Anxiolytic-like effects of median raphe nucleus lesion in the elevated T-maze

The cell bodies of 5-HT containing neurons that innervate the limbic forebrain are mainly found in the dorsal raphe nucleus and in the median raphe nucleus (MRN). To assess the role of the median raphe nucleus in anxiety, rats bearing either electrolytic or 5-HT-selective neurotoxic lesion of the MRN were tested in the elevated T-maze. This apparatus consists of two opposed open arms perpendicular to one enclosed arm. Two tasks are performed in succession by the same rat in one experimental session, namely inhibitory avoidance of the open arm, taken as a measure of conditioned anxiety and one-way escape from the open arm, considered as a measure of unconditioned fear. The test was performed 7 days after the electrolytic lesion (3 mA, 10s) or 14 days after the neurotoxic lesion (5,7-DHT, 8 microg/1 microl). The results showed that while the electrolytic lesion impaired both inhibitory avoidance and one-way escape, the neurotoxic lesion impaired only inhibitory avoidance. Therefore, serotonergic pathways originating in the MRN seem to participate in the modulation of conditioned anxiety but not unconditioned fear. Other neurotransmitter systems that either originate in or pass through the MRN may regulate unconditioned fear.     

Paraventricular nucleus serotonin mediates neurally stimulated adrenocortical secretion

The purpose of this study was to further elucidate the role of serotonin (5-HT) in adrenocortical regulation. The effects of stimulating the frontal cortex and extrahypothalamic limbic structures, on plasma corticosterone (CS) responses, were studied in rats with vehicle or 5,7-dihydroxytryptamine (5,7-DHT) injection into the midbrain raphe nuclei. In another group of rats the neurotoxin was injected locally into the paraventricular nucleus (PVN) in view of its importance in adrenocortical regulation, and the effects of photic and dorsal hippocampal stimulation on plasma CS were studied. 5,7-DHT caused a significant depletion of hypothalamic 5-HT and blocked the rise in plasma CS following the stimulation of the above neural modalities. These studies suggest that the PVN 5-HT mediates the adrenocortical responses following afferent neural stimuli.     

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)     

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.     

Behavioral effects of selective midbrain raphe lesions in the rat.

Lesions were produced in the median (n = 8), dorsal (n = 7) or both (n = 7) midbrain raphe nuclei and their effects on behavior (days 16-54 postoperatively) compared to that of controls (n = 9). In addition, forebrain 5-hydroxytryptamine (5-HT) concentration were determined. Only the median and combined lesion groups showed increased running wheel and open field activity, as well as enhanced reactivity to novel stimuli and environmental change. None of the lesion groups, however, showed changes in home cage activity on postoperative day 21. Although all lesion groups were deficient in the acquisition and retention of one-way avoidance, the deficits were of a greater magnitude in the median and combined lesion groups. The latter two groups, furthermore, were impaired in forced extinction of the one-way avoidance response, but only the combined lesion group evidenced facilitation of two-way avoidance acquistion. Thus, in contrast to the effects of median or combined raphe lesions, lesions in the dorsal raphe nucleus affected few of the behavioral parameters studied. These results suggest that the dorsal raphe nucleus plays a different behavioral role than the median raphe nucleus. The median nucleus appears to be involved in the regulation of activity level, the reaction to novelty and environmental change, and the response to aversive stimuli. Possible mechanisms for the observed behavioral changes are discussed, as well as their apparent similarity to the effects of other mesencephalic and limbic lesions. Lastly, the median, dorsal and combined raphe lesions lowered forebraine 5-HT but 26, 65, and 77%, respectively, versus controls. These reductions differed significantly from each other, and with previously reported data indicate that the dorsal raphe nucleus in the principal origin of forebrain 5-HT. It is suggested, furthermore, that the behavioral effects of midbrain raphe lesions are not due primarily to their associated reduction in forebrain 5-HT.     

Ascending afferent regulation of rat midbrain dopamine neurons

Standard, extracellular single-unit recording techniques were used to examine the electrophysiological and pharmacological responsiveness of midbrain dopamine (DA) neurons to selected, ascending afferent inputs. Sciatic nerve stimulation-induced inhibition of nigrostriatal DA (NSDA) neurons was blocked by both PCPA (5-HT synthesis inhibitor) and 5,7-DHT (5-HT neurotoxin), suggesting mediation by a serotonergic (5-HT) system. Direct stimulation of the dorsal raphe (which utilizes 5-HT as a neurotransmitter and inhibits slowly firing NSDA neurons) inhibited all mesoaccumbens DA (MADA) neurons tested. Paradoxically, DPAT, a 5-HT_1a agonist which inhibits 5-HT cell firing, enhanced sciatic nerve stimulation-induced inhibition of NSDA neurons. MADA neurons were not inhibited by sciatic nerve stimulation and, therefore, could not be tested in this paradigm. In contrast to the dorsal raphe, electrical stimulation of the pedunculopontine tegmental nucleus preferentially excited slowly firing NSDA and MADA neurons. Thus, both excitatory and inhibitory ascending afferents influence the activity of midbrain DA neurons, and intact 5-HT systems are necessary for sciatic nerve stimulation to alter DA cell activity. However, the role that 5-HT plays in mediating peripheral sensory input remains unclear.     

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.     

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.     

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.     

Compensatory responses in the aging hippocampal serotonergic system following neurodegenerative injury with 5,7-dihydroxytryptamine

This study utilized a multidisciplinary approach to examine injury-induced compensatory responses in the aging hippocampal serotonin transporter (5-HTT), a membrane protein implicated in a variety of neurodegenerative disorders. Age-dependent cellular, anatomical, and physiological changes of the 5-HTT were evaluated in female Fischer 344 rats (2 and 17 months) following denervation of the serotonergic afferents (fimbria-fornix and cingulum bundle) to the dorsal hippocampus using the neurotoxicant 5,7-dihydroxytryptamine (5,7-DHT). Seven days following 5,7-DHT administration, a uniform loss of the hippocampal 5-HTT immunoreactivity was observed in both age groups. However, at 21 days 5-HTT immunoreactivity in young 5,7-DHT-treated animals was similar to control levels, indicative of recovery, while older animals exposed to 5,7-DHT did not show recovery of hippocampal 5-HTT expression. 5-HTT binding site density, as determined by quantitative autoradiography ([3H]citalopram), supported the immunohistochemical results by demonstrating a recovery of 5-HTT binding sites in young, but not old animals, at 21 days following the lesion (P < 0.001). Furthermore, cellular electrophysiological function of hippocampal CA1 pyramidal neurons in 3- and 18-month-old F344 rats at 21 days following 5,7-DHT or vehicle treatment were assessed using in vivo microiontophoretic application of serotonin (5-HT). Independent of changes in sensitivity to the inhibitory effects of 5-HT application, the time to recovery of cell firing following application of 5-HT was significantly increased in the 18-month 5,7-DHT group compared to the 18-month vehicle and 3-month 5,7-DHT groups (60 and 59% increases, respectively; P < 0.05). Overall, these series of studies comprise a model which can be used to identify cellular events underlying both the formation of injury-induced compensatory processes in younger animals and the lack thereof with advancing age.     

Brain monoaminergic control of male reproductive behavior. I. Serotonin and the post-ejaculatory refractory period

The present experiment was performed to examine the role of serotonergic mechanisms in the control of copulation and the post-ejaculatory refractory period in the male rat. Disruption of central serotonergic systems in two separate groups of animals was achieved by: (1) selective electrolytic lesions of the midbrain raphe nuclei, or (2) localized intraventricular or intracerebral injection of a specific serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). A third group of animals was tested for sexual behavior following intraperitoneal injection of p-chlorophenylalanine (PCPA), an inhibitor of serotonin synthesis. Both electrolytic and neurochemical lesions localized in the dorsal raphe nucleus produced a highly significant shortening of the ejaculatory latency, and the post-ejaculatory refractory period. Disruption of serotonergic mechanisms following intraventricular injection of 5,7-DHT or systemic administration of PCPA also caused a significant reduction in the length of the refractory period. These results support the hypothesis that central serotonergic systems are normally inhibitory to certain facets of male copulatory behavior and suggest the existence of a serotonergic control system which normally exerts an inhibitory influence over the resumption of mating following ejaculation.     

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.     

Hippocampal serotonin re-uptake and nocturnal locomotor activity after microinjections of 5,7-DHT in the fornix-fimbria

The role of the hippocampal 5-hydroxytryptamine (5-HT) terminals in the control of locomotor activity was investigated by lesioning 5-HT axons in the fimbria with 5,7-dihydroxytryptamine (5,7-DHT). Rats pretreated with desimipramine (10 mg/kg, i.p.) received microinjections of 5,7-DHT (0, 1, 3, 5 or 10 μg in 0.4 μl ascorbic Ringer's solution) into the fornix-fimbria. On the fourteenth to twenty-first nights after operation, nocturnal locomotor activity was measured in photocell cages. Twenty-eight to thirty days after operation degeneration of 5-HT terminals was assessed by measuring in vitro [³H]5-HT re-uptake in slices of dorsal hippocampus, ventral hippocampus and the septum.Groups injected with 5,7-DHT showed hyperactivity in the night period and increased decrements of activity between tests, both of which were related to the dose of neurotoxin. A reduction of [³H]5-HT re-uptake was found in dorsal hippocampus which was related to the dose of 5,7-DHT, but ventral hippocampal and septal [³H]5-HT re-uptake were not systematically reduced. For each rat, levels of dorsal and ventral hippocampal [³H]5-HT re-uptake were negatively correlated with the mean nocturnal activity from the 7 nights of testing. Levels of dorsal, but not ventral hippocampal [³H]5-HT re-uptake were negatively correlated with the mean nightly decrement of activity. No correlations were found between septal [³H]5-HT and these activity measures. These results, indicate that the increase in nocturnal locomotor activity caused by generalized depletion of 5-HT in the brain may be due to disruption of hippocampal 5-HT terminals supplied by the fornix-fimbria.     

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.     

Serotonergic function of aging hippocampal CA3 pyramidal neurons: Electrophysiological assessment following administration of 5,7-dihydroxytryptamine in the fimbria-fornix and cingulum bundle

Serotonergic (5-hydroxytryptamine; 5-HT) neuro-transmission has been implicated in the regulation of cognitive function and this neurotransmitter system may underlie selective neuronal degeneration found in the aging hippocampus. Age-dependent changes in 5-HT function of hippocampal CA3 subfield pyramidal neurons were evaluated in female Fischer 344 rats (2 and 17 months) following denervation of the serotonergic afferents to the dorsal hippocampus using the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). Vehicle (ascorbic saline) or 5,7-DHT was administered bilaterally in the fimbria-fornix/cingulum bundle and dorsal pyramidal cell responses to microiontophoretic application of 5-HT, the 5-HT_1A agonist (±)-8-hydroxy-2-(di-N-propylamino) tetralin, the 5-HT_1A antagonist WAY 100,135 and N-methyl-D-aspartate were recorded at 3 weeks post-lesion. Independent of changes in sensitivity to the inhibitory effects of 5-HT with aging, the time to recovery of cell firing following application of 5-HT was significantly increased in the 18 month 5,7-DHT group compared to the 18 month Vehicle and 3 month 5,7-DHT groups (3.3- and 2.6-fold, respectively). These results demonstrate that serotonergic neurotransmission is altered with aging following a selective neurotoxic insult to the hippocampus. J. Neurosci. Res. 47:58–67, 1997. © 1997 Wiley-Liss, Inc.     

Increased tryptophan hydroxylase mRNA in raphe serotonergic neurons spared by 5,7-dihydroxytryptamine

Neurons expressing the tryptophan hydroxylase (TPH) mRNA within the raphe nuclei of control rats showed a distribution similar to that observed using an antibody for TPH. Numerous packed cells expressing the TPH mRNA were observed in the ventral and dorsal zone of the nucleus raphe dorsalis (NDR) and in the pars dorsalis of the nucleus centralis superior (NCS) whereas fewer and more scattered neurons were found in the pars medialis of NCS. Five days after the intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT), which markedly reduced the serotonin (5-HT) content in the hippocampus, caudate putamen and cortex, the hybridization signal had completely disappeared in the dorsal region of the NDR. In the ventromedial region, above and between the medial longitudinal fasciculus (MLF), which includes the pars dorsalis of NCS, there was a partial decrease of cell number and a marked increase of the grain density over spared neurons. No significant change was noted in the number of TPH-positive cells and hybridization signal in individual neurons of the pars medialis of NCS. Consistent with previous evidence of increased TPH activity in the residual 5-HT terminals, the present study shows that synthesis of the TPH mRNA may be augmented in some neurons surviving the lesion.     

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.     

The dorsal raphe nucleus exerts opposed control on generalized anxiety and panic-related defensive responses in rats

It has been proposed that the ascending dorsal raphe (DR)-serotonergic (5-HT) pathway facilitates conditioned avoidance responses to potential or distal threat, while the DR-periventricular 5-HT pathway inhibits unconditioned flight reactions to proximal danger. Dysfunction on these pathways would be, respectively, related to generalized anxiety (GAD) and panic disorder (PD). To investigate this hypothesis, we microinjected into the rat DR the benzodiazepine inverse receptor agonist FG 7142, the 5-HT(1A) receptor agonist 8-OH-DPAT or the GABA(A) receptor agonist muscimol. Animals were evaluated in the elevated T-maze (ETM) and light/dark transition test. These models generate defensive responses that have been related to GAD and PD. Experiments were also conducted in the ETM 14 days after the selective lesion of DR serotonergic neurons by 5,7-dihydroxytriptamine (DHT). In all cases, rats were pre-exposed to one of the open arms of the ETM 1 day before testing. The results showed that FG 7142 facilitated inhibitory avoidance, an anxiogenic effect, while impairing one-way escape, an anxiolytic effect. 8-OH-DPAT, muscimol, and 5,7-DHT-induced lesions acted in the opposite direction, impairing inhibitory avoidance while facilitating one-way escape from the open arm. In the light/dark transition, 8-OH-DPAT and muscimol increased the time spent in the lighted compartment, an anxiolytic effect. The data supports the view that distinct DR-5-HT pathways regulate neural mechanisms underlying GAD and PD.     

Long-term impairment of anterograde axonal transport along fiber projections originating in the rostral raphe nuclei after treatment with fenfluramine or methylenedioxymethamphetamine

To further evaluate the serotonin (5-HT) neurotoxic potential of substituted amphetamines, we used tritiated proline to examine anterograde transport along ascending axonal projections originating in the rostral raphe nuclei of animals treated 3 weeks previously with (+/-)fenfluramine (FEN, 10 mg/kg, every 2 h x 4 injections; i.p.) or (+/-)3,4-methylenedioxymethamphetamine (MDMA, 20 mg/kg, twice daily for 4 days; s.c.). The documented 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT, 75 microg; ICV; 30 min after pretreatment with pargyline, 50 mg/kg; i.p., and desipramine 25 mg/kg; i.p.), served as a positive control. Along with anterograde axonal transport, we measured two 5-HT axonal markers, 5-HT and 5-hydroxyindoleacetic acid (5-HIAA). Prior treatment with FEN or MDMA led to marked reductions in anterograde transport of labeled material to various forebrain regions known to receive 5-HT innervation. These reductions were associated with lasting decrements in 5-HT axonal markers. In general, decreases in axonal transport were less pronounced than those in 5-HT and 5-HIAA. However, identical changes were observed after 5,7-DHT. These results further indicate that FEN and MDMA, like 5,7-DHT, are 5-HT neurotoxins.     

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.