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The recent cloning of three types of 5-hydroxytryptamine (5-HT, serotonin) receptors substantiates radioligand-based definitions of 5-HT receptors, and provides a framework in which to understand the function and evolution of the receptors. The primary sequences determined by molecular cloning of the 5-HT1c, 5-HT1a and 5-HT2 receptors place each of these 5-HT receptor subtypes into the class of G protein-coupled receptors. These receptors all share similar functional and structural features. Each receptor is positioned in the lipid bilayer with seven membrane-spanning domains and corresponding intracellular and extracellular domains. By analogy to the known functional structures of the beta-adrenergic receptor, the binding site of 5-HT is proposed to be in the membrane domains and the intracellular domain is important for G protein interaction. The primary sequences and the second messenger systems of the receptors indicate the 5-HT2 and 5-HT1c receptors are closely related, whereas the 5-HT1a receptor is more distantly related to the 5-HT2 and 5-HT1c receptors.

Ligands of various chemical classes (e.g., indoles, indazoles, benzamides, carbazoles, and quinolines) have demonstrated high affinity for the 5-HT3 receptor in radiolabeled ligand-binding studies, and have shown 5-HT3 receptor antagonistic activity in functional assays which utilize the excitatory effects of 5-HT on enteric neurons and autonomic afferents. Several 5-HT3 antagonists are currently being evaluated for potential use in the treatment of migraine, schizophrenia, and anxiety, and a few have already demonstrated high efficacy as antiemetics in cancer chemotherapy. The purpose of this presentation is to highlight the significant structure-affinity relationships (SAFIR) and common geometrical features among 5-HT3 receptor ligands, and to describe the three-dimensional pharmacophore for the 5-HT3 recognition site derived from computational techniques. The chemical template containing the recognition elements (functional groups) for the 5-HT3 receptor are: an aromatic or heteroaromatic ring system, a coplanar carbonyl group, and a nitrogen center, interrelated by well-defined distances. Two "binding shapes" or "active shapes" for 5-HT3 ligands have been identified from detailed conformational analyses.

The 5-HT1A receptor has been one of the most studied 5-HT receptor subtypes in terms of its pharmacologic profile. Comparisons of various studies of structure-activity relationships (SAR) at this receptor shows an emerging profile for this receptor's pharmacophore. The present discussion focuses on the findings generated with relatively small molecules that can be considered as analogs of serotonin itself and that illustrate some of the structural properties that are important for high-affinity recognition by the receptor. Most of the SAR work has been based on the affinities of compounds for the receptor as determined by the radioligand-binding technique, which has a significant limitation in that it cannot define the intrinsic activity of compounds at the receptor. This problem can be addressed by functional assays, and an example of SAR at the 5-HT1A receptor-coupled adenylate cyclase system is provided.

The serotonergic neurons of the mammalian brain comprise one of the most expansive chemical systems known. The cell bodies are largely confined to the midline (raphe) region of the brain stem in two general clusters: a superior group that consists of the dorsal raphe nucleus (B-7 and B-6), median raphe nucleus (B-8 and B-5), caudal linear nucleus (rostral B-8), and supralemniscal nucleus (B-9), and an inferior group that consists of nucleus raphe obscurus (B-2), nucleus raphe pallidus (B-1), nucleus raphe magnus (B-3), ventral lateral medulla (B-1/B-3), and the area postrema. The axons from these cells project throughout the neuroaxis from the spinal cord to the olfactory bulb and from the cerebral cortex to the hypothalamus. The development of this giant system begins very early in gestation and is influenced by a variety of growth regulatory factors, including the astroglial protein S-100 beta. Evidence will be presented that the serotonergic system plays a major role in the maturation of the brain by interacting with the 5-HT1A receptors which are most dense during these early developmental periods. The 5-HT1A receptor is located on both neurons and astrocytes, and in the latter cells may serve to stimulate release of S-100 beta. The developmental role of 5-HT appears to become dormant as the brain matures, and during aging and Alzheimer's disease, 5-HT receptors are significantly depressed. However, specific damage to 5-HT fibers in the adult brain by 5,7-dihydroxytryptamine produces a sharp fall in the levels of 5-HT which seems to reactivate the developmental signals in the brain. Not only are the serotonergic fibers encouraged to sprout and expand their territory, but the stimulation of the astrocytic growth factor by a 5-HT1A agonist is reinstated. The ability to recall developmental processes in the adult brain by interrupting the 5-HT fibers may provide important tools for understanding and treating the aged brain.

Molecular modeling studies are useful in as much as they may allow us to understand the activity and selectivity of currently existing agents, and, furthermore, may aid in the design of completely novel therapeutic agents. There are two basic modeling strategies: the ligand-ligand approach and the ligand-receptor approach. Both approaches possess certain inherent advantages and disadvantages and, in addition, make certain assumptions about the agents and/or receptors being investigated. Keeping with the spirit of this minisymposium, we describe these two approaches, their general usefulness, and their limitations. Using serotonin (5-HT) receptors as a focal point, we review and provide novel examples of molecular modeling studies involving both strategies. Presented for the first time are examples of ligand-receptor models to account for the binding of serotonergic agents at 5-HT2 and 5-HT1C receptors.

Three pharmacologically distinct high-affinity [3H]serotonin ([3H]5-HT) binding sites were identified in spinal cord synaptosomes. [3H]5-HT competition studies using selective 5-HT1A receptor ligands indicated that approximately 25% of high-affinity synaptosomal [3H]5-HT binding was inhibited by 5-HT1A-selective compounds, an estimate consistent with [3H](+-)-8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) saturation experiments in which 5-HT1A receptors were directly labeled. [3H]5-HT competition studies using high-affinity 5-HT1B compounds performed in the presence of 100 nM 8-OH-DPAT (to block 5-HT1A receptors) indicated that approximately 26% of all specific, high-affinity [3H]5-HT binding to spinal cord synaptosomes was to 5-HT1B receptors. [3H]5-HT competition studies performed in the presence of 100 nM 8-OH-DPAT and 10 nM RU 24969 (to block 5-HT1A and 5-HT1B receptors, respectively) indicated that the remaining 49% of [3H]5-HT binding did not possess the pharmacologic profile previous reported for 5-HT1C, 5-HT1D, 5-HT1E, 5-HT2, or 5-HT3 receptors. This residual 49% of [3H]5-HT binding to spinal cord synaptosomes observed in the presence of 100 nM 8-OH-DPAT and 10 nM RU 24969 (subsequently referred to as "5-HT1S") displayed high affinity and saturability (KD = 4.7 nM) in association/dissociation and saturation experiments. Addition of 300 microM GTP or the nonhydrolyzable form of GTP, 5'-guanylylimidodiphosphate, inhibited [3H]5-HT binding to 5-HT1S receptors in saturation experiments by 35 and 57%, respectively, whereas ATP was without effect.(ABSTRACT TRUNCATED AT 250 WORDS)

The neurotransmitter serotonin (5HT) activates a variety of second messenger signaling systems and through them indirectly regulates the function of ion channels. Serotonin also activates ion channels directly, suggesting that it may also mediate rapid, excitatory responses. A complementary DNA clone containing the coding sequence of one of these rapidly responding channels, a 5HT3 subtype of the serotonin receptor, has been isolated by screening a neuroblastoma expression library for functional expression of serotonin-gated currents in Xenopus oocytes. The predicted protein product has many of the features shared by other members of the ligand-gated ion channel family. The pharmacological and electrophysiological characteristics of the cloned receptor are largely consistent with the properties of native 5HT3 receptors. Messenger RNA encoding this receptor is found in the brain, spinal cord, and heart. This receptor defines a new class of excitatory ligand-gated channels.

Chemical modification of the 5-HT3 receptors in membranes from NG108-15 hybridoma cells was achieved using protein modifying reagents specific for various amino acid residues: N-bromosuccinimide for tryptophan, dithiothreitol for cystine, sodium tetrathionate for cysteine, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline for aspartic and glutamic acids, diethylpyrocarbonate for histidine, tetranitromethane for tyrosine and 2,3-butanedione for arginine. Among all the reagents tested, N-bromosuccinimide produced the largest alteration in the specific binding of [3H]zacopride onto 5-HT3 receptors. A significant reduction in Bmax (approximately 50%) with no change in Kd were noted on [3H]zacopride specific binding to membranes which were incubated with 40 microM N-bromosuccinimide for 60 min at 25 degrees. The occupancy of 5-HT3 receptor binding sites by various 5-HT3 agonists and antagonists (phenylbiguanide, ondansetron, granisetron, MDL 72222) prevented, at least partially, any subsequent reduction in [3H]zacopride specific binding by N-bromosuccinimide treatment. However, neither m-chloro-phenylbiguanide, among the agonists, nor zacopride, among the antagonists, were able to prevent the effect of N-bromosuccinimide, suggesting that variations might exist in the molecular mechanisms implicated in the binding of 5-HT3 ligands to the recognition site on 5-HT3 receptors. Nevertheless, these data support the suggestion that tryptophan residue(s) are probably involved in the binding of agonists and antagonists onto 5-HT3 receptors in NG108-15 cell membranes.

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) exerts diverse physiological effects in the central and peripheral nervous systems and in smooth muscle by interacting with pharmacologically distinct membrane receptors. We report here the cDNA cloning of the mouse 5-HT1C receptor and its functional expression in Xenopus oocytes. This receptor possesses the unusual feature of containing eight hydrophobic domains capable of forming membrane-spanning alpha-helices, contrary to the usual '7-helix' paradigm for other membrane receptors that function through coupling to GTP-binding proteins. By hybridization analysis of Chinese hamster x mouse somatic cell hybrid lines, the gene for the receptor, designated Htr1c, has been assigned to the mouse X chromosome.

We describe the nucleic acid sequence encoding a human 5-hydroxytryptamine1D (5-HT1D) serotonin receptor and some of the functional characteristics of the gene product. The receptor gene was isolated by hybridization to a probe based on a canine thyroid cDNA (called RDC4) previously isolated by others and believed to encode a heretofore undetermined member of the guanine nucleotide-binding protein (G protein)-linked receptor family. The human clone we isolated, called MA6A, contains an apparently intronless open reading frame encoding a 377-amino acid polypeptide with the seven hydrophobic domains characteristic of G protein-linked receptors. The MA6A deduced amino acid sequence is 88% identical to that for RDC4 and 43% identical to that for the human 5-HT1A receptor. Expression of the human gene product in transfected cell lines results in the appearance of saturable high affinity 5-HT1D-type [3H]5-HT binding. The expressed receptor exhibits features indicative of coupling to Gi proteins, i.e., robust inhibition of forskolin-stimulated cAMP accumulation and formation of a pertussis toxin-sensitive high agonist affinity binding state. These findings may help clarify several ambiguities in the classification and action of serotonin receptor subtypes.

We have isolated a hamster fibroblast cDNA clone that encodes a serotoninergic receptor whose deduced amino acid sequence displays 94% identity with the rat brain serotonin (5-HT) type 2 receptor. When expressed in Xenopus oocytes, the hamster receptor efficiently couples to the phosphoinositide second messenger system and leads to intracellular Ca2+ mobilization in response to 5-HT. To determine the pharmacological properties of this receptor, and to evaluate the role of phospholipase C (PLC) activation in growth modulation by 5-HT, we have expressed it in hamster fibroblasts. Transfected cells that express 5-HT receptors were selected using a novel method based on coexpression of the Na+/H+ antiporter gene as a selectable marker. After co-transfection of the 5-HT receptor and Na+/H+ antiporter cDNAs in fibroblasts lacking antiporter activity (variants of the CCL39 line), 50% of the clones resistant to an acute acid load express functional receptors. The pharmacological profile of the transfected receptor is consistent with it being of the 5-HT2 subtype, and the extent of 5-HT-stimulated PLC activation in independent clones correlates with their relative level of cRNA expression. In cells in where addition of 5-HT leads to strong activation of PLC, and inhibition of adenylate cyclase via endogenous 5-HT1b receptors, 5-HT alone has little effect on DNA synthesis stimulation. Thus we conclude that activation of the PLC signalling pathway in these cells is not sufficient to trigger G0/G1 to S phase transition. Strong activation of PLC via 5-HT2 receptors does however contribute to the synergy observed between 5-HT (Gi-coupled pathway) and fibroblast growth factor (tyrosine kinase-activated pathway) on DNA synthesis reinitiation in transfected cells.

The recent surge of scientific interest in 5-hydroxytryptamine3 (5-HT3) receptors can be attributed largely to the rapid increase in the number of potent and selective 5-HT3 receptor antagonists. The availability of a large variety of chemically diverse 5-HT3 compounds allows for a unique opportunity to apply computer-based modelling techniques to the analysis of the 5-HT3 receptor pharmacophore. The present review summarises recent studies performed in this laboratory. The primary goal of these studies was to elucidate the chemical requirements needed for molecular recognition at the 5-HT3 receptor binding site. A secondary goal was to determine if this type of molecular information could be assessed using computerised chemical databases.

High densities of serotonergic 5-HT1A receptors, in excess of adult levels, were found in the human fetal brain between the 16th and 22nd weeks of gestation, 5-HT1A receptors were measured by quantitative autoradiography using brain sections of fetuses aborted at gestational ages 16-22 weeks. The highest receptor concentrations were detected in the cortex and hippocampus. Two brains obtained from fetuses with Down's syndrome at 22 and 24 weeks gestation exhibited abnormal receptor levels compared to age matched controls. The presence of an early, prenatal peak of 5-HT1A receptors in fetal cortex and hippocampus suggests that these receptors play a role in human brain development and may also be involved in developmental disorders such as Down's syndrome.

The previously proposed models for the recognition and activation of 5-HT and histamine-H2 receptors, which were employed to explain the antagonist activity of LSD at both of these receptors, as well as the selective antagonism for H2 receptors by SKF-10856 and 9,10-dihydro-LSD, are used herein to design a compound to test the H2-receptor model. The design strategy attempts to construct a compound with potentially selective H2 agonism. The design scheme maintains features which were previously used to explain selective recognition of SKF-10856 and 9,10-dihydro-LSD as well as reintroduces the chemical features proposed to be responsible for H2 activation. The existence of the H2 recognition and activation features in the proposed compound is verified, in a previously proposed model, by computational studies of the molecular electrostatic potentials and shifts in the tautomeric preference.

Few site-selective serotonergic agents are currently available; in fact, most of these agents might more appropriately be termed semi-selective in that they typically bind to at least two different populations of serotonin receptors. We describe the application of structure-affinity relationship (SAFIR) studies to the development of high-affinity and/or site-selective serotonergic agents; examples are provided from work conducted in our laboratories. Also discussed is the concept of selectively non-selective agents and their potential preclinical and clinical utility. A case is made for development of selective and nonselective serotonergic agents.

A structure-activity analysis was used to identify selective 5-HT1A versus 5-HT1D receptor agents. An analysis of published data identified 13 drugs which display nanomolar affinity for the 5-HT1A receptor and that have been analyzed at 5-HT1D receptor binding sites. Four agents display greater than or equal to 100-fold selectivity for the 5-HT1A receptor. Two structural features were identified which hypothetically result in selectivity for 5-HT1A versus 5-HT1D binding sites. The linkage of an indole ring to a basic nitrogen atom via the 4 position on the indole ring or the absence of an indole ring are two features which lower the affinity for the 5-HT1D receptor, but do not necessarily lower the affinity for the 5-HT1A receptor. A series of 7 agents (5 indoles, 2 quinolines) was identified which met these hypothetical selectivity criteria. These compounds were then analyzed in radioligand binding studies. These 7 agents display affinities of 1.3-170 nM for the 5-HT1A receptor binding site, and 1,800-13,000 nM for the 5-HT1D receptor binding site. All 7 agents display greater than or equal to 47-fold selectivity for the 5-HT1A versus 5-HT1D site and 4 of the agents are greater than 100-fold selective. Compound No. 1 (N,N'-bis[3-(4-indolyloxy)-2-hydroxypropyl]-(Z)-1,8-diamino-p-meth ane) and compound No. 2 (N8-[3-(4-indolyloxy)-2-hydroxypropyl]-N1-(propioloyl)-(Z)-1 ,8-diamino-p-methane) are the most selective agents yet described for 5-HT1A versus 5-HT1D receptor binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Progress in the field of neuronal receptor research has accelerated during the last few years due to developments in pharmacology and molecular biology. This is particularly true in the case of the serotonin 5-HT1A receptor. In 1983 the very selective, high affinity 5-HT1A agonist 8-OH-DPAT was developed which allowed the pharmacology and distribution of the 5-HT1A receptor in the central nervous system of the rat and man to be extensively characterized. By 1987, the gene encoding this receptor protein was cloned and sequenced, allowing not only elucidation of its structure, but also better insight into the nature of its coupling to transmembrane signal transduction systems. Thus in a short period of time considerable knowledge has accumulated on how serotonin exerts its functions in the central nervous system via the 5-HT1A receptor. In the present review we will briefly discuss some of the latest developments regarding the 5-HT1A receptor.

Specific binding sites with pharmacological properties typical of serotonin 5-HT3 receptors were identified in membranes of the murine hybridoma cell line NG 108-15, using [3H]zacopride as a ligand. Optimal solubilization of these sites (yield, 50%) could be achieved using the detergent 3-[3-(cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS) at 24 mM plus 0.5 M NaCl in 25 mM Tris-HCl, pH 7.4. Specific [3H]zacopride binding to soluble sites in the 100,000-g CHAPS extract was saturable and showed characteristics (Bmax = 425 +/- 81 fmol/mg of protein; KD = 0.19 +/- 0.02 nM) closely related to those of membrane-bound sites (Bmax = 932 +/- 183 fmol/mg of protein; KD = 0.60 +/- 0.03 nM). Determination of association (k+1 = 0.17 nM min-1) and dissociation (k-1 = 0.02 min-1) rate constants for the soluble sites gave a KD value of 0.12 nM, a result consistent with that calculated from saturation studies. As assessed from the displacement potencies (IC50) of 10 different drugs, the pharmacological profile of [3H]zacopride specific binding sites was essentially the same (r = 0.99) in the CHAPS-soluble extract and in cell membranes, although some increase in the affinity for 5-HT3 antagonists (zacopride, ICS 205-930, and MDL 72222) and decrease in the affinity for 5-HT3 agonists (2-methyl-5-hydroxytryptamine and phenylbiguanide) were noted for the soluble sites. Sucrose density gradient sedimentation of the CHAPS-soluble extract gave a Svedberg coefficient of 12S for the material with [3H]zacopride specific binding capacity. Chromatographic analyses using Sephacryl S-400 and wheat germ agglutinin-agarose columns indicated marked enrichment (by 2.5- and 10-fold, respectively) in [3H]zacopride specific binding activity in the corresponding eluates compared with the starting soluble extract, a finding suggesting that both steps are of potential interest for the partial purification of solubilized 5-HT3 receptors. Two soluble materials with apparent molecular masses of approximately 600 and approximately 36 kDa were found to bind [3H]zacopride specifically in the Sephacryl S-400 eluate. Interestingly, molecular mass determination by radiation inactivation of [3H]zacopride binding sites in frozen NG 108-15 cells gave a value of approximately 35 kDa.

We review our results from the first computational simulations of a mechanism by which ligands can activate a 5-HT1A receptor, and relate the findings to information on the structure and function of the authentic receptor. The computational exploration of the recognition and activation mechanisms is carried out inside a protein selected as a model for the receptor based on cognate physicochemical and experimental data. A similar approach is applied to the 5-HT2 receptor. The interaction mechanisms at the two 5-HT receptor subtypes differ in the nature of the forces determining ligand-receptor interactions and the types of receptor activation mechanisms they entail. The main molecular property related to recognition at 5-HT1A receptors was shown to be the directional character of the electrostatic potential generated by the ligands in the molecular region corresponding to the indole in 5-HT. The corresponding recognition site was shown to have properties of a positively-charged (imidazolium) form of the side chain of a His residue. The mechanism of recognition at the 5-HT1A receptor was shown to be electrostatic, and conducive to a triggering of the receptor response through the change in the electronic structure of the imidazolium recognition site when it interacts with an activating ligand (agonist). This effect was shown to induce a proton transfer from the ring to a neighboring residue to which it can be hydrogen-bonded in the resting state. We show how this model for recognition and activation defines in molecular terms the mechanisms underlying the classical pharmacologic properties of agonists, partial agonists, and antagonists. The molecular correlates of pharmacologic efficacy emerge from the calculations of the effect of the ligands on the barriers for proton transfer, and on the energy drive for the proton transfer reaction. A different model is proposed for selective recognition at the 5-HT2 receptors, based on structural details of 5-HT-binding peptides. The recognition site is considered to consist of two aromatic residues separated by a hydrophilic residue. In contrast to the model for 5-HT1A, the recognition is based on the interaction of neutral molecules and the stabilization is provided by dispersion forces. The resulting activation mechanism is based on a structural rearrangement. These detailed descriptions of elements in the ligand-receptor interactions at the two receptor subtypes lead to a new basis for rational design of receptor-selective compounds with predetermined efficacy.

Recently, the family of G protein-coupled serotonin (5-hydroxytryptamine[5-HT]) receptors has begun to yield to molecular analysis. The cloning of the 5-HT1C and 5-HT2 receptors has provided a structural basis for the similarities observed in their pharmacologic properties. Furthermore, pharmacologic characterization of the transfected human 5-HT2 receptor has answered two outstanding questions regarding this receptor. First, the few amino acid differences that exist between the human and the rat genes are sufficient to account for the species differences seen in their pharmacologic properties. Second, the single protein encoded by the human 5-HT2 receptor gene is capable of binding both [3H]DOB and [3H]ketanserin. Analysis of the effects of guanine nucleotides provides further evidence that this single protein binds both ligands, that this receptor has high- and low-affinity states, and that these states are partially interconvertible. Furthermore, the close relationship between the adrenergic receptors and the 5-HT1A receptor has been reaffirmed by the recent cloning of a new adrenergic receptor subtype, alpha 2B, by use of the 5-HT1A receptor sequence. Finally, the detailed level of structural information now available on serotonin receptors has yielded valuable information about the ligand binding site and about the possible functional significance of differing rates of evolutionary change in various parts of the gene.

On account of the postulated existence of 5-HT3 receptor subtypes, the respective physico-chemical and pharmacological properties of specific binding sites for the potent 5-HT3 antagonist [3H]zacopride were compared using membranes from the rat posterior cortex or neuroblastoma-glioma NG 108-15 clonal cells. In both membrane preparations, [3H]zacopride bound to a single class of specific sites with a Kd close to 0.5 nM. However, the Bmax value in NG 108-15 cell membranes (970 +/- 194 fmol/mg protein) was approximately 50 times larger than that in cortical membranes (19 +/- 2 fmol/mg protein). The specific binding of [3H]zacopride was equally affected by temperature, pH and molarity of the assay medium, and equally insensitive to thiol- and disulfide-reagents (N-ethylmaleimide, p-chloromercuribenzene sulfonic acid, dithiothreitol) and GTP in cortical as well as NG 108-15 cell membranes. Determination of the molecular size of [3H]zacopride specific binding sites by radiation inactivation yielded values close to 35 kDa for both membrane preparations. Finally, a highly significant positive correlation (r = 0.979) was found between the respective pKi values of 34 different drugs for their inhibition of [3H]zacopride specific binding to cortical or NG 108-15 cell membranes. Among them, the most potent was S(-)zacopride (pKi = 9.55), followed by BRL 43964, ICS 205-930, quipazine, R(+)zacopride, GR 38032F and MDL 72222. Atypical antidepressants (mianserin, amoxapine) and neuroleptics (clotiapine, loxapine and clozapine) were active in rather low concentrations (pKi less than 6.5), suggesting that recognition of 5-HT3 sites might be relevant to part of the in vivo effects of these drugs. Such identical physico-chemical and pharmacological properties of [3H]zacopride specific binding in cortical and NG 108-15 cell membranes strongly suggest that the same 5-HT3 receptor (subtype?) exists in these two preparations.

The highly selective 5-HT3 receptor antagonist [3H]GR65630 has been used to characterize 5-HT3 receptors in intact N1E-115 neuroblastoma cells. Equilibrium binding analysis demonstrated high-affinity binding to a single class of receptors with a Kd of 0.69 (+/- 0.12) nM and Bmax of 31.4 (+/- 11.4) fmol/10(5) cells, equivalent to approximately 200,000 sites per cell. Specific binding was displaced by low concentrations of 5-HT3-selective ligands, and by the nicotinic antagonist d-tubocurarine.

5-Hydroxytryptamine (5-HT) receptors of the 5-HT1A subtype are localized on serotoninergic cells and dendrites in the raphe nuclei of the brain stem and are believed to regulate synaptic 5-HT release through an inhibitory influence on serotoninergic impulse flow. The effects of 5-HT1A agonists on 5-HT release can, therefore, only be detected by measurement of 5-HT release from intact serotoninergic neurones. Here we review the evidence that the microdialysis technique, when applied to the anaesthetized rat, is able to detect extracellular 5-HT in the brain which derives from serotoninergic neurones and changes in accordance with serotoninergic neuronal activity. We have observed that a range of 5-HT1A agonists, including 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), inhibit 5-HT release in hippocampus, most probably by acting on somatodendritic 5-HT1A autoreceptors in the dorsal raphe nucleus. The inhibitory action of 8-OH-DPAT and several other selective 5-HT1A receptor active drugs on 5-HT release is sensitive to pindolol, further supporting the idea that the 5-HT receptor being measured is of the 5-HT1 subtype. Two drugs, BMY 7378 and NAN-190, which show 5-HT1A antagonist properties in certain models, reduce 5-HT release indicating that they have mixed agonist/antagonist actions at the 5-HT1A receptor. Our data indicate that measurement of 5-HT release in rat brain using the microdialysis technique may be a useful method to probe the pharmacology of the 5-HT1A autoreceptor in vivo.

5-HT1 and 5-HT1A binding sites were measured in brain tissue obtained at postmortem from 19 suicides, with definite evidence of depression, and 19 sex and age-matched controls. Thirteen of the depressed suicides had not been prescribed psychoactive drugs recently (drug-free suicides); six had been receiving antidepressant drugs, alone or in combination with other drugs (antidepressant-treated suicides). No significant differences were found in the number or affinity of 5-HT1 and 5-HT1A binding sites in frontal or temporal cortex between drug-free suicides and controls. The number of 5-HT1 sites was significantly lower (by 20%), affinity unaltered, in hippocampus and the affinity significantly lower (by 33%), number unaltered, in amygdala of drug-free suicides than controls. The number of 5-HT1 binding sites tended to be higher and the affinity lower in the antidepressant-treated compared to drug-free suicides, and significantly so in hippocampus. The present results, together with our previous studies, provide no evidence of altered cortical 5-HT markers in depressed suicides, but further emphasise abnormalities in the hippocampus.

Serotonin (5-HT) plays a major role as a neurotransmitter in the brain and large amounts are found in blood platelets. 5-HT release can be induced by action potentials invading the nerve terminals and by platelet aggregation. The targets of 5-HT are specific receptors mediating a wide variety of central and peripheral effects. For two of the main 5-HT receptor classes, the 5-HT2 and 5-HT3 receptors, selective antagonists are available, but this is not the case for the heterogeneous population of 5-HT1 receptors. In addition, the drugs with antagonistic properties at the 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT1D receptors block other 5-HT receptors or even entirely different receptors (e.g., beta-adrenoceptors); as a rule, they do not discriminate between the four 5-HT receptor subtypes. Identification and characterization of these subtypes is further complicated by the fact that, with the exception of drugs activating 5-HT1A receptors, e.g., 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and urapidil; no subtype-selective agonists are available. Hence, the pharmacological characterization of a 5-HT1 receptor must be based on experiments with several putative 5-HT receptor agonists and antagonists with an overlapping profile of affinities for the various 5-HT1 receptor subclasses. The 5-HT1A receptor is the best-defined subclass and has already been cloned. It has been identified on cell bodies of 5-HT neurons in the raphe nuclei, and it mediates inhibition of cell firing.(ABSTRACT TRUNCATED AT 250 WORDS)