Barium-sensitive conductances in developing Genioglossal (GG) motoneurons of the rat

Activity of GG motoneurons protrude the tongue and dilate the upper airways. Lack of coordinated contraction with the diaphragm can result in obstructive apnea. During development, the input resistance of these motoneurons is reduced by half between first and second postnatal weeks. As a result, more descending synaptic drive is required to activate these cells. Earlier experiments from our lab have shown that cells with the lower resistance at the older ages exhibited a larger increase in resistance when cesium was injected into the cell than did their higher resistance counterparts. This observation suggests a larger role for the cesium-sensitive potassium channels in the low as compared to high resistance cells. In the present study, we investigated the barium-sensitive component of potassium conductances in these developing motoneurons. Membrane properties were measured using sharp electrodes in a slice preparation of the rat brainstem at 1, 2 and 3 weeks of age. Tetrodotoxin (1mM) was added to the perfusate to block action potentials and control values recorded. Then, artificial cerebrospinal fluid containing 2 mM BaCl was washed into the recording chamber and input resistance was calculated from the membranes response to a series of hyperpolarizing current pulses. The cells with larger resistances showed the largest increases in input resistance with external barium. This finding contradicted the hypothesis that a proliferation of barium-sensitive conductances (e.g., leak) are responsible for the decreased resistance found in the lower resistance cells at 2 weeks.