Effects of ethosuximide, a T-type Ca2+ channel blocker, on dorsal horn neuronal responses in rats
Introduction
Neuropathic pain, arising from injury- or disease-evoked damage to the peripheral or central nervous system, often responds poorly to traditional analgesics. Patients often experience sensory deficits, persistent and stimulus-evoked pain (allodynia and hyperalgesia). Peripheral nerve damage provides abnormal input into the central nervous system and then leads to dorsal horn hyperexcitability. Under the influence of both excitatory and inhibitory neurotransmitter systems, the dorsal horn is a site of peripheral input modulation before projection to higher brain centres, thus it controls the stimulus–response relationship. Reduction of this excitability is a possible key to neuropathic pain management.
Animal models of neuropathy have been critical in elucidating its complex causal mechanisms, involving plasticity in nociceptive transmission and modulating systems. The rat spinal nerve ligation model (Kim and Chung, 1992) involves tight ligation of two (L5 and L6) of the three spinal nerves that form the sciatic nerve. Behavioural consequences include thermal hyperalgesia, and mechanical and cooling allodynia Kim and Chung, 1992, Chaplan et al., 1997.
High voltage-activated Ca2+ channels (L-, N-, P/Q- and R-types), consisting of a pore-forming α1 subunit and modulatory accessory subunits, β, α2-δ and γ (Walker and De Waard, 1998), are widely expressed throughout the brain and spinal cord Kerr et al., 1988, Mintz et al., 1992, Gohil et al., 1994. They are activated by relatively strong membrane depolarisation and permit Ca2+ influx in response to action potentials. Consequential secondary actions include neurotransmitter release; thus these channels establish a major link between neuronal excitability and synaptic transmission. For these reasons high voltage-activated Ca2+ channels have been the focus of both acute and persistent pain transmission studies. Animal models have demonstrated the antinociceptive abilities of antagonists specific for L-, N- and P/Q-type Ca2+ channels, highlighting the differential role each subtype plays in nociception, often dependent on the nature of the pain state (Vanegas and Schaible, 2000).
In addition to high voltage-activated Ca2+ channels, kinetically distinct low voltage-activated Ca2+ channels, or T-type channels, also exist both in neuronal and non-neuronal cells. They activate at voltages near the resting membrane potential, inactivate rapidly, deactivate slowly and have a small single channel conductance (Huguenard, 1996). These unique gating properties prohibit T-type channels alone to support neurotransmission, however they permit their involvement in low-amplitude oscillations, neuronal bursting, synaptic signal boosting, Ca2+ entry promotion and lowering threshold for high-threshold spike generation. This Ca2+ current appears to play an important physiological role in near-threshold phenomena and regulation of neuronal excitability.
Until recently, the low voltage-activated current was considered a single entity. However, α1G, α1H and α1I subunits have now been cloned, showing 30% homology to high voltage-activated channel forming α1 subunits Cribbs et al., 1998, Perez-Reyes, 1998, Lee et al., 1999 and hallmark native T-type Ca2+ channel properties when expressed heterologously. In situ hybridisation studies on the rat brain have shown that these channels have unique distributions, including the dorsal horn of the spinal cord and sensory ganglia (Talley et al., 1999). This is complimented by reported T-type currents in primary sensory neurones Carbone and Lux, 1984, Kostyuk et al., 1992, Scroggs and Fox, 1992, Todorovic and Lingle, 1998 and some superficial rat dorsal horn neurones (Ryu and Randic, 1990), an important site for the processing and integration of sensory information, including pain. Unlike the high voltage-activated Ca2+ channels, the involvement of T-type channels in pain-related central sensitisation has been hindered by a scarcity of specific pharmacological agents.
Neuropathic pain and epilepsy both share neuronal hyperexcitability as a common underlying mechanism. There are established antiepileptic drugs that target the generation of neuronal hyperexcitability in the brain and some of these have been proven effective in the treatment of various forms of neuropathic pain Swerdlow and Cundill, 1981, McQuay et al., 1995. The succinimde derivative ethosuximide, or 2-ethyl-2-methylsuccinimide, is an anticonvulsant (Macdonald and McLean, 1986) effective in the treatment of absence epilepsy (Coulter et al., 1989b); a condition characterised by spike-wave rhythm likely generated by T-type Ca2+ current. Ethosuximide has been demonstrated to be a relatively specific T-type channel antagonist in thalamic (Coulter et al., 1989a) and dorsal root ganglion neurones (Kostyuk et al., 1992). This study uses the spinal nerve ligation model, confirmed by behavioural testing, to induce a neuropathic state, subsequent to which electrophysiological studies of dorsal horn spinal neurones were made to investigate the effects of spinally delivered ethosuximide on a wide range of electrical and natural-evoked neuronal activity.
Section snippets
Spinal nerve ligation
Male Sprague–Dawley rats, initially weighing 130–150 g, were used in this study. All experimental procedures were approved by the Home Office and follow the guidelines under the International Association for the study of Pain (Zimmermann, 1983). Selective tight ligation of spinal nerves L5 and L6, and a sham procedure were performed as first described by Kim and Chung (1992). For details, see Chapman et al. (1998).
Behavioural testing
For 2 weeks following surgery, the rats were housed in groups of 4, in plastic
Behavioural studies
During the post-operative period the animals showed normal weight gain and maintained good general health. Rats subjected to spinal nerve ligation exhibited abnormal foot posture ipsilateral to nerve injury whereby toes were held together in a ‘guarding’ behaviour. This did not occur in either the contralateral hindpaw, or in the sham-operated rats. Successful replication of the nerve injury model was confirmed by behavioural testing which demonstrated the development of mechanical and cooling
Discussion
Unilateral tight ligation of L5 and L6 spinal nerves produced reproducible nociceptive syndromes in the lesioned hindpaw. A clear withdrawal reflex with associated aversive behaviours, indicative of the development of mechanical and cooling allodynia, was produced as previously described by Chapman et al. (1998). Thus, all spinal nerve ligated animals used for the electrophysiology and subsequent pharmacology exhibited neuropathic signs; sham-operated did not. This is the first
Acknowledgements
E.M. is supported by the Wellcome Trust 4-year Neuroscience PhD programme.
References (46)
- et al.
Effects of systemic carbamazepine and gabapentin on spinal neuronal responses in spinal nerve ligated rats
Pain
(1998) - et al.
Specific petit mal anticonvulsants reduce calcium currents in thalamic neurons
Neurosci. Lett.
(1989) - et al.
Electrophysiological studies on the effects of intrathecal morphine on nociceptive neurones in the rat dorsal horn
Pain
(1986) - et al.
Neuroanatomical distribution of receptors for a novel voltage-sensitive calcium-channel antagonist, SNX-230 (omega-conopeptide MVIIC)
Brain Res.
(1994) - et al.
Spontaneous discharge originates in the dorsal root ganglion at the onset of a painful peripheral neuropathy in the rat
Neurosci. Lett.
(1992) - et al.
Autoradiographic localization of calcium channels with [125I]omega-conotoxin in rat brain
Eur. J. Pharmacol.
(1988) - et al.
Different action of ethosuximide on low- and high-threshold calcium currents in rat sensory neurons
Neuroscience
(1992) - et al.
P-type calcium channels in rat central and peripheral neurons
Neuron
(1992) - et al.
Effects of antagonists to high-threshold calcium channels upon spinal mechanisms of pain, hyperalgesia and allodynia
Pain
(2000) - et al.
Subunit interaction sites in voltage-dependent Ca2+ channels: role in channel function
Trends Neurosci.
(1998)
Sensory afferent impulses originate from dorsal root ganglia as well as from the periphery in normal and nerve injured rats
Pain
Effect of subcutaneous administration of calcium channel blockers on nerve injury-induced hyperalgesia
Brain Res.
Peripheral and central substrates involved in the rostrad transmission of nociceptive information
Pain
Ethical guidelines for investigations of experimental pain in conscious animals [editorial]
Pain
T-type calcium channel in mammalian CNS neurones
Comp. Biochem. Physiol. A. Physiol.
Depolarization elicits two distinct calcium currents in vertebrate sensory neurones
Pflugers Arch.—Eur. J. Physiol.
Selective N-type neuronal voltage-sensitive calcium channel blocker, SNX-111, produces spinal antinociception in rat models of acute, persistent and neuropathic pain
J. Pharmacol. Exp. Ther.
Use of intrathecal SNX-111, a novel, N-type, voltage-sensitive, calcium channel blocker, in the management of intractable brachial plexus avulsion pain
Clin. J. Pain
A low voltage-activated, fully inactivating Ca channel in vertebrate sensory neurones
Nature
Role of voltage-dependent calcium channel subtypes in experimental tactile allodynia
J. Pharmacol. Exp. Ther.
Efficacy of spinal NMDA receptor antagonism in formalin hyperalgesia and nerve injury evoked allodynia in the rat
J. Pharmacol. Exp. Ther.
Transient enhancement of low-threshold calcium current in thalamic relay neurons after corticectomy
J. Neurophysiol.
Characterization of ethosuximide reduction of low-threshold calcium current in thalamic neurons
Ann. Neurol.
Cited by (99)
Involvement of T-type calcium channels in the mechanism of low dose morphine-induced hyperalgesia in adult male rats
2021, NeuropeptidesCitation Excerpt :Matthew and collages also showed that intraperitoneal administration of the T-type Ca+2 channel blockers ethosuximide, trimethadione and mibefradil dose-dependently reverses capsaicininduced mechanical hyperalgesia. In addition, mibefradil reverses capsaicin-induced mechanical hyperalgesia (Matthews and Dickenson, 2001). It has been also reported that mibefradil exerts an anti-hyperalgesic response on formalin-induced paw flinching test (Takasusuki and Yaksh, 2011).
MicroRNA and chronic pain: From mechanisms to therapeutic potential
2017, Pharmacology and TherapeuticsExploring poisonous mechanism of honeybee, Apis mellifera ligustica Spinola, caused by pyrethroids
2017, Pesticide Biochemistry and PhysiologyCitation Excerpt :After cells were firmly adhered and incubated with Fura-2 AM, they were carefully rinsed twice with HBSS buffer solution, and added with 1 mL HBSS buffer solution containing 0.1% DMSO, and then the calcium ion ratio (F340/F380) was measured and recorded. After stable real-time Measurement was obtained (about 3–5 min), the culture vessel and cells were held still, and the extracellular fluid was replaced with HBSS buffer solution containing 50 μM DL-2-Amino-5-phosphonopentanoic acid (AP-5, NMDA receptor-gated calcium channel antagonist, Sigma-Aldrich,USA), which was specifically used to close the NMDA receptor-gated calcium channel [34–37], or 10 μM nimodipine (L-type calcium channel antagonist, Sigma-Aldrich, USA) buffer solution to close the L-type voltage-gated calcium channel [38–40] or ethosuximide (ETX, T-type calcium channel antagonist, Sigma-Aldrich,USA) buffer solution at a concentration of 50 μg/mL to close the T-type voltage-gated calcium channel [41–45] or 5 μM tetrodotoxin (TTX, a specific voltage-sensitive sodium channel blocker, Sigma-Aldrich, USA) buffer solution to close the voltage-dependent sodium channels. During these processes, the ratio (F340/F380) values were measured and recorded.
Central and peripheral contributions of T-type calcium channels in pain
2022, Molecular BrainThe T-Type Calcium Channel Cav3.2 in Somatostatin Interneurons in Spinal Dorsal Horn Participates in Mechanosensation and Mechanical Allodynia in Mice
2022, Frontiers in Cellular NeuroscienceVoltage-dependent Ca<inf>V</inf>3.2 and Ca<inf>V</inf>2.2 channels in nociceptive pathways
2022, Pflugers Archiv European Journal of Physiology