Sex Differences in Response to Cool Temperatures in Mice

Objectives: Males and females have differing sensitivities to cool temperatures. This project evaluated the role of TRPM8-expressing trigeminal-ganglion neurons (TRGNs) in producing this sex difference in mice.
Methods: TRGNs were isolated from Trpm8^tm1Apat/J knockoutXC57BL/6 heterozygote mice. The TRGNs were acutely dissociated and the EGFP expressing neurons were whole-cell voltage-clamped to examine TRPM8, voltage-gated sodium, potassium, and hyperpolarization-activated cyclic nucleotide-gated currents (I_h). The responses of these currents to temperatures ranging from 30-18 degrees was also evaluated. In another set of experiments SKH1 hairless mice (20 male, 20 female) were tested for their responses to facial stimulation by 33, 18 and 10 degree temperatures in Orofacial Pain Assessment Devices (OPAD, Stoelting co.). In a subset of these animals the I_h inhibitor ZD7288 (5mg/kg, ip) was administered prior to testing at 18 degrees.
Results: TRPM8 expressing TRGNs from males were found to have larger menthol (100μM) evoked currents (N=20 male neurons, N=32 female neurons, p=0.033 t-test) and larger voltage-gated sodium currents than females (N=29 male neurons, N=39 female neurons, p=0.0005, 2-Way ANOVA). Voltage-gated potassium currents were similar for the sexes. I_h was found to be larger in female neurons than males (N=13 female, N=11 male, p=0.013, 2-Way ANOVA). Cooling the neurons from 30 to 18 degrees reduced all of the currents. The I_h current in male neurons at 18 degrees (membrane potential -60mV) approached 0pA, whereas the female I_h current exceeded 50pA. In the behavioral experiments it was found that female mice spent less time in contact with 18 degrees than male mice when attempting to obtain a reward solution of diluted sweetened condensed milk (N=10 male, N=10 female, p<0.05, 2-Way ANOVA, Bonferroni’s test). ZD7288 reduced the difference between male and female responses to 18 degrees.

Conclusions: The enhanced sensitivity of females to cool temperatures is mediated by I_h in TRPM8 expressing TRGNs.