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Inhibition of Muscimol on Morphine-Induced Hyperactivity, Reverse Tolerance and Postsynaptic Dopamine Receptor Supersensitivity

This study was performed to investigate the effect of muscimol on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice. A single administration of morphine induced hyperactivity as measured in mice, and the morphine-induced hyperactivity was inhibited dose-dependently by the administration of the GABAA agonist, muscimol (0.3, 0.5 and 1.0 mg kg–1 i.p.). However, daily repeated administration of morphine caused the development of reverse tolerance against morphine hyperactivity (10 mg kg–1 s.c.). The administration of muscimol inhibited the development of reverse tolerance against morphine hyperactivity (10 mg kg–1 s.c.) in mice that had received chronic administration of morphine. Postsynaptic dopamine receptor supersensitivity, as shown by the enhanced ambulatory activity after administration of apomorphine (2 mg kg–1 s.c.), also developed in reverse-tolerant mice. Muscimol also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine can be inhibited via the activation of GABAA receptors.