Pii: S0304-3940(99)00564-9

Hypothalamic supraoptic nucleus (SON) neurons express nitric oxide synthase (NOS) in an activity-dependent manner. In the present study, the effect of aging on the NOS expression of the SON neurons, as detected by nicotinamide adenine dinucleotide phosphate-diaphorase activity, was studied under normal conditions and under dehydration stress induced by salt loading. In the control rats, the number of stained neurons did not differ between the two age groups. Dehydration resulted in an increase in both the number of staining neurons and in the staining intensity in both 2and 14±16-month-old rats. Furthermore, dehydration-induced NOS expression was signi®cantly higher in the older animals. The results suggest that the response to dehydration, as indicated by increased NOS activity in the supraoptic nucleus, is enhanced in the aging rat. q 1999 Elsevier Science Ireland Ltd. All rights reserved.