Calcium permeability in NMDA receptors in NTS baroreceptive neurons is not decreased at low temperature in Syrian hamsters, a hibernating species

Abstract

Neurons in the nucleus tractus solitarius (NTS) receiving cardio‐respiratory afferent information continue to process signals at 15°C in the Syrian hamster. It is not known if intrinsic adaptations in NMDA receptors limit Ca2+ influx and hence augment neuroprotection in this hibernating species. As an initial step in identifying potential intrinsic adaptations, we tested the hypothesis that the Ca2+/Cs+ permeability ratio of NMDA receptors is decreased in hamsters compared to rats at 15°C. Using fluorescent tracer techniques, we identified 2nd‐order NTS baroreceptive neurons and recorded whole‐cell currents with Cs+ replacement of K+ in the pipette solution. From I‐V plots before and after bath perfusion of NMDA (0.3mM) in the Na+ and K+ free extracellular solution containing 2 mM Ca2+, we determined reversal potentials. Calculated Ca2+/Cs+ permeability ratios were 6.6±0.6 (n = 8 neurons) in euthermic hamsters, 7.1±1.0 (n = 13 neurons) in rats, and 7.7±1.0 (n = 9 neurons) in hibernating hamster (p = 0.782). These data negate our hypothesis and suggest that intrinsic NMDA adaptations do not reduce Ca2+ conductance relative to K+ conductance. Barring other adaptations in the NMDA receptor itself, adaptations of other membrane properties that affect activation of NMDA receptors may take on a major protective role in hamster NTS neurons. (support: R01 HL091763)