Vacuous jaw movements induced by acute reserpine and low-dose apomorphine: Possible model of parkinsonian tremor

Coadministration of the monoamine-depleting agent reserpine with a low dose of apomorphine has been shown to result in high levels of vacuous jaw movements in rats. Two experiments were conducted to study the pharmacologic and motoric characteristics of the vacuous jaw movements induced by 5.0 mg/kg reserpine plus 0.1 mg/kg apomorphine. The first experiment was undertaken to determine whether the vacuous jaw movements induced by reserpine plus apomorphine could be reduced by coadministration of the muscarinic antagonist scopolamine. Injections of scopolamine produced a dose-related decrease in vacuous jaw movements induced by reserpine plus apomorphine, with the two highest doses (0.5 and 1.0 mg/kg scopolamine) producing significant differences relative to the control group that received reserpine plus apomorphine. In the second experiment, a slow-motion videotape system was used to study the temporal characteristics of the vacuous jaw movements induced by reserpine and apomorphine, and to study the effects of 1.0 mg/kg scopolamine on these movements. Most of the vacuous jaw movements shown by rats treated with reserpine and apomorphine occurred in rapid bursts of jaw movement. Analysis of the interresponse times (i.e., time between each jaw movement) showed that most of the jaw movements had a local frequency in the range of 2.86-6.67 Hz. Cotreatment with scopolamine significantly affected several measures of jaw movements. Thus, the vacuous jaw movements induced by reserpine plus apomorphine can be reversed by anticholinergic treatment, and these movements tend to occur as periodic oscillations of the lower jaw with a frequency of 3-7 Hz. Vacuous jaw movements in rats share some characteristics with parkinsonian symptoms and may represent an animal model of parkinsonian tremor.