Dopamine affects two voltage-dependent K+ currents of identified rat lactotroph cells

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Interaction of opiate peptides with dopamine effects on prolactin secretion and membrane electrical properties in anterior pituitary cells from lactating rats

Abstract Met-enkephalin and beta-endorphin induced a partial reversion of the dopamine inhibition of prolactin release from pituitary cells of lactating rats in primary culture. This effect of opiate peptides was dose-dependent with an EC50 of 40 +/- 8 nM and 45 +/- 7 nM and maximal blockade of dopamine inhibition of 60% and 68% for Met-enkephalin and beta-endorphin, respectively. Naloxone antagonized the effect of Met-enkephalin with an EC50 of 22 +/- 12 nM. Furthermore, this Met-enkephalin effect on dopamine inhibition of prolactin secretion appeared non-competitive since it reduced maximal inhibition without affecting the apparent affinity of dopamine. Finally, it should be noted that the two opiate peptides had no effect on spontaneous prolactin release. In electrophysiological experiments, local ejection of dopamine on tested cells induced an hyperpolarization concomitant with an increase of the membrane conductance. Ejection of Met-enkephalin or beta-endorphin alone did not modify the electrical properties of the cells (resting potential, membrane conductance and excitability). In contrast, both peptides blocked in a reversible manner the dopamine-induced electrical responses. These effects were antagonized by naloxone. However, this interaction of opiatepeptides with dopamine electrical response was not observed on all cells tested. We conclude that the blocking effect of opiates on dopamine-induced hyperpolarization may account, at least in part, for the ability of these peptides to interact with dopamine inhibition of prolactin release.

Somatostatin increases voltage-dependent potassium currents in rat somatotrophs

To study the modulatory effects of somatostatin on membrane K+ currents, whole cell voltage-clamp recordings were performed on identified rat somatotrophs in primary culture. In the presence of Co2+ (2 mM) and tetrodotoxin (1 microM) in the bath solution to block Ca2+ and Na+ inward currents, two types of voltage-activated K+ currents were identified on the basis of their kinetics and pharmacology. First, a delayed rectifier K+ current (IK) had a threshold of -20 mV, did not decay during voltage steps lasting 300 ms, and was markedly attenuated by extracellular application of tetraethylammonium (TEA, 10 mM). Second, a transient outward K+ current (IA) was activated at -40 mV (from a holding potential of -80 mV) and persisted despite the presence of TEA. This IA was blocked by 4-aminopyridine (2 mM). Somatostatin (10 nM) increased IK by 75% and IA by 45% without obvious effects on steady-state voltage dependency of activation or inactivation, and these effects were reversible. This increase in K+ currents may contribute in part to the inhibitory effect of somatostatin on growth hormone release.