Involvement of the presynaptic dopamine D2 receptor in the depression of spinal reflex by apomorphine

The relative roles of D1 and D2 dopamine (DA) receptors in mediating apomorphine (APO)-induced changes in the spinal reflex was investigated. Low doses of APO, a DA receptor agonist (0.2 mg kg-1, i.v.), depressed the monosynaptic mass reflex (MMR) in spinalized rats. Pretreatment with the D2-specific antagonist, spiperone, 10 min before APO prevented the APO-induced MMR depression. Pretreatment with the D1 antagonist SCH 23390 failed to prevent the APO-induced depression. Interestingly, SCH 23390 pretreatment preferentially antagonized the depression induced by a high dose of APO (3 mg kg-1, i.v.). Pretreatment with SKF 38393, a selective D1 agonist, completely prevented the APO-induced MMR depression. These results suggest that inhibition of spinal transmission by low dose of APO may be mediated through its action on presynaptic D2 receptors and that D1 and D2 receptors are functionally coupled at the spinal level in modulating the spinal motor output.