Postsynaptic dopamine/adenosine interaction: I. Adenosine analogues inhibit dopamine D2-mediated behaviour in short-term reserpinized mice.

Mice pretreated with reserpine 5 mg/kg (4 h prior to the start of motor activity recording) showed locomotor activation after the administration of the D-2 agonist bromocriptine (5 mg/kg). This bromocriptine-induced locomotor activity was dose dependently inhibited by the co-administration of a D-2 antagonist (sulpiride) and dose dependently potentiated by a D-1 agonist (CY 208-243). The potentiating effect of the D-1 agonist could be inhibited by either a D-1 or a D-2 antagonist (SCH 23390 1 mg/kg or sulpiride 100 mg/kg, respectively). The bromocriptine-induced locomotor activity was not altered by either blockade of D-1 dopaminergic receptors (SCH 23390 1 mg/kg) or by co-administration of a greater dose of reserpine (10 mg/kg) plus the dopamine synthesis inhibitor, alpha-methyl-p-tyrosine (200 mg/kg). The adenosine agonists, L-PIA (a preferentially A-1 adenosine agonist) and NECA (an A-1 and A-2 adenosine agonist with above 10-fold greater affinity for A-2 than L-PIA) inhibited in a dose-dependent manner the effect of bromocriptine, NECA being above ten times more potent than L-PIA. The findings show that bromocriptine stimulates postsynaptic D-2 receptors in dopamine-depleted mice and that this effect can be inhibited by adenosine stimulation. The existence of a postsynaptic D-2/A-2 interaction is suggested, the stimulation of A-2 receptors causing an inhibition of responses elicited by postsynaptic D-2 stimulation.