D1/D5 dopamine receptors in basal ganglia, hippocampus, and cerebral cortex modulate motor, reward, and cognitive behavior. Previous work with recombinant proteins revealed that in cells primed with heterologous Gq/11-coupled G-protein-coupled receptor (GPCR) agonists, the typically Gs-linked D1/D5 receptors can stimulate robust release of calcium from internal stores when coexpressed with calcyon. To learn more about the intracellular signaling mechanisms underlying these D1/D5 receptor regulated behaviors, we explored the possibility that endogenous receptors stimulate internal release of calcium in neurons. We have identified a population of neurons in primary cultures of hippocampus and neocortex that respond to D1/D5 dopamine receptor agonists with a marked increase in intracellular calcium (Cai2+) levels. The D1/D5 receptor stimulated responses occurred in the absence of extracellular Ca2+ indicating the rises in Cai2+ involve release from internal stores. In addition, the responses were blocked by D1/D5 receptor antagonists. Further, the D1/D5 agonist-evoked responses were state dependent, requiring priming with agonists of Gq/11-coupled glutamate, serotonin, muscarinic, and adrenergic receptors or with high external K+ solution. In contrast, D1/D5 receptor agonist-evoked Ca2+ responses were not detected in neurons derived from striatum. However, D1/D5 agonists elevated cAMP levels in striatal cultures as effectively as in neocortical and hippocampal cultures. Further, neither forskolin nor 8-Br-cAMP stimulation following priming was able to mimic the D1/D5 agonist-evoked Ca2+ response in neocortical neurons indicating that increased cAMP levels are not sufficient to stimulate Cai2+ release. Our data suggest that D1-like dopamine receptors likely modulate neocortical and hippocampal neuronal excitability and synaptic function via Ca2+ as well as cAMP-dependent signaling.
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