TRPM4 mediates a subthreshold membrane potential oscillation in respiratory chemoreceptor neurons that drives pacemaker firing and breathing

Abstract

SUMMARY Brainstem networks that control regular tidal breathing depend on excitatory drive, including from tonically active, CO 2 /H + -sensitive neurons of the retrotrapezoid nucleus (RTN). Here, we examine intrinsic ionic mechanisms underlying the metronomic firing activity characteristic of RTN neurons. In mouse brainstem slices, large-amplitude membrane potential oscillations are evident in synaptically isolated RTN neurons after blocking action potentials. The voltage-dependent oscillations are abolished by sodium replacement; blocking calcium channels (primarily L-type); chelating intracellular Ca 2+ ; and inhibiting TRPM4, a Ca 2+ -dependent cationic channel. Likewise, oscillation voltage waveform currents are sensitive to calcium and TRPM4 channel blockers. Extracellular acidification and serotonin (5-HT) evoke membrane depolarization that augments TRPM4-dependent oscillatory activity and action potential discharge. Finally, inhibition of TRPM4 channels in the RTN of anesthetized mice reduces central respiratory output. These data implicate TRPM4 in a subthreshold oscillation that supports the pacemaker-like firing of RTN neurons required for basal, CO 2 -stimulated, and state-dependent breathing.

TRPM4 Contributes to Subthreshold Membrane Potential Oscillations in Multiple Mouse Pacemaker Neurons

“…A similar subthreshold oscillation in RTN chemosensory neurons requires activity of L-type calcium channels and, secondarily, the calcium-activated TRPM4 nonspecific cation channel ( Li et al, 2021 ). In previous work from LC neurons, the subthreshold oscillation (i.e., TTX-insensitive spikes and spikelets) was also attributed to L-type calcium channels but involvement of TRPM4 was not assessed ( Filosa and Putnam, 2003 ; Imber and Putnam, 2012 ; Sanchez-Padilla et al, 2014 ).…”

“…Brain slices were prepared from neonatal Phox2b::GFP mice at ∼4 h after lights on at 7 A.M. [i.e., at ∼11 A.M., around zeitgeber time (ZT)4; Li et al, 2021 ]. Briefly, pups [postnatal day (P)6–P14] were anesthetized with ketamine and xylazine (375 and 25 mg/kg, i.m.…”

“…These voltage-dependent oscillations are revealed after blocking fast voltage-activated sodium (Na V ) channels, and are typically ∼20 mV in amplitude in the 2- to 4-Hz frequency range. Specifically, they have been observed in suprachiasmatic nucleus (SCN) neurons that comprise the circadian clock ( Pennartz et al, 2002 ; Jackson et al, 2004 ), dopaminergic neurons of the substantia nigra pars compacta (SNpc; Yung et al, 1991 ; Nedergaard and Greenfield, 1992 ; Nedergaard et al, 1993 ; Chan et al, 2007 ; Puopolo et al, 2007 ), noradrenergic neurons of the locus coeruleus (LC; Christie et al, 1989 ; Filosa and Putnam, 2003 ; Imber and Putnam, 2012 ; Sanchez-Padilla et al, 2014 ), and most recently in Nmb -expressing respiratory chemoreceptor neurons in the retrotrapezoid nucleus (RTN; Li et al, 2021 ).…”

“…In terms of ion channel pharmacology, it was recently demonstrated that subthreshold oscillations in RTN neurons depends on both L-type calcium channels (i.e., they were sensitive to dihydropyridine antagonists) and on TRPM4 calcium-dependent cationic channels [i.e., they were sensitive to 9-phenanthrol (9-pt) and 4-chloro-2-[[2-(2-chlorophenoxy)acetyl]amino]benzoic acid (CBA); Li et al, 2021 ]. The seemingly similar oscillations in SCN, SNpc, and LC neurons also include contributions from L-type calcium channels but a role for TRPM4 channels has not yet been evaluated ( Yung et al, 1991 ; Nedergaard and Greenfield, 1992 ; Nedergaard et al, 1993 ; Pennartz et al, 2002 ; Filosa and Putnam, 2003 ; Jackson et al, 2004 ; Chan et al, 2007 ; Puopolo et al, 2007 ; Sanchez-Padilla et al, 2014 ; Flourakis et al, 2015 ).…”

ICAN (TRPM4) Contributes to the Intrinsic Excitability of Prefrontal Cortex Layer 2/3 Pyramidal Neurons

“…In our experiments, HFS increased the firing frequency and the ADP amplitude in a Ca 2+ i -dependent manner and the inhibition of TRPM4 reduced but did not eliminate ADP. The remaining ADP amplitude could be attributed to a combination of channels, particularly T-type Ca 2+ channels, since the application of Cd 2+ produced a small reduction in ADP, which is different than what is observed in retrotrapezoid nucleus neurons, where L-type Ca 2+ channels control the activation of TRPM4 and then, the pacemaker firing [58]. In this respect, our results using Cd 2+ suggest the participation of a T-type Ca 2+ channels which are less sensitive to Cd 2+ than other VGCC, opening the possibility that these channels take part in the activation of TRPM4 in an equivalent way as in thalamic reticular nucleus neurons [54]; however, further experiments need to address this hypothesis.…”