmacology. After studying the action of 20 selective and non-selective 5-HT receptor ligands we believe that this response may be mediated by a novel receptor; but its pharmacology is closest to that of receptors in the 5-HT2 receptor family. Like 5-CT, 5-HT (3-300 microM) could evoke an LSD-sensitive response in the presence of the 5-HT2 receptor antagonist, ketanserin and the 5-HT3 receptor antagonist, tropisetron (all 1 microM). 6. We conclude that 5-HT activates three pharmacologically distinct receptors to depolarize the guinea-pig SCG. Low concentrations of 5-HT appear to activate 5-HT2A receptors. Higher concentrations of 5-HT also activate 5-HT3 receptors and a possible novel 5-HT receptor. The novel receptor could be a species homologue of a 5-HT2 receptor or an, as yet, unclassified 5-HT receptor.

The role of 5-HT1D and 5-HT1A receptors in mediating 5-hydroxytryptophan induced myoclonic jerks in guinea pigs

Systemic administration of 5-hydroxytryptophan (5-HTP) to guinea pigs causes species-specific, rhythmic, whole body jerks (myoclonic jerks), the frequency and amplitude of which were measured in an automated apparatus. The brain penetrant 5-HT1D receptor agonist 3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate (SKF 99101H) (3-30 mg/kg i.p.) and the selective 5-HT1A receptor agonist (+/-)8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.3-3 mg/kg s.c.) dose dependently potentiated the frequency and intensity of myoclonic jerks caused by 5-HTP (100 mg/kg). Cotreatment of guinea pigs with 8-OH-DPAT (3 mg/kg s.c.) and SKF 99101H (30 mg/kg i.p.), which were inactive when given alone, gave a marked myoclonic jerk response. Conversely, the myoclonic jerk response to higher doses of 5-HTP (150 mg/kg i.p.) was dose dependently blocked by the 5-HT1D receptor antagonist GR 127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1 ,2,4-oxadiazol-3-yl)[1,1'-biphenyl]4-carboxamide oxalate) (ED50 0.32 mg/kg i.p.) and the selective 5-HT1A receptor antagonist WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride) (ED50 0.33 mg/kg i.p.). The response to 5-HTP (150 mg/kg i.p.) was also blocked by ritanserin (0.01-0.3 mg/kg i.p.). Our data therefore confirm previous reports concerning the effects of 5-HT2A/2C receptor blockade on 5-HTP induced myoclonic jerks and suggest that both 5-HT1D and 5-HT1A receptors play an important role in mediating this behavioural response.

Interaction between a selective 5-HT1A receptor antagonist and an SSRI in vivo: effects on 5-HT cell firing and extracellular 5-HT

1. The acute inhibitory effect of selective 5-hydroxytryptamine (serotonin) reuptake inhibitors (SSRIs) on 5-HT neuronal activity may offset their ability to increase synaptic 5-HT in the forebrain. 2. Here, we determined the effects of the SSRI, paroxetine, and a novel selective 5-HT1A receptor antagonist, WAY 100635, on 5-HT cell firing in the dorsal raphé nucleus (DRN), and on extracellular 5-HT in both the DRN and the frontal cortex (FCx). Extracellular electrophysiological recording and brain microdialysis were used in parallel experiments, in anaesthetized rats. 3. Paroxetine dose-dependently inhibited the firing of 5-HT neurones in the DRN, with a maximally effective dose of approximately 0.8 mg kg-1, i.v. WAY 100635 (0.1 mg kg-1, i.v.) both reversed the inhibitory effect of paroxetine and, when used as a pretreatment, caused a pronounced shift to the right of the paroxetine dose-response curve. 4. Paroxetine (0.8 mg kg-1, i.v.), doubled extracellular 5-HT in the DRN, but did not alter extracellular 5-HT in the FCx. A higher dose of paroxetine (2.4 mg kg-1, i.v.) did increase extracellular 5-HT in the FCx, but to a lesser extent than in the DRN. Whereas 0.8 mg kg-1, i.v. paroxetine alone had no effect on extracellular 5-HT in the FCx, in rats pretreated with WAY 100635 (0.1 mg kg-1), paroxetine (0.8 mg kg-1, i.v.) markedly increased extracellular 5-HT in the FCx. 5. In conclusion, pretreatment with the selective 5-HT1A receptor antagonist, WAY 100635, blocked the inhibitory effect of paroxetine on 5-HT neuronal activity in the DRN and, at the same time, markedly enhanced the effect of paroxetine on extracellular 5-HT in the FCx. These results may be relevant to recent clinical observations that 5-HT1A receptor antagonists in combination with SSRIs have a rapid onset of antidepressant effect.