Insulin potentiates the response to mechanical stimuli in small dorsal root ganglion neurons and thin fibre muscle afferents in vitro

Abstract

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.

Insulin sensitizes mechanosensitive ion channels, which aggravates pain

“…The findings by Hotta et al . () highlight the role of insulin‐mediated sensitization of pain‐mediating ion channels (Figure ) and suggest that treatment efforts should also take this pathophysiology into consideration. Hopefully, these new insights will facilitate the development of more specific and targeted drugs against the pain receptors to provide relief of neuropathic pain for diabetic patients taking insulin.…”

“…In a recent issue of The Journal of Physiology , Hotta et al . () provide experimental evidence that insulin potentiates mechanical stimuli‐induced currents in sensory neurons to lower activation thresholds. Whole‐cell patch clamp recording of isolated dorsal root ganglion (DRG) neurons revealed that brief treatment with 500 mU/mL of insulin for 5 min induced much larger inwards currents upon applying mechanical force by moving the probe forward against the neuronal membrane.…”

“…Once sensitized, these channels give a much more robust response to mechanical stimulation, an effect elegantly demonstrated by Hotta et al . (). Together, these observations indicate that insulin activates insulin receptor signalling by inducing the activation of PI3K, PKC enzymes which phosphorylate and sensitize the nociceptive ion channels towards a lower mechanical stimulus (Figure ).…”

“…Further, using ex vivo muscle–nerve preparations, Hotta et al . () showed that intramuscular administration of insulin sensitizes the associated group IV afferent neurons to lower mechanical stimuli, without increasing the response magnitude (number of spikes evoked by the mechanical stimulation) significantly. Administration of buffer only or insulin along with GSK1838705 did not evoke similar responses, supporting the hypothesis that insulin augments mechanical hypersensitivity in vivo .…”

“…This could explain the decrease in mechanical stimulus threshold for generating inward currents observed by Hotta et al . () in their studies with muscle–nerve preparations. It is noteworthy that strong activation of TRPV1 or TRPV4 channels in neurons can lead to the formation of bulbous enlargements called varicosities, which could lead to chronification of pain, as observed in insulin‐induced diabetic neuropathy.…”