The Leptin, Dopamine and Serotonin Receptors in Hypothalamic POMC-Neurons of Normal and Obese Rodents.

The pro-opiomelanocortin (POMC)-expressing neurons of the hypothalamic arcuate nucleus (ARC) are involved in the control of food intake and metabolic processes. It is assumed that, in addition to leptin, the activity of these neurons is regulated by serotonin and dopamine, but only subtype 2C serotonin receptors (5-HT2CR) was identified earlier on the POMC-neurons. The aim of this work was a comparative study of the localization and number of leptin receptors (LepR), types 1 and 2 dopamine receptors (D1R, D2R), 5-HT1BR and 5-HT2CR on the POMC-neurons and the expression of the genes encoding them in the ARC of the normal and diet-induced obese (DIO) rodents and the agouti mice (A y /a) with the melanocortin obesity. As shown by immunohistochemistry (IHC), all the studied receptors were located on the POMC-immunopositive neurons, and their IHC-content was in agreement with the expression of their genes. In DIO rats the number of D1R and D2R in the POMC-neurons and their expression in the ARC were reduced. In DIO mice the number of D1R and D2R did not change, while the number of LepR and 5-HT2CR was increased, although to a small extent. In the POMC-neurons of agouti mice the number of LepR, D2R, 5-HT1BR and 5-HT2CR was increased, and the D1R number was reduced. Thus, our data demonstrates for the first time the localization of different types of the serotonin and dopamine receptors on the POMC-neurons and a specific pattern of the changes of their number and expression in the DIO and melanocortin obesity.

5-HT1A receptor immunoreactivity in hypothalamic neurons involved in body weight control.

Serotonin (5-hydroxytryptamine; 5-HT) is a regulator of feeding behavior. The effect of serotonin on food intake is believed to be primarily mediated via 5-HT(1A) and 5-HT(2C) receptors, which both are expressed in hypothalamic regions implicated in regulation of feeding behavior. Using an antiserum to the 5-HT(1A) receptor, immunoreactive neurons were observed in the rat supraoptic, paraventricular, arcuate and ventromedial nuclei and lateral hypothalamic area. 5-HT(1A) receptor immunoreactivity was demonstrated in neuropeptide Y-, agouti-related peptide-, proopiomelanocortin- and cocaine- and amphetamine-regulated transcript-containing neurons of the arcuate nucleus. In the lateral hypothalamus, 5-HT(1A) receptor immunoreactivity was observed in melanin-concentrating hormone- and orexin-containing neurons. The results suggest that serotonin via postsynaptic 5-HT(1A) receptors affects the release of peptides regulating food intake.

In vivo 5-HT2A receptor blockade by quetiapine: an R91150 single photon emission tomography study.

BACKGROUND: Atypical antipsychotic drugs are thought to show a high degree of 5-HT2A receptor blockade, which may prevent the emergence of extrapyramidal symptoms. METHOD: 5-HT2A binding was estimated using 123I-5-I-R91150 and single photon emission tomography (SPET) in six schizophrenic subjects treated with quetiapine at a mean (+/-SD) daily dose of 350+/-123 mg for at least 5 weeks and a matched sample of six healthy volunteers. Clinical and side-effect ratings were performed at baseline and at the time of SPET scanning. The reference region approach was used to define a 5-HT2A binding index in the frontal and temporal cortex. RESULTS: Quetiapine treatment resulted in a significant decline in 5-HT2A receptor availability in the frontal cortex (mean 0.98+/-0.09) relative to healthy volunteers (mean 1.33+/-0.16). All patients showed improvements in clinical symptom or side-effect ratings. The mean frontal cortex:cerebellum ratio after quetiapine treatment was significantly negatively correlated with reduction in the Abnormal Involuntary Rating scale and Simpson-Angus scores (P<0.05 Bonferroni corrected), but not with the reduction in the scores from the scale for the assessment of positive symptoms, the scale for the assessment of negative symptoms, the Montgomery-Asberg depression rating scale or patient age. CONCLUSION: Quetiapine treatment results in significant in vivo blockade of cortical 5-HT2A, similar to other atypical antipsychotic drugs. This effect may contribute to its placebo level extrapyramidal side-effect profile.

Distribution of serotonin 5-HT(2A) receptors in afferents of the rat striatum.

Treatment with conventional antipsychotic drugs (APDs) is accompanied by extrapyramidal side effects (EPS), which are thought to be due to striatal dopamine D(2) receptor blockade. In contrast, treatment with atypical APDs is marked by a low incidence or absence of EPS. The reduced motor side effect liability of atypical APDs has been attributed to a high serotonin 5-HT(2A) receptor affinity coupled with a relatively low D(2) affinity. Despite the high density of 5-HT(2A) binding sites in the striatum, there are few detectable 5-HT(2A) mRNA-expressing neurons in the striatum. This suggests that most striatal 5-HT(2A) receptors are heteroceptors located on afferent axons. A combined retrograde tracer-immunohistochemistry method was used to determine the sites of origin of striatal 5-HT(2A)-like immunoreactive axons. 5-HT(2A)-like immunoreactive neurons in both the cortex and globus pallidus were retrogradely labeled from the striatum; very few nigrostriatal or thalamostriatal neurons expressed 5-HT(2A)-like immunoreactivity. Within the striatum, parvalbumin-containing interneurons displayed 5-HT(2A) immunolabeling; these neurons are the targets of cortical and pallidal projections. Our data indicate that cortico- and pallido-striatal neurons are the major source of 5-HT(2A) receptor binding in the striatum, and suggest that cortico- and pallido-striatal neurons are strategically positioned to reduce the motor side effects that accompany striatal D(2) receptor blockade or are seen in parkinsonism.

Sex differences in expression of serotonin receptors (subtypes 1A and 2A) in rat brain: a possible role of testosterone.

Sexual differences in the expression of messenger RNA and in the binding of serotonin receptors (subtypes 1A and 2A) were studied by in situ hybridization and autoradiography ¿[3H]8-hydroxy-2(di-n-propylamino)tetralin and [3H]ketanserin binding) in the rat brain. Serotonin-1A receptor messenger RNA showed distinct expression patterns for female and male rats. Expression of serotonin-1A receptor messenger RNA was greater in males in subregions of the hypothalamus and amygdala, and less in males in subregions of the hippocampus. No significant differences in the distribution of serotonin-1A receptor binding sites were found between the sexes. Serotonin-2A receptor messenger RNA expression was comparable in males and females in all brain regions except the ventromedial hypothalamic nuclei, where lower levels were seen in females. However, the binding of serotonin-2A receptor measured with [3H]ketanserin was significantly higher in females in all regions of the hippocampus. In a separate study, gonadectomy in males significantly increased serotonin-1A messenger RNA content in the cortex, hypothalamus, hippocampus, amygdala and dorsal raphe, and decreased serotonin-2A messenger RNA in ventromedial hypothalamic nuclei only. Almost all gonadectomy-induced changes were reversed by concomitant administration of testosterone. Our data provide evidence for region-specific sex differences in serotonin receptor subtype 1A and 2A transcription and concentration in the rat brain, and further suggest a modulatory role of testosterone in serotonin (particularly subtype 1A) receptor expression. Gender and gonadal steroid effects on central serotonergic systems may underlie the reported sexual dimorphisms in affective state regulation, response to psychopharmacological agonists or pituitary adrenal activation.

Distribution and cellular localization of the serotonin type 2C receptor messenger RNA in human brain.

The regional and cellular distribution of serotonin type 2C receptor messenger RNA was investigated in autopsy samples of human brain by in situ hybridization histochemistry. The main sites of serotonin receptor type 2C messenger RNA expression were the choroid plexus, cerebral cortex, hippocampus, amygdala, some components of the basal ganglia, the substantia nigra, the substantia innominata and the ventromedial hypothalamus, suggesting that this receptor might be involved in the regulation of different brain functions. Interestingly, in all regions examined, the serotonin type 2C receptor messenger RNA was always restricted to subpopulations of cells, suggesting a specific role, perhaps determined by regionality. A comparison of the in situ hybridization results with those previously obtained by means of radioligand binding experiments suggested that in most of the areas analysed the serotonin type 2C receptors were located at axon terminals.

Whole hemisphere autoradiography of the postmortem human brain.

Whole hemisphere autoradiography (WHA) with selective high-affinity radioligands is a new tool in studying the distribution of receptors and of other neuronal components postmortem in brains from controls vs. subjects with psychiatric and neurologic diseases. WHA can be performed with several different isotopes (e.g., 3H, 125I, and 11C), and is in addition to characterization studies also used as a tool in early radioligand development. Moreover, using this technique, high-resolution images are obtained that are complementary to those obtained in vivo with e.g., PET and SPECT. Results on dopamine and serotonin receptor subtypes as well as of their transporters show that WHA is a very suitable technique for the detailed characterization of the distribution in the whole human brain.

5-HT3-like receptors in the rat medial prefrontal cortex: further pharmacological characterization.

The aim of the study was to further characterize the pharmacological properties of 5-hydroxytryptamine (5-HT)3-like receptors in the rat medial prefrontal cortex (mPFC) using combinations of biochemical and electrophysiological approaches. Phenylbiguanide (PBG) and three chlorinated derivatives, ortho-chloro-PBG (oCPBG), meta-chloro-PBG (mCPBG) and para-chloro-PBG (pCPBG), dose-dependently stimulated phosphoionositide (PI) turnover in fronto-cingulate cortical slices. All three chloro-isomers of PBG were equipotent in stimulating PI turnover. SR 57227A ((4-amino)-(6-chloro-2-pyridyl) L-piperidine hydrochloride, a novel compound with high affinity and selectivity for peripheral and central 5-HT3 receptors) dose-dependently stimulated PI turnover in fronto-cingulate cortical slices. The rank order of potency of all the 5-HT3 receptor agonists tested in the PI assay as compared to 5-HT was: 5-HT > 2-Me-5-HT > SR57227A > PBG = mCPBG = oCPBG = mCPBG. 5-HT and 5-HT receptor agonists depressed the firing rate of both spontaneously active and glutamate-activated quiescent mPFC cells in a current (dose)-dependent fashion. The rank order of effectiveness of these compounds was: 5-HT > SR57227A = 2-Me-5-HT = mCPBG = oCPBG = pCPBG = PBG. Unlike its action on the 5-HT3 receptors in the periphery or cultured cell lines, D-tubocurarine chloride appears to be non-specific in blocking the depressant action of 2-Me-5-HT, gamma-aminobutyric acid and dopamine. Our results combined support the view that the pharmacological properties of 5-HT3-like receptors in the mPFC are not identical to those located in peripheral tissues and in cultured cell lines.

alpha-Linolenic acid dietary deficiency alters age-related changes of dopaminergic and serotoninergic neurotransmission in the rat frontal cortex.

The effects of alpha-linolenic acid diet deficiency on rat dopaminergic and serotoninergic neurotransmission systems were investigated in the frontal cortex, striatum, and cerebellum of male rats 2,6,12, and 24 months of age. The diet deficiency induced severe decrease in the 22:6n-3 fatty acid levels in all regions and a compensatory increase in n-6 fatty acid levels. A recovery in the levels of 22:6n-3 was observed in deficient rats between 2 and 12 months of age; however, this recovery was lower in frontal cortex than in striatum and cerebellum. In the striatum and cerebellum, dopaminergic and serotoninergic receptor densities and endogenous dopamine and serotonin levels were affected by aging regardless of the diet. In contrast, a 40-75% lower level of endogenous dopamine in the frontal cortex occurred in deficient rats according to age. The deficiency also induced an 18-46% increase in serotonin 5-HT2 receptor density in the frontal cortex during aging, without variation in endogenous serotonin level, and a 10% reduction in density of dopaminergic D2 receptors. Monoamine oxidase-A and -B activities showed specific age-related variations but regardless of the diet. Our results suggest that a chronically alpha-linolenic-deficient diet specifically affects the monoaminergic systems in the frontal cortex.

Immunocytochemical localization of serotonin1A receptors in the rat central nervous system.

Specific anti-rat 5-hydroxytryptamine1A (serotonin1A) receptor antibodies raised in a rabbit injected with a synthetic peptide corresponding to a highly selective portion of the third intracellular loop of the receptor protein (El Mestikawy et al. [1990] Neurosci. Lett. 118:189-192) were used for immunohistochemical mapping of serotonin1A receptors in the brain and spinal cord of adult rats. The highest density of immunostaining was found in limbic areas (lateral septum, CA1 area of Ammon's horn and dentate gyrus in the hippocampus, and frontal and entorhinal cortices), in the anterior raphe nuclei, and in the interpeduncular nucleus, in agreement with previous autoradiographic studies with selective radioligands showing the enrichment of these regions in serotonin1A receptor binding sites. Serotonin1A receptor-like immunoreactivity was also present, but at a moderate level, in the neocortex, in some thalamic and hypothalamic nuclei, in the nucleus of the solitary tract, in the dorsal tegmentum, in the nucleus of the spinal tract of the trigeminal nerve, and in the superficial layers of the dorsal horn in the spinal cord. In contrast, extrapyramidal areas, including the caudate putamen, the globus pallidus, and the substantia nigra as well as the cerebellum, exhibited very low to no immunostaining by antiserotonin1A receptor antibodies. At the cellular level, both the plasma membrane of neuronal perikarya and fine neuronal processes probably corresponding to dendritic fields were found to bind antiserotonin1A receptor antibodies. Regional differences were noted regarding these two types of immunostaining, because only dendrites bound antibodies within the hippocampus and the lateral septum, whereas both dendrites and neuronal cell bodies were immunoreactive in the medial septum, in the diagonal band of Broca, and in the dorsal and median raphe nuclei. Therefore, differential addressing of serotonin1A receptors could occur from one neuron to another. In general, the distribution and density of serotonin1A receptor-like immunoreactivity in the whole brain and in spinal cord were consistent with the mapping of serotonin1A receptor binding sites and serotonin1A receptor mRNA previously established by immunoautoradiographic and in situ hybridization procedures.

Effects of hypericum extract on the expression of serotonin receptors.

The influence of hypericum extract LI 160 on the expression of serotonin receptors was investigated using a neuroblastoma cell line to establish a model for the regulation of neurotransmitters by immunologically active compounds such as cytokines. The cells were incubated with hypericum extract LI 160 in kinetic form for 2, 4, 6, 8, and 10 hours, then washed. The serotonin receptor expression analysis was compared to that of a placebo control solution. The neuroblastoma cells showed a clearly reduced expression of the serotonin receptors under treatment with hypericum extract. First stimulation experiments with interleukin-1 (IL-1) and hypericum extract suggest that a further reduction of the serotonin receptors is possible when IL-1 is added.

The mouse 5-hydroxytryptamine1B receptor is localized predominantly on axon terminals.

The 5-hydroxytryptamine1B receptor is a serotonin receptor subtype which is expressed predominantly in the basal ganglia. It has been suggested to play a role in movement and appetite control as well as in certain pathological states such as migraine. The recent cloning of the 5-hydroxytryptamine1B gene as well as the discovery of a radioligand that labels in rodents 5-hydroxytryptamine1B and possibly 5-hydroxytryptamine1D alpha receptors (S-CM-G[125I]TNH2) allowed us to compare the distribution of the messenger RNA and of the protein in mouse brain sections. A high 5-hydroxytryptamine1B messenger RNA level is found in the caudate-putamen in medium spiny neurons that project to the globus pallidus and the substantia nigra. In contrast, no messenger RNA is expressed in the globus pallidus and substantia nigra although these structures reveal the highest level of 5-hydroxytryptamine1B binding sites. In the hippocampus, 5-hydroxytryptamine1B messenger RNA is localized in the cell bodies of pyramidal cells of the CA1 field while the protein is found predominantly in the dorsal subiculum, a projection zone for the CA1 pyramidal neurons. In the cerebellum, 5-hydroxytryptamine1B messenger RNA is expressed in the Purkinje cells, which display no receptor binding sites. Conversely, moderate binding is found in the deep nuclei of the cerebellum, the main projection zone of the Purkinje cells. 5-Hydroxytryptamine1B sites are also detected in the superficial gray layer of the superior colliculus and the lateral geniculate nucleus, brain regions containing the terminals of retinal ganglion cells. The soma of these ganglion cells express high levels of 5-hydroxytryptamine1B messenger RNA while no 5-hydroxytryptamine1B binding sites were found in the retina. This study demonstrates that the main brain regions, expressing 5-hydroxytrypamine1B messenger RNA contain low densities of 5-hydroxytryptamine1B binding sites. Conversely, the major projection areas of these anatomical structures do not express detectable levels of 5-hydroxytryptamine1B messenger RNA, but present a high density of binding sites. In addition, our data suggest that the distribution of the 5-hydroxytryptamine1D alpha binding sites is different from that of the 5-hydroxytryptamine1D alpha messenger RNA. These results together with previous lesion studies, indicate that the 5-hydroxytryptamine1B and possibly the 5-hydroxytryptamine1D alpha receptors are localized predominantly on axon terminals, while their expression is low or absent at the somatodendritic level. The 5-hydroxytryptamine1D alpha proteins might therefore contain an addressing signal allowing their transport toward nerve endings.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term fluoxetine, but not desipramine, inhibits the ACTH and oxytocin responses to the 5-HT1A agonist, 8-OH-DPAT, in male rats.

The present studies determined whether serotonin 5-HT1A receptor-mediated function is modified by chronic exposure to antidepressants. Hormone responses to the 5-HT1A agonist, 8-OH-DPAT, were evaluated after long-term exposure to two antidepressants, the 5-HT uptake blocker, fluoxetine, and the norepinephrine uptake blocker, desipramine (DMI). In addition, the density and affinity of 5-HT1A receptors in the hypothalamus and cerebral cortex were measured. Male rats received fluoxetine (10 mg/kg i.p.), DMI (5 mg/kg i.p.) or saline injections once daily for 21 days. 8-OH-DPAT (0-500 micrograms/kg s.c.) was administered 18 h after the final antidepressant injection and 15 min before sacrifice. 8-OH-DPAT significantly increased plasma ACTH, corticosterone, oxytocin and prolactin, but not renin or vasopressin concentrations. Chronic injections of fluoxetine inhibited the ACTH, corticosterone and oxytocin responses to 8-OH-DPAT, suggesting reduced 5-HT1A receptor function. In contrast, chronic DMI did not alter the hormone responses to 8-OH-DPAT. The density and affinity of 5-HT1A receptors in the frontal cortex or hypothalamus were not altered by either fluoxetine or DMI. To verify that the observed effects require prolonged exposure to fluoxetine, rats received a single injection of fluoxetine (10 mg/kg, i.p.), 3 h before 8-OH-DPAT (0-500 micrograms/kg s.c.). Acute fluoxetine did not reduce any of the hormone responses to 8-OH-DPAT. In conclusion, the results suggest that chronic, but not acute, exposure to fluoxetine decreases 5-HT1A receptor function. This effect is not seen in rats chronically exposed to DMI.(ABSTRACT TRUNCATED AT 250 WORDS)

An interdigitated columnar mosaic of cytochrome oxidase, zinc, and neurotransmitter-related molecules in cat and monkey visual cortex.

There is considerable physiological evidence for the compartmentalization of mammalian visual cortex into functional columnar modules, representing features of visual information processing such as eye and orientation specificity. However, anatomical markers of visual cortical compartmentalization have been described only for primate visual cortex. In this report, we describe an interdigitated mosaic of four neuroactive molecules which demarcate two distinct columnar systems in the kitten visual cortex. Serotonin 1C receptors and synaptic zinc were found to demarcate columns within layer IV of kitten visual cortex, which were interdigitated with a second, patchy system characterized by increased levels of cytochrome oxidase and acetylcholinesterase. In primate visual cortex, as well as in the kitten, synaptic zinc was periodically distributed in a manner precisely complementary to cytochrome oxidase. These findings provide an anatomical framework on which unifying hypotheses of the functional organization of columnar systems in mammalian visual cortex can be built.

Localization of the mRNA for the 5-HT2 receptor by in situ hybridization histochemistry. Correlation with the distribution of receptor sites.

32P-labelled oligonucleotides complementary to rat 5-HT2 receptor mRNA were used as probes to study the distribution of cells in rat brain containing the mRNA coding for this receptor by in situ hybridization histochemistry. 5-HT2 receptor binding sites were visualized by autoradiography using [125I]DOI as ligand. Both distributions were comparable, demonstrating that 5-HT2 receptors are expressed by cells intrinsic to the neocortex (lamina Va), claustrum, olfactory bulb and several nuclei of the brainstem.

Central serotonergic modulation of drinking behavior induced by water deprivation: effect of a serotonergic agonist (MK-212) administered intracerebroventricularly.

The present study was carried out to evaluate the participation of the serotonergic system (5-HT) in the modulation of the drinking response induced by water deprivation. Male Wistar rats implanted with a cannula in the 3rd ventricle were injected with the 5-HT1C/5-HT2 agonist 6-chloro-2-[1-piperazinyl]-pyrazine (MK-212) at doses of 0.5, 5, 25, 50 and 125 nmol/2 microliters. MK-212 induced a significant reduction (P less than or equal to 0.05) in water intake over a period of 300 min. This result indicates that the central 5-HT system plays an important role, probably at the level of the periventricular hypothalamus, in the modulation of drinking behavior induced by water deprivation.

Central serotonergic modulation of drinking behavior induced by water deprivation: effect of a serotonergic agonist (MK-212) administered intracerebroventricularly

The present study was carried out to evaluate the participation of the serotonergic system (5-HT) in the modulation of the drinking response induced by water deprivation. Male Wistar rats implanted with a cannula in the 3rd ventricle were injected with the 5-HT1C/5-HT2 agonist 6-chloro-2-[1-piperazinyl]-pyrazine (MK-212) at doses of 0.5, 5, 25, 50 and 125 nmol/2 µl. MK-212 induced a significant reduction (p < ou = 0.05) in water intake over a period of 300 min. This result indicates that the central 5-HT system plays an important role, probably at the level of the periventricular hypothalamus, in the modulation of drinking behavior induced by water deprivation

A semi-quantitative atlas of 5-hydroxytryptamine-1 receptors in the primate brain.

The regional distribution of 5-hydroxytryptamine-1 receptors in the primate brain was studied by semi-quantitative autoradiographic analysis of tritiated ligand binding. Areas showing the highest density of 5-hydroxytryptamine-1 receptors (greater than 200 fmol [3H]5-hydroxytryptamine bound per mg tissue), included the cerebral cortex (laminae I-II), claustrum, posterior cell group of the basal nucleus of Meynert, the infracommissural part of the globus pallidus, cortical amygdaloid nucleus, hippocampal formation (CA1-subiculum region, the anterior CA2, CA3 and CA4 regions and the molecular layer of the dentate gyrus), thalamic nuclei (parafascicular, parataenial, paraventricular and superior central lateral nuclei), substantia nigra pars reticulata, dorsal raphe nucleus and choroid plexus. The distribution of 5-hydroxytryptamine-1 receptors is compared to the distribution of both 5-hydroxytryptamine receptors and terminal fields of serotonergic projections as previously described in subprimates.

Inhibition of monoamine oxidase in monoaminergic neurones in the rat brain by irreversible inhibitors.

The irreversible inhibition of monoamine oxidase (MAO) inside and outside monoaminergic neurones in the rat brain by the suicide inhibitors clorgyline, selegiline (l-deprenyl), pheniprazine, phenelzine, iproniazid, pargyline and the d- and l-enantiomers of tranylcypromine was determined. This was achieved by incubating crude synaptosomal preparations of hypothalamus and striatum from rats treated with the inhibitors 24 hr earlier, with low concentrations of [14C]serotonin (0.1 microM), [14C]-noradrenaline (0.25 microM) and [14C]dopamine (0.25 microM) in the absence and presence of selective uptake inhibitors (citalopram, maprotiline and amfonelic acid, respectively). It was found that all inhibitors inhibited the deamination of serotonin and noradrenaline outside the amine neurons at slightly lower doses than that within these neurones. This could at least in part be due to protection of MAO by the endogenous amines in these neurones. The deamination of dopamine was rather more strongly inhibited inside the neutrons than outside, particularly at higher doses. There was no indication that tranylcypromine or phenelzine was accumulated in the neurones by the membranous amine uptake mechanisms. The rate of the recovery of the deaminating activities inside and outside the serotonergic and noradrenergic neurones in hypothalamus after phenelzine and clorgyline inhibition was the same (50% recovery after 12-15 days), which indicates similar rate of synthesis of MAO in different cell types.

125I-LSD autoradiography confirms the preferential localization of caudate-putamen S2 receptors to the caudal (peripallidal) region.

The in vitro binding of 125I-lysergic acid diethylamide (LSD) to horizontal sections of rat brain was quantified with computer-assisted autoradiography. Specific binding of 125I-LSD to D2 and S2 sites, defined with 5 microM (+)-butaclamol, was 65-94% of the total binding. Identification of S2 sites with 50 nM ketanserin showed that over 90% of the butaclamol-displaced 125I-LSD binding in the frontal, cingulate and parietal neocortex was to S2 sites (22-55 fmol/mg protein). 125I-LSD also labeled a dense population of S2 sites (16 fmol/mg protein) in the caudal caudate-putamen at the level of the globus pallidus which exceeded by 5-fold the concentration of S2 sites (3 fmol/mg protein) in more rostral portions of the caudate-putamen. The peripallidal distribution of S2 sites was identical to that observed previously with the less selective S2 label, [3H]spiperone. The dense concentration of S2 sites in the caudal caudate-putamen and their overlap with D2 binding sites indicates that the peripallidal neostriatum may play an important role in interactions between dopamine and serotonin.