Activation of Human D3 Dopamine Receptor Inhibits P/Q-type Calcium Channels and Secretory Activity in AtT-20 Cells

The D3 dopamine receptor is postulated to play an important role in the regulation of neurotransmitter secretion at both pre- and postsynaptic terminals. However, this hypothesis and the underlying mechanisms remain untested because of the lack of D3-selective ligands, paucity of appropriate model secretory systems, and the weak and inconsistent coupling of D3 receptors to classical signal transduction pathways. The absence of ligands that selectively discriminate between D3 and D2 receptors in vivo precludes the study of D3 receptor function in the brain and necessitates the use of heterologous expression systems. In this report we demonstrate that activation of the human D3 dopamine receptor expressed in the AtT-20 neuroendocrine cell line causes robust inhibition of P/Q-type calcium channels via pertussis toxin-sensitive G-proteins. In addition, using the vesicle trafficking dye FM1-43, we show that D3 receptor activation significantly inhibits spontaneous secretory activity in these cells. Our results not only support the hypothesis that the D3 receptor can regulate secretory activity but also provide insight into the underlying signaling mechanisms. We propose a functional model in which the D3 receptor tightly regulates neurotransmitter release at a synapse by only allowing the propagation of spikes above a certain frequency or burst-duration threshold.