Differentiation of the substrates for analgesia and vocalizations elicited by midbrain stimulation in rats: refractory period estimates.

Stimulation of periaqueductal gray sites resulted in both increased latencies to escape heated water with a tail-flick and audible vocalizations. To determine whether these two responses were the result of stimulation of the same substrates, refractory periods were estimated by delivering paired-pulse trains of stimulation. Stimulation consisted of 10 or 20 s trains of single pulses or pulse pairs. The pulse pair frequency threshold for both behaviors was determined for intra-pulse (C-T) intervals of 0.4-10 ms. The ratio of single to double pulse frequency thresholds provided an indication of relative effectiveness of the paired-pulse stimulation. For analgesia, the paired-pulse effectiveness gradually increased between C-T intervals of 2.0 and 5.0 ms, after which asymptotic effectiveness values were obtained. Thus, the estimated refractory period for analgesia was 2.0-5.0 ms. The refractory period estimate for vocalizations was shorter. Effectiveness values increased with a C-T interval of 1.2 ms and reached asymptote with a C-T interval of 2.0 ms. These results suggest that different fibers with overlapping spatial distributions contribute to analgesia and vocalizations produced by midbrain stimulation.