Measurement of intracranial self-stimulation thresholds using the post-reinforcement pause

A method for measuring the current threshold for intracranial self-stimulation (ICSS) using the post-reinforcement pause (PRP) is described. Rats trained to lever press on a fixed ratio schedule for ICSS in prefrontal cortex, medial forebrain bundle, ventral tegmental area or periaqueductal gray received stimuli of opposite polarity in an alternating fashion. Stimuli of one polarity were sufficient to maintain ICSS responding by themselves (maintaining stimuli). Stimuli of the other polarity were systematically varied in 5-10 microA steps (experimental stimuli). PRPs following both maintaining (Pm) and experimental (Pe) stimuli were measured, and the ratio Pm/Pe was calculated. The PRP threshold was defined as the lowest experimental stimulus current producing PRP ratios significantly less than the ratios produced by all lower current steps. After the PRP threshold for one polarity was determined in 5 daily sessions, the experimental and maintaining stimuli were reversed, and the PRP threshold was measured for the alternate polarity. Rates of ICSS on a continuous reinforcement schedule were subsequently measured at currents around the PRP threshold. Rates increased sharply at PRP threshold, suggesting a correlation between PRP threshold and reinforcement threshold. Similar results were obtained from all four ICSS sites indicating the broad applicability of the PRP threshold method.