Localization and gene expression of serotonin 1A (5HT1A) receptors in human brain postmortem

TAAR1 dependent and independent actions of the potential antipsychotic and dual TAAR1/5-HT(1A) receptor agonist SEP-383856

SEP-383856 (SEP-856) is a novel antipsychotic under clinical development. It displays a unique pattern of receptor interaction, with only weak (partial agonist) activity at dopamine D₂ receptors, yet more potent agonist activity at the trace amine associated receptor (TAAR1) and 5-hydroxytryptamine 1 A receptor (5-HT_1A). Nonetheless, these observations await independent confirmation and more detailed characterization of the in vitro and in vivo actions of SEP-856 at TAAR1 and 5-HT_1A receptors would be instructive. Herein, we employed luminescence complementation technology in heterologous live cell systems, confocal microscopy, voltage clamp electrophysiology, behavioral readouts and TAAR1 knockout (KO) mice to study SEP-856 in further detail. We provide evidence for the ability of SEP-856 to activate TAAR1 at the surface plasma membrane, and show that this interaction results in Gα_s recruitment (pEC₅₀: 6.08 ± 0.22 E_MAX: 96.41% ± 15.26) and by extension, to G-protein inwardly rectifying potassium (GIRK) channel activation. Using TAAR1-KO mice, we find TAAR1 to be indispensable for SEP-856 control of body temperature, baseline locomotion reduction and for "antipsychotic-like" efficacy as characterized by a reversal of dizocilipine (MK-801) mediated disruption of pre-pulse inhibition. Conversely, the inhibition by SEP-856 of MK-801 induced locomotion was unaffected in TAAR1 KO mice. SEP-856 behaved as a low-potency, partial agonist at the 5-HT_1A receptor, while it partially inhibited recruitment of D₂ receptor-coupled Gα and GIRK by DA and acted as a weak partial agonist with low potency at the same receptor when applied alone. Our findings corroborate and extend previous observations on the molecular substrates engaged by this unique, dual TAAR1/5-HT_1A receptor agonist and potential antipsychotic that could prove to have major advantages in the treatment of schizophrenia and other psychotic disorders.

The serotonin system in the hippocampus CA3 involves in effects of CSDS on social recognition in adult female mandarin voles (Microtus mandarinus)

Chronic social defeat stress (CSDS) exacerbated the development of stress-related psychiatric disorders, and the social recognition dysfunction is the core feature of many psychiatric disorders. However, the effects of CSDS on female social recognition and the underlying neural mechanisms remain unclear. Using highly aggressive adult female mandarin voles (Microtus mandarinus) as animal model, the aim of this work is to investigate the effects of CSDS on social recognition in adult female rodents and the neurobiological mechanisms underlying these effects. Our results indicate the CSDS disrupted the normal social recognition in adult female voles. Meanwhile, defeated voles exhibited increased neural activity in the DG, CA1 and CA3 of the hippocampus. Furthermore, CSDS reduced levels of serotonin (5-HT) and serotonin 1A receptors (5-HT_1AR) in the CA3. We also discovered that microinjection of 8-OH-DPAT into the CA3 effectively reversed the social recognition deficits induced by CSDS, and an infusion of WAY-100635 into the CA3 of control female voles impaired social recognition. Moreover, targeted activation of the 5-HT neuron projection from the DRN to CA3 by long-term administration of CNO significantly prevented the CSDS induced social recognition deficits. Taken together, our study demonstrated that CSDS induced social recognition deficits in adult female voles, and these effects were mediated by the action of 5-HT on the 5-HT_1AR in the hippocampus CA3. The projection from the DRN to CA3 may be involved in social recognition deficits induced by CSDS.

Cognitive Control as a 5-HT1A-Based Domain That Is Disrupted in Major Depressive Disorder

Heterogeneity within Major Depressive Disorder (MDD) has hampered identification of biological markers (e.g., intermediate phenotypes, IPs) that might increase risk for the disorder or reflect closer links to the genes underlying the disease process. The newer characterizations of dimensions of MDD within Research Domain Criteria (RDoC) domains may align well with the goal of defining IPs. We compare a sample of 25 individuals with MDD compared to 29 age and education matched controls in multimodal assessment. The multimodal RDoC assessment included the primary IP biomarker, positron emission tomography (PET) with a selective radiotracer for 5-HT_1A [(11C)WAY-100635], as well as event-related functional MRI with a Go/No-go task targeting the Cognitive Control network, neuropsychological assessment of affective perception, negative memory bias and Cognitive Control domains. There was also an exploratory genetic analysis with the serotonin transporter (5-HTTLPR) and monamine oxidase A (MAO-A) genes. In regression analyses, lower 5-HT_1A binding potential (BP) in the MDD group was related to diminished engagement of the Cognitive Control network, slowed resolution of interfering cognitive stimuli, one element of Cognitive Control. In contrast, higher/normative levels of 5-HT_1A BP in MDD (only) was related to a substantial memory bias toward negative information, but intact resolution of interfering cognitive stimuli and greater engagement of Cognitive Control circuitry. The serotonin transporter risk allele was associated with lower 1a BP and the corresponding imaging and cognitive IPs in MDD. Lowered 5HT 1a BP was present in half of the MDD group relative to the control group. Lowered 5HT 1a BP may represent a subtype including decreased engagement of Cognitive Control network and impaired resolution of interfering cognitive stimuli. Future investigations might link lowered 1a BP to neurobiological pathways and markers, as well as probing subtype-specific treatment targets.

Role of hippocampal 5-HT1A receptors in the antidepressant-like phenotype of mice expressing RGS-insensitive Gαi2 protein

A single base mutation in the Gα_i2 protein (G184S) renders this Gα subunit insensitive to the negative modulatory effects of Regulator of G-protein Signaling (RGS) proteins. Mice expressing this RGS insensitive (RGSi) variant of Gα_i2 (RGSi Gα_i2) display a spontaneous antidepressant-like phenotype that is reversible by treatment with the 5-HT1A receptor (5-HT1AR) antagonist WAY100635. Here we test the hypothesis that increased activity of 5-HT1ARs in the hippocampus of RGSi Gα_i2 knock-in mice is responsible for the expression of the observed antidepressant-like behavior. We administered the 5-HT1AR antagonist WAY100635 or the agonist 8-OH-DPAT via bilateral intra-hippocampal infusion cannulae and evaluated antidepressant-like behavior using the tail suspension test (TST). WAY100635 reversed the antidepressant-like phenotype of the RGSi Gα_i2 knock-in mice and 8-OH-DPAT produced an antidepressant-like response in wild type mice that was blocked by systemic WAY100635. Furthermore, intra-hippocampal infusion of the RGS19/4 inhibitor CCG-203769 produced an antidepressant-like effect in female mice. Ex-vivo slice recording confirmed the 5-HT1AR-mediated decrease in hippocampal CA1 pyramidal neuron excitability was enhanced in the RGSi Gα_i2 knock-in mice. There was no change in hippocampal 5-HT1AR expression as measured by ligand binding but there was a compensatory reduction in Gαi proteins. The findings demonstrate that RGS protein control of hippocampal 5-HT1AR signaling is necessary and sufficient to account for the antidepressant-like phenotype in the RGSi Gα_i2 knock-in mice and that RGS proteins highly expressed in the hippocampus should be investigated as targets for novel antidepressant therapies.

Differences in memory function between 5-HT1A receptor genotypes in patients with major depressive disorder

BACKGROUND

While extensive literature on the role of the serotonin receptor 1A (5-HT1A-R) in cognition exists, the findings are largely from animal studies. There has been little research conducted into 5-HT1A-R genotypes and cognitive function in humans. This article evaluates the role of 5-HT1A-R genotypes on the profile of cognitive function in patients with major depressive disorder (MDD).

METHODS

The study sample was 455 MDD patients aged between 18 and 55 years. They had enrolled into a clinical trial and were tested prior to dosing on the baseline study day using the CDR System, an integrated set of 3 attention tests, 2 working memory tests, and 4 episodic memory tests. 5-HT1A-R genotyping for (SNP ID rs6295) had been conducted during the study screening period.

RESULTS

Validated factor scores were derived from the 9 tests. It was found that patients with the C/C genotype for the C(1019)G polymorphism of the 5-HT1A-R were significantly superior in retaining and retrieving information, in both working and episodic memory, than those with either the C/G or the G/G genotypes. No differences were found in measures of attention or in the speed of retrieval of information from memory.

CONCLUSIONS

This is, to our knowledge, the first relationship found between objective tests of cognitive function and 5-HT1A-R genotypes in MDD.

Effect of Pharmacological Interventions on the Fronto-Cingulo-Parietal Cognitive Control Network in Psychiatric Disorders: A Transdiagnostic Systematic Review of fMRI Studies

Executive function deficits, such as working memory, decision-making, and attention problems, are a common feature of several psychiatric disorders for which no satisfactory treatment exists. Here, we transdiagnostically investigate the effects of pharmacological interventions (other than methylphenidate) on the fronto-cingulo-parietal cognitive control network, in order to identify functional brain markers for future procognitive pharmacological interventions. Twenty-nine manuscripts investigated the effect of pharmacological treatment on executive function-related brain correlates in psychotic disorders (n = 11), depression (n = 4), bipolar disorder (n = 4), ADHD (n = 4), OCD (n = 2), smoking dependence (n = 2), alcohol dependence (n = 1), and pathological gambling (n = 1). In terms of impact on the fronto-cingulo-parietal network, the preliminary evidence for catechol-O-methyl-transferase inhibitors, nicotinic receptor agonists, and atomoxetine was relatively consistent, the data for atypical antipsychotics and anticonvulsants moderate, and interpretation of the data for antidepressants was hampered by the employed study designs. Increased activity in task-relevant areas and decreased activity in task-irrelevant areas were the most common transdiagnostic effects of pharmacological treatment. These markers showed good positive and moderate negative predictive value. It is concluded that fronto-cingulo-parietal activity changes can serve as a marker for future procognitive interventions. Future recommendations include the use of randomized double-blind designs and selective cholinergic and glutamatergic compounds.

Cartography of 5-HT1A and 5-HT2A Receptor Subtypes in Prefrontal Cortex and Its Projections

Since the development of chemical neuroanatomical tools in the 1960s, a tremendous wealth of information has been generated on the anatomical components of the serotonergic system, at the microscopic level in the brain including the prefrontal cortex (PFC). The PFC receives a widespread distribution of serotonin (5-hydroxytryptamine, 5-HT) terminals from the median and dorsal raphe nuclei. 5-HT receptors were first visualized using radioligand autoradiography in the late 1980s and early 1990s and showed, in contrast to 5-HT innervation, a differential distribution of binding sites associated with different 5-HT receptor subtypes. Due to the cloning of the different 5-HT receptor subtype genes in the late 1980s and early 1990s, it was possible, using in situ hybridization histochemistry, to localize cells expressing mRNA for these receptors. Double in situ hybridization histochemistry and immunohistochemistry allowed for the chemical characterization of the phenotype of cells expressing 5-HT receptors. Tract tracing technology allowed a detailed cartography of the neuronal connections of PFC and other brain areas. Based on these data, maps have been constructed that reflect our current understanding of the different circuits where 5-HT receptors can modulate the electrophysiological, pharmacological, and behavioral functions of the PFC. We will review current knowledge regarding the cellular localization of 5-HT1A and 5-HT2A receptors in mammalian PFC and their possible functions in the neuronal circuits of the PFC. We will discuss data generated in our laboratory as well as in others, focusing on localization in the pyramidal and GABAergic neuronal cell populations in different mammalian species using molecular neuroanatomy and on the connections with other brain regions.

8-hydroxy-2-(di-n-propylamino)tetralin intervenes with neural cell apoptosis following diffuse axonal injury

Previous studies have reported a neuroprotective effect of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) against traumatic brain injury. In accordance with the Marmarou method, rat models of diffuse axonal injury were established. 8-OH-DPAT was intraperitoneally injected into model rats. 8-OH-DPAT treated rats maintained at constant temperature served as normal temperature controls. TUNEL results revealed that neural cell swelling, brain tissue necrosis and cell apoptosis occurred around the injured tissue. Moreover, the number of Bax-, Bcl-2- and caspase-3-positive cells increased at 6 hours after diffuse axonal injury, and peaked at 24 hours. However, brain injury was attenuated, the number of apoptotic cells reduced, Bax and caspase-3 expression decreased, and Bcl-2 expression increased at 6, 12, 24, 72 and 168 hours after diffuse axonal injury in normal temperature control and in 8-OH-DPAT-intervention rats. The difference was most significant at 24 hours. All indices in 8-OH-DPAT-intervention rats were better than those in the constant temperature group. These results suggest that 8-OH-DPAT inhibits Bax and caspase-3 expression, increases Bcl-2 expression, and reduces neural cell apoptosis, resulting in neuroprotection against diffuse axonal injury. This effect is associated with a decrease in brain temperature.

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.

Neuroimaging over the course of Parkinson's disease: from early detection of the at-risk patient to improving pharmacotherapy of later-stage disease

Brain imaging of striatal dopamine terminal degeneration serves an important role in the clinical management of Parkinson's disease (PD). Imaging biomarkers for interrogating dopaminergic systems are used for clarifying diagnosis when only subtle motor symptoms are present. However, motor dysfunction is not the earliest symptom of PD. There is increasing interest in identifying premotor PD patients, particularly because potential disease-modifying therapies are developed and the clinical imperative becomes early and accurate diagnosis. On the other end of the spectrum of the disease course, during later stages of PD, significant clinical challenges like levo-dopa-induced dyskinesias and medication on-off phenomenon become more prevalent. In this instance, better understanding of altered PD motor pathways suggests the potential utility of novel treatments targeting neuronal systems that are impacted by degenerating dopamine neurons and chronic dopamine replacement treatment. Molecular neuroimaging serves unique roles in both very early PD and later-stage disease, in the former, potentially pushing back the time of diagnosis, and in the latter, elucidating pathology relevant to new drug development.

Agonist activity of N-desmethylclozapine at delta-opioid receptors of human frontal cortex

The clozapine metabolite N-desmethylclozapine (NDMC) has been recently shown to act at different neurotransmitter receptors and to display both antagonist and agonist activities. We have previously reported that in cells over-expressing the recombinant delta-opioid receptor NDMC behaved as partial agonist with high intrinsic activity, but its action at the receptors naturally expressed in human brain remained to be investigated. In the present study, we examined whether NDMC was able to bind to and activate delta-opioid receptors in membranes of post-mortem human frontal cortex. In radioligand binding assays, NDMC competition curves displayed high- (K(i)=26 nM) and low-affinity (K(i)=3 microM) components, whose proportion was regulated by guanine nucleotides in an agonist-like fashion. In functional assays, NDMC stimulated [(35)S]GTPgammaS binding (EC(50)=905 nM) and inhibited cyclic AMP formation (EC(50)=590 nM) as effectively as delta-opioid agonists, whereas clozapine was much less potent and efficacious and clozapine N-oxide was completely inactive. The NDMC agonist activity was potently antagonized by the delta-opioid antagonist naltrindole, but not by the micro-opioid receptor antagonist CTAP (D-phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) or the kappa-opioid antagonist nor-binaltorphimine. Moreover, blockade of either acetylcholine muscarinic, dopamine D(2) or serotonin 5HT(1A) receptors failed to affect NDMC agonist activity. These data demonstrate that at clinically relevant concentrations NDMC behaves as an efficacious agonist at delta-opioid receptors of human frontal cortex.

Serotonin 1A receptors in human and monkey prefrontal cortex are mainly expressed in pyramidal neurons and in a GABAergic interneuron subpopulation: implications for schizophrenia and its treatment

Serotonin 1A (5-HT(1A)) receptors are found in high densities in prefrontal cortex. However, their distribution within cortical cell populations is unknown in both humans and primates. We used double in situ hybridization histochemistry to quantify the percentage of glutamatergic and GABAergic neurons expressing 5-HT(1A) receptors in human and monkey prefrontal cortex. Moreover, in the case of the monkey, we also quantified the parvalbumin and calbindin GABAergic subpopulations expressing this receptor. 5-HT(1A) receptor mRNAs were expressed in about 80% of glutamatergic neurons in external layers II and upper III, and in around 50% in layer VI; they were also present in approximately 20% of GABAergic neurons in both species. Although they were found in up to 43% of the calbindin cell subpopulation they were rarely present in parvalbumin cells in monkey prefrontal cortex. The knowledge of the phenotype of the prefrontal cortex (PFC) cells expressing 5-HT(1A) will help understanding serotonin actions in PFC.

Striatal 5-HT1A receptor stimulation reduces D1 receptor-induced dyskinesia and improves movement in the hemiparkinsonian rat

Convergent evidence suggests that serotonin 5-HT1A receptor (5-HT1AR) agonists reduce l-DOPA-induced dyskinesia by auto-regulating aberrant release of l-DOPA-derived dopamine (DA) from raphestriatal neurons. However, recent findings indicate that 5-HT1AR stimulation also modifies D1 receptor (D1R)-mediated dyskinesia and rotations implicating a previously unexplored extra-raphe mechanism. In order to characterize the contribution of the striatum to these effects, rats with medial forebrain bundle DA lesions were tested for abnormal involuntary movements (AIMs) and rotations following striatal microinfusions of the 5-HT1AR agonist +/-8-OH-DPAT and systemic D1R agonist treatment with SKF81297. Additional rats with multi-site striatal DA lesions were tested for motor disability following systemic or intrastriatal +/-8-OH-DPAT with or without systemic SKF81297. In rats with medial forebrain bundle lesions, striatal infusions of +/-8-OH-DPAT dose-dependently reduced AIMs while conversely increasing rotations. In rats with striatal lesions, +/-8-OH-DPAT alone, both systemic and intrastriatal administration, optimally reversed motor disability. Collectively, these results support an important functional interaction between 5-HT1AR and D1R in the striatum with implications for the improved treatment of Parkinson's disease.

Short-term effects of 3,4-methylen-dioxy-metamphetamine (MDMA) on 5-HT(1A) receptors in the rat hippocampus

The first effects of 3,4-methylen-dioxy-metamphetamine (MDMA, "ecstasy"), on serotonin 1A (5-HT(1A)) receptors in rat hippocampus were determined by means of [(3)H]-8-hydroxy-dipropylamino-tetralin ([(3)H]-8-OH-DPAT) and 5'guanosine-(gamma-[(35)S]-thio)triphosphate ([(35)S]-GTPgammaS) binding as well as inhibition of forskolin (FK)-stimulated adenylyl cyclase (AC) activity. The study was completed by [(35)S]-GTPgammaS functional autoradiography experiments carried out in frontal sections of rat brain, including the hippocampal region. Results showed that MDMA was either able to displace [(3)H]-8-OH-DPAT binding (K(i) congruent with 500 nM) or to reduce the number of specific sites (B(max)) without affecting K(d). The drug also failed to change the [(35)S]-GTPgammaS binding or to inhibit AC velocity, underlying its behavior as a non-competitive 5-HT(1A) receptor antagonist. Further, MDMA (1 or 100 microM), partially antagonized either [(35)S]-GTPgammaS binding stimulation of the agonists 5CT and 8-OH-DPAT or the AC inhibition induced by 5CT and DP-5CT. However, in contrast to binding studies, in AC assays the amphetamine displayed an effect also on EC(50), always being less potent than the reference antagonist WAY100,635. In functional autoradiography, MDMA behaved either as a partial 5-HT(1A) antagonist in limbic areas or, added alone, as an agonist, increasing the coupling signal presumably through 5-HT release from synapses. Interestingly, the selective 5-HT re-uptake inhibitor (SSRI) fluoxetine had no effect on MDMA [(35)S]-GTPgammaS binding activation. This latter finding indicates that the amphetamine can release 5-HT via alternative mechanisms to 5-HT transporter binding, probably via membrane synaptic receptors or vesicular transporters. The release of other transmitters is not excluded. Therefore, our results encourage at extending the study of MDMA biochemical profiles, in the attempt to elucidate those amphetamine-induced pathways with a potential for neurotoxicity or psycho-stimulant activity.

Pharmacology of flibanserin

Flibanserin has preferential affinity for serotonin 5-HT(1A), dopamine D(4k), and serotonin 5-HT(2A) receptors. In vitro and in microiontophoresis, flibanserin behaves as a 5-HT(1A) agonist, a very weak partial agonist on dopamine D(4) receptors, and a 5-HT(2A) antagonist. In vivo flibanserin binds equally to 5-HT(1A) and 5-HT(2A) receptors. However, under higher levels of brain 5-HT (i.e., under stress), flibanserin may occupy 5-HT(2A) receptors in higher proportion than 5-HT(1A) receptors. The effects of flibanserin on adenylyl cyclase are different from those of buspirone and 8-OH-DPAT, two other purported 5-HT(1A) receptor agonists. Flibanserin reduces neuronal firing rate in cells of the dorsal raphe, hippocampus, and cortex with the CA1 region being the most sensitive in the brain. Flibanserin-induced reduction in firing rate in the cortex seems to be mediated through stimulation of postsynaptic 5-HT(1A) receptors, whereas the reduction of the number of active cells seems to be mediated through dopamine D(4) receptor stimulation. Flibanserin quickly desensitizes somatic 5-HT autoreceptors in the dorsal raphe and enhances tonic activation of postsynaptic 5-HT(1A) receptors in the CA3 region. Flibanserin preferentially reduces synthesis and extracellular levels of 5-HT in the cortex, where it enhances extracellular levels of NE and DA. Flibanserin displays antidepressant-like activity in most animal models sensitive to antidepressants. Such activity, however, seems qualitatively different from that exerted by other antidepressants. Flibanserin seems to act via direct or indirect stimulation of 5-HT(1A), DA, and opioid receptors in those animal models. Flibanserin does not display consistent effects in animal models of anxiety and seems to exert potential antipsychotic effects. Flibanserin may induce some sedation but does not induce observable toxic effects at pharmacologically relevant doses.

Effect of the serotonin agonist 8-OH-DPAT on the sensorimotor system of the rat

8-Hydroxy-2-(di-n-propylamino)-tetralin hydrobromide (8-OH-DPAT, 2 mg/kg) is used to induce perseverative behavior in rats in a T-maze as a model for obsessive-compulsive disorder (OCD). Using the open-field test, radiant heat test, and the test with von Frey filaments, we examined whether alterations in sensorimotor functioning could contribute to the perseverative tendencies in this model by measuring differences in left versus right hind paw reactions after 8-OH-DPAT administration (2 mg/kg, sc). Also, the effect of repeated 8-OH-DPAT administration on sensorimotor functioning was tested every third day. 8-OH-DPAT administration induced a significantly decreased sensorimotor performance in the open-field test, an increased threshold for noxious thermal stimulation (increased withdrawal latency, WL, and decreased elevation time, ET) in the radiant heat test, and a decreased nociceptive threshold for mechanical stimulation in the test with von Frey filaments. All changes in sensorimotor functioning were similar for left and right hind paws suggesting that, these changes as measured with the tests in the present study, are not likely to contribute to the perseverative behavior of rats in a T-maze. Further, repeated administration of 8-OH-DPAT had no effect in the radiant heat test and the test with the Frey filaments, but produced a tolerance effect in the open-field test.

Autoradiography of receptor-activated G-proteins in post mortem human brain

The agonist-stimulated guanosine 5'-(gamma-[(35)S]thio)triphosphate binding assay was used to anatomically localize receptor-activated G-proteins by autoradiography in post mortem human brain. The optimal conditions for guanosine 5'-(gamma-[(35)S]thio)triphosphate binding to human brain sections were established in post mortem samples of the prefrontal cortex, hippocampus, basal ganglia, brainstem and cerebellar cortex. An excess of GDP (2mM) was required to decrease basal activity and obtain effective stimulation by specific agonists. guanosine 5'-(gamma-[(35)S]Thio)triphosphate binding was increased after stimulation with specific agonists of different G-protein-coupled receptors. They include cannabinoid (WIN55212-2), mu-opioid ([D-Ala(2),N-Me-Phe(4), Gly(5)-ol]enkephalin), serotonin-1A [(+/-)-8-hydroxy-2-(di-n-propylamino)tetralin] and serotonin-1B/1D (sumatriptan), cholinergic muscarinic receptors (carbachol) and alpha(2)-adrenoceptors (UK14304). Such stimulation reached 1458%, 440%, 188%, 219%, 61% and 339%, respectively, over the basal levels. In tissue sections, the use of the above-mentioned agonists (10(-4)M) showed patterns of anatomical distribution similar to those already described by receptor autoradiography, with high densities over the hippocampus (serotonin-1A receptors), cortex (alpha(2)-adrenoceptors) and striatum (mu-opioid receptors). The highest binding levels were reached with the cannabinoid receptor agonist in most of the analysed brain regions. Carbachol produced only moderate stimulation of those same regions. The blockage of agonist-stimulated guanosine 5'-(gamma-[(35)S]thio)triphosphate binding by selective antagonists verified that the effect was receptor mediated. This technique provides a method to identify modifications of the receptor-mediated activation of G-proteins in post mortem human brain with anatomical resolution. It also provides valuable information on the level of drug efficacy in the human species.

Effects of postmortem delay on serotonin and (+)8-OH-DPAT-mediated inhibition of adenylyl cyclase activity in rat and human brain tissues

The reproducibility of serotonin (5-HT) and (+)8-OH-DPAT-mediated inhibition of adenylyl cyclase activity was assessed in membranes, stimulated by forskolin, of rat frontal cortex postmortem as well as of human fronto-cortical, hippocampal and dorsal raphe tissues obtained from autopsy brains. The results revealed that differences between basal and forskolin-stimulated enzyme activities were still significant after 48 h postmortem in rat cortex and in all human brain regions up to 46 h after death. However, a decrease of about 17 and 26% in forskolin-stimulated adenylyl cyclase activity was observed at 24 and 48 h, respectively, in rat cortex. 5-HT and the 5-HT1A receptor agonist, (+)8-hydroxy-2(di-N-propylamino)tetraline (8-OH-DPAT), were able to inhibit forskolin-stimulated adenylyl cyclase activity in a dose-dependent manner for 48 h after death in rat and human brain. In rat cortex, both 5-HT and (+)8-OH-DPAT potencies (EC50, nM) and efficacies (percent of maximum inhibition capacity, %) varied significantly with postmortem delay. Conversely, in human tissues, postmortem delay and subject age did not modify agonist potencies and efficacies. Furthermore, a regionality of 5-HT potency and efficacy was revealed in the human brain. 5-HT was equally potent in cortex and raphe nuclei, while being more potent but less effective in hippocampus. (+)8-OH-DPAT was more active in hippocampus and raphe nuclei than in cortex. (+)8-OH-DPAT behaved as an agonist in all areas, as its efficacy was similar or greater than those obtained with 5-HT. The (+)8-OH-DPAT dose-response curve was completely reversed by 5-HT1A receptor antagonists in rat cortex and all human brain areas. In conclusion, we suggest here that differences between rat and human brain might exist at the level of postmortem degradation of 5-HT-sensitive adenylyl cyclase activity. In human brain, 5-HT1A receptor-mediated inhibition of adenylyl cyclase seems to be reproducible, suggesting that reliable experiments can be carried out on postmortem specimens from patients with neuropsychiatric disorders.

Apparent absence of aging and gender effects on serotonin 1A receptors in human neocortex and hippocampus

The effects of gender, aging and gender x age on the binding of the 5-HT1A receptor high-affinity agonist [3H]8-hydroxy-2(di-N-propylamino)tetralin ([3H]8-OH-DPAT), were evaluated and compared in tissues of human prefrontal, temporal, parietal, occipital cortex and hippocampus obtained from 21 autopsy subjects. The results revealed no variation with age or gender in either the [3H]8-OH-DPAT maximum binding capacity (Bmax) or dissociation constant (Kd) values. On the other hand, when separate correlations to subject ages were performed for men and women, aging effects on [3H]8-OH-DPAT Bmax and Kd were detected: in men, a significant age-dependent decrease in Kd values was observed in the occipital cortex; in women, the Bmax significantly decreased with aging in the parietal cortex and hippocampus, while increasing in occipito-cortical membranes. Overall, the present study reveals that, although neither gender nor aging 'per se' seem to modify the 5-HT1A receptor binding, gender may reveal region-specific aging effects, i.e. on receptor affinity in men and receptor density in women. Such findings should stimulate further investigation on the hypothesized existence of gender x age-related cross-connections between serotonergic system and hypothalamus-pituitary-gonadal circuits.

Contrasting effects of systemic and intracerebral infusions of the 5-HT1A receptor agonist 8-OH-DPAT on spatial short-term working memory in rats

The present study compared the effects of systemic 8-OH-DPAT (0.05, 0.1 and 1.0 mg/kg) with intra-raphe and intra-hippocampal infusions of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10, 30 100 ng) on delayed non-matching-to-position (DNMP) performance in rats. The highest dose of 8-OH-DPAT administered systemically impaired DNMP performance in a delay-independent manner, increased premature responding and increased response bias. Infusions of 8-OH-DPAT (100 ng) into the median raphe nucleus improved performance accuracy, independent of delay whilst having no effect on any other response measure. Infusions of 8-OH-DPAT into the dorsal raphe nucleus had no effect on performance at any dose tested. Infusions of 8-OH-DPAT into the dorsal hippocampus produced a small impairment in performance which was also independent of delay. However, this decrement in performance accuracy was not accompanied by any changes in other response measures. These findings demonstrate a dissociation between the effects of stimulation of pre- and post-synaptic 5-HT1A receptors on performance of a DNMP task although the changes in performance cannot be accounted for by changes in mnemonic function.

Serotonin1A receptors are increased in postmortem prefrontal cortex in schizophrenia

Serotonin-5-HT1A receptors were measured with [3H]8-hydroxy-2-(di-n-propyl)aminotetralin ([3H]8-OH-DPAT) in postmortem prefrontal cortex of 12 pairs of subjects with schizophrenia and age-matched control subjects. The saturation binding isotherms of [3H]8-OH-DPAT revealed high- and low-affinity binding sites. The density (Bmax) of the high-affinity sites was significantly elevated by an average of 79% in subjects with schizophrenia. The dissociation constant (Kd) of the high-affinity sites in subjects with schizophrenia was not significantly different from the control subjects. These results provide further evidence for a role of the serotonergic system in the pathophysiology of schizophrenia.

Presence of serotonin1A (5-HT1A) receptor mRNA without binding of [3H]-8-OH-DPAT in peripheral blood mononuclear cells

The authors investigated the presence of serotonin receptor type 1A (5-HT1A) as labeled by the specific ligand 3H-8 hydroxy-2-(di-N-propylamino)tetralin (3H-8-OH-DPAT) in saturation experiments, and the expression of the mRNA encoding them, in human peripheral blood mononuclear cells (PBMC). In situ hybridization experiments were performed as well. The results, showing that the binding of [3H]-8-OH-DPAT to lymphocyte membranes increased linearly up to 100 nM without reaching saturation, may indicate that the 3H-8-OH-DPAT was not specifically labeling the 5-HT1A receptor. By contrast, the expression studies revealed 5-HT1A mRNA in PBMC. These findings suggest that, despite the presence of mRNA, 5-HT1A receptors are not expressed in PBMC, at least in healthy controls.

The distribution of 5-HT1A and 5-HT2A receptor mRNA in human brain

We have examined the distribution of 5-HT1A and 5-HT2A receptor mRNAs in post-mortem human hippocampus, neocortex, raphe nuclei, cerebellum and basal ganglia using in situ hybridization histochemistry. Receptor transcripts in brains from two males and two females (mean age +/- S.D. = 70 +/- 4 years; post-mortem interval = 29 +/- 6 h) were visualised with 35S-radiolabelled synthetic oligodeoxyribonucleic acid probes. In the hippocampus, 5-HT1A receptor mRNA was present in all fields, especially CA1. In the parahippocampal gyrus and neocortical regions 5-HT1A receptor mRNA was enhanced in superficial and middle laminae. 5-HT1A receptor mRNA was particularly abundant in the raphe and other serotonergic cell groups of the brainstem. The analysis of emulsion dipped sections showed 5-HT1A receptor mRNA to be concentrated in pyramidal neurons, together with the granule cells of the dentate gyrus. In neocortical areas lamina III pyramidal neurons were more heavily labelled than those in lamina V. There was no evidence of glial expression of 5-HT1A receptor mRNA in grey matter or white matter compartments. 5-HT2A receptor mRNA was present in all neocortical areas examined, where it was located in pyramidal neurons, of lamina V more than in those of lamina III, as well as in putative interneurons, especially within lamina IVc of the striate cortex. 5-HT2A receptor mRNA was observed at minimal levels in the hippocampus and not in the raphe. Neither 5-HT1A nor 5-HT2A receptor mRNA were detected in the cerebellum, substantia nigra or striatum. The ability to detect these transcripts at the regional and cellular level will help reveal important details of the 5-HT receptor system in the human brain. This includes the investigation of their putative roles in the normal chemoarchitecture and in pathophysiological brain processes.