The Role of Noradrenergic and Glutamatergic Signaling in Noradrenergic Neurons in Respiratory Control

Abstract

Central noradrenergic (NA) neurons are a critical component of the brainstem respiratory network that maintains blood CO2 and O2 levels through respiratory chemoreflexes. NA dysfunction is implicated in numerous developmental respiratory disorders such as the fatal Sudden Infant Death Syndrome (SIDS), Rett Syndrome, and Congenital Central Hypoventilation Syndrome (CCHS), in which abnormal chemosensory function is implicated. However, which NA neurons modulate respiratory chemoreflexes and the underlying mechanisms are still not fully understood. Previous studies in our lab utilizing intersectional chemogenetic (hM4D and hM3D DREADD mediated) inhibition and excitation identified multiple rhombomere (transient genetically-defined hindbrain segments arising during early development) specific NA populations that play a role in the adult hypercapnic chemoreflex. NA neurons are typically thought to communicate with synaptic partners through noradrenaline as their primary neurotransmitter. However, the excitatory neurotransmitter, glutamate, is also expressed in subpopulations of NA neurons including those derived from rhombomeres playing a role in the hypercapnic reflex. Prior studies have implicated NA derived glutamatergic signaling in respiratory control. Thus, we seek to determine the relative roles of noradrenergic and glutamatergic signaling in rhombomeric NA populations that are involved in adult respiratory chemoreflexes. To determine the requirement of noradrenergic signaling in the hypercapnic reflex, we aim to conditionally ablate noradrenergic signaling in rhombomeric NA sub-populations by crossing rhombomere specific Credrivers with the THflox/flox(tyrosine hydroxylase) allele to remove dopamine and downstream (nor) adrenaline production in targeted neurons. We also aim to determine if the glutamate producing NA neurons represented a larger functional cohort in breathing control. We therefore used our intersectional strategy to target and chemogenetically activate (hM3D) or inhibite (hM4De) glutamate co-expressing NA neurons (defined by partially overlapping Vglut2Cre; DBHFLPo activity) which are expected to comprise a super-set of rhombomere specific NA neurons that play a role in respiratory chemoreflexes. Whole-body barometric plethysmography was used to measure the respiratory function of the mice under room air (21%O2,79%N2) and hypercapnia (21%O2, 74%N2, 5% CO2). Our preliminary data suggests that removing noradrenergic signaling in a key rhombomeric NA subpopulation does not inhibit the hypercapnic reflex while DREADD mediated inhibition does reduce the hypercapnic reflex. Acute inhibition of glutamate-expressing NA neurons shows a reduction in the hypercapnic ventilatory reflex while acute activation of these same neurons shows a significantly enhanced reflex. Altogether, these preliminary data suggest that glutamate production may delineate a functionally specific NA subpopulation in the hypercapnic reflex and offers additional clues to the respective roles of noradrenaline and glutamatergic signaling in the noradrenergic system.