"Temporal summation decay" in hypothalamic self-stimulation: threshold changes at long intrapair intervals due to axonal subnormal periods.

Rats were trained to bar press for trains of conditioning (C) and test (T) pulses delivered via lateral hypothalamic electrodes. As intrapair (C-T) intervals increased from 10 ms to 100 ms, the frequency of pulses required for self-stimulation increased, similar to results of Smith and Coons (1970). This effect was observed only for electrode placements where self-stimulation was obtained at frequencies below 16 Hz and currents of 600 microA and higher. The effect was larger when the train duration was increased from 0.5 s to 2.0 s. The threshold increase was abolished when the T pulses were greater in current than the C pulses but not when C pulses were larger than T pulses. The larger T pulses also removed relative refractory period effects at a C-T interval of 1.0 ms. Therefore, the increase in required current or frequency at long C-T intervals appears to be due to a decline in axonal excitability (i.e., the subnormal period) rather than a decay in synaptic temporal summation. Possible flaws in other reports of paired-pulse "temporal summation decay" at long C-T intervals using 2 electrodes are discussed.