Depression of the monosynaptic reflex by apomorphine or bromocriptine is not mediated by D1/D2 receptors.

The role of dopamine in spinal motor transmission was investigated using spinal reflexes in acutely spinalized rats. Intravenous administration of a relatively high dose of the dopamine receptor agonist apomorphine-HCl (3 mg/kg) or the D2 receptor agonist bromocriptine mesylate (1 mg/kg) reduced the amplitude of the monosynaptic reflex (MSR). Depression of the MSR by both drugs was antagonized by haloperidol (1 mg/kg), but not by the D2 receptor antagonists YM-09151-2 (0.2 mg/kg) and sulpiride (10 mg/kg), or by a combination of the D1 receptor antagonist SKF 83566 (0.01 mg/kg) and sulpiride (10 mg/kg). Intravenous administration of the selective D1 receptor agonist SKF 77434 (0.1 and 1 mg/kg) and the D2/D3 receptor agonist quinpirole-HCl (0.1 and 1 mg/kg) had no significant effect on the MSR. Simultaneous administration of SKF 77434 and quinpirole had no significant effect on the MSR. These results show that stimulation of D1/D2 receptors has little influence on the MSR, and suggest that descending dopaminergic systems mediating these receptors have little influence on MSR transmission. Apomorphine and bromocriptine may inhibit the MSR via other subtypes of D1/D2 or other, as yet undiscovered, dopamine receptors or via non-dopaminergic mechanisms.