Forelimb placing and hopping reflexes in haloperidol- and morphine-treated cataleptic rats

Although both haloperidol and morphine produce catalepsy, there are fundamental differences in their neurological effects (De Ryck, Schallert, & Teitelbaum, 1980). Haloperidol-treated rats show brisk righting, bracing, and clinging reflexes, effects suggesting that motor subsystems subserving static postural support are dominant over those involved in more phasic locomotor and orienting movements. In contrast, morphine-treated rats show impaired righting, bracing, and clinging, effects suggesting that postural support mechanisms are suppressed. In order to determine whether phasic postural reactions other than righting are also differentially affected by these drugs, forelimb placing and hopping reflexes were evaluated in rats given either haloperidol (0, 0.25, 0.5, 1, and 5 mg/kg) or morphine sulfate (0, 10, 20, and 40 mg/kg). Morphine produced a dose-dependent impairment in all tests. In contrast, haloperidol did not impair contact placing to dorsal stimulation of the limb or chin placing. Hopping and contact placing to lateral stimulation of the limb were impaired by haloperidol, perhaps because stimulation induced a competing tendency to brace. These results provide additional evidence that morphine and haloperidol produce functionally different neurological states.