Titration procedure with rats using a nose poke response and tail shock.

An operant titration procedure that provides relatively stable within-session shock thresholds (0.15--0.25 mA) within a week of training is described. Rats are placed into Plexiglas restraint tubes having a hole at one end through which the animal can poke its nose to break a photobeam; when the photobeam is broken, a response is counted. The tail of the animal is firmly held by a Plexiglas plug and an electrode connected to a programmable titration shocker is attached to the tail. Conditioned nose poke response are made after about 15 min during the first 30 min session. The titration procedure was found to be sensitive to the analgesic effect of morphine. Significant increases in the median shock level tolerated was observed after 3 mg/kg, while response rate was not altered. Dose-related increases in threshold after 6 and 9 mg/kg of morphine were associated with decreases in the rate of nose poking. The technique offers several advantages in the study of chemical induced alterations in pain or reactivity, including rapidity of initial training, relatively short time to establish median shock thresholds, minimum involvement of motor components in the response, and sensitivity to a psychopharmacological tool.