Physiological release of striatal acetylcholine in vivo: modulation by D1 and D2 dopamine receptor subtypes.

Our experiments assessed the modulation of striatal acetylcholine (ACh) output by dopamine (DA) receptor subtypes under physiological conditions using in vivo microdialysis in awake rats. The degree to which the dopaminergic modulation of striatal cholinergic neurons might vary as a function of local extracellular ACh level also was examined by application of varying concentrations of the acetylcholinesterase (AChE), inhibitor neostigmine (NEO) in the microdialysis perfusate. Under physiological conditions (O NEO), the amount of ACh in the dialysates was 25.1 +/- 2.2 fmol/20-microliters sample (n = 20) whereas values of 67.9 +/- 3.5 (n = 35) and 527.7 +/- 56.1 (n = 13) fmol/20-microliters sample were obtained when the applied NEO concentration was 10 and 100 nM, respectively. In the absence of NEO, a low dose of the indirect DA agonist amphetamine (AMPH; 2 mg/kg i.p.) failed to affect striatal ACh output; a higher AMPH dose (10 mg/kg i.p.) significantly decreased the amount of ACh in dialysates. Under physiological conditions, the direct D2-selective agonist quinpirole (3 mg/kg i.p.) decreased extracellular ACh in striatum to nondetectable levels and the direct D1-selective agonist SKF-38393 (10 mg/kg i.p.) produced a significant increase in this measure. Analysis of the changes in striatal ACh output produced by administration of these DA compounds in the absence vs. presence of local NEO revealed that 10 nM NEO did not qualitatively alter the pharmacological responsivity of this system as compared to the physiological condition. However, in the presence of 100 nM NEO, 2 mg/kg AMPH elicited a significant increase in striatal ACh output. At the 100 nM NEO concentration it also was observed that the amplitude of the quinpirole-induced inhibition of ACh efflux did not increase further in proportion to basal ACh levels whereas the amplitude of the increase in ACh output produced by SKF-38393 was linearly related to basal ACh levels across all NEO concentrations. Under conditions where cholinergic pharmacological responsivity was minimally affected (10 nM NEO), the D2 receptor antagonist haloperidol (1 mg/kg i.p.) increased striatal ACh output by 50% and the D1 receptor antagonist SCH-23390 (0.5 mg/kg i.p.) decreased this variable by 41%. Under these conditions, the inhibitory action of quinpirole on ACh output could be reversed by subsequent administration of AMPH (5 mg/kg i.p.) and this effect of AMPH could then be blocked by administration of SCH-23390. Thus, under physiological or low NEO (10 nM) conditions a prevalent D2-mediated inhibition as well as an opposing D1-mediated excitation of striatal ACh output can be demonstrated. At a higher NEO concentration (100 nM), regulation of the striatal ACh system by DA receptor subtypes is differentially affected such that the D2-mediated inhibitory influence no longer predominates over the D1-mediated excitatory drive. Caution should be exercised when interpreting ACh efflux data obtained using microdialysis under conditions of AChE inhibition.