Sodium and Calcium Current-Mediated Pacemaker Neurons and Respiratory Rhythm Generation
Journal of Neuroscience
The breathing motor pattern in mammals originates in brainstem networks. Whether pacemaker neurons play an obligatory role remains a key unanswered question. We performed whole-cell recordings in the preBötzinger Complex in slice preparations from neonatal rodents and tested for pacemaker activity. We observed persistent Na ϩ current (I NaP )-mediated bursting in ϳ5% of inspiratory neurons in postnatal day 0 (P0)-P5 and in P8 -P10 slices. I NaP -mediated bursting was voltage dependent and
... dependent and blocked by 20 M riluzole (RIL). We found Ca 2ϩ current (I Ca )-dependent bursting in 7.5% of inspiratory neurons in P8 -P10 slices, but in P0 -P5 slices these cells were exceedingly rare (0.6%). This bursting was voltage independent and blocked by 100 M Cd 2ϩ or flufenamic acid (FFA) (10 -200 M), which suggests that a Ca 2ϩ -activated inward cationic current (I CAN ) underlies burst generation. These data substantiate our observation that P0 -P5 slices exposed to RIL contain few (if any) pacemaker neurons, yet maintain respiratory rhythm. We also show that 20 nM TTX or coapplication of 20 M RIL ϩ FFA (100 -200 M) stops the respiratory rhythm, but that adding 2 M substance P restarts it. We conclude that I NaP and I CAN enhance neuronal excitability and promote rhythmogenesis, even if their magnitude is insufficient to support bursting-pacemaker activity in individual neurons. When I NaP and I CAN are removed pharmacologically, the rhythm can be maintained by boosting neural excitability, which is inconsistent with a pacemaker-essential mechanism of respiratory rhythmogenesis by the preBötzinger complex.