Specific behaviours of rodents

Specific behaviours, such as burrowing and nest building, are part of the animals’

natural behaviour. They are not, even though highly motivated (VAN DE WEERD et al. 1998; SHERWIN et al. 2004), essential to the immediate survival and therefore deemed “luxury” behaviours (JIRKOF 2014). They will only be performed if the animals’ essential needs are met (HOHLBAUM et al. 2018) and can in view of these

conditions be useful in assessing not only pain states but possibly also a generally reduced well-being (JIRKOF et al. 2013a; JIRKOF et al. 2013b).

Burrowing and nest-building activity have been increasingly investigated in recent years (DEACON 2006; VAN LOO et al. 2007; DEACON 2012; GASKILL et al. 2013;

JIRKOF et al. 2013a; JIRKOF et al. 2013b). Both tests have the distinct advantage of being home-cage-based tests.

Burrowing is performed by most strains of laboratory mice, even though, strain-related differences have been detected (CONTET et al. 2001). Burrowing activity is evaluated by measuring the amount of substrate removed from a provided burrow (DEACON 2012); additionally, the latency to start burrowing can be analysed (JIRKOF et al. 2010). Burrowing has been successfully applied in studies investigating post-laparotomy pain (JIRKOF et al. 2010), the well-being in a DSS colitis model (JIRKOF et al. 2013b) and after repeated anaesthesia (HOHLBAUM et al. 2017).

Nesting activity is a behaviour inherent in both male and female mice, being not solely motivated by maternal instincts but serving as a protection from environmental influences such as temperature and predators (LATHAM and MASON 2004;

DEACON 2006). Nesting performance is evaluated by scoring nest shape, height and amount of untouched nesting material (DEACON 2006). The nesting test has been successfully implemented in studies for assessment of model-dependent severity, having been sensitive to surgery as well as anaesthesia without surgery (GASKILL et al. 2013; JIRKOF et al. 2013a). The time-to-integrate-to-nest test (TINT) also relies on nest-building activity but evaluates if a piece of nesting material will be included into the existing nest within 10 minutes after provision (ROCK et al. 2014).

The TINT has been used to evaluate pain after carotid injury surgery (ROCK et al.

2014) and in a DSS colitis model (HÄGER et al. 2015).

Voluntary wheel running (VWR)

Another specific behaviour of many rodents is voluntary wheel running (VWR) (NOVAK et al. 2012). Wheel running is a controversially discussed phenomenon, occurring both in laboratory and in wild mice (MEIJER and ROBBERS 2014) and

many other animals such as rats and hamsters, domestic cats, bobcat, red fox and pig-tail macaques (KAVANAU 1971). Its motivational basis is not yet fully understood but has been clearly distinguished from general locomotor activity (SHERWIN 1998b;

KOTEJA et al. 1999; DE VISSER et al. 2006). Over the years of research, VWR has been suggested to reflect searching or exploratory behaviour (MATHER 1981;

SHERWIN 1998b) or to be a tool for energy regulation (COLLIER and HIRSCH 1971). It has furthermore been described as a stereotypy (MASON 1991a; RICHTER et al. 2014; MASON and WURBEL 2016), because it seems to fulfil several criteria of stereotypies such as being invariant, goal- and functionless as well as repetitive. This assumption, however, has been contradicted by studies showing VWR is performed in enriched conditions (ROPER and POLIOUDAKIS 1977) or in specially shaped wheels demanding high concentration (KAVANAU 1967). Besides being stereotypic, the voluntary use of a wheel has been proposed to be addictive (BRENÉ et al. 2007), leading to similar behavioural changes as addictive drugs: Werme et al. (1999) found that drug-preferring strains of rats also showed increased running activity, and Brené et al. (2007) also suggested a correlation of running to drug preference in mice. In another study in rats, the intensity of wheel usage remained at a stable level when restricted to one hour access, while it significantly increased when access was granted for six hours (LATTANZIO and EIKELBOOM 2003). This pattern is very similar to the intake pattern of addictive drugs under similarly restricted access (AHMED and KOOB 1998). These studies, therefore, indicate a possibly common mechanism of drug addiction and extensive wheel running. The rewarding effect of VWR, regardless of its nature, has been shown by induction of conditioned place preference (CPP) (LETT et al. 2000; LETT et al. 2001; BELKE and WAGNER 2005).

In line with these findings, VWR has been addressed as ‘self-reinforcing behaviour’

(SHERWIN 1998b).

The effects of VWR on other animal behaviour have also been intensively studied.

Concerning potentially anxiolytic properties, literature is controversial: while several studies point towards reduced anxiety-like behaviour (BINDER et al. 2004; DUMAN et al. 2008), a recent study by Fuss (2010) indicated increased anxiety.

Wheel running in rodents, despite the possibly confounding factors detailed above, has been widely used in a vast variety of studies. One major field is the investigation of exercise-related effects, for example on the regulation of energy homeostasis (PATTERSON and LEVIN 2008), tumour onset and tumour growth (ZHENG et al.

2008), depression (GREENWOOD et al. 2003), stress (ADLARD and COTMAN 2004), neuropathic pain (GRACE et al. 2016) and colitis (LIU et al. 2015). The marked effect on neurogenesis, while varying across strains (CLARK et al. 2011), is also of great interest, and in line with this, a marked increase of brain-derived neurotrophic factor (BDNF) by VWR has repeatedly been demonstrated (ADLARD and COTMAN 2004).

Because rodents show a very steady circadian rhythm of wheel running even in total darkness (PITTENDRIGH and DAAN 1976), another field utilizing VWR is the research of various influences on circadian rhythmicity (DECOURSEY 1960; CHO et al. 2012). A third research area is the severity assessment. Several studies have shown a distinct change in VWR behaviour in response to pain, for example following intraperitoneal (IP) transmitter implantation (HELWIG et al. 2012), after onset of inflammatory pain (COBOS et al. 2012; KANDASAMY et al. 2016) or migraine (KANDASAMY et al. 2017). Additionally, VWR has been able to reveal the impact of non-painful experiences such as an unpredictable chronic mild stress protocol (DEVALLANCE et al. 2017).

Since the amount of running can differently affect the study outcome, the genetic background of the animals has to be considered. By comparing different strains of mice, Festing (1977) and Clark et al. (2011) were able to demonstrate that genetic differences play a great role in VWR performance. Overall, wheel running in rodents is a very simple-to-obtain parameter and, while its complex and underlying mechanism are not yet fully understood, it offers great opportunities for many fields of research, including severity assessment.