Aging of tuberoinfundibular (A-12) dopamine neurons in the C57Bl/6N male mouse.

C57Bl/6N male mice at 4, 12, 20, and 28 months of age were processed for formaldehyde-induced fluorescence, and tuberoinfundibular dopamine (A-12) neurons were examined for qualitative, age-related changes. A-12 perikarya were weakly fluorescent at all ages studied. In mice of 4 and 12 months of age, A-12 terminals formed a brightly fluorescent band that filled the external zone of the median eminence. The zone of A-12 terminal fluorescence within the median eminence was less extensive and reduced in fluorescence intensity in 20-month old mice; in 28-month old mice, only a thin rim of fluorescence remained at the ventral edge of the median eminence. The drop in fluorescence intensity of A-12 terminals within the median eminence of the aged C57Bl/6N male mouse paralleled the decrease in A-12 terminal fluorescence previously reported for the Fischer 344 male rat [26]; however, mouse A-12 perikarya did not exhibit the dramatic increase in fluorescence intensity seen in the rat. The possibility that the divergent aging patterns exhibited in A-12 neurons of these two species might be related to differential resilience of their reproductive axes is discussed.