NEURONAL ACTIVATION OF THE HYPOTHALAMIC MAGNOCELLULAR SYSTEM IN RESPONSE TO OROPHARYNGEAL STIMULI IN THE RAT

The present study was designed to delineate the neuronal site, the nat ure, and the gastrointestinal origin of the stimulation of the hypotha lamic magnocellular system induced by the ingestion of sweetened conde nsed milk, Concomitant localization of the c-fos protein (Fos) with ei ther arginine-vasopressin (AVP) mRNA or oxytocin (OT) mRNA in the para ventricular nucleus of the hypothalamus (PVH) and the supraoptic nucle us (SON) revealed that the hypothalamic neurons containing AVP and OT were activated following ingestion of sweetened condensed milk, Expres sion of c-fos mRNA was also determined in rats implanted with a gastri c cannula that allowed for real, sham, and gastric feeding of sweetene d condensed milk. The results provide evidence that the stimulation of the PVH and SON induced by sweetened condensed milk originate from or opharyngeal stimuli, Indeed, in real-and sham-fed rats, the postprandi al levels of c-fos mRNA in the PVH and SON were significantly higher t han the preprandial values, whereas there was no early postprandial ri se in c-fos mRNA levels within the magnocellular division of the PVH a nd SON after gastric feeding. The results of this study also suggested that the stimulation of the PVH and SON induced by sweetened condense d milk was related to the hypertonicity of the milk, Indeed, ingestion of an hypertonic solution of sucrose with a carbohydrate content clos e to that of sweetened condensed milk led to a stimulation of the PVH and SON that was comparable to that induced by the milk, whereas inges tion of an isotonic solution of sucrose did not trigger any significan t activation of the PVH and SON, Taken together, the present results i ndicate that magnocellular neurosecretory neurons are sensitive to oro pharyngeal stimuli and further support the view of the existence of or opharyngeal osmoreceptors.