Lead-induced alterations in rod-mediated visual functions and cGMP metabolism: new insights

Long-term scotopic (rod-mediated) visual deficits following developmental lead exposure occur in monkeys and hooded rats. This report describes and summarizes previous ERG and biochemical findings, presents new biochemical data aimed at determining the mechanism of inhibition of lead on rod cGMP-PDE, presents an integratory framework for understanding the ERG and cGMP results and speculates on the implications of the present data. A- and b-wave voltage-log intensity and latency-log intensity functions, generated from single-flash ERGs in fully dark-adapted rats, revealed that low and moderate level lead exposure caused decreases in absolute sensitivity and amplitude, and increases in latency. Rod- and cone-mediated flicker fusion frequency measures revealed selective rod deficits in temporal resolution. In addition, the slope of the increment threshold function was decreased, but only at scotopic adapting backgrounds, and dark adaptation was delayed. Prior exposure to lead produced a dose-response inhibition of retinal cGMP-phosphodiesterase (PDE) resulting in an increase in cGMP in dark-adapted and light-adapted states and an increase in the calcium content of rods. In vitro experiments with adult rat retinas incubated with 10(-9) to 10(-4) M Pb2+ revealed a concentration-dependent inhibition of cGMP-PDE which suggested that Pb2+ directly inhibited the rod cGMP-PDE. This was confirmed in experiments conducted with isolated, purified, trypsin-activated bovine rod cGMP-specific PDE exposed to 5 x 10(-8) to 10(-4) M Pb2+. The cGMP data are entirely consistent with the observed ERG changes. The ERG data is relevant to low level pediatric lead poisoning since rat rods are similar to human rods. Finally, since a lesion in the gene that codes for a cAMP-PDE leads to defective learning and memory in the Drosophila dunce flies, it is possible that lead-induced alterations in cyclic nucleotide phosphodiesterases contribute to the long-term CNS deficits produced by developmental lead exposure.