Changes in potentiation and timing of sensorimotor transmission after adaptation of a postsynaptic transient outward current in a simulated cortical neuron.
How adaptation of a postsynaptic transient outward current might affect the efficacy of sensorimotor transmission was investigated. The transmission signals that were studied were a 5 ms conditioned stimulus (CS) and a 60 ms US drawn from intracellularly recorded, depolarizing postsynaptic potentials (PSPs) elicited in pyramidal neurons of the cat motor cortex by a click CS and a glabella tap US, respectively. SPICE, a program used to analyze electrical circuits, was used to simulate the cortical neuron containing the adaptive outward current. Changes in the magnitude and latency of rise to firing threshold of the PSPs were compared i) after presynaptic augmentation of a CS input in the absence of an adaptive postsynaptic current and ii) after decreasing the magnitude of an adaptive postsynaptic current that was rapidly activated by depolarization. Effects of short (6 ms) and long (24 ms) inactivation time constants of the postsynaptic current were also studied. In both presynaptic adaptation and postsynaptic adaptation, the potentiation of the magnitude of the CS-induced PSP was similar, with the latency to threshold being reduced by < or = 1 ms in both cases. The effects on the US PSP differed. Presynaptic adaptation affecting the CS had no effect on the US. Adaptation of the CS by a postsynaptic outward current with a 6 ms inactivation time constant, reduced the latency to threshold of an EPSP from a nearby US synapse by up to 6 ms by augmenting the initial portion of the slowly rising US-induced PSP. Adaptation of a postsynaptic current with a 24 ms inactivation time constant reduced the latency of response to the US PSP by up to 16 ms. When the US synapse was relocated to the soma, the reduction in US latency caused by adaptation of the outward current at the CS synapse was reduced by up to one half. The latency of slowly rising components of integrated synaptic responses to compound CSs of > 5 ms duration from multiple synaptic inputs would be expected to show reductions corresponding to those of the US. We conclude that potentiation of synaptic transmission by adaptation of a postsynaptic outward current can result in reductions of latency of sensorimotor transmission that can significantly affect the timing and accuracy of controlled motor tasks. These effects depend significantly on the locations of the synaptic inputs within the cell.