Effects of Bepridil and Pimozide, Existing Medicines Capable of Blocking T-Type Ca2+ Channels, on Visceral Pain in Mice.

T-Type Ca2+ channels (T-channels), particularly Cav3.2, are now considered as therapeutic targets for treatment of intractable pain including visceral pain. Among existing medicines, bepridil, a multi-channel blocker, used for treatment of arrhythmia and angina, and pimozide, a dopamine D2 receptor antagonist, known as a typical antipsychotic, have potent T-channel blocking activity. We thus tested whether bepridil and pimozide could suppress visceral pain in mice. Colonic and bladder pain were induced by intracolonic administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) and systemic administration of cyclophosphamide (CPA), respectively. Referred hyperalgesia was assessed by von Frey test, and colonic hypersensitivity to distension by a volume load with intracolonic water injection and spontaneous bladder pain were evaluated by observing nociceptive behaviors in conscious mice. The mice exhibited referred hyperalgesia and colonic hypersensitivity to distension on day 6 after TNBS treatment. Systemic administration of bepridil at 10-20 mg/kg or pimozide at 0.1-0.5 mg/kg strongly reduced the referred hyperalgesia on the TNBS-induced referred hyperalgesia and colonic hypersensitivity to distension. CPA treatment caused bladder pain-like nociceptive behavior and referred hyperalgesia, which were reversed by bepridil at 10-20 mg/kg or pimozide at 0.5-1 mg/kg. Our data thus suggest that bepridil and pimozide, existing medicines capable of blocking T-channels, are useful for treatment of colonic and bladder pain, and serve as seeds for the development of new medicines for visceral pain treatment.

Essential role of Cav3.2 T-type calcium channels in butyrate-induced colonic pain and nociceptor hypersensitivity in mice.

Given the role of Cav3.2 isoform among T-type Ca2+ channels (T-channels) in somatic and visceral nociceptive processing, we analyzed the contribution of Cav3.2 to butyrate-induced colonic pain and nociceptor hypersensitivity in mice, to evaluate whether Cav3.2 could serve as a target for treatment of visceral pain in irritable bowel syndrome (IBS) patients. Mice of ddY strain, and wild-type and Cav3.2-knockout mice of a C57BL/6J background received intracolonic administration of butyrate twice a day for 3 days. Referred hyperalgesia in the lower abdomen was assessed by von Frey test, and colonic hypersensitivity to distension by a volume load or chemicals was evaluated by counting nociceptive behaviors. Spinal phosphorylated ERK was detected by immunohistochemistry. Cav3.2 knockdown was accomplished by intrathecal injection of antisense oligodeoxynucleotides. Butyrate treatment caused referred hyperalgesia and colonic hypersensitivity to distension in ddY mice, which was abolished by T-channel blockers and/or Cav3.2 knockdown. Butyrate also increased the number of spinal phosphorylated ERK-positive neurons following colonic distension in the anesthetized ddY mice. The butyrate-treated ddY mice also exhibited T-channel-dependent colonic hypersensitivity to intracolonic Na2S, known to enhance Cav3.2 activity, and TRPV1, TRPA1 or proteinase-activated receptor 2 (PAR2) agonists. Wild-type, but not Cav3.2-knockout, mice of a C57BL/6J background, after treated with butyrate, mimicked the T-channel-dependent referred hyperalgesia and colonic hypersensitivity in butyrate-treated ddY mice. Our study provides definitive evidence for an essential role of Cav3.2 in the butyrate-induced colonic pain and nociceptor hypersensitivity, which might serve as a target for treatment of visceral pain in IBS patients.

Genetic deletion of Cav3.2 T-type calcium channels abolishes H2S-dependent somatic and visceral pain signaling in C57BL/6 mice.

We tested whether genetic deletion of Cav3.2 T-type Ca2+ channels abolishes hydrogen sulfide (H2S)-mediated pain signals in mice. In Cav3.2-expressing HEK293 cells, Na2S, an H2S donor, at 100 µM clearly increased Ba2+ currents, as assessed by whole-cell patch-clamp recordings. In wild-type C57BL/6 mice, intraplantar and intracolonic administration of Na2S evoked mechanical allodynia and visceral nociceptive behavior, respectively, which were abolished by TTA-A2, a T-type Ca2+ channel blocker. In Cav3.2-knockout mice of a C57BL/6 background, Na2S caused neither somatic allodynia nor colonic nociception. Our study thus provides definitive evidence for an essential role of Cav3.2 in H2S-dependent somatic and colonic pain.