Sexual behavior in male rats after intracerebral injection of 5,7-dihydroxytryptamine

Adult intact, or castrated testosterone propionate (TP, 150 μg/kg) treated male rats, were tested for masculine sexual behavior after having been injected with 5,7-dihydroxytryptamine (5,7-DHT, 4 μg/4 ml) intracerebrally either alone or in combination with systemic treatment with protriptyline, a noradrenaline (NA) re-uptake blocking agent. No changes were found in the sexual behavior of intact rats although the brain 5-HT levels were reduced to about one-third of their normal value. By contrast, there was a marked increase in the proportion of rats showing ejaculation patterns in the castrate + TP group after 5,7-DHT lesion than in the vehicle-injected group.Compared to the control group, the 5,7-DHT group showed a reduced uptake of [³H]5-HT and [³H]NA in the hypothalamus. Also the uptake of [³H]amines in the cerebral cortex was lowered although the difference did not attain statistical significance. A statistically significant relationship was found between the behavioral changes and the reduction of [³H]5-HT uptake in the hypothalamus while no such relationship was found between the NA uptake and the behavioral changes.Tistochemical analysis of the site of the 5,7-DHT injections showed that the unspecific damage (nerve cell loss, glial cell infiltration) involved a somewhat larger area in the 5,7-DHT group than in the controls. These unspecific lesions were, however, located outside the region of the large medial 5-HT bundle.The results support the hypothesis that 5-HT serves as a transmitter in the neural processes underlying masculine sexual behavior and, further, points to one component of the ascending 5-HT projections which innervates inter alia the hypothalamus as being of particular importance in this context.     

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.     

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.     

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.     

The effect of unilateral neurotoxic lesions to serotonin fibres in the medial forebrain bundle on the metabolism of [H]DOPA in the telencephalon of the living rat

We used quantitative autoradiography to measure the contribution of the 5-hydroxytryptamine (5-HT, serotonin) innervation of rat telencephalon to the synthesis of dopamine (DA) from exogenous l-DOPA. One week after stereotaxic infusions of 5,7-dihydroxytryptamine (5,7-DHT, 1.6 μg) into the right medial forebrain bundle (MFB), rats received [³H]DOPA (200 μCi, i.v.), which circulated for 90 min. The specific bindings in vitro of the 5-HT uptake site ligand [³H]citalopram and the DA uptake site ligand [¹²⁵I]RTI-55 were measured in cryostat sections from the prosencephalon. In most structures ipsilateral to the lesion, [³H]citalopram specific binding was substantially reduced (50–90%). In the lateral habenula specific binding declined by only 30–40%, reflecting the presence of a 5-HT pathway deviating from the MFB at the mesencephalic flexure. [¹²⁵I]RTI-55 binding in the basal ganglia was reduced by 50% on the side of the 5,7-DHT lesion, but was unperturbed in rats pretreated with desmethylimipramine (DMI). 5,7-DHT infusions decreased the synthesis of [³H]DA from [³H]DOPA in vivo in the basal ganglia by (40–90%). Pretreatment with DMI protected [³H]DA synthesis in the basal ganglia, but not in the olfactory tubercle and amygdala ipsilateral to the lesion. Whereas the 5-HT innervation does not contribute greatly to [³H]DA synthesis in the basal ganglia, a substantial proportion of [³H]DA synthesis in olfactory tubercle and amygdala requires an intact 5-HT innervation.     

The effect of unilateral neurotoxic lesions to serotonin fibres in the medial forebrain bundle on the metabolism of [3H]DOPA in the telencephalon of the living rat

We used quantitative autoradiography to measure the contribution of the 5-hydroxytryptamine (5-HT, serotonin) innervation of rat telencephalon to the synthesis of dopamine (DA) from exogenous l-DOPA. One week after stereotaxic infusions of 5,7-dihydroxytryptamine (5,7-DHT, 1.6 μg) into the right medial forebrain bundle (MFB), rats received [³H]DOPA (200 μCi, i.v.), which circulated for 90 min. The specific bindings in vitro of the 5-HT uptake site ligand [³H]citalopram and the DA uptake site ligand [¹²⁵I]RTI-55 were measured in cryostat sections from the prosencephalon. In most structures ipsilateral to the lesion, [³H]citalopram specific binding was substantially reduced (50–90%). In the lateral habenula specific binding declined by only 30–40%, reflecting the presence of a 5-HT pathway deviating from the MFB at the mesencephalic flexure. [¹²⁵I]RTI-55 binding in the basal ganglia was reduced by 50% on the side of the 5,7-DHT lesion, but was unperturbed in rats pretreated with desmethylimipramine (DMI). 5,7-DHT infusions decreased the synthesis of [³H]DA from [³H]DOPA in vivo in the basal ganglia by (40–90%). Pretreatment with DMI protected [³H]DA synthesis in the basal ganglia, but not in the olfactory tubercle and amygdala ipsilateral to the lesion. Whereas the 5-HT innervation does not contribute greatly to [³H]DA synthesis in the basal ganglia, a substantial proportion of [³H]DA synthesis in olfactory tubercle and amygdala requires an intact 5-HT innervation.     

Thioperamide, an H3 Receptor Antagonist Prevents [3H]Glucose Uptake in Brain of Adult Rats Lesioned as Neonates with 5,7-Dihydroxytryptamine

As a first attempt at exploring an association between histaminergic and serotoninergic neuronal phenotypes in glucose regulation, the influence of the histamine H₃ receptor antagonist thioperamide on glucose uptake by brain was determined in rats in which the serotoninergic innervations of brain was largely destroyed perinatally. Male Wistar rats were initially treated on the 3rd day after birth with the serotoninergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) (75 μg icv) or saline vehicle (10 μl icv). At 8 weeks lesioned and control rats were terminated in order to validate the effectiveness of 5,7-DHT: reduction in 5-HT and 5-HIAA by 83–91% and 69–83% in striatum, frontal cortex, and hippocampus (HPLC/ED method). Other groups of rats were pretreated with thioperamide (5.0 mg/kg ip) or saline vehicle 60 min prior to 6-[³H]-D-glucose (500 μCi/kg ip). Fifteen-min later rats were decapitated and brains were excised and dissected to remove frontal cortex, striatum, hippocampus, thalamus/hypothalamus, pons, and cerebellum. Liquid scintillation spectroscopy was used to determine that [³H]glucose uptake, which was enhanced in 5,7-DHT lesioned rats in cortex (by 88%), hippocampus, thalamus/hypothalamus, pons and cerebellum (each by 47–56%), and in striatum (by 35%). In contrast, thioperamide prevented the enhancement in [³H]glucose uptake in all brain regions of 5,7-DHT neonatally lesioned rats; and [³H]glucose levels were significantly different in all brain regions (except thalamus/hypothalamus) in thioperamide-versus saline-treated rats. These findings indicate a functional association between histaminergic and serotoninergic systems in brain in relation to glucose regulation.     

Plastic responses of neonatal 5-hydroxytryptamine1B receptors to 5,7-dihydroxytryptamine lesions mapped by quantitative autoradiography

We previously found different effects on behavior, serotonin (5-HT) concentrations, 5-HT uptake sites, and 5-HT_1A binding sites of neonatal 5,7-dihydroxytryptamine (5,7-DHT) lesions depending on the route of 5,7-DHT injection. To study the impact of early lesions on 5-HT_1B sites as putative 5-HT terminal autoreceptors, we labelled them autoradiographically with [³H]5-HT 4 months after intraperitoneal (i.p.) or intracisternal (i.c.) 5,7-DHT injection during the first postnatal week and quantitated specific binding in 22 brain regions. Changes were confined to the subiculum and substantia nigra, regions with the most 5-HT_1B-specific binding and projection areas of structures with high mRNA expression. Both routes of 5,7-DHT injection were associated with increases in specific binding in subiculum (24% for i.p. and 47% for i.c. route). In contrast, there was a 32% increase in specific binding in the substantia nigra in rats with lesions made i.c. but not i.p. No significant differences were found in nucleus accumbens, caudate-putamen or other brain areas. In saturation homogenate binding studies of 5-HT_1B sites using [¹²⁵I]iodocyanopindolol 1 month after i.p. injections, neonatal 5,7-DHT lesions did not significantly alter B_max or K_d in the neocortex, striatum, diencephalon or brainstem. These data indicate the differential effects of the route of neonatal 5,7-DHT injections on plasticity of 5-HT_1B receptor recognition sites and suggest the presence of a subpopulation of post-synaptically located 5-HT_1B sites which increases in response to denervation. The data also suggest that sprouting of 5-HT neurons after neonatal 5,7-DHT lesions does not involve 5-HT_1B sites.     

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.     

Ipsilateral alterations in tryptophan hydroxylase activity in rat brain after hypothalamic 5,7-di-hydroxytryptamine lesion

The in vivo relationship between the amounts of tryptophan hydroxylase (TPH) protein and its intrinsic synthetic activity, measured by quantifying the amounts of α-[³H]methyl-5-hydroxytryptamine (α-[³H]M5-HT), is reported in cell body and terminal areas of intact and disturbed serotonergic neurons following a unilateral 5,7-dihydroxytryptamine (5,7-DHT) lesion of the dorsolateral hypothalamus. Five days after the lesion, the relationships between TPH and its synthetic product 5-HT were evaluated on adjacent brain sections in serotonergic cells bodies of the dorsal raphe nucleus (DRN) and nerve fibres of the medial forebrain bundle (MFB). On the side contralateral to the lesion, TPH and α-[³H]M5-HT levels in the intact hemi-DRN exhibited a caudo-rostral distribution and were positively and significantly correlated (P 0.001); the calculated TPH-specific activity was 0.76 nCi of α-[³H]M5-HT formed per U TPH. In the MFB, quantitative measurements of TPH and α-[³H]M5-HT showed no correlation between enzyme and product and no specific activity for TPH could be determined. On the side ipsilateral to the lesion, the density of TPH-immunoreactive fibers was drastically decreased in the dorsolateral hypothalamus where a significant reduction in TPH content (45.5% of control side,P < 0.001) was found. In the overall ipsilateral hemi-DRN, TPH and α-[³H]M5-HT levels, their correlation as well as TPH-specific activity were unaltered by the lesion but a significant increase in α-[³H]M5-HT and TPH contents was observed in the lateral wings of the DRN. The lesion also induced a significant increase in α-[³H]M5-HT and TPH levels (136% and 93.8%,P < 0.001, respectively) in the ipsilateral MFB, which resulted in a positive and significant correlation between these two markers and yielded a TPH-specific activity of 1.0 nCi of α-[³H]M5-HT formed per U TPH. TPH topological area was also significantly increased in the lateral aspect of the ipsilateral MFB 5 days post lesion. These results show that 5-HT synthesis in the intact DRN is proportional to and dependent on TPH activity while in the MFB, 5-HT accumulation appears unrelated to TPH content which is most likely in an inactive enzymatic form. Moreover, the data show that a local disruption of serotonergic terminals in the dorsolateral hypothalamus does not affect 5-HT synthesis in the overall ipsilateral DRN neurons but results in local activation of TPH within the serotonergic projection neurons and the ipsilateral MFB, as evidenced by active de novo synthesis of 5-HT. Altogether the results point to circumscribed activation of compensatory mechanisms in 5-HT synthesis after selective destruction of serotonergic terminals.     

Intrahypothalamic 5,7-dihydroxytryptamine: Temporal analysis of effects on 5-hydroxytryptamine content in brain nuclei and on facilitated lordosis behavior

The long-term relationship between serotonin (5-HT) levels in discrete hypothalamic nuclei and female rat sexual behavior, the lordosis response, was examined following intrahypothalamic injection of 5,7-dihydroxytryptamine (5,7-DHT). One week following 5,7-DHT injection, 5-HT levels in the ventromedial hypothalamic nucleus, dorsomedial nucleus, anterior hypothalamic nucleus and the medial preoptic nucleus were approximately 90% depleted as compared to sham animals. Other hypothalamic and preoptic areas including the arcuate-median eminence, vertical nucleus of diagonal band and lateral septal nucleus showed smaller reductions in 5-HT, from 40 to 70% of sham values. At this time estrogen-dependent lordosis behavior in the lesioned group was facilitated. Behavioral facilitation was greatest at 4 weeks post lesion when depletion of 5-HT in the VMN was maximal. 5-HT levels increased at 57 days after 5,7-DHT treatment in most areas, and by 71 days post lesion, no significant differences in 5-HT levels were found between sham and 5,7-DHT-treated groups. Concomitant with the increases in 5-HT, facilitated lordosis behavior gradually decreased. Loss of behavioral facilitation appeared to be most closely related to increases in content of 5-HT in the ventromedial nucleus. These results further support the hypothesis that 5-HT endings in the hypothalamus exert tonic inhibitory regulation over hormone-dependent lordosis in the female rat. They also indicate that regenerating 5-HT fibers in the hypothalamus can reinstate a normal pattern of hormone-dependent behavioral function.     

Regeneration of serotonergic fibers in the rat hypothalamus following unilateral 5,7-dihydroxytryptamine injection

The time course of degeneration and regeneration of serotonin (5-HT) fibers in the rat hypothalamus was studied with 5-HT immunocytochemistry and [3H]5-HT uptake following unilateral injections of 5,7-dihydroxytryptamine (5,7-DHT) into the dorsolateral hypothalamus. Within 3 days of the lesion, 5-HT fibers in the ipsilateral hypothalamus were swollen and darkly stained for 5-HT. In the contralateral hypothalamus few swollen fibers were apparent and these were generally restricted to the area adjacent to the fornix. Swollen 5-HT fibers were evident in the ipsilateral hypothalamus 3-19 days post-lesion in the medial forebrain bundle (MFB) during which time there was a gradual decrease in their density. In the medial and periventricular areas of the ipsilateral hypothalamus there were essentially no 5-HT fibers 7-30 days post-lesion. Sprouting 5-HT fibers were observed 12-19 days post-lesion. Thirty days post-lesion the density of 5-HT fibers in the MFB appeared normal; however, medial and periventricular areas remained denervated. Fifty days post-lesion there was an apparent bilateral hyperinnervation in the lateral and dorsomedial hypothalamic areas of 5,7-DHT-injected animals as compared to sham-injected animals. The morphological data were paralleled by changes in [3H]5-HT uptake. Seven days post-lesion specific high affinity uptake was reduced to 27% of sham in the ipsilateral hypothalamus and to 53% of sham in the contralateral hypothalamus. By 50 days post-lesion, specific high affinity uptake of [3H]5-HT was 141% of sham in the ipsilateral hypothalamus and 96% of sham in the contralateral hypothalamus.     

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

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

Effects of 5,6-dihydroxytryptamine on nerve terminal serotonin and serotonin uptake in the rat brain

One intraventricular injection of 5,6-dihydroxytryptamine (5,6-DHT) caused the disappearance of fluorescent histochemically detectable 5-hydroxytryptamine (5-HT)-containing terminals and a loss in 5-HT uptake sites. There was an almost complete disappearance of 5-HT-containing nerve terminals in periventricularly located diencephalic areas and in the spinal cord 10–15 days after 75 μg of 5,6-DHT. The noradrenaline and dopamine innervation patterns in the hypothalamus, septum, basal ganglia, and spinal cord appeared normal, except in a narrow zone of the caudate nuclei facing the lateral ventricles, where there was a marked reduction in dopamine fluorescence. These changes were accompanied by 50–87% reductions in the uptake of [³H]5-HT by thin slices of cortex, hypothalamus and spinal cordin vitro. In contrast, the uptake ofL-[³H]noradrenaline was close to normal in hypothalamus and spinal cord slices, and about 35% reduced in the cortex slices. These results are consistent with the idea that intraventricularly administered 5,6-DHT causes extensive axonal degeneration of central serotonin neurones, and that noradrenaline and dopamine neurones are largely unaffected after one injection of 75 μg.     

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.     

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.     

Plasticity and ontogeny of the central 5-HT transporter: effect of neonatal 5,7-dihydroxytryptamine lesions in the rat.

5,7-Dihydroxytryptamine (5,7-DHT) is unique as a serotonin (5-HT) neurotoxin in that i.p. injection of neonatal rats increases concentrations of 5-HT in brainstem while depleting 5-HT in cortex, hippocampus and spinal cord. To study the mechanism of this effect we measured the 5-HT transporter or uptake site, a presynaptic marker, using [3H]paroxetine binding. There were significant regional differences in Bmax of vehicle-injected rats: brainstem, diencephalon > striatum, cortex, spinal cord > hippocampus, cerebellum. There were also regional differences in the ontogeny of bindings sites: at postnatal day 7, [3H]paroxetine sites were 39% of adult levels in cortex compared to 63% in brainstem. Thirty days after 100 mg/kg 5,7-DHT i.p., Bmax of [3H]paroxetine binding was significantly increased in brainstem (+67%) and diencephalon (+136%), whereas it decreased in cortex (-59%), hippocampus (-94%) and spinal cord (-99%), striatum (-41%) and cerebellum (-37%). KD remained unaltered. In dose-response studies (0-200 mg/kg), 50 mg/kg was the threshold dose for Bmax effects and 200 mg/kg was lethal. In weekly time-course studies, changes were apparent 1 week after 5,7-DHT lesions. Binding site increases in diencephalon and brainstem were not maximal until 3 weeks after injection, whereas percent decreases in cortical sites remained unchanged at each week studied. Lesion effects on the ontogeny of [3H]paroxetine binding sites were region-dependent: cortical sites continued to increase with age but spinal sites did not. There was no significant recovery in spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

The effect of intracerebral injections of 5,7-dihydroxytryptamine and 6-hydroxydopamine on the serotonin-immunoreactive cell bodies and fibers in the adult rat hypothalamus

The effect of intracerebral injections of 5,7-dihydroxytryptamine (5,7-DHT) and 6-hydroxydopamine (6-OHDA) on the serotonin (5-HT)-immunoreactive (IR) cell bodies ¹¹ and fibers in the adult rat hypothalamus was studied with 5-HT immunocytochemistry. In rats pretreated with pargyline and l-tryptophan, 5-HT-IR cells were seen in the ventromedial part of the dorsomedial nucleus (DMN) and 5-HT-IR fibers in all hypothalamic areas. In the ventrolateral part of the DMN the 5-HT-IR fibers were of a much finer type than those seen in other hypothalamic areas. Five days after unilateral injection of 5,7-DHT into the dorsolateral hypothalamus, the 5-HT-IR cells were absent from the DMN, and there was a decrease in the number of 5-HT-IR fibers throughout the hypothalamus ipsilateral to the injection. Contralateral to the injection there was evidence of selective 5-HT fiber degeneration but the 5-HT-IR cells and the group of fine fibers in the ventrolateral DMN remained. Unilateral injection of 6-OHDA into the dorsolateral hypothalamus had no effect on the 5-HT-IR fibers or cell bodies in the hypothalamus.Twelve days after unilateral injection of 5,7-DHT into the rostral midbrain, the majority of 5-HT-IR fibers in the ipsilateral hypothalamus had disappeared. The 5-HT-IR cell bodies in the DMN and the group of fine 5-HT-IR fibers in the ventrolateral DMN remained on both sides of the hypothalamus. These results support our previous finding of a group of 5-HT-IR cell bodies in the ventromedial DMN of the hypothalamus, and suggest that these cells innervate the ventrolateral part of the same nucleus. Evidence that these cells constitute a new 5-HT cell group, B-10, is discussed.