Effects of bright light on age-related changes in the locomotor activity of Syrian hamsters.

Syrian hamsters display age-related changes in the expression of circadian rhythms and in responsiveness of the circadian system to photic and non-photic stimuli. This study characterized the effects of age on the locomotor activity rhythm of middle-aged and old hamsters and evaluated the effects of strengthening the entraining light signal. Compared with young (4.5 mo) animals, middle-aged (11.25 mo) and old (16 mo) animals displayed increased daily bouts of activity (P < 0.001) and reduced total daily activity and activity rhythm amplitude (P < 0.05) in 14:10-h light-dark cycles. After the light intensity was increased from 300 to 1,500 lx during the light cycle, middle-aged hamsters demonstrated decreased daily activity bouts (P < 0.05) and increased total daily activity (P < or = 0.01) and activity rhythm amplitude (P < or = 0.001) compared with controls maintained in 300 lx. The pattern of changes in the activity rhythm of old experimental animals was similar to trends observed in middle-aged experimental hamsters, although not as robust. Thus age-related changes in the activity rhythm are occurring by middle age in hamsters, and the provision of stronger entraining signals may lead to more stable circadian organization.

Aging and photoperiod affect entrainment and quantitative aspects of locomotor behavior in Syrian hamsters.

Aging affects the regulation of diurnal and circadian rhythmicity. We tested the hypothesis that the age-related difference in the phase angle of entrainment of the locomotor activity rhythm to a light-dark (LD) cycle would be greater under LD 6:18 than LD 14:10. We also analyzed changes in quantitative aspects of wheel-running behavior according to age group. Young (9-wk-old), middle-aged (11- to 12-mo-old), and old (15- to 17-mo-old) male golden hamsters were entrained to a 14:10 LD cycle followed by re-entrainment to a 6:18 LD cycle. Fourteen days after the start of locomotor recording in LD 14:10 and again after 27 days in LD 6:18, the phase of activity onset, the total number of wheel revolutions performed per day, the peak intensity of wheel-running activity, the duration of the active period, and the level of fragmentation of locomotor activity were quantitated. We also studied the temporal distribution of the largest bout of wheel-running activity among the age groups in both photoperiods. Short days induced testicular regression at a similar rate among young, middle-aged, and old hamsters. The data are discussed in terms of the effects of age on overall circadian organization in the seasonally changing environment.

Aging alters the serotonergic modulation of light-induced phase advances in golden hamsters.

Recent findings have raised the possibility that some of the age-related changes in the circadian system and the response of the circadian pacemaker to environmental stimuli may involve central serotonergic mechanisms. The present study compared the effects ofpretreatment with the serotonin agonist 8-hydroxy-2(di-n-propylamino)tetralin (5 mg/kg ip) on the magnitude of light-induced phase advances in young (2-4 mo) and old (18-20 mo) golden hamsters. The ability of this serotonin agonist to attenuate the photic phase resetting of circadian locomotor rhythmicity in young animals was decreased by 46% in old hamsters (P < 0.05). These results suggest that deficits in the mechanisms for serotonergic control of circadian function may interfere with the optimal adaptation of the senescent organism to its temporal environment.

Aging alters the phase-resetting properties of a serotonin agonist on hamster circadian rhythmicity.

Serotonergic mechanisms are believed to play a considerable role in mediating the effects of photic and nonphotic stimuli on circadian rhythmicity. Because aging is associated with significant changes in the responsiveness of the rodent circadian system to major synchronizing agents in the environment, this study examined the phase-shifting effects of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin [8-OH-DPAT; 2.0, 5.0, and 8.0 mg/kg ip at circadian time 8 (CT 8)], on the 24-h activity rhythm of young (3-4 mo old) and old (18-19 mo old) golden hamsters. Aging was associated with a dramatic attenuation of the phase-shifting effects of 8-OH-DPAT in this species. The results suggest the existence of age-related deficits in the serotonergic control of mammalian circadian rhythmicity, which could interfere with the temporal adaptation of the senescent organism to its environment.

Photoperiodic response in the male laboratory rat.

Normally photoperiodic laboratory rats can be induced to respond reproductively to a change in the length of the day by various experimental manipulations. One such paradigm that results in significant gonadal regression involves the treatment of rats with exogenous testosterone during exposure to short days. Studies were undertaken to assess various aspects of this model system including 1) the testicular response of testosterone-treated rats exposed to various photoperiods, 2) the time course for testicular regression under a short photoperiod, and 3) the role of the pineal gland as a mediator of the effects of day length on the neuroendocrine-gonadal axis. Photoperiods ranging in length from 2 to 22 h/24 h had no effect on testicular size in untreated rats. In contrast, while near normal testicular weights were maintained in laboratory rats treated with testosterone and exposed to 10 or more h of light per day, testicular regression occurred in rats implanted with testosterone-filled capsules and exposed to photoperiods of 8 or fewer h of light per day. Maximal testicular regression was reached in about 9 wk in testosterone-treated rats exposed to 6L:18D. Removal of the pineal gland totally blocked the inhibitory effects of exposure to short day lengths in testosterone-treated rats. These studies define some of the characteristics of an extant, but dormant, system for photoperiodic time measurement in the common laboratory rat and implicate a role for the pineal gland in this system. These experiments offer evidence that neuroendocrine factors that regulate continuous vs. seasonal reproductive patterns are malleable. Such flexibility in the photoperiodic response may also contribute to the evolution of seasonal to non-seasonal species and vice versa.

Photoperiodicity in the male albino laboratory rat.

Animals inhabiting areas where there are drastic changes in the environment often reproduce only during limited time periods to ensure that young are raised in optimal environmental conditions. The lack of a well defined breeding season in many domesticated animals, presumably because the selective pressures for seasonal breeding have been minimized, suggests that the neuroendocrine events controlling seasonal cyclicity have been bred out of these animals. Little is known about the underlying neuroendocrine changes which may occur during the evolution of a species from a seasonal to a nonseasonal breeder. Whereas the changing photoperiod is the primary environmental cue which initiates and/or terminates the reproductive season in many animals, this is not so in the albino rat Rattus norvegicus, a model nonseasonal breeder. Nevertheless, daylength can influence various reproductive parameters in laboratory rats, suggesting that some of the neuroendocrine components that controlled seasonal breeding previously are still extant in this species. To test this hypothesis, we investigated the effect of daylength on the responsiveness of the neuroendocrine--gonadal axis to the negative-feedback effects of testosterone. This paradigm was chosen because of the important role played by photic-induced changes in steroid feedback sensitivity in the control of seasonal reproduction. We report here that although daylength has very little effect on neuroendocrine--gonadal function in the intact male laboratory rat, it seems that some component(s) of a photoperiodic system involving the pineal gland has been preserved.

Effect of testosterone on the hypothalamic-pituitary-gonadal axis of pinealectomized and pineal-intact male rats.

Previous studies indicate that steroid hormones alter pineal biochemistry, and it has been suggested that at least part of the negative feedback effect of steroid hormones on pituitary gonadotropin release may be mediated by the pineal gland. In this study, pinealectomy did not alter the inhibitory effect of testosterone on neuroendocine-gonadal activity in the male rat, suggesting that the pineal gland does not mediate the response of the rat hypothalamic-pituitary axis to testosterone.