Lateral hypothalamic stimulation gates nucleus gigantocellularis-induced aversion via a reward-independent process

Abstract

A monophasic pulse-pair stimulation technique was used behaviorally to infer neurophysiological interaction between ‘reward’ sites in the lateral hypothalamus (LH) and pain-implicated nucleus reticularis gigantocellularis (NGC). Rats lever-pressed for 3 s escapes from an otherwise continuous train of 0.1 ms NGC pulses delivered every 40 ms. When pulses to the LH were interdigitated with those to NGC, inter-response latencies were significantly longer. This occurred whether LH pulses were also interrupted by each lever-press (experiment 1) or not (experiment 2). Further, the same intensities of LH stimulation which inhibited escape supported high rates of lever-pressing for 3 s LH trains during inescapable NGC stimulation even though these LH trains were not of sufficient intensity to support ‘rewarding’ lever-pressing in the absence of NGC stimulation. This finding indicates that escape-inhibition in the first two experiments was not due to interfering motor effects of LH stimulation but most probably due to LH amelioration of NGC-induced aversion. Indeed, when aversion was ameliorated by morphine, lever-pressing for sub-reward-threshold LH stimulation during inescapable NGC stimulation decreased although classical self-stimulation did not. In the escape experiments, inhibition was greatest when each NGC pulse was preceded by an LH pulse at an interval of 0.1 or 10 ms. During inescapable NGC stimulation, rates of lever-pressing for LH trains were greatest when LH pulses preceded NGC pulses again at intervals of 0.1 or 10 ms. This congruence of the temporal bimodal profiles in the response functions of these two different behavioral experiments strongly suggests that the same integrative mechanism underlies both LH inhibition of NGC escape and lever-pressing for LH trains during inescapable stimulation of NGC. Since gastric loading did not alter lever-pressing for sub-reward-threshold LH trains during inescapable NGC stimulation but did inhibit classical self-stimulation, it was concluded that the inferred common integrative mechanism mediates a supraspinal gating of aversion which is independent of the LH self-stimulation reward process.