Dopamine D2 agonist-induced behavioural depression is reversed by dopamine D1 agonists.

The dopamine (DA) D2 agonist bromocriptine produced dose-dependent locomotor depression in mice with intact stores of DA, as measured in automated activity cages. The DA D1 agonist CY208-243, reversed the bromocriptine-induced depression. Using direct observational analysis, another selective DA D2 agonist, quinpirole, induced dose-dependent depression and this was reversed by the D1 agonist SKF38393. The effect of SKF38393 could be blocked by prior pretreatment with SCH23390. It is concluded that DA D2 agonist-induced locomotor depression is mediated via a DA D2 autoreceptor-mediated inhibition of DA release onto postsynaptic DA receptors. This reduction in release probably deprives postsynaptic D1 and D2 receptors of endogenous DA. However, since bromocriptine (and probably quinpirole) in all likelihood occupies both pre- and postsynaptic D2 receptors immediately on injection, and since CY208-243 and SKF38393 (respectively) could reverse the depression, the depression seems to be due specifically to a deprivation of DA at postsynaptic D1 receptors.