Infralimbic prefrontal cortical projections to the autonomic brainstem: Quantification of inputs to cholinergic and adrenergic/noradrenergic nuclei

The ventromedial prefrontal cortex regulates both emotional and physiological processes. The infralimbic cortex (IL), a prefrontal subregion in rodents, integrates behavioral, neuroendocrine, and autonomic responses to stress. However, the organization of cortical inputs to brainstem nuclei that regulate homeostatic responses are not well defined. We hypothesized that IL projections differentially target pre-ganglionic parasympathetic neurons and adrenergic/noradrenergic nuclei. To quantify IL projections to autonomic brainstem nuclei in male rats, we utilized viral-mediated gene transfer to express yellow fluorescent protein (YFP) in IL glutamatergic neurons. YFP-positive projections to cholinergic and adrenergic/noradrenergic nuclei were then imaged and quantified. Cholinergic neurons were visualized by immunohistochemistry for choline acetyltransferase (ChAT), the enzyme responsible for the synthesis of acetylcholine. Adrenergic/noradrenergic neurons were visualized with immunohistochemistry for dopamine beta-hydroxylase (DBH) which converts dopamine to norepinephrine. Our results indicate that IL glutamate neurons innervated the cholinergic dorsal motor nucleus of the vagus with greater density than the nucleus ambiguus. Furthermore, numerous DBH-positive cell groups received IL inputs. The greatest density was to the C2 and A2 regions of the nucleus of the solitary tract with intermediate levels of input to A6 locus coeruleus and throughout the C1 and A1 regions of the ventrolateral medulla. Minimal input was present in the pontine A5. Additionally, IL projections targeted the local GABAergic neurons that regulate activity within preautonomic nuclei. Collectively, our results indicate that IL pyramidal neurons project to vagal preganglionic parasympathetic neurons, presympathetic neurons of the ventrolateral medulla, as well as diffuse homeostatic modulators the nucleus of the solitary tract and locus coeruleus. Ultimately, these findings provide a roadmap for determining circuit-level mechanisms for neural control of homeostasis and autonomic balance.