Dopamine D 4 Receptor Mediated Inhibition of Potassium Current in Neurohypophysial Nerve Terminals 1

Dopamine influences the release of neurohypophysial peptides in vivo. However, the extent to which this effect is caused by a direct dopaminergic action within the neurohypophysis remains unclear. With use of the patch-clamp technique on thin slices of rat posterior pituitary glands, we now provide evidence that dopaminergic agonists inhibit potassium current (IK) in neurohypophysial nerve terminals. Superfusion with the dopamine receptor agonist, (6)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin (PPHT), causes a reversible inhibition of wholeterminal IK under voltage clamp. This effect is concentrationdependent, with a maximal inhibition of 40 6 5% and an EC50 of 1.8 6 1.0 mM. It can be blocked with either a nonselective D2-like antagonist (100 mM eticlopride) or with the highly selective D4 antagonist, RBI-257 (10 mM). U101958 (a derivative of RBI-257) exhibits agonist activity similar to PPHT. Neither SKF 38393 (a D1/D5 agonist) nor quinpirole (a D2/D3 agonist) had any effect on whole-terminal IK in this preparation. Kinetic analysis demonstrated that the amplitude of both the rapidly and slowly inactivating phases of neurohypophysial IK are reduced by D4 receptor activation. These two separate current components have previously been shown to represent current through two distinct potassium channels, an A-current channel and a highconductance Ca-activated K channel. Thus, both channel types can be modulated by D4 receptors. This effect is likely to enhance the release of neurohypophysial peptides in vivo. Antidiuretic hormone and OXT are the main hormonal secretory products of the neurohypophysis. ADH plays an integral role in the maintenance of fluid homeostasis and vascular tone; OXT is a neurohormonal mediator of various reproductive functions (Martin, 1986). It has long been known that dopamine modulates the release of these neuropeptides in vivo (Bridges et al., 1976; Moos and Richard, 1982; Melis et al., 1990). It is unclear, however, whether dopaminergic agents act directly on the neurohypophysis, or if they exert their action on neurons located more proximally within the integrative circuitry of the hypothalamus (Crowley et al., 1991). Direct electrophysiological study of the neurohypophysis should help clarify this issue. Animal studies suggested that the stimulatory effect of apomorphine on neurohypophysial peptide release was mediated through a D2 dopamine receptor (Amico et al., 1992). Although conflicting reports have since implicated other receptor subtypes, much of the available evidence still supports the initial claim that this effect is transduced through a dopaminergic receptor which has “D2-like” pharmacology (Parker and Crowley, 1992; Uvnas-Moberg et al., 1995). Recently, molecular cloning has revealed that the D2-like family of dopamine receptors consists of at least three distinct membrane proteins: D2, D3 and D4 (Gingrich and Caron, 1993). Great effort currently is being made in the development of selective ligands for each of these receptor subtypes. Although several highly selective D4 antagonists are now commercially available, no D4 agonist has yet been described (Strange, 1994; Kebabian, et al., 1997). Dopamine has been shown to modulate potassium channel function in a variety of neuronal preparations (Stack and Surprenant, 1991; Liu et al., 1996). In the past, the small size and inaccessibility of peptidergic nerve terminals has impeded progress in understanding the membrane events governing neurohypophysial hormone secretion. To overcome this problem, several approaches have been developed for making intracellular recordings in the posterior pituitary (Lemos and Nowycky, 1989; Bourque, 1990; Jackson et al., 1991). In this study we have used patch-clamp techniques to investigate the effect of dopamine receptor activation on IK in voltage-clamped nerve terminals in neurohypophysial slices. Received for publication May 13, 1997. 1 Support for this research was provided by National Institutes of Health grant NS30016. 2 Trainee in the Clinical Investigator Pathway and a postdoctoral fellow in the Department of Physiology at the University of Wisconsin. ABBREVIATIONS: aCSF, artificial cerebrospinal fluid; ADH, antidiuretic hormone; IK, potassium current; OXT, oxytocin; PPHT, (6)-2-(Nphenylethyl-N-propyl)-amino-5-hydroxytetralin. 0022-3565/98/2842-0542$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 284, No. 2 Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 284:542–548, 1998 542 at A PE T Jornals on M ay 9, 2017 jpet.asjournals. org D ow nladed from