Inhibition of spinal or hypoglossal motoneurons of the newborn rat by glycine or GABA

The function of GABA or glycine during early postnatal development remains controversial as their action is reported as either excitatory or inhibitory. The present study addressed the question of the functional role of GABA or glycine on rat motoneurons shortly after birth. For this purpose, using in vitro preparations from immature rats (postnatal age, P0-P4 days), we recorded from lumbar spinal motoneurons and hypoglossal motoneurons. All data were obtained under current clamp conditions (recording with potassium methylsulphate containing electrodes) from cells at about -70 mV resting potential. On spinal motoneurons we used the glycinergic and GABAergic recurrent postsysnaptic potential (PSP) mediated by Renshaw cells to assess its impact on excitatory synaptic inputs from dorsal afferent fibres. Despite its depolarizing nature, the recurrent PSP consistently inhibited synaptic excitation of lumbar motoneurons. On hypoglossal motoneurons, exogenously applied GABA or glycine produced depolarization with decreased input resistance. This response was always associated with inhibition of cell firing induced by intracellular current pulses. Even when the membrane potential was repolarized to resting level in the presence of GABA or glycine, hypoglossal motoneurons failed to generate spikes. Conversely, similar depolarization produced by glutamate consistently facilitated spike firing. GABAergic and glycinergic synaptic potentials evoked by focal stimulation of the reticular formation inhibited firing and/or increased firing latency in the majority of hypoglossal motoneurons. These results indicate that, immediately after birth, rat motoneurons were inhibited by synaptically released or exogenously applied GABA or glycine.