Kappa Opioid Receptor Inhibition of Glutamatergic Transmission in the Nucleus Accumbens Shell

Microinjection of kappa-opioid receptor agonists into the nucleus accumbens produces conditioned place aversion. While attention has focused primarily on the inhibition of dopamine release by kappa-receptor agonists as the synaptic mechanism underlying this effect, recent anatomical studies have raised the possibility that regulation of noncatecholaminergic transmission also contribute. We have investigated the effects of kappa-receptor activation on fast excitatory synaptic transmission in an in vitro slice preparation using whole cell voltage-clamp or extracellular field recordings in the shell region of the nucleus accumbens. The kappa-receptor agonist U69593 produces a pronounced, dose-dependent inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) that can be reversed by 100 nM nor-BNI. Furthermore, U69593 causes an increase in the paired-pulse ratio as well as a decrease in the frequency of spontaneous miniature events, suggesting a presynaptic site of action. Despite anatomical evidence for kappa-receptor localization on dendritic spines of nucleus accumbens neurons, no electrophysiological evidence of a postsynaptic effect was found. This presynaptic inhibition of excitatory synaptic transmission in the nucleus accumbens shell provides a novel mechanism that may contribute to the kappa-receptor-mediated aversion observed in intact animals.