Effects of apomorphine on behavioural activity and brain catecholamine synthesis in normal and L-triiodothyronine-treated rats.

The effect of chronic apomorphine treatment on behavioural activity as well as brain dopamine metabolism was studied in normal and neonatally L-triiodothyronine-treated rats. Neonatal hyperthyroidism was accompanied by an increase in spontaneous locomotor activity as well as by enhanced synthesis and release of dopamine as evidenced by increased catecholamine synthesis in crude synaptosomal preparation (P2 pellet), elevated tyrosine hydroxylase activity and higher concentrations of homovanillic acid and 3, 4-dihydroxyphenylacetic acid in striatum of rats. Repeated apomorphine treatment (1 mg/kg/day s.c) for 15 days, beginning from the 15th day of age, produced hypermobility and stereotyped behavior (consisting of sniffing, gnawing, rearing) which appeared to be more pronounced in neonatally hyperthyroid rats than in normal controls. In addition, apomorphine-treated hyperthyroid animals marched in a row with straub tail, and displayed increased aggressiveness and bizarre social behavior consisting of "mock fighting" when left in pairs. In contrast to normal rats, apomorphine-treated hyperthyroid animals displayed marked hyperactivity which was evident even at 24 hours after the last injection of apomorphine. Administration of apomorphine resulted in significant decreases in striatal tyrosine hydroxylase, catecholamine synthesis in crude synaptosomal preparation (P2 pellet) as well as dopamine metabolite levels in brains of both normal and hyperthyroid animals. Our present data showing that apomorphine potentiates behavioural activity in hyperthyroid rats suggest that L-triiodothyronine and apomorphine probably share certain features common to activating dopaminergic neurons in the brain.