Concentration of chicken gonadotropin-releasing hormones I and II in microdissected areas of turkey hen brain during the reproductive cycle.

Chicken gonadotropin-releasing hormones I and II (cGnRH I and II) were measured by radioimmunoassay (RIA) in extracts of microdissected regions of turkey hen brain (preoptic area [POA], region of periventricular nuclei [PHN], septum [SEP], hippocampus [HP], dorsomedial thalamus/habenula [DMT], midbrain central gray [MCG], and caudal lateral hypothalamus [LH]) at five stages of the reproductive cycle: before photostimulation, during egg laying, during incubation, during photorefractoriness, and after return to short daylengths. The highest concentration of cGnRH I occurred in PHN, followed by POA, SEP, DMT, HP, LH, and MCG, in decreasing order, whereas the highest concentration of cGnRH II occurred in SEP, followed by POA, DMT, HP, MCG, PHN, LH. These results agree, with some exceptions, with the distribution of fibers and cells as determined by immunohistochemistry. cGnRH II was from 1.3 to 24 times as abundant as cGnRH I in different brain areas. During incubation, cGnRH I concentrations were significantly elevated in the POA and cGnRH II levels were significantly elevated in HP; few other significant differences were detected. Correlation analysis detected occasional significant positive and negative correlations between cGnRH I and II concentrations in forebrain areas and MCG of laying birds and in PHN and LH of incubating birds. These results demonstrate an approximate correspondence between hormone concentrations measured in tissue extracts by RIA and immunohistochemistry and indicate an abundance of cGnRH II as compared with cGnRH I. cGnRH I and II concentrations did not, however, change in parallel in all brain areas, suggesting that these peptides do not function in an exactly parallel fashion. Thus, an extent to which cGnRH II is involved in gonadotropin release remains unresolved.