Rat strain differences in the acquisition of hippocampal self-stimulation

The rate of acquisition of lever-pressing for electrical stimulation of the hippocampus (HPC) was compared in two strains of rat: barrier-sustained Wistar and Sprague-Dawley. Sprague-Dawley rats initially bar-pressed at very low rates and took a median of 11 days to self-stimulate, according to the criterion used. Wistar rats all reached the same criterion in the first test session. Differential sensitivity to the activating effects of stimulation as an explanation for this difference was ruled out by the observation that both strains decreased response rates at the same rate and to the same level if stimulation was made non-contingent on lever-pressing. Differential threshold for reward was ruled out by the observation that rate-intensity curves yielded the same threshold currents and peak rates in both strains. Finally, it was shown that the rate of development of kindled seizures in the two strains of rats is different: Wistars kindle to full seizures faster than do Sprague-Dawleys. The relationship between the quicker onset of self-stimulation and of kindled seizures in Wistars is discussed.