5-HT depletion with 5,7-DHT, PCA and PCPA in mice: differential effects on the sensitivity to 5-MeODMT, 8-OH-DPAT and 5-HTP as measured by two nociceptive tests

Depletion of 5-hydroxytryptamine (5-HT) in mice was produced by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) or by systemic injections of p-chloroamphetamine (PCA, 3 X 40 or 4 X 40 mg/kg), p-chlorophenylalanine (PCPA, 5 X 400 or 14 X 400 mg/kg) or combined PCA (3 X 40 mg/kg) + PCPA (11 X 400 mg/kg). Neither of the pretreatments altered nociception in the increasing temperature hot-plate test, whereas hyperalgesia was demonstrated in 5,7-DHT lesioned animals in the tail-flick test. 5,7-DHT-pretreatment enhanced the antinociceptive effect of the 5-HT agonists 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-hydroxytryptophan (5-HTP). This effect was observed after 2, 5 and 8 days in the tail-flick test and after 5 and 8 days in the hot-plate test. However, pretreatment with PCPA or PCA failed to alter the antinociception elicited by the 5-HT agonists, although a tendency towards enhancement of antinociception was found after combined treatment with PCA and PCPA. It is suggested that the injection of 5,7-DHT induces denervation supersensitivity of post-synaptic 5-HT receptors. The lack of such supersensitivity after PCPA-pretreatment which induces similar 5-HT depletion to 5,7-DHT, may suggest that other factors than the absence of 5-HT may contribute to the development of denervation supersensitivity. Alternatively, the three 5-HT depleting agents may produce a qualitatively different reduction of 5-HT.     

Involvement of 5-hydroxytryptamine-containing neurons in antinociception produced by injection of morphine into nucleus raphe magnus or onto spinal cord

We studied whether antinociception produced by injection of morphine into the nucleus raphe magnus (NRM) or superfusion onto the spinal cord involved serotonergic neurons that descend from brainstem to spinal cord. Involvement of 5-hydroxytryptamine (5-HT)-containing neurons was determined by correlating morphine-induced analgesia with an increase in turnover of 5-HT and by determining if depletion of cord 5-HT with the neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) could attenuate the antinociceptive effects of morphine. When injected directly into the NRM, 10 micrograms of morphine produced profound analgesia as measured by the paw-pressure technique, and significantly increased the turnover of 5-HT in both posterior medulla and spinal cord. Depletion of cord 5-HT to less than 10% of control concentrations attenuated the antinociceptive effect of morphine injected into the NRM. When various concentrations of morphine (1, 10 or 50 micrograms) were injected directly into the spinal subarachnoid space, a dose-dependent analgesia was observed. No change in 5-HT turnover in spinal cord was observed with any dose of morphine superfused onto the cord. In addition, depletion of cord 5-HT with 5,7-DHT did not alter the analgesic response to either 1 or 10 micrograms of intrathecal morphine. These results suggest that although 5-HT-containing neurons descending from brainstem into spinal cord are involved with analgesia produced by morphine injection into the NRM, they are not involved in the analgesia induced by applying morphine directly to the cord.     

Altered neuronal responsiveness to biogenic amines in rat cerebral cortex after serotonin denervation or depletion

To further investigate monoaminergic mechanisms in cerebral cortex, responsiveness of cortical neurons to microiontophoretic applications of serotonin (5-HT), dopamine (DA) or noradrenaline (NA) was examined in the frontoparietal region of control, 5,7-dihydroxytryptamine (5,7-DHT)- and p-chlorophenylalanine (PCPA)-treated rats anesthetized with urethane. As a rule, 100 nA applications of either one of these biogenic amines induced marked slowings or total interruptions of ‘spontaneous’ firing overlasting the 30s periods of ejection. Given the large amounts of monoamines ejected, it could be inferred that such microiontophoretic applications produced a maximal activation of receptors. In control rats, the responses to 5-HT, DA and NA were of approximately equal duration ( 5 min). Two to 4 weeks after denervation with 5,7-DHT, most neurons (75%) exhibited greatly prolonged responses to 5-HT( 14 min), and marked depressions of firing could be induced by small ejection currents (2 nA) having little or no effect in the controls. In addition, 85% of the units supersensitive to 5-HT showed considerably shortened responses to DA and NA( 1 min). After 2–14 days of depletion with PCPA, there was no change in the responsiveness to 5-HT in spite of a 91% lowering of cortical 5-HT content equivalent to that measured after denervation. Nevertheless, responsiveness to DA and NA was again diminished in a majority (80%) of the units tested. In control or PCPA-treated rats, acute administration of the 5-HT re-uptake blocker fluoxetine increased the duration of depressions induced by 100 nA applications of 5-HT but did not enhance responsiveness to low ejection currents. This suggested that, after 5-HT denervation, the suppression of re-uptake was mainly responsible for the prolongation of 5-HT responses (‘presynaptic’ component of supersentivity), whereas a modification of 5-HT receptors accounted for the greater efficacy of small doses of 5-HT (‘postsynaptic’ component). Responsiveness to the microiontophoretic application of phenylephrine (PHE), a noradrenergic α-agonist, was comparable with that to NA in PCPA- and 5,7-DHT-treated as well as in control rats. Therefore, the hyposensitivity to DA and NA appeared indicative of a desensitization of catecholamine receptors caused by the absence of 5-HT. Such a desensitization may be viewed as an adaptive change resulting from an increased release of endogenous DA and NA. This interpretation would in turn imply that, normally, 5-HT regulates catecholamine release in the neocortex.     

Neonatal 5,7-DHT lesions upregulate [3H]mesulergine-labelled spinal 5-HT1C binding sites in the rat

To delineate the involvement of spinal 5-HT1C receptors in supersensitivity and recovery following neonatal 5,7-DHT lesions, we injected rats on postnatal days 2 and 5 with 5,7-DHT or vehicle by intraperitoneal (IP) or intracisternal (IC) injection. [3H]Mesulergine-labelled sites measured 4 or 14 weeks later exhibited a significant increase (+35% for IP and 27% for IC) in Bmax without changes in Kd or nH. Spinal 5-HT content was significantly reduced (-80 to 89%) by either route of 5,7-DHT injection. These data describe novel upregulation of spinal 5-HT1C receptors in rats with neonatal 5,7-DHT lesions. Spinal 5-HT1C receptor upregulation may contribute to the behavioral supersensitivity to L-5-hydroxytryptophan (L-5-HTP) in rats with 5,7-DHT lesions. It does not explain the behavioral recovery we found previously only after IP 5,7-DHT injection.     

Proposed animal model of attention deficit hyperactivity disorder

Dopamine (DA) neurons are implicated in the hyperlocomotion of neonatal 6-hydroxydopamine (6-OHDA)-lesioned rats, an animal model of attention deficit hyperactivity disorder (ADHD). Because serotonin (5-HT) neurons mediate some DA agonist effects, we investigated the possible role of 5-HT neurons on locomotor activity. Rats were treated at 3 days after birth with vehicle or 6-OHDA (134 μg ICV; desipramine pretreatment, 20 mg/kg IP, 1 h), and at 10 weeks with vehicle or 5,7-dihydroxytryptamine (5,7-DHT; 75 μg ICV; pretreatment with desipramine and pargyline, 75 mg/kg IP, 30 min), to destroy DA and/or 5-HT fibers. Intense spontaneous hyperlocomotor activity was produced in rats lesioned with both 6-OHDA and 5,7-DHT. Locomotor time in this group was 550 ± 17 s in a 600 s session, vs. 127 ± 13 s in the 6-OHDA group and <75 s in 5,7-DHT and intact control groups (p < 0.001). Oral activity dose-effect curves established that 5,7-DHT attenuated DA D₁ receptor supersensitivity and further sensitized 5-HT_2c receptors. Acute treatment with dextroamphetamine (0.25 mg/kg SC) reduced locomotor time in 6-OHDA+5,7-DHT-lesioned rats to 76 ± 37 s (p < 0.001). Striatal DA was reduced by 99% and 5-HT was reduced by 30% (vs. 6-OHDA group). Because combined 6-OHDA (to neonates) and 5,7-DHT (to adults) lesions produce intense hyperlocomotion that is attenuated by amphetamine, we propose this as a new animal model of ADHD. The findings suggest that hyperactivity in ADHD may be due to injury or impairment of both DA and 5-HT neurons.     

Depletion of serotonin in the nervous system of Aplysia reduces the behavioral enhancement of gill withdrawal as well as the heterosynaptic facilitation produced by tail shock

Noxious stimuli, such as electrical shocks to the animal's tail, enhance Aplysia's gill- and siphon-withdrawal reflex. Previous experimental work has indicated that this behavioral enhancement, known as dishabituation (if the reflex has been habituated) or sensitization (if it has not been habituated), might be mediated, at least in part, by the endogenous monoaminergic transmitter serotonin (5-HT). To assess 5-HT's role in dishabituation and sensitization of Aplysia withdrawal reflex, we treated Aplysia with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). We found that 5,7-DHT treatment significantly reduced the dishabituation of the withdrawal reflex produced by tail shock. Treatment with the neurotoxin also blocked the heterosynaptic facilitation of monosynaptic connections between siphon sensory neurons and their follower cells, which contributes to the behavioral enhancement. Analysis by high-performance liquid chromatography indicated that 5,7-DHT treatment significantly reduced 5-HT levels in the Aplysia CNS. Moreover, the neurotoxic effects of 5,7-DHT appeared to be relatively specific for serotonergic pathways. Thus, 5,7-DHT treatment did not disrupt the ability of nonserotonergic facilitatory interneurons, the L29 cells, to facilitate the connections of siphon sensory neurons. Also, 5,7-DHT reduced 5-HT-dependent, but not dopamine-dependent, histofluorescence in Aplysia central ganglia. Finally, 5,7-DHT does not reduce the levels of the facilitatory peptides SCPA and SCPB within the Aplysia CNS. Our results, together with those of Mackey et al. (1989), indicate that 5-HT plays a major role in mediating dishabituation and sensitization of Aplysia's withdrawal reflex.     

Effects of serotonin depleters on the contents of beta-endorphin, alpha-melanotropin and adrenocorticotropin in rat brain and pituitary

Serotonin depleters, 5,7-dihydroxytryptamine (5,7-DHT) and p-chlorophenylalanine (PCPA), were injected into adult male rats and beta-endorphin (beta-EP), alpha-melanotropin (alpha-MSH) and adrenocorticotropin (ACTH) levels in rat brain and pituitary were each estimated by radioimmunoassay combined with a gel column chromatography. (1) 5,7-DHT, injected intracerebroventricularly combined with pargyline, decreased the levels of immunoreactive-beta-EP, -alpha-MSH and -ACTH significantly and concomitantly in hypothalamus, thalamus, and brainstem. (2) PCPA, repeatedly injected intraperitoneally 8 times every 3 days, decreased the levels of these peptides in some of these brain regions. (3) There was no significant change of IR-beta-EP, -alpha-MSH, -ACTH in the anterior and the intermediate-posterior pituitaries after the treatment of 5,7-DHT or PCPA. (4) A single injection of the same dose of PCPA induced no significant effects on these peptide levels in both brain and pituitary. These data suggest that central serotoninergic neurons might affect beta-EP-alpha-MSH-ACTH containing neurons in rat brain.     

Implanted BDNF-producing fibroblasts prevent neurotoxin-induced serotonergic denervation in the rat striatum

Degeneration of serotonergic fibers in the rat striatum was produced by local administration of the serotonergic neurotoxin 5, 7-dihydroxytryptamine (5,7-DHT) or the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP(+)), which is also toxic to serotonergic neurons. One week before neurotoxin administration, fibroblasts engineered to express the human BDNF gene were grafted into the mesencephalon, dorsal to the substantia nigra. Rats implanted with fibroblasts expressing the LacZ gene were used as controls, as well as sham-operated animals (not injected with any neurotoxin). After a survival period of 1 week, the serotonergic innervation of the striatum was assessed by measuring serotonin (5-HT) content and by immunohistochemical detection of 5-HT positive fibers. BDNF-producing cells prevented the striatal 5-HT loss induced by local administration of either 5,7-DHT or MPP(+), as well as the striatal dopamine (DA) loss induced by the latter neurotoxin. Grafting of fibroblasts carrying the BDNF or the Lac-Z gene did not modify striatal 5-HT or DA content in sham-operated animals. In 5, 7-DHT-lesioned rats, implanted or not with control Lac-Z fibroblasts, a striking reduction in the density of 5-HT immunoreactive fibers was observed. By contrast, the density of 5-HT fibers was similar in rats implanted with BDNF-producing fibroblasts as compared to sham-operated controls. The protective effect of BDNF on the damage to serotonergic terminals induced by the two neurotoxins suggests the interest of this neurotrophin in the treatment of behavioral disorders associated to neurodegenerative diseases.     

Brainstem serotonergic hyperinnervation modifies behavioral supersensitivity to 5-hydroxytryptophan in the rat

Rat pups were injected intracisternally (i.c.) or intraperitoneally (i.p.) with 5,7-dihydroxytryptamine (5,7-DHT) or saline and challenged 2 and 14 weeks later with the 5-HT precursor 5-hydroxytryptophan (5-HTP), which evokes behavioral supersensitivity in adult rats, 5,7-DHT induced transient postinjection convulsions in rats injected i.c. but not i.p. Rats with either type of 5,7-DHT lesions displayed supersensitive behavioral responses to 5-HTP. However, rats lesioned by i.p. injections exhibited significantly greater shaking behavior (+1445%) in response to 5-HTP than their i.c. counterparts, who instead showed more forepaw myoclonus (+250%) and head weaving (+270%), the core features of the 5-HT syndrome. Differences in 5-HT syndrome behaviors were already present 2 weeks after lesioning, whereas the difference in shaking behavior was not. After 14 weeks, 5-HT was selectively depleted (-43 to -92%) in hippocampus, spinal cord, and frontal cortex, and differences between i.c. and i.p. 5,7-DHT routes were insignificant except in frontal cortex. Brainstem 5-HT concentrations were significantly increased (+35%) after i.p. 5,7-DHT injections in contrast to reduction (-89%) after i.c. 5,7-DHT; 5-hydroxyindole acetic acid/5-hydroxytryptamine (5-HIAA/5-HT) ratios were decreased (-20%) with either route. These data suggest that brainstem 5-HT hyperinnervation following i.p. 5,7-DHT injection modifies the functional consequences of injury in abating the 5-HT syndrome, but does not result in complete recovery since shaking behavior is enhanced. Loss of presynaptically mediated autoregulation or receptor dysregulation may play a major role in behavioral supersensitivity induced by 5-HTP in rats with 5,7-DHT lesions. To the extent that the 5-HT syndrome is mediated by 5-HT1A receptors and shaking behavior by 5-HT2 sites, differential responses to injury of 5-HT1A and 5-HT2 receptors may contribute to these behavioral differences.     

Regional central serotonin-2 receptor binding and phosphoinositide turnover in rats with 5,7-dihydroxytryptamine lesions

"Denervation supersensitivity" of serotonin (5-HT) receptors has been proposed to explain the behavioral supersensitivity to 5-hydroxytryptophan (5-HTP) which develops after lesions of indoleamine neurons with 5,7-dihydroxytryptamine (5,7-DHT). To examine the possible role of receptor recognition sites and second messenger activity in supersensitivity, we measured regional 5-HT2 receptor ligand binding and 5-HT-stimulated phosphoinositide turnover in adult rats with 5,7-DHT lesions made by intracisternal injection and their saline-treated controls. In [3H]ketanserin binding studies of fresh brain tissue two weeks after 5,7-DHT injection, there were no significant changes in frontal cortex, brainstem, or spinal cord in Bmax, Kd, or nH of 5-HT2 receptors, 5,7-DHT lesions did not affect basal levels of [3H]inositol phosphate (IP) accumulation but significantly increased 5-HT-stimulated [3H]IP accumulation in the brainstem (+27%) and cortex (+23%). Because brainstem rather than cortex is involved in 5-HTP-evoked myoclonus, increased 5-HT-stimulated phosphoinositide hydrolysis in brainstem following 5,7-DHT lesions in the rat may be relevant to serotonergic behavioral supersensitivity.     

Opposite changes in striatal neuropeptide Y immunoreactivity after partial and complete serotonergic depletion in the rat

The aim of this study was to investigate the consequences of partial vs. complete serotonergic (5-HT) depletions on the immunoreactivity of striatal interneurons containing neuropeptide Y (NPY). Taking into account the plasticity of the monoaminergic neurons, the effects of various doses of 5,7-dihydroxytryptamine (5,7-DHT) injected into the anterior raphe nuclei and P-chlorophenylalanine (PCPA) administration were compared in the dorsal (caudate-putamen) and the ventral (nucleus accumbens) striatum. Twenty days after administering 5,7-DHT injections inducing a substantial but partial decrease in the striatal 5-HT concentrations (about 80%), we detected a significant decrease in the number of NPY immunoreactive cells. In contrast, the PCPA inhibition of serotonin synthesis in the neurons spared by the partial lesion or the near-complete neurotoxic lesion induced an increase in the number of striatal NPY neurons. These results suggest that complex adaptive mechanisms are probably responsible for the changes in striatal NPY reactivity observed after a partial lesion and that these neurons can adapt according to the extent of 5-HT depletion. Upon comparing the NPY responses in the dorsal and ventral components of the striatal complex, no main differences were observed; while in the caudate-putamen, the changes were primarily found to occur in the medial zone. This finding is discussed here with reference to the topographical effects of dopaminergic or glutamatergic deafferentation. Finally, these results suggest that a complete interruption of the 5-HT transmission may lead to an increase in the intracellular NPY level, which may be associated with a decrease in the release of the peptide. It can therefore be postulated that serotonergic neurons normally exert a positive influence on NPY striatal neurons. © 1996 Wiley-Liss, Inc.     

The functional significance of neonatal 5,7-dihydroxytryptamine lesions in the rat: response to selective 5-HT1A and 5-HT2,1C agonists

To study the involvement of serotonin (5-HT) receptor subtypes in behavioral supersensitivity following neonatal 5,7-dihydroxytryptamine (5,7-DHT) lesions, we measured acute behavioral responses to a single dose of selective 5-HT1A (8-OH-DPAT) or 5-HT2,1C (DOI) agonist compared to 5-hydroxytryptophan (5-HTP) in rats injected with 5,7-DHT intraperitoneally or intracisternally 14 weeks earlier. Only intraperitoneal 5,7-DHT injection resulted in brainstem 5-HT hyperinnervation, but cortical 5-HT depletions were also less. Effects of DOI, such as shaking behavior and forepaw myoclonus, were enhanced by 5,7-DHT lesions made intracisternally not intraperitoneally, whereas 8-OH-DPAT-evoked behaviors, such as forepaw myoclonus and head weaving, were enhanced more by the intraperitoneal route. The main consequence of intraperitoneal compared to intracisternal 5,7-DHT injection on supersensitivity to 5-HT agonists was increased presynaptic 5-HT1A responses and decreased 5-HT2,1C responses. In contrast, 5-HTP evoked more shaking behavior and less of the serotonin syndrome with the intraperitoneal compared to the intracisternal route of 5,7-DHT injection. Behavioral supersensitivity to 5-HTP, which was attributable to 5-HT1A, 5-HT2,1C, and possibly to other 5-HT receptors, was orders of magnitude greater than that elicited by direct receptor agonists and more clearly differentiated between rats with 5,7-DHT lesions and their controls, and between routes of 5,7-DHT injections, than responses to 5-HT agonists at the dose studied. 5,7-DHT induced dysregulation of 5-HT receptors, including both presynaptic and postsynaptic changes and altered interactions between receptor subtypes, better explains these data than postsynaptic changes alone.     

Potentiation of pancuronium induced neuromuscular blockade by calcium channel blockersin vitro

Summary Microinjection of sodium acetylsalicylate (aspirin, 0.2–0.6 mg) into the preoptic anterior hypothalamic area, but not the dorsal raphe region or the periaqueductal central gray matter, produced a dose-related analgesia in conscious monkeys. The analgesia induced by intrahypothalamic administration of aspirin was antagonized by pretreatment of monkeys with either a serotoninergic receptor blocker (cyproheptadine) or two catecholaminergic receptor blockers (haloperidol and yohimbine). These suggest the existence of a monoaminergic pain-inhibitory mechanism in the preoptic anterior hypothalamic area activated by aspirin.     

Effects of intracisternal vs. intrahypothalamic 5,7-DHT on feeding elicited by hypothalamic infusion of NE.

A variety of evidence has led to suggestions that brain serotonin (5-HT) and norepinephrine (NE) interact within the medial hypothalamus to control food intake. To test the possibility that chronic decrements in 5-HT might enhance NE-induced feeding, adult male rats were prepared with permanently indwelling cannulae aimed at the paraventricular nucleus (PVN), then received either intracisternal (IC) or PVN injections of the 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) vs. its vehicle, 1% ascorbic acid. Over a 4-week period, IC-5,7-DHT rats showed no signs of enhanced daily feeding or drinking. However, in 40-min intake tests, feeding but not drinking was enhanced by injecting 20 nmol NE into the PVN commencing 2 weeks after neurotoxin treatment. Terminal monoamine assays confirmed that IC-5,7-DHT produced large (80-90%) depletions of brain regional 5-HT. A functional index of 5-HT terminal damage was also implied by the impaired short-term feeding responses IC-5,7-DHT rats showed to the systemic administration of the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) when tested between 3 and 4 weeks after IC treatment. Over a comparable 4-week period, PVN-5,7-DHT rats also showed no tendencies to overeat or overdrink on a daily basis. However, in contrast to IC-5,7-DHT rats, they also showed no differences in their feeding or drinking responses to NE injections into the PVN. This was so despite reliable depletions of 5-HT in the hypothalamus (-28%) and hippocampus (-71%). These results support earlier work showing that neither widespread nor localized hypothalamic damage to brain 5-HT neurons produce chronic overeating. However, the data suggest that phasic enhancements of PVN NE activity may trigger enhanced feeding when there is widespread damage to brain 5-HT neurons, although the PVN does not appear to be the brain site mediating this effect.     

Effects of 5,7-dihydroxytryptamine on HRP retrograde transport from hippocampus to midbrain raphe nuclei in the rat

The combination of horseradish peroxidase (HRP) retrograde tracing and specific lesioning using 5,7-dihydroxytryptamine (5,7-DHT) was applied to the midbrain raphe-hippocampal system. Serotonergic fibers from the median raphe nucleus (MRN) of the rat reach the dorsal hippocampus (HIPP) through the cingulum bundle (CB) and the fornix-fimbria (FF). Intracerebral microinjections of 5,7-DHT in these two bundles were made at various times before HRP injections into the dorsal HIPP. After both CB and FF lesion, the number of labeled cells in MRN is reduced to 49.6% at zero time (HRP injected immediately after 5,7-DHT) and to 6.5% after 2 days. There was no significant effect on the number of labeled cells in the locus ceruleus. Selective lesioning of 5-HT fibers in the CB or the FF revealed that raphe-CB-HIPP neurons and raphe-FF-HIPP neurons have a similar distribution pattern in the MRN, but that a dorsal group of neurons at the junction of MRN and dorsal raphe nucleus took the CB route exclusively to innervate the HIPP. The CB pathway was used by more neurons (55% of total number of labeled neurons) than was the FF (21%). An appreciable number of fibers (23%) appear to have branches in both pathways. Our findings are discussed with regard to the recovery of HIPP function seen after long term destruction of 5-HT fibers in the CB.     

Inhibition of neurons in the amygdala by dorsal raphe stimulation: Mediation through a direct serotonergic pathway

This study presents data showing that the dorsal raphe nucleus (DRN) has a marked inhibitory influence upon neurons in the amygdala and that this inhibitory effect is mediated by a direct DRN-amygdala serotonergic pathway. The evidence may be briefly summarized as follows: (1) on the same amygdaloid cells, both iontophoresis of serotonin (5-HT) and electrical stimulation of the DRN markedly inhibited spontaneous single unit activities; (2) the latency of DRN-induced inhibition was relatively short and is compatible with the conduction velocities (which were determined by antidromic activation of the 5-HT pathway) of unmyelinated 5-HT fibers; (3) destruction of 5-HT projections by 5, 7-dihydroxytryptamine (5, 7-DHT) or pharmacological depletion of 5-HT by parachlorophenylalanine (PCPA) prevented the inhibitory responses to DRN stimulation in the great majority of cells studied; (4) in PCPA-pretreated animals, injection of 5-hydroxytryptophan (5-HTP) reversed the PCPA effect, restoring the responses of amygdaloid cells to DRN stimulation. In the amygdala, the presumptive 5-HT antagonists which we tested did not block the inhibitory effects of 5-HT except that intravenously administered LSD blocked the inhibitory responses produced by submaximal DRN stimulation. The implications of these results for the possible functions of 5-HT in the amygdala is discussed.     

Intra-raphe neurokinin-induced hyperactivity: effects of 5,7-dihydroxytryptamine lesions

Rats were implanted with cannulae in the median raphe nucleus (MR). 5,7-Dihydroxytryptamine (5,7-DHT) or vehicle was infused either directly through the MR cannula, or bilaterally into the medial forebrain bundle (MFB). The MR 5,7-DHT lesions completely blocked the hyperactivity elicited by injections into the MR of the neurokinin (NK) 3 agonists, DiMe-C7 and senktide, and the NK-2 agonist, neurokinin A. In contrast, the MFB 5,7-DHT lesions did not affect the locomotor hyperactivity produced by intra-MR administration of DiMe-C7 and senktide, but appeared to attenuate the effects of NKA. The data indicate that intra-raphe neurokinin-induced hyperactivity is mediated by 5-HT neurons, and that 5-HT projections to the forebrain may be involved in the behavioral activation induced by intra-raphe neurokinin A administration, but not that induced by intra-MR NK-3 agonists.     

Effects of serotonergic denervation on the density and plasticity of brain muscarinic receptors in the rat.

This study investigated whether serotonergic lesion may affect density, sensitivity, and plasticity of muscarinic receptors in hippocampus and cerebral cortex. Intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) in rats produced a 90% reduction in cortical and hippocampal 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents. In these brain areas, the 5,7-DHT lesion did not affect the overall density of muscarinic receptors or those of M1 and non-M1 muscarinic receptor subtypes as assayed using [3H]N-methylscopolamine ([3H]NMS), [3H]pirenzepine, and [3H]NMS in the presence of pirenzepine, respectively. In addition, the binding of the muscarinic agonist [3H]oxotremorine-M (OXO-M), taken as an indirect index of coupling efficiency of non-M1 receptors with G-proteins, did not change significantly in cortex and hippocampus of 5,7-DHT-lesioned rats. Similarly, carbachol-induced accumulation of [3H]inositol phosphates (InPs) in hippocampal miniprisms showed no significant differences between tissues from 5,7-DHT-lesioned and sham-operated rats. In sham-operated rats, an intraperitoneal (i.p.) injection of scopolamine (10 mg/kg once daily) during 21 days caused an increased density of [3H]NMS binding sites in cortex (+20%) and hippocampus (+26%). This up-regulation was restricted to non-M1 receptors subtypes. In 5,7-DHT-lesioned rats, chronic scopolamine failed to modify significantly the density of cortical or hippocampal M1 or non-M1 receptors. These results suggest 1) that 5-HT denervation did not affect the density and sensitivity of muscarinic receptors and 2) that the ability of cortical and hippocampal non-M1 receptors to up-regulate following repeated injection of scopolamine requires the integrity of 5-HT neurons terminating in these brain structures.     

The effects of putative 5-hydroxytryptamine receptor active agents ond-amphetamine self-administration in controls and rats with 5,7-dihydroxytryptamine median forebrain bundle lesion

Animals in which 5,7-dihydroxytryptamine (5,7-DHT) was bilaterally injected into the median forebrain bundle (MFB) and sham lesioned animals were allowed access to an apparatus which delivered, upon lever pressing, intravenousd-amphetamine injections. MFB lesioned rats achieved stable self-injections patterns and self-administered more drug per test session than controls. A number of agents known to either directly or indirectly affect 5-hydroxytryptamine (5-HT) receptor function were administered prior tod-amphetamine access. The responses to these pretreatments in lesioned vs non-lesioned rats were markedly different. Pretreatment withl-tryptophan reduced the number ofd-amphetamine self-injections in sham lesioned rats but had no effect in MFB lesioned animals. Fluoxetine pretreatment, likewise, reduced responding in non-lesioned rats and had no observable effect in lesioned animals. Quipazine markedy reduced self-injection in control rats but was not evaluated in the lesioned group. The putative 5-HT antagonists utilized, cyproheptadine and methysergide, unpredictably reduced self-injection frequency of non-lesioned animals in a dose related manner. When MFB lesioned animals were pretreated with cyproheptadine, rapid bursts of lever pressing were observed and 3 of 6 animals thus treated died as a result (presumably amphetamine overdose). In the remaining animals, methysergide produced a similar marked increase in self-injection rate. While these data may suggest that, in some instances, non-serotonergic mechanisms are involved, for the most part it would appear that 5-HT containing neurons are of major import in some aspect ofd-amphetamine self-administration.     

Differential effects of serotonin (5-HT) lesions and synthesis blockade on neuropeptide-Y immunoreactivity and 5-HT1A, 5-HT1B/1D and 5-HT2A/2C receptor binding sites in the rat cerebral cortex

The present study was aimed at comparing the effects of serotonin (5-HT) synthesis blockade using chronic administration of p-chlorophenylalanine (PCPA) and 5,7-dihydroxytryptamine injections of variable volume (3 vs. 6 μl) on the density of NPY immunoreactive (Ir) neurons and binding of [³H]8-OH-DPAT, S-CM-G[¹²⁵I]TNH₂ and [¹²⁵I]DOI to 5-HT_1A, 5-HT_1B/1D, and 5-HT_2A/2C receptors in rat cortical regions. Three weeks after large but partial (89% depletion in 5-HT tissue concentration) lesions of 5-HT neurons no changes in neither NPY immunoreactivity nor 5-HT receptor binding were detected. The complete 5,7-DHT lesions produced increases in the number of NPY-Ir neurons in the upper regions of the cingular (134%), frontal (140%) and parietal cortex (48%) and corresponding decreases in 5-HT_2A/2C binding (16–26%). No changes in 5-HT_1A and 5-HT_1B/1D binding were observed after lesions of this kind. After PCPA treatment, decreases in NPY-Ir neurons density (22–40%) and increases in 5-HT_1A and 5-HT_1B/1D receptor binding sites (20–50%) were distributed in both upper and deeper cortical regions. The lack of effect of the partial lesion suggests that spared 5-HT neurons may exert compensatory mechanisms up to a large extent. The changes in NPY immunoreactivity and 5-HT_2A/2C binding detected in the upper regions of the cortex after complete 5-HT lesions probably result from local cellular rearrangements, whereas blocking 5-HT synthesis has more widespread influence on NPY neurons and on 5-HT_1A and 5-HT_1B/1D receptor subtypes. Moreover, decreases in DOPAC concentrations detected only after complete lesions suggest that the involvement of catecholaminergic transmission may also differentiate 5,7-DHT and PCPA treatments. Altogether, these data suggest that different receptor subtypes might be involved in 5-HT–NPY relationships.