Elsevier

Appetite

Volume 54, Issue 2, April 2010, Pages 320-330
Appetite

Research report
An analysis of licking microstructure in three strains of mice

https://doi.org/10.1016/j.appet.2009.12.007Get rights and content

Abstract

Mouse models of feeding provide a useful tool for elucidating the molecular pathways of energy regulation. The majority of studies in mice have been limited to intake analyses conducted over extended periods of time, which fail to distinguish between a variety of factors that influence nutrient intake. Using licking microstructure analyses we examined both the size and number of licking bursts for water, polycose, sucrose and lecithin in three strains of mice (C57BL/6J, 129Sv/ImJ and C57129F1 hybrids), using pause criteria (250–500, >500 and >1000 ms) that have previously been described in the rat. Burst size and number varied both as a function of tastant concentration and mouse strain; however, these differences were most evident with the >1000 ms pause criterion. Consistent with previous reports, during water consumption C57 mice showed longer mean interlick intervals, a larger number of bursts but reduced burst size relative to the two other strains. F1 mice showed larger burst sizes for polycose, while C57 mice displayed a greater number of bursts for both polycose and sucrose. Both 129 and F1 mice were insensitive to sucrose concentration, whereas C57 mice showed attenuated lecithin intake influenced by a reduction in the size of bursts for this tastant. These results suggest that these strains of mice display differences in the pattern of licking that are most evident with the use of larger pause criteria. These differences in licking behavior might reflect influences of genetic background on pre- and post-ingestive factors controlling intake, the reinforcing properties of each tastant, or native differences in licking style.

Introduction

Animal models provide an essential contribution for understanding the basic parameters that regulate components of energy balance. In mice, a variety of genetic models of obesity have proven useful for elucidating the molecular basis of energy regulation, with the hope that such studies will aid in the identification of new drug targets and novel therapeutic strategies (Robinson, Dinulescu, & Cone, 2000). However, the majority of studies concerning energy regulation and food intake in mice have been limited to intake analyses over longer time frames, which fail to distinguish between a variety of variables that are known to influence intake (Davis and Smith, 1992, Sclafani et al., 1993). Here we describe a detailed analysis of licking microstructure, which provides an advantageous strategy by which to gauge the possible contributions of tastant palatability and post-ingestive inhibitory feedback on consumption (Smith, 2001).

Licking in rodents is a highly stereotyped behavior that involves the rhythmic cycling of tongue extensions and retractions thought to be under the control of a group of neurons in medulla oblongata nuclei V, VII and XII, which collectively function as a central pattern generator (Nakamura and Katakura, 1995, Norgren, 1995, Travers and Norgren, 1991). In microstructure analyses the rate of licking is defined by the interlick intervals (ILIs), where the majority of ILIs fall <250 ms and reflect continuous licking bursts (Davis & Smith, 1992). Longer pauses between bursts of licking appear to be relevant for dissecting the various components of meal intake. Davis (1996), Davis and Perez (1993) and Davis and Smith (1992) used two criteria to divide pauses. The pauses between 250 and 500 ms were thought to reflect brief interruptions of licking, such as lateral tongue movements (Grill & Norgren, 1978), whereas pauses >500 ms indicated longer interruptions of licking due to the active engagement of other competing behavior, such as grooming or leaving the food area. The number of licks occurring before the pause intervals defined the size of the licking bursts. This measure was unaffected by sham-feeding preparations and increased as a function of sucrose concentration, indicative of pre-ingestive influences (Davis & Smith, 1992). By contrast, increases in both burst and pause number were seen at 250–500 and >500 ms with sham-feeding preparations. Under normal feeding conditions, the number of bursts in a meal displayed an inverted U-shaped function of concentration with sucrose, reflecting post-ingestive negative feedback (Davis and Smith, 1992, Smith, 2001). Spector, Klumpp, and Kaplan (1998) conducted a systematic assessment to examine pause criteria for the study of licking behaviors in rats, deciding upon a >1000 ms pause criterion, whose burst size and number were associated with pre- and post-ingestive factors, respectively. These results suggest that during the initial stages of sucrose intake, rats rapidly reinstate licking behavior following termination of a licking burst. This pattern leads to increases in both the number of pauses and bursts in the meal. Typically, as the meal progresses the frequency of these events decreases as influences of post-ingestive inhibitory feedback develop.

In the current experiments we evaluated strains of mice that are commonly used in genetic research: C57BL/6J (C57), 129Sv/ImJ (129), and C57129F1 (F1), a hybrid of the previous two. In particular, we were interested in assessing the pattern of licking in these different strains of mice with respect to microstructural variables of burst size and number using pause criteria that had previously been described in the rat (Davis and Smith, 1992, Spector et al., 1998). Previous studies suggest mouse strain differences in lick rates and its microstructure (Boughter et al., 2007, Glendinning et al., 2008, Horowitz et al., 1977). Horowitz et al. (1977) examined licking for water in C57, DBA, and an F1-hybrid strain. C57 mice exhibited the slowest lick rate, F1 an intermediate, and DBA the highest lick rate. More recent examinations with water confirmed slower lick rates and fewer bursts in C57 mice relative to D2, 129 and SWR strains (Boughter et al., 2007, Dotson and Spector, 2005, Glendinning et al., 2005).

Strain differences in licking have also been reported with nutritive tastants. C57 mice showed elevated initial lick rates for sucrose (Glendinning et al., 2008) and increased sucrose and polycose intake (Sclafani, 2006) relative to 129 mice (Glendinning et al., 2005, Glendinning et al., 2008, Sclafani, 2006). While the taste receptor that mediates glucose polymer taste is unknown, strain differences at low concentrations of sucrose are thought to reflect allelic variations to the T1R3 receptor (Glendinning et al., 2005, Inoue et al., 2007). These results suggest that variations in background genetic strain contribute to factors that influence meal intake. Moreover, targeted gene manipulations of specific neuropeptides (Fintini et al., 2005, Lakaye et al., 2004) have been shown to influence energy regulation and food intake; however, to date only a few studies have used licking microstructure to assess the role of neuropeptides on feeding behavior (e.g., Baird et al., 2006).

Here we examined lick patterns in C57, F1 and 129 mice during consumption of water and three tastants that differ in their nutritive and caloric properties. In addition to the total intake of each tastant, we examined the size and number of licking bursts. As part of our analysis of licking bursts, we compared three pause criteria (250–500, >500 and >1000 ms) used to define such bursts. In Experiment 1, we examined water consumption to provide an initial assessment of native licking style to a non-nutritive solution in these mice. Next, we assessed the patterns of licking of two commonly studied tastants, the complex polysaccharide polycose and the disaccharide sucrose (Experiments 2 and 3, respectively). Finally, we examined licking microstructure for lecithin (Experiment 4), a principal phospholipid that is used extensively in the food industry as a food additive but has yet to be assessed in animal models.

Section snippets

Subjects

A total of 66 male mice were used from three strains: C57BL/6J (C57), 129Sv/ImJ (129), and C57129F1 hybrids (F1), obtained from Jackson Laboratory (Bar Harbor, ME). In Experiment 1, eight F1 and 129, and six C57 mice were used, which prior to behavioral testing weighed 33.2 ± 1.5, 31 ± 1.3 and 32.5 ± 1.8 g, respectively. The same mice were also used in Experiment 2. In Experiment 3, seven C57 (23.7 ± 3.5 g) and F1 mice (26.6 ± 0.8 g), and six 129 mice (23.7 ± 1.1 g) from each strain were used. Finally, in

Total consumption and ILI distributions for C57, F1 and 129 mice

When allowed free access to water, all mice showed comparable intake as evidenced by total licks during the consumption session (Fig. 1; right panel). ANOVA revealed no strain differences (F(2,19) = 0.23, p = 0.79). The interlick intervals (ILIs) for continuous licking of water were also quantified to compare previously established licking topographies (Boughter et al., 2007, Dotson and Spector, 2005) with those seen using the current apparatus. The ILI was defined as the time between the onset of

Discussion

The primary purpose of the current series of experiments was to examine burst size and number in three strains of mice, using a variety of tastants that differed in their caloric and nutritive properties. We also compared three pause criteria previously used to define licking bursts in rats (Davis, 1996, Davis and Perez, 1993, Davis and Smith, 1992, Spector et al., 1998).

Our results with water replicated previous findings (Boughter et al., 2007, Dotson and Spector, 2005), with C57 mice

Acknowledgements

Supported by NCRR grant P40-RR-017688 and NIMH grant R01-MH-60179.

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