5-HT autoreceptors in the regulation of 5-HT release from guinea pig raphe nucleus and hypothalamus

5-HT autoreceptors involved in the regulation of 5-HT release in the guinea pig dorsal raphe nucleus have been studied in comparison with those in the hypothalamus. In vitro release was measured in slices of raphe and hypothalamus prelabelled with [3H]5-HT, superfused with Krebs solution and depolarized electrically. The non-selective 5-HT receptor agonist, 5-carboxamidotryptamine (5-CT) (0.1-10 nM for raphe: 1-100 nM for hypothalamus) and antagonist, methiothepin (10-1000nM), decreased and increased, respectively, the release of [3H]5-HT evoked by electrical stimulation in either of these regions when given alone. The selective 5-HT1B/D receptor antagonist, GR127935 (100-1000 nM), and the 5-HT1D receptor antagonist, ketanserin (300-1000 nM), had no significant effect on this release in either of these regions. Methiothepin and GR127935 (100-1000 nM) shifted to the right the concentration-effect curve of 5-CT in both the raphe and the hypothalamus. At 300 nM, ketanserin shifted to the right the concentration-effect curve of 5-CT in the raphe but did not modify the 5-CT curve in the hypothalamus. In microdialysis experiments ketanserin, applied locally at 10 microM, increased the extracellular levels of 5-HT in the dorsal raphe nucleus of the freely moving guinea pig, whereas 5-HT levels were unchanged in the hypothalamus. Ketanserin at 1 microM did not affect the decrease in 5-HT output induced by the selective 5-HT1B/D receptor agonist, naratriptan (used at 10 microM in raphe and 0.1 microM in hypothalamus), in the raphe or the hypothalamus. In the raphe, WAY100635, a 5-HT1A receptor antagonist, at 1 microM, did not prevent naratriptan (10 microM) from reducing the extracellular levels of 5-HT. These results suggest that, in the conditions used in this study, the release of 5-HT in the dorsal raphe nucleus is possibly modulated in part by 5-HT1B receptors but essentially the control is through 5-HT receptors whose subtype is still to be determined. In the hypothalamus, however, it is clear that only 5-HT1B receptors are involved in the modulation of 5-HT neurotransmission.     

High serotonin and 5-hydroxyindoleacetic acid levels in limbic brain regions in a rat model of depression: normalization by chronic antidepressant treatment

Although alterations in serotonin levels and neurotransmission are associated with depressive disorders and effective antidepressant therapy, the exact cause of these disorders and the mode of action of antidepressant drugs are poorly understood. In a genetic rat model of depression [Flinders sensitive line (FSL) rats], deviations from normal serotonin (5-HT) levels and metabolism in specific brain regions were determined. The levels of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in tissue punches of various brain regions were quantitated simultaneously with an HPLC apparatus coupled to an electrochemical detector. In the nucleus accumbens, prefrontal cortex, hippocampus, and hypothalamus of FSL rats, the levels of 5-HT and 5-HIAA were three- to eightfold higher than in control Sprague-Dawley rats. Significant differences in the levels of 5-HT and 5-HIAA in the striatum and raphe nucleus of the "depressed" and normal rats were not observed. After chronic treatment with the antidepressant desipramine (5 mg/kg/day for 18 days), the immobility score in a swim test, as a measure of a behavioral deficit, and 5-HT levels of the FSL rats became normalized, but these parameters in the control rats did not change. The [5-HIAA]/[5-HT] ratio was lower in the nucleus accumbens and hypothalamus of the FSL than in the control rats, and increased after desipramine treatment only in the nucleus accumbens of the FSL rats. These results indicate that the behavioral deficits expressed in the FSL model for depression correlate with increased 5-HT levels in specific limbic sites and suggest the FSL rats as a novel model for clarification of the molecular mechanism of clinically used antidepressant drugs.     

Prenatal exposure to fluoxetine (Prozac) produces site-specific and age-dependent alterations in brain serotonin transporters in rat progeny: evidence from autoradiographic studies

The present study provides the first autoradiographic evidence of age-dependent regional changes in the density of serotonin (5-HT) transporters in offspring following prenatal exposure to fluoxetine. Pregnant rats received either saline or fluoxetine (10 mg/kg, s.c.) daily from gestational day 13 through 20. The density of [3H]citalopram-labeled 5-HT transporters was determined in forebrain regions and in midbrain raphe nuclei of prepubescent and adult male offspring. Brain regions representing integral components of the limbic system were particularly sensitive to the prenatal treatment. For example, prenatal fluoxetine exposure significantly altered the density of 5-HT transporters in subregions of the hypothalamus (dorsomedial nucleus, -21%; lateral hypothalamus, +21%), hippocampus (CA2, +47%; CA3, +38%), and amygdala (basolateral nucleus, +32%; medial nucleus, +44%) in prepubescent offspring. However, 5-HT transporter density in the dorsal and median raphe was unaltered in this same group of offspring. In adult offspring, 5-HT transporter densities, in all brain regions examined, were not significantly altered by prenatal exposure to fluoxetine. The present study also identifies significant age-related differences in 5-HT transporter densities between prepubescent and adult control offspring. For example, in adult control offspring, densities of 5-HT transporters were significantly greater in the cingulate cortex (+33%), basolateral amygdala (+58%), and CA1 area of the hippocampus (+78%); but significantly lower in the temporal cortex (-65%) and median raphe (-25%). The age-dependent and site-specific alterations in the density of 5-HT transporters suggests that either 5-HT innervation and/or 5-HT neuron function in various forebrain regions may be altered by prenatal exposure to fluoxetine.     

Preferential potentiation of the effects of serotonin uptake inhibitors by 5-HT1A receptor antagonists in the dorsal raphe pathway: role of somatodendritic autoreceptors

5-HT1A autoreceptor antagonists enhance the effects of antidepressants by preventing a negative feedback of serotonin (5-HT) at somatodendritic level. The maximal elevations of extracellular concentration of 5-HT (5-HT(ext)) induced by the 5-HT uptake inhibitor paroxetine in forebrain were potentiated by the 5-HT1A antagonist WAY-100635 (1 mg/kg s.c.) in a regionally dependent manner (striatum > frontal cortex > dorsal hippocampus). Paroxetine (3 mg/kg s.c.) decreased forebrain 5-HT(ext) during local blockade of uptake. This reduction was greater in striatum and frontal cortex than in dorsal hippocampus and was counteracted by the local and systemic administration of WAY-100635. The perfusion of 50 micromol/L citalopram in the dorsal or median raphe nucleus reduced 5-HT(ext) in frontal cortex or dorsal hippocampus to 40 and 65% of baseline, respectively. The reduction of cortical 5-HT(ext) induced by perfusion of citalopram in midbrain raphe was fully reversed by WAY-100635 (1 mg/kg s.c.). Together, these data suggest that dorsal raphe neurons projecting to striatum and frontal cortex are more sensitive to self-inhibition mediated by 5-HT1A autoreceptors than median raphe neurons projecting to the hippocampus. Therefore, potentiation by 5-HT1A antagonists occurs preferentially in forebrain areas innervated by serotonergic neurons of the dorsal raphe nucleus.     

Influence of AMPA/kainate receptors on extracellular 5-hydroxytryptamine in rat midbrain raphe and forebrain

1. The regulation of 5-hydroxytryptamine (5-HT) release by excitatory amino acid (EAA) receptors was examined by use of microdialysis in the CNS of freely behaving rats. Extracellular 5-HT was measured in the dorsal raphe nucleus (DRN), median raphe nucleus (MRN), nucleus accumbens, hypothalamus, frontal cortex, dorsal and ventral hippocampus. 2. Local infusion of kainate produced increases in extracellular 5-HT in the DRN and MRN. Kainate infusion into forebrain sites had a less potent effect. 3. In further studies of the DRN and nucleus accumbens, kainate-induced increases in extracellular 5-HT were blocked by the EAA receptor antagonists, kynurenate and 6,7-dinitroquinoxaline-2,3-dione (DNQX). 4. The effect of infusing kainate into the DRN or nucleus accumbens was attenuated or abolished by tetrodotoxin (TTX), suggesting that the increase in extracellular 5-HT is dependent on 5-HT neuronal activity. In contrast, ibotenate-induced lesion of intrinsic neurones did not attenuate the effect of infusing kainate into the nucleus accumbens. Thus, the effect of kainate in the nucleus accumbens does not depend on intrinsic neurones. 5. Infusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate (AMPA) into the DRN and nucleus accumbens induced nonsignificant changes in extracellular 5-HT. Cyclothiazide and diazoxide, which attenuate receptor desensitization, greatly enhanced the effect of AMPA on 5-HT in the DRN, but not in the nucleus accumbens. 6. In conclusion, AMPA/kainate receptors regulate 5-HT in the raphe and in forebrain sites.     

Detailed mapping of serotonin 5-HT1B and 5-HT1D receptor messenger RNA and ligand binding sites in guinea-pig brain and trigeminal ganglion: clues for function

The similar pharmacology of the 5-HT1B and 5-HT1D receptors, and the lack of selective compounds sufficiently distinguishing between the two receptor subtypes, have hampered functional studies on these receptors. In order to provide clues for differential functional roles of the two subtypes, we performed a parallel localization study throughout the guinea-pig brain and the trigeminal ganglia by means of quantitative in situ hybridization histochemistry (using [35S]-labelled riboprobes probes for receptor messenger RNA) and receptor autoradiography (using a new radioligand [3H]alniditan). The anatomical patterns of 5-HT1B and 5-HT1D receptor messenger RNA were quite different. While 5-HT1B receptor messenger RNA was abundant throughout the brain (with highest levels in the striatum, nucleus accumbens, olfactory tubercle, cortex, hypothalamus, hippocampal formation, amygdala, thalamus, dorsal raphe and cerebellum), 5-HT1D receptor messenger RNA exhibited a more restricted pattern; it was found mainly in the olfactory tubercle, entorhinal cortex, dorsal raphe, cerebellum, mesencephalic trigeminal nucleus and in the trigeminal ganglion. The density of 5-HT(1B/1D) binding sites (combined) obtained with [3H]alniditan autoradiography was high in the substantia nigra, superior colliculus and globus pallidus, whereas lower levels were detected in the caudate-putamen, hypothalamus, hippocampal formation, amygdala, thalamus and central gray. This distribution pattern was indistinguishable from specific 5-HT1B receptor labelling in the presence of ketanserin under conditions to occlude 5-HT1D receptor labelling; hence the latter were below detection level. Relationships between the regional distributions of the receptor messenger RNAs and binding sites and particular neuroanatomical pathways are discussed with respect to possible functional roles of the 5-HT1B and 5-HT1D receptors.     

The effects of prenatal exposure to cocaine on the dopaminergic cells in the rat retina. An immunocytochemical and neurochemical study

5-Hydroxytryptamine1A (5-HT1A) receptors have been visualized at the electron microscopic level in selected areas (dorsal raphe nucleus, hippocampus, septum) of the rat brain using specific anti-peptide antibodies. 5-HT1A receptor immunoreactivity was found almost exclusively in the somatodendritic compartment of neurons and was very rarely observed within processes possibly belonging to glial cells. The immunoenzymatic reaction product was associated exclusively with dendritic spines in the dorsal hippocampus, whereas in the dorsal raphe nucleus and the septal complex, immunoreactivity was found in both dendritic processes and somata. Although some immunolabeling was observed within the cytoplasm of cell bodies, 5-HT1A receptor immunoreactivity was essentially confined to the plasma membrane where it was unevenly distributed. It was frequently associated with synapses (except in the dorsal raphe nucleus), but was also found extrasynaptically in both somata and dendrites. These data suggest that the action of serotonin via 5-HT1A receptor could occur through junctional as well as nonjunctional transmission.     

Release of cerebral 5-hydroxytryptamine evoked by electrical stimulation of the dorsal and median raphe nuclei: effect of a neurotoxic amphetamine

Recent neuroanatomical data suggest that the axons and terminals of serotonergic neurons of the dorsal and median raphe nuclei are morphologically and pharmacologically distinct. Here we attempted to establish a functional in vivo model of serotonergic terminals derived from these nuclei, and then carry out a preliminary comparison of their physiological and pharmacological properties. Brain microdialysis was used to monitor extracellular 5-hydroxytryptamine in the hippocampus (dorsal and median raphe innervation) and frontal cortex (preferential dorsal raphe innervation) of the anaesthetized rat. To distinguish 5-hydroxytryptamine released by terminals of dorsal raphe neurons from that released by median raphe neurons, one or other of these nuclei was stimulated electrically. Electrical stimulation of either the dorsal or median raphe nucleus evoked a release of 5-hydroxytryptamine in the hippocampus. Whereas stimulation of the dorsal raphe nucleus also released 5-hydroxytryptamine in the frontal cortex, stimulation of the median raphe nucleus did not. No release of 5-hydroxytryptamine was evoked when electrodes were located in regions bordering the dorsal raphe nucleus and the median raphe nucleus. The amounts of hippocampal 5-HT released by stimulation of the dorsal or median raphe nucleus were found to be similarly altered by a 5-hydroxytryptamine uptake inhibitor (citalopram) and calcium-free perfusion medium, and also by increasing stimulation frequency (2-10 Hz). Furthermore, the amount of 5-hydroxytryptamine released by electrical stimulation of either the dorsal raphe nucleus or median raphe nucleus was markedly reduced in rats pretreated with p-chloroamphetamine. In summary, our data show that electrical stimulation of the dorsal or median raphe nucleus releases 5-hydroxytryptamine in a regionally specific manner (hippocampus versus frontal cortex), suggesting that serotonergic nerve terminals of the dorsal and median raphe pathways were being activated selectively. Using this model, we found no differences in the responsiveness of dorsal and median raphe pathways to a specific set of physiological and pharmacological manipulations. In particular, our data suggest that the neurotoxic action of p-chloroamphetamine may not be targeted solely on serotonergic axons and terminals of the dorsal raphe nucleus but includes those of the median raphe nucleus.     

Dorsal lesions of the prefrontal cortex: effects on alcohol consumption and subcortical monoaminergic systems

Male Wistar rats were subjected to either bilateral aspiration lesions of the dorsal regions of the prefrontal cortex (PFC) or sham lesions and placed on a 6-week, modified sucrose-fading procedure. At the time of sacrifice, the size of the lesion, both in anterior-posterior and medial-lateral dimensions, was measured. Following sacrifice, levels of dopamine (DA), serotonin (5-HT), norepinephrine (NE), and their metabolites were measured in the midbrain (raphe) and nucleus accumbens (NA). Lesioned animals had reductions in 5-HT in the NA, and DA and NE in the raphe. The lesioned group drank more of a solution of 5% alcohol than controls early in the sucrose fading, and less during the later stages. In the lesioned group, the size of the left- and right-hemisphere lesions predicted 5-HIAA levels in the NA, and 5-HT and 5-HIAA levels in the raphe. A laterality effect was noted, such that the size of left-hemisphere lesions were positively associated with raphe 5-HT and 5-HIAA levels, and negatively associated with 5-HT levels in the NA, while right-hemisphere lesions showed the opposite relationships. In addition, the width of the left-hemisphere lesion predicted some measures of alcohol intake. These results suggest that, in the rat, the dorsal PFC is involved in the regulation of monoamines in subcortical regions known to be important in the regulation of reinforced behaviors, and that this regulation differs between hemispheres and shows a laterality effect. In addition, the dorsal PFC appears to have a subtle involvement in the regulation of alcohol intake.     

Effects of tamoxifen on the cytology of the uterine cervix in breast cancer patients

Knowledge about the central innervation of the lower urinary tract is limited. The spinal cord and the pontine micturition center have been investigated most thoroughly, whereas high centers have received little attention. Pseudorabies virus (PRV), a self-amplifying and transneuronal tracer was injected into the bladder trigone of 21 Sprague-Dawley rats. The animals were killed after 72, 96, and 120 h. The whole CNS was sectioned and immunostained for PRV. CNS centers directly connected to the bladder include the intermedio lateral cell column, the central autonomic nucleus, and the nucleus intercalatus at the spinal cord levels T12-L2 and L6-S2. The raphe pallidus et magnus, the A5 nor-adrenergic area, the pontine micturition center, the locus coeruleus, the periaquaductal gray, the nucleus para- et periventricularis of the hypothalamus, the red nucleus, the medial preoptic area, and the cortex are supraspinal centers connected to the bladder. Lower urinary tract function is a complex multilevel and multineuronal interaction. It involves facilitation and inhibition at many levels of the CNS.     

"High-load" polyethylene glycol-polystyrene (PEG-PS) graft supports for solid-phase synthesis

In the rat dorsal hippocampus and dorsal raphe nucleus, the microiontophoretic application of ergotamine and 5-HT suppressed the firing activity of CA3 pyramidal neurons and 5-HT neurons, an effect antagonized by selective 5-HT1A receptor antagonists. Co-application of ergotamine prevented the inhibitory action of 5-HT on the firing activity of CA3 pyramidal neurons but not of 5-HT neurons, indicating that ergotamine acted as a partial 5-HT1A receptor agonist in the dorsal hippocampus and as a full agonist at 5-HT1A autoreceptors. Ergotamine decreased, in a concentration-dependent manner, the electrically evoked release of [3H]5-HT in preloaded rat and guinea pig hypothalamus slices; this effect was prevented by the nonselective 5-HT receptor antagonist methiothepin but not by the selective 5-HT1B/1D receptor antagonist GR 127935 or the alpha 2-adrenoceptor antagonist idazoxan. Although body temperature in humans remained unchanged following inhaled ergotamine, in the rat, subcutaneously injected ergotamine produced a hypothermia that was prevented by a pretreatment with the 5-HT1A/1B receptor/beta-adrenoceptor antagonist pindolol. Finally in humans, ergotamine did not alter prolactin or adrenocorticotropic hormone levels, but increased growth hormone level, which was prevented by pindolol. Cortisol level was increased in humans by ergotamine, but this enhancement was unaltered by pindolol. In conclusion, the present results suggest that ergotamine acted in the rat brain as a 5-HT1A receptor agonist and as an agonist of terminal 5-HT autoreceptor of a yet undefined subtype. In humans, ergotamine also displayed some 5-HT1A receptor activity but, probably because of lack of receptor selectivity, it did not present the same profile as other 5-HT1A receptor agonists.     

EMD 57445, the selective sigma receptor ligand, has no effect on the 5-hydroxytryptamine system

The behavioral and biochemical effects of EMD 57445, a selective sigma receptor ligand with potential antipsychotic activity, on the 5-hydroxytryptamine (5-HT) system were studied in rats and mice. The drug influence was investigated in three behavioral tests: 8-OH-DPAT (5-HT1A agonist)-induced behavioral syndrome in rats, m-chlorophenylpiperazine (m-CPP, 5-HT1B agonist)-induced hypothermia in mice and L-5-hydroxytryptophan (L-5-HTP)-induced head twitches (5-HT2A stimulation) in rats. EMD 57445 did not show any activity in all three behavioral models. In biochemical studies, no changes in the 5-HT and 5-HIAA levels in rat brain cortex, nucleus accumbens, striatum, hypothalamus and hippocampus were found. The results indicate that EMD 57445 does not interact with 5-HT (5-HT1A, 5-HT1B, 5-HT2A) receptor subpopulations and does not affect 5-HT metabolism.     

Post-lesion up-regulation of 5-HT1B binding sites in the suprachiasmatic nucleus may be reversed after spontaneous or graft-induced serotonin reinnervation

We have previously reported that selective axotomy of serotoninergic neurons produced by an intraventricular injection of 5, 7-dihydroxytryptamine is followed by an increase in 5-HT1B binding sites in the suprachiasmatic nucleus of the hypothalamus. This post-lesion up-regulation is shown here to be spontaneously reversed after long-term survival in spite of an incomplete reinnervation of the nucleus. Recovery may be accelerated by fetal raphe transplants that produce more rapid reinnervation.     

A haplotype and linkage disequilibrium analysis of the hereditary hemochromatosis gene region

Monoclonal antibodies were generated against serotonin (5-HT) and the C-terminal portion of the neuronal form of nitric oxide synthase (nNOS), the enzyme producing nitric oxide in neurons. These antibodies were used to compare the distribution of 5-HT- and nNOS-containing neurons in the raphe nuclei of four animal species (rat, mouse, guinea pig, and cat). It was found that the rat was the only species in which the raphe nuclei contain a substantial number of nNOS-immunoreactive (IR) cell bodies. In this species and as observed by other authors, all mesencephalic raphe nuclei contained nNOS-IR cells, the largest group being located in the nucleus raphe dorsalis. The coexistence of nNOS and 5-HT immunoreactivities in these nuclei was visualized by double labeling. In the medulla, the nuclei raphe magnus and obscurus displayed a rather low number of nNOS-IR neurons. In the other species, nNOS-IR cell bodies were found in very low numbers, whatever raphe nucleus was considered. The rostral pole of the nucleus raphe dorsalis and the nuclei raphe magnus and obscurus contained a few nNOS-IR neurons which did not show any coincidence with the 5-HT neurons. In addition, nNOS-IR axons were rare. It is concluded that in the mouse, guinea pig, and cat the involvement of nitric oxide in functions subserved by 5-HT within the raphe nuclei might be minimal.     

5-HT1A receptor activation: short-term effects on the mRNA expression of the 5-HT1A receptor and galanin in the raphe nuclei

Systemic administration of the 5-HT1A receptor agonist 8-OH-2-(di-n-propylamino)-tetralin (8-OH-DPAT; 0.3 mg/kg, s.c.) was used to explore the effects of activation of 5-HT1A receptors on expression of mRNA coding for 5-HT1A receptor, tryptophan hydroxylase (TPH) and galanin in the ascending raphe nuclei. 8-OH-DPAT increased the hybridization signal of the 5-HT1A receptor by 105% in the dorsal raphe nucleus (B7) 30 min after the injection. No effects were seen at the later time points (2-8 h). In the median raphe nucleus (B8) and the B9 cell group in the medial lemniscus, 8-OH-DPAT induced a marked decrease in labeling 30 min after injection. At 8 h following 8-OH-DPAT injection, the effect had shifted to an increase in 5-HT1A receptor labeling by 68% in the B8 area. Importantly 8-OH-DPAT had no significant effects on the expression of mRNA coding for TPH and galanin. The results suggest an important and differential mechanism for the regulation of 5-HT1A receptor mRNA levels in the dorsal and median raphe nuclei. This regulation may be of importance for the differential control of the activity of the ascending 5-HT neurons, and hence for mood regulation. The results also indicate a dissociation between the effects mediated by 5-HT1A receptor functions and those regulating the coexisting peptide galanin in the dorsal raphe.     

Graft-derived cholinergic reinnervation of the hippocampus prevents a lasting increase of hippocampal noradrenaline concentration induced by septohippocampal damage in rats

Long-Evans female rats sustained aspirative lesions of the septohippocampal pathways and, 2 weeks later, received into the dorsal hippocampus grafts prepared from the septal area (rich in cholinergic neurons; Group Sep) or from the mesencephalic raphe (poor in cholinergic neurons; Group Rap) of rat fetuses. Lesion-only (Group Les) and virtually intact (Group Sham) rats served as controls. Between 9.5 and 10.5 months after grafting surgery, we found the lesions to decrease choline acetyltransferase activity (ChAT), high affinity synaptosomal uptake of [3H]choline (HACU) and serotonin concentration ([5-HT]), as well as to increase the noradrenaline concentration ([NA]) in the dorsal hippocampus. Raphe grafts increased [5-HT] to 456% of normal, but had only weak or no effects on the other lesion-induced modifications in brain neurochemistry. Septal grafts dramatically increased ChAT activity and HACU, enhanced [5-HT], and reduced [NA] to near-normal levels. We also found a significant negative correlation between HACU and [NA] in rats with lesions, whether grafted or not. These data show that grafts providing the denervated hippocampus with a new cholinergic innervation might be able to exert inhibitory effects on the lesion-induced increase of [NA]. Since such an increase is indicative of sympathetic sprouting, the finding of reduced [NA] in rats with graft-derived cholinergic reinnervation of the hippocampus is in line with the hypothesis that hippocampal cholinergic denervation plays a crucial role in the induction of sympathetic sprouting. However, our data do not allow to distinguish whether grafts rich in cholinergic neurons inhibited the sympathetic sprouting itself, or rather reduced the NA content of sprouted fibers.     

Serotonin depletion in the adult rat produces differential changes in highly polysialylated form of neural cell adhesion molecule and tenascin-C immunoreactivity

Levels of immunoreactivity for highly polysialylated neural cell adhesion molecule (PSA-NCAM), NCAM, and tenascin-C (TN-C), were examined in the basal ganglia regions and hypothalamic nuclei of adult rats after serotonergic (5-HT) lesions induced by 5,7-dihydroxytryptamine injections in the dorsal and medial raphe nuclei. Decreases in the density of serotonin fibers were associated with no changes in NCAM and general decreases in PSA-NCAM staining, the time-course of changes being selective for each region. Taken that the confocal analysis indicated that serotonin neurons do not express PSA-NCAM and that similar decreases in PSA-NCAM staining were observed after inhibition of 5-HT synthesis induced by parachlorophenylalanine administration, these results suggest that 5-HT may reduce adhesion by acting on PSA-NCAM expression in its environment, and thus facilitate plasticity in adult brain. Two months after the neurotoxin lesions, a normalization of PSA-NCAM staining was associated with a partial restoration in 5-HT fiber density in the nucleus accumbens and the supraoptic nucleus, suggesting that PSA-NCAM may facilitate sprouting of 5-HT fibers. Since a similar normalization was also detected in the suprachiasmatic nucleus, which remained deprived of serotonin fibers, negative factors are likely to be involved in regeneration processes. Indeed, increases in glial fibrillary acidic protein (GFAP) followed by increases in TN-C were observed in these areas, suggesting that the secretion of TN-C by astrocytes may have negative consequences on the sprouting of 5-HT fibers. Finally, the lack of changes in striatal PSA-NCAM or TN-C staining observed after selective lesions of the dopaminergic pathway induced by intranigral injections of 6-hydroxydopamine indicates that 5-HT has a selective and critical role in adult brain plasticity.     

The dorsal raphe: an important nucleus in pain modulation

The dorsal raphe nucleus (DRN) is an important nucleus in pain modulation. It has abundant 5-HT neurons and many other neurotransmitter and/or neuromodulator containing neurons. Its vast fiber connections to other parts of the central nervous system provide a morphological basis for its pain modulating function. Its descending projections, via the nucleus raphe magnus or directly, modulate the responses caused by noxious stimulation of the spinal dorsal horn neurons. In ascending projections, it directly modulates the responses of pain sensitive neurons in the thalamus. It can also be involved in analgesia effects induced by the arcuate nucleus of the hypothalamus. Neurophysiologic and neuropharmacologic results suggest that 5-HT neurons and ENKergic neurons in the DRN are pain inhibitory, and GABA neurons are the opposite. The studies of the intrinsic synapses between ENKergic neurons, GABAergic neurons, and 5-HT neurons within the DRN throw light on their relations in pain modulation functions, and further explain their functions in pain mediation.     

Studies on the neuronal circuits involved in the discriminative stimulus effects of 5-hydroxytryptamine1A receptor agonists in the rat

Rats were trained to discriminate 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.1 mg/kg i.p.) or 5-methoxy-N,N-dimethyltryptamine (5-OMe-DMT, 1.25 mg/kg i.p.), a selective and nonselective 5-hydroxytryptamine1A (5-HT, serotonin) receptor agonist, respectively, from saline in a two-lever procedure. The selective 5-HT1A receptor agonist ipsapirone substituted completely for 8-OH-DPAT (ED50, 1.52 mg/kg) and 5-OMe-DMT substituted partially for 8-OH-DPAT, whereas 8-OH-DPAT (ED50, 0.07 mg/kg) and ipsapirone (ED50, 4.15 mg/kg) substituted completely for 5-OMe-DMT. These results suggest that the discriminative stimulus properties of both 8-OH-DPAT and 5-OMe-DMT are 5-HT1A receptor mediated, although 5-OMe-DMT may involve an additional interaction with other 5-HT receptor subtypes. 5-OMe-DMT substituted for 8-OH-DPAT after application in the lateral ventricle (ED50, 3.0 micrograms/rat) and the dorsal raphe nucleus (DRN, 1.1 micrograms/rat). After application in the DRN (ED50 range, 1.4-5.0 micrograms/rat) and the median raphe nucleus (2.3 micrograms/rat), and after bilateral application into the CA-4 region of the dorsal hippocampus (4.1 micrograms/rat), 8-OH-DPAT also produced responding on the 8-OH-DPAT lever. Ipsapirone also substituted for 8-OH-DPAT after application into the DRN and the hippocampus (ED50S, 38 and 62 micrograms/rat, respectively). The 5-HT1A mixed agonist-antagonist (1-(2-methoxyphenyl) 4-[4-(2-pthalimido)butyl]piperazine, i.p. NAN-190) attenuated the discriminative stimulus effects of 8-OH-DPAT injected i.p. (0.1 mg/kg), into the DRN (10 micrograms) or into the hippocampus (2 x 10 micrograms).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of chronic m-CPP on locomotion, hypophagia, plasma corticosterone and 5-HT2C receptor levels in the rat

1. The present study examined 5-HT2C receptor agonist-induced behavioural tolerance and 5-HT2C receptor down-regulation in adult rat brain. The effect of chronic subcutaneous infusion of the 5-HT2C receptor agonist, m-chlorophenylpiperazine (m-CPP, 10 mg kg(-1), day(-1)), for 14 days was examined on daily food intake, the ability of acute m-CPP (2.5 mg kg(-1), i.p.) to induce hypolocomotion in a novel arena and elevate plasma corticosterone levels and on ex vivo cortical [3H]-mesulergine binding and hippocampal 5-HT2C receptor protein levels. 2. Before chronic infusion, m-CPP (2.5 mg kg(-1), i.p.) attenuated the number of turns and rears made in a novel open field arena. In contrast, while m-CPP still elicited this hypolocomotion following 14 days, saline infusion, no such hypolocomotion occurred in rats given chronic m-CPP (10 mg kg(-1) day(-1)), indicating that almost complete tachyphylaxis of this behaviour occurred with chronic 5-HT2C receptor agonist injection. 3. During chronic infusion of m-CPP, rats consumed less food per day than saline-treated controls. Acute challenge with m-CPP following two weeks, treatment still attenuated food intake over the next four hours (by 43% and 30%, respectively from that on the previous day) in saline and m-CPP infusion groups, showing that only partial tolerance to 5-HT2C receptor agonist-induced hypophagia occurred. 4. In naive home cage rats, plasma corticosterone was elevated in a dose-dependent manner 35 min after m-CPP injection (0.5, 1 and 3 mg kg(-1), i.p.) but levels were comparable to control values 16 h after m-CPP (2, 5 and 10 mg kg(-1), i.p.). Sixteen hours after a single m-CPP injection (2.5 mg kg(-1), i.p.), plasma corticosterone levels were comparable in a group of rats which had received 14 days infusion of m-CPP or saline. However, following a similar acute m-CPP injection (2.5 mg kg(-1), i.p., - 16 h) in rats previously infused for 14 days with m-CPP, plasma corticosterone levels were lower than those in a separate group which received no chronic infusions (but only acute m-CPP injection), even though the plasma m-CPP levels were comparable in both groups. The data are consistent with the proposal that chronic m-CPP induced some down-regulation of hypothalamic 5-HT2C receptors which contribute, in a tonic manner, to plasma corticosterone secretion under the conditions investigated. 5. Chronic m-CPP infusion reduced the amount of [3H]-mesulergine binding (by 27%, without altering the KD) in membranes prepared from parietal/occipital/temporal cortex (under conditions to exclude binding to 5-HT2A receptors) and 5-HT2C receptor protein-like immunoreactive levels measured by radioimmunoassay in the hippocampus by 38%, confirming that 5-HT2C receptor down-regulation had occurred. 6. Even after 14 days m-CPP infusion only partial behavioural tolerance and 5-HT2C receptor down-regulation were observed, which may vary in different brain regions of the rat. Thus the hypophagia produced by m-CPP may involve activation of 5-HT2C receptors in the hypothalamus, where there is a greater receptor reserve or which are more resistant to agonist-induced down-regulation than 5-HT2C receptors in limbic areas (striatum and nucleus accumbens) mediating m-CPP-induced hypolocomotion.