Luvadaxistat: A Novel Potent and Selective D-Amino Acid Oxidase Inhibitor Improves Cognitive and Social Deficits in Rodent Models for Schizophrenia.

N-methyl-D-aspartate (NMDA) receptor hypofunctionality is a well-studied hypothesis for schizophrenia pathophysiology, and daily dosing of the NMDA receptor co-agonist, D-serine, in clinical trials has shown positive effects in patients. Therefore, inhibition of D-amino acid oxidase (DAAO) has the potential to be a new therapeutic approach for the treatment of schizophrenia. TAK-831 (luvadaxistat), a novel, highly potent inhibitor of DAAO, significantly increases D-serine levels in the rodent brain, plasma, and cerebrospinal fluid. This study shows luvadaxistat to be efficacious in animal tests of cognition and in a translational animal model for cognitive impairment in schizophrenia. This is demonstrated when luvadaxistat is dosed alone and in conjunction with a typical antipsychotic. When dosed chronically, there is a suggestion of change in synaptic plasticity as seen by a leftward shift in the maximum efficacious dose in several studies. This is suggestive of enhanced activation of NMDA receptors in the brain and confirmed by modulation of long-term potentiation after chronic dosing. DAAO is highly expressed in the cerebellum, an area of increasing interest for schizophrenia, and luvadaxistat was shown to be efficacious in a cerebellar-dependent associative learning task. While luvadaxistat ameliorated the deficit seen in sociability in two different negative symptom tests of social interaction, it failed to show an effect in endpoints of negative symptoms in clinical trials. These results suggest that luvadaxistat potentially could be used to improve cognitive impairment in patients with schizophrenia, which is not well addressed with current antipsychotic medications.

Ceramide kinase regulates acute wound healing by suppressing 5-oxo-ETE biosynthesis and signaling via its receptor OXER1.

The sphingolipid, ceramide-1-phosphate (C1P), has been shown to promote the inflammatory phase and inhibit the proliferation and remodeling stages of wound repair via direct interaction with group IVA cytosolic phospholipase A2, a regulator of eicosanoid biosynthesis that fine-tunes the behaviors of various cell types during wound healing. However, the anabolic enzyme responsible for the production of C1P that suppresses wound healing as well as bioactive eicosanoids and target receptors that drive enhanced wound remodeling have not been characterized. Herein, we determined that decreasing C1P activity via inhibitors or genetic ablation of the anabolic enzyme ceramide kinase (CERK) significantly enhanced wound healing phenotypes. Importantly, postwounding inhibition of CERK enhanced the closure rate of acute wounds, improved the quality of healing, and increased fibroblast migration via a "class switch" in the eicosanoid profile. This switch reduced pro-inflammatory prostaglandins (e.g., prostaglandin E2) and increased levels of 5-hydroxyeicosatetraenoic acid and the downstream metabolite 5-oxo-eicosatetraenoic acid (5-oxo-ETE). Moreover, dermal fibroblasts from mice with genetically ablated CERK showed enhanced wound healing markers, while blockage of the murine 5-oxo-ETE receptor (oxoeicosanoid receptor 1) inhibited the enhanced migration phenotype of these cell models. Together, these studies reinforce the vital roles eicosanoids play in the wound healing process and demonstrate a novel role for CERK-derived C1P as a negative regulator of 5-oxo-ETE biosynthesis and the activation of oxoeicosanoid receptor 1 in wound healing. These findings provide foundational preclinical results for the use of CERK inhibitors to shift the balance from inflammation to resolution and increase the wound healing rate.

Discovery of TAK-041: a Potent and Selective GPR139 Agonist Explored for the Treatment of Negative Symptoms Associated with Schizophrenia.

The orphan G-protein-coupled receptor GPR139 is highly expressed in the habenula, a small brain nucleus that has been linked to depression, schizophrenia (SCZ), and substance-use disorder. High-throughput screening and a medicinal chemistry structure-activity relationship strategy identified a novel series of potent and selective benzotriazinone-based GPR139 agonists. Herein, we describe the chemistry optimization that led to the discovery and validation of multiple potent and selective in vivo GPR139 agonist tool compounds, including our clinical candidate TAK-041, also known as NBI-1065846 (compound 56). The pharmacological characterization of these GPR139 agonists in vivo demonstrated GPR139-agonist-dependent modulation of habenula cell activity and revealed consistent in vivo efficacy to rescue social interaction deficits in the BALB/c mouse strain. The clinical GPR139 agonist TAK-041 is being explored as a novel drug to treat negative symptoms in SCZ.

GABAB receptor modulation-to B or not to be B a pro-cognitive medicine?

Metabotropic GABAB receptors clearly modify cognitive performance in preclinical animal models, yet translation to treating human disease has been elusive. Compared to their ionotropic GABAA receptor counterpart GABAB receptors not only regulate postsynaptic excitability but also regulate diverse synaptic inputs by presynaptically inhibiting neurotransmitter release. As such, the choice of agonist, antagonist, -ve or +ve modulator as well as CNS exposure level, timing of delivery and longevity of action strongly influence the probability of unlocking the procognitive potential of GABAB receptors in human disease. Quantitative clinical analysis of target/mechanistic engagement of GABAB receptors within cognitive circuits at the level of distinct pre-synaptic and post-synaptic subpopulations is required to determine the optimal pharmacological/dosing profile for different cognitive disorders.

Pro-inflammatory immune-to-brain signaling is involved in neuroendocrine responses to acute emotional stress.

Activation of the hypothalamo-pituitary-adrenal (HPA) axis by inflammatory stressors (e.g., bacterial lipopolysaccharide) is thought to involve vascular transduction of circulating cytokines, with perivascular macrophages (PVMs) along with endothelia, effecting activation of HPA control circuitry via inducible (cyclooxygenase-2- or COX-2-dependent) prostaglandin synthesis. To test the stressor-specificity of this mechanism, we examined whether ablation of PVMs or pharmacologic blockade of COX activity affected HPA responses to a representative emotional stressor, restraint. Exposing rats to a single 30min acute restraint episode provoked increased plasma levels of at least one proinflammatory cytokine, IL-6, microglial activation and multiple indices of cerebrovascular activation, including COX-2 expression and increased brain prostaglandin E2 levels at 0-2h after stress. Pretreatment with the nonselective COX inhibitor, indomethacin, either icv (10µg in 5µl) or iv (1mg/kg) significantly reduced restraint-induced Fos expression in the paraventricular hypothalamic nucleus (PVH) by 45%, relative to vehicle-injected controls. A 75% reduction of the PVH activational response was seen in rats exposed to acute restraint 5-7days after ablation of brain PVMs by icv injection of liposomes encapsulating the bisphosphonate drug, clodronate. Basal plasma levels of ACTH and corticosterone were not altered in clodronate liposome-injected rats, but the peak magnitude of restraint-induced HPA secretory responses was substantially reduced, relative to animals pretreated with saline-filled liposomes. These findings support an unexpectedly prominent role for inducible prostaglandin synthesis by PVMs in HPA responses to acute restraint, a prototypic emotional stressor.

Dendritic cells and multiple sclerosis: disease, tolerance and therapy.

Multiple sclerosis (MS) is a devastating neurological disease that predominantly affects young adults resulting in severe personal and economic impact. The majority of therapies for this disease were developed in, or are beneficial in experimental autoimmune encephalomyelitis (EAE), the animal model of MS. While known to target adaptive anti-CNS immune responses, they also target, the innate immune arm. This mini-review focuses on the role of dendritic cells (DCs), the professional antigen presenting cells of the innate immune system. The evidence for a role for DCs in the appropriate regulation of anti-CNS autoimmune responses and their role in MS disease susceptibility and possible therapeutic utility are discussed. Additionally, the current controversy regarding the evidence for the presence of functional DCs in the normal CNS is reviewed. Furthermore, the role of CNS DCs and potential routes of their intercourse between the CNS and cervical lymph nodes are considered. Finally, the future role that this nexus between the CNS and the cervical lymph nodes might play in site directed molecular and cellular therapy for MS is outlined.

Lipopolysaccharide-induced tau phosphorylation and kinase activity--modulation, but not mediation, by corticotropin-releasing factor receptors.

Clinical studies suggest that exposure to stress can increase risk for Alzheimer's disease (AD). Although the precise links between stress and vulnerability to develop AD remain uncertain, recent animal work suggests that stress may promote susceptibility to AD pathology by activating tau kinases and inducing tau phosphorylation (tau-P). Our previous findings indicate the differential involvement of corticotropin-releasing factor receptor (CRFR) types 1 and 2 in regulating tau-P in the hippocampus induced by acute restraint, an emotional stressor. To assess the generality of CRFR involvement in stress-induced tau-P and tau kinase activity, the present study extends our investigation to a well-characterized physiological stressor, i.e. immune challenge induced by bacterial lipopolysaccharide (LPS). Acute systemic administration of LPS (100 µg/kg) robustly increased hippocampal (but not isocortical or cerebellar) tau-P, peaking at 40-120 min postinjection and abating thereafter. Assessments of the genotype dependence of this effect yielded results that were distinct from the restraint model. Treatment with LPS increased phosphorylation in wild-type, single and double CRFR knockouts with only subtle variation, which included a reliable exaggeration of tau-P responses in CRFR1-deficient mice. Parallel analyses implicated glycogen synthase kinase-3 and cyclin-dependent kinase-5 as likely cellular mediators of LPS-induced tau-P. Conversely, our data suggest that temperature-dependent fluctuations in tau protein phosphatase 2A (PP2A) may not play a role in this context. Thus, neither the strict CRFR1 dependence of restraint-induced tau-P nor the exaggeration of these responses in CRFR2 null mice generalize to the LPS model. CRFR mediation of stress-induced hippocampal tau-P may be limited to emotional stressors.

Dual roles for perivascular macrophages in immune-to-brain signaling.

Cytokines produced during infection/inflammation activate adaptive central nervous system (CNS) responses, including acute stress responses mediated by the hypothalamo-pituitary-adrenal (HPA) axis. The mechanisms by which cytokines engage HPA control circuitry remain unclear, though stimulated release of prostanoids from neighboring vascular cells has been implicated in this regard. How specific vascular cell types, endothelial cells (ECs) versus perivascular cells (PVCs; a subset of brain-resident macrophages), participate in this response remains unsettled. We exploited the phagocytic activity of PVCs to deplete them in rats by central injection of a liposome-encapsulated proapoptotic drug. This manipulation abrogated CNS and hormonal indices of HPA activation under immune challenge conditions (interleukin-1) that activated prostanoid synthesis only in PVCs, while enhancing these responses to stimuli (lipopolysaccharide) that engaged prostanoid production by ECs as well. Thus, PVCs provide both prostanoid-mediated drive to the HPA axis and an anti-inflammatory action that constrains endothelial and overall CNS responses to inflammatory insults.

Brain perivascular macrophages and the sympathetic response to inflammation in rats after myocardial infarction.

Inflammation is associated with increased sympathetic drive in cardiovascular diseases. Blood-borne proinflammatory cytokines, markers of inflammation, induce cyclooxygenase 2 (COX-2) activity in perivascular macrophages of the blood-brain barrier. COX-2 generates prostaglandin E(2), which may enter the brain and increase sympathetic nerve activity. We examined the contribution of this mechanism to augmented sympathetic drive in rats after myocardial infarction (MI). Approximately 24 hours after acute MI, rats received an intracerebroventricular injection (1 microL/min over 40 minutes) of clodronate liposomes (MI+CLOD) to eliminate brain perivascular macrophages, liposomes alone, or artificial cerebrospinal fluid. A week later, COX-2 immunoreactivity in perivascular macrophages and COX-2 mRNA and protein had increased in hypothalamic paraventricular nucleus of MI rats treated with artificial cerebrospinal fluid or liposomes alone compared with sham-operated rats. In MI+CLOD rats, neither perivascular macrophages nor COX-2 immunoreactivity was seen in the paraventricular nucleus, and COX-2 mRNA and protein levels were similar to those in sham-operated rats. Prostaglandin E(2) in cerebrospinal fluid, paraventricular nucleus neuronal excitation, and plasma norepinephrine were less in MI+CLOD rats than in MI rats treated with artificial cerebrospinal fluid or liposomes alone but more than in sham-operated rats. Intracerebroventricular CLOD had no effect on interleukin 1beta and tumor necrosis factor-alpha mRNA and protein in the paraventricular nucleus or plasma interleukin-1beta and tumor necrosis factor-alpha, which were increased in MI compared with sham-operated rats. In normal rats, pretreatment with intracerebroventricular CLOD reduced (P<0.05) the renal sympathetic, blood pressure, and heart rate responses to intracarotid artery injection of tumor necrosis factor-alpha (0.5 microg/kg); intracerebroventricular liposomes had no effect. The results suggest that proinflammatory cytokines stimulate sympathetic excitation after MI by inducing COX-2 activity and prostaglandin E(2) production in perivascular macrophages of the blood-brain barrier.

Cerebrovascular cyclooxygenase-1 expression, regulation, and role in hypothalamic-pituitary-adrenal axis activation by inflammatory stimuli.

Systemic injection of lipopolysaccharide (LPS) is a widely used model of immune/inflammatory challenge, which can invoke a host of CNS responses, including activation of the hypothalamic-pituitary-adrenal (HPA) axis. Inducible vascular prostaglandin E(2) (PGE(2)) synthesis by endothelial (ECs) and/or perivascular cells (PVCs) (a macrophage-derived vascular cell type) is implicated in the engagement of HPA and other CNS responses, by virtue of their capacity to express cyclooxygenase-2 (COX-2) and microsomal PGE(2) synthase-1. Evidence from genetic and pharmacologic studies also supports a role for the constitutively expressed COX-1 in inflammation-induced activation of the HPA axis, although histochemical evidence to support relevant localization(s) and regulation of COX-1 expression is lacking. The present experiments fill this void in showing that COX-1 immunoreactivity (IR) and mRNA are detectable in identified PVCs and parenchymal microglia under basal conditions and is robustly expressed in these and ECs 1-3 h after intravenous injection of LPS (2 microg/kg). Confocal and electron microscopic analyses indicate distinct cellular/subcellular localizations of COX-1-IR in the three cell types. Interestingly, COX-1 expression is enhanced in ECs of brain PVC-depleted rats, supporting an anti-inflammatory role of the latter cell type. Functional involvement of COX-1 is indicated by the observation that central, but not systemic, pretreatment with the selective COX-1 inhibitor SC-560 attenuated the early phase of LPS-induced increases in adrenocorticotropin and corticosterone secretion. These findings support an involvement of COX-1 in bidirectional interplay between ECs and PVCs in initiating vascular PGE(2) and downstream HPA response to proinflammatory challenges.

How T-cell-dependent and -independent challenges access the brain: vascular and neural responses to bacterial lipopolysaccharide and staphylococcal enterotoxin B.

Bacterial lipopolysaccharide (LPS) is widely used to study immune influences on the CNS, and cerebrovascular prostaglandin (PG) synthesis is implicated in mediating LPS influences on some acute phase responses. Other bacterial products, such as staphylococcal enterotoxin B (SEB), impact target tissues differently in that their effects are T-lymphocyte-dependent, yet both LPS and SEB recruit a partially overlapping set of subcortical central autonomic cell groups. We sought to compare neurovascular responses to the two pathogens, and the mechanisms by which they may access the brain. Rats received iv injections of LPS (2 microg/kg), SEB (1mg/kg) or vehicle and were sacrificed 0.5-3h later. Both challenges engaged vascular cells as early 0.5h, as evidenced by induced expression of the vascular early response gene (Verge), and the immediate-early gene, NGFI-B. Cyclooxygenase-2 (COX-2) expression was detected in both endothelial and perivascular cells (PVCs) in response to LPS, but only in PVCs of SEB-challenged animals. The non-selective COX inhibitor, indomethacin (1mg/kg, iv), blocked LPS-induced activation in a subset of central autonomic structures, but failed to alter SEB-driven responses. Liposome mediated ablation of PVCs modulated the CNS response to LPS, did not affect the SEB-induced activational profile. By contrast, disruptions of interoceptive signaling by area postrema lesions or vagotomy (complete or hepatic) markedly attenuated SEB-, but not LPS-, stimulated central activational responses. Despite partial overlap in their neuronal and vascular response profiles, LPS and SEB appear to use distinct mechanisms to access the brain.

Cellular and molecular bases of the initiation of fever.

All phases of lipopolysaccharide (LPS)-induced fever are mediated by prostaglandin (PG) E2. It is known that the second febrile phase (which starts at approximately 1.5 h post-LPS) and subsequent phases are mediated by PGE2 that originated in endotheliocytes and perivascular cells of the brain. However, the location and phenotypes of the cells that produce PGE2 triggering the first febrile phase (which starts at approximately 0.5 h) remain unknown. By studying PGE2 synthesis at the enzymatic level, we found that it was activated in the lung and liver, but not in the brain, at the onset of the first phase of LPS fever in rats. This activation involved phosphorylation of cytosolic phospholipase A2 (cPLA2) and transcriptional up-regulation of cyclooxygenase (COX)-2. The number of cells displaying COX-2 immunoreactivity surged in the lung and liver (but not in the brain) at the onset of fever, and the majority of these cells were identified as macrophages. When PGE2 synthesis in the periphery was activated, the concentration of PGE2 increased both in the venous blood (which collects PGE2 from tissues) and arterial blood (which delivers PGE2 to the brain). Most importantly, neutralization of circulating PGE2 with an anti-PGE2 antibody both delayed and attenuated LPS fever. It is concluded that fever is initiated by circulating PGE2 synthesized by macrophages of the LPS-processing organs (lung and liver) via phosphorylation of cPLA2 and transcriptional up-regulation of COX-2. Whether PGE2 produced at the level of the blood-brain barrier also contributes to the development of the first phase remains to be clarified.

CNS activational responses to staphylococcal enterotoxin B: T-lymphocyte-dependent immune challenge effects on stress-related circuitry.

Staphylococcal enterotoxin B (SEB) is a bacterial superantigen that engages the immune system in a T-lymphocyte-dependent manner and induces a cytokine profile distinct from that elicited by the better-studied bacterial pathogen analog, lipopolysaccharide (LPS). Because of reports of SEB recruiting central nervous system (CNS) host defense mechanisms via pathways in common with LPS, we sought to further characterize central systems impacted by this agent. Rats were treated with SEB at doses of 50-5,000 mug/kg, and killed 0.5-6 hours thereafter. SEB injection produced a discrete pattern of Fos induction in brain that peaked at 2-3 hours postinjection and whose strength was dose-related. Induced Fos expression was predominantly subcortical and focused in a set of interconnected central autonomic structures, including aspects of the bed n. of the stria terminalis, central amygdala and lateral parabrachial nuclei; functionally related (and LPS-responsive) cell groups in the n. solitary tract, ventrolateral medulla, and paraventricular hypothalamic n. (PVH) were, by contrast, weakly responsive. SEB also activated cell groups in the limbic forebrain (lateral septal n, medial prefrontal cortex) and hypothalamic GABAergic neurons, which could account for its failure to elicit reliable increases in Fos-ir or corticotropin-releasing factor (CRF) mRNA in the PVH. SEB nevertheless did provoke reliable pituitary-adrenal secretory responses. The identification of subsets of central autonomic and limbic forebrain structures that are sensitive to SEB provides a basis for a systems-level understanding of the physiological and behavioral effects attributed to the superantigen. Core SEB-responsive cell groups exclude a medullary-PVH circuit implicated in pituitary-adrenal responses to LPS.

Neuroprotection induced by the adenosine A2A antagonist CSC in the 6-OHDA rat model of parkinsonism: effect on the activity of striatal output pathways.

In Parkinson's disease (PD), the striatal dopamine depletion and the following overactivation of the indirect pathway of the basal ganglia leads to very early disinhibition of the subthalamic nucleus (STN) that may contribute to the progression of PD by glutamatergic overstimulation of the dopaminergic neurons in the substantia nigra. Adenosine A2A antagonism has been demonstrated to attenuate the overactivity of the striatopallidal pathway. To investigate whether neuroprotection exerted by the A2A antagonist 8-(3-chlorostyryl)caffeine (CSC) correlates with a diminution of the striatopallidal pathway activity, we have examined the changes in the mRNA encoding for enkephalin, dynorphin, and adenosine A2A receptors by in situ hybridization induced by subacute systemic pretreatment with CSC in rats with striatal 6-hydroxydopamine(6-OHDA) administration. Animals received CSC for 7 days until 30 min before 6-OHDA intrastriatal administration. Vehicle-treated group received a solution of dimethyl sulfoxide. CSC pretreatment partially attenuated the decrease in nigral tyrosine hydroxylase immunoreactivity induced by 6-OHDA, whereas no modification of the increase in preproenkephalin mRNA expression in the dorsolateral striatum was observed. The neuroprotective effect of the adenosine A2A antagonist CSC in striatal 6-OHDA-lesioned rats does not result from a normalization of the increase in striatal PPE mRNA expression in the DL striatum, suggesting that other different mechanisms may be involved.

Expression of serotonin 5-HT2C receptors in GABAergic cells of the anterior raphe nuclei.

We have used double in situ hybridization to examine the cellular localization of 5-HT2C receptor mRNA in relation to serotonergic and GABAergic neurons in the anterior raphe nuclei of the rat. In the dorsal and median raphe nuclei 5-HT2C receptor mRNA was not detected in serotonergic cells identified as those expressing serotonin (5-HT) transporter mRNA. In contrast, 5-HT2C receptor mRNA was found in most GABAergic cells, recognized by the presence of glutamic acid decarboxylase mRNA. Such 5-HT2C receptor-positive GABAergic neurons were mainly located in the intermediolateral and lateral portions of the dorsal raphe and lateral part of the median raphe. The present data give anatomical support to a previous hypothesis that proposed a negative-feedback loop involving reciprocal connections between GABAergic interneurons bearing 5-HT2A/2C receptors and 5-HT neurons in the dorsal raphe and surrounding areas. According to this model, the excitation of GABAergic interneurons through these 5-HT2C (and also 5-HT2A) receptors would result in the suppression of 5-HT cell firing.

Expression of serotonin1A and serotonin2A receptors in pyramidal and GABAergic neurons of the rat prefrontal cortex.

Serotonergic 5-HT1A and 5-HT2A receptors are abundantly expressed in prefrontal cortex (PFC) and are targets of atypical antipsychotic drugs. They mediate, respectively, inhibitory and excitatory actions of 5-HT. The transcripts for both receptors are largely (approximately 80%) colocalized in rat and mouse PFC, yet their quantitative distribution in pyramidal and GABAergic interneurons is unknown. We used double in situ hybridization histochemistry to estimate the proportion of pyramidal and GABAergic neurons expressing these receptor transcripts in rat PFC. The number of GABAergic interneurons (expressing GAD mRNA) was a 22% of glutamatergic neurons (expressing vGluT1 mRNA, considered as putative pyramidal neurons). 5-HT2A receptor mRNA was present in a large percentage of pyramidal neurons (from 55% in prelimbic cortex to 88% in tenia tecta), except in layer VI, where it was localized only in 30% of those neurons. 5-HT2A receptor mRNA was present in approximately 25% of GAD-containing cells except in layer VI (10%). Likewise, approximately 60% of glutamatergic cells contained the 5-HT1A receptor transcript. We also found that approximately 25% of GAD-expressing cells contained the 5-HT1A receptor mRNA. These data help to clarify the role of 5-HT in prefrontal circuits and shed new light to the cellular elements involved in the action of atypical antipsychotics.

Co-expression and in vivo interaction of serotonin1A and serotonin2A receptors in pyramidal neurons of prefrontal cortex.

The prefrontal cortex plays a key role in the control of higher brain functions and is involved in the pathophysiology and treatment of schizophrenia. Here we report that approximately 60% of the neurons in rat and mouse prefrontal cortex express 5-HT(1A) and/or 5-HT2A receptor mRNAs, which are highly co-localized (approximately 80%). The electrical stimulation of the dorsal and median raphe nuclei elicited 5-HT1A-mediated inhibitions and 5-HT2A-mediated excitations in identified pyramidal neurons recorded extracellularly in rat medial prefrontal cortex (mPFC). Opposite responses in the same pyramidal neuron could be evoked by stimulating the raphe nuclei at different coordinates, suggesting a precise connectivity between 5-HT neuronal subgroups and 5-HT1A and 5-HT2A receptors in pyramidal neurons. Microdialysis experiments showed that the increase in local 5-HT release evoked by the activation of 5-HT2A receptors in mPFC by DOI (5-HT2A/2C receptor agonist) was reversed by co-perfusion of 5-HT1A agonists. This inhibitory effect was antagonized by WAY-100635 and the prior inactivation of 5-HT1A receptors in rats and was absent in mice lacking 5-HT1A receptors. These observations help to clarify the interactions between the mPFC and the raphe nuclei, two key areas in psychiatric illnesses and improve our understanding of the action of atypical antipsychotics, acting through these 5-HT receptors.

An autoradiographic study of the influence of pindolol upon [35S]GTPgammaS binding in rat, guinea pig and human brain.

The 5-HT1A/beta-adrenoceptor ligand (+/-)pindolol has been used in clinical trials to enhance the antidepressant effect of selective serotonin (5-HT) reuptake inhibitors (SSRIs). The accelerating effect of (+/-)pindolol is thought to derive from its blockade of the SSRI-induced, 5-HT1A autoreceptor-mediated inhibition of serotonergic cell firing and 5-HT release. However, controversial results have been reported in regard to its ability to antagonize the effect of 5-HT at such receptors. In the present study, we have analysed the effect of (+/-)pindolol on receptor-mediated G-protein activation by measuring guanylyl 5'-[gamma-[35S]thio]-triphosphate ([35S]GTPgammaS) binding onto tissue sections from the hippocampus and dorsal raphe nucleus from rat, guinea pig and human brain. In these regions, enriched in 5-HT1A receptors, (+/-)pindolol antagonized the stimulation of [35S]GTPgammaS binding induced by 5-HT in a concentration-dependent manner. We found that in both rat and human brain the calculated pEC50 values were higher in the dorsal raphe nucleus than in hippocampus. This suggests a higher potency of (+/-)pindolol at somatodendritic 5-HT1A receptors compared to post-synaptic 5-HT1A sites. In the absence of 5-HT, (+/-)pindolol (up to 10(-3) M) did not modify [35S]GTPgammaS binding, which remained at basal levels, indicating that, in this assay, (+/-)pindolol acts as a neutral antagonist rather than a partial agonist as it has been observed in other experimental models. The present data are relevant for the understanding of the neurobiological basis of pindolol acceleration of the action of SSRI antidepressants.

5-ht5B receptor mRNA in the raphe nuclei: coexpression with serotonin transporter.

We used double-label in situ hybridization to examine the cellular localization of 5-ht(5B) receptor mRNA in relation to serotonin transporter mRNA in the rat dorsal raphe (DR) and central superior nucleus (CS, median raphe nucleus). 5-ht(5B) receptor mRNA hybridization signal was often found on serotonin transporter mRNA-positive neuron profiles. The degree of cellular colocalization of these mRNAs notably varied among the different regions of the raphe nuclei. In the DR, cell bodies showing 5-ht(5B) receptor mRNA expression were abundant in the medial portions of the nucleus, all of them being also labeled for serotonin transporter mRNA. In contrast, in the ventrolateral regions (lateral wings) of the DR, we observed serotonin transporter mRNA-positive cells, but they were devoid of 5-ht(5B) receptor mRNA signal. In the CS, the level of coexpression of 5-ht(5B) receptor mRNA with serotonin transporter mRNA was high in the intermediate portions of the nucleus; however, we were unable to detect specific 5-ht(5B) receptor mRNA hybridization signal in its caudal extent. Our results support the presence of 5-ht(5B) receptor in serotonergic neurons in the DR and CS, suggesting an autoreceptor role for this receptor subtype.