naltrindole and Inflammation

Opioids are well known for their robust analgesic effects. Chronic activation of mu opioid receptors (MOPs) is, however, accompanied by various unwanted effects such as analgesic tolerance. Among other mechanisms, interactions between MOPs and delta opioid receptors (DOPs) are thought to play an important role in morphine-induced behavioral adaptations. Interestingly, certain conditions such as inflammation enhance the function of the DOP through a MOP-dependent mechanism. Here, we investigated the role of DOPs during the development of morphine tolerance in an animal model of chronic inflammatory pain. Using behavioral approaches, we first established that repeated systemic morphine treatment induced morphine analgesic tolerance in rats coping with chronic inflammatory pain. We then observed that blockade of DOPs with subcutaneous naltrindole (NTI), a selective DOP antagonist, significantly attenuated the development of morphine tolerance in a dose-dependent manner. We confirmed that this effect was DOP mediated by showing that an acute injection of NTI had no effect on morphine-induced analgesia in naive animals. Previous pharmacological characterizations revealed the existence of DOP subtype 1 and DOP subtype 2. As opposed to NTI, 7-benzylidenenaltrexone and naltriben were reported to be selective DOP subtype 1 and DOP subtype 2 antagonists, respectively. Interestingly, naltriben but not 7-benzylidenenaltrexone was able to attenuate the development of morphine analgesic tolerance in inflamed rats. Altogether, our results suggest that targeting of DOP subtype 2 with antagonists provides a valuable strategy to attenuate the analgesic tolerance that develops after repeated morphine administration in the setting of chronic inflammatory pain.

Topics: Analgesics, Opioid; Animals; Benzylidene Compounds; Chronic Pain; Disease Models, Animal; Dose-Response Relationship, Drug; Drug Tolerance; Freund's Adjuvant; Hindlimb; Inflammation; Male; Morphine; Naltrexone; Narcotic Antagonists; Rats, Sprague-Dawley; Receptors, Opioid, delta

Peripherally-restricted opiate compounds attenuate hyperalgesia in experimental models of inflammatory pain, but have little discernable effect on nociceptive behavior in normal animals. This suggests that activation of opioid receptors on peripheral sensory axons contributes to decreased afferent activity after injury. Previously, we reported that direct application of morphine to cutaneous receptive fields decreased mechanical and heat-evoked responses in a population of C-fiber nociceptors in inflamed skin. Consistent with reported behavioral studies, direct application of morphine had no effect on fiber activity in control skin. The aim of the present study was to determine whether mechanical responsiveness of nociceptors innervating inflamed skin was attenuated by direct activation of delta opioid receptors (DORs) on peripheral terminals. An ex vivo preparation of rat plantar skin and tibial nerve was used to examine effects of a selective DOR agonist, deltorphin II, on responsiveness of single fibers innervating inflamed skin. Electrical recordings were made eighteen hours after injection of complete Freund's adjuvant into the hindpaw. Deltorphin II produced an inhibition of the mechanical responsiveness of single fibers innervating inflamed skin; an effect blocked by the DOR-selective antagonist, naltrindole. The population of units responsive to deltorphin II was identified as consisting of C fiber mechanical nociceptors.

Topics: Action Potentials; Analysis of Variance; Animals; Dose-Response Relationship, Drug; Electric Stimulation; Freund's Adjuvant; Inflammation; Male; Naltrexone; Narcotic Antagonists; Nerve Fibers; Neurons, Afferent; Oligopeptides; Physical Stimulation; Rats; Rats, Sprague-Dawley; Receptors, Opioid, delta; Skin

Electroacupuncture (EA) has been widely accepted and applied as an important acupuncture-related technique for acupuncture analgesia (AA) research. The involvement of opioid peptides and receptors in acute AA has been shown via pre-EA application of opioid receptor/peptide antagonists. In this study, we intended to reproducibly institute acupoint position and needling excluding influences from anaesthesia or restrainers on rats with complete Freund's adjuvant (CFA) hind paw inflammatory pain, as well as to explore opioid-dependency and anti-inflammatory effects in sustained acupuncture analgesia.. Accurate position and needling approach on acupoint GB30 was modelled by computer-based three-dimensional (3D) images and followed by an optimal EA treatment protocol (100 Hz, 2-3 mA, 20 min) at 0 and 24 h post-CFA in conscious free-moving rats. Opioid receptor antagonists, naloxone (NLX) and naltrindole (NTI) were applied intraplantarly post-EA at late phase (96 h) of CFA. Nociceptive thresholds were assessed by paw pressure threshold (Randall-Sellito) or paw withdrawal latency (Hargreaves), and anti-inflammatory effects were evaluated by measurement of plantar temperature and paw volume.. EA elicited significant sustained mechanical and thermal antinociception up to 144 h. Mechanical antinociception of EA was suppressed by peripheral intraplantar application of NLX and NTI. EA also reduced paw temperature and volume during the same time frame indicating anti-inflammatory effects.. By employing a reproducible EA treatment model on GB30 in free-moving rats, we demonstrated the involvement of peripheral opioid receptors mediated EA-induced long-term antinociception. Future studies should examine the specific neuroimmunological connection of EA-induced sustained antinociception in inflammation.

Topics: Acupuncture Analgesia; Acupuncture Points; Animals; Disease Models, Animal; Electroacupuncture; Freund's Adjuvant; Hyperalgesia; Inflammation; Male; Naloxone; Naltrexone; Pain Measurement; Rats; Rats, Wistar; Receptors, Opioid; Time

In this study we analyzed the mechanisms underlying celecoxib-induced analgesia in a model of inflammatory pain in rats, using the intracerebroventricular (i.c.v.) administration of selective opioid and cannabinoid antagonists.. Analgesic effects of celecoxib were prevented by selective μ-(β-funaltrexamine) and δ-(naltrindole), but not κ-(nor-binaltorphimine) opioid antagonists, given i.c.v. 30 min before celecoxib. Similar pretreatment with AM 251, but not SR 144528, cannabinoid CB(1) and CB(2) receptor antagonists, respectively, prevented celecoxib-induced analgesia. The fatty acid amide hydrolase inhibitor, URB 597, also prevented celecoxib-induced analgesia.. Our data provided further evidence for the involvement of endogenous opioids and revealed a new cannabinoid component of the mechanism(s) underlying celecoxib-induced analgesia.

Topics: Analgesics; Animals; Carrageenan; Celecoxib; Central Nervous System; Cyclooxygenase 2; Cyclooxygenase 2 Inhibitors; Inflammation; Male; Naltrexone; Pain; Piperidines; Pyrazoles; Rats; Rats, Sprague-Dawley; Receptor, Cannabinoid, CB1; Receptors, Opioid, delta; Receptors, Opioid, mu; Sulfonamides

Several works reveal that nitric oxide could enhance the peripheral antinociception induced by opioids during acute inflammation. Nonetheless, the role of nitric oxide in the local antinociceptive effects of delta-opioid receptor (DOR) agonists during chronic peripheral inflammation is not known. The aim of this study is to evaluate whether nitric oxide would enhance the local antinociceptive effects of a DOR agonist during chronic inflammatory pain in mice. Chronic inflammatory pain was induced by the subplantar administration of complete Freund's adjuvant (CFA; 30 microl) and thermal hyperalgesia assessed by plantar test. In C57BL/6J mice, we evaluated the local antinociceptive effects of a DOR agonist, [D-Pen2,5]-enkephalin (DPDPE) and a nitric oxide donor, DETA NONOate DETA/NO 2,2'-(hydroxynitrosohydrazino) Bis-Ethanamine (NOC-18) alone or combined (DPDPE plus NOC-18) at 1, 4, 7, and 10 days after CFA injection. The reversibility of the peripheral antinociceptive effects of DPDPE, alone or combined with NOC-18, was assessed with the local administration of selective (naltrindole) and non-selective (naloxone methiodide) DOR antagonists. The local administration of DPDPE or NOC-18 alone dose-dependently inhibited the thermal hyperalgesia induced by peripheral inflammation. Moreover, the co-administration of NOC-18 with DPDPE significantly increased the antinociceptive effects produced by DPDPE from 1 to 10 days of CFA-induced inflammatory pain (P < 0.05). These effects were completely blocked by naltrindole and naloxone methiodide. Our results demonstrate that nitric oxide might enhance the local antinociceptive effects of a DOR agonist during chronic inflammatory pain by interaction with peripheral DOR, representing a useful strategy for an efficient antinociceptive treatment of peripheral inflammatory pain.

Topics: Analgesics, Opioid; Animals; Chronic Disease; Disease Models, Animal; Drug Therapy, Combination; Enkephalin, D-Penicillamine (2,5)-; Freund's Adjuvant; Hot Temperature; Hyperalgesia; Inflammation; Male; Mice; Mice, Inbred C57BL; Naltrexone; Narcotic Antagonists; Nitric Oxide; Nitric Oxide Donors; Nitroso Compounds; Pain Measurement; Receptors, Opioid, delta; Time Factors

The aim of the present study was to evaluate the antinociceptive effect of the novel pyrazoline methyl ester: 4-methyl-5-trifluoromethyl-5-hydroxy-4,5-dihydro-1H-pyrazole methyl ester (MPF4).. The effect of MPF4 was assessed in two models of pain: arthritic pain caused by Complete Freund's Adjuvant (CFA) and postoperative pain caused by surgical incision in mice.. MPF4 given intraperitoneally (1.0 mmol/kg, i.p.) produced marked antinociception in inflammatory allodynia caused by CFA. The antinociceptive effect produced by MPF4 was reversed with the pre-treatment of animals with naloxone or naltrindole. Oral administration of MPF4 (1.0 mmol/kg, p.o), dipyrone (1.0 mmol/kg, p.o.) and morphine (0.026 mmol/kg, p.o.) also produced an anti-allodynic effect. However, none of the compounds evaluated reversed the paw edema produced by CFA. Moreover, MPF4, dipyrone and morphine also produced an anti-allodynic effect in the surgical incisional pain model. The maximal inhibitions obtained with preemptive drug treatment were 66+/-7%, 73+/-9% and 88+/-8% for MPF4 (1.0 mmol/kg, p.o.), dipyrone (1.0 mmol/kg, p.o.) and morphine (0.026 mmol/kg, p.o.), respectively. The maximal inhibitions obtained with curative drug treatment were 53+/-9%, 83+/-7% and 84+/-7%, for MPF4, dipyrone and morphine, respectively. Unlike indomethacin, MPF4 did not induce gastric lesions at the dose that caused the highest antinociception (1.0 mmol/kg, p.o). The anti-allodynic action of MPF4, dipyrone and morphine was not associated with impairment of motor activity.. The results of the present study suggest that MPF4 represents a potential target for the development of new drugs to treat persistent inflammatory pain.

Topics: Analgesics; Animals; Anti-Inflammatory Agents, Non-Steroidal; Arthritis, Experimental; Dipyrone; Freund's Adjuvant; Indomethacin; Inflammation; Male; Mice; Morphine; Motor Activity; Naloxone; Naltrexone; Narcotic Antagonists; Pain; Pain, Postoperative; Postural Balance; Pyrazoles; Stomach Ulcer

The present study investigated the role of peripheral opioid receptors in mustard oil-induced nociceptive behavior and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing between 300 and 400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for the intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 microl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle. Intramuscularly-administered morphine significantly reduced shaking behavior but not MO-induced inflammation. Intramuscular pretreatment with naloxone, an opioid receptor antagonist, reversed antinociception produced by intramuscularly-administered morphine, while intracisternal administration of naloxone did not affect the antinociception of peripheral morphine. Pretreatment with d-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a mu opioid receptor antagonist, but not naltrindole, a delta opioid receptor antagonist, nor norbinaltorphimine (nor-BNI), a kappa opioid receptor antagonist, reversed intramuscularly-administered morphine-induced antinociception. These results indicate that intramuscularly-administered morphine produces antinociception in craniofacial muscle nociception and that this intramuscularly-administered morphine-induced antinociception is mediated by a peripheral mu opioid receptor. Our observations further support the clinical approach of administering opioids in the periphery for the treatment of craniofacial muscle nociception.

Topics: Analgesics; Anesthesia, General; Animals; Inflammation; Injections; Injections, Intramuscular; Male; Masseter Muscle; Morphine; Mustard Plant; Naloxone; Naltrexone; Narcotic Antagonists; Nociceptors; Pain; Plant Oils; Psychomotor Performance; Rats; Rats, Sprague-Dawley; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu; Single-Blind Method; Somatostatin

It has been shown that inflammation of rat paws elicits accumulation of opioid peptide beta-endorphin-containing immune cells in the inflamed subcutaneous tissue, contributing to immunocyte-produced pain suppression. However, the possible mechanisms involved in the pharmacological application of beta-endorphin in rat paw inflammation have not been investigated. The present study was set up to explore the effects of intraplantar injection of beta-endorphin on Concanavalin A-induced paw edema in two inbred rat strains, Albino Oxford (AO) and Dark Agouti (DA). Both high dose-induced suppression and low dose-induced potentiation of edema development in AO and DA rats, respectively, were blocked with antagonists specific for delta (naltrindole) and kappa (nor-binaltorphimine) opioid receptors. beta-endorphin in vitro decreased phagocytosis and increased nitric oxide (NO) production in air pouch granulocytes obtained from AO rats. However, in cells from DA rat strain beta-endorphin modulated both phagocytosis and NO production in a concentration-dependent manner. It could be concluded that the strain-dependent opposing effects of beta-endorphin on paw inflammation are mediated through delta and kappa opioid receptors and probably involve changes in the production of reactive oxygen species by inflammatory cells. Our results point to the importance of genotype for pharmacological manipulations and the development of inflammation.

Topics: Animals; beta-Endorphin; Concanavalin A; Dose-Response Relationship, Drug; Edema; Female; Granulocytes; Hindlimb; Inflammation; Male; Naltrexone; Narcotic Antagonists; Neurotransmitter Agents; Nitric Oxide; Phagocytosis; Rats; Rats, Inbred Strains; Receptors, Opioid, delta; Receptors, Opioid, kappa; Species Specificity

The present study was conducted on rats with inflammation induced by subcutaneous injection of carrageenan into the left hindpaw. Intrathecal administration of oxytocin produced dose-dependent increases in the hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats with inflammation. The antinociceptive effect of oxytocin was blocked by intrathecal administration of atosiban, a selective oxytocin antagonist, indicating that oxytocin receptor mediates oxytocin-induced antinociception in the spinal cord. The oxytocin-induced antinociceptive effect was attenuated by intrathecal administration of the opioid antagonist naloxone, suggesting an involvement of the endogenous opioid system in oxytocin-induced antinociception in the spinal cord of rats with inflammation. Furthermore, the antinociceptive effect of oxytocin was attenuated by intrathecal injections of the mu-receptor antagonist beta-funaltrexamine and the kappa-receptor antagonist nor-binaltorphimine, but not by the delta-receptor antagonist naltrindole, illustrating that mu- and kappa-receptors, but not delta-receptor, are involved in oxytocin-induced antinociception in the spinal cord of rats with inflammation. Moreover, intrathecal administration of atosiban alone induced a hyperalgesia in rats with inflammation, indicating that endogenous oxytocin is involved in the transmission and regulation of nociceptive information in the spinal cord of rats with inflammation. The present study showed that both exogenous and endogenous oxytocin displayed antinociception in the spinal cord in rats with inflammation, and mu- and kappa-receptors were involved in oxytocin-induced antinociception.

Topics: Analgesics; Animals; Carrageenan; Hot Temperature; Inflammation; Injections, Spinal; Male; Naloxone; Naltrexone; Narcotic Antagonists; Oxytocin; Pain; Pain Measurement; Physical Stimulation; Rats; Rats, Wistar; Spinal Cord; Vasotocin

The effect of intraplantarly (i.pl.)-injected methionine-enkephalin (ME) on Concanavalin A (Con A)-induced paw edema in Dark Agouti (DA) and Albino Oxford (AO) rats was investigated. ME suppressed edema in DA rats, which was antagonized with naloxone (non-selective opioid receptor antagonist) and naltrindole (delta opioid receptors antagonist). Potentiating effect of ME in AO rats was blocked by naloxone, nor-binaltorphimine (kappa opioid receptors antagonist) and beta-funaltrexamine (mu opioid receptors antagonist). Dexamethasone suppressed edema in both rat strains. These findings suggest that strain-dependent differences in the effects of ME on inflammation in DA and AO rats could be related to diversity in opioid receptors expression in these strains.

Topics: Animals; Dexamethasone; Dose-Response Relationship, Drug; Edema; Enkephalin, Methionine; Glucocorticoids; Inflammation; Kinetics; Male; Naloxone; Naltrexone; Narcotic Antagonists; Rats; Species Specificity; Time Factors

1. Nicotine can activate primary afferent nociceptors, one result of which is to increase neurogenic plasma extravasation. In this study we have demonstrated a novel proinflammatory effect of sub-nanomolar nicotine, mediated by peripheral action at sensory neurons. This action is normally masked by adrenal medulla-derived delta-opioid receptor agonists. 2. While neurogenic plasma extravasation in the knee joint of the rat was not increased by intra-articular perfusion of nicotine (10(-8) M), perfusion of nicotine, at concentrations as low as 10(-10) M, combined with naloxone to block opioid receptors (or naltrindole to block delta-opioid receptors) was able to enhance bradykinin-induced plasma extravasation. This pro-inflammatory effect of intra-articular nicotine was mimicked by subcutaneous nicotine which was abolished by intra-articularly-administered hexamethonium, a nicotinic receptor antagonist. 3. Following denervation of the adrenal medulla, intra-articular nicotine, alone at 10(-8) M, enhanced plasma extravasation, which was no longer enhanced by naloxone. 4. Destruction of primary afferents by neonatal treatment with capsaicin or blockade of sensory neurotransmitter by neurokinin-1 receptor antagonist RP-87,580 abolished the pro-inflammatory effect of nicotine. 5. The effect of nicotine we describe in promoting inflammation is exerted at extremely low concentrations and therefore could have relevance to smokers, patients receiving medicinal nicotine as therapy and even second-hand smokers. Since receptor mechanisms on peripheral terminals of nociceptors may also be present on central terminals, actions of the endogenous nicotinic agonist acetylcholine, at central terminals of primary afferents or at other sites in the central nervous system, may be similarly modulated by opioids.

Topics: Adrenal Medulla; Analysis of Variance; Animals; Animals, Newborn; Bradykinin; Capsaicin; Dose-Response Relationship, Drug; Hexamethonium; Inflammation; Knee Joint; Male; Naloxone; Naltrexone; Narcotic Antagonists; Neurokinin-1 Receptor Antagonists; Neurons, Afferent; Nicotine; Nociceptors; Opioid Peptides; Rats; Rats, Sprague-Dawley; Receptors, Neurokinin-1; Receptors, Opioid; Sciatic Nerve; Time Factors

We previously reported that the morphine-induced place preference was attenuated under inflammation produced by the unilateral injection of 2.5 % formalin (50 microl) into the hind paw of rats. In the present study, to elucidate the mechanism of this attenuation, the effects of pretreatment with delta- and kappa-opioid receptor antagonists, naltrindole (NTI) and nor-binaltorphimine (nor-BNI), on the development of the morphine-induced place preference under inflammation were examined in rats. Nor-BNI, but not NTI, eliminated the suppression of the morphine-induced place preference in inflamed groups. These results suggest that endogenous kappa-opioid systems may be activated in the presence of chronic inflammatory nociception; as a result, the development of morphine's rewarding effect may be suppressed under inflammation.

Topics: Animals; Chronic Disease; Conditioning, Psychological; Formaldehyde; Hindlimb; Inflammation; Male; Morphine; Naltrexone; Pain; Rats; Rats, Sprague-Dawley; Receptors, Opioid, delta; Receptors, Opioid, kappa; Reward; Time Factors

This study performed in freely moving rats evaluated the ability of specific opioid receptor antagonists to reverse the inhibitory effects of morphine on carrageenin-induced c-Fos expression in the spinal cord. Our study focused on the superficial dorsal horn (laminae I-II), which is the main termination site of nociceptive primary afferent fibers and is rich in opioid receptors. In order to replicate clinical routes of administration, all agents were administered intravenously (i.v.). As previously demonstrated, pre-administered i.v. morphine (3 mg/kg) produced a marked decrease (58+/-5%) in the number of Fos-LI neurones measured at 2 h after intraplantar (i.pl.) carrageenin (6 mg/150 microl) and yet was without influence on peripheral oedema. This decrease in c-Fos expression was completely blocked by combined administration of morphine with the mu-opioid receptor antagonist, [D-Phe-Cys-Tyr-D-Orn-Thr-Pen-Thr-NH2] (CTOP-1+1 mg/kg). Naltrindole (NTI-1+1 mg/kg), a delta-opioid receptor antagonist partially blocked the effects of systemic morphine, so that the inhibitory effects of morphine after NTI injection are now 40+/-4%. However, this effect of NTI was weak since the depressive effects of morphine were still highly significant (p<0.001). In contrast, nor-binaltorphimine (nor-BNI-1+1 mg/kg), a kappa-opioid receptor antagonist, had no significant effect on the effects of morphine. These results indicate the major contribution of mu-opioid receptors to the antinociceptive effects of systemic morphine at the level of the superficial dorsal horn. The observed effect of NTI is not necessarily related to a direct action of morphine on delta-opioid receptors and some possible actions of this antagonist are discussed.

Topics: Analgesics, Opioid; Animals; Carrageenan; Hindlimb; Inflammation; Injections, Intravenous; Male; Morphine; Naltrexone; Narcotic Antagonists; Proto-Oncogene Proteins c-fos; Rats; Rats, Sprague-Dawley; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu; Somatostatin; Spinal Cord

The role of neuropeptide FF (NPFF) in the modulation of spinal nociception was studied in rats with carrageenan inflammation in the hind paw. Normally no NPFF-ir neuronal cell bodies are found in the spinal cord. During inflammation NPFF-neurons were seen in an area receiving innervation from the inflamed hind limb, but in rats pretreated with morphine no NPFF-ir neurons were found. NPFF or IgG from NPFF immunoserum administered intrathecally had no effect in thermal and mechanical nociceptive tests. Morphine produced significant antinociception in both tests in the inflamed paw, but the effect was not modified by NPFF. These findings differ from the effects of intrathecal administration of NPFF and opioids in acute thermal tests when no inflammation is present. The role of NPFF in the modulation of nociception in the spinal cord may be markedly changed during acute inflammation.

Topics: Animals; Carrageenan; Hot Temperature; Inflammation; Male; Morphine; Naltrexone; Narcotic Antagonists; Neurons; Neuropeptides; Oligopeptides; Pain; Rats; Rats, Sprague-Dawley; Spinal Cord

Carrageenan-induced inflammation of the rat hindpaw has been used as a model for persistent pain of inflammatory origin. The induction of inflammation resulting from carrageenan injection in the rat hindpaw has been shown to elicit an increase in the antinociceptive potency of morphine, an effect postulated to be related to reduced levels of spinal cholecystokinin (CCK). Recent findings have related the anti-opioid effect of CCK to a decrease in activation of delta-opioid receptors. For this reason, we have examined the effects of the delta-opioid antagonist naltrindole (NTI) on the modulation of morphine antinociceptive potency resulting from carrageenan-induced inflammation. Rats with carrageenan-induced hindpaw inflammation received several doses of morphine in the absence or presence of NTI and were tested in the hot plate (HP) and tail flick (TF) tests. These results were compared to those of non-carrageenan injected rats. Morphine was significantly more potent in inflamed, than in control, rats in both tests. While NTI did not affect morphine antinociceptive potency in control rats in either test, this opioid delta antagonist blocked the increase in morphine potency resulting from carrageenan inflammation in nearly every case. The blockade of the enhancement of morphine potency was such that the effect of a given dose of morphine was similar in control rats and carrageenan-injected rats with NTI. We suggest that carrageenan-induced inflammation may alter endogenous enkephalin levels, perhaps by a decrease in CCK availability. The enhancement of morphine antinociceptive potency may result from the well-established synergism seen following the activation of opioid delta receptors by enkephalins.

Topics: Animals; Carrageenan; Hindlimb; Inflammation; Male; Morphine; Naltrexone; Nociceptors; Rats; Rats, Sprague-Dawley; Receptors, Opioid, delta