Estrogen dependence of cholinergic systems that regulate lordosis in cycling female rats

Previous evidence indicated that physostigmine, an acetylcholinesterase inhibitor, facilitated lordosis behavior when administered intraventricularly to cycling female rats on proestrus prior to the onset of natural sexual receptivity, but not when administered to rats on mid-diestrus or diestrus II. In the present experiments, intraventricular infusion of physostigmine (10 micrograms bilaterally) facilitated lordosis on mid-diestrus and diestrus II if females were primed with two injections of estradiol (0.2, 0.1, or 0.05 micrograms) administered 20 h and 32 h prior to infusion of physostigmine. Despite unequal levels of endogenous progesterone, physostigmine facilitated lordosis equally on mid-diestrus and diestrus II following estradiol priming. Finally, intraventricular infusion of the muscarinic receptor blocker scopolamine (20 micrograms bilaterally) reduced the incidence of lordosis in females that displayed lordosis on mid-diestrus following estrogen priming. Results confirm that cholinergic mechanisms influence sexual behavior displayed by cycling female rats. Data further indicate that sufficient estrogen stimulation is necessary for cholinergic neurons to facilitate lordosis. However, progesterone does not play a major role in the regulation of lordosis by cholinergic systems.