Memory fixation in the goldfish.

Abstract

shortly after training, but within minutes to hours it becomes insusceptible to these agents.l-3 This change in susceptibility to disruption suggests that memory becomes fixed after training. Correlations between changes in brainl metabolism and deficits in fixed memory produced by various agents may provide insights into the biochenical basis of memory formation. Puromycin, an antibiotic compound which inhibits protein synthesis, blocks memory in mice4 and in goldfish5, C when given after training. The extent and duration of inhibition of protein synthesis in goldfish brain produced by intracranial injectiolns of different amounts of puromycin has been studied.7 In the present investigations we measured the memory deficits obtained by injecting different amounts of puromycin at different times after training. We have also found evidence which suggests that puronycin specifically disrupts t,he fixation of memory. Materials and Methods.--The training procedure and apparatus have beenl described.t Goldfish were placed in individual shuttle boxes in which light was paired with repetitive electrical shock (0.2 sec of shock of 3 vac, 0.1 ma, at the rate of 40/1min). To avoid the shock, fish had to swim over a hurdle from the light to the dark end of the box. The trial cycle was 20 sec of light alone, 20 sec of light paired with the shock, followed by 20 sec of darknless. A correct response was scored when the fish swam over the hurdle before the onset of the shock. All fish were given 20 trials in a 40-min session on day 1 of an experiment and 10 trials in a 20-min session on day 4. Intracranial (IC) injections of 10 ul of saline or of puromycin dihydrochloride (Nutritional Biochemicals Corp.) in 10 ul of saline were made with a 30-gauge needle. The solutions were injected inito the cranial cavity over the tectum. at specified times on day 1. On both days 1 and 4, fish were placed in the shuttle boxes in darkness 5 min before the first trial. The trials were given in blocks of 5 separated by 5 min rest in darkness. Six fish were run simultaneously in individual shuttle boxes, and responses were recorded by direct observation. Mleasurement of memory: Memory is inferred from ann increase in correct responses between blocks of 10 trials. In the development of procedures in earlier work, we evaluated differences in menory between groups of fish by comparing the mnean day-4 scores. We subsequently found an alternate method based on a regression analysis of day-i scores on day-4 scores for uninjected (control) fish. The retention score of a fish is obtained by subtracting the score it (A) on day 4 from the score which was (P) for that fish from its day-i score. The retention score of a group of fish is evaluated by a t-test of the achieved and predicted (A vs. P) scores. Discrepancies in the results of the two methods have been minor, but the retention scores of groups of fish have tended to give the most consistent measures of memory.