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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 (ERα) 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 ERα across the rostrocaudal axis of the PAG; and (2) to determine whether AR and/or ERα were colocalized on PAG neurons projecting to the RVM in male and female rats. AR and ERα immunoreactive neurons (AR-IR, ERα-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 ERα was comparable between thesexes. In both sexes, approximately 25-50% of AR-IR neurons and 20-50% of ERα-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.


This article was published in the Journal of Chemical Neuroanatomy and is available to subscribers here: Copyright © 2008 Elsevier B.V.

The pre-print version is posted here with permission of the author.