DRL interresponse-time distributions: quantification by peak deviation analysis

Peak deviation analysis is a quantitative technique for characterizing interresponse-time distributions that result from training on differential-reinforcement-of-low-rate schedules of reinforcement. It compares each rat's obtained interresponse-time distribution to the corresponding negative exponential distribution that would have occurred if the rat had emitted the same number of responses randomly in time, at the same rate. The comparison of the obtained distributions with corresponding negative exponential distributions provides the basis for computing three standardized metrics (burst ratio, peak location, and peak area) that quantitatively characterize the profile of the obtained interresponse-time distributions. In Experiment 1 peak deviation analysis quantitatively described the difference between the interresponse-time distributions of rats trained on variable-interval 300-s and differential-reinforcement-of-low-rate 72-s schedules of reinforcement. In Experiment 2 peak deviation analysis differentiated between the effects of the psychomotor stimulant d-amphetamine, the anxiolytic compound chlordiazepoxide, and the antidepressant desipramine. The results suggest that peak deviation analysis of interresponse-time distributions may provide a useful behavioral assay system for characterizing the effects of drugs.

Effects of caffeine and PD 116,600 on the differential-reinforcement-of-low rate 72-S (DRL 72-S) schedule of reinforcement

Caffeine and PD 116,600 were found to decrease the reinforcement rate and increase the response rate in rats performing under a differential-reinforcement-of-low rate 72-s (DRL 72-s) schedule of reinforcement. In contrast, antidepressant drugs previously have been found to increase the reinforcement and decrease the response rate. Caffeine has been found to test similar to antidepressant drugs on at least one other behavioral screen, but caffeine does not possess clinical antidepressant properties. These results provide further support for the DRL 72-s schedule as a behavioral screen for antidepressant drugs.

The N-methyl-D-aspartate antagonist MK-801 protects against serotonin depletions induced by methamphetamine, 3,4-methylenedioxymethamphetamine and p-chloroamphetamine

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 blocks the ability of D-methamphetamine (MA) to deplete striatal dopamine (DA). We now report that MK-801 attenuates decreases in serotonin (5-HT) concentration induced by MA and two other amphetamine analogues, 3,4-methylenedioxymethamphetamine (MDMA) and p-chloroamphetamine (PCA). Rats were injected with saline (1.0 ml/kg) or MK-801 (0.5, 1.0 or 2.5 mg/kg) followed by either saline (1.0 mg/kg), MA (4, 2 or 1 injection(s); 10.0, 20.0 or 40.0 mg/kg), MDMA (20.0 or 40.0 mg/kg) or PCA (5.0 or 10.0 mg/kg). In some experiments, two injections of MK-801 or saline were used. Seventy-two hours after the last injection rats were sacrificed and concentrations of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA) and DA were determined in hippocampus and striatum. MA caused a depletion of 5-HT to 33% of control in hippocampus and to 50% of control in striatum after the 4 x 10.0 mg/kg dose regimen. When MK-801 (2.5 mg/kg) was co-administered with MA, concentrations of 5-HT did not differ from control levels in either brain region. MDMA depleted 5-HT to approximately 58% of control in hippocampus and 66% of control in striatum at the 40 mg/kg dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of salbutamol upon performance on an operant screen for antidepressants

The beta adrenergic (beta) agonist salbutamol increased reinforcement rates and decreased response rates on a differential-reinforcement-of-low-rate (DRL) 72-S schedule. These changes in DRL 72-S schedule performance are also produced by most clinically used antidepressants. The effects of salbutamol on a DRL 72-S schedule were dose-dependently antagonized by the beta antagonist metoprolol, but not changed by the 5HT antagonist methysergide. Additionally, neither salbutamol nor the antagonism of salbutamol by metoprolol caused disruption of DRL 72-S schedule performance. These results indicate that stimulation of beta receptors, and not of 5HT receptors, mediates salbutamol antidepressant-like effects on a DRL 72-S schedule.

Fluoxetine attenuates the DL-fenfluramine-induced increase in extracellular serotonin as measured by in vivo dialysis

Rats with hippocampal dialysis probes were treated with DL-fenfluramine (FEN), fluoxetine, or FEN with fluoxetine pre-treatment. FEN (12.5 mg/kg) increased extracellular serotonin (5-HT) from 0.4 +/- 0.04 to 25.2 +/- 4.16 pg/10 microliters. Fluoxetine (10.0 mg/kg) increased extracellular 5-HT levels from 0.4 +/- 0.05 to 2.4 +/- 0.33 pg/10 microliters. FEN-induced increases in extracellular 5-HT were attenuated by 66% with fluoxetine pre-treatment. This result supports the view that the 5-HT releasing properties of FEN are mediated by the 5-HT uptake transporter.

The NMDA receptor antagonist MK-801 does not protect against serotonin depletions caused by high doses of DL-fenfluramine

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK-801) has been shown to block methamphetamine (MA) induced damage to the dopamine (DA) and serotonin (5HT) systems of the brain. DL-Fenfluramine (FEN) is another potential neurotoxin but its long-term depletions are more selective to the 5HT system. To determine whether MK-801 protects against damage induced by FEN, we treated rats with FEN (4 injections of 12.5 mg/kg, at 1 h intervals) in conjunction with either saline or MK-801 (2 injections of 2.5 mg/kg, administered 15 min before and 90 min after the first FEN injection). Two weeks post-treatment, MK-801 alone caused a small but significant decrease in 5HT tissue concentrations in striatum and amygdala. FEN significantly reduced 5HT in all 8 brain regions studied. MK-801 + FEN did not protect against FEN-induced 5HT depletions in nucleus accumbens/olfactory tubercle, septum, frontal cortex, somatosensory cortex or hippocampus. MK-801 + FEN enhanced 5HT depletions in striatum, hypothalamus and amygdala. The differential protective effect of MK-801 between MA and FEN are discussed in terms of a possible dopaminergic mechanism.

Effects of repeated injections of cocaine on catecholamine receptor binding sites, dopamine transporter binding sites and behavior in rhesus monkey

In order to determine if repeated injections of cocaine produced long-lasting alterations in catecholaminergic binding sites, rhesus monkeys were treated with saline (1.0 ml/15 kg) or cocaine (3.0-4.0 mg/kg) four times daily for 14 consecutive days and sacrificed two weeks after the last injection. The densities of dopamine D1 receptor binding sites, dopamine transporter binding sites and beta adrenergic receptor binding sites were significantly decreased in caudate nucleus to 51%, 17% and 61% of control, respectively, two weeks after repeated cocaine injections. There were no differences in D2 receptor binding site densities in the caudate, nor were there differences in binding sites between groups in the other brain regions examined: prefrontal cortex (D1, D2, dopamine transporter, beta), nucleus accumbens (D1, D2, dopamine transporter) and substantia nigra (D2). Behavioral observation showed that the cocaine-treated monkeys became sensitized to the repeated injections. Early in the regimen, these animals displayed stereotypic grooming, buccal movements and visual checking after each injection that differed significantly from the saline-treated animals. As the regimen progressed, the frequency of grooming decreased while the frequencies of visual tracking and splayed legs increased in a manner consistent with the development of behavioral sensitization. Together, these findings suggest that the caudate nucleus may be more sensitive than other dopamine-containing brain regions to long-lasting pre- and post-synaptic effects of repeated cocaine administration, and that the changes seen in dopaminergic neurons may be related to behavioral sensitization.

Fenfluramine-induced increases in extracellular hippocampal serotonin are progressively attenuated in vivo during a four-day fenfluramine regimen in rats

Rats were administered 8 injections of 12.5 mg/kg fenfluramine over a 4-day period. Extracellular hippocampal serotonin levels were monitored in vivo during the 4-day treatment period. Predrug baseline serotonin levels were 0.6 +/- 0.17 pg/5 microliters; 60 min after the first fenfluramine injection extracellular serotonin levels were increased to 28.06 +/- 5.2 pg/5 microliters. Fenfluramine-induced increases in serotonin were substantially reduced on the 2nd through 4th days of the regimen. Baseline serotonin levels were increased on days 2 through 4 of the treatment regimen. In a separate group of animals post-mortem tissue concentrations of serotonin were measured 2 weeks after 1,2,4, or 8 injections of 12.5 mg/kg fenfluramine. There were decreases in serotonin tissue concentrations which were related to the number of fenfluramine injections administered. The in vivo dialysis and post-mortem tissue assay results are consistent with the view that fenfluramine is neurotoxic.

A comparison of trazodone and fluoxetine: implications for a serotonergic mechanism of antidepressant action

Trazodone is an atypical antidepressant drug that is commonly referred to as a serotonin (5-hydroxytryptamine; 5-HT) uptake inhibitor. However, the most potent pharmacological effect of trazodone appears to be antagonist action at 5-HT2/1C receptors. This is in contrast to fluoxetine, for which inhibition of 5-HT uptake is the most potent pharmacological action. The effects of trazodone and fluoxetine on several antidepressant drug screens are mediated by antagonist action at 5-HT2 receptors and inhibition of 5-HT uptake, respectively. While fluoxetine is an effective agent for the treatment of major depression, obsessive-compulsive disorder (OCD) and panic disorder, trazodone does not appear to be effective in the treatment of OCD and panic disorder. In addition, trazodone and fluoxetine differ in humans with respect to their effects on sleep and weight. Taken together, the preclinical and clinical data suggest that trazodone acts as an antidepressant via antagonist action at 5-HT2/1C receptors, while fluoxetine likely acts as an antidepressant via inhibition of 5-HT uptake.

A quantitative interresponse-time analysis of DRL performance differentiates similar effects of the antidepressant desipramine and the novel anxiolytic gepirone

We describe an interresponse-time analysis of performance on a differential-reinforcement-of-low-rate 72-s schedule. This analysis compares the obtained interresponse-time distribution of individual rats to a corresponding random interresponse-time distribution. The random interresponse-time distribution is a negative exponential probability function; it predicts the relative distribution of interresponse times if the rat emitted the same number of responses randomly (i.e., with a constant probability) with respect to time. The analysis provides quantitative measures of peak location and dispersion of the interresponse times toward random performance. In Experiment 1, an unexpected outcome of this analysis was that the rats would have obtained more reinforcers had they responded at the same rate but randomly. Based on the interresponse-time analysis in Experiment 1, it was shown that rats trained on the differential-reinforcement-of-low-rate 72-s schedule could increase the number of reinforcers obtained in two ways: first, by a coherent shift of the interresponse-time distribution toward longer durations and, second, by dispersal of the interresponse times toward a random interresponse-time distribution. Experiment 2 applied the analysis described in Experiment 1 to the effects of desipramine and gepirone. Both drugs decreased response rate and increased reinforcement rate, but their effects on the distribution of interresponse times were different. The increase in reinforcement rate observed with desipramine was accompanied by a coherent shift of the reinforcement rate observed with gepirone was accompanied by dispersal of the interresponse-time distribution toward the random negative exponential prediction.

Repeated injection of cocaine potentiates methamphetamine-induced toxicity to dopamine-containing neurons in rat striatum

Rats were treated with daily injections of saline or cocaine (20 mg/kg i.p. x 15 days) and, beginning 24 h after the last injection, administered saline or methamphetamine (6.25-50 mg/kg s.c., BID x 4 days) and sacrificed two weeks later. Repeated daily administration of cocaine potentiated the long-lasting striatal dopamine depletions caused by methamphetamine. The results suggest that human use of cocaine may increase the likelihood of neurotoxicity resulting from ingestion of high doses of methamphetamine.

Comparison of the effects of mianserin and its enantiomers and metabolites on a behavioral screen for antidepressant activity

The behavioral effects of racemic mianserin, its (+) and (-) enantiomers, and its metabolites desmethylmianserin and 8-hydroxymianserin were evaluated on the differential-reinforcement-of-low-rate 72-s (DRL 72-s) schedule, a screen known to be sensitive to and specific for the antidepressant properties of drugs. Racemic mianserin produced the antidepressant-like effect (increased reinforcement rate, decreased response rate) at 5 and 10 mg/kg. The mianserin enantiomers showed the antidepressant-like effect beginning at lower doses [(+) mianserin; 0.6 mg/kg; (-) mianserin: 2.5 mg/kg]. The mianserin metabolites showed no clear dose-related effect at doses up to 10 mg/kg. It is concluded that the antidepressant-like effects of mianserin are due to the activity of the parent compound rather than to its metabolites, and that they may be primarily attributable to the (+) enantiomer. The greater potency of (+)-mianserin may be related to its higher affinity for the 5-HT2 receptor.

Effects of repeated injections of cocaine on D1 and D2 dopamine receptors in rat brain

In order to determine if chronic administration of cocaine produced long-lasting alterations in dopamine receptor binding, rats were treated with single daily injections of cocaine (0, 10, or 20 mg/kg) for 15 consecutive days and killed either 20 min or 2 weeks after the last injection. The density of D1 binding sites in frontal cortex was either unchanged (10 mg/kg) or slightly increased (20 mg/kg) 20 min after the last daily injection, but was decreased 2 weeks later. D1 sites in striatum were decreased both immediately and 2 weeks after the injection regimen. Decreases in D1 binding site density in nucleus accumbens were observed only immediately after the last injection. In contrast to these effects on D1 binding sites, D2 binding sites were decreased in striatum and frontal cortex and increased in the nucleus accumbens 20 min after repeated cocaine, but were unaffected 2 weeks after repeated cocaine. Computer-assisted analysis of the saturation isotherms revealed that chronic administration of cocaine did not affect the affinity (Kd) of the radioligands used to label D1 or D2 sites. These findings suggest that repeated administration of cocaine results in long-term decreases in D1 binding sites in striatum and frontal cortex and transient decreases in D2 binding sites. Furthermore, cocaine caused opposite, transient effects on D1 and D2 site density in nucleus accumbens.

The effects of monoamine uptake inhibitors and methamphetamine on neostriatal 6-hydroxydopamine (6-OHDA) formation, short-term monoamine depletions and locomotor activity in the rat

Monoamine uptake inhibitors block the neurotoxic effects of methamphetamine (MA) upon dopaminergic and serotonergic neurons in the rat. The neurotoxic effects of MA upon dopaminergic neurons have previously been suggested to be mediated via formation of 6-hydroxydopamine (6-OHDA) from endogenous stores of dopamine (DA). In the present experiments, administration of the DA uptake inhibitor amfonelic acid (AFA, 10 mg/kg, i.p.) did not block the formation of 6-OHDA in rats treated with a single s.c. 100 mg/kg dose of MA. Consistent with the lack of effect by AFA on MA-induced 6-OHDA formation, neither AFA (10 mg/kg, i.p.) nor the DA and 5-hydroxytryptamine (5-HT) uptake inhibitor mazindol (40 mg/kg i.p., MAZ) blocked the depletion seen in neostriatal DA levels 1, 2 or 8 h following administration of a single 100 mg/kg dose of MA. In fact, AFA enhanced the DA depletions 2 and 8 h following MA administration. AFA also enhanced the MA-induced increase in locomotor activity in rats and this effect was blocked by lesions of dopaminergic neurons with i.v.t. (intraventricular) 6-OHDA in desipramine-pretreated rats. These results suggest that DA uptake inhibitors do not prevent the neurotoxic effect of MA on DA neurons by either preventing entry of MA into the cell or blocking the efflux of DA out of the cell. Instead, the DA uptake inhibitors appear to prevent the neurotoxic effect of MA upon dopaminergic neurons by blocking entry of 6-OHDA into the cell.

alpha-Methyl-p-tyrosine pretreatment partially prevents methamphetamine-induced endogenous neurotoxin formation

Depletion of brain dopamine (DA) by pretreatment with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMT) has been shown to prevent the long-term neurotoxic effects of methamphetamine (MA). In addition, it has recently been reported that the neurotoxins 6-hydroxydopamine (6-OHDA) and 5,6-dihydroxytryptamine (5,6-DHT) are formed endogenously in neostriatum and hippocampus, respectively, following a single neurotoxic dose of MA. We, therefore, have examined the ability of AMT pretreatment to prevent the MA-induced formation of 6-OHDA and 5,6-DHT. We report that AMT pretreatment significantly decreases the frequency with which 6-OHDA and 5,6-DHT are detected following MA administration. Neurotoxin formation is compared with brain levels of DA and 5-hydroxytryptamine (5-HT) 2 weeks after MA administration. It is concluded that the ability of AMT to attenuate both 6-OHDA formation and long-term depletions of DA is due to a decrease in the MA-releasable pool of DA. The effect of AMT on MA-induced depletions of 5-HT is less clear and may involve additional factors.

Pargyline increases 6-hydroxydopamine levels in the neostriatum of methamphetamine-treated rats

Neostriatal 6-hydroxydopamine (6-OHDA) was detected in 6 of 13 rats pretreated 2 or 4 hr earlier with methamphetamine (MA; 100 mg/kg, SC) and pargyline (25 mg/kg, IP, 30 min before MA injection). Neostriatal 6-OHDA was detected in 2 of 16 rats treated 2 or 4 hr earlier with MA. These results suggest that pargyline pretreatment may enhance formation of 6-OHDA from endogenous stores of dopamine (DA) following MA administration. Alternatively, these results suggest that pargyline pretreatment may protect endogenously formed 6-OHDA from oxidative deamination by monoamine oxidase. Enhancement of MA-induced neostriatal 6-OHDA levels may be the mechanism by which pargyline enhances the long-term neurotoxic effects of MA upon dopaminergic nerve terminals. These observations support the hypothesis that MA toxicity to DA-containing fibers is caused by the conversion of released DA into 6-OHDA.

Dopamine uptake inhibitors block long-term neurotoxic effects of methamphetamine upon dopaminergic neurons

A single large dose (100 mg/kg, s.c.) of methamphetamine (MA) is known to exert neurotoxic effects on dopaminergic neurons. The potency at which a series of dopamine (DA) uptake inhibitors blocked MA-induced neostriatal depletions (amfonelic acid (AFA) much greater than mazindol (MAZ) greater than or equal to bupropion (BUP) greater than benztropine (BENZ)) was similar to their potency at blocking 6-hydroxydopamine (6-OHDA) neurotoxicity in rats. Amfonelic acid was able to block long-term neostriatal DA depletions when given 8 h, but not 16 h, after a single large MA dose. These results suggest that an intact and functional DA uptake site is necessary for the development of MA-induced long-term DA depletions.

alpha-Methyl-p-tyrosine partially attenuates p-chloroamphetamine-induced 5-hydroxytryptamine depletions in the rat brain

alpha-Methyl-p-tyrosine (AMT) partially attenuates the long-term p-chloroamphetamine (pCA)-induced 5-hydroxytryptamine (5-HT) depletions. Pretreatment of rats with the tyrosine hydroxylase inhibitor AMT before treatment with the serotonin neurotoxin pCA decreased the extent of 5-HT depletion in the two brain regions examined. In these experiments, rats were administered AMT (150 mg/kg) 1 and 5 hours prior to an injection of pCA (5, 10, or 15 mg/kg). AMT reduced the pCA-induced 5-HT depletions in the striatum and to a lesser extent in the hippocampus. Furthermore, the attenuation of neurotoxicity was dependent on dose of pCA, with greater AMT effects at higher doses of pCA. AMT-pretreated rats were still significantly depleted of brain 5-HT following all doses of pCA. However, at the higher doses of pCA, the AMT-pretreated rats were significantly less depleted than saline-pretreated, pCA-treated rats. These results suggest that the neurotoxic effects of high doses of pCA on 5-HT-containing nerve terminals may be in part dependent on the availability of newly synthesized dopamine (DA).

Antidepressant-like effects of trazodone on a behavioral screen are mediated by trazodone, not the metabolite m-chlorophenylpiperazine

Trazodone is an atypical antidepressant drug (i.e. blocks neither monoamine uptake nor monoamine oxidase) which tests as an antidepressant drug on the differential-reinforcement-of-low-rate 72-s (DRL 72-s) schedule of reinforcement by increasing the reinforcement rate and decreasing the response rate. m-Chlorophenylpiperazine (m-CPP) is a 5-HT1B and 5-HT1C agonist, weak 5-HT2 antagonist, and trazodone metabolite. It has been suggested that formation of m-CPP is responsible for the antidepressant action of trazodone. Administration of m-CPP (1-10 mg/kg i.p.) 60, 30 or 10 min before the behavioral session did not mimic the reinforcement rate-increasing effects of trazodone (10-20 mg/kg i.p.) on rats performing under the DRL 72-s schedule of water reinforcement. Pretreatment with proadifen (50 mg/kg i.p.), an inhibitor of trazodone metabolism, caused a greater than 30-fold leftward shift in the dose-response curve for both the reinforcement rate and the response rate. These results suggest that the parent compound and not the trazodone metabolite m-CPP, mediates the antidepressant-like effects of trazodone on DRL 72-s behavior.