ATP-dependent K+ channel blockers antagonize morphine- but not U-504,88H-induced antinociception

The antihyperalgesic effect of docosahexaenoic acid in streptozotocin-induced neuropathic pain in the rat involves the opioidergic system

Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid that has shown an antinociceptive effect in multiple pain models, such as inflammatory and neuropathic pain by chronic constriction injury in rats; however, its mechanism of action is still not well-understood. Reports suggest that DHA activates opioid signaling, but there is no information on this from a model of neuropathic pain. As a result, the aims of this study were (1) to determine the antihyperalgesic and antiallodynic effect of peripheral DHA administration, and (2) to evaluate the participation of the opioid receptors in the antihyperalgesic effect of DHA on streptozotocin-induced neuropathic pain in the rat. Female Wistar rats were injected with streptozotocin (50 mg/kg, i.p.) to induce hyperglycemia. The formalin, Hargreaves, and von Frey filaments tests were used to assess the nociceptive activity. Intraplantar administration of DHA (100-1000 μg/paw) or gabapentin (562-1778 μg/paw) decreased formalin-evoked hyperalgesia in diabetic rats, in a dose-dependent manner. Furthermore, DHA (562 μg/paw) and gabapentin (1000 μg/paw) reduced thermal hyperalgesia and allodynia. Local peripheral administration of naloxone (non-selective opioid receptor antagonist; 100 μg/paw), naltrindole (selective δ receptor antagonist; 1 μg/paw), and CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2, μ receptor antagonist; 20 μg/paw) prevented formalin-evoked hyperalgesia in diabetic rats but not by GNTI (guanidinonaltrindole, κ receptor antagonist;1 µg/paw). It is suggested that peripheral DHA shows an antihyperalgesic effect in neuropathic pain in the rat. Furthermore, δ and μ receptors are involved in the antihyperalgesic peripheral effect of DHA in diabetic rats.

Role of L-arginine/NO/cGMP/KATP channel signaling pathway in the central and peripheral antinociceptive effect of thymoquinone in rats

Objective(s):

Growing evidence demonstrates that L-arginine/NO/cGMP/K_ATP channel pathway has a modulatory role in pain perception. Previous studies have shown that thymoquinone exerts antinociceptive effects; however, the mechanisms underlying antinociception induced by thymoquinone have not been fully clarified. The aim of the present study was to evaluate the role of L-arginine/NO/cGMP/K_ATP channel pathway in the central and peripheral antinociceptive effect of thymoquinone in rats.

Materials and Methods:

Rats were pretreated intraplantarly (IPL) or intracerebroventricularly (ICV) with L-arginine (the NO precursor), l-NAME (an NO synthase inhibitor), SNAP (an NO donor), methylene blue (a guanylyl cyclase inhibitor), glibenclamide (the blocker of K_ATP channel), and tetraethylammonium (TEA, a K_v channel blocker) before the injection of thymoquinone.

Results:

Local ipsilateral (20 and 40 μg, IPL) but not contralateral and ICV (4 and 8 μg) administration of thymoquinone caused a dose-dependent and significant antinociception in both early and late phases of the formalin test. Pretreatment of rats with L-arginine (100 μg, IPL or ICV) and SNAP (200 μg, IPL or ICV) increased while l-NAME (100 μg, IPL or 1 μg, ICV) and methylene blue (400 μg, IPL or ICV) decreased the antinociceptive effects of thymoquinone in the formalin test. The administration of TEA (IPL or ICV) did not modify but glibenclamide (50 μg, IPL or ICV) significantly abolished the peripheral and central antinociceptive effects of thymoquinone in both phases of the formalin test.

Conclusion:

The results of the present study indicate that L-arginine/NO/cGMP/K_ATP channel pathway participates in the central and peripheral antinociceptive effect of thymoquinone.

Lysophosphatidic acid induces anxiety-like behavior via its receptors in mice

Lysophosphatidic acid (LPA) is a potent bioactive lipid mediator with diverse biological properties. We previously found altered expression of the LPA-related genes in rodents after treatment with sertraline, which is widely used to treat anxiety disorders and depression. However, little is known about the behavioral effects of LPA. In the present study, we investigated the behavioral effects of intracerebroventricular injection of LPA in adult mice. LPA did not significantly affect spontaneous locomotor activity, suggesting that LPA does not induce hyperactivity, ataxia, or sedation. We next investigated the emotional effects of LPA via the hole-board test. LPA significantly increased the number of head-dips in a dose- and time-related manner. A significant induction of head-dip counts occurred 15 and 30 min after LPA administration. To clarify the involvement of LPA receptors, we examined the effect of the non-selective LPA1-4 receptor antagonist, 1-bromo-3(S)-hydroxy-4-(palmitoyloxy)butyl-phosphonate (BrP-LPA) co-administered with LPA. BrP-LPA dose-dependently inhibited LPA-induced head-dip counts. We next investigated anxiety-like behavior via the elevated plus-maze test. LPA significantly reduced the percentage of time spent in the open arms and BrP-LPA dose-dependently inhibited this anxiety-like behavior. In conclusion, LPA induced anxiety-like behavior in mice via LPA receptors. Our results suggest that LPA signaling plays an important role in regulating anxiety in mice.

Flupirtine, a re-discovered drug, revisited

Flupirtine was developed long before K(V)7 (KCNQ) channels were known. However, it was clear from the beginning that flupirtine is neither an opioid nor a nonsteroidal anti-inflammatory analgesic. Its unique muscle relaxing activity was discovered by serendipity. In the meantime, broad and intensive research has resulted in a partial clarification of its mode of action. Flupirtine is the first therapeutically used K(V)7 channel activator with additional GABA(A)ergic mechanisms and thus the first representative of a novel class of analgesics. The presently accepted main mode of its action, potassium K(V)7 (KCNQ) channel activation, opens a series of further therapeutic possibilities. One of them has now been realized: its back-up compound, the bioisostere retigabine, has been approved for the treatment of epilepsy.

Sex differences in the contribution of ATP-sensitive K+ channels in trigeminal ganglia under an acute muscle pain condition

In this study, we examined whether functional subunits of the ATP-dependent K+ channel (KATP) are expressed in trigeminal ganglia (TG), which contains sensory neurons that innervate oral and facial structures. We also investigated whether direct activation of the KATP effectively attenuates mechanical hypersensitivity in the context of an acute orofacial muscle pain condition. The KATP expression in TG and behavioral studies were conducted in age matched male and female Sprague-Dawley rats. RT-PCR experiments showed that the mRNAs for the inwardly rectifying pore-forming subunits, Kir6.1 and Kir6.2, as well as the regulatory sulfonylurea subunits, SUR1 and SUR2, were reliably detected in TG. Subsequent western blot analysis confirmed that proteins for all four subunits are expressed in TG, and showed that Kir6.2 is expressed at a significantly higher level in male TG compared to that of female rats. This observation was confirmed by the immunohistochemical demonstration of higher percentages of Kir6 positive masseter afferents in female rats. Masseteric injection of capsaicin evokes a time dependent increase in masseter sensitivity to noxious mechanical stimulation. A specific KATP agonist, pinacidil, dose-dependently attenuated the capsaicin-induced mechanical hypersensitivity in male rats. The dose of pinacidil (20 μg) that completely blocked the capsaicin responses in male rats was ineffective in female rats regardless of their estrus phases. Only at the highest dose (300 μg) we used, pinacidil was partially effective in female rats. Similarly, another KATP agonist, diazoxide which targets different KATP subunits also showed sex specific responses in attenuating capsaicin-induced masseter hypersensitivity. These data suggested that sex differences in functional KATP expression in TG may underlie sex specific responses to KATP agonists. The present study provided novel information on sex differences in KATP expression in TG and its contribution under an orofacial muscle pain condition.

The acute antinociceptive effect of HBO₂ is mediated by a NO-cyclic GMP-PKG-KATP channel pathway in mice

Previous research has found that hyperbaric oxygen (HBO(2)) produces an acute antinociceptive effect that is dependent on nitric oxide (NO). The present study was undertaken to determine whether HBO(2)-induced acute antinociception might involve a NO-cyclic GMP-protein kinase G-ATP-sensitive potassium (K(ATP)) channel pathway. Male NIH Swiss mice were subjected to a 5-min HBO(2) treatment (100% oxygen at 3.5 absolute atmospheres) and antinociception was assessed over the next 6 min still under HBO(2) using the acetic acid abdominal constriction test. Pretreatment with 2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3-oxide (carboxy-PTIO, an NO scavenger), 1H-[1,2,4]-oxadiazolo-[4,3-a]quinoxalin-1-one) (a soluble guanylyl cyclase-inhibitor, Rp-8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphorothioate (a protein kinase G-inhibitor) or glibenclamide (an ATP-sensitive potassium channel-inhibitor) all led to antagonism of the HBO(2)-induced acute antinociception in a dose-dependent manner. These findings suggest that HBO(2)-induced acute antinociception might be due to activation of a NO-cyclic GMP-protein kinase G-K(ATP) channel pathway.

Intrathecal Nicorandil and Small-Dose Morphine Can Induce Spastic Paraparesis After a Noninjurious Interval of Spinal Cord Ischemia in the Rat

We investigated the interaction between nicorandil, a K(+)ATP channel opener, and morphine on motor function after a noninjurious interval of spinal cord ischemia in the rat. Spinal ischemia was induced by aortic occlusion for 6 min with a balloon catheter in Sprague-Dawley rats. All animals received intrathecal (IT) injection of morphine (1-60 microg) 1 h after ischemia. In addition to IT injection of morphine, group M (control), group MN (combination of morphine and nicorandil), and group MNG (combination of morphine, nicorandil, and glibenclamide) received IT saline, nicorandil (10 microg), and both glibenclamide (10 microg) and nicorandil (10 microg) after 150 min of reperfusion, respectively. A quantal bioassay for the effect of IT morphine on neurological function after ischemia was performed to calculate 50% effective dose values (ED50) for inducing paraparesis at 3 h of reperfusion. The ED50 in group M and group MN was 15.1 +/- 4.9 microg and 2.9 +/- 1.0 microg of IT morphine, respectively (P < 0.05). In Group MNG, the dose-response curve shifted back to the right and the ED50 for inducing paraparesis was 11.6 +/- 4.7 microg of IT morphine. The present study demonstrates that IT small-dose morphine combined with nicorandil induces spastic paraparesis after noninjurious interval of spinal cord ischemia in the rat.

Potassium channels and pain: present realities and future opportunities

Four families of potassium channels with different structures, functional characteristics and pharmacological sensitivity, are distinguished in neurons: voltage-gated (K(v)), calcium-activated (K(Ca)), inward rectifier (K(ir)) and two-pore (K(2P)) K(+) channels. During the last 15 years, numerous studies have demonstrated that the opening of some of these K(+) channels plays an important role in the antinociception induced by agonists of many G-protein-coupled receptors (alpha(2)-adrenoceptors, opioid, GABA(B), muscarinic M(2), adenosine A(1), serotonin 5-HT(1A) and cannabinoid receptors), as well as by other antinociceptive drugs (nonsteroidal antiinflammatory drugs [NSAIDs], tricyclic antidepressants, etc.) and natural products. Several specific types of K(+) channels are involved in antinociception. The most widely studied are the ATP-sensitive K(+) channels (K(ATP)), members of the K(ir) family, which participate in the antinociception induced by many drugs that activate them in both the central and the peripheral nervous system. The opening of G-protein-regulated inwardly rectifying K(+) channels (GIRK or K(ir)3), K(v)1.1 and two types of K(Ca) channels, the small- and large-conductance calcium-activated K(+) channels (SK and BK channels, respectively), also play a role in the antinociceptive effect of different drugs and natural products. Recently, drugs that open K(+) channels by direct activation (such as openers of neuronal K(v)7 and K(ATP) channels) have been shown to produce antinociception in models of acute and chronic pain, which suggests that other neuronal K(+) channels (e.g. K(v)1.4 channels) may represent an interesting target for the development of new K(+) channel openers with antinociceptive effects.

Study of the involvement of K+ channels in the peripheral antinociception of the kappa-opioid receptor agonist bremazocine

The involvement of the nitric oxide (NO)/cyclic GMP pathway in the molecular mechanisms of antinociceptive drugs like morphine has been previously shown by our group. Additionally, it is known that the desensitisation of nociceptors by K(+) channel opening should be the final target for several analgesic drugs including nitric oxide donors and exogenous micro-opioid receptor agonists. In our previous study, we demonstrated that bremazocine, a kappa-opioid receptor agonist, induces peripheral antinociception by activating nitric oxide/cyclic GMP pathway. In the current study, we assessed whether bremazocine is capable to activate K(+) channels eliciting antinociception. Bremazocine (20, 40 and 50 microg) dose-dependently reversed the hyperalgesia induced in the rat paw by local injection of carrageenan (250 microg) or prostaglandin E(2) (2 microg), measured by the paw pressure test. Using the selective kappa-opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 200 microg/paw), it was confirmed that bremazocine (50 microg/paw) acts specifically on the kappa-opioid receptors present at peripheral sites. Prior treatment with the ATP-sensitive K(+) channel blockers glibenclamide (40, 80 and 160 microg) and tolbutamide (40, 80 and 160 microg) did not antagonise the antinociceptive effect of bremazocine (50 microg). The same results were obtained when we used prostaglandin E(2) (2 microg) as the hyperalgesic stimulus. The supposed participation of other types of K(+) channels was tested using the Ca(2+)-activated K(+) channel blockers dequalinium (12.5, 25 and 50 microg) and charybdotoxin (0.5, 1 and 2 microg) and different types of the non-selective K(+) channel blockers tetraethylammonium (25, 50 and 100 microg) and 4-aminopyridine (10, 25 and 50 microg). None of the K(+) channel blockers reversed the antinociceptive effect of bremazocine. On the basis of these results, we suggest that K(+) channels are not involved in the peripheral antinociceptive effect of bremazocine, although this opioid receptor agonist induces nitric oxide/cGMP pathway activation.

Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of alpha 2 adrenoceptors and GABAB but not kappa-opioid receptors in the tail flick test in mice

We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the mu-opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein-coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25-4 mg/kg), baclofen (2-32 mg/kg) or U50,488H (2-16 mg/kg) (agonists of alpha(2) adrenoceptors, GABA(B) and kappa-opioid receptors, respectively) produced dose-dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose-dependent way by the protein phosphatase inhibitors okadaic acid (0.001-10 pg/mouse, i.c.v.) and cantharidin (0.001-10 ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L-norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine- or baclofen-induced antinociception over the wide range of doses used (0.001-1000 pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of kappa-opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein-coupled receptors in mice.

Role of Na(+), K(+)-ATPase in morphine-induced antinociception

We evaluated the modulation by Na+,K+-ATPase inhibitors of morphine-induced antinociception in the tail-flick test and [3H]naloxone binding to forebrain membranes. The antinociception induced by morphine (1-32 mg/kg, s.c.) in mice was dose-dependently antagonized by ouabain (1-10 ng/mouse, i.c.v.), which produced a significant shift to the right of the morphine dose-response curve. The i.c.v. administration of three Na+,K+-ATPase inhibitors (ouabain at 0.1-100, digoxin at 1-1000, and digitoxin at 10-10000 ng/mouse) dose-dependently antagonized the antinociceptive effect of morphine (4 mg/kg, s.c.) in mice, with the following order of potency: ouabain > digoxin > digitoxin. This effect cannot be explained by any interaction at opioid receptors, since none of these Na+,K+-ATPase inhibitors displaced [3H]naloxone from its binding sites, whereas naloxone did so in a concentration-dependent manner. The antinociception induced by morphine (5 mg/kg, s.c.) in rats was antagonized by the i.c.v. administration of ouabain at 10 ng/rat, whereas it was not significantly modified by intrathecally administered ouabain (10 and 100 ng/rat). These results suggest that the activation of Na+,K+-ATPase plays a role in the supraspinal, but not spinal, antinociceptive effect of morphine.

Effects of serine/threonine protein phosphatase inhibitors on morphine-induced antinociception in the tail flick test in mice

The aim of this study was to evaluate the effects of serine/threonine protein phosphatase (PP) inhibitors on morphine-induced antinociception in the tail flick test in mice, and on [3H]naloxone binding to the forebrain crude synaptosome fraction. Neither okadaic acid nor cantharidin (1-10000 nM) displaced [3H]naloxone from its specific binding sites, which indicates that they do not interact at the opioid receptor level. The i.c.v. administration of very low doses of okadaic acid (0.001-1 pg/mouse) and cantharidin (0.001-1 ng/mouse), which inhibit PP2A, produced a dose-dependent antagonism of the antinociception induced by morphine (s.c.). However, L-nor-okadaone (0.001 pg/mouse-1 ng/mouse, i.c.v.), an analogue of okadaic acid lacking activity against protein phosphatases, did not affect the antinociceptive effect of morphine. On the other hand, high doses of okadaic acid (10 ng/mouse, i.c.v.) and cantharidin (1 microg/mouse, i.c.v.), which also block PP1, and calyculin-A (0.1 fg/mouse-1 ng/mouse, i.c.v.), which inhibits equally both PP1 and PP2A, did not modify the morphine-induced antinociception. These results suggest that the activation of type 2A serine/threonine protein phosphatases may play a role in the antinociceptive effect of morphine, and that PP1 might counterbalace this activity.

Neurotransmitter mechanisms in gabapentin antinociception

The present study was done to investigate the neurotransmitter mechanisms involved in gabapentin antinociception in healthy albino rats. The formalin test was used to asses antinociception. Gabapentin (10-120 mg/kg s.c.) decreased the pain score in the formalin test. In order to study the putative neurotransmitter mechanisms involved in gabapentin action, the effect of gabapentin (30 mg/kg s.c.) alone and in rats pretreated with different receptor blockers, e.g. bicuculine, saclofen, naloxone, mecamylamine, atropine, DL-p-chlorophenylalanine methyl ester hydrochloride, glibenclamide, theophylline, and L-arginine was assessed. Gabapentin decreased the pain score, and the ED(50) of gabapentin was 36.8 +/- 8.2 (30.2-43.1) mg/kg s.c. Pretreatment with different receptor blockers did not modify gabapentin (30 mg/kg s.c.) antinociception except for L-arginine which increased the pain score from 1.68 +/- 0.29 (gabapentin) to 2.29 +/- 0.41. Results suggest the involvement of L-arginine nitric oxide pathways in gabapentin antinociception.

Involvement of potassium channels in spinal antinociceptions induced by fentanyl, clonidine and bethanechol in rats

In the central nervous systems, intracellular and extracellular movement of potassium ions plays an important role in regulating neuronal excitability and the release of neurotransmitters. The purpose of our study was to determine whether nicorandil (adenosine triphosphate-sensitive K+ channel opener) exerts antinociceptive effects by itself or in combination with fentanyl, clonidine and bethanechol and whether glibenclamide (adenosine triphosphate-sensitive K+ channel blocker) and charybdotoxin (Ca2+-activated K+ channel blocker) may antagonize the antinociceptive action of fentanyl, clonidine and bethanechol. Antinociceptive effects were assessed using the tail-flick test in rats. Nicorandil (100 microg) and antinociceptively ineffective doses of fentanyl (1 microg), clonidine (2.5 microg) or bethanechol (10,ug) were coadministered intrathecally (i.t.). Glibenclamide (100 microg) or charybdotoxin (2.5 ng) were administered i.t. at 5 min before each effective dose of fentanyl (2.5 microg), clonidine (10 microg) or bethanechol (40 microg). The present findings demonstrated that i.t. administration of nicorandil alone exerted no influence on the tail-flick latency. However, concomitant administrations of antinociceptively inactive doses of fentanyl, clonidine or bethanechol with nicorandil elicited significant suppression of the thermonociceptive response. Also, each antinociception induced by fentanyl, clonidine or bethanechol was partially antagonized by both glibenclamide and charybdotoxin. These findings showed that activation of the K+ channel might enhance the antinociceptive effects of fentanyl, clonidine and bethanechol.

Elevation of atrial natriuretic peptide levels in aqueous humor of the rabbit by kappa opioid receptor agonists

Following the topical administration of three kappa agonists (bremazocine, spiradoline and ICI 204448) to rabbit eyes, aqueous humor samples were analyzed for levels of atrial natriuretic peptide (ANP). Bremazocine (BRE) and spiradoline (SPR) elevated aqueous ANP levels in a dose-dependent manner. In contrast, ICI had no significant effect on ANP levels in aqueous humor. Nor-binaltorphimine (nor-BNI), a selective kappa opioid receptor antagonist, was used to assess kappa opioid receptor involvement and glibenclamide, an ATP-sensitive K+ channel blocker, was used to test for the role of ATP-sensitive potassium channels in ANP release. Pretreatment with nor-BNI antagonized the increases in ANP levels observed with both BRE and SPR. Likewise, glibenclamide suppressed the stimulation of ANP secretion by bremazocine. In summary, BRE and SPR increased ANP levels in aqueous humor of rabbits, in part, via activation of K+(ATP) channels that are assumed to be associated with kappa opioid receptors.

Changes in [(3)H]glibenclamide binding to mouse forebrain membranes during morphine tolerance

The characteristics of specific binding of the ATP-sensitive K(+) (K(ATP)) channel blocker [3H]glibenclamide to forebrain membranes (P(2) fraction, 4 degrees C) obtained from morphine-naive and -tolerant mice were evaluated. Morphine tolerance was induced by osmotic minipumps that released 45 mg/kg/day of morphine subcutaneously for 6 days. This treatment enhanced the antinociceptive ED(50) of morphine without changing its E(max). In morphine-naive animals, (1) both the association and the dissociation of [3H]glibenclamide were biphasic; (2) [3H]glibenclamide was displaced by other sulfonylureas (order of potency: glibenclamide>glipizide&z.Gt;tolbutamide) with pseudo-Hill coefficients lower than unity and biphasic Hofstee plots; and (3) Scatchard plots of saturation experiments were curvilinear, showed a Hill coefficient of 0.81+/-0.04 and suggested the presence of two binding sites with a K(D) of 0.13 and 3.17 nM and a B(max) of 12.30 and 84.47 fmol/mg protein, respectively. By contrast, in membranes obtained from morphine-tolerant animals, (1) the Scatchard plots showed only one population of binding sites with a K(D) of 0.87 nM and a B(max) of 77.99 fmol/mg protein, and the Hill coefficient was very close to unity (0.96+/-0.1); (2) competition experiments (using glibenclamide as displacer) showed a pseudo-Hill coefficient of 0.99+/-0.04; and (3) dissociation experiments showed only one phase of dissociation. These results suggest that [3H]glibenclamide binds to two different sites in membranes obtained from morphine-naive animals, but to only one site in morphine-tolerant animals. Consequently, it seems that morphine tolerance in mice involves adaptive changes in K(ATP) channels.

Antinociceptive action of epidural K+(ATP) channel openers via interaction with morphine and an alpha(2)- adrenergic agonist in rats

Potassium (K(+)) channels may play some role in the analgesic actions of mu-opioid agonists and alpha(2)-adrenergic agonists (alpha(2) agonists). We examined whether the adenosine triphosphate-sensitive K(+)(K(+)(ATP)) channel openers, levcromakalim and nicorandil, (given epidurally), might have antinociceptive effects in a tail flick test in adult male Sprague-Dawley rats implanted with a lumbar epidural catheter. The interactions with morphine and an alpha(2) agonist were also examined. The epidural administration of levcromakalim (10 microg, 100 microg) or nicorandil (10 microg, 100 microg) alone did not produce antinociception, but 100 microg levcromakalim or nicorandil did potentiate the antinociceptive effect induced by epidural morphine. Epidural glibenclamide (10 microg), a K(+)(ATP) channel blocker, or naloxone (10 microg) antagonized this potentiation. Systemic administration of levcromakalim or nicorandil (at the same dose as that given into the epidural space) did not potentiate the epidural morphine-induced analgesia. A combination of epidural dexmedetomidine (1 microg) and morphine (1 microg) (each at a subantinociceptive dose) had a significant antinociceptive effect, and epidural glibenclamide (10 microg) partly antagonized this antinociception. These data suggest that levcromakalim and nicorandil potentiate the analgesic action of both morphine and dexmedetomidine, probably via an activation of K(+)(ATP) channels at the spinal cord level.

Possible role of potassium channels in mu-receptor-mediated inhibition and muscarinic autoinhibition in acetylcholine release from myenteric plexus of guinea pig ileum

It is known that mu-agonists inhibit electrical field stimulation (EFS)-evoked ACh release from longitudinal muscle myenteric plexus (LMMP) preparation of guinea pig ileum when muscarinic autoinhibition does not fully work. In the present study, the possible role of K+ channels in the mechanisms of mu-agonists-induced inhibition and autoinhibition of ACh release was studied. In the presence of atropine, which blocks the autoinhibition, non-selective K+ channel blockers, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), reversed the inhibitory effect of mu-agonists, morphine and [D-Ala2, N-Me-Phe4, Gly5-ol] enkephalin, on EFS-evoked ACh release, but not that of kappa-agonist U-50,488. Apamin, iberiotoxin or glibenclamide did not affect the inhibition of ACh release by morphine. On the other hand, in the absence of atropine (under the autoinhibition working condition), 4-AP increased EFS-evoked ACh release, but atropine did not further increase ACh release in the presence of 4-AP. In contrast, although TEA did not affect EFS-evoked ACh release, atropine increased ACh release in the presence of TEA. These results suggest that the inhibitory effects of mu-agonists and muscarinic autoinhibition on the ACh release are associated with activation of different types of K+ channels in the guinea pig LMMP preparations: the former is associated with 4-AP- and TEA-sensitive K+ channels and the latter is associated with 4-AP- but not TEA-sensitive K+ channels.

Antisenses to opioid receptors attenuate ATP-gated K(+) channel opener-induced antinociception

The ATP-gated K(+) channel openers diazoxide, levcromakalim and morphine induce cell hyperpolarization by opening the K(+) channels and enhancing K(+) efflux. This hyperpolarization decreases intracellular Ca(2+) levels, lessening neurotransmitter release thus leading to antinociception. Previous findings implicate the release of endogenous opioids as the mediator of the antinociceptive effects of ATP-gated K(+) channel openers. Antisense oligodeoxynucleotides to the opioid receptor clones, which decrease the number of available receptors, were used to demonstrate the involvement of endogenous opioids in diazoxide- and levcromakalim-induced antinociception. Antisense to all three opioid receptors attenuated the effect of diazoxide, suggesting that diazoxide is inducing the release of endogenous opioids activating the mu(MOR-1)-, delta(DOR-1)-, and kappa(KOR-1)-opioid receptors. Antisense to the mu-opioid receptor clone and delta-opioid receptor clone attenuated levcromakalim-induced antinociception, indicating that endogenous opioids acting at the mu- and delta-opioid receptors are potential candidates for the mediation of the antinociceptive effects of levcromakalim.

Morphine tolerance-induced modulation of [3H]glyburide binding to mouse brain and spinal cord

Modulation by opioids of ATP-gated potassium channels, which regulate in part intracellular calcium levels, was measured by the binding of [3H]glyburide. Scatchard analyses generated a KD for whole brain of vehicle-pretreated mice of 288 pM with a Bmax of 694 fmol/mg protein. In the spinal cord the KD was 0.94 nM and the Bmax was 184 fmol/mg protein. Acute morphine decreased the KD in brain and spinal cord with no change in Bmax. Morphine tolerance increased the KD in brain and spinal cord 2.6- and 2.9-fold, respectively, concurrent with 1.6- and 3.4-fold increases in Bmax. Modulation by morphine of glyburide-sensitive binding sites may contribute at least in part to tolerance to morphine via alterations in intracellular calcium levels in neurons.

Implication of ATP-sensitive K+ channels in various stress-induced analgesia (SIA) in mice

Exposure to footshock (2 mA, 1-sec duration, 0.2 Hz for 15 min; FS), forced swimming (water at 20 degrees C for 3 min, SW) or psychological stress (using a communication box for 5 min, PSY) produced antinociceptive effects (stress-induced analgesia, SIA). Intracerebroventricular (i.c.v.) injection of glibenclamide (10-40 micrograms/mouse), an ATP-sensitive K+ (KATP) channel blocker, antagonized FS-SIA, while SW- and PSY-SIA were unaffected by the compound. Cromakalim (0.1-10 micrograms/mouse, i.c.v.), a KATP-channel opener, did not affect FS-, SW- or PSY-SIA. Thus, we provided evidence that central KATP channels participate in the production of FS-SIA but not production of SW- or PSY-SIA; and we suggest that glibenclamide, through closing of KATP channels, suppresses mu-opioid receptor functions, which subsequently leads to the inhibition of FS-SIA since antinociception is produced by the activation of mu-receptors.

Glibenclamide antagonizes the inhibitory effect of morphine on gall bladder emptying

Egg yolk-induced gall-bladder emptying in mice was used to investigate the effect of glibenclamide and minoxidil (ATP-dependent K(+)-channel modulators) on biliary tract effects of morphine. The inhibitory effect of morphine (1-4 mg kg-1, i.p.) on egg yolk-induced gall-bladder emptying was completely blocked by pretreatment with naloxone (2 mg kg-1, i.p.) or glibenclamide (0.65 mg kg-1, i.p.) whereas, pretreatment with minoxidil (0.65 mg kg-1, i.p.) did not modify the inhibitory effect of morphine on gall-bladder emptying. Our results suggest that biliary-tract actions of morphine are mediated through glibenclamide-sensitive K+ channels similar to those involved in the analgesic action of morphine.

Effects of K+ channel blockers and openers on antinociception induced by agonists of 5-HT1A receptors

The modulation by K+ channel-acting drugs of the antinociceptive effect of several 5-HT1A receptor agonists was examined with the hot plate test in mice. All the 5-HT1A receptor agonists tested induced dose-dependent antinociception, the order of potency being (+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT) > buspirone > or = lesopitron > or = tandospirone. The blockers of ATP-sensitive K+ channels (KATP) gliquidone and glipizide (1-4 and 16-64 micrograms/mouse i.c.v., respectively) reduced the antinociceptive effect of 8-OH-DPAT, whereas cromakalim (32-64 micrograms/mouse i.c.v.), an opener of KATP channels, enhanced the effect. In contrast, 4-aminopyridine (25-250 ng/mouse i.c.v.) and tetraethylammonium (10-20 micrograms/mouse i.c.v.), which antagonize several non-ATP-dependent K+ conductances, were inactive. The same results were found with other agonists of 5-HT1A receptors (lesopitron, buspirone and tandospirone): gliquidone inhibited whereas cromakalim increased their antinociceptive effects. None of the K+ channel-acting drugs modified the binding of [3H]8-OH-DPAT to hippocampal membranes, whereas all the 5-HT1A receptor agonists displaced the ligand. These results suggest that ATP-sensitive K+ conductances are involved in the antinociception induced by agonists of 5-HT1A receptors.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Morphine-3-glucuronide: evidence to support its putative role in the development of tolerance to the antinociceptive effects of morphine in the rat

Antinociceptive tolerance to morphine (MOR) was induced in groups of Sprague-Dawley rats receiving continuous intravenous infusions of morphine sulphate administered by 3 different MOR dosing regimes. At appropriate intervals throughout each infusion period, antinociceptive testing was performed using the tail-flick latency test and blood samples were collected. Groups of saline (SAL)-infused control rats also underwent antinociceptive testing and blood sample collection. Complete antinociceptive tolerance developed during each MOR infusion period and was characterized by a marked decline in the degree of antinociception from values greater than 90% of the maximum possible effect (%MPE) to pre-dosing baseline values. By contrast, %MPE values in SAL-infused control animals and in sham-operated rats were not significantly different from pre-dosing values throughout the infusion period, indicating that the experimental procedures themselves did not contribute to the development of antinociceptive tolerance to MOR. In addition, the rate of MOR tolerance development was inversely proportional to the MOR infusion rate. A very significant inverse relationship was observed between the mean degree of antinociception (%MPE) and the mean plasma molar concentration ratio, [morphine-3-glucuronide]/[MOR], for each of the 3 MOR dosing regimes and for the cumulated data. This relationship showed that near-maximum antinociception was attainable at ratio values less than approximately 0.50, whilst at ratio values above approximately 1.5, little or no antinociception was observed. Although %MPE was highly inversely correlated with the mean plasma morphine-3-glucuronide (M3G) concentrations for rats receiving regimes A and B, this was not the case for rats receiving regime C where antinociceptive tolerance was partially reversed by an increase in the morphine infusion rate part-way through the infusion period. In addition, a poor relationship was observed between %MPE and the mean plasma MOR concentration, possibly due to the confounding presence of M3G in all samples. Thus, we may conclude from this study in Sprague-Dawley rats that irrespective of the rate of antinociceptive tolerance development, the level of antinociception achievable appears to be highly inversely correlated with the mean [M3G]/[MOR] plasma molar concentration ratio and poorly correlated with the plasma MOR concentration, consistent with the notion that it is perhaps the balance between the excitatory effects of M3G and the inhibitory effects of MOR at the functional level which is the important determinant. Further research is required in carefully conducted studies in cancer patients to evaluate the possible contribution of the MOR metabolites, M3G and morphine-6-glucuronide (MbG), to increasing dosing requirements of MOR.(ABSTRACT TRUNCATED AT 400 WORDS)

The 'glibenclamide-shift' of centrally-acting antinociceptive agents in mice

Morphine-induced antinociception is antagonized by the K(+)-channel blocker glibenclamide (glyburide; Glib), implicating ATP-sensitive (KATP) K+ channels in the analgesic effect of opioids. The present study examined the generality of this conclusion by measuring the effect of Glib on supraspinal (intracerebroventricular; i.c.v.) antinociception produced by representative mu-opioids and the non-opioids pilocarpine and two alpha 2-adrenoceptor agonists (clonidine and tizanidine) using the mouse tail-flick test. Concurrent administration of Glib (40 micrograms, i.c.v.) produced a significant rightward shift of the dose-response curve of morphine, levorphanol, methadone, pilocarpine, clonidine and tizanidine; a modest, but not statistically significant, rightward shift of the dose-response curves of the mu-selective peptides DAMGO ([D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin) and PL017 ([N-Me-Phe3,D-Pro4]-morphiceptin); and no shift of the dose-response curves of alfentanil, carfentanil, fentanyl, sufentanil, or beta-endorphin. Glib produced a leftward shift of the dose-response curve of etorphine. These data support the involvement of KATP-type K+ channels in mediation of supraspinal antinociception, differentiate Glib-sensitive and Glib-insensitive opioid agonists, and reveal fundamental differences among antinociceptive agents in the extent of demonstrable utilization of this transduction pathway.

Subgroups among mu-opioid receptor agonists distinguished by ATP-sensitive K+ channel-acting drugs

1. We evaluated the effects of the i.c.v. administration of different K+ channel blockers (gliquidone, 4-aminopyridine and tetraethylammonium) and an opener of K+ channels (cromakalim) on the antinociception induced by several mu-opioid receptor agonists in a tail flick test in mice. 2. The s.c. administration of all agonists of mu-opioid receptors tested (morphine, 1-16 mg kg-1; metadone, 1-6 mg kg-1; buprenorphine, 0.04-0.64 mg kg-1; fentanyl, 0.02-0.32 mg kg-1 and levorphanol, 0.2-3.2 mg kg-1) elicited a dose-dependent antinociceptive effect. 3. The ATP-sensitive K+ channel blocker, gliquidone (0.06-16 micrograms per mouse, i.c.v.) antagonized the antinociception induced by buprenorphine, morphine and metadone. In contrast, gliquidone (0.25-160 micrograms per mouse) did not modify the antinociceptive effects of fentanyl and levorphanol. 4. Cromakalim (4-64 micrograms per mouse, i.c.v.), an opener of ATP-sensitive K+ channels, enhanced the antinociception produced by buprenorphine, morphine, and methadone, and did not significantly modify the antinociceptive effects of fentanyl and levorphanol. 5. The i.c.v. administration of the K+ channel blockers tetraethylammonium (10 micrograms per mouse) or 4-aminopyridine (25 ng per mouse) did not significantly modify the antinociception induced by any mu-opioid receptor agonist tested. 6. These results suggest that the opening of ATP-sensitive K+ channels is involved in the antinociceptive effect of morphine, buprenorphine and methadone, but not in that of fentanyl or levorphanol. Consequently, we suggest that at least two subgroups can be distinguished among mu-opioid receptor agonists, each inducing antinociception through different effector mechanisms.

Kappa-opioid receptor mediated antinociception in rats is dependent on the functional state of dihydropyridine-sensitive calcium channels

The modulatory effect of the dihydropyridine Ca2+ channel antagonist nimodipine on the analgesic action of the kappa-opioid receptor agonist U-69,593 was analyzed using the tail-flick test in rats. The antinociceptive effect of U-69,593 (0.25-4 mg/kg) was antagonized by L-type Ca2+ channel blockade with nimodipine (200 microgram/kg, i.p.), the ED50 being increased from 1.4 to 7.3 mg/kg. On the contrary, when an increase in the density of these channels was induced by means of chronic and simultaneous treatment with nimodipine (1 microgram/h, 7 days) and sufentanil (2 micrograms/h, 8 days), the analgesic effect of U-69,593 was potentiated by 5-fold. Our results suggest a functional coupling between kappa-opioid receptors and L-type Ca2+ channels in nociception.

ATP-sensitive K+ channel openers inhibit morphine withdrawal

We studied the effects of two different ATP-sensitive K+ channel openers on naloxone-precipitated withdrawal in morphine-dependent mice. The i.c.v. administration of cromakalim and diazoxide (both at 5-40 micrograms/mouse) dose-dependently inhibited several signs of morphine withdrawal (number of jumps and episodes of forepaw tremors, and body weight loss). At present it is impossible to specify the exact mechanism(s) involved in this effect. However, considering that morphine opens K+ channels in neurons, it is tempting to suggest that K+ channel openers can mimic the effects of morphine on neuronal K+ currents, and as a consequence can act as substitutes for this drug during morphine withdrawal.

Differential effects of K+ channel blockers on antinociception induced by alpha 2-adrenoceptor, GABAB and kappa-opioid receptor agonists

1. The effects of several K+ channel blockers (sulphonylureas, 4-aminopyridine and tetraethylammonium) on the antinociception induced by clonidine, baclofen and U50,488H were evaluated by use of a tail flick test in mice. 2. Clonidine (0.125-2 mg kg-1, s.c.) induced a dose-dependent antinociceptive effect. The ATP-dependent K+ (KATP) channel blocker gliquidone (4-8 micrograms/mouse, i.c.v.) produced a dose-dependent displacement to the right of the clonidine dose-response line, but neither 4-aminopyridine (4-AP) (25-250 ng/mouse, i.c.v.) nor tetraethylammonium (TEA) (10-20 micrograms/mouse, i.c.v.) significantly modified clonidine-induced antinociception. 3. The order of potency of sulphonylureas in antagonizing clonidine-induced antinociception was gliquidone > glipizide > glibenclamide > tolbutamide, which is the same order of potency as these drugs block KATP channels in neurones of the CNS. 4. Baclofen (2-16 mg kg-1, s.c.) also induced a dose-dependent antinociceptive effect. Both 4-AP (2.5-25 ng/mouse, i.c.v.) and TEA (10-20 micrograms/mouse, i.c.v.) dose-dependently antagonized baclofen antinociception, producing a displacement to the right of the baclofen dose-response line. However, gliquidone (8-16 micrograms/mouse, i.c.v.) did not significantly modify the baclofen effect. 5. None of the K+ channel blockers tested (gliquidone, 8-16 micrograms/mouse; 4-AP, 25-250 ng/mouse and TEA, 10-20 micrograms/mouse, i.c.v.), significantly modified the antinociception induced by U50,488H (8 mg kg-1, s.c.). 6. These results suggest that the opening of K+ channels is involved in the antinociceptive effect of alpha 2 and GABAB, but not kappa-opioid, receptor agonists. The K+ channels opened by alpha2-adrenoceptor agonists seem to be ATP-dependent channels, whereas those opened by GABAB receptor agonists are not.

Gliquidone, an ATP-dependent K+ channel antagonist, antagonizes morphine-induced hypermotility

The effect of gliquidone, an ATP-dependent K+ (KATP) channel blocker, on morphine-induced hypermotility in mice was studied. Morphine (5-40 mg/kg s.c.) dose dependently increased ambulatory activity. Gliquidone (10 micrograms/mouse i.c.v.) induced a parallel displacement to the right of the morphine dose-response curve. Moreover, gliquidone (10 and 40 micrograms/mouse i.c.v.) produced a dose-dependent antagonism of morphine (20 mg/kg s.c.)-induced hypermotility. These results suggest that KATP channels are involved in morphine-induced hypermotility. The present data, together with those of previous studies showing antagonism by KATP channel blockers of morphine-induced antinociception and hyperthermia, further indicate that the opening of KATP channels plays an important role in the mechanism of action of morphine.