Evidence for multiple dopamine receptors involved in the modulation of acetylcholine release in the striatum

The involvement of heterogeneous dopamine (DA) receptors in the regulation of striatal cholinergic neurotransmission was investigated by measuring the release of acetylcholine (ACh) from isolated striatum of the rat. The DA agonists apomorphine (3.7 X 10(-7)-7.4 X 10(-4) mol/l) and piribedil (3.3 X 10(-6)-9.9 X 10(-5) mol/l) exerted a biphasic action: in low concentrations they enhanced, while in high concentrations they inhibited, the ouabain-induced release of ACh. The enhancement of ACh release elicited by low concentrations of DA agonists was reversed by a noncataleptogenic dose of pimozide. 6-hydroxydopamine (6-OHDA) pretreatment also prevented the stimulatory action of apomorphine or piribedil on ACh release. In 6-OHDA pretreated striatum, apomorphine caused a biphasic inhibition of ACh release in a concentration dependent manner. DPI (3,4-dihydroxyphenylamino/-2-imidazoline, 2.5 X 10(-5)-2.5 X 10(-4) mol/l) increased the release of ACh in a monophasic manner and did not induce inhibition of transmitter output in 6-OHDA pretreated striatum. Pretreatment of rats with increasing doses of haloperidol or pimozide (0.037-15 mg/kg) exerted a biphasic action on striatal ACh release; low doses decreased and high doses enhanced the transmitter output and the elevation was accompanied by the appearance of catalepsy. From these experiments it is concluded that multiple DA receptors are involved in the regulation of striatal cholinergic neurotransmission. Among these, the presynaptic DA autoreceptors located on nigrostriatal nerve endings are the most sensitive to both agonists and antagonists. DA receptors located on cholinergic interneurons of the striatum, which are postsynaptic in relation to nigrostriatal neurons, possess high and low affinity to apomorphine. Stimulation of DA autoreceptors enhances, while that of postsynaptic DA receptors leads to inhibition of ACh release.