Genomic responses to 5-HT1A or 5-HT2A/2C receptor activation is differentially regulated in four regions of rat brain

Classical Psychedelics as Therapeutics in Psychiatry - Current Clinical Evidence and Potential Therapeutic Mechanisms in Substance Use and Mood Disorders

Classical psychedelics, primarily psilocybin and lysergic acid diethylamide (LSD), have been used and extensively studied in Western medicine as part of substance-assisted psychotherapy in the 1950s and 1960s. Modern clinical research is currently gaining momentum and provides new evidence for the safety and efficacy of classical psychedelics (primarily psilocybin, but also LSD and ayahuasca) in the treatment of different psychiatric conditions, including substance use and mood disorders.In this review article, we outline common pathological mechanisms of substance use disorders (SUD) and unipolar depression. Next, the current literature on the effects of psychedelics is summarized in order to generate hypotheses regarding their potential therapeutic mechanisms of action in treating these psychiatric conditions. Finally, we review and discuss clinical trials published since 2011 investigating the effects of psychedelics in SUD and depression.While results from those modern clinical trials are promising, most of them do not meet the methodological requirements to allow firm conclusions on the clinical efficacy of psychedelics. Larger, blinded, randomized controlled trials (RCT) with clearly defined patient groups and well-defined primary endpoints are needed. Additionally, the therapeutic mechanisms of classical psychedelics are currently unknown. This review presents hypotheses derived from preclinical and human studies that need to be tested in future trials to better understand the clinical potential of psychedelic substances in modern psychiatry.

Inhibitor of Striatal-Enriched Protein Tyrosine Phosphatase, 8-(Trifluoromethyl)-1,2,3,4,5-Benzopentathiepin-6-Amine hydrochloride (TC-2153), Produces Antidepressant-Like Effect and Decreases Functional Activity and Protein Level of 5-HT2A Receptor in the Brain

The serotoninergic 5-HT_2A receptor is involved in the mechanism of depression and antidepressant drugs action. Earlier we showed that striatal-enriched protein tyrosine phosphatase (STEP) inhibitor - 8-(trifluoromethyl)-1,2,3,4,5-benzopentathiepin-6-amine hydrochloride (TC-2153) affects both the brain serotoninergic system and the brain-derived neurotropic factor that are known to be involved in the psychopathology of depression. In the present study we investigated the effects of chronic TC-2153 administration on behavior in the standard battery of tests as well as the effects of acute and chronic TC-2153 treatment on the brain 5-HT_2A receptors in mice. We obtained a prominent antidepressant-like effect of chronic TC-2153 treatment in the forced swim test without any adverse side effects on locomotor activity, anxiety, exploration, motor skill and obsessive-compulsive-like behavior. Moreover, both acute and chronic TC-2153 administration inhibited the functional activity of 5-HT_2A receptors estimated by the number of 2,5-dimethoxy-4-iodoamphetamine (DOI, agonist of 5-HT_2A receptors)-induced head-twitches. TC-2153 treatment also attenuated the DOI-induced c-fos expression in cortical and hippocampal neurons and reduced the 5-HT_2A receptor protein level in the hippocampus and frontal cortex, but not in the striatum. Taken together, our combined data demonstrate that the antidepressant effect of STEP inhibitor TC-2153 could be mediated by its inhibitory properties towards the 5-HT_2A receptor-mediated signaling.

The Effects of Hallucinogens on Gene Expression

The classic serotonergic hallucinogens, or psychedelics, have the ability to profoundly alter perception and behavior. These can include visual distortions, hallucinations, detachment from reality, and mystical experiences. Some psychedelics, like LSD, are able to produce these effects with remarkably low doses of drug. Others, like psilocybin, have recently been demonstrated to have significant clinical efficacy in the treatment of depression, anxiety, and addiction that persist for at least several months after only a single therapeutic session. How does this occur? Much work has recently been published from imaging studies showing that psychedelics alter brain network connectivity. They facilitate a disintegration of the default mode network, producing a hyperconnectivity between brain regions that allow centers that do not normally communicate with each other to do so. The immediate and acute effects on both behaviors and network connectivity are likely mediated by effector pathways downstream of serotonin 5-HT2A receptor activation. These acute molecular processes also influence gene expression changes, which likely influence synaptic plasticity and facilitate more long-term changes in brain neurochemistry ultimately underlying the therapeutic efficacy of a single administration to achieve long-lasting effects. In this review, we summarize what is currently known about the molecular genetic responses to psychedelics within the brain and discuss how gene expression changes may contribute to altered cellular physiology and behaviors.

Psychedelics Recruit Multiple Cellular Types and Produce Complex Transcriptional Responses Within the Brain

There has recently been a resurgence of interest in psychedelics, substances that profoundly alter perception and cognition and have recently demonstrated therapeutic efficacy to treat anxiety, depression, and addiction in the clinic. The receptor mechanisms that drive their molecular and behavioral effects involve activation of cortical serotonin 5-HT_2A receptors, but the responses of specific cellular populations remain unknown. Here, we provide evidence that a small subset of 5-HT_2A-expressing excitatory neurons is directly activated by psychedelics and subsequently recruits other select cell types including subpopulations of inhibitory somatostatin and parvalbumin GABAergic interneurons, as well as astrocytes, to produce distinct and regional responses. To gather data regarding the response of specific neuronal populations, we developed methodology for fluorescence-activated cell sorting (FACS) to segregate and enrich specific cellular subtypes in the brain. These methods allow for robust neuronal sorting based on cytoplasmic epitopes followed by downstream nucleic acid analysis, expanding the utility of FACS in neuroscience research.

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Psychedelics activate distinct transcription across cell types, including excitatory neurons, inhibitory neurons, and astrocytes

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Psychedelics induce internalization of 5-HT_2A receptors throughout the cortex and claustrum

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FACS can separate neuronal subpopulations that require non-nuclear markers

Psychedelic drugs are known to act through the 5-HT_2A receptor to produce many of their effects, however, the precise cellular populations in the brain which respond to this class of drugs remain unknown. We use flow cytometric analyses, immunohistochemistry, and gene expression analyses to identify small populations of specific cells in the brain that are activated by the psychedelic drug, (R)-DOI. The methodology used in these studies will be useful to determine the molecular effects of any manipulation or disease on particular brain cells.

Nucleus incertus contributes to an anxiogenic effect of buspirone in rats: Involvement of 5-HT1A receptors

The nucleus incertus (NI), a brainstem structure with diverse anatomical connections, is implicated in anxiety, arousal, hippocampal theta modulation, and stress responses. It expresses a variety of neurotransmitters, neuropeptides and receptors such as 5-HT1A, D2 and CRF1 receptors. We hypothesized that the NI may play a role in the neuropharmacology of buspirone, a clinical anxiolytic which is a 5-HT1A receptor partial agonist and a D2 receptor antagonist. Several preclinical studies have reported a biphasic anxiety-modulating effect of buspirone but the precise mechanism and structures underlying this effect are not well-understood. The present study implicates the NI in the anxiogenic effects of a high dose of buspirone. Systemic buspirone (3 mg/kg) induced anxiogenic effects in elevated plus maze, light-dark box and open field exploration paradigms in rats and strongly activated the NI, as reflected by c-Fos expression. This anxiogenic effect was reproduced by direct infusion of buspirone (5 μg) into the NI, but was abolished in NI-CRF-saporin-lesioned rats, indicating that the NI is present in neural circuits driving anxiogenic behaviour. Pharmacological studies with NAD 299, a selective 5-HT1A antagonist, or quinpirole, a D2/D3 agonist, were conducted to examine the receptor system in the NI involved in this anxiogenic effect. Opposing the 5-HT1A agonism but not the D2 antagonism of buspirone in the NI attenuated the anxiogenic effects of systemic buspirone. In conclusion, 5-HT1A receptors in the NI contribute to the anxiogenic effect of an acute high dose of buspirone in rats and may be functionally relevant to physiological anxiety.

Circadian rhythms of clock gene expression in the cerebellum of serotonin-deficient Pet-1 knockout mice

Serotonin plays an important role in the central regulation of circadian clock function. Serotonin levels are generally higher in the brain during periods of high activity, and these periods are in turn heavily regulated by the circadian clock located in the suprachiasmatic nucleus. However, the role of serotonin as a regulator of circadian rhythms elsewhere in the brain has not been extensively examined. In this study, we examined circadian rhythms of clock gene expression in the cerebellum in mice lacking the Pet-1 transcription factor, which results in a developed brain that is deficient in serotonin neurons. If serotonin helps to synchronize rhythms in brain regions other than the suprachiasmatic nucleus, we would expect to see differences in clock gene expression in these serotonin deficient mice. We found minor differences in the expression of Per1 and Per2 in the knockout mice as compared to wild type, but these differences were small and of questionable functional importance. We also measured the response of cerebellar clocks to injections of the serotonin agonist 8-OH-DPAT during the early part of the night. No effect on clock genes was observed, though the immediate-early gene Fos showed increased expression in wild type mice but not the knockouts. These results suggest that serotonin is not an important mediator of circadian rhythms in the cerebellum in a way that parallels its regulation of the circadian clock in the suprachiasmatic nucleus.

Clozapine effects on adenylyl cyclase activity and serotonin type 1A receptors in human brain post-mortem

Although the pharmacological profile of the atypical antipsychotic clozapine has been extensively studied in animal models, little information is available on its effects in the human brain. In particular, much interest is focused on the understanding of clozapine activity on serotonin (5-HT) neurotransmission, particularly on 5-HT receptor of type 1A (5-HT(1A)) that seems to play a pivotal role in the control of the 5-HT system. The present work, therefore, aimed at evaluating the effects of clozapine and its major metabolite, norclozapine, on the modulation of adenylyl cyclase (AC) velocity via 5-HT(1A) receptors in human post-mortem brain regions, in particular the prefrontal cortex, hippocampus and raphe nuclei. Concomitantly, the ability of the two compounds to displace the specific binding of the 5-HT(1A) receptor agonist [³H]-8-hydroxy-(2-di-N-propylamino) tetralin ([³H]-8-OH-DPAT) was evaluated in the same brain areas. The results showed that both clozapine and norclozapine, although with a 20-fold lower affinity, displaced [³H]8-OH-DPAT binding in all of the brain regions analysed, suggesting their interaction with 5-HT(1A) receptors. At the same time, clozapine and, to a lesser extent, norclozapine were found to inhibit the forskolin (FK)-stimulated AC system, while decreasing cyclic adenosine monophosphate (cAMP) concentrations in the hippocampus only. The receptor characterisation of the clozapine effect on AC observed in the hippocampus by the use of antagonists showed a mixed profile, involving not only the 5-HT(1A) receptor but also a muscarinic (M) receptor subtype, most likely the M₄ one. These findings, while considering all the limitations due to the use of post-mortem tissues, are strongly suggestive of a region-dependent pharmacological action of clozapine in the human brain that may explain its peculiar clinical effects and open up research towards novel targets for future antipsychotic drugs.

Fos expression in response to dopamine D3-preferring phenylpiperazine drugs given with and without cocaine

WC 44 and WC 10 are phenylpiperazines with low (23 fold) to moderate (42 fold) selectivity for dopamine D3 receptors (D3Rs) over D2Rs, respectively. WC 44 is a full D3R agonist in the forskolin-stimulated adenylyl cyclase (AC) assay, whereas WC 10 has little efficacy. In contrast to their opposite effects in the AC assay, these drugs often produce similar behavioral effects, suggesting that the AC assay does not predict the efficacy of these drugs in vivo. Here, we examined whether Fos protein expression induced by these drugs would be more consistent with their behavioral effects in vivo. Rats received either vehicle, WC 10 (5.6 mg/kg, i.p.), WC 44 (10.0 mg/kg, i.p), cocaine (10.0 mg/kg, i.p.), or cocaine with WC 10 (5.6 mg/kg, i.p.) or with WC 44 (10.0 mg/kg, i.p). Locomotion was monitored for 90 min and the brains were harvested for immunohistochemistry. Both WC 10 and WC 44 decreased spontaneous and cocaine-induced locomotion. Both compounds also increased Fos expression relative to saline in the dorsal striatum and nucleus accumbens core and shell, and relative to cocaine alone in the nucleus accumbens shell. The findings suggest that even though these compounds have different efficacy in the AC bioassy, they produce similar brain activation and attenuation of cocaine hyperlocomotion. Together with our previous research demonstrating that these compounds down-shift the cocaine self-administration dose-effect function, the findings support the idea that D3R-selective compounds may be useful for cocaine dependence medications development.

Multiple controls exerted by 5-HT2C receptors upon basal ganglia function: from physiology to pathophysiology

Serotonin2C (5-HT2C) receptors are expressed in the basal ganglia, a group of subcortical structures involved in the control of motor behaviour, mood and cognition. These receptors are mediating the effects of 5-HT throughout different brain areas via projections originating from midbrain raphe nuclei. A growing interest has been focusing on the function of 5-HT2C receptors in the basal ganglia because they may be involved in various diseases of basal ganglia function notably those associated with chronic impairment of dopaminergic transmission. 5-HT2C receptors act on numerous types of neurons in the basal ganglia, including dopaminergic, GABAergic, glutamatergic or cholinergic cells. Perhaps inherent to their peculiar molecular properties, the modality of controls exerted by 5-HT2C receptors over these cell populations can be phasic, tonic (dependent on the 5-HT tone) or constitutive (a spontaneous activity without the presence of the ligand). These controls are functionally organized in the basal ganglia: they are mainly localized in the input structures and preferentially distributed in the limbic/associative territories of the basal ganglia. The nature of these controls is modified in neuropsychiatric conditions such as Parkinson's disease, tardive dyskinesia or addiction. Most of the available data indicate that the function of 5-HT2C receptor is enhanced in cases of chronic alterations of dopamine neurotransmission. The review illustrates that 5-HT2C receptors play a role in maintaining continuous controls over the basal ganglia via multiple diverse actions. We will discuss their interest for treatments aimed at ameliorating current pharmacotherapies in schizophrenia, Parkinson's disease or drugs abuse.

Functional anatomy of 5-HT2A receptors in the amygdala and hippocampal complex: relevance to memory functions

The amygdaloid complex and hippocampal region contribute to emotional activities, learning, and memory. Mounting evidence suggests a primary role for serotonin (5-HT) in the physiological basis of memory and its pathogenesis by modulating directly the activity of these two areas and their cross-talk. Indeed, both the amygdala and the hippocampus receive remarkably dense serotoninergic inputs from the dorsal and median raphe nuclei. Anatomical, behavioral and electrophysiological evidence indicates the 5-HT2A receptor as one of the principal postsynaptic targets mediating 5-HT effects. In fact, the 5-HT2A receptor is the most abundant 5-HT receptor expressed in these brain structures and is expressed on both amygdalar and hippocampal pyramidal glutamatergic neurons as well as on γ-aminobutyric acid (GABA)-containing interneurons. 5-HT2A receptors on GABAergic interneurons stimulate GABA release, and thereby have an important role in regulating network activity and neural oscillations in the amygdala and hippocampal region. This review will focus on the distribution and physiological functions of the 5-HT2A receptor in the amygdala and hippocampal region. Taken together the results discussed here suggest that 5-HT2A receptor may be a potential therapeutic target for those disorders related to hippocampal and amygdala dysfunction.

Induction of the plasticity-associated immediate early gene Arc by stress and hallucinogens: role of brain-derived neurotrophic factor

Exposure to stress and hallucinogens in adulthood evokes persistent alterations in neurocircuitry and emotional behaviour. The structural and functional changes induced by stress and hallucinogen exposure are thought to involve transcriptional alterations in specific effector immediate early genes. The immediate early gene, activity regulated cytoskeletal-associated protein (Arc), is important for both activity and experience dependent plasticity. We sought to examine whether trophic factor signalling through brain-derived neurotrophic factor (BDNF) contributes to the neocortical regulation of Arc mRNA in response to distinct stimuli such as immobilization stress and the hallucinogen 2,5-dimethoxy-4-iodoamphetamine (DOI). Acute exposure to either immobilization stress or DOI induced Arc mRNA levels within the neocortex. BDNF infusion into the neocortex led to a robust up-regulation of local Arc transcript expression. Further, baseline Arc mRNA expression in the neocortex was significantly decreased in inducible BDNF knockout mice with an adult-onset, forebrain specific BDNF loss. The induction of Arc mRNA levels in response to both acute immobilization stress or a single administration of DOI was significantly attenuated in the inducible BDNF knockout mice. Taken together, our results implicate trophic factor signalling through BDNF in the regulation of cortical Arc mRNA expression, both under baseline conditions and following stress and hallucinogen exposure. These findings suggest the possibility that the regulation of Arc expression via BDNF provides a molecular substrate for the structural and synaptic plasticity observed following stimuli such as stress and hallucinogens.

F15063, a potential antipsychotic with dopamine D(2)/D(3) receptor antagonist and 5-HT(1A) receptor agonist properties: influence on immediate-early gene expression in rat prefrontal cortex and striatum

Brain region-specific modulation of immediate-early gene (IEG) may constitute a marker of antipsychotic drug-like activity. We investigated the effects of the putative antipsychotic drug N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine (F15063), a compound that targets both dopamine D(2) and serotonin 5-HT(1A) receptors, in comparison with haloperidol and clozapine on rat mRNA expression of IEG i.e. the zinc-fingered transcription factors c-fos, fosB, zif268, c-jun and junB, two transcription factors of the nuclear receptor family nur77 and nor1, and the effector IEG arc. F15063 (10 mg/kg) and clozapine (10 mg/kg), but not haloperidol (0.63 mg/kg), induced c-fos and fosB mRNA expression in prefrontal cortex, a region associated with control of cognition and negative symptoms of schizophrenia. In striatum, only c-fos, fosB, junB and nur77 were induced by clozapine whereas all IEG mRNAs were increased by haloperidol and F15063 (from 2.5 mg/kg) with similar high efficacy despite a total absence of F15063-induced catalepsy. However, at 0.63 mg/kg, F15063 induced a lower degree of striatal IEG mRNA expression than haloperidol and pretreatment with the serotonin 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl-N-(2-pyridinyl)cyclohexane carboxamide trihydrochloride (WAY100635) (0.63 mg/kg) increased the level of IEG mRNA induction by F15063. Furthermore, (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)-8-OH-DPAT] at 0.16 mg/kg decreased haloperidol-induced striatal IEG mRNA expression although it exerted no effects on its own. These results are consistent with an activation of serotonin 5-HT(1A) receptors by F15063, thus reducing D(2) blockade-induced striatal IEG mRNA. Furthermore, the substantial F15063-induced expression of IEGs such as c-fos in striatum is not related to cataleptogenic activity and may act more as a marker of efficacious dopamine D(2) receptor blockade.

Role of G(q) protein in behavioral effects of the hallucinogenic drug 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane

Extensive evidence suggests that 5-HT2 receptors may play a role in mental disorders including schizophrenia. In addition, several studies indicate that G(q)-coupled 5-HT(2A) receptors are likely targets for the initiation of events leading to the hallucinogenic behavior elicited by lysergic acid diethylamide (LSD), (+/-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), and related drugs. However, 5-HT(2A) receptors couple to other G proteins in addition to G(q) protein. To evaluate the role of the G(q) signaling pathway in DOI-induced behaviors, we utilized two behavioral models of 5-HT(2A) receptor activation: induction of head-twitches by DOI, a common response to hallucinogenic drugs in rodents, and DOI elicited anxiolytic-like effects in the elevated plus maze. Experimental subjects were genetically modified mice [Galpha(q)(-/-)] in which the G(q) alpha gene was eliminated. Galpha(q)(-/-) mice exhibited a decrease in DOI-induced head-twitches, when compared to wild-type littermates. In addition, the DOI-induced increase in anxiolytic-like behavior was abolished in Galpha(q)(-/-) mice. These results, combined with our finding that DOI-induced FOS expression in the medial prefrontal cortex was also eliminated in Galpha(q)(-/-) mice, suggests a key role for G(q) protein in hallucinogenic drug effects.

Antidepressant treatment reduces Fos-like immunoreactivity induced by swim stress in different columns of the periaqueductal gray matter

Antidepressant treatment attenuates behavioral changes induced by uncontrollable stress. The periaqueductal gray matter (PAG) is proposed to be a brain site involved in the behavioral responses to uncontrollable stress and antidepressant effects. The main goal of the present study was to investigate the effect of antidepressant treatment on the pattern of neural activation of the PAG along its mediolateral and rostrocaudal subregions after a forced swim stress episode. Male Wistar rats were sub-acutely treated with desipramine (a selective noradrenaline re-uptake blocker, three injections of 10 mg/kg in 24 h) or clomipramine (a non-selective serotonin and noradrenaline re-uptake blocker, three injections of 10 mg/kg in 24 h) and submitted to the forced swimming test (FST). Two hours after the test their brain were removed for Fos immunohistochemistry. Fos-like immunoreactivity (FLI) in rostral, intermediate and caudal portions of dorsomedial (dmPAG), dorsolateral (dlPAG), lateral (lPAG) and ventrolateral (vlPAG) PAG were quantified by a computerized system. The FST session increased FLI in most parts of the PAG. Previous treatment with desipramine or clomipramine reduced FLI in all columns of the PAG. FLI in the PAG correlated positively with to the immobility time and negatively with to climbing behavior scored during the test. These results indicate that neurons in the PAG are activated by uncontrollable stress. Moreover, inhibitory action of antidepressants on this activity may be associated with the anti-immobility effects of these drugs in the FST.

Rats subjected to extended L-tryptophan restriction during early postnatal stage exhibit anxious-depressive features and structural changes

Serotonin transmission dysfunction has been suggested to play an important role in depression and anxiety. This study reports the results of a series of experiments in which rats were subjected to extended maize-based tortilla diets during early postnatal stages. This diet contains only approximately 20% of the L-tryptophan in normal diets of laboratory rodents. Compared with controls, experimental rats displayed a significant increase of immobility counts in the forced swimming test and exhibited anxiety-like behavior in the elevated plus maze test after 1 month of diet treatment. Low levels of serotonin contents were found in prefrontal cortex, striatum, hippocampus, and brainstem using high-performance liquid chromatography. Immunocytochemical reactions against 5-Bromo-2'-deoxyuridine revealed a significant decrease in the proliferation rate for the subgranular zone of dentate gyrus. c-Fos expression after the forced swimming test was found reduced in prefrontal cortex, dentate gyrus, CA1, and hilus of hippocampus and amygdala. Moreover, dendrite arbor atrophy and decreased spine density were evident in Golgi-Cox-impregnated CA1 pyramidal neurons. Abnormal dendrite swelling in dentate gyrus granule cells was also observed. These findings indicate an involvement of hyposerotoninergia in emotional disturbance produced by L-tryptophan restriction during critical developmental stages and suggest that neuroplasticity changes might underlie these changes.

Investigation of the SSRI augmentation properties of 5-HT2 receptor antagonists using in vivo microdialysis

Recent evidence that 5-HT(2) receptors exert a negative influence on central 5-hydroxytryptamine (5-HT) neurones suggests that 5-HT(2) receptor antagonists may augment the effects of serotonin selective reuptake inhibitors (SSRIs). The present study investigated whether pre-treatment with 5-HT(2) receptor antagonists enhances the effect of SSRI administration on hippocampal extracellular 5-HT of freely moving rats. Administration of the SSRI citalopram at a low (2mg kg(-1)) and higher (4 mg kg(-1)) dose, increased dialysate 5-HT by 5- and 8-fold, respectively. Pre-treatment with the 5-HT(2) receptor antagonist ketanserin (4 mg kg(-1)) augmented the effect of 4 mg kg(-1) but not 2mg kg(-1) citalopram. The effect of 4 mg kg(-1) citalopram was also augmented by pre-treatment with either the 5-HT(2C) receptor antagonist SB 242084 (0.5mg kg(-1)) or the 5-HT(2A) receptor antagonist MDL 100907 (0.5mg kg(-1)). As with citalopram, fluoxetine elevated dialysate 5-HT at both a low (5mg kg(-1)) and higher (20mg kg(-1)) dose. However, neither dose of fluoxetine was augmented by ketanserin (4 mg kg(-1)). These results confirm recent findings that 5-HT(2) receptor antagonists augment the effect of citalopram on extracellular 5-HT, and indicate the involvement of 5-HT(2C) and possibly 5-HT(2A) receptors. The lack of augmentation of fluoxetine might reflect the intrinsic 5-HT(2) receptor antagonist properties of this drug.

Effects of chronic paroxetine pretreatment on (+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin induced c-fos expression following sexual behavior

Chronic treatment with the selective serotonin reuptake inhibitor paroxetine impairs the functioning of 5-HT(1A) receptors involved in ejaculation. This could underlie the development of delayed ejaculation often reported by men treated with paroxetine. The neurobiological substrate linking the effects of selective serotonin reuptake inhibitor-treatment and 5-HT(1A) receptor activation with ejaculation was investigated. Male Wistar rats that were pretreated with paroxetine (20 mg/kg/day p.o.) or vehicle for 22 days and had received an additional injection with the 5-HT(1A) receptor agonist 8-OH-DPAT ((+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.4 mg/kg s.c.) or saline on day 22, 30 min prior to a sexual behavior test, were perfused 1 h after the sexual behavior test. Brains were processed for Fos-, and oxytocin immunohistochemistry. The drug treatments markedly changed both sexual behavior and the pattern and number of Fos-immunoreactive cells in the brain. Chronic pretreatment with paroxetine caused delayed ejaculation. Acute injection with 8-OH-DPAT facilitated ejaculation in vehicle-pretreated rats, notably evident in a strongly reduced intromission frequency, whereas 8-OH-DPAT had no effects in paroxetine-pretreated rats. Chronic treatment with paroxetine reduced Fos-immunoreactivity in the locus coeruleus, and prevented the increase in Fos-immunoreactive neurons induced by 8-OH-DPAT in the oxytocinergic magnocellular part of the paraventricular nucleus as well as in the locus coeruleus. Since oxytocin and noradrenalin facilitate ejaculation, the alterations in Fos-IR in these areas could connect selective serotonin reuptake inhibitor treatment and 5-HT(1A) receptor activation to ejaculation. Chronic paroxetine treatment and 8-OH-DPAT changed c-fos expression in a number of other brain areas, indicating that Fos-immunohistochemistry is a useful tool to find locations where selective serotonin reuptake inhibitors and 8-OH-DPAT exert their effects.

Hallucinogens

Hallucinogens (psychedelics) are psychoactive substances that powerfully alter perception, mood, and a host of cognitive processes. They are considered physiologically safe and do not produce dependence or addiction. Their origin predates written history, and they were employed by early cultures in a variety of sociocultural and ritual contexts. In the 1950s, after the virtually contemporaneous discovery of both serotonin (5-HT) and lysergic acid diethylamide (LSD-25), early brain research focused intensely on the possibility that LSD or other hallucinogens had a serotonergic basis of action and reinforced the idea that 5-HT was an important neurotransmitter in brain. These ideas were eventually proven, and today it is believed that hallucinogens stimulate 5-HT(2A) receptors, especially those expressed on neocortical pyramidal cells. Activation of 5-HT(2A) receptors also leads to increased cortical glutamate levels presumably by a presynaptic receptor-mediated release from thalamic afferents. These findings have led to comparisons of the effects of classical hallucinogens with certain aspects of acute psychosis and to a focus on thalamocortical interactions as key to understanding both the action of these substances and the neuroanatomical sites involved in altered states of consciousness (ASC). In vivo brain imaging in humans using [(18)F]fluorodeoxyglucose has shown that hallucinogens increase prefrontal cortical metabolism, and correlations have been developed between activity in specific brain areas and psychological elements of the ASC produced by hallucinogens. The 5-HT(2A) receptor clearly plays an essential role in cognitive processing, including working memory, and ligands for this receptor may be extremely useful tools for future cognitive neuroscience research. In addition, it appears entirely possible that utility may still emerge for the use of hallucinogens in treating alcoholism, substance abuse, and certain psychiatric disorders.

Fenfluramine evokes 5-HT2A receptor-mediated responses but does not displace [11C]MDL 100907: small animal PET and gene expression studies

The in vivo binding of the 5-HT(2A) receptor-selective positron emission tomography (PET) ligand [(11)C]MDL 100907 and its sensitivity to endogenous 5-HT were quantified in rat brain using quad-HIDAC, a novel high-resolution PET camera for small animals. Specific binding of [(11)C]MDL 100907, estimated using volume of interest (VOI) to cerebellum ratios, corresponded well with both the known distribution of 5-HT(2A) receptors and tissue:cerebellum ratios obtained using ex vivo dissection. Specific binding was blocked by predosing with either nonradioactive MDL 100907 (0.2 or 0.4 mg/kg i.v.) or the 5-HT(2A/2C) receptor antagonist ketanserin (2 mg/kg i.v.), but was unaffected in rats pretreated with the 5-HT releasing agent, fenfluramine (10 mg/kg i.p.). In parallel studies, the same dose of fenfluramine was shown to be sufficient to cause an increase in the expression of the immediate early genes (IEG) c-fos and Arc mRNA in cortical regions with high 5-HT(2A) receptor density. This increase was blocked by MDL 100907 (0.2 mg/kg i.v.), confirming a 5-HT(2A) receptor-mediated effect. The results demonstrate that PET with [(11)C]MDL 100907 is insensitive to an increased concentration of synaptic 5-HT, implying that the ligand can be used clinically to monitor 5-HT(2A) receptor function or dysfunction in disease or during therapy, without the need to consider concomitant changes in neurotransmitter concentration.

Serotonin in the dorsal periaqueductal gray modulates inhibitory avoidance and one-way escape behaviors in the elevated T-maze

The dorsal periaqueductal gray has been implicated in the modulation of escape behavior, a defensive behavior that has been related to panic disorder. Intra-dorsal periaqueductal gray injection of serotonin or drugs that mimic its effects inhibits escape induced by electrical or chemical stimulation of this brainstem area. In this study, we investigate whether intra-dorsal periaqueductal gray injection of 5-HT receptor agonists attenuates escape generated by an ethologically based model of anxiety, the elevated T-maze. This test also allows the measurement of inhibitory avoidance, which has been related to generalized anxiety disorder. The effects of the 5-HT receptor agonists were compared in animals with or without a previous exposure to the open arms of the elevated T-maze. In these two test conditions, intra-dorsal periaqueductal gray injection of the endogenous agonist serotonin or the 5-HT(2B/2C) receptor agonist m-chlorophenylpiperazine (mCPP) enhanced inhibitory avoidance, suggesting an anxiogenic effect. The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) impaired this response, suggesting an anxiolytic effect, and the preferential 5-HT2A receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) was ineffective. All these agonists inhibited escape behavior. Apart from mCPP, the effect on escape was detected only in animals pre-exposed to the open arm. None of the drugs tested affected locomotion in the open-field test. Taken altogether, our findings suggest that 5-HT1A and 5-HT2c receptors in the dorsal periaqueductal gray exert opposed control on inhibitory avoidance, implicating these receptors in anxiety conditioning. As previously observed in tests employing the aversive stimulation of the dorsal periaqueductal gray, 5-HT1A and 5-HT2A receptors in this brain area are involved in escape inhibition. Therefore, in different animal models, the activation of these two subtypes of receptors in the dorsal periaqueductal gray consistently attenuates the expression of a panic-related behavior.

Lysergic acid diethylamide-induced Fos expression in rat brain: role of serotonin-2A receptors

Lysergic acid diethylamide (LSD) produces altered mood and hallucinations in humans and binds with high affinity to serotonin-2A (5-HT(2A)) receptors. Although LSD interacts with other receptors, the activation of 5-HT(2A) receptors is thought to mediate the hallucinogenic properties of LSD. The goal of this study was to identify the brain sites activated by LSD and to determine the influence of 5-HT(2A) receptors in this activation. Rats were pretreated with the 5-HT(2A) receptor antagonist MDL 100907 (0.3 mg/kg, i.p.) or vehicle 30 min prior to LSD (500 microg/kg, i.p.) administration and killed 3 h later. Brain tissue was examined for Fos protein expression by immunohistochemistry. LSD administration produced a five- to eight-fold increase in Fos-like immunoreactivity in medial prefrontal cortex, anterior cingulate cortex, and central nucleus of amygdala. However, in dorsal striatum and nucleus accumbens no increase in Fos-like immunoreactivity was observed. Pretreatment with MDL 100907 completely blocked LSD-induced Fos-like immunoreactivity in medial prefrontal cortex and anterior cingulate cortex, but only partially blocked LSD-induced Fos-like immunoreactivity in amygdala. Double-labeled immunohistochemistry revealed that LSD did not induce Fos-like immunoreactivity in cortical cells expressing 5-HT(2A) receptors, suggesting an indirect activation of cortical neurons. These results indicate that the LSD activation of medial prefrontal cortex and anterior cingulate cortex is mediated by 5-HT(2A) receptors, whereas in amygdala 5-HT(2A) receptor activation is a component of the response. These findings support the hypothesis that the medial prefrontal cortex, anterior cingulate cortex, and perhaps the amygdala, are important regions involved in the production of hallucinations.

Similar ultrastructural distribution of the 5-HT(2A) serotonin receptor and microtubule-associated protein MAP1A in cortical dendrites of adult rat

As visualized by light and electron microscopic immunocytochemistry, the distribution of the neuronal serotonin-2A (5-HT(2A)) receptor is mainly intracellular throughout adult rat brain. This localization is particularly striking in the pyramidal cells of cerebral cortex, the dendrites of which are intensely immunoreactive, but without any labeling of their spines. In view of recent yeast two-hybrid and biochemical results suggesting an association of 5-HT(2A) receptors with the cytoskeletal microtubule-associated protein MAP1A, the respective subcellular distributions of the receptors and of MAP1A were compared by quantitative electron microscopic immunocytochemistry in dendrites of adult rat frontoparietal cortex. Counts of silver-intensified immunogold particles revealed a higher density of 5-HT(2A) receptors in smaller rather than larger dendrites, and an apportionment between pre-defined compartments representing the plasma membrane and the cytoplasm that was proportional to the relative surface area of these compartments. MAP1A immunoreactivity also predominated in smaller versus larger dendrites, but with a slightly lower proportion of labeling in the plasma membrane versus cytoplasmic compartment. The co-localization of 5-HT(2A) receptors and MAP1A protein in the same dendrites could be demonstrated in double immunolabeling experiments. These results confirmed the predominantly somato-dendritic, intracellular localization of 5-HT(2A) receptors in cerebral cortex, showed their higher concentration in distal as opposed to proximal dendrites, and suggested their potential association to the cytoskeleton in cortical neurons in vivo. Such a distribution of 5-HT(2A) receptors reinforces our earlier hypothesis that 5-HT(2A) receptors participate in intraneuronal signaling processes involving the cytoskeleton, and raises the possibility that their activation could be dependent upon that of another co-localized, plasma membrane-bound, 5-HT receptor.

Effects of acute and chronic fluoxetine treatments on restraint stress-induced Fos expression

Chronic treatment with antidepressants has been shown to attenuate behavioral changes induced by uncontrollable stress. The mechanisms and brain sites of this effect, however, remain controversial. The objective of the present work was to investigate the effects of chronic and acute treatment with fluoxetine (FLX), a selective serotonin reuptake blocker, on Fos expression in animals submitted to restraint stress. Male Wistar rats (n = 3-9/group) received, during 1 or 21 days, intraperitoneal. Injections of vehicle (saline + 0.2% Tween-80, 1 ml/kg) or FLX (10 mg/kg). One hour after the last injection they were forced restrained for 2 h and sacrificed immediately after. Non-stressed animals were sacrificed 2 h after the last injection. The brains were removed and processed for immunohistochemistry. Fos-like immunoreactivity (FLI) was quantified by a computer system. In acutely treated animals FLX decreased stress-induced FLI in the medial amygdala (MeA), bed nucleus of the stria terminalis (BNST), ventrolateral part, and dorsolateral periaqueductal gray (PAG). After chronic treatment, however, the drug induced a significant increase in FLI in the BNST (ventrolateral and medial parts), lateral septal nucleus (LSN, dorsal part), dorsal raphe nucleus (DRN), and locus coeruleus in restrained group. In non-restrained animals chronic treatment with FLX increased FLI in the MeA, BNST (ventrolateral and dorsolateral parts), LSN (dorsal and intermediate parts), dorsolateral and dorsomedial PAG and in the DRN. The results suggest that chronic fluoxetine treatment induce plastic changes that result in a different regional pattern of Fos expression.

5-HT2A receptors stimulate ACTH, corticosterone, oxytocin, renin, and prolactin release and activate hypothalamic CRF and oxytocin-expressing cells

The 5-HT(2A/2C) agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI) stimulates hypothalamic neurons to increase the secretion of several hormones. This study addressed two questions: 1) are the neuroendocrine effects of DOI mediated via activation of 5-HT(2A) receptors; and 2) which neurons are activated by 5-HT(2A) receptors. The 5-HT(2A) antagonist (+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol (MDL 100,907; 0.001, 0.01, or 0.1 mg/kg, s.c.) was administered before rats were challenged with DOI (2.5 mg/kg, i.p.). MDL 100,907 produced a dose-dependent inhibition (ED(50) congruent with 0.001 mg/kg) of the effect of DOI on plasma levels of ACTH, corticosterone, oxytocin, prolactin, and renin without altering basal hormone levels. Complete blockade of the effect of DOI was achieved for all hormones at MDL 100,907 doses of 0.01-0.1 mg/kg. In a parallel experiment, DOI was injected 2 hr before killing to determine its effects on the expression of Fos, the product of the immediate early gene c-fos. DOI induced an increase in Fos immunoreactivity in corticotropin-releasing factor (CRF) and in oxytocin-expressing neurons but not in vasopressin-containing neurons in the hypothalamic paraventricular nucleus or CRF cells in the amygdala. Pretreatment with MDL 100,907 (0.1 mg/kg, s.c.) blocked the DOI-induced increase in Fos expression in all regions including the hypothalamus, amygdala (central and corticomedial), bed nucleus of the stria terminalis, and prefrontal cortical regions. The combined neuroanatomical and pharmacological observations suggest that the neuroendocrine responses to DOI are mediated by activation of neurons in the hypothalamic paraventricular nucleus and associated circuitry. Furthermore, selective activation of 5-HT(2A) receptors mediates the hormonal and Fos-inducing effects of DOI.

DOI-Induced activation of the cortex: dependence on 5-HT2A heteroceptors on thalamocortical glutamatergic neurons

Administration of the hallucinogenic 5-HT(2A/2C) agonist 1-[2, 5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) induces expression of Fos protein in the cerebral cortex. To understand the mechanisms subserving this action of DOI, we examined the consequences of pharmacological and surgical manipulations on DOI-elicited Fos expression in the somatosensory cortex of the rat. DOI dose-dependently increased cortical Fos expression. Pretreatment with the selective 5-HT(2A) antagonist MDL 100,907 completely blocked DOI-elicited Fos expression, but pretreatment with the 5-HT(2C) antagonist SB 206,553 did not modify DOI-elicited Fos expression. These data suggest that DOI acts through 5-HT(2A) receptors to increase cortical Fos expression. However, we found that DOI did not induce Fos in cortical 5-HT(2A) immunoreactive neurons but did increase expression in a band of neurons spanning superficial layer V to deep III, within the apical dendritic fields of layer V 5-HT(2A)-immunoreactive cells. This band of Fos immunoreactive neurons was in register with anterogradely labeled axons from the ventrobasal thalamus, which have previously been shown to be glutamatergic and express the 5-HT(2A) transcript. The effects of DOI were markedly reduced in animals pretreated with the AMPA/KA antagonist GYKI 52466, and lesions of the ventrobasal thalamus attenuated DOI-elicited Fos expression in the cortex. These data suggest that DOI activates 5-HT(2A) receptors on thalamocortical neurons and thereby increases glutamate release, which in turn drives Fos expression in cortical neurons through an AMPA receptor-dependent mechanism. These data cast new light on the mechanisms of action of hallucinogens.

Serotonergic regulation of mRNA expression of Arc, an immediate early gene selectively localized at neuronal dendrites

Arc (activity regulated, cytoskeleton associated protein) is an effector immediate early gene that is selectively localized in the neuronal dendrites. Elevation of brain 5-HT by the combined administration of the monoamine oxidase inhibitor, tranylcypromine (TCP, 5 mg/kg, i.p.), and the 5-HT precursor L-tryptophan (L-TP, 100 mg/kg, i.p.), increased Arc mRNA abundance in the cingulate, orbital, frontal and parietal cortices as well as in the striatum but a reduction was observed in the CA1 region of the hippocampus. The 5-HT releasing agent p-chloroamphetamine (PCA, 5 mg/kg, s.c.) also increased Arc mRNA in the cortical and striatal areas. Depleting brain 5-HT with the tryptophan hydroxylase inhibitor, p-chlorophenylalanine (pCPA, 300 mg/kg, i.p. for two days), on the other hand, significantly attenuated the increase in Arc mRNA induced by tranylcypromine and L-tryptophan (TCP/L-TP). Pretreatment with the 5-HT2 receptor antagonist ketanserin (2 mg/kg, i.p.) significantly attenuated the effect of TCP/L-TP in the cortex but only partially in striatum and did not affect the reduction in the CA1 region. The 5-HT2 agonist DOI (0.2, 1 and 2 mg/kg, i.p.) dose-dependently increased Arc mRNA abundance in cortical areas with a pattern similar to that of TCP/L-TP and PCA. DOI, however, had much weaker effects on Arc mRNA in the striatum and did not have any significant effect in the CA1, CA3 and the dentate gyms (DG) of the hippocampus. Pretreatment with ketanserin completely blocked the effect of DOI on Arc expression. These data suggest that Arc mRNA expression can be induced in the cortex by increases in extracellular 5-HT and that 5-HT2 receptors play a major part in mediating such effects. Additional 5-HT receptors as well as other neurotransmitters may also be involved, particularly in the striatum and in CA1 subfield of the hippocampus. Overall, our data suggest that expression of Arc mRNA is highly responsive to changes in brain 5-HT functions, and may provide a sensitive marker of postsynaptic 5-HT2(2A and 2C) receptor functions.

Effect of 5-HT(1A) receptor ligands on Fos-like immunoreactivity in rat brain: evidence for activation of noradrenergic transmission

This study investigated the effects of 8-OH-DPAT and various other 5-HT(1A) receptor agonists on brain noradrenergic transmission using Fos-like immunoreactivity (Fos-LI) as a marker of neural activation. Administration of 8-OH-DPAT (0.1 and 1 mg/kg) induced a marked and dose-related increase in the number of cells positive for Fos-LI in the locus coeruleus (LC), the main source of noradrenergic projections to the forebrain. This effect was also induced by the non-selective, partial 5-HT(1A) receptor agonist buspirone (10 mg/kg). The effect of both 8-OH-DPAT (0.1 mg/kg) and buspirone (10 mg/kg) on Fos-LI in the LC was blocked by pretreatment with the selective 5-HT(1A) receptor antagonist WAY 100635 (1 mg/kg). The active S(-)-enantiomer of the partial 5-HT(1A) receptor agonist (+/-)-MDL 75005EF (1 mg/kg) also induced the expression of Fos-LI in the LC, whereas the inactive R(+)-enantiomer of (+/-)-MDL 73005EF at the same dose did not. In addition to the LC, 8-OH-DPAT (0.1 mg/kg) also induced a marked increase in Fos-LI in various forebrain areas including the medial prefrontal cortex (infralimbic and cingulate cortical areas). More detailed analysis of the Fos response to 8-OH-DPAT in the medial prefrontal cortex revealed that the effect was attenuated by pretreatment with a combination of the beta(1)- and beta(2)-adrenoceptor antagonists ICI 118551 (4 mg/kg) and metoprolol (4 mg/kg), but not the alpha(1)-adrenoceptor antagonist prazosin (5 mg/kg). Taken together, the present findings provide immunocytochemical evidence that 5-HT(1A) receptor agonists activate noradrenergic neurones in the LC and that this leads to increased noradrenergic transmission at postsynaptic sites in the forebrain (specifically medial prefrontal cortex).

c-Fos proteins, induced by the serotonin receptor agonist DOI, are not expressed in 5-HT2A positive cortical neurons

In the present study, we tried to find out whether the expression of c-Fos proteins induced by DOI, an agonist of 5-HT2A/2C receptor subtypes is colocalized with 5-HT2A receptor protein in cortical neurons. 5-HT2A receptor protein was found in two major neuronal elements: dendritic processes (seen in layers II/III-V) and less abundantly in cell bodies (layer V). In our experiment, DOI (8 mg/kg) induced a robust appearance of c-Fos proteins mainly in neuronal nuclei of the upper part of layer V/IV, and a moderate amount of sparsely distributed nuclei in deep cortical layers (V and VI). It was found that c-Fos proteins never occurred in cortical neurons, which were immunopositive for the presence of 5-HT2A receptor protein. It is concluded that the induction of c-Fos proteins expression by DOI though initiated by activation of 5-HT2A receptors, requires the involvement of intermediate neurotransmitter(s). Additionally, our study indicates that the appearance of DOI-induced c-Fos proteins cannot be used as a simple and direct marker of localization and site of activation of 5-HT2A receptors.

Cellular and subcellular distribution of the serotonin 5-HT2A receptor in the central nervous system of adult rat

Light and electron microscope immunocytochemistry with a monoclonal antibody against the N-terminal domain of the human protein was used to determine the cellular and subcellular localization of serotonin 5-HT2A receptors in the central nervous system of adult rat. Following immunoperoxidase or silver-intensified immunogold labeling, neuronal, somatodendritic, and/or axonal immunoreactivity was detected in numerous brain regions, including all those in which ligand binding sites and 5-HT2A mRNA had previously been reported. The distribution of 5-HT2A-immunolabeled soma/dendrites was characterized in cerebral cortex, olfactory system, septum, hippocampal formation, basal ganglia, amygdala, diencephalon, cerebellum, brainstem, and spinal cord. Labeled axons were visible in every myelinated tract known to arise from immunoreactive cell body groups. In immunopositive soma/dendrites as well as axons, the 5-HT2A receptor appeared mainly cytoplasmic rather than membrane bound. Even though the dendritic labeling was generally stronger than the somatic, it did not extend to dendritic spines in such regions as the cerebral and piriform cortex, the neostriatum, or the molecular layer of the cerebellum. Similarly, there were no labeled axon terminals in numerous regions known to be strongly innervated by the immunoreactive somata and their axons (e.g., molecular layer of piriform cortex). It was concluded that the 5-HT2A receptor is mostly intracellular and transported in dendrites and axons, but does not reach into dendritic spines or axon terminals. Because it has previously been shown that this serotonin receptor is transported retrogradely as well as anterogradely, activates intracellular transduction pathways and intervenes in the regulation of the expression of many genes, it is suggested that one of its main functions is to participate in retrograde signaling systems activated by serotonin.

Phencyclidine induces D-1 dopamine receptor mediated Fos-like immunoreactivity in discretely localised populations of striatopallidal and striatoentopeduncular neurons in the rat

Phencyclidine (PCP), a non-competitive antagonist of the NMDA subtype of glutamate receptor, which also acts as an indirect dopamine agonist and at sigma sites, can induce a long lasting psychotic state when taken acutely. It is well established that PCP is toxic to specific limbic structures and we have recently demonstrated that it induces apoptosis of a subpopulation of striatal neurons. These neurons lie predominantly in the dorsomedial striatum and project to the globus pallidus. The mechanisms mediating this neuronal death are unclear though manipulations of dopamine transmission can induce striatal c-fos expression and continuous c-fos expression has been implicated in the molecular cascades controlling apoptosis. We accordingly undertook a series of experiments to determine the action of PCP on striatal Fos-like immunoreactivity (FLI). PCP (80 mg/kg, s.c.) elicited FLI in three distinct striatal areas, namely dorsomedial, dorsolateral and the nucleus accumbens. The level of PCP-induced FLI was consistently attenuated by the co-administration of the D-1 antagonist, SCH 23390. Vehicle injections also induced modest levels of FLI in the dorsomedial striatum and the nucleus accumbens which again were attenuated by SCH 23390. The type of striatal neuron in which PCP-induced FLI was determined by the use of a retrograde anatomical tracer. A colloidal gold tracer was thus injected into the major areas of termination of striatal projection neurons prior to the administration of PCP. This procedure demonstrated that the majority of the FLI positive striatal cells were striatopallidal neurons, though some FLI positive striatoentopeduncular neurons were also seen. The potential pharmacological mechanisms underlying the results are discussed. It is argued that the complex pattern of PCP-induced striatal FLI might be accounted for by a differential action upon extracellular dopamine levels whereby they are elevated in some striatal areas and simultaneously reduced in others.

The 5-HT1A and 5-HT2A/2C receptor antagonists WAY-100635 and ritanserin do not attenuate D-fenfluramine-induced fos expression in the brain

D-Fenfluramine is a serotonin (5-hydroxytryptamine, 5-HT) releaser and reuptake inhibitor. It is used to study the neurochemical control of feeding and has been used to treat obesity. It has also been employed as a pharmacological tool to study changes in serotonergic function in psychiatric patients. Brain sites activated by D-fenfluramine via the release of 5-HT have been mapped via the expression of the immediate early gene c-fos. Studies in our laboratory have indicated that D-fenfluramine induces Fos in the hypothalamus and cortex through 5-HT release. The present study investigated whether 5-HT released by D-fenfluramine induces Fos expression in the brain by activating 5-HT1A or 5-HT2A/2C receptors. Rats were pretreated either with WAY-100635, a 5-HT1A antagonist, or ritanserin, a 5-HT2A/2C antagonist, prior to d-fenfluramine injection. Blockade of either 5-HT1A or 5-HT2A/2C receptors was not sufficient to suppress the Fos response to D-fenfluramine in any region of the brain examined, including the cingulate cortex, frontal cortex, caudate-putamen, paraventricular nucleus of the hypothalamus, amygdala, and brainstem. These results indicate that Fos response elicited by D-fenfluramine may be mediated by other receptors, in addition to the 5-HT1A or 5-HT2A/2C receptors.