omega-agatoxin-iva and Pain

We have recently shown that injection of the P/Q-type (Ca(v)2.1/alpha(1A)) calcium channel blocker, omega-agatoxin IVA, into the periaqueductal gray (PAG) facilitates meningeal dural stimulation-evoked trigeminal nociceptive processing. We injected the GABA(A) antagonist bicuculline into the PAG in addition to the agatoxin and observed bicuculline's effect on neurons responding to dural stimulation recorded in the trigeminal nucleus caudalis of rats in order to determine if P/Q channel-mediated changes acted through GABAergic mechanisms. The inhibition of trigeminal nociceptive neurons characteristic of bicuculline administered into the PAG was maintained in the presence of blocked PAG P/Q-type calcium channels. This suggests the PAG descending pain modulatory pathway is not affected by P/Q-type calcium channel blockade at the postsynaptic GABAergic inhibitory interneuron and the facilitation produced by agatoxin is mediated by another mechanism. These findings have implications for disorders involving the PAG or P/Q-type channels, such as migraine, in particular for the development of preventative treatments, suggesting GABAergic and voltage-gated calcium channels could be separately modulated.

Topics: Animals; Bicuculline; Calcium Channel Blockers; Calcium Channels, P-Type; Calcium Channels, Q-Type; Dose-Response Relationship, Drug; Electrophysiology; Male; Microinjections; Migraine Disorders; Neural Inhibition; Neurons; Nociceptors; omega-Agatoxin IVA; Pain; Periaqueductal Gray; Rats; Rats, Sprague-Dawley; Reproducibility of Results; Sensitivity and Specificity; Trigeminal Caudal Nucleus

Although considerable evidence indicates neuronal Ca channels play significant roles in pain perception, their possible importance in hypersensitization after acute inflammation has not been investigated.. Using carrageenan for inducing hypersensitization, the authors investigated the analgesic effects of intrathecally administered N- and P/Q-type channel blockers, omega-conotoxin GVIA and omega-agatoxin IVA, respectively, and also examined the level of N-type channel expression.. Acute inflammation, produced by carrageenan injection in a rat hind paw, caused mechanical hypersensitivity that resolved within several days. Injection of prostaglandin E2 into the same hind paw after resolution caused a markedly prolonged mechanical allodynia lasting more than 4 h. Similar but less potent prolonged allodynia was also induced in the contralateral hind paws. Intrathecal administration of omega-conotoxin GVIA (0.03-0.3 microg) produced dose-dependent inhibition of the allodynia in both control and carrageenan-preconditioned rats. However, the potency of omega-conotoxin GVIA was significantly lower in carrageenan-preconditioned paws than in those in the contralateral and saline-preconditioned paws. In contrast, omega-agatoxin IVA (0.01-0.1 microg) did not reduce the allodynia. Significant up-regulation of N-type channel expression was observed in both dorsal root ganglia and the spinal cord ipsilateral to the carrageenan-preconditioned hind paw.. The results suggest an aggravating role of the N-type channel in pain sensation and a selective plastic change of this channel expression that could underlie the mechanism of hypersensitization after acute inflammation.

Topics: Animals; Calcium Channels, N-Type; Carrageenan; Dinoprostone; Dose-Response Relationship, Drug; Inflammation; Male; omega-Agatoxin IVA; omega-Conotoxin GVIA; Pain; Rats; Rats, Sprague-Dawley

The importance of voltage-dependent Ca2+ channels (VDCCs) in pain transmission has been noticed gradually, as several VDCC blockers have been shown to be effective in inhibiting this process. In particular, the N-type VDCC has attracted attention, because inhibitors of this channel are effective in various aspects of pain-related phenomena. To understand the genuine contribution of the N-type VDCC to the pain transmission system, we generated mice deficient in this channel by gene targeting. We report here that mice lacking N-type VDCCs show suppressed responses to a painful stimulus that induces inflammation and show markedly reduced symptoms of neuropathic pain, which is caused by nerve injury and is known to be difficult to treat by currently available therapeutic methods. This finding clearly demonstrates that the N-type VDCC is essential for development of neuropathic pain and, therefore, controlling the activity of this channel can be of great importance for the management of neuropathic pain.

Topics: Acetic Acid; Acoustic Stimulation; Animals; Anxiety; Behavior, Animal; Calcium Channel Blockers; Calcium Channels, N-Type; Formaldehyde; Ganglia, Spinal; Gene Targeting; Hyperalgesia; Mice; Mice, Knockout; Motor Activity; Neurons; Nociceptors; omega-Agatoxin IVA; Pain; Periaqueductal Gray; Spinal Nerves

High voltage calcium channels are implicated in nociceptive transmission after nerve injury, capsaicin or formalin injection. The purpose of this study was to investigate the role of calcium channels in secondary heat hyperalgesia associated with acute joint inflammation. After induction of acute inflammation (knee joint injection of kaolin and carrageenan), decreased paw withdrawal latency (PWL) to radiant heat (i.e., secondary heat hyperalgesia), increased guarding of the limb and increased joint circumference occurs. Spinal administration (through a microdialysis fiber placed in dorsal horn) of an N-type calcium channel blocker (MVIIA, SNX 111, ziconotide, 0.001-0.1 mM), before induction of inflammation, prevents the decrease in PWL. Treatment with SNX 111 4 hr after inflammation reverses heat hyperalgesia. A small reduction in spontaneous pain-related behaviors (guarding of the limb) occurs after pre- or post-treatment with SNX 111. Spinal blockade of P/Q-type calcium channels (with omega-agatoxin IVA) had no effect on the decrease in PWL to radiant heat when administered after induction of inflammation. However, pre-treatment with omega-agatoxin IVA prevents secondary heat hyperalgesia. omega-Agatoxin IVA has no effect on spontaneous pain-related behaviors whether administered before or after induction of inflammation. In contrast, pre or post-treatment with nifedipine (L-type calcium channel blocker, 0.01-1.0 mM), had no effect on heat hyperalgesia or spontaneous pain-related behaviors induced by acute inflammation. There were no differences in joint circumference between groups with any treatment. Thus, N-type calcium channels contribute to both the development and maintenance of secondary heat hyperalgesia while P-type calcium channels are only involved during development of hyperalgesia.

Topics: Animals; Arthritis; Calcium Channel Blockers; Calcium Channels; Hot Temperature; Hyperalgesia; Male; Microdialysis; Nifedipine; omega-Agatoxin IVA; omega-Conotoxins; Pain; Peptides; Rats; Rats, Sprague-Dawley; Spider Venoms

Intradermal capsaicin injection in humans results in primary hyperalgesia to heat and mechanical stimuli applied near the injection site, as well as secondary mechanical hyperalgesia (increased pain from noxious stimuli) and mechanical allodynia (pain from innocuous stimuli) in an area surrounding the site of primary hyperalgesia. This study in rats tested the hypothesis that the secondary hyperalgesia and allodynia observed following intradermal injection of capsaicin was dependent upon activation of voltage sensitive calcium channels in the spinal cord. Responses to application of von Frey filaments of 10 mN and 90 mN bending forces were tested in all rats before and after injection of capsaicin into the plantar surface of a hindpaw. Animals were pretreated with L-type (nifedipine), N-type (omega-conotoxin GVIA) or P-type (omega-agatoxin IVA) calcium channels blockers through a microdialysis fiber implanted in the spinal dorsal horn prior to the injection of capsaicin. None of the calcium channel blockers had any affect on normal sensory or motor responses. However, all three blockers dose dependently prevented the development of secondary mechanical hyperalgesia and allodynia. The threshold to mechanical stimulation with von Frey filaments was also increased significantly in animals treated with these calcium channel blockers when compared to articial cerebrospinal fluid control animals. These data suggest that calcium channels are important for the development of mechanical hyperalgesia and allodynia that occurs following capsaicin injection.

Topics: Animals; Behavior, Animal; Calcium Channel Blockers; Capsaicin; Hyperalgesia; Injections, Intradermal; Male; Microdialysis; Neurons; omega-Agatoxin IVA; omega-Conotoxin GVIA; Pain; Pain Threshold; Peptides; Physical Stimulation; Rats; Rats, Sprague-Dawley; Spider Venoms

High threshold voltage-dependent P- and Q-type calcium channels are involved in neurotransmitter release. In order to investigate the role of P- and Q-type calcium channels in the mechanosensory (nociceptive) processing in the spinal cord, their participation in the responses of spinal wide-dynamic-range neurons to innocuous and noxious mechanical stimulation of the knee and ankle joints was studied in 30 anaesthetized rats. The knee was either normal or acutely inflamed by kaolin/carrageenan. During the topical application of omega-agatoxin IVA (P-type channel antagonist, 0.1 microM) onto the dorsal surface of the spinal cord, the responses to innocuous and noxious pressure applied to the normal knee were increased to respectively 124 +/- 42% and 114 +/- 23% of predrug values (mean +/- SD, P < 0.05, 14 neurons). By contrast, in rats with an inflamed knee, the responses to innocuous and noxious pressure applied to the knee were reduced to respectively 72 +/- 19 and 73 +/- 22% of baseline (mean +/- SD, P < 0.01, 13 neurons). In the same neurons, omega-agatoxin IVA slightly increased the responses to pressure on the non-inflamed ankle whether the knee was normal or inflamed. Thus P-type calcium channels seem to acquire a predominant importance in the excitation of spinal cord neurons by mechanosensory input from inflamed tissue and hence in the generation of inflammatory pain. By contrast, the Q-type channel antagonist, omega-conotoxin MVIIC (1 or 100 microM), had no significant effect upon responses to innocuous or noxious pressure applied to either normal or inflamed knees (25 neurons).

Topics: Animals; Calcium Channel Blockers; Calcium Channels; Calcium Channels, N-Type; Electrophysiology; Inflammation; Joints; Male; Neurons; omega-Agatoxin IVA; Pain; Physical Stimulation; Rats; Rats, Wistar; Spider Venoms; Spinal Cord