Assessment of incising ethology in the absence and presence of jaw muscle hyperalgesia in a mouse home cage environment

Abstract

Introduction Assessment of oral motor behavior in a mouse is challenging due to the lack of currently available techniques that are non-invasive and allow long-term assessment in a home cage environment. The purpose of this study was to evaluate incising behavior using mouse chow attached to a three-dimensional force transducer that was mounted on the existing home cage. In addition, a persistent hyperalgesia condition was introduced to evaluate the sensitivity of the technique to identify incising behavioral changes. Methods Incising activity of CD-1 male and female mice (n=48) was evaluated over a 24 hour recording session during four baseline and six longitudinal hyperalgesia assessment sessions using custom written software. A pre-clinical persistent pain model was used to induce hyperalgesia in the masseter muscle by repetitive acidic saline injections. Sex and age differences were evaluated for multiple incising variables during both light and dark cycles during baseline and hyperalgesia conditions. Results Significant sex differences were found for multiple incising variables but not for age. Discrete incising frequencies were identified in the range of 4.6-10.4 Hz and were reproducibly found in both female and male mice. A significant shift to lower incising frequencies was observed after repetitive acidic saline injections compared to neutral saline injections. This shift to lower frequencies of incising returned to baseline levels after approximately four weeks but was statistically longer in female compared to male mice. Significant differences were also found for chow intake (reduced) and weight change during the hyperalgesia condition. No significant differences were found for total number of incisions or number of incising episodes per day or incising force. Conclusions The findings from this study support the use of recording three dimensional incising forces as a sensitive measure of incising behavior. This novel technique allowed the identification of specific incising variables that were differentially affected in female and male mice during a persistent hyperalgesia. The data were collected in the home cage environment with minimal bias such as experimenter interaction. Similar to other dental pain studies, mice were able to maintain normal incising activity levels per day (total incisions, total number of incising episodes) even in the presence of hyperalgesia.

Mouse preferential incising force orientation changes during jaw closing muscle hyperalgesia and is sex dependent

“…Animals that were used in this study were the same ones as described in a previous report [10]. A total of 34 CD-1 mice (Charles Rivers) consisting of 17 males and 17 females (age 3–9 months) were housed in the same animal room with four male or female mice residing in the same home cage.…”

“…The data acquisition procedures for this study have already been described in a previous publication [10]. Briefly, audio, video and incising force recordings were simultaneously acquired from four cages during a 24 hr period.…”

“…As previously described [10], mild to moderate masseter muscle hyperalgesia was elicited in 13 males and 13 females by repetitive injections (20µl, separated by five days) of acidic saline (pH = 4.0) into the center of the muscle on the left side. The pain that was elicited was a persistent pain that lasted at least 3–4 weeks depending on the sex of the mouse.…”

“…The breakdown of different types and hardness of foods is important to maintain the necessary food intake for survival. Assessing relative contributions from non-noxious or noxious sensory inputs on masticatory motor control has been a topic of research for many years [2–10]. However, little is known regarding the effect of noxious inputs such as muscle or joint pain on freely motivated behavior because the effects of pain on masticatory behavior have been difficult to evaluate in a non-invasive approach and over an extended period of time.…”

“…Recently, we described a technique to non-invasively record incising forces without experimenter intervention and for long periods of time (24 hrs) in the home cage environment [10]. In an experimental jaw closing muscle pain condition, female mice were found to respond more robustly to repetitive acidic saline injections into the masseter muscle than male mice with a longer duration of the response and a shift to lower incising frequencies.…”