Blockade of Brain Stem Gap Junctions Increases Phrenic Burst Frequency and Reduces Phrenic Burst Synchronization in Adult Rat

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Abstract
Recent investigations have examined the influence of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in vitro using transverse medullary slice and en bloc brain stem-spinal cord preparations obtained from neonatal (1–5 days postnatal) mice. Gap junction proteins, however, have been identified in both neurons and glia in brain stem regions implicated in respiratory control in both neonatal and adult rodents. Here, we used an in vitro arterially perfused rat preparation to examine the role of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents. We recorded rhythmic inspiratory motor activity from one or both phrenic nerves before and during pharmacological blockade (i.e., uncoupling) of brain stem gap junctions using carbenoxolone (100 μM), 18α-glycyrrhetinic acid (25–100 μM), 18β-glycyrrhetinic acid (25–100 μM), octanol (200–300 μM), or heptanol (200 μM). During perfusion with a gap junction uncoupling agent, we observed an increase in the frequency of phrenic bursts (∼95% above baseline frequency; P < 0.001) and a decrease in peak amplitude of integrated phrenic nerve discharge ( P < 0.001). The increase in frequency of phrenic bursts resulted from a decrease in both T I ( P < 0.01) and T E ( P < 0.01). In addition, the pattern of phrenic nerve discharge shifted from an augmenting discharge pattern to a “bell-shaped” or square-wave discharge pattern in most experiments. Spectral analyses using a fast Fourier transform (FFT) algorithm revealed a reduction in the peak power of both the 40- to 50-Hz peak (corresponding to the MFO) and 90- to 110-Hz peak (corresponding to the HFO) although spurious higher frequency activity (≥130 Hz) was observed, suggesting an overall loss or reduction in inspiratory-phase synchronization. Although additional experiments are required to identify the specific brain stem regions and cell types (i.e., neurons, glia) mediating the observed modulations in phrenic motor output, these findings suggest that gap junction communication modulates generation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents in vitro.