Regulation of dopamine release in vitro from the posterior pituitary by opioid peptides

Opioid peptides are present in both the posterior pituitary (PP) and stalk-median eminence (SME). Their effects on the dopaminergic neurons in the SME are well documented, but little is known concerning their role in the regulation of dopamine (DA) release from the PP. The objectives of this study were (1) to develop an in vitro method suitable for examining the regulation of endogenous DA release from PP and SME, and (2) to describe and compare the effects of selected opioid peptides on potassium-evoked DA release from these tissues. Tissues were dissected from ovariectomized rats and incubated in media. After equilibration, two pulses of 28 mM potassium (K+), 3 min each, were delivered 30 min apart. Test substances were administered 20 min before the second K+ stimulus. DA in the media was determined by high-performance liquid chromatography. Potassium at 28 and 56 mM elicited a marked increase in DA release from the PP and SME; this was abolished by the removal of calcium. The opioid receptor antagonist, naloxone, significantly increased the release of DA from both PP and SME by 55%. Dynorphin A elicited a significant inhibition of DA release from PP and SME by 33 and 50%, respectively. In contrast, methionine enkephalinamide decreased DA release from the SME by 50%, but was without effect in the PP. The release of DA from both PP and SME was significantly inhibited by beta-endorphin, and this was reversed by naloxone. However, beta-endorphin was fourfold more effective in the SME. N-acetyl-beta-endorphin did not alter DA release.

(1) we have developed a simple and sensitive in vitro method for studying the effects of hormones and drugs on the release of endogenous DA from PP and SME; (2) tuberoinfundibular dopaminergic and tuberohypophyseal dopaminergic nerve terminals are subjected to a similar inhibitory control by endogenous opioid peptides, and (3) exogenously applied opioid peptides exert differential effects on the release of DA from SME and PP which could be attributable to a dissimilar distribution of opioid receptor subtypes in these two tissues.