Abstract

These experiments were designed to test two hypotheses: 1) the tolerance induced by morphine pellet implantation in guinea pigs will result in subsensitivity of cells in the locus ceruleus (LC), not only to morphine, but to another agonist acting on a different receptor and transduction system, namely the γ-aminobutyric acid_A receptor agonist, muscimol; and 2) The nonspecific (heterologous) tolerance would be associated with a partial depolarization of the tolerant cells and a decrease in the contribution of electrogenic Na⁺/K⁺ pumping. Extracellular recording from LC neurons in brain slices from animals implanted with either morphine or placebo pellets established that the tolerant preparations were subsensitive to both morphine and muscimol. Immunocytochemical analysis identified the α₃-subunit as the primary isoform of the Na⁺/K⁺ pump in the cells under investigation. Whole-cell patch clamp recording of neurons in brain slices demonstrated that, with electrodes containing 20 mM Na⁺(approximating [Na]_i), tolerant cells were significantly depolarized by a mean of 6.7 mV. Dialysis with antibody specific for the α₃-isoform from patch pipettes produced depolarization of both control and tolerant cells. However, the depolarizing effect of the antibody was less in tolerant cells, suggesting a lesser degree of electrogenic Na⁺ pumping. Furthermore, the presence of antibody reduced the membrane potentials of tolerant and placebo cells to equal values, suggesting that the diffusion potentials were not different. In contrast, antibody specific for the α₁-subunit isoform in the pipettes had no effect on membrane potential in either control or tolerant cells. In conclusion, both hypotheses were supported.