Use of bullfrog tadpoles (Rana catesbeiana) to examine the mechanisms of lead neurotoxicity

Sublethal exposure to lead elicits changes in behavior, particularly learning. Previously, we had shown that bullfrog tadpoles exposed to 1 mg Pb/liter for 7 days exhibit learning deficits in a discriminate avoidance learning assay. The precise mechanisms involved in these lead-induced learning deficits are not understood, but CNS monoamine neurotransmitter systems have been implicated in the learning process. In the present study, we exposed bullfrog tadpoles to 1.7 +/- 0.2 mg Pb/liter for 7 days and then compared concentrations of neurotransmitters from whole brain samples with those of controls. Serotonin or 5-hydroxytryptamine (5-HT) was significantly decreased in the lead exposed group while the serotonin metabolite, 5-hydroxyindoleacetic acid, (5-HIAA) was similar to controls. No changes were observed in the catecholamines epinephrine and norepinephrine following lead exposure. The ratio of 5-HIAA/S-HT was significantly increased in lead exposed animals as a result of the decrease in 5-HT, suggesting a decrease in 5-HT biosynthesis rather than an increase in 5-HT metabolism. These findings are the first to suggest that lead exposure in bullfrog tadpoles affects the monoamine neurotransmitters that are implicated in the learning process. The results of the present study, in conjunction with previous evidence of learning deficits following lead exposure, offer the possibility of correlating lead exposure with learning deficits and alterations in CNS neurotransmitters in bullfrog tadpoles. The use of this tadpole model shows promise as a means to examine and understand the mechanisms involved in lead neurotoxicity.