Serotonin neurons grafted to the adult rat hippocampus. I. Time course of growth as studied by immunohistochemistry and biochemistry

The maturation and growth of fetal serotonergic raphe neurons have been studied immunohistochemically and biochemically between 1 week and 5 months after grafting to the hippocampal formation in 5,7-dihydroxytryptamine-pretreated adult rats. The average number of surviving neurons in each group was 1800, which is equivalent to approximately 20% of the potential number of serotonin neurons contained in the grafted cell suspension. The fetal raphe cells, which were taken from 12-14-day-old embryos, had developed strong serotonin immunoreactivity at 1 week after transplantation, and the number of serotonin cells present at 1 week was similar to that found at later time points. Fiber outgrowth was demonstrable already at 1 week but the serotonin-positive fibers were restricted to the areas close to the graft. Single fibers, however, could be traced for distances of up to 500-800 microns into the host hippocampus and dentate gyrus. At later time points, the graft-derived serotonin-immunoreactive fiber network extended to cover the entire hippocampal formation. At the longest postoperative time point, 7 weeks and 5 months, some of the animals exhibited extensive hyperinnervation patterns throughout the dorsal parts of the hippocampus and the dentate gyrus. Consistent with these immunohistochemical observations, supranormal serotonin levels developed with time after transplantation in the grafted hippocampi from an average of 5% of normal at 1 week, to 28% of normal at 3 weeks, 146% of normal at 7 weeks, and 216% of normal at 5 months. Although the recovery of 5-hydroxyindoleacetic acid (5-HIAA) paralleled that of serotonin (5-HT), the increase in the metabolite concentrations was less than that of the amine, indicating a change in the turnover or metabolism of serotonin in the grafted neurons over time. Thus, the 5-HIAA:5-HT ratio was higher than normal at 3 weeks post-grafting (when the host hippocampus was only partially reinnervated); it was similar to normal at 7 weeks, and it tended to be lower than normal in the hyperinnervated specimens at 5 months' survival. A regression analysis revealed a significant inverse correlation between the hippocampal 5-HT concentration and the 5-HIAA:5-HT ratio in the graft-reinnervated hippocampal formation. In conclusion, the grafted serotonergic raphe neurons, in contrast to other types of aminergic neurons, exhibited a prominent tendency to form extensive hyperiinnervation patterns in the previously dennervated host target.(ABSTRACT TRUNCATED AT 400 WORDS)     

Serotonin release from mesencephalic raphe neurons grafted to the 5,7-dihydroxytryptamine-lesioned rat hippocampus: effects of behavioral activation and stress

Transplants of fetal midbrain raphe neurons into the adult brain have been shown to promote recovery of complex behavioral deficits in several experimental models, but the mechanisms underlying these effects are only partially understood. In the present study, we have used a well-characterized model system to ascertain whether midbrain raphe graft can display behaviorally relevant changes in transmitter release and/or metabolism. Fetal mesencephalic raphe neurons were grafted unilaterally into the hippocampus previously deprived of its innate serotonergic innervation by intraventricular injections of 5,7-dihydroxytryptamine. The contralateral hippocampus remained as a nongrafted, lesioned control. Microdialysis probes were implanted in the hippocampus 5-7 months postgrafting. Under baseline conditions, extracellular levels of serotonin were similar to normal in the grafted hippocampi, but undetectable on the contralateral, nongrafted side. Levels of the serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA), were markedly higher than normal in the grafted hippocampi, but dramatically reduced on the contralateral nongrafted side. Handling stimulation (gentle stroking of a rat's fur and tail for 15 min) induced a 64% increase in serotonin output in the intact rats and a small but significant 12% increase in the grafted animals. Non-noxious tail-pinch (15 min) enhanced serotonin release by 86% in the intact rats and 28% in the grafted ones. Extracellular 5-HIAA levels remained unchanged during both handling and tail-pinch in both the intact and the grafted rats. Forced immobilization of the rats for 15 min induced a transient 124% increase in extracellular serotonin levels in the intact rats and a significant 19% increase in the grafted animals, whereas swimming in temperate water (25-30 degrees C; 15 min) induced no detectable changes in serotonin output in any of the groups. 5-HIAA levels remained unchanged during forced immobilization, but were significantly reduced during the swimming session in both the intact (-38%) and grafted (-15%) animals. The present results indicate that median raphe grafts can become functionally integrated in the denervated host hippocampus and respond by altered indole output when the animal is exposed to different types of environmental challenges.     

Serotonin may stimulate granule cell proliferation in the adult hippocampus, as observed in rats grafted with foetal raphe neurons

The long-term effects of hippocampal serotonergic denervation and reinnervation by foetal raphe tissue were examined in the dentate gyrus where neurons are continously born in the adult. Complete lesion of serotonin neurons following injections of 5, 7-dihydroxytryptamine in the dorsal and medial raphe nuclei produced long-term decreases in the number of newly generated granule cells identified with 5-Bromo-2'-deoxyuridine (BrdU) and the polysialylated form of neural cell adhesion molecule (PSA-NCAM) immunostaining, as observed in 2-month-survival rats. The raphe grafts, but not the control grafts of embryonic spinal tissue, reversed the postlesion-induced decreases in the density of BrdU- and PSA-NCAM-labelled cells detected in the granule layer. Inhibition of serotonin synthesis in animals with raphe grafts reversed back to lesion-induced changes in granule cell proliferation. Furthermore, extensive serotonergic reinnervation of the dentate gyrus in the area proximal to the raphe graft could be associated with supranormal density of BrdU-labelled cells. These results indicate that serotonin may be considered a positive regulatory factor of adult granule cell proliferation. Finally, the lack of effect of embryonic nonserotonergic tissue grafted to serotonin-deprived rats suggests that neurotrophic factors may not be involved in the effects of serotonin on adult neurogenesis.     

Intracerebral microdialysis as a tool to monitor transmitter release from grafted cholinergic and monoaminergic neurons

In the present study the microdialysis technique has been used as a tool for the study of functional regulation of intracerebrally grafted cholinergic and monoaminergic neurons as well as for the analysis of graft-host interactions. Fetal noradrenergic, serotonergic, dopaminergic, and cholinergic neurons were transplanted into the hippocampus or striatum previously denervated of their intrinsic monoaminergic or cholinergic afferents. After a few months survival, when the grafts had reinnervated the surrounding target, dialysis probes were implanted into the graft-reinnervated region. Although the graft-derived fiber and terminal density varied substantially from one animal to another the transmitters in the extracellular space were maintained at near-normal levels, not only under baseline conditions, but also during K(+)-induced depolarization, transmitter-selective uptake blockade, and tetrodotoxin. This suggests that the grafted neurons possess efficient autoregulatory properties despite their ectopic location. The results also show that monoamine release in the graft-reinnervated host target is impulse-dependent, and that the neurons are spontaneously functionally active at the synaptic level. Electrical stimulation of the lateral habenula (which has previously been identified as a powerful activator of the intrinsic hippocampal cholinergic and noradrenergic afferents) produced a similar increase in the release of these transmitters in the intact and grafted hippocampi. A complex environmental stimulus, such as handling, induced a consistent increase in acetylcholine but not noradrenaline release in the hippocampus. These findings suggest that grafted cholinergic and noradrenergic neurons can be functionally activated by host brain inputs.     

Neurotensin counteracts apomorphine-induced inhibition of dopamine release as studied by microdialysis in rat neostriatum

Microdialysis in the neostriatum of the halothane-anesthetized male rats was used to study the effect of neurotensin on the release of dopamine and its metabolites in the absence or presence of systemic apomorphine treatment. Perfusate levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were assayed by high-performance liquid chromatography in combination with electrochemical detection. Perfusion with neurotensin (1000 nM but not 10 nM) increased the dialysate levels of dopamine without affecting those of DOPAC and HVA. Systemic treatment with apomorphine (0.05 and 0.5 mg/kg, s.c.) reduced the dialysate levels of dopamine, DOPAC and HVA in a dose-related way. Neurotensin (10 nM but not 1 nM) counteracted the inhibitory effect of apomorphine on dialysate levels of dopamine without affecting those of DOPAC and HVA. The results indicate a facilitatory effect of neurotensin on dopamine release in rat neostriatum. It is suggested that activation of neurotensin receptors may cause a reduction in the affinity of dopamine autoreceptors, since the low dose of neurotensin is able to counteract the inhibitory effect of apomorphine on dopamine release.     

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.     

Fetal raphe neurons grafted into the hippocampus develop normal adult physiological properties

Embryonic midbrain raphe was grafted into serotonin-deficient adult rat hippocampus. Serotonin-containing neurons in the graft survive for at least 6 months after grafting. Grafted neurons develop physiological properties, not present on the day of grafting, identical to those of normal adult serotonin-containing neurons. These include (a) high input resistance and slow membrane time constant, (b) lack of inward rectification in response to hyperpolarizing current pulses and (c) a potent, 4-aminopyridine-sensitive transient outward rectification. The grafted neurons innervate the host tissue with axons that have a slow conduction velocity and refractoriness. It is suggested that grafted CNS neurons may possess normal physiological properties.     

Serotonin preferentially hyperpolarizes capsaicin-sensitive C type sensory neurons by activating 5-HT1A receptors.

The effects of serotonin (5-HT) were investigated by intracellular recording from 179 dorsal root ganglion (DRG) cells classified by conduction velocity. Bath applied 5-HT depolarized 82% and hyperpolarized 4% of the A-type cells. In C-type cells, 5-HT depolarized only 41%, but hyperpolarized 39% of the cells. The depolarizing responses were of two types; an increase or decrease in R(in), mediated by 5-HT2or3 receptors, respectively. These receptors were observed in both A- and C-type cells. Hyperpolarizing responses were largely confined to A(delta)- and C-type cells. Carboxamidotryptamine and 8-OH-dipropylamino-tetralin were full agonists in eliciting hyperpolarization, and metitepin, spiperone and spiroxitrine behaved as competitive antagonists. This indicated that hyperpolarization was mediated by a 5-HT1A receptor. A 5-HT1A&3 receptor were found co-localized on some C-type cells. A strong depolarizing response to capsaicin was observed in the subgroup of C-type neurons that were also hyperpolarized by 5-HT. Thus a co-localization of capsaicin and 5-HT1A receptors was also observed.     

Antagonist-induced increase in 5-HT1A-receptor expression in adult rat hippocampus and cortex

Many receptor antagonists function as reverse agonists on the signaling transduction pathway, but little is known about the action of these drugs on the regulation of receptor expression. Serotonin 1A (5-HT1A) receptor expression in 5-HT and serum-free fetal hippocampal cultures is increased in the presence of a specific 5-HT1A-receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635). To study the plasticity of postsynaptic 5-HT1A receptors in the presence of antagonist in vivo, adult Sprague Dawley rats were injected i.p. either once or twice daily with a dose of WAY 100635 (3 mg/kg) over a period of 3 days. The 5-HT1A receptor expression was detected by immunocytochemistry and light microscopy, and the receptor immunoreactivity (IR) in hippocampus subregions was quantitatively assessed by using a comparative computer-assisted morphometric analysis. Following the daily injections of WAY 100635, a significant increase in 5-HT1A receptor labeling in hippocampal neurons was recorded. This marked increase in 5-HT1A receptor expression, which occurred within 4 h after a single injection of WAY 100635, is evident on the somata membrane and dendritic processes of hippocampal and cortex layer V neurons. By contrast, no increase in 5-HT1A receptor-IR was observed after multiple daily injections at a low dose (1 mg/kg) of WAY 100635. Our study shows that a single or multiple daily injections of WAY 100635 can result in an increase in 5-HT1A receptor-IR. This increase in labeling is consistent with an enhanced expression of the receptor protein. The action of this "inverse agonist" may have clinical importance in disorders such as depression, epilepsy, and Alzheimer's disease in which 5-HT1A receptor levels are deficient.     

Serotonin 5-HT1A receptors might control the output of cortical glutamatergic neurons in rat cingulate cortex

The present study was designed to investigate the distribution of serotonin 5-HT1A receptor protein (5-HT1A-immunoreactivity) and its localization within cortical pyramidal neurons of the rat cingulate cortex. This experimental direction was inspired by recent data showing the role of 5-HT1A receptors in the pathology of schizophrenia, and in the mechanism of action of novel antipsychotic drugs as well as by the importance of the cingulate cortex in regulation of cognitive functions. It was found that 5-HT1A-immunoreactivity was densely distributed in neuronal eyelash-like elements, and their size, shape and spatial orientation may suggest concentration of 5-HT1A-immunopositive material in the proximal fragments of axons of cortical neurons. Moreover, it was observed that these 5-HT1A-immunopositive fragments were present predominately on proximal fragments of axons of pyramidal neurons, which was evidenced by double labeling experiments using glutamate and non-phosphorylated neurofilament H as markers of the cortical pyramidal cells. The 5-HT1A receptor immunoreactivity was localized distally to the inhibitory GABAergic terminals of chandelier and basket cells surrounding the pyramidal cell bodies and occasionally surrounding short initial segment of axonal hillock of pyramidal neurons. These anatomical data indicate that 5-HT1A receptors might control the excitability and propagation of information transmitted by the pyramidal cells. Moreover, our results indicate that drugs operating via 5-HT1A receptors in the cingulate cortex might control from this level the release of glutamate in the subcortical structures. Finally, the 5-HT1A receptors present in the cingulate cortex, as demonstrated in the present study, may constitute an important target for drugs used to repair dysfunction of glutamate neurotransmission, which is observed for example in schizophrenia.     

Acetylcholine release in the hippocampus: regulation by monoaminergic afferents as assessed by in vivo microdialysis

The role of monoamines in the functional regulation of the septo-hippocampal cholinergic system was studied using in vivo microdialysis of acetylcholine (ACh) release in the hippocampus of awake unrestrained rats. Systemic administration of the dopamine receptor agonist apomorphine (2.0 mg/kg) resulted in a 170% increase in hippocampal ACh overflow. Similarly the catecholamine-releasing agent amphetamine (2.5 mg/kg) produced a 400% increase in ACh overflow. The effect induced by amphetamine, but not that of apomorphine, was blocked in animals pretreated with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT). The effect of amphetamine on ACh release was reduced by 75% after a 6-hydroxydopamine (6-OHDA) lesion of the ventral tegmental area (VTA) but was not affected by 6-OHDA lesions of the noradrenergic dorsal and ventral bundles. However, baseline ACh overflow was increased by 130% by the dorsal and ventral bundle lesions. The serotonin-releasing agent p-chloroamphetamine (2.5 mg/kg) produced a 160% increase in hippocampal ACh release, and this effect was enhanced after a 5,7-dihydroxytryptamine (5,7-DHT) lesion of the serotonin projection system. The results show that surgical or pharmacological manipulations of the ascending brainstem monoaminergic systems, which innervate wide areas of the forebrain, including the septum and the hippocampal formation, have pronounced effects on septo-hippocampal cholinergic activity. Thus, the present data provide support for the view that information regarding behavioral state and arousal is conveyed to the septo-hippocampal system via ascending monoaminergic systems.     

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

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

Physiological effects of selective 5-HT1a and 5-HT1b ligands in rat hippocampus: comparison to 5-HT

The responses of CA1 neurons to topical application of serotonin (5-HT) and selective 5-HT1a and 5-HT1b agonists were examined with intracellular recording in the hippocampal slice. 5-HT produced a uniform hyperpolarizing response associated with an increase in K conductance as previously reported. In addition a marked reduction was recorded in slow afterhyperpolarization (AHP) which follows a burst discharge. 8-OH-DPAT, ipsapirone and LY165,163 partially mimicked the hyperpolarizing response to 5-HT when first applied to the slice. However, these 5-HT1a ligands antagonized responses to subsequent applications of 5-HT. Topical application of the 5-HT1b ligand TFMPP on the slice did not produce the direct or antagonistic action seen with the 5-HT1a ligands. It is suggested that the physiological response to 5-HT in the rat hippocampus is mediated by a 5-HT1a receptor. The currently available 5-HT1a ligands show a low agonist potential and a high antagonist action towards the responses of hippocampal neurons to 5-HT. Definite classification of the hyperpolarizing response to 5-HT awaits development of more specific ligands having a pure agonistic activity.     

Serotonin 5-HT2A and 5-HT5A receptors are expressed by different motoneuron populations in rat Onuf's nucleus

Motoneurons of Onuf's nucleus innervate the pelvic striated muscles, which play a crucial role in erection, ejaculation, and urinary continence. Serotonergic descending projections from the brain are involved in the modulation of Onuf's motoneuron activity. However, conflicting results regarding the effects of spinal serotonin (5-HT) on pelvi-perineal functions have been reported. They may be partly accounted for by the multiplicity of neuronal targets and receptor subtypes on which 5-HT is acting. In order to provide comparative data regarding 5-HT receptor expression in various groups of Onuf's motoneurons, we used retrograde tracing techniques from different pelvic muscles combined with immunocytochemistry of 5-HT2A and 5-HT5A receptors in male and female rats. In males, 5-HT2A receptor immunolabeling was very dense in motoneurons innervating the ischiocavernosus muscle. By contrast, in female rats, 5-HT2A receptor expression in Onuf's nucleus was very weak. In both genders, 5-HT5A receptor immunoreactivity was found in motoneurons innervating the external urethral sphincter. In males, a moderate or low 5-HT5A immunolabeling was observed in motoneurons innervating the bulbospongiosus and ischiocavernosus muscles, respectively. These data show a preferential localization of 5-HT2A and 5-HT5A receptors to motoneurons controlling the striated muscles located at the penile crus and sphincter muscles, respectively, suggesting a specific serotoninergic control on different pelvic functions. In addition, the subcellular distribution of receptors suggests a different mode of action of 5-HT, paracrine at 5-HT2A receptors and synaptic at 5-HT5A receptors. This might have implications for pharmacological research targeting different pelvic functions e.g., micturition and ejaculation.     

5-Fluorouracil chemotherapy affects spatial working memory and newborn neurons in the adult rat hippocampus

Chemotherapy-associated memory deficits in adults are prevalent with systemic treatment utilizing 5-fluorouracil (5-Fu). 5-Fu disrupts cell proliferation and readily crosses the blood–brain barrier. Proliferating cells within the adult dentate gyrus of the hippocampus give rise to new neurons involved in memory and learning and require neurotrophic factors such as brain-derived neurotrophic factor (BDNF) to nurture this process of adult neurogenesis. Some of these proliferating cells are anatomically and functionally supported by vascular endothelial cells. We propose that systemically administered 5-Fu chemotherapy will cause deficits in hippocampal memory that are associated with altered BDNF levels and proliferating cells (particularly vascular-associated cells) in the dentate gyrus. This was tested by determining the effect of 5-Fu on spatial working memory as modelled by the object location recognition test. Numbers of vascular-associated (VA) and non-vascular-associated (NVA) proliferating cells in the dentate gyrus were measured using double-labelling immunohistochemistry with markers of proliferation (Ki67) and endothelial cells (RECA-1). 5-Fu-induced changes in hippocampal BDNF and doublecortin (DCX) protein levels were quantified using Western immunoblotting. 5-Fu chemotherapy caused a marginal disruption in spatial working memory and did not alter the total proliferating cell counts or the percentage of VA and NVA proliferating cells in the dentate gyrus. In contrast, 5-Fu significantly reduced BDNF and DCX levels in the hippocampus, indicating alterations in neurotrophin levels and neurogenesis. These findings highlight the usefulness of animal models of 'chemobrain' for understanding the mechanisms that underlie chemotherapy-associated declines in cognitive performance and memory.     

Does in vivo voltammetry in the hippocampus measure 5-HT release?

In vivo voltammetry enables catecholamine and 5-hydroxytryptamine (5-HT) release to be directly determined in the brains of conscious unrestrained animals. This study concerned the validation of the measurement of 5-HT release using scanning voltammetry at chronically implanted carbon paste/epoxy resin glass microelectrodes in the rat hippocampus. Automated recordings of the current produced by appling a ramp (100 mV/s) potential (0–1 V) to the electrode at 5 min intervals were made over a period of several hours. A current peak at 0.35–0.4 V corresponded to that produced by solutions of 5-HT in vitro.The effect of a number of drugs known to affect 5-HT synthesis and release was then studied in vivo.p-Chloroamphetamine (PCA) caused an increase in the peak which correlated with a ‘5-HT behavioural syndrome’.p-Chlorophenylalanine caused a reduction in the peak and prevented both the increase seen with PCA and the PCA induced behaviour. Fluoxetine andl-tryptophan also led to increases in signal; all these results were consistent with the signal reflecting extraneuronal 5-HT. However 5-hydroxyindoleacetic acid (5HIAA) also oxidizes at 0.35–0.4 V in vitro. Probenecid, which blocks 5HIAA egress from the brain caused a large increase in the hippocampal signal. The monoamine oxidase inhibitors pargyline and nialamide, which increase 5-HT and reduce 5HIAA levels, had no significant effect on the signal. These results, together with the greater increase in signal following probenecid than tryptophan and the delayed increase after tryptophan, are interpreted as implying thatextraneuronal 5HIAA and 5-HT contribute approximately equally to the hippocampal signal. We conclude that this method gives information distinct from and complementary to biochemical estimation, and offers great scope for the investigation of the role of 5-hydroxyindole release in drug and environmental effects on behaviour.     

Effects of locus coeruleus lesions on the release of endogenous dopamine in the rat nucleus accumbens and caudate nucleus as determined by intracerebral microdialysis.

Bilateral 6-hydroxydopamine lesions of the rat locus coeruleus (a) depleted forebrain norepinephrine levels by 67%, (b) reduced the basal release of dopamine in the nucleus accumbens and caudate nucleus by 26% and 19%, respectively, and (c) reduced (+)-amphetamine-induced release in the nucleus accumbens and caudate nucleus. The locus coeruleus appears to exert a tonic excitatory influence on striatal and limbic dopamine release in vivo.     

Serotonin receptors in the human brain. II. Characterization and autoradiographic localization of 5-HT1C and 5-HT2 recognition sites

The presence, pharmacological properties and anatomical distribution of serotonin-1C and serotonin-2 receptor subtypes were studied in the human brain by both radioligand binding and autoradiographic procedures. Frontal cortex, hippocampus and choroid plexus from human brains obtained at autopsy without history of neurological diseases were used in this study. [3H]5-HT and [3H]mesulergine were used to label 5-HT1C recognition sites while [3H]ketanserin was used to label 5-HT2 receptors. The pharmacological profile of 5-HT1C sites which are very concentrated in the choroid plexus, was extremely similar to that of pig and rat 5-HT1C sites. These receptors were also detected in the hippocampus and the cortex from human brain. The general distribution of 5-HT1C sites in human and rat brain was similar although slight differences were observed. Human 5-HT2 receptors were concentrated in cortical areas but also found in the hippocampus. The pharmacological profile of these receptors was extremely similar in human and pig brain tissue, but differed in certain respects to that found in rat brain 5-HT2 receptors. The anatomical distribution of 5-HT2 receptors is similar in human and rat brain with some differences at the microscopic level. The importance of species differences in the development of 5-HT2 compounds is discussed.