Neurons In Median Raphe Nucleus Research Articles

Sex-specific modulation of the medial prefrontal cortex by glutamatergic median raphe neurons.

A substantial proportion of raphe neurons are glutamatergic. However, little is known about how these glutamatergic neurons modulate the forebrain. We investigated how glutamatergic median raphe nucleus (MRN) input modulates the medial prefrontal cortex (mPFC), a critical component of fear circuitry. We show that vesicular glutamate transporter 3 (VGLUT3)-expressing MRN neurons activate VGLUT3- and somatostatin-expressing neurons in the mPFC. Consistent with this modulation of mPFC GABAergic neurons, activation of MRN (VGLUT3) neurons enhances GABAergic transmission in mPFC pyramidal neurons and attenuates fear memory in female but not male mice. Serotonin plays a key role in MRN (VGLUT3) neuron-mediated GABAergic plasticity in the mPFC. In agreement with these female-specific effects, we observed sex differences in glutamatergic transmission onto MRN (VGLUT3) neurons and in mPFC (VGLUT3) neuron-mediated dual release of glutamate and GABA. Our results demonstrate a cell type-specific modulation of the mPFC by MRN (VGLUT3) neurons and reveal a sex-specific role of this neuromodulation in mPFC synaptic plasticity.

Food intake increased after injection of adrenaline into the median raphe nucleus of free-feeding rats

The present study examined the effects of local injections of adrenaline (AD) or noradrenaline (NA) in equimolar doses (6, 20, and 60 nmol) into the median raphe nucleus (MRN) on ingestive and non-ingestive behaviors of free-feeding rats. The results showed that the treatment with AD at doses of 20 and 60 nmol increased food intake. While the hyperphagic response evoked by 60 nmol dose of AD was accompanied by a reduction of the latency to start feeding and an increase in the frequency of feeding, the 20 nmol dose of AD was unable to change these behavioral aspects of feeding response. The meal size and non-ingestive behaviors were not affected by AD treatment in the MRN. While water intake remained unchanged after the treatment with 20 nmol of AD in the MRN, this dose decreased the latency to start drinking. Feeding and drinking behaviors were not affected by treatment with NA in the MRN. These data suggest that adrenergic receptors of MRN participate in mechanisms that control food intake initiation or appetite. In addition, our results also indicate that the availability of energetic substrate could affect the adrenergic influence on MRN neurons since previous data indicated that the injection of AD into the MRN of food restricted rats decreased food intake.

Intra-median raphe nucleus (MRN) infusions of muscimol, a GABA-A receptor agonist, reinstate alcohol seeking in rats: role of impulsivity and reward

We previously found that the inhibition of median raphe nucleus (MRN) 5-HT transmission by local injections of a 5-HT1A agonist 8-OH-DPAT or corticotrophin-releasing factor (CRF) mimic the effect of foot shock stress to reinstate alcohol seeking. In this study, we further explored the role of the MRN by examining the effect of inhibition of MRN neurons, by injecting the GABA-A receptor agonist muscimol, on the reinstatement of alcohol seeking. Male rats were trained to lever press for 12% alcohol. Cannulae were implanted aimed at the MRN; some rats were also given intra-MRN injections of 5,7-DHT to destroy ascending 5-HT neurons. After retraining, alcohol responding was extinguished for 14 days. Subsequently, we tested the effect of muscimol injections into the MRN (0, 12.5, 25, 50 ng) on reinstatement. We also tested the effect of MRN muscimol injections on a measure of reward, conditioned place preference (CPP) and performance in the five-choice serial reaction time task (5-CSRTT), which tests a variety of psychological processes including sustained attention and impulsivity. MRN muscimol injections strongly reinstated alcohol seeking and this effect was not reversed by the depletion of 5-HT with 5,7-DHT. MRN muscimol injections did not induce a CPP, but did significantly impair multiple aspects of performance on the 5-CSRTT, including a marked increase in premature, or impulsive, responding. Together with our previous findings, these results suggest that the inhibition of MRN projection neurons provokes alcohol seeking. Results from the 5-CSRTT suggest that increased impulsivity may contribute to these effects.

Effects of lidocaine reversible inactivation of the median raphe nucleus on long-term potentiation and recurrent inhibition in the dentate gyrus of rat hippocampus

Considering the fact that median raphe nucleus (MRN) constitutes one of the inputs of the hippocampus, the effects of reversible inactivation of MRN on long-term potentiation (LTP) and recurrent inhibition in the dentate gyrus (DG) of rat hippocampus, in vivo, were examined. Rats were anesthetized with urethane (1.5 g/kg, i.p.). MRN was temporarily suppressed by intra-MRN injection of lidocaine (0.5 μl, 2%). For LTP induction, eight episodes of high frequency stimuli (100 Hz) were delivered to the perforant path (PP), each consisting of 10 stimuli at 100 Hz. Population spikes (PS) and population excitatory post synaptic potentials ( PEPSP) in DG were recorded 10 min before, and 5, 10, 20, 40, 60 and 120 min after tetanization. MRN inactivation itself had no effect on the amplitude of baseline responses. The PS amplitude and PEPSP slope in rats, injected with intra-MRN lidocaine, 5 min before tetanization, were not different from the control group. However, at 120 min PS amplitude was significantly higher than control. Lidocaine injection 5 min after tetanic stimuli caused a significant decrease in PS amplitude (10, 20 and 60 min) and PEPSP slope (20 and 40 min) after tetanization. The data showed that inactivation of MRN has no effect on LTP induction in the DG of hippocampus but it does affect its maintenance, and this effect depends on the pre- or post-tetanic inactivation. In the last part of this study, in order to investigate the effect of MRN on the efficacy of recurrent inhibition in the perforant–dentate synapses, paired pulse was applied to the PP at 10 and 20 ms interpulse intervals. Inactivation of MRN increased the amount of recurrent inhibition in the DG with 20 ms interpulse interval. This observation indicates that MRN inhibits the recurrent inhibition mechanism, which is in accordance with the suggested role of MRN neurons on inhibition of hippocampal GABAergic interneurons.

Morphological Study of 5-HT Neurons and Astroglial Cells on Brain of Adult Rats Perinatal or Chronically Exposed to 2,4-Dichlorophenoxyacetic Acid

2,4-D is a chlorophenoxyherbicide used worldwide. We have studied the morphological alterations of 5-HT neurons and glial cells in the mesencephalic nuclei of adult rats exposed to 2,4-D both perinatally (during pregnancy and lactation) and chronically (during pregnancy, lactation and after weaning) with quantitative methods. Pregnant rats were daily exposed to 70 mg/kg of 2,4-D from gestation day (GD) 16 to post-natal day (PND) 23 through diet. After weaning, pups were assigned to one of two sub-groups: T1 (fed with untreated diet until PND 90) and T2 (maintained with 2,4-D diet until PND 90). Brain sections were immunocytochemically stained using polyclonal anti-5-HT, anti-GFAP and anti-S-100 protein antibodies as cells markers. 2,4-D exposure during pregnancy and lactancy (T1 group) produced an increase in 5-HT neuronal area and immunoreactivity (IR) in the mesencephalic nuclei studied. However, with the chronical 2,4-D exposure (T2 group) only the 5-HT neuronal area from the dorsal raphe nucleus (DRN) was increased, suggesting an adaptable response of 5-HT neurons in median raphe nucleus (MRN). The presence of reactive astrocytes in mesencephalic nuclei and in hippocampus were also different for the two 2,4-D exposure designs, showing the existence of a correspondence between neuronal changes and astrogliosis. Results support evidences that 2,4-D alters the serotoninergic system and that 5-HT neurons of each mesencephalic nuclei show different responses to the 2,4-D exposure designs which are parallel to astrogliosis.

Sexual dimorphism in number and proportion of neurons in the human median raphe nucleus

The number and proportion of neurons in the median raphe nuclei stained by the Golgy–Cox and Nissl methods was compared in males and females infants. When subjects are matched by age and cause of death the number and proportion of fusiform, ovoid and multipolar cells differs significantly between sexes at different ages.

Alteration of the serotonergic nervous system in fatal familial insomnia

Fatal familial insomnia (FFI) is a unique hereditary prion disease with characteristic disturbances of sleep. We studied the serotonergic system in 8 FFI-affected subjects by immunohistochemistry for the serotonin-synthesizing enzyme, tryptophan hydroxylase (TH). Quantification of neurons in median raphe nuclei showed no total neuronal loss in FFI but a substantial increase of TH+ neurons (approximately 62%) in FFI subjects compared with controls. Our data indicate an alteration of the serotonergic system that might represent the functional substrate of some typical symptoms of FFI.

In Syrian hamsters, 5-HT fibres within the suprachiasmatic nuclei are necessary for the expression of 8-OH-DPAT induced phase-advance of locomotor activity rhythm

8-Hydroxy-2-[di-n-propylamino]-tetralin) (8-OH-DPAT), a 5-HT 1A/7 receptor agonist, has a chronobiological effect on the circadian system. To identify how the 8-OH-DPAT exerts this effect, we specifically destroyed the serotonergic (5-HT) fibres connecting the median raphe nuclei (RN) to the suprachiasmatic nuclei (SCN) of the hypothalamus by using microinjections of a neurotoxin 5,7-dihydroxytryptamine into the SCN. After administration of 8-OH-DPAT (0.1 ml, 5 mg/kg) at circadian time 7, the control and the `partially-lesioned' animals showed a large phase-advance whereas in the `well-lesioned' hamsters the phase-advances were significantly reduced or absent. The present study demonstrates that, in the Syrian hamster, the 5-HT fibres connecting the RN to the SCN are essential for the phase-shifting action of peripheral 8-OH-DPAT injections, and that the drug does very probably not exert its chronobiological effect directly onto SCN neurons but through receptors localized on median raphe nucleus neurons.

Inhibition of median and dorsal raphe neurones following administration of the selective serotonin reuptake inhibitor paroxetine

Acute systemic injection of selective serotonin reuptake inhibitors (SSRIs) decreases 5-HT neuronal firing in the dorsal raphe nucleus (DRN). Recent data, however, question whether these drugs also inhibit the firing of 5-HT neurones in the median raphe nucleus (MRN). Using in vivo extracellular electrophysiological recording techniques in the chloral hydrate anaesthetised rat, we have tested the effect of acute administration of the SSRI, paroxetine, on 5-HT neuronal activity in the MRN and DRN. Presumed 5-HT neurones in the MRN displayed the same electrophysiological characteristics as those in the DRN, the only detectable difference being that MRN neurones showed a significantly (p < 0.001) slower mean (+/- SEM(n)) spontaneous firing rate (MRN, 5.6 +/- 0.9 (14) spikes/10 s; DRN, 13.5 +/- 1.6 (24) spikes/10 s). Paroxetine caused a dose-related (0.1-0.8 mg/kg i.v.) inhibition of all MRN neurons tested (n = 8), producing a complete cessation of cell-firing at the highest doses. DRN neurones (n = 9) responded in a similar fashion. Furthermore, paroxetine inhibited MRN and DRN neurones with almost identical potency (MRN ED50 259 +/- 57 micrograms/kg i.v.: DRN ED50 243 +/- 49 micrograms/kg i.v.). In the majority of cells tested, the effect of paroxetine was reversed by the 5-HT1A receptor antagonists spiperone or (+)WAY100135, implicating the involvement of the 5-HT1A autoreceptor. The selective 5-HT1A receptor agonist 8-OH-DPAT also inhibited the firing of MRN (n = 5) and DRN (n = 12) neurones and with equal potency (MRN ED50, 1.32 +/- 0.40 microgram/kg i.v.: DRN ED50, 1.19 +/- 0.23 microgram/kg i.v.).(ABSTRACT TRUNCATED AT 250 WORDS)

Transplanted raphe and hippocampal fetal neurons do not displace afferent inputs to the dorsal hippocampus from serotonergic neurons in the median raphe nucleus of the rat

To determine if fetal transplants can substitute for or suppress intrinsic serotonergic (5-HT) innervation, we studied the relationship between transplanted and the endogenous raphe neurons projecting to the hippocampus. Fetal raphe transplants produced a 5-HT hyperinnervation of dorsal hippocampus in adult rats. Yet, transplants of fetal raphe tissue did not affect the number of median raphe nucleus (MRN) neurons, approximately 300, which retrogradely transported HRP from the hippocampus. This provides evidence that transplanted 5-HT neurons can co-exist with intrinsic 5-HT nerve terminals in the target area for at least one month. In the second part of this study, fetal hippocampal tissue was transplanted into the host hippocampus. Intrinsic 5-HT immunoreactive fibers innervated the transplanted fetal tissue. Nevertheless, the number of MRN neurons innervating the host tissue as revealed by HRP retrograde transport remained unchanged. Changes in the innervation pattern and 5-HT level in the dorsal hippocampus occur following transplantation of fetal tissue. These changes are discussed and suggest that both the target tissue and afferent neurons readjust to accommodate extrinsic transplanted tissue.

Induced homotypic collateral sprouting of serotonergic fibers in the hippocampus of rat

The organization of the raphe cells efferent to the dorsal hippocampus (DHipp) has been shown to consist of two homologous groups of cells in the median raphe nucleus (MRN). Their axons project by way of the cingulum bundle (CB) and the fornix-fimbria (FF)58. A group of neurons located in the interfascicular part of the dorsal raphe nucleus (IFN) was found to use the CB route exclusively. Quantitation of the HRP-labeledd cells in the MRN neurons projecting to the DHipp decreased by 55% and 92% 3 days following CB and CB-FF 5, 7-DHT injections respectively.There was a significant increase in HRP-labeled neurons in the MRN at 21 and 42 days compared to 3 days after CB injection (a 2% decrease at 21 days and 38% increase at 42 days compared to sham injected). No change in the number of labeled MRN cells was seen at these long-term time points after combined CB-FF 5, 7-DHT microinjections. The labeled cells in the MRN increased in size in the long survival group of CB lesioned animals. The labeled cells localized in the IFN which were lost after CB 5, 7-DHT injections did not reappear after loong-term survival. These results indicate that CB 5-HT fibers do not regenerate into the DHipp, but FF 5-HT fibers increase their terminal territory by collateral sprouting in response to homotypic denervation.