Effects of chronic morphine administration on the mu and delta opioid responses in the CA1 region of the rat hippocampus.

Extracellular recording of population spike amplitudes in the hippocampal CA1 region were compared in slices from control and chronically morphine-treated rats. Morphine treatment resulted in a decrease in the maximal excitation produced by both mu and delta selective agonists, [N-MePhe3,D-Pro4]-morphiceptin and [D-Pen2,L-Pen5]-enkephalin. The opioid antagonist naloxone did not produce apparent signs of withdrawal in hippocampal slices from morphine-treated rats as shown by a lack of change in the evoked population spike in the presence of 500 nM naloxone. Brain slices from morphine-treated rats that were maintained in the absence of morphine showed signs of tolerance reversal when monitored over an 8-hr period after dissection. If morphine-tolerant slices were maintained in vitro in the presence of 5 microM morphine (the concentration found by high-performance liquid chromatography to be present in the cerebrospinal fluid of morphine-treated rats), there was no significant reversal of tolerance. Furchgott analysis of the [N-MePhe3,D-Pro4]-morphiceptin concentration-response curve shifts induced by the irreversible opioid receptor antagonist beta-chlornaltrexamine revealed an apparent 50% spare receptor reserve for the mu selective agonist in slices from drug-naive rats. beta-Chlornaltrexamine (20 nM) treatment and chronic morphine exposure resulted in a similar reduction in the maximal response to [N-MePhe3,D-Pro4]morphiceptin. This observation indicates that the development of morphine tolerance resulted in an elimination of the spare opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)