Local opioid withdrawal in rat single periaqueductal gray neurons in vitro

Opioid dependence in ventrolateral periaqueductal gray (PAG) neurons was studied by using intracellular recordings from brain slices. In slices from morphine-dependent rats maintained in morphine (5 microM) in vitro, action potential frequencies of opioid-sensitive neurons did not differ from untreated control neurons but were greater than in control neurons maintained in morphine in vitro, indicating development of tolerance. Naloxone (100 nM or 1 microM) depolarized 25 of 51 neurons from morphine-dependent rats maintained in morphine in vitro, 19 of which previously had been classified as opioid-sensitive. Action potential frequencies in the presence of naloxone were greater than in control neurons in the absence of opioids, as well as in control neurons in the presence of both morphine and naloxone, demonstrating opioid withdrawal. In slices from control animals, opioid-induced hyperpolarizations and naloxone-induced depolarizations (in the presence of morphine) reversed polarity near expected EK (-111 +/- 3 mV and -113 +/- 3 mV, respectively). In contrast, the reversal potential of naloxone-induced depolarizations was more negative than expected in neurons from dependent animals (-143 +/- 9 mV), indicating that the depolarization was not attributable simply to antagonism of a K-conductance increase. Naloxone-induced depolarizations were not inhibited by tetrodotoxin (1 microM), bicuculline (30 microM), 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM), or prazosin (300 nM), suggesting no involvement of major synaptic neurotransmitters. Clonidine (1 microM) and baclofen (30 microM) overcame naloxone-induced depolarizations. These results demonstrate development of both tolerance and withdrawal in PAG neurons and suggest induction of a novel opioid-sensitive current that could be involved in withdrawal behavior.