科研之友
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摘要
Access to strongly preferred resources improves animals' welfare. Ethologists often assess strengths of preference by making resource-access costly, and observing animals' responses as costs increase (e.g. identifying the cost reached before animals cease to pay). Such costs typically comprise operant tasks (wherein the number of responses required is progressively increased), or aversive barriers such as variably weighted doors. However, operant training can be time-consuming, while pushing weighted doors can reflect non-motivational confounds like strength (and be prone to ceiling effects beyond which animals simply cannot move the weight). Here, for mice, we therefore validate a new technique for imposing costs that avoids these issues: crossing a mildly electrified grid. We used enriched cages to motivate 108 trio-housed C57BL/6, BALB/c and DBA/2 females to cross grids set at steadily increasing currents (0-0.6 mA, increased by 0.02mA-0.04 mA every 48 h). Both starting cages and enriched cages contained food, water, bedding and nesting material, so that mice never had to experience the current unless they chose to, and 98/108 mice traversed the grid from the outset. The hypothesis that stronger currents represent greater obstacles made several testable predictions that were all supported by our data. First, mice steadily ceased crossing as currents increased (only 16/98 still crossing at 0.6 mA, the maximum imposed). Second, videos of 13 cages showed that if mice did cross, they did so with longer latencies at higher currents (F-1,F-329 = 61.7, p < 0.0001). Third, data from six focal DBAs in this subset revealed fewer crossings per 48 h as currents increased (F-1,F-47 = 16.0, p = 0.0001), accompanied by correspondingly longer visits to the enriched cage (F-1,F-87 = 143.3, p < 0.0001). Next, such effects were stable within individuals (e.g. the last and penultimate latencies to cross co-varied: F-1,F-329 = 61.7, p < 0.0001), and different measures of motivation inter-correlated in a consistent manner (thus as crossings declined, median visit durations increased [F-1,F-89 = 66.9, p < 0.0001] and latencies to cross tended to increase [F-1,F-89 = 1.78, p = 0.095]; while median visit durations and latencies to cross also positively covaried [F-1,F-76 = 3.2, p = 0.04]). Finally, the Maximum Prices Paid (MPPs) also differentiated between strains as expected from differences in nociception (BALE/cs and DBA/2s being more sensitive to shock than C57BL/6s; respective median MPPs of 0.1, 0.12, and 0.44 mA). Thus mice discriminate between varying intensities of electric current in a floor-grid, treating increasing currents as more aversive in an internally-consistent, graded manner. Mildly electrified grids can therefore be valid, useful tools for imposing access costs and thence measuring strengths of preference in laboratory mice.
- 出版日期2018-1
