Intrinsic burst generation of preinspiratory neurons in the medulla of brainstem-spinal cord preparations isolated from newborn rats

In brainstem-spinal cord preparations isolated from newborn rats, intrinsic burst-generating properties of preinspiratory (Pre-I) neurons in the rostral ventrolateral medulla, which have been suggested to be primary respiratory rhythm-generating neurons, were studied by perforated whole-cell recordings using the antibiotic nystatin. Nystatin causes small pores to be formed in the cells, through which pass small monovalent ions. For blockade of chemical synaptic transmission, perfusate Ca²⁺ concentration was lowered to 0.2 mM and the Mg²⁺ concentration was increased to 5 mM. In Iow-Ca²⁺, high-Mg²⁺ solution (referred to here as low Ca), 10 of 55 Pre-I neurons generated rhythmic bursts (burst type), 14 fired tonically (tonic type), and 31 were silent (silent type). Burst-type neurons showed periodic depolarization of 5–12 mV in low Ca, at a rate of 12±6.5/min. Hyperpolarization of the membrane caused decrease in or disappearance of the periodic depolarization and prolongation of the cycle period. Thus, the burst generations were voltage dependent. The firing frequency of tonictype neurons was 2.3±1.6 Hz and was decreased by hyperpolarization. In 6 of these neurons, the firing patterns changed to burst patterns during continuous hyperpolarization. Membrane depolarization by continuous outward current injection into some silent-type neurons (3 of 11 tested) induced bursting activity. Activity of C4 and Pre-I neurons was completely silent with 0.1–1 M tetrodotoxin (TTX) added to the standard perfusate. In low Ca, burst-type neurons (n=3) were also silent with 1 M TTX perfusion. Inspiratory neurons either became silent (n=4) or fired tonically (n=1) in low Ca. The present study by perforated whole-cell recordings confirmed that some Pre-I neurons possess intrinsic burst-generating properties, which were not attributable to phasic synaptic inputs.