INTRINSIC BURST GENERATION OF PREINSPIRATORY NEURONS IN THE MEDULLA OF BRAIN-STEM-SPINAL CORD PREPARATIONS ISOLATED FROM NEWBORN RATS

In brainstem-spinal cord preparations isolated from newborn rats, intr insic burst-generating properties of preinspiratory (Pre-I) neurons in the rostral ventrolateral medulla, which have been suggested to be pr imary respiratory rhythm-generating neurons, were studied by ''perfora ted'' whole-cell recordings using the antibiotic nystatin. Nystatin ca uses small pores to be formed in the cells, through which pass small m onovalent ions. For blockade of chemical synaptic transmission, perfus ate Ca2+ concentration was lowered to 0.2 mM and the Mg2+ concentratio n was increased to 5 mM. In low-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 (sil ent type). Burst-type neurons showed periodic depolarization of 5-12 m V in low Ca, at a rate of 12+/-6.5/min. Hyperpolarization of the membr ane caused decrease in or disappearance of the periodic depolarization and prolongation of the cycle period. Thus, the burst generations wer e voltage dependent. The firing frequency of tonic-type neurons was 2. 3+/-1.6 Hz and was decreased by hyperpolarization. In 6 of these neuro ns, the firing patterns changed to burst patterns during continuous hy perpolar ization. Membrane depolarization by continuous outward curren t injection into some silent-type neurons (3 of II tested) induced bur sting activity. Activity of C4 and Pre-I neurons was completely silent with O.1-1 mu M tetrodotoxin (TTX) added to the standard perfusate. I n low Ca, burst-type neurons (n=3) were also silent with I mu M TTX pe rfusion. Inspiratory neurons either became silent (n=4) or fired tonic ally (n=1) in low Ca. The present study by ''perforated'' whole-cell r ecordings confirmed that some Pre-I neurons possess intrinsic burst-ge nerating properties, which were not attributable to phasic synaptic in puts.