Reversal of supersensitive apomorphine-induced rotational behavior in mice by continuous exposure to apomorphine

Mice with unilateral lesions of the nigrostriatal dopaminergic neurons, induced by an intrastriatal injection of 6-hydroxydopamine, respond to an acute injection of apomorphine by rotating in a direction contralateral to the lesion. The dose of apomorphine that produced a half-maximal turning response was 0.4 mumol/kg s.c.; maximal turning responses were seen at 2 mumol/kg s.c. This behavioral supersensitivity may be due to an increased density of dopaminergic receptors. To determine whether continuous stimulation of these receptors with a dopaminergic agonist would reverse the dopaminergic behavioral supersensitivity, mice were exposed to continuous infusion of apomorphine via a s.c. implanted osmotic minipump containing 150 mumol of apomorphine in 0.2 ml and then challenged with an acute injection of apomorphine (2 mumol/kg s.c.) at varying times after the implant. Initially, the mice displayed spontaneous rotational behavior beginning by 1 hr after implantation, but this spontaneous behavior diminished over time and was absent by 4 days after implantation. The turning response of mice challenged with an acute injection of apomorphine was significantly reduced at 1 day after the chronic implantation and was totally absent at 2 and 4 days after implantation. This effect of continuous exposure to apomorphine was found to be concentration- and time-dependent; mice implanted with pumps containing 75 mumol of apomorphine required 8 days of exposure for tolerance to develop. Upon removal of the apomorphine-containing implants, the supersensitive rotational behavior returned over a 7-day period, with 50% recovery occurring at 2 days.(ABSTRACT TRUNCATED AT 250 WORDS)