Diabetic neuropathy enhances voltage-activated Ca2+ channel activity and its control by M4 muscarinic receptors in primary sensory neurons

Painful neuropathy is one of the most serious complications of diabetes and remains difficult to treat. The muscarinic acetylcholine receptor (mAChR) agonists have a profound analgesic effect on painful diabetic neuropathy. Here we determined changes in T-type and high voltage-activated Ca2+ channels (HVACCs) and their regulation by mAChRs in dorsal root ganglion (DRG) neurons in a rat model of diabetic neuropathy. The HVACC currents in large neurons, T-type currents in medium and large neurons, the percentage of small DRG neurons with T-type currents, and the Cav3.2 mRNA level were significantly increased in diabetic rats compared with those in control rats. The mAChR agonist oxotremorine-M significantly inhibited HVACCs in a greater proportion of DRG neurons with and without T-type currents in diabetic than in control rats. In contrast, oxotremorine-M had no effect on HVACCs in small and large neurons with T-type currents and in most medium neurons with T-type currents from control rats. The M-2 and M-4 antagonist himbacine abolished the effect of oxotremorine-M on HVACCs in both groups. The selective M-4 antagonist muscarinic toxin-3 caused a greater attenuation of the effect of oxotremorine-M on HVACCs in small and medium DRG neurons in diabetic than in control rats. Additionally, the mRNA and protein levels of M-4, but not M-2, in the DRG were significantly greater in diabetic than in control rats. Our findings suggest that diabetic neuropathy potentiates the activity of T-type and HVACCs in primary sensory neurons. M-4 mAChRs are up-regulated in DRG neurons and probably account for increased muscarinic analgesic effects in diabetic neuropathic pain.