NEUROPEPTIDE COEXPRESSION IN THE MAGNOCELLULAR NEUROSECRETORY-SYSTEM OF THE FEMALE RAT - EVIDENCE FOR DIFFERENTIAL MODULATION BY ESTROGEN

Single- and double-immunohistochemical staining methods were used to a ssay the effect of estrogen on the expression of po-existing peptides in the magnocellular neurosecretory system of the female rat. It was c onfirmed in colchicine-treated, ovariectomized animals that immunoreac tive corticotropin releasing factor and cholecystokinin co-exist in su bsets of oxytocinergic neurons; in addition, dynorphin immunoreactivit y was detected in a substantial majority of oxytocin containing magnoc ellular neurons. Consistent with previous studies, magnocellular vasop ressinergic cells were found to display angiotensin II-, dynorphin- an d galanin-immunoreactivities. Comparable results occurred in colchicin e-treated ovariectomized rats independent of whether or not the animal s received replacement injections of estradiol benzoate or vehicle. Ov ariectomized rats that were not pretreated with colchicine showed enha nced staining (increased cell number and staining intensity of both ce ll bodies and terminals in the posterior pituitary) for each of the pe ptides that was found to co-exist in vasopressinergic neurons after tr eatment with estradiol; staining for vasopressin was similar in steroi d- and oil-treated animals. Perikaryal staining for peptides co-locali zed with oxytocin was not discernibly different in estradiol- vs vehic le-treated animals, while in the posterior lobe, differential effects of hormone replacement on oxytocin, cholecystokinin, and corticotropin -releasing factor immunostaining of terminals were apparent. Perikarya l staining for co-existing peptides in gonadally intact animals killed at the estrus or the diestrus II phases of the estrous cycle provided a pattern of results compatible with those seen in ovariectomized ani mals treated with estradiol or oil, respectively. These observations s uggest that circulating gonadal steroids affect co-existing peptide ex pression differentially in oxytocinergic vs vasopressinergic neurons. All peptides examined that could be colocalized in vasopressinergic ce lls showed evidence of enhanced expression in the presence of estrogen , while at least two of these co-localized with oxytocin appeared driv en in the opposite direction. The results in normally cycling rats ind icate that this kind of influence may be manifest under normal physiol ogical conditions.