Esperanza Del Pozo

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

Analgesic effects of several calcium channel blockers in mice

The possibility that calcium channel blockers might produce antinociception and increase morphine analgesia was examined using the acetic acid writhing test in mice. Subcutaneous injections of diltiazem, verapamil, nicardipine, flunarizine and cinnarizine produced a dose-dependent antinociception. This activity of diltiazem was stereospecific; d-cis-diltiazem was more potent than 1-cis-diltiazem. All the calcium channel blockers studied increased morphine analgesia and displaced to the left the morphine dose-response curve. This effect of diltiazem was also stereospecific. These results suggest that calcium channel blockers can induce analgesia and increase morphine analgesia, possibly through a decrease in cellular calcium availability.

Sigma-1 receptors are essential for capsaicin-induced mechanical hypersensitivity: Studies with selective sigma-1 ligands and sigma-1 knockout mice

ABSTRACT We evaluated the role of σ1 receptors on capsaicin‐induced mechanical hypersensitivity and on nociceptive pain induced by punctate mechanical stimuli, using wild‐type and σ1 receptor knockout (σ1‐KO) mice and selective σ1 receptor‐acting drugs. Mutation in σ1‐KO mice was confirmed by PCR analysis of genomic DNA and, at the protein level, by [3H](+)‐pentazocine binding assays. Both wild‐type and σ1‐KO mice not treated with capsaicin showed similar responses to different intensities of mechanical stimuli (0.05–8 g force), ranging from innocuous to noxious, applied to the hind paw. This indicates that σ1 gene inactivation does not modify the perception of punctate mechanical stimuli. The intraplantar (i.pl.) administration of capsaicin induced dose‐dependent mechanical allodynia in wild‐type mice (markedly reducing both the threshold force necessary to induce paw withdrawal and the latency to paw withdrawal induced by a given force). In contrast, capsaicin was completely unable to induce mechanical hypersensitivity in σ1‐KO mice. The high‐affinity and selective σ1 antagonists BD‐1063, BD‐1047 and NE‐100, administered subcutaneously (s.c.), dose‐dependently inhibited mechanical allodynia induced by capsaicin (1 μg,i.pl.), yielding ED50 (mg/kg) values of 15.80 ± 0.93, 29.31 ± 1.65 and 40.74 ± 7.20, respectively. The effects of the σ1 antagonists were reversed by the σ1 agonist PRE‐084 (32 mg/kg, s.c.). None of the drugs tested modified the responses induced by a painful mechanical punctate stimulus (4 g force) in nonsensitized animals. These results suggest that σ1 receptors are essential for capsaicin‐induced mechanical hypersensitivity, but are not involved in mechanical nociceptive pain.

Tetrodotoxin inhibits the development and expression of neuropathic pain induced by paclitaxel in mice

&NA; We evaluated the effect of low doses of systemically administered tetrodotoxin (TTX) on the development and expression of neuropathic pain induced by paclitaxel in mice. Treatment with paclitaxel (2 mg/kg, i.p., once daily during 5 days) produced long‐lasting (2–4 weeks) heat hyperalgesia (plantar test), mechanical allodynia (electronic Von Frey test) and cold allodynia (acetone drop method), with maximum effects observed on days 7, 10 and 10–14, respectively. Acute subcutaneous treatment with 1 or 3 μg/kg of TTX reduced the expression of mechanical allodynia, whereas higher doses (3 or 6 μg/kg) were required to reduce the expression of cold allodynia and heat hyperalgesia. In contrast, TTX (3 or 6 μg/kg, s.c.) did not affect the response to the same thermal and mechanical stimuli in control animals, which indicates that the antihyperalgesic and antiallodynic effects of TTX were not due to unspecific inhibition of the perception of these stimuli. Administration of TTX (6 μg/kg, s.c.) 30 min before each of the 5 doses of paclitaxel did not modify the development of heat hyperalgesia produced by the antineoplastic, but abolished the development of mechanical and cold allodynia. Coadministration of a lower dose of TTX (3 μg/kg) also prevented the development of mechanical allodynia. No signs of TTX‐induced toxicity or motor incoordination were observed. These data suggest that low doses of TTX can be useful to prevent and treat paclitaxel‐induced neuropathic pain, and that TTX‐sensitive subtypes of sodium channels play a role in the pathogenesis of chemotherapy‐induced neuropathic pain.

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.

Irreversible blockade of sigma-1 receptors by haloperidol and its metabolites in guinea pig brain and SH-SY5Y human neuroblastoma cells

We evaluated the effect of haloperidol (HP) and its metabolites on [3H](+)‐pentazocine binding to σ1 receptors in SH‐SY5Y human neuroblastoma cells and guinea pig brain P1, P2 and P3 subcellular fractions. Three days after a single i.p. injection in guinea pigs of HP (but not of other σ1 antagonists or (−)‐sulpiride), [3H](+)‐pentazocine binding to brain membranes was markedly decreased. Recovery of σ1 receptor density to steady state after HP‐induced inactivation required more than 30 days. HP‐metabolite II (reduced HP, 4‐(4‐chlorophenyl)‐α‐(4‐fluorophenyl)‐4‐hydroxy‐1‐piperidinebutanol), but not HP‐metabolite I (4‐(4‐chlorophenyl)‐4‐hydroxypiperidine), irreversibly blocked σ1 receptors in guinea pig brain homogenate and P2 fraction in vitro. We found similar results in SH‐SY5Y cells, which suggests that this process may also take place in humans. HP irreversibly inactivated σ1 receptors when it was incubated with brain homogenate and SH‐SY5Y cells, but not when incubated with P2 fraction membranes, which suggests that HP is metabolized to inactivate σ1 receptors. Menadione, an inhibitor of the ketone reductase activity that leads to the production of HP‐metabolite II, completely prevented HP‐induced inactivation of σ1 receptors in brain homogenates. These results suggest that HP may irreversibly inactivate σ1 receptors in guinea pig and human cells, probably after metabolism to reduced HP.

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.

Changes of quantal transmitter release caused by gadolinium ions at the frog neuromuscular junction

1 The actions of the trivalent cation, gadolinium (Gd3+), were studied on frog isolated neuromuscular preparations by conventional electrophysiological techniques. 2 Gd3+(450 μm) applied to normal or formamide‐treated cutaneous pectoris nerve‐muscle preparations induced, after a short delay, a complete block of neuromuscular transmission. The reversibility of the effect was dependent on the time of exposure. 3 Gd3+(5–450 μm) had no consistent effect on the resting membrane potential of the muscle fibres. 4 Gd3+(5–40 μm) applied to preparations equilibrated in solutions containing high Mg2+and low Ca2+reduced the mean quantal content of endplate potentials (e.p.ps) in a dose‐dependent manner. Under those conditions, 3,4‐diaminopyridine (10 μm) consistently reversed the depression of evoked quantal release. 5 The calcium current entering motor nerve terminals, revealed after blocking presynaptic potassium currents with tetraethylammonium (10 mm) in the presence of elevated extracellular Ca2+(8 mm), was markedly reduced by Gd3+(0.2–0.5 mm). 6 Gd3+(40–200 μm) increased the frequency of spontaneous miniature endplate potentials (m.e.p.ps) in junctions bathed either in normal Ringer solution or in a nominally Ca2+‐free medium supplemented with 0.7 μm tetrodotoxin. This effect may be due to Gd3+entry into the nerve endings since it is not reversed upon removal of extracellular Gd3+with chelators (1 mm EGTA or EDTA). Gd3+also enhanced the frequency of me.p.ps appearing after each nerve stimulus in junctions bathed in a medium containing high Mg2+and low Ca2+ 7 Gd3+, in concentrations higher than 100 μm, decreased reversibly the amplitude of m.e.p.ps suggesting a postsynaptic action. 8 It is concluded that the block of nerve‐impulse evoked quantal release caused by Gd3+is related to its ability to block the calcium current entering the nerve endings, supporting the view that Gd3+blocks N‐type Ca2+channels; while the enhancement of spontaneous quantal release is probably the result of Gd3+entry into motor nerve endings. Besides its dual prejunctional effects on quantal release it is suggested that Gd3+exerts a postsynaptic action on the endplate acetylcholine receptor‐channel complex.

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.

Effects of calcium channel blockers on neuromuscular blockade induced by aminoglycoside antibiotics

The effects of several calcium channel blockers (nifedipine, verapamil and diltiazem) on rat phrenic-hemidiaphragm preparations were studied. The calcium channel blockers were used either alone or associated with two aminoglycoside antibiotics, neomycin and streptomycin. All drugs investigated produced a concentration-dependent decrease in indirectly elicited diaphragmatic contractions. The order of potency was: verapamil greater than neomycin congruent to nifedipine greater than diltiazem greater than streptomycin. Moreover, neomycin-induced neuromuscular blockade was significantly increased by nifedipine (1 and 10 microM), verapamil (1 and 10 microM) and diltiazem (10 microM), whereas the streptomycin-induced neuromuscular blockade was increased only by nifedipine (1 and 10 microM) and verapamil (10 microM).