The T‐type calcium channel antagonist, Z944, reduces spinal excitability and pain hypersensitivity

Abstract

Background and Purpose: T-type voltage-gated calcium channels are an emerging therapeutic target for neurological disorders including epilepsy and pain. Inhibition of T-type channels reduces the excitability of peripheral nociceptive sensory neurons and reverses pain hypersensitivity in male rodent pain models. However, administration of peripherally restricted T-type antagonists failed to show efficacy in multiple clinical and preclinical pain trials, suggesting that inhibition of peripheral T-type channels alone may be insufficient for pain relief. Experimental Approach: We utilized the selective and CNS-penetrant T-type channel antagonist, Z944, in electrophysiological, calcium imaging and behavioural paradigms to determine its effect on lamina I neuron excitability and inflammatory pain behaviours. Key Results: Voltage-clamp recordings from lamina I spinal neurons of adult rats revealed that approximately 80% of neurons possess a low threshold T-type current, which was blocked by Z944. Due to this highly prevalent T-type current, Z944potently blocked action-potential evoked somatic and dendritic calcium transients in lamina I neurons. Moreover, application of Z944 to spinal cord slices attenuated action potential firing rates in over half of laminae I/II neurons. Finally, we found that intraperitoneal injection of Z944 (1-10 mgÁkg À1 ) dose-dependently reversed mechanical allodynia in the complete Freund's adjuvant model of persistent inflammatory pain, with a similar magnitude and time course of analgesic effects between male and female rats. Conclusion and Implications:T-type calcium channels critically shape the excitability of lamina I pain processing neurons and inhibition of these channels by the clinical stage antagonist Z944 potently reverses pain hypersensitivity across sexes.

Central and peripheral contributions of T-type calcium channels in pain

“…Within lamina I neurons, functional T-type currents have also been identified, as confirmed through Z944 inhibition (Fig. 2 ) [ 27 ]. Similar to lamina II, T-type channels appear to be present in most, but not all lamina I neurons, and use of pharmacological inhibitors has demonstrated that T-type channels contribute to neuronal excitability as well as action potential afterdepolarizations [ 27 , 28 , 97 ].…”

“…c T-type calcium channels contribute to action potential-induced calcium current in the soma and dendrites of rat lamina I neurons. Adapted with permission from Harding et al [ 27 ] …”

“…Further classification based on protein markers for different subpopulations have revealed that Ca v 3.2 is present in both excitatory and inhibitory interneurons, including subpopulations known to synapse onto lamina I projection neurons [ 29 , 91 ]. T-type channel expression in these neurons increases their excitability, as demonstrated by reduction of neuronal firing of lamina II neurons upon pharmacological inhibition of T-type channels or genetic ablation of Ca v 3.2 [ 27 , 29 , 93 , 95 ].…”

The Mechanisms of Plasticity of Nociceptive Ion Channels in Painful Diabetic Neuropathy

“…Although T-channel antagonists applied intrathecally reversed pain hypersensitivity in various rodent models of pain ( 49 , 57 , 62 , 70 ), the presence of T-type mediated calcium currents in a subset of lamina II neurons was discovered only recently ( 71 , 72 ). Harding et al ( 73 ) discovered that T-type channels are available abundantly in lamina I neurons, and are dominant mediators of action potential-evoked calcium signals that represent actively backpropagating action potentials ( 73 ), shaping the excitability of neurons and affecting short-term and long-term plasticity in the spinal cord. Furthermore, Z944, a selective centrally penetrant T-channel antagonist, reduced superficial dorsal horn excitability in vitro , and reversed tactile allodynia in an inflammatory pain model in rats of both sexes.…”

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

“…Z944 is another high-affinity T-type channel blocker that is effective against Ca v 3.1, Ca v 3.2, and Ca v 3.3 with little affinity for other Ca 2+ channel types ( 411 ). Its effectiveness in murine pain models may reflect it actions on spinal and thalamic neurons ( 412 , 413 ). So far, the results of phase 1 and phase 2 trials appear promising ( 101 ) ( Table 1 ).…”