T-type calcium channels participate in intrinsic and synaptic activity of PKCγ neurons of the dorsal horn of the spinal cord during chronic pain

The disinhibition of the excitatory PKCγ interneurons plays a central role during mechanical allodynia in the dorsal horn of the spinal cord, routing harmless information to nociceptive pathways. The T-type calcium channel Cav3.2, necessary for mechanical and cold allodynia, is found in most PKCγ neurons of the spinal cord. In this study, the role of Cav3.2 in PKCγ neurons was studied after its pharmacological inhibition and its conditional deletion (KO) in Cav3.2 ^GFP-Flox KI x PKCγ-CreERT2 x Ai14 mice in normal conditions and in the spared-nerve-injury (SNI) model of neuropathic pain. Conditional deletion of Cav3.2 increased the hind-paw basal mechanical sensitivity before surgery, and decreased mechanical pain 7 days, but not 28 days, after surgery. At the cellular level, Cav3.2 participated in the low-threshold currents of PKCγ neurons and the T-type calcium current of PKCγ neurons was decreased in KO mice as compared to wild-type (WT). This loss did not convert into proportional alterations in subthreshold properties including “rebound” potentials, suggesting the involvement of other T-type channels. Action potential kinetics and firing properties seemed similar in WT and KO mice too, but rebound potentials were diminished in the SNI model in WT but not in KO mice. In addition, the modulations of firing properties induced by T-type channel pharmacological blocker Z944 observed in WT mice were absent in KO mice and after SNI. Furthermore, the pairing of action potentials was modified after SNI in WT mice, and not in KO mice. At the synaptic level, excitatory currents were lowered 7- and 28-days after surgery, while inhibitory currents were lowered only at 28 days. These changes were not found in Cav3.2-ablated neurons. Miniature currents analysis indicated that Cav3.2 was involved in both excitatory and inhibitory synaptic transmissions at the level of PKCγ neurons. Surprisingly, Z944 did not mimic the effects of Cav3.2 ablation in PKCγ neurons, suggesting distinct and eventually opposite roles of other T-type calcium channels. Altogether, our results show that Cav3.2 is not mandatory for firing of PKCγ neurons of the dorsal horn of the spinal cord, but that it participates to the SNI-induced changes in their intrinsic and synaptic activity, including changes in their excitatory and inhibitory controls.