Effects of concomitant motor reactions on the measurement of rewarding efficacy of brain stimulation.

In self-stimulation behavior, the rate-frequency (R-F) function relates bar-pressing performance to the number of cathodal pulses of constant intensity, delivered in a train of fixed duration. The lateral position of the R-F function depends on the rewarding efficacy of the stimulation; a shift of the function toward larger pulse numbers after some experimental manipulation indicates a decrease in the efficacy of the stimulation. Because self-stimulation is often accompanied by stimulation-contingent motoric reactions, it is required to show that such reactions do not alter the estimates of rewarding efficacy of the stimulation. We describe an experiment in which the presence and severity of motoric reactions were controlled experimentally by simultaneous stimulation through a second electrode, located in a motoric brain region. Rats were implanted with one hypothalamic (LH) electrode (which elicited self-stimulation) and one reticular (RF) electrode (which elicited head and body movements). The rate-frequency function for each LH electrode was obtained under a single-pulse condition (LH electrode alone) and under a paired-pulse condition repeated three times, in which each LH pulse was accompanied by three different intensities of an RF pulse. Despite its severe effect on the slope and the asymptotic rate of R-F function, the interfering reaction failed to shift the R-F function to any significant degree. We concluded that these interfering reactions do not alter the estimates of neuronal density obtained through application of the curve-shift paradigm.