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.     

Ontogenetic Serotoninergic Lesioning Alters Histaminergic Activity in Rats in Adulthood

The aim of this study was to determine histamine content in the brain and the effect of histamine receptor antagonists on behavior of adult rats lesioned as neonates with the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). At 3 days after birth Wistar rats were pretreated with desipramine (20 mg/kg ip) before bilateral icv administration of 5,7-DHT (37.5 μg base on each side) or saline—ascorbic (0.1%) vehicle (control). At 10 week levels of 5-HT and its metabolite 5-hydroxyindole acetic acid (5-HIAA) were determined in frontal cortex, striatum, and hippocampus by an HPLC/ED technique. In the hypothalamus, frontal cortex, hippocampus and medulla oblongata, the level of histamine was analyzed by an immunoenzymatic method. Behavioral observations (locomotion, exploratory-, oral-, and stereotyped activity) were performed, and effects of DA receptor agonists (SKF 38393, apomorphine) and histamine receptor antagonists S(+)chlorpheniramine (H₁), cimetidine (H₂), and thioperamide (H₃) were determined. We confirmed that 5,7-DHT profoundly reduced contents of 5-HT and 5-HIAA in the brain in adulthood. Histamine content was also reduced in all examined brain regions. Moreover, in 5,7-DHT-lesioned rats the locomotor and oral activity responses to thioperamide were altered, and apomorphine-induced stereotype was intensified. From the above, we conclude that an intact central serotoninergic system modulates histamine H₃ receptor antagonist effects on the dopaminergic neurons in rats.     

Endogenous levels of serotonin and 5-hydroxyindoleacetic acid in specific areas of the pigeon CNS: effects of serotonin neurotoxins

Endogenous levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were determined by high-performance liquid chromatography (HPLC) in specific regions of the pigeon central nervous system (CNS). High endogenous 5-HT levels in the visual wulst and brainstem and medium 5-HT content in the optic lobes were found. The cerebellum and retina showed low endogenous 5-HT levels. Similar endogenous 5-HIAA levels were measured in the visual wulst, optic lobes and brainstem, whereas the 5-HIAA content of the cerebellum and retina was significantly lower. The effects of para-chloroamphetamine (p-CA) and 5,7-dihydroxytryptamine (5,7-DHT) on the 5-HT and 5-HIAA content of the same regions were studied. Six days after p-CA treatment, the 5-HT content of the visual wulst, optic lobes, brainstem and the 5-HIAA content of the optic lobes and cerebellum markedly decreased. Nine days after 5,7-DHT administration, the 5-HT and 5-HIAA content of the visual wulst and optic lobes was significantly reduced. At longer survival times, serotonergic systems were differentially affected depending on both the neurotoxin treatment and the specific brain regions examined. The 5-HT content of the pigeon retina was not modified by p-CA treatment, whereas 5,7-DHT intravitreally injected caused a pronounced 5-HT depletion. Our results demonstrate that selective neurotoxins for serotonergic systems can provide a useful denervation tool for the study of serotonergic function in the pigeon CNS.     

Behavioral and biochemical interactions of 5,7-dihydroxytryptamine with various drugs when administered intracisternally to adult and developing rats.

Intracisternal administration of 200 mug of 5,7-dihydroxytryptamine (5,7-DHT) caused a prolonged reduction of brain serotonin which was accompanied by a depletion of brain norepinephrine. The depletion of norepinephrine was found to be antagonized by agents that inhibit uptake of norepinephrine as well as by several monoamine oxidase inhibitors. Intracisternal injections of 5,7-DHT (75 or 100 mug) to 7-day-old neonatal rats reduced brain serotonin and norepinephrine and produced a significant reduction of adult body weight. As in adults, pretreatment of neonatal rats with pargyline or desipramine prevented 5,7-DHT induced depletion of norepinephrine without affecting depletion of serotonin. Behaviorally, treatment of adult rats with 5,7-DHT facilitated acquisition of an active avoidance task and enhanced muricidal behavior. 5,7-DHT treatment was also found to enhance the depressant effects of 5-hydroxytryptophan on a fixed-ratio barpress response, suggesting that 5,7-DHT treated rats are supersensitive to serotonin in the central nervous system.     

Brainstem serotonergic hyperinnervation modifies behavioral supersensitivity to 5-hydroxytryptophan in the rat

Rat pups were injected intracisternally (i.c.) or intraperitoneally (i.p.) with 5,7-dihydroxytryptamine (5,7-DHT) or saline and challenged 2 and 14 weeks later with the 5-HT precursor 5-hydroxytryptophan (5-HTP), which evokes behavioral supersensitivity in adult rats, 5,7-DHT induced transient postinjection convulsions in rats injected i.c. but not i.p. Rats with either type of 5,7-DHT lesions displayed supersensitive behavioral responses to 5-HTP. However, rats lesioned by i.p. injections exhibited significantly greater shaking behavior (+1445%) in response to 5-HTP than their i.c. counterparts, who instead showed more forepaw myoclonus (+250%) and head weaving (+270%), the core features of the 5-HT syndrome. Differences in 5-HT syndrome behaviors were already present 2 weeks after lesioning, whereas the difference in shaking behavior was not. After 14 weeks, 5-HT was selectively depleted (-43 to -92%) in hippocampus, spinal cord, and frontal cortex, and differences between i.c. and i.p. 5,7-DHT routes were insignificant except in frontal cortex. Brainstem 5-HT concentrations were significantly increased (+35%) after i.p. 5,7-DHT injections in contrast to reduction (-89%) after i.c. 5,7-DHT; 5-hydroxyindole acetic acid/5-hydroxytryptamine (5-HIAA/5-HT) ratios were decreased (-20%) with either route. These data suggest that brainstem 5-HT hyperinnervation following i.p. 5,7-DHT injection modifies the functional consequences of injury in abating the 5-HT syndrome, but does not result in complete recovery since shaking behavior is enhanced. Loss of presynaptically mediated autoregulation or receptor dysregulation may play a major role in behavioral supersensitivity induced by 5-HTP in rats with 5,7-DHT lesions. To the extent that the 5-HT syndrome is mediated by 5-HT1A receptors and shaking behavior by 5-HT2 sites, differential responses to injury of 5-HT1A and 5-HT2 receptors may contribute to these behavioral differences.     

Evidence for a bulbospinal serotonergic pressor pathway in the rat brain

The cardiovascular role of spinal serotonin (5-HT) neurones descending from 5-HT cells near the ventrolateral surface of the medulla oblongata was investigated by stimulating these cells in normal animals and in animals with selective chemical ablation of 5-HT nerves. These laterally placed 5-HT nerves fall within the B1 and B3 groups in the medulla and were identified using immunohistochemistry. 5,7-Dihydroxytryptamine (5,7-DHT) was injected into the lateral cerebral ventricle (i.c.v.) to produce a generalized destruction of central 5-HT pathways, with preliminary intraperitoneal administration of desipramine to prevent depletion of noradrenaline stores. In other experiments, 5,7-DHT was injected directly into the cervical spinal cord, after preliminary treatment with desipramine, to produce selective destruction of spinal 5-HT nerves, confirmed both biochemically and immunohistochemically. Electrical stimulation near the lateral 5-HT cells in the B1 and B3 cell groups elicited pressor responses in control (vehicle-injected) rats; the increase in mean arterial pressure was proportional to the intensity and to the frequency of stimulation. Microinjections of kainic acid or l-glutamate at the same sites also produced an increase in mean arterial pressure. Selective destruction of 5-HT nerves, whether produced by i.c.v. or intra-spinal administration of 5,7-DHT, reduced the magnitude of the pressor response to electrical stimulation by over 50%. These experiments suggest the activity of 5-HT nerve cells adjacent to the ventrolateral surface of the medulla oblongata and projecting to the intermediolateral cell column serves to elevate arterial pressure and maintain vasomotor tone.     

Effects of indolic neurotoxins on enteric serotonergic neurons

The effects of the indolic neurotoxins 5,6- and 5,7-dihydroxytryptamine (5,6-DHT; 5,7-DHT) on the enteric nervous system were examined. 5,6-DHT, in moderate dosage, 40 mg/kg, decreased uptake of tritiated 5-hydroxytryptamine (³H-5-HT) but not that of tritiated norepinephrine (³H-NE). However, selectivity of the neurotoxins for enteric serotonergic rather than adrenergic axons was enhanced by pretreating animals with desmethylimipramine to inhibit the catecholamine uptake mechanism. When this was done, 5,7-DHT was found to prevent the development of 5-HT histofluorescence (following injection of L-tryptophan) without affecting histofluorescence of NE. In contrast, 6-hydroxydopamine virtually abolished NE histofluorescence and uptake of ³H-NE without affecting development of 5-HT histofluorescence or uptake of ³H-5-HT. Electron microscopy revealed that 5,7-DHT induced dose-dependent lesions of axonal varicosities in the enteric nervous system. Early lesions, 1–4 hours following injection, resembled cytolysosomes and consisted of membrane enclosed regions of opaque cytoplasm containing synaptic vesicles. Affected varicosities contained a mixed population of large dense cored (～120nm) and small lucent vesicles (～70nm) but none exhibited pre- or postsynaptic membrane specializations. After 24 hours terminals degenerated and were engulfed by surrounding supporting cells. It is concluded that peripheral serotonergic neurons resemble central serotonergic neurons in susceptibility to the toxic effects of indolic neurotoxins. These neurotoxins are useful anatomical markers of serotonergic terminal varicosities.     

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

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

Experimentally induced supersensitivity of neocortical neurons to microiontophoretically administered serotonin

Central serotonergic fiber systems of the rat were selectively lesioned by intraventricular injection of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). At various times thereafter, the sensitivity of rostral cortical neurons to microiontophoretically administered serotonin (5-HT) was compared in groups of lesioned and sham-operated animals pretreated with the 5-HT uptake inhibitor CGP 6085. Twenty-four hours after the injection of 5,7-DHT, at which time the cortical 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels were both reduced by 40%, there was no significant difference in the sensitivity of cortical neurons to 5-HT. However, 3 days after such treatment, when the cortical 5-HT and 5-HIAA levels were reduced by 52% and 53% respectively, pronounced supersensitivity to 5-HT was noted. The depressant action of 5-HT on neuronal firing was potentiated with regard to both maximal firing depression and duration of the firing inhibition. A similar potentiation of the 5-HT responses was observed 7 days after lesioning. Supersensitivity thus appears to develop between 1 and 3 days after the injection of 5,7-DHT. Seven days after lesioning, the sensitivity of rostral cortical neurons to gamma-aminobutyric acid was unchanged compared to that observed in sham-operated animals.     

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.     

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.     

Involvement of noradrenergic and 5-hydroxytryptaminergic systems in allylnitrile-induced head twitching

Allylnitrile induces in rats persistent behavioral abnormalities, including head twitching, following a single administration. We studied the role of 5-hydroxytryptamine (5-HT) and noradrenaline (NA) systems in the brain of rats in inducing and maintaining the head twitching. Allynitrile (1.49 mmol/kg) induced 5-HT system activation in all areas of the brain studied 1–4 days after oral administration, and a reduction in the content of NA in the hippocampus, cortex and hypothalamus 1 day after dosing, in the hippocampus, cortex, hypothalamus and midbrain 2 days after dosing, and in the hypothalamus 4 days after dosing. Allylnitrile induced no change in the content of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA) or NA 7–28 days after dosing. Pretreatment with 5,7-dihydroxytryptamine (5,7-DHT) suppressed the allylnitrile-induced head twitching, and decreased the contents of 5-HT and 5-HIAA in almost all areas of the brain throughout the observation period, as well as the ratio of 5-HIAA/5-HT in the medulla oblongata plus pons from 1 to 30 days after dosing with allylnitrile. No change in NA was observed in any areas of the brain. Pretreatment withN-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) increased the head twitching induced by allylnitrile, and decreased the content of NA in all areas of the brain throughout the observation period, without any change in the contents of 5-HT or 5-HIAA or in the ratio of 5-HIAA/5-HT. The present results suggest the involvement of 5-HT and NA systems in allylnitrile-induced head twitching.     

Effect ofP-chloroamphetamine and 5,7-dihydroxytryptamine on rotation and dopamine turnover

Injections of p-chloroamphetamine (PCA) or 5,7-dihydroxytryptamine (DHT) (after pretreatment with desmethylimipramine) into the median raphe nucleus (MRN) caused depletions of 5-HT and 5-HIAA in the cortex and striatum, and a decrease of cortical 5-HT uptake without affecting NE uptake. Unilateral injections of these neurotoxins into the MRN caused contralateral rotation, which was blocked by haloperidol. The size of the lesion correlated with the rate of rotation and the decrease in 5-HT turnover in the cortex. We also found a significant correlation between the rate of rotation and the decrease in cortical 5-HT turnover, and the increase in striatal DA turnover. Moreover, there was a significant correlation between the decrease in cortical 5-HT turnover and the increase in striatal DA turnover. It was found that injections of DHT into the SN produced similar behavioral and biochemical changes as did the MRN lesions. In this model, amphetamine and apomorphine produce turning in the same direction, whereas they have opposite effects after lesions of the nigrostriatal pathway where postsynaptic DA supersensitivity occurs. Presynaptic changes appear to determine turning in this model. An inhibitory role of the serotonergic MRN on the dopaminergic nigrostriatal pathway mediated via the substantia nigra (SN) is suggested.     

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.     

Selective increase of offensive behavior in the rat following intrahypothalamic 5,7-DHT-induced serotonin depletion

Cerebral serotonin (5-HT) depletions usually increase aggressive behaviors and more specifically facilitate elicitation of offensive behaviors. In order to localize the brain structures involved in this effect, 5,7-dihydroxytryptamine (5,7-DHT), a neurotoxin of 5-HT neurons, was injected into the ascending serotonergic pathway within the lateral hypothalamus, thus depleting 5-HT only in the forebrain structures. The effects of such treatment on offensive and defensive as well as social and non-social behaviors were studied in resident rats confronted with untreated intruders. Pretreatment with desipramine protected noradrenergic neurons. The content of 5-HT fell to 25% of controls, whereas noradrenaline was maintained at 90% in the forebrain anterior to the injection site. Ethological analysis of both resident's and intruder's behavior showed that offensive items were increased in 5,7-DHT-treated residents, whereas defensive items were increased in their non-treated partners; non-social activities were unchanged. Control of mouse-killing behavior during a 2-h test in the same animals showed a clear increase in elicitation of killing in 5,7-DHT-injected rats. These results confirm that the inhibitory control of serotonin is exerted specifically on offensive aggression. They suggest that forebrain structures are involved in this control.     

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.     

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.     

Serotonin 2A receptor mRNA levels in the neonatal dopamine-depleted rat striatum remain upregulated following suppression of serotonin hyperinnervation.

Sixty days after bilateral dopamine (DA) depletion (>98%) with 6-hydroxydopamine (6-OHDA) in neonatal rats, serotonin (5-HT) content doubled and 5-HT(2A) receptor mRNA expression rose 54% within the rostral striatum. To determine if striatal 5-HT(2A) receptor mRNA upregulation is dependent on increased 5-HT levels following DA depletion, neonatal rats received dual injections of 6-OHDA and 5,7-dihydroxytryptamine (5,7-DHT) which suppressed 5-HT content by approximately 90%. In these 6-OHDA/5,7-DHT-treated rats, striatal 5-HT(2A) receptor mRNA expression was still elevated (87% above vehicle controls). Comparative analysis of 5-HT(2C) receptor mRNA expression yielded no significant changes in any experimental group. These results demonstrate that upregulated 5-HT(2A) receptor biosynthesis in the DA-depleted rat is not dependent on subsequent 5-HT hyperinnervation.     

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.