Further studies of glucocorticoid effects on spinal cord function: Single and repetitive monosynaptic transmission and apparent transmitter turnover

The effects of an intensive triamcinolone regimen were studied on the spinal monosynaptic (2N) input-output relationship, the maintenance of 2N response during repetitive stimulation at moderate frequencies (2-10 Hz) and primary afferent (Ia) transmitter turnover in acute C-1 spinal cats. Stimuli were applied to the triceps surae nerves and recordings made at the L7 dorsal and ventral roots on the first posttreatment day. In treated cats, first a shift in the 2N input-output curve occurred such that significantly less Ia activation was required to initiate a liminal 2N discharge, but the slopes of the input-output curves were identical, however. Secondly, the treated animals exhibited less rundown in the 2N response amplitude during trains of 10 stimuli at 5 or 10 Hz. Thirdly, a comparative analysis of the extent of the amplitude decline in terms of apparent transmitter turnover parameters revealed that while the fractional release of transmitter per impulse by the spinal Ia terminals was not affected, the rate of replenishment of the store of transmitter immediately available for release was more than doubled in treated animals. The decrease in Ia activation necessary to evoke a threshold 2N response, without an increase in the fractional release of transmitter represents an improvement in the safety factor for terminal impulse invasion. Moreover, the enhanced maintenance of repetitive transmission reflects an increased rate of transmitter mobilization. These direct facilitatory effects on central 2N transmission may have implications for the interpretation of glucocorticoid effects in central neurologic disorders and trauma.