Test Stimulus

The Randall–Selitto (RS) paw pressure test (Ugo Basile, Varese, Italy) was used to measure analgesia or hyperalgesia. An increasing pressure was applied to the rat hindpaw until there was a withdrawal response (cutoff at 500 g). The pressure applied when the rat withdrew its paw was recorded as the paw withdrawal threshold (PWT). In our model it serves as the test stimulus to measure the DNIC response following application of the conditioning stimulus. Prior to minipump implantation, the Randall-Selitto baseline PWT measurement was taken. On day 7 after pump implantation, a second PWT measurement was taken. After a further 14 days and before bright light stress, rats were evaluated to confirm recovery from OIH demonstrated by a return to baseline PWT. On day 21, two hours after BLS, a pre-capsaicin injection baseline measure was taken, followed by the immediate injection of capsaicin into the left forepaw and the beginning of the DNIC timecourse measurements. PWT was measured three times for each paw at each timepoint and averaged prior to data analysis.