Kindling-induced changes in morphine analgesia and catalepsy: Evidence for independent opioid systems

Repeated stimulation of the amygdaloid complex in rats results in the kindling of epileptic seizures. During the process of kindling and prior to the appearance of full behavioral convulsions naloxone-sensitive changes in responsiveness to noxious stimulation occur, which disappear with repeated stimulation. When full behavioral convulsions appear, a short period of post-ictal behavioral depression (PID) can be seen immediately following convulsions. Naloxone either attenuates or completely blocks the occurrence of the PID.In order to test further the opioid nature of these phenomena, the development of tolerance to PD with repeated stimulation and the development of cross-tolerance to the analgesic and motor depressant effects of morphine were tested in kindled animals. Repeated elicitation of full behavioral convulsions resulted in a progressive decrease of PID duration across days. PID was also decreased in morphine tolerant animals. Repeated convulsions also induced cross-tolerance with both morphine analgesia and morphine-induced catalepsy. In contrast, animals tested following 10 days of amygdaloid stimulation which did not cause full behavioral convulsions to develop, showed cross-tolerance to the analgesic but not the motor depressant effect of morphine. The possibility that two different opioid systems, which are independent of one another are activated during different phases of kindling is discussed.     

Involvement of spinal opioid systems in footshock-induced analgesia: Antagonism by naloxone is possible only before induction of analgesia

Footshock reliably produces analgesia in rats which is mediated either by opiate or non-opiate systems. It has recently been demonstrated that a critical factor determining the involvement of endogenous opioids is the body region shocked; front paw shock produces a naloxone-reversible analgesia and hind paw shock produces an analgesia which fails to be attenuated by this opiate antagonist. The present study demonstrated that a crucial opiate site for the production of front paw footshock-induced analgesia (FSIA) exists within the spinal cord. One microgram of naloxone delivered directly to the lumbosacral cord immediately prior to shock significantly attenuated this analgesia. However, the efficacy of naloxone antagonism was order-dependent in that naloxone failed to antagonize fron paw FSIA if delivered immediately after shock; naloxone could prevent but could not reverse the analgesic state. The body region shocked was again observed to be a critical factor determining the involvement of endogenous opioids since 1 microgram of spinal naloxone failed to antagonize hind paw FSIA. These results were discussed in light of recent evidence proposing a neuromodulatory role of opioids within the spinal cord.     

Stress-induced analgesia in frogs: evidence for the involvement of an opioid system

Immobilization of frogs for 1 h induces analgesia which is blocked but not reversed by low doses of naloxone. After 9 days of daily immobilization for 1 h, the treatment fails to cause analgesia thus indicating that tolerance has developed. Animals tolerant to i immobilization-induced analgesia do not show cross-tolerance to the analgesic action of morphine. The development of tolerance to this form of stress-induced analgesia and the ability of naloxone to prevent its occurrence indicate the involvement of opioid pathways. The lack of cross tolerance to morphine suggests that μ receptors are not involved.     

Evidence for opioid and non-opioid forms of stimulation-produced analgesia in the rat

This study compares stimulation-produced analgesia (SPA) elecited from two different midline regions of the midbrain of the rat. Dorsal electrode placements were in the caudal periaqueductal gray matter; ventral placements lay within or subjacent to the dorsal raphe n. SPA thresholds were measured by the tail-flick method both during and immediately after the period of brain stimulation. Thresholds were consistently higher in the post-stimulation test. SPA from dorsal and ventral regions differed in the following ways: (1) Post-stimulation analgesia was significantly more difficult to obtain in ventral than in dorsal regions, whereas during-stimulation analgesia did not vary as a function of electrode location; (2) Although a continuous distribution of thresholds was seen for ventral placements, thresholds for dorsal placements tended to be either high or low on both during- and post-stimulation tests; (3) Naloxone (0.01–10 mg/kg) reliably elevated SPA thresholds for ventral but not dorsal stimulation placements. We conclude that different substrates of SPA lie in close proximity to one another in the medial midbrain of the rat. This portion of the midbrain appears to mediate both opioid and non-opioid mechanisms of analgesia.     

Stimulation-produced and stress-induced analgesia: cross-tolerance between opioid forms

Electrical stimulation of medial brainstem sites produces potent analgesia in rats that is either opioid- or non-opioid-mediated depending on the specific brain region stimulated. Footshock stress also causes opioid and non-opioid forms of analgesia in rats depending on the exact parameters of footshock administered. We now report that opioid, but not non-opioid, stress analgesia demonstrates cross-tolerance with opioid, but not non-opioid, stimulation-produced analgesia. This finding suggest that opioid forms of stimulation-produced and stress-induced analgesia share a common substrate.     

Footshock-induced analgesia: Its opioid nature depends on the strain of rat

Previous studies have indicated that stressful footshock can induce both opioid, naloxone-sensitive, and non-opioid, naloxone-insensitive forms of analgesia, depending on stimulation parameters used with 30 min of intermittent footshock (3 mA, 1 s on, 5 s off) producing opioid analgesia and 3 min of continuous shock (3 mA) producing non-opioid analgesia. Using a local strain of Charles River (CR)-derived rats we conducted a parametric investigation of footshock-induced analgesia applying both AC and DC scrambled shock ranging from 1 to 4 mA, continuous shock of 1, 3 and 5 min in duration and intermittent shock lasting 1, 3, 5, 10, 20, 30 and 80 min. All shock parameters produced potent analgesia. In no case did 10 mg/kg of naloxone block this analgesia. Varying the dose of the antagonist (0.1-10 mg/kg) and testing the animals at different points in the diurnal cycle did not result in the emergence of naloxone-sensitive anangesia. Based on the assumption that non-opioid systems may mask the activity of opioid analgesia systems, we attempted to either enhance opioid analgesia by: preventing enkephalin degradation by the use of D-phenylalanine; increasing the entry of blood-borne opioids into the brain by the use of DMSO; and the attenuation of non-opioid analgesia by the use of reserpine. In no case did a naloxone-sensitive component of analgesia emerge. To test whether the animals possess an intact opioid analgesia system, both electrical stimulation of, and injection of opiates into the periaqueductal gray (PAG) were examined. Both procedures produced analgesia which was reversed by naloxone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of central muscarinic cholinergic mechanisms in opioid stress analgesia

Stress has been shown capable of differentially activating opioid- and non-opioid-mediated endogenous analgesia systems. In this study, the muscarinic cholinergic antagonist, scopolamine, but not the centrally inactive methylscopolamine, blocks opioid, but not non-opioid stress analgesia. Additionally, naltrexone, an opiate antagonist, attenuates analgesia induced by oxotremorine, a cholinergic agonist. These findings support the existence of a muscarinic cholinergic synapse in a central nervous system opioid pain-inhibitory pathway.     

Body region shocked need not critically define the neurochemical basis of stress analgesia

Both opioid and non-opioid forms of stress-induced analgesia have been demonstrated in rats, although the conditions leading to their selective activation are still being investigated. We have shown that variations in shock intensity, duration or temporal pattern can determine whether opioid or non-opioid stress analgesia occurs. Others have suggested that body region shocked is the critical determinant, analgesia from front paw shock being opioid and that from hind paw shock non-opioid. We now report that either opioid or non-opioid stress analgesia can be evoked from either front or hind paws depending only on footshock intensity when duration and temporal pattern are held constant.     

Effects of naloxone and hypophysectomy on electroconvulsive shock-induced analgesia

Powerful analgesia follows electroconvulsive shock in both hypophysectomized and sham-operated rats. Antagonism of this analgesia by naloxone implicates opioid peptides in its mediation, its occurrence in hypophysectomized animals implicating opioids of central nervous system rather than pituitary origin. Because naloxone only partially reduces electroconvulsive shock analgesia in hypophysectomized rats, the participation of another, non-opioid analgesia substrate also seems indicated.     

Test-specific analgesia elicited by kindled seizures in rats

Analgesia following kindled seizures in rats was studied using a test thought to measure pain integration at 3 levels of the CNS. Seizures caused an elevation of the threshold at which tail shocks elicited multiple squeaks, without affecting single squeak or tail withdrawal thresholds. Opioid involvement was indicated by naloxone's partial blockade of this effect.     

Opiate vs non-opiate footshock-induced analgesia (FSIA): The body region shocked is a critical factor

Previous work has demonstrated that footshock can elicit either opiate or non-opiate analgesia. The present study has demonstrated that one critical factor determining the involvement of endogenous opioids is the body region shocked. Using 90 s shock, front paw shock produced an opiate analgesia which was significantly antagonized by as little as 0.1 mg/kg systemic naloxone and morphine tolerance. In the latter experiment, a parallel recovery of the analgesic potencies of both front paw shock and morphine was observed following 2 weeks of opiate abstinence. In contrast, hind paw shock produced a non-opiate analgesia which failed to be attenuated by 20 mg/kg systemic naloxone and showed no cross-tolerance to morphine. Since identical shock parameters were used for front paw and hind paw shock in the systemic naloxone experiments, stress per se clearly cannot be the crucial factor determining the involvement of endogenous opioids in footshock-induced analgesia. These results were discussed with respect to clinical treatments of pain which utilize somatosensory stimulation.     

Two opioid forms of stress analgesia: Studies of tolerance and cross-tolerance

We have previously reported that stress analgesia sensitive to and insensitive to opiate antagonists can be differentially produced in rats by varying the severity or temporal pattern of inescapable footshock. In these studies, we give further evidence for the opioid and non-opioid bases of these paradigms of stress analgesia. We find that naloxone-sensitive analgesia demonstrates tolerance with repeated stress and cross-tolerance with morphine, whereas naloxone-insensitive analgesia demonstrates neither of these characteristics. Moreover, different forms of opioid, but not non-opioid, stress analgesia manifest cross-tolerance with each other. These data are discussed in terms of the similarities and differences between two forms of opioid stress analgesia.     

Multiple opioid system involvement in the mediation of parasitic-infection induced analgesia

Although parasite modification of host behaviour is well established, little is known about the mechanisms underlying such effects. The present study examined the relationships between subclinical infection with the enteric sporozon parasite,Eimeria vermiformis, nociceptive responses and endogenous opioid systems in male mice. Infected mice displayed significant analgesia which increased through the prepatent period [oocyst formation (pre-infective); days 1–7 post-infection (PI)], reached a maximum with the onset of patency (onset oocyst shedding and infectivity; days 7–8 PI) and declined during patency (oocyst shedding), with response latencies declining to basal levels with the cessation of oocyst production and infectivity (day 15 PI). The increasing nociception during the prepatent period (day 4 PI) was associated with k opioid mechanisms, being reduced by the k antagonist, nor-binaltorphimine, and insensitive to either the σ antagonist, ICI 174,864, or the general, predominately μ antagonist, naloxone. Maximum analgesia (day 7 PI) associated with the onset of patency (infectivity) was sensitive to both the k and μ antagonists, but insensitive to the σ antagonist, while the declining analgesia during patency (day 10 PI) was reduced by the μ and σ antagonists, but was insensitive to the k antagonist. These results indicate that μ, σ and k opioid systems are involved in the mediation of su subclinical parasitic infection-induced analgesia and likely other associated parasite-induced modifications of host behaviour.     

The role of stimulus intensity and stress in opioid-mediated analgesia

Rats exposed to a Pavlovian conditioning paradigm developed naloxone-reversible analgesia only when the intensity of a noxious unconditioned stimulus was suprathreshold and the level of stress was augmented. The time course of the onset of this conditioned analgesia was reproduced by systemic administration of morphine. These findings suggest that both a minimal level of stimulus intensity and stress are necessary for the activation of endogenous opioid-mediated analgesia.     

Neuraminidase inhibitor, oseltamivir blocks GM1 ganglioside-regulated excitatory opioid receptor-mediated hyperalgesia, enhances opioid analgesia and attenuates tolerance in mice

The endogenous glycolipid GM1 ganglioside plays a critical role in nociceptive neurons in regulating opioid receptor excitatory signaling demonstrated to mediate "paradoxical" morphine hyperalgesia and to contribute to opioid tolerance/dependence. Neuraminidase (sialidase) increases levels of GM1, a monosialoganglioside, in these neurons by enzymatic removal of sialic acid from abundant polysialylated gangliosides. In this study, acute treatment of mice with the neuraminidase inhibitor, oseltamivir enhanced morphine analgesia. Acute oseltamivir also reversed "paradoxical" hyperalgesia induced by an extremely low dose of morphine, unmasking potent analgesia. In chronic studies, co-administration of oseltamivir with morphine prevented and reversed the hyperalgesia associated with morphine tolerance. These results provide the first evidence indicating that treatment with a neuraminidase inhibitor, oseltamivir, blocks morphine's hyperalgesic effects by decreasing neuronal levels of GM1. The present study further implicates GM1 in modulating morphine analgesia and tolerance, via its effects on the underlying excitatory signaling of Gs-coupled opioid receptors. Finally, this work suggests a remarkable, previously unrecognized effect of oseltamivir-which is widely used clinically as an antiviral agent against influenza-on glycolipid regulation of opioid excitability functions in nociceptive neurons.     

Diazepam reduces stress-induced analgesia in humans

The analgesic effects of a repetitive stress induced by anticipation of pain (noxious footshock) were studied on both the threshold of a nociceptive flexion reflex and the corresponding pain sensation after a 4-day-treatment of diazepam vs placebo (cross-over and double-blind study) in normal volunteers. During diazepam, the stressor stimulus produced a weaker depression on both nociceptive reflex and pain sensation than that observed during placebo. Furthermore, the reversal effect by naloxone was much more marked during placebo than during diazepam. These data clearly suggest a possible moderating action of benzodiazepine brain type receptors upon the endogenous opiate systems involved in the phenomenon of stress-induced analgesia in humans.     

Systemic morphine blocks the seizures induced by intracerebroventricular (i.c.v.) injections of opiates and opioid peptides

Intracerebroventricular (i.c.v.) injections of the endorphins and of morphine in rats produce highly characteristic, naloxone sensitive, electrographic seizures. In contrast, systemic injections of morphine have been shown to exert a marked anticonvulsant effect. The present study demonstrates that systemic morphine pretreatment can prevent the occurrence of electrographic seizures induced by i.c.v. morphine, Leu-enkephalin and β-endorphin and that the anti-epileptic effect of morphine can be reversed by naloxone. Male albino rats, previously prepared for chronic i.c.v. injections and EEG recording, were pretreated with 0–100 mg/kg of intraperitoneal (i.p.) morphine. Thirty five minutes later morphine (520 nmol), Leu-enkephalin (80 nmol) or β-endorphin (5 nmol) were injected i.c.v. Pretreatment with i.p. morphine blocked the occurrence of seizures induced by morphine and both endogenous opioids. Lower doses of systemic morphine (50 mg/kg) were necessary to block i.c.v. morphine seizures than the dose (100 mg/kg) necessary to block seizures induced by i.c.v. Leu-enkephalin and β-endorphin. Naloxone (1 mg/kg) administered 25 min following 50 mg/kg of i.p. morphine and preceding the injections of i.c.v. morphine reversed the antiepileptic effect of systemic morphine. These results demonstrate the possible existence of two opiate sensitive systems, one with excitatory-epileptogenic effects and the other possessing inhibitory-antiepileptic properties. The possible relationship between these findings and the known heterogeneity of opiate receptors and opiate actions is discussed.     

Bacitracin produces analgesia by increasing brain immunoreactive ß-endorphin (ß-E) content

The effects of protease inhibitor bacitracin on brain ß-endorphin content and analgesia, were examined in vivo. Male Sprague-Dawley rats were injected with bacitracin intracerebroventricularly and sacrificed by microwave irradiation 15 and 30 min after injection. Brain ß-endorphin levels were 27% higher in bacitracin treated rats than in controls. A second group of bacitracin injected rats was subjected to continuous intermittent 55 °C hot plate exposure. Bacitracin-injected rats exhibited total analgesia 15 min after bacitracin injection. At 30 min, this analgesic effect subsided. Control rats exhibited no analgesia. Bacitracin induced analgesia was naloxone reversible at a low dose of naloxone (1 mg/kg). At a higher dose of naloxone (10 mg/kg), bacitracin induced analgesia was only partially antagonized. These results suggest that bacitracin induced analgesia might be due to the elevated levels of brain ß-endorphin caused by a decrease in its breakdown by bacitracin.     

Reduction of stress-induced analgesia but not of exogenous opioid effects in mice lacking CB1 receptors

CB1 cannabinoid receptors are widely distributed in the central nervous system where they mediate most of the cannabinoid-induced responses. Here we have evaluated the interactions between the CB1 cannabinoid receptors and the endogenous opioid system by assaying a number of well-characterized opioid responses, e.g. antinociception and stress-mediated effects, on mutant mice in which the CB1 receptor gene was invalidated. The spontaneous responses to various nociceptive stimuli (thermal, mechanical and visceral pain) were not changed in mutant CB1 mice. Furthermore, the absence of the CB1 cannabinoid receptor did not modify the antinociceptive effects induced by different opioid agonists: morphine (preferential mu opioid agonist), D-Pen2-D-Pen5-enkephalin (DPDPE) and deltorphin II (selective delta opioid agonists), and U-50,488H (selective kappa opioid agonist) in the hot-plate and tail-immersion tests. In contrast, the stress-induced opioid mediated responses were modified in CB1 mutants. Indeed, these mutants did not exhibit antinociception following a forced swim in water at 34 degrees C and presented a decrease in the immobility induced by the previous exposure to electric footshock. However, the antinociception induced by a forced swim in water at 10 degrees C was preserved in CB1 mutants. These results indicate that CB1 receptors are not involved in the antinociceptive responses to exogenous opioids, but that a physiological interaction between the opioid and cannabinoid systems is necessary to allow the development of opioid-mediated responses to stress.