Acute effects of intravenous glucocorticoid on cat spinal motor neuron electrical properties

The acute effects of a single large intravenous dose of methylprednisolone were examined on the resting membrane potentials and the antidromic action potentials characteristics in cat lumbar spinal motor neurons via intracellular recording. A 30 mg/kg glucocorticoid dose was found to cause a 3.5 mV hyperpolarization of the resting membrane potential. An examination of the effects of the single dose on the conduction and generation of an antidromic action potential revealed an increased conduction velocity along the myelinated motor axon, a decreased conduction rate through the unmyelinated initial axon segment, an increased threshold for antidromic soma-dendritic activation, a decreased action potential zero overshoot and a faster repolarization. All of these effects were greatest after the 30 mg/kg methylprednisolone dose except for the increase in the antidromic soma-dendritic threshold which was even greater after 60 mg/kg. The 30 mg/kg dose also caused a stimulus-bound repetitive discharge in a third of the motor neurons which appeared to arise in the myelinated motor axon or initial axon segment. The neurophysiological implications of these acute glucocorticoid actions are discussed.