Opioid effects on [3H]norepinephrine release from dissociated embryonic locus coeruleus cell cultures

The acute and chronic effects of opioid exposure on [3H]norepinephrine ([3H]NE) release were examined in cell cultures of embryonic rat locus coeruleus (LC). Initial morphological and biochemical characterization of the cultures indicated that the cells exhibited properties similar to those observed in situ. Specific [3H]NE uptake was saturable with a Km value of 222 +/- 52 nM. [3H]NE accumulated by LC cells was released in response to 20 mM K+ stimulation, in a calcium-dependent manner. Both components of neurotransmitter release, spontaneous and K+ evoked, were significantly inhibited by beta-endorphin, with the latter being maintained in the presence of tetrodotoxin. The pharmacology of the opioid effect was consistent with that of mu-receptor activation. The effect of chronic exposure to the mu-selective agonist fentanyl (1 microM) was examined following 4 days of drug treatment. Although there was no significant effect of fentanyl on K(+)-evoked [3H]NE release, these cells were tolerant to the acute inhibitory effect of beta-endorphin. These results indicate that this is an appropriate system for examining the effects of acute and chronic opioid treatment on noradrenergic cells in vitro. In addition, this system may be useful as a CNS model for examining mechanisms that underlie tolerance and dependence following chronic opioid exposure.