TRPV1 properties in thoracic dorsal root ganglia neurons are modulated by intraperitoneal capsaicin administration in the late phase of type-1 autoimmune diabetes

Abstract

Pharmacological therapies in type 1 diabetes for efficient control of glycemia and changes in pain alterations due to diabetic neuropathy are a continuous challenge. Transient receptor potential vanilloid type 1 (TRPV1) from dorsal root ganglia (DRG) neurons is one of the main pharmacological targets in diabetes, and its ligand capsaicin can be a promising compound for blood-glucose control. Our goal is to elucidate the effect of intraperitoneal (i.p.) capsaicin administration in type 1 diabetic mice against TRPV1 receptors from pancreatic DRG primary afferent neurons. A TCR(+/-)/Ins-HA(+/-) diabetic mice (dTg) was used, and patch-clamp and immunofluorescence microscopy measurements have been performed on thoracic T(9)-T(12) DRG neurons. Capsaicin (800 μg/kg, i.p. three successive days) administration in the late-phase diabetes reduces blood-glucose levels, partly reverses the TRPV1 current density and recovery time constant, without any effect on TRPV1 expression general pattern, in dTg mice. A TRPV1 hypoalgesia profile was observed in late-phase diabetes, which was partly reversed to normoalgesic profile upon capsaicin i.p. administration. According to the soma dimensions of the thoracic DRG neurons, a detailed analysis of the TRPV1 expression upon capsaicin i.p. treatment was done, and the proportion of large A-fiber neurons expressing TRPV1 increased in dTg capsaicin-treated mice. In conclusion, the benefits of low-dose capsaicin intraperitoneal treatment in late-phase type-1 diabetes should be further exploited.

Fig. 1

Blood-glucose levels (mg/dl) for Balb/c and dTg mice vehicle or capsaicin i.p. treated. Capsaicin dose was 800 μg/kg (1/10 from LD₅₀), and the intraperitoneal administration was done for three successive days. Glycemia levels are measured after 24 h from the last treatment. Number of mice (N) in each group was mentioned in parentheses above each column in the graph, and the levels of statistical significance are marked by asterisks (*p < 0.05; **p < 0.01)

Fig. 2

TRPV1 expression in primary cultures of thoracic DRG neurons from Balb/c and dTg mice, vehicle and capsaicin-treated. Representative fluorescence images (excitation: 530–550 nm; beam splitter: 570 nm; emission >570 nm) of immuno-labeled receptors with polyclonal anti-TRPV1 antibodies (Abcam) for: a Balb/c vehicle-treated, b Balb/c capsaicin-treated, c dTg vehicle-treated, d dTg capsaicin-treated mice. Scale bar 20 μm ×40 air objective, e Fluorescence intensities (evaluated as average gray level on the surface of each cell) are represented as mean ± SD for each experimental variant. Number of analyzed cells is mentioned in parentheses, f Fluorescence intensity distribution with respect to the neuronal soma-size diameter (S small neurons, diameter <25 μm; M medium neurons, 25–40 μm; L large neurons, diameter >40 μm). Number of analyzed cells in each size subgroup is mentioned in parentheses and the levels of statistical significance are marked by asterisks (*p < 0.05, **p < 0.01)

Fig. 3

Distribution of the immunofluorescence intensity versus soma-size diameter of thoracic DRG sensory neurons in: a Balb/c vehicle-treated, b Balb/c capsaicin-treated, c dTg vehicle-treated, d dTg capsaicin-treated mice. The soma-size subgroups are delimited by vertical green bars. The pie plots show the percentage of each size subgroup, S small neurons (<25 μm), M medium neurons (25–40 μm)m, and L large neurons (>40 μm) (Color figure online)

Fig. 4

Capsaicin-induced TRPV1 currents in small and medium thoracic DRG sensory neurons from age-matched (16 weeks) Balb/c and dTg mice, vehicle and capsaicin–treated. a. Whole-cell recordings of inward-currents through TRPV1 channels elicited by 1 μM capsaicin (10 s). Voltage-clamp mode was used. Holding potential is − 70 mV. Green lines are marking the limits for fitting the recovery time constant for the TRPV1 capsaicin-activated current. Scale bar 100 pA, 10 s, b. Current densities of capsaicin-activated TRPV1 channels, c. Recovery time constant for capsaicin-activated TRPV1 currents. The number in parenthesis represents the number of cells, and the asterisks represent the significance levels (*p < 0.05, **p < 0.01, ***p < 0.001)