Diet-Induced Obesity Differentially Regulates Intestinal Nutrient Sensing by Enteric Neurons: Evidence of Direct Neuronal Activation by Luminal Nutrients

Objectives: Background: Our published data shows that blocking intestinal cholinergic neurotransmission in diet-induced obese (DIO) mice resulted in significant weight loss and robust improvements in hyperglycemia, hyperlipidemia, and glucose tolerance. Metabolic benefits occurred independent of weight loss, implying potential direct roles of enteric neurons in nutrient sensing. Hypothesis: We hypothesized that high-fat feeding alters the activity of enteric neurons in the small intestine. Methods: Lean and DIO mice were administered orogastric gavage of glucose, oleic acid, or saline. Two hours later, whole mounts were prepared from submucosal and longitudinal layer myenteric plexus (LMMP) of the proximal and distal small intestines. Immunohistochemistry analyses were performed to evaluate expression of neuronal activation marker, c-FOS. Electrophysiological studies were performed in primary neuronal cultures isolated from both layers and intracellular calcium transients were measured as surrogate for real-time neuronal activity. Results: Both glucose and oleic acid led to robust increase in percentage of c-FOS positive nuclei (over total HuC/D positive cell bodies) in the proximal submucosal intestinal layers of lean mice (87%, p=0.0003; 78%, p=0.002). However, significant blunting was observed in the DIO mice (-20%, p=0.03). A similar pattern was observed in the distal small intestine. Nutrient-mediated activation was less pronounced in the LMMPs of lean and DIO mice (p=0.06). Interestingly, DIO mice had lower baseline activity in proximal and distal small intestines (p=0.008). Primary neurons isolated from the submucosal layer had 2-3-fold increase in fluorescence intensity induced by calcium entry (ΔF/F0), confirming neuronal activation by glucose (p=0.003) and oleic acid (p=0.02), respectively. Conclusions: Enteric neurons of the submucosal layer are more sensitive to nutrient stimulation than the deeper myenteric layer. Obesity blunts glucose and oleic acid mediated neuronal activation along the entire small intestine. This is the FIRST study, to our knowledge, that provides evidence of direct activation of enteric neurons by nutrients and lays the foundation for potential therapeutic strategies targeting neural mediation of intestinal nutrient sensing. Funding Sources: MWU Intramural Funds.