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Accumulating evidence indicated that minocycline, a glial cell modulator, is able to modify a variety of morphine effects. Here, we investigated minocycline effects on electrical activity of nucleus accumbens (NAc) neurons using single unit recording in urethane-anesthetized rats. In addition, we investigated whether minocycline can modify the effects of morphine on NAc neural activity during reinstatement of morphine-seeking behavior. Minocycline increased the NAc firing activity in intact animals. Electrophysiological recording in morphine-treated animals was performed, following the acquisition of morphine-induced conditioned place preference (5 mg/kg, s.c., 3 days) and a drug-free extinction period. In acutely minocycline- treated animals, the neurons were recorded for 40 minutes following a single injection of either minocycline (50 μg/5 μl, i.c.v.) or saline. Then a priming dose of morphine (1 mg/kg, s.c.) was injected while the recording was continued for an additional 40 minutes. Minocycline significantly increased the firing rates of neurons and significantly modified morphine inhibitory effects on NAc neurons. In subchronically minocycline-treated groups, the rats were given daily injections of minocycline (50 μg/5 μl, i.c.v) during the extinction period. Then, on the reinstatement day, NAc neurons were recorded for 10 minutes, the priming dose of morphine was administered and the recording was continued for 45 minutes. Our results showed the failure of minocycline to significantly modify the inhibitory effects of morphine. In conclusion, our findings indicated that minocycline modifies morphine-induced decreases in the firing rates of NAc neurons in the reinstatement phase. [ABSTRACT FROM AUTHOR]