Muscarinic receptors acting at pre- and post-synaptic sites differentially regulate dopamine/DARPP-32 signaling in striatonigral and striatopallidal neurons.

Muscarinic receptors, activated by acetylcholine, play critical roles in the functional regulation of medium spiny neurons in the striatum. However, the muscarinic receptor signaling pathways are not fully elucidated due to their complexity. In this study, we investigated the function of muscarinic receptors in the striatum by monitoring DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of M(r) 32 kDa) phosphorylation at Thr34 (the PKA-site) using mouse striatal slices. Treatment of slices with a non-selective muscarinic receptor agonist, oxotremorine (10 µM), rapidly and transiently increased DARPP-32 phosphorylation. The increase in DARPP-32 phosphorylation was completely abolished either by a dopamine D(1) receptor antagonist (SCH23390), tetrodotoxin, genetic deletion of M5 receptors, muscarinic toxins for M1 and M4 receptors, or 6-hydroxydopamine lesioning of dopaminergic neurons, whereas it was enhanced by nicotine. Analysis in D(1)-DARPP-32-Flag/D(2)-DARPP-32-Myc transgenic mice revealed that oxotremorine increases DARPP-32 phosphorylation selectively in D(1)-type/striatonigral, but not in D(2)-type/striatopallidal, neurons. When D(1) and D(2) receptors were blocked by selective antagonists to exclude the effects of released dopamine, oxotremorine increased DARPP-32 Thr34 phosphorylation only in D(2)-type/striatopallidal neurons. This increase required activation of M1 receptors and was dependent upon adenosine A(2A) receptor activity. The results demonstrate that muscarinic receptors, especially M5 receptors, act at presynaptic dopaminergic terminals, regulate the release of dopamine in cooperation with nicotinic receptors, and activate D(1) receptor/DARPP-32 signaling in the striatonigral neurons. Muscarinic M1 receptors expressed in striatopallidal neurons interact with adenosine A(2A) receptors and activate DARPP-32 signaling.

Role of adrenoceptors in the regulation of dopamine/DARPP-32 signaling in neostriatal neurons.

Studies in animal models of Parkinson's disease have revealed that degeneration of noradrenaline neurons is involved in the motor deficits. Several types of adrenoceptors are highly expressed in neostriatal neurons. However, the selective actions of these receptors on striatal signaling pathways have not been characterized. In this study, we investigated the role of adrenoceptors in the regulation of dopamine/dopamine- and cAMP-regulated phosphoprotein of M(r) 32 kDa (DARPP-32) signaling by analyzing DARPP-32 phosphorylation at Thr34 [protein kinase A (PKA)-site] in mouse neostriatal slices. Activation of beta(1)-adrenoceptors induced a rapid and transient increase in DARPP-32 phosphorylation. Activation of alpha(2)-adrenoceptors also induced a rapid and transient increase in DARPP-32 phosphorylation, which subsequently decreased below basal levels. In addition, activation of alpha(2)-adrenoceptors attenuated, and blockade of alpha(2)-adrenoceptors enhanced dopamine D(1) and adenosine A(2A) receptor/DARPP-32 signaling. Chemical lesioning of noradrenergic neurons mimicked the effects of alpha(2)-adrenoceptor blockade. Under conditions of alpha(2)-adrenoceptor blockade, the dopamine D(2) receptor-induced decrease in DARPP-32 phosphorylation was attenuated. Our data demonstrate that beta(1)- and alpha(2)-adrenoceptors regulate DARPP-32 phosphorylation in neostriatal neurons. G(i) activation by alpha(2)-adrenoceptors antagonizes G(s)/PKA signaling mediated by D(1) and A(2A) receptors in striatonigral and striatopallidal neurons, respectively, and thereby enhances D(2) receptor/G(i) signaling in striatopallidal neurons. alpha(2)-Adrenoceptors may therefore be a therapeutic target for the treatment of Parkinson's disease.