Place Learning Impairment in Chronically Tryptophan-Restricted Rats

Acute pharmacological studies have shown that the relationship between serotonin and place learning is very unclear and when serotoninergic neurotransmission is manipulated, little or no effect on place learning has been observed. Since tryptophan is the precursor of the serotonin synthesis, female rats were chronically given a tryptophan-restricted diet from the time they were weaned until they were 60 days old and their place learning ability was assessed in the Morris' water maze, during the dark phase and under red lighting. Animals should have solved two 5 attempt blocks separated by a change in diametral position of the goal, at 21, 40 and 60 days of age. Experimental animals took longer distances than control animals, mainly in the second attempt block both at 40 and 60 days of age. Also, a remarkable irregularity on place learning curves of experimental animals was seen, possibly due to an apparent loss of maze solving strategy. These animals were also incapable of efficiently adjusting their external signals-related cognitive map when the goal was moved to a different position and, on the other hand, two different patterns were seen in maze solving at 60 days of age in these same animals. These findings strongly suggest that indirect lowering of brain serotonin levels through chronic restriction of dietary tryptophan may be responsible for the spatial learning deficits observed.