Identification of Serotonergic Neuronal Modules that Affect Aggressive Behavior

Escalated aggression can have devastating societal consequences, yet underlying neurobiological mechanisms are poorly understood. Here, we show significantly increased inter-male mouse aggression when neurotransmission is constitutively blocked from either of two subsets of serotonergic, Pet1+ neurons: one identified by dopamine receptor D1(Drd1a)::cre-driven activity perinatally, and the other by Drd2::cre from pre-adolescence onward. Blocking neurotransmission from other Pet1+ neuron subsets of similar size and/or overlapping anatomical domains had no effect on aggression compared with controls, suggesting subtype-specific serotonergic neuron influences on aggression. Using established and novel intersectional genetic tools, we further characterized these subtypes across multiple parameters, showing both overlapping and distinct features in axonal projection targets, gene expression, electrophysiological properties, and effects on non-aggressive behaviors. Notably, Drd2::cre marked 5-HT neurons exhibited D2-dependent inhibitory responses to dopamine in slices, suggesting direct and specific interplay between inhibitory dopaminergic signaling and a serotonergic subpopulation. Thus, we identify specific serotonergic modules that shape aggression.

Sex differences in corticotropin-releasing factor receptor-1 action within the dorsal raphe nucleus in stress responsivity

BACKGROUND

Women are twice as likely as men to suffer from stress-related affective disorders. Corticotropin-releasing factor (CRF) is an important link between stress and mood, in part through its signaling in the serotonergic dorsal raphe (DR). Development of CRF receptor-1 (CRFr1) antagonists has been a focus of numerous clinical trials but has not yet been proven efficacious. We hypothesized that sex differences in CRFr1 modulation of DR circuits might be key determinants in predicting therapeutic responses and affective disorder vulnerability.

METHODS

Male and female mice received DR infusions of the CRFr1 antagonist, NBI 35965, or CRF and were evaluated for stress responsivity. Sex differences in indices of neural activation (cFos) and colocalization of CRFr1 throughout the DR were examined. Whole-cell patch-clamp electrophysiology assessed sex differences in serotonin neuron membrane characteristics and responsivity to CRF.

RESULTS

Males showed robust behavioral and hypothalamic-pituitary-adrenal axis responses to DR infusion of NBI 35965 and CRF, whereas females were minimally responsive. Sex differences were also found for both CRF-induced DR cFos and CRFr1 co-localization throughout the DR. Electrophysiologically, female serotonergic neurons showed blunted membrane excitability and divergent inhibitory postsynaptic current responses to CRF application.

CONCLUSIONS

These studies demonstrate convincing sex differences in CRFr1 activity in the DR, where blunted female responses to NBI 35965 and CRF suggest unique stress modulation of the DR. These sex differences might underlie affective disorder vulnerability and differential sensitivity to pharmacologic treatments developed to target the CRF system, thereby contributing to a current lack of CRFr1 antagonist efficacy in clinical trials.

Optogenetic modulation of descending prefrontocortical inputs to the dorsal raphe bidirectionally bias socioaffective choices after social defeat

It has been well established that modulating serotonin (5-HT) levels in humans and animals affects perception and response to social threats, however the circuit mechanisms that control 5-HT output during social interaction are not well understood. A better understanding of these systems could provide groundwork for more precise and efficient therapeutic interventions. Here we examined the organization and plasticity of microcircuits implicated in top-down control of 5-HT neurons in the dorsal raphe nucleus (DRN) by excitatory inputs from the ventromedial prefrontal cortex (vmPFC) and their role in social approach-avoidance decisions. We did this in the context of a social defeat model that induces a long lasting form of social aversion that is reversible by antidepressants. We first used viral tracing and Cre-dependent genetic identification of vmPFC glutamatergic synapses in the DRN to determine their topographic distribution in relation to 5-HT and GABAergic subregions and found that excitatory vmPFC projections primarily localized to GABA-rich areas of the DRN. We then used optogenetics in combination with cFos mapping and slice electrophysiology to establish the functional effects of repeatedly driving vmPFC inputs in DRN. We provide the first direct evidence that vmPFC axons drive synaptic activity and immediate early gene expression in genetically identified DRN GABA neurons through an AMPA receptor-dependent mechanism. In contrast, we did not detect vmPFC-driven synaptic activity in 5-HT neurons and cFos induction in 5-HT neurons was limited. Finally we show that optogenetically increasing or decreasing excitatory vmPFC input to the DRN during sensory exposure to an aggressor's cues enhances or diminishes avoidance bias, respectively. These results clarify the functional organization of vmPFC-DRN pathways and identify GABAergic neurons as a key cellular element filtering top-down vmPFC influences on affect-regulating 5-HT output.

Effects of chronic sleep fragmentation on wake-active neurons and the hypercapnic arousal response

STUDY OBJECTIVES

Delayed hypercapnic arousals may occur in obstructive sleep apnea. The impaired arousal response is expected to promote more pronounced oxyhemoglobin desaturations. We hypothesized that long-term sleep fragmentation (SF) results in injury to or dysfunction of wake-active neurons that manifests, in part, as a delayed hypercapnic arousal response.

DESIGN

Adult male mice were implanted for behavioral state recordings and randomly assigned to 4 weeks of either orbital platform SF (SF4wk, 30 events/h) or control conditions (Ct4wk) prior to behavioral, histological, and locus coeruleus (LC) whole cell electrophysiological evaluations.

MEASUREMENTS AND RESULTS

SF was successfully achieved across the 4 week study, as evidenced by a persistently increased arousal index, P < 0.01 and shortened sleep bouts, P < 0.05, while total sleep/wake times and plasma corticosterone levels were unaffected. A multiple sleep latency test performed at the onset of the dark period showed a reduced latency to sleep in SF4wk mice (P < 0.05). The hypercapnic arousal latency was increased, Ct4wk 64 ± 5 sec vs. SF4wk 154 ± 6 sec, P < 0.001, and remained elevated after a 2 week recovery (101 ± 4 sec, P < 0.001). C-fos activation in noradrenergic, orexinergic, histaminergic, and cholinergic wake-active neurons was reduced in response to hypercapnia (P < 0.05-0.001). Catecholaminergic and orexinergic projections into the cingulate cortex were also reduced in SF4wk (P < 0.01). In addition, SF4wk resulted in impaired LC neuron excitability (P < 0.01).

CONCLUSIONS

Four weeks of sleep fragmentation (SF4wk) impairs arousal responses to hypercapnia, reduces wake neuron projections and locus coeruleus neuronal excitability, supporting the concepts that some effects of sleep fragmentation may contribute to impaired arousal responses in sleep apnea, which may not reverse immediately with therapy.

Developmental effects of serotonin 1A autoreceptors on anxiety and social behavior

The serotonin 1A receptor (5-HT1A) has a major role in modulating the effects of serotonin on mood and behavior. Previous studies have shown that knockout of 5-HT1A selectively in the raphe leads to higher levels of anxiety during adulthood. However, it remains unclear whether this phenotype is due to variation in receptor levels specifically during development or throughout life. To test the hypothesis that developmental sensitivity may underlie the effects of 5-HT1A on anxiety, we used an inducible transgenic system to selectively suppress 5-HT1A levels in serotonergic raphe neurons from post-natal days (P) 14 to P30, with a maximal reduction of 40% at P21 and return to regular levels by P30. This developmental decrease in receptor levels has long-lasting consequences, increasing anxiety and decreasing social investigation in adulthood. In addition, post-natal knockdown of autoreceptors leads to long-term increases in the excitability of serotonergic neurons, which may represent a mechanism underlying the effects of post-natal receptor variation on behavior later in life. Finally, we also examined the interplay between receptor variation and juvenile exposure to stress (applied from P14 to P21). Similar to receptor knockdown, juvenile exposure to stress led to increased anxiety phenotypes but did not exacerbate 5-HT1A knockdown-mediated anxiety levels. This work indicates that the effects of 5-HT1A autoreceptors on anxiety and social behaviors are developmentally mediated and suggests that natural variations in the expression of 5-HT1A may act during development to influence individual anxiety levels and contribute to susceptibility to anxiety disorders.

Vasopressin indirectly excites dorsal raphe serotonin neurons through activation of the vasopressin1A receptor

The neuropeptide vasopressin (AVP; arginine-vasopressin) is produced in a handful of brain nuclei located in the hypothalamus and extended amygdala and is released both peripherally as a hormone and within the central nervous system as a neurotransmitter. Central projections have been associated with a number of functions including regulation of physiological homeostasis, control of circadian rhythms, and modulation of social behavior. The AVP neurons located in the bed nucleus of the stria terminalis and medial amygdala (i.e., extended amygdala) in particular have been associated with affiliative social behavior in multiple species. It was recently demonstrated that in the mouse AVP projections emanating from extended amygdala neurons innervate a number of forebrain and midbrain brain regions including the dorsal raphe nucleus (DR), the site of origin of most forebrain-projecting serotonin neurons. Based on the presence of AVP fibers in the DR, we hypothesized that AVP would alter the physiology of serotonin neurons via AVP 1A receptor (V1AR) activation. Using whole-cell electrophysiology techniques, we found that AVP increased the frequency and amplitude of excitatory post-synaptic currents (EPSCs) in serotonin neurons of male mice. The indirect stimulation of serotonin neurons was AMPA/kainate receptor dependent and blocked by the sodium channel blocker tetrodotoxin, suggesting an effect of AVP on glutamate neurons. Further, the increase in EPSC frequency induced by AVP was blocked by selective V1AR antagonists. Our data suggest that AVP had an excitatory influence on serotonin neurons. This work highlights a new target (i.e., V1AR) for manipulating serotonin neuron excitability. In light of our data, we propose that some of the diverse effects of AVP on physiology and behavior, including social behavior, may be due to activation of the DR serotonin system.

Serotonergic neuron regulation informed by in vivo single-cell transcriptomics

Despite the recognized importance of the dorsal raphe (DR) serotonergic (5-HT) nuclei in the pathophysiology of depression and anxiety, the molecular components/putative drug targets expressed by these neurons are poorly characterized. Utilizing the promoter of an ETS domain transcription factor that is a stable marker of 5-HT neurons (Pet-1) to drive 5-HT neuronal expression of YFP, we identified 5-HT neurons in live acute slices. We isolated RNA from single 5-HT neurons in the ventromedial and lateral wings of the DR and performed single-cell RNA-Seq analysis identifying >500 G-protein coupled receptors (GPCRs) including receptors for classical transmitters, lipid signals, and peptides as well as dozens of orphan-GPCRs. Using these data to inform our selection of receptors to assess, we found that oxytocin and lysophosphatidic acid 1 receptors are translated and active in costimulating, with the α1-adrenergic receptor, the firing of DR 5-HT neurons, while the effects of histamine are inhibitory and exerted at H3 histamine receptors. The inhibitory histamine response provides evidence for tonic in vivo histamine inhibition of 5-HT neurons. This study illustrates that unbiased single-cell transcriptomics coupled with functional analyses provides novel insights into how neurons and neuronal systems are regulated.

Social stress alters inhibitory synaptic input to distinct subpopulations of raphe serotonin neurons

Anxiety disorders are among the most prevalent psychiatric disorders, yet much is unknown about the underlying mechanisms. The dorsal raphe (DR) is at the crux of the anxiety-inducing effects of uncontrollable stress, a key component of models of anxiety. Though DR serotonin (5-HT) neurons play a prominent role, anxiety-associated changes in the physiology of 5-HT neurons remain poorly understood. A 5-day social defeat model of anxiety produced a multifaceted, anxious phenotype in intruder mice that included increased avoidance behavior in the open field test, increased stress-evoked grooming, and increased bladder and heart weights when compared to control mice. Intruders were further compared to controls using electrophysiology recordings conducted in midbrain slices wherein recordings targeted 5-HT neurons of the ventromedial (vmDR) and lateral wing (lwDR) subfields of the DR. Though defining membrane characteristics of 5-HT neurons were unchanged, γ-aminobutyric-acid-mediated (GABAergic) synaptic regulation of 5-HT neurons was altered in a topographically specific way. In the vmDR of intruders, there was a decrease in the frequency and amplitude of GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs). However, in the lwDR, there was an increase in the strength of inhibitory signals due to slower sIPSC kinetics. Synaptic changes were selective for GABAergic input, as glutamatergic synaptic input was unchanged in intruders. The distinct inhibitory regulation of DR subfields provides a mechanism for increased 5-HT output in vmDR target regions and decreased 5-HT output in lwDR target regions, divergent responses to uncontrollable stress that have been reported in the literature but were previously poorly understood.

Direct activation of sleep-promoting VLPO neurons by volatile anesthetics contributes to anesthetic hypnosis

BACKGROUND

Despite seventeen decades of continuous clinical use, the neuronal mechanisms through which volatile anesthetics act to produce unconsciousness remain obscure. One emerging possibility is that anesthetics exert their hypnotic effects by hijacking endogenous arousal circuits. A key sleep-promoting component of this circuitry is the ventrolateral preoptic nucleus (VLPO), a hypothalamic region containing both state-independent neurons and neurons that preferentially fire during natural sleep.

RESULTS

Using c-Fos immunohistochemistry as a biomarker for antecedent neuronal activity, we show that isoflurane and halothane increase the number of active neurons in the VLPO, but only when mice are sedated or unconscious. Destroying VLPO neurons produces an acute resistance to isoflurane-induced hypnosis. Electrophysiological studies prove that the neurons depolarized by isoflurane belong to the subpopulation of VLPO neurons responsible for promoting natural sleep, whereas neighboring non-sleep-active VLPO neurons are unaffected by isoflurane. Finally, we show that this anesthetic-induced depolarization is not solely due to a presynaptic inhibition of wake-active neurons as previously hypothesized but rather is due to a direct postsynaptic effect on VLPO neurons themselves arising from the closing of a background potassium conductance.

CONCLUSIONS

Cumulatively, this work demonstrates that anesthetics are capable of directly activating endogenous sleep-promoting networks and that such actions contribute to their hypnotic properties.

Glutamatergic input is selectively increased in dorsal raphe subfield 5-HT neurons: role of morphology, topography and selective innervation

Characterization of glutamatergic input to dorsal raphe (DR) serotonin (5-HT) neurons is crucial for understanding how the glutamate and 5-HT systems interact in psychiatric disorders. Markers of glutamatergic terminals, vGlut1, 2 and 3, reflect inputs from specific forebrain and midbrain regions. Punctate staining of vGlut2 was homogeneous throughout the mouse DR whereas vGlut1 and vGlut3 puncta were less dense in the lateral wing (lwDR) compared with the ventromedial (vmDR) subregion. The distribution of glutamate terminals was consistent with the lower miniature excitatory postsynaptic current frequency found in the lwDR; however, it was not predictive of glutamatergic synaptic input with local activity intact, as spontaneous excitatory postsynaptic current (sEPSC) frequency was higher in the lwDR. We examined the morphology of recorded cells to determine if variations in dendrite structure contributed to differences in synaptic input. Although lwDR neurons had longer, more complex dendrites than vmDR neurons, glutamatergic input was not correlated with dendrite length in the lwDR, suggesting that dendrite length did not contribute to subregional differences in sEPSC frequency. Overall, glutamatergic input in the DR was the result of selective innervation of subpopulations of 5-HT neurons and was rooted in the topography of DR neurons and the activity of glutamate neurons located within the midbrain slice. Increased glutamatergic input to lwDR cells potentially synergizes with previously reported increased intrinsic excitability of lwDR cells to increase 5-HT output in lwDR target regions. Because the vmDR and lwDR are involved in unique circuits, subregional differences in glutamate modulation may result in diverse effects on 5-HT output in stress-related psychopathology.

The presynaptic component of the serotonergic system is required for clozapine's efficacy

Clozapine, by virtue of its absence of extrapyramidal side effects and greater efficacy, revolutionized the treatment of schizophrenia, although the mechanisms underlying this exceptional activity remain controversial. Combining an unbiased cheminformatics and physical screening approach, we evaluated clozapine's activity at >2350 distinct molecular targets. Clozapine, and the closely related atypical antipsychotic drug olanzapine, interacted potently with a unique spectrum of molecular targets. This distinct pattern, which was not shared with the typical antipsychotic drug haloperidol, suggested that the serotonergic neuronal system was a key determinant of clozapine's actions. To test this hypothesis, we used pet1(-/-) mice, which are deficient in serotonergic presynaptic markers. We discovered that the antipsychotic-like properties of the atypical antipsychotic drugs clozapine and olanzapine were abolished in a pharmacological model that mimics NMDA-receptor hypofunction in pet1(-/-) mice, whereas haloperidol's efficacy was unaffected. These results show that clozapine's ability to normalize NMDA-receptor hypofunction, which is characteristic of schizophrenia, depends on an intact presynaptic serotonergic neuronal system.

Cellular correlates of anxiety in CA1 hippocampal pyramidal cells of 5-HT1A receptor knockout mice

RATIONALE

5-HT(1A) receptor knockout (1AKO) mice have a robust anxiety phenotype. Tissue-specific "rescue" strategies and electrophysiology have implicated a critical role for postsynaptic 5-HT(1A) receptors, particularly in the CA1 region of the hippocampus.

OBJECTIVES

In this study, we evaluated differences in membrane properties and synaptic activity in CA1 hippocampal pyramidal cells between 1AKOs and wild-type (WT) controls to better understand the cellular correlates of anxiety in this mouse model.

METHODS

Whole-cell patch-clamp recordings were conducted in CA1 pyramidal cells in hippocampal brain slices from 1AKOs and WTs that had previously been screened for anxiety with the elevated-plus maze. Spontaneous miniature inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs) and stimulus-evoked eIPSCs and eEPSCs were recorded in addition to the effect of the benzodiazepine agonist diazepam or the inverse agonist FG 7142 on γ-aminobutyric acid (GABA)ergic eIPSCs.

RESULTS

Evoked EPSC amplitude was greater in 1AKOs than WTs. When subjects were pooled across genotypes, anxiety measures correlated with eEPSC amplitude, indicating enhanced postsynaptic glutamate synaptic activity under conditions of synaptic activation in anxious subjects. While GABA synaptic activity and sensitivity to diazepam were not affected by genotype or correlated with anxiety, sensitivity to the anxiogenic FG 7142 was smaller in anxious subjects.

CONCLUSIONS

These data indicate enhanced postsynaptic glutamate receptor sensitivity and decreased GABAergic inhibition by a benzodiazepine inverse agonist in CA1 hippocampal neurons of anxious mice are produced by deletion of the 5-HT(1A) receptor. These data provide new information about interactions between 5-HT, GABA, and glutamate systems during the expression of chronic anxiety.

Swim stress differentially blocks CRF receptor mediated responses in dorsal raphe nucleus

Modulation of the serotonergic (5-HT) neurotransmitter system arising from the dorsal raphe nucleus (DR) is thought to support the behavioral effects of swim stress, i.e., immobility. In vivo pharmacological and anatomical studies suggest that corticotropin-releasing factor (CRF) and γ-aminobutyric acid (GABA) synaptic transmission closely interact to set the response of the DR to swim stress. To investigate the cellular basis of these physiological mechanisms the effects of ovine CRF (oCRF) on GABA(A)-dependent miniature inhibitory postsynaptic currents (mIPSCs) in 5-HT and non-5-HT DR neurons in acute mesencephalic slices obtained from rats either naïve or 24h after a 15 min swim stress session were tested. In this study, the effect of swim stress alone was to decrease the holding current, i.e., hyperpolarize the neuron, and to increase the amplitude and charge of mIPSCs recorded from non-5-HT neurons. Ovine CRF (10 nM) induced an increase in mIPSC frequency in 5-HT neurons recorded from naïve rats, an effect that was suppressed by swim stress. The inward current elicited by oCRF in both 5-HT and non-5-HT neurons was also blocked by swim stress. Ovine CRF increased mIPSCs amplitude and charge in both 5-HT and non-5-HT neurons, but this effect was not modified by swim stress. In concert with our previous findings that swim stress decreased input resistance, action potential threshold and action potential duration and increased glutamatergic synaptic activity the overall primary effect of swim stress is to increase the excitability of 5-HT neurons. These data provide a mechanism at the cellular level for the immobility induced by swim stress and identifies critical components of the raphe circuitry responsible for the altered output of 5-HT neurons induced by swim stress.

Increased intrinsic excitability of lateral wing serotonin neurons of the dorsal raphe: a mechanism for selective activation in stress circuits

The primary center of serotonin (5-HT) projections to the forebrain is the dorsal raphe nucleus (DR), a region known for its role in the limbic stress response. The ventromedial subregion of the DR (vmDR) has the highest density of 5-HT neurons and is the major target in experiments that involve the DR. However, studies have demonstrated that a variety of stressors induce activation of neurons that is highest in the lateral wing subregion (lwDR) and includes activation of lwDR 5-HT neurons. Despite the functional role that the lwDR is known to play in stress circuits, little is known about lwDR 5-HT neuron physiology. Whole cell patch clamp electrophysiology in mice revealed that lwDR 5-HT cells have active and passive intrinsic membrane properties that make them more excitable than vmDR 5-HT neurons. In addition, lwDR 5-HT neurons demonstrated faster in vitro firing rates. Finally, within the vmDR there was a positive correlation between rostral position and increased excitability, among several other membrane parameters. These results are consistent with stressor induced patterns of activation of 5-HT neurons that includes, in addition to lwDR neurons, a small subset of rostral vmDR neurons. Thus increased intrinsic excitability likely forms a major part of the mechanism underlying the propensity to be activated by a stressor. The membrane properties identified in lwDR recordings may thereby contribute to a unique role of lwDR 5-HT neurons in adaptive responses to stress and in the pathobiology of stress-related mood disorders.

5-HT1A autoreceptor levels determine vulnerability to stress and response to antidepressants

Most depressed patients don't respond to their first drug treatment, and the reasons for this treatment resistance remain enigmatic. Human studies implicate a polymorphism in the promoter of the serotonin-1A (5-HT(1A)) receptor gene in increased susceptibility to depression and decreased treatment response. Here we develop a new strategy to manipulate 5-HT(1A) autoreceptors in raphe nuclei without affecting 5-HT(1A) heteroreceptors, generating mice with higher (1A-High) or lower (1A-Low) autoreceptor levels. We show that this robustly affects raphe firing rates, but has no effect on either basal forebrain serotonin levels or conflict-anxiety measures. However, compared to 1A-Low mice, 1A-High mice show a blunted physiological response to acute stress, increased behavioral despair, and no behavioral response to antidepressant, modeling patients with the 5-HT(1A) risk allele. Furthermore, reducing 5-HT(1A) autoreceptor levels prior to antidepressant treatment is sufficient to convert nonresponders into responders. These results establish a causal relationship between 5-HT(1A) autoreceptor levels, resilience under stress, and response to antidepressants.

Serotonergic transcriptional programming determines maternal behavior and offspring survival

Central serotonergic signaling influences many physiological processes, but a requirement for reproductive success has not been demonstrated. Using mouse dams with a specific disruption in serotonin neuron development, we found that serotonergic function is required for the nurturing and survival of offspring. Full rescue of survival depended on the mother's expression level of the upstream serotonergic transcriptional cascade. Thus, intrinsic transcriptional programming of maternal serotonergic activity determines the quality of nurturing and whether or not the organism survives.

Cellular effects of swim stress in the dorsal raphe nucleus

Swim stress regulates forebrain 5-hydroxytryptamine (5-HT) release in a complex manner and its effects are initiated in the serotonergic dorsal raphe nucleus (DRN). The purpose of this study was to examine the effects of swim stress on the physiology of DRN neurons in conjunction with 5-HT immunohistochemistry. Basic membrane properties, 5-HT(1A) and 5-HT(1B) receptor-mediated responses and glutamatergic excitatory postsynaptic currents (EPSCs) were measured using whole-cell patch clamp techniques. Rats were forced to swim for 15min and 24h later DRN brain slices were prepared for electrophysiology. Swim stress altered the resting membrane potential, input resistance and action potential duration of DRN neurons in a neurochemical-specific manner. Swim stress selectively elevated glutamate EPSC frequency in 5-HT DRN neurons. Swim stress non-selectively reduced EPSC amplitude in all DRN cells. Swim stress elevated the 5-HT(1B) receptor-mediated inhibition of glutamatergic synaptic activity that selectively targeted 5-HT cells. Non-5-HT DRN neurons appeared to be particularly responsive to the effects of a milder handling stress. Handling elevated EPSC frequency, reduced EPSC decay time and enhanced a 5-HT(1B) receptor-mediated inhibition of mEPSC frequency selectively in non-5-HT DRN cells. These results indicate that swim stress has both direct, i.e., changes in membrane characteristics, and indirect effects, i.e., via glutamatergic afferents, on DRN neurons. These results also indicate that there are distinct local glutamatergic afferents to neurochemically specific populations of DRN neurons, and furthermore that these distinct afferents are differentially regulated by swim stress. These cellular changes may contribute to the complex effects of swim stress on 5-HT neurotransmission and/or the behavioral changes underlying the forced swimming test model of depression.

Suppression of conditioning to ambiguous cues by pharmacogenetic inhibition of the dentate gyrus

Serotonin receptor 1A knockout (Htr1a(KO)) mice show increased anxiety-related behavior in tests measuring innate avoidance. Here we demonstrate that Htr1a(KO) mice show enhanced fear conditioning to ambiguous conditioned stimuli, a hallmark of human anxiety. To examine the involvement of specific forebrain circuits in this phenotype, we developed a pharmacogenetic technique for the rapid tissue- and cell type-specific silencing of neural activity in vivo. Inhibition of neurons in the central nucleus of the amygdala suppressed conditioned responses to both ambiguous and nonambiguous cues. In contrast, inhibition of hippocampal dentate gyrus granule cells selectively suppressed conditioned responses to ambiguous cues and reversed the knockout phenotype. These data demonstrate that Htr1a(KO) mice have a bias in the processing of threatening cues that is moderated by hippocampal mossy-fiber circuits, and suggest that the hippocampus is important in the response to ambiguous aversive stimuli.

Selective 5-HT receptor inhibition of glutamatergic and GABAergic synaptic activity in the rat dorsal and median raphe

The dorsal (DR) and median (MR) raphe nuclei contain 5-hydroxytryptamine (5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain. The DR and MR have differential roles in mediating stress, anxiety and depression. Glutamate and GABA activity sculpt putative 5-HT neuronal firing and 5-HT release in a seemingly differential manner in the MR and DR, yet isolated glutamate and GABA activity within the DR and MR has not been systematically characterized. Visualized whole-cell voltage-clamp techniques were used to record excitatory and inhibitory postsynaptic currents (EPSC and IPSC) in 5-HT-containing neurons. There was a regional variation in action potential-dependent (spontaneous) and basal [miniature (m)] glutamate and GABAergic activity. mEPSC activity was greater than mIPSC activity in the DR, whereas in the MR the mIPSC activity was greater. These differences in EPSC and IPSC frequency indicate that glutamatergic and GABAergic input have distinct cytoarchitectures in the DR and MR. 5-HT(1B) receptor activation decreased mEPSC frequency in the DR and the MR, but selectively inhibited mIPSC activity only in the MR. This finding, in concert with its previously described function as an autoreceptor, suggests that 5-HT(1B) receptors influence the ascending 5-HT system through multiple mechanisms. The disparity in organization and integration of glutamatergic and GABAergic input to DR and MR neurons and their regulation by 5-HT(1B) receptors may contribute to the distinction in MR and DR regulation of forebrain regions and their differential function in the aetiology and pharmacological treatment of psychiatric disease states.

Identifying genes in monoamine nuclei that may determine stress vulnerability and depressive behavior in Wistar-Kyoto rats

The Wistar-Kyoto (WKY) rat is stress sensitive and exhibits depressive-like behavior. The locus coeruleus (LC)-norepinephrine and dorsal raphe (DR)-serotonin systems mediate certain aspects of the stress response and have been implicated in depression. Microarray technology was used to identify gene expression differences in the LC and DR between WKY vs Sprague-Dawley (SD) rats that might account for the WKY phenotype. RNA was isolated from microdissected LC and DR, amplified, and hybridized to microarrays (1 array/subject, n = 4/group). Significance of microarray (SAM) analysis revealed increased expression of 66 genes in the LC and 19 genes in the DR and decreased expression of 33 genes in the DR of WKY rats. Hierarchical clustering identified differences in gene expression profiles of WKY vs SD rats that generally concurred with SAM. Notably, genes that encoded for enzymes involved in norepinephrine turnover, amino-acid receptors, and certain G-protein-coupled receptors were elevated in the LC of WKY rats. The DR of WKY rats showed decreased expression of genes encoding several potassium channels and neurofilament genes. The chromosomal locations of 15 genes that were differentially expressed in WKY rats were near loci identified as contributing to depressive-like behaviors in the rat. The specific genes revealed by the present analysis as being differentially expressed in WKY rats may contribute to their unique behavioral profile and suggest targets that confer susceptibility to stress-related psychiatric disorders.

Two populations of glutamatergic axons in the rat dorsal raphe nucleus defined by the vesicular glutamate transporters 1 and 2

Most glutamatergic neurons in the brain express one of two vesicular glutamate transporters, vGlut1 or vGlut2. Cortical glutamatergic neurons highly express vGlut1, whereas vGlut2 predominates in subcortical areas. In this study immunohistochemical detection of vGlut1 or vGlut2 was used in combination with tryptophan hydroxylase (TPH) to characterize glutamatergic innervation of the dorsal raphe nucleus (DRN) of the rat. Immunofluorescence labeling of both vGlut1 and vGlut2 was punctate and homogenously distributed throughout the DRN. Puncta labeled for vGlut2 appeared more numerous then those labeled for vGlut1. Ultrastructural analysis revealed axon terminals containing vGlut1 and vGlut2 formed asymmetric-type synapses 80% and 95% of the time, respectively. Postsynaptic targets of vGlut1- and vGlut2-containing axons differed in morphology. vGlut1-labeled axon terminals synapsed predominantly on small-caliber (distal) dendrites (42%, 46/110) or dendritic spines (46%, 50/110). In contrast, vGlut2-containing axons synapsed on larger caliber (proximal) dendritic shafts (> 0.5 microm diameter; 48%, 78/161). A fraction of both vGlut1- or vGlut2-labeled axons synapsed onto TPH-containing dendrites (14% and 34%, respectively). These observations reveal that different populations of glutamate-containing axons innervate selective dendritic domains of serotonergic and non-serotonergic neurons, suggesting they play different functional roles in modulating excitation within the DRN.

Median and dorsal raphe neurons are not electrophysiologically identical

The dorsal (DR) and median raphe (MR) nuclei contain 5-hydroxytryptamine (serotonin, 5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain limbic areas that control emotional behavior. In the past, the electrophysiological identification of neurochemically identified 5-HT neurons has been limited. Recent technical developments have made it possible to re-examine the electrophysiological characteristics of identified 5-HT- and non-5-HT-containing neurons. Visualized whole cell electrophysiological techniques in combination with fluorescence immunohistochemistry for 5-HT were used. In the DR, both 5-HT- and non-5-HT-containing neurons exhibited similar characteristics that have historically been attributed to putative 5-HT neurons. In contrast, in the MR, the 5-HT-and non-5-HT-containing neurons had very different characteristics. Interestingly, the MR 5-HT-containing neurons had a shorter time constant and larger afterhyperpolarization (AHP) amplitude than DR 5-HT-containing neurons. The 5-HT(1A) receptor-mediated response was also measured. The efficacy of the response elicited by 5-HT(1A) receptor activation was greater in 5-HT-containing neurons in the DR than the MR, whereas the potency was similar, implicating greater autoinhibition in the DR. Non-5-HT-containing neurons in the DR were responsive to 5-HT(1A) receptor activation, whereas the non-5-HT-containing neurons in the MR were not. These differences in the cellular characteristics and 5-HT(1A) receptor-mediated responses between the MR and DR neurons may be extremely important in understanding the role of these two 5-HT circuits in normal physiological processes and in the etiology and treatment of pathophysiological states.

Distinguishing characteristics of serotonin and non-serotonin-containing cells in the dorsal raphe nucleus: electrophysiological and immunohistochemical studies

The membrane properties and receptor-mediated responses of rat dorsal raphe nucleus neurons were measured using intracellular recording techniques in a slice preparation. After each experiment, the recorded neuron was filled with neurobiotin and immunohistochemically identified as 5-hydroxytryptamine (5-HT)-immunopositive or 5-HT-immunonegative. The cellular characteristics of all recorded neurons conformed to previously determined classic properties of serotonergic dorsal raphe nucleus neurons: slow, rhythmic activity in spontaneously active cells, broad action potential and large afterhyperpolarization potential. Two electrophysiological characteristics were identified that distinguished 5-HT from non-5-HT-containing cells in this study. In 5-HT-immunopositive cells, the initial phase of the afterhyperpolarization potential was gradual (tau=7.3+/-1.9) and in 5-HT-immunonegative cells it was abrupt (tau=1.8+/-0.6). In addition, 5-HT-immunopositive cells had a shorter membrane time constant (tau=21.4+/-4.4) than 5-HT-immunonegative cells (tau=33.5+/-4.2). Interestingly, almost all recorded neurons were hyperpolarized in response to stimulation of the inhibitory 5-HT(1A) receptor. These results suggested that 5-HT(1A) receptors are present on non-5-HT as well as 5-HT neurons. This was confirmed by immunohistochemistry showing that although the majority of 5-HT-immunopositive cells in the dorsal raphe nucleus were double-labeled for 5-HT(1A) receptor-IR, a small but significant population of 5-HT-immunonegative cells expressed the 5-HT(1A) receptor. These results underscore the heterogeneous nature of the dorsal raphe nucleus and highlight two membrane properties that may better distinguish 5-HT from non-5-HT cells than those typically reported in the literature. In addition, these results present electrophysiological and anatomical evidence for the presence of 5-HT(1A) receptors on non-5-HT neurons in the dorsal raphe nucleus.

Anatomical evidence for presynaptic modulation by the delta opioid receptor in the ventrolateral periaqueductal gray of the rat

The delta opioid receptor (DOR) modulates nociception and blood pressure in the periaqueductal gray (PAG). To examine the cellular basis for DOR effects, the ultrastructural distribution of DOR immunoreactivity was examined in the caudal ventrolateral PAG. DOR immunoreactivity was located predominantly in axon terminals that formed asymmetric (excitatory-type) synaptic contacts. However, rather then localized to the plasma membrane of synaptic boutons, immunolabeling for the DOR was intracellular, often associated with large dense-core vesicles. This finding suggests that dense-core vesicles may play a role in targeting the DOR, as vesicle fusion would shift the distribution of the DOR to the plasma membrane. To investigate the neural circuits in which DOR may function, dual-immunolabeling was used to determine the relationship of the DOR to an endogenous ligand, enkephalin, and to a potential target, GABAergic neurons. Approximately a third (38 of 127) of DOR containing axons had enkephalin immunoreactivity, indicating DOR may act in part as a presynaptic autoreceptor. Although single axon terminals containing immunoreactivity for both DOR and GABA were not detected, some DOR-immunolabeled axon terminals (26 of 86) contacted soma or dendrites containing GABA. These data suggest that the DOR may act in part as an autoreceptor to regulate synaptic input to GABAergic as well as non-GABAergic PAG neurons. Furthermore, the exposure of the DOR to the extracellular space may be contingent upon dense-core vesicle fusion with the plasma membrane.

The polysialic acid moiety of the neural cell adhesion molecule is involved in intraretinal guidance of retinal ganglion cell axons

We have characterized the antigen recognized by mab10, a monoclonal antibody that has been shown to modify outgrowth of thalamic and cortical axons in vitro, and investigated the influence of this antibody on axonal growth in the chicken retina in vivo. Immunopurification, peptide sequencing, and biochemical characterization proved the epitope recognized by mab10 to be polysialic acid (PSA), associated with the neural cell adhesion molecule (NCAM). Intravitreal injections of antibody-secreting hybridoma cells were combined with whole-mount studies using the fluorescent tracer 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI). Pathfinding at the optic fissure was affected, resulting in a failure of axons to exit into the nerve. Misprojections also occurred in more peripheral areas of the retina; however, axons eventually oriented toward the center. Similar projection errors were observed after enzymatic removal of PSA by injecting endoneuraminidase N (endo N). Quantitative measurements of the optic nerve diameter as well as the width of the optic fiber layer confirmed that many axons failed to leave the retina and grew back in the optic fiber layer of the retina. Our findings suggest that NCAM-linked PSA is involved in guiding ganglion cell axons in the retina and at the optic fissure.

Corticosteroids regulate 5-HT(1A) but not 5-HT(1B) receptor mRNA in rat hippocampus

The role of mineralocorticoid and glucocorticoid receptors (MR and GR, respectively) in the regulation of serotonin receptors has not been clearly delineated. There is no consensus regarding the regulation of 5-HT(1A) receptors, and corticosteroid regulation of 5-HT(1B) mRNA has not been previously studied. We compared the effects of long-term (two week) adrenalectomy (no MR or GR activation) and several hormone replacement protocols designed to stimulate MR selectively (ALDO), MR and GR (HCT), and continuous MR with cyclical GR activation (SHAM adrenalectomy). 5-HT(1A) and 5-HT(1B) mRNAs were measured by in situ hybridization in hippocampus and raphe nuclei. None of the experimental manipulations altered 5-HT(1B) mRNA levels in the hippocampus or dorsal raphe, and also had no effect on 5-HT(1A) mRNA in dorsal or median raphe. However, 5-HT(1A) mRNA levels were regulated in a complex manner in the different subfields of hippocampus. We conclude that both MR and GR play an integrated role in regulating 5-HT(1A) mRNA levels in hippocampus while having no effect on 5-HT(1B) mRNA levels under these conditions.

Corticosteroids alter the 5-HT(1A) receptor-mediated response in CA1 hippocampal pyramidal cells

The hippocampus, prolonged excessive corticosterone secretion, and the 5-hydroxytryptamine (5-HT) neurotransmitter system are implicated in the etiology and treatment of psychiatric disorders. Corticosterone regulates CA1 hippocampal physiology and the 5-HT(1A) receptor-effector pathway; however the effect of chronic stress levels of corticosterone is unknown. Bilateral adrenalectomy (ADX), adrenalectomy with high dose corticosterone replacement (HCT), or surgical sham (SHAM) treatments were for 2 weeks. Standard intracellular recording techniques were used in hippocampal slices to measure active and passive cellular properties and 5-HT(1A) receptor-mediated responses in CA1 pyramidal cells. The magnitude and half-decay time of the slow after-hyperpolarization (sAHP) were decreased and the membrane time constant (tau) was increased by HCT treatment. The E(max) and EC(50), but not the slope, of the concentration-response curve for 5-HT activation of the 5-HT(1A) receptor were reduced in cells recorded from HCT versus SHAM treated rats. The net effect of treatment with stress levels of corticosterone was to increase the excitability of the CA1 hippocampal pyramidal cell through changes in membrane properties and 5-HT(1A) receptor-mediated response.

5-Hydroxytryptamine(7) receptor activation decreases slow afterhyperpolarization amplitude in CA3 hippocampal pyramidal cells

The 5-hydroxytryptamine(7) (5-HT(7)) receptor was originally defined by molecular biology techniques. The 5-HT(7) receptor protein and mRNA are found in brain areas, such as the CA3 subfield of the hippocampus, that are involved in various neuropsychiatric disease states. No functional response has previously been attributed to activation of the 5-HT(7) receptor in any of these brain areas. Calcium spike-induced slow afterhyperpolarizations (sAHP) were recorded from CA3 hippocampal pyramidal cells using intracellular recording techniques in a brain slice preparation maintained in vitro. A concentration-dependent inhibition of the sAHP amplitude was obtained when 5-HT was used as the agonist. To identify whether the 5-HT(7) receptor was one of the receptors mediating the inhibition of the sAHP amplitude, 5-HT agonists and antagonists were tested in the presence of WAY-100635 and GR-113808 to block 5-HT(1A) and 5-HT(4) receptor activation, respectively. The rank order potency of the agonists was 5-carboxyamidotryptamine (5-CT) > 5-HT > 5-methoxytryptamine (5-MeOT). Other agonists with high affinity at 5-HT(2), 5-HT(3), 5-HT(1B), 5-HT(1D), or 5-HT(6) receptors did not produce any response when tested at 10 microM. Ritanserin, mesulergine, and SB-269770 were competitive antagonists of the 5-CT inhibition of sAHP amplitude, with affinity (pA(2)) values of 6.8, 7. 9, and 8.8, respectively. Methiothepin was also an effective antagonist but was insurmountable. Other antagonists with affinity for the 5-HT(2), 5-HT(3), or 5-HT(6) receptor had no effect. Based on the rank order potency of the agonists and antagonists, one of the receptors that mediates the decrease in sAHP amplitude in CA3 hippocampal pyramidal cells was concluded to be the 5-HT(7) receptor.

Corticosteroids alter G protein inwardly rectifying potassium channels protein levels in hippocampal subfields

Corticosterone or cortisol, stress hormones in rat and human, respectively, alter neurotransmitter receptor-mediated responses in the brain. Corticosterone could alter these responses by modifying any component of the receptor-effector pathway. Many of these receptors are linked to guanine nucleotide regulatory proteins (G proteins) which, in turn, can activate second messenger systems and/or ion channels, such as G protein inwardly rectifying potassium channels (GIRK). The aim of these experiments was to determine whether corticosterone treatment altered the levels of GIRK proteins in rat hippocampus. Corticosterone treatment selectively altered the levels of GIRK1 and GIRK2 (measured on immunoblots) depending on the subfield of the hippocampus examined. These data lend credence to the hypothesis that corticosterone differentially alters neurotransmitter receptor-mediated responses dependent on the brain area.

Corticosteroids alter 5-hydroxytryptamine1A receptor-effector pathway in hippocampal subfield CA3 pyramidal cells

Corticosteroids influence neuron activity in the hippocampus through the activation of mineralocorticoid and glucocorticoid receptors. For example, corticosteroids modulate the responses elicited by the activation of several different neurotransmitter receptors on hippocampal pyramidal cells. However, the effects of corticosteroids on the serotonin (5-HT) receptors systems in subfield CA3 are not completely known. Therefore, we used single-electrode voltage clamp techniques to examine the actions of chronic corticosteroid treatment on the 5-HT1A receptor-effector pathway in rat hippocampal subfield CA3 pyramidal cells. Activation of the 5-HT1A receptor increases the conductance of an inward rectifying potassium channel, increasing outward current. The treatment groups used in this investigation were: adrenalectomy, selective mineralcorticoid receptor activation with aldosterone, mineralcorticoid receptor and glucocorticoid receptor activation with high levels of corticosterone and SHAM. Corticosteroids altered the characteristics of the 5-HT concentration-response curve for the 5-HT1A receptor. The effective concentration at 50% of maximum value was smaller in cells from the adrenalectomy treatment group compared to the other treatment groups. The maximum response was smaller in cells from the high corticosterone treatment group compared to SHAM and adrenalectomy treatment group animals. G protein function was also altered by corticosterone treatment. Less current was elicited by guanosine 5'-0-13-thiotriphosphate in cells from the high corticosterone treatment group compared to the other treatment groups and in cells from the SHAM treatment group compared to adrenalectomy treatment group animals. Corticosteroid treatment did not alter the current-voltage relationship, the conductance or the reversal potential of the potassium current linked to the 5-HT1A receptor. We conclude that corticosteroids alter the 5-HT1A receptor-mediated-response in hippocampal subfield CA3 neurons at site(s) downstream of the receptor.

Corticosteroids influence the action potential firing pattern of hippocampal subfield CA3 pyramidal cells

Corticosteroids regulate gene expression through the activation of mineralocorticoid and glucocorticoid receptors. The hippocampus contains the highest density of mineralocorticoid and glucocorticoid receptors in the central nervous system. The modulation of neuron excitability by corticosteroids in hippocampal subfield CA1 is well documented. However, it is not known whether corticosteroids produce different effects across the various hippocampal subfields. Therefore, we used intracellular recording techniques to examine the actions of chronic corticosteroid treatment (2 weeks) on the electrophysiological properties of rat hippocampal subfield CA3 pyramidal cells. The treatment groups used in this investigation were: adrenalectomy (ADX), selective mineralocorticoid receptor activation with aldosterone (ALD), mineralocorticoid and glucocorticoid receptor activation with high levels of corticosterone (HCT), and SHAM. Corticosteroid treatment altered the percentage of nonburst and burst firing neurons. The percentages of nonbursting cells were 74 and 62% in tissue from ADX and HCT animals compared to 42 and 41% in ALD and SHAM animals, respectively. The corticosteroid-induced effect on the ratio of nonbursting to bursting cells does not appear to be secondary to changes in the cell's membrane input resistance, resting potential, time constant, action potential, slow-or fast-afterhyperpolarizing potential properties. Based on these results we conclude that corticosteroids are important for maintaining the ratio of nonburst and burst firing pyramidal neurons in subfield CA3. These novel results are distinct from those previously reported for subfield CA1, suggesting that corticosteroids have different effects across hippocampal subfields.

Levetiracetam (ucb LO59) affects in vitro models of epilepsy in CA3 pyramidal neurons without altering normal synaptic transmission

Previous behavioural and electrophysiological studies have indicated that levetiracetam (ucb LO59) acts as an anticonvulsant drug in vivo. The purpose of the present study was to investigate the effects of levetiracetam on normal synaptic transmission and epileptiform activity in vitro. Intracellular recordings were obtained from the CA3 subfield of the rat hippocampal slice preparation. Levetiracetam in a concentration of 10 microM did not influence basic cell properties or normal synaptic transmission evoked by subthreshold and suprathreshold stimuli to the commissural pathway. However, it strongly inhibited the development of epileptiform bursting by the gamma-aminobutyric acid (GABA)A-receptor antagonist bicuculline (1-30 microM). Levetiracetam also decreased the size of bursts previously established by bicuculline. In experiments in which the glutamate-receptor agonist N-Methyl-D-Aspartate (NMDA) was used to generate spontaneous bursting, levetiracetam had no effect on the size of the bursts but decreased bursting frequency. The difference in effects of levetiracetam on bicuculline- and NMDA-induced bursting appeared to be dependent on the convulsant used, since in the presence of 10 microM bicuculline, levetiracetam decreased the size of NMDA-bursts to the same extent as the size of synaptically evoked bicuculline-bursts but had little effect on bursting frequency. The results show that under our experimental conditions, levetiracetam did not alter the components of normal synaptic transmission. However, levetiracetam at the concentrations studied inhibited epileptiform bursting induced by bicuculline and NMDA in vitro in a manner consistent with the profile of an antiepileptogenic drug.

Molecular cloning and characterization of human podocalyxin-like protein. Orthologous relationship to rabbit PCLP1 and rat podocalyxin

Human renal cortex and heart cDNA libraries were screened for a human homolog of rabbit PCLP1 using the rabbit PCLP1 cDNA as a probe. Clones spanning 5869 base pairs with an open reading frame coding for a 528-amino acid peptide were obtained. The putative peptide contains a potential signal peptide and a single membrane-spanning region. The extracellular domain contains multiple potential sites for N- and O-linked glycosylation and 4 cysteines for potential disulfide bonding similar to rabbit PCLP1. On Northern blot a major transcript is seen at 5.9 kilobases. Antibodies to this protein show a doublet at 160/165 kDa on Western blots of human glomerular extract and a pattern of intense glomerular staining and vascular endothelial staining on immunofluorescence of human kidney sections. Comparison of the rabbit and human peptide sequences shows a high degree of identity in the transmembrane and intracellular domains (96%) with a lower degree of identity in the extracellular domain (36%). An antibody to the intracellular domain reacted across species (human, rabbit, and rat) and recognized both rabbit PCLP1 and rat podocalyxin. An interspecies Southern blot probed with a cDNA coding for the intracellular domain showed strong hybridization to all vertebrates tested in a pattern suggesting a single copy gene. We conclude that this cDNA and putative peptide represent the human homolog of rabbit PCLP1 and rat podocalyxin.

Fluoxetine selectively alters 5-hydroxytryptamine1A and gamma-aminobutyric acidB receptor-mediated hyperpolarization in area CA1, but not area CA3, hippocampal pyramidal cells

Fluoxetine is a 5-hydroxytryptamine (5-HT, serotonin)-selective reuptake inhibitor (SSRI) and is one of the main drugs used for the treatment of depression. Because it takes 2 to 3 weeks of treatment before clinical efficacy is manifest, the acute actions of fluoxetine cannot account for the clinical actions of the drug. The chronic effects of fluoxetine have not been completely delineated. The experiments detailed here investigate the chronic effects of fluoxetine on 5-HT and gamma-aminobutyric acid (GABA) receptor-mediated actions using intracellular recording techniques in hippocampal brain slices. Rats were treated with fluoxetine for 3 weeks via osmotic minipumps implanted s.c. Fluoxetine and norfluoxetine plasma levels were determined. The hippocampal pyramidal cell characteristics and the 5-HT1A and GABA(B) receptor-mediated hyperpolarization were measured in the CA1 and the CA3 subfields. The 5-HT4 receptor-mediated decrease in the slow afterhyperpolarization amplitude was also recorded in area CA1. The time constant, magnitude of the change in resistance during 300-ms hyperpolarizing current pulses and half-decay time of the sAHP were altered by chronic fluoxetine treatment in area CA1 pyramidal cells. No changes were seen in any of the active or passive membrane properties of the CA3 hippocampal pyramidal cells. Fluoxetine treatment increased the potency of 5-HT for the 5-HT1A receptor-mediated hyperpolarization in area CA1, but not area CA3, and decreased the potency of baclofen for the GABA(B) receptor-mediated hyperpolarization in area CA1, but not area CA3. The characteristics of the concentration-response curve for the 5-HT-mediated decrease in sAHP amplitude in area CA1 were not altered by fluoxetine treatment. Chronic fluoxetine selectively and differentially altered the cell characteristics and the 5-HT1A and GABA(B) receptor-mediated responses in area CA1 of the hippocampus, which forms the final common output of the hippocampus.

Corticosterone alters G protein alpha-subunit levels in the rat hippocampus

The hypothalamic-pituitary-adrenal axis regulates the synthesis and secretion of corticosteroid hormones. The hippocampus, a component of the limbic system, contains the highest concentration of corticosteroid receptors in the brain and may play an important role in regulating hypothalamic-pituitary-adrenal axis activity and mediating physiological responses to stress. The corticosteroid hormone corticosterone alters the response elicited by activation of several different G protein-linked neurotransmitter receptors in the hippocampus. In the present study we used Western blot and immunohistochemical techniques to determine the effects of chronic adrenalectomy (ADX), low basal (CT) and high (HCT) corticosterone treatments on Gs, Gi1 and 2 and Go alpha-subunit levels and intracellular location in the rat hippocampus. CT treatment increased Gs alpha-subunit levels and HCT treatment increased the levels of Gs, Gi1 and 2 and Go alpha-subunits when compared to sham as detected on Western blots. No change in the intracellular location of the G protein alpha-subunits was detected using immunohistochemistry. Based on our results, we conclude that corticosterone alters G protein alpha-subunit levels in the rat hippocampus without altering their intracellular location. These results provide an important piece of information towards understanding how corticosteroids alter G protein-linked neurotransmitter receptor-mediated responses.

Androgens selectively modulate C-fos messenger RNA induction in the rat hippocampus following novelty

We have previously shown that androgen receptors are found in high concentrations in hippocampal CA1 pyramidal cells. To begin to explore the possible roles for androgen receptors in this area of the brain, we studied the effects of endogenous and exogenous androgen on the behaviourally induced expression of cellular immediate early gene messenger RNAs. Adult male Fischer 344 rats were either gonadectomized, gonadectomized and given two Silastic capsules of dihydrotestosterone propionate at the time of surgery, or left intact. Three weeks later, animals were placed into a novel open field for 20 min. This behavioural paradigm caused region- and gene-specific increases of c-fos, jun-B, c-jun and zif268 messenger RNA in the hippocampus as determined by semi-quantitative in situ hybridization histochemistry. The removal of circulating androgen by gonadectomy potentiated, whereas dihydrotestosterone treatment of castrates attenuated, the behaviourally induced expression of c-fos messenger RNA in the CA1 region of the hippocampus. No changes in c-fos messenger RNA expression were detected in the CA3 or dentate gyrus regions where androgen receptor levels are low. Androgen status did not affect either the basal or stimulated expression of Jun-B, c-Jun or zif268 messenger RNA in any of the three cellular regions of the hippocampus examined. These results implicate androgen receptors in modulating the active response of hippocampal neurons to a behaviourally relevant stimulus. Since the products of cellular immediate genes can function to alter an array of downstream genes, the modulation of these genes in the hippocampus by gonadal hormones may have important ramifications for hippocampal function.

Differential immunohistochemical labeling of Gs, G(il) and 2, and G(o) alpha-subunits in rat forebrain

The anatomical and morphological distribution of the G proteins G(o), G(i1) and 2, and Gs alpha-subunits in rat forebrain sections was determined using immunohistochemical techniques. Diffuse G(o) labeling occurred in the neuropil throughout the cortex, superficial layers of the entorhinal cortex, thalamus, several white matter fiber tracts, and hippocampus. G(i1) and 2 immunoreactivity was also located in the neuropil but produced a more fibrous pattern. Fibrous labeling of G(i1) and 2 was observed in the cortex, amygdala, hippocampal subfield CA3, and several white matter fiber tracts. Both G(o) and G(i1) and 2 labeling was present in the pencil fibers within the striatum and lateral geniculate nucleus. Gs labeling, in contrast to G(o) and G(i1) and 2, was generally cytoplasmic. Cytoplasmic Gs labeling was observed in the thalamus, habenula, dentate, geniculate nucleus, hypothalamus, and hippocampus. Intense Gs labeling was observed in the striatum parenchyma, choroid plexus, and infundibular stem. Based on our results, we conclude that the G proteins G(o), G(i1) and 2, and Gs are anatomically distributed differently throughout the brain. The diffuse neuropil labeling of G(o), fibrous neuropil labeling of G(i1) and 2, and cytoplasmic labeling of Gs strongly suggests that the G proteins are also differentially distributed morphologically within a neuron. The differential anatomical and cellular location of G proteins in the CNS may contribute to the coupling specificity between neurotransmitter receptors and G proteins.

Androgens modulate glucocorticoid receptor mRNA, but not mineralocorticoid receptor mRNA levels, in the rat hippocampus

Androgen, mineralocorticoid and glucocorticoid receptors (AR, MR and GR, respectively) are ligand-activated transcription factors that alter gene expression and have a wide variety of effects in the central nervous system. High levels of AR, MR and GR mRNA have been found in the CA1 pyramidal cell region of the rat hippocampus and all 3 of these proteins bind to a similar hormone response element in DNA suggesting the possibility of common receptor function or cross-talk between these receptors at the level of transcription. To begin to investigate this hypothesis, we examined the regulation of AR, MR and GR mRNA expression in the rat hippocampus following treatment with androgens in combination with gonadectomy and/or adrenalectomy. Three-month-old male Sprague-Dawley rats were either castrated for 3 weeks, castrated and immediately implanted with 2 Silastic capsules filled with the non-aromatizable androgen, dihydrotestosterone, or left gonadally intact. Four days prior to sacrifice, these animals were either adrenalectomized or sham operated. GR, MR and AR mRNA were measured in the hippocampal subfields using in situ hybridization. In the CA1 region, dihydrotestosterone treatment of castrates decreased GR mRNA levels to 69 percent of levels found in gonadally intact rats and prevented the adrenalectomy-induced increases in GR mRNA observed in the gonadally intact and castrated animals. No changes in GR mRNA were observed in the CA3 region or dentate gyrus, where AR expression is low or absent. There was no effect of androgen treatment on MR mRNA levels nor did gonadectomy or androgen replacement alter the increases in MR mRNA following adrenalectomy. AR mRNA levels in the CA1 region were unchanged across all treatment groups. In vitro binding studies revealed almost complete nuclear occupancy of hippocampal AR in dihydrotestosterone-treated castrates. No appreciable in vitro binding of dihydrotestosterone to hippocampal MR or GR (Ki approximately 1500 nM) was observed which suggests that androgen regulation of GR mRNA in the hippocampus is occurring through AR binding. These data demonstrate a functional similarity of androgens and glucocorticoids in the regulation of GR mRNA levels in an area where AR and GR are colocalized. Androgen-mediated downregulation of GR expression may prove to be an important event in the adaptive responses of CA1 pyramidal cells to hormonal stimuli.

Androgen modulates N-methyl-D-aspartate-mediated depolarization in CA1 hippocampal pyramidal cells

Previously, research elucidating steroid hormone actions in the central nervous system has focused on their role in sexual reproduction and maintaining homeostasis. The hippocampus is a target of steroid modulation and is involved in the development of emotional behavior and memory storage. Area CA1 of the hippocampus contains a high density of androgen receptor (AR) and N-methyl-D-aspartate (NMDA) receptors. NMDA receptors underlie excitatory synaptic transmission and excitotoxicity in CA1 neurons. The effects of AR activation on the neurophysiology of hippocampal pyramidal neurons is unknown. Standard intracellular recording techniques in hippocampal slices were used to investigate the effects of the non-aromatizable androgen, 5-alpha-dihydrotestos-terone-proprionate (DHTP), on CA1 pyramidal cell characteristics and NMDA receptor-mediated responses. Male Sprague-Dawley rats were unoperated, sham-operated (SHAM), gonadectomized (GDX), or gonadectomized with DHTP replacement therapy (GDX + DHTP). Neuronal AR was saturated by DHTP treatment as determined by binding studies and immunocytochemistry. Chronic DHTP treatment increased the action potential duration and decreased the fast afterhyperpolarization (fAHP) amplitude. To test the effect of DHTP on glutamate receptor-mediated responses, hippocampal slices were exposed to increasing concentrations of NMDA. In pyramidal cells from SHAM and GDX-treated animals, 30 microM NMDA induced an irreversible depolarization; the membrane potential of pyramidal cells from GDX + DHTP-treated animals recovered to baseline. The effect of DHTP was time dependent, implicating protein synthetic mechanisms. Our findings demonstrate that androgens can influence pyramidal cell characteristics and neurotransmitter-evoked actions in hippocampal CA1 pyramidal cells.

Corticosterone alters 5-HT1A receptor-mediated hyperpolarization in area CA1 hippocampal pyramidal neurons

The 5-hydroxytryptamine1A (5-HT1A) receptor in the CA1 region of the hippocampus is linked through a G protein to an inwardly rectifying potassium conductance. Activation of the 5-HT1A receptor results in a membrane hyperpolarization and decreases neuronal firing rate. The hippocampus contains a high concentration of the mineralocorticoid (MR) and glucocorticoid (GR) corticosterone (CT) receptor subtypes. Some laboratories have reported that CT modulates 5-HT1A receptor binding density and mRNA levels in area CA1 of the hippocampus; however, others have reported no change. Previous electrophysiological studies have demonstrated that acute (1 to 4 hour) MR activation in slices from adrenalectomized (ADX) rats attenuates the 5-HT1A receptor-mediated hyperpolarization, while acute MR+ GR or GR activation alone did not alter the 5-HT1A response. Our results confirm that the 5-HT1A response was attenuated 2 to 8 hours following MR activation. However, we found that GR activation alone decreased the potency, but not the maximal response to 5-HT. Chronic (2-week) treatment with basal levels of CT did not alter the 5-HT1A response. Administration of high concentrations of CT in vitro to neurons from chronically treated ADX rats decreased the magnitude of the 5-HT1A receptor-mediated hyperpolarization. We conclude that the 5-HT1A receptor-effector system in CA1 hippocampal pyramidal neurons is modulated by CT in a dose- and time-dependent manner.

Distribution and hormonal regulation of androgen receptor (AR) and AR messenger ribonucleic acid in the rat hippocampus

The actions of androgens in both peripheral and central tissues are linked in part to their ability to specifically bind and activate androgen receptors (ARs). ARs have been well studied in the rat hypothalamus and peripheral reproductive tissues, where they are directly involved in endocrine feedback mechanisms and reproduction. Previous studies revealed relatively high levels of AR and AR messenger RNA (mRNA) in the rat hippocampus; however, the action of androgen in this brain region remains unclear. To begin to address this issue, we used a multidisciplinary approach to quantitate hippocampal AR and AR mRNA levels and investigate their regulation after various hormonal manipulations. In vitro binding assays revealed a single, saturable, high affinity binding site for androgen in hippocampal cytosols. The expression of AR mRNA in the intact adult male rat hypothalamus and hippocampus was demonstrated using reverse transcription-polymerase chain reaction and quantified using a ribonuclease protection assay. Comparable levels of AR mRNA were found in the hippocampus and hypothalamus. In addition, in situ hybridization analysis revealed a unique distribution of AR mRNA in the hippocampus. AR mRNA was found predominately in the CA1 pyramidal cells, which form the major signal output of the hippocampal trisynaptic circuit. Reverse transcription-polymerase chain reaction of total RNA from microdissected hippocampal regions confirmed this distribution. Ribonuclease protection assay demonstrated a significant decrease in the AR mRNA content of the hippocampus in animals killed 4 days after castration or in intact rats after four daily injections of the AR antagonist, flutamide (15 mg/animal), compared to that in intact controls (P < 0.01). In contrast, a 35% increase (P < 0.05) in the hippocampal AR mRNA content was found in old (22-month-old) compared to young (5-month-old) male rats. In both cases, [3H]dihydrotestosterone binding to the cytosolic preparation did not parallel the changes observed in the AR mRNA content. Taken together, these data demonstrate that hippocampal cells containing AR can respond to circulating androgen to alter AR gene expression. Furthermore, AR mRNA autoregulation appears to be both age and tissue specific and does not directly follow the regulatory patterns described for other steroid hormone receptors found in the hippocampus.

Modulation of the 5-hydroxytryptamine4 receptor-mediated response by short-term and long-term administration of corticosterone in rat CA1 hippocampal pyramidal neurons

Corticosterone (CT) treatment decreases the magnitude of the 5-hydroxytryptamine (5-HT)1A receptor-mediated hyperpolarization in rat CA1 hippocampal pyramidal neurons. In the present study, we examined the short- and long-term effects of CT on the functionally excitatory 5-HT4 receptor-mediated decrease in the amplitude of the slow afterhyperpolarization (sAHP) that follows a calcium spike and the concomitant decrease in sAHP half decay time. Rats were adrenalectomized (ADX) 2 weeks before the experiment. Data for concentration-response curves were obtained with sharp electrode current clamp recordings in the CA1 pyramidal cell layer of hippocampal slices. Significant changes were found in the 5-HT4 receptor-mediated decrease in sAHP amplitude. The Emax of the 5-HT4 response was significantly increased in cells from ADX rats when the superfusion medium contained 1 nM CT. Short-term administration of 100 nM CT did not alter the 5-HT4 response. Chronic treatment with low concentrations of CT decreased the Emax of the 5-HT4 response. Treatment with CT concentrations that mimic conditions of chronic stress decreased the Emax of the 5-HT4 response and shifted the EC50 to the right. Based on these results we conclude that the magnitude and the potency of the 5-HT4 receptor-mediated decrease in sAHP amplitude is altered by CT. Because the short- and long-term effects of CT treatment are not the same, the actions of CT are time and concentration dependent. CT modulation of the 5-HT4 response is different from its modulation of the 5-HT1A response.

Chronic corticosterone treatment maintains synaptic activity of CA1 hippocampal pyramidal cells: acute high corticosterone administration increases action potential number

The hypothalamic-pituitary-adrenocortical (HPA) axis controls the levels of plasma corticosterone (CT) in the rat and the levels of cortisol in man. CT is important in maintaining homeostasis and regulating energy production. Homeostasis is maintained by basal activation of the hippocampal-HPA axis. In response to stress CT secretion is increased. CT activation of receptors in the hippocampus provides feedback inhibition of the HPA axis to return the system to basal activity. There are two types of CT receptors: the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). CT has a 10-fold higher affinity for MR than GR. Normal basal levels of CT occupy the majority of the MR. During the diurnal surge of CT and following the presentation of a stressful stimulus, the MR and GR are both maximally occupied. To begin to understand how CT influences the hippocampal-HPA axis, intracellular recording techniques in the hippocampal brain slice preparation were used to determine how high concentrations of CT may alter cell characteristics and/or evoked synaptic activity. Two treatment groups were used, i.e., adrenalectomized (ADX) and ADX with CT pellet replacement (ADX+CT) that produced plasma blood levels equal to that seen in a normal rat in the morning. Acute administration of 100 nM CT decreased action potential threshold and the number of action potentials elicited by a depolarizing current pulse in cells from both the ADX and ADX+CT treated rats. The amplitude of the evoked excitatory postsynaptic potentials (EPSP) or inhibitory postsynaptic potentials (IPSP) declined in cells recorded from ADX animals and ADX rats acutely treated with high concentrations of CT (ADX/CT).(ABSTRACT TRUNCATED AT 250 WORDS)

Baclofen concentration-response curves differ between hippocampal subfields

The hippocampus contains interneurons that release gamma-aminobutyric acid (GABA). GABA hyperpolarizes hippocampal CA1 and CA3 pyramidal cells through activation of GABAB postsynaptic receptors. GABAB and 5-hydroxytryptamine1A (5-HT1A) receptors share effector mechanism(s). Agonist potency and the maximal hyperpolarization produced by 5-HT1A receptor activation is different between the CA1 and CA3 subfields. We determined that baclofen, a selective GABAB agonist, was more potent and produced a greater maximal response in area CA3 than in CA1. The larger magnitude of the response can be attributed partly to the larger input resistance of CA3 neurons. GABAB receptor-effector coupling differences between area CA1 and CA3 are proposed as the mechanism underlying the baclofen response incongruities.