Neurokinin receptor-expressing pre-botzinger complex neurons in neonatal mice studied in vitro

Breathing in mammals depends on inspiratory-related neural activity generated in the pre-Bötzinger complex (preBötC), where neurokinin receptor-expressing neurons (NKR(+)) have been hypothesized to play a critical rhythmogenic role. Currently, the extent to which the preBötC is populated by rhythmogenic NKR(+) neurons and whether neurons without neurokinin receptor expression (NKR(-)) share similar electrical properties with NKR(+) neurons are not well understood. These interrelated problems must be resolved to understand the widespread excitatory effects of neuropeptides and the mechanism of respiratory rhythmogenesis. We recorded and imaged inspiratory neurons in neonatal mouse slices that isolate the preBötC and generate respiratory motor output in vitro. Using tetramethylrhodamine conjugated to the endogenous NKR agonist substance P (TMR-SP) to tag neurons that express NKRs, we show that early inspiratory neurons with small whole cell capacitance (C(M)) are 36% TMR-SP(+) and 64% TMR-SP(-). Also, late inspiratory neurons with large C(M) are 67% TMR-SP(+) and 33% are TMR-SP(-). Thus NKR(+) and NKR(-) neurons exhibit the same phenotypic properties, which suggests that they may share functional roles also. Substance P (SP) alone evoked a voltage-insensitive inward current (I(SP)) that reversed at -19 mV and was associated with an increase in membrane conductance in both NKR(+) and NKR(-) neurons. Gap junctions may be needed to confer SP sensitivity to neurons that appear to lack NKR expression. We propose that cell death in NKR(+) preBötC neurons, by targeted lesion or neurodegeneration, may impair breathing behavior by killing less than one half of the rhythmogenic preBötC neurons and a large number of respiratory premotoneurons.