Eric T. Pleim

Correlation of dopamine release in the nucleus accumbens with masculine sexual behavior in rats

Extracellular dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens (N. Acc.) were measured by in vivo microdialysis during male sexual activity. DA and DOPAC were significantly increased during copulation, but not during mild tail pinch. These results are consistent with studies showing increases in N. Acc. DA associated with positively reinforcing environmental stimuli.

Facilitation of sexual receptivity in hamsters by simultaneous progesterone implants into the VMH and ventral mesencephalon

Intracranial implantation experiments have shown that the ventromedial hypothalamus (VMH) is the most sensitive site for the facilitation of female sexual behavior by progesterone in estrogen-primed rats. However, similar implantation techniques have been much less successful in hamsters. Several lines of evidence indicate that both hypothalamic and midbrain structures are important for hamster lordosis. Therefore we compared the effect of progesterone (P) implants administered simultaneously to VMH and ventral midbrain on opposite sides of the brain to the effects of bilateral implants to each of these sites separately. Ovariectomized female hamsters were stereotaxically implanted with 24-gauge thin-wall guide tubes according to one of five patterns. Bilaterally symmetrical cannulae were aimed at VMH or ventral mesencephalon (vMES) or asymmetrical implants were aimed at one of the following pairs of sites, on opposite sides of the brain: VMH-vMES, VMH-preoptic area (VMH-POA), or anterior hypothalamus-anterior mesencephalon (AH-aMES). After recovery from surgery, females were primed with 10 micrograms estradiol benzoate and given pellets of P or cholesterol through a 30-gauge injector in the targeted sites. Latency, frequency, and duration of lordosis were recorded in 10-min tests with sexually active male hamsters. Sexual receptivity was significantly facilitated by simultaneous contralateral P implants into the VMH-vMES. P implants in any other combination of sites did not significantly facilitate lordosis compared to cholesterol control implants, nor did bilateral administration of this dose of P in either VMH or vMES have a reliable effect. The results support the hypothesis that P action is required in both VMH and vMES to reliably stimulate receptivity in hamsters.

A contributory role for midbrain progesterone in the facilitation of female sexual behavior in rats

Progesterone (P) facilitation of sexual receptivity in rodents has been achieved by intracranial administration to the ventral hypothalamus; the preoptic area; and midbrain areas such as central gray, mesencephalic reticular formation, and ventral tegmental nucleus. In our laboratory, by far the most effective site in rats has been the ventromedial nucleus of the hypothalamus (VMN). However, several reports of sensitivity to P in the midbrain of rats and other rodent species led us to investigate whether stimulation of the ventral midbrain of female rats might contribute to facilitation of sexual receptivity. Ovariectomized Long-Evans rats received one cannula aimed at the VMN, and another aimed at the contralateral ventral mesencephalon. P in both cannulae, following a priming dose of estradiol, caused significantly higher lordosis quotients (LQ) than blank tubes. Controls with bilateral cannulae in the VMN responded when both tubes were filled with P, but did not respond to unilateral VMN P stimulation. P in the VMN and contralateral anterior preoptic area did not result in a greater degree of receptivity than did the empty tubes. These studies indicate that although progesterone stimulation in the midbrain alone is not sufficient to facilitate receptivity in female rats with our methods, the midbrain may play an auxiliary role. P implants in the midbrain appear to facilitate receptivity in the case of VMN implant treatments that are subthreshold for stimulating lordosis. The results are discussed in light of similar studies in other rodent species, and in the context that more than one brain site may be important in the natural stimulation of sexual receptivity by gonadal hormones.

Subtle behavioural changes produced in rat pups by in utero exposure to haloperidol

Prenatal exposure to a dopamine receptor blocking agent such as haloperidol (given to the mother at a dose of 0.5 mg/kg s.c. from day 4 to day 15 of gestation) produced subtle behavioural changes in rat pups. Haloperidol decreased the rate of ultrasonic vocalization in 4-day-old male pups removed from the nest. The changes in ultrasonic emission elicited by in utero exposure to this neuroleptic were markedly different from those produced by its administration during the early postnatal period. Moreover, adult male rats treated prenatally with haloperidol exhibited a significant increase in the intensity of ultrasonic 22 kHz post-ejaculatory calls emitted during sexual behaviour. The duration of the period of the 22 kHz calls emission was also significantly increased by haloperidol treatment. These results confirm that ultrasonic vocalization in rats is a sensitive indicator of subtle changes in adverse treatments administered during development.

Facilitation of receptive behavior in estrogen-primed female rats by the anti-progestin, RU 486

The progestin receptor antagonist RU 38486 (henceforth referred to as RU 486) was tested for facilitative effects on female receptive behavior in ovariectomized Long-Evans rats primed with 2 micrograms estradiol benzoate (EB). RU 486 (0, 0.5, 1.6, or 5.0 mg) was administered 48 hr after estrogen priming. The lordosis quotient (LQ) and lordosis score (LS) were assessed 4 hr after RU 486 administration in a standardized test consisting of a 10-mount test by a stimulus male. A significant dose effect was found by both LQ and LS, with those subjects receiving 5 mg of RU 486 being significantly more receptive than vehicle control animals. Thus RU 486 acted as a weak progestin agonist under testing conditions typical for assessment of progestin facilitation of female sexual behavior in rats. Low levels of proceptive behavior (hops and darts) were seen in a minority of the tests, and did not vary systematically as a function of the dose of RU 486 administered. We also examined the effects of RU 486 given before progesterone (P) on receptivity in a blocking paradigm and confirmed previous reports that the antagonist significantly attenuates facilitation of sexual behavior when given in combination with P. A progestin receptor assay of the cytosols of the hypothalamus-preoptic area in estrogen-primed female rats treated with 5 mg RU 486 revealed a significantly greater depletion of available cytosolic P receptors than when rats were treated with a similarly facilitating dose of P (100 micrograms). The results suggest a possible dual mode of action for RU 486--a weak, receptor-mediated agonistic effect on sexual behavior when given alone to estrogen-primed rats, and a competitive blocking effect on receptivity when administered with P.

Mediation of rat postejaculatory 22 kHz ultrasonic vocalization by dopamine D2 receptors

We investigated the role of dopamine receptor subtypes in the regulation of ultrasonic vocalization and masculine copulatory behavior. Intact sexually experienced male Long-Evans rats were treated with saline, selective dopamine D1 (SKF 38393) and D2 (LY 171555) receptor agonists and with selective dopamine D1 (SCH 23390) and D2 (raclopride) receptor antagonists 15 and 30 min before the 30-min test session, respectively. Mating stimuli were ovariectomized female rats injected SC with estradiol benzoate (8 micrograms/0.1 ml/rat) and progesterone (200 micrograms/0.1 ml/rat), 48 and 4 hr before the test session, respectively. We found a decrease in the number of intromissions required to reach ejaculation in animals treated with SKF 38393 (10 mg/kg/IP), LY 171555 (doses ranging from 0.01 to 0.5 mg/kg/SC) and with raclopride (0.1 mg/kg/SC). LY 171555 reduced the postejaculatory vocalization (PEV) in a dose-dependent fashion with complete suppression at the highest dose. No other parameters of sexual behavior were affected by this treatment. Raclopride, a dopamine D2 receptor antagonist, antagonized the suppressive effects of the D2 agonist LY 171555 on the PEV (and also decreased the number of intromissions to reach ejaculation), whereas SCH 23390, a dopamine D1 receptor antagonist, did not. Raclopride, given alone at the dose of 0.5 mg/kg/SC, almost completely suppressed all behavioral activity, whereas the lower dose (0.1 mg/kg) decreased intromission frequency and increased the length of the 22 kHz PEV. Therefore, we suggest that 22 kHz PEV is under the control of dopamine D2 receptors.

The relative effectiveness of progestins for facilitation and inhibition of sexual receptivity in hamsters

In a series of experiments, we studied the biphasic actions of 4 progestins on sexual receptivity in female hamsters. Based on previous reports, we selected doses of progesterone (P), desoxycorticosterone acetate (DOCA), 5-alpha-dihydroprogesterone (DHP) and 20-alpha-hydroxyprogesterone (20-OHP) that were expected to have initial facilitative and/or subsequent inhibitory actions on sexual receptivity. Estrogen-primed ovariectomized female hamsters were given one of 4 doses of a progestin at hour 44 post EB, and were tested for facilitation (with tape over their vaginas to prevent intromissions) 4 hours later. At hour 68, all animals received 500 micrograms P and were tested for the inhibitory effects of the test progestin 4 hours later. In additional experiments, similar procedures were followed with an alternate vehicle (1% Tween-80 instead of sesame oil) and a longer initial progestin-to-facilitation test interval (8 rather than 4 hours) in attempts to improve bioavailability of the steroids. P and DOCA were clearly the most behaviorally effective progestins tested, and they were equally potent for facilitation and inhibition in an oil vehicle at the 4 hour interval. DHP given in oil caused some inhibition but no facilitation. 20-OHP had no behavioral effects at the doses tested. The results are discussed in light of previous results with these steroids in hamsters and other rodent species. Also, the differing patterns of effectiveness across conditions provide some, although qualified, support for our view that the biphasic actions of progestins may be controlled by separate mechanisms. However, an alternate explanation is also discussed.

Site-specific inhibition of receptivity by intracranial anisomycin in hamsters

The ventromedial nucleus of the hypothalamus (VMH) has been implicated in the mediation of the hormonal control of female rodent sexual behavior. However, in hamsters, progesterone (P) has been found to have effects on sexual receptivity in other diencephalic and mesencephalic sites as well. Progesterone is thought to exert its behavioral effects by altering protein synthesis in CNS target neurons. We tested the effects of 30 gauge implants of the protein synthesis inhibitor anisomycin in the preoptic area (POA), VMH, and ventral mesencephalon (VMES) 30 minutes before 500 micrograms P SC, on the facilitation of lordosis in ovariectomized estrogen-primed female hamsters. The same animals were tested one week later with estrogen and progesterone treatment but without anisomycin. Anisomycin reduced sexual receptivity (lordosis) when placed in the VMH or VMES, but not when delivered to the POA. The results confirm the importance of the VMH in the mediation of progesterone facilitation of female sexual behavior, but also provide evidence that ventral midbrain structures may play a role in female sexual receptivity in hamsters. These two structures may be important for different aspects of lordosis. Progesterone effects in both sites appear to be protein synthesis dependent.

Effects of neonatal exposure to the antiprogestin mifepristone, RU 486, on the sexual development of the rat

RU 38486 (RU 486, mifepristone) is a potent progesterone receptor antagonist that has been used in humans in the pharmacologic induction of abortion. The effects of exposure to RU 486 during the neonatal period of the rat has not been previously reported. We examined the consequences of such exposure in the context of sexual development. Long-Evans rat pups were subcutaneously injected with either 100 micrograms RU 486, 300 micrograms RU 486, 500 micrograms progesterone (P), or 50 micrograms testosterone propionate (TP) in 0.05 ml sesame oil, or oil vehicle alone within 8 hours of birth, and 24 and 48 hours later. Treatment with either dose of RU 486 significantly advanced the onset of vaginal opening in females and attenuated defeminization of the lordosis response measured in males castrated as adults. As expected, TP-treated subjects were masculinized and defeminized, with females displaying fused vaginas and neither males nor females demonstrating lordosis behavior. Treatment with P caused no significant alterations in either the timing of vaginal opening or sexual behavior. These results indicate that RU 486 has clear developmental effects in the rat. Since this may well be a result of progesterone receptor blockade, further research is needed to clarify the processes involved.

Facilitation of Sexual Receptivity by Ventromedial Hypothalamic Implants of the Antiprogestin RU 486

RU 486 is known primarily as an antagonist to progestins and glucocorticoids. However, RU 486 has also been shown to have agonistic progestational properties in biochemical and behavioral studies. In the current study, RU 486 was implanted directly into the ventromedial hypothalamus (VMH) to test for facilitative action on the receptive behavior of female ovariectomized Long-Evans rats primed with 5 micrograms of estradiol benzoate. Cannulae containing RU 486, progesterone (P), or empty cannulae were implanted 48 hr after estrogen priming. The lordosis quotient and the lordosis score were assessed 4 hr after the cannulae were lowered by a standardized test consisting of 10 mounts by a stimulus male. P and RU 486 significantly facilitated receptivity compared to blank implants in terms of lordosis quotient and lordosis score, with no significant difference between the hormone treatments. While only a single dose of each treatment was given in the current study, RU 486 facilitated lordosis when implanted to the VMH as well as progesterone in contrast to our previous results where the steroids were administered systemically.