Somatic Ca2+ transients do not contribute to inspiratory drive in preBotzinger complex neurons

PreBotzinger Complex (preBotC) neurons are postulated to underlie respiratory rhythm generation. The inspiratory phase of the respiratory cycle in vitro results from preBotC neurons firing synchronous bursts of action potentials (APs) on top of 10-20 mV, 0.3-0.8 s inspiratory drive potentials. Is the inspiratory drive in individual neurons simply the result of the passive integration of inspiratory-modulated synaptic currents or do active processes modulate these currents ? As somatic Ca2+ is known to increase during inspiration, we hypothesized that it affects inspiratory drive. We combined whole cell recording in an in vitro slice preparation with Ca2+ microfluorometry to detect single inspiratory neuron somatic Ca2+ transients with high temporal resolution (similar to mu s). In neurons loaded with either Fluo-4 or Oregon Green BAPTA 5 N, we observed Ca2+ transients associated with each AP. During inspiration, significant somatic Ca2+ influx was a direct consequence of activation of voltage-gated Ca2+ channels by APs. However, when we isolated the inspiratory drive potential in active preBotC neurons (by blocking APs with intracellular QX-314 or by hyperpolarization), we did not detect somatic Ca2+ transients; yet, the parameters of inspiratory drive were the same with or without APs. We conclude that, in the absence of APs, somatic Ca2+ transients do not shape the somatic inspiratory drive potential. This suggests that in preBotC neurons, substantial and widespread somatic Ca2+ influx is a consequence of APs during the inspiratory phase and does not contribute substantively to the inspiratory drive potential. Given evidence that the Ca2+ buffer BAPTA can significantly reduce inspiratory drive, we hypothesize that dendritic Ca2+ transients amplify inspiratory-modulated synaptic currents.