Physostigmine facilitation of lordosis in naturally cycling female rats.

Both systemic and intracerebral administrations of the cholinergic muscarinic antagonist, scopolamine, have been shown to inhibit naturally occurring sexual behavior in intact, cycling female rats. The present study examined the facilitative effects of the acetylcholinesterase inhibitor, physostigmine (eserine), on sexual behavior in intact, cycling female rats. Cycling was determined by daily monitoring of sexual behavior and vaginal cytology. When administered during either early proestrus or proestrus, physostigmine activated lordosis 15 min and 1 hr after intraventricular infusion (10 micrograms bilaterally). However, infusion of physostigmine failed to facilitate lordosis 15 min after administration during either diestrus I, mid-diestrus, or diestrus II. The administration of this cholinergic agent did not interrupt cyclicity patterns. Because estrogen levels are highest during proestrus and cholinergic facilitation appears to be limited to this time, it is suggested that estrogen priming of central cholinergic systems is necessary for the cholinergic regulation of sexual behavior in intact, cycling female rats.