Scopolamine reversal of tremor produced by low doses of physostigmine in rats: evidence for a cholinergic mechanism.

In order to provide evidence for the involvement of cholinergic mechanisms in a low-dose physostigmine-induced tremor, a novel technique for measuring forelimb tremor in rats was used. Rats that were administered physostigmine (0.05 and 0.1 mg/kg, s.c.), scopolamine (0.1 and 0.2 mg/kg, s.c.), and combinations of the two drugs, pressed a force-sensing operandum while a computer measured force output and performed Fourier analyses on resulting force-time waveforms. When given alone, both drugs decreased task engagement but mutually antagonized this effect when given together. Physostigmine increased tremors as well as peak force. Scopolamine decreased tremor and force when administered alone and reversed physostigmine-induced increases in force and tremor. Physostigmine's low-dose induction of increased tremor during rats' skilled forelimb use appears to have a prominent cholinergic component.