María Ocaña

An ATP-dependent potassium channel blocker antagonizes morphine analgesia

We showed that glibenclamide, a blocker of ATP-dependent potassium channels in the CNS, antagonizes morphine-induced analgesia in mice

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

1 We evaluated the effects of the i.e.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 μ‐opioid receptor agonists in a tail flick test in mice.

Differential effects of K+ channel blockers on antinociception induced by α2-adrenoceptor, GABAB and κ-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 μg/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 μg/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 μg/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 μg/mouse, i.c.v.) did not significantly modify the baclofen effect. 5 None of the K+ channel blockers tested (gliquidone, 8–16 μg/mouse; 4‐AP, 25–250 ng/mouse and TEA, 10–20 μg/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 antinoceptive effect of α2 and GABAB, but not κ‐opioid, receptor agonists. The K+ channels opened by α2‐adrenoceptor agonists seem to be ATP‐dependent channels, whereas those opened by GABAB receptor agonists are not.

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

The effects of four ATP-dependent K+ channel blockers (hypoglycemic sulfonylureas) against morphine- and U50488H-induced antinociception were evaluated using the tail flick test in mice. None of the sulfonylureas tested significantly modified tail flick latency in control animals. However, i.c.v. pretreatment with gliquidone (0.4-1.6 micrograms/mouse), glipizide (2.5-10 micrograms/mouse), glibenclamide (10-40 micrograms/mouse) or tolbutamide (20-80 micrograms/mouse) dose dependently antagonized morphine-induced antinociception approximately equieffectively, the only difference being in potency: gliquidone > glipizide > glibenclamide > tolbutamide. This effect of sulfonylureas was very specific, since none antagonized the antinociception elicited by U50488H even at doses twice as great as the dose that induced maximum antagonism of morphine antinociception. Because morphine, but not U50488H, opens K+ channels in neurons and because the order of potency of the different sulfonylureas for blocking ATP-dependent K+ channels in neurons and for antagonizing morphine antinociception is the same, we suggest that morphine antinociception is mediated by the opening of ATP-dependent K+ channels.

Analgesic effects of centrally administered aminoglycoside antibiotics in mice

The possible analgesic effects of i.c.v. administration of several aminoglycoside antibiotics were evaluated in mice using hot plate and tail flick tests. Neomycin (10-80 micrograms/mouse), gentamicin (40-160 micrograms/mouse) and kanamycin (80-320 micrograms/mouse) produced dose-dependent increases in the latencies to forepaw licking and jumping in hot plate test. These drugs also produced dose-dependent increases in the percentage of animals showing analgesia in tail flick test. The order of potency of these aminoglycoside antibiotics in both tests was neomycin greater than gentamicin greater than kanamycin, which is exactly the same order that these drugs show as N-type calcium channel blockers. Bearing in mind this fact and the well known analgesic activity of several drugs which decrease neuronal calcium availability, we suggest that the mechanism of aminoglycoside-induced antinociception may be related to the capacity of these antibiotics to block N-type calcium channels and decrease neuronal calcium availability.

Role of ATP-sensitive K+ channels in antinociception induced by R-PIA, an adenosine A1 receptor agonist

The influence of several K+ channel-acting drugs on antinociception induced by the adenosine A1 receptor agonist (−)-N6-(2-phenylisopropyl)-adenosine (R-PIA) was evaluated with a tail flick test in mice. The subcutaneous administration of R-PIA (0.5–8 mg/kg) induced a dose-dependent antinociceptive effect. The ATP-sensitive K+ (KATP) channel blocker gliquidone (2–8 μg/mouse, i.c.v.) produced a dose-dependent displacement to the right of the R-PIA dose-response line, whereas the KATP channel opener cromakalim (32 μg/mouse, i.c.v.) shifted it to the left. Several KATP channel blockers dose-dependently antagonized the antinociceptive effect of R-PIA, the order of potency being gliquidone > glipizide > glibenclamide (i.e., the same order of potency shown by these drugs in blocking KATP channels in neurons). In contrast, the K+ channel blockers 4-aminopyridine and tetraethylammonium did not antagonize the effect of R-PIA. These data suggest that antinociception produced by adenosine A1 receptor agonists is mediated by the opening of ATP-sensitive K+ channels. The present results, together with those of previous studies, further support a role for K+ channel opening in the antinociceptive effect of agonists of receptors coupled to Gi/Go proteins.

Changes in morphine-induced activation of cerebral Na+,K+-ATPase during morphine tolerance: Biochemical and behavioral consequences

There is ample evidence of the biological changes produced by the sustained activation of opioid receptors. We evaluated the adaptive changes of cerebral Na(+),K(+)-ATPase in response to the sustained administration of morphine (minipumps, 45mg/kg/day, 6 days) in CD-1 mice and the functional role of these changes in opioid antinociception. The antinociceptive effect of morphine as determined with tail-flick tests was reduced in morphine-tolerant mice. There were no significant changes in the density of high-affinity Na(+),K(+)-ATPase α subunits labeled with [(3)H]ouabain in forebrain membranes from morphine-tolerant compared to those of morphine-naive animals. Western blot analysis showed that there were no significant differences between groups in the changes in relative abundance of α(1) and α(3) subunits of Na(+),K(+)-ATPase in the spinal cord or forebrain. However, the morphine-induced stimulation of Na(+),K(+)-ATPase activity was significantly lower in brain synaptosomes from morphine-tolerant mice (EC(50)=1.79±0.10μM) than in synaptosomes from morphine-naive mice (EC(50)=0.69±0.12μM). Furthermore, adaptive alterations in the time-course of basal Na(+),K(+)-ATPase activity were observed after sustained morphine treatment, with a change from a bi-exponential decay model (morphine-naive mice) to a mono-exponential model (morphine-tolerant mice). In behavioral studies the antinociceptive effects of morphine (s.c.) in the tail-flick test were dose-dependently antagonized by ouabain (1 and 10ng/mouse, i.c.v.) in morphine-naive mice, but not in morphine-tolerant mice. These findings suggest that during morphine tolerance, adaptive cellular changes take place in cerebral Na(+),K(+)-ATPase activity which are of functional relevance for morphine-induced antinociception.

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.

The antinociceptive effect of morphine is reversed by okadaic acid in morphine-naive but not in morphine-tolerant mice.

The activation of specific subtypes of serine/threonine protein phosphatases (PPs) plays a role in the antinociceptive effect of acute morphine, but it is not known whether these enzymes are involved in morphine-induced antinociception in morphine-tolerant animals. We evaluated the effects of both okadaic acid (a selective inhibitor of some serine/threonine PPs) and its inactive analogue L-norokadaone on the antinociception induced by morphine in morphine-naive and -tolerant female mice in the tail-flick test. Okadaic acid (0.01 and 1 pg/mouse, i.c.v.), but not L-norokadaone (1 pg/mouse, i.c.v.), antagonized in a dose-dependent way the antinociception induced by morphine (1-16 mg/kg, s.c.) in morphine-naive animals. However, both okadaic acid (0.01 and 1 pg/mouse, i.c.v.) and L-norokadaone (1 pg/mouse, i.c.v.) were unable to modify the antinociceptive effect of morphine in morphine-tolerant mice. These results suggest that in morphine-induced thermal analgesia, the role of serine/threonine PPs highly sensitive to okadaic acid is different in morphine-tolerant and morphine-naive female mice.

Differential potentiation by calcium antagonists of neuromuscular blockade induced by pancuronium and succinylcholine in cats in vivo

The effects of several calcium antagonists (verapamil, nicardipine and two diltiazem isomers, d-cis and l-cis diltiazem) alone and associated to non-depolarizing (pancuronium) and depolarizing (succinylcholine) neuromuscular blockers, were evaluated on sciatic nerve-tibialis anterior muscle preparations from cats in vivo. The calcium antagonists used (at 0.1 and 0.5mg/kg iv) did not modify the height of muscular twitches elicited indirectly. However, these agents potentiated in a dose-dependent way the neuromuscular blockade induced by iv pancuronium (2–40μg/kg) and succinylcholine (6–200μg/kg). The order of potency in increasing the effects of pancuronium was nicardipine ≫ d-cis diltiazem ⩾ verapamil, whereas the order of potency in enhancing succinylcholine effects was d-cis diltiazem ⩾ verapamil ≫ nicardipine. The effects of diltiazem were stereoselective, thus the potentiation induced by d-cis diltiazem was significantly greater in all cases than that induced by l-cis diltiazem, which suggests that calcium channel blockade plays a role in these interactions. However, other mechanisms such as calcium antagonists-induced nicotinic receptor desensitization may also be involved.