David Shurtleff

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)

Rapid demand curves for behavioral economics

A method for determining the relationship between food consumption and the price of food (demand function) in behavioral economic experiments is described. Although previous methods have generally required as long as 40 days, the present method can generate a complete demand function within 7 days, and therefore may be more suitable for use in the evaluation of drugs, toxins, and physiological/anatomical interventions. Moreover, measures of circadian rhythmicity, post-reinforcement pause durations, and interresponse times can also be generated. Three experiments tested the stability of the method in a variety of procedural manipulations: repeated exposure to the procedure, increasing versus random daily food price, and size of daily changes in food price. The procedure generated demand functions that were similar to those that require more extended testing, and the demand functions were not generally affected by procedural manipulations. Body weight, which can also affect consumption, generally decreases with increases in the price of food; so this variable should be recorded and used as a covariate in analyzing demand functions. With the exception of circadian rhythmicity, the other measures were stable across procedural variations and showed expected changes as a function of food price: postreinforcement pause durations increased as price increased, but interresponse times did not.

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.

Tyrosine pretreatment alleviates suppression of schedule-controlled responding produced by corticotropin releasing factor (CRF) in rats

Disruption of performance observed when animals are exposed to physical stressors which deplete brain catecholamines can be alleviated by pretreatment with the catecholamine precursor tyrosine. Central administration of the stress hormone corticotropin releasing factor (CRF) has been shown to affect a variety of behaviors and also to potently increase the release of central catecholamines. Since CRF-induced disruption of behavior may involve CRF-induced depletion of brain catecholamines, the present study examined whether tyrosine would alleviate suppression of schedule-controlled responding in rats resulting from ICV administration of CRF. Administration of CRF (1.0 microgram-10 micrograms) produced dose-dependent suppression of response rate and total number of earned reinforcers in rats responding on a multiple fixed-interval 60 s/fixed-ratio 20 schedule for food reinforcement. Pretreatment with 200 mg/kg tyrosine (IP) administered with ICV saline decreased response rate but did not lower total reinforcers, whereas 400 mg/kg of tyrosine decreased both. Injection of 400 mg/kg tyrosine reduced, but did not completely restore, CRF-induced suppression of behavior. The 200 mg/kg tyrosine dose was less effective in alleviating CRF-induced suppression of performance. These data indicate that pretreatment with the catecholamine precursor tyrosine can partially ameliorate performance decrements resulting from CRF administration.

Loudness bisection and masking in the rat (Rattus norvegicus)

The bisection method of animal psychophysical scaling was examined as a measurement procedure. The critical assumptions of bisection scaling, as described by Pfanzagl (1968), were tested to determine if a valid equal-interval scale could be derived. A valid scale was derived in which loudness for the rat (Rattus norvegicus; n = 13) was a power function of sound pressure for 4-kHz tones. Masking noise reduced the discriminability of tonal stimuli but did not affect the bisection point. This result is consistent with an interval scale representation of loudness and demonstrates scale meaningfulness. Loudness bisection data that have been reported in the literature for 3 species (humans, rats, and pigeons) are in substantial agreement with our results.

Thermal stress modulates temporal patterns of responding on a multiple DRL-FR schedule.

To investigate temporal changes in behavior induced by moderate cold temperatures, rats performing on a multiple differential-reinforcement-of-low-rate (DRL) fixed-ratio (FR) schedule were exposed to ambient temperatures of 2, 8, 16, and 24 degrees C. DRL response rates markedly increased with decreasing cold temperatures, while FR response rates remained unchanged. In addition, as ambient temperatures decreased, the interresponse time (IRT) distribution of DRL responses shifted toward shorter times and short IRT bursts increased. Compared with cold effects, exposure to 38 degrees C heat induced decreases in both DRL and FR response rates which were associated with increases in long IRTs. Decreases in reinforcement frequency was associated only with the DRL schedule in cold, and with both DRL and FR schedules during heat exposures. The distinct effects of cold and heat on both DRL and FR responding suggest that the increases in DRL response rates and shifts in IRT distribution are unique to cold, and are not due to general effects of nonspecific thermal change in the ambient environment.