Enhanced spatial discrimination learning in rats following 5,7-DHT-induced serotonergic deafferentation of the hippocampus.

Learning in rats trained in the Stone 14-unit T-maze, a complex, positively reinforced spatial discrimination task was assessed following cytotoxic (5,7-dihydroxytryptamine; 5,7-DHT) deafferentation of the serotonergic inputs to the hippocampus. Serotonergic deafferentation was accomplished by infusing the cytotoxin in to the fornix-fimbria/cingulum bundle. Lesioned rats reached criterion (i.e. learned) in significantly fewer trials and made significantly fewer errors throughout training than either vehicle-injected or sham-operated controls. This represents the first time that the effects of selective chronic destruction of serotonergic inputs to the hippocampus have been investigated. The present results provide, therefore, evidence in support of a neuromodulatory role for serotonin (5-HT) within the rat hippocampus in the mediation of the processes underlying learning and memory for this task. Other studies are, therefore, warranted in order to determine whether hippocampal 5-HT also plays a role in the mediation of the processes underlying learning and memory in other types of tasks.