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.     

Activity, avoidance learning and regional 5-hydroxytryptamine following intra-brain stem 5,7-dihydroxytryptamine and electrolytic midbrain raphe lesions in the rat.

Rats underwent one of the following treatments: (1) electrocoagulation of both the dorsal and median midbrain raphe nuclei; (2) 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) injection (10 mug, as the salt, in 5 mul vehicle) into the vicinity of each midbrain raphe nucleus; (3) intra-brain stem vehicle (5 mul of 0.2% ascorbic acid in isotonic saline) injections; or, (4) a control operation. Open field activity and one-way avoidance conditioning were examined on postoperative days 16-23. Regional central 5-hydroxytryptamine (5-HT) and catecholamine (CA) concentrations were determined 25-27 days postoperatively. Regional 5-HT levels were greatly reduced following 5,7-DHT administration and electrolytic raphe lesions. The 5,7-DHT rats also showed a reduction in spinal 5-HT content. Central CA concentrations were not affected. Variation in the pattern of regional 5-HT changes after 5,7-DHT treatment was observed but appeared to be related to the adequacy of the dorsal raphe (B7) injection. Only the electrolytic raphe lesion animals, however, showed increased locomotor activity and retarded acquisition and forced-extinction of the one-way avoidance response. In contrast, no significant differences were observed in the open field and avoidance behavior of the 5,7-DHT, vehicle, and control groups. The hyperactivity and impaired one-way avoidance performance observed after electrolytic midbrain raphe lesions are not related simply to reductions in regional forebrain 5-HT and may well be due to damage of non-serotonergic neural systems. Clearly, the behavioral effects of central 5-HT depletion depend on the method employed. The role of 5-HT in regulating activity level and mediating avoidance behavior, furthermore, remains to be determined.     

Regional 5-hydroxytryptamine following selective midbrain raphe lesions in the rat

Lesions were produced in either the dorsal or median raphe nucleus and regional 5-hydroxytryptamine (5-HT), 5-hydroxyindole acetic acid (5-HIAA), and norepinephrine (NE) determined 26–30 days post-operatively. Only lesions in the dorsal raphe nucleus produced a fall (54%) in striatal 5-HT, while only lesions in the median raphe nucleus reduced (62%) hippocampal 5-HT. Reductions in the 5-HT and 5-HIAA contents of the remaining portion of the telencephalon and of the diencephalon following dorsal raphe lesions were twice as great as after median raphe lesions. The lowered 5-HT concentration (50%) of the telecephalon (excluding hippocampus and striatum) after dorsal raphe lesions, furthermore, was twice as large as that of the diecephalon (24%). Only dorsal raphe lesions produced a fall in brain stem 5-HT content. Neither lesion affected spinal (cervical-thoracic) 5-HT, nor NE in any of the brain areas assayed. It would appear that the dorsal and median raphe nuclei project 5-HT fibers into the forebrain but not to the spinal cord. The marger number of 5-HT fibers seems to originate in the dorsal raphe nucleus, which also seems to send a greater number of its 5-HT fibers to telecephalic than diencephalic structures. And, lastly, hippocampal and striatal 5-HT inputs apparently originate chiefly in the median and dorsal raphe nuclei, respectively.     

Effect of hypothalamic injections of 5,7-dihydroxytryptamine on elicitation of mouse-killing in rats

An overall and marked serotonin (5-HT) depletion of the brain was found to facilitate initiation of mouse-killing behavior in the rat, whereas more selective 5-HT depletions within forebrain structures such as the septum, hippocampus, cingular cortex and amygdala, did not have such an effect. In order to further investigate the topography of the 5-HT pathways and terminals thought to be involved in an inhibitory control over this behavior, localized lesions of the serotonergic system(s) were performed by means of bilateral 5,7-dihydroxytryptamine (5,7-DHT) injections (5 μg/μl) into the hypothalamus in naive rats. 5,7-DHT injections into the medial hypothalamus did not affect the initiation of mouse-killing behavior, whereas the reflexive startle responses to air puffs were increased. The animals' open-field behavior remained unchanged. Forebrain 5-HT content was reduced by 50% in this group. 5,7-DHT injections into the lateral hypothalamus increased the proportion of killers to 46% as compared to 10% in the control group, in spite of a reduced activity in the open-field and unchanged startle responses. Forebrain 5-HT content was reduced by 88%. As the lateral hypothalamus contains afferents from both the dorsal and the median raphe nuclei, it is likely that 5-HT terminals modulate some hypothalamic mechanism involved in the control of mouse-killing behavior.     

Autoradiographic visualization of NK-3 tachykinin binding sites in the rat brain, utilizing [3H]senktide.

The autoradiographic distribution of the selective NK-3 tachykinin agonist [3H]senktide was investigated in rat brain. [3H]Senktide bound with high affinity (KD less than 2.5 nM) and high specificity (greater than 75%) to cerebral cortex and numerous subcortical sites, including the substantia nigra pars compacta. In addition, moderately dense binding was seen in the median but not the dorsal raphe nucleus, and this was disrupted by 5,7-dihydroxytryptamine (5,7-DHT)-induced destruction of 5-HT neurons. 5,7-DHT lesions did not affect the binding of [3H]senktide to forebrain regions, suggesting that 5-HT terminals are devoid of NK-3 receptors.     

Denervation of serotonergic fibers in the hippocampus induced a trophic factor which enhances the maturation of transplanted serotonergic neurons but not norepinephrine neurons

The trophic effects of specific denervation on the growth and survival of fetal serotonergic (5-HT) or norepinephrinergic (NE) neurons grafted into the hippocampus were assessed by means of two transplantation paradigms. In the first, fetal raphe cells (containing 5-HT neurons) were transplanted into the control hippocampus. In the second, the transplantation was performed 2 weeks after the specific removal of 5-HT afferents to the hippocampus with 5,7-dihydroxytryptamine (5,7-DHT). We found that a month after transplantation, the number of 5-HT immunoreactive neurons was not significantly different between the two experimental paradigms. However, transplanted raphe neurons had 400% more 5-HT synaptosomal high-affinity uptake and 380% higher content of 5-HT in the hippocampus with prior 5,7-DHT lesion than in control hippocampus. Furthermore, immunocytochemistry showed that the transplanted 5-HT neurons had denser processes and varicosities in the hippocampus with lesion than in control hippocampus. The somatic area of the neurons with these denser processes and varicosities was 42% larger than that of control group. A greater 5-HT level could be achieved if transplanted neurons in the control hippocampus were treated with the supernatant extracted from the hippocampus with 5,7-DHT lesion. In contrast, the NE level of the implanted fetal locus ceruleus (containing NE neurons) was not significantly higher in the 5-HT denervated hippocampus than in control hippocampus a month after transplantation. These results suggest that 5-HT denervation in the hippocampus induces a trophic substance which promotes the maturation rather than survival of 5-HT neurons but not NE neurons.     

Effect of the selective lesion of serotoninergic neurons on the regional distribution of 5-HT1A receptor mRNA in the rat brain.

The effects of the selective lesion of serotoninergic neurons by an intra-raphe administration of 5,7-dihydroxytryptamine on the 5-HT1A receptor protein and the 5-HT1A receptor mRNA were examined in various regions of the rat brain using specific antibodies and an antisense riboprobe, respectively. Twenty one days after the treatment, the 5-HT1A receptor protein was no longer detected within the dorsal raphe nucleus but was still present in the hippocampus and entorhinal cortex. Quantitative in situ hybridization showed an 85% decrease in the levels of 5-HT1A receptor mRNA within the dorsal raphe nucleus, but no significant change in the hippocampus, interpeduncular nucleus and entorhinal cortex of 5,7-dihydroxytryptamine-treated rats. These data demonstrate that 5-HT1A receptors are synthesized by serotoninergic neurons in the dorsal raphe nucleus, and by neurons located postsynaptically with regard to serotoninergic projections in other areas. The unchanged levels of 5-HT1A receptor mRNA in the hippocampus, interpeduncular nucleus and entorhinal cortex three weeks after the extensive lesion of serotoninergic neurons are consistent with the absence of 5-HT1A receptor up regulation already reported under this condition.     

Antidepressant-like action of 8-OH-DPAT, a 5-HT1A agonist, in the learned helplessness paradigm: evidence for a postsynaptic mechanism

In animal models of depression, the 5-HT1A agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), buspirone, gepirone and ipsapirone administered i.p. have been shown to mimic the behavioural effects of antidepressants. For instance, in the present study, using the learned helplessness paradigm, 8-OH-DPAT dose-dependently reversed helpless behaviour. To assess the possible role of pre- or postsynaptic 5-HT1A receptors in this effect, the ability of 8-OH-DPAT to reduce helpless behaviour was investigated following (1) i.p. administration (0.125 or 0.25 mg/kg/day) in rats whose ascending 5-HT neurons were partially destroyed by previous 5,7-dihydroxytryptamine (5,7-DHT) injection (5 micrograms free base in 0.6 microliter) into the raphe nuclei or (2) after local microinjection (0.1 or 1.0 microgram in 0.5 microliter) into the raphe nuclei or into the septum. The reversal of helpless behaviour by 8-OH-DPAT (i.p.) was still observed in 5,7-DHT-treated rats with telencephalic 5-HT uptake reduced by 50-75% depending on the region. 8-OH-DPAT microinjected into the raphe nuclei did not reverse helpless behaviour; in contrast, 8-OH-DPAT microinjected into the septum reversed helpless behaviour. These results suggest that the ability of 8-OH-DPAT to reverse helpless behaviour probably involved the stimulation of postsynaptic rather than presynaptic 5-HT1A receptors.     

A thyrotropin-releasing hormone-containing system in the rat dorsal horn separate from serotonin

In this study, we report the identification of a thyrotropin-releasing hormone (TRH)-containing system in the dorsal horn of the rat spinal cord. This system is distinct from the TRH and serotonin (5-hydroxytryptamine, 5-HT) cotransmitter supraspinal system that has projections to the intermediolateral (IML) and ventral columns. Spinal cord sections from untreated rats, and those treated with colchicine or 5,7-dihydroxytryptamine (5,7-DHT) were processed using peroxidase-antiperoxidase (PAP) immunocytochemistry with nickel intensification. Results of the 5,7-DHT treatment were verified by quantifying TRH and 5-HT by radioimmunoassay (RIA) and high performance liquid chromatography (HPLC), respectively. Prominent immunocytochemical staining for TRH in the dorsal horn was seen in varicose fibers mainly in lamina II and superficial lamina III of the dorsal horn of the spinal cord of control rats. A few fibers were seen ascending into lamina I. A moderate number of fibers that were immunoreactive for 5-HT were primarily in laminae I and II. The distribution of TRH- and 5-HT-containing neurites in the IML and the ventral horn agreed with previously published reports. Rats treated with colchicine showed many small round TRH immunoreactive cells that were limited to laminae II/III of the dorsal horn. TRH immunoreactivity in the dorsal horn and IML was resistant to the effects of the selective serotonin neurotoxin, 5,7-DHT, while the ventral horn was depleted of TRH staining. Serotonin was almost completely eliminated in all spinal cord laminae. Quantitative biochemical studies showed significant, but non-parallel reductions of TRH and 5-HT in cervical, thoracic and lumbar spinal cord. These studies demonstrate the existence of TRH-containing cell bodies and terminals in the dorsal horn of the rat spinal cord. These findings provide evidence that a TRH-containing system exists in the dorsal horn of the rat and that it is distinct from the descending medullary raphe system that contains 5-HT; suggest that a population of TRH-containing fibers that project to the IML may not contain 5-HT; and confirm previously published results that 5-HT and TRH coexist in terminals in the ventral horn of the spinal cord.     

Noradrenergic innervation of serotonergic neurons in the dorsal raphe: demonstration by electron microscopic autoradiography

In this study, noradrenergic (NE) terminals in the dorsal raphe were identified by [3H]NE electron microscopic (EM) autoradiography. Lesioning of NE terminals by treatment with the selective catecholamine neurotoxin, 6-hydroxydopamine produced a marked decrease in NE-labelled terminals. [3H]5-HT EM autoradiography of the dorsal raphe produced labelling of cell bodies, dendrites and axons but labelled terminals with synaptic junctions were not observed. Serotonergic (5-HT) neurons were identified at an early stage of degeneration following treatment with the selective 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). When both [3H]NE autoradiography and 5,7-DHT lesioning were combined, a majority of NE-labelled terminals, which formed synaptic specializations, innervated degenerating dendrites. These findings suggest that NE terminals directly innervate 5-HT cells in the dorsal raphe.     

Effects of 5,7-dihydroxytryptamine on serotonin1 and serotonin2 receptors throughout the rat central nervous system using quantitative autoradiography

The effects of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), on serotonin1 (5-HT1) and 5-HT2 receptors were investigated using the high degree of resolution provided by quantitative autoradiography in an effort to determine the synaptic location of these receptors. 5,7-DHT treatment resulted in a decrease in 5-HT1 binding in the dentate gyrus and CA3c/4 of the anterior hippocampus and in the dorsal raphe nucleus, whereas no changes were observed in the posterior hippocampus nor in many other brain structures. 5-HT2 receptors exhibited no changes in any brain area examined in response to 5,7-DHT treatment, despite over 90% serotonin depletion in most of the forebrain nuclei examined. The results indicate that at least some of the 5-HT1 sites labelled by [3H]5-HT in the hippocampus and dorsal raphe nucleus are presynaptic, whereas 5-HT2 receptors are probably postsynaptic. In addition, the distribution profiles of 5-HT1 and 5-HT2 binding sites were compared in the rat central nervous system at various anatomical levels. 5-HT1 binding sites were identified using [3H]5-HT, while 5-HT2 binding sites were labelled with [3H]ketanserin. Both receptor subtypes displayed distinctly different localization patterns, which, in most cases was the inverse of the other pattern. In the brainstem it is significant that 5-HT2 receptors are concentrated in the facial nucleus and the motor nucleus of the trigeminal nerve, areas known to influence head and facial movement. The serotonin-mediated head-shake response occurs when 5-HT2 receptors are activated. In contrast, 5-HT1 receptors are distributed throughout the brainstem and in specific portions of the spinal cord. These areas are thought to control the serotonin behavioral syndrome and this behavior is 5-HT1A-mediated. All raphe nuclei were devoid of 5-HT2 receptors; only 5-HT1 receptor were found in these nuclei. Correlations with serotonin terminal distribution patterns are discussed. The pattern of 5-HT2 receptor distribution was also compared with the pattern of alpha 1 receptors, using [3H]prazosin in order to determine whether [3H]ketanserin significantly labels alpha 1 receptors. Although some similarities exist, overlap of binding did not occur in other nuclei, indicating that alpha 1 contamination of this system is probably negligible.     

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 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.     

Supranormal levels of serotonin and its metabolite after raphe cell transplantation in serotonin-denervated rat hippocampus

Transplantation of fetal raphe cells (14 days of gestation) into the adult rat hippocampus, 2 weeks following serotonin (5-HT)-denervation with intracisternal injection of 5,7-dihydroxytryptamine, can restore 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels in the hippocampus to far beyond normal values. Transplantation into the unilateral hippocampus produces asymmetrical turning behavior after administration of the 5-HT releasor, p-chloroamphetamine (IP), comparable to the behavior reported for rats with 5-HT denervation of the unilateral hippocampus. The effect is blocked by prior depletion of 5-HT with p-chlorophenylalanine (IP). The asymmetry in 5-HT levels are correlated with the behavioral change. These data indicate that a large amount of 5-HT is released from nerve terminals of transplanted raphe cells, and suggest that the supranormal levels of 5-HT and 5-HIAA after raphe cell transplantation are neurochemical correlates of 5-HT hyper-innervation of the hippocampus which has been reported previously.     

Developmental regulation of the serotonergic transmitter phenotype in rostral and caudal raphe neurons by transforming growth factor-betas.

Serotonergic (5-HT) neurons of the CNS develop as two separate clusters, a rostral and a caudal group, within the brain stem raphe. We show here that the transforming growth factors -beta2 and -beta3 (TGF-beta) and the TGF-beta type II receptor are expressed in the embryonic rat raphe, when 5-HT neurons develop and differentiate. To investigate putative roles of TGF-betas in the regulation of 5-HT neuron development we have generated serum-free cultures isolated either from the rostral or the caudal embryonic rat raphe, respectively. In cultures from the caudal E14 raphe saturating concentrations (5 ng/ml) of TGF-beta2 and -beta3 augmented numbers of tryptophan hydroxylase (TpOH) -immunoreactive neurons and cells specifically taking up 5,7-dihydroxytryptamine (5,7-DHT) by about 1.7-fold over a period of 4 days. Treatment with TGF-betas also increased uptake of 3H-5HT uptake about 1.7-fold. Alterations in 5-HT neuron numbers were due to the induction of serotonergic markers rather than increased survival, as shown by the efficacy of delayed short-term treatments. Comparing rostral and caudal raphe cultures from different embryonic ages suggests that distinct effects of TGF-betas reflect the responsiveness of 5-HT neurons at different ages rather than of different origins.     

5,7-DHT lesion of the dorsal raphe nuclei impairs object recognition but not affective behavior and corticosterone response to stressor in the rat

Previous studies with acute tryptophan depletion, leading to transient central 5-HT reductions, showed no effects on affective behavior but impaired object memory. In the present study, the behavioral effects of a 5,7-dihydroxytryptamine (5,7-DHT) lesion in the dorsal raphe were evaluated in animal models of anxiety (open field test), depression (forced swimming test), behavioral inhibition (discrete fixed interval test) and cognition (object recognition task). The corticosterone response to a stress condition was examined at several intervals after 5,7-DHT treatment. The substantial reduction in neuronal 5-HT markers in the dorsal raphe did not affect anxiety-related, depressive-like or impulsive behavior. Compared to the SHAM group, the lesioned rats showed a lower response latency to obtain a reward, indicating a quick and accurate reaction to a stimulus. No differences were found in the progressive ratio test for food motivation. A marked impairment in object recognition was found. The 5,7-DHT treatment did not affect the corticosterone response to a stressful situation. Overall, these results corroborate studies with acute tryptophan depletion suggesting a role of 5-HT in object memory, but not affective behavior.     

Effect of GM1 ganglioside on neonatally neurotoxin induced degeneration of serotonin neurons in the rat brain

The effect of exogenous GM1 ganglioside on the 5,7-dihydroxytryptamine (5,7-HT; a selective serotonin neurotoxin) induced alteration of the postnatal development of central 5-hydroxytryptamine (5-HT; serotonin) neurons has been investigated using neuro-chemical and immunocytochemical techniques. Neonatal 5,7-HT (50 mg/kg s.c.) treatment is known to lead to a marked and a permanent degeneration of distant 5-HT nerve terminal projections (e.g. in cerebral cortex, hippocampus and spinal cord), while projections close to the 5-HT perikarya in the mesencephalon and pons-medulla increase their nerve density. These regional alterations are reflected by decreases and increases, respectively, of endogenous 5-HT, [3H]5-HT uptake in vitro and number of 5-HT nerve terminals demonstrated by immunocytochemistry. Treatment of newborn rats with GM1 (4 X 30 mg/kg s.c.; 24 h interval) had no significant effect on the postnatal development of 5-HT neurons. GM1 administration had furthermore no effect on the 5,7-HT induced alteration of the regional 5-HT levels and [3H]5-HT uptake in the cerebral cortex acutely, indicating that GM1 did not significantly interfere with the primary neurodegenerative actions of 5,7-HT. At the age of 1 month a clear counteracting effect of GM1 was observed, in particular of the 5,7-HT induced 5-HT denervations. The 5-HT levels in the frontal and occipital cortex were reduced to 25 and 20% of control after 5,7-HT alone, while these values were 70 and 40%, respectively, after 5,7-HT and GM1 treatment. A similar antagonizing effect of GM1 was found in the frontal cortex when measuring [3H]5-HT uptake. GM1 treatment also caused a minor reduction of the 5,7-HT induced increase of the 5-HT levels in striatum and mesencephalon. Quantitation of 5-HT nerve terminal density in sections processed for 5-HT immunocytochemistry using an automatic image analysis system showed markedly more nerve terminals in the frontal and occipital cortex after 5,7-HT + GM1 compared to 5,7-HT treatment alone. Minor counteracting effects of GM1 were noted in the hippocampus and spinal cord (thoracic-lumbar) as evaluated by chemical 5-HT assay, although substantial counteracting effects were observed locally in these areas by quantitative immunocytochemistry.(ABSTRACT TRUNCATED AT 400 WORDS)     

Early post-natal administration of 5,7-dihydroxytryptamine destroys 5-HT neurons but does not affect spatial memory.

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) may play an important role in learning and memory. It has also been suggested that 5-HT abnormalities may mediate some aspects of the cognitive disorders associated with Korsakoff syndrome and Alzheimer's Disease. The effect of intracisternally applied 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) on learning and memory in rodents was evaluated. Three-day-old rat pups were treated with pargyline (40 mg/kg, i.p.) followed by 5,7-DHT (50 micrograms/pup) and returned to the dam for a month. At 75 days of age, rats were tested on a learning set problem in the Morris water maze for 5 days followed by 30 days of testing in a 12-arm radial maze with 8 of the 12 arms baited. In the Morris water maze, the latency to locate the hidden platform did not differ significantly for 5,7-DHT treated and control rats (F less than 1.0). Similarly, 5,7-DHT treated rats performed comparably to controls on the 12-arm radial maze (F less than 1.0). At 106 days of age the assay of tryptophan hydroxylase activity in the dorsal raphe nuclei and hippocampus showed marked reduction (86%, 78%, respectively) in 5,7-DHT treated animals compared to vehicle injected controls. Immunocytochemical analysis was consistent with the biochemical results. In 5,7-DHT treated animals there was severe loss of neurons that bind 5-HT antibody in the dorsal and medial raphe nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonin neurons grafted to the adult rat hippocampus. II. 5-HT release as studied by intracerebral microdialysis

Extracellular levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were monitored by microdialysis in the hippocampal formation previously denervated of its serotonergic input by an intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT), and in 5,7-DHT denervated hippocampi reinnervated by grafted fetal rat serotonin neurons. Two weeks after 5,7-DHT lesion, baseline 5-HT release was reduced to levels below detection, and KCl- and p-chloro-amphetamine-evoked release was reduced by 90-95%. In the chronically denervated hippocampus (3 months after lesion), baseline 5-HT release had recovered to near-normal levels, but KCl- and p-chloroamphetamine-evoked release remained severely impaired. Addition of the 5-HT re-uptake blocker indalpine to the perfusion medium induced a 5-6-fold increase in serotonin overflow in the normal hippocampus, while the serotonin overflow in the 5,7-DHT denervated hippocampus remained unaffected. The intrahippocampal fetal raphe transplants restored 5-HT release to near-normal levels, not only under baseline conditions but also in the presence of re-uptake blockade. Both KCl- and p-chloroamphetamine-induced release had recovered in the grafted hippocampus and the responses were even greater than those seen in normal animals. In both normal and grafted hippocampus addition of the sodium channel blocker tetrodotoxin reduced 5-HT overflow to the level seen in the denervated hippocampus. The new hippocampal serotonin innervation, established by the grafts, was markedly denser than normal, and the tissue 5-HT and 5-HIAA levels were 3-4-fold higher than normal in the grafted hippocampi. The 5-HIAA level in the perfusate collected from the grafted hippocampi showed a similar increase above normal, whereas 5-HT release was maintained within the normal range, both under baseline conditions and in the presence of re-uptake blockade. The results indicate that the grafted serotonergic raphe neurons are spontaneously active at the synaptic level, despite their ectopic location. The ability of the grafted neurons to maintain 5-HT release within the normal range suggests that local regulatory mechanisms at the terminal level can compensate for abnormalities in the graft-derived innervation density.     

In vivo binding properties of [carbonyl-11C]WAY-100635: effect of endogenous serotonin

[Carbonyl-(11)C]WAY-100635 has been reported to be a useful ligand for the investigation of 5-HT(1A) receptor imaging in vivo. However, the cellular distribution and the influence of endogenous serotonin (5-HT) on in vivo binding have not been fully examined. In this study, we investigated the effect of 5,7-dihydroxytryptamine-produced destruction of 5-HT neurons, reserpine-induced 5-HT depletion, and fenfluramine-induced 5-HT increase on [carbonyl-(11)C]WAY-100635 binding in vivo. There was no significant change in the uptake of [carbonyl-(11)C]WAY-100635 in the slice of 5-HT denervated rat brain except in the raphe nucleus, where 5-HT cell bodies exist. There was no obvious effect of enhanced 5-HT release by fenfluramine or decreased release by reserpine on [carbonyl-(11)C]WAY-100635 binding in the dissected brain region. No significant effect was observed in the time course of [carbonyl-(11)C]WAY-100635 in the hippocampus and frontal cortex measured by PET. These results indicated that the in vivo binding of [carbonyl-(11)C]WAY-100635 in the hippocampus and cerebral cortex mainly reflects postsynaptic 5-HT(1A) receptor binding, and that this binding is not sensitive to endogenous 5-HT.