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Contribution of Ca²⁺-dependent conductances to membrane potential fluctuations of medullary respiratory neurons of newborn rats in vitro

Hiroshi Onimaru, Klaus Ballanyi* and Ikuo Homma

Ca²⁺-dependent conductances were studied in respiratory interneurons in the brainstem-spinal cord preparation of newborn rats. -Conotoxin-GVIA attenuated evoked postsynaptic potentials, spontaneous or evoked inspiratory spinal nerve activity and blocked spike afterhyperpolarization. Furthermore, -conotoxin-GVIA augmented rhythmic drive potentials of pre-inspiratory and inspiratory neurons and increased respiratory-related spike frequency of pre-inspiratory cells with no effect on inspiratory hyperpolarization. In contrast, -agatoxin-IVA depressed drive potentials of pre-inspiratory and inspiratory neurons and attenuated inspiratory hyperpolarization and spike frequency of pre-inspiratory cells. It did not affect spike shape and exerted only minor, non-significant, attenuating effects on spontaneous or evoked nerve bursts or evoked postsynaptic potentials. Nifedipine diminished drive potentials and spike frequency of pre-inspiratory neurons and shortened drive potentials in some cells. -Conotoxin-MVIIC attenuated drive potentials and intraburst firing rate of pre-inspiratory neurons and decreased substantially respiratory frequency. Respiratory rhythm disappeared following combined application of -conotoxin-GVIA, -conotoxin-MVIIC, -agatoxin-IVA and nifedipine. Apamin potentiated drive potentials and abolished spike afterhyperpolarization, whereas charybdotoxin and tetraethylammonium prolonged spike duration without effect on shape of drive potentials. The results show that specific sets of voltage-activated L-, N- and P/Q-type Ca²⁺ channels determine the activity of particular subclasses of neonatal respiratory neurons, whereas SK- and BK-type K⁺ channels attenuate drive potentials and shorten spikes, respectively, independent of cell type. We hypothesize that modulation of spontaneous activity of pre-inspiratory neurons via N-, L- and P/Q-type Ca²⁺ channels is important for respiratory rhythm or pattern generation.

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