‘Priming’ phenomena in the expression of D1 and D2 receptor supersensitivity

Contralateral turning can be evoked in rats with unilateral lesion of the nigro-striatal dopaminergic projection by the administra, tion of drugs which stimulate DA transmission. Two types of DA-receptors are currently distinguished (Garau et al., 1978; Kebabian and Calne, 1979); the D-1 type, stimulated preferentially by SKF 38393 and blocked by SCH 23390 and the D-2 type, stimulated, among other drugs, by LY 171555 and specifically blocked by various substituted benzamides. In unilaterally 6-hydroxydopamine (6-OHDA)-denervated rats, supersensitivity appears to develop specifically for each DA-receptor type both biochemically and behaviourally (Creese et al., 1977; Arnt and Hyttel, 1985). We have recently found that previous stimulation of D-2 receptors exerts a ‘priming’ effect on the ability of D-1 receptor stimulation to induce contraversive turning in the unilaterally DA-denervated rat (Morelli et al., 1987), indicating that the behavioural expression of D-1 receptor supersensitivity depends on previous stimulation of D-2 receptors. We now present evidence that the D-1 and D-2 receptor stimulation influence the expression of D-1 and D-2 dependent behavioural supersensitivit