Role of thermosensitive transient receptor potential (TRP) channels in thermal preference of male and female mice

Abstract

Transient Receptor Potential (TRP) ion channels are important for sensing environmental temperature. In rodents, TRPV4 senses warmth (25-34 °C), TRPV1 senses heat (>42 °C), TRPA1 putatively senses cold (<17 °C), and TRPM8 senses cool-cold (18-26 °C). We investigated if knockout (KO) mice lacking these TRP channels exhibited changes in thermal preference. Thermal preference was tested using a dual hot-cold plate with one thermoelectric surface set at 30 °C and the adjacent surface at a temperature of 15-45 °C in 5 °C increments. Blinded observers counted the number of times mice crossed through an opening between plates and the percentage of time spent on the 30 °C plate. In a separate experiment, observers blinded as to genotype also assessed the temperature at the location on a thermal gradient (1.83 m, 4-50 °C) occupied by the mouse at 5- or 10-min intervals over 2 h. Male and female wildtype mice preferred 30 °C and significantly avoided colder (15-20 °C) and hotter (40-45 °C) temperatures. Male TRPV1KOs and TRPA1KOs, and TRPV4KOs of both sexes, were similar, while female WTs, TRPV1KOs, TRPA1KOs and TRPM8KOs did not show significant thermal preferences across the temperature range. Male and female TRPM8KOs did not significantly avoid the coldest temperatures. Male mice (except for TRPM8KOs) exhibited significantly fewer plate crossings at hot and cold temperatures and more crossings at thermoneutral temperatures, while females exhibited a similar but non-significant trend. Occupancy temperatures along the thermal gradient exhibited a broad distribution that shrank somewhat over time. Mean occupancy temperatures (recorded at 90-120 min) were significantly higher for females (30-34 °C) compared to males (26-27 °C) of all genotypes, except for TRPA1KOs which exhibited no sex difference. The results indicate (1) sex differences with females (except TRPA1KOs) preferring warmer temperatures, (2) reduced thermosensitivity in female TRPV1KOs, and (3) reduced sensitivity to cold and innocuous warmth in male and female TRPM8KOs consistent with previous studies.

Keywords: Cold avoidance; TRP channel knockout1; Thermal gradient; Thermal preference; Warm avoidance.

Fig. 1.

Thermal preference comparisons by genotype and temperature differential. Each graph plots the percent time spent on the 30 °C plate for each temperature difference. A: wildtype (WT) males. B: WT females. C: TRPV1KO males. D: TRPV1KO females. E: TRPA1KO males. F: TRPV1KO females. G: TRPV4KO males. H: TRPV4KO females. I: TRPM8KO males. J: TRPM8KO females. *: temperature difference statistically significant (p < 0.05, Kruskal-Wallis ANOVA with Dunns post-hoc test). #: significantly different from WT of same sex at same temperature difference (p < 0.05, Kruskal-Wallis ANOVA with Dunns post-hoc test for differences vs. WT at the given temperature differential). Numbers of mice: WT males 10, females 6; TRPV1KO males: 6, females 5; TRPA1KO males 6, females 8; TRPV4KO males 7, females 5; TRPM8KO males 7, females 7. Some of the temperature differentials have fewer data points due to attrition (range: 3–10). Bars plot means ± SEM.

Fig. 2.

Plate crossings by genotype and temperature differential. Each graph plots the number of crossings between thermoelectric plates for each genotype and temperature differential. A: WT males. B: WT females. C: TRPV1KO males. D: TRPV1KO females. E: TRPA1KO males. F: TRPV1KO females. G: TRPV4KO males. H: TRPV4KO females. I: TRPM8KO males. J: TRPM8KO females. *: temperature difference statistically significant (p < 0.05, Kruskal-Wallis ANOVA with Dunns post-hoc test). #: significantly different from WT of same sex at same temperature difference (p < 0.05). Numbers of mice: WT males 10, females 6; TRPV1KO males: 6, females 5; TRPA1KO males 6, females 8; TRPV4KO males 7, females 5; TRPM8KO males 7, females 7. Some of the temperature differentials have fewer data points due to attrition (range: 3–10). Bars plot means ± SEM.

Fig. 3.

Graph plots number of crossings vs. time with both plates set at 30 °C for WT mice. A: males. ●: individual data; ●: means. Thick dashed line shows linear fit of means. B: females. ▴: individual data; ●: means. Thick dashed line shows linear fit of means. Error bars omitted for clarity.

Fig. 4.

Distribution of occupancy temperatures. Mice were placed on a linear thermal gradient and the temperature at the site of occupancy of each mouse was measured every 5 min (or 10 min for TRPM8KOs). Each graph plots the number of occupancy temperatures sampled for each mouse every 5 or 10 min in the time period 90–120 min after placement on the gradient. Each group included 5–10 individuals. A: WTs. B: TRPV1KOs. C: TRPA1KOs. D: TRPV4KOs. E: TRPM8KOs. Numbers of mice: male WT: 10, female WT: 6, male TRPV1KO: 6, female TRPV1KO: 6, male TRPA1KO: 6, female TRPA1KO: 9, male TRPV4KO: 6, female TRPV4KO: 5, male TRPM8KO: 7, female TRPM8KO: 7.

Fig. 5.

Mean occupancy temperatures on the thermal gradient for males and females of various genotypes, measured 90–120 min after being placed on the gradient. A: WTs. B: TRPV1KOs. C: TRPA1KOs. D: TRPV4KOs. E: TRPM8KOs. *: p < 0.05, unpaired t-test. Error bars: SEM. Numbers of mice: male WT: 10, female WT: 6, male TRPV1KO: 6, female TRPV1KO: 6, male TRPA1KO: 6, female TRPA1KO: 9, male TRPV4: 6, female TRPV4KO: 5, male TRPM8KO: 7, female TRPM8: 7.