Age-related changes in sexual function and steroid-hormone receptors in the medial preoptic area of male rats

Abstract

Testosterone is the main circulating steroid hormone in males, and acts to facilitate sexual behavior via both reduction to dihydrotestosterone (DHT) and aromatization to estradiol. The mPOA is a key site involved in mediating actions of androgens and estrogens in the control of masculine sexual behavior, but the respective roles of these hormones is not fully understood. As males age they show impairments in sexual function, and a decreased facilitation of behavior by steroid hormones compared to younger animals. We hypothesized that an anatomical substrate for these behavioral changes is a decline in expression and/or activation of hormone receptor-sensitive cells in the mPOA. We tested this by quantifying and comparing numbers of AR- and ERα-containing cells, and Fos as a marker of activated neurons, in the mPOA of mature (4-5months) and aged (12-13months) male rats, assessed one hour after copulation to one ejaculation. Numbers of AR- and ERα cells did not change with age or after sex, but the percentage of AR- and ERα-cells that co-expressed Fos were significantly up-regulated by sex, independent of age. Age effects were found for the percentage of Fos cells that co-expressed ERα (up-regulated in the central mPOA) and the percentage of Fos cells co-expressing AR in the posterior mPOA. Interestingly, serum estradiol concentrations positively correlated with intromission latency in aged but not mature animals. These data show that the aging male brain continues to have high expression and activation of both AR and ERα in the mPOA with copulation, raising the possibility that differences in relationships between hormones, behavior, and neural activation may underlie some age-related impairments.

Keywords: Aging; Androgen receptor; Estradiol; Estrogen receptor alpha; Male; Preoptic area.

Fig 1

A) Representative micrograph of double-labeling of ERα- and Fos- in the mPOA. ERα-immunoreactive cells are labeled with a green fluorophore as indicated with a green arrow. Fos-immunoreactive cells are labeled with a red fluorophore as indicated with a red arrow. Colocalized ERα-and Fos- immunoreactive cells appear yellow, and an example shown with the orange arrow. B) Representative micrograph of double-labeling of AR-(green) and Fos (red), similarly labeled as in A. Counting of single- and double-labeled cells was done in 30 × 400 um² sections of the mPOA at anterior (C, Bregma 0.12mm), central (D, Bregma -0.36mm), and posterior (E, Bregma -0.96) levels. Coordinates are shown with respect to Bregma from Paxinos and Watson, 2007. Scale bar = 10 um.

Fig 2

Total numbers of ERα (A-C), AR (D-F) and Fos (G-I) immunoreactive cell counts are shown in mature and aged male rats from the sex and no-sex groups in the three mPOA sub-regions. There were no main effects or interactions for ERα or AR in any of the sub-regions. Fos-positive cells were higher in all of the mPOA sub-regions of animals that had sex. Abbreviations here and in other figures are: MAT-S, mature-sex; MAT-NS, mature–no sex; AG-S, aged-sex; AG-NS, aged–no sex. Data here and subsequently are mean + SEM.

Fig 3

The percentage of ERα or AR cells that co-express Fos are shown from anterior to posterior. For ERα-Fos double labeling (A-C), there were main effects of sex (increased compared to no sex) in the anterior and central mPOA (p < 0.001). In the central mPOA (B) there was also a significant interaction of age and sex, with AG-S group having a greater percentage of double labeled cells than any other group. In addition, the MAT-S group was greater than the MAT-NS for this endpoint. AR-Fos double labeling (D-F) was significantly higher in sex than no-sex males across the mPOA. P-values for interactions are indicated as: **, p < 0.01, *** p < 0.001.

Fig 4

The percentage of Fos-positive cells also expressing ERα or AR are shown from anterior to posterior. For ERα-Fos double labeling, the central mPOA (B) had a significant interaction of age and sex, with the AG-S group having more double-labeled cells than any other groups. In the posterior mPOA (C), there was significant main effect of sex, with fewer cells in the sex than the no-sex groups. For AR-Fos double labeling, in the central mPOA (E) animals that had sex had more double-labeled cells compared to no-sex animals. In the posterior mPOA (F), main effects of both age and sex were found, with higher numbers in sex vs. no-sex rats, and an age-related decrease. P-values for interactions are indicated as: *, p < 0.05; **, p < 0.01.

Fig 5

Behavioral measures are shown for MAT-S and AG-S groups. A) Numbers of mounts were significantly higher in MAT-S than AG-S males (p < 0.05). Numbers of Intromissions, and latencies to mount, intromit, or ejaculate (B) were unaffected.

Fig 6

Serum estradiol concentrations are shown, measured 1 hour post-copulation or handling. There was a main effect of both sex and age (p < 0.05), with AG animals having higher estradiol concentrations than MAT animals, and animals that had had sex prior to sacrifice having higher estradiol concentrations than those that did not.

Fig 7

Correlations between serum estradiol and mount latency (A) and intromission latency (B) are shown separately for MAT-S and AG-S male rats. In AG-S males, mount latency had a non-significant trend for a positive correlation with estradiol (r = 0.723, p = 0.066). Intromission latency had a significant positive correlation with estradiol (r = 0.873, p < 0.01) in the AG-S males.