Richard R. Allen

Chronic toxicity/carcinogenicity studies of sulphamethazine in Fischer 344/N rats: Two-generation exposure

Fischer 344 rats were given 10, 40, 600, 1200 or 2400 ppm sulphamethazine (SMZ) in the diet to determine the toxicity and potential carcinogenicity of SMZ. There were 225 rats of each sex in the control groups and 135 of each sex in each dose group. Animals were killed after 3, 12, 18 or 24 months of continuous dosing. Body weights, feed consumption, clinical observations, organ weights and histopathology data were collected. A slight decrease in body-weight gain was observed in the high-dose groups compared with the controls. No difference in feed consumption was found between the control and dosed rats. Mortality was inversely related to SMZ dose, especially in females, that is mortality was highest in the controls and decreased as the dose of SMZ increased. A statistically significant dose-related increase in the incidence of follicular cell adenocarcinomas of the thyroid gland was observed in the animals killed after 24 months. The incidences of non-neoplastic lesions of the thyroid gland in treated animals were significantly higher among treated animals than among controls; these lesions included follicular cell hyperplasia, follicular cell focal cellular change and multilocular cysts. The incidences of retinal atrophy, atrophy of the acinar pancreas (males), and dilatation of the uterine lumen also increased with increasing SMZ dose.

Acute behavioral effects of MK-801 in rhesus monkeys: Assessment using an operant test battery

The acute effects of MK-801, a selective, noncompetitive NMDA receptor antagonist, were assessed using an operant test battery (OTB) of complex food-reinforced tasks that are thought to depend upon relatively specific brain functions such as motivation to work for food (progressive ratio, PR), learning (incremental repeated acquisition, IRA), color and position discrimination (conditioned position responding, CPR), time estimation (temporal response differentiation, TRD), and short-term memory and attention (delayed matching-to-sample, DMTS). Endpoints included response rates (RR), accuracies (ACC), and percent task completed (PTC). MK-801 (0.003-0.075 mg/kg, IV), given 15 min pretesting, produced significant dose-dependent decreases in measures of IRA and TRD performance at doses > or = 0.03 mg/kg. In both tasks, MK-801 produced significant decreases in accuracy at doses lower than those required to affect response rate. MK-801 also produced statistically significant decreases in PR, CPR, and DMTS measures, but only at higher doses (> or = 0.056 mg/kg) that caused significant decreases in both response rates and accuracies. These results indicate that, in monkeys, performance of operant tasks designed to model learning and time estimation is more sensitive to the disruptive effects of MK-801 than performance of tasks that model motivation, color, and position discrimination, and short-term memory and attention.

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.

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.

The effect of chronic cocaine exposure during pregnancy on the acquisition of operant behaviors by rhesus monkey offspring

To explore possible long-term effects of gestational cocaine exposure in a nonhuman primate model, pregnant rhesus monkeys were treated from about 1 month of gestation until term with either 0 (N = 3), 0.3 (N = 3), 1.0 (N = 3), or escalating doses up to 8.5 (N = 3) mg/kg (IM), three times per day, 5 consecutive days per week. Despite these differences in cocaine exposure, the experimental groups did not differ significantly with respect to the postnatal growth of offspring over an 18-month period following birth. Beginning at 6 months of age, the behavior of offspring was monitored using an operant test battery that included five food-reinforced tasks designed to model aspects of learning, color and position discrimination, time estimation, short-term memory and attention, and motivation. Although the acquisition of each operant behavior by offspring progressed significantly during training between 6 and 18 months of age, this acquisition was not differentially affected by gestational cocaine exposure. It was concluded that, in a rhesus monkey model, chronic cocaine exposure during pregnancy had no significant effect on the offsprings acquisition of operant behaviors.

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.

Chronic toxicity/carcinogenicity studies of sulphamethazine in B6C3F1 mice

A chronic feeding study was carried out in B6C3F1 mice with sulphamethazine (SMZ). The test substance was administered in the diet at dose levels of 0 (control), 300, 600, 1200, 2400 and 4800 ppm for 24 months. Mice were killed after 12, 18 and 24 months of continuous dosing. Body weights and food consumption were measured weekly, and mortality was recorded daily. All animals received a complete necropsy and histopathological examination, the results of which were analysed statistically. A slight decrease in body-weight gain was noted for mice of all dose groups with females showing the greater effect. Food consumption based on g food/g average body weight was relatively constant among the controls and various dose groups. The mortality rate for males and females of the control groups (8 and 8%, respectively) was higher than that for males and females of some of the higher dose groups. Neoplastic lesions associated with the ingestion of SMZ in the diet included follicular cell adenomas of the thyroid gland. At the 24-month necropsy, the incidence of this lesion for males and females of the 4800-ppm dose groups was 33 and 26%, respectively. Non-neoplastic dose-related lesions observed in both males and females included follicular cell hyperplasia (diffuse and focal) of the thyroid gland, haematopoietic cell proliferation of the spleen and pigmentation of the spleen. In females, pigmentation of the lymph nodes and hyperplasia of the mammary gland were also noted.

Acute effects of caffeine on several operant behaviors in rhesus monkeys

The acute effects of 1,3-trimethylxanthine (caffeine) were assessed using an operant test battery (OTB) of complex food-reinforced tasks that are thought to depend upon relatively specific brain functions, such as motivation to work for food (progressive ratio, PR), learning (incremental repeated acquisition, IRA), color and position discrimination (conditioned position responding, CPR), time estimation (temporal response differentiation, TRD), and short-term memory and attention (delayed matching-to-sample, DMTS). Endpoints included response rates (RR), accuracies (ACC), and percent task completed (PTC). Caffeine sulfate (0.175-20.0 mg/kg, IV), given 15 min pretesting, produced significant dose-dependent decreases in TRD percent task completed and accuracy at doses > or = 5.6 mg/kg. Caffeine produced no systematic effects on either DMTS or PR responding, but low doses tended to enhance performance in both IRA and CPR tasks. Thus, in monkeys, performance of an operant task designed to model time estimation is more sensitive to the disruptive effects of caffeine than is performance of the other tasks in the OTB.

Prenatal Ethanol Exposure in Rats: Long-Lasting Effects on Learning

Pregnant Sprague-Dawley rats were fed a liquid diet containing either 0% (group C), 18% (group L), or 36% (group H) ethanol-derived calories (EDC) from gestational day 1 to 20. Male offspring were assessed under a conditioned taste aversion paradigm (PND 35-45), in a complex maze (PND 68-80), and for operant behavior (temporal response differentiation and motivation to work for food, PND 140-198). Although conditioned taste aversion was fully acquired by all groups, retention of the conditioned taste aversion response was impaired in group H animals. Importantly, deficits in the acquisition of timing behavior were found in group H (group L not tested), confirming that this operant task is quite sensitive in detecting prenatal drug effects and demonstrating that neurological effects of prenatal ethanol exposure persist into late adulthood.

Chronic Exposure to NMDA Receptor and Sodium Channel Blockers during Development in Monkeys and Rats

Abstract: The effects of chronic administration of MK‐801 (NMDA‐receptor antagonist) and remacemide (sodium channel blocker) on monkey learning of several brain function tasks was assessed in juveniles (nine months old). Low (LO) and high (HI) doses of both drugs were given orally each day for 18 months. There were no adverse effects of any treatment on tests of short‐term memory or motivation. HI doses of both MK‐801 and remacemide delayed acquisition of a visual discrimination task (the remacemide effect was much greater). HI doses of remacemide alone severely disrupted learning task acquisition and this effect lasted for several months after dosing. Thus, in monkeys, chronic blockade of NMDA receptors is relatively well tolerated, whereas blockade of sodium channels (perhaps in conjunction with NMDA receptor blockade) has long‐term—perhaps permanent—consequences. To further explore the roles of NMDA receptors and sodium channels in these effects, MK‐801, phenytoin (sodium channel blocker), or both were administered to rats and the acquisition of tasks similar to those used in the monkey study were assessed. Dosing began at weaning and continued for nine months. Throughout the study, HI MK‐801 subjects exhibited impaired performance in all tasks. Some effects of MK‐801 were blocked completely by phenytoin. In the rat, blockade of sodium channels was well tolerated but blockade of NMDA receptors had significant and long‐term (permanent?) adverse consequences. These data contrast markedly with those obtained for the monkey and suggest, at least for some drug classes, that the rat might not be a good predictor of effects in primates.