Inhibition by endomorphin-1 and endomorphin-2 of excitatory transmission in adult rat substantia gelatinosa neurons

Abstract

Intrathecally-administered endomorphin-1 and endomorphin-2 produce antinociceptive effects which are different from each other. In order to elucidate a cellular basis for this result, we examined the effects of endomorphin-1 and endomorphin-2 on holding currents and spontaneous glutamatergic excitatory transmission in substantia gelatinosa neurons of adult rat spinal cord slices by use of the whole-cell patch-clamp technique. In about half of the neurons examined, endomorphin-1 and endomorphin-2 produced an outward current having a similar amplitude (25–27 pA at 1 μM) at −70 mV with almost the same value of effective concentration producing half-maximal response (0.19–0.21 μM). Both of them reversed at a potential close to the equilibrium potential for K +, and had the slope conductance that was larger at negative (−120 to −140 mV) than positive potentials (−60 to −90 mV). The endomorphin-1 and endomorphin-2 currents were reduced in amplitude by K +-channel inhibitors, Ba 2+ (100 μM) and 4-aminopyridine (1 mM), and also by μ-opioid receptor antagonist d-Phe-Cys-Tyr- d-Trp-Arg-Thr-Pen-Thr-NH 2 (1 μM) to a similar extent. The endomorphin-2 but not endomorphin-1 current amplitude was increased by dipeptidyl peptidase IV inhibitor diprotin A (30 μM). One micromolar endomorphin-1 and endomorphin-2 reduced the frequency of spontaneous excitatory postsynaptic current with a similar time course and extent without altering its amplitude; these actions were not in the presence of d-Phe-Cys-Tyr- d-Trp-Arg-Thr-Pen-Thr-NH 2 (1 μM). We conclude that endomorphin-1 and endomorphin-2 hyperpolarize membranes by opening inwardly-rectifying K + channels and attenuate the spontaneous release of l-glutamate from nerve terminals in the substantia gelatinosa, both of which are mediated by μ-opioid receptors, in a manner quantitatively similar to each other. The difference in antinociceptive effects between endomorphin-1 and endomorphin-2 could not be attributed to a distinction in their effects on excitatory transmission in substantia gelatinosa neurons, and may be explained by a difference in their enzymatic degradation.