The maternal hormone in the male brain: Sexually dimorphic distribution of prolactin signalling in the mouse brain

Abstract

Research of the central actions of prolactin is highly focused on females, but this hormone has also documented roles in male physiology and behaviour. Here, we provide the first description of the pattern of prolactin-derived signalling in the male mouse brain, employing the immunostaining of phosphorylated signal transducer and activator of transcription 5 (pSTAT5) after exogenous prolactin administration. Next, we explore possible sexually dimorphic differences by comparing pSTAT5 immunoreactivity in prolactin-supplemented males and females. We also assess the role of testosterone in the regulation of central prolactin signalling in males by comparing intact with castrated prolactin-supplemented males. Prolactin-supplemented males displayed a widespread pattern of pSTAT5 immunoreactivity, restricted to brain centres showing expression of the prolactin receptor. Immunoreactivity for pSTAT5 was present in several nuclei of the preoptic, anterior and tuberal hypothalamus, as well as in the septofimbrial nucleus or posterodorsal medial amygdala of the telencephalon. Conversely, non-supplemented control males were virtually devoid of pSTAT5-immunoreactivity, suggesting that central prolactin actions in males are limited to situations concurrent with substantial hypophyseal prolactin release (e.g. stress or mating). Furthermore, comparison of prolactin-supplemented males and females revealed a significant, female-biased sexual dimorphism, supporting the view that prolactin has a preeminent role in female physiology and behaviour. Finally, in males, castration significantly reduced pSTAT5 immunoreactivity in some structures, including the paraventricular and ventromedial hypothalamic nuclei and the septofimbrial region, thus indicating a region-specific regulatory role of testosterone over central prolactin signalling.

Fig 1. Mapping of pSTAT5 immunoreactivity and prolactin receptor expression in the brains of male and female mice.

Semi-schematic camera lucida drawings of coronal sections of the mouse brain showing the distribution patterns of pSTAT5-ir in female and male mice (left side) and the pattern of PRLR expression in the brain of an adult male mouse specimen (right side), as determined by PRLR expression data obtained from the Allen Brain Institute (2004 Allen Institute for Brain Science. Allen Mouse Brain Atlas. Available from: mouse.brain-map.org experiment 72340223, http://mouse.brain-map.org/experiment/show/72340223). In the left side, pink dots represent pSTAT5 expression exclusive of ovariectomized, steroid-primed females, whereas dark blue dots encode overlapping expression of pSTAT5-ir in both the female and male specimens. Black dots in the right illustrate PRLR expression in the male mouse brain. Each dot represents approximately 4 cell nuclei labelled for pSTAT5 or equivalent number of cells expressing the PRLR. Shaded areas indicate the counting frames designed for the AVPe/VMPO region (Fig 1A), the Pa (Fig 1E), and the Arc, VMHvl, VMHc and VMHdm, DM and MePD (all frames in Fig 1F), as part of the quantitative analysis performed in this work (see below). Approximate distance to bregma is indicated for each section.

Fig 2. Representative examples of pSTAT5 immunoreactivity in the brain of females, intact males and castrated male mice.

Photomicrographs illustrating pSTAT5 labelling in representative brain sections in an ovariectomized, steroid-primed female supplemented with PRL (leftmost column), an intact, PRL-supplemented male (left-center column), a castrated, PRL supplemented male (right-center column) and a intact control male lacking PRL supplementation (rightmost column). Sections correspond to the caudal septum (A-D), the posterior medial amygdaloid region (E-H), the preoptic hypothalamus (I-L), the paraventricular hypothalamic region (M-P) and the tuberal hypothalamus (Q-T). The approximate distance to bregma is indicated in each section. Scale bars correspond to 100 μm.

Fig 3. Quantitative analysis of pSTAT5 immunoreactivity in selected brain regions of female and male mice.

Assessment of pSTAT5-ir density (pSTAT5-positive cell nuclei/mm²) in the major brain regions with expression of pSTAT5-ir in both male and female mice. Bar histograms show mean interhemispheric pSTAT5-ir density ± SEM in gonadally-intact, vehicle-treated males (Male+PRL; n = 6; black) and ovariectomized females treated with estradiol and progesterone (Female+PRL; n = 6; grey). Counting frames for each of the analyzed nuclei are included in Fig 1. Statistical analysis was applied independently to each brain region (independent t-test for parametric data or Mann-Whitney test for non-parametric data, see Results). *P ≤ 0.05; **P ≤ 0.01; (*) P ≤ 0.06.

Fig 4. Effect of testosterone withdrawal in pSTAT5 immunoreactivity of the male mouse brain.

Assessment of pSTAT5-ir density (pSTAT5-positive cell nuclei/mm²) in gonadally-intact male mice and castrated male mice within the major brain regions showing pSTAT5 expression in the male mouse brain. Bar histograms show mean interhemispheric pSTAT5-ir density ± SEM in intact, PRL-supplemented males (Males+PRL; n = 6; black) and castrated, PRL-supplemented males (Castrated+PRL; n = 6; white). Counting frames for each of the analyzed nuclei are enclosed in Fig 2. Statistical analysis was applied independently to each brain region (independent t-test for parametric data or Mann-Whitney test for non-parametric data). *P ≤ 0.05; **P ≤ 0.01.