Repeated cocaine administration causes persistent enhancement of D1 dopamine receptor sensitivity within the rat...

The rewarding effects of cocaine are mediated primarily by the mesoaccumbens dopamine (DA) system, which projects from A10 DA cell bodies within the ventral tegmental area to the nucleus accumbens (NAc). This pathway is also intricately involved in the locomotor stimulating effect of cocaine and the progressive increases (sensitization) in this behavior observed after repeated administration of cocaine and other psychomotor stimulants. By using single-cell electrophysiological recording and microiontophoretic techniques, we demonstrated previously that repeated cocaine administration (10 mg/kg i.p., twice daily, 14 days) renders impulse-regulating somatodendritic A10 DA autoreceptors subsensitive, thereby increasing impulse flow within the mesoaccumbens DA system. In striking contrast, inhibitory responses of NAc neurons to iontophoretic DA were significantly increased in cocaine-treated rats tested 16 to 24 hr after the last cocaine injection. In the present study, iontophoretic application of selective D1 (SKF 38393) and D2 (quinpirole) DA receptor agonists was utilized to determine the extent to which each of these DA receptor subtypes is altered by repeated cocaine administration. After 2 weeks of twice daily cocaine (10 mg/kg i.p.) injections, significant increases in the inhibitory responses of NAc neurons to SKF 38393, but not quinpirole, were observed. In addition, this D1 receptor sensitization was still evident when animals were tested either 7 days or 1 month after the final cocaine injection. After 2 months of withdrawal from cocaine treatment, D1 receptor sensitivity in the NAc had returned to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)