Interoception Network in the Rat Brain.

The brain is never truly at rest. Even in the absence of external stimuli or cognitive tasks, the central nervous system continuously receives peripheral signals from visceral organs, such as the heart, lungs, and stomach, and sends motor commands to regulate organ physiology. This bidirectional brain-body interaction, also known as interoception, engages neural pathways via a hierarchical set of brain regions, including the nucleus of the solitary tract, hypothalamus, paraventricular nuclei of the thalamus, insular cortex, and anterior cingulate cortex, among others. However, it is largely unclear to what extent interoceptive signaling shapes ongoing fluctuations and correlations of brain activity. To address this question, we recorded resting state functional magnetic resonance imaging data from 34 anesthetized rats, examined intrinsic correlations (or functional connectivity), and tested their dependence on the body's digestive state and peripheral nerve integrity. We observed reciprocal functional connectivity among brain regions situated along established interoceptive pathways, revealing a cohesive network, which we refer to as the interoception network. This network showed stronger functional connectivity in the fed condition (digestive phase) compared to the fasted condition (inter-digestive phase), suggesting its dependence on distinct states of gastrointestinal interoception without apparent cardiac or respiratory confounds. Importantly, we found that the interoception network relied on the integrity of the vagus nerve, a primary component of the peripheral nervous system for visceral sensation and parasympathetic control. When vagal signaling was surgically severed by bilateral cervical vagotomy, functional connectivity within the interoception network was notably reduced. Given these findings, we conclude that resting state functional connectivity is not sustained by the central nervous system alone, but relies on interoceptive signaling mediated through peripheral nerves that connect the brain and viscera.