THE EFFECT OF PROGESTERONE ON OXYTOCIN MESSENGER-RNA IN HYPOTHALAMIC NEURONS OF ESTROGEN-TREATED FEMALE RATS STUDIED WITH QUANTITATIVE INSITU HYBRIDIZATION HISTOCHEMISTRY

Using in situ hybridization histochemistry with tritiated synthetic oligodeoxynucleotide probes and a computer-assisted image analysis system, we examined possible progesterone effects on oxytocin messenger RNA (mRNA) in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the female estrogen-primed rat. Conditions of combined estrogen and progesterone exposures were used to address questions of relevance to the combined effect of these two hormones on female reproductive behavior. Hybridization signals for oxytocin mRNA were shown by control maneuvers to be quite specific, and were observed in neurons of the dorsolateral part of the SON and in the peripheral part of the PVN. Statistic analysis of the Student t-test and Kolmogorov-Smirnov test showed that progesterone treatment of estrogen-primed ovariectomized female rats did not affect hybridization for oxytocin mRNA in neurons of the SON, whereas prolonged progesterone exposure did significantly increase the number of silver grains in neurons of the PVN. Among the subdivisions of the PVN, the posterior ventromedial part showed a particularly strong increase in oxytocin mRNA levels. These results suggest that progesterone, working in concert with estrogen priming, has an effect on oxytocin mRNA levels in that portion of the PVN, which might be relevant for the control of female reproductive behavior.