Hypoxia augments TRPM3-mediated calcium influx in vagal sensory neurons

Abstract

Transient receptor potential melastatin 3 (TRPM3) channels contribute to nodose afferent and brainstem nucleus tractus solitarii (nTS) activity. Exposure to short, sustained hypoxia (SH) and chronic intermittent hypoxia (CIH) enhances nTS activity, although the mechanisms are unknown. We hypothesized TRPM3 may contribute to increased neuronal activity in nTS-projecting nodose ganglia viscerosensory neurons, and its influence is elevated following hypoxia. Rats were exposed to either room air (normoxia), 24-h of 10 % O₂ (SH), or CIH (episodic 6 % O₂ for 10d). A subset of neurons from normoxic rats were exposed to in vitro incubation for 24-h in 21 % or 1 % O₂. Intracellular Ca²⁺ of dissociated neurons was monitored via Fura-2 imaging. Ca²⁺ levels increased upon TRPM3 activation via Pregnenolone sulfate (Preg) or CIM0216. Preg responses were eliminated by the TRPM3 antagonist ononetin, confirming agonist specificity. Removal of extracellular Ca²⁺ also eliminated Preg response, further suggesting Ca²⁺ influx via membrane-bound channels. In neurons isolated from SH-exposed rats, the TRPM3 elevation of Ca²⁺ was greater than in normoxic-exposed rats. The SH increase was reversed following a subsequent normoxic exposure. RNAScope demonstrated TRPM3 mRNA was greater after SH than in Norm ganglia. Incubating dissociated cultures from normoxic rats in 1 % O₂ (24-h) did not alter the Preg Ca²⁺ responses compared to their normoxic controls. In contrast to in vivo SH, 10d CIH did not alter TRPM3 elevation of Ca²⁺. Altogether, these results demonstrate a hypoxia-specific increase in TRPM3-mediated calcium influx.