Continuous recording of locomotor activity in groups of rats: postweaning maturation

Sex Differences in Pubertal Circadian and Ultradian Rhythmic Development Under Semi-naturalistic Conditions

Biological rhythms in core body temperature (CBT) provide informative markers of adolescent development under controlled laboratory conditions. However, it is unknown whether these markers are preserved under more variable, semi-naturalistic conditions, and whether CBT may therefore prove useful in a real-world setting. To evaluate this possibility, we examined fecal steroid concentrations and CBT rhythms from pre-adolescence (p26) through early adulthood (p76) in intact male and female Wistar rats under natural light and climate at the Stephen Glickman Field Station for the Study of Behavior, Ecology and Reproduction. Despite greater environmental variability, CBT markers of pubertal onset and its rhythmic progression were comparable with those previously reported in laboratory conditions in female rats and extend actigraphy-based findings in males. Specifically, sex differences emerged in CBT circadian rhythm (CR) power and amplitude prior to pubertal onset and persisted into early adulthood, with females exhibiting elevated CBT and decreased CR power compared with males. Within-day (ultradian rhythm [UR]) patterns also exhibited a pronounced sex difference associated with estrous cyclicity. Pubertal onset, defined by vaginal opening, preputial separation, and sex steroid concentrations, occurred later than previously reported under lab conditions for both sexes. Vaginal opening and increased fecal estradiol concentrations were closely tied to the commencement of 4-day oscillations in CBT and UR power. By contrast, preputial separation and the first rise in testosterone concentration were not associated with adolescent changes to CBT rhythms in male rats. Together, males and females exhibited unique temporal patterning of CBT and sex steroids across pubertal development, with tractable associations between hormonal concentrations, external development, and temporal structure in females. The preservation of these features outside the laboratory supports CBT as a strong candidate for translational pubertal monitoring under semi-naturalistic conditions in females.

Acute Sleep Deprivation Reduces Oscillatory Network Inhibition in the Young Rat Basolateral Amygdala

The amygdala is concerned with the emotional memory consolidation, and is known as a stress-vulnerable region of the brain. Slow network oscillation is considered to play roles in memory consolidation during sleep. We investigated the relationship between the sleep and oscillation in the basolateral nucleus (BL) of the amygdala, in which burst firing is preferentially observed during sleep and the slow inhibitory oscillation is recorded from projection neuron. We examined whether sleep deprivation (SD) alters the properties of the network inhibition by whole-cell recordings from BL projection neurons and interneurons of the slice preparation of the juvenile rats. The level of the oscillatory network inhibition, measured as summed power of the spectral density between 0.1 and 3 Hz of the synaptic currents in the projection neurons, was significantly attenuated by acute (3 h) SD in older (P20-24) but not in younger (P15-19) animals. This reduction was mainly derived from the reduced peak amplitude of periodic IPSC bursts. In inhibitory interneurons in BL, spontaneous firings were reduced in older SD rats. The spike threshold of interneurons was increased and the power of the periodic excitatory transmission was reduced in the SD rats. Moreover, a reduction in input resistance in projection neurons was observed in SD rats without significant difference in the excitability which was measured by the spike number induced by depolarizing currents. These results suggest that SD stress affects the network oscillatory property accompanied by changes of individual neuronal excitability and synaptic communications.

Adolescent sleep patterns in humans and laboratory animals

This article is part of a Special Issue "Puberty and Adolescence". One of the defining characteristics of adolescence in humans is a large shift in the timing and structure of sleep. Some of these changes are easily observable at the behavioral level, such as a shift in sleep patterns from a relatively morning to a relatively evening chronotype. However, there are equally large changes in the underlying architecture of sleep, including a >60% decrease in slow brain wave activity, which may reflect cortical pruning. In this review we examine the developmental forces driving adolescent sleep patterns using a cross-species comparison. We find that behavioral and physiological sleep parameters change during adolescence in non-human mammalian species, ranging from primates to rodents, in a manner that is often hormone-dependent. However, the overt appearance of these changes is species-specific, with polyphasic sleepers, such as rodents, showing a phase-advance in sleep timing and consolidation of daily sleep/wake rhythms. Using the classic two-process model of sleep regulation, we demonstrate via a series of simulations that many of the species-specific characteristics of adolescent sleep patterns can be explained by a universal decrease in the build-up and dissipation of sleep pressure. Moreover, and counterintuitively, we find that these changes do not necessitate a large decrease in overall sleep need, fitting the adolescent sleep literature. We compare these results to our previous review detailing evidence for adolescent changes in the regulation of sleep by the circadian timekeeping system (Hagenauer and Lee, 2012), and suggest that both processes may be responsible for adolescent sleep patterns.

The neuroendocrine control of the circadian system: adolescent chronotype

Scientists, public health and school officials are paying growing attention to the mechanism underlying the delayed sleep patterns common in human adolescents. Data suggest that a propensity towards evening chronotype develops during puberty, and may be caused by developmental alterations in internal daily timekeeping. New support for this theory has emerged from recent studies which show that pubertal changes in chronotype occur in many laboratory species similar to human adolescents. Using these species as models, we find that pubertal changes in chronotype differ by sex, are internally generated, and driven by reproductive hormones. These chronotype changes are accompanied by alterations in the fundamental properties of the circadian timekeeping system, including endogenous rhythm period and sensitivity to environmental time cues. After comparing the developmental progression of chronotype in different species, we propose a theory regarding the ecological relevance of adolescent chronotype, and provide suggestions for improving the sleep of human adolescents.

Changes in circadian rhythms during puberty in Rattus norvegicus: developmental time course and gonadal dependency

During puberty, humans develop a later chronotype, exhibiting a phase-delayed daily rest/activity rhythm. The purpose of this study was to determine: 1) whether similar changes in chronotype occur during puberty in a laboratory rodent species, 2) whether these changes are due to pubertal hormones affecting the circadian timekeeping system. We tracked the phasing and distribution of wheel-running activity rhythms during post-weaning development in rats that were gonadectomized before puberty or left intact. We found that intact peripubertal rats had activity rhythms that were phase-delayed relative to adults. Young rats also exhibited a bimodal nocturnal activity distribution. As puberty progressed, bimodality diminished and late-night activity phase-advanced until it consolidated with early-night activity. By late puberty, intact rats showed a strong, unimodal rhythm that peaked at the beginning of the night. These pubertal changes in circadian phase were more pronounced in males than females. Increases in gonadal hormones during puberty partially accounted for these changes, as rats that were gonadectomized before puberty demonstrated smaller phase changes than intact rats and maintained ultradian rhythms into adulthood. We investigated the role of photic entrainment by comparing circadian development under constant and entrained conditions. We found that the period (τ) of free-running rhythms developed sex differences during puberty. These changes in τ did not account for pubertal changes in entrained circadian phase, as the consolidation of activity at the beginning of the subjective night persisted under constant conditions in both sexes. We conclude that the circadian system continues to develop in a hormone-sensitive manner during puberty.

Comparison of infant and adult rats in exploratory activity, diurnal patterns, and responses to novel and anxiety-provoking environments

Infant rats emerge from the maternal nest at Postnatal Day 17-18 to have their first critical environmental experiences; they may be particularly sensitive to experiences or experimental interventions that can affect their adult capacity. The authors address open questions on 2 components of normative environmental exploration, locomotor activity and response to anxiety-provoking locations, in Postnatal Day 18 infant and Postnatal Day 60 adult rats. The authors compare diurnal patterns of locomotor activity, wheel running, novel and familiar open-field activity, and 2 measures of anxiety. Infants have an equivalent capacity to adults for locomotor activity and wheel running and a fundamentally adult-like diurnal rhythm, except that they do not anticipate light-dark transitions, are more perturbable at their most somnolent, and are more or less active during specific limited phases than adults. Infants initially have a lower rate of locomotor activity in novel environments and have a greater willingness to be active in anxiety-provoking locations. Such differences may allow enhanced gathering of environmental information by the infant and are important to consider in the design of experiments using infants.

Evidence against alpha2-adrenoceptor involvement in the regulation of rat melatonin synthesis by ambient lighting

This study was carried out to clarify the role of alpha2-adrenoceptors in the regulation of pineal melatonin synthesis. Medetomidine, a selective alpha2-adrenoceptor agonist, was previously found to be a potent suppressor of nocturnal melatonin levels in rats. Medetomidine and alpha2-adrenoceptor antagonists atipamezole and yohimbine were injected into rats in different conditions, and their pineal melatonin contents were measured by radioimmunoassay. Experiment 1: Blocking the alpha2-adrenoceptors and possible non-adrenergic binding sites with atipamezole did not counteract the light-induced suppression of nocturnal melatonin. These receptors are, thus, not essential for the suppression of melatonin by light. Experiment 2: Blocking the alpha2-adrenoceptors with atipamezole or yohimbine did not sensitize the pineal melatonin synthesis to daytime darkness in the light/dark-entrained rats. The binding sites are not involved in keeping the daytime melatonin levels low, even in darkness. Experiment 3: The rats were sensitized to daytime darkness by keeping them for seven days in constant light. The dark-elicited melatonin rise was suppressed by a lower dose of medetomidine than the normal nocturnal rise in light/dark-entrained rats, while atipamezole had no effect. The results showed that alpha2-adrenoceptor insufficiency is not involved in the constant light-induced pineal supersensitivity. In summary, the experiments indicated that the physiological regulation of melatonin synthesis by ambient lighting in rats does not depend on alpha2-adrenergic mechanisms.

Differential effects of prenatal cocaine and retinoic acid on activity level throughout day and night

Prenatal cocaine exposure is associated with disrupted state control and lowered activity levels. Prenatal retinoic acid excess also influences activity levels in laboratory rats. Activity level is usually monitored during a brief period in young offspring. The effects of these drugs on pup activity levels throughout the day is unknown. There is also little information on the long-lasting effects of these teratogens in adult animals. We compared the daily activity of rats which were prenatally exposed to cocaine or retinoic acid (RA). Appropriate control groups were also used. The offspring were evaluated for activity levels in a neophobic situation and for a 22-h period in same-sex groups of 3 littermates. As both pups and adults, the cocaine groups were hypoactive while the RA group was hyperactive when first placed into the testing cage (neophobic situation). Similarly, during the remainder of the 22-h testing period, the pup and adult cocaine animals exhibited reduced activity levels while the RA animals exhibited elevated activity levels. Thus, prenatal cocaine and retinoic acid exposures affected offspring activity levels differently, both drugs have long-lasting neurobehavioral effects that persist into adulthood, and effects are influenced by time-of-day. Strain-dependent differences and mechanisms of action are discussed.

Locomotor activity and melatonin rhythms in rats under non-24-h lighting cycles

The adjustment of pineal melatonin and locomotor activity rhythms to 10:10-h light:dark (LD) or 14:14-h LD cycles was studied in male Wistar rats. Both lighting conditions were thought to be outside the limits of entrainment of the rest-activity rhythm in this species. We assumed that the rhythm of pineal melatonin synthesis might be more adaptable. As expected, the locomotor activity rhythm was not adjusted to the 10:10-h LD cycles. Under these conditions, a free-running component (25 h) became dominant. Under the 14:14-h LD cycles, however, an unexpected adaptation occurred within 10 days. The profiles of the pineal melatonin contents measured on days 5 and 30 under the 10:10-h LD and on day 7 under the 14:14-h LD schedule were in line with the estimated free-running oscillations, but the profile on day 21 under the 14:14-h LD schedule was not. This melatonin pattern fitted the LD-adjusted activity rhythm. Thus, the melatonin rhythm did not adapt better than the activity rhythm to the exotic LD cycles. Instead, parallel changes were found.

A multivariate assessment of spontaneous locomotor activity in the Mongolian gerbil (Meriones unguiculatus): influences of age and sex

A multivariate assessment of the spontaneous locomotor activity of male and female Mongolian gerbils (Meriones unguiculatus) was obtained using a Digiscan automated animal activity monitoring system. Spontaneous motor activity data were collected over 1 h (5-min samples) for groups of male and female gerbils ranging from 26-341 days of age (26, 38, 62, 116, 151, 172, 196, 247, and 341). Variables examined included: total distance travelled, average distance per movement, average speed, number of horizontal movements, time in horizontal movement, time per horizontal movement, number of vertical movements, time in vertical movement, and time per vertical movement. Age had a significant effect on spontaneous activity; all measures of horizontal activity increased from preadulthood (26 and 38 days) and remained relatively constant thereafter for adults (62+ days). Vertical activity (rearing) measures were found to increase from the 62-day-old group to the 151- and 172-day-old groups and then decrease among the older groups (196+ days). Across the 12 samples, within sessions, all horizontal and vertical activity measures (except average speed) declined for both males and females. Habituation was more rapid for the preadults than for the adults on all horizontal measures except average distance per movement. No consistent sex differences in locomotor activity were found.

Pineal melatonin and locomotor activity of rats under gradual illuminance transitions

The locomotor activity and pineal melatonin patterns of adult male rats were compared under two different lighting regimes. The animals were kept 8 days under 12/12 h light/dark cycles with abrupt or slowly decreasing and increasing transitions (twilight periods about 2 h). The onsets of high activity and melatonin rise were phase-locked in the two conditions and related to about half-maximal illuminance level of the gradual dusk. The high activity of the control rats stopped 30-60 min before the abrupt light onset and the rats under the gradual lighting transitions ceased the locomotor activity at about 1 hour before the half-maximal illuminance. The melatonin peak levels were found 4 h before the abrupt lights-on time. Under the slow illuminance transitions the average melatonin peak was related to the illuminance level between maximum and minimum in the morning. Thus, both the melatonin rhythm and the rest-activity rhythm under the gradual dawn and dusk were adjusted according to about half-maximal illuminances in the present conditions.