An AMPK-dependent hypoxia-responsive subnucleus of the nucleus tractus solitarius coordinates the hypoxic ventilatory response and protects against apneoa in mice

Abstract

Abstract Functional magnetic resonance imaging (fMRI) suggests that the hypoxic ventilatory response is facilitated by the AMP-activated protein kinase (AMPK), not at the carotid bodies, but within a subnucleus (Bregma − 7.5 to -7.1mm) of the nucleus tractus solitarius that exhibits rightsided bilateral asymmetry. Here, we map this subnucleus using cFos expression as a surrogate for neuronal activation and mice in which the genes encoding the AMPK-α1 (Prkaa1) and AMPKα2 (Prkaa2) catalytic subunits were deleted in catecholaminergic cells by Cre expression via the tyrosine hydroxylase promoter. Comparative analysis of brainstem sections, relative to controls, revealed that AMPKα1/α2 deletion inhibited, with rightsided bilateral asymmetry, cFos expression in and thus activation of a neuronal cluster that partially spanned three interconnected anatomical nuclei adjacent to the area postrema: SolDL (Bregma − 7.44mm to -7.48mm), SolDM (Bregma − 7.44mm to --7.48mm) and SubP (Bregma − 7.48mm to -7.56mm). This approximates the volume identified by fMRI. Moreover, these nuclei are known to be in receipt of carotid body afferent inputs, and catecholaminergic neurons of SubP and SolDL innervate aspects of the ventrolateral medulla responsible for respiratory rhythmogenesis. Accordingly, AMPKα1/α2 deletion attenuated hypoxiaevoked increases in minute ventilation, blocked active expiration, decreased sigh frequency and increased apnoea frequency. The metabolic status of these AMPKα1/α2 knockouts and the brainstem and spinal cord catecholamine levels were equivalent to controls. We conclude, that within the brainstem an AMPK-dependent, hypoxia-responsive subnucleus partially spans SubP, SolDM and SolDL, namely SubSolHΙe, and is critical to coordination of active expiration, the hypoxic ventilatory response and defence against apnoea.