Influence of omega-conotoxin on morphine analgesia and withdrawal syndrome in rats

Blocking the R-type (Cav2.3) Ca2+ channel enhanced morphine analgesia and reduced morphine tolerance

Morphine is the drug of choice to treat intractable pain, although prolonged administration often causes undesirable side-effects including analgesic tolerance. It is speculated that voltage-dependent Ca(2+) channels (VDCCs) play a key role in morphine analgesia and tolerance. To examine the subtype specificity of VDCCs in these processes, we analysed mice lacking N-type (Ca(v)2.2) or R-type (Ca(v)2.3) VDCCs. Systemic morphine administration or exposure to warm water swim-stress, known to induce endogenous opioid release, resulted in greater analgesia in Ca(v)2.3(-/-) mice than in controls. Moreover, Ca(v)2.3(-/-) mice showed resistance to morphine tolerance. In contrast, Ca(v)2.2(-/-) mice showed similar levels of analgesia and tolerance to control mice. Intracerebroventricular (i.c.v.) but not intrathecal (i.t.) administration of morphine reproduced the result of systemic morphine in Ca(v)2.3(-/-) mice. Furthermore, i.c.v. administration of an R-type channel blocker potentiated morphine analgesia in wild-type mice. Thus, the inhibition of R-type Ca(2+) current could lead to high-efficiency opioid therapy without tolerance.

The role of T-type calcium channels in morphine analgesia, development of antinociceptive tolerance and dependence to morphine, and morphine abstinence syndrome

Involvement of T-type voltage dependent Ca2+ channels (VDCCs) on morphine antinociception, in the development of tolerance and dependence to morphine, and naloxone-precipitated abstinence syndrome in morphine dependent mice was examined by using mibefradil, a T-type VDCCs blocker. Mice were rendered tolerant and dependent on morphine by subcutaneous (s.c.) implantation of a morphine pellet containing 75 mg of morphine base for 72 hr. The tail-flick test was used to assess the nociceptive threshold. Coadministration of acute mibefradil (10 mg/kg, i.p.) with morphine enhanced the antinociceptive effects of acute morphine. Repeated mibefradil administration (10 mg/kg, i.p., just before, 24 and 48 hr after morphine pellet implantation) completely blocked the development of tolerance to the antinociceptive effect of morphine and even by this effect reached supersensitivity to morphine. However, repeated mibefradil treatment did not alter the development of dependence to morphine assessed by the A(50) values of naloxone (s.c.) required to precipitate withdrawal jumping 72 hr after morphine pellet. But, acute mibefradil (10, 30, and 50 mg/kg, i.p.) dose dependently decreased the expression of morphine abstinence syndrome when given directly 30 min prior to naloxone (0,05 mg/kg, s.c.) 72 hr after morphine pellet. These results indicate a critical role of T-type VDCCs in morphine antinociception, the development of tolerance to the antinociceptive effects of morphine and in morphine abstinence syndrome.

Role of calcium channels in the spinal transmission of nociceptive information from the mesentery

Opioids, alpha(2)-adrenoceptor agonists and blockers of voltage-gated calcium channels (VGCCs) have been attributed antinociceptive activity in various experimental set-ups. The present study tested the ability of morphine, clonidine and drugs acting at various VGCCs to inhibit the transmission of noxious stimuli from the mesentery at the level of the spinal cord. In rats under barbiturate anaesthesia traction of 20 g was applied to a bundle of mesenteric blood vessels. This caused immediate transient changes of mean arterial pressure that were taken as indication of nociception. Similar reflexes were elicited by applying 0.6% acetic acid to the same bundle of vessels. The reflexes were dose-dependently reduced by intrathecal administration of morphine or clonidine, but were left unaltered by intrathecal administration of verapamil, Bay-K 8644 or omega-conotoxin MVIIA. Neither verapamil nor Bay-K 8644 influenced clonidine-induced analgesia. Conotoxin markedly enhanced the effectiveness of all doses of clonidine against both types of mesenteric stimuli. Verapamil, Bay-K 8644, as well as conotoxin reduced the ability of morphine to inhibit mechanically evoked reflexes, while there was no statistically significant effect in chemonociception. These data suggest that, at the spinal level, both morphine and clonidine are effective drugs to decrease the cardiovascular changes caused by acute mesenteric pain. In the dorsal spinal cord neither L-type nor N-type VGCCs are responsible on their own for the transmission of noxious stimuli from the mesentery. Inhibition of N-type channels markedly augments the action of clonidine, whereas blocking either VGCC seems to inhibit antinociceptive mechanisms induced by morphine. It is suggested that in patients the combined administration of clonidine with omega-conotoxin MVIIA might lead to effective pain control with reduced side effects.

Cholera and pertussis toxins inhibit differently hypothermic and anti-opioid effects of neuropeptide FF

In mice pretreated intracerebroventricularly (i.c.v.) with pertussis or cholera toxins, effects of neuropeptide FF (NPFF), on hypothermia and morphine-induced analgesia, were assessed. NPFF and a potent NPFF agonist, 1DMe (0.005-22 nmol) injected into the lateral ventricle decreased morphine analgesia and produced naloxone (2.5 mg x kg(-1), s.c.)-resistant hypothermia after administration into the third ventricle. Cholera toxin (CTX 1 microg, i.c.v.) pretreatment (24 or 96 h before) inhibited the effect of 1DMe on body temperature, but failed to reverse its anti-opioid activity in the tail-flick test. CTX reduced hypothermia induced by a high dose of morphine (8 nmol, i.c.v.) but not the analgesic effect due to 3 nmol morphine. Pertussis toxin (PTX) pretreatment inhibited both morphine-hypothermia and -analgesia but did not modify hypothermia induced by 1DMe. The present results suggest that NPFF-induced hypothermia depends on the stimulation of Gs (but not Gi) proteins. In contrast, anti-opioid effects resulting from NPFF-receptor stimulation do not involve a cholera toxin-sensitive transducer protein.

Involvement of calcium in pain and antinociception

Calcium ions are widely recognized to play a fundamental role in the regulation of several biological processes. Transient changes in cytoplasmic calcium ion concentration represent a key step for neurotransmitter release and the modulation of cell membrane excitability. Evidence has accumulated for the involvement of calcium ions also in nociception and antinociception, including the analgesic effects produced by opioids. The combination of opioids with drugs able to interfere with calcium ion functions in neurons has been pointed out as a useful alternative for safer clinical pain management. Alternatively, drugs that reduce the flux of calcium ions into neurons have been indicated as analgesic alternatives to opioids. This article reviews the manners by which calcium ions penetrate cell membranes and the changes in these mechanisms caused by opioids and calcium antagonists regarding nociceptive and antinociceptive events.

The combined effects of N-type calcium channel blockers and morphine on A delta versus C fiber mediated nociception

Intrathecal mu opiates produce analgesia presynaptically by inhibiting calcium ion influx and postsynaptically by increasing potassium flux. Mu receptors are expressed on presynaptic terminals of unmyelinated (C), but not myelinated (A delta) nociceptors. Thus, mu-opioids such as morphine may act presynaptically to inhibit C, but not A delta, neurotransmission, and postsynaptically on dorsal horn cells that receive input from A delta and/or C fiber nociceptors. N-type calcium ion channel blockers, such as omega-conotoxin GVIA (omega-CTX), produce analgesia by impeding flux of calcium ions into A delta and C fiber nociceptor terminals. Thus, morphine and omega-CTX attenuated C fiber nociception additively, possibly indicating the same presynaptic site of action. Conversely, morphine and omega- CTX were supraadditively analgesic on an A delta test, indicating that these agents probably have different sites of action. We conclude that although intrathecal application of either morphine or omega-CTX attenuates both A delta and C fiber mediated nociception in rats, the combined effects are quite different for the two fiber types. Specifically, although coadministration of morphine with omega-CTX produces an additive, apparently presynaptic antinociception for C fiber-mediated responses, the combination produces a clearly supraadditive, and likely synergistic effect on A delta mediated nociception, probably by acting at pre and postsynaptic sites, respectively.

IMPLICATIONS

This study demonstrates that combined spinal administration of mu opioids and N-type calcium channel blockers may be useful in providing analgesia for A delta mediated (first, sharp) pain while minimizing the side effects of both drugs.

Interactions of intrathecally administered ziconotide, a selective blocker of neuronal N-type voltage-sensitive calcium channels, with morphine on nociception in rats

Ziconotide is a selective, potent and reversible blocker of neuronal N-type voltage-sensitive calcium channels (VSCCs). Morphine is an agonist of mu-opioid receptors and inhibits N-type VSCC channels via a G-protein coupling mechanism. Both agents are antinociceptive when they are administered intrathecally (spinally). The present study investigated the acute and chronic (7-day) interactions of intrathecally administered ziconotide and morphine on nociception in several animal models of pain. In the acute study, intrathecal bolus injections of morphine and ziconotide alone produced dose-dependent inhibition of formalin-induced tonic flinch responses and withdrawal responses to paw pressure. The combination of ziconotide and morphine produced an additive inhibition of formalin-induced tonic flinch responses and a significant leftward shift of the morphine dose-response curve in the paw pressure test. After chronic (7-day) intrathecal infusion, ziconotide enhanced morphine analgesia in the formalin test. In contrast, chronic intrathecal morphine infusion produced tolerance to analgesia, but did not affect ziconotide antinociception. Antinociception produced by ziconotide alone was the same as that observed when the compound was co-administered with morphine to morphine-tolerant rats. In the hot-plate and tail immersion tests, chronic intrathecal infusion of morphine lead to rapid tolerance whereas ziconotide produced sustained analgesia with no loss of potency throughout the infusion period. Although ziconotide in combination with morphine produced an apparent synergistic analgesic effects during the initial phase of continuous infusion, it did not prevent morphine tolerance to analgesia. These results demonstrate that (1) acute intrathecal administrations of ziconotide and morphine produce additive or synergistic analgesic effects; (2) chronic intrathecal morphine infusion results in tolerance to analgesia but does not produce cross-tolerance to ziconotide; (3) chronic intrathecal ziconotide administration produces neither tolerance nor cross-tolerance to morphine analgesia; (4) intrathecal ziconotide does not prevent or reverse morphine tolerance.

Role of intracellular calcium on the modulation of naloxone-precipitated withdrawal jumping in morphine-dependent mice by diabetes

The role of intracellular calcium in the modifications of naloxone-precipitated withdrawal jumping in morphine-dependent mice by diabetes was examined. Naloxone-precipitated withdrawal jumping was significantly less in morphine-dependent diabetic mice than in morphine-dependent non-diabetic mice. Intracerebroventricular (i.c.v. ) pretreatment with ryanodine attenuated naloxone-precipitated withdrawal jumping in morphine-dependent non-diabetic mice. However, naloxone-precipitated withdrawal jumping in morphine-dependent diabetic mice was not affected by i.c.v. pretreatment with ryanodine. Moreover, i.c.v. pretreatment with thapsigargin, a Ca2+-ATPase inhibitor, enhanced naloxone-precipitated withdrawal jumping in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice. The noradrenaline (NA) turnover in the frontal cortex in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice, was significantly increased by naloxone injection. Naloxone-induced enhancement of NA turnover in morphine-dependent non-diabetic mice, but not in morphine-dependent diabetic mice, was blocked by i.c.v. pretreatment with ryanodine. In contrast to ryanodine, thapsigargin enhanced naloxone-induced enhancement of NA turnover in morphine-dependent non-diabetic mice. These results suggest that increased intracellular calcium augmented naloxone-precipitated withdrawal jumping and the turnover rate of NA in the frontal cortex in morphine-dependent non-diabetic mice. Furthermore, it seems likely that the attenuation of naloxone-precipitated withdrawal jumping in morphine-dependent diabetic mice may be due, in part, to the dysfunction of intracellular calcium store.

Effects of intrathecally administered aminoglycoside antibiotics, calcium-channel blockers, nickel and calcium on acetic acid-induced writhing test in mice

1. Antinociceptive effects of intrathecally administered aminoglycoside antibiotics, calcium-channel blockers, nickel and calcium ions on the acetic acid-induced writhing test in mice were examined. 2. Neomycin (0.5-20.0 micrograms/mouse) gentamicin (5-40 micrograms/mouse), nicardipine, diltiazem and verapamil (0.5-80.0 micrograms/mouse) and calcium ions (0.02-1.0 mumol/mouse) exerted a dose-dependent antinociceptive activity on the acetic acid-induced writhing test. Nickel ions (2.5, 5.0 and 10.0 mumol/mouse) were found ineffective in this test. 3. These results suggest that N- and L-type, but not T-type, voltage-dependent calcium channels are implicated in the spinal processing of nociceptive information.

Omega-agatoxin IVA blocks spinal morphine/clonidine antinociceptive synergism

Involvement of P-type voltage-dependent Ca2+ channels in spinal morphine- or clonidine-induced antinociception and in the synergistic interaction between morphine and clonidine was examined in the present studies. Coadministration of the selective P-type antagonist, omega-agatoxin IVA (25 ng) intrathecally (i.t.) to mice along with morphine or clonidine enhanced the tail flick antinociception of each agonist 5-6-fold. The greater-than-additive (synergistic) interaction that occurred when morphine and clonidine were coadministered i.t. decreased to an additive interaction in the presence of omega-agatoxin IVA. In mice pretreated with pertussis toxin (10 ng) to inactivate G proteins, omega-agatoxin IVA did not alter the morphine/clonidine synergism. Surprisingly, omega-agatoxin IVA reversed the additive morphine/clonidine interaction that occurs in morphine-tolerant mice back to synergism. These results suggest that functional P-type Ca2+ channels play an essential role in the antinociceptive synergism between spinal morphine and clonidine.

Behavioral and analgesic effects induced by administration of nifedipine and nimodipine

Evidence exists that calcium antagonists can have effects on neural function. The aim of this work is to analyze the effect of two dihydropyridines, nifedipine and nimodipine, administered for 11 days on the behavior and pain sensitivity of rats. Nociception was tested using the tail electric stimulation test, and behavior parameters using a holeboard. Our results show that chronic administration of nifedipine or nimodipine induces analgesia that can be evaluated by tail withdrawal. However, neither the vocalization nor the vocalization after discharge were modified, so the analgesia may be mediated by spinal mechanisms. Rats treated with nifedipine or nimodipine exhibited a dose-dependent tendency to avoid the center of the field without modification of other parameters, suggesting an increased emotivity in the rats. This conclusion is supported by the fact that anxiogenic or anxiolytic drugs modify the pattern of locomotion without significant changes in other parameters related with the motility. The results from this study suggest the view of a complex mechanism of action underlying nifedipine- and nimodipine-mediated behavioral effects.

Effect of omega-conotoxin and verapamil on antinociceptive, behavioural and thermoregulatory responses to opioids in the rat

This study with the rat evaluated the contribution of omega-conotoxin GVIA-(omega-CgTx) and verapamil-sensitive Ca2+ channels in behavioural, antinociceptive and thermoregulatory responses to intracerebroventricular (i.c.v.) injection of [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAMGO), [D-Pen2,D-Pen5]enkephalin (DPDPE) and dynorphin A-(1-17), which are selective agonists for putative mu, delta and kappa-opioid receptors, respectively. The rats treated with omega-CgTx (8-32 pmol i.c.v.) showed transient, dose-dependent shaking behaviour, hyperalgesia and hypothermia which gradually disappeared within 4 h. The behaviour of the rats was normal by 24 h. Histological examination of brain sections showed morphological alterations of neurons in the hippocampus, medial-basal hypothalamus and pyriform cortex. antinociception, catalepsy and thermoregulatory responses elicited by DAMGO (0.4 and 2.0 nmol) were significantly prolonged and potentiated by verapamil (20 pmol i.c.v. 15 min before) or omega-CgTx (8 pmol 24 h before). Antinociception and hypothermia induced by DPDPE were antagonized by verapamil and omega-CgTx, whereas only omega-CgTx prevented the behavioural arousal observed after DPDPE. Similarly, hypothermia induced by dynorphin A-(1-17) (5.0 nmol) and by the kappa-opioid receptor agonist U50,488H (215 nmol) was antagonized by the two Ca2+ channel blockers but only omega-CgTx prevented the barrel rolling and bizarre postures caused by the opioid peptide.