Intramuscular insulin administration potentiates sympathetic and pressor responses to capsaicin in rats
Authors: Fukazawa Ayumi , Hori Amane , Hotta Norio , Estrada Juan , Katanosaka Kimiaki , Mizumura Kazue , Sato Jun , Ishizawa Rie , Kim Han‐Kyul , Iwamoto Gary A. , Vongpatanasin Wanpen , Mitchell Jere H. , Smith Scott A. , Mizuno Masaki 
Published: May 2022

Insulin centrally activates the sympathetic nervous system. However, little is known about the effects of insulin on peripheral primary sensory neurons. Transient receptor potential vanilloid 1 (TRPV1), of which capsaicin (CAP) is an agonist, is widely expressed in small dorsal root ganglia (DRG) neurons subserving skeletal muscle thin‐fiber afferents expressing sensory nociceptors and/or metabolically sensitive skeletal muscle receptors. Evidence suggests that insulin increases the sensitivity of these sensory receptors as well as translocation of TRPV1 from cytosol to plasma membrane in DRG neurons. We therefore hypothesized that insulin potentiates the responsiveness of thin fiber muscle afferents to TRPV1 activation.PurposeThe aim of this investigation was 1) to examine whether TRPV1 and insulin receptors (IR) are co‐localized in small‐diameter (< 30 µm) DRG neurons subserving thin‐fiber afferents (C‐fibers or A‐delta fibers), and 2) to determine the effects of insulin on renal sympathetic nerve activity (RSNA) and mean arterial blood pressure (MAP) responses to stimulation of TRPV1 by injection of CAP into the arterial supply of the hindlimb in decerebrated rats.MethodsL4‐L6 DRGs were harvested from Sprague‐Dawley rats (n=3, 19 weeks, body weight: 518 ± 6 g). The percentage of neurons that expressed IR and/or isolectin B4 binding (IB4, a C‐fiber neuronal marker) in TRPV1 positive small DRGs neurons were assessed. RSNA and MAP responses to intra‐arterial administration of CAP (0.3 µg/100 µL) were measured in decerebrate Sprague‐Dawley rats (10 ± 1 weeks, body weight: 330 ± 43 g) before and after intramuscular injection of either insulin (5 U/mL, 30 µL) or saline (vehicle control, 30 µL). To avoid insulin/saline spill over into the systemic circulation, a vascular occluder placed around the abdominal aorta was inflated immediately prior to intramuscular administration of insulin or saline.ResultsImmunohistochemical staining demonstrated that 22 % of TRPV1 positive DRG neurons expressed IR and IB4 binding. Intramuscular insulin administration did not change blood glucose as well as plasma insulin levels assessed from the jugular vein. The RSNA response to CAP after insulin administration was significantly greater than post injection of vehicle (vehicle: 25 ± 24, n=9, vs. insulin: 54 ± 32 %, n=8, P=0.04). While the MAP response to CAP was not significantly changed by vehicle injection (before: 26 ± 13 vs. after: 24 ± 12 mmHg, n=18), insulin administration significantly increased MAP responsiveness (before: 24 ± 1 vs. after: 29 ± 15 mmHg, n=22, P=0.01).ConclusionThe data demonstrate that 1) TRPV1 and IR co‐localize on small DRG neurons and 2) intramuscular insulin administration significantly augments sympathetic and pressor responses to chemical stimulation of skeletal muscle afferents. These findings suggest that insulin potentiates TRPV1 responsiveness at the muscle tissue level, possibly contributing to insulin‐induced sympathoexcitation.