Presynaptic 5-HT(1B) receptor-mediated serotonergic inhibition of glutamate transmission in the bed nucleus of the stria terminalis

The role and mechanism of 5-HTDRN-BNST neural circuit in anxiety and fear lesions

Central 5-hydroxytryptaminergic dorsal raphe nucleus (5-HTDRN)-bed nucleus of stria terminalis (BNST) neural circuit dysfunction is one of the important neurobiological basis of anxiety and fear disorders. Under stress, 5-hydroxytryptamine (5-HT) neurons act on BNST receptors to attenuate anxiety and fear responses or enhance anxiety and fear. In BNST, corticotropin releasing factor neurons play a role in regulating emotions by reversely regulating excitatory or inhibitory 5-HT neurons. The composition of 5-HTDRN-BNST neural circuit, the pathological changes of 5-HTDRN-BNST neural circuit function damage under stress, and the effects of 5-HTDRN-BNST neural circuit on anxiety disorder, panic disorder and post-traumatic stress disorder were analyzed and are summarized in this paper. The characteristics of functional changes of the neural circuit and its effects on brain functional activities provide a basis and ideas for the treatment of anxiety and fear disorders through the regulation of 5-HTDRN-BNST neural circuit, and they also provide a new perspective for understanding the pathological mechanism of such diseases.

How Oxidative Stress Induces Depression?

Depression increasingly affects a wide range and a large number of people worldwide, both physically and psychologically, which makes it a social problem requiring prompt attention and management. Accumulating clinical and animal studies have provided us with substantial insights of disease pathogenesis, especially central monoamine deficiency, which considerably promotes antidepressant research and clinical treatment. The first-line antidepressants mainly target the monoamine system, whose drawbacks mainly include slow action and treatment resistant. The novel antidepressant esketamine, targeting on central glutamatergic system, rapidly and robustly alleviates depression (including treatment-resistant depression), whose efficiency is shadowed by potential addictive and psychotomimetic side effects. Thus, exploring novel depression pathogenesis is necessary, for seeking more safe and effective therapeutic methods. Emerging evidence has revealed vital involvement of oxidative stress (OS) in depression, which inspires us to pursue antioxidant pathway for depression prevention and treatment. Fully uncovering the underlying mechanisms of OS-induced depression is the first step towards the avenue, thus we summarize and expound possible downstream pathways of OS, including mitochondrial impairment and related ATP deficiency, neuroinflammation, central glutamate excitotoxicity, brain-derived neurotrophic factor/tyrosine receptor kinase B dysfunction and serotonin deficiency, the microbiota-gut-brain axis disturbance and hypothalamic-pituitary-adrenocortical axis dysregulation. We also elaborate on the intricate interactions between the multiple aspects, and molecular mechanisms mediating the interplay. Through reviewing the related research progress in the field, we hope to depict an integral overview of how OS induces depression, in order to provide fresh ideas and novel targets for the final goal of efficient treatment of the disease.

Serotonin modulation of hippocampal functions: From anatomy to neurotherapeutics

The hippocampal region receives a dense serotoninergic innervation originating from both medial and dorsal raphe nuclei. This innervation regulates hippocampal activity through the activation of distinct receptor families that are expressed in excitatory and inhibitory neurons, terminals of several afferent neurotransmitter systems, and glial cells. Preclinical and clinical studies indicate that hippocampal dysfunctions are involved in learning and memory deficits, dementia, Alzheimer's disease, epilepsy and mood disorders such as anxiety, depression and post-traumatic syndrome disorder, whereas the hippocampus participates also in the therapeutic mechanisms of numerous medicines. Not surprisingly, several drugs acting via 5-HT mechanisms are efficacious to some extent in some diseases and the link between 5-HT and the hippocampus although clear remains difficult to untangle. For this reason, we review reported data concerning the distribution and the functional roles of the 5-HT receptors in the hippocampal region in health and disease. The impact of the 5-HT systems on the hippocampal function is such that the research of new 5-HT mechanisms and drugs is still very active. It concerns notably drugs acting at the 5-HT_1A,_2A,_2C,_4,6 receptor subtypes, in addition to the already existing drugs including the selective serotonin reuptake inhibitors.

5-HT1A receptor-mediated attenuation of synaptic transmission in rat medial vestibular nucleus impacts on vestibular-related motor function

KEY POINTS

Chemogenetic activation of medial vestibular nucleus-projecting 5-HT neurons resulted in deficits in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. The 5-HT_1A receptor mediates the vestibular-related behavioural effects of 5-HT in the vestibular nucleus. 5-HT_1A receptor activation attenuated evoked excitatory postsynaptic currents and evoked inhibitory postsynaptic currents via a presynaptic mechanism in the vestibular nucleus.

ABSTRACT

While the anxiolytic effects of serotonergic neuromodulation are well studied, its role in sensorimotor coordination and postural control is unclear. In this study, we show that an increase of serotonin (5-hydroxytryptamine, 5-HT) at the medial vestibular nucleus (MVN), a brainstem centre for vestibulospinal coordination, by either direct cannula administration or chemogenetic stimulation of MVN-projecting serotonergic neurons, adversely affected performance of rats in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. Application of the 5-HT₁ and 5-HT₇ receptor co-agonist 8-hydroxy-2-(di-n-propylamino) tetralin recapitulated the effect of 5-HT, while co-administration of the specific 5-HT_1A receptor antagonist WAY 100135 effectively abolished all 5-HT-induced behavioural deficits. This indicated that 5-HT_1A receptors mediated the effects of 5-HT in the rat MVN. Using whole-cell patch-clamp recording, we demonstrated that 5-HT_1A receptor activation attenuated both evoked excitatory and evoked inhibitory postsynaptic currents through a presynaptic mechanism in the rat MVN. The results thus highlight the 5-HT_1A receptor as the gain controller of vestibular-related brainstem circuits for posture and balance.

Stimulation of 5-HT receptors in anterodorsal BNST guides fear to predictable and unpredictable threat

Through pharmacological manipulation of the serotonergic (5-Hydroxytryptamin, 5-HT) system, combined with behavioral analysis, we tested the hypothesis that fear responses to predictable and unpredictable threat are regulated through stimulation of 5-HT receptors (5-HT-R) in the anterodorsal section of the bed nucleus of the stria terminalis (adBNST). Local adBNST application of 5-HT1A-R antagonist WAY100635 and 5-HT1B-R antagonist NAS-181 before fear retrieval enhanced freezing, 24 h after predictable fear conditioning. In contrast, increased fear responses to unpredictable threat were blocked by 5-HT1A-R agonist Buspirone (given before conditioning or retrieval) and 5-HT1B-R agonist CP-94253 (applied before training). Prolonged fear responses were also blocked by local application of the 5-HT2A-R antagonist R-96544 before fear retrieval, and conversely, local application of the 5-HT2A-R agonist NBOH-2C-CN hydrochloride before fear retrieval enhanced freezing 24 h after predictable conditioning, indicating augmented fear responses. Activation of inhibitory 5-HT1A- or 5-HT1B-Rs and the blockade of the excitatory 5-HT2A-R before unpredictable fear conditioning significantly reduced freezing during retrieval. The results from this study suggest that modulation of inhibitory 5-HT1A/1B-R and/or excitatory 5-HT2A-R activity in the adBNST may represent potential targets for the development of new treatment strategies in anxiety disorders. In addition, this study supports the validity and reliability of the mouse model of modulated fear to predictable and unpredictable threats to study mechanisms of fear and anxiety in combination with pharmacological manipulations.

Inflammation induces developmentally regulated sumatriptan inhibition of spinal synaptic transmission

BACKGROUND AND PURPOSE

While triptans are used to treat migraine, there is evidence that they also reduce inflammation-induced pain at the spinal level. The cellular mechanisms underlying this spinal enhancement are unknown. We examined whether inflammation alters sumatriptan modulation of synaptic transmission in the rat spinal dorsal horn.

EXPERIMENTAL APPROACH

Three to four days following intraplantar injection of complete Freund's adjuvant (CFA) or saline, whole cell recordings of evoked glutamatergic EPSCs were made from lumbar lamina I-II dorsal horn neurons in rat spinal slices KEY RESULTS: In 2- to 3-week-old animals, sumatriptan reduced the amplitude of evoked EPSCs and this was greater in slices from CFA, compared to saline-injected rats. In CFA-injected animals, sumatriptan increased the paired pulse ratio of evoked EPSCs and reduced the rate of spontaneous miniature EPSCs. The 5-HT_1B and 5-HT_1D agonists CP9 3129 and PNU109291 both inhibited evoked EPSCs in CFA but not saline-injected rats. By contrast, the 5-HT_1A agonist R(+)-8-OH-DPAT inhibited evoked EPSCs in saline but not CFA-injected rats. In CFA-injected rats, the sumatriptan-induced inhibition of evoked EPSCs was reduced by the 5-HT_1B and 5-HT_1D antagonists NAS181 and BRL-15572. Intriguingly, the difference in sumatriptan inhibition between CFA and saline-injected animals was only observed in animals less than 4 weeks old.

CONCLUSION AND IMPLICATIONS

These findings indicate that inflammation induces a developmentally regulated 5-HT_1B/1D presynaptic inhibition of excitatory transmission into the rat superficial dorsal horn. Thus, triptans could potentially act as spinal analgesic agents for inflammatory pain in the juvenile setting.

Dopaminergic afferents from midbrain to dorsolateral bed nucleus of stria terminalis inhibit release and expression of corticotropin-releasing hormone in paraventricular nucleus

Dopaminergic (DAergic) neurons of the midbrain ventral tegmental area (VTA) are known to regulate the hypothalamic-pituitary-adrenal (HPA) axis but have no direct projections to the paraventricular nucleus (PVN) of the hypothalamus. This study investigated whether VTA DAergic afferents modulate glutamatergic transmission-dependent GABAergic neurons in dorsolateral bed nucleus of stria terminalis (dlBNST) to affect the activity of the HPA-axis. Herein, we demonstrate that systemic administration of the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or the VTA-injection of 1-methyl-4-phenylpyridinium ion (MPP+) in male mice (MPTP-mice and MPP+mice) caused a decline of tyrosine hydroxylase positive (TH+) cells in VTA with a reduction in TH+fibers in the dlBNST. MPTP-mice and MPP+mice displayed a clear increase in serum levels of corticosterone (CORT) and adrenocorticotropic hormone, corticotropin-releasing hormone (CRH) expression, and CRH neuron activity in PVN. The presynaptic glutamate release, glutamatergic synaptic transmission and induction of long-term potentiation in dlBNST of MPTP-mice were suppressed, and these effects were rescued by a D1-like DAergic receptor (D1R) agonist and mimicked in control dlBNST by blockade of D1R. MPTP-mice exhibited low expression of glutamic acid decarboxylase and dysfunction of the excitatory-dependent GABAergic circuit in dlBNST, and these effects were recovered by the administration of D1R agonist. Furthermore, either dlBNST-injection of D1R agonist or PVN-injection of GABA_A receptor (GABA_A R) agonist could correct the increased secretion and expression of CRH in MPTP-mice. The results indicate that the DAergic afferents from VTA enhance excitatory-dependent activation of GABAergic neurons in dlBNST, which suppress the activity of the HPA-axis.

Serotonergic control of GABAergic inhibition in the lateral amygdala

Much evidence implicates the serotonergic regulation of the amygdala in anxiety. Thus the present study was undertaken to characterize the influence of serotonin (5-HT) on principal neurons (PNs) of the rat lateral amygdala (LA), using whole cell recordings in vitro. Because inhibition is a major determinant of PN activity, we focused on the control of GABAergic transmission by 5-HT. IPSCs were elicited by local electrical stimulation of LA in the presence of glutamate receptor antagonists. We found that 5-HT reduces GABA_A inhibitory postsynaptic currents (IPSCs) via presynaptic 5-HT_1B receptors. While the presynaptic inhibition of GABA release also attenuated GABA_B currents, this effect was less pronounced than for GABA_A currents because 5-HT also induced a competing postsynaptic enhancement of GABA_B currents. That is, GABA_B currents elicited by pressure application of GABA or baclofen were enhanced by 5-HT. In addition, we obtained evidence suggesting that 5-HT differentially regulates distinct subsets of GABAergic synapses. Indeed, GABA_A IPSCs were comprised of two components: a relatively 5-HT-insensitive IPSC that had a fast time course and a 5-HT-sensitive component that had a slower time course. Because the relative contribution of these two components varied depending on whether neurons were recorded at proximity versus at a distance from the stimulating electrodes, we speculate that distinct subtypes of local-circuit cells contribute the two contingents of GABAergic synapses. Overall, our results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.NEW & NOTEWORTHY We report that 5-HT, acting via presynaptic 5-HT_1B receptors, attenuates GABA_A IPSCs by reducing GABA release in the lateral amygdala (LA). In parallel, 5-HT enhances GABA_B currents postsynaptically, such that GABA_B inhibitory postsynaptic currents (IPSCs) are relatively preserved from the presynaptic inhibition of GABA release. We also found that the time course of 5-HT-sensitive and -insensitive GABA_A IPSCs differ. Together, these results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.

The 5-HT1B receptor - a potential target for antidepressant treatment

Major depressive disorder (MDD) is the leading cause of disability worldwide. The serotonin hypothesis may be the model of MDD pathophysiology with the most support. The majority of antidepressants enhance synaptic serotonin levels quickly, while it usually takes weeks to discern MDD treatment effect. It has been hypothesized that the time lag between serotonin increase and reduction of MDD symptoms is due to downregulation of inhibitory receptors such as the serotonin 1B receptor (5-HT1BR). The research on 5-HT1BR has previously been hampered by a lack of selective ligands for the receptor. The last extensive review of 5-HT1BR in the pathophysiology of depression was published 2009, and based mainly on findings from animal studies. Since then, selective radioligands for in vivo quantification of brain 5-HT1BR binding with positron emission tomography has been developed, providing new knowledge on the role of 5-HT1BR in MDD and its treatment. The main focus of this review is the role of 5-HT1BR in relation to MDD and its treatment, although studies of 5-HT1BR in obsessive-compulsive disorder, alcohol dependence, and cocaine dependence are also reviewed. The evidence outlined range from animal models of disease, effects of 5-HT1B receptor agonists and antagonists, case-control studies of 5-HT1B receptor binding postmortem and in vivo, with positron emission tomography, to clinical studies of 5-HT1B receptor effects of established treatments for MDD. Low 5-HT1BR binding in limbic regions has been found in MDD patients. When 5-HT1BR ligands are administered to animals, 5-HT1BR agonists most consistently display antidepressant-like properties, though it is not yet clear how 5-HT1BR is best approached for optimal MDD treatment.

Serotonin gating of cortical and thalamic glutamate inputs onto principal neurons of the basolateral amygdala

The basolateral amygdala (BLA) is a key site for crossmodal association of sensory stimuli and an important relay in the neural circuitry of emotion. Indeed, the BLA receives substantial glutamatergic inputs from multiple brain regions including the prefrontal cortex and thalamic nuclei. Modulation of glutamatergic transmission in the BLA regulates stress- and anxiety-related behaviors. Serotonin (5-HT) also plays an important role in regulating stress-related behavior through activation of both pre- and postsynaptic 5-HT receptors. Multiple 5-HT receptors are expressed in the BLA, where 5-HT has been reported to modulate glutamatergic transmission. However, the 5-HT receptor subtype mediating this effect is not yet clear. The aim of this study was to use patch-clamp recordings from BLA neurons in an ex vivo slice preparation to examine 1) the effect of 5-HT on extrinsic sensory inputs, and 2) to determine if any pathway specificity exists in 5-HT regulation of glutamatergic transmission. Two independent input pathways into the BLA were stimulated: the external capsule to mimic cortical input, and the internal capsule to mimic thalamic input. Bath application of 5-HT reversibly reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs) induced by stimulation of both pathways. The decrease was associated with an increase in the paired-pulse ratio and coefficient of variation of eEPSC amplitude, suggesting 5-HT acts presynaptically. Moreover, the effect of 5-HT in both pathways was mimicked by the selective 5-HT_1B receptor agonist CP93129, but not by the 5-HT_1A receptor agonist 8-OH DPAT. Similarly the effect of exogenous 5-HT was blocked by the 5-HT_1B receptor antagonist GR55562, but not affected by the 5-HT_1A receptor antagonist WAY 100635 or the 5-HT₂ receptor antagonists pirenperone and MDL 100907. Together these data suggest 5-HT gates cortical and thalamic glutamatergic inputs into the BLA by activating presynaptic 5-HT_1B receptors.

Characterization of serotonin-induced inhibition of excitatory synaptic transmission in the anterior cingulate cortex

Excitatory synaptic transmission in central synapses is modulated by serotonin (5-HT). The anterior cingulate cortex (ACC) is an important cortical region for pain perception and emotion. ACC neurons receive innervation of projecting serotonergic nerve terminals from raphe nuclei, but the possible effect of 5-HT on excitatory transmission in the ACC has not been investigated. In the present study, we investigated the role of 5-HT on glutamate neurotransmission in the ACC slices of adult mice. Bath application of 5-HT produced dose-dependent inhibition of evoked excitatory postsynaptic currents (eEPSCs). Paired pulse ratio (PPR) was significantly increased, indicating possible presynaptic effects of 5-HT. Consistently, bath application of 5-HT significantly decreased the frequency of spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs). By contrast, amplitudes of sEPSCs and mEPSCs were not significantly affected. After postsynaptic application of G protein inhibitor GDP-β-S, 5-HT produced inhibition of eEPSCs was significantly reduced. Finally, NAN-190, an antagonist of 5-HT_1A receptor, significantly reduced postsynaptic inhibition of 5-HT and abolished presynaptic inhibition. Our results strongly suggest that presynaptic as well as postsynaptic 5-HT receptor including 5-HT1A subtype receptor may contribute to inhibitory modulation of glutamate release as well as postsynaptic responses in the ACC.

The antihypertensive drug pindolol attenuates long-term but not short-term binge-like ethanol consumption in mice

Alcohol dependence is a debilitating disorder with current therapies displaying limited efficacy and/or compliance. Consequently, there is a critical need for improved pharmacotherapeutic strategies to manage alcohol use disorders (AUDs). Previous studies have shown that the development of alcohol dependence involves repeated cycles of binge-like ethanol intake and abstinence. Therefore, we used a model of binge-ethanol consumption (drinking-in-the-dark) in mice to test the effects of compounds known to modify the activity of neurotransmitters implicated in alcohol addiction. From this, we have identified the FDA-approved antihypertensive drug pindolol, as a potential candidate for the management of AUDs. We show that the efficacy of pindolol to reduce ethanol consumption is enhanced following long-term (12 weeks) binge-ethanol intake, compared with short-term (4 weeks) intake. Furthermore, pindolol had no effect on locomotor activity or consumption of the natural reward sucrose. Because pindolol acts as a dual beta-adrenergic antagonist and 5-HT_1A/1B partial agonist, we examined its effect on spontaneous synaptic activity in the basolateral amygdala (BLA), a brain region densely innervated by serotonin and norepinephrine-containing fibres. Pindolol increased spontaneous excitatory post-synaptic current frequency of BLA principal neurons from long-term ethanol-consuming mice but not naïve mice. Additionally, this effect was blocked by the 5-HT_1A/1B receptor antagonist methiothepin, suggesting that altered serotonergic activity in the BLA may contribute to the efficacy of pindolol to reduce ethanol intake following long-term exposure. Although further mechanistic investigations are required, this study demonstrates the potential of pindolol as a new treatment option for AUDs that can be fast-tracked into human clinical studies.

Attenuation of the anxiogenic effects of cocaine by 5-HT1B autoreceptor stimulation in the bed nucleus of the stria terminalis of rats

RATIONALE

Cocaine produces significant aversive/anxiogenic actions whose underlying neurobiology remains unclear. A possible substrate contributing to these actions is the serotonergic (5-HT) pathway projecting from the dorsal raphé (DRN) to regions of the extended amygdala, including the bed nucleus of the stria terminalis (BNST) which have been implicated in the production of anxiogenic states.

OBJECTIVES

The present study examined the contribution of 5-HT signaling within the BNST to the anxiogenic effects of cocaine as measured in a runway model of drug self-administration.

METHODS

Male Sprague-Dawley rats were fitted with bilateral infusion cannula aimed at the BNST and then trained to traverse a straight alley once a day for a single 1 mg/kg i.v. cocaine infusion delivered upon goal-box entry on each of 16 consecutive days/trials. Intracranial infusions of CP 94,253 (0, 0.25, 0.5, or 1.0 μg/side) were administered to inhibit local 5-HT release via activation of 5-HT_1B autoreceptors. To confirm receptor specificity, the effects of this treatment were then challenged by co-administration of the selective 5-HT_1B antagonist NAS-181.

RESULTS

Intra-BNST infusions of the 5-HT_1B autoreceptor agonist attenuated the anxiogenic effects of cocaine as reflected by a decrease in runway approach-avoidance conflict behavior. This effect was reversed by the 5-HT_1B antagonist. Neither start latencies (a measure of the subject's motivation to seek cocaine) nor spontaneous locomotor activity (an index of motoric capacity) were altered by either treatment.

CONCLUSIONS

Inhibition of 5-HT_1B signaling within the BNST selectively attenuated the anxiogenic effects of cocaine, while leaving unaffected the positive incentive properties of the drug.

The Cyclic Vomiting Syndrome Threshold: A Framework for Understanding Pathogenesis and Predicting Successful Treatments

Cyclic vomiting syndrome (CVS) is an uncommon, idiopathic disorder defined by recurrent, sudden-onset attacks of repetitive retching and vomiting that are separated by symptom-free intervals. CVS was long regarded as a disorder primarily experienced by children but is now known to present de novo in adulthood. Adult CVS has garnered more research attention over the past 20 years, and these efforts have identified some acute and prophylactic treatments for this disorder. However, CVS still lacks a unifying disease model, and this has hindered the development of new therapies. Here adult CVS is reframed as a neurogenic disorder, driven by various endophenotypic factors that shape patterns of activity within the neural circuits required for disease expression. The concept of the "CVS threshold" is put forth in parallel with exploring the remarkable similarity of adult CVS with features of chronic migraine, epilepsy, and panic disorder. Because of such shared neural mechanisms and overlapping endophenotypes, many therapies that have been developed for these other disorders could also be useful in managing CVS. This review seeks to achieve three primary aims: (1) to develop a comprehensive, explanatory framework for adult CVS pathogenesis, (2) to use this framework for identifying potentially novel therapies for CVS, and (3) to describe future research directions that are needed to move the field forward.

Development-dependent behavioral change toward pups and synaptic transmission in the rhomboid nucleus of the bed nucleus of the stria terminalis

Sexually naïve male C57BL/6 mice aggressively bite unfamiliar pups. This behavior, called infanticide, is considered an adaptive reproductive strategy of males of polygamous species. We recently found that the rhomboid nucleus of the bed nucleus of the stria terminalis (BSTrh) is activated during infanticide and that the bilateral excitotoxic lesions of BSTrh suppress infanticidal behavior. Here we show that 3-week-old male C57BL/6 mice rarely engaged in infanticide and instead, provided parental care toward unfamiliar pups, consistent with observations in rats and other rodent species. This inhibition of infanticide at the periweaning period is functional because the next litter will be born at approximately the time of weaning of the previous litter through maternal postpartum ovulation. However, the mechanism of this age-dependent behavioral change is unknown. Therefore, we performed whole-cell patch clamp recordings of BSTrh and compared evoked neurotransmission in response to the stimulation of the stria terminalis of adult and 3-week-old male mice. Although we were unable to detect a significant difference in the amplitudes of inhibitory neurotransmission, the amplitudes and the paired-pulse ratio of evoked excitatory postsynaptic currents differed between adult and 3-week-old mice. These data suggest that maturation of the synaptic terminal in BSTrh that occurred later than 3 weeks after birth may mediate by the adaptive change from parental to infanticidal behavior in male mice.

Stress Modulation of Opposing Circuits in the Bed Nucleus of the Stria Terminalis

The anterior bed nucleus of the stria terminalis (BNST) has been recognized as a critical structure in regulating trait anxiety, contextual fear memory, and appetitive behavior, and is known to be sensitive to stress manipulations. As one of the most complex structures in the central nervous system, the intrinsic circuitry of the BNST is largely unknown; however, recent technological developments have allowed researchers to begin to untangle the internal connections of the nucleus. This research has revealed the possibility of two opposing circuits, one anxiolytic and one anxiogenic, within the BNST, the relative strength of which determines the behavioral outcome. The balance of these pathways is critical in maintaining a normal physiological and behavioral state; however, stress and drugs of abuse can differentially affect the opposing circuitry within the nucleus to shift the balance to a pathological state. In this review, we will examine how stress interacts with the neuromodulators, corticotropin-releasing factor, norepinephrine, dopamine, and serotonin to affect the circuitry of the BNST as well as how synaptic plasticity in the BNST is modulated by stress, resulting in long-lasting changes in the circuit and behavioral state.

Attenuated methamphetamine-induced locomotor sensitization in serotonin transporter knockout mice is restored by serotonin 1B receptor antagonist treatment

Repeated administration of methamphetamine (METH) enhances acute locomotor responses to METH administered in the same context, a phenomenon termed as 'locomotor sensitization'. Although many of the acute effects of METH are mediated by its influences on the compartmentalization of dopamine, serotonin systems have also been suggested to influence the behavioral effects of METH in ways that are not fully understood. The present experiments examined serotonergic roles in METH-induced locomotor sensitization by assessing: (a) the effect of serotonin transporter (SERT; Slc6A4) knockout (KO) on METH-induced locomotor sensitization; (b) extracellular monoamine levels in METH-treated animals as determined by in-vivo microdialysis; and (c) effects of serotonin (5-HT) receptor antagonists on METH-induced behavioral sensitization, with focus on effects of the 5-HT1B receptor antagonist SB 216641 and a comparison with the 5-HT2 receptor antagonist ketanserin. Repeated METH administration failed to induce behavioral sensitization in homozygous SERT KO (SERT-/-) mice under conditions that produced substantial sensitization in wild-type or heterozygous SERT KO (SERT+/-) mice. The selective 5-HT1B antagonist receptor SB 216641 restored METH-induced locomotor sensitization in SERT-/- mice, whereas ketanserin was ineffective. METH-induced increases in extracellular 5-HT (5-HTex) levels were substantially reduced in SERT-/- mice, although SERT genotype had no effect on METH-induced increases in extracellular dopamine. These experiments demonstrate that 5-HT actions, including those at 5-HT1B receptors, contribute to METH-induced locomotor sensitization. Modulation of 5-HT1B receptors might aid therapeutic approaches to the sequelae of chronic METH use.

Serotonergic Systems in the Pathophysiology of Ethanol Dependence: Relevance to Clinical Alcoholism

Alcoholism is a progressive brain disorder that is marked by increased sensitivity to the positive and negative reinforcing properties of ethanol, compulsive and habitual use despite negative consequences, and chronic relapse to alcohol drinking despite repeated attempts to reduce intake or abstain from alcohol. Emerging evidence from preclinical and clinical studies implicates serotonin (5-hydroxytryptamine; 5-HT) systems in the pathophysiology of alcohol dependence, suggesting that drugs targeting 5-HT systems may have utility in the treatment of alcohol use disorders. In this Review, we discuss the role of 5-HT systems in alcohol dependence with a focus on 5-HT interactions with neural circuits that govern all three stages of the addiction cycle. We attempt to clarify how 5-HT influences circuit function at these different stages with the goal of identifying neural targets for pharmacological treatment of this debilitating disorder.

Activation of 5-HT1A receptors in the preBötzinger region has little impact on the respiratory pattern

The preBötzinger (preBötC) complex has been suggested as the primary site where systemically administered selective serotonin agonists have been shown to reduce or prevent opioid-induced depression of breathing. However, this hypothesis has not been tested pharmacologically in vivo. This study sought to determine whether 5-HT1A receptors within the preBötC and ventral respiratory column (VRC) mediate the tachypneic response induced by intravenous (IV) (±)-8-Hydroxy-2-diproplyaminotetralin hydrobromide (8-OH-DPAT) in a decerebrated dog model. IV 8-OH-DPAT (19 ± 2 μg/kg) reduced both inspiratory (I) and expiratory (E) durations by ∼ 40%, but had no effect on peak phrenic activity (PPA). Picoejection of 1, 10, and 100 μM 8-OH-DPAT on I and E preBötC neurons produced dose-dependent decreases up to ∼ 40% in peak discharge. Surprisingly, microinjections of 8-OH-DPAT and 5-HT within the VRC from the obex to 9 mm rostral had no effect on timing and PPA. These results suggest that the tachypneic effects of IV 8-OH-DPAT are due to receptors located outside of the areas we studied.

5HT(1B) receptor-mediated pre-synaptic depression of excitatory inputs to the rat lateral habenula

Accumulating lines of evidence indicate that the lateral habenula (LHb), which reciprocally interacts with raphe nuclei (RN), displays hyperactivity including synaptic potentiation of excitatory inputs to the LHb during a depressed state. Despite the potential importance of glutamatergic excitatory synapses in depression-like behavior, modulation of these LHb synapses by monoamines such as serotonin (5HT) is not fully understood at the cellular and molecular level. Therefore, we used whole cell voltage-clamp recording to examine the molecular mechanisms by which 5HT modulates glutamatergic transmission in the LHb. The present study provides the first evidence that glutamatergic transmission of LHb synapses is inhibited by activation of the 5HT(1B) receptor at the pre-synapse in both acute depression (5HT-AD) and long-term depression (5HT-LTD). We further show that 5HT-AD results from the activation of Shaker-type K(+) channels whereas 5HT-LTD depends on inhibition of the adenylyl cyclase-cAMP (AC-cAMP) pathway with an increase in pre-synaptic Ca(2+) release from ryanodine-sensitive internal stores in an NO-dependent manner.

The role of serotonin, vasopressin, and serotonin/vasopressin interactions in aggressive behavior

Aggression control has been investigated across species and is centrally mediated within various brain regions by several neural systems that interact at different levels. The debate over the degree to which any one system or region affects aggressive responding, or any behavior for that matter, in some senses is arbitrary considering the plastic and adaptive properties of the central nervous system. Nevertheless, from the reductionist point of view, the compartmentalization of evolutionarily maladaptive behaviors to specific regions and systems of the brain is necessary for the advancement of clinical treatments (e.g., pharmaceutical) and novel therapeutic methods (e.g., deep brain stimulation). The general purpose of this chapter is to examine the confluence of two such systems, and how their functional interaction affects aggressive behavior. Specifically, the influence of the serotonin (5HT) and arginine vasopressin (AVP) neural systems on the control of aggressive behavior will be examined individually and together to provide a context by which the understanding of aggression modulation can be expanded from seemingly parallel neuromodulatory mechanisms, to a single and highly interactive system of aggression control.

Corticotropin-releasing factor-related peptides, serotonergic systems, and emotional behavior

Corticotropin-releasing factor (CRF) is a 41-amino acid neuropeptide that is involved in stress-related physiology and behavior, including control of the hypothalamic-pituitary-adrenal (HPA) axis. Members of the CRF family of neuropeptides, including urocortin 1 (UCN 1), UCN 2, and UCN 3, bind to the G protein-coupled receptors, CRF type 1 (CRF₁) and CRF₂ receptors. In addition, CRF binding protein (CRFBP) binds both CRF and UCN 1 and can modulate their activities. There are multiple mechanisms through which CRF-related peptides may influence emotional behavior, one of which is through altering the activity of brainstem neuromodulatory systems, including serotonergic systems. CRF and CRF-related peptides act within the dorsal raphe nucleus (DR), the major source for serotonin (5-HT) in the brain, to alter the neuronal activity of specific subsets of serotonergic neurons and to influence stress-related behavior. CRF-containing axonal fibers innervate the DR in a topographically organized manner, which may contribute to the ability of CRF to alter the activity of specific subsets of serotonergic neurons. CRF and CRF-related peptides can either increase or decrease serotonergic neuronal firing rates and serotonin release, depending on their concentrations and on the specific CRF receptor subtype(s) involved. This review aims to describe the interactions between CRF-related peptides and serotonergic systems, the consequences for stress-related behavior, and implications for vulnerability to anxiety and affective disorders.

Mechanisms in the bed nucleus of the stria terminalis involved in control of autonomic and neuroendocrine functions: a review

The bed nucleus of the stria terminalis (BNST) is a heterogeneous and complex limbic forebrain structure, which plays an important role in controlling autonomic, neuroendocrine and behavioral responses. The BNST is thought to serve as a key relay connecting limbic forebrain structures to hypothalamic and brainstem regions associated with autonomic and neuroendocrine functions. Its control of physiological and behavioral activity is mediated by local action of numerous neurotransmitters. In the present review we discuss the role of the BNST in control of both autonomic and neuroendocrine function. A description of BNST control of cardiovascular and hypothalamus-pituitary-adrenal axisactivity at rest and during physiological challenges (stress and physical exercise) is presented. Moreover, evidence for modulation of hypothalamic magnocellular neurons activity is also discussed. We attempt to focus on the discussion of BNST neurochemical mechanisms. Therefore, the source and targets of neurochemical inputs to BNST subregions and their role in control of autonomic and neuroendocrine function is discussed in details.

Central CRF neurons are not created equal: phenotypic differences in CRF-containing neurons of the rat paraventricular hypothalamus and the bed nucleus of the stria terminalis

Corticotrophin-releasing factor (CRF) plays a key role in initiating many of the endocrine, autonomic, and behavioral responses to stress. CRF-containing neurons of the paraventricular nucleus of the hypothalamus (PVN) are classically involved in regulating endocrine function through activation of the stress axis. However, CRF is also thought to play a critical role in mediating anxiety-like responses to environmental stressors, and dysfunction of the CRF system in extra-hypothalamic brain regions, like the bed nucleus of stria terminalis (BNST), has been linked to the etiology of many psychiatric disorders including anxiety and depression. Thus, although CRF neurons of the PVN and BNST share a common neuropeptide phenotype, they may represent two functionally diverse neuronal populations. Here, we employed dual-immunofluorescence, single-cell RT-PCR, and electrophysiological techniques to further examine this question and report that CRF neurons of the PVN and BNST are fundamentally different such that PVN CRF neurons are glutamatergic, whereas BNST CRF neurons are GABAergic. Moreover, these two neuronal populations can be further distinguished based on their electrophysiological properties, their co-expression of peptide neurotransmitters such as oxytocin and arginine-vasopressin, and their cognate receptors. Our results suggest that CRF neurons in the PVN and the BNST would not only differ in their response to local neurotransmitter release, but also in their action on downstream target structures.

Activation of 5-hydroxytryptamine1B/1D/1F receptors as a mechanism of action of antimigraine drugs

INTRODUCTION

The introduction of the triptans (5-hydroxytryptamine (5-HT)1B/1D receptor agonists) was a great improvement in the acute treatment of migraine. However, shortcomings of the triptans have prompted research on novel serotonergic targets for the treatment of migraine.

AREAS COVERED

In this review the different types of antimigraine drugs acting at 5-HT receptors, their discovery and development are discussed. The first specific antimigraine drugs were the ergot alkaloids, consisting of ergotamine, dihydroergotamine and methysergide, which are agonists at 5-HT receptors, but can also bind α-adrenoceptors and dopamine receptors. In the 1990s, the triptans became available on the market. They are 5-HT1B/1D receptor agonists, showing fewer side effects due to their receptor specificity. In the last years, compounds that bind specifically to 5-HT1D, 5-HT1F and 5-HT7 receptors have been explored for their antimigraine potential. Furthermore, the serotonergic system seems to act in tight connection with the glutamatergic as well as the CGRP-ergic systems, which may open novel therapeutic avenues.

EXPERT OPINION

Although the triptans are very effective in treating migraine attacks, their shortcomings have stimulated the search for novel drugs. Currently, the focus is on 5-HT1F receptor agonists, which seem devoid of vascular side effects. Moreover, novel compounds that affect multiple transmitter and/or neuropeptide systems that are involved in migraine could be of therapeutic relevance.

Long-term ω-3 fatty acid supplementation induces anti-stress effects and improves learning in rats

Chronic stress leads to secretion of the adrenal steroid hormone corticosterone, inducing hippocampal atrophy and dendritic hypertrophy in the rat amygdala. Both alterations have been correlated with memory impairment and increased anxiety. Supplementation with ω-3 fatty acids improves memory and learning in rats. The aim of this study was to evaluate the effects of ω-3 supplementation on learning and major biological and behavioral stress markers. Male Sprague-Dawley rats were randomly assigned to three experimental groups: 1) Control, 2) Vehicle, animals supplemented with water, and 3) ω-3, rats supplemented with ω-3 (100 mg of DHA+25 mg of EPA). Each experimental group was divided into two subgroups: one of which was not subjected to stress while the other was subjected to a restraint stress paradigm. Afterwards, learning was analyzed by avoidance conditioning. As well, plasma corticosterone levels and anxiety were evaluated as stress markers, respectively by ELISA and the plus-maze test. Restraint stress impaired learning and increased both corticosterone levels and the number of entries into the open-arm (elevated plus-maze). These alterations were prevented by ω-3 supplementation. Thus, our results demonstrate that ω-3 supplementation had two beneficial effects on the stressed rats, a strong anti-stress effect and improved learning.

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.

Intrinsic connections in the anterior part of the bed nucleus of the stria terminalis

Intrinsic connections in the anterior portion of the bed nucleus of the stria terminalis (BNST-A) were studied using patch recordings and ultraviolet (UV) glutamate uncaging (GU) in vitro. UV light was delivered at small BNST-A sites in a grid-like pattern while evoked responses were monitored in different BNST-A regions. Three sectors were distinguished in the BNST-A using fiber bundles readily identifiable in transilluminated slices: the anterior commissure, dividing the BNST-A into dorsal and ventral (BNST-AV) regions, and the intra-BNST component of the stria terminalis, subdividing the dorsal portion into medial (BNST-AM) and lateral (BNST-AL) regions. Overall, GU elicited GABAergic inhibitory postsynaptic potentials (IPSPs) more frequently than excitatory postsynaptic potentials. The incidence of intraregional connections was higher than interregional links. With respect to the latter, asymmetric connections were seen between different parts of the BNST-A. Indeed, while reciprocal connections were found between the BNST-AL and BNST-AM, BNST-AL to BNST-AM connections were more frequent than in the opposite direction. Similarly, while GU in the BNST-AM or BNST-AL often elicited IPSPs in BNST-AV cells, the opposite was rarely seen. Within the BNST-AM, connections were polarized, with dorsal GU sites eliciting IPSPs in more ventrally located cells more frequently than the opposite. This trend was not seen in other regions of the BNST. Consistent with this, most BNST-AM cells had dorsally directed dendrites and ventrally ramified axons, whereas this morphological polarization was not seen in other parts of the BNST-A. Overall, our results reveal a hitherto unsuspected level of asymmetry in the connections within and between different BNST-A regions, implying a degree of interdependence in their activity.

5-HT(1B) receptors inhibit glutamate release from primary afferent terminals in rat medullary dorsal horn neurons

BACKGROUND AND PURPOSE

Although 5-HT(1B) receptors are expressed in trigeminal sensory neurons, it is still not known whether these receptors can modulate nociceptive transmission from primary afferents onto medullary dorsal horn neurons.

EXPERIMENTAL APPROACH

Primary afferent-evoked EPSCs were recorded from medullary dorsal horn neurons of rat horizontal brain stem slices using a conventional whole-cell patch clamp technique under a voltage-clamp condition.

KEY RESULTS

CP93129, a selective 5-HT(1B) receptor agonist, reversibly and concentration-dependently decreased the amplitude of glutamatergic EPSCs and increased the paired-pulse ratio. In addition, CP93129 reduced the frequency of spontaneous miniature EPSCs without affecting the current amplitude. The CP93129-induced inhibition of EPSCs was significantly occluded by GR55562, a 5-HT(1B/1D) receptor antagonist, but not LY310762, a 5-HT(1D) receptor antagonist. Sumatriptan, an anti-migraine drug, also decreased EPSC amplitude, and this effect was partially blocked by either GR55562 or LY310762. On the other hand, primary afferent-evoked EPSCs were mediated by the Ca(2+) influx passing through both presynaptic N-type and P/Q-type Ca(2+) channels. The CP93129-induced inhibition of EPSCs was significantly occluded by ω-conotoxin GVIA, an N-type Ca(2+) channel blocker.

CONCLUSIONS AND IMPLICATIONS

The present results suggest that the activation of presynaptic 5-HT(1B) receptors reduces glutamate release from primary afferent terminals onto medullary dorsal horn neurons, and that 5-HT(1B) receptors could be, at the very least, a potential target for the treatment of pain from orofacial tissues.

Yohimbine depresses excitatory transmission in BNST and impairs extinction of cocaine place preference through orexin-dependent, norepinephrine-independent processes

The alpha2 adrenergic receptor (α(2)-AR) antagonist yohimbine is a widely used tool for the study of anxiogenesis and stress-induced drug-seeking behavior. We previously demonstrated that yohimbine paradoxically depresses excitatory transmission in the bed nucleus of the stria terminalis (BNST), a region critical to the integration of stress and reward pathways, and produces an impairment of extinction of cocaine-conditioned place preference (cocaine-CPP) independent of α(2)-AR signaling. Recent studies show yohimbine-induced drug-seeking behavior is attenuated by orexin receptor 1 (OX(1)R) antagonists. Moreover, yohimbine-induced cocaine-seeking behavior is BNST-dependent. Here, we investigated yohimbine-orexin interactions. Our results demonstrate yohimbine-induced depression of excitatory transmission in the BNST is unaffected by alpha1-AR and corticotropin-releasing factor receptor-1 (CRFR(1)) antagonists, but is (1) blocked by OxR antagonists and (2) absent in brain slices from orexin knockout mice. Although the actions of yohimbine were not mimicked by the norepinephrine transporter blocker reboxetine, they were by exogenously applied orexin A. We find that, as with yohimbine, orexin A depression of excitatory transmission in BNST is OX(1)R-dependent. Finally, we find these ex vivo effects are paralleled in vivo, as yohimbine-induced impairment of cocaine-CPP extinction is blocked by a systemically administered OX(1)R antagonist. These data highlight a new mechanism for orexin on excitatory anxiety circuits and demonstrate that some of the actions of yohimbine may be directly dependent upon orexin signaling and independent of norepinephrine and CRF in the BNST.

Differential distribution of serotonin receptor subtypes in BNST(ALG) neurons: modulation by unpredictable shock stress

The bed nucleus of the stria terminalis (BNST) plays a critical role in regulating the behavioral response to stress. Stressors that activate the BNST also activate serotonergic (5-HT) systems. Hence, maladaptive changes of 5-HT receptor expression may contribute to stress-induced anxiety disorders. The BNST contains three neuronal types, Type I-III neurons. However, little is known about 5-HT receptor subtypes mRNA expression in these neurons, or whether it can be modulated by stress. Whole-cell patch clamp recording from Type I-III neurons was used in conjunction with single cell reverse transcriptase polymerase chain reaction (RT-PCR) to characterize 5-HT receptor mRNA expression, and examine the effects of stress on this expression. We report that Type I neurons expressed mRNA transcripts predominantly for 5-HT(1A) and 5-HT(7) receptors. Type II neurons expressed transcripts for every 5-HT receptor except the 5-HT(2C) receptor. Type II neurons were divided into three sub-populations: Type IIA in which transcripts for 5-HT(3) and 5-HT(7) receptors predominate, Type IIB that mainly express 5-HT(1B) and 5-HT(4) receptor transcripts, and Type IIC in which transcripts for 5-HT(1A) and 5-HT(2A) receptors predominate. Type III neurons were also subdivided into two sub-populations; one that predominantly expressed transcripts for 5-HT(1A), 5-HT(1B) and 5-HT(2A) receptors, and another that mainly expressed transcripts for 5-HT(2C) receptor. Unpredictable shock stress (USS) caused a long-lasting increase in anxiety-like behavior, and a concomitant decrease in 5-HT(1A) transcript expression in Type I-III neurons, as well as an up-regulation of a transcriptional repressor of 5-HT(1A) gene expression, deformed epidermal autoregulatory factor 1 (Deaf-1). Significantly USS decreased 5-HT(1A) protein level, and increased the level of Deaf-1. USS also increased 5-HT(1B) transcript expression in Type III neurons, as well as 5-HT(7) expression in Type I and II neurons. These data suggest that cell type-specific disruption of 5-HT receptor expression in BNST(ALG) neurons may contribute to stress-induced anxiety disorders.

A serotonergic discrimination favoring synaptic inputs that accompany robust spike firing in lateral amygdala neurons

The amygdala and serotonergic innervations thereunto are considered to cooperatively modulate affective behaviors. By whole-cell recording, the present study examined effects of serotonin (5-HT) on synaptic transmission in the rat basolateral amygdala (BLA) complex, which is the amygdalar entrance for sensory information. Application of 5-HT-attenuated excitatory postsynaptic currents at synapses from the lateral amygdala (LA) to the BLA proper, and also at synapses from putative thalamic afferents to LA principal neurons, both depending on 5-HT(2) receptors. This reduction of synaptic responses was confirmed in the BLA under current clamp. In the LA, by contrast, synaptic potentials were not reduced, but enhanced by 5-HT. With 5-HT bath-applied, a prolonged depolarization was induced in LA neurons by strong synaptic stimulation, which appears similar to a slow after-depolarization (sADP) induced by injecting depolarizing currents. Occurrence of such current-induced sADP was confirmed in LA neurons. Both the synaptically-activated prolonged depolarization and the current-induced sADPs depended on 5-HT(2) receptor activation and postsynaptic calcium increase, suggesting that the same postsynaptic intrinsic mechanisms are involved. Reduction of potassium currents was identified as a major ionic mechanism for this sADPs. We thus revealed that 5-HT usually reduces overall synaptic transmission in the whole BLA complex, but enables sADPs to occur, thereby increasing synaptic responsiveness of LA neurons in a positive feedback manner. With this duality of 5-HT actions in operation, a weak input to the BLA complex would be usually eliminated, but could be selected were it associated with sufficiently large depolarization.

Serotonin receptors in hippocampus

Serotonin is an ancient molecular signal and a recognized neurotransmitter brainwide distributed with particular presence in hippocampus. Almost all serotonin receptor subtypes are expressed in hippocampus, which implicates an intricate modulating system, considering that they can be localized as autosynaptic, presynaptic, and postsynaptic receptors, even colocalized within the same cell and being target of homo- and heterodimerization. Neurons and glia, including immune cells, integrate a functional network that uses several serotonin receptors to regulate their roles in this particular part of the limbic system.

Neuroinformatics analysis of molecular expression patterns and neuron populations in gray matter regions: the rat BST as a rich exemplar

The rat bed nuclei of the stria terminalis (BST) is an important part of the cerebral nuclei, both structurally and functionally. However, the literature is rather scarce and more importantly, often contradictory. In this paper we review the literature related to neuron populations reported in different rat BST parts, and to a set of more than 50 expressed molecules. The information related to neuron populations and molecules detected in the BST was expertly collated manually in a publicly available neuroinformatics system, the Brain Architecture Knowledge Management System (BAMS; http://brancusi.usc.edu/bkms). Using the tools implemented in BAMS, we organized the collated information, and further analyzed it statistically. The result of our analysis over the set of >50 expressed molecules confirms the BST parcellation scheme proposed by Swanson in 2004, with two exceptions. We present and discuss these results, and propose refined parcellation ventrally in the BST. We also review and discuss the presence of cholinergic neurons in the BST, and of neuron populations that express serotonin receptors. This review is one of the most comprehensive for the rat BST published in the literature, and it was possible only by using neuroinformatics tools.

Presynaptic muscarinic M(2) receptors modulate glutamatergic transmission in the bed nucleus of the stria terminalis

The anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) serves as an important relay station in stress circuitry. Limbic inputs to the BNST(ALG) are primarily glutamatergic and activity-dependent changes in this input have been implicated in abnormal behaviors associated with chronic stress and addiction. Significantly, local infusion of acetylcholine (ACh) receptor agonists into the BNST trigger stress-like cardiovascular responses, however, little is known about the effects of these agents on glutamatergic transmission in the BNST(ALG). Here, we show that glutamate- and ACh-containing fibers are found in close association in the BNST(ALG). Moreover, in the presence of the acetylcholinesterase inhibitor, eserine, endogenous ACh release evoked a long-lasting reduction of the amplitude of stimulus-evoked EPSCs. This effect was mimicked by exogenous application of the ACh analog, carbachol, which caused a reversible, dose-dependent, reduction of the evoked EPSC amplitude, and an increase in both the paired-pulse ratio and coefficient of variation, suggesting a presynaptic site of action. Uncoupling of postsynaptic G-proteins with intracellular GDP-β-S, or application of the nicotinic receptor antagonist, tubocurarine, failed to block the carbachol effect. In contrast, the carbachol effect was blocked by prior application of atropine or M(2) receptor-preferring antagonists, and was absent in M(2)/M(4) receptor knockout mice, suggesting that presynaptic M(2) receptors mediate the effect of ACh. Immunoelectron microscopy studies further revealed the presence of M(2) receptors on axon terminals that formed asymmetric synapses with BNST neurons. Our findings suggest that presynaptic M(2) receptors might be an important modulator of the stress circuit and hence a novel target for drug development.

Neuronal localization of 5-HT2A receptor immunoreactivity in the rat hippocampal region

The 5-HT2A receptor subtype (5-HT2Ar) plays an important role in the modulation of the hippocampal region activity and it has been associated with learning and memory processes. In the present study, the 5-HT2Ar was immunohistochemically localized in the rat hippocampal region, which includes the hippocampal formation and the parahippocampal region. In the hippocampal formation (dentate gyrus, hippocampus proper and subiculum) and entorhinal cortex, the colocalization of the 5-HT2Ar with the inhibitory transmitter γ-aminobutyric acid (GABA) was studied using double immunofluorescence confocal microscopy. The patterns of immunostaining were very different in non-injected and colchicine-injected rats. In untreated rats, the immunoreactivity could be attributed especially to neuropil. Interestingly, in non-injected rats, the 5-HT2Ar immunoreactivity was located in the mossy fibers, suggesting that serotonin acts presynaptically via this receptor subtype directly on glutamate axons. Pretreatment with colchicine increased the number of 5-HT2Ar-immunoreactive somata. Morphological and double immunofluorescence analyses indicated that the 5-HT2Ar was located on both the excitatory and the inhibitory neurons of the rat hippocampal region. The results of the present study suggest that the 5-HT2Ar could participate in the hippocampal neurotransmission by acting on different neuronal populations.

Serotonin control of sleep-wake behavior

Based on electrophysiological, neurochemical, genetic and neuropharmacological approaches, it is currently accepted that serotonin (5-HT) functions predominantly to promote wakefulness (W) and to inhibit REM (rapid eye movement) sleep (REMS). Yet, under certain circumstances the neurotransmitter contributes to the increase in sleep propensity. Most of the serotonergic innervation of the cerebral cortex, amygdala, basal forebrain (BFB), thalamus, preoptic and hypothalamic areas, raphe nuclei, locus coeruleus and pontine reticular formation comes from the dorsal raphe nucleus (DRN). The 5-HT receptors can be classified into at least seven classes, designated 5-HT(1-7). The 5-HT(1A) and 5-HT(1B) receptor subtypes are linked to the inhibition of adenylate cyclase, and their activation evokes a membrane hyperpolarization. The actions of the 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptor subtypes are mediated by the activation of phospholipase C, with a resulting depolarization of the host cell. The 5-HT(3) receptor directly activates a 5-HT-gated cation channel which leads to the depolarization of monoaminergic, aminoacidergic and cholinergic cells. The primary signal transduction pathway of 5-HT(6) and 5-HT(7) receptors is the stimulation of adenylate cyclase which results in the depolarization of the follower neurons. Mutant mice that do not express 5-HT(1A) or 5-HT(1B) receptor exhibit greater amounts of REMS than their wild-type counterparts, which could be related to the absence of a postsynaptic inhibitory effect on REM-on neurons of the laterodorsal and pedunculopontine tegmental nuclei (LDT/PPT). 5-HT(2A) and 5-HT(2C) receptor knock-out mice show a significant increase of W and a reduction of slow wave sleep (SWS) which has been ascribed to the increase of catecholaminergic neurotransmission involving mainly the noradrenergic and dopaminergic systems. Sleep variables have been characterized, in addition, in 5-HT(7) receptor knock-out mice; the mutants spend less time in REMS that their wild-type counterparts. Direct infusion of the 5-HT(1A) receptor agonists 8-OH-DPAT and flesinoxan into the DRN significantly enhances REMS in the rat. In contrast, microinjection of the 5-HT(1B) (CP-94253), 5-HT(2A/2C) (DOI), 5-HT(3) (m-chlorophenylbiguanide) and 5-HT(7) (LP-44) receptor agonists into the DRN induces a significant reduction of REMS. Systemic injection of full agonists at postsynaptic 5-HT(1A) (8-OH-DPAT, flesinoxan), 5-HT(1B) (CGS 12066B, CP-94235), 5-HT(2C) (RO 60-0175), 5-HT(2A/2C) (DOI, DOM), 5-HT(3) (m-chlorophenylbiguanide) and 5-HT(7) (LP-211) receptors increases W and reduces SWS and REMS. Of note, systemic administration of the 5-HT(2A/2C) receptor antagonists ritanserin, ketanserin, ICI-170,809 or sertindole at the beginning of the light period has been shown to induce a significant increase of SWS and a reduction of REMS in the rat. Wakefulness was also diminished in most of these studies. Similar effects have been described following the injection of the selective 5-HT(2A) receptor antagonists volinanserin and pruvanserin and of the 5-HT(2A) receptor inverse agonist nelotanserin in rodents. In addition, the effects of these compounds have been studied on the sleep electroencephalogram of subjects with normal sleep. Their administration was followed by an increase of SWS and, in most instances, a reduction of REMS. The administration of ritanserin to poor sleepers, patients with chronic primary insomnia and psychiatric patients with a generalized anxiety disorder or a mood disorder caused a significant increase in SWS. The 5-HT(2A) receptor inverse agonist APD-125 induced also an increase of SWS in patients with chronic primary insomnia. It is known that during the administration of benzodiazepine (BZD) hypnotics to patients with insomnia there is a further reduction of SWS and REMS, whereas both variables tend to remain decreased during the use of non-BZD derivatives (zolpidem, zopiclone, eszopiclone, zaleplon). Thus, the association of 5-HT(2A) antagonists or 5-HT(2A) inverse agonists with BZD and non-BZD hypnotics could be a valid alternative to normalize SWS in patients with primary or comorbid insomnia.

A transcriptomic analysis of type I-III neurons in the bed nucleus of the stria terminalis

The activity of neurons in the anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) plays a critical role in anxiety- and stress-related behaviors. Histochemical studies have suggested that multiple distinct neuronal phenotypes exist in the BNST(ALG). Consistent with this observation, the physiological properties of BNST(ALG) neurons are also heterogeneous, and three distinct cell types can be defined (Types I-III) based primarily on their expression of four key membrane currents, namely I(h), I(A), I(T), and I(K(IR)). Significantly, all four channels are multimeric proteins and can comprise of more than one pore-forming α subunit. Hence, differential expression of α subunits may further diversify the neuronal population. However, nothing is known about the relative expression of these ion channel α subunits in BNST(ALG) neurons. We have addressed this lacuna by combining whole-cell patch-clamp recording together with single-cell reverse transcriptase polymerase chain reaction (scRT-PCR) to assess the mRNA transcript expression for each of the subunits for the four key ion channels in Type I-III neurons of the BNST(ALG.) Here, cytosolic mRNA from single neurons was probed for the expression of transcripts for each of the α subunits of I(h) (HCN1-HCN4), I(T) (Ca(v)3.1-Ca(v)3.3), I(A) (K(v)1.4, K(v)3.4, K(v)4.1-K(v) 4.3) and I(K(IR)) (Kir2.1-Kir2.4). An unbiased hierarchical cluster analysis followed by discriminant function analysis revealed that a positive correlation exists between the physiological and genetic phenotype of BNST(ALG) neurons. Thus, the analysis segregated BNST(ALG) neurons into 3 distinct groups, based on their α subunit mRNA expression profile, which positively correlated with our existing electrophysiological classification (Types I-III). Furthermore, analysis of mRNA transcript expression in Type I-Type III neurons suggested that, whereas Type I and III neurons appear to represent genetically homologous cell populations, Type II neurons may be further subdivided into three genetically distinct subgroups. These data not only validate our original classification scheme, but further refine the classification at the molecular level, and thus identifies novel targets for potential disruption and/or pharmacotherapeutic intervention in stress-related anxiety-like behaviors.

Double-dissociation of the catecholaminergic modulation of synaptic transmission in the oval bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BST) is a cluster of nuclei within the extended amygdala, a forebrain macrostructure with extensive projection to motor nuclei of the hindbrain. The subnuclei of the BST coordinate autonomic, neuroendocrine, and somato-motor functions and receive robust neuromodulatory monoaminergic afferents, including 5-HT-, noradrenaline (NA)-, and dopamine (DA)-containing terminals. In contrast to 5-HT and NA, little is known about how DA modulates neuronal activity or synaptic transmission in the BST. DA-containing afferents to the BST originate in the ventral tegmental area, the periaqueducal gray, and the retrorubral field. They form a fairly diffuse input to the dorsolateral BST with dense terminal fields in the oval (ovBST) and juxtacapsular (jxBST) nuclei. The efferent-afferent connectivity of the BST suggests that it may play a key role in motivated behaviors, consistent with recent evidence that the dorsolateral BST is a target for drugs of abuse. This study describes the effects of DA on synaptic transmission in the ovBST. Whole cell voltage clamp recordings were performed on ovBST neurons in brain slices from adult rats in the presence or absence of exogenous DA and receptor-targeted agonists and antagonists. The results showed that DA selectively and exclusively reduced inhibitory synaptic transmission in the ovBST in a dose-dependent and D2-like dopamine receptor-dependent manner. DA also modulated excitatory synaptic transmission in a dose-dependent dependent manner. However, this effect was mediated by α2-noradrenergic receptors. Thus these data reveal a double dissociation in catecholaminergic regulation of excitatory and inhibitory synaptic transmission in the ovBST and may shed light on the mechanisms involved in neuropathological behaviors such as stress-induced relapse to consumption of drugs of abuse.

Inhibition by 5-HT of the synaptic responses evoked by callosal fibers on cortical neurons in the mouse

We have studied the modulation by 5-HT of the synaptic excitatory responses evoked by callosal fibers on cortical pyramidal neurons. We have used a mouse brain slice preparation that preserves the callosal fibers and allows their selective activation. EPSCs evoked by callosal stimulation (ccEPSCs) were recorded with patch electrodes from pyramidal neurons identified visually. We observed that 5-HT (10-40 μM) inhibited the ccEPSCs peak amplitude in 64% of the neurons; 5-HT had no effect in the remaining neurons. 5-HT also increased the frequency and amplitude of spontaneous EPSCs. This inhibition was accompanied with an increase in the coefficient of variation of the fluctuations of the ccEPSCs amplitude and with an increase in the ratio of the amplitudes of paired ccEPSCs. Agonists of 5-HT receptor subtypes 5-HT(1A) (8-OH-DPAT) and 5-HT(2A) (DOI) mimicked the effect of 5-HT; also, the effect of 8-OH-DPAT and DOI was blocked in the presence of specific blockers of 5-HT(1A) (WAY 100135) and 5-HT(2A) (MDL 11,939) receptors. Application of 5-HT did not change the amplitude of currents evoked by direct application of glutamate to neurons in which 5-HT decreased the amplitude of ccEPSC. The effects of 5-HT on ccEPSCs and on the synaptic currents evoked by intracortical stimulation were not correlated; this suggests that the effect of 5-HT was specific to particular synaptic inputs to a neuron. These results demonstrate the presynaptic modulation of the callosal synaptic responses by 5-HT and the implication of 5-HT(1A) and 5-HT(2A) receptors in this effect.

Serotoninergic control of glycinergic inhibitory postsynaptic currents in rat hypoglossal motoneurons

This report presents the results of a study of the frequency potentiation of inhibitory postsynaptic currents (IPSCs) in hypoglossal motoneurons and its modulation by serotonin. A release-site model of synaptic plasticity was used to characterize the frequency-related potentiation of evoked IPSCs. Data were obtained to determine if the frequency potentiation of IPSCs occurs as a consequence of a low baseline quantal content of evoked IPSCs using whole cell patch-clamp recordings from hypoglossal motoneurons in the neonatal rat brainstem slice preparation. In these motoneurons, EPSCs and GABAergic IPSCs were blocked by the application of CNQX, AP-5 and bicuculline. Glycinergic IPSCs were evoked by threshold stimulation of inhibitory neurons in the nucleus of Roller, which is located ventro-lateral to the hypoglossal nucleus. IPSC responses to trains of stimuli were recorded in control solutions and in solutions containing serotonin, which is known to reduce IPSPs in this preparation. The amplitude of non-potentiated IPSCs was reduced and their frequency potentiation was enhanced when serotonin was added to the bath. These data were examined using a release-site model of synaptic plasticity in which facilitation is attributed to a time-dependent increase in the probability of transmitter release; depression is attributed to a time-dependent decrease in the number of sites available for release. Using this model, the effect of serotonin on frequency potentiation was explained by a combination of a reduction in the baseline probability of transmitter release and an increase in the time constant of decay of the increase in probability of release that follows a stimulus.