Tyrosine ameliorates a cold-induced delayed matching-to-sample performance decrement in rats

Abstract

Exposure to cold stress has been shown to impair short-term, or working, memory which may be related to a reduction in brain catecholamines. Administration of the catecholamine precursor tyrosine may alleviate a cold-stress-induced memory impairment by preventing a deficit in brain catecholamine levels. To test this hypothesis, eight rats performed a delayed matching-to-sample (DMTS) task at an ambient temperature of either 2°C (cold) or 22°C, following intraperitoneal administration of saline or tyrosine (50, 100 or 200 mg/kg). Rats administered saline prior to 22°C exposure demonstrated a characteristic delay gradient in which accuracy decreased as the delay interval between sample and comparison stimuli increased from 1 to 16 s. Consistent with previous research, and relative to 22°C exposure sessions, matching accuracy during 2°C exposure sessions was reduced, which is attributed to the effect of cold on short-term, or working, memory. In particular, during cold exposure sessions matching accuracy was significantly reduced at the longer delay intervals, relative to matching accuracy at 22°C. Additional analysis of cumulative matching errors within sessions showed that during exposure to cold, errors occurred at a constant rate throughout the session, indicating rats' performance was equally debilitated by the stressor over the entire session. During cold exposure sessions, the higher doses of 100 and 200 mg/kg tyrosine significantly improved overall matching accuracy relative to saline, but did not completely reverse the effect of cold exposure, as overall matching accuracy did not increase entirely to levels obtained at 22°C. A linear slope analysis of cumulative errors within cold sessions indicated that, relative to saline, the higher doses of tyrosine also significantly reduced errors, but did not reduce these errors to levels obtained during exposure to 22°C. It appears that supplemental tyrosine was effective in partially ameliorating the effects of cold stress on DMTS performance, possibly by preventing a cold-stress-induced reduction in brain catecholamine levels.