Aging alters the rhythmic expression of vasoactive intestinal polypeptide mRNA but not arginine vasopressin mRNA in the suprachiasmatic nuclei of female rats

Abstract

Our laboratory has shown that the ability of the suprachiasmatic nuclei (SCN) to regulate a number of rhythmic processes may be compromised by the time females reach middle age. Therefore, we examined the effects of aging on the rhythmic expression of two neuropeptides synthesized in the SCN, vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP), using in situ hybridization. Because both VIP and AVP are outputs of the SCN, we hypothesized that age-related changes in rhythmicity are associated with alterations in the patterns of expression of these peptides. We found that VIP mRNA levels exhibited a 24 hr rhythm in young females, but by the time animals were middle-aged, this rhythm was gone. The attenuation of rhythmicity was associated with a decline in the level of mRNA per cell and in the number of cells in the SCN producing detectable VIP mRNA. AVP mRNA also showed a robust 24 hr rhythm in young females. However, in contrast to VIP, the AVP rhythm was not altered in the aging animals. The amount of mRNA per cell and the number of cells expressing AVP mRNA also was not affected with age. Based on these results we conclude that (1) various components of the SCN are differentially affected by aging; and (2) age-related changes in various rhythms may be attributable to changes in the ability of the SCN to transmit timing information to target sites. This may explain why the deterioration of various rhythmic processes occurs at different rates and at different times during the aging process.

Fig. 1.

Overall levels of VIP mRNA per cell in young, middle-aged, and old females (mean ± SEM). The 3 (age) × 7 (time of day) ANOVA revealed a significant effect of age (F_(2,101) = 5.99; p < 0.004) on mRNA levels per cell. VIP gene expression was significantly lower in middle-aged and old females compared with young (p < 0.05).

Fig. 2.

This photomicrograph shows VIP labeling in the SCN of young (A), middle-aged (B), and old (C) females at 1200 hr. VIP labeling was seen predominantly in the ventrolateral portion of the SCN in all animals. However, as animals age, there appears to be a decrease in the number of VIP-expressing cells in the medial portion of the nucleus. 3V, Third ventricle;OC, optic chiasm. Scale bar, 25 μm.

Fig. 3.

VIP-expressing cells per section (mean ± SEM) in young, middle-aged, and old females. The two-way ANOVA analyzing the effects of age and time of day on the number of VIP-expressing cells revealed a main effect of age (F_(2,101) = 4.53; p < 0.02), with the number of VIP-expressing cells per section being higher in young than in middle-aged or old females (p < 0.05).

Fig. 4.

VIP mRNA levels per cell (mean ± SEM) over time in young (•), middle-aged (○), and old (▾) females. One-way ANOVA revealed that VIP mRNA was rhythmic in young females (F_(6,43) = 3.03; p < 0.02), with VIP gene expression being higher at 2400, 1200, and 2300 hr than at other times of day (*p < 0.05). VIP mRNA levels did not significantly fluctuate over the day in middle-aged or old females. The black bar under thex-axis represents the dark phase of the cycle, and thewhite bar represents the light phase.

Fig. 5.

AVP-expressing cells per section (mean ± SEM) in young, middle-aged, and old females. Aging did not alter the number of AVP-expressing cells in the SCN of females (F_(2,108) = 0.928; p = 0.399).

Fig. 6.

These photomicrographs show AVP mRNA labeling in the SCN of young (A), middle-aged (B), and old (C) females at 1200 hr. AVP expression was seen primarily within the dorsomedial portion of the SCN in all groups of animals. 3V, Third ventricle; OC, optic chiasm. Scale bar, 25 μm.

Fig. 7.

AVP-expressing cells per section (mean ± SEM) over time in young (•), middle-aged (○), and old females (▾). The two-way ANOVA examining the effects of age and time of day on AVP cell number revealed a significant effect of time (F_(6,108) = 11.19; p < 0.001), with the number of AVP-expressing cells gradually increasing between 0300 and 1600 hr and then declining to baseline levels by 2300 hr (different letters are significantly different from each other; p < 0.05). The black bar under the x-axis represents the dark phase of the cycle, and the white bar represents the light phase.

Fig. 8.

AVP mRNA levels per cell (mean ± SEM) over time in young (•), middle-aged (○), and old females (▾). The 3 (age) × 7 (time of day) ANOVA revealed a main effect of time on AVP mRNA levels (F_(6,108) = 18.73;p < 0.001), with AVP mRNA levels increasing between 0300 and 1600 hr and then declining to baseline levels by 2300 hr (different letters are significantly different from each other; p < 0.05). The black bar under the x-axis represents the dark phase of the cycle, and the white bar represents the light phase.