Role of Oestrogen α Receptors in Sociosexual Behaviour in Female Rats Housed in a Seminatural Environment.

The present study investigated the role of oestrogen receptor (ER)α in the ventromedial nucleus of the hypothalamus (VMN), the preoptic area (POA), the medial amygdala (MePD) and the bed nucleus of stria terminalis (BNST) in sociosexual behaviour in female rats. This was conducted in two sets of experiments, with the VMN and POA investigated in the first set, and the MePD and BNST in the second set. The VMN and POA received intense projections from the MePD and BNST. We used a short hairpin RNA encoded within an adeno-associated viral vector directed against the gene for ERα to reduce the number of ERα in the VMN or POA (first set of experiments) or in the BNST or MePD (second set of experiments) in female rats. The rats were housed in groups of four ovariectomised females and three males in a seminatural environment for 8 days. Compared with traditional test set-ups, the seminatural environment provides an arena in which the rats can express their full behavioural repertoire, which allowed us to investigate multiple aspects of social and sexual behaviour in groups of rats. Behavioural observation was performed after oestrogen and progesterone injections. A reduction of ERα expression in the VMN or POA diminished the display of paracopulatory behaviours and lordosis responses compared to controls, whereas the lordosis quotient remained unaffected. This suggests that ERα in the VMN and POA play an important role in intrinsic sexual motivation. The reduction in ERα did not affect the social behaviour of the females, although the males sniffed and pursued the females with reduced ERα less than the controls. This suggests that the ERα in the VMN and POA is involved in the regulation of sexual attractiveness of females. The ERα in the MePD and BNST, on the other hand, plays no role in sociosexual behaviour.

Testosterone or oestradiol implants in the medial preoptic area induce mating in noncopulating male rats.

Noncopulating (NC) male rats are those males that do not mount, intromit or ejaculate when repeatedly tested with receptive females. The lack of sexual behaviour in these males is not associated with alterations in testosterone or oestradiol (E2) plasma concentrations. Instead, it has been shown that androgen receptors are higher and oestrogen receptors are lower in the medial preoptic area (MPOA) of NC male rats than those observed in copulating (C) male rats. We have also observed reduced aromatase activity in the MPOA (but not in other brain regions) of NC male rats. The aim of the present study was to determine whether testosterone or E2 implants in the MPOA of NC male rats could induce sexual behaviour. Accordingly, in Experiment 1, we evaluated the long-term effects of testosterone or E2 implants in the MPOA, the ventromedial nucleus of the hypothalamus or the medial amygdala with respect to inducing sexual behaviour in castrated C male rats. Male rats were bilaterally implanted with a guide cannula, either empty or containing testosterone or E2. Starting 1 week later, all male rats were mated once weekly for 5 months. As described previously, the site where hormone implants most consistently induced sexual behaviour in castrated C male rats was the MPOA. Experiment 1 extended these findings showing that the males continued mating even 5 months after the implant. In the second experiment, NC males were implanted in the MPOA with a guide cannula empty or filled with testosterone or E2. One week after the testosterone or E2 implant, the percentage of males that mounted and intromitted started to increase and, 5 weeks after the implant, 50% of the subjects displayed mounts and intromissions. All NC males implanted with testosterone ejaculated consistently from week 11 after the implant until the end of testing (5 months), whereas all subjects implanted with E2 ejaculated from week 16 after the implant until the end of testing. These results support the hypothesis that, in the MPOA of NC male rats, there is a hormonal alteration associated with the lack of sexual behaviour.

Behavioral characterization of non-copulating male mice.

Non-copulating (NC) males are those animals that do not mate in spite of repeated testing with sexually receptive females. They have been observed in several species including rats and mice. The present experiment was designed to perform a detailed behavioral characterization of NC male mice. Thus, we evaluated their sexual incentive motivation for a sexually receptive female or a sexually active male, olfactory preference for volatile and non-volatile odors from females or males, and olfactory discrimination between female and male volatile odors and food related odors (milk versus vinegar). We compared the activity of the accessory olfactory system (AOS) in copulating (C) and NC males in response to estrous bedding using the induction of Fos-immunoreactivity (Fos-IR) as a measure of neuronal activation. We also determined if estradiol or dopamine treatment could induce sexual behavior in NC males. Finally, we compared the testis weight and the number of penile spines in C, NC, and gonadectomized males. In the sexual incentive motivation test C males spend significantly more time in the female incentive zone than in the male incentive zone. On the other hand, NC males spend the same amount of time in both incentive zones. In tests of olfactory preference, NC males spent less time investigating estrous odors than C males. As well, NC males discriminate urine from conspecifics but they spend less time smelling these odors than C males. In addition, no increase in Fos expression is observed in NC males when they are exposed to odors from estrous females. Our data also suggest that the deficits observed in NC males are not due to lower circulating levels of gonadal hormones, because estradiol supplementation does not induce sexual behavior in these animals, and their testis weight and the number of penile spines are normal. The results suggest that NC males are not sexually motivated by the receptive females and their odors.