A review of seasonal/circannual effects of laboratory rodent behavior

Adolescent morphine exposure induced long-term cognitive impairment and prefrontal neurostructural abnormality in adulthood in male mice

Opioids abuse in adolescence is becoming a pressing public health issue. While evidence suggests that exposure to opioids during adolescence leads to lasting alterations in brain development, the long-term cognitive implications in adulthood remain uncertain. We developed a male mouse model of adolescent morphine exposure and used the 5-choice serial reaction time task (5-CSRTT), along with the open field, novel object recognition, Y maze and Barnes maze tests, to assess changes in cognitive behavior. We found that exposure to morphine during adolescence led to deficits in multidimensional cognitive functions in mice, including attention, information processing speed, and behavior inhibition. Notable, these impairments persisted into adulthood. Furthermore, the morphine-exposed mice exhibited decreased learning efficiency and spatial memory. Adolescent morphine exposure also induced significant and persistent morphological changes and synaptic abnormalities in medial prefrontal cortex (mPFC) neurons, which may be responsible for cognitive impairments in adulthood. Together, our study suggests that opioid exposure during adolescence profoundly affects cognitive development and emphasizes that opioid-induced disruption of neurons in adolescence may link mPFC-associated cognitive impairments in adulthood.

Increased incidence of weight-loss-associated humane endpoints in rats administered buprenorphine slow-release LAB formulation following traumatic brain injury: a retrospective study

Traumatic brain injury (TBI) remains a significant global public health epidemic with adverse health and cost implications. Due to its complex, heterogeneous nature and wide-ranging impacts, definitive TBI treatments remain elusive. As such, continued laboratory research using animal models is warranted. In accordance with guidelines set forth for the humane treatment of research animals, TBI animal models are often administered analgesics for pain management. The choice of drug, timing, dose, and formulation of analgesic can vary depending on the study's unique needs and can potentially and unintentionally influence experimental results. In TBI studies utilizing rats as animal models, buprenorphine is a common analgesic administered. In addition to pain management in such studies, investigators must also monitor the research animals post-operatively and make the decision for humane euthanasia before intended experimental survival timepoint if the animals are assessed to be excessively suffering. This study investigated the differences in adult, male Sprague Dawley rats used for various TBI studies that reached weight-loss-induced humane endpoints following a single administration of buprenorphine slow-release LAB (bup-SR-LAB) or buprenorphine slow-release HCl (bup-SR-HCl). Our findings indicate that TBI-induced rats receiving bup-SR-LAB in conjunction with a secondary surgical insult such as artificial intracranial pressure elevation and/or osmotic pump implantation reach a weight-loss-induced humane euthanasia endpoint more often compared to sham-injured rats. When stratifying into the same groups, we did not find this pattern to hold true for rats administered bup-SR-HCl. Overall, this study contributes to the limited body of literature addressing different analgesic formulations' effects on laboratory animals.

On hidden factors and design-associated errors that may lead to data misinterpretation: An example from preclinical research on the potential seasonality of neonatal seizures

Unintentional misinterpretation of research in published biomedical reports that is not based on statistical flaws is often underrecognized, despite its possible impact on science, clinical practice, and public health. Important causes of such misinterpretation of scientific data, resulting in either false positive or false negative conclusions, include design-associated errors and hidden (or latent) variables that are not easily recognized during data analysis. Furthermore, cognitive biases, such as the inclination to seek patterns in data whether they exist or not, may lead to misinterpretation of data. Here, we give an example of these problems from hypothesis-driven research on the potential seasonality of neonatal seizures in a rat model of birth asphyxia. This commentary aims to raise awareness among the general scientific audience about the issues related to the presence of unintentional misinterpretation in published reports.

Effects of season, daytime, sex, and stress on the incidence, latency, frequency, severity, and duration of neonatal seizures in a rat model of birth asphyxia

Neonatal seizures are common in newborn infants after birth asphyxia. They occur more frequently in male than female neonates, but it is not known whether sex also affects seizure severity or duration. Furthermore, although stress and diurnal, ultradian, circadian, or multidien cycles are known to affect epileptic seizures in adults, their potential impact on neonatal seizures is not understood. This prompted us to examine the effects of season, daytime, sex, and stress on neonatal seizures in a rat model of birth asphyxia. Seizures monitored in 176 rat pups exposed to asphyxia on 40 experimental days performed over 3 years were evaluated. All rat pups exhibited seizures when exposed to asphyxia at postnatal day 11 (P11), which in terms of cortical development corresponds to term human babies. A first examination of these data indicated a seasonal variation, with the highest seizure severity in the spring. Sex and daytime did not affect seizure characteristics. However, when rat pups were subdivided into animals that were exposed to acute (short-term) stress after asphyxia (restraint and i.p. injection of vehicle) and animals that were not exposed to this stress, the seizures in stress-exposed rats were more severe but less frequent. Acute stress induced an increase in hippocampal microglia density in sham-exposed rat pups, which may have an additive effect on microglia activation induced by asphyxia. When seasonal data were separately analyzed for stress-exposed vs. non-stress-exposed rat pups, no significant seasonal variation was observed. This study illustrates that without a detailed analysis of all factors, the data would have erroneously indicated significant seasonal variability in the severity of neonatal seizures. Instead, the study demonstrates that even mild, short-lasting postnatal stress has a profound effect on asphyxia-induced seizures, most likely by increasing the activity of the hypothalamic-pituitary-adrenal axis. It will be interesting to examine how postnatal stress affects the treatment and adverse outcomes of birth asphyxia and neonatal seizures in the rat model used here.

Seasonal Variation of Laboratory Animals as a Consideration for Research Reproducibility

Laboratory rodents are generally maintained under standardized conditions in order to control the effects of extrinsic factors on research. However, despite attempts to standardize conditions, variability can nonetheless confound efforts directed toward research reproducibility. Here we explore some of the existing literature on the potential impact of seasonal variability as an extrinsic factor that can potentially impact research results. We discuss the influence of seasonal changes in association with an internal clock mechanism that might account for such variation, noting that the mechanisms and interactions of seasonal and internal time-keeping remain largely undetermined. Finally, we speculate that seasonal changes experienced by personnel who handle animals may influence the animals in ways that result in physiologic and behavioral changes.

Dynamic changes to signal allocation rules in response to variable social environments in house mice

Urine marking is central to mouse social behavior. Males use depletable and costly urine marks in intrasexual competition and mate attraction. We investigate how males alter signaling decisions across variable social landscapes using thermal imaging to capture spatiotemporal marking data. Thermal recording reveals fine-scale adjustments in urinary motor patterns in response to competition and social odors. Males demonstrate striking winner-loser effects in scent mark allocation effort and timing. Competitive experience primes temporal features of marking and modulates responses to scent familiarity. Males adjust signaling effort, mark latency, and marking rhythm, depending on the scent identities in the environment. Notably, recent contest outcome affects how males respond to familiar and unfamiliar urine. Winners increase marking effort toward unfamiliar relative to familiar male scents, whereas losers reduce marking effort to unfamiliar but increase to familiar rival scents. All males adjust their scent mark timing after a contest regardless of fight outcome, and deposit marks in more rapid bursts during marking bouts. In contrast to this dynamism, initial signal investment predicts aspects of scent marking days later, revealing the possibility of alternative marking strategies among competitive males. These data show that mice flexibly update their signaling decisions in response to changing social landscapes.

Spike-Wave Seizures, NREM Sleep and Micro-Arousals in WAG/Rij Rats with Genetic Predisposition to Absence Epilepsy: Developmental Aspects

The current study was done in Wistar Albino Glaxo Rijswijk (WAG/Rij) rats, which are genetically prone to develop spontaneous spike-wave discharges (SWDs) and are widely used as a genetic model of absence epilepsy. Here, we examined functional links between sleep and spike-wave epilepsy in aging WAG/Rij rats using advanced techniques of EEG analysis. SWDs, periods of NREM sleep and micro-arousals were automatically detected in three-channel epidural EEG recorded in freely moving WAG/Rij rats consequently at the age 5, 7 and 9 months. We characterized the developmental profile of spike-wave epilepsy in drug-naïve WAG/Rij rats and defined three epi-phenotypes—severe, mild and minor epilepsy. Age-related changes of SWDs were associated with changes in NREM sleep. Several signs of NREM sleep fragmentation were defined in epileptic WAG/Rij rats. It seems that spike-wave epilepsy per se promotes micro-arousals during NREM sleep. However, subjects with a higher number of micro-arousals (and NREM sleep episodes) at the age of 5 months were characterized by a reduction of SWDs between 5 and 7 months of age.

Photoperiod-induced alterations in biomarkers of oxidative stress and biochemical pathways in rats of different ages: Focus on individual physiological reactivity

Effects of photoperiodicity caused by both the age and individual physiological reactivity estimated by resistance to hypobaric hypoxia on the levels of lipid peroxidation, protein oxidation (aldehydic and ketonic derivatives), total antioxidant capacity, activities of antioxidant enzymes (superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase), and biochemical parameters of aerobic and anaerobic pathways in hepatic tissue depending on the blood melatonin level were studied. The study was carried out on 96 6- and 21-month-old male rats divided into hypoxia resistance groups (LR, low resistance, HR, high resistance). The analyses were conducted at four photoperiods: winter (January), spring (March), summer (July), and autumn (October). Our results indicate a significant effect of melatonin, i.e. over 80%, revealed by the complete statistical model of the studied biomarkers of oxidative stress and oxygen-dependent parameters of metabolism. The effects of melatonin vary with age and between photoperiods, which in turn was determined by individual physiological reactivity. In terms of the photoperiods, the melatonin content in the group of the adult animals with low resistance to hypoxia decreased from winter to summer. In a group of old animals in comparison with adults, the melatonin content in all the studied photoperiods was much lower as well, regardless of their hypoxia resistance. In the group of old animals with low resistance to hypoxia, the melatonin content decreased throughout the photoperiods as follows: winter, autumn, summer, and spring. As can be concluded, spring is a critical period for old animals, particularly those with low hypoxia resistance. The important role of melatonin in these processes was also confirmed by our correlation analysis between oxidative stress biomarkers, energy-related metabolites, and antioxidant enzymes in the hepatic tissue of rats of different ages, with different resistance to hypoxia, and in different photoperiods. The melatonin concentration in the blood of highly resistant rats was higher than in those with low resistance to hypoxia. Melatonin determines the individual constitutional level of resistance to hypoxia and is responsible for individual enzymatic antioxidative responses, depending on the four photoperiods. Our studies have shown that melatonin levels are related to the redox characteristics of antioxidant defenses against lipid peroxidation and oxidative modification of proteins in old rats with low resistance to hypoxia, compared to a group of highly resistant adults. Finally, the melatonin-related mechanisms of antioxidative protection depend on metabolic processes in hepatic tissue and exhibit photoperiodical variability in adult and old rats.

Behavior Testing in Rodents: Highlighting Potential Confounds Affecting Variability and Reproducibility

Rodent models of brain disorders including neurodevelopmental, neuropsychiatric, and neurodegenerative diseases are essential for increasing our understanding of underlying pathology and for preclinical testing of potential treatments. Some of the most important outcome measures in such studies are behavioral. Unfortunately, reports from different labs are often conflicting, and preclinical studies in rodent models are not often corroborated in human trials. There are many well-established tests for assessing various behavioral readouts, but subtle aspects can influence measurements. Features such as housing conditions, conditions of testing, and the sex and strain of the animals can all have effects on tests of behavior. In the conduct of behavior testing, it is important to keep these features in mind to ensure the reliability and reproducibility of results. In this review, we highlight factors that we and others have encountered that can influence behavioral measures. Our goal is to increase awareness of factors that can affect behavior in rodents and to emphasize the need for detailed reporting of methods.

Major oscillations in spontaneous home-cage activity in C57BL/6 mice housed under constant conditions

The mouse is the most important mammalian model in life science research and the behavior of the mouse is a key read-out of experimental interventions and genetic manipulations. To serve this purpose a solid understanding of the mouse normal behavior is a prerequisite. Using 14-19 months of cumulative 24/7 home-cage activity recorded with a non-intrusive technique, evidence is here provided for a highly significant circannual oscillation in spontaneous activity (1-2 SD of the mean, on average 65% higher during peak of highs than lows; P = 7E-50) of male and female C57BL/6 mice held under constant conditions. The periodicity of this hitherto not recognized oscillation is in the range of 2-4 months (average estimate was 97 days across cohorts of cages). It off-sets responses to environmental stimuli and co-varies with the feeding behavior but does not significantly alter the preference for being active during the dark hours. The absence of coordination of this rhythmicity between cages with mice or seasons of the year suggest that the oscillation of physical activity is generated by a free-running intrinsic oscillator devoid of external timer. Due to the magnitude of this rhythmic variation it may be a serious confounder in experiments on mice if left unrecognized.

An individual based, multidimensional approach to identify emotional reactivity profiles in inbred mice

BACKGROUND

Despite extensive environmental standardization and the use of genetically and microbiologically defined mice of similar age and sex, individuals of the same mouse inbred strain commonly differ in quantitative traits. This is a major issue as it affects the quality of experimental results. Standard analysis practices summarize numerical data by means and associated measures of dispersion, while individual values are ignored. Perhaps taking individual values into account in statistical analysis may improve the quality of results.

NEW METHOD

The present study re-inspected existing data on emotional reactivity profiles in 125 BALB/cJ and 129 mice, which displayed contrasting patterns of habituation and sensitization when repeatedly exposed to a novel environment (modified Hole Board). Behaviors were re-analyzed on an individual level, using a multivariate approach, in order to explore whether this yielded new information regarding subtypes of response, and their expression between and within strains.

RESULTS

Clustering individual mice across multiple behavioral dimensions identified two response profiles: a habituation and a sensitization cluster.

COMPARISON WITH EXISTING METHOD(S)

These retrospect analyses identified habituation and sensitization profiles that were similar to those observed in the original data but also yielded new information such as a more pronounced sensitization response. Also, it allowed for the identification of individuals that deviated from the predominant response profile within a strain.

CONCLUSIONS

The present approach allows for the behavioral characterization of experimental animals on an individual level and as such provides a valuable contribution to existing approaches that take individual variation into account in statistical analysis.

Detecting Behavioral Deficits in Rats After Traumatic Brain Injury

With the increasing incidence of traumatic brain injury (TBI) in both civilian and military populations, TBI is now considered a chronic disease; however, few studies have investigated the long-term effects of injury in rodent models of TBI. Shown here are behavioral measures that are well-established in TBI research for times early after injury, such as two weeks, until two months. Some of these methods have previously been used at later times after injury, up to one year, but by very few laboratories. The methods demonstrated here are a short neurological assessment to test reflexes, a Beam-Balance to test balance, a Beam-Walk to test balance and motor coordination, and a working memory version of the Morris water maze that can be sensitive to deficits in reference memory. Male rats were handled and pre-trained to neurological, balance, and motor coordination tests prior to receiving parasagittal fluid percussion injury (FPI) or sham injury. Rats can be tested on the short neurological assessment (neuroscore), the beam-balance, and the Beam-Walk multiple times, while testing on the water maze can only be done once. This difference is because rats can remember the task, thus confounding the results if repeated testing is attempted in the same animal. When testing from one to three days after injury, significant differences are detected in all three non-cognitive tasks. However, differences in the Beam-Walk task were not detectable at later time points (after 3 months). Deficits were detected at 3 months in the Beam-Balance and at 6 months in the neuroscore. Deficits in working memory were detected out to 12 months after injury, and a deficit in a reference memory first appeared at 12 months. Thus, standard behavioral tests can be useful measures of persistent behavioral deficits after FPI.

Evaluation of the safety of conventional lighting replacement by artificial daylight

Background

Short morning exposure to high illuminance visible electromagnetic radiations termed as artificial daylight is beneficial for the mental health of people living in geographical areas with important seasonal changes in daylight illuminance. However, the commercial success of high illuminance light sources has raised the question of the safety of long hour exposure.

Methods

We have investigated the effect of the replacement of natural daylight by artificial daylight in Swiss mice raised under natural lighting conditions. Mice were monitored for neurotoxicity and general health changes. They were submitted to a battery of conventional tests for mood, motor and cognitive functions' assessment on exposure day (ED) 14 and ED20. Following sacrifice on ED21 due to marked signs of neurotoxicity, the expression of markers of inflammation and apoptosis was assessed in the entorhinal cortex and neurons were estimated in the hippocampal formation.

Results

Signs of severe cognitive and motor impairments, mood disorders, and hepatotoxicity were observed in animals exposed to artificial daylight on ED20, unlike on ED14 and unlike groups exposed to natural daylight or conventional lighting. Activated microglia and astrocytes were observed in the entorhinal cortex, as well as dead and dying neurons. Neuronal counts revealed massive neuronal loss in the hippocampal formation.

Conclusions

These results suggest that long hour exposure to high illuminance visible electromagnetic radiations induced severe alterations in brain function and general health in mice partly mediated by damages to the neocortex-entorhinal cortex-hippocampus axis. These findings raise caution over long hour use of high illuminance artificial light.

Adult neurogenesis reduction by a cytostatic treatment improves spatial reversal learning in rats

Adult neurogenesis in the dentate gyrus adds a substantial number of new functional neurons to the hippocampus network in rodents. To date, however, the function of these new granule cells remains unclear. We conducted an experiment to assess the contribution of adult neurogenesis in the dentate gyrus to acquisition and reversal learning in a task that predominantly requires generalization of a rule. Young adult male Long-Evans rats were repeatedly administered either a cytostatic temozolomide or saline for a period of four weeks (3 injections per week). Post treatment, animals were injected with bromodeoxyuridine to quantify adult neurogenesis in the dentate gyrus. For behavioral assessment we used hippocampus-dependent active place avoidance with reversal in a Carousel maze. Animals first learned to avoid a 60° sector on the rotating arena. Afterwards, sector was relocated to the opposite side of the rotating arena (reversal). The administration of temozolomide significantly improved the reversal performance compared to saline-treated rats. Our results suggest a significant, level-dependent, improvement of reversal learning in animals with reduced adult neurogenesis in hippocampus.

Voluntary running decreases nonexercise activity in lean and diet-induced obese mice

PURPOSE

Determine whether voluntary wheel running triggers compensatory changes in nonexercise activity in lean and high-fat diet fed mice.

METHODS

C57Bl/6 mice received a control (C) or a high-fat diet (H) and half of them had free access to a running wheel 5days/week (CE and HE, respectively) for 10weeks. Energy intake, nonexercise activity (global activity, distance covered and average speed of displacement in the home cage) and energy expenditure (EE) were evaluated at weeks 5 and 10 during the 2days without the wheels.

RESULTS

High-fat diet increased weight gain in H (110%) and HE (60%) groups compared to C and CE groups, respectively, with no effect of exercise. Wheel running increased energy intake (26% CE, 11% HE in week 5; 7% CE, 45% HE in week 10) and decreased distance covered (26% for both CE and HE in week 5; 35% CE and 13% HE in week 10) and average speed (35% CE and 13% HE in week 5; 45% CE and 18% HE in week 10) compared to the respective nonexercised groups. In week 10 there was an interaction between diet and exercise for global activity, which was reduced nearly 18% in CE, H, and HE groups compared to C. Access to a running wheel increased EE in week 5 (11% CE and 16% HE) but not in week 10, which is consistent with the period of highest running (number of turns: weeks 1-5 nearly 100%>weeks 6-10 for CE and HE groups). EE was reduced in H (19%) and HE (12%) groups compared to C and CE, in week 10.

CONCLUSION

Voluntary running causes a compensatory decrease in nonexercise activity and an increase in energy intake, both contributing to the lack of effect of exercise on body mass.

Circannual changes in stress and feeding hormones and their effect on food-seeking behaviors

Seasonal fluctuations in food availability show a tight association with seasonal variations in body weight and food intake. Seasonal variations in food intake, energy storage, and expenditure appear to be a widespread phenomenon suggesting they may have evolved in anticipation for changing environmental demands. These cycles appear to be driven by changes in external daylength acting on neuroendocrine pathways. A number of neuroendocrine pathways, two of which are the endocrine mechanisms underlying feeding and stress, appear to show seasonal changes in both their circulating levels and reactivity. As such, variation in the level or reactivity to these hormones may be crucial factors in the control of seasonal variations in food-seeking behaviors. The present review examines the relationship between feeding behavior and seasonal changes in circulating hormones. We hypothesize that seasonal changes in circulating levels of glucocorticoids and the feeding-related hormones ghrelin and leptin contribute to seasonal fluctuations in feeding-related behaviors. This review will focus on the seasonal circulating levels of these hormones as well as sensitivity to these hormones in the modulation of food-seeking behaviors.