Research reportAging selectively suppresses vasoactive intestinal peptide messenger RNA expression in the suprachiasmatic nucleus of the Syrian hamster
Introduction
Circadian rhythms govern the behavior and physiological processes of virtually all organisms. By facilitating adjustments to the daily changes in illumination, temperature and other conditions, circadian rhythms enhance survival. In fact, robust and coordinated circadian rhythms favor good health, longevity and optimal cognitive ability in a variety of species, including humans [10], [20], [38], [39], [58].
Aging brings about many changes in circadian rhythms. One age-related change is a reduction in amplitude, a common feature of melatonin rhythms in old rodents [43], [44]. During aging, circadian rhythms also become disrupted or fragmented, and their phase relationship to environmental time signals is altered [11], [31], [40], [48], [61]. For example, locomotor activity rhythms in old hamsters exhibit fragmentation and an earlier onset of the active phase in relationship to the onset of darkness [11], [40], [48], [61]. Also, circulating cortisol rhythms in old men are dampened and phase-advanced compared to the rhythms in young men [51]. During aging, the circadian pacemaker becomes more resistant to phase resetting signals, such as injections of triazolam or 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) [41], [55]. Finally, aging may induce a dissociation of circadian rhythms within an individual [47]. Studies of circadian rhythms in drinking behavior, body temperature, and neuronal activity in old rats showed that some individuals exhibited a loss of all three rhythms, while other experienced a loss of only one or two of these rhythms [47]. Thus, aging disrupts the coordinated expression of circadian rhythms.
The neural basis for these age-related alterations in circadian rhythms may involve functional changes in the circadian pacemaker in the hypothalamic suprachiasmatic nucleus (SCN). The SCN responds to timing signals received from afferent pathways and drives overt circadian rhythms through its efferent connections [25], [37], [57]. The SCN consists of neurons that synthesize a variety of neuropeptides, including vasoactive intestinal peptide (VIP), arginine vasopressin (AVP), and somatostatin (SS), and appear to play multiple roles in circadian timekeeping [7], [23], [52]. VIP and AVP have been identified in several efferent pathways from the SCN and thus may be involved in the regulation of overt circadian rhythms [25], [57]. AVP and SS have been shown to modulate the function of the SCN. AVP neurons provide endogenous excitatory tone to the SCN [34]. Somatostatin administration inhibits SCN neuronal firing in vitro [22]. Somatostatin neurons are also involved in regulating the phase of circadian rhythms. Depletion of somatostatin induces phase advances in locomotor activity rhythms and SCN electrical activity rhythms, and permits rhythmic release of VIP from the SCN in vitro [15], [16]. The demonstration that VIP neurons receive synaptic inputs from the retinohypothalamic tract, the geniculohypothalamic tract and the serotonergic projection from the median raphe nucleus suggests that the VIP neurons are involved in entrainment of the circadian pacemaker [18], [19], [21], [29]. This concept is supported by findings that VIP microinjections in the SCN region induce phase-dependent phase shifts in locomotor activity rhythms [1], [2], [42].
Because neuropeptide neurons play important roles in regulating circadian timekeeping, alterations in their function may contribute to age-related changes in circadian rhythms. For example, aging modulates the VIP mRNA rhythm in the rat SCN [28], [30]. However, it remains to be determined if aging changes the rhythmic expression of VIP mRNA or other neuropeptide mRNA in the SCN of the Syrian hamster, a well-characterized model of age-related changes in behavioral circadian rhythms [45], [54], [62]. Therefore, the current study tested the hypothesis that aging alters the SCN rhythmic expression of messenger RNA for VIP, AVP or SS in Syrian hamsters. In order to assess each mRNA species at the site of greatest expression within the SCN, a preliminary study was conducted to delineate the regional distribution of VIP mRNA, AVP mRNA and SS mRNA.
Section snippets
Animals and tissue preparation
Male Syrian hamsters obtained from Harlan Labs (Harlan, HsdHan:AURA) were maintained in the Department of Laboratory Animal Research at the University of Kentucky under a 14-h light, 10-h dark photoperiod (lights on from 06:00 to 20:00 h) for at least 1–2 weeks. Food (Teklad, Amway) and water were available continuously. Three age groups were studied: young (3–5 months old), middle-aged (12–15 months old) and old (19–22 months old). The hamsters were sacrificed at five different times of day,
Regional distribution of neuropeptide mRNA expression in the SCN
The Syrian hamster SCN expressed all three mRNA species investigated: VIP, AVP and SS (Fig. 1). VIP mRNA expression, which was localized to the ventral SCN, exhibited a statistically significant variation throughout the rostral to caudal axis of the SCN, characterized by greatest expression in the middle and no detectable expression at either pole (Fig. 2). The expression of AVP mRNA and SS mRNA expression in the dorsal region of the SCN were homogeneous along the rostral to caudal axis of the
Discussion
In order to elucidate the neural basis for age-related changes in circadian rhythms, this study investigated the effect of aging on expression of neuropeptide mRNAs in the SCN, the major mammalian circadian pacemaker. The results showed that aging decreased VIP mRNA expression without affecting AVP mRNA or SS mRNA expression. These findings support the hypothesis that aging modulates SCN expression of neuropeptide messenger RNA.
The selectivity of this effect for VIP suggests that it may be
Acknowledgements
We thank Anthony Deveraux for assistance with these experiments and Dr Kathryn Scarbrough for helpful consultation concerning in situ hybridization for VIP mRNA expression. NIH Grant AG-13418 supported these studies.
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