Down's Syndrome (DS), the phenotypic expression of human trisomy 21, is presumed to result from overexpression of certain genes residing on chromosome 21 at the segment 21q22-the Down locus. The "housekeeping" enzyme CuZn-superoxide dismutase (CuZnSOD) is encoded by a gene from that region and its activity is elevated in DS patients. Moreover, the recent discovery that familial ALS is associated with mutations in the gene encoding CuZnSOD, focused attention on the entanglement of oxygen-free radicals in cell death and neuronal disorders. To investigate the involvement of CuZnSOD gene dosage in the etiology of the syndrome we have developed both cellular and animal models which enabled us to investigate the physiological consequences resulting from overexpression of the CuZnSOD gene. Rat PC12 cells expressing elevated levels of transfected human CuZnSOD gene were generated. These transformants (designated PC12-hSOD) closely resembled the parental cells in their morphology, growth rate, and response to nerve growth factor, but showed impaired neurotransmitter uptake. The lesion was localized to the chromaffin granule transport mechanism. These results show that elevation of CuZnSOD activity interferes with the transport of biogenic amines into chromaffin granules. Since neurotransmitter uptake plays an important role in many processes of the central nervous system, CuZnSOD gene-dosage may contribute to the neurobiological abnormalities of Down's Syndrome. As an approach to the development of an animal model for Down's Syndrome, several strains of transgenic mice which carry the human CuZnSOD gene have been prepared. These animals express the transgene as an active enzyme with increased activity from 1.6 to 6.0-fold in the brains of four transgenic strains and to an equal or lesser extent in several other tissues. To investigate the contribution of CuZnSOD gene dosage in the neuropathological symptoms of Down's Syndrome, we analyzed the tongue muscle of the transgenic-CuZnSOD mice. The tongue neuromuscular junctions (NMJ) in the transgenic animals exhibited significant pathological changes; withdrawal and destruction of some terminal axons and the development of multiple small terminals. The ratio of terminal axon area to postsynaptic membranes decreased, and secondary folds were often complex and hyperplastic. The morphological changes in the transgenic NMJ were similar to those previously seen in the transgenic NMJ and were similar to those previously seen in muscles of aging mice and rats as well as in tongue muscles of patients with Down's Syndrome. The findings suggest that CuZnSOD gene dosage is involved in the pathological abnormalities of tongue NMJ observed in Down's Syndrome patients.(ABSTRACT TRUNCATED AT 400 WORDS)