Variations in opioid peptide levels during the estrous cycle in Sprague-Dawley rats

• The estrous cycle, with its various hormonal conditions, may provide us with the means of understanding how endocrine states relate to opioid mechanisms. There has been increasing experimental support for interaction between sex steroids and opioid peptides in the central nervous system. Here, we describe fluctuations in endogenous brain immunoreactive (ir) peptide levels during various phases of the estrous cycle in the female Sprague-Dawley rat. Ir levels of dynorphin A, dynorphin B, Leu-enkephalin-Arg(6), Met-enkephalin-Arg(6)Phe(7) and nociceptin/orphanin FQ were measured in the pituitary gland and in 10 areas of the brain during the diestrus, proestrus and estrus phase. In several areas of the brain, basal levels of endogenous opioid peptides showed variation during the course of the estrous cycle. Significant differences were found between the diestrus state and the proestrus and/or estrus conditions, particularly in the nucleus accumbens, caudate putamen and the substantia nigra. The ir levels of the endogenous peptide nociceptin/orphanin FQ became altered in only one of the areas measured, indicating less variance during the estrous cycle. Correlation analyses revealed that significant associations between dynorphin A or dynorphin B and Leu-enkephalin-Arg(6) were found more often during estrus than during the diestrus and proestrus conditions. The ratio between the ir levels of Leu-enkephalin-Arg(6), a cleavage product of the enzymatic conversion of dynorphin peptides into shorter peptides in vivo, and dynorphin peptides was calculated. The significantly lower ratio between Leu-enkephalin-Arg(6) and dynorphin B in diestrus than in proestrus and estrus also indicates cyclic fluctuations in the enzymatic cleavage of dynorphin. These findings are discussed in relation to the possible role of interactions between sex steroids and opioid peptide mechanisms during the normal estrous cycle.

Subject headings

MEDICIN OCH HÄLSOVETENSKAP  -- Medicinska och farmaceutiska grundvetenskaper -- Farmaceutiska vetenskaper (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Basic Medicine -- Pharmaceutical Sciences (hsv//eng)

MEDICIN OCH HÄLSOVETENSKAP  -- Medicinska och farmaceutiska grundvetenskaper (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Basic Medicine (hsv//eng)

Keyword

Dynorphin

Leu-enkephalin-Arg6

Met-enkephalin-Arg6Phe7

Nociceptin/Orphanin FQ

Neuropeptides

Radioimmunoassay

Peptide processing

Diestrus

Proestrus

Estrus

PHARMACY

FARMACI

Publication and Content Type

ref (subject category)

art (subject category)