David L. Frederick

Effects of MDMA on complex brain function in laboratory animals.

This review surveys experiments that have examined the effects of acute and chronic MDMA exposure on schedule-controlled operant behaviors thought to engender responses that reflect the expression of complex brain functions. Such functions include time estimation, short-term memory, learning, motivation, and color and position discrimination. Recent experiments conducted in the Behavioral Toxicology Laboratory at the National Center for Toxicological Research concerning MDMAs acute and long-term effects on rhesus monkey performance in an operant test battery are compared to previous studies involving the effects of MDMA on operant behaviors. Results of these experiments suggest that when given acutely, MDMA disrupts complex brain functions associated with learning and time estimation more than those associated with short-term memory and visual discrimination, and that behavioral tasks requiring relatively high rates of responding are particularly sensitive to the disruptive effects of MDMA. Repeated exposure to doses of MDMA sufficient to produce long-lasting changes in brain neurotransmitter systems results in residual effects (e.g. tolerance, sensitivity) on behavioral task performance when subjects are subsequently challenged with acute MDMA, whereas baseline (non-challenged) performance of these tasks after such exposure generally remains unchanged. Although the experiments described herein were conducted on a relatively small number of non-human subjects, they raise the possibility that long-term effects on cognitive processes may also occur in humans exposed to repeated or acute high doses of MDMA.

Behavioral and neurochemical effects of chronic methylenedioxymethamphetamine (MDMA) treatment in rhesus monkeys

Effects of chronic treatment with the putative serotonergic neurotoxicant MDMA were assessed in rhesus macaques using behavior in an operant test battery (OTB) designed to model aspects of time estimation, short-term memory, motivation, learning, and color and position discrimination. After an initial acute dose-response assessment, escalating doses of MDMA (0.10-20.0 mg/kg, im, twice daily, for 14 consecutive days at each dose) were administered, followed by three additional acute dose-response assessments. In general, tolerance to MDMAs acute effects was evident in all OTB tasks by the second week of repeated exposure to each individual MDMA dose and as doses escalated. Baseline OTB performance after chronic treatment was not significantly altered. Residual behavioral tolerance to MDMAs acute effects, however, was evident in all OTB tasks but was least pronounced in the motivation task. Monkeys were sacrificed (21 months after chronic treatment) and brains were dissected into several regions for neurochemical analyses. Serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were analyzed via HPLC. Although MDMA-treated monkeys tended to have lower 5-HT concentrations in the frontal cortex, chronic MDMA treatment had no significant effects on 5-HT concentrations in any brain area sampled. Hippocampal 5-HIAA concentration, 5-HT uptake sites, and turnover of 5-HT of MDMA-treated monkeys were significantly lower than control values. DA concentrations in the CN of MDMA-treated monkeys were significantly greater than control values. No significant effects on DA concentrations were noted in any other brain area sampled. The absence of significant decreases in 5-HT and the general increase in DA concentrations are dissimilar to neurochemical effects reported after a short course of MDMA treatment at relatively high doses. These data suggest that chronic administration of gradually increasing doses of MDMA results in long-lasting tolerance to the drugs acute effects on the complex brain functions modeled in the OTB. It is uncertain, however, if such tolerance is related to the observed decreases in uptake sites and turnover of 5-HT in the hippocampus of these monkeys.

Effects of selective dopamine D1- and D2-agonists and antagonists on timing performance in rats

Dopamine (DA) D1- and D2-agonists and antagonists were administered at fixed doses to assess putative dopaminergic involvement in timing behavior in rats performing under a peak-interval schedule. Significant shifts in response distributions to the left (consistent with the overestimation of the passage of time) were observed after treatment with the D1- and D2-agonists SKF 38393 and quinpirole, respectively. Both DA antagonists, eticlopride (D2) and SCH 23390 (D1), shifted the response distributions to the right (consistent with the underestimation of the passage of time), but neither drug produced statistically significant shifts. Based on percent shift in peak time from predrug baseline values, no significant differences were detected between agents as a function of their reported affinities for the D1- or D2-receptors. Results indicate the need for a systematic evaluation of each drug at various doses and a more detailed examination of the use of temporal schedules in predicting the efficacy of psychotherapeutic agents.

Plasma Levels of Parent Compound and Metabolites after Doses of Either d-Fenfluramine or d-3,4-Methylenedioxymethamphetamine (MDMA) that Produce Long-Term Serotonergic Alterations

Plasma levels of parent compounds and metabolites were determined in adult rhesus monkeys after doses of either 5mg/kg d-fenfluramine (FEN) or 10mg/kg d-3, 4-methylenedioxymethamphetamine (MDMA) i.m. twice daily for four consecutive days. These treatment regimens have been previously shown to produce long-term serotonin (5-HT) depletions. Peak plasma levels of 2.0+/-0.4 microM FEN were reached within 40min after the first dose of FEN, and then declined rapidly, while peak plasma levels (0.4+/-0.1 microM) of the metabolite norfenfluramine (NFEN) were not reached until 6h after dosing. After the seventh (next to last) dose of FEN, peak plasma levels of FEN were 35% greater than after the first dose while peak NFEN-levels were 500% greater. The t(1/2) for FEN was 2.6+/-0.3h after the first dose and 3.2+/-0.2h after the seventh. The estimated t(1/2) for NFEN was more than 37.6+/-20.5h. Peak plasma levels of 9.5+/-2.5 microM MDMA were reached within 20min after the first dose of MDMA, and then declined rapidly, while peak plasma levels (0.9+/-0.2 microM) of the metabolite 3,4-methylenedioxyamphetamine (MDA) were not reached until 3-6h after dosing. After the seventh (next to last) dose of MDMA, peak plasma levels of MDMA were 30% greater than the first dose while peak MDA levels were elevated over 200%. The t(1/2) for MDMA was 2.8+/-0.4h after the first and 3.9+/-1.1h after the seventh dose. The estimated t(1/2) for MDA was about 8.3+/-1.0h. Variability in plasma levels of MDMA and MDA between subjects was much greater than that for FEN and NFEN. This variability in MDMA and MDA exposure levels may have lead to variability in the subsequent disruption of some behaviors seen in these same subjects. There were 80% reductions in the plasma membrane-associated 5-HT transporters 6 months after either the FEN or MDMA dosing regimen indicating that both treatments produced long-term serotonergic effects.

Acute behavioral effects of phencyclidine on rhesus monkey performance in an operant test battery

The effects of phencyclidine (PCP; a noncompetitive NMDA antagonist) were assessed in rhesus monkeys using performance in an operant test battery (OTB) consisting of five food-reinforced tasks thought to engender responses dependent upon aspects of time estimation, short-term memory, motivation, learning, and color and position discrimination. End-points included percent task completed (PTC), response rate or latency, and response accuracy. Testing occurred 15 min after IV injections of PCP (0.00, 0.003, 0.01, 0.03, 0.1, 0.13, 0.18, and 0.3 mg/kg). PCP disrupted performance of all tasks at 0.30 mg/kg. PTC was significantly decreased in the time estimation, motivation, and learning tasks at doses > or = 0.13 mg/kg. PTC for the short-term memory and color and position discrimination tasks was significantly decreased at 0.18 mg/kg and above. Response rate was significantly decreased at 0.13 mg/kg and above in the motivation and learning tasks and at 0.18 mg/kg and above in the time estimation, short-term memory, and color and position discrimination tasks. Response accuracy was significantly decreased in the time estimation, short-term memory, and learning tasks at doses > or = 0.13 mg/kg, while accuracy in the color and position discrimination task was decreased only at 0.30 mg/kg. PCPs effects on OTB performance were generally nonspecific, in that the time estimation, short-term memory, learning, and motivation tasks were all equally sensitive, with the color and position discrimination task being the least sensitive. These results are different than those obtained from earlier studies on the effects of MK-801, a more selective noncompetitive NMDA antagonist.

Acute effects of dexfenfluramine (d-FEN) and methylenedioxymethamphetamine (MDMA) before and after short-course, high-dose treatment.

ABSTRACT: The acute behavioral effects of methylenedioxymethamphetamine (MDMA) and dexfenfluramine (d‐FEN) were assessed in six rhesus monkeys using performance in the National Center for Toxicological Research (NCTR) Operant Test Battery (OTB); three additional animals served as controls for neurochemical endpoints. The OTB consists of five food‐reinforced tasks designed to model aspects of learning, short‐term memory and attention, time estimation, motivation, and color and position discrimination. Shortly after the acute effects of each drug were determined, three of the monkeys received a short‐course, high‐dose exposure (2×/day × 4 days, intramuscular (i.m.) injections) of MDMA (10 mg/kg), while three monkeys were exposed to an identical regimen of d‐FEN (5 mg/kg). Approximately one month later, the acute effects of each drug were again determined. In monkeys exposed to high‐dose d‐FEN, the sensitivities of the OTB tasks to acute disruption by either MDMA or d‐FEN were essentially unchanged. Conversely, monkeys treated with high‐dose MDMA were less sensitive to the acute behavioral effects of both drugs, although such an effect was seen more frequently for d‐FEN and was OTB task specific. Thus a residual behavioral tolerance to the acute behavioral effects of MDMA and d‐FEN was noted after high‐dose MDMA exposure, but not after high‐dose d‐FEN exposure. These findings are surprising, as similar neurochemical effects (i.e., significant decreases of ca. 50% in serotonin in frontal cortex and hippocampus) were observed in all monkeys approximately six months after short‐course, high‐dose MDMA or d‐FEN treatment.

Acute effects of methylenedioxymethamphetamine (MDMA) on several complex brain functions in monkeys

The effects of MDMA were assessed in rhesus macaques using behavior in an operant test battery (OTB) consisting of five food-reinforced tasks designed to model aspects of time estimation, short-term memory, and attention, motivation, learning, and color and position discrimination. Testing occurred 30 min after intramuscular, injections of MDMA (0.0, 0.1, 0.3, and 1.0 mg/kg). The behavioral endpoints monitored included percent task completed, response rate or latency, and response accuracy. Percent task completed was significantly decreased in the time estimation, learning, and motivation tasks at 1.0 mg/kg as compared to saline controls. Response accuracies in the time estimation and learning tasks were also decreased at 1.0 mg/kg. Response rate was decreased at 1.0 mg/kg in the motivation task but was not significantly affected in any other tasks. No behavioral endpoints were significantly affected in the short-term memory and attention and color and position discrimination tasks at any dose tested. Results indicate that time estimation, motivation, and learning are more sensitive to the acute effects of MDMA than are short-term memory and attention and color and position discrimination.

The effects of perinatal hypoxia on the behavioral, neurochemical, and neurohistological toxicity of the metabolic inhibitor 3-nitropropionic acid

Abstract3-nitropropionic acid (3-NPA) neurotoxicity and long-term effects of perinatal hypoxia were evaluated in 18 adult rats. Hypoxia-insulted (I) and noninsulted (NI) rats were delivered by cesarean section. Hypoxic insult was effected by submerging dissected uterine horns in warmed saline for 15 min. NI rats were delivered from the adjacent nonsubmerged horns. At postnatal day 90, I and NI rats were trained to perform tasks thought to measure behaviors dependent upon aspects of time estimation (TE), motivation, and learning. At 12 months of age, rats were injected i.p. with escalating doses of 3-NPA (5 mg/kg/day to a maximum of 30 mg/kg/day) immediately after each test session and sacrificed at the end of treatment. Additional male rats were used as untreated controls. Although 3-NPA produced a dose-dependent impairment of performance in each task, the effects were qualitatively similar for each group. A significant difference between I and NI rats was, however, observed in the TE task where NI rats completed less of the task at high doses of 3-NPA compared to I rats. Compared to untreated controls, dopamine concentrations were decreased in caudate nucleus of both I and NI rats after 3-NPA. Specific areas most frequently damaged included cerebral cortex, hippocampal subfield CA1, thalamus, caudate nucleus, and the cerebellum. Lesions usually were less extensive in the I rather than NI members of a littermate pair, suggesting a possible protective effect of perinatal hypoxia against subsequent 3-NPA neurotoxicity.

The effects of total and subtotal prefrontal cortex lesions on the timing ability of the rat

Two peak procedure experiments studied the effects of different prefrontal cortex lesions on temporal discrimination in rats. In Experiment 1, lesions were made to either the medial frontal cortex (including medial precentral, anterior cingulate, prelimbic, and infralimbic cortex) or the ventral frontal cortex (including orbital area and agranular insular area) to determine the contribution of each to time estimation. In Experiment 2, lesions were made to the entire prefrontal cortex (including the medial frontal cortex and the ventral frontal cortex). In both experiments, rats were trained with a 40-sec peak interval and tested for 14 postoperative sessions. In Experiment 1, peak time and response distributions showed timing behavior unaffected by either lesion. In Experiment 2, ablations of the prefrontal cortex (PFC) produced a leftward shift in the temporal discrimination functions, indicating that these animals expected the time of reinforcement to be early. Also, these discrimination functions were substantially flattened, which was interpreted as a general decline in timing ability. These results are inconsistent with those of earlier studies. It is concluded that PFC functions as a unit with respect to timing behavior.

Acute effects of LSD on rhesus monkey operant test battery performance.

The acute effects of LSD were assessed in rhesus macaques using behavior in several complex tasks designed to model aspects of time estimation, short-term memory and attention, motivation, learning, and color and position discrimination. The end points monitored included percent task completed, response rate, and accuracy. LSD (0.0003-0.03 mg/kg intravenously) significantly decreased percent task completed and accuracy in the time estimation task at doses < or = 0.003 mg/kg, but did not significantly affect response rate in this task at any dose tested. Accuracy in the short-term memory task was significantly decreased at the highest dose tested (0.03 mg/kg), but no other end points were affected in this task. Response rate was decreased in both the motivation and learning tasks at doses (0.01 and 0.003 mg/kg, respectively) lower than those affecting other end points. In the color and position discrimination task, only response rate was affected (0.01 and 0.03 mg/kg). These data demonstrate that in rhesus monkeys, performance of tasks believed to depend on aspects of time estimation and motivation are more sensitive to the acute disruptive effects of LSD than are tasks thought to model learning, short-term memory, and color and position discrimination.