Immunohistochemical evidence for the presence of gamma-aminobutyric acid and serotonin in one nerve cell. A study on the raphe nuclei of the rat using antibodies to glutamate decarboxylase and serotonin

Constitutive depletion of brain serotonin differentially affects rats' social and cognitive abilities

Central serotonin appears a promising transdiagnostic marker of psychiatric disorders and a modulator of some of their key behavioral symptoms. In adult male Tph2 -/- rats, constitutively lacking central serotonin, we tested individual's cognitive, social and non-social abilities and characterized group's social organization under classical and ethological testing conditions. Using unsupervised machine learning, we identified the functions most dependent on serotonin. Although serotonin depletion did not affect cognitive performances in classical testing, in the home-cage it induced compulsive aggression and sexual behavior, hyperactive and hypervigilant stereotyped behavior, reduced self-care and exacerbated corticosterone levels. This profile recalled symptoms of impulse control and anxiety disorders. Serotonin appeared essential for behavioral adaptation to dynamic social environments. Our animal model challenges the essential role of serotonin in decision-making, flexibility, impulsivity, and risk-taking. These findings highlight the importance of studying everyday life functions within the dynamic social living environment to model complexity in animal models.

Serotonergic regulation of the dopaminergic system: Implications for reward-related functions

Serotonin is a critical neuromodulator involved in development and behavior. Its role in reward is however still debated. Here, we first review classical studies involving electrical stimulation protocols and pharmacological approaches. Contradictory results on the serotonergic' involvement in reward emerge from these studies. These differences might be ascribable to either the diversity of cellular types within the raphe nuclei or/and the specific projection pathways of serotonergic neurons. We continue to review more recent work, using optogenetic approaches to activate serotonergic cells in the Raphe to VTA pathway. From these studies, it appears that activation of this pathway can lead to reinforcement learning mediated through the excitation of dopaminergic neurons by serotonergic neurons co-transmitting glutamate. Finally, given the importance of serotonin during development on adult emotion, the effect of abnormal early-life levels of serotonin on the dopaminergic system will also be discussed. Understanding the interaction between the serotonergic and dopaminergic systems during development and adulthood is critical to gain insight into the specific facets of neuropsychiatric disorders.

5-HT neurons of the medullary raphe contribute to respiratory control in toads

Air-breathing vertebrates undergo respiratory adjustments when faced with disturbances in the gas composition of the environment. In mammals, the medullary raphe nuclei are involved in the neuronal pathway that mediates the ventilatory responses to hypoxia and hypercarbia. We investigate whether the serotoninergic neurons of the medullary raphe nuclei of toads (Rhinella diptycha) play a functional role in respiratory control during resting conditions (room air), hypercarbia (5% CO₂), and hypoxia (5% O₂). The raphe nuclei were located and identified based on the location of the serotoninergic neurons in the brainstem. We then lesioned the medullary raphe (raphe pallidus, obscurus and magnus) with anti-SERT-SAP and measured ventilation in both control and lesioned groups and we observed that serotonin (5-HT) specific chemical lesions of the medullary raphe caused reduced respiratory responses to both hypercarbia and hypoxia. In summary, we report that the serotoninergic neurons of the medullary raphe of the cururu toad Rhinella diptycha participate in the chemoreflex responses during hypercarbia and hypoxia, but not during resting conditions. This current evidence in anurans, together with the available data in mammals, brings insights to the evolution of brain sites, such as the medullary raphe, involved in the ventilatory chemoreflex in vertebrates.

Embracing diversity in the 5-HT neuronal system

Neurons that synthesize and release 5-hydroxytryptamine (5-HT; serotonin) express a core set of genes that establish and maintain this neurotransmitter phenotype and distinguish these neurons from other brain cells. Beyond a shared 5-HTergic phenotype, these neurons display divergent cellular properties in relation to anatomy, morphology, hodology, electrophysiology and gene expression, including differential expression of molecules supporting co-transmission of additional neurotransmitters. This diversity suggests that functionally heterogeneous subtypes of 5-HT neurons exist, but linking subsets of these neurons to particular functions has been technically challenging. We discuss recent data from molecular genetic, genomic and functional methods that, when coupled with classical findings, yield a reframing of the 5-HT neuronal system as a conglomeration of diverse subsystems with potential to inspire novel, more targeted therapies for clinically distinct 5-HT-related disorders.

A Brainstem-Spinal Cord Inhibitory Circuit for Mechanical Pain Modulation by GABA and Enkephalins

Pain thresholds are, in part, set as a function of emotional and internal states by descending modulation of nociceptive transmission in the spinal cord. Neurons of the rostral ventromedial medulla (RVM) are thought to critically contribute to this process; however, the neural circuits and synaptic mechanisms by which distinct populations of RVM neurons facilitate or diminish pain remain elusive. Here we used in vivo opto/chemogenetic manipulations and trans-synaptic tracing of genetically identified dorsal horn and RVM neurons to uncover an RVM-spinal cord-primary afferent circuit controlling pain thresholds. Unexpectedly, we found that RVM GABAergic neurons facilitate mechanical pain by inhibiting dorsal horn enkephalinergic/GABAergic interneurons. We further demonstrate that these interneurons gate sensory inputs and control pain through temporally coordinated enkephalin- and GABA-mediated presynaptic inhibition of somatosensory neurons. Our results uncover a descending disynaptic inhibitory circuit that facilitates mechanical pain, is engaged during stress, and could be targeted to establish higher pain thresholds. VIDEO ABSTRACT.

Depression-like behavior in rat: Involvement of galanin receptor subtype 1 in the ventral periaqueductal gray

The neuropeptide galanin coexists in rat brain with serotonin in the dorsal raphe nucleus and with noradrenaline in the locus coeruleus (LC), and it has been suggested to be involved in depression. We studied rats exposed to chronic mild stress (CMS), a rodent model of depression. As expected, these rats showed several endophenotypes relevant to depression-like behavior compared with controls. All these endophenotypes were normalized after administration of a selective serotonin reuptake inhibitor. The transcripts for galanin and two of its receptors, galanin receptor 1 (GALR1) and GALR2, were analyzed with quantitative real-time PCR using laser capture microdissection in the following brain regions: the hippocampal formation, LC, and ventral periaqueductal gray (vPAG). Only Galr1 mRNA levels were significantly increased, and only in the latter region. After knocking down Galr1 in the vPAG with an siRNA technique, all parameters of the depressive behavioral phenotype were similar to controls. Thus, the depression-like behavior in rats exposed to CMS is likely related to an elevated expression of Galr1 in the vPAG, suggesting that a GALR1 antagonist could have antidepressant effects.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

Presynaptic partners of dorsal raphe serotonergic and GABAergic neurons

The serotonin system powerfully modulates physiology and behavior in health and disease, yet the circuit mechanisms underlying serotonin neuron activity are poorly understood. The major source of forebrain serotonergic innervation is from the dorsal raphe nucleus (DR), which contains both serotonin and GABA neurons. Using viral tracing combined with electrophysiology, we found that GABA and serotonin neurons in the DR receive excitatory, inhibitory, and peptidergic inputs from the same specific brain regions. Embedded in this overall similarity are important differences. Serotonin neurons are more likely to receive synaptic inputs from anterior neocortex while GABA neurons receive disproportionally higher input from the central amygdala. Local input mapping revealed extensive serotonin-serotonin as well as GABA-serotonin connectivity with a distinct spatial organization. Covariance analysis suggests heterogeneity of both serotonin and GABA neurons with respect to the inputs they receive. These analyses provide a foundation for further functional dissection of the serotonin system.

Modulation of firing and synaptic transmission of serotonergic neurons by intrinsic G protein-coupled receptors and ion channels

Serotonergic neurons project to virtually all regions of the central nervous system and are consequently involved in many critical physiological functions such as mood, sexual behavior, feeding, sleep/wake cycle, memory, cognition, blood pressure regulation, breathing, and reproductive success. Therefore, serotonin release and serotonergic neuronal activity have to be precisely controlled and modulated by interacting brain circuits to adapt to specific emotional and environmental states. We will review the current knowledge about G protein-coupled receptors and ion channels involved in the regulation of serotonergic system, how their regulation is modulating the intrinsic activity of serotonergic neurons and its transmitter release and will discuss the latest methods for controlling the modulation of serotonin release and intracellular signaling in serotonergic neurons in vitro and in vivo.

Distinct neurochemical and functional properties of GAD67-containing 5-HT neurons in the rat dorsal raphe nucleus

The serotonergic (5-HTergic) system arising from the dorsal raphe nucleus (DRN) is implicated in various physiological and behavioral processes, including stress responses. The DRN is comprised of several subnuclei, serving specific functions with distinct afferent and efferent connections. Furthermore, subsets of 5-HTergic neurons are known to coexpress other transmitters, including GABA, glutamate, or neuropeptides, thereby generating further heterogeneity. However, despite the growing evidence for functional variations among DRN subnuclei, relatively little is known about how they map onto neurochemical diversity of 5-HTergic neurons. In the present study, we characterized functional properties of GAD67-expressing 5-HTergic neurons (5-HT/GAD67 neurons) in the rat DRN, and compared with those of neurons expressing 5-HTergic molecules (5-HT neurons) or GAD67 alone. While 5-HT/GAD67 neurons were absent in the dorsomedial (DRD) or ventromedial (DRV) parts of the DRN, they were selectively distributed in the lateral wing of the DRN (DRL), constituting 12% of the total DRL neurons. They expressed plasmalemmal GABA transporter 1, but lacked vesicular inhibitory amino acid transporter. By using whole-cell patch-clamp recording, we found that 5-HT/GAD67 neurons had lower input resistance and firing frequency than 5-HT neurons. As revealed by c-Fos immunohistochemistry, neurons in the DRL, particularly 5-HT/GAD67 neurons, showed higher responsiveness to exposure to an open field arena than those in the DRD and DRV. By contrast, exposure to contextual fear conditioning stress showed no such regional differences. These findings indicate that 5-HT/GAD67 neurons constitute a unique neuronal population with distinctive neurochemical and electrophysiological properties and high responsiveness to innocuous stressor.

Disruption of raphé serotonergic neural projections to the cortex: a potential pathway contributing to remote loss of brainstem neurons following neonatal hypoxic-ischemic brain injury

Neuronal injury is a key feature of neonatal hypoxic-ischemic (HI) brain injury. However, the mechanisms underpinning neuronal losses, such as in the brainstem, are poorly understood. One possibility is that disrupted neural connections between the cortex and brainstem may compromise the survival of neuronal cell bodies in the brainstem. We investigated whether brainstem raphé serotonergic neurons that project to the cortex are lost after HI. We also tested if neuroinflammation has a role in disrupting brainstem raphé projections. Postnatal day 3 (P3) rats underwent unilateral carotid artery ligation followed by hypoxia (6% oxygen for 30 min). A retrograde tracer, choleratoxin b, was deposited in the motor cortex on P38. On P45 we found that retrogradely labelled neurons in the dorsal raphé dorsal, ventrolateral, interfascicular, caudal and ventral nuclei were lost after P3 HI. All retrogradely labelled neurons in the raphé nuclei were serotonergic. Numbers of retrogradely labelled neurons were also reduced in the ventromedial thalamus and basolateral amygdala. Minocycline treatment (45 mg/kg 2 h post-HI, 22.5 mg/kg daily P4-P9) attenuated losses of retrogradely labelled neurons in the dorsal raphé ventrolateral, interfascicular and ventral raphé nuclei, and the ventromedial thalamus. These results indicate that raphé neurons projecting to the cortex constitute a population of serotonergic neurons that are lost after P3 HI. Furthermore, neuroinflammation has a role in the disruption of raphé and thalamic neural projections. Future studies investigating the cellular mechanisms of axonal degeneration may reveal new targets for interventions to prevent neuronal losses after neonatal HI.

Collateralized dorsal raphe nucleus projections: a mechanism for the integration of diverse functions during stress

The midbrain dorsal raphe nucleus (DR) is the origin of the central serotonin (5-HT) system, a key neurotransmitter system that has been implicated in the expression of normal behaviors and in diverse psychiatric disorders, particularly affective disorders such as depression and anxiety. One link between the DR-5-HT system and affective disorders is exposure to stressors. Stress is a major risk factor for affective disorders, and stressors alter activity of DR neurons in an anatomically specific manner. Stress-induced changes in DR neuronal activity are transmitted to targets of the DR via ascending serotonergic projections, many of which collateralize to innervate multiple brain regions. Indeed, the collateralization of DR efferents allows for the coordination of diverse components of the stress response. This review will summarize our current understanding of the organization of the ascending DR system and its collateral projections. Using the neuropeptide corticotropin-releasing factor (CRF) system as an example of a stress-related initiator of DR activity, we will discuss how topographic specificity of afferent regulation of ascending DR circuits serves to coordinate activity in functionally diverse target regions under appropriate conditions.

Functional organization of the dorsal raphe efferent system with special consideration of nitrergic cell groups

The serotonin (5HT) system of the brain is involved in many CNS functions including sensory perception, stress responses and psychological disorders such as anxiety and depression. Of the nine 5HT nuclei located in the mammalian brain, the dorsal raphe nucleus (DRN) has the most extensive forebrain connectivity and is implicated in the manifestation of stress-related psychological disturbances. Initial investigations of DRN efferent connections failed to acknowledge the rostrocaudal and mediolateral organization of the nucleus or its neurochemical heterogeneity. More recent studies have focused on the non-5HT contingent of DRN cells and have revealed an intrinsic intranuclear organization of the DRN which has specific implications for sensory signal processing and stress responses. Of particular interest are spatially segregated subsets of nitric oxide producing neurons that are activated by stressors and that have unique efferent projection fields. In this regard, both the midline and lateral wing subregions of the DRN have emerged as prominent loci for future investigation of nitric oxide function and modulation of sensory- and stressor-related signals in the DRN and coinciding terminal fields.

Raphe serotonin neurons are not homogenous: electrophysiological, morphological and neurochemical evidence

The median (MR) and dorsal raphe (DR) nuclei contain the majority of the 5-hydroxytryptamine (5-HT, serotonin) neurons that project to limbic forebrain regions, are important in regulating homeostatic functions and are implicated in the etiology and treatment of mood disorders and schizophrenia. The primary synaptic inputs within and to the raphe are glutamatergic and GABAergic. The DR is divided into three subfields, i.e., ventromedial (vmDR), lateral wings (lwDR) and dorsomedial (dmDR). Our previous work shows that cell characteristics of 5-HT neurons and the magnitude of the 5-HT(1A) and 5-HT(1B) receptor-mediated responses in the vmDR and MR are not the same. We extend these observations to examine the electrophysiological properties across all four raphe subfields in both 5-HT and non-5-HT neurons. The neurochemical topography of glutamatergic and GABAergic cell bodies and nerve terminals were identified using immunohistochemistry and the morphology of the 5-HT neurons was measured. Although 5-HT neurons possessed similar physiological properties, important differences existed between subfields. Non-5-HT neurons were indistinguishable from 5-HT neurons. GABA neurons were distributed throughout the raphe, usually in areas devoid of 5-HT neurons. Although GABAergic synaptic innervation was dense throughout the raphe (immunohistochemical analysis of the GABA transporters GAT1 and GAT3), their distributions differed. Glutamate neurons, as defined by vGlut3 anti-bodies, were intermixed and co-localized with 5-HT neurons within all raphe subfields. Finally, the dendritic arbor of the 5-HT neurons was distinct between subfields. Previous studies regard 5-HT neurons as a homogenous population. Our data support a model of the raphe as an area composed of functionally distinct subpopulations of 5-HT and non-5-HT neurons, in part delineated by subfield. Understanding the interaction of the cell properties of the neurons in concert with their morphology, local distribution of GABA and glutamate neurons and their synaptic input, reveals a more complicated and heterogeneous raphe. These results provide an important foundation for understanding how specific subfields modulate behavior and for defining which aspects of the circuitry are altered during the etiology of psychological disorders.

Brain serotonin receptors and transporters: initiation vs. termination of escalated aggression

RATIONALE

Recent findings have shown a complexly regulated 5-HT system as it is linked to different kinds of aggression.

OBJECTIVE

We focus on (1) phasic and tonic changes of 5-HT and (2) state and trait of aggression, and emphasize the different receptor subtypes, their role in specific brain regions, feed-back regulation and modulation by other amines, acids and peptides.

RESULTS

New pharmacological tools differentiate the first three 5-HT receptor families and their modulation by GABA, glutamate and CRF. Activation of 5-HT(1A), 5-HT(1B) and 5-HT(2A/2C) receptors in mesocorticolimbic areas, reduce species-typical and other aggressive behaviors. In contrast, agonists at 5-HT(1A) and 5-HT(1B) receptors in the medial prefrontal cortex or septal area can increase aggressive behavior under specific conditions. Activation of serotonin transporters reduce mainly pathological aggression. Genetic analyses of aggressive individuals have identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT(1B), 5-HT transporter, Pet1, MAOA) or indirectly (e.g., Neuropeptide Y, αCaMKII, NOS, BDNF). Dysfunction in genes for MAOA escalates pathological aggression in rodents and humans, particularly in interaction with specific experiences.

CONCLUSIONS

Feedback to autoreceptors of the 5-HT(1) family and modulation via heteroreceptors are important in the expression of aggressive behavior. Tonic increase of the 5-HT(2) family expression may cause escalated aggression, whereas the phasic increase of 5-HT(2) receptors inhibits aggressive behaviors. Polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT modulate aggression, often requiring interaction with the rearing environment.

Behavioral and pharmacogenetics of aggressive behavior

Serotonin (5-HT) has long been considered as a key transmitter in the neurocircuitry controlling aggression. Impaired regulation of each subtype of 5-HT receptor, 5-HT transporter, synthetic and metabolic enzymes has been linked particularly to impulsive aggression. The current summary focuses mostly on recent findings from pharmacological and genetic studies. The pharmacological treatments and genetic manipulations or polymorphisms of aspecific target (e.g., 5-HT1A receptor) can often result in inconsistent results on aggression, due to "phasic" effects of pharmacological agents versus "trait"-like effects of genetic manipulations. Also, the local administration of a drug using the intracranial microinjection technique has shown that activation of specific subtypes of 5-HT receptors (5-HT1A and 5-HT1B) in mesocorticolimbic areas can reduce species-typical and other aggressive behaviors, but the same receptors in the medial prefrontal cortex or septal area promote escalated forms of aggression. Thus, there are receptor populations in specific brain regions that preferentially modulate specific types of aggression. Genetic studies have shown important gene-environment interactions; it is likely that the polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT (e.g., MAOA) determine the vulnerability to adverse environmental factors that escalate aggression. We also discuss the interaction between the 5-HT system and other systems. Modulation of 5-HT neurons in the dorsalraphe nucleus by GABA, glutamate and CRF profoundly regulate aggressive behaviors. Also, interactions of the 5-HT system with other neuropeptides(arginine vasopressin, oxytocin, neuropeptide Y, opioid) have emerged as important neurobiological determinants of aggression. Studies of aggression in genetically modified mice identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT1B, 5-HT transporter, Pet1, MAOA) or indirectly[e.g., BDNF, neuronal nitric oxide (nNOS), aCaMKII, Neuropeptide Y].The future agenda delineates specific receptor subpopulations for GABA, glutamate and neuropeptides as they modulate the canonical aminergic neurotransmitters in brainstem, limbic and cortical regions with the ultimate outcome of attenuating or escalating aggressive behavior.

Chemical neuroanatomy of the dorsal raphe nucleus and adjacent structures of the mouse brain

Serotonin neurons play a major role in many normal and pathological brain functions. In the rat these neurons have a varying number of cotransmitters, including neuropeptides. Here we studied, with histochemical techniques, the relation between serotonin, some other small-molecule transmitters, and a number of neuropeptides in the dorsal raphe nucleus (DRN) and the adjacent ventral periaqueductal gray (vPAG) of mouse, an important question being to establish possible differences from rat. Even if similarly distributed, the serotonin neurons in mouse lacked the extensive coexpression of nitric oxide synthase and galanin seen in rat. Although partly overlapping in the vPAG, no evidence was obtained for the coexistence of serotonin with dopamine, substance P, cholecystokinin, enkephalin, somatostatin, neurotensin, dynorphin, thyrotropin-releasing hormone, or corticotropin-releasing hormone. However, some serotonin neurons expressed the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase (GAD). Work in other laboratories suggests that, as in rat, serotonin neurons in the mouse midline DRN express the vesicular glutamate transporter 3, presumably releasing glutamate. Our study also shows that many of the neuropeptides studied (substance P, galanin, neurotensin, dynorphin, and corticotropin-releasing factor) are present in nerve terminal networks of varying densities close to the serotonin neurons, and therefore may directly or indirectly influence these cells. The apparently low numbers of coexisting messengers in mouse serotonin neurons, compared to rat, indicate considerable species differences with regard to the chemical neuronatomy of the DRN. Thus, extrapolation of DRN physiology, and possibly pathology, from rat to mouse, and even human, should be made with caution.

GABA(B) receptor modulation of serotonin neurons in the dorsal raphé nucleus and escalation of aggression in mice

The serotonin (5-HT) system in the brain has been studied more than any other neurotransmitter for its role in the neurobiological basis of aggression. However, which mechanisms modulate the 5-HT system to promote escalated aggression is not clear. We here explore the role of GABAergic modulation in the raphé nuclei, from which most 5-HT in the forebrain originates, on escalated aggression in male mice. Pharmacological activation of GABA(B), but not GABA(A), receptors in the dorsal raphé nucleus (DRN) escalated aggressive behaviors. In contrast, GABA agonists did not escalate aggressive behaviors after microinjection into the median raphé nucleus. The aggression-heightening effect of the GABA(B) agonist baclofen depended on the activation of 5-HT neurons in the DRN because it was blocked by coadministration of the 5-HT(1A) agonist 8-OH-DPAT [((+/-)-8-hydroxy-2-(di-n-propylamino)tetralin) hydrobromide] (DPAT), which acts on autoreceptors and inhibits 5-HT neural activity. In vivo microdialysis showed that GABA(B) activation in the DRN increased extracellular 5-HT level in the medial prefrontal cortex. This may be attributable to an indirect action via presynaptic GABA(B) receptors. The presynaptic GABA(B) receptors suppress Ca(2+) channel activity and inhibit neurotransmission, and the coadministration of N-type Ca(2+) channel blocker facilitated the effect of baclofen. These findings suggest that the indirect disinhibition of 5-HT neuron activity by presynaptic GABA(B) receptors on non-5-HT neurons in the DRN is one of the neurobiological mechanisms of escalated aggression.

GABA(A) receptors in the dorsal raphé nucleus of mice: escalation of aggression after alcohol consumption

RATIONALE

The dorsal raphé nucleus (DRN), the origin for serotonin (5-HT) in forebrain areas, has been implicated in the neural control of escalated aggression. Gamma aminobutyric acid type-A (GABA(A)) and type-B (GABA(B)) receptors are expressed in the DRN and modulate 5-HT neuronal activity, and both play a role in the behavioral effect of alcohol.

OBJECTIVE

The purpose of this study is to examine the interaction between drugs acting on GABA receptors in the DRN and alcohol in their effects on aggressive behaviors.

METHOD

Male CFW mice, housed with a female, were trained to self-administer ethanol (1.0 g/kg) or water via an operant conditioning panel in their home cage. Immediately after they drank either ethanol or water, the animals were microinfused with a GABAergic drug into the DRN, and their aggressive behaviors were assessed 10 min later. Muscimol (0.006 nmol), a GABA(A) receptor agonist, escalated alcohol-heightened aggression but had no effect in the absence of ethanol. This effect of muscimol was prominent in the animals that showed alcohol-heightened aggression, but not the animals that reduced or did not change aggressive behavior after ethanol infusion compared to water. On the other hand, the GABA(B) agonist baclofen (0.06 nmol) increased aggressive behavior similarly in both water and ethanol conditions. Antagonists of the GABA(A) and GABA(B) receptors, bicuculline (0.006 nmol) and phaclofen (0.3 nmol) respectively, did not suppress heightened-aggressive behavior induced by ethanol self-administration.

CONCLUSION

GABA(A) receptors in the DRN are one of the neurobiological targets of alcohol-heightened aggression. Activation of the GABA(B) receptors in the DRN also produced escalated aggression, but that is independent of the effect of alcohol.

Neuroanatomic relationships between the GABAergic and serotonergic systems in the developing human medulla

gamma-Amino butyric (GABA) critically influences serotonergic (5-HT) neurons in the raphé and extra-raphé of the medulla oblongata. In this study we hypothesize that there are marked changes in the developmental profile of markers of the human medullary GABAergic system relative to the 5-HT system in early life. We used single- and double-label immunocytochemistry and tissue receptor autoradiography in 15 human medullae from fetal and infant cases ranging from 15 gestational weeks to 10 postnatal months, and compared our findings with an extensive 5-HT-related database in our laboratory. In the raphé obscurus, we identified two subsets of GABAergic neurons using glutamic acid decarboxylase (GAD65/67) immunostaining: one comprised of small, round neurons; the other, medium, spindle-shaped neurons. In three term medullae cases, positive immunofluorescent neurons for both tryptophan hydroxylase and GAD65/67 were counted within the raphé obscurus. This revealed that approximately 6% of the total neurons counted in this nucleus expressed both GAD65/67 and TPOH suggesting co-production of GABA by a subset of 5-HT neurons. The distribution of GABA(A) binding was ubiquitous across medullary nuclei, with highest binding in the raphé obscurus. GABA(A) receptor subtypes alpha1 and alpha3 were expressed by 5-HT neurons, indicating the site of interaction of GABA with 5-HT neurons. These receptor subtypes and KCC2, a major chloride transporter, were differentially expressed across early development, from midgestation (20 weeks) and thereafter. The developmental profile of GABAergic markers changed dramatically relative to the 5-HT markers. These data provide baseline information for medullary studies of human pediatric disorders, such as sudden infant death syndrome.

SDF-1alpha/CXCL12 enhances GABA and glutamate synaptic activity at serotonin neurons in the rat dorsal raphe nucleus

The serotonin (5-hydroxytryptamine; 5-HT) system has a well-characterized role in depression. Recent reports describe comorbidities of mood-immune disorders, suggesting an immunological component may contribute to the pathogenesis of depression as well. Chemokines, immune proteins which mediate leukocyte trafficking, and their receptors are widely distributed in the brain, mediate neuronal patterning, and modulate various neuropathologies. The purpose of this study was to investigate the neuroanatomical relationship and functional impact of the chemokine stromal cell-derived factor-1alpha/CXCL12 and its receptor, CXCR4, on the serotonin dorsal raphe nucleus (DRN) system in the rat using anatomical and electrophysiological techniques. Immunohistochemical analysis indicates that over 70% of 5-HT neurons colocalize with CXCL12 and CXCR4. At a subcellular level, CXCL12 localizes throughout the cytoplasm whereas CXCR4 concentrates to the outer membrane and processes of 5-HT neurons. CXCL12 and CXCR4 also colocalize on individual DRN cells. Furthermore, electrophysiological studies demonstrate CXCL12 depolarization of 5-HT neurons indirectly via glutamate synaptic inputs. CXCL12 also enhances the frequency of spontaneous inhibitory and excitatory postsynaptic currents (sIPSC and sEPSC). CXCL12 concentration-dependently increases evoked IPSC amplitude and decreases evoked IPSC paired-pulse ratio selectively in 5-HT neurons, effects blocked by the CXCR4 antagonist AMD3100. These data indicate presynaptic enhancement of GABA and glutamate release at 5-HT DRN neurons by CXCL12. Immunohistochemical analysis further shows CXCR4 localization to DRN GABA neurons, providing an anatomical basis for CXCL12 effects on GABA release. Thus, CXCL12 indirectly modulates 5-HT neurotransmission via GABA and glutamate synaptic afferents. Future therapies targeting CXCL12 and other chemokines may treat serotonin related mood disorders, particularly depression experienced by immune-compromised individuals.

Distinct transcriptomes define rostral and caudal serotonin neurons

The molecular architecture of developing serotonin (5HT) neurons is poorly understood, yet its determination is likely to be essential for elucidating functional heterogeneity of these cells and the contribution of serotonergic dysfunction to disease pathogenesis. Here, we describe the purification of postmitotic embryonic 5HT neurons by flow cytometry for whole-genome microarray expression profiling of this unitary monoaminergic neuron type. Our studies identified significantly enriched expression of hundreds of unique genes in 5HT neurons, thus providing an abundance of new serotonergic markers. Furthermore, we identified several hundred transcripts encoding homeodomain, axon guidance, cell adhesion, intracellular signaling, ion transport, and imprinted genes associated with various neurodevelopmental disorders that were differentially enriched in developing rostral and caudal 5HT neurons. These findings suggested a homeodomain code that distinguishes rostral and caudal 5HT neurons. Indeed, verification studies demonstrated that Hmx homeodomain and Hox gene expression defined an Hmx(+) rostral subtype and Hox(+) caudal subtype. Expression of engrailed genes in a subset of 5HT neurons in the rostral domain further distinguished two subtypes defined as Hmx(+)En(+) and Hmx(+)En(-). The differential enrichment of gene sets for different canonical pathways and gene ontology categories provided additional evidence for heterogeneity between rostral and caudal 5HT neurons. These findings demonstrate a deep transcriptome and biological pathway duality for neurons that give rise to the ascending and descending serotonergic subsystems. Our databases provide a rich, clinically relevant resource for definition of 5HT neuron subtypes and elucidation of the genetic networks required for serotonergic function.

Serotonin and GABA are colocalized in restricted groups of neurons in the larval sea lamprey brain: insights into the early evolution of neurotransmitter colocalization in vertebrates

Colocalization of the classic neurotransmitters serotonin (5-HT) and gamma-aminobutyric acid (GABA) (or the enzyme that synthesizes the latter, glutamate decarboxylase) has been reported in a few neurons of the rat raphe magnus-obscurus nuclei. However, there are no data on the presence of neurochemically similar neurons in the brain of non-mammalian vertebrates. Lampreys are the oldest extant vertebrates and may provide important data on the phylogeny of neurochemical systems. The colocalization of 5-HT and GABA in neurons of the sea lamprey brain was studied using antibodies directed against 5-HT and GABA and confocal microscopy. Colocalization of the neurotransmitters was observed in the diencephalon and the isthmus. In the diencephalon, about 87% of the serotonergic cells of the rostral tier of the dorsal thalamus (close to the zona limitans) exhibited GABA immunoreactivity. In addition, occasional cells double-labelled for GABA and 5-HT were observed in the hypothalamic tuberal nucleus and the pretectum. Of the three serotonergic isthmic subgroups already recognized in the sea lamprey isthmus (dorsal, medial and ventral), such double-labelled cells were only observed in the ventral subgroup (about 61% of the serotonergic cells in the ventral subgroup exhibited GABA immunoreactivity). An equivalence between these lamprey isthmic cells and the serotonergic/GABAergic raphe cells of mammals is suggested. Present findings suggest that serotonergic/GABAergic neurons are more extensive in lampreys than in the rat and probably appeared before the separation of agnathans and gnathostomes. Cotransmission by release of 5-HT and GABA by the here-described lamprey brain neurons is proposed.

Intergeniculate leaflet and suprachiasmatic nucleus organization and connections in the golden hamster

The intergeniculate leaflet (IGL) is a distinct subdivision of the lateral geniculate complex which receives retinal input and projects upon a circadian pacemaker, the suprachiasmatic nucleus (SCN). In the present study, we have analyzed the organization of the IGL and its connections in the hamster, a species commonly used in circadian rhythm studies. The location of the IGL is defined by the presence of retinal afferents demonstrated by anterograde transport of cholera toxin-HRP, neuropeptide Y-containing neurons and axons, cells retrogradely labeled from the regions of the SCN and contralateral IGL, and substance P-containing axons. It is a long nucleus extending the entire rostrocaudal axis of the geniculate. The most rostral IGL lies between the lateral dorsal thalamus, ventrolateral part, and the horizontal cerebral fissure. It then enlarges ventral to the rostral dorsal lateral geniculate, medial to the optic tract. The mid-portion of the leaflet is a thin lamina intercalated between the dorsal and ventral geniculate nuclei. The extended caudal portion of the nucleus lies lateral and ventral to the medial geniculate and is contiguous with the zona incerta and the lateral terminal nucleus. The IGL contains populations of neuropeptide Y (NPY+) and enkephalin (ENK+) neurons which project to the retinorecipient portion of the SCN. In addition to the immunoreactive perikarya, the IGL contains plexuses of NPY+, ENK +, substance P-, serotonin-, and glutamic acid decarboxylase-immunoreactive axons.

Retrograde transport studies demonstrate that, in addition to the NPY+ neurons, there is a population of non-NPY+ neurons projecting upon the SCN. There is also a reciprocal projection upon the IGL from neurons in the SCN region, particularly the retrochiasmatic area. The hamster SCN differs from the rat in containing a distinct subdivision of substance P-immunoreactive neurons.

Ipsilateral and contralateral serotonergic projections from dorsal and median raphe nuclei to the forebrain in rats: immunofluorescence quantitative analysis

The objective of this study was to clarify the 5-HT projections from the right and left sides of the dorsal (DRD), ventral (DRV) and lateral (DRL) subdivisions of the middle level of the dorsal (DR) raphe nucleus and median (MR) raphe nucleus to the lateral septum (LS), preoptic area (POA) or ventromedial hypothalamus (VMH), which are important neural substrates for neuroendocrine regulation of reproduction. A retrograde neural tracer, Fluoro-Gold (FG), was infused into the right side of these regions in ovariectomized rats and the numbers of FG and/or 5-HT immunopositive cells in the right and left sides of the raphe nuclei were counted. It was found that the POA and VMH received more 5-HT projections than the LS from the DR and MR. In the subdivisions of the DR, 70% of all 5-HT projections from the DR to these 3 areas originated from the DRL. Furthermore, ipsilateral projections from the DR to the POA and VMH but not to the LS were dominant, compared to the contralateral projections. A right-left difference was not seen among the MR 5-HT projections. Thus, laterality of the projections is thought to be strong in the 5-HT clusters located far from the midline of the midbrain raphe nuclei.

Characterization of rat rostral raphe primary cultures: multiplex quantification of serotonergic markers

Previous reports establishing raphe cultures typically yield less than 1% serotonin (5-HT)-positive neurons and are impractical for transcriptional studies. In this study, we have established primary cultures enriched in 5-HT neurons and quantified the proportion of cells expressing serotonergic and non-serotonergic markers. We have also shown the feasibility of using the multiplex real-time PCR technique to measure the relative amounts of RNA for some of these markers. Rostral raphe cells derived from E13-15 rat embryos were cultured for 7 days and analyzed by quantitative immunofluorescence and western blot analysis. In these cultures, approximately 8% of neurons were immunopositive for serotonergic markers (5-HT or tryptophan hydroxylase (TPH)). The percentage of cells labeled for GFAP (glial marker), tyrosine hydroxylase (catecholaminergic), and GAD65/67 (GABAergic) was 5, 1, and 54%, respectively. Transcription factors REST/NRSF and Deaf-1 were present in 9 and 98% of cells, respectively. Multiplex quantitative RT-PCR (Q-PCR) analysis was done for TPH2, 5-HT1A receptor or Deaf-1 RNAs paired with GAPDH RNA as control. Using this approach, standard curves for each RNA were obtained over 200-fold concentration range of dilution with r2 values >0.99. The relative abundances determined by Q-PCR are consistent with the expression of TPH2>Deaf-1>5-HT1A receptor RNA in serotonergic raphe cells. The standard error of TPH2 RNA levels between cultures was <20%, indicating a consistent purity of 5-HT neurons. Thus, we have generated a highly consistent and reproducible model system that is enriched in 5-HT neurons and that will be valuable in future investigation of serotonergic regulation.

Ventilatory effects of muscimol microdialysis into the rostral medullary raphé region of conscious rats

We hypothesized that inhibition of the rostral medullary raphe region (MRR), a putative central chemoreceptor location, with the GABA(A) receptor agonist muscimol would decrease ventilatory responses to hypercapnia and hypoxia in conscious rats, and that its known effect at this site on body temperature might alter its effect upon these ventilatory responses. At ambient temperatures of 24.5-26.5 degrees C (Cool), microdialysis of 1mM muscimol into the MRR significantly decreased body temperature by approximately 0.5 degrees C, increased the ventilatory response to 7% CO(2) and decreased the response to 10% O(2). At ambient temperatures of 29.5-30.5 degrees C (Warm), 1 mM muscimol microdialysis no longer decreased body temperature and increased the ventilatory response to hypercapnia and to hypoxia. Muscimol did not significantly affect the VE/VO2 ratio at either temperature. Muscimol significantly increased the hypercapnic ventilatory responses in Cool and Warm conditions and the hypoxic response in Warm conditions, which indicates the presence of an inhibitory effect of rostral MRR neurons sensitive to muscimol. In the Cool condition the ventilatory response to hypoxia is inhibited but appropriately so for the lower VO2 .

Serotonin as a modulator of glutamate- and GABA-mediated neurotransmission: implications in physiological functions and in pathology

The neurotransmitter serotonin (5-HT), widely distributed in the central nervous system (CNS), is involved in a large variety of physiological functions. In several brain regions 5-HT is diffusely released by volume transmission and behaves as a neuromodulator rather than as a "classical" neurotransmitter. In some cases 5-HT is co-localized in the same nerve terminal with other neurotransmitters and reciprocal interactions take place. This review will focus on the modulatory action of 5-HT on the effects of glutamate and gamma-amino-butyric acid (GABA), which are the principal neurotransmitters mediating respectively excitatory and inhibitory signals in the CNS. Examples of interaction at pre-and/or post-synaptic levels will be illustrated, as well as the receptors involved and their mechanisms of action. Finally, the physiological meaning of neuromodulatory effects of 5-HT will be briefly discussed with respect to pathologies deriving from malfunctioning of serotonin system.

The wake-promoting peptide orexin-B inhibits glutamatergic transmission to dorsal raphe nucleus serotonin neurons through retrograde endocannabinoid signaling

The wake-promoting neuropeptides orexins (hypocretins) play a crucial role in controlling neuronal excitability and synaptic transmission in the CNS. In this study, using whole-cell patch-clamp recordings in an acute dorsal raphe nucleus (DRN) slice preparation, we report that orexin B (Orx-B) depresses the evoked glutamate-mediated synaptic currents in DRN 5-HT neurons. The Orx-B-induced depression is accompanied by an increase in the paired-pulse ratio and the coefficient of variance, suggesting a presynaptic site of action. Orx-B also reduces the frequency but not the amplitude of miniature EPSCs, indicating that depression of glutamatergic transmission is mediated by a decrease in glutamate release. Surprisingly, the Orx-B-induced inhibition of glutamatergic transmission is abolished by postsynaptic inhibition of G-protein signaling with GDPbetaS, suggesting that this effect is signaled by postsynaptic orexin receptors and expressed presynaptically, presumably through a retrograde messenger. Interestingly, the Orx-B-induced depression of glutamate release is mimicked and occluded by the cannabinoid receptor agonist WIN 55,212-2, and is abolished by the CB1 cannabinoid receptor antagonist AM 251. These results imply that the Orx-B-induced depression of glutamatergic transmission to DRN 5-HT neurons is mediated by retrograde endocannabinoid release. Examination of downstream signaling pathways involved in this response indicates that the effect of Orx-B requires the activation of phospholipase C and DAG lipase enzymatic pathways but not a rise in postsynaptic intracellular calcium. Therefore, our findings reveal a previously unsuspected mechanism by which postsynaptic orexin receptors can modulate glutamatergic synaptic transmission to DRN 5-HT neurons.

Facilitatory effects of WAY-100635, a 5-HT1A receptor antagonist, on lordosis in female rats

Effects of the serotonin (5-HT) receptor 1A antagonist, WAY-100635, on lordosis in female rats were examined. Ovariectomized rats were implanted with a silicon tube containing estradiol and behavioral tests were performed. Next, 5, 10 or 20 mg/kg bw WAY-100635 or saline was injected subcutaneously in female rats with a lordosis quotient (LQ) from 10 to 30 and the behavioral test was performed again. As a result, the mean LQs in 10 or 20 mg WAY-100635-treated groups were higher than in the saline-treated group (P < 0.01 and P < 0.05, 10 or 20 mg groups versus saline, respectively). In the experiment on the time-course of change in LQ after injection with 10 mg WAY-100635, the mean LQ was increased (P < 0.01, versus saline) 15 min after the injection and high levels persisted for 1 h. This finding shows that WAY-100635 has the potency to enhance lordosis behavior acutely in female rats with a low estrous state. In order to investigate relationships between the 5-HT(1A) receptor and the GABA(B) receptor in regulating lordosis, 10 mg baclofen, a GABA(B) receptor agonist, was injected and this was followed 1 h later by the injection of 10 mg/kg WAY-100635. Mean LQ decreased after the injection of baclofen (P < 0.0001, versus placebo-treated control), but the decrease in LQs was not reversed by injection with WAY-100635.

Effects of 5-HT1A-Receptor Agonist, 8-OH-DPAT, and GABAB-Receptor Agonist, Baclofen, on Lordosis in Female Rats With Lesions in Either the Dorsal Raphe Nucleus or Septum

The inhibitory effects of 8-OH-DPAT, a 5-HT1A-receptor agonist, and baclofen, a GABAB-receptor agonist, on lordosis were examined in estrogen and progesterone-treated ovariectomized rats with lesions in either the dorsal raphe nucleus (DRN) or septum and in rats with either sham lesions or no lesions. The first behavior test series was carried out 6 days after implantation of the rats with silicon tubes containing estradiol. Four hours after injection with 0.5 mg progesterone, behavioral tests were performed before and 30 min after an injection with 1 mg/kg body weight 8-OH-DPAT. As a result, the mean lordosis quotient (LQ)s were changed from 100 to less than 20 before and after the injection in all groups. These results suggest that 8-OH-DPAT acts on areas other than the DRN and the septum, leading to a decrease in lordosis. Two weeks after implantation with estradiol, the next behavioral test series was carried out after injection with progesterone. Behavioral tests were performed before and after an injection with 10 mg baclofen. The results showed that the mean LQs decreased after the injection in all groups, but the mean LQ in the DRN lesion group was higher than that in the sham groups. These results indicate that baclofen may act partially on the DRN in inhibiting lordosis in female rats.

Serotonergic and Nonserotonergic Dorsal Raphe Neurons Are Pharmacologically and Electrophysiologically Heterogeneous

The dorsal raphe nucleus (DRN) projects serotonergic axons throughout the brain and is involved in a variety of physiological functions. However, it also includes a large population of cells that contain other neurotransmitters. To clarify the physiological and pharmacological differences between the serotonergic and nonserotonergic neurons of the DRN, their postsynaptic responses to 5-hydroxytryptamine (5-HT, serotonin) and to selective activation of 5-HT1A or 5-HT2A/C receptors and their action potential characteristics were determined using in vitro patch-clamp recordings. The slices containing these neurons were then immunostained for tryptophan hydroxylase (TPH), a marker of serotonergic neurons. It was found that subpopulations of both serotonergic and nonserotonergic neurons responded to 5-HT with outward (i.e., inhibitory) and inward (i.e., excitatory) currents, responded to both 5-HT1A and 5-HT2A/C receptor activation with outward and inward currents, respectively, and displayed overlapping action potential characteristics. These findings suggest that serotonergic and nonserotonergic neurons in the DRN are both heterogeneous with respect to their individual pharmacological and electrophysiological characteristics. The findings also suggest that the activity of the different populations of DRN neurons will display heterogeneous changes when the serotonergic tone in the DRN is altered by neurological disorders or by drug treatment.

GABA receptor modulation of 5-HT neuronal firing: characterization and effect of moderate in vivo variations in glucocorticoid levels

Evidence from electrophysiological studies suggests that 5-HT neuronal firing in the dorsal raphe nucleus (DRN) may be regulated by both GABA(A) and GABA(B) receptors. Here, we addressed the question of whether the activity of individual 5-HT neurons is regulated by both GABA(A) and GABA(B) receptors. In addition, we examined the concentration-response relationships of GABA(A) and GABA(B) receptor activation and determined if GABA receptor regulation of 5-HT neuronal firing is altered by moderate alterations in circulating corticosterone. The activity of 5-HT neurons in the DRN of the rat was examined using in vitro extracellular electrophysiology. The firing of all individual neurons tested was inhibited by both the GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo-[5,4-c]-pyridin-3-ol hydrochloride (THIP) (25 microM) and the GABA(B) receptor agonist baclofen (1 microM). Responses to THIP (5, 10, 25 microM) and baclofen (1, 3, 10 microM) were concentration dependent and attenuated by the GABA(A) and GABA(B) receptor antagonists, bicuculline (50 microM) and phaclofen (200 microM), respectively. To examine the effects of corticosterone on the sensitivity of 5-HT neurons to GABA receptor activation, experiments were conducted on adrenalectomized animals with corticosterone maintained for two weeks at either a low or moderate level within the normal diurnal range. These changes in corticosterone levels had no significant effects on the 5-HT neuronal response to either GABA(A) or GABA(B) receptor activation. The data indicate that the control of 5-HT neuronal activity by GABA is mediated by both GABA(A) and GABA(B) receptors and that this control is insensitive to moderate changes in circulating glucocorticoid levels.

Differential expression of 5HT-1A, alpha 1b adrenergic, CRF-R1, and CRF-R2 receptor mRNA in serotonergic, gamma-aminobutyric acidergic, and catecholaminergic cells of the rat dorsal raphe nucleus

The dorsal raphe nucleus (DR) has a topographic neuroanatomy consistent with the idea that different parts of this nucleus subserve different functions. Here we use dual in situ hybridization to describe the rostral-caudal neurochemical distribution of three major cell groups, serotonin (5-hydroxytryptamine; 5-HT), gamma-aminobutyric acid (GABA), and catecholamine, and their relative colocalization with each other and mRNA encoding four different receptor subtypes that have been described to influence DR responses, namely, 5HT-1A, alpha(1b) adrenergic (alpha(1b) ADR), and corticotropin-releasing factor type 1 (CRF-R1) and 2 (CRF-R2) receptors. Serotonergic and GABAergic neurons were distributed throughout the rostral-caudal extent of the DR, whereas catecholaminergic neurons were generally restricted to the rostral half of the nucleus. These phenotypes essentially represent distinct cell populations, because the neurochemical markers were rarely colocalized. Both 5HT-1A and alpha(1b) ADR mRNA were highly expressed throughout the DR, and the vast majority of serotonergic neurons expressed both receptors. A smaller percentage of GABAergic neurons also expressed 5HT-1A or alpha(1b) ADR mRNA. Very few catecholaminergic cells expressed either 5HT-1A or alpha(1b) ADR mRNA. CRF-R1 mRNA was detected only at very low levels within the DR, and quantitative colocalization studies were not technically feasible. CRF-R2 mRNA was mainly expressed at the middle and caudal levels of the DR. At midlevels, CRF-R2 mRNA was expressed exclusively in serotonin neurons, whereas, at caudal levels, approximately half the CRF-R2 mRNA was expressed in GABAergic neurons. The differential distribution of distinct neurochemical phenotypes lends support to the idea of functional differentiation of the DR.

Sensorimotor-related discharge of simultaneously recorded, single neurons in the dorsal raphe nucleus of the awake, unrestrained rat

Multi-channel, multi-neuron recording procedures were used to monitor simultaneously the spike train activity of single neurons (n=7-16 cells/animal) in the dorsal raphe (DR) nucleus of the awake, freely moving rat. Putative serotonergic and non-serotonergic neurons were distinguished from one another on the basis of established criteria, i.e. waveform shape and duration, firing pattern and firing frequency. As a group, presumed serotonergic neurons exhibited low tonic discharge rates, depressed firing after serotonin (5HT)-1a agonist administration, and, except for the transition from sleep to waking, a general insensitivity to specific sensory or motor events. By contrast, non-serotonergic cells in midline and lateral wing sub-regions of the nucleus displayed responses to a variety of sensorimotor events including locomotion, grooming, head movement, chewing, auditory stimuli, and whisker movement (both passive and active). However, within this latter group, the sensorimotor response repertoire of individual cells was not uniform. Likewise, non-5HT cells with diverse response profiles were identified in both medial and lateral sub-regions of the nucleus. Cells categorized as non-serotonergic also had varied responses to 5HT1a agonist administration. These results emphasize the diverse input/output relationships of individual DR neurons and underscore the need for a more comprehensive analysis of such properties under waking conditions in order to obtain a better understanding of the role of the DR nucleus in brain function.

The modulation by 5-HT of glutamatergic inputs from the raphe pallidus to rat hypoglossal motoneurones, in vitro

Decreases in the activity of 5-HT-containing caudal raphe neurones during sleep are thought to be partially responsible for the resultant disfacilitation of hypoglossal motoneurones. Whilst 5-HT has a direct excitatory action on hypoglossal motoneurones as a result of activation of 5-HT2 receptors, microinjection of 5-HT2 antagonists into the hypoglossal nucleus reduces motor activity to a much lesser extent compared to the suppression observed during sleep suggesting other transmitters co-localised in caudal raphe neurones may also be involved. The aim of the present study was therefore to characterise raphe pallidus inputs to hypoglossal motoneurones. Whole cell recordings were made from hypoglossal motoneurones in vitro. 5-HT evoked a direct membrane depolarisation (8.45 +/- 3.8 mV, P < 0.001) and increase in cell input resistance (53 +/- 40 %, P < 0.001) which was blocked by the 5-HT2 antagonist, ritanserin (2.40 +/- 2.7 vs. 7.04 +/- 4.6 mV). Stimulation within the raphe pallidus evoked a monosynaptic EPSC that was significantly reduced by the AMPA/kainate antagonist, NBQX (22.8 +/- 16 % of control, P < 0.001). In contrast, the 5-HT2 antagonist, ritanserin, had no effect on the amplitude of these EPSCs (106 +/- 31 % of control, P = n.s.). 5-HT reduced these EPSCs to 50.0 +/- 13 % of control (P < 0.001), as did the 5-HT1A agonist, 8-OH-DPAT (52.5 +/- 17 %, P < 0.001) and the 5-HT1B agonist, CP 93129 (40.6 +/- 29 %, P < 0.01). 8-OH-DPAT and CP 93129 increased the paired pulse ratio (1.38 +/- 0.27 to 1.91 +/- 0.54, P < 0.05 & 1.27 +/- 0.08 to 1.44 +/- 0.13, P < 0.01 respectively) but had no effect on the postsynaptic glutamate response (99 +/- 4.4 % and 100 +/- 2.5 %, P = n.s.). They also increased the frequency (P < 0.001), but not the amplitude, of miniature glutamatergic EPSCs in hypoglossal motoneurones. These data demonstrate that raphe pallidus inputs to hypoglossal motoneurones are predominantly glutamatergic in nature, with 5-HT decreasing the release of glutamate from these projections as a result of activation of 5-HT1A and/or 5-HT1B receptors located on presynaptic terminals.

Differential expression of nitric oxide in serotonergic projection neurons: neurochemical identification of dorsal raphe inputs to rodent trigeminal somatosensory targets

The dorsal raphe (DR) is invested with nitric oxide synthase (NOS)-expressing profiles. To characterize the connections of NO-containing cells and further assess neurochemical relationships maintained by DR, the transmitter identity of the raphe projection to the trigeminal somatosensory system was examined. Rats were injected with retrograde tracer into vibrissae-related target areas or with anterograde tracer into DR. NADPH-diaphorase (NADPHd) histochemistry or NOS-immunostaining was combined with serotonin (5HT)- or serotonin transporter (SERT)-immunolabeling to examine: 1) the presence of NO in 5HT-containing axons from DR; 2) the distribution of NO-containing fibers with respect to other nitrergic profiles in the somatosensory system; and 3) the propensity for individual projection neurons in specific subdivisions of DR to colocalize 5HT and NO. Results confirm that "barrel-like" patches can be identified in several adult trigeminal relay nuclei by NADPHd histochemistry and demonstrate that fibers from DR contain 5HT and NO. Observations include a high percentage of cortical midline projection neurons which contained NADPHd (70-80%) and coexpressed 5HT. In contrast, approximately 40% of retrogradely labeled DR-thalamus cells in the lateral wing demonstrated NADPHd or 5HT expression, but not both in the same neuron. Colocalization of NADPHd and 5HT within individual DR projection neurons indicates that: i) DR is a source of nitrergic input to trigeminal structures, and ii) NO and 5HT may be simultaneously released to influence information-processing within somatosensory targets. Disparities in NADPHd expression between retrogradely labeled DR neuronal subpopulations further suggest functional differences in the impact of NO on cortical and subcortical targets.

GABAB receptor mRNA in the raphe nuclei: co-expression with serotonin transporter and glutamic acid decarboxylase

We have used double-label in situ hybridization techniques to examine the cellular localization of GABAB receptor mRNA in relation to serotonin transporter mRNA and glutamic acid decarboxylase mRNA in the rat dorsal raphe, median raphe and raphe magnus nuclei. The degree of cellular co-localization of these markers notably varied among the different nuclei. In the dorsal raphe, cell bodies showing GABAB receptor mRNA were very abundant, the 85% being also labelled for serotonin transporter mRNA, and a low proportion (5%) showing glutamic acid decarboxylase mRNA. In the median raphe, the level of co-expression of GABAB receptor mRNA with serotonin transporter mRNA was significantly lower. Some cells were also identified that contained GABAB receptor mRNA in the absence of either one of the other mRNA species studied. Our results support the presence of GABAB receptors in serotonergic as well as GABAergic neurones in the dorsal and median raphe, providing the anatomical basis for the reported dual inhibitory/disinhibitory effect of the GABAB agonist baclofen on serotonergic function.

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.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Control of the serotonergic system by the medial prefrontal cortex: potential role in the etiology of PTSD and depressive disorders

The prefrontal cortex is involved in an array of higher brain functions that are altered in psychiatric disorders. Serotonergic neurons of the midbrain rapbe nuclei innervate the prefrontal cortex and are the cellular target for drugs used to treat mood disorders such as the selective serotonin (5-HT) reuptake inhibitors. Anatomical evidence supports the existence of projections from the medial prefrontal cortex (mPFC) to the dorsal raphe nucleus (DR). We report on a functional control of the activity of DR 5-HT neurons by projection neurons in the mPFC. The stimulation of the mPFC elicits two types of responses in DR 5-HT neurons, orthodromic excitations and inhibitions. Excitations are mediated by AMPA/KA and NMDA receptors whereas inhibitions are mediated by GABA(A) and 5-HT(1A) receptors. The activation of a subgroup of 5-HT neurons increases 5-HT release which subsequently activates 5-HT(1A) autoreceptors on other 5-HT neurons. GABA(A)-mediated inhibitions involve GABAergic elements in the DR or adjacent areas. Pyramidal neurons of the mPFC co-express postsynaptic 5-HT(1A) (inhibitory) and 5-HT(2A) (excitatory) receptors. Consistent with the above observations, the selective activation of both receptors in mPFC reduced and increased, respectively, the firing activity of DR 5-HT neurons and the 5-HT release in mPFC. Overall, these data indicate that the activity of the 5-HT system is strongly controlled by the mPFC. Thus, the abnormal prefrontal function in post-traumatic stress disorder and depressive patients may induce a disregulation of 5-HT neurons projecting to other brain areas that can underlie the existing symptomatology in these psychiatric disorders.

Localization of 5-HT(2A) receptors on dopamine cells in subnuclei of the midbrain A10 cell group

Considerable evidence suggests that a dysfunction of the dopamine and serotonin (5-hydroxytryptamine or 5-HT) neurotransmitter systems contributes to a diverse range of pathological conditions including schizophrenia, depression and drug abuse. Recent electrophysiological and behavioral studies suggest that 5-HT modulates dopaminergic neurons in the ventral tegmental area via activation of 5-HT(2A) receptors. It is currently unknown if 5-HT(2A) receptors mediate their actions on dopaminergic neurons in the ventral tegmental area via direct or indirect mechanisms. This study investigated whether 5-HT(2A) receptors were localized on dopamine cells within the A10 dopamine subnuclei of the rat, including the ventral tegmental area. We discovered that 5-HT(2A) receptor-like immunoreactivity colocalized with tyrosine hydroxylase, a marker for dopamine neurons, throughout the A10 dopamine cell population. Colocalization was most prominent in rostral and mid A10 regions, including the paranigral, parabrachial, and interfascicular subnuclei. Though more rare, non-dopaminergic neurons also expressed 5-HT(2A) receptor immunoreactivity in the ventral tegmental area. Additionally, although a dense population of 5-HT(2A) immunoreactive cells was observed in the rostral dorsal raphe nucleus, rarely were these cells immunoreactive for tyrosine hydroxylase. The linear raphe A10 dopamine subdivisions also displayed a low degree of 5-HT(2A) receptor and tyrosine hydroxylase colocalization. These findings provide an anatomical basis for the physiological modulation of dopamine neurons in the rostral ventral tegmental area either directly, by 5-HT(2A) receptors localized on dopamine cells, or indirectly, through a non-dopaminergic mechanism. Interestingly, 5-HT(2A) receptors were expressed on dopamine neurons in several A10 subnuclei that project to mesolimbic forebrain regions implicated in drug addiction, and recent evidence indicates that ventral tegmental area 5-HT(2A) receptor activation may modulate reward-related behavior in rodents. 5-HT(2A) receptors were also expressed on dopamine cells in A10 subnuclei that project to forebrain areas that have been implicated in schizophrenia, and atypical antipsychotic drugs have high affinities for 5-HT(2A) receptors. Thus, findings in this study could have important implications for understanding 5-HT and dopamine circuitry dysfunction in schizophrenia.

GABAergic Interneurons are the Major Postsynaptic Targets of Median Raphe Afferents in the Rat Dentate Gyrus

The termination pattern of median raphe axons was studied in the rat dentate gyrus using Phaseolus vulgaris leucoagglutinin as an anterograde tracer, in combination with postembedding immunostaining for gamma-amino-butyric acid (GABA), and pre-embedding immunostaining for calbindin D28k, parvalbumin and GABA. Postembedding immunogold staining for GABA revealed that the majority (73.7%) of anterogradely labelled median raphe boutons make synaptic contacts with GABA-immunoreactive postsynaptic targets, mainly with dendritic shafts and perikarya. Pre-embedding immunocytochemical double staining for the anterograde tracer and GABA confirmed the electron microscopic results and showed that varicose median raphe axons establish multiple contacts with fusiform interneurons in the hilus and different types of basket cells in the granule cell layer. Some of the innervated cells were shown to contain calbindin D28k, whereas GABAergic interneurons containing another calcium-binding protein, parvalbumin, were never seen to receive multiple contacts from axons of raphe origin. Our results suggest that serotonergic median raphe fibres influence the firing of dentate granule cells via local inhibitory interneurons. The mechanism of using these interneurons with extensive local connections as monosynaptic targets may explain the great efficacy of this pathway in the control of hippocampal electrical activity.

Conditioned-fear stress increases Fos expression in monoaminergic and GABAergic neurons of the locus coeruleus and dorsal raphe nuclei

Many studies have demonstrated that physical or psychological stress can increase Fos expression in brainstem monoaminergic nuclei. Little is known, however, about the extent to which stress increases the expression of Fos in monoaminergic and nonmonoaminergic neurons in the brainstem. We examined the effects of conditioned-fear (CF) stress following mild footshock (FS) as unconditioned stress on Fos expression in the monoaminergic and GABAergic neurons of the ventral tegmental area (VTA), locus coeruleus (LC), and dorsal raphe nucleus (DR) in rats. The CF stress significantly increased the number of Fos-positive (Fos+) cells in both the LC and DR, whereas it did not increase the number in the VTA. Using a double-labeling technique, we combined Fos immunostaining with that for tyrosine hydroxylase (TH), serotonin (5-HT), or GABA for histochemical identification of the CF stress-induced Fos+ neurons. The percentage of TH/Fos double-labeled cells resulting from CF stress was 63% of the Fos+ cells in the LC, whereas 52% of the Fos+ cells contained 5-HT in the DR. We also found that approximately 60% of the CF stress-induced Fos+ cells were GABAergic neurons in these brain regions. These results indicate that CF stress induces intense Fos expression in the noradrenergic LC and serotonergic DR neurons, but not in the dopaminergic VTA neurons. They also indicate that not only monoaminergic neurons but also GABAergic neurons within the LC and DR are activated by the stress.

Co-localization of histamine with GABA but not with galanin in the human tuberomamillary nucleus

The presence of GABA and galanin in histaminergic neurons was previously reported in the rodent brain but whether such co-localizations also occur in the human brain was not known. We used in situ hybridization histochemistry and immunohistochemistry to study the co-localization of histamine with GABA and galanin in neurons of the tuberomamillary nucleus of adult human posterior hypothalamus. On consecutive formalin-fixed paraffin-embedded sections, co-localization was assessed using the in situ hybridization for L-histidine decarboxylase mRNA and immunocytochemistry for glutamate decarboxylase-67 kDa or galanin in the two profiles of same cell. The pattern of distribution and number of histaminergic neurons identified by in situ hybridization of the synthesizing enzyme gene transcripts were in accordance with data reported for histamine immunoreactivity. The great majority of neurons within the main divisions of the tuberomamillary nucleus containing L-histidine decarboxylase mRNA was also immunoreactive for glutamate decarboxylase-67 kDa. The range of co-localization of the two markers varied from 72% in the lateral part, to 75-87% in the medial part and 83-88% in the ventral part. In contrast, no cell containing L-histidine decarboxylase mRNA was immunoreactive for galanin. We conclude that tuberomamillary neurons in human co-express histamine with GABA but, unlike the neurons in rodents, do not express galanin, indicating that neurotransmitter co-localization patterns differ in the two species.

Vesicular glutamate transporter DNPI/VGLUT2 mRNA is present in C1 and several other groups of brainstem catecholaminergic neurons

The mouse glutamate vesicular transporter VGLUT2 has recently been characterized. The rat homolog of VGLUT2, differentiation-associated Na(+)/P(i) cotransporter (DNPI), was examined using a digoxigenin-labeled DNPI/VGLUT2 cRNA probe in the present study to determine which, if any, of the various groups of pontine or medullary monoaminergic neurons express DNPI/VGLUT2 mRNA and, thus, are potentially glutamatergic. DNPI/VGLUT2 mRNA was widely distributed within the brainstem and seemed exclusively neuronal. By using a double in situ hybridization method, the presence of the mRNA for DNPI/VGLUT2 and glutamic acid decarboxylase (GAD)-67 was mutually exclusive. By combining DNPI/VGLUT2 mRNA detection and conventional immunohistochemistry, DNPI/VGLUT2 mRNA was undetectable in lower brainstem cholinergic and serotonergic cells, but it was present in several tyrosine hydroxylase-immunoreactive (TH-ir) cell groups. DNPI/VGLUT2 mRNA was detected in most of the adrenergic neurons of the C1, C2, and C3 groups (75-80% of TH-ir neurons), in the A2 noradrenergic group (80%), and in vast numbers of area postrema cells. Within the A1 region, many fewer TH-ir cells contained DNPI/VGLUT2 (16%). Finally, DNPI/VGLUT2 mRNA was undetectable in the pontine noradrenergic cell groups (A5 and A6/locus coeruleus). In conclusion, the general pattern of DNPI/VGLUT2 expression and its exclusion from GABAergic, cholinergic, and serotonergic neurons supports the notion that DNPI/VGLUT2 mRNA identifies a subset of glutamatergic neurons in the lower brainstem. Within this region several catecholaminergic cell groups appear to be glutamatergic, including but not limited to the adrenergic cell groups C1-C3. Based on the present evidence, the noradrenergic cell groups of the pons (A5 and A6) do not contain either known vesicular glutamate transporter and are most likely not glutamatergic.

Control of dorsal raphe serotonergic neurons by the medial prefrontal cortex: Involvement of serotonin-1A, GABA(A), and glutamate receptors

Anatomical evidence indicates that medial prefrontal cortex (mPFC) neurons project to the dorsal raphe nucleus (DR). In this study, we functionally characterized this descending pathway in rat brain. Projection neurons in the mPFC were identified by antidromic stimulation from the DR. Electrical stimulation of the mPFC mainly inhibited the activity of DR 5-HT neurons (55 of 66). Peristimulus time histograms showed a silence of 150 +/- 9 msec poststimulus (latency, 36 +/- 1 msec). The administration of WAY-100635 and picrotoxinin partly reversed this inhibition, indicating the involvement of 5-HT(1A) and GABA(A) receptors. In rats depleted of 5-HT with p-chlorophenylalanine, the electrical stimulation of mPFC mainly activated 5-HT neurons (31 of 40). The excitations (latency, 17 +/- 1 msec) were antagonized by MK-801 and NBQX. Likewise, MK-801 prevented the rise in DR 5-HT release induced by electrical stimulation of mPFC. The application of 8-OH-DPAT in mPFC significantly inhibited the firing rate of DR 5-HT neurons and, in dual-probe microdialysis experiments, reduced the 5-HT output in mPFC and DR. Furthermore, the application of WAY-100635 in mPFC significantly antagonized the reduction of 5-HT release produced by systemic 8-OH-DPAT administration in both areas. These results indicate the existence of a complex regulation of DR 5-HT neurons by mPFC afferents. The stimulus-induced excitation of some 5-HT neurons by descending excitatory fibers releases 5-HT, which inhibits the same or other DR neurons by acting on 5-HT(1A) autoreceptors. Afferents from the mPFC also inhibit 5-HT neurons through the activation of GABAergic interneurons. Ascending serotonergic pathways may control the activity of this descending pathway by acting on postsynaptic 5-HT(1A) receptors.

Physiological and anatomic evidence for functional subclasses of serotonergic raphe magnus cells

Serotonergic cells in the medullary nucleus raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) project to the spinal cord where they are likely to modulate nociceptive transmission. Previous studies have suggested that these cells are physiologically and anatomically heterogeneous. In the present investigation, we examined whether subclasses of serotonergic RM and NRMC cells can be delineated based on their response to a visceral stimulus, and whether any such subclasses are morphologically distinct. Most RM and NRMC serotonergic cells tested (81 of 116) responded to retraction of the descending aorta into a polyethylene tube (the snare stimulus) with 57% of all cells tested excited and 13% inhibited. Responses of serotonergic cells to the snare outlasted the stimulus, were not reflective of evoked cardiovascular changes, and were observed in sino-aortic deafferented rats, evidence that the snare stimulus does not influence serotonergic cell discharge through activation of baroreceptors. Because serotonergic cells responsive to the snare were also responsive to mechanical brushing within the retroperitoneum, the snare is likely to change serotonergic cell discharge by means of the activation of mechanosensitive visceral afferents. Intracellular labeling of physiologically characterized serotonergic RM and NRMC cells showed that cells that were responsive to the snare stimulus had simpler axonal collateralization patterns than cells that were unresponsive to the snare stimulus. This association between morphological and physiological properties provides additional evidence that subpopulations of serotonergic cells exist and serve varied physiological functions.