Effect of surgical deafferentation of the hypothalamic supraoptic nucleus on urinary vasopressin levels in twenty-month-old rats

Indices of water balance (daily water consumption, urine volume, and urine osmolality), and vasopressin (VP) urinary excretion were measured in 3-month-old (m.o.) and 20 m.o. F344 rats after neurosurgical lesion of noradrenergic afferents to the supraoptic nucleus (SON) in the medial forebrain bundle (mfb), in order to assess whether morphological denervation (21) was functionally effective. Lesion of the mfb was accomplished using a mechanical knife cut placed stereotaxically medial and caudal to SON. The experiment was performed three times. Morphological evaluation consistently indicated a marked depletion of noradrenergic afferents in SON after lesion placement. The effect of the lesion on daily excreted VP was assessed by paired t-test, using means of equal numbers of urinary VP measurements representing 13–15 days each pre- and postsurgically for each animal. The postsurgical interval for assessing the functional effect of the lesion was chosen based upon morphological evidence for mfb regeneration at 14 days (21). Three m.o. rats showed comparable urinary VP levels before (mean = 328.4 ± 97.3 pg/24 hr/100 g b.wt., for 10 rats for 2 weeks) and after (354.7 ± 63.0 pg/24 hr/100 g b.wt.) lesion placement. In 20 m.o. rats, overall mean postsurgical VP excretion was 199.4 ± 44.8 pg for 15 rats for 2 weeks, compared with a presurgical mean of 343.2 ± 86.2 pg/24 hr/100 g b.wt. Mean urine VP was lower in 20 m.o. rats in each of three experiments; VP levels after lesion in 3 m.o. rats was lower in 2, but higher in one experiment; mean differences were not statistically significant for treatment (lesion) effect. Interaction of age and lesion was significant, F(1,12)=5.866, p < 0.01, for one experiment, and approached significance, F(1,23)=1.897, p < 0.182, for combined experiments, despite large interexperiment variance. Using paired t-test for individual rats (thus, eliminating individual variance, but weighting magnitude of change) in each age group for three experiments, there was no effect of the lesion in young rats, but a marked VP decrease ( p < 0.001) occurred in 20 m.o. animals. Ranked comparison (Mann-Whitney U-test) of 3 m.o. and 20 m.o. postsurgical VP showed a significant ( p < 0.05) difference in response to SON deafferentation. Urine volume after lesioning increased ( p < 0.01 by paired t-test) in both groups; decreased urine osmolality ( p < 0.01) persisted for 2 weeks postsurgically in 20 m.o., but not in young, rats. In 3 m.o. rats, evidence