Neural substrate for brain stimulation reward in the rat: Cathodal and anodal strength-duration properties.

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Abstract
The trade-off between current strength and duration of a stimulating pulse was studied for the rewarding and priming effects of brain stimulation reward (BSR). With cathodal pulses, strength-duration functions for BSR had chronaxies of 0.8-3 ms. No differences were observed between the results for rewarding and priming effects. With anodal pulses, strength-duration curves were parallel to the cathodal curves at pulse durations of 0.1-5 ms, but at pulse durations greater than 5 ms the anodal curves showed a greater drop in required current intensity than did the cathodal curves. The parallel portion of the anodal curves was interpreted as due to anode-make excitation, and the drop at longer pulse durations was interpreted as due to anode-break excitation. Cathodal strength-duration functions for the motor effect elicited through the BSR electrodes had chronaxies of 0.15-0.48 ms. Measurements of the latency of the muscle twitch confirmed that anode-make and anode-break excitation occurred, the latter becoming evident at pulse durations as brief as 0.3-0.4 ms. The results provide quantitative characterization of cathodal and anodal strength-duration properties of the neural substrate for BSR and are discussed in terms of their value in guiding electrophysiological investigation of that substrate.