Automated Behavioral Experiments in Mice Reveal Periodic Cycles of Task Engagement within Circadian Rhythms

Dark exposure reduces high-frequency hearing loss in C57BL/6J mice

Plastic changes in the brain are primarily limited to early postnatal periods. Recovery of adult brain plasticity is critical for the effective development of therapies. A brief (1-2 week) duration of visual deprivation (dark exposure, DE) in adult mice can trigger functional plasticity of thalamocortical and intracortical circuits in the primary auditory cortex suggesting improved sound processing. We tested if DE enhances the ability of adult mice to detect sounds. We trained and continuously evaluated the behavioral performance of mice in control and DE conditions using automated home-cage training. Consistent with age-related peripheral hearing loss present in C57BL/6J mice, we observed decreased performance for high-frequency sounds with age, which was reduced by DE. In CBA mice with preserved peripheral hearing, we also found that DE enhanced auditory performance in low and mid frequencies over time compared to the control.

A systematic review of the development and application of home cage monitoring in laboratory mice and rats

BACKGROUND

Traditionally, in biomedical animal research, laboratory rodents are individually examined in test apparatuses outside of their home cages at selected time points. However, the outcome of such tests can be influenced by various factors and valuable information may be missed when the animals are only monitored for short periods. These issues can be overcome by longitudinally monitoring mice and rats in their home cages. To shed light on the development of home cage monitoring (HCM) and the current state-of-the-art, a systematic review was carried out on 521 publications retrieved through PubMed and Web of Science.

RESULTS

Both the absolute (~ × 26) and relative (~ × 7) number of HCM-related publications increased from 1974 to 2020. There was a clear bias towards males and individually housed animals, but during the past decade (2011-2020), an increasing number of studies used both sexes and group housing. In most studies, animals were kept for short (up to 4 weeks) time periods in the HCM systems; intermediate time periods (4-12 weeks) increased in frequency in the years between 2011 and 2020. Before the 2000s, HCM techniques were predominantly applied for less than 12 h, while 24-h measurements have been more frequent since the 2000s. The systematic review demonstrated that manual monitoring is decreasing in relation to automatic techniques but still relevant. Until (and including) the 1990s, most techniques were applied manually but have been progressively replaced by automation since the 2000s. Independent of the year of publication, the main behavioral parameters measured were locomotor activity, feeding, and social behaviors; the main physiological parameters were heart rate and electrocardiography. External appearance-related parameters were rarely examined in the home cages. Due to technological progress and application of artificial intelligence, more refined and detailed behavioral parameters have been investigated in the home cage more recently.

CONCLUSIONS

Over the period covered in this study, techniques for HCM of mice and rats have improved considerably. This development is ongoing and further progress as well as validation of HCM systems will extend the applications to allow for continuous, longitudinal, non-invasive monitoring of an increasing range of parameters in group-housed small rodents in their home cages.

Challenges and advanced concepts for the assessment of learning and memory function in mice

The mechanisms underlying the formation and retrieval of memories are still an active area of research and discussion. Manifold models have been proposed and refined over the years, with most assuming a dichotomy between memory processes involving non-conscious and conscious mechanisms. Despite our incomplete understanding of the underlying mechanisms, tests of memory and learning count among the most performed behavioral experiments. Here, we will discuss available protocols for testing learning and memory using the example of the most prevalent animal species in research, the laboratory mouse. A wide range of protocols has been developed in mice to test, e.g., object recognition, spatial learning, procedural memory, sequential problem solving, operant- and fear conditioning, and social recognition. Those assays are carried out with individual subjects in apparatuses such as arenas and mazes, which allow for a high degree of standardization across laboratories and straightforward data interpretation but are not without caveats and limitations. In animal research, there is growing concern about the translatability of study results and animal welfare, leading to novel approaches beyond established protocols. Here, we present some of the more recent developments and more advanced concepts in learning and memory testing, such as multi-step sequential lockboxes, assays involving groups of animals, as well as home cage-based assays supported by automated tracking solutions; and weight their potential and limitations against those of established paradigms. Shifting the focus of learning tests from the classical experimental chamber to settings which are more natural for rodents comes with a new set of challenges for behavioral researchers, but also offers the opportunity to understand memory formation and retrieval in a more conclusive way than has been attainable with conventional test protocols. We predict and embrace an increase in studies relying on methods involving a higher degree of automatization, more naturalistic- and home cage-based experimental setting as well as more integrated learning tasks in the future. We are confident these trends are suited to alleviate the burden on animal subjects and improve study designs in memory research.

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.

Early peripheral activity alters nascent subplate circuits in the auditory cortex

Cortical function can be shaped by sensory experience during a critical period. The onset of the critical period is thought to coincide with the onset of thalamocortical transmission to the thalamo-recipient layer 4 (L4). In early development, subplate neurons (SPNs), and not L4 neurons, are the first targets of thalamic afferents. SPNs are transiently involved in early development and are largely eliminated during development. Activation of L4 by thalamic afferents coincides with the opening of ear canal (~P11 in mice) and precedes the later critical period. Here, we show in mice that abolishing peripheral function or presenting sound stimuli even before P11 leads to bidirectionally altered functional connectivity of SPNs in auditory cortex. Thus, early sensory experience can sculpt subplate circuits before thalamocortical circuits to L4 are mature. Our results show that peripheral activity shapes cortical circuits in a sequential manner and from earlier ages than has been appreciated.