Tyrosine pretreatment reverses hypothermia-induced behavioral depression.

Cold exposure accelerates the firing frequency of norepinephrine (NE) neurons, enhancing NE release and leading to NE depletion in specific regions of the brain. The accelerated firing activates the enzyme tyrosine-hydroxylase, making it more tyrosine sensitive. The reduction of brain NE is accompanied by a behavioral depression on the open field test. Two experiments were performed on adult male rats. First, it was determined whether systematic lowering of core body temperature produced behavioral depression in the swim test. Second, treatment with the NE precursor tyrosine was employed in an attempt to prevent hypothermia-induced behavioral depression. In Experiment 1, two levels of hypothermia were highly effective in producing behavioral depression in rats forced to swim in a narrow cylinder. In Experiment 2, treatment with tyrosine (400 mg/kg, IP) thirty minutes prior to the hypothermia procedure completely reversed the behavioral depression found in Experiment 1. Tyrosine administration did not significantly influence the rate of deep body cooling during the hypothermia treatment.