INVOLVEMENT OF THE PRESYNAPTIC DOPAMINE D2 RECEPTOR IN THE DEPRESSIONOF SPINAL REFLEX BY APOMORPHINE

THE relative roles of D1 and D2 dopamine (DA) receptors in mediating a pomorphine (APO)-induced changes in the spinal reflex was investigated . Low doses of APO, a DA receptor agonist (0.2 mg kg(-1), i.v.), depre ssed the monosynaptic mass (MMR) in spinalized rats. Pretreatment with D2-specific antagonist, spiperone, 10 min before APO prevented the AP O-induced MMR depression. Pretreatment with the D1 antagonist SCH 2339 0 failed to prevent the APO-induced depression. Interestingly, SCH 233 90 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 depress ion. These results suggest that inhibition of spinal transmission by l ow dose of APO may be mediated through its action on presynaptic D2 re ceptors and that D1 and D2 receptors are functionally coupled at the s pinal level in modulating the spinal motor output.