Synaptic control of motoneuron excitability in rodents: From months to milliseconds

1. Motoneurons (MN) shape motor patterns by transforming inputs into action potential output, This transformation, excitability, is determined by an i nteraction between synaptic inputs and intrinsic membrane properties. Excit ability is not static, but changes over multiple time scales. The purpose o f the present paper is to review our recent data on synaptic factors import ant in the dynamic control of MN excitability over time scales ranging from weeks to milliseconds. 2. Developmental changes in modulation of MN excitability are well establis hed. Noradrenergic potentiation of hypoglossal (XII) MN inspiratory activit y in rhythmically active medullary slice preparations from rodents increase s during the first two postnatal weeks. This is due to increasing alpha(1)- and beta-adrenoceptor excitatory mechanisms and to a decreasing inhibitory mechanism mediated by alpha(2)-adrenoceptors. Over a similar period, ATP p otentiation of XII inspiratory activity does not change. 3. Motoneuron excitability may also change on a faster time scale, such as between different behaviours or different phases of a behaviour. Examinatio n of this has been confounded by the fact that excitatory synaptic drives u nderlying behaviour can obscure smaller concurrent changes in excitability. Using the rhythmically active neonatal rat brain-stem-spinal cord preparat ion, we blocked excitatory inspiratory drive to phrenic MN (PMN) to reveal a reduction in PMN excitability specific to the inspiratory phase that: (i) arises from an inhibitory GABAergic input; (ii) is not mediated by recurre nt pathways; and (iii) is proportional to and synchronous with the excitato ry inspiratory input. We propose that the proportionality of the concurrent inhibitory and excitatory drives provides a means for phase-specific modul ation of PMN gain. 4. Modulation across such diverse time scales emphasizes the active role th at synaptic factors play in controlling MN excitability and shaping behavio ur.