Original article 1 Effects of intrathecal injection of T-type calcium channel blockers in the rat formalin test Jen-Kun Cheng a,c,d,e , Chia-Shiang Lin c , Chien-Chuan Chen c , Jia-Rung Yang d and Lih-Chu Chiou a,b T-type Ca 2+ channels have been implicated in the induction of long-term potentiation, a synaptic plasticity involved in the central sensitization that contributes to the generation of inflammatory pain, in spinal sensory neurons. In this study, we examined the effects of intrathecal T-type Ca 2+ channel blockers, mibefradil, ethosuximide and NiCl 2 , in the rat formalin test, an inflammatory pain model. Biphasic characteristic nociceptive behaviors were induced by intraplantar injection of formalin (5% formaldehyde, 50 ll) in Sprague–Dawley rats and monitored at 0–9 min (phase 1) and 10–60 min (phase 2) after formalin injection. Intrathecal pretreatment with mibefradil (50–500 lg) and NiCl 2 (1–10 lg) dose-dependently decreased the flinch numbers and biting and licking time in both phases. The ID 50 s of mibefradil in inhibiting the phases 1 and 2 flinch responses were 74.3 ± 4.6 and 100.9 ± 8.7 lg, respectively, and those of NiCl 2 were 2.7 ± 1.1 and 3.3 ± 0.1 lg, respectively. Ethosuximide, at the doses up to 1200 lg, however, did not affect the nociceptive responses in both phases. It is suggested that spinal T-type Ca 2+ channels may play a role in formalin-induced inflammatory pain. The ineffectiveness of ethosuximide is discussed. Behavioural Pharmacology 18:1–8  c 2007 Lippincott Williams & Wilkins. Behavioural Pharmacology 2007, 18:1–8 Keywords: ethosuximide, formalin test, intrathecal, mibefradil, nickel, rat a Institute of Pharmacology, b Department of Pharmacology, College of Medicine, National Taiwan University, c Department of Anesthesiology, d Medical Research, Mackay Memorial Hospital and e Department of Anesthesiology, Taipei Medical University, Taipei, Taiwan. Correspondence to Professor Lih-Chu Chiou, PhD, Department of Pharmacology, College of Medicine, National Taiwan University, No. 1, Jen-Ai Rd., Section 1, Taipei 100, Taiwan. E-mail: lcchiou@ha.mc.ntu.edu.tw Received 20 September 2006 Accepted as revised 1 November 2006 Introduction Voltage-dependent Ca 2+ channels are classified by their activation voltages into high and low-voltage-activated channels. The former are further divided into L-(Ca V 1.x), P/Q-(Ca V 2.1), N-(Ca V 2.2) and R-(Ca V 2.3) types based on their activation kinetics, pharmacological sensitivities and a 1 -subunit sequences. The latter consists of T-type (Ca V 3.x) Ca 2+ channels, by which the currents mediated are tiny and transient (Catterall et al., 2002). In the spinal cord, all types of Ca 2+ channels exist and might be involved in spinal neurotransmission and neuronal activity (Heinke et al., 2004). In-vivo studies showed that intrathecal administration of high-voltage-activated Ca 2+ channel blockers induced antinociceptive responses in various animal pain models (Cheng and Chiou, 2006). For example, N-type blockers were effective in neuro- pathic (Chaplan et al., 1994b), inflammatory (Bowersox et al., 1996) and postoperative (Atanassoff et al., 2000) pain models, L-type blockers were effective in tail-flick and colorectal distension tests (Hara et al., 1998), and P/Q-type blocker could prevent inflammation-induced heat hyperalgesia (Sluka, 1998). On the other hand, diverse results were reported regarding the role of spinal T-type Ca 2+ channels in the pain regulation. Intrathecal injection of mibefradil or ethosuximide, the T-type Ca 2+ channel blockers (Todorovic and Lingle, 1998), did not affect the nociceptive responses in spinal nerve ligation (Dogrul et al., 2003) or postoperative (Cheng et al., 2006) pain models, whereas Matthews and Dickenson (2001) observed that local application of ethosuximide onto the spinal cord decreased the sensory neuronal activity in response to nociceptive paw stimulation in both spinal nerve-ligated and naive rats. Intrathecal mibefradil was found to potentiate morphine-induced analgesic effect in mouse tail-flick test (Dogrul et al., 2001). On the other hand, downregulation of T-type Ca 2+ channels in dorsal root ganglia (DRG) was reported in the chronic constric- tion injury pain model (McCallum et al., 2003). Ikeda et al. (2003) reported that T-type Ca 2+ channels of the neurokinin-1 (NK-1) receptor-containing projection sensory neurons in the spinal lamina I are essential for the generation of long-term potentiation (LTP), a form of synaptic plasticity involved in the central sensitization of spinal hyperalgesia (Willis, 2002). This suggests that T-type Ca 2+ channels might be involved in the central sensitization of spinal hyperalgesia. The formalin-induced nociceptive response in rats is believed to be an inflammatory pain and involves central sensitization in the spinal cord (Abbott et al., 1995). LTP of 0955-8810  c 2007 Lippincott Williams & Wilkins Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.