In vitro evidence that the reduction in mesencephalic dopaminergic neurons in the weaver heterozygote is not due to a failure in target cell interaction.

The murine weaver (wv) mutation is characterized by a genetically determined loss of several neuronal populations, which include the nigrostriatal dopaminergic neurons. Animals homozygous for the wv gene exhibit marked deficits in dopaminergic morphological and neurochemical parameters. The wv gene shows incomplete dominance in that heterozygous (wv/+) mice exhibit moderate reductions in midbrain dopaminergic neuron number. It is unclear whether the dopaminergic neuronal loss in homozygous and heterozygous animals results from an effect of the wv gene solely on the dopaminergic neurons or is due to a failure of interaction of dopaminergic neurons with target cells of the striatum. This issue has been addressed utilizing three-dimensional reaggregate tissue cultures to determine whether the wv gene acts directly on the mesencephalic dopaminergic neurons. Embryonic mesencephalon and striatum from wv/+ and wild-type (+/+) brains were dissociated and the cells recombined into four mesencephalic-striatal aggregate combinations: (1) mesencephalic(+/+)-striatal(+/+)aggregates; (2) mesencephalic(wv/+)-striatal(wv/+)aggregates; (3) mesencephalic(wv/+)-striatal(+/+)aggregates; and (4) mesencephalic(+/+)-striatal(wv/+)aggregates. At 29 days and 57 days of culture, the number of dopaminergic neurons and dopamine content from mesencephalic-striatal aggregates consisting of mixed genotype or from only wv/+ tissue were quantitated and compared with that from mesencephalic-striatal cultures prepared from +/+ tissue alone. At both culture time points, aggregates containing wv/+ mesencephalon coaggregated with either wv/+ or +/+ striatum contained fewer dopaminergic neurons than mesencephalic-striatal cultures composed of only +/+ cells. Coaggregation of +/+ mesencephalon with wv/+ striatum did not have a detrimental effect on dopaminergic cell number. The findings demonstrate that the difference in the number of mesencephalic dopaminergic neurons between wv/+ and +/+ animals seen in vivo can be reproduced in three-dimensional reaggregate culture. Since the coculture of +/+ striatum with wv/+ mesencephalon did not appear to rescue wv/+ dopaminergic neurons in the aggregates as compared to wv/+ striatum and, wv/+ striatum proved to be a perfectly adequate target for +/+ mesencephalic dopaminergic neurons, it appears that the effect of the wv gene is on the dopaminergic neurons themselves.