Met-enkephalin release from slices of the rat striatum and globus pallidus: stimulation by excitatory amino acids.

The present study evaluated the effects of high K+ and four excitatory amino acids (EAAs) on the release of met-enkephalin-like immunoreactivity (ME-i.r.) from slices of the rat striatum and globus pallidus. High K+ (15-50 mM) increased the release of ME-i.r. in a concentration-dependent manner in both regions, the release response in the globus pallidus being consistently greater than in the striatum. This release was highly Ca(++)-dependent and was significantly enhanced in the absence of external Mg++. D-2-Amino-7-phosphonoheptanoic acid (0.5 mM), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, did not alter this enhanced action of K+, suggesting that the activation of NMDA receptors by an endogenous agonist did not contribute to the enhancement. Exposure of pallidal or striatal slices to four EAA receptor agonists, NMDA, L-glutamate, kainate (KA) and quisqualate, increased the release of ME-i.r. above the base line, an effect that was Ca(++)-dependent. Both L-glutamate and NMDA, at concentrations of 1 and 5 mM, produced a graded increase in the ME-i.r. release, but a higher concentration (10 mM) produced a lower release. In both regions the NMDA (5 mM)-evoked release was effectively inhibited by Mg++ (1.2 mM), 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) (5 microM), a competitive NMDA receptor antagonist and thienylcyclohexylpiperidine (10 microM), a noncompetitive NMDA receptor antagonist. Tetrodotoxin (0.3 microM), a Na+ channel blocker, did not affect the NMDA-evoked release of ME-i.r. in the striatum, but decreased it by 52% in the globus pallidus.(ABSTRACT TRUNCATED AT 250 WORDS)