CALCIUM-DEPENDENT RESPONSES IN NEURONS OF THE ISOLATED RESPIRATORY NETWORK OF NEWBORN RATS

1. Membrane potentials (E(m)) and currents (I-m) were measured using w hole-cell patch clamp techniques in inspiratory (Insp, types I-III), p reinspiratory (Pre-I) and tonic expiratory (Exp) neurons of the ventra l respiratory group (VRQ) in the isolated brainstem-spinal cord prepar ation of 0- to 4-day-old rats. 2. After blocking on-going synaptic act ivity with 0.2-0.5 mu M tetrodotoxin (TTX), Ca2+-dependent responses w ere analysed using patch pipettes containing 120 mM Cs+ and 20 mM tetr aethylammonium (TEA) to block K+ conductances. 3. In all cells studied , all-or-none high voltage-activated (HVB) Ca2+ spikes with an activat ion threshold of -33 +/- 6.9 mV (n = 37) were evoked by depolarizing c urrent pulses. 4. In less than 15% of Insp and Pre-I cells and in 20% of Exp neurons, termination of hyperpolarizing pulses led to low volta ge-activated (LVA) Ca2+ spikes with a threshold potential of between - 70 and -60 mV (n = 7). 5. In more than 50% of Insp III and Pre-I neuro ns, depolarizing pulses evoked graded 'plateau' potentials with 1 an a mplitude of 5-20 mV. Slow voltage ramp commands revealed that this typ e of Ca2+ response was due to an inward current with a mean activation threshold of -42 +/- 2.1 mV (n = 5). These intermediate voltage-activ ated (IVA) plateau potentials persisted for several seconds after term ination of depolarizing current pulses and decreased in amplitude at m ore negative holding potentials. 6. The HVA and LVA Ca2+ spikes as wel l as the IVA plateau potentials and the underlying inward current were potentiated after extracellular addition of 2 mM Ba2+ whereas 1-2 mM Co2+ led to blockade of these responses. 7. Nifedipine (10 mu M) selec tively suppressed HVA Ca2+ potentials whereas 0.2-0.4 mu M omega-agato xin-IVA reduced the IVA response without major effects on HVA Ca2+ spi kes. omega-Conotoxin-GVIA (2 mu M) led to a partial blockade of both I VA and HVA potentials. 8. After extracellular application of TTX, Ba2 and/or TEA, HVA and LVA Ca2+ spikes as well as IVA plateau potentials were also revealed using patch pipettes containing K+ instead of Csand TEA. 9. The results indicate that neonatal respiratory neurons hav e a complex set of Ca2+-dependent membrane conductances. The relevance of these conductances for initiation and maintenance of respiratory b ursts is discussed.