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.     

Phencyclidine-induced locomotor hyperactivity is enhanced in mice after stereotaxic brain serotonin depletion

The aim of this study was to investigate the role of forebrain serotonin projections in behavioural models with relevance to schizophrenia. Mice received stereotaxic micro-injections of the serotonin neurotoxin 5,7-dihydroxytryptamine into the median raphe nucleus (MRN). Two weeks later, MRN-lesioned mice were hyperactive at baseline and showed enhanced locomotor hyperactivity induced by phencyclidine. In contrast, no lesion effect was observed on the locomotor hyperactivity induced by amphetamine treatment or on prepulse inhibition. Lesioned mice showed a 68% depletion of serotonin in the hippocampus and 31% depletion in the striatum. These data confirm previous studies in rats that selective serotonin depletion in the brain enhances the effect of phencyclidine, but not amphetamine, on locomotor activity. This enhanced action of phencyclidine is likely to be mediated by the absence of serotonin-mediated behavioural inhibition in the hippocampus, leaving the psychostimulant effects of phencyclidine unopposed. Taken together with previous studies in rats, these studies in mice suggest that serotonin release in the dorsal hippocampus constitutes a behavioural inhibitory pathway normally involved in dampening excessive behavioural stimulation. Dysfunction of this pathway could be involved in psychosis and its stimulation could be a potential mechanism of action of antipsychotic drugs.     

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.     

The dorsal and medial raphe projections to the substantia nigra in the rat: Electrophysiological, biochemical and behavioural observations

Electrophysiological experiments have been performed in urethane anaesthetized rats to investigate the projections from the dorsal (DRN) and medial raphe nuclei (MRN) to the substantia nigra. The biochemical and behavioural effects following discrete electrolytic lesions in the dorsal and medial raphe have also been investigated.Stimulation of the DRN produced predominantly inhibition of spontaneous activity of single neurones in the substantia nigra though some neurones were also excited. Bilateral stimulation of the substantia nigra produced antidromic spikes in DRN and MRN neurones.Lesions of the DRN and MRN produced a significant reduction in substantia nigra 5-HT concentration. Additionally, DRN lesions reduced striatal 5-HT, while MRN lesions reduced hippocampal 5-HT. Both lesions increased substantia nigra HVA concentration but did not affect DA concentration. Neither DRN nor MRN lesions affected striatal HVA, although DA levels were significantly elevated after 14 days.Animals with DRN lesions explored more than controls or MRN-lesioned animals. However, this behaviour was transient and was not observed after 14 days. On the other hand, MRN-lesioned animals were significantly hyperactive.These observations suggest that the substantia nigra receives a direct monosynaptic inhibitory input from the DRN and MRN and that these pathways use 5-HT as a neurotransmitter serving to tonically inhibit dopaminergic neurones. While 5-HT and dopamine appear to be involved in the control of motor behaviour, the precise relationship between these serotoninergic and dopaminergic systems in this respect is unclear.     

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.     

Phenotype and function of raphe projections to the suprachiasmatic nucleus

The circadian clock, located in the suprachiasmatic nucleus (SCN), receives a major afferent from the median raphe nucleus (MRN). In the Syrian hamster, only about 50% of the cells giving rise to this afferent contain serotonin. There is mixed evidence as to whether the serotonergic portion of this projection is involved in non‐photic phase shifting of circadian locomotor rhythms. In order to better characterize the non‐serotonergic projections, we conducted retrograde tract tracing using the beta subunit of cholera toxin combined with multi‐label immunohistochemistry. Similar to previous findings, almost half of the retrogradely labeled cells contained serotonin. Additionally, approximately 30% of the retrogradely labeled cells contained vesicular glutamate transporter 3 (VGLUT3), but not serotonin. Surprisingly, some dorsal raphe cholera toxin labeling was also noted, particularly in animals with central‐SCN injections. To determine if the non‐serotonergic projections were important for non‐photic phase shifts elicited by MRN stimulation, the MRN was electrically stimulated in animals pretreated with SCN injection of either the serotonin neurotoxin 5,7‐dihydroxytryptamine or vehicle control. Intact animals phase advanced to midday electrical stimulation of the raphe while lesioned animals did not. Together, these results show that although some of the non‐serotonergic raphe projections to the SCN contain VGLUT3, it is the serotonergic raphe innervation of the SCN that is critical for non‐photic phase shifting elicited by MRN stimulation.     

Divergent innervation of the olfactory bulb by distinct raphe nuclei

The raphe nuclei provide serotonergic innervation widely in the brain, thought to mediate a variety of neuromodulatory effects. The mammalian olfactory bulb (OB) is a prominent recipient of serotonergic fibers, particularly in the glomerular layer (GL), where they are thought to gate incoming signals from the olfactory nerve. The dorsal raphe nucleus (DRN) and the median raphe nucleus (MRN) are known to densely innervate the OB. The majority of such projections are thought to terminate in the GL, but this has not been explicitly tested. We sought to investigate this using recombinant adeno-associated viruses (rAAV)-mediated expression of green fluorescent protein (GFP)-synaptophysin targeted specifically to neurons of the DRN or the MRN. With DRN injections, labeled fibers were found mostly in the granule cell layer (GCL), not the GL. Conversely, dense labeling in the GL was observed with MRN injections, suggesting that the source of GL innervation is the MRN, not the DRN, as previously thought. The two raphe nuclei thus give dual innervation within the OB, with distinct innervation patterns. J. Comp. Neurol. 523:805–813, 2015. © 2015 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.     

Effect of lesions of raphe nuclei on the activity of catecholaminergic and serotonergic neurones in various brain regions of the ratin vivo

Summary The dorsal raphe nucleus (DRN), the median raphe nucleus (MRN) and the B9 cell groups were lesioned separately. Then, the concentrations of 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), dopamine (DA), 5-hydroxytryptophan (5-HTP) and dopa were measured in various brain regions. The levels of the two latter compounds presumably reflect the activity of serotonergic and Catecholaminergic neurones respectively under the experimental conditions. The findings suggest close functional interrelationships between the two types of neurones. Lesions of cell bodies in the DRN cause decreased activity of Catecholaminergic neurones in the striatum, the brainstem, the hippocampus, and the hypothalamus. Lesions of the MRN cause increased activity of Catecholaminergic mechanisms in the hypothalamus which may be explained by a functional disinhibition after destruction of the MRN. The other brain regions investigated are not affected. Lesions of the B9 cell group elicit increased activity of catecholaminergic neurones in the brainstem and hippocampus.     

Hippocampal mediation of raphe lesion- and PCPA-induced hyperactivity in the rat.

These experiments provide a direct test of the hypothesis that median raphe lesion- or PCPA-induced hyperactivity in the rat is mediated specifically by the hippocampus. Previous studies had shown that a marked depletion of hippocampal serotonin accompanied median lesion-induced hyperactivity. In the present studies, aspiration of the anterodorsal hippocampus of adult male rats prior to median raphe lesions or PCPA administration abolished the ability of both of these treatments to produce locomotor hyperactivity in animals chronically housed in tilt cages. Control lesions of the overlying dorsal cortex and corpus callosum were ineffective in blocking the hyperactivity produced by these two treatments. The possibility that serotonin depletion-induced hyperactivity was dependent on the pituitary was excluded by the fact that PCPA effectively elevated the activity of hypophysectomized rats. These data indicate that serotonin depletion-induced hyperactivity in the rat is mediated by the hippocampus.     

Local treatments of dorsal raphe nucleus induce changes in serotonergic activity in rat major cerebral arteries.

BACKGROUND AND PURPOSE: Rat major cerebral arteries seem to receive serotonergic fibers originating from the dorsal raphe nucleus (DRN), but little is known about their function. The aim of our present work was to establish a functional relationship between this brain stem nucleus and the cerebral blood vessels by studying the effects of several treatments in the DRN on cerebrovascular serotonergic activity. METHODS: Serotonin, clomipramine, 8-OH-DPAT, and WAY-100635 were administered in DRN. A stereotaxically localized electrode allowed the electrical stimulation of this brain stem nucleus. Serotonergic activity was appraised in major cerebral arteries, striatum, and hippocampus from 5-hydroxytryptophan accumulation after aromatic L-amino acid decarboxylase inhibition with NSD-1015. RESULTS: Serotonin significantly decreased serotonergic activity in major cerebral arteries and striatum without affecting it in hippocampus. This reduction was blocked by previous injection of WAY-100635 in DRN. Local administration of 8-OH-DPAT or clomipramine elicited an effect similar to that of serotonin, whereas that of WAY-100635 did not modify serotonergic activity in either of the tissues. Electrical stimulation of DRN significantly increased serotonergic activity in major cerebral arteries and striatum but not in hippocampus. CONCLUSIONS: These results confirm the presence of a serotonergic innervation in rat major cerebral arteries functionally related to DRN. 5-HT(1A) receptor activation partly mediates the action of serotonin in DRN. A serotonergic tone acting on these somatodendritic receptors was not clearly found.     

Median and Dorsal Raphe Serotonergic Neurons Control Moderate Versus Compulsive Cocaine Intake

Background

Reduced expression of the serotonin transporter (SERT) promotes anxiety and cocaine intake in both humans and rats. We tested the hypothesis that median raphe nucleus (MRN) and dorsal raphe nucleus (DRN) serotonergic projections differentially mediate these phenotypes.

Serotonergic cells of the rat raphe nuclei express mRNA of tyrosine kinase B (trkB), the high-affinity receptor for brain derived neurotrophic factor (BDNF)

Here we have studied the distribution of mRNA for tyrosine kinase B (trkB), the high-affinity receptor for brain-derived neurotrophic factor (BDNF) amongst serotonergic cell bodies of the raphe nuclei and their ascending projections into the dorsal hippocampus in the rat brain. Previous studies have shown that BDNF has got trophic action on serotonergic neurons. In the present study, we provide evidence that serotonergic neurons express mRNA for the functional receptor of BDNF, trkB. Intracerebro-ventricular (i.c.v.) injection of the 5-HT-specific neurotoxin, 5,7-dihydroxytryptamine, which lesions serotonergic cell bodies in the raphe nuclei as well as their ascending projections into the dorsal hippocampus, caused a dramatic loss of trkB mRNA from serotonergic cell bodies of the dorsal raphe nucleus. In contrast, there was no change in the abundance of trkB mRNA within the dorsal hippocampus. These findings provide direct evidence for the expression of trkB mRNA by serotonergic neurons and suggest distinct mechanisms of action of BDNF upon serotonergic neurons at the levels of their cell bodies and terminal projection sites.     

A role for nitric oxide in serotonin neurons of the midbrain raphe nuclei

Neuronal nitric oxide synthase (nNOS) catalyses the production of the neurotransmitter nitric oxide. nNOS is expressed in the dorsal raphe nucleus (DRN), a source of ascending serotonergic projections. In this study, we examined the distribution nNOS and the function of nitric oxide in the DRN and adjacent median raphe nucleus (MRN) of the rat. We hypothesized that nNOS is differentially expressed across the raphe nuclei and that nitric oxide influences the firing activity of a subgroup of 5‐HT neurons. Immunohistochemistry revealed that, nNOS is present in around 40% of 5‐HT neurons, throughout the DRN and MRN, as well as in some non‐5‐HT neurons immediately adjacent to the DRN and MRN. The nitric oxide receptor, soluble guanylyl cyclase, was present in all 5‐HT neurons examined in the DRN and MRN. In vitro extracellular electrophysiology revealed that application of the nitric oxide donor, diethylamine NONOate (30–300 µM) inhibited 60%–70% of putative 5‐HT neurons, excited approximately 10% of putative 5‐HT neurons and had no effect on the rest. The inhibitory response to nitric oxide was blocked by [1H‐[1,2,4]oxadiazolo‐[4, 3‐a]quinoxalin‐1‐one (ODQ, 30 or 100 µM), indicating mediation by soluble guanylyl cyclase. Juxtacellular labelling revealed that nitric oxide inhibits firing in both putative 5‐HT neurons which express nNOS and those which do not express nNOS. Our data are consistent with the notion that nitric oxide acts as both a trans‐synaptic and autocrine signaller in 5‐HT neurons in the DRN and MRN and that its effects are widespread and primarily inhibitory.     

Estrogen selectively increases tryptophan hydroxylase-2 mRNA expression in distinct subregions of rat midbrain raphe nucleus: association between gene expression and anxiety behavior in the open field.

BACKGROUND: Ovarian steroids modulate anxiety behavior, perhaps by regulating the serotonergic neurons in the midbrain raphe nucleus. The regulation of the brain-specific isoform of rat tryptophan hydroxylase (TPH2) by ovarian hormones has not yet been investigated. Therefore, we examined the effects of estrogen and progesterone on TPH2 mRNA in the rat dorsal and median raphe nuclei (DRN and MRN, respectively) and whether TPH2 mRNA levels correlated with anxiety behavior. METHODS: Ovariectomized rats were treated for two weeks with placebo, estrogen or estrogen plus progesterone, exposed to the open field test, and TPH2 mRNA was quantified by in situ hybridization histochemistry. RESULTS: Estrogen increased TPH2 mRNA in the mid-ventromedial and caudal subregions of the DRN and the caudal MRN. Combined estrogen and progesterone treatment did not change TPH2 mRNA relative to ovariectomized controls. TPH2 mRNA in caudal DRN was associated with lower anxiety-like behavior, whereas TPH2 mRNA in rostral dorsomedial DRN was associated with increased anxiety-like behavior. CONCLUSIONS: These results suggest that estrogen may increase the capacity for serotonin synthesis in discrete subgroups of raphe neurons, and reinforce previous observations that different subregions of DRN contribute to distinct components of anxiety behavior.     

Effects of excitotoxic median raphe lesion on working memory deficits produced by the dorsal hippocampal muscarinic receptor blockade in the inhibitory avoidance in rats

The experiments investigated the interactions between median raphe nucleus (MRN) serotonergic and septo-hippocampal muscarinic cholinergic systems in the modulation of forming and storing performances of working memory. Rats with ibotenic acid-induced MRN-lesion bilaterally received scopolamine (2-4 microg/each side) infusion into the dentate gyrus of the dorsal hippocampus and were tested in a single trial step-through inhibitory avoidance. Initial preference to the dark compartment (escape latency) was taken as the measure of non-mnemonic behaviours and response latency to enter the dark compartment immediately after the foot-shock was used to measure working memory. The high-dose scopolamine infusion 10 min before the training decreased escape latencies in the sham-lesioned rats, whereas had no effect in the MRN-lesioned rats. Although MRN lesion per se did not alter response latency, it alleviated pre-training scopolamine-induced decrease, but aggravated post-training scopolamine-induced reduction in this parameter. These results suggest that the antagonistic interactive processes between MRN-serotonergic and hippocampal cholinergic systems modulate non-mnemonic component of working memory formation, whereas the storing performance of working memory is modulated by the synergistic interactions between these systems in the hippocampus, mainly in the dentate gyrus.     

Quantitative mapping of tryptophan hydroxylase-2, 5-HT1A, 5-HT1B, and serotonin transporter expression across the anteroposterior axis of the rat dorsal and median raphe nuclei

Depression and anxiety disorders are among the leading causes of morbidity, mortality, and disability in the United States. Impaired serotonin neurotransmission appears to be a central mechanism inducing depressive and anxiety symptoms. Most serotonergic innervation of the forebrain arises from the median raphe nucleus (MRN) and dorsal raphe nucleus (DRN). The DRN displays a complex internal morphology, with distinct subregions varying across the anteroposterior (A-P) axis. However, many studies have considered the DRN as a whole or used easily confused terminology to describe position. Given the large differences in receptor expression, electrophysiological properties, and connectivity between various subregions of the DRN, it appears probable that they have distinct functional roles in the regulation of behavior. To foster uniform definitions of locations within these nuclei, we have quantitatively mapped gene expression in DRN and MRN for tryptophan hydroxylase-2 (Tph2), the serotonin transporter, as well as 5-HT1A and 5-HT1B receptors. These quantitative studies revealed differences in the density of expression of each gene in the ventromedial, dorsomedial, and dorsolateral subnuclei of the DRN, as well as distinct variation in expression across the A-P axis. These findings provide additional evidence that subregions of the DRN are heterogeneous and need to be considered independently. In addition, a fine scale map of Tph2 expression suggests definitions for categorical divisions of the DRN across the A-P axis. These are based on distinct morphological patterns of Tph2 expression and may be more reflective of physiology than the classic terminology dividing the DRN into equal thirds.     

Regulation of contextual conditioning by the median raphe nucleus

The median raphe nucleus (MRN) has been suggested as the origin of a behavioral inhibition system that projects to the septum and hippocampus. Electrical stimulation of this mesencephalic area causes behavioral and autonomic manifestations characteristic of fear such as, freezing, defecation and micturition. In this study we extend these observations by analyzing the behavioral and autonomic responses of rats with lesions in the MRN submitted to a contextual conditioning paradigm. The animals underwent electrolytic or sham lesions of the median raphe nucleus. One day (acute) or 7 days (chronic) later they were tested in an experimental chamber where they received 10 foot-shocks (0.7 mA, 1 s with 20-s interval). The next day, sham and MRN-lesioned animals were tested again either in the same or in a different experimental chamber. During this, the duration of freezing, rearings, bouts of micturition and number of fecal boli were recorded. Sham-operated rats placed in the same chamber showed more freezing than rats exposed to a different context. This freezing behavior was clearly suppressed in rats with acute or chronic lesions in the MRN. MRN lesions also reduced the bouts of micturition and number of fecal boli. These rats showed a reduced number of rearings than sham-lesioned rats. This effect is probably the result of the displacement effect provoked by freezing since no significant differences in the number of rearings could be observed between these animals and the NMR-lesioned rats tested in an open field. This lesion produced higher horizontal locomotor activity in this test than the controls (sham-lesioned rats). These results point to the importance of the median raphe nucleus in the processing of fear conditioning with freezing being the most salient feature of it. Behavioral inhibition is also under control of MRN but its neural substrate seems to be dissociated from that of contextual fear.     

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.     

When injected into the fimbria-fornix/cingular bundle, not in the raphe, 5,7-dihydroxytryptamine prevents amphetamine-induced hyperlocomotion

The locomotor effects of acute amphetamine treatment (1 mg/kg, i.p.) were assessed in Long-Evans rats after 5,7-dihydroxytryptamine (5, 7-DHT) injections into the fimbria-fornix/cingular bundle (FiFx/CB; 4 microg/side), or the dorsal and median raphe (Raphe; 10 microg). In control rats, amphetamine induced a significant increase of home-cage activity for about 2 h. This effect was similar in Raphe rats, but was absent in FiFx/CB rats. The raphe lesions reduced serotonin concentrations by 50% in the dorsal hippocampus, 75% in the ventral hippocampus and 58% in the fronto-parietal cortex. After FiFx/CB lesions, the reduction amounted 50, 61 and only 25%, in each of these regions, respectively. In the fronto-partietal cortex, dopamine concentration was significantly decreased in Raphe (-27%) and FiFx/CB rats (-65%). The results suggest that a serotonergic denervation of the hippocampus by injections of 5,7-DHT into the FiFx/CB pathways hampers the stimulating effects of amphetamine on locomotor activity. This effect might be related to the reduced dopaminergic tone in the fronto-parietal cortex.     

Whole body sodium depletion modifies AT1 mRNA expression and serotonin content in the dorsal raphe nucleus

Angiotensin II (Ang II) acts on Ang II type 1 (AT1) receptors located in the organum vasculosum and subfornical organ (SFO) of the lamina terminalis as a main facilitatory mechanism of sodium appetite. The brain serotonin (5‐HT) system with soma located in the dorsal raphe nucleus (DRN) provides a main inhibitory mechanism. In the present study, we first investigated the existence of Ang II AT1 receptors in serotonergic DRN neurones. Then, we examined whether whole body sodium depletion affects the gene expression of the AT1a receptor subtype and the presumed functional significance of AT1 receptors. Using confocal microscopy, we found that tryptophan hydroxylase‐2 and serotonin neurones express AT1 receptors in the DRN. Immunofluorescence quantification showed a significant reduction in 5‐HT content but no change in AT1 receptor expression or AT1/5‐HT colocalisation in the DRN after sodium depletion. Whole body sodium depletion also significantly increased Agtr1a mRNA expression in the SFO and DRN. Oral treatment with the AT1 receptor antagonist losartan reversed the changes in Agtr1a expression in the SFO but not the DRN. Losartan injection into either the DRN or the mesencephalic aqueduct had no influence on sodium depletion‐induced 0.3 mol L^‐1 NaCl intake. The results indicate the expression of Agtr1a mRNA in the DRN and SFO as a marker of sodium depletion. They also suggest that serotonergic DRN neurones are targets for Ang II. However, the function of their AT1 receptors remains elusive.