Role of midbrain raphe in stress-induced renin and prolactin secretion

Stress-induced changes in renin and prolactin secretion were studied using a conditioned emotional response paradigm. Three minutes after being placed in a chamber, the stressed animals received a brief electric shock (1.0 mA for 10 s through the grid floor), then were returned to their home cage. This procedure was repeated for 3 consecutive days. On the fourth day, the rats were placed in the chamber for 3 min, but instead of receiving shock, they were removed and sacrificed. Control animals were treated in the same manner, except that they never received foot shock. The sham-operated stressed rats evidenced significant elevations in plasma renin activity (270%) and prolactin level (550%). Electrolytic lesions in the dorsal raphe nucleus blocked the stress-induced increase in plasma renin activity but did not affect the stress-induced increase in prolactin secretion. Electrolytic lesions in the median raphe nucleus did not affect prolactin levels in either control or stressed animals. However, median raphe lesions led to a significant increase in plasma renin activity in non-stressed rats and potentiated the stress-induced elevation in plasma renin activity. These results suggest that neurons within the dorsal and median raphe nuclei are involved in the regulation of renin but not prolactin secretion during stress. The results also suggest that median raphe neurons play a role in basal renin secretion.     

Role of midbraib raphe in stress-induced renin and prolactin secretion

Stress-induced changes in renin and prolactin secretion were studied using a conditioned emotional response paradigm. Three minutes after being placed in a chamber, the stressed animals received a brief electric shock (1.0 mA for 10 s through the grid floor), then were returned to their home cage. This procedure was repeated for 3 consecutive days. On the fourth day, the rats were placed in the chamber for 3 min, but instead of receiving shock, they were removed and sacrificed. Control animals were treated in the same manner, except that they never received foot shock. The sham-operated stressed ratsevidenced significant elevations in plasma renin acitivity (270%) and prolactin level (550%). Electrolytic lesions in the dorsal raphe nucleus blocked the stress-induced increase in plasma renin activity but did not affect the stress-induced increase in prolactin secretion. Electrolytic lesions in the median raphe nucleus did not affect prolactin levels in either control or stressed animals. However, median raphe lesions led to a significant increase in plasma renin activity in non-stressed rats and potentiated the stress-induced elevation in plasma renin activity. These results suggest that neurons within the dorsal and median raphe nuclei are involved in the regulation of renin but not prolactin secretion during stress. The results also suggest that median raphe neurons play a role in basal renin secretion.     

Further studies on the activation of rat median raphe serotonergic neurons by inescapable sound stress

Previous studies, using a biochemical measure of serotonergic neuronal function, show that inescapable, randomly presented sound pulses activate serotonergic neurons in the rat median raphe but not dorsal raphe nucleus. The present study reveals that this activation also occurs in serotonin projection areas, in hippocampus, nucleus accumbens and cortex but not in caudate nucleus. The selectivity of this response is examined by comparing the response to sound stress with that produced by morphine, a treatment known to selectively activate dorsal raphe but not median raphe serotonergic neurons. Two approaches are used in Sprague-Dawley rat to measure the activation of serotonergic neurons: (1) determination ex vivo of accumulation of 5-hydroxytryptophan (5-HTP) in tissue from the dorsal and median raphe nuclei, hippocampus, cortex, caudate nucleus, and nucleus accumbens following in vivo inhibition of aromatic amino acid decarboxylase; and (2) measurement of extracellular serotonin levels in hippocampus, caudate nucleus, and nucleus accumbens. Sound stress increases 5-HTP accumulation in median raphe nucleus, hippocampus, cortex, and nucleus accumbens, but not dorsal raphe nucleus or caudate nucleus. Sound stress also enhances extracellular serotonin levels in hippocampus and nucleus accumbens, but not caudate nucleus. In contrast, the morphine treatment enhances 5-HTP accumulation in dorsal raphe nucleus, cortex and caudate nucleus, but not in median raphe nucleus, hippocampus or nucleus accumbens. Furthermore, it increases extracellular serotonin levels in only the caudate nucleus. The combined effects of sound stress and morphine on 5-HTP accumulation are identical to those obtained by each treatment individually. These findings provide further support for the presence of serotonergic neurons within the median raphe nucleus that have a unique response profile. These neurons may have an important role in responses or adaptations to stress.     

Serotonergic lesions alter cocaine-induced locomotor behavior and stress-activation of the mesocorticolimbic dopamine system

The aim of this study was to examine the effects of serotonergic lesions to the dorsal raphe on midbrain dopaminergic systems. 5,7-Dihydroxytryptamine lesions of the dorsal raphe resulted in a substantial loss of serotonin in the medial prefrontal cortex (about 75%) and the nucleus accumbens (about 50%), while no change in DA levels or DA metabolism were noted in either region at 12 days postlesion. A transient basal locomotor activation was noted in the lesioned animals compared to the sham controls 7 to 12 days after the lesions. The locomotor response to an acute dose of cocaine was also enhanced in 5,7-dihydroxytryptamine lesioned rats, however, no change in the time course or magnitude of the behavioral locomotor response to repeated cocaine administration was observed. Restraint for 30 min increased DA metabolism in both the NAS and mPFC of sham rats, as expected. However, in 5,7-dihydroxytryptamine lesioned rats, restraint stress enhanced the usual stress-induced increase in DA metabolism by about 50 and 150% in the medial prefrontal cortex and nucleus accumbens, respectively. Our results indicate the 5,7-dihydroxytryptamine lesions of the dorsal raphe lower serotonin in both the mPFC and NAS leading to an enhanced responsiveness of the DA projections in both regions. This effect may be explained by a loss of sensitivity of DA receptors in 5,7-dihydroxytryptamine denervated rats. This interpretation implies that the stimulated, but not basal, release of DA in the mPFC and NAS is dependent on serotonin tone. © 1996 Wiley-Liss, Inc.     

Evidence that the medial and dorsal raphe nuclei mediate serotonergically-induced increases in prolactin release from the pituitary

Electrolytic lesions of the medial (MR) or dorsal (DR) raphe nucleus significantly antagonized serum prolactin elevations produced by 5-hydroxytryptophan (5-HTP) in rats pretreated with fluoxetine or citalopram, (serotonin (5-HT) uptake blockers). Lesioned animals in which total blockade of serum prolactin elevations was observed also had total blockade of 5-HT accumulation in the median eminence. However, the increase in serum prolactin levels produced by 5-HTP plus 5-HT reuptake blockade in lesioned rats was not significantly different from sham-operated rats if as little as 15–20% of control median eminence accumulation was present. Serum prolactin elevations produced by quipazine, a direct acting 5-HT agonist, were not significantly affected by MR lesions. On the basis of these results, we suggest that: (1) serum prolactin elevations following 5-HT reuptake blockade plus 5-HTP are correlated with 5-HT concentration in the median eminence; (2) lesion-induced antagonism of 5-HTP-induced prolactin elevation is critically dependent upon complete blockade of median eminence 5-HT accumulation; and (3) 5-HT neurons arising from cell bodies located in the MR and DR are necessary for endogenous serotonergically mediated effects on prolactin secretion in the rat.     

Cardiovascular effects of α-adrenergic agents in the dorsal raphe nucleus of the rat

Local injection of norepinephrine (NE) or phenylephrine into the lateral aspect of the dorsal raphe nucleus results in an increase in blood pressure and heart rate in the urethane-anesthetized rat. The increases in blood pressure and heart rate are blocked by prior injection of phentolamine into the dorsal raphe nucleus and are significantly reduced by i.v. mecamylamine. Selective lesion of serotonergic neurons in the dorsal raphe nucleus by 5,7-dihydroxytryptamine significantly reduces the pressor response to phenylephrine, but does not affect the increase in heart rate in response to phenylephrine. These data are consistent with a central α-adrenergic mechanism in the dorsal raphe that elevates blood pressure at least partly by an action on serotonergic neurons and elevates heart rate by a mechanism involving non-serotonergic neurons in the dorsal raphe area.     

Serotonergic innervation of the lateral hypothalamus: Evidence from synaptosomal uptake studies

Uptake of tritiated serotonin by synaptosomes prepared from rat lateral hypothalamus was examined. Uptake of serotonin into lateral hypothalamic synaptosomes occurred by both saturable and non-saturable processes. The saturable process was a high affinity transport with kinetic parameters that agree closely with those previously reported for serotonin uptake into synaptosomes prepared from other brain regions. Fluoxetine, a selective inhibitor of uptake into serotonergic neurons, was a potent inhibitor of serotonin uptake into lateral hypothalamic synaptosomes. Desipramine and benztropine, noradrenergic and dopaminergic uptake inhibitors respectively, were much less effective. Damage to the ascending serotonergic system, by either electrolytic lesion of the dorsal or median raphe nucleus, or by 5,7 dihydroxytryptamine injections into the midbrain serotonergic pathways, significantly reduced the uptake of serotonin by lateral hypothalamic synaptosomes. Taken together, these data provide further evidence for a serotonergic terminal field within the lateral hypothalamus.     

An analysis of neuronal elements within the median nucleus of the raphe that mediate lesion-induced increases in locomotor activity

Electrolytic lesions of the median nucleus of the raphe (MR) are known to result in large increases in motor activity. The present studies were concerned with identifying the neuronal elements within or near the MR (e.g. fibers of passage, serotonergic or non-serotonergic cells) which, when destroyed, lead to these increases in ambulation. Group of rats were given either an electrolytic MR lesion or were injected locally with the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or the excitotoxin ibotenic acid, and their subsequent locomotor activities were compared in the open field and in photocell cages.In a 5 min open test, rats with either electrolytic or ibotenate lesions of the MR were more active compared to all other groups, although rats in the former group were also more active than those in the latter. In a longer activity test conducted in photocell cages, rats with electrolytic lesions were more active than all other groups during the first 20 min period, after which their activity did not differ from the ibotenate group; however, both of these groups were hyperactive compared to all others. Both electrolytic and ibotenate groups showed exaggerated hyperactivity in response tod-amphetamine. Lesions produced by 5,7-DHT failed to significantly increase either spontaneous ord-aphetamine-induced locomotor activity. When the results from all of the activity measures are considered, it appears that damage to at least two different neuronal populations may be responsible for the hyperactivity observed in MR lesioned rats. However, damage to the serotonergic system does not appear to contribute to these locomotor effects.     

Differential role of serotonin projections from the dorsal and median raphe nuclei in phencyclidine-induced hyperlocomotion and fos-like immunoreactivity in rats

Altered brain serotonin activity is implicated in schizophrenia. We have previously shown differential involvement of serotonergic projections from the dorsal or median raphe nucleus in phencyclidine‐induced hyperlocomotion in rats, a behavioral model of aspects of schizophrenia. Here we further investigated the effects of serotonergic lesions of the raphe nuclei on phencyclidine‐induced hyperlocomotion by parallel assessment of Fos‐like immunoreactivity (FLI), a marker of neuronal activation in the brain. Male Sprague‐Dawley rats were anesthetized with pentobarbitone and stereotaxically microinjected with 5 μg of the serotonergic neurotoxin, 5,7‐dihydroxytryptamine (5,7‐DHT), into either the dorsal raphe (DRN) or median raphe nucleus (MRN). Two weeks after the surgery, rats with lesions of the MRN, but not those with lesions of the DRN, showed significant enhancement of the hyperlocomotion induced by injection of 2.5 mg/kg of phencyclidine. Rats with MRN lesions also showed significantly higher levels of FLI in the polymorphic layer of the dentate gyrus in the dorsal hippocampus (PoDG) when compared with sham‐operated controls. Rats with lesions of the DRN showed significantly higher levels of FLI in the nucleus accumbens (NAcc). These results indicate that FLI in the PoDG, but not the NAcc, correlates with enhanced phencyclidine‐induced locomotor hyperactivity in MRN‐lesioned rats. These results support our previous studies suggesting a role of serotonergic projections from the MRN to the dorsal hippocampus in some of the symptoms of schizophrenia. Synapse, 2012. © 2012 Wiley Periodicals, Inc.     

Extent of colocalization of serotonin and GABA in the neurons of the rat raphe nuclei

Previous investigations of the distribution of neurons containing both serotonin and GABA in the brainstem raphe nuclei have yielded discrepant results amongst different authors. This study attempted to clarify the distribution as well as the proportions of raphe and other brainstem neurons that contain both neurotransmitters. All the nine serotonergic cell groups known to be present in the brainstem were examined with an indirect immunofluorescence method using antibodies against serotonin and glutamic acid decarboxylase in colchicine-treated rats. Sections were incubated either simultaneously or sequentially for the two immunolabels. Brainstem neurons that were labelled for both markers were generally infrequent. Of all the serotonin cell groups in the brainstem, the nucleus raphe magnus contained the most double-labelled cells (a mean of 3.6% of a total of 625–1155 serotonin-immunoreactive cells counted in this nucleus), followed by the nucleus raphe obscurus (1.5% of a total of 220–550 serotonin-immunoreactive neurons counted). The dorsal, median and pontine raphe nuclei as well as the supralemniscal nucleus (the B9 group) contained very few double-labelled cells, which comprised a mean of 0.1–0.7% of all serotonin-immunoreactive cells in each of these nuclei. No double labelled cells were present in the caudal linear raphe nucleus or the nucleus raphe pallidus, nor in the B4 group. These results suggest that only a very small percentage of serotonergic neurons in the medullary raphe nuclei (raphe magnus and raphe obscurus) also contain GABA, whereas such cells are virtually absent in the midbrain raphe nuclei or in the non-raphe serotonergic cell groups in the brainstem.     

Influence of inhibitory and excitatory inputs on serotonin efflux differs in the dorsal and median raphe nuclei

The dorsal (DRN) and median raphe nuclei (MRN) are two major sources of serotonergic projections to forebrain that are involved in regulation of behavioral state and motor activity, and implicated in affective disorders such as depression and schizophrenia. To investigate afferent influences on serotonergic neurons, this study compared the role of endogenous GABA and glutamate in the DRN and MRN using microdialysis and measurement of locomotor activity in freely behaving rats. Local infusion of the GABA(A) receptor antagonist bicuculline increased serotonin (5-HT) efflux in the DRN but not the MRN. In contrast, infusion of glutamate receptor antagonists produced larger decreases in 5-HT efflux in the MRN compared with the DRN. Moreover, glutamate receptor antagonists attenuated the increase in 5-HT efflux produced by GABA receptor blockade in the DRN. Thus, the disinhibitory effect of GABA blockers could be ascribed in part to an enhanced influence of glutamate. Measurements of locomotor activity indicate that changes in 5-HT were not simply correlated with behavioral activity induced by drug infusion. In summary, the role of inhibitory and excitatory afferents was strikingly different in the DRN and MRN. GABA afferents were the predominant tonic influence on serotonergic neurons in the DRN. In contrast, glutamatergic but not GABAergic afferents had a strong tonic influence on serotonergic neurons in the MRN.     

Serotonergic lesion of median raphe nucleus alters nerve growth factor content and vulnerability of cholinergic septohippocampal neurons in rat.

About 45% of the serotonergic raphe neurons are reported to express nerve growth factor (NGF) receptors. We therefore investigated whether selective serotonergic lesions of the median or dorsal raphe nuclei are associated with changes in NGF protein levels of the brain and whether the loss of serotonergic function alters the vulnerability of cholinergic septohippocampal neurons. In adult rats the hippocampal NGF content changed in a biphasic way after lesion of the median raphe nucleus by 5,7-dihydroxytryptamine (5,7-DHT), with a significant increase after 2-3 weeks of up to 35%, followed by a significant reduction of 22% below control levels after 7 weeks, and a return to control levels within the following 4 weeks. By contrast, the decrease in hippocampal serotonin and 5-hydroxyindoleacetic acid remained throughout the observation period of 11 weeks, being still reduced to 15 and 30% of the control levels, respectively. In the frontal cortex the partial loss of the serotonergic innervation projecting from the median raphe was associated 5 weeks after 5,7-DHT injection with an increase in NGF protein of 39.7+/-9.6% (P<0.05), which remained elevated up to 11 weeks. At 9 weeks after 5,7-DHT, the lesion of the septohippocampal cholinergic neurons induced by the cholinotoxin ethylcholine aziridinium (AF64A) was exaggerated (P<0.05) as compared to AF64A-treated rats with intact serotonergic innervation. The present data indicate that a serotonergic lesion of the median raphe nucleus results in biphasic changes of NGF protein content and in a delayed increase in the vulnerability of septohippocampal cholinergic neurons.     

Female sexual behaviors in male rats with dorsal raphe nucleus lesions: Treatment with p-chlorophenylalanine

The effects of the serotonin-synthesis inhibitor, p-chlorophenylalanine (PCPA) on female sexual behaviors were examined in male rats with or without lesions (DRL) of the dorsal raphe nucleus, which contains a large number of serotonergic cell bodies. Estrogen-primed castrated males without brain surgery (control) showed extremely low levels of lordosis compared with females. On the other hand, DRL males displayed lordosis response more frequently than control males, but the lordosis quotient (LQ) in this group was lower than that in females. As well as DRL males, all PCPA-treated males showed lordosis, the mean LQ being comparable to the DRL group. Thus, the destruction of the dorsal raphe nucleus or the deprivation of serotonin by PCPA treatment facilitates manifestation of lordosis behavior in male rats. However, synergistic effect of DRL and PCPA treatments on female sexual behaviors have not been observed. The mean LQ in PCPA-treated male rats with DRL was almost the same as in DRL males or PCPA-treated males. These results suggest that the possible site of action of PCPA in regulating female sexual behavior in male rats is the serotonergic neurons in the dorsal raphe nucleus. Furthermore, the lordosis-facilitating effect of DRL is due to destruction of the serotonergic cell bodies in the dorsal raphe nucleus.     

Hyperinnervation of the striatum by dorsal raphe afferents after dopamine-depleting brain lesions in neonatal rats

Dopamine-depleting brain lesions produced in neonatal rats by intraventricular injection of 6-hydroxydopamine (6-OHDA) are known to increase striatal levels of serotonin by adulthood. We now report that such lesions lead to an increased innervation of the striatum by serotonin-containing neurons from the dorsal raphe nucleus. This hyperinnervation was revealed as a 300% increase in the number of retrogradely labeled neurons in the dorsal raphe after horseradish peroxidase was injected into the adult striatum. The degree of hyperinnervation was reduced sharply when serotonin-depleting brain lesions were given to adult animals that had received 6-OHDA as neonates.     

Effect of 5-HT depletion of the hippocampus on neuronal transmission from perforant path through dentate gyrus

Stimulation of the perforant pathway (pp) elicits a characteristic evoked action potential (EAP) in the granule cell layer of the dentate gyrus. The EAP was recorded in rats depleted of hippocampal serotonin (5-HT) by prior injection of p-chloroamphetamine (PCA) or 5,7-dihydroxy-tryptamine (5,7-DHT) as well as in untreated animals during two behavioral states, slow-wave sleep (SWS) and still-alert behavior (SAL). As reported previously, in untreated rats the amplitude of the EAP response was significantly greater during SWS than SAL. Stimulation of the median raphe nucleus (MR) prior to stimulating the pp (prestimulation) augmented the EAP response, but only during SWS. In contrast, in animals injected with PCA or 5,7-DHT there was no difference of the amplitude of the EAP during SWS and SAL. However, the augmentation of the EAP during SWS produced by prestimulation of the median raphe was still present. It is concluded that 5-HT innervation of the dentate gyrus may be involved in the behavioral modulation of the EAP response. Modulation of the EAP following prestimulation of the MR appears to be effected by a non-serotonergic input to the dentate gyrus originating in, or coursing through, the median raphe.     

Functional dissociation between serotonergic pathways in dorsal and ventral hippocampus in psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition in rats

Altered hippocampal function and brain serotonin activity are implicated in the development and symptoms of schizophrenia. We have previously shown that lesions of the median raphe nucleus, but not the dorsal raphe nucleus, produced a marked enhancement of locomotor hyperactivity induced by phencyclidine and disruption of prepulse inhibition. The dorsal and ventral hippocampus receive serotonin projections predominantly from the median raphe nucleus and dorsal raphe nucleus, respectively. Therefore, we investigated the effect of local lesions of serotonin projections into the dorsal and ventral hippocampus on psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition. Male Sprague-Dawley rats were anaesthetized with pentobarbitone and stereotaxically microinjected with 5 microg of the serotonergic neurotoxin 5,7-dihydroxytryptamine into either the dorsal or the ventral hippocampus. Two weeks after surgery, dorsal hippocampus-lesioned rats showed a 100% enhancement of the locomotor hyperactivity caused by phencyclidine treatment and a slight but significant reduction of the effect of amphetamine. Prepulse inhibition was significantly disrupted in lesioned rats and serotonin levels in the dorsal hippocampus were reduced by 80%. Rats with lesions of the ventral hippocampus showed 85% depletion of serotonin and partial disruption of prepulse inhibition, but no significant changes in the effect of phencyclidine or amphetamine. These results suggest that serotonin projections from the median raphe nucleus to the dorsal hippocampus play an important role in locomotor hyperactivity and prepulse inhibition in rats, animal models of aspects of schizophrenia. This suggests that these serotonin projections may be involved in the pathophysiology of schizophrenia symptomology.     

Neurotoxic effects of p-chloroamphetamine on the serotoninergic innervation of the trigeminal motor nucleus: a retrograde transport study

The rat forebrain receives projections from both dorsal and median raphe nuclei. It has recently been shown that serotoninergic axons arising from the dorsal raphe nucleus, but not those from the median raphe nucleus, degenerate following systemic administration of p-chloroamphetamine (PCA). The present study was conducted to determine (i) whether the motor nucleus of the trigeminal nerve is innervated by overlapping projections from multiple serotonin cell groups and (ii) whether a particular subset of serotoninergic axon terminals in the trigeminal motor nucleus are sensitive to the neurotoxic effects of PCA. Retrograde transport was used in combination with immunofluorescence to identify the serotonin-positive cells that project to the trigeminal motor nucleus both in control rats and in rats previously treated with PCA. In untreated rats, an average of 95 retrogradely labeled serotonin-positive neurons were found in the dorsal raphe nucleus, 135 in the nucleus raphe obscurus, 132 in the nucleus raphe pallidus and 63 in the ventrolateral medulla. After treatment with PCA, there was a marked decrease (-77%) in the number of retrogradely labeled serotoninergic neurons in the dorsal raphe nucleus, whereas the number of labeled neurons was unchanged in the raphe obscurus and raphe pallidus. These results demonstrate that PCA selectively lesions serotonin axon terminals arising from the dorsal raphe nucleus, while sparing projections from the raphe obscurus and raphe pallidus to the trigeminal motor nucleus. This conclusion is in agreement with previous findings that in the forebrain only axons from the dorsal raphe are vulnerable to PCA. The data provide further evidence that serotoninergic axons originating in the dorsal raphe nucleus differ from other serotoninergic axons in their pharmacological properties and that the dorsal raphe may contain a functionally unique subset of serotonin neurons.     

The identification of serotonergic neurons in the nucleus raphe magnus by conduction velocity

Histochemical studies have described serotonergic neurons in the nucleus raphe magnus, and since their axons are small, they would be expected to have low conduction velocities. However, previous studies have reported few slow-conducting units in the nucleus. In these studies, raphe-spinal neurons, detected by antidromic activation, were found to exhibit a wide range of conduction velocities, including numbers of slow-conducting units. In a second set of experiments, the number of raphe-spinal units found in control rats and those treated with 5,7-dihydroxytryptamine, a serotonin neurotoxin, were compared. Two groups of slow-conducting units were reduced in treated animals and those units, conducting between 0.7–1.0 m/sec and 3.1–6.0 m/sec, were presumed to be serotonergic. These neurons comprised about 40% of the total found in the nucleus.     

Opioid receptor-like 1 (NOP) receptors in the rat dorsal raphe nucleus: evidence for localization on serotoninergic neurons and functional adaptation after 5,7-dihydroxytryptamine lesion

A high density of opioid receptor-like 1 (ORL1) receptor (also referred to as NOP receptor) is found in limbic areas and in regions containing monoamines, which are implicated in emotional activity and physiopathology of depression and anxiety. We aimed at defining precisely the localization of ORL1 receptors in dorsal raphe nucleus, by means of a lesion strategy and autoradiographic studies. In control rats, [3H]nociceptin and nociceptin-stimulated [35S]GTPgammaS bindings were found to be correlated in several brain regions. We performed in rats a selective destruction of serotoninergic neurons by surgical stereotaxic injection of 5,7-dihydroxytryptamine (5,7-DHT) in dorsal raphe nucleus. This led to a marked decrease in serotonin contents in striata and frontal cortices (about -60%) and in autoradiographic [3H]citalopram binding in posterior regions. In dorsal raphe nucleus, [3H]nociceptin binding was decreased to the same extent as [3H]citalopram binding, whereas it was unchanged in the other regions studied. Nevertheless, in the dorsal raphe, nociceptin-stimulated [35S]GTPgammaS binding was decreased to a lesser extent than [3H]nociceptin binding in 5,7-DHT-lesioned rats. The ratio between nociceptin-stimulated [35S]GTPgammaS binding and [3H]nociceptin binding was significantly increased in 5,7-DHT-lesioned rats compared with controls in this region. These data demonstrate 1) that ORL1 receptors are located on serotoninergic neurons in the dorsal raphe nucleus and 2) that, after a lesion, the functionality of remaining ORL1 receptors appears to be up-regulated, which could correspond to a compensatory mechanism.     

The serotonergic inhibition of slowly bursting cells in the intergeniculate leaflet of the rat

Electrophysiological studies combined with local neurotoxic lesions were conducted on anaesthetized rats in order to determine whether the dorsal raphe nucleus (DRN) inhibits the intergeniculate leaflet (IGL) of the lateral geniculate nucleus by means of innervation by serotonin-containing fibres. In the control animals, electrical stimulation of the DRN induced the long-latency and long-lasting inhibition of the neuronal firing of the IGL cells that are characterized by rhythmic, slow-bursting activity in light conditions. The electrical destruction of the DRN resulted in an increase in the firing rate of the recorded IGL cells, whilst at the same time not affecting the rhythmic, bursting pattern of the activity. In the second group of animals, local neurotoxic lesion of serotonergic fibres was performed by injection of the toxin 5,7-dihydroxytryptamine into the IGL. After 10 days of postoperative recovery, electrophysiological experiments were performed on the toxin-treated rats. In these animals, electrical stimulation as well as electrical lesion of the DRN did not induce any change in the firing of the slowly bursting cells in the 5,7-dihydroxytryptamine-injected IGL. The results obtained provide evidence that inhibition of the IGL slowly bursting cells, by innervation from the dorsal raphe, is mediated by the release of serotonin. Furthermore, the observed serotonergic inhibition of the light-dependent activity of slowly bursting cells can contribute to the neuronal mechanism gating the information that flows through this nucleus to the vestibular, visuomotor, circadian and sleep/arousal systems, with which the IGL is strongly interconnected.