Mechanisms for communication between the gut and brain

Numerous studies have indicated that variations in the intestinal microflora can influence brain activity and may greatly affect various psychosis. Whether this influence is due to direct neuronal circuitry or more global systemic influences is unknown. The current study attempts to identify the mechanism by which the intestine influences brain activity by optogenetically manipulating the activity of the vagal nerve to a subsection of the jejunum. Methods: Normal mouse behavior was recorded during behavioral activities (i.e., elevated plus maze, open-field, and forced swim tests) for a period of two weeks. Mice were then surgically given 3-5 injections (0.01-0.03 μl) of either an optogenetic vector (AAV-CaMKIIa-eNpHR3.0-EYFP) or saline control into the mesenteric wall of the jejunum. During surgery the cervical vagus was identified, and a fiber optic ferrule implanted to allow illumination of the vagus nerve. After surgical recovery, behavioral tests were initiated for 2 weeks to assess variations in behavior from the initial control that may be caused by the surgery. These tests were then utilized as the control behavioral activity for each animal. Four weeks post-surgery, behavioral experiments were initiated in which vagal afferents to the injected regions of the jejunum were either deactivated with light stimulation of the vagus or in the normal (non-deactivated) state. Results were then compared for each animal. Results: Mice activity was consistent across pre-surgical control, post-surgical control, and post-surgical normal state behavioral tests. In the saline condition, animals with light activation of the vagus showed no variation in their behavioral activity from that of the other control states. In the optogenetic light deactivation condition, animal behavior was dramatically different. In each behavioral test, animals showed a freezing behavior for the extent of time that the vagus was deactivated. Within the forced swim experiments, frozen animals did not keep their head above water and tended to sink to the bottom of the tank (requiring multiple rescues during each session). Conclusions: Optogenetic deactivation of the vagal afferents of the jejunum had a dramatic and immediate influence on animal behavior. Therefore, this pathway provides an optimal mechanism by which the enteric system can influence brain activity and thus behavior.