Abnormal trigeminal sensory processing in obese mice

Abstract

Obesity is associated with several pain disorders including headache. The effects of obesity on the trigeminal nociceptive system, which mediates headache, remain unknown. We used 2 complementary mouse models of obesity (high-fat diet and leptin deficiency) to examine this. We assessed capsaicin-induced nocifensive behavior and photophobia in obese and control mice. Calcium imaging was used to determine the effects of obesity on the activity of primary trigeminal afferents in vitro. We found that obese mice had a normal acute response to a facial injection of capsaicin, but they developed photophobic behavior at doses that had no effect on control mice. We observed higher calcium influx in cultured trigeminal ganglia neurons from obese mice and a higher percentage of medium to large diameter capsaicin-responsive cells. These findings demonstrate that obesity results in functional changes in the trigeminal system that may contribute to abnormal sensory processing. Our findings provide the foundation for in-depth studies to improve the understanding of the effects of obesity on the trigeminal system and may have implications for the pathophysiology of headache disorders.

Fig. 1

Experimental groups to determine the relative contribution of obesity and high fat diet to photophobia.

Fig. 2

Effects of diet-induced obesity on nocifensive behaviors in C57BL/6J mice (10 min test). Intradermal capsaicin in the whisker pad (A) Increases ipsilateral facial wipes and (B) Decreases rearing events in both diet groups equally. *p < 0.05 **p < 0.0001 versus vehicle. (vehicle n = 15–17/group, capsaicin n = 10–12/group)

Fig. 3

Effects of diet-induced obesity on basal photophobic behavior and locomotor parameters in C57BL/6J mice. (A) More obese HFD mice (n = 80) than RD mice (n = 74) are photophobic at baseline (i.e. they spent less than 50% time in light). (B–D) Locomotor parameters (B, percentage of time resting, C, percentage of time moving, and D, cm/s) exhibited by mice for both zones at baseline. There were no post-hoc differences between the diets in percentage of time resting or percentage of time moving in either zone. Obese HFD mice traveled less distance relative to the time they spent in each zone than RD mice. *p < 0.05 versus control mice.

Fig. 4

Effects of trigeminal stimulation with capsaicin on photophobic behavior and locomotor activity in obese HFD mice. (A) Percentage of time in light decreases after facial injection of 0.01% capsaicin in obese HFD (n = 44) but not RD (n = 38) mice. (B–D) Locomotor parameters: B, percentage of time resting increases after 0.01% capsaicin in obese HFD mice in the dark compartment only; C, percentage of time moving decreases after 0.01% capsaicin in obese HFD mice in the dark compartment only; and D, distance travelled relative to the time spent in each side (cm/s) decreases after 0.01% capsaicin in obese HFD mice in the dark compartment only. (E, F) Correlation of % time in light after capsaicin with body weight in RD and obese HFD mice respectively. (E) For RD mice, there is no significant correlation between body weight and % time in light (R² = 0.01, p = 0.46). (F) For obese HFD mice, there is a significant correlation of body weight with % time in light (R² = 0.25, P = 0.0001). *p < 0.05 versus vehicle for the indicated diet. ⁺p < 0.05 versus control mice.

Fig. 5

Effects of trigeminal stimulation with capsaicin on photophobic behavior and locomotor activity in lean HFD (n = 17) and RD (n = 16) mice. (A) Percentage of time in light is similar in both diet groups for all treatments. (B–D) Locomotor parameters (B, percentage of time resting, C, percentage of time moving, and D, cm/s) are similar in both diet groups for all treatments. *p < 0.05 versus vehicle for the indicated diet.

Fig. 6

Effects of trigeminal stimulation with capsaicin on photophobic behavior and locomotor activity in ob/ob (n = 16) and age-matched control mice (n = 13). (A) ob/ob spend less time in light than controls and display a significant decrease following 0.1% capsaicin injection compared with vehicle. (B–D) Locomotor parameters (B, percentage of time resting, C, percentage of time moving, and D, cm/s) of ob/ob and control mice indicate that ob/ob mice move less in general and that their movement is altered by capsaicin injection. * p < 0.05 versus vehicle for ob/ob mice. ⁺p < 0.05 versus control mice.

Fig. 7

Serum corticosterone levels in obese HFD and RD mice under various testing and treatment conditions. (A) Baseline corticosterone levels are similar in both diet groups and they increase after light avoidance test (n = 12–22). (B) Corticosterone levels are higher in obese HFD mice 18 hours after capsaicin injection but before behavioral testing (n = 5–7). (C) Corticosterone levels after the light avoidance test are higher in obese HFD mice regardless of treatment (n = 5–7). *p < 0.05

Fig. 8

Calcium imaging of primary trigeminal ganglia cultures from obese HFD (n = 6) and RD (n = 6) mice. (A, B) Percentage of TRPV1+ cells from six obese HFD (A) and six RD (B) coverslips and their distribution by cell diameter. (C, D) Histograms of cell diameter size distribution. (C) TRPV1+ neurons from obese HFD mice exhibit rightward shift in cell diameter compared to RD. (D) TRPV1− neurons from both diets exhibit normal distribution of cell diameter. (E) Change in relative calcium level in response to capsaicin is higher in neurons from obese HFD mice. (F) Change in relative calcium levels in response to 60mM KCl is higher in TRPV1+ and – neurons from obese HFD mice. These values are normalized to the initial baseline signal before capsaicin treatment. (G, H) Representative coverslips from obese HFD and RD mice. *p < 0.05 versus control diet.