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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 nature, and the gastrointestinal origin of the stimulation of the hypothalamic magnocellular system induced by the ingestion of sweetened condensed milk. Concomitant localization of the c‐fos protein (Fos) with either arginine‐vasopressin (AVP) mRNA or oxytocin (OT) mRNA in the paraventricular nucleus of the hypothalamus (PVH) and the supraoptic nucleus (SON) revealed that the hypothalamic neurons containing AVP and OT were activated following ingestion of sweetened condensed milk. Expression of c‐fos mRNA was also determined in rats implanted with a gastric cannula that allowed for real, sham, and gastric feeding of sweetened condensed milk. The results provide evidence that the stimulation of the PVH and SON induced by sweetened condensed milk originate from oropharyngeal stimuli. Indeed, in real‐and sham‐fed rats, the postprandial levels of c‐fos mRNA in the PVH and SON were significantly higher than the preprandial values, whereas there was no early postprandial rise in c‐fos mRNA levels within the magnocellular division of the PVH and SON after gastric feeding. The results of this study also suggested that the stimulation of the PVH and SON induced by sweetened condensed milk was related to the hypertonicity of the milk. Indeed, ingestion of an hypertonic solution of sucrose with a carbohydrate content close 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 ingestion of an isotonic solution of sucrose did not trigger any significant activation of the PVH and SON. Taken together, the present results indicate that magnocellular neurosecretory neurons are sensitive to oropharyngeal stimuli and further support the view of the existence of oropharyngeal osmoreceptors.