TYROSINE ENHANCES BEHAVIORAL AND MESOCORTICOLIMBIC DOPAMINERGIC RESPONSES TO AVERSIVE-CONDITIONING

Tyrosine is a precursor in the biosynthesis of catecholamines and, whe n administered systemically, has been shown to enhance the in vivo rat e of tyrosine hydroxylation in the medial prefrontal cortex. Additiona lly, exogenous tyrosine has been demonstrated to enhance the pharmacol ogically-induced increase in dopamine metabolism seen following admini stration of haloperidol or the anxiogenic B-carboline, FG-7142. In thi s report, we examine the effect of a physiologically relevant dose of tyrosine (25 mg/kg) on biochemical and behavioral consequences of aver sive conditioning. Rats were conditioned to fear a tone by pairing it with footshock, so that when challenged with the tone alone, rats resp onded with immobility, defecation, and elevated dopamine metabolism in the medial prefrontal cortex and nucleus accumbens. When tyrosine was administered on the test day (tones alone), the rats displayed an eve n greater elevation of dopamine metabolism in the nucleus accumbens an d prolonged immobility to the tone, compared to the saline/conditioned controls. Tyrosine did not alter mobility or dopamine utilization in the nucleus accumbens in nonconditioned controls. However, dopamine me tabolism in the medial prefrontal cortex of nonconditioned rats treate d with tyrosine was increased to levels similar to those in the condit ioned groups. This may be accounted for by handling and by exposure to an unfamiliar environment necessary for nonconditioned controls. We c onclude that exogenous tyrosine is able to 1) elevate stress-induced d opamine metabolism in the nucleus accumbens, 2) alter dopamine utiliza tion in the medial prefrontal cortex of handled, nonconditioned contro ls, and 3) enhance fear-induced immobilization. These data suggest a r ole for dietary tyrosine in biochemical and behavioral responses to av ersive stimuli. (C) 1996 Wiley-Liss, Inc.