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.     

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.     

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.     

Identification of and interactions between noradrenergic and serotonergic neurites in the myenteric plexus

Interactions between enteric noradrenergic and serotonergic neurites in the myenteric plexus were examined. The influence of exogenous norepinephrine (NE) and endogenously released NE on the release of 3H-serotonin (3H-5-HT) from electrically stimulated, everted segments of guinea pig small intestine were analyzed. In addition, methods were employed to permit the ultrastructural identification of enteric serotonergic and noradrenergic neurites. These included electron microscopic radioautographic localization of 3H-5-HT in gut from animals treated with 6-hydroxydopamine (6-HD; 100 mg/kg) and examination of NaMnO4-fixed tissue from animals given desmethylimipramine and the indolic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). Exogenous NE antagonized the stimulated release of 3H-5-HT; this action was mediated through alpha adrenoceptors. Evidence was obtained, however, that endogenous NE liberated from sympathetic postganglionic nerve terminals had a different action on the stimulated release of 3H-5-HT from that of exogenous NE. Endogenous NE appeared to facilitate 3H-5-HT release through an action on beta adrenoceptors. Terminals identified as probably serotonergic by radioautographic labeling with 3H-5-HT or by loading with 5,7-DHT were most often found to end on neuronal somata or proximal dendrites. Synaptic specializations were found by radioautography in these locations. Terminals, dendrites, and cell bodies that took up 5,7-DHT could be identified in NaMnO4-fixed material that also permitted the simultaneous recognition of noradrenergic varicosities. Apparent noradrenergic-serotonergic axoaxonic contacts were found by means of this double-labeling technique. Together with the results of the experiments on the release of 3H-5-HT, these anatomical observations suggest that noradrenergic axons form facilitatory axoaxonic synapses with enteric serotonergic neurons.     

Localization of GABAA and GABAB receptor subtypes on serotonergic neurons

The effect of selective destruction of serotonin (5-HT)-containing neurons with 5,7-dihydroxytryptamine (5,7-DHT) on [3H] muscimol and (-)-[3H]baclofen binding was investigated in various rat brain regions. Ten days after intracerebroventricular 5,7-DHT, serotonin levels and [3H]imipramine binding were markedly decreased. 5,7-DHT reduced [3H]muscimol binding only in the mesencephalon, and (-)-[3H]baclofen binding was unmodified in all the areas considered. These results suggest that except in the mesencephalon GABA receptors may not be localized on serotonergic nerve terminals.     

5,7-Dihydroxytryptamine lesions enhance and serotonergic grafts normalize the evoked overflow of acetylcholine in rat hippocampal slices

Adult rats were subjected to intracerebroventricular injections of 5,7-dihydroxytryptamine (5,7-DHT; 150 micro g) and, 15 days later, to intrahippocampal grafts of fetal raphe cell suspensions. About 11 months later, we assessed baseline and electrically evoked release of tritium ([3H]) in hippocampal slices, preloaded with tritiated ([3H])choline or [3H]serotonin (5-HT), in the presence or absence of the 5-HT1B receptor agonist CP-93,129 and the 5-HT receptor antagonist methiothepine. HPLC determinations of monoamine concentrations were also performed. The lesions reduced the concentration of 5-HT (-90%) and the accumulation (-80%) as well as the evoked release (-90%) of [3H]5-HT. They also decreased the inhibitory effects of CP-93,129 on the evoked release of [3H]5-HT. Most interestingly, they facilitated the evoked release of [3H]acetylcholine (+20%). In slices from rats subjected to lesions and grafts, the responsiveness of the serotonergic autoreceptors (presumably located on the terminals of the grafted neurons) and the release of acetylcholine were close to normal. These results confirm that grafts rich in serotonergic neurons may partially compensate for the dramatic effects of 5,7-DHT lesions on serotonergic hippocampal functions. The lesion-induced reduction of the 5-HT1B autoreceptor-mediated inhibition of evoked 5-HT release may be an adaptation enhancing serotonergic transmission in the (few) remaining terminals. The facilitated release of acetylcholine is probably caused by a reduced serotonergic tone on the inhibitory 5-HT1B heteroreceptors of the cholinergic terminals. When related to data in the literature, this facilitation may be of particular interest in terms of transmitter-based strategies developed to tackle cognitive symptoms related to neurodegenerative diseases.     

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.     

Intrahypothalamic 5,7-dihydroxytryptamine facilitates feminine sexual behavior and decreases [H]imipramine binding and 5-HT uptake

5,7-Dihydroxytryptamine (5,7-DHT) injected into the hypothalamus facilitated feminine sexual behavior in ovariectomized, estrogen-treated female rats beginning 9 days post-lesion. 5,7-DHT treatment was associated with decreased [³H]5-HT but not [³H]NE uptake in the whole hypothalamus and with decreased [³H]-imipramine binding in some hypothalamic nuclei. These data provide the first demonstration using chemical lesions that 5-HT neurons may exert tonic inhibition on hormone-mediated feminine sexual behavior.     

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)     

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.     

Autoradiographic quantification of serotonin1A receptors in rat brain following antidepressant drug treatment

There is growing evidence that the serotonergic (5-HT) system is involved in the pathogenesis and treatment of major depression. The 5-HT receptor subtype involved in the enhancing effect of antidepressant treatments, however, has not been identified. The present study was undertaken to quantify 5-HT1A sites in the rat brain by autoradiography and membrane binding, using the selective ligand [3H]8-hydroxy-N,N-dipropyl-2-aminotetralin (8-OH-DPAT), following long-term antidepressant treatment. Following a 21-day treatment with amitriptyline (10 mg/kg/day), there was a significant increase of [3H]8-OH-DPAT binding measured by autoradiography in the dorsal hippocampus, but there was no change in the nucleus raphe dorsalis; whole brain membrane binding revealed an increase in the number of binding sites, with no change in the affinity for [3H]8-OH-DPAT. Conversely, fluoxetine (10 mg/kg/day), a selective blocker of 5-HT reuptake, and gepirone (10 mg/kg/day), a 5-HT1A agonist, both administered for 21 days, significantly reduced [3H]8-OH-DPAT binding measured by autoradiography in the nucleus raphe dorsalis without altering hippocampal binding sites. The control active treatment with diazepam (2 mg/kg/day) did not alter [3H]8-OH-DPAT binding in the hippocampus or in the nucleus raphe dorsalis. All groups were compared to a 21-day vehicle-treated control group. These results are fully consistent with previous electrophysiological and behavioral studies and suggest that alterations of 5-HT1A receptors might underlie the enhancement of 5-HT neurotransmission by antidepressant treatments.     

Opioid receptor-like 1 (NOP) receptors in the rat dorsal raphe nucleus: evidence for localization on serotoninergic neurons and functional adaptation after 5,7-dihydroxytryptamine lesion

A high density of opioid receptor-like 1 (ORL1) receptor (also referred to as NOP receptor) is found in limbic areas and in regions containing monoamines, which are implicated in emotional activity and physiopathology of depression and anxiety. We aimed at defining precisely the localization of ORL1 receptors in dorsal raphe nucleus, by means of a lesion strategy and autoradiographic studies. In control rats, [3H]nociceptin and nociceptin-stimulated [35S]GTPgammaS bindings were found to be correlated in several brain regions. We performed in rats a selective destruction of serotoninergic neurons by surgical stereotaxic injection of 5,7-dihydroxytryptamine (5,7-DHT) in dorsal raphe nucleus. This led to a marked decrease in serotonin contents in striata and frontal cortices (about -60%) and in autoradiographic [3H]citalopram binding in posterior regions. In dorsal raphe nucleus, [3H]nociceptin binding was decreased to the same extent as [3H]citalopram binding, whereas it was unchanged in the other regions studied. Nevertheless, in the dorsal raphe, nociceptin-stimulated [35S]GTPgammaS binding was decreased to a lesser extent than [3H]nociceptin binding in 5,7-DHT-lesioned rats. The ratio between nociceptin-stimulated [35S]GTPgammaS binding and [3H]nociceptin binding was significantly increased in 5,7-DHT-lesioned rats compared with controls in this region. These data demonstrate 1) that ORL1 receptors are located on serotoninergic neurons in the dorsal raphe nucleus and 2) that, after a lesion, the functionality of remaining ORL1 receptors appears to be up-regulated, which could correspond to a compensatory mechanism.     

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.     

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.     

Temporal effects of intrahypothalamic 5,7-dihydroxytryptamine: relationship between serotonin levels and [3H]serotonin binding

The relationship between serotonin (5-HT) levels and [3H]5-HT binding in discrete hypothalamic areas was examined in separate groups of animals at various times, following unilateral intrahypothalamic injection of 5,7-dihydroxytryptamine (5,7-DHT). Seven days post-5,7-DHT lesion, 5-HT levels were significantly decreased in both the ipsilateral and contralateral ventromedial and dorsomedial hypothalamic nuclei (VMN, DMN). In the lateral hypothalamic area (LHA), 5-HT levels were significantly decreased only ipsilaterally. Fifty days postlesion, 5-HT levels in the ipsilateral VMN remained significantly below sham, while the DMN and LHA returned to sham values. Seven days after 5,7-DHT there was a significant increase in [3H]5-HT labeling densities in the ipsilateral and contralateral ventromedial hypothalamic area as well as in the ipsilateral LHA. In contrast, in the dorsomedial hypothalamic area there was no increase in [3H]5-HT binding. Fifty days postlesion, no significant differences in [3H]5-HT binding between 5,7-DHT and sham were observed in any areas examined. This data provides further evidence for the regeneration of 5-HT fibers in the hypothalamus and demonstrates that the relationship between [3H]5-HT binding and 5-HT levels varies from one hypothalamic area to another.     

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.     

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.     

Temporal effects of intrahypothalamic 5,7-dihydroxytryptamine: relationship between serotonin levels and [H]serotonin binding

The relationship between serotonin (5-HT) levels and [³H]5-HT binding in discrete hypothalamic areas was examined in separate groups of animals at various times, following unilateral intrahypothalamic injection of 5,7-dihydroxytryptamine (5,7-DHT). Seven days post-5,7-DHT lesion, 5-HT levels were significantly decreased in both the ipsilateral and contralateral ventromedial and dorsomedial hypothalamic nuclei (VMN, DMN). In the lateral hypothalamic area (LHA), 5-HT levels were significantly decreased only ipsilaterally. Fifty days postlesion, 5-HT levels in the ipsilateral VMN remained significantly below sham, while the DMN and LHA returned to sham values. Seven days after 5,7-DHT there was a significant increase in [³H]5-HT labeling densities in the ipsilateral and contralateral ventromedial hypothalamic area as well as in the ipsilateral LHA. In contrast, in the dorsomedial hypothalamic area there was no increase in [³H]5-HT binding. Fifty days postlesion, no significant differences in [³H]5-HT binding between 5,7-DHT and sham were observed in any areas examined. This data provides further evidence for the regeneration of 5-HT fibers in the hypothalamus and demonstrates that the relationship between [³H]5-HT binding and 5-HT levels varies from one hypothalamic area to another.     

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.     

Ultrastructural, biochemical and electrophysiological changes induced by 5,6-dihydroxytryptamine in the CNS of the snail Helix pomatia L.

The serotonin neurotoxin, 5,6-dihydroxytryptamine (5,6-DHT), was injected into the body cavity of snails. Changes induced in the central nervous system (CNS) by the neurotoxin were studied by morphological, electrophysiological and biochemical techniques for up to 90 days following injection. The neurotoxin induced a variety of ultrastructural alterations during the early phase (1st to 6th days) following treatment. On day 6 after treatment, membranous structures first appeared in the synaptic-like areas and apparently migrated to cell bodies where they were detected by day 14. Their number increased with time. Neurotoxin-induced structural alterations were found in neuronal processes and cell bodies of the serotonergic metacerebral giant cells injected intracellularly with horseradish peroxidase and in serotonin immunoreactive axons. These findings suggest that the toxin-induced alterations are rather selective for the serotonin-containing neuronal elements. The neurotoxin decreased the concentration of 5-HT in and [3H]5-HT uptake into cerebral and pedal ganglia, with a maximum effect between the 3rd and 5th day following drug administration. 5-HT levels and 5-HT uptake returned to normal by 19-21 days after treatment. The concentration of dopamine and of [3H]DA uptake capacity were reduced between 3-5 days after injection of 5,6-DHT by 6-7 days following treatment. The transmission from identified serotonergic synapses to targets was reduced beyond day 5 after 5,6-DHT administration. By 15 days after treatment, synaptic transmission between the metacerebral giant cell (MGC) and buccal followers was blocked. Transmission recovered by day 21 after 5,6-DHT. Comparison of the time-course of functional and structural recovery indicates that while functional recovery takes place within 21 days after treatment, certain structural alterations, e.g. the membranous structures and dense particles, remain in the nerve fibres and cell bodies. These may serve as specific intracellular markers of the serotonin-containing neuronal elements long after functional recovery from the effect of 5,6-DHT.