Sensitivity changes to dopaminergic agents in fine motor control of rhesus monkeys after repeated methamphetamine administration

Long-term behavioral and neurochemical effects of repeated methamphetamine (MA) administration were investigated in rhesus monkeys trained to perform a fine motor task requiring control of exerted force for a specified time. Rhesus monkeys were trained to extend their arms into a tube to press a lever with a force between 25 and 40 g for 5 sec in order to receive 1.5 ml of water. The effects of intramuscular administration of MA, apomorphine (APO) and haloperidol (HAL) on responding were compared before and after a 2-week period of repeated MA administration. During this period, MA was given in 4 divided doses starting at a total daily dose of 4 mg/kg/day and increasing to 40 mg/kg/day. Tolerance to MA, increased sensitivity to HAL and no consistent sensitivity change to APO were observed when dose-response functions were redetermined starting 1 month after the repeated MA administration. One month after these determinations were completed, the brains of the monkeys were analyzed for changes in monoamines. Significant depletions of dopamine in the caudate nucleus and serotonin in the frontal cortex were seen. It is hypothesized that the sensitivity changes to the drugs on performance were related to the dopamine depletion.

A parametric description of amphetamine's effect on response rate: changes in reinforcement efficacy and response topography

A mathematical model was used to describe the effects of amphetamine on the rate of a reinforced response in the rat. The model provides measures of reinforcement efficacy and response topography for behavior maintained by variable-interval reinforcement schedules. In this study the measured behavior was a lever press, the reinforcer was water, and the variable-interval schedules provided five different rates of reinforcement, ranging from about 20 to 660/h. In each session the rats were exposed to each of the five schedules, and as reinforcement rate increased, the rate of lever pressing increased in a negatively accelerated manner that was closely approximated by the equation for a rectangular hyperbola. Amphetamine changed response rate and the parameters of the best-fitting hyperbolas. The 0.25-1.0-mg/kg doses increased response rate, and the parameter changes supported the interpretation that the increases were due primarily to an increase in reinforcement efficacy. The 2.0- and 3.0-mg/kg doses decreased response rates maintained by low reinforcement rates and increased response rates maintained by high reinforcement rates, and the parameter changes supported the interpretation that at higher doses amphetamine produced counteracting changes in reinforcement efficacy and response topography: reinforcement efficacy decreased, whereas response topography changed so as to increase response rates.

Behavioral screen for antidepressants: the effects of drugs and electroconvulsive shock on performance under a differential-reinforcement-of-low-rate schedule

Those antidepressant drugs that are in wide clinical use decrease response rate and increase reinforcement rate when administered to rats performing on a differential-reinforcement-of-low-rate 72-s (DRL 72-s) schedule. Drugs that are not antidepressants do not have this effect. In this experiment, the following were examined for their effects on a DRL 72-s schedule: trazodone, zimelidine, fluoxetine, and bupropion (atypical antidepressants); electroconvulsive shock (ECS, which is an effective treatment for depression); and haloperidol and clozapine (antipsychotic drugs). Trazodone (3.12-25.00 mg/kg), fluoxetine (10-20 mg/kg), and ECS decreased response rate and increased reinforcement rate. Zimelidine (20 mg/kg) increased reinforcement rate and nonsignificantly decreased response rate. At doses between 2.5 and 40 mg/kg, bupropion had no effect on reinforcement rate or response rate, but at 60 mg/kg response rate was increased and reinforcement rate was nonsignificantly decreased. At the higher dose, the effects of bupropion resemble those of a psychomotor stimulant. Haloperidol (0.04 mg/kg) and clozapine (2.5-10.0 mg/kg) decreased response rate and reinforcement rate. These results suggest that the DRL 72-s schedule may be useful for testing the antidepressant potential of new drugs.

Effect of the experimental antidepressant AHR-9377 on performance during differential reinforcement of low response rate

The effect of acute administration of AHR-9377 on performance under a differential-reinforcement-of-low-rate 72-s (DRL 72-s) schedule was assessed. AHR-9377 reduced response rates and increased reinforcement rates of rats responding under the DRL 72-s schedule; these effects were similar to those of established antidepressant drugs. AHR-9377 also decreased the time of immobility in the forced-swimming test, an empirical model of depression. These results are consistent with the contention that AHR-9377 possesses antidepressant activity.

Formation of 6-hydroxydopamine in caudate nucleus of the rat brain after a single large dose of methylamphetamine

We now report that 6-hydroxydopamine (0.39 +/- 0.31 nanograms/mg of tissue at 2 hr) is formed in the rat caudate nucleus after a single injection of methylamphetamine (100 mg/kg). The same dose of methylamphetamine causes approximately 50% depletion of caudate dopamine 2 weeks after the injection. We suggest that the formation of 6-hydroxydopamine from endogenous dopamine is responsible for the neurotoxicity to dopamine terminals seen after methylamphetamine administration.

Further evidence that amphetamines produce long-lasting dopamine neurochemical deficits by destroying dopamine nerve fibers

Methamphetamine and amphetamine were continuously administered to rats for 3 days by means of subcutaneously implanted osmotic minipumps. The total daily dose of each drug was approximately 4 mg/day. Dopamine, norepinephrine and serotonin determinations two weeks later indicated that both amphetamines produced a selective striatal dopamine depletion. Anatomical studies indicated that this depletion was associated with striatal nerve fiber degeneration. To determine whether this fiber degeneration induced by amphetamines was dopaminergic, the long-lasting dopamine depletion produced by methamphetamine was antagonized with alpha-methyl-para-tyrosine. This prevented the appearance of nerve fiber degeneration after methamphetamine. These findings suggest that amphetamines produce a long-term striatal dopamine depletion by destroying striatal dopamine nerve fibers.

Time-dependent changes in hypothalamic dopamine metabolism during feeding in the rat

The accumulation of the dopamine metabolite dihydroxyphenylacetic acid (DOPAC), but not the serotonin metabolite 5-hydroxyindoleacetic acid, in the hypothalamus is increased in rats during the second, third and fourth hours of a four hour period of access to food following a 20 hour period of food deprivation. This metabolic change does not correlate with duration of access to food or with amount of food consumed. These results suggest that increased hypothalamic dopamine metabolism during feeding is not related in any simple way to either the onset or termination of feeding.

Altered effects of desipramine on operant performance after 6-hydroxydopamine-induced depletion of brain dopamine or norepinephrine

Performance maintained by differential-reinforcement-of-low-rate operant schedules has been found to be sensitive to antidepressant drugs. Tricyclic antidepressants, monoamine oxidase inhibitors and atypical antidepressants reduce response rate and increase reinforcement rate under long differential-reinforcement-of-low-rate schedules. In order to study the neurochemical mechanism by which the tricyclic antidepressant desipramine alters differential-reinforcement-of-low-rate performance, the effect of desipramine was determined before and after brain catecholamine depletion was induced by 6-hydroxydopamine administration. Before lesioning, desipramine reduced response rate and increased reinforcement rate in a dose-dependent manner. Brain norepinephrine depletion (produced by 6-hydroxydopamine injection into the dorsal noradrenergic bundle) attenuated the ability of desipramine to reduce response rate, but did not alter its ability to increase reinforcement rate, but did not alter its ability to increase reinforcement rate. Brain dopamine depletion, (produced by i.c.v. 6-hydroxydopamine administration after pargyline and desipramine pretreatment) attenuated the ability of desipramine to increase reinforcement rate. These results suggest that the sedative effect of desipramine could be mediated by its interaction with central norepinephrine neurons and that the reinforcement rate-increasing effect may involve central dopamine neurons.

Effects of methamphetamine on atropine-induced conditioned gustatory avoidance

The repeated administration of high doses of methamphetamine (MA) has been shown to cause monoaminergic damage in rhesus monkeys and rats. In view of the purported interaction between central cholinergic and monoaminergic systems, rhesus monkeys and rats previously exposed to high doses of MA were tested in conditioned gustatory avoidance studies with atropine (a muscarinic blocker) as the unconditioned stimulus. It was observed that both rhesus monkeys and rats previously exposed to high doses of MA exhibited less of an atropine-induced avoidance than control monkeys and rats. To control for the nonspecific effects of prior exposure to stimulants, an additional group of rats previously exposed to high doses of methylphenidate ( a stimulant shown to not cause catecholaminergic damage) was tested in the same paradigm. The methylphenidate treated rats showed no change in sensitivity to atropine in the conditioned gustatory avoidance paradigm as compared to control rats which indicated that prior exposure to the nonspecific effects of a stimulant without monoaminergic alterations does not alter the sensitivity of atropine's avoidance-inducing properties. The results of these experiments imply that atropine's avoidance-inducing properties may in part be mediated through the monoaminergic system.

Increased transport of 3,4-dihydroxyphenylacetic acid from brain during performance of operant behavior in the rat

Although the performance of positively reinforced operant behavior is accompanied by increased turnover of brain dopamine in the rat, the accumulation of 3,4-dihydroxyphenylacetic acid (DOPAC), the major metabolic product of dopamine in brain, is not increased during operant performance. The hypothesis that the lack of increased DOPAC accumulation stems from an increase in the rate at which DOPAC is eliminated from the brain was tested by measuring the relative rate of DOPAC transport from brain in rats performing operant behavior and in control rats. The transport of DOPAC was estimated following inhibition of monoamine oxidase with pargyline. The pargyline-induced depletions of DOPAC in the caudate putamen and amygdala were significantly greater in rats that performed operant behavior than in control rats. In the caudate putamen, the concentrations of homovanillic acid and 5-hydroxyindoleacetic acid were also depleted by pargyline to a greater extent in operant performing rats than in controls. These results suggest that in addition to the increased turnover of dopamine in central neurons during operant behavior, the rate of elimination of acidic amine metabolites from brain is also accelerated.

Nerve terminal degeneration after a single injection of D-amphetamine in iprindole-treated rats: relation to selective long-lasting dopamine depletion

A single injection of D-amphetamine has recently been shown to produce long-lasting dopamine (DA) deficits in rats pretreated with iprindole, an agent which interferes with the metabolism of amphetamine and prolongs its half-life. The basis for these persistent DA deficits has not been determined. The present results suggest that amphetamine produces prolonged DA depletions in iprindole-treated rats by destroying DA nerve terminals.

Increased dopamine metabolism in the rat neostriatum after toxic doses of d-methylamphetamine

Rats were given repeated large doses of d-methylamphetamine (d-MA) which have been shown to destroy a fraction of nigrostriatal dopamine (DA) nerve terminals. Two to three weeks later, the synthesis of DA in the remaining neostriatal DA terminals was examined by measuring the rate of accumulation of 3,4-dihydroxyphenylalanine (DOPA) after inhibition of L-aromatic amino acid decarboxylase by 3-dihydroxybenzylhydrazine (NSD 1015). The release of DA from surviving DA terminals was also examined by measuring the steady-state level of 3,4-dihydroxyphenylacetic acid (DOPAC). Both the rate constant of the synthesis of DA and the DOPAC/DA ratio were increased in the neostriatum of rats with large depletions of neostriatal DA. These findings suggest that both synthesis and release of neostriatal DA are enhanced in rats in which d-methylamphetamine destroyed a large number of nigrostriatal DA terminals.

Fluoxetine increases long-lasting neostriatal dopamine depletion after administration of d-methamphetamine and d-amphetamine

Repeated administration of large doses of d-methamphetamine produce long-lasting depletion of brain dopamine (DA) and serotonin (5-HT), as well as persistent decreases in the activity of their respective biosynthetic enzymes, tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH). The present results indicate that the inhibitor of 5-HT uptake fluoxetine, prevented the long-term depletion of 5-HT produced by large doses of methamphetamine (15 mg/kg X 5, 6 hr apart) in the neostriatum and hippocampus, while simultaneously augmenting the depletion of DA produced by this drug in the neostriatum. Fluoxetine also enhanced the prolonged neostriatal depletion of DA produced by a comparable regimen of d-amphetamine. In these doses (15 mg/kg X 5,6 hr apart), d-amphetamine did not produce long-lasting depletion of 5-HT in either the neostriatum or hippocampus. Larger depletion of DA after the amphetamines had been administered in the fluoxetine pretreated animal were associated with a transient increase in the brain levels of methamphetamine and amphetamine. This suggests that fluoxetine may inhibit the metabolism of amphetamines.

Alpha-methyltyrosine attenuates and reserpine increases methamphetamine-induced neuronal changes

The repeated administration of methamphetamine to rats has been shown to cause a long-lasting depletion of dopamine in various brain regions. In the first study, the effects of pretreatment with alphamethyltyrosine (AMT) or reserpine on the long-lasting methamphetamine-induced dopamine depletion were examined. In the second study, the effects of AMT and reserpine on central dopamine levels were measured in rats previously treated with methamphetamine. Pretreatment with AMT attenuated the long-lasting dopamine depletion induced by methamphetamine, whereas, pretreatment with reserpine increased the depletion. The acute effects of AMT and reserpine on brain dopamine were not altered when administered two weeks after the last methamphetamine injection.

Locomotor hyperactivity in neonatal rats following electrolytic lesions of mesocortical dopamine neurons

These experiments examined the effects on locomotor activity of brain lesions that destroyed either mesocortical or nigrostriatal dopamine (DA) neuronal projections in neonatal rats. Electrolytic lesions of the medial ventral tegmental area in 4-day-old rats reduced the content of DA within the frontal cortex and septum by 42-57% and resulted in a 2-fold increase in locomotor activity during days 22-24 of life. In contrast, bilateral electrolytic lesions of the substantia nigra in 4-day-old rats reduced the content of DA within the caudate putamen by 68%, but failed to alter locomotor activity during days 22-24 of life. These results suggest that loss of mesocortical DA neurons may underlie the locomotor hyperactivity seen following brain DA-depleting 6-hydroxydopamine injections in neonatal rats and that these mesocortical DA neurons may normally influence the ontogeny of locomotion in the rat.

Differential-reinforcement-of-low-rate 72-second schedule: selective effects of antidepressant drugs

The effects of antidepressant drugs in rats responding under a differential-reinforcement-of-low-rate 72-sec schedule were assessed. Seven clinically used tricyclic antidepressant drugs (imipramine, desipramine, chlorimipramine, protriptyline, nortriptyline, amitriptyline and doxepin), two atypical antidepressants (iprindole and mianserin) and a monoamine oxidase inhibitor (tranylcypromine) dose-dependently reduced response rate and increased reinforcement rate. Nomifensine, an atypical antidepressant which has been reported to have psychomotor stimulant properties and abuse potential, increased response rate and decreased reinforcement rate. Chlorpromazine, an antipsychotic agent, and diphenhydramine, an antihistamine, have been reported to produce effects similar to antidepressants in several behavioral tests, but neither of these drugs mimicked the actions of antidepressants on responding under a differential-reinforcement-of-low-rate 72-sec schedule. Chlorpromazine decreased response rate but did not increase reinforcement rate. Diphenhydramine did not have consistent effects but tended to decrease reinforcement rate. These findings suggest that behavior maintained by the differential-reinforcement-of-low-rate schedule may be selectively affected by antidepressants that have no psychomotor stimulant properties.

Impaired acquisition of an operant response in young rats depleted of brain dopamine in neonatal life

In an attempt to examine the ability of brain dopamine (DA) depletion to alter learning ability in the developing rat, the rate of acquisition of a positively reinforced lever pressing response was examined in rats during days 30-45 of life following treatment with desmethylimipramine (DMI, 20 mg/kg IP) and 6-hydroxydopamine (6-OHDA, 35 micrograms intraventricularly) at 3 and 6 days of age, respectively. The 6-OHDA treatment produced a 40%-70% reduction of brain DA without altering growth rate, water intake, or locomotor activity. On the average, water-deprived control rats achieved the criterion for acquisition (50 reinforced lever presses/h) on a fixed-ratio 1 schedule of water reinforcement after 3.1 +/- 0.5 sessions (mean +/- SEM). In contrast, nearly one-fourth of the DMI + 6-OHDA-treated rats failed to acquire the response after 16 sessions and the remaining 6-OHDA-treated rats required more than twice as long as controls for acquisition (7.8 +/- 0.7 sessions). These results suggest that brain DA depletion in neonatal life can impair the acquisition of an operant response during development and that this deficit is independent of changes in growth rate or locomotor activity.

The effects of intracerebroventricular administration of 5,6-dihydroxytryptamine to neonatal rats

5,6-Dihydroxytryptamine was administered intracerebroventricularly to neonatal rats on days 3 and 6 (25, 50, 75 or 100 micrograms) or on day 12 (50, 75 or 100 micrograms) after birth. Administration on days 3 and 6 increased serotonin (5-HT) in the diencephalon, decreased it in the telencephalon and produced variable results in the brainstem. The dose of 100 micrograms eliminated the peak in locomotor activity that normally occurs at day 15. Administration on day 12 produced increased 5-HT levels in the diencephalon and the brainstem. The dose of 100 micrograms delayed the decrease in activity after the peak. Administration on days 3 and 6 or on day 12 produced non-specific damage throughout the brain, although dopamine levels were normal. The behavioral effects did not correspond with the neurochemical effects and probably were due to the pronounced non-specific effects of the treatment.

Neurochemical similarities between d,l-cathinone and d-amphetamine

Cathinone, the principal alkaloid of Khat, was compared to the psychomotor stimulant d-amphetamine on a number of neurochemical measures. Like d-amphetamine, d,l-cathinone released and blocked the uptake of tritiated dopamine (DA) in synaptosomal preparations. In addition, repeated high doses of d,l-cathinone produced long-lasting DA depletions in various rat brain regions and decreased the number of synaptosomal DA uptake sites in a manner similar to that seen after repeated d-amphetamine administration. Importantly, this DA neurotoxic effect of d,l-cathinone, like that of d-amphetamine, is selective since regional brain levels of norepinephrine (NE) or serotonin (5-HT) are not altered on a long-term basis by repeated administration of d,l-cathinone. These findings are discussed with reference to the current practice of Khat leaf chewing by people in north-eastern Africa.

Possible involvement of serotonergic neurons in the reduction of locomotor hyperactivity caused by amphetamine in neonatal rats depleted of brain dopamine

This experiment attempted to determine the mechanism by which amphetamine reduces locomotor hyperactivity in neonatal rats given brain dopamine (DA)-depleting 6-hydroxydopamine (6-OHDA) injections. Brain DA neurons were destroyed selectively in neonatal rats by intraventricular (i.v.t.) injections of 6-OHDA following desmethylimipramine (DMI) pretreatment. Control rats received DMI and i.v.t. injections of the 6-OHDA vehicle solution. Rats given the 6-OHDA treatment displayed 7-fold increases in locomotor activity compared to controls during days 16-55 of life. Throughout this period, amphetamine (1 mg/kg) reduced locomotor hyperactivity in 6-OHDA-treated rats but increased locomotor activity in control rats. The reduction of hyperactivity caused by amphetamine (0.5-4 mg/kg) was dose-related and was not accompanied by stereotyped behavior. Like amphetamine, methylphenidate (4 mg/kg) reduced locomotor hyperactivity in rats given 6-OHDA. The DA antagonist, spiroperidol (50-200 micrograms/kg) failed to attenuate the hyperactivity-reducing effect of amphetamine in 6-OHDA-treated rats at doses which abolished the stimulant effect of amphetamine in control rats. However, the serotonin antagonist methysergide (0.5-4 mg/kg) produced dose-dependent antagonism of the effect of amphetamine in 6-OHDA-treated rats. Pretreatment with propranolol ((5 mg/kg), phentolamine (5 mg/kg), atropine (0.5 mg/kg) or naloxone (10 mg/kg) failed to alter the reduction in locomotor hyperactivity caused by amphetamine. The serotonin releasing agent, fenfluramine (3 mg/kg), and the serotonin agonist, quipazine (0.5-4 mg/kg), both reduced locomotor hyperactivity in 6-OHDA-treated rats while not altering locomotion in control rats. These results confirm previous observations that amphetamine reduces locomotor hyperactivity caused by neonatal 6-OHDA administration and suggest that this effect is mediated by increased serotonergic neurotransmission.

Dopamine nerve terminal degeneration produced by high doses of methylamphetamine in the rat brain

Numerous recent studies indicate that when amphetamines are administered continuously or in high doses, they exert long-lasting toxic effects on dopamine (DA) neurons in the central nervous system (CNS). Specifically, it has been shown that amphetamines can decrease the content of brain DA, reduce the number of synaptosomal DA uptake sites and selectively depress the in vitro activity of neostriatal tyrosine hydroxylase (TH). To date, however, anatomical evidence of DA neuronal destruction following amphetamines has not been reported. In this study, chemical methods were used in conjunction with the Fink-Heimer method which allows for the selective silver impregnation of degenerating nerve fibers, in order to determine whether methylamphetamine, a potent psychomotor stimulant often abused by man, causes actual DA neural degeneration. It has been found that methylamphetamine induces terminal degeneration along with correlative DA neurochemical deficits in the neostriatum and nucleus accumbens; furthermore, that in cresyl violet-stained sections of the substantia nigra (SN), pars compacta, and ventral tegmental area (VTA), there is no evidence of cell body loss in rats in which 50-60% of neostriatal DA terminals have been destroyed.

Effects of monoamine oxidase inhibitors on performance during differential reinforcement of low response rate

The effects of three monoamine oxidase inhibitors (MAOI) on performance under a differential-reinforcement-of-low-rate 72-s schedule (DRL 72-s) for water reinforcement were determined. All three drugs (isocarboxazid, iproniazid, phenelzine) reduced response rate and increased reinforcement rate in rats performing under the DRL schedule. Drugs from other classes (alcohol, chlordiazepoxide, morphine, pentobarbital) did not produce similar effects. The ability of MAOI to increase reinforcement rate under a DRL 72-s schedule is similar to that recently reported for tricyclic antidepressants and the two atypical antidepressants mianserin and iprindole. These findings support the contention that the DRL schedule may be useful as a test for identifying new antidepressants and for elucidating the neurochemical effects of antidepressants that are responsible for their therapeutic actions.

Altered sensitivity to d-methylamphetamine, apomorphine, and haloperidol in rhesus monkeys depleted of caudate dopamine by repeated administration of d-methylamphetamine

The long-term neurochemical and behavioral effects of repeated d-methylamphetamine (d-MA) administration were investigated using four male rhesus monkeys trained to lever-press for food on a DRL-40s schedule of reinforcement. Dose-response curves for d-MA (0.0625-2.0 mg/kg), apomorphine (0.025-0.4 mg/kg), and haloperidol (0.005-0.04 mg/kg) on responding showed that repeated d-MA administration (0.5-16.0 mg/kg/day) decreased sensitivity to d-MA and to apomorphine but increased sensitivity to haloperidol. At 3-6 months after the last injection of d-MA, a 48.1% decrease in caudate dopamine (DA) was observed, with the frontal cortex, midbrain, and pons-medulla showing no significant change. A trend toward increasing concentrations of norepinephrine was noted in the same brain areas, but only in the frontal cortex did this change reach significance. Specific binding of 3H-spiroperidol to caudate membrane preparations was not changed, while the Vmax of the caudate DA re-uptake process declined 32%, with no change in Km. These results suggest that exposure of DA neurons in the caudate nucleus to high concentrations of d-MA can lead to nerve terminal degeneration.