Enhanced responses to muscarine in rats treated chronically with atropine.

Rats were treated for 14 or 30 days with 10 mg/kg sc atropine (AT) daily. 24 h after the last dose of At, 50 micrograms of muscarine (MU) was injected intracerebroventricularly (ivtr.). Long-term atropinization enhanced the behavior induced by MU. These data support our previous suggestion that chronic blockade of central muscarinic receptors induces their hypersensitivity.