Dietary tryptophan supplements attenuate amphetamine self-administration in the rat

Previously, it had been shown that lesions of cerebral 5-hydroxytryptamine (5-HT)-containing neurons and injections of drugs affecting 5-HT synthesis or receptor mediated function would alter amphetamine self-administration in the rat. The present study sought to ascertain whether diets enriched in L-tryptophan (L-TRY), the amino acid precursor to 5-HT, would: elevate cerebral 5-HT concentrations and affect amphetamine self-administration behavior. Diets containing 2.0 and 4.0% L-TRY increased cerebral 5-HT concentrations above those of rats on normal rat chow (0.26% L-TRY). The 4.0% diet elevated brain 5-HT to the same degree in rats exposed to the diet for 1, 2 or 3 days. When normal diets were restored, brain 5-HT concentrations rapidly returned to normal. Animals trained to self-administer d-amphetamine, when given access to the L-TRY enriched diets, significantly reduced their daily amphetamine self-injection during exposure periods. When normal rat chow was restored a delay in recovery to pre-diet amphetamine self-administration was observed: 1 day with the 2.0% L-TRY diet and 2 days with the 4.0% L-TRY diet. The 4.0% L-TRY diet failed to alter saline-frustration responding indicating the diet did not produce decrements in motor performance. When animals were placed on the 4.0% L-TRY diet and allowed access to amphetamine for 1 day then exposed to saline, a profound decrease in saline-frustration responding was observed.(ABSTRACT TRUNCATED AT 250 WORDS)