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

Cellular basis for opioid potentiation in the rostral ventromedial medulla of rats with persistent inflammatory nociception

Direct inhibition of pain facilitatory neurons in the rostral ventromedial medulla (RVM) is one mechanism by which mu opioid receptor (MOPr) agonists are proposed to produce antinociception. The antinociceptive and anti-hyperalgesic effects of the MOPr agonist DAMGO are enhanced after intraplantar injection of complete Freund's adjuvant (CFA). This study therefore examined whether CFA treatment similarly enhanced the ability of DAMGO to induce outward currents in spinally projecting RVM neurons. It further examined whether the electrophysiological properties of RVM neurons are altered by CFA treatment. Whole-cell patch clamp recordings were made from three types of serotonergic as well as non-serotonergic spinally projecting RVM neurons obtained from control rats and rats 4h or four days after CFA. Persistent, but not acute inflammatory nociception increased the percentage of Type 2 non-serotonergic neurons that responded to DAMGO from 17% to 57% and the percentage of Type 3 serotonergic neurons that responded to DAMGO from 5% to 55%. These same two populations of RVM neurons exhibited significant differences in their passive membrane properties or spontaneous discharge rate. The outward currents produced by the GABA(B) receptor agonist baclofen were not enhanced, suggesting that the enhancement does not reflect global changes in levels of G(i/o) or activity of G-protein regulated inwardly rectifying potassium channels. These results provide a cellular basis for the enhanced anti-hyperalgesic and antinociceptive effects of MOPr agonists under conditions of persistent inflammatory nociception. These results also provide intriguing, albeit indirect, evidence for two different populations of pain facilitatory neurons in the RVM.

The structure of the dorsal raphe nucleus and its relevance to the regulation of sleep and wakefulness

Serotonergic (5-HT) cells in the rat dorsal raphe nucleus (DRN) appear in topographically organized groups. Based on cellular morphology, expression of other neurotransmitters, afferent and efferent connections and functional properties, 5-HT neurons of the DRN have been grouped into six cell clusters. The subdivisions comprise the rostral, ventral, dorsal, lateral, caudal and interfascicular parts of the DRN. In addition to 5-HT cells, neurons containing γ-aminobutyric acid (GABA), glutamate, dopamine, nitric oxide and the neuropeptides corticotropin-releasing factor, substance P, galanin, cholecystokinin, neurotensin, somatostatin, vasoactive intestinal peptide, neuropeptide Y, thyrotropin-releasing hormone, growth hormone, leu-enkephalin, met-enkephalin and gastrin have been characterized in the DRN. Moreover, numerous brain areas have neurons that project to the DRN and express monoamines (norepinephrine, histamine), amino acids (GABA, glutamate), acetylcholine or neuropeptides (orexin, melanin-concentrating hormone, corticotropin-releasing factor and substance P) that directly or indirectly, through local circuits, regulate the activity of 5-HT cells. The 5-HT cells predominate along the midline of the rostral, dorsal and ventral subdivisions of the DRN and outnumber the non-5-HT cells occurring in the raphe nucleus. The GABAergic and glutamatergic neurons are clustered mainly in the lateral and dorsal subdivisions of the DRN, respectively. The 5-HT(1A) receptor is located on the soma and the dendrites of 5-HT neurons and at postsynaptic sites (outside the DRN). It is expressed, in addition, by non-5-HT cells of the DRN. The 5-HT(1B) receptor is located at presynaptic and postsynaptic sites (outside the boundaries of the DRN). It has been described also in the ventromedial DRN where it is expressed by non-5-HT cells. The 5-HT(2A) and 5-HT(2C) receptors are located within postsynaptic structures. At the level of the DRN the 5-HT(2A) and 5-HT(2C) receptor-containing cells are predominantly GABAergic interneurons and projection neurons. Within the boundaries of the DRN the 5-HT(3) receptor is expressed by, among others, glutamatergic interneurons. 5-HT(7) receptors in the DRN are not localized to serotonergic neurons but, at least in part, to GABAergic cells and terminals. The complex structure of the DRN may have important implications for neural mechanisms underlying 5-HT modulation of wakefulness and REM sleep.

Effect of the CB(1) receptor antagonists rimonabant and AM251 on the firing rate of dorsal raphe nucleus neurons in rat brain slices

BACKGROUND AND PURPOSE

Previous studies have suggested a regulation of 5-hydroxytryptamine (5-HT) neurons by the endocannabinoid system. The aim of our work was to examine the effect of two CB(1) receptor antagonists, SR141716A (rimonabant, Sanofi-Synthélabo Recherche, Montpellier, France) and N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251, Tocris Cookson, Bristol, UK), on the firing rate of dorsal raphe nucleus (DRN) neurons.

EXPERIMENTAL APPROACH

Single-unit extracellular recordings were performed to study the effect of CB(1) receptor antagonists in slices of the DRN from rat brain.

KEY RESULTS

Rimonabant (1 microM) and AM251 (1 microM) decreased the firing rate of about 50% of all the recorded DRN 5-HT cells. The GABA(A)receptor antagonist picrotoxin (20 microM) (Sigma) prevented and also reversed the inhibitory effect of rimonabant (1 microM) and AM251 (1 microM), suggesting that CB(1) receptors regulate 5-HT neurons through the GABAergic system. However, the CB(1)/CB(2) receptor agonist R-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)-methyl]pyrrolol[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate salt (10 microM) (WIN55212-2, Sigma, St. Louis, MO, USA) failed to change the firing activity of non-5-HT (presumably GABAergic) neurons in the DRN. The endocannabinoid N-(2-hydroxyethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (anandamide, Tocris Cookson) (10 microM) also inhibited the firing activity of a number of 5-HT neurons, but this inhibition was not blocked by rimonabant (1 microM) or AM251 (1 microM), and the stable analogue R-(+) N-(2-hydroxy-1methylethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (methanandamide, Tocris Cookson) (10 microM) did not mimic this effect. The selective CB(1) receptor agonist arachidonoyl-2-chloroethylamide (ACEA) (1 microM) only slightly increased the firing rate of DRN 5-HT cells.

CONCLUSIONS AND IMPLICATIONS

These results suggest a tonic/constitutive regulation of DRN 5-HT neurons by the endocannabinoid system, which may occur through a CB(1) receptor-mediated inhibition of the GABAergic system. The inhibitory effect of anandamide may be mediated through a CB(1) receptor-independent mechanism.

Differential expression of membrane conductances underlies spontaneous event initiation by rostral midline neurons in the embryonic mouse hindbrain

Spontaneous activity is expressed in many developing CNS structures and is crucial in correct network development. Previous work using [Ca(2+)](i) imaging showed that in the embryonic mouse hindbrain spontaneous activity is initiated by a driver population, the serotonergic neurons of the nascent raphe. Serotonergic neurons derived from former rhombomere 2 drive 90% of all hindbrain events at E11.5. We now demonstrate that the electrical correlate of individual events is a spontaneous depolarization, which originates at the rostral midline and drives events laterally. Midline events have both a rapid spike and a large plateau component, while events in lateral tissue comprise only a smaller amplitude plateau. Lateral cells have a large resting conductance and are highly coupled via neurobiotin-permeant gap junctions, while midline cells are significantly less gap junction-coupled and uniquely express a T-type Ca(2+) channel. We propose that the combination of low resting conductance and expression of T-type Ca(2+) current is permissive for midline neurons to acquire the initiator or driver phenotype, while cells without these features cannot drive activity. This demonstrates that expression of specific conductances contributes to the ability to drive spontaneous activity in a developing network.

Influence of sex and corticotropin-releasing factor pathways as determinants in serotonin sensitivity

Stress sensitivity and sex are predictive factors in affective disorder susceptibility. Serotonin (5-HT) pathway recruitment by corticotropin-releasing factor (CRF) during stress is necessary in adaptive coping behaviors, but sex differences in such responses have not been investigated. Using selective 5-HT reuptake inhibitor (SSRI) administration to acutely elevate 5-HT in a genetic model of stress sensitivity, we examined behavioral and physiological responses in male and female stress-sensitive CRF receptor-2-deficient (R2KO) mice. Chronic SSRI treatment was used to confirm outcomes were specific to acute 5-HT elevation and not antidepressant efficacy. We hypothesized that R2KO mice would show a greater sensitivity to acute changes in 5-HT and that, because females typically are more stress sensitive, R2KO females would be the most responsive. Our results supported this hypothesis because females of both genotypes and R2KO males showed a greater sensitivity to an acute 10 mg/kg dose of citalopram in a tail suspension test, displaying decreased immobile time and increased latency to immobility. Furthermore, acute citalopram promoted significant anxiogenic-like effects that were specific to R2KO females in the elevated plus maze and light-dark box tests. Chronic citalopram did not produce these behavioral changes, supporting specificity to acute 5-HT modulation. Mechanistically, females had decreased hippocampal 5-HT transporter (SERT) levels, whereas R2KO mice showed reduced SERT in the prefrontal cortex, supporting a possible intersection of sex and genotype where R2KO females would have the lowest SERT to be blocked by the SSRI. This sensitivity to 5-HT-mediated anxiety in females may underlie a heightened vulnerability to stress-related affective disorders.

Aggressive encounters alter the activation of serotonergic neurons and the expression of 5-HT1A mRNA in the hamster dorsal raphe nucleus

Serotonergic (5-HT) neurons in the dorsal raphe nucleus (DRN) have been implicated in stress-induced changes in behavior. Previous research indicates that stressful stimuli activate 5-HT neurons in select subregions of the DRN. Uncontrollable stress is thought to sensitize 5-HT neurons in the DRN and allow for an exaggerated 5-HT response to future stimuli. In the current study, we tested the hypothesis that following aggressive encounters, losing male Syrian hamsters would exhibit increased c-Fos immunoreactivity in 5-HT DRN neurons compared to winners or controls. In addition, we tested the hypothesis that losers would have decreased 5-HT1A mRNA levels in the DRN compared to winners or controls. We found that a single 15-min aggressive encounter increased c-Fos expression in 5-HT and non-5-HT neurons in losers compared to winners and controls. The increased c-Fos expression in losers was restricted to ventral regions of the rostral DRN. We also found that four 5-min aggressive encounters reduced total 5-HT1A mRNA levels in the DRN in losers compared to winners and controls, and that differences in mRNA levels were not restricted to specific DRN subregions. These results suggest that social defeat activates neurons in select subregions of the DRN and reduces message for DRN 5-HT1A autoreceptors. Our results support the hypothesis that social stress can activate 5-HT neurons in the DRN, reduce 5-HT1A autoreceptor-mediated inhibition, and lead to hyperactivity of 5-HT neurons.

Failure to mount adaptive responses to stress results in dysregulation and cell death in the midbrain raphe

Stress is a common trigger in affective disorder onset, yet the mechanism and predisposing factors of vulnerability remain unknown. Effective disease prevention requires a critical balance of responses within the serotonergic raphe nucleus, including a coordination of corticotropin-releasing factor (CRF) actions at both of its receptors, CRF receptor-1 and CRF receptor-2. Mice deficient in CRF receptor-2 (R2KO) were used as a model of maladaptive stress responsivity to examine the physiological and molecular markers of stress dysregulation within the raphe in the absence of this receptor. After chronic stress, R2KO mice failed to display the robust stress-mediated adaptations characteristic of control mice, including elevations in tryptophan hydroxylase-2 and CRF receptor-1 expression and concordant increases in behavioral arousal. As a further indication of failed homeostatic mechanisms, R2KO mice displayed indices of cell death in the raphe after stress exposure, with elevations in proapoptotic factors but a failure to mount adaptive increases in antiapoptotic factors found in control mice. In vitro electrophysiological characterization of the specific influence of CRF on the raphe revealed both basal differences and a failure to respond to CRF administration in R2KO mice. These results support a requirement for homeostatic maintenance in response to stress in the raphe, where dysregulation may be a critical predictor of affective disorder onset.

Role of serotonergic input to the ventrolateral medulla in expression of the 10-Hz sympathetic nerve rhythm

We studied the changes in inferior cardiac sympathetic nerve discharge (SND) produced by unilateral microinjections of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists into the ventrolateral medulla (VLM) of urethane-anesthetized, baroreceptor-denervated cats. Microinjection of the 5-HT2 receptor antagonist LY-53857 (10 mM) into either the rostral or caudal VLM significantly reduced (P < or = 0.05) the 10-Hz rhythmic component of basal SND without affecting its lower-frequency, aperiodic component. The selective depression of 10-Hz power was accompanied by a statistically significant decrease in mean arterial pressure (MAP). Microinjection of LY-53857 into the VLM also attenuated the increase in 10-Hz power that followed tetanic stimulation of depressor sites in the caudal medullary raphé nuclei. Microinjection of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)2-amino-propane (DOI; 10 microM) into the VLM selectively enhanced 10-Hz SND, and intravenous DOI (1 mg/kg) partially reversed the reduction in 10-Hz SND produced by 5-HT2 receptor blockade in the VLM. Microinjection of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT; 10 mM), into either the rostral or caudal VLM also selectively attenuated 10-Hz SND and significantly reduced MAP. The reduction in 10-Hz SND produced by 8-OHDPAT was partially reversed by intravenous WAY-100635 (1 mg/kg), which selectively blocks 5-HT1A receptors. These results support the view that serotonergic inputs to the VLM play an important role in expression of the 10-Hz rhythm in SND.

Corticotropin-releasing factor 1 and 2 receptors in the dorsal raphé differentially affect serotonin release in the nucleus accumbens

Corticotropin-releasing factor (CRF) is a neurohormone that mediates stress, anxiety, and affects serotonergic activity. Studies have shown that CRF has dose-dependent opposing effects on serotonergic activity. This effect has been hypothesized to be differentially mediated by CRF(1) and CRF(2) receptors in the dorsal raphé nucleus. We directly tested this hypothesis by using in vivo microdialysis to determine the effects of CRF and CRF receptor antagonists in the dorsal raphé nucleus on serotonin (5-HT) release in the nucleus accumbens, a brain region implicated in the neuropathology of stress-related psychiatric disorders. Male urethane-anesthetized rats were implanted with a microdialysis probe into the nucleus accumbens, and CRF (0, 100 or 500 ng) was infused into the dorsal raphé. Infusion of CRF into the dorsal raphé nucleus had dose-dependent opposite effects, with 100 ng of CRF significantly decreasing 5-HT levels in the nucleus accumbens and 500 ng CRF significantly increasing accumbal 5-HT levels. In subsequent experiments, the raphé was pre-treated with the CRF(1) receptor antagonist antalarmin (0.25 microg) or the CRF(2) receptor antagonist antisauvagine-30 (ASV-30; 2 microg) prior to CRF infusion. Antagonism of CRF(1) receptors in the dorsal raphé nucleus abolished the decrease in accumbal 5-HT levels elicited by 100 ng CRF, and CRF(2) receptor antagonism in the raphé blocked the increase in accumbal 5-HT levels elicited by 500 ng CRF. These results suggest that the opposing effects of dorsal raphé CRF on 5-HT release in the nucleus accumbens are dependent on differential activation of CRF(1) and CRF(2) receptors in the dorsal raphé nucleus.

Early postnatal stress affects the serotonergic function in the median raphe nuclei of adult rats

Recent studies have focused on the serotonergic mechanism mediated via serotonin (5-HT) receptors underlying regulation of emotional stress during the developmental period. The present study was undertaken to elucidate whether early postnatal stress affects rat brain development and influences the serotonergic function in the midbrain median raphe nuclei (MRN) and dorsal raphe nuclei (DRN) in the adult, focusing on the response to unconditioned fear stress. Rats received aversive foot shock (FS) stimuli at the third week of the postnatal period (3wFS), but not those at the second week (2wFS), had increased percentage of time spent on open arms, estimated by the elevated plus maze test, at the postadolescent period (10-12 weeks old). The anxiolytic behavior observed in 3wFS was similar to that in rats having electrolytic lesion of the MRN, but not the DRN. In addition, the number of MRN 5-HT-immunoreactive cells in 3wFS remarkably was reduced compared to the non-FS control and 2wFS groups. These data suggest that aversive stress at the third week is attributable to the serotonergic function in the MRN underlying regulation of unconditioned fear stress. In other words, the "critical period" appears to be the time of neural circuit development of the MRN serotonergic system, which may be implicated in lifelong susceptibility to emotional stress.

Interleukin-1 inhibits firing of serotonergic neurons in the dorsal raphe nucleus and enhances GABAergic inhibitory post-synaptic potentials

In vitro electrophysiological data suggest that interleukin-1 may promote non-rapid eye movement sleep by inhibiting spontaneous firing of wake-active serotonergic neurons in the dorsal raphe nucleus (DRN). Interleukin-1 enhances GABA inhibitory effects. DRN neurons are under an inhibitory GABAergic control. This study aimed to test the hypothesis that interleukin-1 inhibits DRN serotonergic neurons by potentiating GABAergic inhibitory effects. In vitro intracellular recordings were performed to assess the responses of physiologically and pharmacologically identified DRN serotonergic neurons to rat recombinant interleukin-1beta. Coronal slices containing DRN were obtained from male Sprague-Dawley rats. The impact of interleukin-1 on firing rate and on evoked post-synaptic potentials was determined. Evoked post-synaptic potentials were induced by stimulation with a bipolar electrode placed on the surface of the slice ventrolateral to DRN. Addition of interleukin-1 (25 ng/mL) to the bath perfusate significantly decreased firing rates of DRN serotonergic neurons from 1.3 +/- 0.2 Hz (before administration) to 0.7 +/- 0.2 Hz. Electrical stimulation induced depolarizing evoked post-synaptic potentials in DRN serotonergic neurons. The application of glutamatergic and GABAergic antagonists unmasked two different post-synaptic potential components: a GABAergic evoked inhibitory post-synaptic potentials and a glutamatergic evoked excitatory post-synaptic potentials, respectively. Interleukin-1 increased GABAergic evoked inhibitory post-synaptic potentials amplitudes by 30.3 +/- 3.8% (n = 6) without affecting glutamatergic evoked excitatory post-synaptic potentials. These results support the hypothesis that interleukin-1 inhibitory effects on DRN serotonergic neurons are mediated by an interleukin-1-induced potentiation of evoked GABAergic inhibitory responses.

Morphological and physiological properties of serotonergic neurons in dissociated cultures from the postnatal rat dorsal raphe nucleus

We have developed dissociated primary cultures of the dorsal raphe nucleus from postnatal 9-12-day-old rats. The nucleus was dissected out from brain slices, dissociated, and cultured over a glial feeder layer. Serotonin immunocytochemistry revealed that 62% of cultured neurons were serotonergic. There was no significant difference in diameters between serotonergic and non-serotonergic neurons. With the whole-cell patch-clamp method, cultured neurons were tested for responses to 8-hydroxydipropylaminotetraline (8-OH-DPAT, a selective agonist for 5-HT(1A)), and then treated with serotonin immunocytochemistry. Ninety-two percent of neurons responding to 8-OH-DPAT were serotonergic. These results were used to identify serotonergic neurons. In most cases, serotonergic neurons did not show spontaneous firings of action potentials. Constant current depolarizations elicited trains of action potentials that usually did not show marked adaptation. Application of 8-OH-DPAT inhibited action potential firing. The current-voltage relation of the 8-OH-DPAT-induced current indicated an inward rectification with its reversal potential near E(K). Serotonergic neurons were depolarized by phenylephrine, bombesin, and gastrin-releasing peptide. This culture system will serve as a useful tool for elucidating the cellular, physiological, and molecular properties of brain serotonergic neurons.

Serotonergic raphe magnus cell discharge reflects ongoing autonomic and respiratory activities

Serotonergic cells are located in a restricted number of brain stem nuclei, send projections to virtually all parts of the CNS, and are critical to normal brain function. They discharge tonically at a rate modulated by the sleep-wake cycle and, in the case of medullary serotonergic cells in raphe magnus and the adjacent reticular formation (RM), are excited by cold challenge. Yet, beyond behavioral state and cold, endogenous factors that influence serotonergic cell discharge remain largely mysterious. The present study in the anesthetized rat investigated predictors of serotonergic RM cell discharge by testing whether cell discharge correlated to three rhythms observed in blood pressure recordings that averaged >30 min in length. A very slow frequency rhythm with a period of minutes, a respiratory rhythm, and a cardiac rhythm were derived from the blood pressure recording. Cross-correlations between each of the derived rhythms and cell activity revealed that the discharge of 38 of the 40 serotonergic cells studied was significantly correlated to the very slow and/or respiratory rhythms. Very few serotonergic cells discharged in relation to the cardiac cycle and those that did, did so weakly. The correlations between serotonergic cell discharge and the slow and respiratory rhythms cannot arise from baroreceptive input. Instead we hypothesize that they are by-products of ongoing adjustments to homeostatic functions that happen to alter blood pressure. Thus serotonergic RM cells integrate information about multiple homeostatic activities and challenges and can consequently modulate spinal processes according to the most pressing need of the organism.

Characterization of 4-(2-hydroxyphenyl)-1-[2'-[N-(2''-pyridinyl)-p-fluorobenzamido]ethyl]piperazine (p-DMPPF) as a new potent 5-HT1A antagonist

BACKGROUND AND PURPOSE

The identification of potent and selective radioligands for the mapping of 5-HT receptors is interesting both for clinical and experimental research. The aim of this study was to compare the potency of a new putative 5-HT(1A) receptor antagonist, p-DMPPF, (4-(2-hydroxyphenyl)-1-[2'-[N-(2''-pyridinyl)-p-fluorobenzamido]ethyl]piperazine) with that of the well-known 5-HT(1A) antagonists, WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide) and its fluorobenzoyl analogue, p-MPPF (4-(2-methoxyphenyl)-1-[2'-[N-(2''-pyridinyl)-p-fluorobenzamido]ethyl]piperazine).

EXPERIMENTAL APPROACH

Single cell extracellular recordings of dorsal raphe (DR) neurones were performed in rat brain slices. The potency of each compound at antagonizing the effect of the 5-HT(1A) agonist, 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)-tetraline], was quantified using the Schild equation. The pharmacological profile of p-DMPPF was defined using competition binding assays.

KEY RESULTS

Consistently with a 5-HT(1A) receptor antagonist profile, incubation of slices with an equimolar (10 nM) concentration of each compound markedly reduced the inhibitory effect of 8-OH-DPAT on the firing rate of DR neurones, causing a significant rightward shift in its concentration-response curve. The rank order of potency of the antagonists was WAY-100635>p-DMPPF>or=p-MPPF. The sensitivity of DR neurones to the inhibitory effect of 8-OH-DPAT was found to be heterogeneous. The binding experiments demonstrated that p-DMPPF is highly selective for 5-HT(1A) receptors, with a K(i) value of 7 nM on these receptors.

CONCLUSIONS AND IMPLICATIONS

The potency of the new compound, p-DMPPF, as a 5-HT(1A) antagonist is similar to that of p-MPPF in our electrophysiological assay. Its selectivity towards 5-HT(1A) receptors makes it a good candidate for clinical development.

Selective anterograde tracing of the individual serotonergic and nonserotonergic components of the dorsal raphe nucleus projection to the vestibular nuclei

It is well known that the dorsal raphe nucleus (DRN) sends serotonergic and nonserotonergic projections to target regions in the brain stem and forebrain, including the vestibular nuclei. Although retrograde tracing studies have reported consistently that there are differences in the relative innervation of different target regions by serotonergic and nonserotonergic DRN neurons, the relative termination patterns of these two projections have not been compared using anterograde tracing methods. The object of the present investigation was to trace anterogradely the individual serotonergic and nonserotonergic components of the projection from DRN to the vestibular nuclei in rats. To trace nonserotonergic DRN projections, animals were pretreated with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), and then, after 7 days, the anterograde tracer biotinylated dextran amine (BDA) was iontophoretically injected into the DRN. In animals treated with 5,7-DHT, nonserotonergic BDA-labeled fibers were found to descend exclusively within the ventricular plexus and to terminate predominantly within the periventricular aspect of the vestibular nuclei. Serotonergic DRN projections were traced by injecting 5,7-DHT directly into DRN, and amino-cupric-silver staining was used to visualize the resulting pattern of terminal degeneration. Eighteen hours after microinjection of 5,7-DHT into the DRN, fine-caliber degenerating serotonergic terminals were found within the region of the medial vestibular nucleus (MVN) that borders the fourth ventricle, and a mixture of fine- and heavier-caliber degenerating serotonergic terminals was located further laterally within the vestibular nuclear complex. These findings indicate that fine-caliber projections from serotonergic and nonserotonergic DRN neurons primarily innervate the periventricular regions of MVN, whereas heavier-caliber projections from serotonergic DRN neurons innervate terminal fields located in more lateral regions of the vestibular nuclei. Thus, serotonergic and nonserotonergic DRN axons target distinct but partially overlapping terminal fields within the vestibular nuclear complex, raising the possibility that these two DRN projection systems are organized in a manner that permits regionally-specialized regulation of processing within the vestibular nuclei.

Chronic low dose ovine corticotropin releasing factor or urocortin II into the rostral dorsal raphe alters exploratory behavior and serotonergic gene expression in specific subregions of the dorsal raphe

Corticotropin releasing factor (CRF) family peptides play key roles in integrating neural responses to stress. Both major CRF receptors have been pharmacologically identified in the dorsal raphe nucleus (DRN), a stress sensitive and internally heterogeneous nucleus supplying many forebrain regions with serotonergic input. Despite the involvement of chronic stress and serotonergic dysfunction in human mood and anxiety disorders, little is known about the effects of chronic CRF receptor activation on the DRN. We infused ovine CRF (1 ng/h), urocortin II (UCNII, 1 ng/h), or vehicle alone into rat DRN over 6 days. During infusion, animals were allowed to freely explore an open field for 15 min on each of 2 days, with the addition of a novel object on the second day. Following behavioral testing, 5-HT1A, 5-HT1B, 5-HT transporter (SERT), and tryptophan hydroxylase-2 (Tph2) expression was examined through the DRN by in situ hybridization. Ovine CRF infusion resulted in significantly decreased novel object touches, climbs, as well as increased latency to first novel object contact. UCNII had a similar but less dramatic effect, decreasing only climbing behavior. Both ovine CRF and UCNII blunted the decrease in corner time expected on re-exposure to the open field. Both peptides also produced regionally specific changes in gene expression: 5-HT1A expression was increased 30% in the mid-rostral ventromedial DRN, while SERT was decreased by 30% in the mid-caudal shell dorsomedial DRN. There also appeared to be a shift in the relative level of Tph2 expression between the ventromedial and core dorsomedial DRN at the mid-rostral level. Changes in 5-HT1A, SERT, and relative Tph2 mRNA abundance were correlated with novel object exploration. These findings suggest chronic intra-DRN administration of CRF agonists decreases exploratory behavior, while producing subregionally limited changes in serotonergic gene expression. These studies may be relevant to mechanisms underlying behavioral changes after chronic stress.

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.

Neurochemical identification of stereotypic burst-firing neurons in the rat dorsal raphe nucleus using juxtacellular labelling methods

Recent electrophysiological studies have discovered evidence of heterogeneity of 5-hydroxytryptamine (5-HT) neurons in the mesencephalic raphe nuclei. Of particular interest is a subpopulation of putative 5-HT neurons that display many of the electrophysiological properties of presumed 5-HT-containing neurons (regular and slow firing of single spikes with a broad waveform) but fire spikes in short, stereotyped bursts. In the present study we investigated the chemical identity of these neurons in rats utilizing in vivo juxtacellular labelling methods. Of ten dorsal raphe nucleus (DRN) neurons firing short stereotyped bursts within an otherwise regular firing pattern, all exhibited immunoreactivity for either 5-HT (n = 6) or the 5-HT synthesizing enzyme, tryptophan hydroxylase (TRH; n = 2) or both (n = 2). Supporting pharmacological experiments demonstrated that the burst firing DRN neurons demonstrated equal sensitivity to 5-HT(1A) agonism and alpha(1)-adrenoceptor antagonism to single spiking DRN neurons that we have previously identified as 5-HT-containing. Collectively these data provide direct evidence that DRN neurons that exhibit stereotyped burst firing activity are 5-HT containing. The presence of multiple types of electrophysiologically distinct midbrain 5-HT neurons is discussed.

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.

Differential effects of exposure to low-light or high-light open-field on anxiety-related behaviors: relationship to c-Fos expression in serotonergic and non-serotonergic neurons in the dorsal raphe nucleus

Serotonergic systems arising from the mid-rostrocaudal and caudal dorsal raphe nucleus (DR) have been implicated in the facilitation of anxiety-related behavioral responses to anxiogenic drugs or aversive stimuli. In this study we attempted to determine a threshold to engage serotonergic neurons in the DR following exposure to aversive conditions in an anxiety-related behavioral test. We manipulated the intensity of anxiogenic stimuli in studies of male Wistar rats by leaving them undisturbed (CO), briefly handling them (HA), or exposing them to an open-field arena for 15-min under low-light (LL: 8-13 lx) or high-light (HL: 400-500 lx) conditions. Rats exposed to HL conditions responded with reduced locomotor activity, reduced time spent exploring the center of the arena, a lower frequency of rearing and grooming, and an increased frequency of facing the corner of the arena compared to LL rats. Rats exposed to HL conditions had small but significant increases in c-Fos expression within serotonergic neurons in subdivisions of the rostral DR. Exposure to HL conditions did not alter c-Fos responses in serotonergic neurons in any other DR subdivision. In contrast, rats exposed to the open-field arena had increased c-Fos expression in non-serotonergic cells throughout the DR compared to CO rats, and this effect was particularly apparent in the dorsolateral part of the DR. We conclude that exposure to HL conditions, compared to LL conditions, increased anxiety-related behavioral responses in an open-field arena but this stimulus was at or below the threshold required to increase c-Fos expression in serotonergic neurons.

The progesterone metabolite allopregnanolone potentiates GABA(A) receptor-mediated inhibition of 5-HT neuronal activity

The dorsal raphe nucleus (DRN) is the origin of much of the 5-HT innervation of the forebrain. The activity of DRN 5-HT neurons is regulated by a number of receptors including GABA(A) and 5-HT(1A) inhibitory receptors and by excitatory alpha(1)-adrenoceptors. Using in vitro electrophysiological recording we investigated the action of progesterone and its metabolite, allopregnanolone on receptor-mediated responses of DRN 5-HT neurons. Neither allopregnanolone nor progesterone affected the alpha(1)-adrenoceptor agonist-induced firing. Allopregnanolone also had no effect on the inhibitory response to 5-HT. However, allopregnanolone significantly potentiated the inhibitory responses to GABA(A) receptor agonists. Progesterone did not enhance GABA(A) receptor-meditated inhibitory responses. Thus, the neuroactive metabolite of progesterone, allopregnanolone, has the ability to cause potentiation of GABA(A)-mediated inhibition of DRN 5-HT neurons. This effect on 5-HT neurotransmission may have relevance for mood disorders commonly associated with reproductive hormone events, such as premenstrual dysphoric disorder and postpartum depression.

Gender-dependent regulation of G-protein-gated inwardly rectifying potassium current in dorsal raphe neurons in knock-out mice devoid of the 5-hydroxytryptamine transporter

Agonists at G-protein-coupled receptors in neurons of the dorsal raphe nucleus (DRN) of knock-out mice devoid of the serotonin transporter (5-HTT(-/-)) exhibit lower efficacy to inhibit cellular discharge than in wild-type counterparts. Using patch-clamp whole-cell recordings, we found that a G-protein-gated inwardly rectifying potassium (GIRK) current is involved in the inhibition of spike discharge induced by 5-HT1A agonists (5-carboxamidotryptamine (5-CT) and (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT); 50 nM-30 microM) in both wild-type and 5-HTT(-/-) female and male mice. These effects were mimicked by 5'-guanylyl-imido-diphosphate (Gpp(NH)p; 400 microM) dialysis into cells with differences between genders. The 5-HTT(-/-) knock-out mutation reduced the current density induced by Gpp(NH)p in females but not in males. These data suggest that the decreased response of 5-HT1A receptors to agonists in 5-HTT(-/-) mutants reflects notably alteration in the coupling between G-proteins and GIRK channels in females but not in males. Accordingly, gender differences in central 5-HT neurotransmission appear to depend-at least in part-on sex-related variations in corresponding receptor-G protein signaling mechanisms.

Serotonin modulation of cell excitability and of [3H]GABA and [3H]D-aspartate efflux in primary cultures of rat cortical neurons

The effects of 5-hydroxytryptamine (5-HT) on neuronal excitability, evaluated as depolarization-induced firing rate, and on amino acid release, measured as electrically-evoked [(3)H]GABA and [(3)H]d-aspartate efflux, were investigated in rat primary cortical neuronal cultures. 5-HT displayed a concentration-dependent, bimodal effect on neuronal excitability: at 3-10microM it increased excitability through 5-HT(2A) receptors, and was blocked by the selective 5-HT(2A) antagonist MDL 100907, whereas at 30-100microM it reduced excitability through 5-HT(1A) receptors, and was, in turn, blocked by the selective 5-HT(1A) antagonist WAY 100135. The electrically-evoked [(3)H]GABA efflux was concentration-dependently inhibited by 5-HT (pEC(50)=4.74) and such inhibition was prevented by WAY 100135, but not by GR 55562, a selective 5-HT(1D/B) receptor antagonist. Conversely, 5-HT concentration-dependently increased stimulus-evoked [(3)H]d-aspartate efflux (pEC(50)=4.71). The increase was facilitated by methiothepin and was reversed into inhibition by ICS 205930, a selective 5-HT(3) receptor antagonist. In the presence of ICS 205930, the inhibition induced by 5-HT was prevented by the selective 5-HT(1D/B) receptor antagonist GR 55562, but not by WAY 100135. These findings suggest that 5-HT inhibits GABA release through 5-HT(1A) receptors and exerts a dual modulation on glutamate release, mostly facilitatory (through 5-HT(3) receptors) but also inhibitory (through 5-HT(1D/B) receptors), leading to a prevalently positive modulation of the excitatory signal by amino acid neurotransmitter containing neurons.

Simultaneous inhibition of caudal medullary raphe and retrotrapezoid nucleus decreases breathing and the CO2 response in conscious rats

The medullary raphe (MR) and the retrotrapezoid nucleus (RTN) in the ventral medulla are two of many central chemoreceptor sites. We examine their combined function in conscious rats by focal inhibition using microdialysis. Inhibition of RTN neurons with the GABA(A) receptor agonist muscimol, with simultaneous dialysis of artificial cerebrospinal fluid (ACSF) in or near to the caudal MR, causes hypoventilation (decrease in the ratio of minute ventilation to oxygen consumption, V(E)/V(O2)) and reduces the ventilatory response to 7% CO(2) by 24%. Inhibition of caudal MR serotonergic neurons with the 5-HT(1A) receptor agonist (R)-(+)-8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), with simultaneous dialysis of ACSF in or near to the RTN, causes hypoventilation but has no significant effect on the CO(2) response. Inhibition of both the RTN and the caudal MR simultaneously produces enhanced hypoventilation and a 51% decrease in the CO(2) response. The effects of treatment on the CO(2) response are similar in wakefulness and in non-rapid eye movement sleep. Comparison of the effect of 8-OH-DPAT microdialysed into a more rostral portion of the MR, where the CO(2) response is reduced by 22%, demonstrates heterogeneity within the MR of the function of serotonergic neurons in breathing. We conclude that serotonergic neurons within the caudal MR provide a non-CO(2)-dependent tonic drive to breathe and potentiate the effects of RTN neurons that contribute to a resting chemical 'drive to breathe' as well as the response to added CO(2). These effects of caudal MR serotonergic neurons could be at a chemoreceptor site, e.g. the RTN, or at 'downstream' sites involved in rhythm and pattern generation.

Activity-dependent heterogeneous populations of nitric oxide synthase neurons in the rat dorsal raphe nucleus

The brainstem dorsal raphe nucleus (DRN) contains an abundant distribution of nitric oxide (NO) synthase (NOS)-containing neuronal profiles in two distinct populations: faint- and intense-immunoreactive cells in midline (ventromedial and dorsomedial) and lateral wing subregions, respectively. This study tested the hypothesis that different functional dynamics underlie the topography of NOS-containing cells in the DRN rostrocaudal and mediolateral neuraxis by using a capsaicin challenge paradigm (50 mg/kg, subcutaneous). Compared with vehicle, capsaicin significantly and preferentially increased nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d, an index of constitutive NOS) reactivity in the rostral midline and caudal lateral wing subregions. Furthermore, capsaicin activated more Fos-positive cells than vehicle within all subregions of the DRN but with a caudal versus rostral predominance in activation pattern. In addition, a high proportion of capsaicin-induced Fos cells in the midline but almost none in lateral wing stained for NADPH-d. These observations suggest the existence of two functionally distinct populations of NOS neurons in the DRN. Furthermore, capsaicin increased galanin immunoreactivity with predominant staining in cell soma and fiber processes in midline and lateral wing subregions of the nucleus, respectively. The total capsaicin-induced galanin immunoreactivity was higher in rostral versus caudal DRN, and a high proportion of galanin-positive cells in the midline also contained NADPH-d and neuronal NOS, thus suggesting a potential NO-galanin interaction in these neurons. The differential pattern of Fos/NADPH-d colocalization across the nucleus suggests that midline and lateral wing NOS neurons of the DRN express their neuromodulatory actions on discrete efferent targets via different intracellular mechanisms.

Electrophysiological diversity of the dorsal raphe cells across the sleep-wake cycle of the rat

Through their widespread projections to the entire brain, dorsal raphe cells participate in many physiological functions and are associated with neuropsychiatric disorders. In previous studies, the width of action potentials was used as a criterion to identify putative serotonergic neurons, and to demonstrate that cells with broad spikes were more active in wakefulness, slowed down their activity in slow wave sleep and became virtually silent during paradoxical sleep. However, recent studies reported that about half of these presumed serotonergic cells were not immunoreactive for tyrosine hydroxylase. Here, we re-examine the electrophysiological properties of dorsal raphe cells across the sleep-wake cycle in rats by the extracellular recording of a large sample of single units (n = 770). We identified two major types of cells, which differ in spike waveform: a first population characterized by broad, mostly positive spikes, and a second one displaying symmetrical positive-negative spikes with a large distribution of spike durations (0.6-3.2 ms). Although we found classical broad-spike cells that were more active in wakefulness, we also found that about one-third of these cells increased or did not change their firing rate during sleep compared with wakefulness. Moreover, 62% of the latter cells were active in paradoxical sleep when most of raphe cells were silent. Such a diversity in the neuronal firing behaviour is important in the light of the recent controversy regarding the neurochemical identity of dorsal raphe cells exhibiting broad spikes. Our results also suggest that the dorsal raphe contains subpopulations of neurons with reciprocal activity across the sleep-wake cycle.

Serotonergic neuron diversity: identification of raphe neurons with discharges time-locked to the hippocampal theta rhythm

The serotonergic system plays a key role in the regulation of brain states, and many of the known features of serotonergic neurons appear to match this function. Midbrain raphe nuclei provide a diffuse projection to all regions of the forebrain, and raphe neurons exhibit a slow metronome-like activity that sets the ambient levels of serotonin across the sleep-wake cycle. Serotonergic cells have also been implicated, however, in a variety of more specific functions that can hardly be related to their low-rate monotonous patterns of discharges. The amazing variety of serotonergic receptors and their type-specific distribution on cortical neurons also raise the possibility of a more intimate coordination between the activity of serotonergic neurons and their target cortical circuits. Here we report an unexpected diversity in the behavior of immunohistochemically identified serotonergic neurons. Two outstanding subpopulations were identified by using the in vivo juxtacellular recording and labeling technique. The first subpopulation of serotonergic cells exhibited the classic clock-like activity with no apparent short timescale interaction with the hippocampal electroencephalogram. The other subpopulation discharged action potentials that were phase-locked to the hippocampal theta rhythm, the oscillatory pattern associated with acquisition of information and memory formation. These results indicate that the ascending serotonergic system comprises cells involved in complex information processing beyond the regulation of state transitions. The heterogeneity of serotonergic neuron behavior can also help to explain the complexity of symptoms associated with serotonergic dysfunction.

Serotonergic modulation of the limbic system

The limbic system is composed of cortical as well as subcortical structures, which are intimately interconnected. The resulting macrostructure is responsible for the generation and expression of motivational and affective states. Especially high levels of serotonin are found in limbic forebrain structures. Serotonin projections to these structures, which arise from serotonergic cell body groups in the midbrain, form a dense plexus of axonal processes. In many areas of the limbic system, serotonergic neurotransmission can best be described as paracrine or volume transmission, and thus serotonin is believed to play a neuromodulatory role in the brain. Serotonergic projections to limbic structures, arising primarily from the dorsal and median raphe nuclei, compose two distinct serotonergic systems differing in their topographic organization, electrophysiological characteristics, morphology, as well as sensitivity to neurotoxins and perhaps psychoactive or therapeutic agents. These differences may be extremely important in understanding the role of these two serotonergic systems in normal brain function and in mental illness. Central serotonergic neurons or receptors are targets for a variety of therapeutic agents used in the treatment of disorders of the limbic system.

Electrophysiological heterogeneity of spinally projecting serotonergic and nonserotonergic neurons in the rostral ventromedial medulla

This study examined the passive membrane and action potential properties of serotonergic and nonserotonergic neurons in the rostral ventromedial medulla (RVM) of the rat using whole cell patch-clamp recording techniques in the slice. Serotonergic neurons were identified by immunoreactivity for tryptophan hydroxylase (TrpH). Spinally projecting neurons were retrogradely labeled with 1'-dioactadecyl-3,3,3',3'-tetramethylindocarbodyanine perchlorate (DiI). Three types of neurons were identified within both spinally projecting serotonergic and nonserotonergic populations. Type 1 neurons exhibited irregular or sporadic spontaneous activity interspersed with periods of quiescence. Type 2 neurons were not spontaneously active and were additionally discriminated by a more negative resting membrane potential and a larger-amplitude action potential. Type 3 neurons fired repetitively without pause. Serotonergic neurons had a higher membrane resistance and greater action potential half-width than their nonserotonergic counterparts and rarely exhibited a fast afterhyperpolarization. Serotonergic type 3 neurons also fired more slowly and regularly than nonserotonergic type 3 neurons. Comparison of electrophysiological and immunohistochemical characteristics suggested that the smallest type 3 serotonergic neurons had an increased risk of immunohistochemical "misclassification" due to failure to detect TrpH, possibly due to more complete dialysis of intracellular contents during lengthy recordings. This risk was minimal for type 1 or 2 serotonergic neurons. The three different types of spinally projecting serotonergic neurons also differed markedly in their responsiveness to the mu opioid receptor agonist D-Ala2, NMePhe4, Gly5-ol]enkephalin. These results provide important new electrophysiological and pharmacological evidence for a significant heterogeneity among spinally projecting serotonergic RVM neurons. They may also provide a basis for resolving the controversy concerning the role of serotonergic RVM neurons in opioid analgesia.

Single-unit responses of serotonergic medullary raphe neurons to cardiovascular challenges in freely moving cats

Single-unit activity of serotonergic neurons in the nuclei raphe obscurus (NRO) and raphe pallidus (NRP) were recorded in conjunction with heart rate in freely moving cats in response to systemic administration of vasoactive drugs and to graded haemorrhage. Bolus administration of phenylephrine hydrochloride and sodium nitroprusside (20 microg/kg, i.v.) produced a marked, transient reflex bradycardia (-42 b.p.m.) and tachycardia (+60 b.p.m.), respectively. The activity of NRO/NRP serotonergic neurons remained unchanged after phenylephrine and nitroprusside administration. The administration of hydralazine (1 mg/kg, i.v.), a long-acting vasodilator, produced sustained tachycardia (+60 b.p.m.), which was not accompanied by changes in neuronal activity, despite prolonged reflex activation of the sympathetic nervous system. The initial withdrawal of up to 15% of total blood volume increased heart rate (+12 b.p.m.), whereas the removal of 22.5% of total blood decreased heart rate (-44 b.p.m.). The activity of NRO/NRP serotonergic neurons remained unaltered throughout graded haemorrhage trials, despite the changes in sympathetic outflow. Thus, serotonergic NRO and NRP neurons appear to be insensitive to alterations in blood pressure and baroreceptor activity, and this lack of responsiveness does not support a specific role for these cells in cardiovascular regulation. Furthermore, these neurons do not appear to be involved in physiological mechanisms underlying alterations in autonomic outflow invoked by hypertension and hypotension. Taken within the context of our previous work, the present data suggest that medullary serotonergic neurons may modulate autonomic outflow, but only in relation to their primary role in motor control.

A genetic approach to access serotonin neurons for in vivo and in vitro studies

Serotonin (5HT) is a critical modulator of neural circuits that support diverse behaviors and physiological processes, and multiple lines of evidence implicate abnormal serotonergic signaling in psychiatric pathogenesis. The significance of 5HT underscores the importance of elucidating the molecular pathways involved in serotonergic system development, function, and plasticity. However, these mechanisms remain poorly defined, owing largely to the difficulty of accessing 5HT neurons for experimental manipulation. To address this methodological deficiency, we present a transgenic route to selectively alter 5HT neuron gene expression. This approach is based on the ability of a Pet-1 enhancer region to direct reliable 5HT neuron-specific transgene expression in the CNS. Its versatility is illustrated with several transgenic mouse lines, each of which provides a tool for 5HT neuron studies. Two lines allow Cre-mediated recombination at different stages of 5HT neuron development. A third line in which 5HT neurons are marked with yellow fluorescent protein will have numerous applications, including their electrophysiological characterization. To demonstrate this application, we have characterized active and passive membrane properties of midbrain reticular 5HT neurons, which heretofore have not been reported to our knowledge. A fourth line in which Pet-1 loss of function is rescued by expression of a Pet-1 transgene demonstrates biologically relevant levels of transgene expression and offers a route for investigating serotonergic protein structure and function in a behaving animal. These findings establish a straightforward and reliable approach for developing an array of tools for in vivo and in vitro studies of 5HT neurons.

Firing of 5-HT neurones in the dorsal and median raphe nucleus in vitro shows differential alpha1-adrenoceptor and 5-HT1A receptor modulation

The median raphe nucleus and dorsal raphe nucleus together are the major source of ascending 5-HT projections. Here, using in vitro extracellular single unit electrophysiology we examined the responses of individual neurones in the rat median raphe nucleus and dorsal raphe nucleus to alpha(1)-adrenoceptor and 5-HT(1A) receptor activation and made comparisons between the two nuclei. In the presence of the alpha(1)-adrenoceptor agonist phenylephrine (1microM) all spontaneously active neurones recorded in the median and dorsal raphe nuclei fired slowly (<5Hz) and regularly. Most were inhibited by 5-HT (10-50microM), although a few were excited by 5-HT. 5-HT-induced inhibition was attenuated by the 5-HT(1A) receptor antagonist, WAY100635 (100nM). Compared to those in the dorsal raphe nucleus, the neurones in the median raphe nucleus which were inhibited by 5-HT had: (1) lower basal firing rates in the continuous presence of phenylephrine (1microM), (2) smaller excitatory responses to higher concentrations of phenylephrine (3-10microM), (3) smaller excitatory responses to brief application of norepinephrine (10-100microM) and (4) smaller inhibitory responses to 5-HT (10-50microM). The lower sensitivity of median raphe neurones to alpha(1)-adrenoceptor excitation and 5-HT(1A) receptor inhibition will have consequences for 5-HT neurotransmission in forebrain regions innervated by the two nuclei.

Does Serotonin Influence Aggression? Comparing Regional Activity before and during Social Interaction

Serotonin is widely believed to exert inhibitory control over aggressive behavior and intent. In addition, a number of studies of fish, reptiles, and mammals, including the lizard Anolis carolinensis, have demonstrated that serotonergic activity is stimulated by aggressive social interaction in both dominant and subordinate males. As serotonergic activity does not appear to inhibit agonistic behavior during combative social interaction, we investigated the possibility that the negative correlation between serotonergic activity and aggression exists before aggressive behavior begins. To do this, putatively dominant and more aggressive males were determined by their speed overcoming stress (latency to feeding after capture) and their celerity to court females. Serotonergic activities before aggression are differentiated by social rank in a region-specific manner. Among aggressive males baseline serotonergic activity is lower in the septum, nucleus accumbens, striatum, medial amygdala, anterior hypothalamus, raphe, and locus ceruleus but not in the hippocampus, lateral amygdala, preoptic area, substantia nigra, or ventral tegmental area. However, in regions such as the nucleus accumbens, where low serotonergic activity may help promote aggression, agonistic behavior also stimulates the greatest rise in serotonergic activity among the most aggressive males, most likely as a result of the stress associated with social interaction.

Medullary serotonergic neurones modulate the ventilatory response to hypercapnia, but not hypoxia in conscious rats

Serotonergic neurones in the mammalian medullary raphe region (MRR) have been implicated in central chemoreception and the modulation of the ventilatory response to hypercapnia, and may also be involved in the ventilatory response to hypoxia. In this study, we ask whether ventilatory responses across arousal states are affected when the 5-hydroxytryptamine 1A receptor (5-HT1A) agonist (R)-(+)-8-hydroxy-2(di-n-propylamino)tetralin (DPAT) is microdialysed into the MRR of the unanaesthetized adult rat. Microdialysis of 1, 10 and 30 mM DPAT into the MRR significantly decreased absolute ventilation values(VE) during 7% CO2 breathing by 21%, 19% and 30%, respectively, in wakefulness compared to artificial cerebrospinal fluid (aCSF) microdialysis, due to decreases in tidal volume (VT) and not in frequency (f), similar to what occurred during non-rapid eye movement (NREM) sleep. The concentration-dependence of the hypercapnic ventilatory effect might be due to differences in tissue distribution of DPAT. DPAT (30 mM) changed room air breathing pattern by increasing f and decreasing VT. As evidenced by a sham control group, repeated experimentation and microdialysis of aCSF alone had no effect on the ventilatory response to 7% CO2 during wakefulness or sleep. Unlike during hypercapnia, microdialysis of 30 mM DPAT into the MRR did not change the ventilatory response to 10% O2. Additionally, 10 and 30 mM DPAT MRR microdialysis decreased body temperature, and 30 mM DPAT increased the percentage of experimental time in wakefulness. We conclude that serotonergic activity in the MRR plays a role in the ventilatory response to hypercapnia, but not to hypoxia, and that MRR 5-HT1A receptors are also involved in thermoregulation and arousal.

Activation of slowly conducting medullary raphé-spinal neurons, including serotonergic neurons, increases cutaneous sympathetic vasomotor discharge in rabbit

Neurons in the rostral medullary raphé/parapyramidal region regulate cutaneous sympathetic nerve discharge. Using focal electrical stimulation at different dorsoventral raphé/parapyramidal sites in anesthetized rabbits, we have now demonstrated that increases in ear pinna cutaneous sympathetic nerve discharge can be elicited only from sites within 1 mm of the ventral surface of the medulla. By comparing the latency to sympathetic discharge following stimulation at the ventral raphé site with the corresponding latency following stimulation of the spinal cord [third thoracic (T3) dorsolateral funiculus] we determined that the axonal conduction velocity of raphé-spinal neurons exciting ear pinna sympathetic vasomotor nerves is 0.8 ± 0.1 m/s ( n = 6, range 0.6–1.1 m/s). Applications of the 5-hydroxytryptamine (HT)2A antagonist trans-4-((3 Z)3-[(2-dimethylaminoethyl)oxyimino]-3-(2-fluorophenyl)propen-1-yl)-phenol, hemifumarate (SR-46349B, 80 μg/kg in 0.8 ml) to the cerebrospinal fluid above thoracic spinal cord (T1-T7), but not the lumbar spinal cord (L2-L4), reduced raphé-evoked increases in ear pinna sympathetic vasomotor discharge from 43 ± 9 to 16 ± 6% ( P < 0.01, n = 8). Subsequent application of the excitatory amino acid (EAA) antagonist kynurenic acid (25 μmol in 0.5 ml) substantially reduced the remaining evoked discharge (22 ± 8 to 6 ± 6%, P < 0.05, n = 5). Our conduction velocity data demonstrate that only slowly conducting raphé-spinal axons, in the unmyelinated range, contribute to sympathetic cutaneous vasomotor discharge evoked by electrical stimulation of the medullary raphé/parapyramidal region. Our pharmacological data provide evidence that raphé-spinal neurons using 5-HT as a neurotransmitter contribute to excitation of sympathetic preganglionic neurons regulating cutaneous vasomotor discharge. Raphé-spinal neurons using an EAA, perhaps glutamate, make a substantial contribution to the ear sympathetic nerve discharge evoked by raphé stimulation.

Oestrogen and testosterone modulate the firing activity of dorsal raphe nucleus serotonergic neurones in both male and female rats

Women are twice as likely to suffer from mood disorders than men. Moreover, a growing body of evidence suggests a reciprocal modulation between sex steroids and the serotonin (5-HT) system. A previous study from our laboratory has shown that the progesterone metabolites 5beta-pregnane-3,20-dione (5beta-DHP) and 5alpha-pregnan-3alpha-ol,20-one (3alpha,5alpha-THP), as well as dehydroepiandrosterone (DHEA), increase the firing activity of dorsal raphe nucleus (DRN) 5-HT neurones in female rats. The present study was undertaken to assess the effects of these steroids in male rats, as well as the effects of testosterone and 17beta-oestradiol (17beta-E) in both sexes, and finally to evaluate gender differences in the modulation of the 5-HT neuronal firing activity by these different neuroactive steroids. Male rats were treated i.c.v., for 7 days, with a dose of 50 microg/kg/day of one of the following steroids: progesterone, 5beta-DHP, 3alpha,5alpha-THP, DHEA, testosterone, 17beta-hydroxy-5alpha-androstan-3-one (5alpha-DHT) and 17beta-E. Some rats also received a 3-day administration of testosterone (50 microg/kg/day, i.c.v). Females were treated in the same fashion with testosterone and 17beta-E. Extracellular unitary recordings of 5-HT neurones, obtained in vivo in the DRN of these rats, revealed that testosterone and 17beta-E increased the firing activity of 5-HT neurones in both males and females. In males, the effect of testosterone could already be seen after 3 days of treatment. Neither castration nor any treatment with other steroids significantly modified the firing rate of male 5-HT neurones. Taken together with previous findings, the results of the present study indicate both similarities and differences between sexes in the modulation of 5-HT neurones by some steroids. This could prove important in understanding gender differences in mood disorders.

Somatodendritic autoreceptor regulation of serotonergic neurons: dependence on L-tryptophan and tryptophan hydroxylase-activating kinases

The somatodendritic 5-HT(1A) autoreceptor has been considered a major determinant of the output of the serotonin (5-HT) neuronal system. However, recent studies in brain slices from the dorsal raphe nucleus have questioned the relevance of 5-HT autoinhibition under physiological conditions. In the present study, we found that the difficulty in demonstrating 5-HT tonic autoinhibition in slice results from in vitro conditions that are unfavorable for sustaining 5-HT synthesis. Robust, tonic 5-HT(1A) autoinhibition can be restored by reinstating in vivo 5-HT synthesizing conditions with the initial 5-HT precursor l-tryptophan and the tryptophan hydroxylase co-factor tetrahydrobiopterin (BH(4)). The presence of tonic autoinhibition under these conditions was revealed by the disinhibitory effect of a low concentration of the 5-HT(1A) antagonist WAY 100635. Neurons showing an autoinhibitory response to L-tryptophan were confirmed immunohistochemically to be serotonergic. Once conditions for tonic autoinhibition had been established in raphe slice, we were able to show that 5-HT autoinhibition is critically regulated by the tryptophan hydroxylase-activating kinases calcium/calmodulin protein kinase II (CaMKII) and protein kinase A (PKA). In addition, at physiological concentrations of L-tryptophan, there was an augmentation of 5-HT(1A) receptor-mediated autoinhibition when the firing of 5-HT cells activated with increasing concentrations of the alpha(1) adrenoceptor agonist phenylephrine. Increased calcium influx at higher firing rates, by activating tryptophan hydroxylase via CaMKII and PKA, can work together with tryptophan to enhance negative feedback control of the output of the serotonergic system.

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.

Expression of estrogen receptors (alpha, beta) and androgen receptor in serotonin neurons of the rat and mouse dorsal raphe nuclei; sex and species differences

Sex steroids have been inferred to be involved in the regulation of affective status at least partly through the serotonergic (5-HT) system, particularly in the dorsal raphe nucleus (DRN), which innervates enormous projections to the cerebral cortex and limbic system. In the present study, the expression of estrogen receptors-alpha and -beta (ERalpha, ERbeta), androgen receptor (AR) and 5-HT was examined immunohistochemically in the rat and mouse DRN in both sexes. The results showed that large numbers of ERalpha- and/or ERbeta-immunoreactive (ERalpha-I, ERbeta-I) cells were found in the DRN of both male and female mice, whereas only small numbers of ERalpha-I cells and no ERbeta-I cells were seen in the rat DRN of each sex. With respect to AR-immunoreactive (AR-I) cells, moderate numbers of such cells were present only in male rats and mice, and no or very few could be observed in female ones. The ERalpha-I, ERbeta-I, and AR-I cells were mainly distributed in the rostral DRN. In double-immunostaining, many 5-HT-I neurons were found to show ERalpha and/or ERbeta expression specifically in the rostral DRN (particularly dorsal, ventral and interfascicular parts) of mice of both sexes, but not in that of rats. In contrast, only a few 5-HT neurons were observed to show AR expression in the DRN of both rodents. The current results strongly suggest that sex steroids can modulate the affective regulation of the serotonergic system through ERalpha and/or ERbeta in 5-HT neurons of the mouse rostral DRN (but not so much through AR), and that such effects might be different depending on the sex and species, as shown by the prominent sex differences in AR expression and prominent species differences in ERalpha and ERbeta expression.

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.

Activation of 5-HT1A receptors in raphe pallidus inhibits leptin-evoked increases in brown adipose tissue thermogenesis

To elucidate the central neural pathways contributing to the thermogenic component of the autonomic response to intravenous administration of leptin, experiments were conducted in urethane-chloralose-anesthetized, ventilated rats to address 1) the role of neurons in the rostral ventromedial medulla, including raphe pallidus (RPa), in the leptin-evoked stimulation of brown adipose tissue (BAT) sympathetic nerve activity (SNA); and 2) the potential thermolytic effect of 5-hydroxytryptamine1A (5-HT1A) receptors on RPa neurons that influence BAT thermogenesis. Leptin (1 mg/kg) administration increased BAT SNA by 1,219% of control, BAT temperature by 2.8°C, expired CO2 by 1.8%, heart rate by 90 beats/min, and mean arterial pressure by 12 mmHg. Microinjection of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) into RPa resulted in a prompt and sustained reversal of the leptin-evoked stimulation of BAT SNA, BAT thermogenesis, and heart rate, with these variables returning to their pre-leptin control levels. Subsequent microinjection of the selective 5-HT1A receptor antagonist WAY-100635 into RPa reversed the BAT thermolytic effects of 8-OH-DPAT, returning BAT SNA and BAT temperature to the elevated levels after leptin. In conclusion, activation of neurons in RPa, possibly BAT sympathetic premotor neurons, is essential for the increases in BAT SNA, BAT thermogenesis, and heart rate stimulated by intravenous administration of leptin. Neurons in RPa express 5-HT1A receptors whose activation leads to reversal of the BAT thermogenic and the cardiovascular responses to intravenous leptin, possibly through hyperpolarization of local sympathetic premotor neurons. These results contribute to our understanding of central neural substrates for the augmented energy expenditure stimulated by leptin.

Activity of dorsal raphe cells across the sleep-waking cycle and during cataplexy in narcoleptic dogs

Cataplexy, a symptom associated with narcolepsy, represents a unique dissociation of behavioural states. During cataplectic attacks, awareness of the environment is maintained, as in waking, but muscle tone is lost, as in REM sleep. We have previously reported that, in the narcoleptic dog, noradrenergic cells of the locus coeruleus cease discharge during cataplexy. In the current study, we report on the activity of serotonergic cells of the dorsal raphe nucleus. The discharge patterns of serotonergic dorsal raphe cells across sleep-waking states did not differ from those of dorsal raphe and locus coeruleus cells recorded in normal rats, cats and monkeys, with tonic discharge in waking, reduced activity in non-REM sleep and cessation of activity in REM sleep. However, in contrast with locus coeruleus cells, dorsal raphe REM sleep-off neurones did not cease discharge during cataplexy. Instead, discharge continued at a level significantly higher than that seen in REM sleep and comparable to that seen in non-REM sleep. We also identified several cells in the dorsal raphe whose pattern of activity was the opposite of that of the presumed serotonergic cells. These cells were maximally active in REM sleep and minimally active in waking and increased activity during cataplexy. The difference between noradrenergic and serotonergic cell discharge profiles in cataplexy suggests different roles for these cell groups in the normal regulation of environmental awareness and muscle tone and in the pathophysiology of narcolepsy.

Neurochemical and anatomical identification of fast- and slow-firing neurones in the rat dorsal raphe nucleus using juxtacellular labelling methods in vivo

GABA neurones in the dorsal raphe nucleus (DRN) influence ascending 5-hydroxytryptamine (5-HT) neurones but are not physiologically or anatomically characterised. Here, in vivo juxtacellular labelling methods in urethane-anaesthetised rats were used to establish the neurochemical and morphological identity of a fast-firing population of DRN neurones, which recent data suggest may be GABAergic. Slow-firing, putative 5-HT DRN neurones were also identified for the first time using this approach. Fast-firing, DRN neurones were successfully labelled with neurobiotin (n=10) and the majority (n=8/10) were immunoreactive for the GABA synthetic enzyme glutamic acid decarboxylase. These neurones were located in the DRN (mainly lateral regions), and consistently fired spikes with short width (1.1+/-0.1 ms) and high frequency (12.1+/-2.0 Hz). In most cases spike trains were regular but displayed low frequency oscillations (1-2 Hz). These neurones were morphologically heterogeneous but commonly had branching axons with varicosities and dendrites that extended across DRN subregions and the midline. Slow-firing DRN neurones were also successfully labelled with neurobiotin (n=24). These neurones comprised a population of neurones immunopositive for 5-HT and/or tryptophan hydroxylase (n=12) that fired broad spikes (2.2+/-0.2 ms) with high regularity and low frequency (1.7+/-0.2 Hz). However, a slow-firing, less regular population of neurones immunonegative for 5-HT/tryptophan hydroxylase (n=12) was also apparent. In summary, this study chemically identifies fast- and slow-firing neurones in the DRN and establishes for the first time that fast-firing DRN neurones are GABAergic. The electrophysiological and morphological properties of these neurones suggest a novel function involving co-ordination between GABA and 5-HT neurones dispersed across DRN subregions.