Reciprocal innervation between serotonergic and GABAergic neurons in raphe nuclei of the rat

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.

Modulation of sympathetic and somatomotor function by the ventromedial medulla

The ventromedial medulla is implicated in a variety of functions including nociceptive and cardiovascular modulation and the control of thermoregulation. To determine whether single microinjections into the ventromedial medulla elicit changes in one or multiple functional systems, the GABA(A) receptor antagonist bicuculline was microinjected (70 nl, 5-50 ng) into the ventromedial medulla of lightly anesthetized rats, and cardiovascular, respiratory, and nociceptive measures were recorded. Bicuculline microinjection into either the midline raphe or the laterally adjacent reticular nucleus simultaneously increased interscapular brown adipose tissue temperature, heart rate, blood pressure, expired [CO(2)], and respiration rate and elicited shivering. Bicuculline microinjection also decreased the noxious stimulus-evoked changes in heart rate and blood pressure, decreased the frequency of heat-evoked sighs, and suppressed the cortical desynchronization evoked by noxious stimulation. Although bicuculline suppressed the motor withdrawal evoked by noxious tail heat, it enhanced the motor withdrawal evoked by noxious paw heat, evidence for specifically patterned nociceptive modulation. Saline microinjections into midline or lateral sites had no effect on any measured variable. All bicuculline microinjections, midline or lateral, evoked the same set of physiological effects, consistent with the lack of a topographical organization within the ventromedial medulla. Furthermore, as predicted by the isodendritic morphology of cells in the ventromedial medulla, midline bicuculline microinjection increased the number of c-fos immunoreactive cells in both midline raphe and lateral reticular nuclei. In summary, 70-nl microinjections into ventromedial medulla activate cells in multiple nuclei and elicit increases in sympathetic and somatomotor tone and a novel pattern of nociceptive modulation.

Organization of projections from the raphe nuclei to the vestibular nuclei in rats

Previous anatomic and electrophysiological evidence suggests that serotonin modulates processing in the vestibular nuclei. This study examined the organization of projections from serotonergic raphe nuclei to the vestibular nuclei in rats. The distribution of serotonergic axons in the vestibular nuclei was visualized immunohistochemically in rat brain slices using antisera directed against the serotonin transporter. The density of serotonin transporter-immunopositive fibers is greatest in the superior vestibular nucleus and the medial vestibular nucleus, especially along the border of the fourth ventricle; it declines in more lateral and caudal regions of the vestibular nuclear complex. After unilateral iontophoretic injections of Fluoro-Gold into the vestibular nuclei, retrogradely labeled neurons were found in the dorsal raphe nucleus (including the dorsomedial, ventromedial and lateral subdivisions) and nucleus raphe obscurus, and to a minor extent in nucleus raphe pallidus and nucleus raphe magnus. The combination of retrograde tracing with serotonin immunohistofluorescence in additional experiments revealed that the vestibular nuclei receive both serotonergic and non-serotonergic projections from raphe nuclei. Tracer injections in densely innervated regions (especially the medial and superior vestibular nuclei) were associated with the largest numbers of Fluoro-Gold-labeled cells. Differences were observed in the termination patterns of projections from the individual raphe nuclei. Thus, the dorsal raphe nucleus sends projections that terminate predominantly in the rostral and medial aspects of the vestibular nuclear complex, while nucleus raphe obscurus projects relatively uniformly throughout the vestibular nuclei. Based on the topographical organization of raphe input to the vestibular nuclei, it appears that dense projections from raphe nuclei are colocalized with terminal fields of flocculo-nodular lobe and uvula Purkinje cells. It is hypothesized that raphe-vestibular connections are organized to selectively modulate processing in regions of the vestibular nuclear complex that receive input from specific cerebellar zones. This represents a potential mechanism whereby motor activity and behavioral arousal could influence the activity of cerebellovestibular circuits.

GABAergic dysfunction in mood disorders

The authors review the available literature on the preclinical and clinical studies involving GABAergic neurotransmission in mood disorders. Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter present almost exclusively in the central nervous system (CNS), distributed across almost all brain regions, and expressed in interneurons modulating local circuits. The role of GABAergic dysfunction in mood disorders was first proposed 20 years ago. Preclinical studies have suggested that GABA levels may be decreased in animal models of depression, and clinical studies reported low plasma and CSF GABA levels in mood disorder patients. Also, antidepressants, mood stabilizers, electroconvulsive therapy, and GABA agonists have been shown to reverse the depression-like behavior in animal models and to be effective in unipolar and bipolar patients by increasing brain GABAergic activity. The hypothesis of reduced GABAergic activity in mood disorders may complement the monoaminergic and serotonergic theories, proposing that the balance between multiple neurotransmitter systems may be altered in these disorders. However, low GABAergic cortical function may probably be a feature of a subset of mood disorder patients, representing a genetic susceptibility. In this paper, we discuss the status of GABAergic hypothesis of mood disorders and suggest possible directions for future preclinical and clinical research in this area.

Involvement of presynaptic 5-HT(1A) and benzodiazepine receptors in the anticonflict activity of 5-HT(1A) receptor antagonists

In the present paper, we examined the effect of lesions of 5-hydroxytryptamine (5-HT) neurons, produced by p-chloroamphetamine (p-CA; 2 x 10 mg/kg), and the influence of flumazenil (Ro 15-1788, 10 mg/kg), a benzodiazepine receptor antagonist, on the anticonflict activity of N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide (WAY 100635) and trans-1-(2-methoxy-phenyl)-4-[4-succinimidocyclohexyl]piperazine (MP 349), pre- and postsynaptic 5-HT(1A) receptor antagonists, and 1-(2-methoxyphenyl)-4-(4-succinimidobutyl)piperazine (MM 77), a postsynaptic 5-HT(1A) receptor antagonist, in the Vogel conflict drinking test in rats. Diazepam was used as a reference compound. WAY 100635 (0.5-1 mg/kg), MP 349 (0.25-0.5 mg/kg), MM 77 (0.03-0.25 mg/kg) and diazepam (2.5-5 mg/kg) significantly increased the number of shocks accepted during experimental sessions in the conflict drinking test. In p-chloroamphetamine-pretreated rats, neither WAY 100635 (1 mg/kg) nor MP 349 (0.25 mg/kg) induced an anticonflict effect, whereas MM 77 (0.06 mg/kg) did produce it. Flumazenil fully blocked the anticonflict effects of WAY 100635 (1 mg/kg) and diazepam (5 mg/kg), and it partly but significantly reduced the anticonflict effects of MP 349 (0.25 mg/kg) and MM 77 (0.06 mg/kg). p-Chloroamphetamine and flumazenil alone were inactive in the conflict drinking test. The present results suggest that, first, the anticonflict effect of the 5-HT(1A) receptor antagonists, WAY 100635 and MP 349, but not MM 77, is linked to the presynaptically located 5-HT(1A) receptors, and second, benzodiazepine receptors are indirectly involved in such effects of WAY 100635, MP 349 and MM 77, due maybe to a possible interaction between the 5-HT and the gamma-aminobutyric acid (GABA)/benzodiazepine systems.

Gender and gonadal status modulation of dorsal raphe nucleus serotonergic neurons. Part II. Regulatory mechanisms

In the companion paper, we showed that the spontaneous firing activity of DRN 5-HT neurons is significantly higher in male (M) than in freely cycling female (CF) rats. Moreover, during pregnancy, it increased in parallel to circulating levels of progesterone, peaking at day 17 of pregnancy (P17). In this second part, we assessed the role of three regulatory mechanisms potentially involved in these modifications of the 5-HT neurons firing activity. During pregnancy, the ED(50) for the response to LSD was decreased by about 70%, indicating a partial desensitization of 5-HT(1A) autoreceptors, which is consistent with the 5-HT neurons higher firing activity. The GABAergic tonic inhibition of 5-HT neurons was assessed using the responses to GABA, bicuculline and isoniazid. Together, they indicate a lower GABAergic tonic inhibition in males and P17 as compared to CF, which is in agreement with their greater 5-HT neurons firing rate. Finally, the efficacy of the long feedback loop, involving postsynaptic 5-HT(1A) receptors, did not seem affected by gender, ovariectomy or pregnancy since the response to systemic 8-OH-DPAT was similar. These results constitute strong evidence of mechanisms by which gender and hormonal fluctuations can modulate the 5-HT neurons function and influence vulnerability to mood disorders.

Mechanisms of nicotine actions on dorsal raphe serotoninergic neurons

Nicotine, locally administered into the dorsal raphe nucleus (DRN) of rat midbrain slices, increased the discharge rate of 70% of serotoninergic neurons, decreased it in 30% and induced reciprocal oscillatory increases in serotonin (5-hydroxytryptamine, 5-HT) and gamma-aminobutyric acid (GABA) release. All of nicotine's stimulatory effects were maximal at 2.15 microM. Bicuculline, a GABA(A) receptor antagonist, increased the firing rate in 64% of serotoninergic neurons, decreased it in 36% and augmented serotonin and GABA release. Bicuculline increased nicotine's stimulatory effects on firing rate but did not reverse the inhibitory ones. N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinil-cyclohexanecarboxamide (WAY-100635), a 5-HT(1A) receptor antagonist, increased the firing rate of 88% of serotoninergic neurons, as well as serotonin and GABA release and reversed nicotine's inhibitory action on serotoninergic neurons. These data suggest that nicotine decreases the firing rate of one third of serotoninergic neurons through serotonin release and increases the firing rate of the remaining two thirds, due to stronger stimulatory than indirect inhibitory effects.

Sculpting the visual map: the distribution and function of serotonin-1A and serotonin-1B receptors in the optic tectum of the frog

Agonists of serotonin (5-HT)-1 receptors modulate the synaptic strength of the connection between retinal ganglion cells and neurons of the frog optic tectum in brain slices (Brain Res. 1998;781:167-181). We have now used autoradiographic receptor binding techniques to determine the location of 5-HT1A and 5-HT1B binding sites in the laminated optic tectum. 5-HT1A binding sites, as labeled with [3H]8-hydroxy-dipropylaminotetralin (8-OH-DPAT), were highest in the superficial, retinorecipient layers of the tectum, intermediate in layers 6 and 7 and low in the remaining layers. Binding densities in all of these layers were unaffected by optic nerve lesion. 5-HT1B binding sites were visualized using [125I]iodocyanopindolol (ICYP). Binding densities were highest in the plexiform layers 5 and 7 and intermediate in layers 6 and 8. Binding sites were present at low levels in layer 9; however, optic nerve lesion resulted in a strong upregulation of these sites in this layer. Pharmacological manipulation of receptor activation resulted in changes in the activity-dependent visual map that is created at the tectum by retinal ganglion cell terminals. Chronic treatment of the tectum with SB-224289, a selective antagonist of 5-HT1B receptors, disrupted the topographic map. In contrast, exposure to WAY-100635, a selective antagonist of 5-HT1A receptors, refined it. We conclude that both 5-HT1A and 5-HT1B receptors are present in the adult frog tectum and that changes in their activation levels can produce changes in retinotectal transmission levels that drive visual plasticity in opposite directions.

Evidence for a role of GABA interneurones in the cortical modulation of midbrain 5-hydroxytryptamine neurones

Recent electrophysiological studies demonstrate that the ventral medial prefrontal cortex has a powerful inhibitory influence on 5-hydroxytryptamine (5-HT) neurones in the dorsal raphe nucleus. Here we utilised a combination of anatomical and electrophysiological methods to characterise the cellular substrate underlying this effect.Anterograde tracing (Phaseolus vulgaris leucoagglutinin) using electron microscopy demonstrated a pathway from the ventral medial prefrontal cortex that makes neuronal contacts throughout the dorsal raphe nucleus. These contacts were predominantly asymmetrical synapses adjoining GABA immunoreactive dendrites and spines. In vivo extracellular recordings were made in the dorsal raphe nucleus of the anaesthetised rat from a subpopulation of non-5-HT neurones. These neurones were fast-firing, irregular and with short spike width, properties strongly reminiscent of immunochemically identified GABA interneurones in other brain regions. Recordings of classical 5-HT neurones were also included. Electrical stimulation of the ventral medial prefrontal cortex elicited a rapid onset (16 ms latency), orthodromic excitation of the non-5-HT neurones (13/25 neurones). This stimulation also caused a pronounced inhibition of most 5-HT neurones tested, with a longer latency (30 ms), and this was partially blocked by locally applied bicuculline. These data provide the first evidence that the ventral medial prefrontal cortex influences the activity of large numbers of raphe 5-HT neurones by targeting a local network of GABA neurones. This circuitry predicts that physiological and pathological changes in the ventral medial prefrontal cortex will impact on significant parts of the forebrain 5-HT system.