Environmental context and drug history modulate amphetamine-induced c- fos mrna expression in the basal ganglia, central extended amygdala, and associated limbic forebrain

Abstract

The context in which amphetamine is administered modulates its ability to induce both behavioral sensitization and immediate early gene expression. When given in a novel test environment amphetamine produces greater levels of c- fos and arc mRNA expression in many brain regions relative to when it is given in the home cage. The purpose of the current study was to determine if environment and drug history interact to influence amphetamine-induced c- fos mRNA expression. Rats with a unilateral 6-hydroxydopamine lesion were treated for 7 days with saline or 0.5 mg/kg of d-amphetamine (i.v.) in a distinct and relatively novel test environment (Novel), or in their home cage (Home). Following a 10–12-day withdrawal period, a challenge injection of either saline or 0.5 mg/kg d-amphetamine was administered. In situ hybridization histochemistry was used to examine c- fos mRNA expression in several regions of the basal ganglia, the central extended amygdala, and limbic forebrain. In most brain regions amphetamine given in the Novel environment produced greater c- fos mRNA expression than when given it was given at Home, and drug history had no effect on amphetamine-induced c- fos mRNA expression. However, within the subthalamic nucleus, substantia nigra reticulata, and central nucleus of the amygdala prior experience with amphetamine in the Novel but not Home environment enhanced the effect of an amphetamine challenge injection on c- fos mRNA expression. In contrast, there was a decrease in c- fos mRNA expression in amphetamine-pretreated animals, regardless of environmental context, in the ventral portion of the far caudal striatum. Reexposure to an environment previously paired with amphetamine produced a conditioned increase in c- fos mRNA expression in portions of the caudate-putamen, the subthalamic nucleus, the nucleus accumbens shell and a conditioned decrease in c- fos mRNA expression in the central nucleus of the amygdala. We conclude that environmental context and drug history interact to alter the basal ganglia and central extended amygdala circuitry engaged by subsequent exposure to amphetamine, or exposure to an environment previously paired with amphetamine.