Delta opiate receptors mediate tail-shock induced antinociception at supraspinal levels.

Previous work has demonstrated that 3 pharmacologically and neuroanatomically distinct analgesia systems can be sequentially activated by increasing numbers of transcutaneous tail-shock. To date, the categorization of the early (after 2 tail-shocks) and late (after 80-100 tail-shocks) analgesias as opiate-mediated has been based on the ability of systemic naltrexone and morphine tolerance to block these effects. In contrast, the analgesia observed after 5-40 tail-shocks is unaffected by these manipulations, leading to its categorization as non-opiate. The preceding companion paper and the present work were aimed at identifying the neuroanatomical loci at which opiates exert their analgesic effects in this tail-shock paradigm and, further, to identify which opiate receptor subtypes are involved. The 8 experiments included in the present paper examined the effect of microinjecting either naltrexone (a relatively non-selective opiate receptor antagonist), binaltorphimine (kappa receptor antagonist), Cys2-Tyr3-Orn5-Pen7-amide (CTOP) (mu receptor antagonist), or naltrindole (delta receptor antagonist) either into the third ventricle or over the frontal cortex. Taken together, these experiments demonstrate that the late (80-100 shock) opiate analgesia is mediated by delta opiate receptors located within subcortical structures rostral to the 4th ventricle. No evidence for supraspinal opiate involvement in the early (2 shock) opiate analgesia was found.     

Opiate and non-opiate analgesia induced by inescapable tail-shock: Effects of dorsolateral funiculus lesions and decerebration

Previous studies have demonstrated that inescapable tail-shock can produce either non-opiate or opiate short-term analgesia, dependent on the number of shocks delivered. Additionally, extended exposure to inescapable tail shock can produce long-term, opiate analgesic effects. Several lines of investigation suggest that the psychological dimension of perceived controllability may powerfully influence these phenomena in that each form of opiate analgesia can only be produced following exposure to inescapable, rather than equal amounts and distribution of escapable, shock. This has suggested that these opiate analgesias result from the organism's learning that it has no control over shock. Although it has been assumed that the opiate and non-opiate analgesias induced by tail shock may be subserved by neural circuitry similar to that mediating morphine analgesia and other forms of environmentally induced analgesia, no direct evidence exists to support this assumption. The present study sought to provide an initial attempt at defining the neural circuitry involved in these phenomena by examining the effect of bilateral dorsolateral funiculus (DLF) lesions and decerebration. These experiments revealed that pathways within the spinal cord DLF are critical for the production of short-term non-opiate analgesia, short-term opiate analgesia, and long-term opiate analgesia since bilateral DLF lesions abolished all three pain inhibitory effects. Additionally, it was found that decerebration did not attenuate either the short-term non-opiate or short-term opiate analgesia induced by inescapable tail shock. Combining the observations that these non-opiate and opiate short-term effects are not reduced by decerebration yet are abolished by DLF lesions clearly delimits the source of descending pain inhibition as being within the caudal brainstem.     

A permissive role of corticosterone in an opioid form of stress-induced analgesia: blockade of opiate analgesia is not due to stress-induced hormone release

The 100 inescapable tail-shock paradigm produces three sequential analgesic states as the number of shocks increases: an early opioid analgesia (after 2 shocks) that is attenuated by systemic naltrexone, a middle analgesia (after 5–40 shocks) that is unaffected by systemic naltrexone, and a late opioid analgesia (after 80–100 shocks) that is attenuated by systemic naltrexone. In order to determine whether the absence of adrenal hormones would affect any of these analgesias, we tested adrenalectomized (ADX) versus sham-operated control rats 2 weeks post-surgery. Pain threshold was assessed using the tail-flick (TF) test. ADX attenuated both the early (2 shock) and late (80–100 shock) opiate analgesias and failed to reduce the naltrexone-insensitive analgesia after 5–40 shocks. We demonstrated that a loss of adrenomedullary catecholamines does not underlie the ADX-induced attenuation of opioid analgesia since sympathetic blockade using systemic chlorisondamine (6 mg/kg) failed to reduce analgesia at any point in the shock session. It was further shown that stress levels of adrenal hormones are not critical since (a) analgesia was unaffected when animals were tested 48 h after ADX, (b) 2 shocks do not produce a surge in corticosterone (CORT) over and above levels observed in animals restrained and TF tested in preparation for shock, and (c) basal CORT replacement in drinking water fully restored analgesia in ADX rats. These experiments demonstrate that basal CORT, rather than adrenomedullary substances, is critical to the expression of analgesia. The function of CORT here is not linked to a shock-induced surge of the steroid. CORT appears to play a permissive role in the expression of analgesia. Potential effects of the absence of corticosteroids on neurotransmitter biosynthesis important in analgesia production are discussed.     

Parallel activation of multiple spinal opiate systems appears to mediate 'non-opiate' stress-induced analgesias.

Pain is powerfully modulated by circuitries within the CNS. Two major types of pain inhibitory systems are commonly believed to exist: opiate (those that are blocked by systemic opiate antagonists and by systemic morphine tolerance) and non-opiate (those that are not). We used intrathecal delivery of mu, delta, and kappa opiate receptor antagonists to examine 3 well-accepted non-opiate stress-induced analgesias. Combined blockade of all 3 classes of opiate receptors antagonized all of the 'non-opiate' analgesias. Further experiments demonstrated that blocking mu and delta or mu and kappa was sufficient to abolish 'non-opiate' analgesias. Combined blockade of kappa and delta receptors was without effect. The clear conclusion is that all endogenous analgesia systems may in fact be opiate at the level of the spinal cord. Phenomena previously thought to be non-opiate appear to involve parallel activation of multiple spinal opiate processes. These findings suggest the need for a fundamental shift in conceptualizations regarding the organization and function of pain modulatory systems in particular, and opiate systems in general.     

The nature of conditioned anti-analgesia: spinal cord opiate and anti-opiate neurochemistry

The central nervous system contains circuitry that inhibits pain sensitivity (analgesia), as well as circuitry that opposes pain inhibition (anti-analgesia). Activation of analgesia systems and anti-analgesia systems can each be brought under environmental control using classical conditioning procedures. Analgesia can be produced by cues present before and during aversive events such as electric shock, while active inhibition of analgesia comes to be produced by cues never present immediately before or during shock and therefore signal safety. We have recently reported that these analgesia and anti-analgesia systems interact at the level of the spinal cord. A series of 3 experiments were performed to examine how such interactions occur. First, potential opioid mediation of conditioned analgesia was investigated using systemic and intrathecal (i.t.) delivery of opiate antagonists. Conditioned analgesia was found to be mediated by activation of spinal μ and δ opiate receptors. Second, analgesia produced by each of these receptor subtypes was challenged by environmental signals for safety. Analgesias produced by μ and δ opiate agonists were each abolished by safety signals. Third, antagonists/antisera directed against several putative anti-opiate neurotransmitters were tested i.t. to identify which mediate conditioned anti-analgesia at the level of the spinal cord. A cholecystokinin antagonist abolished conditioned anti-analgesia. In contrast, neuropeptide FF antiserum and a κ opiate antagonist were without effect.     

Chronic antagonist infusion does not increase morphine antinociception in rat spinal cord

Chronic spinal infusion of the opiate antagonist naloxone or naltrexone fail to influence the antinociceptive effect of subsequent intrathecal morphine on the hot plate test in rats compared to saline-infused controls. These results contrast the functional supersensitivity to morphine seen after long-term systemic opiate antagonist administration and support the hypothesis that dopaminergic interactions, lacking in the spinal cord, are necessary for antagonist-induced opioid receptor upregulation.     

The neural basis of footshock analgesia: The role of specific ventral medullary nuclei

Previous studies have demonstrated that brief front paw shock produces opiate analgesia while brief hind paw shock produces non-opiate analgesia in rats. Additionally, front paw shock and hind paw shock can produce an opiate-mediated classically conditioned analgesia; that is, when shock is delivered to an animal, environmental cues become associated with this stimulus such that these cues become capable of producing potent opiate analgesia in the absence of shock. Investigations of the neural bases of these phenomena have revealed that front paw shock and classical conditioning lead to activation of supraspinal sites which mediate analgesia via descending pathways lying solely within the dorsolateral funiculus (DLF) of the spinal cord. Hind paw footshock induced analgesia (FSIA) is also mediated by a descending DLF pathway but is unlike front paw FSIA or classically conditioned analgesia in that it involves intraspinal pathways as well. The aim of the present series of experiments was to identify the supraspinal origin of the centrifugal DLF pathway mediating front paw (opiate) FSIA, hind paw (non-opiate) FSIA, and classically conditioned (opiate) analgesia. These studies examined the effect of electrolytic lesions of the nucleus raraphe magnus (NRM), nucleus reticularis paragigantocellularis (PGC), and combined lesions of these two areas (nucleus raphe alatus, NRA) on these environmentally-induced analgesias. The results of this work indicate that the NRA is the origin of the spinal cord DLF pathway mediating front paw (opiate) FSIA and classically conditioned (opiate) analgesia. Hind paw (non-opiate) FSIA is also mediated, in part, by the NRA but must involve another, yet unidentified, brainstem site(s) as well.     

Changes in responsiveness to mu and kappa opiates following a series of convulsions

After a series of seven electroconvulsive shocks, mice (C57BL/6J) showed a marked change in their response to opiates. Although very large doses of mu agonists induce convulsions in normal control mice, our evidence indicated that this was accomplished through nonopiate mechanisms: they could not be blocked by naltrexone and the pattern of drug potencies (codeine greater than morphine greater than levorphanol) was not consistent with an opiate response. In contrast, after electroconvulsive shock small doses of mu agonists induced convulsions that could be blocked by naltrexone and the pattern of drug potency (levorphanol greater than morphine greater than codeine) was consistent with an opiate mechanism. Kappa drugs, on the other hand, produced convulsions in both control and ECS animals, although there was an enhanced responsiveness in the latter. Furthermore, the convulsions produced by kappa drugs were blocked by naltrexone and showed stereoselectivity in both control and ECS animals. The changes in responsiveness to mu and kappa opiates cannot be explained on the basis of a general increase in seizure susceptibility, as sensitivity to the nonopiate convulsant, strychnine, was not enhanced after electroconvulsive shock. The results point to a qualitative change in response to mu agonists after electroconvulsive shock, but only a change in sensitivity to kappa agonists.     

Classical conditioning of front paw and hind paw footshock induced analgesia (FSIA): Naloxone reversibility and descending pathways

Opiate and non-opiate footshock induced analgesia (FSIA) can be differentially elicited dependent upon the body region shocked. As measured by the spinally-mediated tail flick test, hind paw shock produces non-opiate analgesia whereas front paw shock produces opiate analgesia. The present series of experiments utilized cord lesions and transections to identify descending and intraspinal pathways mediating front paw and hind paw FSIA. The results of these studies indicate that front paw shock leads to activation of supraspinal sites which mediate analgeshi via descending pathways lying solely within the dorsolateral funiculus (DLF) of the spinal cord; direct intraspinal pathways are not involved. Hind paw FSIA is also mediated by a descending DLF pathway but is unlike front paw FSIA in that it involves intraspinal pathways as well. This work provides further parallels between the analgesias produced by morphine, electrical brain stimulation and environmental stimuli.     

Endogenous opioid immunoreactivity in rat spinal cord following traumatic injury

It has been postulated that endogenous opioids play a pathophysiological role in spinal cord injury, based on the therapeutic effects of the opiate receptor antagonist naloxone in certain experimental models. The high doses of naloxone required to exert a therapeutic action suggest that naloxone's effects may be mediated by non-mu opiate receptors, such as the kappa receptor. This notion is supported by recent pharmacological studies demonstrating that an opiate antagonist more active at kappa sites is effective and far more potent than naloxone in improving outcome after spinal cord injury. Moreover, dynorphin--postulated to be the endogenous ligand for the kappa receptor--is unique among opioids in producing hindlimb paralysis following intrathecal administration in the rat. In the present studies we have examined changes in endogenous opioid immunoreactivity following traumatic spinal cord injury in the rat. Dynorphin A was found to increase progressively with graded injury; changes were restricted to the injury segment and adjacent areas and were time dependent. Dynorphin A-(1-8) showed no marked changes. Methionine and leucine enkephalin were either unaltered or reduced at the injury site; changes were not well localized and were not clearly related to the injury variables. These findings provide further support for a potential pathophysiological role of prodynorphin-derived peptides in spinal cord injury.     

Muscarinic cholinergic mediation of opiate and non-opiate environmentally induced analgesias

Previous studies have demonstrated that brief front paw shock and brief hind paw shock produce prolonged opiate and non-opiate analgesia, respectively. Additionally, opiate analgesia can be classically conditioned by using either front paw shock or hind paw shock as the unconditioned stimulus. However, beyond this point little is known regarding the neurochemistry of these phenomena. The present series of studies examined the potential involvement of nicotinic and muscarinic cholinergic systems in these 3 forms of environmentally induced analgesia. These experiments demonstrate that muscarinic cholinergic sites within the central nervous system are critically involved in the mediation of both hind paw (non-opiate) foot shock-induced analgesia (FSIA) and classically conditioned (opiate) analgesia since scopolamine, but not equimolar methylscopolamine, significantly attenuated analgesia. Furthermore, the primary muscarine site(s) appears to exist at a supraspinal, rather than spinal, level since delivery of scopolamine directly to the lumbosacral cord produced, at most, only a slight decrease in analgesia. Nicotinic systems do not appear to be importantly involved in any of these forms of environmentally induced analgesias since mecamylamine had no effect on either front paw FSIA or hind paw FSIA and, at most, produced only a slight reduction in classically conditioned analgesia. These data and a review of the literature suggest that the critical cholinergic sites involved in hind paw FSIA exist within the caudal brainstem whereas cholinergic sites within more rostral brain levels probably mediate classically conditioned analgesia.     

Cholecystokinin antagonists selectively potentiate analgesia induced by endogenous opiates

We have recently observed that exogenous sulfated cholecystokinin octapeptide (CCK) can antagonize various forms of opiate analgesia and that the CCK receptor blocker proglumide potentiates morphine analgesia. These observations, plus the similarity in the distribution of CCK and opiate systems, suggest that endogenous CCK may act as a physiological opiate antagonist. We have extended these initial studies by examining the effect of CCK antagonists on opiate analgesia produced by release of endogenous opiates (front paw footshock induced analgesia) and by intrathecal administration of D-Ala-methionine enkephalinamide, a stable analogue of an endogenous opiate. Additionally, the specificity of proglumide's effect was examined by testing the effect of this drug on various forms of non-opiate analgesia. This series of experiments demonstrate that CCK antagonists can markedly potentiate analgesia induced by endogenous opiates and provide strong support for the hypothesis that endogenous CCK systems can oppose the analgesic effects of opiates. Potentiation of analgesia by CCK receptor blockers appears to be selective for opiate systems since proglumide typically attenuated or had no effect on various forms of non-opiate analgesia. These data suggest that CCK blockers may be clinically useful for enhancing the analgesic effects of procedures such as acupuncture, which may be mediated by release of endogenous opiates.     

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.     

Analgesic effects of μ antagonists after naloxone non-reversible stress-induced analgesia

Three antagonists at the mu opiate receptor site: naloxone, naltrexone and diprenorphine, and one agonist-antagonist compound nalorphine, at doses usually not analgesic elicited analgesia in rats when administered after non-naloxone-reversible shock-induced analgesia had disappeared. The chi receptor antagonist, MR 2266, and the delta antagonist, ICI 154129, were all ineffective. This effect was no longer present when non-naloxone-reversible shock-induced analgesia was inhibited by the administration of the chi receptor antagonist, MR 2266. These results suggest that the mu opiate receptor may change its conformation under particular conditions such as continuous inescapable shock.     

Opiate, enkephalin, and endorphin analgesia: relations to a single subpopulation of opiate receptors

Differences in the receptor mechanisms of opiate analgesia and respiratory depression have been studied with three novel irreversible opiates. A single injection of the irreversible agonist oxymorphazone produces analgesia in mice, lasting over 24 hours. Conversely, the irreversible antagonist naloxazone dramatically reduces the analgesic effectiveness of a variety of opiate alkaloids and enkephalin analogs for over a day. Despite this marked reduction in analgesia after naloxazone treatment, morphine lethality (LD50) is unchanged in similarly treated mice. Receptor binding studies show that naloxazone irreversibly and selectively blocks a subpopulation of opiate receptors (the mu1 sites) to which all classes of opiates and enkephalins bind with highest affinity, whereas the drug has little to no effect on their lower-affinity sites (mu, and delta). The return of high-affinity receptor (mu1) binding to normal levels corresponds closely to the return of analgesic sensitivity and possibly represents receptor turnover in the central nervous system. These studies suggest that both opiate and opioid peptide analgesia is mediated through a single receptor subpopulation distinct from those involved with respiratory depression, and raise the possibility of specific opiate analgesics without respiratory depression.     

Adrenal medullary tissue transplants in the rat spinal cord reduce pain sensitivity

Adrenal chromaffin cells contain and release several neuroactive substances which induce analgesia when injected directly into the spinal cord (e.g. opioid peptides and catecholamines). Furthermore, the release of these substances can be induced by nicotine. In order to determine whether adrenal medullary tissue transplanted to the spinal cord can produce alterations in pain sensitivity, pieces of dissected rat adrenal medulla were placed in the subarachnoid space of rat spinal cords. Stimulation by a low dose of nicotine induced potent analgesia in animals with adrenal medullary transplants, but not in animals with control transplants. Furthermore, this analgesia was reversed to pre-nicotine levels by the opiate antagonist naloxone. Thus adrenal medullary transplants in the spinal cord may provide a permanent and locally available source of opioid peptides for the relief of intractable pain.     

Effect of an opiate antagonist on movement disorders.

Animal experiments suggest that opiate peptides might play a role in extrapyramidal function. This hypothesis was tested by administering the opiate antagonist, naltrexone, in doses sufficient to antagonize exogenous opiates, to patients with parkinsonism and Huntington's disease. No improvement in the clinical features of either disorder was noted.     

Comparison of "selective" opiate receptor antagonists on the isolated mouse vas deferens

The selectivity and relative potencies of opiate receptor antagonists were compared on the mouse vas deferens preparation. ICI-174864 was found to be a highly selective antagonist at delta opiate receptors equal in potency to naltrexone in blocking the actions of delta agonists. Although less potent than naltrexone, beta-funaltrexamine (beta-FNA) and Mr-1452, like naltrexone, were less selective in that they blocked the actions of mu, delta and kappa agonists. The relative potencies of beta-FNA and Mr-1452 in antagonizing the three types of agonists also were similar to naltrexone.     

Morphine can produce analgesia via spinal kappa opioid receptors in the absence of mu opioid receptors

Previous studies have demonstrated the virtual lack of analgesia in mu opioid receptor knockout mice after systemic administration of morphine. Thus, it has been suggested that analgesic actions of morphine are produced via the mu opioid receptor, despite its ability to bind to kappa and delta receptors in vitro. However, it is not clear whether the results of these studies reflect the effect of morphine in the spinal cord. In the present study, we report study of the analgesic actions of spinally-administered morphine and other opioid receptor agonists in mu opioid receptor knockout and wild type mice. Morphine produced a dose-dependent antinociceptive effect in the tail flick test in the knockout mice, although higher doses were needed to produce antinociception than in wild type mice. The antinociceptive effect of morphine was completely blocked by naloxone (a non-selective opioid antagonist) and nor-binaltorphimine (nor-BNI, a selective kappa-opioid receptor antagonist), but not by naltrindole (a selective delta-opioid receptor antagonist). U-50,488H (a selective kappa-opioid receptor agonist) also produced a dose-dependent antinociceptive effect in knockout mice but presented lower analgesic potency in knockout mice than in wild type mice. Analgesic effects of [d-Pen2,d-Pen5]enkephalin (DPDPE, a selective delta-opioid receptor agonist) were observed in wild type mice but abolished in knockout mice. SNC80 (a selective delta-opioid receptor agonist) was not antinociceptive even in wild type mice. The present study demonstrated that morphine can produce thermal antinociception via the kappa opioid receptor in the spinal cord in the absence of the mu opioid receptor. Lower potency of U50,488H in mu opioid receptor knockout mice suggests interaction between kappa and mu opioid receptors at the spinal level.     

Beneficial effect of the opioid receptor antagonist naltrexone on hyper-sensitivity induced by spinal cord ischemia in rats: disassociation with MK-801

In this study we examined the effect of the long-acting opioid antagonist naltrexone on the allodynia-like effect of spinal ischemia in rats. The spinal cord ischemia was induced at midthoracic level by a recently developed photochemical technique using laser irradiation and photoactivatable intravascular dyes. An allodynia-like sensory disturbance, where the animals reacted by vocalization to non-noxious mechanical stimuli in the flank area, was consistently seen during several days after ischemia. Pretreatment with 10 and 20 mg/kg, but not 5 mg/kg naltrexone i.v. 10 min before irradiation decreased the incidence of allodynia. However, even the effect of the highest dose of naltrexone (20 mg/kg) was incomplete, which is in contrast to the effect of the NMDA receptor antagonist MK-801, which has been tested in the same model and found to completely prevent the incidence of allodynia at a dose of 0.5 mg/kg. Pretreatment with sub- or suprathreshold doses of naltrexone (5 and 20 mg/kg respectively) combined with a subthreshold dose of MK-801 (0.1 mg/kg) did not produce a synergistic effect. When naltrexone (20 mg/kg) was administered 10 min after induction of ischemia, it was totally ineffective in decreasing the occurrence and severity of allodynia. In contrast, MK-801 (0.5 mg/kg) still had a good protective effect when injected as this time. Histological examination showed slight morphological damage in the spinal cord in 38% of control rats after 1 min laser irradiation without pretreatment with naltrexone. No morphological abnormalities were observed in rats after pretreatment with naltrexone (20 mg/kg). The results suggest that opioid receptor antagonists and NMDA receptor antagonists prevent a consequence of transient spinal cord ischemia through different mechanisms. High doses of opioid antagonists may have anti-ischemic effects by improving local spinal cord microcirculation and therefore may have a role in preventing ischemia after traumatic spinal cord injury. On the other hand, the NMDA receptor may have a role in the secondary neuronal death resulting from ischemia. Thus, NMDA receptor antagonists may contribute to the prevention of tissue damage by antagonizing the excitotoxic action of glutamate and/or aspartate released by ischemia into the spinal cord. Finally, since only high doses of naltrexone had an effect in the present study, we cannot rule out the possiblity that this drug may act through non-opioid mechanisms.