Androgen and estrogen (alpha) receptor localization on periaqueductal gray neurons projecting to the rostral ventromedial medulla in the male and female rat

Abstract

The periaqueductal gray (PAG) is involved in many gonadal steroid-sensitive behaviors, including responsiveness to pain. The PAG projects to the rostral ventromedial medulla (RVM), comprising the primary circuit driving pain inhibition. Morphine administered systemically or directly into the PAG produces greater analgesia in male compared to female rats, while manipulation of gonadal hormones alters morphine potency in both sexes. It is unknown if these alterations are due to steroidal actions on PAG neurons projecting to the RVM. The expression of androgen (AR) and estrogen (ERalpha) receptors in the PAG of female rats and within this descending inhibitory pathway in both sexes is unknown. The present study used immunohistochemical techniques (1) to map the distribution of AR and ERalpha across the rostrocaudal axis of the PAG; and (2) to determine whether AR and/or ERalpha were colocalized on PAG neurons projecting to the RVM in male and female rats. AR and ERalpha immunoreactive neurons (AR-IR, ERalpha-IR) were densely distributed within the caudal PAG of male rats, with the majority localized in the lateral/ventrolateral PAG. Females had significantly fewer AR-IR neurons, while the quantity of ERalpha was comparable between the sexes. In both sexes, approximately 25-50% of AR-IR neurons and 20-50% of ERalpha-IR neurons were retrogradely labeled. This study provides direct evidence of the expression of steroid receptors in the PAG and the descending pathway driving pain inhibition in both male and female rats and may provide a mechanism whereby gonadal steroids modulate pain and morphine potency.

Figure 1

Distribution of cells in the PAG immunoreactive for AR (red circles) and ERα (black circles) in male (left side of plots) and female rats (right side of plots) at six rostrocaudal levels (A-F) of the PAG.

Figure 2

Bar graphs display the mean number (± S.E.M.) of AR immunoreactive cells (green bars) and ERα immunoreactive cells (purple bars) across six rostrocaudal levels of the PAG. Cell counts were combined for the dorsomedial and lateral/ventrolateral subdivisions of PAG. # denotes a significant sex difference in mean # of steroid receptors.

Figure 3

Color photomicrograph showing a low (A,C) and high (B,D) power example of single- and double-labeled AR and FG immunoreactive cells in the lateral PAG (bregma -8.00) of a male (A-B) and female rat (C-D). Scale bar = 100 μm for low power images; scale bar = 50 μm for high power images.

Figure 4

Distribution of cells in the PAG retrogradely labeled from the rostral ventromedial medulla (black circles) and immunoreactive for AR (red stars) and ERα (open stars) in male (left side of plots) and female rats (right side of plots) at six rostrocaudal levels of the PAG.

Figure 5

Bar graphs display the mean (± S.E.M.) %AR/FG+, %FG/AR+, %ERα/FG+, and %FG/ERα+ immunoreactive neurons for the dorsomedial combined with lateral/ventrolateral regions of PAG across six rostrocaudal levels of the PAG.

Figure 6

Color photomicrograph showing a low (A,C) and high (B,D) power example of single- and double-labeled ERα and FG immunoreactive cells in the lateral PAG (bregma -8.00) of a male (A-B) and female rat (C-D). Scale bar = 100 μm for low power images; scale bar = 50 μm for high power images.