John Schrot

Tyrosine Reverses a Cold-Induced Working Memory Deficit in Humans

Acute exposure to cold stress has been shown to impair short-term, or working, memory, which may be related to reduction in, or disruption of, sustained release of brain catecholamines. Administering a supplemental dose of the catecholamine precursor tyrosine may alleviate cold stress-induced memory impairments by preventing cold-induced deficits in brain catecholamine levels. The present experiment determined whether administration of tyrosine would prevent a cold-induced working memory deficit, using a computer-based delayed matching-to-sample (DMTS) memory task. Eight male volunteers performed the DMTS task for 30 min at an ambient temperature of either 4 degrees C (cold) or 22 degrees C following a 30-min preexposure period and 2 h after ingesting 150 mg/kg of L-tyrosine or placebo. Subjects demonstrated a decline in matching accuracy on the DMTS task as delay interval increased, such that matching accuracy following a 16-s delay between sample and comparison stimuli was lower than that following a delay of 2 or 8 s. Consistent with previous research, and relative to 22 degrees C exposure sessions, matching accuracy during 4 degrees C exposure sessions was reduced significantly following placebo administration, which is attributed to the effect of cold exposure on short-term, or working, memory. Administration of tyrosine significantly improved matching accuracy at the longest delay interval most affected by cold exposure, such that matching accuracy in the cold following tyrosine was at the same level as matching accuracy following placebo or tyrosine administration at 22 degrees C. Tyrosine administered prior to 22 degrees C exposure had no effect on DMTS performance.(ABSTRACT TRUNCATED AT 250 WORDS)

Cold-Induced Impairment of Delayed Matching in Rats

Exposure to moderate, nonhypothermic cold temperature has been reported to affect a variety of behavioral and neural functions. To elucidate the effects of mild cold stress on short-term (working) memory, Long-Evans rats were exposed to an ambient temperature of either 2 degrees or 23 degrees C while performing a delayed matching task. At the beginning of each trial, rats were required to respond on one of two levers cued by a light. Following a delay of 2, 8, or 16 s, a response on the lever previously cued produced food reinforcement. Relative to performance at 23 degrees C, exposure to 2 degrees C occasioned no change in matching accuracy at the 2-s delay, a modest decrement at the 8-s delay, and a larger decrement at the 16-s delay. The cold exposure did not decrease colonic temperature. In addition to accuracy decrements, matching response times were consistently shorter during cold exposures. Cold-induced impairments were absent during removal of the memory component from the task, indicating the observed cold effects on memory were not due to impaired attentional, sensory, or motor processes. These data suggest that mild cold stress may impair active maintenance of information in working memory but not processes related to reference memory.

Glucose Attenuates Cold-Induced Impairment of Delayed Matching-to-Sample Performance in Rats,

Administration of glucose has been shown to enhance performance in a variety of test situations in which memory is impaired by some amnestic treatment. In the present study, the effects of glucose were examined on a working-memory deficit produced when rats performed a delayed matching-to-sample (DMTS) task while being exposed to ambient cold air. In the DMTS task, the rats were required to respond on one of two levers cued by an illuminated light above the lever on the front wall of an operant chamber. Following a variable delay ranging from 1 to 16 sec, both lights were illuminated and the rats were required to correctly respond on the lever previously cued for a food reward. They responded on the back wall lever during the delay interval to prevent position bias. Glucose (10–500 mg/kg) or saline, administered (i.p.) in a mixed sequence, were given 1 h before a 75-min session in which the rats performed the DMTS task (180 trials). During test sessions, the ambient air temperature was either 23°C or 2°C. Administration of glucose during exposure to 23°C did not systematically affect matching accuracy or other performance measures. In the rats pretreated with saline, exposure to 2°C produced a downward shift in the delay gradient in that matching accuracy was impaired at all delay intervals when compared with performance at 23°C. Glucose produced dose-dependent improvement of matching accuracy with short, but not long, interpolated delays. Selective modulation of cold-induced impairment of working memory at the short delays suggests that glucose preferentially enhances stimulus acquisition during exposure to cold stress.

Alteration of response patterning by d-amphetamine on repeated acquisition in rats

The acute effects of d-amphetamine on response patterning in a repeated acquisition baseline were investigated with rats. Each session the animals acquired a different four-member response sequence on three levers. Each sequence (trial) completion produced a food pellet. Errors produced a brief timeout that was reset by responses made during the timeout. Acute doses of d-amphetamine (0.5-4.0 mg/kg) and saline were administered 30 min presession. The response patterns analyzed were perseverative responses to a single lever (runs), and a response to each lever in either a left-to-right or right-to-left direction (traverses). The trial position, frequency, and lever location of error and timeout responses that occurred in the context of runs and traverses were studied. In contrast to control sessions, higher doses of d-amphetamine produced increases in the number of error and timeout responses emitted. The majority of these responses occurred as runs; traverse responding did not exceed control levels. Furthermore, the run error and timeout responding tended to occur early in the session and on a single response lever. The results are consistent with the view that d-amphetamine disrupts stimulus control and produces perseverative responding which may account for previous reports of disruption in repeated acquisition tasks following d-amphetamine administration.

Response Patterning in a Repeated Acquisition Baseline with Rats

A response-pattern analysis was performed on data obtained from rats trained on a baseline of repeated acquisition of behavioral chains. The response patterns identified and analyzed were perseverative responses to a single lever (runs) and a response to each lever in either a left-to-right or right-to-left direction (traverses). The analysis indicated that runs and traverses accounted for between 50 and 70% of all error response and between 80 and 100% of all timeout responses. In addition, the number of traverses varied according to the composition of a sequence: higher frequencies occurred when the sequence required a traverse for completion, e.g., LRCL, than when it did not, e.g., LRLC. The differentiation of sequence type, however, was not systematically related to the level of error or timeout responding. The results suggest that the generation and maintenance of response patterns influences control of the repeated-acquisition process.

DRL schedule-induced alcohol ingestion

This study documents the efficacy of a differential reinforcement of low rates (DRL) barpress procedure in producing schedule-induced drinking of alcohol solutions in rat. Solution intakes were a monotonically decreasing function of alcohol concentration in the range of 20% to 10%. Conversely, the amount of absolute ethanol consumed increased as a direct function of alcohol concentration. Peak absolute alcohol ingestion with the 10% concentration was 1.5-1.8 g per rat. Within-session changes in barpress rates and interresponse time (IRT) distributions occurred coincident with intrasession declines in alcohol drinking.

Repeated Acquisition of Four-Member Response Sequences in Rats

A procedure for studying the repeated acquisition of four-member response sequences in rats was developed. Early in training incorrect responses constituted approximately 60% of the total responses emitted during a session, and no evidence of within-session acquisition was observed After 100 training sessions incorrect responses constituted approximately 25% of total responses and within-session acquisition was observed, as evidenced by runs of up to 14 errorless sequence completions during the latter portions of training sessions. The behavior generated after extended training was similar to behavior maintained by repeated acquisition baselines for pigeons and monkeys in over-all levels of error and in the pattern of within-session acquisition.