Abstract
Removal of a small segment of tail at weaning is a common method used to obtain tissue for the isolation of genomic DNA to identify genetically modified mice. When genetically manipulated mice are used for pain research, this practice could result in confounding changes to the animals' responses to noxious stimuli. In this study, we sought to systematically investigate whether tail biopsy representative of that used in standard genotyping methods affects behavioral responses to a battery of tests of nociception. Wild-type littermate C57BL/6J and 129S6 female and male mice received either tail biopsies or control procedural handling at Day 21 after birth and were then tested at 6–9 weeks for mechanical and thermal sensitivity. C57BL/6J mice were also tested in the formalin model of inflammatory pain. In all tests performed (von Frey, Hargreaves, modified Randall Selitto, and formalin), C57BL/6J tail-biopsied animals' behavioral responses were not significantly different from control animals. In 129S6 animals, tail biopsy did not have a significant effect on behavioral responses in either sex to the von Frey and the modified Randall-Selitto tests of mechanical sensitivity. Interestingly, however, both sexes exhibited small but significant differences between tail biopsied and control responses to a radiant heat stimulus. These results indicate that tail biopsy for genotyping purposes has no effect on nocifensive behavioral responses of C57BL/6J mice, and in 129S6 mice, causes only a minor alteration in response to a radiant heat stimulus while other nocifensive behavioral responses are unchanged. The small effect seen is modality- and strain-specific.
Highlights
With the increasing use of genetically modified mice in the study of nociceptive processing, the practice of genotyping has become an integral component of pain research
In the 129S6 strain, a main effect of sex was observed in mechanical sensitivity of the paw (p,0.01) with females again exhibiting lower withdrawal thresholds than males
In the formalin test, there was a main effect of sex in C57BL/6J mice (p,0.0001) with females spending more time exhibiting nocifensive behavior than males
Summary
With the increasing use of genetically modified mice in the study of nociceptive processing, the practice of genotyping has become an integral component of pain research. In order to obtain DNA for genetic analysis, typically, a small piece of tissue is removed from the mouse, as with a tail biopsy, an ear punch, or a toe clip, by 3 weeks after birth. This is a common practice, the resulting impact on behavioral assays of nociception is unknown. Injury to the nervous system during the postnatal sensitive period of development can significantly alter the nociceptive circuitry, thereby inducing long-lasting changes in nociceptive thresholds The polarity of this change is directly dependent on the type of injury and the sensory modality in question, with certain injuries leading to increased sensitivity [1–3,7] and others resulting in decreased sensitivity in adulthood [4–7]. Studies of early-life injury and nociception in adulthood have focused on injuries restricted to the hind paws or viscera, and have not yet examined the consequences of injury to the tail, likely the most common early-life injury experienced by laboratory mice
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