The sympathetic nervous system is controlled by transient receptor potential vanilloid 1 in the regulation of body temperature

Abstract

Transient receptor potential vanilloid 1 (TRPV1) is involved in sensory nerve nociceptive signaling. Recently, it has been discovered that TRPV1 receptors also regulate basal body temperature in multiple species from mice to humans. In the present study, we investigated whether TRPV1 modulates basal sympathetic nervous system (SNS) activity. C57BL6/J wild-type (WT) mice and TRPV1 knockout (KO) mice were implanted with radiotelemetry probes for measurement of core body temperature. AMG9810 (50 mg/kg) or vehicle (2% DMSO/5% Tween 80/10 ml/kg saline) was injected intraperitoneally. Adrenoceptor antagonists or vehicle (5 ml/kg saline) was injected subcutaneously. In WT mice, the TRPV1 antagonist, AMG9810, caused significant hyperthermia, associated with increased noradrenaline concentrations in brown adipose tissue. The hyperthermia was significantly attenuated by the β-adrenoceptor antagonist propranolol, the mixed α-/β-adrenoceptor antagonist labetalol, and the α1-adrenoceptor antagonist prazosin. TRPV1 KO mice have a normal basal body temperature, indicative of developmental compensation. d-Amphetamine (potent sympathomimetic) caused hyperthermia in WT mice, which was reduced in TRPV1 KO mice, suggesting a decreased sympathetic drive in KOs. This study provides new evidence that TRPV1 controls thermoregulation upstream of the SNS, providing a potential therapeutic target for sympathetic hyperactivity thermoregulatory disorders.

Keywords: TRPV1; brown adipose tissue; thermogenesis.

Figure 1.

AMG9810-induced hyperthermia is dependent on spontaneous activity. A) Core body temperature (top) and activity (bottom) recordings over 360 min following AMG9810 (50 mg/kg, i.p.) or vehicle (2% DMSO, 5% Tween 80 in saline) in TRPV1 WT mice. B) Temperature (top panel) and activity (bottom panel) profile of AMG9810 in TRPV1 KO mice (n = 7–8 per group). Arrows in (A) and (B) denote treatments. C) A 3 h average activity posttreatment (n = 7–8 per group). D) Administration of AMG9810 or vehicle to anesthetized WT mice (n = 5–6 per group). Results are means ± sem. *P < 0.05 and ***P < 0.001 vs. vehicle control using 2-way ANOVA and Bonferroni’s.

Figure 2.

Involvement of the SNS in TRPV1 antagonist-induced hyperthermia. NA concentrations measured in brain (A), BAT (B), and skin (C) samples of WT mice 1 h posttreatment with AMG9810 (50 mg/kg, i.p.) or vehicle (2% DMSO, 5% Tween 80 in saline) (n = 5). D) Immunoblot of UCP1 and loading control, VDAC, in BAT mitochondrial-extracted protein of AMG9810- or vehicle (Veh)-treated mice (1 h). E) Densitometric analysis of the relative expression of UCP1/VDAC (n = 3–4). F) Immunoblot of UCP3 and loading control, VDAC, in skeletal muscle (GM) mitochondrial-extracted protein of AMG9810- or vehicle-treated mice (1 h). G) Densitometric analysis of the relative expression of UCP3/VDAC (n = 4). Results are means ± sem. *P < 0.05 and **P < 0.01 vs. vehicle control using 2-tailed Student’s t test.

Figure 3.

Contribution of adrenoceptors to TRPV1 antagonist-induced hyperthermia. A) Core body temperature recordings over 300 min following pretreatment with propranolol (5 mg/kg, s.c.; n = 7) or control (saline, 5 ml/kg, s.c.; n = 13). B) The hyperthermic index was analyzed as area under the temperature curve during the 0 to 180 min period for saline and propranolol-pretreated mice. C) Core body temperature recordings over 300 min following pretreatment with labetalol (30 mg/kg, s.c.; n = 8) or control (saline, 5 ml/kg, s.c.; n = 13). D) Respective hyperthermic index calculated for (C). E) Core body temperature recordings over 300 min following pretreatment with prazosin (0.1 mg/kg, s.c.; n = 7) or control (saline, 5 ml/kg, s.c.; n = 13) followed by AMG9810 (50 mg/kg, i.p.) or vehicle (2% DMSO, 5% Tween 80 in saline) in WT mice. Arrows in (A), (C), and (E) denote treatments. F) The respective hyperthermic index calculated for (E). Results are means ± sem. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. vehicle control; ^#P < 0.05, ^##P < 0.01, and ^###P < 0.001 vs. AMG9810-treated mice using 2-way ANOVA and Bonferroni’s post hoc test.

Figure 4.

Investigating a role for β-adrenoceptors in TRPV1 antagonist-induced hyperthermia. A) Core body temperature recordings over 300 min following pretreatment with metoprolol (20 mg/kg, s.c.; n = 6) or control (saline, 5 ml/kg, s.c.; n = 13). B) The hyperthermic index was analyzed as area under the temperature curve during the 0 to 180 min period for saline and metoprolol-pretreated mice. C) Core body temperature recordings over 300 min following pretreatment with ICI-118,551 (10 mg/kg, s.c.; n = 6) or control (saline, 5 ml/kg, s.c.; n = 13). D) Respective hyperthermic index calculated for (C). E) Core body temperature recordings over 300 min following pretreatment with SR59230A (2.5 mg/kg, s.c.; n = 3) or control (2% DMSO in saline, 5 ml/kg, s.c.; n = 4). Arrows in (A), (C), and (E) denote treatments. Results are means ± sem. **P < 0.01, and ***P < 0.001 vs. vehicle control; ^##P < 0.01 vs. AMG9810-treated mice using 2-way ANOVA and Bonferroni’s post hoc test.

Figure 5.

Core body temperature and activity profile of TRPV1 KO mice. Normal circadian rhythm of temperature (A) and activity (B) of TRPV1 WT mice (closed circles) and TRPV1 KO mice (open squares) over 24 h using radio-telemetry (n = 9). Shaded areas in (A) and (B) denote the dark phase. Average 12-h light-phase activity levels (C) and average 12-h dark-phase activity levels (D), measured in TRPV1 WT and KO mice (n = 9). Results are means ± sem. **P < 0.01 vs. WT using 2-tailed Student’s t test; NS, not significant.

Figure 6.

Amphetamine-induced hyperthermia is attenuated in TRPV1 KO mice. A) Core body temperature recording following d-amphetamine (10 mg/kg, s.c.; n = 7) or vehicle (saline, 5 ml/kg, s.c.; n = 5) administration to TRPV1 WT and KO mice. B) The hyperthermic index was analyzed as area under the temperature curve during the 120 to 290 min period for vehicle and d-amphetamine-treated mice. C) Activity levels following d-amphetamine (10 mg/kg, s.c.; n = 7) or vehicle (saline, 5 ml/kg, s.c.; n = 5) administration to TRPV1 WT and KO mice. D) Respective activity index analyzed as area under the activity curve during the 120 to 290 min period. Results are means ± sem. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. control; ^###P < 0.001 vs. genotype using 2-tailed Student’s t test and 2-way ANOVA and Bonferroni’s post hoc test.

Figure 7.

Suppressed sympathetic activity in TRPV1 KO mice. NA concentrations measured in brain (A), BAT (B), and skin (C) samples of TRPV1 WT and KO mice (n = 7–8). D) Immunoblot of UCP1 and loading control VDAC in BAT mitochondrial protein of TRPV1 WT and KO mice. E) Densitometric analysis of the relative expression of UCP1/VDAC (n = 5–6). F) Immunoblot of UCP3 and loading control VDAC in skeletal muscle mitochondrial protein of TRPV1 WT and KO mice. G) Densitometric analysis of the relative expression of UCP3/VDAC (n = 5–6). Results are means ± sem. *P < 0.05 vs. WT using 2-tailed Student’s t test.

Figure 8.

TRPV1 antagonist-induced hyperthermia involves BAT-mediated thermogenesis. Proposed schematic illustrates acute BAT activation following TRPV1 antagonist-induced hyperthermia. Sensory nerves and sympathetic nerves innervate BAT, suggested to mediate feedback loops serving a proposed pathway for crosstalk. Brown adipocytes have been shown to express TRP channels, including TRPV1 (58). AMG9810 transiently disrupts this feedback loop (1), causing hyperthermia mediated via an increase in sympathetic activity and NA release (2). NA stimulates β-adrenoceptors and initiates downstream signaling resulting in heat generation. SN-SS, sensory nerve-sympathetic system; TG, triglyceride; FFA, free fatty acid.