Alpha-2 adrenoreceptor-mediated decrease in gamma-aminobutyric acid outflow in cortical slices and synaptosomes during morphine tolerance.

Morphine tolerance has proven to be accompanied by alterations in the efficiency of many neuronal signals, as well as by an inversion of the noradrenergic signal response of cortical acetylcholine terminals and gamma-aminobutyric acid (GABA) neurons in vivo (decreased acetylcholine and increased GABA release in normal animals, vice versa in tolerant). The latter observation may be relevant in interpreting morphine withdrawal, because the noradrenergic neuron firing rate increases dramatically during its course. This study was designed as an attempt to anatomically localize the inversion of the GABA response to norepinephrine. Because this phenomenon is observed in cortical slices and synaptosomes, it can be postulated that it occurs in the neocortex at the level of the intracortical GABA nerve terminals. Pharmacological analysis demonstrates that although the stimulation observed in controls is alpha-1 adrenoreceptor-mediated, the inhibition in tolerant animals is exerted via alpha-2 adrenoreceptors. Therefore, an increase in number or an improved coupling to the transduction system of alpha-2 adrenoreceptors is hypothesized. This observation gives a clue to a molecular interpretation of the inversion of GABA response to norepinephrine in tolerant animals, which may be of heuristic value in terms of biological interpretation of morphine tolerance.