Insulin Confers Differing Effects on Neurite Outgrowth in Separate Populations of Cultured Dorsal Root Ganglion Neurons: The Role of the Insulin Receptor

Abstract

Apart from its pivotal role in the regulation of carbohydrate metabolism, insulin exerts important neurotrophic and neuromodulator effects on dorsal root ganglion (DRG) neurons. The neurite outgrowth-promoting effect is one of the salient features of insulin’s action on cultured DRG neurons. Although it has been established that a significant population of DRG neurons express the insulin receptor (InsR), the significance of InsR expression and the chemical phenotype of DRG neurons in relation to the neurite outgrowth-promoting effect of insulin has not been studied. Therefore, in this study by using immunohistochemical and quantitative stereological methods we evaluated the effect of insulin on neurite outgrowth of DRG neurons of different chemical phenotypes which express or lack the InsR. Insulin, at a concentration of 10 nM, significantly increased total neurite length, the length of the longest neurite and the number of branch points of cultured DRG neurons as compared to neurons cultured in control medium or in the presence of 1 μM insulin. In both the control and the insulin exposed cultures, ∼43% of neurons displayed InsR-immunoreactivity. The proportions of transient receptor potential vanilloid type 1 receptor (TRPV1)-immunoreactive (IR), calcitonin gene-related peptide (CGRP)-IR and Bandeiraea simplicifolia isolectin B4 (IB4)-binding neurons amounted to ∼61%, ∼57%, and ∼31% of DRG neurons IR for the InsR. Of the IB4-positive population only neurons expressing the InsR were responsive to insulin. In contrast, TRPV1-IR nociceptive and CGRP-IR peptidergic neurons showed increased tendency for neurite outgrowth which was further enhanced by insulin. However, the responsiveness of DRG neurons expressing the InsR was superior to populations of DRG neurons which lack this receptor. The findings also revealed that besides the expression of the InsR, inherent properties of peptidergic, but not non-peptidergic nociceptive neurons may also significantly contribute to the mechanisms of neurite outgrowth of DRG neurons. These observations suggest distinct regenerative propensity for differing populations of DRG neurons which is significantly affected through insulin receptor signaling.

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

“…; Lázár et al . ). Hence, because small DRG neurons and group IV muscle afferents were used in the present study, the probability that we recorded from DRG neurons/afferent fibres expressing insulin receptors was high.…”

“…In the present study, we did not expressly identify whether insulin receptors were expressed in the DRG neurons and thin fibre muscle afferents from which we recorded. However, it has been reported that insulin receptors are expressed on DRG neurons in relatively high proportions (Baiou et al 2007;Lázár et al 2018), as well as peripheral nerves (Sugimoto et al 2002;Grote et al 2013). Moreover, insulin receptors are reported to be predominantly expressed in small nociceptive neurons (Baiou et al 2007;Lázár et al 2018).…”

“…However, it has been reported that insulin receptors are expressed on DRG neurons in relatively high proportions (Baiou et al 2007;Lázár et al 2018), as well as peripheral nerves (Sugimoto et al 2002;Grote et al 2013). Moreover, insulin receptors are reported to be predominantly expressed in small nociceptive neurons (Baiou et al 2007;Lázár et al 2018). Hence, because small DRG neurons and group IV muscle afferents were used in the present study, the probability that we recorded from DRG neurons/afferent fibres expressing insulin receptors was high.…”

“…Recent studies have demonstrated that insulin activates some ion channels reported as molecular candidates for mechanosensitive receptors in the peripheral terminals of primary sensory neurons for which the cell bodies are in the dorsal root ganglion (DRG) (Uchida & Tominaga, 2011;Colsoul et al 2013;Uchida et al 2018). In addition, insulin receptors have been known to be expressed on DRG neurons (Sugimoto et al 2002;Baiou et al 2007;Grote et al 2013;Lázár et al 2018) and peripheral nerves (Sugimoto et al 2002;Grote et al 2013). Taken together, it is hypothesized that insulin potentiates the response of thin fibre afferents to mechanical stimuli resulting in an increase in feedback signals to central regulatory centres in the medulla, leading to an augmentation in sympathetic nerve activity.…”

Modulation of Sensory Nerve Function by Insulin: Possible Relevance to Pain, Inflammation and Axon Growth

“…In recent years, electrophysiological, immunohistochemical and functional studies demonstrated the localization and functional significance of InsRs in primary sensory neurons (PSNs), in particular nociceptive PSNs expressing the transient receptor potential vanilloid type 1 (TRPV1) receptor [19][20][21][22][23][24][25]. The present review attempts to summarize experimental data on the diverse roles of insulin and InsRs in physiological and pathophysiological processes involving PSNs.…”