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Key Points: Insulin is known to activate the sympathetic nervous system centrally. A mechanical stimulus to tissues activates the sympathetic nervous system via thin fibre afferents. Evidence suggests that insulin modulates putative mechanosensitive channels in the dorsal root ganglion neurons of these afferents. In the present study, we report the novel finding that insulin augments the mechanical responsiveness of thin fibre afferents not only at dorsal root ganglion, but also at muscle tissue levels. Our data suggest that sympathoexcitation is mediated via the insulin-induced mechanical sensitization peripherally. The present study proposes a novel physiological role of insulin in the regulation of mechanical sensitivity in somatosensory thin fibre afferents., Abstract: Insulin activates the sympathetic nervous system, although the mechanism underlying insulin-induced sympathoexcitation remains to be determined. A mechanical stimulus to tissues such as skin and/or skeletal muscle, no matter whether the stimulation is noxious or not, activates the sympathetic nervous system via thin fibre afferents. Evidence suggests that insulin modulates putative mechanosensitive channels in the dorsal root ganglion (DRG) neurons of these afferents. Accordingly, we investigated whether insulin augments whole-cell current responses to mechanical stimuli in small DRG neurons of normal healthy mice. We performed whole-cell patch clamp recordings using cultured DRG neurons and observed mechanically-activated (MA) currents induced by mechanical stimuli applied to the cell surface. Local application of vehicle solution did not change MA currents or mechanical threshold in cultured DRG neurons. Insulin (500 mU mL -1 ) significantly augmented the amplitude of MA currents (P < 0.05) and decreased the mechanical threshold (P < 0.05). Importantly, pretreatment with the insulin receptor antagonist, GSK1838705, significantly suppressed the insulin-induced potentiation of the mechanical response. We further examined the impact of insulin on thin fibre muscle afferent activity in response to mechanical stimuli in normal healthy rats in vitro. Using a muscle-nerve preparation, we recorded single group IV fibre activity to a ramp-shaped mechanical stimulation. Insulin significantly decreased mechanical threshold (P < 0.05), although it did not significantly increase the response magnitude to the mechanical stimulus. In conclusion, these data suggest that insulin augments the mechanical responsiveness of small DRG neurons and potentially sensitizes group IV afferents to mechanical stimuli at the muscle tissue level, possibly contributing to insulin-induced sympathoexcitation., (© 2019 The Authors. The Journal of Physiology © 2019 The Physiological Society.)

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. [ABSTRACT FROM AUTHOR]

Key points The exercise pressor reflex (EPR) is exaggerated in type 2 diabetes mellitus (T2DM), but the underlying central nervous system aberrations have not been fully delineated. Stimulation of muscle afferents within working skeletal muscle activates the EPR, by sending information to neurons in the brainstem, where it is integrated and results in reflexively increased mean arterial pressure (MAP) and sympathetic nerve activity. Brain insulin is known to regulate neural activity within the brainstem. We hypothesize that brain insulin injection in T2DM rats attenuates the augmented EPR, and that T2DM is associated with decreased brain insulin. Using male Sprague–Dawley rats, T2DM and control rats were generated via an induction protocol with two low doses of streptozotocin (35 and 25 mg/kg, i.p.) in combination with a 14–23‐week high‐fat diet or saline injections and a low‐fat diet, respectively. After decerebration, MAP and renal sympathetic nerve activity (RSNA) were evaluated during EPR stimulation, evoked by electrically induced muscle contraction via ventral root stimulation, before and after (1 and 2 h post) intracerebroventricular (i.c.v.) insulin microinjections (500 mU, 50 nl). i.c.v. insulin decreased peak MAP (ΔMAP Pre (36 ± 14 mmHg) vs. 1 h (21 ± 14 mmHg) vs. 2 h (11 ± 6 mmHg), P < 0.05) and RSNA (ΔRSNA Pre (107.5 ± 40%), vs. 1 h (75.4 ± 46%) vs. 2 h (51 ± 35%), P < 0.05) responses in T2DM, but not controls. In T2DM rats, cerebrospinal fluid insulin was decreased (0.41 ± 0.19 vs. 0.11 ± 0.05 ng/ml, control (n = 14) vs. T2DM (n = 4), P < 0.01). The results demonstrated that insulin injections into the brain normalized the augmented EPR in brain hypoinsulinaemic T2DM rats, indicating that the EPR can be regulated by brain insulin. The reflexive increase in blood pressure and sympathetic nerve activity mediated by the autonomic nervous system during muscle contractions is also known as the exercise pressor reflex. The exercise pressor reflex is dangerously augmented in type 2 diabetes, in both rats and humans. In type 2 diabetic rats both cerebrospinal fluid insulin and phosphoinositide 3‐kinase signalling within cardiovascular brainstem neurons decrease in parallel. Brain insulin injections decrease the magnitude of the reflexive pressor and sympathetic responses to hindlimb muscle contraction in type 2 diabetic rats. Partial correction of low insulin within the central nervous system in type 2 diabetes may treat aberrant exercise pressor reflex function. [ABSTRACT FROM AUTHOR]

Insulin not only regulates glucose and/or lipid metabolism but also modulates brain neural activity. The nucleus tractus solitarius (NTS) is a key central integration site for sensory input from working skeletal muscle and arterial baroreceptors during exercise. Stimulation of the skeletal muscle exercise pressor reflex (EPR), the responses of which are buffered by the arterial baroreflex, leads to compensatory increases in arterial pressure to supply blood to working muscle. Evidence suggests that insulin signaling decreases neuronal excitability in the brain, thus antagonizing insulin receptors (IRs) may increase neuronal excitability. However, the impact of brain insulin signaling on the EPR remains fully undetermined. We hypothesized that antagonism of NTS IRs increases EPR function in normal healthy rodents. In decerebrate rats, stimulation of the EPR via electrically induced muscle contractions increased peak mean arterial pressure (MAP) responses 30 min following NTS microinjections of an IR antagonist (GSK1838705, 100 μM; Pre: Δ16 ± 10 mmHg vs. 30 min: Δ23 ± 13 mmHg, n = 11, p =.004), a finding absent in sino‐aortic baroreceptor denervated rats. Intrathecal injections of GSK1838705 did not influence peak MAP responses to mechano‐ or chemoreflex stimulation of the hindlimb muscle. Immunofluorescence triple overlap analysis following repetitive EPR stimulation increased c‐Fos overlap with EPR‐sensitive nuclei and IR‐positive cells relative to sham operation (p <.001). The results suggest that IR blockade in the NTS potentiates the MAP response to EPR stimulation. In addition, insulin signaling in the NTS may buffer EPR stimulated increases in blood pressure via baroreflex‐mediated mechanisms during exercise. [ABSTRACT FROM AUTHOR]