Sigma-1 receptor agonism exacerbates immune-driven nociception: Role of TRPV1 + nociceptors

The analgesic effects of sigma-1 antagonists are undisputed, but the effects of sigma-1 agonists on pain are not well studied. Here, we used a mouse model to show that the administration of the sigma-1 agonists dextromethorphan (a widely used antitussive drug), PRE-084 (a standard sigma-1 ligand), and pridopidine (a selective drug being investigated in clinical trials for the treatment of neurodegenerative diseases) enhances PGE2-induced mechanical hyperalgesia. Superficial plantar incision induced transient weight-bearing asymmetry at early time points, but the mice appeared to recover at 24 h, despite noticeable edema and infiltration of neutrophils (a well-known cellular source of PGE2) at the injured site. Sigma-1 agonists induced a relapse of weight bearing asymmetry in a manner dependent on the presence of neutrophils. The effects of sigma-1 agonists were all reversed by administration of the sigma-1 antagonist BD-1063 in wild-type mice, and were absent in sigma-1 knockout mice, supporting the selectivity of the effects observed. The proalgesic effects of sigma-1 agonism were also abolished by the TRP antagonist ruthenium red and by in vivo resiniferatoxin ablation of TRPV1 + peripheral sensory neurons. Therefore, sigma-1 agonism exacerbates pain-like responses in mice with a mild inflammatory state through the action of TRPV1 + nociceptors. We also show that sigma-1 receptors are present in most (if not all) mouse and human DRG neurons. If our findings translate to humans, further studies will be needed to investigate potential proalgesic effects induced by sigma-1 agonism in patients treated with sigma-1 agonists.

The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors

BACKGROUND AND PURPOSE

Peripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca²⁺ -sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the μ opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown.

EXPERIMENTAL APPROACH

We tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous μ receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, μ- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors.

KEY RESULTS

Sigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to μ receptors, suggesting that sigma-1R participate in TRPV1-μ receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist.

CONCLUSION AND IMPLICATIONS

Sigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing μ receptor activity.

Paclitaxel antitumor effect improvement in lung cancer and prevention of the painful neuropathy using large pegylated cationic liposomes

Paclitaxel (PTX), a drug widely used in lung cancer, has serious limitations including the development of peripheral neurotoxicity, which may lead to treatment discontinuation and therapy failure. The transport of PTX in large cationic liposomes could avoid this undesirable effect, improving the patient's prognosis. PTX was encapsulated in cationic liposomes with two different sizes, MLV (180-200 nm) and SUV (80-100 nm). In both cases, excellent biocompatibility and improved internalization and antitumor effect of PTX were observed in human and mice lung cancer cells in culture, multicellular spheroids and cancer stem cells (CSCs). In addition, both MLV and SUV with a polyethylene glycol (PEG) shell, induced a greater tumor volume reduction than PTX (56.4 % and 57.1 % vs. 36.7 %, respectively) in mice. Interestingly, MLV-PEG-PTX did not induce either mechanical or heat hypersensitivity whereas SUV-PEG-PTX produced a similar response to free PTX. Analysis of PTX distribution showed a very low concentration of the drug in the dorsal root ganglia (DRG) with MLV-PEG-PTX, but not with SUV-PEG-PTX or free PTX. These results support the hypothesis that PTX induces peripheral neuropathy by penetrating the endothelial fenestrations of the DRG (80-100 nm, measured in mice). In conclusion, our larger liposomes (MLV-PEG-PTX) not only showed biocompatibility, antitumor activity against CSCs, and in vitro and in vivo antitumor effect that improved PTX free activity, but also protected from PTX-induced painful peripheral neuropathy. These advantages could be used as a new strategy of lung cancer chemotherapy to increase the PTX activity and reduce its side effects.

Urinary bladder sigma-1 receptors: A new target for cystitis treatment

No adequate treatment is available for painful urinary bladder disorders such as interstitial cystitis/bladder pain syndrome, and the identification of new urological therapeutic targets is an unmet need. The sigma-1 receptor (σ1-R) modulates somatic pain, but its role in painful urological disorders is unexplored. The urothelium expresses many receptors typical of primary sensory neurons (e.g. TRPV1, TRPA1 and P2X3) and high levels of σ1-R have been found in these neurons; we therefore hypothesized that σ1-R may also be expressed in the urothelium and may have functional relevance in this tissue. With western blotting and immunohistochemical methods, we detected σ1-R in the urinary bladder in wild-type (WT) but not in σ1-R-knockout (σ1-KO) mice. Interestingly, σ1-R was located in the bladder urothelium not only in mouse, but also in human bladder sections. The severity of histopathological (edema, hemorrhage and urothelial desquamation) and biochemical alterations (enhanced myeloperoxidase activity and phosphorylation of extracellular regulated kinases 1/2 [pERK1/2]) that characterize cyclophosphamide-induced cystitis was lower in σ1-KO than in WT mice. Moreover, cyclophosphamide-induced pain behaviors and referred mechanical hyperalgesia were dose-dependently reduced by σ1-R antagonists (BD-1063, NE-100 and S1RA) in WT but not in σ1-KO mice. In contrast, the analgesic effect of morphine was greater in σ1-KO than in WT mice. Together these findings suggest that σ1-R plays a functional role in the mechanisms underlying cyclophosphamide-induced cystitis, and modulates morphine analgesia against urological pain. Therefore, σ1-R may represent a new drug target for urinary bladder disorders.

Sigma-1 receptors control neuropathic pain and macrophage infiltration into the dorsal root ganglion after peripheral nerve injury

Neuron-immune interaction in the dorsal root ganglia (DRG) plays a pivotal role in the neuropathic pain development after nerve injury. Sigma-1 receptor (Sig-1R) is expressed by DRG neurons but its role in neuropathic pain is not fully understood. We investigated the effect of peripheral Sig-1R on neuroinflammation in the DRG after spared (sciatic) nerve injury (SNI) in mice. Nerve injury induced a decrease in NeuN staining along with the nuclear eccentricity and ATF3 expression in the injured DRG. Sig-1R was present in all DRG neurons examined, and after SNI this receptor translocated to the periphery of the soma and the vicinity of the nucleus, especially in injured ATF3 + neurons. In WT mice, injured DRG produced the chemokine CCL2, and this was followed by massive infiltration of macrophages/monocytes, which clustered mainly around sensory neurons with translocated Sig-1R, accompanied by robust IL-6 increase and mechanical allodynia. In contrast, Sig-1R knockout (Sig-1R-KO) mice showed reduced levels of CCL2, decreased macrophage/monocyte infiltration into DRG, and less IL-6 and neuropathic mechanical allodynia after SNI. Our findings point to an important role of peripheral Sig-1R in sensory neuron-macrophage/monocyte communication in the DRG after peripheral nerve injury; thus, these receptors may contribute to the neuropathic pain phenotype.

Intracolonic Mustard Oil Induces Visceral Pain in Mice by TRPA1-Dependent and -Independent Mechanisms: Role of Tissue Injury and P2X Receptors

Both TRPA1 and purinergic P2X receptors have been proposed as potential targets for the treatment of visceral pain. We found that the intracolonic administration of a low dose mustard oil (0.5%), a well-known TRPA1 agonist, produced nociceptive responses and abdominal wall referred mechanical hyperalgesia, without inducing apparent tissue damage. Both nociceptive responses and referred hyperalgesia were abolished by the ablation of TRPV1-expressing neurons (and the consequent ablation of TRPA1+ nociceptors) by resiniferatoxin (RTX) treatment, and by the TRPA1 antagonist AP18. However, a higher dose of mustard oil (2.5%) damaged the colonic epithelium and induced pERK activation in the spinal cord, and these processes were clearly independent of TRPV1-expressing neurons ablated by RTX. This higher dose of mustard oil induced nociceptive responses and referred mechanical hyperalgesia which were insensitive or only slightly sensitive to resiniferatoxin or AP18, but were markedly reduced by the P2X antagonist TNP-ATP, which is known to inhibit nociceptive actions induced by ATP released from injured tissues. In conclusion, whereas a low dose of intracolonic mustard oil induces visceral pain in a manner fully dependent on TRPA1 actions, when a high dose of this chemical irritant is used, visceral pain becomes mostly independent of TRPA1 activation but clearly enhanced by ATP purportedly released by the damaged colonic epithelium. Therefore, TRPA1 inhibition is not sufficient to substantially decrease visceral pain during tissue injury, whereas purinergic antagonism appears to be a more effective strategy.

Modality-specific peripheral antinociceptive effects of μ-opioid agonists on heat and mechanical stimuli: Contribution of sigma-1 receptors

Morphine induces peripherally μ-opioid-mediated antinociception to heat but not to mechanical stimulation. Peripheral sigma-1 receptors tonically inhibit μ-opioid antinociception to mechanical stimuli, but it is unknown whether they modulate μ-opioid heat antinociception. We hypothesized that sigma-1 receptors might play a role in the modality-specific peripheral antinociceptive effects of morphine and other clinically relevant μ-opioid agonists. Mechanical nociception was assessed in mice with the paw pressure test (450 g), and heat nociception with the unilateral hot plate (55 °C) test. Local peripheral (intraplantar) administration of morphine, buprenorphine or oxycodone did not induce antinociception to mechanical stimulation but had dose-dependent antinociceptive effects on heat stimuli. Local sigma-1 antagonism unmasked peripheral antinociception by μ-opioid agonists to mechanical stimuli, but did not modify their effects on heat stimulation. TRPV1+ and IB4+ cells are segregated populations of small neurons in the dorsal root ganglia (DRG) and the density of sigma-1 receptors was higher in IB4+ cells than in the rest of small nociceptive neurons. The in vivo ablation of TRPV1-expressing neurons with resiniferatoxin did not alter IB4+ neurons in the DRG, mechanical nociception, or the effects of sigma-1 antagonism on local morphine antinociception in this type of stimulus. However, it impaired the responses to heat stimuli and the effect of local morphine on heat nociception. In conclusion, peripheral opioid antinociception to mechanical stimuli is limited by sigma-1 tonic inhibitory actions, whereas peripheral opioid antinociception to heat stimuli (produced in TRPV1-expressing neurons) is not. Therefore, sigma-1 receptors contribute to the modality-specific peripheral effects of opioid analgesics.

Sigma-1 receptors control immune-driven peripheral opioid analgesia during inflammation in mice

Sigma-1 antagonism potentiates the antinociceptive effects of opioid drugs, so sigma-1 receptors constitute a biological brake to opioid drug-induced analgesia. The pathophysiological role of this process is unknown. We aimed to investigate whether sigma-1 antagonism reduces inflammatory pain through the disinhibition of the endogenous opioidergic system in mice. The selective sigma-1 antagonists BD-1063 and S1RA abolished mechanical and thermal hyperalgesia in mice with carrageenan-induced acute (3 h) inflammation. Sigma-1-mediated antihyperalgesia was reversed by the opioid antagonists naloxone and naloxone methiodide (a peripherally restricted naloxone analog) and by local administration at the inflamed site of monoclonal antibody 3-E7, which recognizes the pan-opioid sequence Tyr-Gly-Gly-Phe at the N terminus of most endogenous opioid peptides (EOPs). Neutrophils expressed pro-opiomelanocortin, the precursor of β-endorphin (a known EOP), and constituted the majority of the acute immune infiltrate. β-endorphin levels increased in the inflamed paw, and this increase and the antihyperalgesic effects of sigma-1 antagonism were abolished by reducing the neutrophil load with in vivo administration of an anti-Ly6G antibody. The opioid-dependent sigma-1 antihyperalgesic effects were preserved 5 d after carrageenan administration, where macrophages/monocytes were found to express pro-opiomelanocortin and to now constitute the majority of the immune infiltrate. These results suggest that immune cells harboring EOPs are needed for the antihyperalgesic effects of sigma-1 antagonism during inflammation. In conclusion, sigma-1 receptors curtail immune-driven peripheral opioid analgesia, and sigma-1 antagonism produces local opioid analgesia by enhancing the action of EOPs of immune origin, maximizing the analgesic potential of immune cells that naturally accumulate in painful inflamed areas.

Mild Social Stress in Mice Produces Opioid-Mediated Analgesia in Visceral but Not Somatic Pain States

Visceral pain has a greater emotional component than somatic pain. To determine if the stress-induced analgesic response is differentially expressed in visceral versus somatic pain states, we studied the effects of a mild social stressor in either acute visceral or somatic pain states in mice. We show that the presence of an unfamiliar conspecific mouse (stranger) in an adjacent cubicle of a standard transparent observation box produced elevated plasma corticosterone levels compared with mice tested alone, suggesting that the mere presence of a stranger is stressful. We then observed noxious visceral or somatic stimulation-induced nociceptive behavior in mice tested alone or in mildly stressful conditions (ie, beside an unfamiliar stranger). Compared with mice tested alone, the presence of a stranger produced a dramatic opioid-dependent reduction in pain behavior associated with visceral but not somatic pain. This social stress-induced reduction of visceral pain behavior relied on visual but not auditory/olfactory cues. These findings suggest that visceral pain states may provoke heightened responsiveness to mild stressors, an effect that could interfere with testing outcomes during simultaneous behavioral testing of multiple rodents.

PERSPECTIVE

In mice, mild social stress due to the presence of an unfamiliar conspecific mouse reduces pain behavior associated with noxious visceral but not somatic stimulation, suggesting that stress responsiveness may be enhanced in visceral pain versus somatic pain states.

Sigma-1 receptor inhibition reverses acute inflammatory hyperalgesia in mice: role of peripheral sigma-1 receptors

RATIONALE

Sigma-1 (σ1) receptor inhibition ameliorates neuropathic pain by inhibiting central sensitization. However, it is unknown whether σ1 receptor inhibition also decreases inflammatory hyperalgesia, or whether peripheral σ1 receptors are involved in this process.

OBJECTIVE

The purpose of this study was to determine the role of σ1 receptors in carrageenan-induced inflammatory hyperalgesia, particularly at the inflammation site.

RESULTS

The subcutaneous (s.c.) administration of the selective σ1 antagonists BD-1063 and S1RA to wild-type mice dose-dependently and fully reversed inflammatory mechanical (paw pressure) and thermal (radiant heat) hyperalgesia. These antihyperalgesic effects were abolished by the s.c. administration of the σ1 agonist PRE-084 and also by the intraplantar (i.pl.) administration of this compound in the inflamed paw, suggesting that blockade of peripheral σ1 receptors in the inflamed site is involved in the antihyperalgesic effects induced by σ1 antagonists. In fact, the i.pl. administration of σ1 antagonists in the inflamed paw (but not in the contralateral paw) was sufficient to completely reverse inflammatory hyperalgesia. σ1 knockout (σ1-KO) mice did not develop mechanical hyperalgesia but developed thermal hypersensitivity; however, the s.c. administration of BD-1063 or S1RA had no effect on thermal hyperalgesia in σ1-KO mice, supporting on-target mechanisms for the effects of both drugs. The antiedematous effects of σ1 inhibition do not account for the decreased hyperalgesia, since carrageenan-induced edema was unaffected by σ1 knockout or systemic σ1 pharmacological antagonism.

CONCLUSIONS

σ1 receptors play a major role in inflammatory hyperalgesia. Targeting σ1 receptors in the inflamed tissue may be useful for the treatment of inflammatory pain.

Genetic inactivation and pharmacological blockade of sigma-1 receptors prevent paclitaxel-induced sensory-nerve mitochondrial abnormalities and neuropathic pain in mice

BACKGROUND

Paclitaxel, a widely-used antineoplastic drug, produces a painful peripheral neuropathy that in rodents is associated with peripheral-nerve mitochondrial alterations. The sigma-1 receptor (σ1R) is a ligand-regulated molecular chaperone involved in mitochondrial calcium homeostasis and pain hypersensitivity. This receptor plays a key role in paclitaxel-induced neuropathic pain, but it is not known whether it also modulates mitochondrial abnormalities.In this study, we used a mouse model of paclitaxel-induced neuropathic pain to test the involvement of the σ1R in the mitochondrial abnormalities associated with paclitaxel, by using genetic (σ1R knockout mice) and pharmacological (σ1R antagonist) approaches.

RESULTS

Paclitaxel administration to wild-type (WT) mice produced cold- and mechanical-allodynia, and an increase in the frequency of swollen and vacuolated mitochondria in myelinated A-fibers, but not in C-fibers, of the saphenous nerve. Behavioral and mitochondrial alterations were marked at 10 days after paclitaxel-administration and had resolved at day 28. In contrast, paclitaxel treatment did not induce allodynia or mitochondrial abnormalities in σ1R knockout mice. Moreover, the prophylactic treatment of WT mice with BD-1063 also prevented the neuropathic pain and mitochondrial abnormalities induced by paclitaxel.

CONCLUSIONS

These results suggest that activation of the σ1R is necessary for development of the sensory nerve mitochondrial damage and neuropathic pain produced by paclitaxel. Therefore, σ1R antagonists might have therapeutic value for the prevention of paclitaxel-induced neuropathy.

Antiallodynic and analgesic effects of maslinic acid, a pentacyclic triterpenoid from Olea europaea

The effects of maslinic acid (1), a pentacyclic triterpenoid obtained from Olea europaea, were studied in several tests for nociception in mice. Systemic administration of 1 reduced acetic acid-induced writhing, the inflammatory phase of formalin-induced pain, and capsaicin-induced mechanical allodynia. However, it did not induce motor incoordination in the rotarod test. The topical administration of 1 also reduced the inflammatory phase of the formalin test, indicating that at least some of its effects are mediated peripherally. The present results demonstrate for the first time that maslinic acid induces antinociceptive and antiallodynic effects.

Pharmacological properties of S1RA, a new sigma-1 receptor antagonist that inhibits neuropathic pain and activity-induced spinal sensitization

BACKGROUND AND PURPOSE

The sigma-1 (σ(1) ) receptor is a ligand-regulated molecular chaperone that has been involved in pain, but there is limited understanding of the actions associated with its pharmacological modulation. Indeed, the selectivity and pharmacological properties of σ(1) receptor ligands used as pharmacological tools are unclear and the demonstration that σ(1) receptor antagonists have efficacy in reversing central sensitization-related pain sensitivity is still missing.

EXPERIMENTAL APPROACH

The pharmacological properties of a novel σ(1) receptor antagonist (S1RA) were first characterized. S1RA was then used to investigate the effect of pharmacological antagonism of σ(1) receptors on in vivo nociception in sensitizing conditions and on in vitro spinal cord sensitization in mice. Drug levels and autoradiographic, ex vivo binding for σ(1) receptor occupancy were measured to substantiate behavioural data.

KEY RESULTS

Formalin-induced nociception (both phases), capsaicin-induced mechanical hypersensitivity and sciatic nerve injury-induced mechanical and thermal hypersensitivity were dose-dependently inhibited by systemic administration of S1RA. Occupancy of σ(1) receptors in the CNS was significantly correlated with the antinociceptive effects. No pharmacodynamic tolerance to the antiallodynic and antihyperalgesic effect developed following repeated administration of S1RA to nerve-injured mice. As a mechanistic correlate, electrophysiological recordings demonstrated that pharmacological antagonism of σ(1) receptors attenuated the wind-up responses in spinal cords sensitized by repetitive nociceptive stimulation.

CONCLUSIONS AND IMPLICATIONS

These findings contribute to evidence identifying the σ(1) receptor as a modulator of activity-induced spinal sensitization and pain hypersensitivity, and suggest σ(1) receptor antagonists as potential novel treatments for neuropathic pain.

Sigma-1 receptors do not regulate calcium influx through voltage-dependent calcium channels in mouse brain synaptosomes

Several lines of evidence suggest that σ(1) receptors regulate intracellular calcium concentration [Ca(2+)](i). However, no previous studies have demonstrated a consistent role for these receptors in the modulation of extracellular calcium entry through plasmalemmal voltage-dependent calcium channels (VDCCs). To search for evidence of such a role we compared [Ca(2+)](i) under basal conditions and after depolarization with KCl in fura-2-loaded synaptosomes from wild-type and σ(1) receptor knockout (σ(1)R-KO) mice. We also tested the effects of the selective σ(1) receptor agonists PRE-084 and (+)-pentazocine and antagonists BD-1047 and NE-100 on the increase in [Ca(2+)](i) induced by depolarization with 60mM KCl. Mibefradil, a nonselective blocker of VDCCs, was used as a positive control. Basal [Ca(2+)](i) and the increase in [Ca(2+)](i) caused by KCl-induced depolarization were similar in brain synaptosomes from both wild-type and σ(1)R-KO mice. Mibefradil (1-30 μM) and all σ(1) receptor ligands studied (3-100 μM) inhibited the KCl-induced increase in [Ca(2+)](i) in a concentration-dependent way. The order of maximum inhibition for the ligands compared here was NE-100>BD-1047=PRE 084>(+)-pentazocine. There were no appreciable differences in their effects between wild-type and σ(1)R-KO mice. These findings indicate that σ(1) receptors are not involved in calcium influx through VDCCs or in the inhibitory effects of these σ(1) ligands on Ca(2+) channels.

Antagonism by haloperidol and its metabolites of mechanical hypersensitivity induced by intraplantar capsaicin in mice: role of sigma-1 receptors

RATIONALE

We evaluated the effects of haloperidol and its metabolites on capsaicin-induced mechanical hypersensitivity (allodynia) and on nociceptive pain induced by punctate mechanical stimuli in mice.

RESULTS

Subcutaneous administration of haloperidol or its metabolites I or II (reduced haloperidol) dose-dependently reversed capsaicin-induced (1 microg, intraplantar) mechanical hypersensitivity of the hind paw (stimulated with a nonpainful, 0.5-g force, punctate stimulus). The order of potency of these drugs to induce antiallodynic effects was the order of their affinity for brain sigma-1 (sigma(1)) receptor ([(3)H](+)-pentazocine-labeled). Antiallodynic activity of haloperidol and its metabolites was dose-dependently prevented by the selective sigma(1) receptor agonist PRE-084, but not by naloxone. These results suggest the involvement of sigma(1) receptors, but discard any role of the endogenous opioid system, on the antiallodynic effects. Dopamine receptor antagonism also appears unlikely to be involved in these effects, since the D(2)/D(3) receptor antagonist (-)-sulpiride, which had no affinity for sigma(1) receptors, showed no antiallodynic effect. None of these drugs modified hind-paw withdrawal after a painful (4 g force) punctate mechanical stimulus in noncapsaicin-sensitized animals. As expected, the control drug gabapentin showed antiallodynic but not antinociceptive activity, whereas clonidine exhibited both activities and rofecoxib, used as negative control, showed neither.

CONCLUSION

These results show that haloperidol and its metabolites I and II produce antiallodynic but not antinociceptive effects against punctate mechanical stimuli and suggest that their antiallodynic effect may be due to blockade of sigma(1) receptors but not to dopamine receptor antagonism.

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 [(3)H](+)-pentazocine binding to sigma(1) receptors in SH-SY5Y human neuroblastoma cells and guinea pig brain P(1), P(2) and P(3) subcellular fractions. Three days after a single i.p. injection in guinea pigs of HP (but not of other sigma(1) antagonists or (-)-sulpiride), [(3)H](+)-pentazocine binding to brain membranes was markedly decreased. Recovery of sigma(1) receptor density to steady state after HP-induced inactivation required more than 30 days. HP-metabolite II (reduced HP, 4-(4-chlorophenyl)-alpha-(4-fluorophenyl)-4-hydroxy-1-piperidinebutanol), but not HP-metabolite I (4-(4-chlorophenyl)-4-hydroxypiperidine), irreversibly blocked sigma(1) receptors in guinea pig brain homogenate and P(2) 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 sigma(1) receptors when it was incubated with brain homogenate and SH-SY5Y cells, but not when incubated with P(2) fraction membranes, which suggests that HP is metabolized to inactivate sigma(1) receptors. Menadione, an inhibitor of the ketone reductase activity that leads to the production of HP-metabolite II, completely prevented HP-induced inactivation of sigma(1) receptors in brain homogenates. These results suggest that HP may irreversibly inactivate sigma(1) receptors in guinea pig and human cells, probably after metabolism to reduced HP.

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.

Antinociceptive effects of haloperidol and its metabolites in the formalin test in mice

RATIONALE

Formalin-induced pain is reduced in sigma-1 (sigma1) receptor knockout mice; therefore, we hypothesized that haloperidol and its metabolites I and II, which have affinity for sigma1 receptors, may modulate formalin-induced pain.

RESULTS

Intraplantar administration of formalin (2.5%) to CD-1 mice produced a biphasic period of pain. Haloperidol (0.03-1 mg/kg, s.c.) and reduced haloperidol (metabolite II, 0.25-8 mg/kg, s.c.) dose-dependently inhibited both phases of formalin-induced pain. Haloperidol metabolite I (4-128 mg/kg, s.c.) also produced dose-dependent antinociception in the second phase of the formalin test, but was less potent and effective against first-phase pain. Haloperidol metabolite III (16 and 128 mg/kg) and (-)sulpiride (200 mg/kg), which have no affinity for sigma1 receptors, did not produce significant antinociception in either phase of the formalin test. The order of potency of the drugs to produce their antinociceptive effect [haloperidol>metabolite II>metabolite I>>metabolite III= (-)sulpiride=inactive] correlated with their affinity for sigma1 receptors, but not with their affinity for sigma2 or dopamine D2 receptors. Naloxone (1 mg/kg, s.c.) did not antagonize the antinociception induced by haloperidol and its metabolites. None of the antinociceptive drugs in the formalin test produced any antinociception in the tail flick test.

CONCLUSION

These results suggest that the antinociceptive effect of haloperidol and its metabolites in the formalin test is not due to unspecific/generalised inhibition of nociception or modulation of opioid receptors, and that it may be related, at least partially, to the ability of these drugs to interact with sigma1 receptors.

Phenytoin differentially modulates the affinity of agonist and antagonist ligands for sigma 1 receptors of guinea pig brain

We evaluated the effects of phenytoin (DPH) on the affinity for sigma-1 (sigma(1)) receptors of agonist or antagonist sigma(1) ligands in guinea pig brain. Heterologous competition experiments showed that DPH (250 microM and 1 mM) concentration-dependently increased the affinity of the sigma(1) agonists dextromethorphan, (+)-SKF-10,047, (+)-3-PPP, and PRE-084. However, neither DPH 250 microM nor 1 mM increased (in fact, they slightly decreased) the affinity of the sigma(1) receptor antagonists haloperidol, BD 1063, NE-100, progesterone, and BD 1047. These findings suggest that allosteric modulation by DPH of the affinity of sigma(1) receptor ligands depends on the agonist or antagonist characteristics of the ligand. Therefore, determining in vitro the differential modulation by DPH of sigma(1) ligand affinity appears to constitute a procedure that can predict the pharmacological profile of different sigma(1) ligands.

Formalin-induced pain is reduced in sigma(1) receptor knockout mice

The role of sigma1 receptors in non-acute pain has not been explored. In this study we show that both phases of formalin-induced pain were reduced by approximately 55% in sigma1 receptor knockout mice in comparison to wild-type animals. These results suggest that the tonic pain induced by formalin is altered in mice lacking sigma1 receptors, and highlight the potential usefulness of further studies of the role of sigma1 receptors in models of non-acute pain.

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.

Differences in the allosteric modulation by phenytoin of the binding properties of the σ1 ligands [3H](+)-pentazocine and [3H]NE-100

The present study evaluated the effects of phenytoin (DPH) on the binding to synaptosomal fraction membranes from guinea pig brain of the prototypic sigma1 (sigma1) receptor agonist [3H](+)-pentazocine and the putative sigma1 antagonist [3H]NE-100. Equilibrium and binding kinetics studies were done. The order of affinity of 12 sigma1 ligands for binding sites labeled with [3H](+)-pentazocine correlated well with their order of affinity for sites labeled with [3H]NE-100, suggesting that both radioligands label the same receptor. Phenytoin increased the binding of [3H](+)-pentazocine, enhancing its affinity (K(D) value) for sigma1 receptors and decreasing its dissociation rate from these receptors. The maximal number of receptors (B(max) value) labeled with [3H](+)-pentazocine was not changed. In contrast, phenytoin decreased the specific binding and maximal number of receptors labeled with [3H]NE-100, and increased its dissociation rate from sigma1 receptors. The affinity of this radioligand for sigma1 receptors was not modified. In conclusion, phenytoin behaved as a positive allosteric modulator on the binding of [3H](+)-pentazocine, whereas it negatively modulated the binding of [3H]NE-100. These results add evidence in favor of the use of phenytoin in vitro to distinguish between agonists and antagonists of sigma1 receptors.

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.

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.

The NMDA receptor antagonist dizocilpine (MK-801) stereoselectively inhibits morphine-induced place preference conditioning in mice

The effect of the non-competitive NMDA receptor antagonist dizocilpine (MK-801) on conditioned place preference induced by morphine was studied in mice. As expected, morphine (1-8 mg/kg, i.p.) elicited a significant preference for the drug-paired compartment. Pretreatment of mice with (+)-dizocilpine (0.1 and 0.2 mg/kg, i.p.), the more active dizocilpine enantiomer, dose-dependently reversed the conditioned place preference produced by morphine (4 mg/kg, i.p.), whereas (-)-dizocilpine (0.2 mg/kg, i.p.) did not modify morphine-induced effects. In contrast, both enantiomers of dizocilpine (at a dose of 0.2 mg/kg, i.p.) elicited a conditioned place preference. These data suggest that (1) NMDA receptors play a role in morphine-induced place preference, and (2) dizocilpine-reinforcing properties in the place preference paradigm do not seem to be dependent on NMDA receptor blockade.

Cromakalim differentially enhances antinociception induced by agonists of alpha(2)adrenoceptors, gamma-aminobutyric acid(B), mu and kappa opioid receptors

The influence of the ATP-sensitive K+(KATP) channel opener cromakalim on the antinociception induced by agonists of several receptors coupled to pertussis toxin-sensitive G proteins, clonidine (alpha2 adrenoceptor), baclofen (gamma-aminobutyric acid(B) receptor), morphine (mu opioid receptor) and U50,488H (kappa opioid receptor), was evaluated with a tail-flick test in mice. The subcutaneous administration of clonidine (0.12-2 mg/kg), morphine (0.5-16 mg/kg), baclofen (2-16 mg/kg) and U50,488H (2-16 mg/kg) induced a dose-dependent antinociceptive effect. Cromakalim (8-64 microgram/mouse intracerebroventricularly [i.c.v.]) did not change tail-flick latency in control animals but produced a dose-dependent enhancement of the antinociception induced by clonidine and morphine, and shifted their dose-response curves to the left. These effects of cromakalim were antagonized dose dependently by the K(ATP) channel blocker gliquidone (0.1-8 microgram/mouse i.c.v.). On the other hand, cromakalim (16-64 microgram/mouse i.c.v.) did not significantly enhance the antinociception induced by baclofen and U50,488H and did not shift their dose-response curves. These results suggest that opening of the K(ATP) channels plays an important role in the antinociception mediated by alpha(2) adrenoceptors and mu opioid receptors, but not in that induced by gamma-aminobutyric acid(B) and kappa opioid receptors.

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.

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.

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 micrograms/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 micrograms/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.

Electron probe microanalysis of gentamicin-induced changes on ionic composition of the vestibular gelatinous membrane

Gentamicin-induced changes in ionic composition in the otolithic membrane of adult OF1 mice were evaluated in the gelatinous layers of the saccule and utricle by quantitative electron probe X-ray microanalysis. The otolithic membranes were plunge-frozen and freeze-dried to prevent the redistribution of elements. Quantitative analysis was carried out with an energy dispersive detector using the peak-to-background (P/B) ratio method and different salts dissolved in dextran as standards to calibrate the P/B ratio against the concentration of the elements P, S and K in the microprobe. Gentamicin selectively decreased the concentrations of P (P < 0.001) and S (P < 0.01) in the gelatinous membrane of the saccule, and had no effect in the utricle. The concentration of K also increased in the utricular gelatinous membrane (P < 0.05). The mechanism of ototoxicity in the gelatinous membrane is unknown, but the ability of aminoglycosides to block calcium channels may induce disturbances in the ionic equilibrium of the endolymphatic fluid, and thus affect the biochemical composition of the gelatinous membrane. This technique can be useful to evaluate the distribution of ions in the process of drug-induced ototoxicity.

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.

Gentamicin ototoxicity in otoconia: quantitative electron probe X-ray microanalysis

Chronic gentamicin ototoxicity was evaluated in the otolithic membrane of adult OF1 mice at the otoconial layer of the saccule and utricle by quantitative electron probe X-ray microanalysis of Ca and K. The otolithic membranes were plunge-frozen and freeze-dried. The analysis was carried out with an energy dispersive detector using the peak-to-back-ground ratio method and different inorganic salts of Ca and K as standards to calibrate the microprobe. Ca and K in the otoconia are related via a linear function in both the saccule and the utricle. This association is not maintained after exposure to gentamicin, which suggests that this aminoglycoside antibiotic interferes with the Ca-K equilibrium in the otoconia. A dose of 200 mg/kg gentamicin twice a day for 5 days did not affect Ca in the mineral phase of the otoconia, but did increase K in both saccular (p < 0.05) and utricular (p < 0.01) otoconia. These increases in K may reflect a modification in the composition of the endolymph, resulting from cellular damage at the plasma membrane.

Role of L-type calcium channels on yohimbine-precipitated clonidine withdrawal in vivo and in vitro

This study was designed to elucidate the possible participation of L-type calcium channels in the expression of clonidine-withdrawal precipitated by yohimbine in clonidine-dependent animals. Mice implanted for 5 days with osmotic minipumps containing the alpha 2-adrenoceptor agonist clonidine showed symptoms of a withdrawal syndrome (jerks, headshakes, defecations and weight loss) when yohimbine, an alpha 2-adrenoceptor antagonist, was injected. Similarly, isolated rat ilea incubated with clonidine in vitro showed a withdrawal contracture when yohimbine was added to the organ bath. The effects of L-type calcium channel blockers (verapamil and diltiazem) and the stimulant Bay K 8644 on these two different types of withdrawal responses were evaluated. A dose-dependent decrease in yohimbine-precipitated clonidine withdrawal in vivo was observed when verapamil (10-40 mg/kg, s.c. and 120 micrograms/mouse, i.c.v.) or diltiazem (5-20 mg/kg, s.c. and 160 micrograms/mouse, i.c.v.) were administered to mice dependent on clonidine. No effect was found after Bay K 8644 (0.5-5 mg/kg, s.c. and 1-5 micrograms/mouse) was injected under these conditions. In vitro, both verapamil (0.1-5 microM) and D-cis-diltiazem (1-50 microM) concentration-dependently reduced the height of the yohimbine-precipitated withdrawal contracture in rat ileum incubated with clonidine. Furthermore, the effect of diltiazem was stereospecific, as D-cis-diltiazem 10 microM markedly inhibited clonidine withdrawal, whereas the same concentration of L-cis-diltiazem had no effect. In contrast, the calcium channel stimulant Bay K 8644 (0.1-1 microM) increased the height of the ileum withdrawal contracture.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

ATP-dependent 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.

Centrally administered aminoglycoside antibiotics antagonize naloxone-precipitated withdrawal in mice acutely dependent on morphine

The effects of i.c.v. administration of several aminoglycoside antibiotics on naloxone-precipitated morphine withdrawal symptoms were evaluated in mice acutely dependent on morphine. Neomycin (10-40 micrograms/mouse), gentamicin (40-160 micrograms/mouse) and kanamycin (80-320 micrograms/mouse) produced a dose-dependent reduction of the number of precipitated jumps, forepaw tremors and head shakes. The order of potency of the aminoglycoside antibiotics on all withdrawal symptoms was neomycin > gentamicin > kanamycin, which is the same order that these drugs show as N-type calcium channel blockers. The capacity of several drugs that decrease neuronal calcium availability (such as lanthanum and L-type calcium channel blockers) to antagonize opiate withdrawal is well known. In the light of these findings, our results suggest that the mechanism of aminoglycoside-induced inhibition of morphine abstinence may be related to the capacity of these antibiotics to block N-type calcium channels, and to decrease neuronal calcium availability.

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 0.5 mg/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 (2-40 micrograms/kg) and succinylcholine (6-200 micrograms/kg). The order of potency in increasing the effects of pancuronium was nicardipine much greater than d-cis diltiazem greater than or equal to verapamil, whereas the order of potency in enhancing succinylcholine effects was d-cis diltiazem greater than or equal to verapamil much greater than 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.

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.

Differential effects of L-type calcium channel blockers and stimulants on naloxone-precipitated withdrawal in mice acutely dependent on morphine

The effects of L-type calcium channel blockers and stimulants on naloxone-precipitated withdrawal in mice acutely dependent on morphine were evaluated. Verapamil (10-80 mg/kg), diltiazem (20-120 mg/kg) and nicardipine (20-160 mg/kg), when administered subcutaneously, produced a dose-dependent reduction in forepaw tremor and weight loss during the abstinence reaction; jumping was also reduced by all three drugs, although the effect was not statistically significant in the case of nicardipine. By contrast, the calcium agonist Bay K 8644 (0.5-2 mg/kg, SC) increased forepaw tremor and weight loss, although this latter effect did not reach statistical significance. The effects of the calcium channel active drugs on the rotarod test were also explored, no correlation appearing with the results observed in abstinence (except for the jumping response), which suggests that the withdrawal results are not influenced by motor incoordination or unspecific CNS depression. These findings suggest that L-type calcium channels probably play an important role in withdrawal after acute morphine dependence. Taken together with other observations in chronic models, these results show that calcium channels are similarly involved in morphine abstinence after acute and chronic dependence, in contrast to the differences in the content and uptake of neuronal calcium induced by morphine under both conditions.

Reversion by calcium but not by Bay K 8644 of neuromuscular blockade induced by nicardipine

We compared the effects of nicardipine on indirectly and directly elicited twitches in phrenic-hemidiaphragm preparations from rats, and the effects of calcium and Bay K 8644 on nicardipine-induced neuromuscular blockade. Nicardipine (5-250 microM) decreased both directly and indirectly elicited muscular contractions in a concentration-dependent way. The inhibitory effect was greater when the preparation was indirectly stimulated. Nicardipine antagonism of indirectly elicited contractions was reversed in an apparently competitive way by calcium (1-4 mM). In contrast, the calcium channel agonist Bay K 8644 (0.1 and 1 microM) had no effects per se, nor did it reverse neuromuscular blockade induced by nicardipine. Interestingly, Bay K 8644, at 10 microM, decreased the height of indirectly elicited muscular contractions. These results suggest that the calcium antagonist nicardipine affects neuromuscular transmission to a greater degree than muscular contraction, this effect being antagonized by calcium. In contrast, the calcium agonist Bay K 8644 was unable to produce any effect as stimulator of calcium channels at the neuromuscular level, indicating that this drug may exhibit tissue specificity.

Analgesic effects of diltiazem and verapamil after central and peripheral administration in the hot-plate test

1. The analgesic effects of diltiazem and verapamil, both per se and together with morphine, were studied using subcutaneous (s.c.) and intracerebroventricular (i.c.v.) administrations, in the hot-plate test in mice. 2. The i.c.v. injection of verapamil (15-120 micrograms/mouse) and diltiazem (60-120 micrograms/mouse) induced dose-dependent analgesic effects. 3. The i.c.v. administration of verapamil (30-120 micrograms/mouse) and diltiazem (60-120 micrograms/mouse) significantly enhanced, in a dose-dependent way, the analgesic effects of morphine and produced a parallel displacement to the left of the morphine log dose-response line. 4. When these calcium channel blockers were administered subcutaneously at doses of 40 and 80 mg/kg, they exerted no analgesic actions, but dose-dependently potentiated the analgesic effects of morphine, producing a parallel shift to the left of the morphine log dose-response line. 5. These results suggest that inhibition of calcium entry through calcium channels induced by verapamil and diltiazem may play a role in analgesia development.

Comparison of the effects of calcium and the calcium channel stimulant Bay k 8644 on neomycin-induced neuromuscular blockade

The effects of calcium and the calcium channel stimulant Bay k 8644 on neomycin-induced neuromuscular blockade were evaluated using rat phrenic nerve-hemidiaphragm preparations in vitro. neomycin showed maximum potency in inducing neuromuscular blockade when the calcium concentration in the bath was 1 mM. Higher calcium concentrations (1.5, 2 and 4 mM) produced a gradual decrease in neomycin potency, manifested as a progressive shift to the right of neomycin concentration-response curves. Bay k 8644 (0.1 and 1 microM) did not significantly modify indirectly elicited diaphragm contractions per se, nor did it antagonize neomycin-induced neuromuscular blockade. A higher concentration of Bay k 8644 (10 microM) antagonized indirectly elicited contractions. In conclusion, our results show that Bay k 8644, at concentrations causing calcium channel stimulation at the cardiac and vascular level, did not exert stimulant effects in a nerve-skeletal muscle preparation, suggesting that its action may be tissue-selective to a certain degree.

Neuromuscular blockade induced by flunarizine alone and in combination with pancuronium, suxamethonium or neomycin: studies in isolated rat phrenic-hemidiaphragm preparations

The in vitro effects of flunarizine on indirectly- and directly-elicited contractions in rat phrenic-hemidiaphragm preparations were studied. The interactions of flunarizine with non-depolarizing and depolarizing neuromuscular blocking drugs (pancuronium and suxamethonium) and with an aminoglycoside antibiotic (neomycin) were also evaluated. Flunarizine induced a slowly developing concentration-dependent reduction of indirectly-elicited diaphragm twitch height, but only slightly reduced directly-elicited contractions. Flunarizine 1 and 5 mumol.l-1 produced a concentration-dependent enhancement of pancuronium-induced neuromuscular blockade, whereas suxamethonium blockade was significantly increased by flunarizine 5 mumols.l.-1 only. Moreover, both flunarizine 1 and 5 mumols.l-1 also increased the neuromuscular blockade induced by neomycin. In conclusion, flunarizine induced neuromuscular blockade and enhanced the effects of several neuromuscular blocking agents to varying degrees in vitro.

Differential effects of calcium channel blockers and stimulants on morphine withdrawal in vitro

The effects of calcium channel blockers and stimulants on naloxone-precipitated morphine withdrawal in morphine-dependent ileum were evaluated in vitro. Both verapamil and diltiazem (0.01-1 microM) inhibited the naloxone-induced morphine withdrawal contractures in a concentration-dependent way. The effect of diltiazem was stereo-specific. On the other hand, the calcium channel stimulant, Bay k 8644 (0.01 microM), significantly increased the naloxone-induced contractures of morphine-dependent ileum. These results suggest a role for calcium channels in morphine withdrawal in vitro.