FR DISCRIMINATION-TRAINING EFFECTS IN SHR AND MICROENCEPHALIC RATS

Fixed-ratio (FR) discrimination learning in adult male spontaneously h ypertensive rats (SHR), methylazoxymethanol-induced microencephalic Sp rague-Dawley (MAM), and Sprague-Dawley control rats was examined. SHR and MAM rats had little problem learning incrementally more difficult FR discriminations (FR1 vs. FR16, FR4 vs. FR16, and FR8 vs. FR16) that resulted in parallel increases in errors in all animals, and displaye d only modest learning deficits during a subsequent FR4 vs. FR16 posit ion reversal. When training involved nonincremental changes in difficu lty (FR8 vs. FR16, FR4 vs. FR16, FR8 vs. FR16, FR12 vs. FR16, and FR14 vs. FR16), SHR and MAM rats evidenced relatively large learning defic its during the initial FR8 vs. FR16 discrimination but had no difficul ty with the last two discriminations. Furthermore, training selectivel y and significantly elevated hippocampal weight in MAM rats. These fin dings: a) question prior suggestions that MAM and SHR model separate h uman developmental disabilities; b) indicate that manifestation of lea rning deficits in even markedly brain-damaged organisms depends on ini tial task difficulty and can be overcome by experience; and c) are the first indicating that training-induced antagonism of prenatally induc ed hippocampal hypoplasia and its consequences is possible.