Roles of opioid receptor subtypes on the antinociceptive effect of intrathecal sildenafil in the formalin test of rats

Endogenous opioid system dysregulation in depression: implications for new therapeutic approaches

The United States is in the midst of an opioid addiction and overdose crisis precipitated and exacerbated by use of prescription opioid medicines. The majority of opioid prescriptions are dispensed to patients with comorbid mood disorders including major depressive disorder (MDD). A growing body of research indicates that the endogenous opioid system is directly involved in the regulation of mood and is dysregulated in MDD. This involvement of the endogenous opioid system may underlie the disproportionate use of opioids among patients with mood disorders. Emerging approaches to address endogenous opioid dysregulation in MDD may yield novel therapeutics that have a low or absent risk of abuse and addiction relative to µ-opioid agonists. Moreover, agents targeting the endogenous opioid system would be expected to yield clinical benefits qualitatively different from conventional monaminergic antidepressants. The development of safe and effective agents to treat MDD-associated endogenous opioid dysregulation may represent a distinct and currently underappreciated means of addressing treatment resistant depression with the potential to attenuate the on-going opioid crisis.

The synthetic peptide PnPP-19 induces peripheral antinociception via activation of NO/cGMP/KATP pathway: Role of eNOS and nNOS

BACKGROUND

and purpose: The peptide PnPP-19, derived from the spider toxin PnTx2-6 (renamed as δ-CNTX-Pn1c), potentiates erectile function by activating the nitrergic system. Since NO has been studied as an antinociceptive molecule and PnPP-19 is known to induce peripheral antinociception, we intended to evaluate whether PnPP-19 could induce peripheral antinociception through activation of this pathway.

EXPERIMENTAL APPROACH

Nociceptive thresholds were measured by paw pressure test. PGE₂ (2 μg/paw) was administered intraplantarly together with PnPP-19 and inhibitors/blockers of NOS, guanylyl cyclase and K_ATP channels. The nitrite concentration was accessed by Griess test. The expression and phosphorylation of eNOS and nNOS were determined by western blot.

KEY RESULTS

PnPP-19 (5, 10 and 20 μg/paw) induced peripheral antinociception in rats. Administration of NOS inhibitor (L-NOarg), selective nNOS inhibitor (L-NPA), guanylyl cyclase inhibitor (ODQ) and the blocker of K_ATP (glibenclamide) partially inhibited the antinociceptive effect of PnPP-19 (10 μg/paw). Tissue nitrite concentration increased after PnPP-19 (10 μg/paw) administration. Expression of eNOS and nNOS remained the same in all tested groups, however the phosphorylation of nNOS Ser852 (inactivation site) increased and phosphorylation of eNOS Ser1177 (activation site) decreased after PGE₂ injection. Administration of PnPP-19 reverted this PGE₂-induced effect.

CONCLUSIONS AND IMPLICATIONS

The peripheral antinociceptive effect induced by PnPP-19 is resulting from activation of NO-cGMP-K_ATP pathway. Activation of eNOS and nNOS might be required for such effect. Our results suggest PnPP-19 as a new drug candidate to treat pain and reinforce the importance of nNOS and eNOS activation, as well as endogenous NO release, for induction of peripheral antinociception.

PnPP-19, a spider toxin peptide, induces peripheral antinociception through opioid and cannabinoid receptors and inhibition of neutral endopeptidase

BACKGROUND AND PURPOSE

The synthetic peptide PnPP-19 has been studied as a new drug candidate to treat erectile dysfunction. However, PnTx2-6, the spider toxin from which the peptide was designed, induces hyperalgesia. Therefore, we intended to investigate the role of PnPP-19 in the nociceptive pathway.

EXPERIMENTAL APPROACH

Nociceptive thresholds were measured by paw pressure test. PnPP-19 was administered intraplantarly alone or with selective cannabinoid or opioid receptor antagonists. The hydrolysis of PnPP-19 by neutral endopeptidase (NEP) (EC 3.4.24.11), an enzyme that cleaves enkephalin, was monitored by HPLC and the cleavage sites were deduced by LC-MS. Inhibition by PnPP-19 and Leu-enkephalin of NEP enzyme activity was determined spectrofluorimetrically.

KEY RESULTS

PnPP-19 (5, 10 and 20 μg per paw) induced peripheral antinociception in rats. Specific antagonists of μ opioid receptors (clocinnamox), δ opioid receptors (naltrindole) and CB1 receptors (AM251) partly inhibited the antinociceptive effect of PnPP-19. Inhibition of fatty acid amide hydrolase by MAFP or of anandamide uptake by VDM11 enhanced PnPP-19-induced antinociception. NEP cleaved PnPP-19 only after a long incubation, and Ki values of 35.6 ± 1.4 and 14.6 ± 0.44 μmol·L(-1) were determined for PnPP-19 and Leu-enkephalin respectively as inhibitors of NEP activity.

CONCLUSIONS AND IMPLICATIONS

Antinociception induced by PnPP-19 appears to involve the inhibition of NEP and activation of CB1, μ and δ opioid receptors. Our data provide a greater understanding of the antinociceptive effects of PnPP-19. This peptide could be useful as a new antinociceptive drug candidate.

Effect of sildenafil, a selective phosphodiesterase 5 inhibitor, on the anticonvulsant action of some antiepileptic drugs in the mouse 6-Hz psychomotor seizure model

Sildenafil, a selective phosphodiesterase 5 inhibitor (PDE5), has been recently reported to have both pro- and anticonvulsant action in various experimental models of seizures and epilepsy. Furthermore, it affects anticonvulsant action of some antiepileptic drugs (AEDs) in mice seizure tests and both pharmacodynamic and pharmacokinetic interactions were noted. The present study was carried out to investigate influence of sildenafil on the threshold for 6 Hz-induced psychomotor seizures in mice. Effect of sildenafil on activity of some AEDs, i.e., phenobarbital (PB), clonazepam (CZP), ethosuximide (ETS), valproic acid (VPA), tiagabine (TGB), oxcarbazepine (OXC) and levetiracetam (LEV), in 6 Hz test was also examined. Moreover, combination of sildenafil with LEV was investigated in terms of influence on motor coordination (determined by the chimney test), muscular strength (evaluated in the grip-strength test) and long-term memory (assessed in the passive avoidance task) in mice. To determine the type of pharmacological interaction between sildenafil and LEV, free plasma and total brain concentrations of this AED were determined by LC-MS/MS method. Sildenafil at a dose ranging from 10 to 40 mg/kg statistically increased psychomotor seizure threshold in mice. Moreover, sildenafil enhanced the anticonvulsant action of all the studied AEDs in this test. Interactions between this PDE5 inhibitor and PB, CZP, ETS, TGB and OXC seem to be pharmacodynamic. Since sildenafil increased free plasma and total brain concentration of LEV, interactions between these drugs have pharmacokinetic nature. This kind of interaction was also noted between sildenafil and VPA. Neither LEV (2.32 mg/kg) nor its co-administration with sildenafil (40 mg/kg) produced any significant changes in motor coordination, muscular strength and long-term memory in mice.

The role of adrenergic and cholinergic receptors on the antinociception of sildenafil in the spinal cord of rats

The role played by spinal adrenergic and cholinergic receptors in the antinociceptive effects of intrathecal sildenafil in formalin-induced nociception was examined. Intrathecal catheters were inserted into the subarachnoid space of male Sprague-Dawley rats, and nociception was assessed using the formalin test, consisting of a subcutaneous injection of 50μL of 5% formalin solution into the hind paw. We examined the effects of an alpha 1 adrenergic receptor antagonist (prazosin), an alpha 2 adrenergic receptor antagonist (yohimbine), a muscarinic acetylcholine receptor antagonist (atropine), and a nicotinic acetylcholine receptor antagonist (mecamylamine) on sildenafil-induced antinociception. Intrathecal sildenafil (3, 10, and 30μg) suppressed, in a dose-dependent manner, formalin-induced flinching during phases 1 and 2 of the test. Intrathecal sildenafil (30μg) could not show any effects against intrathecal prazosin (3μg), yohimbine (10μg), atropine (10μg), and mecamylamine (10μg) pretreatment during both phases of the formalin test. These results suggest that intrathecal sildenafil effectively attenuated the pain evoked by formalin injection. Additionally, spinal alpha 1, alpha 2, muscarinic and nicotinic receptors might play a role in sildenafil-induced antinociception.

Apelin-13 limits infarct size and improves cardiac postischemic mechanical recovery only if given after ischemia

We studied whether apelin-13 is cardioprotective against ischemia/reperfusion injury if given as either a pre- or postconditioning mimetic and whether the improved postischemic mechanical recovery induced by apelin-13 depends only on the reduced infarct size or also on a recovery of function of the viable myocardium. We also studied whether nitric oxide (NO) is involved in apelin-induced protection and whether the reported ischemia-induced overexpression of the apelin receptor (APJ) plays a role in cardioprotection. Langendorff-perfused rat hearts underwent 30 min of global ischemia and 120 min of reperfusion. Left ventricular pressure was recorded. Infarct size and lactate dehydrogenase release were determined to evaluate the severity of myocardial injury. Apelin-13 was infused at 0.5 μM concentration for 20 min either before ischemia or in early reperfusion, without and with NO synthase inhibition by N(G)-nitro-l-arginine (l-NNA). In additional experiments, before ischemia also 1 μM apelin-13 was tested. APJ protein level was measured before and after ischemia. Whereas before ischemia apelin-13 (0.5 and 1.0 μM) was ineffective, after ischemia it reduced infarct size from 54 ± 2% to 26 ± 4% of risk area (P < 0.001) and limited the postischemic myocardial contracture (P < 0.001). l-NNA alone increased postischemic myocardial contracture. This increase was attenuated by apelin-13, which, however, was unable to reduce infarct size. Ischemia increased APJ protein level after 15-min perfusion, i.e., after most of reperfusion injury has occurred. Apelin-13 protects the heart only if given after ischemia. In this protection NO plays an important role. Apelin-13 efficiency as postconditioning mimetic cannot be explained by the increased APJ level.

Role of PKG-L-type calcium channels in the antinociceptive effect of intrathecal sildenafil

Sildenafil increases the cyclic guanosine monophosphate (cGMP) by inhibition of a phosphodiesterase 5, thereby leading to an antinociceptive effect. The increased cGMP may exert the effect on an L-type calcium channel through the activation of protein kinase G (PKG). The purpose of this study was to examine the possible involvement of a PKG-L-type calcium channel on the effect of sildenafil at the spinal level. Catheters were inserted into the intrathecal space of male SD rats. Pain was induced by applying 50 µL of a 5% formalin solution to the hindpaw. The sildenafil-induced effect was examined after an intrathecal pretreatment of a PKG inhibitor (KT 5823), or a L-type calcium channel activator (FPL 64176). Intrathecal sildenafil produced an antinociceptive effect during phase 1 (0~10 min interval) and phase 2 (10~60 min interval) in the formalin test. Intrathecal KT 5823 and FPL 64176 attenuated the antinociceptive effect of sildenafil during both phases. Sildenafil is effective against both acute pain and the facilitated pain state at the spinal level. In addition, the inhibition of an L-type calcium channel by activation of the PKG may contribute to the antinocieptive mechanism of sildenafil in the spinal cord.

Roles of opioid receptor subtype in the spinal antinociception of selective cyclooxygenase 2 inhibitor

BACKGROUND

Selective inhibitors of cyclooxygenase (COX)-2 are commonly used analgesics in various pain conditions. Although their actions are largely thought to be mediated by the blockade of prostaglandin (PG) biosynthesis, evidences suggesting endogenous opioid peptide link in spinal antinociception of COX inhibitor have been reported. We investigated the roles of opioid receptor subtypes in the spinal antinociception of selective COX-2 inhibitor.

METHODS

To examine the antinociception of a selective COX-2 inhibitor, DUP-697 was delivered through an intrathecal catheter, 10 minutes before the formalin test in male Sprague-Dawley rats. Then, the effect of intrathecal pretreatment with CTOP, naltrindole and GNTI, which are µ, δ and κ opioid receptor antagonist, respectively, on the analgesia induced by DUP-697 was assessed.

RESULTS

Intrathecal DUP-697 reduced the flinching response evoked by formalin injection during phase 1 and 2. Naltrindole and GNTI attenuated the antinociceptive effect of intrathecal DUP-697 during both phases of the formalin test. CTOP reversed the antinociception of DUP-697 during phase 2, but not during phase 1.

CONCLUSIONS

Intrathecal DUP-697, a selective COX-2 inhibitor, effectively relieved inflammatory pain in rats. The δ and κ opioid receptors are involved in the activity of COX-2 inhibitor on the facilitated state as well as acute pain at the spinal level, whereas the µ opioid receptor is related only to facilitated pain.

Roles of adenosine and serotonin receptors on the antinociception of sildenafil in the spinal cord of rats

PURPOSE

The phosphodiesterase 5 inhibitor sildenafil has antinociceptive effects, mediated by an increase in cGMP. This study examined the role of spinal adenosine and serotonin receptors played in the antinociceptive effects of intrathecal sildenafil.

MATERIALS AND METHODS

Intrathecal catheters were inserted into the subarachnoid space of Sprague-Dawley male rats as a drug delivery device. Pain was induced by injecting formalin into the plantar surface of rats and observing nociceptive behavior (flinching response) for 60 minutes. Then, the effects of intrathecal adenosine and serotonin receptor antagonists on the antinociceptive activity of intrathecal sildenafil were examined.

RESULTS

Intrathecal sildenafil suppressed the flinching response in a dose-dependent manner during phases 1 and 2 in the formalin test. Both CGS 15943 and dihydroergocristine decreased the antinociceptive effects of sildenafil during phases 1 and 2 in the formalin test.

CONCLUSION

Intrathecal sildenafil effectively attenuated the pain evoked by formalin injection. Both adenosine and serotonin receptors may be involved in the antinociceptive action of sildenafil at the spinal level.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Synergistic antinociception of intrathecal sildenafil with clonidine in the rat formalin test

Spinal sildenafil (phosphodiesterase 5 inhibitor) and clonidine (alpha-2 adrenoceptor agonist) have shown antinociception. The author examined the properties of drug interaction after concurrent administration of intrathecal sildenafil-clonidine, and further clarified the reciprocity of sildenafil and clonidine. Catheters were inserted into the intrathecal space of male Sprague-Dawley rats. The formalin test was used as a nociceptive test, which was induced by subcutaneous injection of 50 microl of 5% formalin solution into the hindpaw. The pharmacological interaction was characterized using an isobolographic analysis. Intrathecal sildenafil and clonidine dose-dependently suppressed the flinching response observed during phase 1 and phase 2 in the formalin test. Isobolographic analysis revealed a synergistic interaction after intrathecal delivery of sildenafil-clonidine in both phases. Intrathecal yohimbine antagonized the antinociceptive action of intrathecal sildenafil during both phases in the formalin test. However, intrathecal ODQ failed to antagonize the antinociceptive action of intrathecal clonidine. These results suggest that sildenafil and clonidine, and the mixture of the two are effective against acute pain and facilitated pain state at the spinal level. Furthermore, synergism was noted after delivery of sildenafil-clonidine mixture. The antinociception of sildenafil can be modulated by spinal alpha-2 adrenoceptor, while the effect of clonidine is independent on the guanyly cyclase.