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Hori A, Hotta N, Fukazawa A, Estrada JA, Katanosaka K, Mizumura K, Sato J, Ishizawa R, Kim HK, Iwamoto GA, Vongpatanasin W, Mitchell JH, Smith SA, Mizuno M. Insulin potentiates the response to capsaicin in dorsal root ganglion neurons in vitro and muscle afferents ex vivo in normal healthy rodents. J Physiol 2022; 600:531-545. [PMID: 34967443 PMCID: PMC8810710 DOI: 10.1113/jp282740] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2021] [Accepted: 12/23/2021] [Indexed: 02/03/2023] Open
Abstract
Systemic insulin administration evokes sympathoexcitatory actions, but the mechanisms underlying these observations are unknown. We reported that insulin sensitizes the response of thin-fibre primary afferents, as well as the dorsal root ganglion (DRG) that subserves them, to mechanical stimuli. However, little is known about the effects of insulin on primary neuronal responses to chemical stimuli. TRPV1, whose agonist is capsaicin (CAP), is widely expressed on chemically sensitive metaboreceptors and/or nociceptors. The aim of this investigation was to determine the effects of insulin on CAP-activated currents in small DRG neurons and CAP-induced action potentials in thin-fibre muscle afferents of normal healthy rodents. Additionally, we investigated whether insulin potentiates sympathetic nerve activity (SNA) responses to CAP. In whole-cell patch-clamp recordings from cultured mice DRG neurons in vitro, the fold change in CAP-activated current from pre- to post-application of insulin (n = 13) was significantly (P < 0.05) higher than with a vehicle control (n = 14). Similar results were observed in single-fibre recording experiments ex vivo as insulin potentiated CAP-induced action potentials compared to vehicle controls (n = 9 per group, P < 0.05). Furthermore, insulin receptor blockade with GSK1838705 significantly suppressed the insulin-induced augmentation in CAP-activated currents (n = 13) as well as the response magnitude of CAP-induced action potentials (n = 9). Likewise, the renal SNA response to CAP after intramuscular injection of insulin (n = 8) was significantly (P < 0.05) greater compared to vehicle (n = 9). The findings suggest that insulin potentiates TRPV1 responsiveness to CAP at the DRG and muscle tissue levels, possibly contributing to the augmentation in sympathoexcitation during activities such as physical exercise. KEY POINTS: Evidence suggests insulin centrally activates the sympathetic nervous system, and a chemical stimulus to tissues activates the sympathetic nervous system via thin fibre muscle afferents. Insulin is reported to modulate putative chemical-sensitive channels in the dorsal root ganglion neurons of these afferents. In the present study, it is demonstrated that insulin potentiates the responsiveness of thin fibre afferents to capsaicin at muscle tissue levels as well as at the level of dorsal root ganglion neurons. In addition, it is demonstrated that insulin augments the sympathetic nerve activity response to capsaicin in vivo. These data suggest that sympathoexcitation is peripherally mediated via insulin-induced chemical sensitization. The present study proposes a possible physiological role of insulin in the regulation of chemical sensitivity in somatosensory thin fibre muscle afferents.
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Affiliation(s)
- Amane Hori
- Graduate School of Life and Health Sciences, Chubu University, Kasugai 487-850, Japan;,Japan Society for the Promotion of Science, Tokyo 102-8472, Japan
| | - Norio Hotta
- Graduate School of Life and Health Sciences, Chubu University, Kasugai 487-850, Japan;,College of Life and Health Sciences, Chubu University, Kasugai 487-850, Japan
| | - Ayumi Fukazawa
- Department of Applied Clinical Research, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Juan A. Estrada
- Department of Applied Clinical Research, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Kimiaki Katanosaka
- Graduate School of Life and Health Sciences, Chubu University, Kasugai 487-850, Japan;,College of Life and Health Sciences, Chubu University, Kasugai 487-850, Japan
| | - Kazue Mizumura
- Department of Physiology, Nihon University School of Dentistry, Tokyo 101-8310, Japan
| | - Jun Sato
- Graduate School of Life and Health Sciences, Chubu University, Kasugai 487-850, Japan;,College of Life and Health Sciences, Chubu University, Kasugai 487-850, Japan
| | - Rie Ishizawa
- Department of Applied Clinical Research, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Han-Kyul Kim
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Gary A. Iwamoto
- Department of Cell Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Wanpen Vongpatanasin
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Jere H. Mitchell
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Scott A. Smith
- Department of Applied Clinical Research, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Masaki Mizuno
- Department of Applied Clinical Research, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
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