Giovane S. Galdino

Participation of endogenous opioids in the antinociception induced by resistance exercise in rats

Exercise is a low-cost intervention that promotes health and contributes to the maintenance of the quality of life. The present study was designed to investigate the influence of different resistance exercise protocols on the nociceptive threshold of rats. Female Wistar rats were used to perform exercises in a weight-lifting exercise model. The following groups were examined (N = 6 per group): untrained rats (control group); an acute protocol group consisting of rats submitted to 15 sets of 15 repetitions of resistance exercise (acute group); rats exercised with 3 sets of 10 repetitions, three times per week for 12 weeks (trained group), and a group consisting of trained rats that were further submitted to the acute protocol (trained-acute group). The nociceptive threshold was measured by the paw-withdrawal test, in which the withdrawal threshold (escape reaction) was measured by an apparatus applying force to the plantar surface of the animal paw. The opioid antagonist naloxone (2 mg/kg) was administered subcutaneously 10 min before the exercise protocols. The trained group demonstrated antinociception only up to day 45 of the 12-week training period. A significant increase (37%, P < 0.05) in the nociceptive threshold was produced immediately after exercise, decreasing to 15% after 15 min, when the acute exercise protocol was used. Naloxone reversed this effect. These data show that the acute resistance exercise protocol was effective in producing antinociception for 15 min. This antinociceptive effect is mediated by the activation of opioid receptors.

The endocannabinoid system mediates aerobic exercise-induced antinociception in rats.

Exercise-induced antinociception is widely described in the literature, but the mechanisms involved in this phenomenon are poorly understood. Systemic (s.c.) and central (i.t., i.c.v.) pretreatment with CB₁ and CB₂ cannabinoid receptor antagonists (AM251 and AM630) blocked the antinociception induced by an aerobic exercise (AE) protocol in both mechanical and thermal nociceptive tests. Western blot analysis revealed an increase and activation of CB₁ receptors in the rat brain, and immunofluorescence analysis demonstrated an increase of activation and expression of CB₁ receptors in neurons of the periaqueductal gray matter (PAG) after exercise. Additionally, pretreatment (s.c., i.t. and i.c.v.) with endocannabinoid metabolizing enzyme inhibitors (MAFP and JZL184) and an anandamide reuptake inhibitor (VDM11) prolonged and intensified this antinociceptive effect. These results indicate that exercise could activate the endocannabinoid system, producing antinociception. Supporting this hypothesis, liquid-chromatography/mass-spectrometry measurements demonstrated that plasma levels of endocannabinoids (anandamide and 2-arachidonoylglycerol) and of anandamide-related mediators (palmitoylethanolamide and oleoylethanolamide) were increased after AE. Therefore, these results suggest that the endocannabinoid system mediates aerobic exercise-induced antinociception at peripheral and central levels.

Ketamine activates the L-arginine/Nitric oxide/cyclic guanosine monophosphate pathway to induce peripheral antinociception in rats.

BACKGROUND: The involvement of the l-arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway in antinociception has been implicated as a molecular mechanism of antinociception produced by several antinociceptive agents, including &mgr;-, &kgr;-, or &dgr;-opioid receptor agonists, nonsteroidal analgesics, cholinergic agonist, and &agr;2C adrenoceptor agonist. In this study, we investigated whether ketamine, a dissociative anesthetic N-methyl-d-aspartate receptor antagonist, was also capable of activating the l-arginine/NO/cGMP pathway and eliciting peripheral antinociception. METHODS: The rat paw pressure test was used, with hyperalgesia induced by intraplantar injection of prostaglandin E2. All drugs were locally administered into the right hindpaw of male Wistar rats. RESULTS: Ketamine (10, 20, 40, 80 &mgr;g/paw) elicited a local antinociceptive effect that was antagonized by the nonselective NOS inhibitor l-NOARG (12, 18, and 24 &mgr;g/paw) and by the selective neuronal NOS inhibitor l-NPA (12, 18, and 24 &mgr;g/paw). In another experiment, we used the inhibitors l-NIO and l-NIL (24 &mgr;g/paw) to selectively inhibit endothelial and inducible NOS, respectively. These 2 drugs were ineffective at blocking the effects of the peripheral ketamine injection. In addition, the level of nitrite in the homogenized paw indicated that exogenous ketamine is able to induce NO release. The soluble guanylyl cyclase inhibitor ODQ (25, 50, and 100 &mgr;g/paw) blocked the action of ketamine, and the cGMP-phosphodiesterase inhibitor zaprinast (50 &mgr;g/paw) enhanced the antinociceptive effects of low-dose ketamine (10 &mgr;g/paw). CONCLUSIONS: Our results suggest that ketamine stimulates the l-arginine/NO/cyclic GMP pathway via neuronal NO synthase to induce peripheral antinociceptive effects.

Acute resistance exercise induces antinociception by activation of the endocannabinoid system in rats.

BACKGROUND:Resistance exercise (RE) is also known as strength training, and it is performed to increase the strength and mass of muscles, bone strength, and metabolism. RE has been increasingly prescribed for pain relief. However, the endogenous mechanisms underlying this antinociceptive effect are still largely unexplored. Thus, we investigated the involvement of the endocannabinoid system in RE-induced antinociception. METHODS:Male Wistar rats were submitted to acute RE in a weight-lifting model. The nociceptive threshold was measured by a mechanical nociceptive test (paw pressure) before and after exercise. To investigate the involvement of cannabinoid receptors and endocannabinoids in RE-induced antinociception, cannabinoid receptor inverse agonists, endocannabinoid metabolizing enzyme inhibitors, and an anandamide reuptake inhibitor were injected before RE. After RE, CB1 cannabinoid receptors were quantified in rat brain tissue by Western blot and immunofluorescence. In addition, endocannabinoid plasma levels were measured by isotope dilution-liquid chromatography mass spectrometry. RESULTS:RE-induced antinociception was prevented by preinjection with CB1 and CB2 cannabinoid receptor inverse agonists. By contrast, preadministration of metabolizing enzyme inhibitors and the anandamide reuptake inhibitor prolonged and enhanced this effect. RE also produced an increase in the expression and activation of CB1 cannabinoid receptors in rat brain tissue and in the dorsolateral and ventrolateral periaqueductal regions and an increase in endocannabinoid plasma levels. CONCLUSIONS:The present study suggests that a single session of RE activates the endocannabinoid system to induce antinociception.

Involvement of the nitric oxide/CGMP/KATP pathway in antinociception induced by exercise in rats

AIMS Physical exercise is responsible for increasing the nociceptive threshold. The present study aimed to investigate the involvement of the nitric oxide/(C)GMP/K(ATP) pathway in antinociception induced by acute aerobic exercise (AAc) in rats. MAIN METHODS Wistar rats performed exercise in a rodent treadmill, according to an AAc protocol. The nociceptive threshold was measured by mechanical and thermal nociceptive tests (paw-withdrawal, tail-flick and face-flick). To investigate the involvement of the NO/(C)GMP/K(ATP) pathway the following nitric oxide synthase (NOS) unspecific and specific inhibitors were used: N-nitro-l-arginine (NOArg), Aminoguanidine, N(5)-(1-Iminoethyl)-l-ornithine dihydrocloride (L-NIO), N(omega)-Propyl-l-arginine (L-NPA); guanylyl cyclase inhibitor, 1H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one (ODQ); and K(ATP) channel blocker, Glybenclamide; all administered subcutaneously at a dose of 2mg/kg 10min before exercise started. Plasma and cerebrospinal fluid (CSF) nitrite levels were determined by spectrophotometry. KEY FINDINGS In the paw-withdrawal, tail-flick and face-flick tests, the AAc protocol produced antinociception, which lasted for more than 15min. This effect was significantly reversed (P<0.05) by NOS specific and unspecific inhibitors, guanylyl cyclase inhibitor (ODQ) and K(ATP) channel blocker (Glybenclamide). Acute exercise was also responsible for increasing nitrite levels in both plasma and cerebrospinal fluid. SIGNIFICANCE Taken together, these results suggest that the NO/(C)GMP/K(ATP) pathway participates in antinociception induced by exercise.

The Nitric oxide/CGMP/KATP pathway mediates systemic and central antinociception induced by resistance exercise in rats.

Resistance exercise (RE) is characterized to increase strength, tone, mass, and/or muscular endurance and also for produces many beneficial effects, such as blood pressure and osteoporosis reduction, diabetes mellitus control, and analgesia. However, few studies have investigated endogenous mechanisms involved in the RE-induced analgesia. Thus, the aim of this study was evaluate the role of the NO/CGMP/KATP pathway in the antinociception induced by RE. Wistar rats were submitted to acute RE in a weight-lifting model. The nociceptive threshold was measured by mechanical nociceptive test (paw-withdrawal). To investigate the involvement of the NO/CGMP/KATP pathway the following nitric oxide synthase (NOS) non-specific and specific inhibitors were used: N-nitro-l-arginine (NOArg), Aminoguanidine, N5-(1-Iminoethyl)-l-ornithine dihydrocloride (l-NIO), Nω-Propyl-l-arginine (l-NPA); guanylyl cyclase inhibitor, 1H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one (ODQ); and KATP channel blocker, Glybenclamide; all administered subcutaneously, intrathecally and intracerebroventricularly. Plasma and cerebrospinal fluid (CSF) nitrite levels were determined by spectrophotometry. The RE protocol produced antinociception, which was significantly reversed by NOS specific and unspecific inhibitors, guanylyl cyclase inhibitor (ODQ) and KATP channel blocker (Glybenclamide). RE was also responsible for increasing nitrite levels in both plasma and CSF. These finding suggest that the NO/CGMP/KATP pathway participates in antinociception induced by RE.

Involvement of the L-arginine/nitric oxide/cyclic guanosine monophosphate pathway in peripheral antinociception induced by N-palmitoyl-ethanolamine in rats

N‐palmitoyl‐ethanolamine (PEA) is an endogenous substance that was first identified in lipid tissue extracts. It has been classified as a CB2 receptor agonist. Exogenous PEA has the potential to become a valid treatment for neuropathic and inflammatory pain. In spite of the well‐demonstrated antiinflammatory properties of PEA, its involvement in controlling pain pathways remains poorly characterized. The participation of the L‐arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway in peripheral antinociception has been established by our group to the μ‐, κ‐ or δ‐opioid receptor agonists, nonsteroidal analgesics, α2C‐adrenoceptor agonists, and even nonpharmacological electroacupuncture. The aim of this study was to verify whether the peripheral antinociception effects of PEA involve the activation of this pathway. All drugs were locally administered to the right hind paw of male Wistar rats. The paw pressure test was used, with hyperalgesia induced by intraplantar injection of prostaglandin E2. PEA elicited a local peripheral antinociceptive effect that was antagonized by the nonselective NO synthase (NOS) inhibitor L‐NOARG and the selective neuronal NOS (nNOS) inhibitor L‐NPA. Selective inhibition of endothelial (eNOS) and inducible (iNOS) NOS via L‐NIO and L‐NIL, respectively, was ineffective at blocking the effects of a local PEA injection. In addition, the dosage of nitrite in the homogenized paw, as determined by colorimetric assay, indicated that exogenous PEA is able to induce NO release. The soluble guanylyl cyclase inhibitor ODQ antagonized the PEA effect, whereas the cGMP‐phosphodiesterase inhibitor zaprinast potentiated the antinociceptive effect of low‐dose PEA. This study provides evidence that PEA activates nNOS, thus initiating the NO/cGMP pathway and inducing peripheral antinociceptive effects.

Ang-(1-7) activates the NO/cGMP and ATP-sensitive K+ channels pathway to induce peripheral antinociception in rats.

Angiotensin-(1-7) is a bioactive component of the renin-angiotensin system that is formed endogenously and induces nitric oxide release in several tissues. The L-arginine/NO/cyclic GMP pathway and ATP-sensitive K+ channels have been proposed as the mechanism of action for the peripheral antinociception of several groups of drug and endogenous substances, including opioids, non-steroidal analgesics, acetylcholine and others. The aim of the present study was to investigate the involvement of the L-arginine/NO/cGMP and KATP+ pathway on antinociception induced by angiotensin-(1-7). Paw pressure in rats was used to induce hyperalgesia via an intraplantar injection of prostaglandin E2 (2 μg/paw). Ang-(1-7) (2, 3 and 4 μg/paw) elicited a local peripheral antinociceptive effect that was antagonized by the nonselective NO synthase (NOS) inhibitor L-NOarg and the selective neuronal NOS (nNOS) inhibitor L-NPA. The selective inhibition of endothelial (eNOS) and inducible (iNOS) NOS by L-NIO and L-NIL, respectively, was ineffective at blocking the effects of a local Ang-(1-7) injection. In addition, the level of nitrite in the homogenized paw tissue, as determined by a colorimetric assay, indicated that exogenous Ang-(1-7) is able to induce NO release. The soluble guanylyl cyclase inhibitor ODQ and the specific blocker of ATP-sensitive K+ channels glibenclamide (40, 80 and 160 μg/paw) antagonized the Ang-(1-7) response. The results provide evidence that Ang-(1-7) most likely induces peripheral antinociceptive effects via the L-arginine/NO/cGMP pathway and KATP+ pathway activation.

Effect of dietary caloric restriction on the nociceptive threshold of rats that underwent aerobic and resistance exercise training

The purpose of this study was to evaluate the effect that exercise and caloric restriction have on the nociceptive threshold of rats. Male Wistar rats were divided into two groups: one group that was fed ad libitum (FED) and another group that was subjected to dietary caloric restriction (CR). The CR group received 50% of the food the FED group received, for 4 weeks. Both groups were submitted to aerobic (AE) and resistance (RE) exercise training protocols performed in a rodent treadmill and in a weight-lifting exercise model, respectively. Mechanical and thermal nociceptive thresholds were measured by tail-flick and paw-withdrawal tests, respectively. Both exercise protocols produced antinociception, but there was no difference found between the FED and CR groups after either 1 or 4 weeks. Additionally, although dietary caloric restriction alone did not result in antinociception, it did increase the running time of animals during aerobic exercise and increased the load lifted in resistance exercise after 4 weeks. These results indicate that caloric restriction for 1 or 4 weeks did not alter the nociceptive threshold, but could play an important role in improvement of physical performance.

Evaluation of exercise and potassium chloride supplementation on blood pressure and nociceptive threshold in hypertensive rats

Hypertensive subjects present an increased nociceptive threshold, and the lack or delay of pain perception may impede detection of angina and myocardial infarction. Nutritional interventions, like potassium chloride (KCl) diet supplementation, and exercises are common nonpharmacological indications for treating hypertension. Spontaneous hypertensive rats (SHR) and normotensive male Wistar rats were submitted to a combination of exercise and KCl diet supplementation. Exercise reduced the nociceptive threshold in SHR; however, this effect was inhibited by KCl supplementation. Exercise and KCl supplementation did not alter systolic blood pressure. Reduction of the nociceptive threshold by exercise may be important for the detection of angina and myocardial infarction in hypertensive individuals.