Effects of age on behavioral responses to dopamine agonists in the rat

This study served to examine the differential effects of age (rats aged 2 or 12 months) on behavioral responses induced by bromocriptine and apomorphine. Intraperitoneal (i.p.) injection of bromocriptine or apomorphine produced a lower frequency of yawning responses, in 12-month-old rats than in 2-month-old rats. Apomorphine produced a more pronounced stereotyped behavior in 12-month-old rats than in 2-month-old rats. Apomorphine, at 0.1 mg/kg administered after bromocriptine (1.0-20 mg/kg) potentiated yawning behavior. The frequency of yawning in 2-month-old rats was pronounced at 2.5 mg/kg of bromocriptine but only at 5 mg/kg 12-month-old rats. Apomorphine (0.1 mg/kg) did not produce perioral behavior in 2-month-old rats but did in 12-month-old rats. The apomorphine (1.0 mg/kg)-induced stereotypy was stimulated dose dependently by bromocriptine in 2-month-old-rats but not in 12-month-old rats. Bromocriptine did not produce this behavior when administered alone. Pretreatment of 2-month-old rats with reserpine, a catecholamine depletor, plus alpha-methyl-p-tyrosine, a tyrosine hydroxylase inhibitor, inhibited the yawning induced by bromocriptine but potentiated that induced by apomorphine. Such treatment did not significantly alter either bromocriptine or apomorphine-induced yawning responses in 12-month-old rats. The apomorphine-induced stereotypy in 2-month-old rats was markedly potentiated by catecholamine depletion but was not affected in 12-month-old rats. These results suggest that the increasing effect on stereotypy and decreasing effects on yawning in the 12-month-old rats seem to result in an alteration of potency and of the ratio of D-2 versus D-1 receptor activity.