D-, L- and DL-fenfluramine cause long-lasting depletions of serotonin in rat brain

D-Fenfluramine (1.6-12.5 mg/kg), L-fenfluramine (1.6-25 mg/kg), and DL-fenfluramine (1.6-25 mg/kg) injected s.c. twice daily for 4 consecutive days produced dose-related depletions of serotonin (5-HT) levels in somatosensory cortex, striatum, hypothalamus, and hippocampus of rats (n = 5-8/group) sacrificed two weeks after the last injection. While the results indicate that long-lasting effects of racemic fenfluramine are due to both stereoisomers, the magnitude of depletions caused by the isomers varied with dose, suggesting that they have different neurochemical effects.

Long-term central 5-HT depletions resulting from repeated administration of MDMA enhances the effects of single administration of MDMA on schedule-controlled behavior of rats

The behavioral effect of single administration of +/- 3,4-methylene-dioxymethamphetamine (MDMA) on rats performing on the differential-reinforcement-of-low-rate 72-second schedule (DRL 72-sec) was compared before and after a period of repeated administration of MDMA known to deplete 5-hydroxytryptamine (5-HT) levels in the brain. Single administration of MDMA decreased reinforcement rate (1, 2, 4, 6 mg/kg) and increased response rate (4,6 mg/kg) of rats performing on the DRL 72-sec schedule. This effect is typical of amphetamines and other psychomotor stimulants. Four weeks after repeated administration of MDMA (6 mg/kg twice daily for 4 days) there was an increase in sensitivity to the effect of single administration of MDMA. Doses of 2, 4 and 6 mg/kg of MDMA resulted in increases in response rate that were significantly greater after repeated MDMA administration than before. Doses of 0.5, 2, and 6 mg/kg of MDMA resulted in decreases of reinforcement rate that were significantly greater after repeated MDMA administration than before. Repeated administration of MDMA resulted in long-term depletion of serotonin levels by 30-50% in the amygdala, neostriatum, hippocampus and the frontal cortex. Norepinephrine and dopamine (DA) levels were not significantly different from control in any of the brain regions analyzed. The behavioral and neurochemical results suggest that serotonergic neurons normally exert an inhibitory action upon the psychomotor stimulant effects of MDMA. Since the psychomotor stimulant effects of amphetamines appear to be mediated primarily by the dopamine system, these results provide evidence that 5-HT and DA may represent opposing systems in the DRL schedule-controlled behavior.

Selective 5-hydroxytryptamine2 antagonists have antidepressant-like effects on differential-reinforcement-of-low-rate 72-second schedule

The effects of eleven 5-hydroxytryptamine antagonists with varying selectivity for the 5-hydroxytryptamine2 (5-HT2) relative to the 5-HT1 binding site were assessed in rats responding under a differential-reinforcement-of-low rate 72-sec (DRL 72-s) schedule of reinforcement. Three drugs with a 1000-fold selectivity for the 5-HT2 binding site (ketanserin, ritanserin, pipamperone) increased the reinforcement rate and decreased the response rate similar to antidepressant drugs. The two drugs with roughly the same affinity for both 5-HT1 and 5-HT2 binding sites (methysergide and metergoline) did not increase the reinforcement rate. The maximal increase in the reinforcement rate after 5-HT antagonist administration was positively correlated with the selectivity of the 5-HT antagonists for the 5-HT2 versus the 5-HT1 binding site. The increase in the reinforcement rate after administration of 5-HT antagonists was not correlated with the affinity of the 5-HT antagonists for the alpha-1 adrenergic, alpha-2 adrenergic, histamine-1 or dopamine-2 receptors. The 1000-fold selective 5-HT2 antagonist xylamidine, which does not pass the blood-brain barrier, did not increase the reinforcement rate or decrease the response rate. Thus, selective antagonism of central 5-HT2 relative to 5-HT1 receptors results in antidepressant-like effects on the DRL 72-s schedule. Furthermore, the specificity of the DRL 72-s schedule as a screen for antidepressant drugs was strengthened by the observation that the alpha-1 adrenergic antagonist, prazosin, did not increase the reinforcement rate despite significant decreases in the response rate.

Evidence for involvement of 5-hydroxytryptamine1 receptors in antidepressant-like drug effects on differential-reinforcement-of-low-rate 72-second behavior

Previous work in this laboratory has suggested that antagonist action of 5-hydroxytryptamine2 (5-HT2) receptors and agonist action of 5-HT1 receptors results in antidepressant-like effects (increased reinforcement rate and decreased response rate) in rats performing under the differential-reinforcement-of-low-rate 72-sec schedule (DRL 72-s) of reinforcement. Serotonergic mediation of antidepressant drug effects on DRL 72-s behavior was assessed with a series of 5-HT agonists, and blockade of the effects of the antidepressant drugs clorgyline and fluoxetine (which presumably indirectly stimulate 5-HT1 receptors) was attempted in separate experiments with the 5-HT1 and 5-HT2 antagonist methysergide and the 5-HT neurotoxin 5,7-dihydroxytryptamine. Direct 5-HT1A agonists 8-hydroxy-2-(di-n-propylamino)tetralin and 5-methoxy-N,N-dimethyltryptamine and the 5-HT precursor 5-hydroxytryptophan all increased the reinforcement rate. The 5-HT1B and 5-HT1C agonists m-chlorophenylpiperazine and 1-(m-trifluoromethylphenyl)piperazine did not increase the reinforcement rate. The 5-HT2 agonist and 5-HT3 antagonist quipazine also did not increase the reinforcement rate. The monoamine oxidase inhibitor clorgyline and the 5-HT uptake inhibitor fluoxetine increased the reinforcement rate and decreased the response rate as seen with other antidepressant drugs on the DRL 72-s schedule. Methysergide antagonized the reinforcement rate increasing effects of both clorgyline and fluoxetine. Depletion of brain 5-HT with i.v.t. 5,7-dihydroxytryptamine blocked the antidepressant-like effects of clorgyline. These results suggest that central 5-HT1A receptors are involved in mediating the antidepressant-like effects of some drugs on DRL 72-s behavior. These results provide evidence that stimulation of 5-HT1A receptors and antagonism of 5-HT2 receptors lead to an antidepressant-like effect on the DRL 72-s schedule and implies that these two receptors may be important in mediating clinical drug effects in depression.

Variability among brain regions in the specificity of 6-hydroxydopamine (6-OHDA)-induced lesions

6-Hydroxydopamine (6-OHDA; 200 micrograms, 150 micrograms or 110 micrograms) or vehicle was infused stereotaxically into the lateral ventricles of rats, usually following pretreatment with desmethylimipramine (DMI). Various brain regions were then assayed for dopamine (DA), serotonin (5-HT) and norepinephrine (NE). As expected, 6-OHDA depleted DA in all brain regions examined. Unexpectedly, however, the two highest doses of 6-OHDA significantly decreased 5-HT levels in the hippocampus and increased 5-HT levels in the striatum. In addition, despite pretreatment with doses of DMI commonly considered adequate to block 6-OHDA-induced depletion of NE, all doses of 6-OHDA tested significantly reduced NE levels in the hippocampus, hypothalamus and septum. We interpret our data as suggesting that some brain regions are susceptible to nonspecific toxic effects of 6-OHDA at doses commonly employed. Furthermore, these nonspecific effects may or may not occur, depending on seemingly minor variations in experimental technique.

Evidence that both intragastric and subcutaneous administration of methylenedioxymethylamphetamine (MDMA) produce serotonin neurotoxicity in rhesus monkeys

It has been demonstrated that repeated, subcutaneous administration of 3,4-methylenedioxymethamphetamine (MDMA) to rats, guinea pigs, and squirrel monkeys produces long-lasting depletions of serotonin (5-hydroxytryptamine; 5-HT) in several brain regions. Since evidence of degenerating 5-HT neurons has been observed in the rat brain following MDMA injections, it is likely that these depletions are due to neurotoxicity similar to that observed with other substituted amphetamines. The purpose of the present study was to determine if MDMA produces similar evidence of neurotoxicity in rhesus monkeys when administered by either the intragastric (i.g.) or subcutaneous (s.c.) route. Administration of MDMA (5.0 mg/kg/12 h x 4 days) by either i.g. or s.c. routes depleted 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in various brain regions 2 weeks after the last injection. Further, a significant decrease in [3H]5-HT uptake sites in the hippocampus was observed in monkeys treated with MDMA by the i.g. route. Reductions in uptake sites did not achieve statistical significance when drug was administered s.c. The results suggest that repeated administration of MDMA produces long-lasting, potentially neurotoxic effects on central 5-HT neurons in primates and does so when given orally.

Long-term effects of chronic methamphetamine administration in rhesus monkeys

Biochemical and neuropathological effects of exposure to a high dose regimen of methamphetamine were evaluated in rhesus monkeys approximately 4 years after the last drug injection. Concentrations of dopamine and serotonin in caudate were below control levels as were concentrations of serotonin in several other brain regions. These changes were more severe in a monkey that was exposed twice to the drug regimen. A decrease in caudate synaptosomal uptake of both neurotransmitters as well as neuropathological changes were evident in that monkey. Although it is possible that partial recovery occurred, these results strongly suggest that methamphetamine-induced neurotoxicity may be permanent.

Antidepressant-like effects of (+)-oxaprotiline on a behavioral screen

(+)-Oxaprotiline (1.25-10 mg/kg), a highly selective and stereospecifically acting norepinephrine (NE) uptake inhibiting drug, increased the reinforcement rate, decreased the response rate, and enhanced temporal discrimination in rats performing under a differential-reinforcement-of-low-rate 72-s schedule of reinforcement similar to other antidepressant drugs. (-)-Oxaprotiline did not affect the reinforcement rate, response rate or temporal discrimination. Since the most prominent known difference between the oxaprotiline enantiomers is the greater potency for inhibition of norepinephrine (NE) uptake by the (+) enantiomer, the effects of (+)-oxaprotiline in the present studies is probably due to inhibition of NE or epinephrine uptake. The present work also predicts that the therapeutic effects of oxaprotiline in the treatment of affective disorders is due to the (+) enantiomer.

Neurotoxicity in dopamine and 5-hydroxytryptamine terminal fields: a regional analysis in nigrostriatal and mesolimbic projections

In summary, we have shown that MA is toxic to both 5-HT and DA cells and we have proposed a mechanism that would account for this response, namely, the conversion of the transmitters to neurotoxins. In addition, brain depletions of DA seem regionally specific with larger depletions occurring in some areas than in others. The depletions, however, do not seem to depend entirely on the nuclei of origin, that is, substantia nigra versus VTA. 5-HT was depleted by different amounts in the various regions examined and the 5-HT depletions, although proportional to the DA depletions, were consistently greater. The reasons for this differential sensitivity of the 5-HT and DA systems to the toxic effect of MA is speculative, but may be related to the differential formation of toxins due to the differing availability of oxygen and superoxides at serotonergic and dopaminergic synapses.

Effect of depletion of brain serotonin by repeated fenfluramine on neurochemical and anorectic effects of acute fenfluramine

Fenfluramine is an anorectic agent in clinical use that is believed to act by enhancing 5-hydroxytryptamine (5-HT) neurotransmission. Tolerance to the anorectic properties of fenfluramine develops rapidly and long-lasting depletions of brain 5-HT have been reported to occur after repeated administration. It is possible that tolerance to fenfluramine may be related to the 5-HT depletions. Rats (n = 96), previously allowed to drink sweetened condensed milk during daily 15-min sessions, were treated with fenfluramine (6.25 mg/kg/12 hr x 4 days) or saline. Two or 8 weeks later rats were administered fenfluramine acutely (0, 1.25, 6.25 or 12.5 mg/kg; n = 6/group), tested for milk intake and sacrificed 2 hr later. Brains were removed and regions assayed for dopamine, 5-HT and metabolites. Acute administration of fenfluramine produced a dose-dependent decrease in milk intake and 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels in cortex, hypothalamus and hippocampus. Tolerance to the effects of acute fenfluramine on milk intake was observed in rats at 2 weeks (ED50 = 3.24 vs. 6.37 mg/kg; saline vs. fenfluramine pretreatment, respectively) and, to a lesser extent, at 8 weeks (ED50 = 2.99 vs. 4.04 mg/kg; saline vs. fenfluramine pretreatment) after the 4-day regimen of fenfluramine. Levels of 5-HT and 5-HIAA in somatosensory cortex, hypothalamus, striatum and hippocampus were depleted significantly 2 weeks after the last daily fenfluramine injection. The acute 5-HT depleting effect of fenfluramine was markedly attenuated in these regions 2 weeks after the 4-day regimen of fenfluramine.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of morphine-induced analgesia after repeated injections of methylenedioxymethamphetamine

Repeated administration of methylenedioxymethamphetamine (MDMA) to rats results in long-term depletion of serotonin (5-hydroxytryptamine; 5-HT) in several brain regions. Because of the apparent role of 5-HT in morphine-induced antinociception, the present experiment was designed to determine the effects of repeated MDMA injections on morphine-induced analgesia. Rats (n = 48) received 8 s.c. injections (one every 12 h for 4 days) of MDMA (20 mg/kg) or saline (1.0 ml/kg). Two weeks after the last injection, the groups were divided into 4 subgroups that received either saline, or morphine 2.5, 3.55 or 5.0 mg/kg (s.c.). Nociception was assayed before and after saline or morphine administration by the method of tail immersion in warm water (55 degrees C). The day after analgesia testing, the animals were sacrificed, brains and spinal cords removed and 5-HT, norepinephrine (NE) and dopamine (DA) levels in various brain and spinal cord regions were assayed. The analgesic effect of morphine was enhanced in rats that had received repeated MDMA injections. MDMA selectively depleted 5-HT in the cortex, hippocampus, striatum, brainstem and in the cervical portion of spinal cord. However, 5-HT levels were not changed in the thoracic and lumbar segments of the spinal cord. Thus, a functional consequence of repeated MDMA administration in rats was to enhance morphine-induced antinociception in association with reductions in brain and cervical spinal cord 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of long-term monoamine depletions following repeated or continuous exposure to cocaine

Cocaine was administered to rats for prolonged periods either by repeated injections (10 mg/kg twice daily for 10 days and 12.5 mg/kg 8 times daily for 10 days) or by continuous intravenous infusion (100 mg/kg/day for 21 days). None of the regimens produced long-lasting depletions of dopamine (DA), serotonin (5-HT), or major metabolites in striatum, hippocampus, hypothalamus, or somatosensory cortex. These results suggest that prolonged exposure to cocaine does not produce neurotoxicity like that observed with d-amphetamine or d-methylamphetamine.

Effects of selective 5-hydroxytryptamine-2 and nonselective 5-hydroxytryptamine antagonists on the differential-reinforcement-of-low-rate 72-second schedule

The effects of antidepressant drugs, administered with and without a 5-hydroxytryptamine (5-HT) antagonist that is not selective for 5-HT receptor subtypes, were assessed in rats responding under a differential-reinforcement-of-low-rate 72-second (DRL 72-s) schedule of reinforcement. The increases in reinforcement rate seen after low dose tricyclic antidepressant drug administration were antagonized by the 5-HT antagonist methysergide. Methysergide did not antagonize either the increases in reinforcement rate or the decreases in response rate induced by the atypical antidepressant trazodone or the putative antidepressant clenbuterol. In addition, the effects of 5-HT receptor antagonists with varying selectivity for the 5-HT2 relative to the 5-HT1 receptor subtypes were assessed when administered alone. The rank order potency series for the maximal increase in the reinforcement rate after administration of the 5-HT antagonists was ketanserin greater than pipamperone greater than trazodone greater than cyproheptadine greater than cinanserin greater than metergoline greater than methysergide, in close agreement with the selectivity of these drugs for the 5-HT2 relative to the 5-HT1 receptor subtypes. In addition, the specificity of the DRL 72-s schedule was further assessed with the alpha adrenergic antagonists phentolamine and phenoxybenzamine which both decreased the response rate but did not increase the reinforcement rate as do antidepressant drugs. These results suggest that the therapeutic effect of atypical antidepressants such as trazodone and mianserin may be related to selective antagonism of 5-HT2 receptors. Furthermore, agonist action at a 5-HT1 receptor and antagonist action at 5-HT2 receptors both appear to contribute to antidepressant-like activity on the DRL 72-s schedule.

Selective inhibition of MAO-A, not MAO-B, results in antidepressant-like effects on DRL 72-s behavior

The effects of monoamine oxidase inhibitors (MAOIs) that selectively inhibit the MAO-A or MAO-B forms of MAO were studied in rats performing under a differential-reinforcement-of-low-rate 72-s (DRL 72-s) schedule of reinforcement. Clorgyline and CGP11'305A, irreversible and reversible MAO-A inhibitors, respectively, increased the reinforcement rate, decreased the response rate, and enhanced temporal discrimination. The irreversible MAO-B inhibitor (-)-deprenyl did not produce similar effects. Pargyline did not increase the reinforcement rate at low doses that selectively inhibit MAO-B, but did increase the reinforcement rate at doses that inhibit MAO-A by more than 90%. The present results are in accord with clinical data demonstrating that MAO-A inhibitors are effective therapeutic agents in treating depression while MAO-B inhibitors are of questionable antidepressant efficacy. The present findings provide further evidence that the DRL 72-s schedule may be useful both as a screen for identifying new antidepressants and for investigating the neurochemical effects of antidepressant drugs that are responsible for their therapeutic effects.

Endogenously produced 5,6-dihydroxytryptamine may mediate the neurotoxic effects of para-chloroamphetamine

Para-chloroamphetamine (PCA) has been used to deplete brain serotonin (5-HT) in numerous studies of serotonergic involvement in various behaviors and physiological functions. PCA is believed to cause long-lasting depletions of 5-HT by causing the selective degeneration of serotonergic nerve terminals, but the mechanism by which it exerts this neurotoxic effect is not understood. In this experiment, 5,6-dihydroxytryptamine (5,6-DHT), a serotonergic neurotoxin, was detected by high performance liquid chromatography in the rat hippocampus 0.5-4 h after a single 15 mg/kg i.p. injection of PCA. 5,6-DHT was also detected in the somatosensory cortex following PCA administration, but much less frequently than in the hippocampus. Degenerating nerve terminals were observed in the striatum and somatosensory cortex in silver-stained brain sections from rats injected with PCA 1 or 2 days prior to sacrifice. Laminae III and IV of the somatosensory cortex also contained degenerating neuronal perikarya. The neurochemical and histological effects of PCA are very similar to those produced by a large dose of methylamphetamine (MA) in that both drugs are toxic to serotonergic nerve terminals and neuronal perikarya in the somatosensory cortex. We hypothesize that the formation of 5,6-DHT, perhaps from endogenous 5-HT, may mediate the toxic effects of PCA, MA and other amphetamine-related drugs on serotonergic neurons and on a subpopulation of cortical neurons.

Biochemical and histological evidence that methylenedioxymethylamphetamine (MDMA) is toxic to neurons in the rat brain

(+/-)-3,4-Methylenedioxymethylamphetamine (MDMA) was administered s.c. to rats (10, 20 or 40 mg/kg b. wt.) and guinea pigs (20 mg/kg) twice a day for 4 days, 2 weeks before decapitation. Norepinephrine, dopamine and serotonin (5-HT) levels were assayed in the hippocampus, hypothalamus, striatum and neocortex. In rats, MDMA produced dose-dependent reductions in 5-HT in all brain regions examined. The highest dose also reduced norepinephrine and/or dopamine in some regions. The 20-mg/kg dose of MDMA depleted 5-HT in all regions of the guinea pig brain assayed. In both species, repeated administration of 20 mg/kg of MDMA reduced the Vmax but not the Km of 5-HT uptake 2 weeks after administration. A single 40-mg/kg injection of MDMA depleted 5-HT 2 and 8 weeks after administration to rats in all regions of the brain examined except the hypothalamus. Administration of 80 mg/kg of MDMA twice a day for 2 days to rats depleted striatal 5-HT and dopamine. Brain sections from rats injected with MDMA according to this dosage regimen were stained by the Fink-Heimer method. Degenerating axon terminals and cell bodies were observed in the striatum and somatosensory cortex, respectively. These findings suggest that MDMA is toxic to serotonergic and, to a lesser extent, catecholaminergic neurons. Some neurons that do not contain these transmitters (neocortical neurons) are also affected.

5,6-Dihydroxytryptamine, a serotonergic neurotoxin, is formed endogenously in the rat brain

Methamphetamine (MA) in high doses produces long-term toxic effects on the serotonergic system in the rat brain, including depletions of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid and reductions in 5-HT reuptake and tryptophan hydroxylase activity. In this study, the formation of 5,6-dihydroxytryptamine (5,6-DHT), a serotonergic neurotoxin, was observed in the rat hippocampus after a single 100 mg/kg injection of MA. The 5,6-DHT was detected by reverse-phase high-performance liquid chromatography with electrochemical detection in tissue samples taken 0.5-4 h after MA administration; the highest levels of 5,6-DHT (0.032 ng/mg wet tissue) were detected at 1 h. Following administration of MA, 5-HT was also depleted in the neocortex, but 5,6-DHT was not detected as frequently in this brain region as in the hippocampus. Comparisons were made between the long-term hippocampal 5-HT depletions seen either after an injection of MA or after intraventricular 5,6-DHT infusions and the levels of 5,6-DHT measured in the hippocampus shortly after each treatment. The amount of 5,6-DHT produced after MA administration appears to be adequate to cause the observed long-term 5-HT depletions. We suggest that 5,6-DHT formed from 5-HT may mediate the neurotoxic effects of MA on serotonergic nerve terminals.

Behavioral and neurochemical evaluation of phenylpropanolamine

(+/-)-Phenylpropanolamine (PPA), a widely available anorectic and decongestant, was evaluated in several behavioral paradigms in rhesus monkeys and for central nervous system neurotoxicity in rats. PPA (1-30 mg/kg intragastric) reduced food intake in rhesus monkeys but was not self-administered i.v. (0.3-10 mg/kg/injection) by monkeys experienced in drug self-administration. PPA (30-100 mg/kg intragastric) resulted in amphetamine-like responding in two of four monkeys trained in a drug discrimination paradigm to discriminate d-amphetamine from saline. In rats, a 4-day injection regimen of high doses of PPA (200 and 400 mg/kg/day) resulted in approximately a 20% depletion of dopamine in the frontal cortex but failed to deplete dopamine, norepinephrine or serotonin in any other brain region studied. Thus, PPA is an effective anorectic in rhesus monkeys that, based upon drug discrimination results, would be expected to have limited amphetamine-like subjective effects and only at doses well in excess of effective anorectic doses. However, based upon self-administration results, PPA would not be predicted to have amphetamine-like dependence potential. Moreover, repeated administration of PPA did not produce the severe central nervous system neurotoxicity associated with many other amphetamine congeners.

Effects of serotonergic agonists and antagonists on the locomotor activity of neonatal rats

The locomotor activity of neonatal rats was measured after treatment with serotonin agonists or antagonists. Treatment with the serotonin agonists 5-hydroxytryptophan or quipazine resulted in the elimination of the peak in activity which normally results from increases in activity from days 10 to 15 of life followed by decreases from days 15 to 20 of life. The drug-induced decreases in activity occurred at doses that did not alter locomotor activity after day 17, when most of the peak in activity had passed. The dose of 5 mg/kg of the serotonergic antagonist methysergide eliminated the peak in activity without changing locomotor activity after the peak had passed. The antagonists methiothepin and cinanserin only produced decreases in locomotor activity which did not appear to be related to the peak in activity. The serotonergic agonist data are compatible with the hypothesis that the development of the serotonin system contributed to the inhibition of locomotor activity. The methiothepin and cinanserin data neither confirm nor dispute the hypothesis, as their effects may have been either nonserotonergic or on serotonin receptors that were different than those acted on by the agonists.

alpha-Methyltyrosine blocks methylamphetamine-induced degeneration in the rat somatosensory cortex

Neurochemical and histological studies suggest that methylamphetamine (MA) administered continuously or in high doses is toxic to dopaminergic and serotonergic nerve terminals. Degeneration of the dopaminergic or serotonergic cell bodies themselves has not been reported, however. In the present study, administration of a single 100 mg/kg dose of MA was toxic to a subpopulation of neurons in the somatosensory cortex, an area of the brain which does not contain catecholaminergic or serotonergic cell bodies. This dose of MA also produced a long-lasting depletion of serotonin (5-HT) but not norepinephrine in the somatosensory cortex. Dopamine levels in the somatosensory cortices of control animals were virtually undetectable and therefore were not studied further. Administration of alpha-methyltyrosine (alpha-MT), a catecholamine synthesis inhibitor, prior to the injection of MA blocked both the depletion of 5-HT and the degeneration of cortical perikarya produced by MA alone. Since the MA-induced depletion of 5-HT and the MA-induced degeneration of cortical perikarya are correlated, we suggest that the serotonergic system may be involved in the toxic effects of MA on the cortical neurons.

Hypothalamic catecholamine metabolism is increased by acute water imbalance

Disruption of water balance alters the metabolism of norepinephrine (NE) and dopamine (DA) in specific regions of the hypothalamus in the rat. Rats received one of the following treatments: hypertonic saline injection (1 M NaCl, 15 ml/kg), polyethylene glycol (40% polyethylene glycol in normal saline, 15 ml/kg), intragastric water load (10 ml), or ligation of the inferior vena cava. Catecholamine metabolism was determined by measuring the concentrations of NE and DA in the hypothalamus after catecholamine synthesis inhibition by alpha-methyl-p-tyrosine methyl-ester hydrochloride (200 mg/kg). No two treatments affected catecholamine metabolism in the same region of the hypothalamus. Intracellular dehydration by hypertonic saline increased NE metabolism in the paraventricular nucleus. Caval ligation, which stimulates the renal renin-angiotensin system, specifically increased NE metabolism in the preoptic area. Water loading increased the metabolism of NE and DA in the dorsomedial/ventromedial region. The effectiveness of the various treatments in increasing catecholamine metabolism was independent of the magnitude of their effects on blood pressure or water intake. The results suggest that there are multiple noradrenergic systems in the hypothalamus which respond to different types of water balance disruption.

Chlorpromazine and pimozide alter reinforcement efficacy and motor performance

This study evaluated the effects of chlorpromazine and pimozide on reinforced responding. In each session, rats were exposed to a series of five variable-interval reinforcement schedules. The response requirement was a lever press, the reward was a small portion of water, and the reinforcement rate varied from about 20 to 660 reinforcers per hour. Response rate was a negatively accelerated function of reinforcement rate, and the relationship between the two variables was described by the equation for a rectangular hyperbola (the matching law). One parameter of the hyperbola is equivalent to the asymptotic response rate and the other parameter is equivalent to the rate of reinforcement that maintains a one-half asymptotic response rate. Chlorpromazine (0.75-3.0 mg/kg) and pimozide (0.1-0.4 mg/kg) dose-dependently decreased response rates. At low doses, the response rate decreases were, for the most part, restricted to the low reinforcement rate schedules. In contrast, the highest dose tested decreased response rates at both low and high reinforcement rates. The patterns of response rate decreases resulted in dose-dependent changes in the parameters of the matching law equation. The shifts in the matching law parameters were discussed in terms of the motoric and motivational interpretations of neuroleptic-induced response rate changes.

Methamphetamine, physostigmine, atropine and mecamylamine: effects on force lever performance

Dose response functions for d-methamphetamine (MA), physostigmine, atropine, and mecamylamine on force lever performance (a measure of motor control) were determined in three rhesus monkeys. The rhesus monkeys were then treated with a repeated high dose regimen of MA, and the effects of the four drugs were redetermined. Following the completion of the behavioral studies, the monkeys were killed and brain monoamine concentrations were measured. It was found that each of the four drugs produced differential effects on force lever performance indices. Following the MA regimen, the MA-treated monkeys were less sensitive to the effects of MA on force lever performance but showed no change in sensitivity to any of the cholinergic agents. The monkeys were subsequently shown to have decreased brain dopamine and serotonin levels.

Water deprivation increases anterior hypothalamic norepinephrine metabolism in the rat

Rats were limited to 10 min of access to water per day. After 1 week, concentrations and rate of metabolism of dopamine, norepinephrine and epinephrine were determined in hypothalamic and limbic areas associated with regulation of water homeostasis. Chronic water deprivation caused hypovolemia, hypotension and ingestion of a large volume of water when water became available. Norepinephrine metabolism was consistently increased in samples containing the anterior hypothalamic nucleus, but no other catecholamine in any other brain area was significantly affected by the deprivation schedule. We conclude that the anterior hypothalamic nucleus is involved in the response to chronic disruption of water balance in the rat.

The effect of water-deprivation on locomotor activity in rats treated with 6-hydroxydopamine

Male albino rats treated with 6-hydroxydopamine (6-OHDA) became more hyperactive than did vehicle-treated controls when both groups were water-deprived. Rats were treated with vehicle, 150 or 250 micrograms of 6-OHDA intraventricularly, after pretreatment with desmethylimpramine (25 mg/kg) and pargyline (50 mg/kg). Eleven days after these treatments, under ad libitum water conditions, the 6-OHDA-treated rats were slightly hypoactive. After several days of water-deprivation all three groups showed increased mean locomotor activity levels, but rats treated with 6-OHDA showed a much greater increase than did vehicle-treated rats. Under subsequent ad libitum and deprivation conditions, locomotor activity decreased and increased, respectively, in all 3 groups. Again, the changes in activity levels of the 6-OHDA-treated groups were greater than those of the vehicle-treated group. In addition, rats treated with 250 micrograms 6-OHDA seemed to become sensitized to the novel environment of the stabilimeter rather than habituating to it. Dopamine (DA) levels were decreased as a result of the injections of 6-OHDA, and significant correlations were found between DA levels in the caudate-putamen and nucleus accumbens, and locomotor activity levels. These results, as well as those obtained by others, suggest that there is an interaction among DA levels, deprivational states, and locomotor activity levels in rats.