Dopamine D 2 receptors and spinal cord excitation in mice

Abstract

Spinal cord excitation was induced in mice by morphine and the effects of dopamine D 1 and D 2 receptor antagonists on the Straub tail reaction were investigated. The dopamine D 2 receptor antagonist, sulpiride (25–100 mg/kg i.p.), or haloperidol (0.25–1.0 mg/kg dose dependently inhibited the Straub tail reaction induced by subcutaneously injected morphine. A low dose of apomorphine (50 μg/kg s.c.) also reduced the Straub tail reaction. The dopamine D 1 receptor antagonist, SCH-23390 (25–100 μg/kg i.p.), had no significant effect. Sulpiride (50 mg/kg i.p.) significantly inhibited the Straub tail reaction induced by intrathecally injected morphine (6 μg/mouse). Intrathecal injection of apomorphine (12.5–25 μg/mouse) induced the Straub tail reaction dose dependently. The Straub tail reaction induced by intrathecally injected apomorphine was significantly inhibited by SCH-23390 had no significant effect on the Straub tail reaction induced by intrathecally injected morphine or apomorphine. These results support the proposal that the dopamine response involved in the Straub tail reaction is mediated by postsynaptic dopamine D 2 receptors in the spinal cord of mice.