Synapse
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Chronoamperometry was used in combination with monoamine-selective electrodes to monitor, in nucleus accumbens (NAcc) and prefrontal cortex (PFC) of freely behaving rats, changes in dopamine (DA)-like electrochemical signals elicited by unilateral ventral tegmental microinjections of the selective mu-opioid receptor agonist D-Ala, N-Me-Phe-Gly-Ol-Enkephalin (DAMGO; 0.01, 0.1, and 1.0 nmol). The results show that DAMGO dose-dependently increased electrochemical signals both in Nacc and PFC within a few minutes of injection. While DAMGO elicited signal increases of comparable amplitudes in both regions, the increases recorded in PFC were significantly longer lasting than those in NAcc; at the highest dose tested (1.0 nmol), DAMGO produced signal increases that lasted (mean +/- sem) 129 +/- 7.3 min in PFC and 96 +/- 12.5 min in NAcc. ⋯ However, differences in the time courses of DAMGO-induced contraversive circling and signal increases in NAcc suggest that the behavioral stimulant effect of ventral tegmental mu-opioid receptor activation may not be mediated exclusively by meso-NAcc DA neurons. The results of this study suggest that enkephalins modulate the activity of meso-PFC DA neurons and that behaviorally relevant activation of mu-opioid receptors in the ventral tegmental area increases DA transmission in PFC to a same, if not to a greater extent as in NAcc. These findings are discussed in relation to evidence indicating that the response of meso-NAcc DA neurons to a variety of stimuli, including drugs of abuse, is indirectly regulated by a DA-sensitive neurons in PFC.