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

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

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.     

Intracisternal 5,7-dihydroxytryptamine lesions in neonatal and adult rats: comparison of response to 5-hydroxytryptophan

There have been few previous studies of the functional significance of 5,7-dihydroxytryptamine (5,7-DHT) lesions made in neonatal rats. To study the role of serotonin (5-HT) in recovery of function, rat pups and adult rats were injected intracisternally with 5,7-DHT or saline and challenged acutely with the 5-HT precursor 5-hydroxytryptophan (5-HTP) 4 weeks later as a test of behavioral supersensitivity. Compared to 5,7-DHT lesions in adults, neonatal lesions induced significantly greater 5-HT depletions in brainstem, but 5-HT depletions in other regions were not significantly different in the two groups. Rats with early 5,7-DHT lesions displayed supersensitive behavioral responses to 5-HTP, consisting of all the component myoclonic-serotonergic behaviors seen in rats with 5,7-DHT lesions made as adults. However, there was significantly less 5-HTP-evoked head weaving, truncal myoclonus and shaking behavior in rats treated with 5,7-DHT as neonates. Body weight was reduced both in rats with early and late 5,7-DHT lesions, but reduction persisted in rats with early lesions. These data indicate overall similarity with some differences between neurochemical and behavioral effects of early and late 5,7-DHT lesions made by the intracisternal route. They suggest that recovery mechanisms did not occur or failed to reverse the neurochemical or behavioral consequences of early 5,7-DHT lesions.     

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.     

The effects of SP-111, a water-soluble THC derivative, on neuronal activity in the rat brain

The serotonin precursor, 5-hydroxytryptophan (5-HTP), can induce a behavioral syndrome characterized by rigidity, splayed feet, tremor, head weaving, salivation and forepaw treading. This response to 5-HTP was markedly potentiated in adult rats treated intracisternally with 5,7-dihydroxytryptamine (5,7-DHT) during development. Prevention of the 5,7-DHT-induced reduction of brain norepinephrine with pargyline or desipramine did not diminish the potentiation of 5-HTP, suggesting that noradrenergic fibers are not contributing to the altered 5-HTP response. It was also found that treatments with 5,7-DHT potentiated the release of prolactin and the disruption of responding in a fixed-ratio operant task induced by 5-HTP. Other experiments indicated that 5,7-DHT treatments potentiated 5-HTP without affecting the action of L-dihydroxyphenylalanine. In addition, administration of the decarboxylase inhibitor, R0-4-4602, at a dose that inhibits enzyme activity in brain, blocked the 5-HTP-induced behavioral syndrome in 5,7-DHT-treated rats, indicating that 5-HTP must be converted to serotonin for 5-HTP to alter behavior. Thus, the present studies indicate that destruction of serotonergic fibers during development can produce permanent changes in central serotonergic mechanisms.     

Inhibition of 5,7-dihydroxytryptamine-induced supersensitivity to 5-hydroxytryptophan in mice by treatment with cycloheximide.

Intracisternal injection of 5,7-dihydroxytryptamine (5,7-DHT) following treatment with desmethylimipramine induced development of behavioral supersensitivity to the intraperitoneally administered serotonin precursor 5-hydroxytryptophan (5-HTP) in the mouse. This behavioral syndrome, characterized by tremor and muscle twitches (myoclonus), showed a clear dose-response relationship with 5,7-DHT as well as with 5-HTP. Mice lesioned with a low dose of 5,7-DHT (20 micrograms) or a placebo were treated repeatedly with a protein synthesis inhibitor, sycloheximide (45 mg/kg, s.c., every 12 h for up to 10 days). This treatment resulted in a reversible decrease of cerebral protein synthesis varying between 70 and 20% with time between treatments. The myoclonic response to 5-HTP in animals pretreated with 5,7-DHT and by cycloheximide showed a decrease in intensity within 24 h when evaluated quantitatively by an electronic activity monitor, the results of which were confirmed by direct observation. Cycloheximide also exerted a similar, though smaller, effect following full development of sensitivity to 5-HTP over 10 days. These effects may de mediated by inhibition of rapidly turning over serotonin receptor proteins, although their interpretation is somewhat obscured by possible toxic effects of cycloheximide.     

Serotonergic mediation of DRL 72s behavior: receptor subtype involvement in a behavioral screen for antidepressant drugs.

BACKGROUND: The functioning of the brain serotonin system has been implicated in the action of antidepressant drugs. The behavior of rats performing the Differential Reinforcement of Low Rate-72 sec (DRL 72s) has been used as a screen for drugs with antidepressant activity. Many antidepressant drugs alter serotonergic function. Hence, experiments were designed to investigate the role of the brain serotonin system in the performance of DRL 72s behavior. METHODS: Rats were trained to perform a DRL 72s, and then depleted (LESION) of brain serotonin (5-HT) using intracerebroventricular 5,7-dihydroxytryptamine (5,7-DHT). Control rats (SHAM) were injected with the 5,7-DHT vehicle. RESULTS: The 5,7-DHT-treated rats showed a higher response rate, a decrease in the number of reinforcements, and a shift in the interresponse time (IRT) distribution toward shorter IRTs when compared to SHAM and prelesion performance. The behavioral deficit in the 5,7-DHT rats persisted for 17 weeks. Postmortem assays indicated extensive depletion of 5-HT in all the assayed brain regions of the LESION rats. The effects of the serotonergic agonists 8-hydroxy-2-di-N-propylaminotetralin (8-OH-DPAT), 5-methoxy-dimethyltryptamine (5-MeODMT), buspirone, and 5-hydroxytryptophan (5-HTP) were assessed. 5-MeODMT and 8-OH-DPAT resulted in greater improvement of DRL 72s performance in the LESION rats than in the SHAM rats. Buspirone failed to ameliorate the behavioral deficit in the LESION rats and produced a behavioral deficit in the SHAM rats. 5-HTP improved performance in the SHAM rats and in the LESION rats. CONCLUSIONS: These results support the contention that the brain 5-HT system is involved in the mediation of antidepressant drug effects.     

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

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

Behavioral effects of 8-OH-DPAT: studies using the Microtaxic ventricular injector

This study introduces the Microtaxic Ventricular Injector, a plastic mold that allows for the rapid administration of drugs into the ventricular system of adult rats. The Microtaxic Ventricular Injector was used to destroy serotonin (5-HT) neurons by administering the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT; 100 micrograms/10 microliters) into the lateral cerebroventricles. Injection of the 5-HT neurotoxin produced a 79% depletion of 5-HT in the cortex and an 86% depletion of 5-HT in the hippocampus. In addition, 5,7-DHT treatment produced a two-fold shift to the left of the dose-response curve of the 5-HT1A agonists 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) or 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) in producing the 5-HT syndrome indicating the development of denervation supersensitivity following the destruction of 5-HT neurons. In addition, the behavioral effects of 8-OH-DPAT were studied when administered to different CNS sites using the Microtaxic Ventricular Injector. 8-OH-DPAT (10 micrograms) injected into the fourth ventricle produced the 5-HT syndrome in 100% of the rats tested within a 3 min time period. In contrast, 8-OH-DPAT injected into the lateral ventricle produced the syndrome in only 33% of the rats tested and with a 6-9 min delay until this effect occurred. These results indicate the greater potency of 8-OH-DPAT at producing the 5-HT syndrome when administered in ventricular sites that are close to its locus of action in the brainstem/spinal cord region. These experiments demonstrate the usefulness and reliability of the Microtaxic Ventricular Injector as an instrument for rapidly injecting drugs directly into different cerebroventricular sites.     

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)     

Effects of 5,7-dihydroxytryptamine depletion of tissue serotonin levels on extracellular serotonin in the striatum assessed with in vivo microdialysis: relationship to behavior.

Effects of i.c.v. administration of 5,7-dihydroxytryptamine (5,7-DHT) on biochemistry and behavior were studied in awake Sprague-Dawley rats. It was found that 5,7-DHT depletion of striatal tissue levels of serotonin (5-HT) does not diminish extracellular levels until substantial depletions occur. This finding is similar to those observed after 6-hydroxydopamine lesions of the brain dopamine systems. Although varying amounts of 5,7-DHT produced serotonin depletions in striatal tissue, decreases in extracellular levels were only observed at tissue depletions greater than 60% compared to saline-injected control subjects. Thus, the effects of serotonin lesions which produce only moderate depletions may not be the result of decreased extracellular serotonin, but instead may be the result of compensatory changes in remaining neurons which maintain normal extracellular serotonin concentrations. Different degrees of striatal serotonin depletion were associated with opposite behavioral effects. Moderate levels of serotonin depletion (50-75%) produced evidence of increased anxiety, while these effects were no longer seen in rats with more severe 5-HT depletions (>75%).     

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.     

Denervation supersensitivity to serotonin in rat forebrain: Single cell studies

To investigate the development of denervation supersensitivity to serotonin (5-hydroxytryptamine, 5-HT) in the amygdala (AMYG) and the ventral lateral geniculate nucleus (vLGN), single cell recordings, microiontophoretic, histochemical and biochemical techniques were used in the present study. 5-HT projections to the vLGN and the AMYG were destroyed by 5,7-dihydroxytryptamine (5,7-DHT, a relatively selective toxin for 5-HT neurons) injected directly into the lateral ventricle or the ascending 5-HT pathway in the ventromedial tegmentum area. Enhanced responsiveness of cells to the inhibitory effect of microiontophoretically applied 5-HT (ionto-5-HT) began to develop within 24 h and approached a maximum 7 days after 5,7-DHT pretreatment. In general, the time courses for the reduction in both the density of 5-HT fluorescent varicosities and synaptosomal 5-HT uptake activity paralleled the time course for the development of denervation supersensitivity to 5-HT. During the first 2 days after 5,7-DHT, the enhanced sensitivity was selective for 5-HT; responses to D-lysergic acid diethylamide (LSD), norepinephrine (NE) and gamma-aminobutyric acid (GABA) were unchanged. Seven or more days after 5,7-DHT there was a marked increase of the responsiveness of neurons in the vLGN and the AMYG to both 5-HT and LSD (a 5-HT agonist which is not a substrate for the high affinity 5-HT uptake system). At these later times, the responsiveness of cells in the AMYG to NE and to a lesser extent GABA was also increased. In contrast to the marked supersensitivity seen after 5,7-DHT induced denervation, chronic administration of parachlorophenylalanine, a 5-HT synthesis inhibitor, failed to induce 5-HT supersensitivity.     

Proposed animal model of attention deficit hyperactivity disorder

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

Monoaminergic denervation of the rat hippocampus: Microiontophoretic studies on pre- and postsynaptic supersensitivity to norepinephrine and serotonin

The responsiveness of hippocampal CA₃ pyramidal neurons to microiontophoretic applications of serotonin (5-HT), norepinephrine (NE), γ-aminobutyric acid (GABA) and isoproterenol (ISO) was assessed in rats following 5,7-dihydroxy-tryptamine (5,7-DHT) and 6-hydroxydopamine (6-OHDA) pretreatments and bilateral locus coeruleus lesions. The intraventricular administration of 200 μg (free base) of 5,7-DHT and of 6-OHDA produced 89% and 93% decreases of 5-HT and NE respectively. None of these pretreatments modified the initial responsiveness to, or recovery from iontophoretic application of 5-HT. In 6-OHDA pretreated and locus-lesioned rats, the initial effectiveness of NE was not altered but its effect was markedly prolonged. However, there was no such prolongation of the effect of ISO which is not a substrate for the high affinity NE reuptake. The effect of GABA was not affected by these pretreatments. Acute pharmacological blockade of the NE reuptake with desipramine (5 mg/kg, i.p.) similarly induced a prolongation of the effect of iontophoretically applied NE, while fluoxetine (10 mg/kg, i.p.) a 5-HT reuptake blocker, failed to alter the recovery of pyramidal cells from iontophoretic application of 5-HT.

It is concluded that 5-HT denervation induces neither pre- nor postsynaptic types of supersensitivity in hippocampal pyramidal cells, contrasting with the previously shown supersensitivity of ventral lateral geniculate and amygdaloid neurons following 5-HT denervation. NE denervation fails to induce a postsynaptic type of supersensitivity but leads to a marked prolongation of the response to NE indicative of a presynaptic mechanism. These results underscore the necessity for regional studies of neurotransmitters and drug action.     

3,4-methylenedioxymethamphetamine (MDMA) administration to rats decreases brain tissue serotonin but not serotonin transporter protein and glial fibrillary acidic protein

Previous experiments conducted in this laboratory showed that administration of high-dose D-fenfluramine (D-FEN) and p-chloroamphetamine (PCA) decreased 5-HT transporter (SERT) binding and tissue 5-HT by 30-60% in caudate and whole brain tissue 2 days and 2 weeks after drug administration. However, protein expression as determined by Western blot analysis did not change in either tissue or time point, except for a 30% decrease in the caudate 2 days after PCA administration. In the present study, we studied the effect of MDMA and 5,7-dihydroxytryptamine (5,7-DHT) on tissue 5-HT levels and the protein expression level of SERT and glial fibrillary acidic protein (GFAP), a validated neurotoxicity marker. HYPOTHESIS: MDMA administration decreases SERT expression. METHODS: Two weeks after MDMA administration (7.5 mg/kg i.p., q 2 h x 3 doses) or 2 weeks after i.c.v. administration of 5,7,-DHT (150 microg/rat), male Sprague-Dawley rats were sacrificed and the caudate, cortex, and hippocampal tissue collected. Western blots for SERT and GFAP were generated using published methods. Tissue 5-HT levels were determined by HPLC coupled to electrochemical detection. RESULTS: MDMA treatment decreased tissue 5-HT in cortex, hippocampus, and caudate by about 50%. However, MDMA treatment had no significant effect on expression level of SERT and GFAP in any brain region. In contrast, 5,7-DHT reduced tissue 5-HT by more than 90%, decreased SERT protein expression by 20-35%, and increased GFAP by 30-39%. CONCLUSION: These data suggest the MDMA treatment regimen used here does not cause degeneration of 5-HT nerve terminals. Viewed collectively with our previous results and other published data, these data indicate that MDMA-induced persistent 5-HT depletion may occur in the absence of axotomy.     

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.     

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.     

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)     

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.