Gary P. Dohanich

Alteration of muscarinic binding in specific brain areas following estrogen treatment

Systemic administration of estradiol benzoate (EB) to ovariectomized female rats significantly altered cholinergic muscarinic binding of [3H]quinuclidinyl benzilate in discrete brain regions. Specifically, EB increased muscarinic binding in the medial basal hypothalamus (MBH) and decreased muscarinic binding in the medial preoptic area (POA). These alterations were dose-dependent and appeared to reflect changes in the number of muscarinic binding sites. Treatment with EB failed to significantly affect binding in several control areas in females or in the MBH and POA of males.

Cholinergic brain mechanisms and the hormonal regulation of female sexual behavior in the rat

Cholinergic muscarinic stimulation of the medial preoptic area or the mesencephalic reticular formation with carbachol or bethanechol facilitated lordosis in ovariectomized female rats treated with estrogen. Adrenalectomy did not abolish the facilitative influence of cholinergic stimulation in the preoptic area. Implants of carbachol in the neocortex failed to increase lordosis.

Inhibition of lordotic behavior in female rats following intracerebral infusion of anticholinergic agents

Cholinergic antagonists were bilaterally infused into forebrain areas of ovariectomized female rats brought into sexual receptivity by administration of estrogen and progesterone. Infusion of hemicholinium-3 (HG-3, 5 or 7.5 micrograms/cannula), an acetylcholine synthesis inhibitor believed to interfere with choline uptake, decreased the incidence of lordotic behavior displayed by females. This inhibition was prevented by infusion of choline chloride (120 micrograms/cannula) along with HC-3. Atropine sulfate, an acetylcholine receptor blocker, also reduced lordotic behavior in females following intracerebral infusion (20 micrograms/cannula). These results lend support to the suggestion that lordotic behavior is facilitated by central cholinergic activity.

Pharmacological and anatomical aspects of cholinergic activation of female sexual behavior.

Forebrain infusion of cholinergic agonists activated the sexual response, lordosis, in ovariectomized female rats that had been primed with a low dose of estrogen. Carbachol, an agonist with both muscarinic and nicotinic properties, and oxotremorine, an agonist with a primarily muscarinic action, produced dose-related increases in the frequency of lordosis elicited by stimulus male rats. This facilitation of lordosis was prevented when females were pretreated systemically with atropine or scopolamine, two muscarinic receptor antagonists. These results indicate that the effect of carbachol and oxotremorine on lordosis is mediated by cholinergic muscarinic receptors. The location of these receptors within the brain has not been identified. Ventricular infusion of carbachol was as effective as infusion directly into the medial preoptic area (POA) and more effective than infusion directly into the ventromedial hypothalamus (VMH). Furthermore, when carbachol or oxotremorine was delivered to the POA through cannulae angled to avoid traversing the lateral ventricles, no facilitation of lordosis was observed. These data suggest that muscarinic receptors stimulated by central infusion of cholinergic agonists may not be located in either the POA or the VMH, two regions traditionally implicated in the regulation of lordosis.

Brain areas implicated in cholinergic regulation of sexual behavior

Abstract Sexual behavior in female rats was facilitated by infusion of a cholinergic agent into specific brain regions. Carbachol, a cholinergic receptor agonist, increased the incidence of lordosis in estrogen-primed female rats following bilateral infusion (0.5 μg/cannula) into either the medial preoptic area or the ventromedial hypothalamus. The behavioral response was highest 15 min after carbachol infusion and returned to control levels within 90 min after infusion. Carbachol failed to activate lordosis following infusion into the mesencephalic reticular formation or frontal cortex. These results indicate that the potential of a brain area to respond to cholinergic stimulation may be related to the ability of that area to concentrate estrogen.

Inhibition of estrogen-activated sexual behavior by androgens

Experiments to determine the potential of androgen to inhibit estrogen-activated female sexual behavior in rats were conducted. Treatment with either testosterone propionate (0.8 or 1.6 mg/day) or dihydrotestosterone propionate (0.2, 0.4, or 0.8 mg/day) significantly reduced the incidence of lordosis in ovariectomized females receiving estradiol benzoate (1 microgram/day). A similar suppression of estrogen-activated lordosis by testosterone was observed in castrated male rats. Flutamide, an androgen-receptor blocker, prevented the inhibition of lordosis by testosterone in females, indicating that the interaction of testosterone or a metabolite with an androgen receptor may be an important feature of this inhibition. Furthermore, the ability of dihydrotestosterone to inhibit lordosis at lower doses than testosterone suggests that the conversion of testosterone to dihydrotestosterone may also be necessary. These experiments demonstrate the potential of testosterone to inhibit the occurrence of female sexual behavior in rats, in contrast to its established facilitative effect on this behavior.

Cholinergic regulation of female sexual behavior in rats demonstrated by manipulation of endogenous acetylcholine

Infusions of the cholinesterase inhibitor, eserine, into the lateral ventricle of the brain facilitated lordosis in ovariectomized, estrogen primed female rats. Lordosis was also facilitated by infusions of 5 or 10 micrograms acetylcholine when this was accompanied by a low dose of eserine. Systemic treatment with the muscarinic antagonist, atropine (2 mg), completely blocked the facilitative effects of eserine and acetylcholine. In female rats made highly receptive by treatment with estrogen and progesterone, lordosis was inhibited by infusion of scopolamine into the lateral ventricles. These results agree with previous findings that sexual receptivity is regulated in part by endogenous cholinergic activity.

Hormonally mediated lordosis in female rats: Actions of flutamide and an aromatization inhibitor

Abstract Agents which interfere with androgen metabolism or receptor binding diminished androgen-induced, but not estrogen-induced lordotic behavior in female rats. The non-steroidal antiandrogen, flutamide, reduced the amount of lordosis displayed by females receiving 250 μg testosterone propionate (TP) plus progesterone (P). Flutamide (10 mg) did not affect the lordotic behavior of animals receiving estradiol benzoate (0.5 μg or 0.25 μg EB) plus P. Similarly, the aromatization inhibitor 1,4,6-androstatriene-3,17-dione (ATD; 10 mg) blocked TP plus P induced lordosis but failed to alter EB (10 μg) plus P induced lordosis. These results suggest that androgen-induced lordosis occurs after the testosterone molecule becomes bound to a receptor and is enzymatically converted to estrogenic metabolites.

Prenatal antiandrogen feminizes behavioral but not neurochemical response to estrogen

In ovariectomized female rats, estrogen activates high levels of female sexual behavior and induces an increase in the number of cholinergic muscarinic binding sites in the medial basal hypothalamus, an area known to regulate this behavior. Male rats normally show low levels of female sexual behavior and no alteration in muscarinic binding in response to estrogen treatment. An experiment was conducted to determine if a prenatal treatment that feminizes the sexual behavior of male rats would also establish the potential for estrogen to increase muscarinic binding in the male hypothalamus. Results indicate that prenatal exposure to the antiandrogen, flutamide, enhanced estrogen-activated female sexual behavior in male rats but failed to reverse the inability of estrogen to increase muscarinic binding in the medial basal hypothalamus.