Classical conditioning in the fetal rat: reinforcing properties of dynorphin A (1-13).
This study examined the reinforcing properties of dynorphin A (1-13) in a single-trial classical conditioning paradigm in the E20 rat fetus. Injection of dynorphin into the cisterna magna increased fetal motor activity and reduced facial wiping in a test of perioral cutaneous responsiveness. Dynorphin was effective as an unconditioned stimulus (US) in a classical conditioning paradigm using an artificial nipple conditioned stimulus (CS) and dynorphin A (1-13) US. The association between CS and US was dependent on activity in the kappa opioid system. Re-exposure to the artificial nipple CS after a single pairing of the nipple with dynorphin resulted in conditioned activation of the kappa opioid system. Dynorphin A (1-13) functions as a reinforcer for classical conditioning in the rat fetus after intracisternal or intrahemispheric injection, with the conditioned response depending on route of administration and site of injection.