Dopamine neuron membrane physiology: Characterization of the transient outward current (I A ) and demonstration of a common signal transduction pathway for I A and I K

Dopamine neurons derived from the mesencephalon of embryonic rats were maintained in primary culture, identified and studied with whole‐cell patch recording techniques. These neurons demonstrated a rapidly activating and inactivating voltage‐dependent outward current which required the presence of K + ions. This current was termed I A because of its transient nature. It was elicited by step depolarizations from holding potentials more negative than ‐50 mV and exhibited steady‐state inactivation at a membrane potential more positive than ‐40 mV and half‐maximal inactivation observed at ‐65 mV. This current rapidly achieved peak activation in less than 8 msec and decayed with a time constant (τ) of 58±5 msec. This current was observed in the presence of tetraethylammonium but was readily blocked by 4‐aminopyridine (2‐4 mM). This current was also observed to be modulated by stimulation of D 2 dopamine receptors (DA autoreceptors) located on the dopamine neurons. Thus, both DA and the D 2 receptor agonist quinpirole enhanced the peak I A observed, while the partial D 1 receptor agonist SKF 38393 was without effect. The enhancement of I A was confirmed to be due to the activation of D2 receptors as the effects of either DA or quinpirole were blocked by the D 2 receptor antagonists eticlopride and sulpiride, but not by the D 1 receptor antagonist SCH 23390. Since we have previously demonstrated that the I K present: in these cells is also enhanced by D 2 receptor stimulation, we investigated the signal transduction pathways involved in coupling DA autoreceptors to both I A and I K . The response of both these potassium currents to DA autoreceptor stimulation was completely abolished by the preincubation of cultures with pertussis toxin, indicating the possible involvement of the G proteins G i and G O . In an attempt to further characterize which G protein may be involved, additional experiments were performed. The ability of DA autoreceptor stimulation to augment both currents was also blocked completely when G protein activation was prevented by the intracellular application of GDPßS (100 μM). In contrast, irreversible activation of G proteins by intracellular application of the nonhydrolyzable GTP analog GTPγS (100 μM) mimicked the effects of DA autoreceptor stimulation on both I A and I K . In addition, the intracellular application of a polyclonal antibody that was selective for the β‐subunit of G O completely abolished the DA autoreceptor modulation of both currents while preimmune serum was without effect. Taken together, these data demonstrate that the enhancement of I A and I K in response to stimulation of DA autoreceptors is dependent upon the activation of G O and appears to involve a G Oα subunit. © 1994 Wiley‐Liss, Inc.