Effects of early hippocampal lesions on trace, delay, and long-delay eyeblink conditioning in developing rats

The effects of bilateral hippocampal aspiration lesions on later acquisitio n of eyeblink conditioning were examined in developing Long-Evans rat pups. Lesions on postnatal day (PND) 10 were followed by evaluation of trace eye blink conditioning (Experiment 1) and delay eyeblink conditioning (Experime nt 2) on PND 25. Pairings of a tone conditioned stimulus (CS) and periocula r shock unconditioned stimulus (US, 100 ms) were presented in one of three conditioning paradigms: trace (380 ras CS, 500 ms trace interval, 880 ms in terstimulus interval [ISI]), standard delay (380 ms CS, 280 ms ISI), or lon g delay (980 ms CS, 880 ms ISI). The results of two experiments indicated t hat hippocampal lesions impaired trace eyeblink conditioning more than eith er type of delay conditioning. In light of our previous work on the ontogen y of trace, delay and long-delay eyeblink conditioning , (Ivkovich, Paczkow ski, & Stanton, 2000, showing that trace and long-delay eyeblink conditioni ng had similar ontogenetic profiles, the current data suggest that during o ntogeny hippocampal maturation may be. more important for the short-term me mory component than for the long-ISI component of trace eyeblink conditioni ng. The late. development of conditioning over long ISIs may depend on a se parate process such as protracted development of cerebellar Cortex. (C) 200 1 Academic Press.