Calcium Ions are Involved in Modulation of Melittin-induced Nociception in Rat: I. Effect of Voltage-gated Calcium Channel Antagonist

Melittin-induced nociceptive responses are mediated by selective activation of capsaicin-sensitive primary afferent fibers and are modulated by excitatory amino acid receptor, cyclooxygenase, protein kinase C and serotonin receptor. The present study was undertaken to investigate the peripheral and spinal actions of voltage-gated calcium channel antagonists on melittin-induced nociceptive responses. Changes in mechanical threshold and number of flinchings were measured after intraplantar (i.pl.) injection of melittin (30microg/paw) into mid-plantar area of hindpaw. L-type calcium channel antagonists, verapamil [intrathecal (i.t.), 6 or 12microg; i.pl.,100 & 200microg; i.p., 10 or 30 mg], N-type calcium channel blocker, omega-conotoxin GVIA (i.t., 0.1 or 0.5microg; i.pl., 5microg) and P-type calcium channel antagonist, omega-agatoxin IVA (i.t., 0.5microg; i.pl., 5microg) were administered 20 min before or 60 min after i.pl. injection of melittin. Intraplantar pre-treatment and i.t. pre- or post-treatment of verapamil and omega-conotoxin GVIA dose-dependently attenuated the reduction of mechanical threshold, and melittin-induced flinchings were inhibited by i.pl. or i.t. pre-treatment of both antagonists. P-type calcium channel blocker, omega-agatoxin IVA, had significant inhibitory action on flinching behaviors, but had a limited effect on melittin-induced decrease in mechanical threshold. These experimental findings suggest that verapamil and omega-conotoxin GVIA can inhibit the development and maintenance of melittin-induced nociceptive responses.

Nimodipine as a potential pharmacological tool for characterizing R-type calcium currents

Nimopidine, one of dihydropyridine derivatives, has been widely used to pharmacologically identify L-type Ca currents. In this study, it was tested if nimodipine is a selective blocker for L-type Ca currents in sensory neurons and heterologous system. In mouse dorsal root ganglion neurons (DRG), low concentrations of nimodipine (10 muM) further reduced the "residual" currents in DRG neurons from alpha1E knock-out mice, after blocking L-, N- and P/Q-type Ca currents with 10 muM nimodipine, 1 muM omega-conotoxin GVIA and 200 nM omega-agatoxin IVA, indicating inhibitory effects of nimodipine on R-type Ca currents. Nimodipine (>10 muM) also produced the inhibition of both low-voltage-activated calcium channel currents in DRG neurons and alpha1B and alpha1E subunit based Ca channel currents in heterologous system. These results suggest that higher nimodipine (>10 muM) is not necessarily selective for L-type Ca currents. While care should be taken in using nimodipine for pharmacologically defining L-type Ca currents from native macroscopic Ca currents, nimodipine (>10 muM) could be a useful pharmacological tool for characterizing R-type Ca currents when combined with toxins blocking other types of Ca channels.

Calcium channel blockers suppress the responses of rat dorsal horn cell to nociceptive input

Calcium ions are implicated in a variety of physiological functions, including enzyme activity, membrane excitability, neurotransmitter release, and synaptic transmission, etc. Calcium antagonists have been known to be effective for the treatment of exertional angina and essential hypertension. Selective and nonselective voltage-dependent calcium channel blockers also have inhibitory action on the acute and tonic pain behaviors resulting from thermal stimulation, subcutaneous formalin injection and nerve injury. This study was undertaken to investigate the effects of iontophoretically applied Ca++ and its antagonists on the responses of WDR (wide dynamic range) cells to sensory inputs. The responses of WDR cells to graded electrical stimulation of the afferent nerve and also to thermal stimulation of the receptive field were recorded before and after iontophoretical application of Ca++, EGTA, Mn++, verapamil, omega-conotoxin GVIA, omega-conotoxin MVIIC and omega-agatoxin IVA. Also studied were the effects of a few calcium antagonists on the C-fiber responses of WDR cells sensitized by subcutaneous injection of mustard oil (10%). Calcium ions and calcium channel antagonists (Mn++, verapamil, omega-conotoxin GVIA & omega-agatoxin IVA) current-dependently suppressed the C-fiber responses of WDR cells without any significant effects on the A-fiber responses. But omega-conotoxin MVIIC did not have any inhibitory actions on the responses of WDR cell to A-fiber, C-fiber and thermal stimulation. Iontophoretically applied EGTA augmented the WDR cell responses to C-fiber and thermal stimulations while spinal application of EGTA for about 20 ~ 30 min strongly inhibited the C-fiber responses. The augmenting and the inhibitory actions of EGTA were blocked by calcium ions. The WDR cell responses to thermal stimulation of the receptive field were reduced by imtophoretical application of Ca++, verapamil, omega -agatoxin IVA, and omega-conotoxin GVIA but not by omega-conotoxin MVIIC. The responses of WDR cells to C-fiber stimulation were augmented after subcutaneous injection of mustard oil (10%, 0.15 ml) into the receptive field and these sensitized C-fiber responses were strongly suppressed by iontophoretically applied Ca++, verapamil, omega-conotoxin GVIA and omega-agatoxin IVA. These experimental findings suggest that in the rat spinal cord, L-, N-, and P-type, but not Q-type, voltage-sensitive calcium channels are implicated in the calcium antagonist-induced inhibition of the normal and the sensitized responses of WDR cells to C-fiber and thermal stimulation, and that the suppressive effect of calcium and augmenting action of EGTA on WDR cell responses are due to changes in excitability of the cell.