Restraint stress enhances morphine-induced analgesia in the rat without changing apparent affinity of receptor

Antagonism of morphine analgesia (tail-flick assay) by naloxone was assessed quantitatively by in vivo "apparent" pA2 determination in unstressed rats and in rats subjected to restraint stress. Restrained rats had a higher baseline tail-flick latency than did unstressed (unrestrained) animals, and were more sensitive to the analgesic effect of morphine, as reflected in lower morphine ED50s. There was no significant difference between apparent pA2 values of unstressed and restrained rats using pA2 regression line analysis. This suggests that while stress enhances the analgesic effect of morphine, it does not appreciably alter opiate receptor affinity for naloxone under the conditions of this study.     

Antinociceptive effects of intraventricular or systemic administration of vasopressin in the rat

The effects of intraventricular administration of lysine-vasopressin on pain sensitivity in the rat were determined in the tail-flick test. Vasopressin (16–100 μg) was found to induce potent and dose-dependent antinociceptive actions, lasting up to one hour. An additional experiment demonstrated that analgesia induced by vasopressin was not blocked by naloxone, suggesting that this analgesia is independent of opiate receptor systems. Vasopressin was also found to be equally effective in elevating tail-flick latency after systemic administration. These results, together with others, suggest a possible role of vasopressin systems in the regulation of pain sensitivity.     

Stereospecific reversal of nitrous oxide analgesia by naloxone

The opiate antagonist naloxone was found to block nitrous oxide analgesia in a stereospecific fashion. Using a modified hotplate test in mice, the (-)-enantiomer of naloxone (which has a KD of approximately 1 nM for opiate receptors) antagonized the analgesic actions of nitrous oxide in a dose-dependent (2.5-20 mg/kg) fashion. In contrast, the (+)-enantiomer (KD approximately 10,000 nM) had no effect on nitrous oxide analgesia at the highest dose tested (40 mg/kg). These data strongly suggest that nitrous oxide analgesia is mediated via opiate receptors and is consistent with the hypotheses that this effect occurs either through the release of endogenous opioids or by physical perturbation of the opiate receptors.     

Opiate properties of SKF 525A.

SKF 525A, a classical inhibitor of microsomal drug-metabolizing enzymes, is structurally similar to the diphenylpropylamine analgesics, and certain reported effects in animals resemble those produced by opiate drugs. In an opiate radioreceptor assay, SKF 525A was 50 times less potent than methadone in the absence of sodium and 10 times less potent in the prescence of sodium. The nature of the sodium effect indicates SKF 525A to have less opiate agonist character than does methadone. In mice, 2 mg of SKF 525A given intraperitoneally induced less profound analgesia on a hot plate (44 degrees C) than did 0.1 mg of methadone. Analgesia by SKF 525A was prevented by pretreatment of the mice with naloxone. In rats, 50 microgram of SKF 525A given intracerebroventricularly was analgesic.     

Antinociceptive effects of intrathecally administered human beta-endorphin in the rat and cat

Rats chronically implanted with intrathecal catheters displayed a dose-dependent increase in the hot-plate and tail-flick response latencies following the injection of human beta-endorphin into the lumbar spinal subarachnoid space through the indwelling catheter. beta-Endorphin was approximately 25 times more potent than morphine on a molar basis. Matching morphine and beta-endorphin doses such that approximately equal submaximal submaximal effects occurred, it was observed that the antinociception produced by beta-endorphin lasted approximately three times longer than that produced by morphine. Experiments with intrathecal injection of beta-endorphin into the spinal subarachnoid space of cats fitted with intrathecal catheters also revealed a potent antinociceptive effect which was completely antagonized by naloxone. In the rats, naloxone administered systemically in doses of 10--100 microgram/kg produced a parallel shift in the dose-response curves of both nociceptive measures suggesting a competitive antagonism. Using a dose ratio analysis, an in vivo pA2 of 7.1 for naloxone was obtained. These data and those derived from previous work based on the pA2 suggest that the interaction of morphine, certain pentapeptides, and beta-endorphin is the same with regard to the spinal opiate receptor population mediating behaviorally defined analgesia.     

Persistent pain model reveals sex difference in morphine potency

Central or systemic administration of agonists directed at the mu or delta opiate receptors generally produce a greater degree of analgesia in males than in females. To date, most studies examining sex-based differences in opioid analgesia have used acute noxious stimuli (i.e., tail-flick and hot plate test); thus the potential dimorphic response of centrally acting opiates in the alleviation of persistent inflammatory pain is not well established. In the present study, right hind paw withdrawal latency (PWL) to radiant thermal stimuli was measured in intact male and cycling female Sprague-Dawley rats before and after unilateral hind paw injection of the inflammatory agent complete Freund's adjuvant (CFA). Control animals received intraplantar injection of saline. Twenty four hours after CFA or saline injection, animals received either saline or morphine bisulfate (0.5-15 mg/kg sc). Separate groups of control or inflamed animals were tested on their responsiveness to morphine at 7, 14, and 21 days post-CFA or saline. No sex differences were noted for baseline PWLs, and females displayed slightly less thermal hyperalgesia at 24 h post-CFA. At all morphine doses administered, both the antihyperalgesic effects of morphine in the inflamed animals and the antinociceptive effects of morphine in control animals were significantly greater in males compared with females. Similarly, in males, the antihyperalgesic effects of morphine increased significantly at 7-21 days post-CFA; no significant shift in morphine potency was noted for females. These studies demonstrate sex-based differences in the effects of morphine on thermal hyperalgesia in a model of persistent inflammatory pain.     

Comparative effects of opioid peptides on respiration and analgesia in rats

Receptor mechanisms for opiate induced respiratory depression and analgesia (tail-flick) were studied by the ED50 ratios and the apparent pA2 values of the interactions of naloxone with the mu-agonists morphine and D-ala2-me-phe4-met (O)ol5-enkephalin (FK-33824), and the delta-agonists D-ala2-D-leu5-enkephalin (DADL) and tyr-D-ser-gly-phe-leu-thr. The apparent pA2 values of morphine, FK-33824 and DADL for analgesia were similar, whereas the apparent pA2 values of the mu-agonists for respiratory depression were significantly lower than those of the delta-agonists. The ratio between the ED50 of FK-33824 in analgesia and respiratory depression was much lower than that of DADL. It is concluded that different receptors mediate the opiate-induced respiratory depression. One difficulty with the delta-receptors being maximally involved in this action is the high degree of antagonism shown by naloxone on the respiratory effects of the delta-agonists.     

Magnetic fields inhibit opioid-mediated ‘analgesic’ behaviours of the terrestrial snail,Cepaea nemoralis

1. The terrestrial snail, Cepaea nemoralis, when placed on a warmed surface (40 degrees C) displays a thermal avoidance behaviour that entails an elevation of the anterior portion of the fully extended foot. The latency of this nociceptive response was increased by the prototypical mu and specific kappa opiate agonists, morphine and U-50, 488H, respectively, in a manner indicative of anti-nociception and the induction of 'analgesia'. Pretreatment with the prototypical opiate antagonist, naloxone, blocked the morphine- and reduced the U-50, 488H-induced analgesia. Naloxone had no effects on the thermal response latencies of saline treated animals. 2. Exposure to either cold (7 degrees C) or warm (38 degrees C) temperature stress increased the nociceptive thresholds of Cepaea in a manner indicative of the induction of 'stress-induced analgesia'. The warm stress-induced analgesia was opioid mediated, being blocked by naloxone, whereas, the cold stress-induced analgesia was insensitive to naloxone. 3. Exposure for 15-30 min to 0.5 Hz weak rotating magnetic fields (1.5-8.0 G) significantly reduced the analgesic effects of the mu and kappa opiate agonists in a manner similar to that observed with naloxone. The magnetic stimuli also inhibited the endogenous opioid mediated warm stress-induced analgesia and significantly reduced the cold stress-induced analgesia. The magnetic stimuli had no evident effects on the nociceptive responses of saline-treated animals. The dihydropyridine (DHP) and non-DHP calcium channel antagonists diltiazem, verapamil. and nifedipine differentially and significantly reduced, while the DHP calcium channel agonist, BAY K8644, significantly enhanced the inhibitory effects of the magnetic fields on morphine-induced analgesia.     

Comparative effects of somatostatin and enkephalins on the guinea pig ileum and the rat vas deferens

The action of somatostatin (SRIF (somatotrophin release inhibiting factor)) was compared with that of Met-enkephalin (Tyr-Gly-Gly-Phe-Met) in the electrically stimulated guinea pig ileum myenteric plexus longitudinal muscle and with that of an enkephalin analogue (FK 33-824 (Tyr-D-Ala-Gly-MePhe-Met-(O)-ol)) in the rat vas deferens. In both tissues SRIF produced a twitch inhibition which was not antagonized by naloxone and which showed a long-lasting tachyphylaxis. The enkephalins tested produced a naloxone-antagonizable inhibition of twitch in both tissues but no tachyphylaxis. Therefore we conclude that SRIF is not acting at opiate receptors in these tissues.     

A tetrapeptide isolated from hamster embryo with central opiate properties on gastrointestinal motility but not on pain perception

The effects of centrally administered kentsin (H-Thr-Pro-Arg-Lys-OH) on intestinal motility and on pain perception were investigated in rats chronically equipped with lateral ventricle catheters. Intestinal motility was recorded electromyographically from electrodes placed on the duodeno-jejunum; analgesia was evaluated by the hot-plate and tail-flick tests. Kentsin (4.0 ug/kg), injected intracerebroventricularly (ICV) 2 hours after the beginning of a meal, restores the "fasted" i.e. the migrating myoelectric complex of intestinal motility, while a 5 times higher dose administered subcutaneously was inactive. The ICV effect of kentsin was blocked by previous ICV administration of naloxone (400 ug/kg). In contrast, kentsin administered ICV (40 ug/kg) or SC (200 ug/kg) did not affect significantly (P greater than 0.05) the time latency in the two analgesic tests during 90 minutes after its administration and did not significantly modify the analgesic effects of (D5-Ala2, Met5) enkephalinamide. We conclude that kentsin when centrally administered acts on opiate receptors to alter gastrointestinal motility but without effects on pain perception.     

beta-Endorphin controls vasopressin release during foot shock-induced stress in the rat

This study tested the possibility that beta-endorphin is involved in the regulation of vasopressin release during stress induced by inescapable electric foot shock. To this end, a specific anti-beta-endorphin antiserum or a control serum lacking the specific anti-beta-endorphin antibodies was administered to male rats. Plasma vasopressin concentrations, measured by radioimmunoassay, were not affected by brief foot shock stress in control rats, but were raised significantly by the stress in animals which had received an intracerebroventricular (i.c.v.) injection of the anti-beta-endorphin antiserum. In contrast, when the same volume of the anti-beta-endorphin antiserum was injected into a tail vein, foot shock stress produced only a slight effect on vasopressin release. I.c.v. injection of the antiserum changed neither basal nociceptive threshold nor stress-induced analgesia as revealed by the tail-flick latency. Vasopressin release induced by an osmotic stimulus was not influenced by the anti-beta-endorphin antiserum given i.c.v. The opiate antagonist naloxone or the glucocorticoid dexamethasone raised plasma vasopressin concentration in stressed rats which had received the control serum (i.c.v.); however, after i.c.v. injection of the anti-beta-endorphin antiserum neither naloxone nor dexamethasone elevated the plasma vasopressin concentration beyond the level reached by the anti-beta-endorphin antiserum (i.c.v.) alone. These results suggest that beta-endorphin inhibits the release of vasopressin during foot shock-induced stress in the rat.     

Vasopressin analgesia: specificity of action and non-opioid effects

Recent neuroanatomical and behavioral evidence has indicated that vasopressin (VP) increases pain thresholds. In the present study intracerebroventricular (ICV) administration of both arginine VP (AVP: 75-500 ng) and 1-deamino-8-D-arginine vasopressin (DDAVP: 150-500 ng) elevated tail flick latencies. Oxytocin (OXY, ICV), also elevated tail-flick latencies (150-1000 ng); however this increase was accompanied by "barrel-roll" seizure activity. VP analgesia was eliminated by pretreatment with 1-deamino-penicillamine-2(O-methyl)tyrosine-AVP (dPTyr(me)AVP: 500 ng, ICV), a VP antagonist, but not naloxone (1 or 10 micrograms, ICV), suggesting that VP modulates nonciceptive thresholds through its own binding sites. Conversely, pretreatment with naloxone (1 micrograms, ICV) but not dPTyr(me)AVP (1 microgram, ICV) attenuated the analgesic efficacy of systemic morphine (10 mg/kg), further dissociating VP and central opiate analgesic processes. Finally, systemic pretreatment with dexamethasone potentiated VP analgesia. These data support the notion that VP is a specific non-opioid pain inhibitor.     

Cholera toxin and pertussis toxin on opioid- and alpha 2-mediated supraspinal analgesia in mice.

Cholera toxin, an agent that impairs the function of Gs transducer proteins, was injected (0.5 microgram/mouse, icv) and the antinociceptive activity of opioids and clonidine was studied 24h later in the tail-flick test. In these animals, an enhancement of the analgesic potency of morphine, beta-endorphin and clonidine could be observed. Cholera toxin did not modify the antinociception evoked by the enkephalin derivatives DAGO and DADLE. Pertussis toxin that catalyses the ADP ribosylation of alpha subunits of Gi/Go regulatory proteins was given icv (0.5 microgram/mouse). This treatment reduced the analgesic effect of opioids and clonidine. However, while the analgesia elicited by DAGO, DADLE and clonidine was greatly decreased, the effect of morphine and beta-endorphin was reduced to a moderate extent. It is concluded that Gi/Go regulatory proteins functionally coupled to opioid and alpha 2 receptors are implicated in the efficacy displayed by opioids and clonidine to produce supraspinal analgesia. Moreover, these two receptors are susceptible to regulation by a process that might involve a Gs protein.     

Effect of naloxone on analgesia induced by food deprivation

Naloxone (4 mg/kg) or saline was administered to animals under food deprived and non-deprived conditions prior to testing pain sensitivity in the tail flick test. Food deprived animals exhibited significantly elevated latencies in comparison to latencies observed under non-deprived conditions. This analgesia was diminished by treatment with the opiate receptor antagonist, naloxone. These findings suggest that analgesia induced by food deprivation is mediated in part by opiate receptor systems.     

Prostaglandin hyperalgesia, V: a peripheral analgesic receptor for opiates

Prostaglandin E2 injected in the rat paw causes hyperalgesia which is antagonized by local injections of opiate and opiate antagonists. In the present investigation in rats it is shown that naloxone has an analgesic effect at doses as low as 2 micrograms/site, injected into the rat hind paw. At a dose that has no analgesic effect (1 microgram/site) naloxone antagonized the analgesia produced by either local or systemic administration of morphine. Local administration of levorphanol (50 micrograms/site) caused a 50% reduction in the intensity of the hyperalgesia induced by prostaglandin E2. A dose four times greater of its isomer, dextrorphan, had little analgesic effect. The present results support the suggestion that this peripheral analgesia is the result of an action of opiates in receptors located at the nociceptors.     

Brain renin angiotensin system (RAS) in stress-induced analgesia and impaired retention.

Physiological stress is known to produce analgesia and memory disruption. Brain renin angiotensin system (RAS) has been reported to participate in stress response and plays a role in the processing of sensory information. Angiotensin receptors (AT), particularly AT1 subtypes have been reported to be distributed in brain areas that are intimately associated with stress response. The purpose of present study was to examine the modulation of AT1 receptor in the immobilization stress and angiotensin II (AngII)-induced analgesia and impaired retention, and to determine whether resultant behavioral changes involve common sensory signals. Result of present experiments showed that immobilization stress in mice and rats, and intracerebroventricular (ICV) administration of AngII (10 and 20 ng) in rats produced an increase in tail-flick latency. Similarly, post training administration of AngII or immobilization stress produced impairment of retention tested on plus-maze learning and on passive avoidance step-down task. Both these responses were sensitive to reversal by prior treatment with losartan (10 and 20 mg/kg), an AT1 AngII receptor antagonist. On the other hand, naloxone, an opiate antagonist preferentially attenuated the stress and AngII-induced analgesia and retention deficit induced by immobilization stress, but failed to reverse the AngII induced retention deficit. These results suggest immobilization stress-induced analgesia and impaired retention involves the participation of brain RAS. Further, failure of naloxone to reverse AngII-induced retention impairment shows. AngII-induced behavioral changes are under control of different sensory inputs.     

Tail-flick latency and self-mutilation following unilateral deafferentation in rats

Unilateral deafferentation induced by transection of the C(4)-C(8) dorsal roots of spinal cord, followed by a complex of abnormal self-mutilating behavior, is interpreted as an animal model of chronic nociception. The objective of our study was to test the differences in tail-flick latency between intact control and unilaterally deafferented animals and to assess the changes in their acute nociceptive sensation. The initial hypothesis was that deafferentation-induced painful sensation might cause stress-induced analgesia that should be manifested as prolonged tail-flick latency. The experiment was carried out on 11 male and 10 female adult Wistar rats. The tail-flick latency was repeatedly measured over a period of 10 consecutive weeks both in the preoperative baseline period and following multiple cervical dorsal rhizotomy. Contrary to our hypothesis, unilateral deafferentation was followed by a significant shortening of the tail-flick latency both in males and females. In deafferented animals, compared to the controls, variations of tail-flick latency were reduced. In individual animals after deafferentation, concurrent dynamic changes were observed in self-mutilating behavior, in a loss and regaining of body weight, and in tail-flick latency. Our data suggest that changes in tail-flick latency may be interpreted in terms of central sensitization and that tail-flick latency might be considered as a useful marker of chronic nociception.     

Comparison of the behavioral effects of β-endorphin and enkephalin analogs

The behavioral effects of β-endorphin, enkephalin analogs, morphine and etorphine were briefly compared. In the tail-flick test in mice and in the wet shake test in rats, β-endorphin and D-Ala²-D-Leu⁵-enkephalin had equal antinociceptive activity; D-Ala² -Met-enkephalinamide and D-Leu⁵-enkephalin were less active. The order of activity of the enkephalin analogs and opiate alkaloids for stimulating locomotor activity in mice paralleled their analgesic activities; β-endorphin, however, had only minimal stimulatory actions. Morphine sulfate, 50 μg injected into the periaqueductal gray, produced hyperactivity but this effect was not observed with etorphine or opioid peptides. By contrast, “wet dog” shakes was observed with the opioid peptides but not with either opiate alkaloid. These heterogenous behavioral responses, which were all antagonized by naloxone, indicate that multiple types of receptors mediate the effects of opiates in the central nervous system.     

Identification of a novel frog RFamide and its effect on the latency of the tail-flick response of the newt

Neuropeptide FF, one of the mammalian PQRFamides, has been reported to affect the latency of the tail-flick response in rat. We intended to examine the nociceptive effect by the peptide PQRFamides from the comparative aspect. Using the dot immunoblot method with antiserum to FMRFamide as an assay system, a peptide (frog's nociception-related peptide, fNRP) which has the C-terminal sequence PQRFamide was isolated from the brain of the frog, Rana catesbeiana. The determined sequence, SIPNLPQRF-NH(2), is the same as that named first (frog growth hormone-releasing peptide-gene-related peptide-1: fGRP-RP-1, which is encoded in the cDNA of the fGRP precursor. Since the peptide was isolated from the frog brain, we tested another amphibian, the newt, which has a tail, by the hot beam tail-flick test. Intraperitoneal injection of fNRP significantly increased the latency of the pain response (tail-flick) 90 min after administration. The effect was blocked by simultaneous administration of 5 mM naloxone. The result provides evidence for the interaction of fNRP and opioid steps in the analgesia pathways in the newt.     

The effect of opiates and opiate antagonists on heat latency response in the parasitic nematode Ascaris suum

The effects of the opiates morphine and morphine-6-glucuronide (M6G), the mu opioid receptor specific antagonist D-Phe-Cys-Tyr-D-Trp-Om-Thr-Pen-Thr-NH(2) (CTOP), and the general opiate antagonist naloxone on the latency of response to thermal stimulation were determined in the parasitic nematode Ascaris suum. Thermal detection and avoidance behaviors of the worms were evaluated with a tail flick analgesia meter using a modification of a technique employed for nociception experiments in rodents. Morphine and M6G were shown to have a dose dependent analgesic effect on A. suum's latency of response to heat with morphine being the most potent. The analgesic effect of morphine was reversed by naloxone but not CTOP. Neither naloxone nor CTOP was able to block the analgesia of M6G. CTOP but not naloxone had significant analgesic effects on its own. These findings are generally consistent with previous results on the effects of opiates and nitric oxide release from A. suum tissue. Apparently these nematodes possess opioid receptors that effect nociception.