Abstract
Oxytocin is a potent anorexigen and is believed to have a role in satiety signaling. We developed rat models to study the activity of oxytocin neurons in response to voluntary consumption or oral gavage of foods using c-Fos immunohistochemistry and in vivo electrophysiology. Using c-Fos expression as an indirect marker of neural activation, we showed that the percentage of magnocellular oxytocin neurons expressing c-Fos increased with voluntary consumption of sweetened condensed milk (SCM). To model the effect of food in the stomach, we gavaged anesthetized rats with SCM. The percentage of supraoptic nucleus and paraventricular nucleus magnocellular oxytocin-immunoreactive neurons expressing c-Fos increased with SCM gavage but not with gastric distention. To further examine the activity of the supraoptic nucleus, we made in vivo electrophysiological recordings from SON neurons, where anesthetized rats were gavaged with SCM or single cream. Pharmacologically identified oxytocin neurons responded to SCM gavage with a linear, proportional, and sustained increase in firing rate, but cream gavage resulted in a transient reduction in firing rate. Blood glucose increased after SCM gavage but not cream gavage. Plasma osmolarity and plasma sodium were unchanged throughout. We show that in response to high-sugar, but not high-fat, food in the stomach, there is an increase in the activity of oxytocin neurons. This does not appear to be a consequence of stomach distention or changes in osmotic pressure. Our data suggest that the presence of specific foods with different macronutrient profiles in the stomach differentially regulates the activity of oxytocin neurons.
Highlights
In the paraventricular nucleus (PVN), the percentage of magnocellular oxytocin neurons expressing c-Fos significantly increased with voluntary sweetened condensed milk (SCM) consumption in comparison with controls [Fig. 1(c) and 1(e)]
There was no significant difference in c-Fos expression in parvocellular PVN oxytocin neurons between the control and SCM consumption groups [Fig. 1(d)]
We studied the involvement of oxytocin neurons in the response to food consumption or gavage using c-Fos immunoreactivity and in vivo electrophysiology
Summary
We developed rat models to study the activity of oxytocin neurons in response to voluntary consumption or oral gavage of foods using c-Fos immunohistochemistry and in vivo electrophysiology. The percentage of supraoptic nucleus and paraventricular nucleus magnocellular oxytocin-immunoreactive neurons expressing c-Fos increased with SCM gavage but not with gastric distention. We show that in response to high-sugar, but not high-fat, food in the stomach, there is an increase in the activity of oxytocin neurons This does not appear to be a consequence of stomach distention or changes in osmotic pressure. Expression of the immediate early gene c-Fos in magnocellular oxytocin neurons of the SON is increased by food intake and associated stimuli, such as refeeding after fasting [11], palatable food consumption [12], changes in osmotic pressure [13], gastric distention [14], and systemic administration of the satiety mediators
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