Differential effects of dopamine agonists in mature and immature rats

It has been suggested that because of a relatively slow maturation of the central cholinergic system, that the neonatal rat (i.e. less than 2 weeks old) may be used as an in vivo model, with minimal cholinergic influence, for studying the effects of dopaminergic compounds. d-Amphetamine, 1-DOPA, and SK and F 38393-A produce a syndrome in neonatal rat similar to the stereotyped behavior characteristically produced by dopamine agonists. This neonatal syndrome includes increased motor activity and sniffing plus licking and/or biting. Apomorphine and bromocryptine do not produce this behavior in neonatal rats. The potencies of d-amphetamine, 1-DOPA, and SK and F 38393-A in producing this syndrome decrease with the age of the rat. Increases in motor activity, or sniffing plus licking and/or biting by d-amphetamine 1-DOPA, or SK and F 38393-A, in thirty and/or sixty day old rats are significantly enhanced by scopolamine pretreatment. Apomorphine- or bromocryptine-induced stereotyped behavior in thirty and sixty day old rats is not affected by scopolamine. These data suggest that the effects of certain dopaminergic agonists, which produce the neonatal syndrome, are modulated in the adult rat by cholinergic activity. The ineffectiveness of apomorphine in the neonate, however, suggest that the neonatal syndrome may not be related exclusively to dopamine, or that the dopaminergic system involved with this behavior is not sensitive to apomorphine.