Cellular depolarization of neurons in the locus ceruleus region of the guinea pig associated with the development of tolerance to opioids

These experiments were designed to test two hypotheses: 1) the tolerance in duced by morphine pellet implantation in guinea pigs will result in subsens itivity of cells in the locus ceruleus (LC), not only to morphine, but to a nother agonist acting on a different receptor and transduction system, name ly the gamma -aminobutyric acid(A) receptor agonist, muscimol; and 2) The n onspecific (heterologous) tolerance would be associated with a partial depo larization of the tolerant cells and a decrease in the contribution of elec trogenic Na+/K+ pumping. Extracellular recording from LC neurons in brain s lices from animals implanted with either morphine or placebo pellets establ ished that the tolerant preparations were subsensitive to both morphine and muscimol. Immunocytochemical analysis identified the a.-subunit as the pri mary isoform of the Na+/K+ pump in the cells under investigation. Whole-cel l patch clamp recording of neurons in brain slices demonstrated that, with electrodes containing 20 mM Na+ (approximating [Na](i)), tolerant cells wer e significantly depolarized by a mean of 6.7 mV. Dialysis with antibody spe cific for the alpha (3)-isoform from patch pipettes produced depolarization of both control and tolerant cells. However, the depolarizing effect of th e antibody was less in tolerant cells, suggesting a lesser degree of electr ogenic Na+ pumping. Furthermore, the presence of antibody reduced the membr ane potentials of tolerant and placebo cells to equal values, suggesting th at the diffusion potentials were not different. In contrast, antibody speci fic for the alpha (1)-subunit isoform in the pipettes had no effect on memb rane potential in either control or tolerant cells. In conclusion, both hyp otheses were supported.