Purified antichick Thy-1 IgG abolishes intermediate and long-term memory

One- to 2-day-old chicks administered purified antichick Thy-1 IgG showed substantial amnesia for a single-trial passive discriminated avoidance task. Amnesia was clearly evident at 30 min after learning, during the intermediate stage of a three-stage model of memory processing, and persisted for at least 3 h. A similar effect was also observed with Fab and F(ab′)₂ fragments. Fc fragments, the non-IgG fraction of ascites, and saline yielded normal retention levels at all times. These results contrast with our earlier reports that both polyclonal and monoclonal antibodies to chick Thy-1 inhibited long-term memory formation only. The present findings are interpreted as possibly representing a dual effect of the purified IgG.