Dopamine Receptor-mediated Ca2+ Signaling in Striatal Medium Spiny Neurons

Abstract

Inositol 1,4,5-trisphosphate (InsP(3)) and cAMP are the two second messengers that play an important role in neuronal signaling. Here, we investigated the interactions of InsP(3)- and cAMP-mediated signaling pathways activated by dopamine in striatal medium spiny neurons (MSN). We found that in approximately 40% of the MSN, application of dopamine elicited robust repetitive Ca(2+) transients (oscillations). In pharmacological experiments with specific agonists and antagonists, we found that the observed Ca(2+) oscillations were triggered by activation of D1 class dopamine receptors (DARs). We further demonstrated that activation of phospholipase C was required for induction of dopamine-induced Ca(2+) oscillations and that maintenance of dopamine-evoked Ca(2+) oscillations required both Ca(2+) influx and Ca(2+) mobilization from internal Ca(2+) stores. In "priming" experiments with a type 2 5-hydroxytryptamine receptor agonist, we have shown a likely role for calcyon in coupling D1 class DARs with Ca(2+) oscillations in MSN. In experiments with the DAR-specific agonist SKF83959, we discovered that phospholipase C activation alone could not account for dopamine-induced Ca(2+) oscillations. We further demonstrated that direct activation of protein kinase A by 8-bromo-cAMP or inhibition of protein phosphatase-1 (PP1) or calcineurin (PP2B) resulted in elevation of basal Ca(2+) levels in MSN, but not in Ca(2+) oscillations. In experiments with competitive peptides, we have shown an importance of type 1 InsP(3) receptor association with PP1alpha and with AKAP9.protein kinase A for dopamine-induced Ca(2+) oscillations. In experiments with MSN from DARPP-32 knock-out mice, we demonstrated a regulatory role of DARPP-32 in dopamine-induced Ca(2+) oscillations. Our results indicate that, following D1 class DAR activation, InsP(3) and cAMP signaling pathways converge on the type 1 InsP(3) receptor, resulting in Ca(2+) oscillations in MSN.