Inspiratory drive to hypoglossal (XII) motoneurones is mediated predominantly via AMPA receptors (Funk et al. 1993). The effect of inhibition upon inspiratory drive currents, and its functional significance for respiratory-related motor output is unclear. Here, we describe phasic inhibition in XII motoneurons, its effect on inspiratory drive currents, and modulation of this inhibition by protein kinase A (PKA).
Neonatal rats were anaesthetized by hypothermia and rapidly decerebrated. A 700 µm medullary slice that generates a respiratory-related rhythm (Smith et al. 1991) was cut. Slices were superperfused with ACSF (28 °C, [K+] 9 mM). The activity of the hypoglossal nerve was amplified, rectified, integrated and used to define the inspiration. All measurements were normalized to the duration of the corresponding inspiratory period and expressed as a percentage of that period. Whole-cell voltage-clamp recordings (Vh = -70 mV, -100 mV for reversal) were made from XII motoneurones. IPSCs were reversed immediately after whole-cell patch formation. Drugs were either included in the patch electrode or bath applied.
Motoneurones were divided into three groups based upon the level of inhibitory drive: non-inhibited (NIM) (n = 6), late-inspiratory inhibited (LIM) (n = 15) and inspiratory inhibited (IIM) (n = 7). In LIMs inspiratory drive currents were truncated (56 ± 28 %) compared with the NIMs (101 ± 5 %, P < 0.01) with no significant difference in the duration of nerve activity. Inhibition in LIMs persisted 131 ± 33 % into post-inspiration. IIMs were inhibited through the inspiratory period in the apparent absence of excitatory drive currents. Bicuculline (200 µM) abolished inspiratory and post-inspiratory inhibition, increased the duration and amplitude of inspiratory drive currents in LIMs (n = 9, P < 0.01), whereas in NIMs it did not markedly alter the currents. In IIMs bicuculline revealed excitatory currents (n = 4), indicating that phasic inhibition gated out excitatory drive currents.
Intracellular dialysis of the catalytic subunit of PKA (250 units ml-1) (n = 7) and Sp-cAMP (n = 6) both increased inhibitory charge transfer, the amplitude of inspiratory phased inhibitory currents and expiratory phased IPSCs (P < 0.01). Mini-analysis of IPSCs suggested that these effects are mediated postsynaptically.
Phasic GABAergic inhibition is common in XII motoneurones and is important in gating and shaping motoneurone output. These inputs are modulated by PKA. Results are given as means ± S.D., P < 0.05 is considered significant (unpaired t tests).
This work was supported by NIH grant no. NS24742.