EXPRESSION OF FOS-RELATED ANTIGENS, OXYTOCIN, DYNORPHIN AND GALANIN IN THE PARAVENTRICULAR AND SUPRAOPTIC NUCLEI OF LACTATING RATS

The magnocellular oxytocin neurons within the paraventricular and supr aoptic nuclei (PVN and SON) of the hypothalamus are important relays i n the milk ejection reflex in lactating animals, and are activated by suckling. It has been suggested that proto-oncogene transcription fact ors such as Fos/Jun act as early nuclear transducers of sensory stimul i in neurons. Therefore, we have studied with immunohistochemistry Fos -related antigens (FRAs) as a marker for neuronal activity in the PVN and SON during suckling in lactating rats. In nonlactating rats, only few cells exhibiting FRAs were observed in these nuclei. Also in lacta ting rats subjected to continuous suckling Fos-like activity was low. In contrast, lactating rats separated from their pups for 4 h and then exposed to suckling for 1 h expressed strong Fos-like immunoreactivit y, both in vasopressin and oxytocin neurons. Using in situ hybridizati on and immunohistochemistry we have also investigated the expression o f the mRNAs for oxytocin, dynorphin, galanin and galanin message-assoc iated peptide and of oxytocin and dynorphin in the PVN of lactating an d nonlactating rats. In lactating rats, an increase in oxytocin and dy norphin and their mRNAs was observed, whereas mRNAs for galanin and ga lanin message-associated peptide were downregulated. With the help of immunohistochemistry and double-staining methods, a substantial coexis tence between oxytocin- and dynorphin-like immunoreactivities was show n in magnocellular neurons. These results indicate that FRAs are activ ated in the PVN in the beginning of a suckling period, while this resp onse cannot be seen after continuous stimulation. Furthermore, in the PVN of lactating rats, an upregulation of oxytocin and dynorphin occur s while galanin expression decreases. Finally, the coexistence between oxytocin and dynorphin is more pronounced in lactating rats and nonla ctating female rats than has previously been described in male rats.