Release of immunoreactive Met-enkephalin from the spinal cord by intraventricular beta-endorphin but not morphine in anesthetized rats

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

The differential effect of morphine and beta-endorphin administered intracerebroventricularly on pERK and pCaMK-II expression induced by various nociceptive stimuli in mice brains

The present study was performed to characterize the differential molecular mechanisms of morphine and beta-endorphin which are injected intracerebroventiricularly in mice. In the immunoblot assay, the increases of phosphorylated extracellular signal-regulated protein kinase (pERK) as well as phosphorylated calcium/calmodulin-dependent protein kinase IIalpha (pCaMK-IIalpha) expression induced by noxious stimuli were attenuated by intracerebroventricular (i.c.v.) beta-endorphin pretreatment in the hypothalamus, but not by i.c.v. morphine pretreatment. In addition to these immunoblot results, immunohistochemical study also showed that the attenuation of pERK or pCaMK-IIalpha immunoreactivity elicited by i.c.v. pretreatment of beta-endorphin mainly occurred in the paraventricular nucleus of the hypothalamus (PVN). We also investigated the effect of morphine and beta-endorphin on pERK and pCaMK-IIalpha expression in the locus coeruleus (LC). I.c.v. injection of morphine significantly increased pERK as well as pCaMK-IIalpha expression in the locus coeruleus, while beta-endorphin increased only pCaMK-IIalpha in the LC. In addition, beta-endorphin significantly attenuated pERK expression induced by SP i.t. injection. These results suggest that the antinociceptive effects of supraspinally administered morphine and beta-endorphin are involved with differentially intracellular signal transduction molecules-pERK, pCaMK-IIalpha in the PVN and the LC.

Effects of veratridine and high potassium on micro-opioid receptor internalization in the rat spinal cord: stimulation of opioid release versus inhibition of internalization

Veratridine and high K+-induced micro-opioid receptor (MOR) internalization in rat spinal cord slices by evoking opioid release. Veratridine induced up to 75% MOR internalization but showed an atypical concentration-response: its effect increased steeply from 5 microM to 10 microM, and declined thereafter to disappear at 100 microM. At 100 microM, veratridine also inhibited of MOR internalization induced by exogenous endomorphin-2. This inhibition was caused by Na+ entry, since the Na+ ionophore monensin (50 microM) also inhibited endomorphin-induced MOR internalization. In contrast, veratridine induced neurokinin 1 receptor internalization (by evoking substance P release) without any inhibition at high concentrations. KCl evoked up to 80% MOR internalization, which disappeared in the presence of lidocaine or in the absence of peptidase inhibitors, indicating that it involved neuronal firing and peptide release. Unlike veratridine, KCl did not inhibit MOR internalization at high concentrations. However, both KCl and veratridine evoked more MOR internalization when applied for 2 min than for 20 min because of a direct inhibition of MOR internalization with the longer incubation times. These results show that short incubations with 20 microM veratridine or 30 mM KCl are optimal stimuli to evoke opioid release and MOR internalization in the spinal cord.

Release of [Met5]enkephalin from the spinal cord by intraventricularly administered endomorphin-2, but not endomorphin-1 in the anesthetized rat

Effects of intraventricular injection of endomorphin-1, endomorphin-2 and beta-endorphin on the release of immunoreactive [Met(5)]enkephalin from the spinal cord were studied in pentobarbital anesthetized rats. Intraventricular injection of endomorphin-2, but not endomorphin-1, caused an increased release of immunoreactive [Met(5)]enkephalin in the spinal perfusates. Beta-endorphin given intraventricularly also increased the release of immunoreactive [Met(5)]enkephalin in an amount 15-fold higher than that produced by endomorphin-2. The increase of the release of immunoreactive [Met(5)]enkephalin induced by endomorphin-2 was blocked by mu-opioid receptor antagonist CTOP. Our result suggests that endomorphin-2 stimulates another subtype of mu-opioid receptor different from that acted by endomorphin-1 at the supraspinal site and subsequently increases the release of [Met(5)]enkephalin from the spinal cord.

Evidence for epsilon-opioid receptor-mediated beta-endorphin-induced analgesia

Among the opioid receptors, which have been pharmacologically classified as mu, delta, kappa and epsilon, the existence of the epsilon receptor has been controversial, and this receptor is generally not recognized as a member of the opioid peptide receptor family because it has not been precisely characterized. However, results from pharmacological, physiological and opioid receptor binding studies clearly indicate the presence of epsilon-opioid receptors, which are distinct from mu-, delta- or kappa-opioid receptors. This putative epsilon-opioid receptor is stimulated supraspinally by the endogenous opioid peptide beta-endorphin, which induces the release of Met-enkephalin, which, in turn, acts on spinal delta2-opioid receptors to produce antinociception. In this article, this beta-endorphin-sensitive epsilon-opioid receptor-mediated descending pain control system, which is distinct from that activated by the mu-opioid receptor agonist morphine, is described and the physiological role of the beta-endorphin-mediated system in pain control activated by cold-water swimming and intraplantar injection of formalin is discussed.

Supraspinal NMDA and non-NMDA receptors are differentially involved in the production of antinociception by morphine and beta-endorphin administered intracerebroventricularly in the formalin pain model

Our previous studies have demonstrated that supraspinal glutamate receptors are differentially involved in the antinociception induced by morphine and beta-endorphin given intracerebroventricularly (i.c.v.) in the tail-flick and hot-plate tests. The formalin pain test was used in the present study. Injection of mice with formalin solution (2%, 10 microl) into the hindpaw intraplantarly produced the first (0-5 min) and second (20-40 min) phases of formalin responses. The formalin responses in the both phases were attenuated dose-dependently by morphine (0.125-1 microg) or beta-endorphin (0.125-1 microg) administered i.c.v. 5 min before. The antinociceptive effect of morphine was slightly more potent in the second phase whereas the effect of beta-endorphin was more pronounced in the first phase. MK-801 (0.1-1 microg), a non-competitive NMDA receptor antagonist, and CNQX (0.05-0.5 microg), a non-NMDA antagonist, given i.c.v., produced antinociceptive effect in the both phases, but only in a partial manner. Both MK-801 (0.05 microg) and CNQX (0.01 microg), at the dose which had no intrinsic effect, reversed the antinociceptive effect of beta-endorphin (1 microg) observed during the second, but not the first, phase partially but significantly. However, the antinociceptive effect of morphine (1 microg) was not affected by the same dose of MK-801 or CNQX given i.c.v. Our results indicate that, at the supraspinal level, both NMDA and non-NMDA receptors are involved in the production of antinociception induced by supraspinally administered beta-endorphin, but not morphine, in the formalin pain model.

Analysis of sex and gonadectomy differences in beta-endorphin antinociception elicited from the ventrolateral periaqueductal gray in rats

Male rats exhibit significantly greater antinociception following central administration of morphine than female rats. The present study examined potential differences in beta-endorphin (5.2-26 microg) antinociception elicited from the ventrolateral periaqueductal gray in adult sham-operated and gonadectomized male and female rats. Male rats displayed significantly greater peak (30 min) tail-flick latencies across the entire range of beta-endorphin doses administered into the ventrolateral periaqueductal gray than female rats tested during the estrous phase of the estrous cycle. Adult gonadectomy failed to appreciably change the pattern of this effect in either males of females. Thus, antinociception elicited from the ventrolateral periaqueductal gray by beta-endorphin, like morphine, is sensitive to sex differences.

Differential modulation by baclofen on antinociception induced by morphine and beta-endorphin administered intracerebroventricularly in the formalin test

Our previous studies have demonstrated that supraspinal GABAergic receptors are differentially involved in the antinociception induced by morphine and beta-endorphin given intracerebroventricularly (i.c.v.) in the tail-flick and hot-plate tests. These two models employed a phasic, thermal nociceptive stimulus. The present study was designed to examine the possible involvement of supraspinal GABAergic receptors in opioid-induced antinociception in the formalin test. Morphine (1 microg) and beta-endorphin (1 microg) given i.c.v. displayed the almost complete inhibitory effects against the hyperalgesic response in both phases. Muscimol (75-100 ng) and baclofen (5-10 ng) injected i.c.v. produced the hypoalgesic response in the both phases. The hypoalgesic response induced by muscimol and baclofen observed during the second phase was more pronounced than that observed during the second phase. Baclofen (2.5 ng), at the dose which did not affect the hyperalgesic response, resulted in a significant reversal of the i.c.v. administered beta-endorphin-induced hypoalgesic response observed during the second, but not the first, phase. However, the hypoalgesic response induced by i.c.v. administered morphine was not changed by the same dose of muscimol or baclofen injected i.c.v. Our results indicate that, at the supraspinal level, GABA(B)receptors appear to be involved in the modulation of antinociception induced by supraspinally administered beta-endorphin, but not morphine, in the formalin test model.

Actions of NMDA and cholinergic receptor antagonists in the rostral ventromedial medulla upon beta-endorphin analgesia elicited from the ventrolateral periaqueductal gray

Analgesia elicited by morphine in the ventrolateral periaqueductal gray is mediated in part by NMDA and cholinergic receptors in the rostral ventromedial medulla because selective receptor antagonists applied to the latter structure reduced morphine analgesia elicited from the former structure. Previous studies have demonstrated that morphine and beta-endorphin employ different anatomical and neurochemical pathways in exerting their supraspinal analgesic effects. The present study evaluated whether pretreatment with either competitive (AP7, 3-10 microg) or non-competitive (MK-801, 3-10 microg) NMDA antagonists, or muscarinic (scopolamine, 5 microg) or nicotinic (mecamylamine, 1 microg) cholinergic antagonists administered into the rostral ventromedial medulla altered beta-endorphin (15 microg) analgesia elicited from the ventrolateral periaqueductal gray as measured by the tail-flick and jump tests in rats. Whereas AP7 produced minimal (11%) and transient (30 min) reductions in beta-endorphin analgesia on the jump test, MK-801 produced minimal (9%) and transient (30 min) reductions in beta-endorphin analgesia on the tail-flick test. Whereas mecamylamine failed to reduce beta-endorphin analgesia on either measure, scopolamine produced small (23%) and transient (30 min) reductions in beta-endorphin analgesia on the tail-flick test. Each of these antagonists administered into the rostral ventromedial medulla at comparable or lower doses virtually eliminated morphine analgesia elicited from the ventrolateral periaqueductal gray. The opioid mediation of beta-endorphin analgesia in the ventrolateral periaqueductal gray was confirmed by its sensitivity to naltrexone (1-20 microg) pretreatment into the same structure. These data provide further evidence for dissociations between the descending neuroanatomical and neurochemical circuitry mediating the supraspinal analgesic responses induced by morphine and beta-endorphin, and indicate that the latter response is mediated by either non-cholinergic and non-NMDA synapses within the rostral ventromedial medulla, and/or by brainstem sites outside of the rostral ventromedial medulla.

Isolation and partial characterization of an opioid-like 88 kDa hibernation-related protein

Previous studies show that infusion of hibernating woodchuck albumin (HWA) induces hibernation in summer-active ground squirrels and results in profound behavioral and physiological depression in primates. These effects are reversed by the administration of opiate antagonists, suggesting that the putative hibernation induction trigger (HIT) may act through opioid receptors. We have demonstrated that both HIT-containing plasma and the synthetic alpha opioid D-Ala2-D-Leu5-enkephalin (DADLE), which mimics the activity of HIT in hibernators, extend tissue survival time of a multi-organ autoperfusion system by 3-fold. In this study we present the first data showing biological activity with a much more highly purified plasma fraction from hibernating woodchucks, identified as the hibernation-related factor (HRF). Both the HRF and DADLE show opiate-like contractile inhibition in the mouse vas deferens (Mvd) bioassay. We also have preliminary evidence in an isolated rabbit heart preparation indicating that the HRF and DADLE act similarly to restore left ventricular function following global myocardial ischemia. Furthermore, we have partially sequenced an alpha 1-glycoprotein-like 88 kDa hibernation-related protein (p88 HRP) present in this fraction, which may prove to be the blood-borne HIT molecule.

Evidence for the existence of the beta-endorphin-sensitive "epsilon-opioid receptor" in the brain: the mechanisms of epsilon-mediated antinociception

Recently, mu-, delta- and kappa-opioid receptors have been cloned and relatively well-characterized. In addition to three major opioid receptor types, more extensive studies have suggested the possible existence of other opioid receptor types that can be classified as non-mu, non-delta and non-kappa. Based upon anatomical and binding studies in the brain, the sensitive site for an endogenous opioid peptide, beta-endorphin, has been postulated to account for the unique characteristics of the opioid receptor defined as a putative epsilon-opioid receptor. Many epsilon-opioid receptors are functionally coupled to G-proteins. The functional epsilon-opioid receptors in the brain are stimulated by bremazocine and etorphine as well as beta-endorphin, but not by selective mu-, delta- or kappa-opioid receptor agonists. Epsilon-opioid receptor agonists injected into the brain produce profound antinociception. The brain sites most sensitive to epsilon-agonist-induced antinociception are located in the caudal medial medulla such as the nucleus raphe obscures, nucleus raphe pallidus and the adjacent midline reticular formation. The stimulation of epsilon-opioid receptors in the brain facilitates the descending enkephalinergic pathway, which probably originates from the brainstem terminating at the spinal cord. The endogenous opioid Met-enkephalin, released in the spinal cord by activation of supraspinal epsilon-opioid receptors, stimulates spinal delta2-opioid receptors for the production of antinociception. It is noteworthy that the epsilon-opioid receptor-mediated pain control system is different from that of other opioid systems. Although there appears to be no epsilon-selective ligand currently available, these findings provide strong evidence for the existence of the putative epsilon-opioid receptor and its unique function in the brain.

Opioid supraspinal analgesic synergy between the amygdala and periaqueductal gray in rats

Analgesia can be elicited following microinjections of morphine, mu-selective agonists and beta-endorphin into the amygdala. These analgesic responses are mediated by opioid synapses in the periaqueductal gray (PAG) since general (naltrexone), mu (beta-funaltrexamine) and delta2 (naltrindole isothiocyanate) opioid antagonists administered into the PAG significantly reduce both morphine and beta-endorphin analgesia elicited from the amygdala. Supraspinal multiplicative opiate analgesic interactions have been observed between the PAG and rostroventromedial medulla (RVM), the PAG and locus coeruleus (LC), and the RVM and LC. The present study further examined the relationship between the amygdala and PAG in analgesic responsiveness by determining whether multiplicative analgesic interactions occur following paired administration of subthreshold doses of morphine into both structures, beta-endorphin into both structures, morphine into one structure and beta-endorphin into the other structure, or morphine and beta-endorphin into one structure. Co-administration of subthreshold doses of morphine into both the amygdala and PAG results in a profound synergistic interaction on the jump test, but not the tail-flick test. Co-administration of subthreshold doses of beta-endorphin into both structures also results in a profound test-specific synergistic interaction. In both cases, the magnitude of the interaction was similar regardless of the site receiving the fixed dose of the opioid, and the site receiving the variable dose of the opioid. Co-administration of beta-endorphin (1 microg) into the amygdala and morphine (1 microg) into the PAG produced a potent interaction, but co-administration of morphine (1 microg) into the amygdala and beta-endorphin (1 microg) into the PAG failed to produce interactive effects. Finally, co-administration of morphine (1 microg) and beta-endorphin (1 microg) into either the amygdala alone or the PAG alone failed to produce an interaction, indicating the importance of regional opioid activation. These data are discussed in terms of the test-specificity of nociceptive processing in the amygdala, in terms of the multiple modulatory mechanisms mediating beta-endorphin analgesia in the PAG, and in terms of whether the interactions are either mediated by anatomical connections between the amygdala and PAG or by mechanisms initiated by these two sites converging at another site or sites.

[Met]enkephalin in the spinal cord is involved in the antinociception induced by intracerebroventricularly-administered etorphine in the mouse

We have recently reported that the antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of both mu- and epsilon-opioid receptors and the activation of both monoaminergic and opioidergic descending pain control systems. [Xu J. Y. et al. (1992) J. Pharmac. exp. Ther. 263, 246-252]. Since the opioid epsilon-receptor-mediated antinociception induced by beta-endorphin is mediated by the release of [Met]enkephalin and subsequent stimulation of delta-opioid receptors in the spinal cord, the present studies were designed to determine if beta-endorphin-like action is also involved in etorphine-induced antinociception. The tail-flick test was used to assess the antinociceptive response performed in male ICR mice. Etorphine at doses from 5 to 20 ng given i.c.v. produced a dose-dependent inhibition of the tail-flick response. The inhibition of the tail-flick response induced by etorphine given i.c.v. was antagonized by intrathecal pretreatment for 60 min with antiserum against [Met]enkephalin (10 microg), but not with antiserum against [Leu]enkephalin (10 microg) or dynorphin A (1-13) (10 microg). Desensitization of delta-opioid receptors in the spinal cord by intrathecal pretreatment with [Met]enkephalin (5 microg) for 60 min attenuated i.c.v. administered etorphine-induced tail-flick inhibition. However, intrathecal pretreatment with [Leu]enkephalin (5 microg) or dynorphin A (1-17) (0.1 microg) for 60 min did not attenuate i.c.v. administered etorphine-induced tail-flick inhibition. The results indicate that antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of the epsilon-opioid receptor at the supraspinal sites and by the release of [Met]enkephalin, which subsequently stimulates delta-opioid receptors in the spinal cord.

Effects of intrathecal injection of nimodipine, omega-conotoxin GVIA, calmidazolium, and KN-62 on the antinociception induced by cold water swimming stress in the mouse

The present study was designed to determine if spinal calcium channels, calmodulin, and calcium/calmodulin-dependent protein kinase II were involved in the production of antinociception induced by cold water swimming stress (CWSS). The effects of intrathecal (i.t.) injection of nimodipine, omega-conotoxin GVIA, calmidazolium, or (S)-5-isoquinolinesulfonic acid, 4-[2-[(5-isoquinolinyl-sulfonyl)methylamino]-3-oxo-3-(4-phenyl-1-piperaz inyl)-propyl]phenyl ester (KN-62) on CWSS-induced antinociception were studied in ICR mice. The antinociception was assessed by the tail-flick test. CWSS produced inhibition of the tail-flick response. Various doses of nimodipine (10-40 ng), omega-conotoxin GVIA (5-40 ng), calmidazolium (10-40 ng), or KN-62 (5-40 ng) injected i.t. alone did not show any antinociceptive effect in the tail-flick test. I.t. pretreatment with omega-conotoxin GVIA, calmidazolium, or KN-62 dose dependently attenuated the CWSS-induced inhibition of the tail-flick response. However, i.t. pretreatment with nimodipine did not affect the inhibition of the tail-flick response induced by CWSS. Our results suggest that spinal N-type calcium channel, calmodulin and calcium/calmodulin-dependent protein kinase II may be involved in the production of antinociception induced by CWSS. On the other hand, CWSS-induced antinociception appears not to be mediated via the spinal L-type calcium channel.

Differential effects of adenosine receptor antagonists injected intrathecally on antinociception induced by morphine and beta-endorphin administered intracerebroventricularly in the mouse

A previous study reported that beta-endorphin and morphine administered supraspinally produce antinociception by activating different descending pain inhibitory systems. The present study was designed to investigate the blocking effects of A1 or A2 adenosine receptors in the spinal cord on antinociception induced by supraspinally administered mu- and epsilon-opioid receptor agonists. The effects of 1,3-dipropyl-8-(2-amino-4-chloro-phenyl)-xanthine (PACPX; an A1 adenosine receptor antagonist) or 3,7-dimethyl-1-propargylxanthine (DMPX; an A2 adenosine receptor antagonist) on the antinociception induced by morphine (a mu-opioid receptor agonist) or beta-endorphin (an epsilon-opioid receptor agonist) administered intracerebroventricularly (i.c.v.) were studied. The antinociception was assayed by the tail-flick test. DMPX at doses of 1-40 micrograms (which administered intrathecally alone did not affect the latencies of tail-flick thresholds), attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered morphine (0.5 microgram) or beta-endorphin (1 microgram). PACPX at doses of 1-40 micrograms (which administered intrathecally alone did not affect the latencies of tail-flick thresholds), attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin but not morphine. These results suggest that A2 but not A1 adenosine receptors in the spinal cord may be involved in the antinociception induced by supraspinally administered morphine, while the antinociception induced by supraspinally administered beta-endorphin appears to be mediated by spinal A1 and A2 adenosine receptors. These results support the hypothesis that morphine and beta-endorphin administered supraspinally produce antinociception by different neuronal mechanisms.

Inhibition of spinal nitric oxide synthase by N(omega)-nitro-L-arginine blocks the release of Met-enkephalin and antinociception induced by supraspinally administered beta-endorphin in the rat

The antinociception induced by beta-endorphin given supraspinally has been demonstrated previously to be mediated by the release of Met-enkephalin acting on delta2-opioid receptors in the spinal cord. The present study was designed to determine the role of nitric oxide in the spinal cord on beta-endorphin-induced release of Met-enkephalin and antinociception. The experiments were performed in pentobarbital-anesthetized rats. The release of Met-enkephalin was performed using a spinal cord perfusion technique and the Met-enkephalin released in the spinal perfusates was measured by radioimmunoassay. Antinociception was assessed by the tail-flick test. beta-Endorphin (2 microg) given intraventricularly induced the release of Met-enkephalin from the spinal cord. The release of Met-enkephalin was dose-dependently attenuated by N(omega)-nitro-L-arginine (0.1 nM-1 microM) added into spinal perfusates and the attenuation was reversed by intrathecally applied L-arginine. The stereoisomer N(omega)-nitro-D-arginine given intrathecally, however, did not inhibit the release of Met-enkephalin induced by intraventricularly administered beta-endorphin. beta-Endorphin (4 microg) given intraventricularly produced antinociception in rats pretreated intrathecally with saline. The antinociception induced by beta-endorphin was blocked by intrathecally administered N(omega)-nitro-L-arginine (5 microg) and the blockade of antinociception was reversed by intrathecal injection of L-arginine (50 microg). N(omega)-Nitro-D-arginine (5 microg) given intrathecally did not block the intraventricularly administered beta-endorphin-induced antinociception. N(omega)-Nitro-L-arginine (10 microg) given intraventricularly did not affect intraventricularly administered beta-endorphin-induced Met-enkephalin release nor did it affect intraventricular beta-endorphin-induced antinociception, indicating that the effect of N(omega)-nitro-L-arginine is not at supraspinal sites. Intrathecal pretreatment with N(omega)-nitro-L-arginine did not affect intrathecally administered [D-Ala2]deltorphin II-induced antinociception. Our results indicate that N(omega)-nitro-L-arginine given intrathecally attenuates intraventricular beta-endorphin-administered inhibition of the tail-flick response by presynaptically inhibiting the release of Met-enkephalin.

The effect of pretreatment with a delta 2-opioid receptor antisense oligodeoxynucleotide on the recovery from acute antinociceptive tolerance to delta 2-opioid receptor agonist in the mouse spinal cord

1. An intrathecal (i.t.) injection of a selective delta 2-opioid receptor agonist, [D-Ala2]deltorphin II, produced an acute antinociceptive tolerance to the antinociceptive effect of a subsequent i.t. challenge of [D-Ala2]deltorphin II. This acute tolerance lasted 3 to 9 h and completely subsided by 12 h. The experiments were designed to examine the effect of pretreatment with an antisense oligodeoxynucleotide to delta 2-opioid receptor mRNA (delta-AS oligo) on the recovery from tolerance to [D-Ala2]deltorphin II-induced antinociception in male ICR mice. 2. Pretreatment with delta-AS oligo (1.63 to 163 pmol, i.t.), but not mismatched oligo (MM oligo) (163 pmol), prevented the recovery from acute tolerance to [D-Ala2]deltorphin II-induced antinociception in a dose-dependent manner. However, treatment with delta-AS oligo (163 pmol) did not prevent the recovery from tolerance to either the mu-opioid receptor agonist [D-Ala2,NMePhe4,Gly(ol)5]enkephalin (DAMGO) or the kappa-opioid receptor agonist U50,488H, indicating subtype specificity in the mechanism by which delta-AS oligo inhibits recovery from delta 2-opioid tolerance. 3. Treatment with [D-Ala2]deltorphin II (i.t.) significantly reduced the binding of [tyrosyl-3,5-(3)H(N)]-Tyr-D-Ser-Gly-Phe-Leu-Thr ([3H]-DSLET), a delta 2-opioid receptor agonist ligand, in the spinal cord 3 h after treatment, but binding returned to control levels by 24 h after treatment. However, [3H]-DSLET binding in the spinal cord remained significantly reduced at 24 h if delta-AS oligo (163 pmol) was coadministered with [D-Ala2]deltorphin II (6.4 nmol). 4. Based on these findings, it is concluded that a single stimulation of spinal cord delta 2-opioid receptors by intrathecally-administered [D-Ala2]deltorphin II induces a long-lasting desensitization of delta 2-opioid receptors to [D-Ala2]deltorphin II. Recovery from delta 2-opioid receptor-mediated antinociceptive tolerance apparently depends on replenishment by newly synthesized delta 2-opioid receptor protein rather than immediate reversal of delta 2-opioid receptors.

Pretreatment with protein kinase C activator phorbol 12,13-dibutyrate attenuates the antinociception induced by mu- but not epsilon-opioid receptor agonist in the mouse

The effects of pretreatment with a protein kinase C activator, phorbol 12,13-dibutyrate, on antinociception induced by i.c.v.-administered mu-opioid receptor agonist (D-Ala2, NMePhe4, Gly(ol)5) enkephalin (DAMGO) or morphine and epsilon-opioid receptor agonist beta-endorphin were studied in male ICR mice. The tail-flick responses were used for antinociceptive tests. I.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 30 or 60 but not 10 min attenuated antinociception induced by i.c.v.-administered DAMGO. I.c.v. pretreatment with phorbol 12,13-dibutyrate (10 and 50 pmol) for 60 min caused a dose-dependent attenuation of DAMGO (19.5 pmol)- or morphine (6.0 nmol)-induced antinociception. The dose-response curve for DAMGO-induced antinociception was shifted to the right by 7.3-fold by i.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 60 min. However, the i.c.v.-administered beta-endorphin-induced antinociception was not affected by the same pretreatment with phorbol 12,13-dibutyrate. The attenuation of i.c.v.-administered DAMGO- and morphine-induced antinociception by phorbol 12,13-dibutyrate was reversed by concomitant i.c.v. pretreatment with a selective protein kinase C inhibitor calphostin C. These results suggest that activation of protein kinase C by phorbol 12,13-dibutyrate leads to the desensitization of mu-, but not epsilon-opioid receptor-mediated antinociception. These findings also provide additional evidence for differential intracellular modulation on antinociceptive action of mu- and epsilon-opioid receptor agonists.

Opioid antagonists in the periaqueductal gray inhibit morphine and beta-endorphin analgesia elicited from the amygdala of rats

In addition to brainstem sites of action, analgesia can be elicited following amygdala microinjections of morphine and mu-selective opioid agonists. The present study examined whether opioid analgesia elicited by either morphine or beta-endorphin in the amygdala could be altered by either the general opioid antagonist, naltrexone, the mu-selective antagonist, beta-funaltrexamine (BFNA) or the delta 2 antagonist, naltrindole isothiocyanate (Ntii) in the periaqueductal gray (PAG). Both morphine (2.5-5 micrograms) and beta-endorphin (2.5-5 micrograms) microinjected into either the baso-lateral or central nuclei of the amygdala significantly increased tail-flick latencies and jump thresholds in rats. The increases were far more pronounced on the jump test than on the tail-flick test. Placements dorsal and medial to the amygdala were ineffective. Naltrexone (1-5 micrograms) in the PAG significantly reduced both morphine (tail-flick: 70-75%; jump: 60-81%) and beta-endorphin (tail-flick: 100%; jump: 93%) analgesia elicited from the amygdala, indicating that an opioid synapse in the PAG was integral for the full expression of analgesia elicited from the amygdala by both agonists. Both BFNA (68%) and Ntii (100%) in the PAG significantly reduced morphine, but not beta-endorphin analgesia in the amygdala on the tail-flick test. Ntii in the PAG was more effective in reducing morphine (60%) and beta-endorphin (79%) analgesia in the amygdala on the jump test than BFNA (15-24%). Opioid agonist-induced analgesia in the amygdala was unaffected by opioid antagonists administered into control misplacements in the lateral mesencephalon, and the small hyperalgesia elicited by opioid antagonists in the PAG could not account for the reductions in opioid agonist effects in the amygdala. These data indicate that PAG delta 2, and to a lesser degree, mu opioid receptors are necessary for the full expression of morphine and beta-endorphin analgesia elicited from the amygdala.

Nicotine enhances morphine- and beta-endorphin-induced antinociception at the supraspinal level in the mouse

The effect of nicotine administered supraspinally on antinociception induced by supraspinally administered opioids was examined in ICR mice. The intracerebroventricular (i.c.v.) injection of nicotine alone at doses from 1 to 12 micrograms produced only a minimal inhibition of the tail-flick response. Morphine (0.2 micrograms), beta-endorphin (0.1 microgram), D-Pen2.5-enkephalin (DPDPE; 0.5 microgram), trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide (U50, 488H; 6 micrograms) caused only slight inhibition of the tail-flick response. Nicotine dose dependently enhanced inhibition of the tail-flick response induced by i.c.v. administered morphine (0.2 microgram) or beta-endorphin (0.1 microgram). The degree of enhancing effect of nicotine toward beta-endorphin-induced inhibition of the tail-flick response was greater than toward morphine-induced inhibition of the tail-flick response. However, i.c.v. administered nicotine at the same doses was not effective in enhancing the inhibition of the tail-flick response induced by DPDPE (0.5 microgram) or U50, 488H (6 micrograms) administered i.c.v. Our results suggest that stimulation of supraspinal nicotinic receptors may enhance antinociception induced by morphine (a mu-opioid receptor agonist) and beta-endorphin (an epsilon-opioid receptor agonist) administered supraspinally. However, the activation of nicotinic receptors at supraspinal sites may not be involved in enhancing the antinociception induced by DPDPE (a delta-opioid receptor agonist) or U50, 488H (a kappa-opioid receptor agonist) administered supraspinally.

An immunocytochemical mapping of beta-endorphin (1-27) in the cat diencephalon

The distribution of beta-endorphin (1-27) immunoreactive cell bodies and fibres was studied in the diencephalon of the cat using an indirect immunoperoxidase technique. In the thalamus, almost all the immunoreactive fibres were found in the midline region and in nuclei located near the midline, whereas in the hypothalamus fibres containing beta-endorphin (1-27) were visualized extending by the whole structure. The hypothalamus showed a higher density of beta-endorphin (1-27) immunoreactive fibres than the thalamus, as well as immunoreactive cell bodies, since in the thalamus no beta-endorphin (1-27) immunoreactive neuron was located. The densest network of immunoreactive fibres was observed in the epithalamus (nucleus periventricularis anterior) and in the hypothalamic nuclei arcuatus, hypothalami ventromedialis, suprachiasmaticus, periventricularis hypothalami, hypothalamus dorsomedialis, area hypothalamica dorsalis, hypothalamus anterior, filiformis, hypothalamus posterior and regio praeoptica. In the hypothalamus, a high density of perikarya containing beta-endorphin (1-27) was observed in the nucleus arcuatus and a low density in the nucleus hypothalami ventromedialis. The distribution of beta-endorphin (1-27) immunoreactive fibres and perikarya is compared with the location of other neuropeptides in the cat diencephalon. Our findings reveal that b-endorphin (1-27) immunoreactive structures are widely distributed in the cat diencephalon, suggesting that the peptide might be involved in several physiological functions.

Activation of a NO-cyclic GMP system by NO donors potentiates beta-endorphin-induced antinociception in the mouse

Nitric oxide (NO) donors such as sodium nitroprusside (SNP, 0.01-1 micrograms) or 3-morpholino-sydnonimine (SIN-1, 0.1-10 micrograms) administered intracerebroventricularly (i.c.v) produced a dose-dependent potentiation of beta-endorphin-induced antinociception assessed by the tail-flick test in ICR mice. The same i.c.v. treatment with SNP or SIN-1 did not affect the antinociception induced by mu-, delta-, or kappa-opioid receptor agonists. The goal of the present study was to determine if the potentiation of the beta-endorphin-induced antinociception by NO donors is mediated by the activation of NO-cGMP system. Co-administration of hemoglobin (30-120 micrograms) or methylene blue (1.25-5 micrograms), but not N omega-nitro-L-arginine (1-5 micrograms) given i.c.v. dose-dependently attenuated the potentiating effects of SNP or SIN-1 on beta-endorphin-induced antinociception. However, the same i.c.v. treatments of mice with hemoglobin, methylene blue or N omega-nitro-L-arginine did not directly affect the i.c.v. administered beta-endorphin-induced antinociception. On the other hand, the treatment of mice with a combination of NO donor (SNP, 0.1 micrograms or SIN-1, 1 microgram) and zaprinast (a cGMP phosphodiesterase inhibitor, 1 microgram) further potentiated beta-endorphin-induced antinociception. These results indicate that the potentiating effect of SNP or SIN-1 on beta-endorphin-induced antinociception is mediated by the increased production of NO-cyclic GMP in the brain. However, the NO-cGMP system is not directly involved in the beta-endorphin-induced antinociception.

The inhibition of ornithine decarboxylase activity in developing rat tissues by central nervous system beta-endorphin is mediated by mu-opioid receptors, but not by delta- or epsilon-opioid receptors

Our laboratory has previously shown that intracerebroventricular (i.c.v.) administration of beta-endorphin suppresses brain and liver ornithine decarboxylase activity (ODC; a growth regulatory enzyme) in preweanling rats. This investigation examined, in 6-day-old rats, the relative participation of brain mu-, delta- and epsilon-opioid receptors in beta-endorphin's ODC effects, by comparing tissue ODC responses to beta-endorphin given alone i.c.v. and in the presence of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; mu-opioid receptor antagonist), N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI-174,864; delta-opioid receptor antagonist) or beta-endorphin-(1-27) (epsilon-opioid receptor antagonist). Administration of 0.5 microgram of beta-endorphin alone significantly decreased brain and liver ODC activity 4 h after injection, and the effect was completely blocked by coinjection of CTOP (0.075 micrograms) but not by ICI-174,864 (0.75 or 3.75 micrograms) or beta-endorphin-(1-27) (3.75 or 7.5 micrograms). The blockade of endogenous opioid:opioid receptor interactions by either CTOP (at doses > 0.075 microgram) or ICI-174,864 alone was accompanied by increased levels of basal ODC activity. The results obtained demonstrate that i.c.v. beta-endorphin downregulates ODC expression in central as well as in peripheral tissues by interacting with brain mu-opioid receptors, but not with delta- or epsilon-opioid receptors or mu/delta-opioid receptor complexes. Also, they indicate that endogenous opioid systems have a tonic inhibitory influence on ODC activity which is mediated, at least in part, by mu- and delta-opioid receptors.

Stimulation of spinal delta-opioid receptors in mice selectively enhances the attenuation of delta-opioid receptor-mediated antinociception by antisense oligodeoxynucleotide

Intrathecal (i.t.) pretreatment of male ICR mice with antisense oligodeoxynucleotide to delta-opioid receptor mRNA once a day for 1-3 days caused a time-dependent attenuation of i.t. administered [D-Ala2]deltorphin II-induced antinociception as measured by the tail-flick test. The attenuation of the antinociception induced by i.t. administered [D-Ala2]deltorphin II, a delta-opioid receptor agonist, was enhanced by i.t. pretreatment for 1 day with [D-Ala2]deltorphin II, but not [D-Ala2,N MePhe4,Gly(ol)5]enkephalin (DAMGO), a mu-opioid receptor agonist, or U50,488H, a kappa-opioid receptor agonist, given together with antisense oligodeoxynucleotide to delta-opioid receptor mRNA. The present results indicate that stimulation of spinal delta-opioid receptors by i.t. injection of [D-Ala2]deltorphin II selectively causes a loss of delta-opioid receptor-mediated antinociception in mice pretreated with antisense oligodeoxynucleotide to delta-opioid receptor mRNA.

Spinal delta 2-, but not delta 1-, mu-, or kappa-opioid receptors are involved in the tail-flick inhibition induced by beta-endorphin from nucleus raphe obscurus in the pentobarbital-anesthetized rat

The antinociception induced by beta-endorphin given supraspinally has been previously demonstrated to be mediated by the release of [Met5]enkephalin acting on delta-opioid receptors in the spinal cord. The present study was designed to determine what type of opioid receptors in the spinal cord is involved in beta-endorphin-induced antinociception in the rat. Antinociception was induced by beta-endorphin (0.6 nmol) given into nucleus raphe obscurus and was assessed by the tail-flick test in pentobarbital-anesthesized rats. Naltriben (0.6-6.0 nmol), a selective delta 2-opioid receptor antagonist, given intrathecally dose-dependently attenuated beta-endorphin-induced inhibition of the tail-flick response. On the other hand, 7-benzylidene naltrexone (2.1-64.3 nmol), CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2, 0.09-2.8 nmol), or nor-binaltorphimine (1.4-40.8 nmol), selective delta 1-, mu-, and kappa-opioid receptor antagonists, respectively, did not block beta-endorphin-induced antinociception. The results of present study in rats are consistent with previous experiments in mice indicating that spinal delta 2-, but not delta 1-, mu- or kappa-opioid receptors are involved in beta-endorphin-induced inhibition of the tail-flick response.

BDNF produces analgesia in the formalin test and modifies neuropeptide levels in rat brain and spinal cord areas associated with nociception

Previous studies have demonstrated an antinociceptive effect of brain-derived neurotrophic factor (BDNF) following infusion into the midbrain, near the periaqueductal grey and dorsal raphe nuclei. BDNF administration attenuated the behavioural response in the tail-flick and hot-plate tests, two models employing a phasic, thermal high-intensity nociceptive stimulus; the present studies extend our previous findings to include a model of moderate, continuous pain resulting from a chemical stimulus, the formalin test. Midbrain infusion of BDNF decreased the behavioural paw flinch response to subcutaneous formalin injection in both the early and late phases of the test. As our previous studies showed that BDNF-induced analgesia was reversible by naloxone, we have examined the effects of BDNF administration on brain and spinal cord levels of neuropeptides involved in the modulation of nociceptive information, including the endogenous opioid peptides, met-enkephalin and beta-endorphin, as well as substance P and neuropeptide Y (NPY). At the site of infusion, within the PAG and dorsal raphe, BDNF increased the level of beta-endorphin by 63%, but had no effect on substance P, metenkephalin or NPY levels. In the dorsal spinal cord, substance P (113% increase), beta-endorphin (97% increase) and NPY (64% increase) were elevated, although ventral spinal cord levels of these peptides remained unchanged. These studies demonstrate a modulatory effect of BDNF on relevant neuropeptides within areas of the brain and spinal cord involved in the processing of nociceptive information.

Involvement of nitric oxide in intracerebroventricular beta-endorphin-induced neuronal release of methionine-enkephalin

Previous work has suggested that the antinociceptive effect of nitrous oxide (N2O) in rats is mediated, at least in part, by beta-endorphin (beta-EP) and that centrally administered beta-EP stimulates release of methionine-enkephalin (ME) in the rat spinal cord. Since inhibition of central nitric oxide (NO) production has been found to suppress N2O antinociception, we examined the possible involvement of NO in the release of spinal cord ME by i.c.v. beta-EP. Urethane-anesthetized, male Sprague-Dawley rats were intrathecally (i.t.) perfused with artificial cerebrospinal fluid (aCSF) and fractions of perfusate were assayed for immunoreactive (i.r.) ME. The beta-EP-induced increase in ME concentration in the i.t. perfusate was significantly suppressed by perfusing the animal with aCSF containing 100 microM L-NG-nitro arginine (L-NOARG), an inhibitor of NO synthase (NOS). The further addition of 50 microM L-arginine (L-ARG), but not D-arginine (D-ARG), to the aCSF reversed the suppression of the ME change by L-NOARG. However, the potency of L-ARG decreased with increasing concentrations of L-ARG. On the other hand, increasing the concentration of L-NOARG in the aCSF to 250 microM failed to produce a greater suppression of the beta-EP-induced increase in ME. These findings suggest that NO may mediate the beta-EP-induced release of ME in the spinal cord and that interference with this mechanism might be an explanation for the antagonism of N2O antinociception in rats by NOS inhibitors.

Stress-induced behavioral responses and multiple opioid systems in the brain

Various stressor produce a wide range of behavioral responses such as analgesia, catalepsy and motor suppression, which are sensitive to opioid receptor antagonists. These behavioral responses in stress are accompanied by changes in the contents of opioid peptides, the mRNAs encoding their precursors and opioid receptor binding in the brain. In the present article, experimental data concerning stress-induced analgesia and motor suppression is reviewed and discussed in relation to a possible involvement of different opioid systems in the various observed behavioral responses in stress. Pharmacological studies with subtype-selective antagonists have demonstrated that not only mu- but also delta- and/or kappa-opioid receptors are involved in opioid-mediated stress-induced analgesia. There are two types of stress-induced analgesia referred to as opioid-mediated and non-opioid mediated forms. It has been proposed that the intensity and temporal pattern of stressor may be a critical factor determining the nature of stress-induced analgesia. Accumulated evidence demonstrate that these two forms of pain inhibitory systems interact each other according to a collateral inhibition model. Recent studies show that parallel activation of multiple opioid receptors mediates non-opioid froms of stress-induced analgesia. Dynorphins, by acting at kappa-opioid receptors, may play a pivotal role in the expression of stress-induced motor suppression, whereas enkephalins may act to attenuate this response.

Non-NMDA receptor antagonist attenuates antinociception induced by morphine but not beta-endorphin, D-Pen2-D-Pen5-enkephalin, and U50, 488H administered intracerebroventricularly in mice

The antinociception induced by morphine but not beta-endorphin, D-Pen2-D-Pen5-enkephalin (DPDPE), or U50, 488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide) administered intracerebroventricularly (i.c.v.) has been previously demonstrated to be mediated by the N-Methyl-D-Aspartic Acid (NMDA) receptor. The present study was designed to determine if non-NMDA receptors are involved in opioid-induced antinociception. Antinociception was assessed by the tail-flick test in male ICR mice. Various doses of CNQX (6-Cyano-7-nitroquinoxaline-2,3-dione), a competitive non-NMDA receptor antagonist, (0.001 to 0.5 microgram) injected intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. CNQX (0.01 to 1 micrograms, i.c.v.) dose-dependently attenuated the inhibition of the tail-flick response induced by morphine (1 microgram). However, inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin (1 microgram), DPDPE (10 micrograms), or U50, 488H was not affected by i.c.v. administered CNQX. It is concluded that non-NMDA receptors are involved in i.c.v. morphine-induced antinociception. However, non-NMDA receptors are not involved in i.c.v. administered beta-endorphin-, DPDPE-, and U50, 488H-induced antinociception at the supraspinal level.

Effects of intraventricular injection of morphine and beta-endorphin on serotonin release from the spinal cord in rats

Effects of intraventricular (third ventricle) injection of morphine and beta-endorphin on the release of serotonin (5-HT; 5-hydroxytryptamine) and 5-HIAA (5-hydroxy indolacetic acid) from the spinal cord were studied using urethane anesthetized spinally perfused rats. Intraventricular injection of morphine (25 micrograms) increased the 5-HT level in the perfusate about threefold. The increase of 5-HT release reached at peak between 30 and 60 min after the first injection of morphine. However, the levels of 5-HIAA, a metabolite of 5-HT, was not significantly altered by intraventricular injection of morphine. Furthermore, second intraventricular injection of morphine at the same dose did not increase 5-HT level in the spinal perfusate. In contrast to the results with morphine, beta-endorphin (10 micrograms) administered intraventricularly did not alter the release of 5-HT and 5-HIAA from the spinal cord. In addition, acute antinociceptive tolerance to intraventricular morphine induced by a prior intraventricular injection of morphine was studied in pentobarbital anesthetized rats. Acute tolerance was induced by intraventricular pretreatment with morphine (20 micrograms) for 120 min and the same dose of morphine was injected intraventricularly. The tail-flick test was used as an antinociceptive test. Pretreatment of rats with morphine intraventricularly reduced inhibition of the tail-flick response to intraventricularly injected morphine. The results support our previous hypothesis that beta-endorphin and morphine administered supraspinally activate separate descending systems. Spinopetal serotonergic descending pathway is selectively activated by intraventricularly injected morphine but not beta-endorphin.(ABSTRACT TRUNCATED AT 250 WORDS)

[D-Pen2-D-Pen5]enkephalin, a delta opioid agonist, given intracerebroventricularly in the mouse produces antinociception through medication of spinal GABA receptors

Intracerebroventricular (ICV) administration of [D-Pen2-D-Pen5]enkephalin (DPDPE), a delta opioid receptor agonist, activates a descending antinociceptive pathway that inhibits the tail-flick response in mice. Involvement of spinal GABA receptors in this response was studied by giving GABA antagonist intrathecally. First, antinociception produced by intrathecally administered isoguvacine, a GABAA agonist, was inhibited by intrathecal bicuculline (GABA receptor antagonist) or picrotoxin (chloride channel antagonist). Then, antinociception induced by ICV DPDPE was antagonized by intrathecal picrotoxin and bicuculline in a dose-and time-dependent manner. Second, intrathecal administration of 2-hydroxysaclofen, a GABAB antagonist (which inhibited antinociception induced by a GABAB agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA and B antagonists given together intrathecally produced a greater than additive antagonistic effect against ICV DPDPE-induced antinociception. Thus, the delta agonist action of DPDPE in the brain leads to activation of descending spinal pathways which involve mediation by spinal GABAA and GABAB receptors in the antinociceptive response.

Role of nitric oxide/cyclic GMP in i.c.v. administered beta-endorphin- and (+)-cis-dioxolane-induced antinociception in the mouse

(+)-cis-Dioxolane (0.5-2 micrograms), a muscarinic receptor agonist, given intracerebroventricularly (i.c.v.) produced a dose-dependent inhibition of the tail-flick response in male ICR mice. (+)-cis-Dioxolane given i.c.v. at a subanalgesic dose (0.25 micrograms), selectively potentiated the antinociceptive response induced by i.c.v. administered beta-endorphin, an epsilon-opioid receptor agonist, but not morphine or [D-Ala2,NMePhe4,Gly5-ol]enkephalin (DAMGO), mu-opioid receptor agonists, [D-Pen2,D-Pen5]enkephalin (DPDPE), a delta receptor agonist, or trans(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide methane sulfonate (U50,488H), a kappa-opioid receptor agonist. The antinociceptive response induced by (+)-cis-dioxolane given i.c.v. was attenuated by i.c.v. treatment with N omega-nitro-L-arginine (1 microgram), hemoglobin (120 micrograms) or methylene blue (10 micrograms). The antinociception induced by carbachol given i.c.v. was also antagonized by the i.c.v. treatment with N omega-nitro-L-arginine (1 microgram). However, the same treatment with N omega-nitro-L-arginine, hemoglobin or methylene blue did not affect the beta-endorphin-induced antinociception. The potentiation of beta-endorphin-induced antinociception by (+)-cis-dioxolane was reversed by i.c.v. treatment with N omega-nitro-L-arginine (1 microgram), hemoglobin (120 micrograms) or methylene blue (10 micrograms). On the other hand, the antinociceptive response induced by (+)-cis-dioxolane (1 microgram) given i.c.v. was potentiated by i.c.v. administered L-arginine (20 micrograms) but not D-arginine (20 micrograms). Dibutyryl cyclic GMP at 0.5-2.0 micrograms given i.c.v. produced an antinociceptive response and at subanalgesic dose (0.1 microgram) potentiated i.c.v. beta-endorphin-induced antinociception.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of intrathecally injected galanin on antinociception induced by beta-endorphin and morphine administered intracerebroventricularly in mice

The effects of intrathecal (i.t.) and intracerebroventricular (i.c.v.) treatments with galanin on inhibition of the tail-flick and paw-licking hot-plate responses induced by beta-endorphin and morphine administered i.c.v. were studied in ICR mice. Galanin (100 ng) given i.t. effectively antagonized inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin (1 microgram) but not morphine (1 microgram). However, the same dose of galanin given i.t. did not affect inhibition of the hot-plate response induced by beta-endorphin and morphine administered i.c.v. Intrathecal treatment with various doses of galanin (0.1-100 ng) dose-dependently antagonized the inhibition of the tail-flick response induced by beta-endorphin administered i.c.v. Galanin (100 ng) in combination with beta-endorphin (1 microgram) or morphine (1 microgram) given i.c.v. did not affect beta-endorphin- or morphine-induced inhibition of the tail-flick and hot-plate responses. It is concluded that galanin given i.t. selectively attenuates i.c.v. beta-endorphin-induced inhibition of the tail-flick response by inhibiting descending epsilon-opioid system activated by supraspinally applied beta-endorphin.

Role of the spinal cord in intracisternal beta-endorphin-evoked suppression of liver DNA synthesis in 10-day-old rats

Previously, we have shown that intracisternal (i.c.) administration of beta-endorphin (an opioid peptide naturally occurring in the brain) to preweanling rats markedly decreases DNA synthesis (an index of cell proliferation) in both brain and liver. This observation is consistent with our hypothesis that endogenous CNS beta-endorphin plays an important role in controlling postnatal growth. The current research specifically undertook to investigate, in 10-day-old rats, whether or not i.c. beta-endorphin-evoked suppression of liver DNA synthesis is actually mediated by spinal opioid receptors and/or by descending endorphinergic pathways. In contrast to the i.c. route of administration, beta-endorphin given directly into the spinal subarachnoid space via intrathecal (i.t.) injection did not alter liver DNA synthesis, yet was able to evoke profound antinociceptive responses. This demonstrates that intracisternally applied beta-endorphin exerts its effect on liver DNA by acting at supraspinal sites, and not by directly stimulating spinal opioid receptors after diffusion from its intracerebral site of injection. As it is possible that beta-endorphin's supraspinal actions may activate a descending inhibitory endorphinergic pathway to reduce DNA synthesis, we conducted studies in rat pups administered naloxone intrathecally. Naloxone i.t. was completely ineffective in preventing beta-endorphin i.c. from inhibiting liver DNA synthesis. On the other hand, i.t. coinjection of naloxone plus beta-endorphin was able to block the analgesic response, while their i.c. coinjection reversed the DNA effect. The results from these studies indicate that opioid systems within the spinal cord do not play a major role in mediating CNS beta-endorphins regulation of DNA synthesis in peripheral tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Antisense oligodeoxynucleotide to a delta-opioid receptor given intrathecally blocks i.c.v. administered beta-endorphin-induced antinociception in the mouse

The antinociception induced by supraspinally administered beta-endorphin is mediated by the release of Met-enkephalin and subsequent stimulation of delta-opioid receptors from the spinal cord in mice. Repeated intrathecal administration of an antisense oligodeoxynucleotide against delta-opioid receptors selectively attenuated i.c.v. administered beta-endorphin-induced antinociception without any effect on the antinociception induced by mu-opioid receptor agonists, morphine and DAMGO, or kappa-opioid receptor agonists, U50,488H. A random sequence oligodeoxynucleotide was inactive against beta-endorphin-induced antinociception. The study confirms previous findings that the antinociception induced by beta-endorphin is mediated by the stimulation of the delta-opioid receptors in the spinal cord.

Partial antinociceptive cross-tolerance to intracerebroventricular beta-endorphin in mice tolerant to systemic morphine

The effects of subcutaneous morphine pellet-implantation on antinociception induced by intracerebroventricular (i.c.v.) administration of beta-endorphin or morphine and intrathecal (i.t.) administration of morphine, [D-Pen2,D-Pen5]enkephalin (DPDPE), [D-Ala2,NMePhe4,Gly5-ol]enkephalin (DAMGO), serotonin or norepinephrine were studied in male ICR mice. The tail-flick and hot-plate responses were used for antinociceptive tests. The ED50 values for i.c.v. administered morphine for antinociception in morphine pellet-implanted mice were increased from 3.3- and 2.2-fold at 0 h to 14.2- and 19.0-fold at 4 h and declined to 4.8- and 3.0-fold at 8 h after pellet removal in the tail-flick and hot-plate tests, respectively. On the other hand, the ED50 values for i.c.v. administered beta-endorphin for antinociception were only slightly increased (1.7- to 5.1-fold increases) throughout the same time course. The inhibition of the tail-flick response induced by i.t. injection of morphine, DPDPE and serotonin, but not norepinephrine or DAMGO, was attenuated in morphine pellet-implanted mice. These findings are consistent with previous studies indicating that different neuronal mechanisms are involved in morphine- and beta-endorphin-induced antinociception.

Effect of diabetes on the antinociceptive effect of beta-endorphin

We examined whether streptozotocin-induced diabetes can modulate beta-endorphin-induced antinociception in mice. While beta-endorphin administered i.c.v. produced a dose-dependent inhibition of the tail-flick response in both diabetic and non-diabetic mice, the antinociceptive response was greater in diabetic mice than in non-diabetic mice. The ED50 value of beta-endorphin administered i.c.v. in diabetic mice was significantly lower than that in non-diabetic mice. The antinociceptive effects of beta-endorphin administered i.c.v. in both diabetic and non-diabetic mice were significantly antagonized by s.c. administration of naltrindole, a selective delta-opioid receptor antagonist. beta-Endorphin administered i.t. also produced a dose-dependent antinociception in both diabetic and non-diabetic mice. However, the ED50 value of kappa-opioid receptor antagonist. On the other hand, the antinociceptive potency of DPDPE, a selective delta-opioid agonist, administered i.t. is significantly increased in diabetic mice, as compared with non-diabetic mice, whereas, the antinociceptive potency of U-50,488H, a kappa-opioid receptor agonist, administered i.t. is significantly less than in non-diabetic mice. These results suggest that diabetes may modulate beta-endorphin-induced antinociception differently at the spinal and supraspinal levels.

Increase of nitric oxide by L-arginine potentiates beta-endorphin- but not mu-, delta- or kappa-opioid agonist-induced antinociception in the mouse

L-Arginine pretreated i.c.v. produced a time- and dose-dependent potentiation of beta-endorphin-induced inhibition of the tail-flick response in ICR mice. However, the inhibition of the tail-flick response induced by morphine, DAMGO ([D-Ala2,NMePhe4,Gly5-ol]enkephalin), DPDPE ([D-Pen2,D-Pen5]enkephalin or U50,488H given i.c.v. was not potentiated by i.c.v. pretreated L-arginine. The results indicate that L-arginine selectively potentiates antinociception induced by epsilon-opioid receptor agonist, but not mu-, delta- or kappa-opioid receptor agonist. L-Arginine pretreated i.t. did not potentiate i.c.v. administered beta-endorphin-induced inhibition of the tail-flick response, indicating that the potentiating effect of L-arginine on beta-endorphin-induced antinociception is located at the supraspinal sites but not at the spinal sites.

Inhibition of spinal opioid antinociception by intrathecal beta-endorphin1-27 in the rat

1. The effects of intrathecal (i.t.) administration of beta-endorphin and two shorter fragments, human and ovine beta-endorphin1-27, were examined for antinociceptive activity in the tail-flick and paw-pressure tests in the rat. Additionally, the ability of ovine beta-endorphin1-27 to influence the action of i.t. beta-endorphin, morphine and [D-Pen2-D-Pen5]enkephalin (DPDPE) was also examined in these tests. 2. After i.t. injection, beta-endorphin produced potent dose-related antinociception in the tail-flick and paw-pressure tests. Shorter endorphins produced much weaker effects. The order of antinociceptive efficacy was beta-endorphin > human beta-endorphin1-27 > ovine beta-endorphin1-27. 3. Administration of ovine beta-endorphin1-27 (0.72, 1.44 nmol, i.t.) significantly attenuated the antinociceptive effect of beta-endorphin (2.88 nmol, i.t.) in the tail-flick and paw-pressure tests. 4. Both i.t. morphine and DPDPE produced dose-related antinociception in the tail-flick and paw-pressure tests. The potency of DPDPE was lower than that of morphine in both tests; however, the effect of DPDPE was weaker in the paw-pressure test. 5. Administration of ovine beta-endorphin1-27 (1.44 nmol, i.t.) significantly attenuated the antinociceptive effect of morphine (14.9 nmol, i.t.) in both tests and the effect of DPDPE (38.7 nmol) in the tail-flick test. 6. The results show that beta-endorphin1-27 acts as an opioid antagonist at the spinal level in the rat. Its ability to inhibit the action of morphine and DPDPE suggests that it may attenuate beta-endorphin action by an interaction with mu- and/or delta-opioid receptors.

Antitussive effect of beta-endorphin is mediated by mu-opioid receptors, but not by kappa- or epsilon-opioid receptors

The present study examined the opioid receptors involved in the antitussive effect of beta-endorphin in mice. beta-Endorphin injected i.c.v. depresses coughs dose dependently in doses from 0.1 to 1 microgram. Blockade of mu-opioid receptors by pretreatment with beta-funaltrexamine significantly reduced the antitussive potency of i.c.v. beta-endorphin. However, the antitussive effect of beta-endorphin was not antagonized by nor-binaltorphimine, a kappa-opioid receptor antagonist. Moreover, i.c.v. injection of beta-endorphin-(1-27), an epsilon-opioid receptor antagonist, did not affect the antitussive effect of beta-endorphin. The results indicate that the antitussive effect of beta-endorphin is mediated by activation of mu-opioid receptors, but not of kappa- or epsilon-opioid receptors.

A review of the role of anti-opioid peptides in morphine tolerance and dependence

Studies on the mechanisms of tolerance and dependence have mostly focused on changes at the receptor level. These experiments, conducted with model systems ranging from clonal cell lines to whole animals, have identified a number of important adaptive mechanisms which occur at the receptor level. However, none of these adaptive mechanisms can completely account for the phenomena which serve to define the state of morphine tolerance and dependence, especially the observation that as an animal becomes more tolerant to morphine, less naloxone is required to trigger withdrawal. The data reviewed in this paper provide strong support for the hypothesis that the brain synthesizes and secretes neuropeptides which act as part of a homeostatic system to attenuate the effects of morphine and endogenous opioid peptides. According to this model, administration of morphine releases anti-opioid peptides (AOP), which then attenuate the effects of morphine. As more morphine is given, more AOP are released, thereby producing tolerance to the effects of morphine. Cessation of morphine administration, or administration of naloxone, produces a relative excess of anti-opioid, which is in part responsible for the withdrawal syndrome. Since endogenous and exogenous antagonists might together produce synergistic effects, less naloxone might be required to trigger withdrawal in the presence of higher levels of AOPs. Although the study of AOP is in its infancy, a deeper understanding of the central nervous system (CNS) anti-opioid systems may lead to new treatments for chronic pain, substance abuse, and psychiatric disorders.

Spinal delta 2 but not delta 1 opioid receptors are involved in intracerebroventricular beta-endorphin-induced antinociception in the mouse

The antinociception induced by beta-endorphin given intracerebroventricularly (i.c.v.) has been previously demonstrated to be mediated by the release of Met-enkephalin and subsequent stimulation of delta receptors in the spinal cord for antinociception. The present study was designed to determine what type of opioid receptor, delta 1 or delta 2, in the spinal cord is involved in i.c.v. beta-endorphin-induced antinociception. Antinociception was assessed by the tail-flick test in male ICR mice. NTB (0.2-20 nmol) and NTI (0.22-2.2 nmol), selective delta 2 receptor antagonists, given intrathecally (i.t.) dose-dependently attenuated i.c.v. beta-endorphin-induced inhibition of the tail-flick response. On the other hand, BNTX (0.02-2.2 nmol), a selective delta 1 receptor antagonist, given i.t., did not block i.c.v. beta-endorphin-induced antinociception. The tail-flick inhibition induced by DAMGO, a mu receptor agonist, or U50,488H, a kappa receptor agonist, was not blocked by i.t. BNTX, NTB or NTI. It is concluded that delta 2 but not delta 1 receptors in the spinal cord are involved in i.c.v. beta-endorphin-induced antinociception.

Pentobarbital attenuates antinociception induced by i.c.v. morphine but not beta-endorphin in the mouse

The effects of pentobarbital anesthesia (45 mg/kg i.p.) on the inhibition of the tail-flick response induced by beta-endorphin and morphine injected intracerebroventricularly (i.c.v.) and intrathecally (i.t.) were studied in male ICR mice. Pentobarbital anesthesia attenuated the inhibition of the tail-flick response induced by morphine but not beta-endorphin given i.c.v. However, the tail-flick inhibition induced by morphine given i.t. was not attenuated by pentobarbital. beta-Endorphin-(1-27) (3 micrograms) given i.c.v. or naloxone (2 micrograms) given i.t. blocked inhibition of the tail-flick response induced by morphine given i.c.v. only in pentobarbital-anesthetized mice but not in conscious mice. beta-Funaltrexamine (beta-FNA, 2.5 micrograms) given i.c.v. or yohimbine (2 micrograms) and methysergide (2 micrograms) injected i.t. blocked the morphine (i.c.v.)-induced inhibition of the tail-flick response in conscious mice but not in pentobarbital-anesthetized mice. The results indicate that pentobarbital attenuates the morphine-induced inhibition of the tail-flick response by inhibiting descending noradrenergic and serotonergic pathways and uncovers a descending opioid system. The tail-flick inhibition induced by supraspinal morphine is mediated by stimulation of mu-opioid receptors in conscious mice and epsilon-opioid receptors in pentobarbital-anesthetized mice. The epsilon-opioid receptor-mediated descending system activated by supraspinally injected beta-endorphin is not attenuated by pentobarbital anesthesia.

The tail-flick inhibition induced by beta-endorphin administered intrathecally is mediated by activation of kappa- and mu-opioid receptors in the mouse

The inhibition of the tail-flick response induced by beta-endorphin given i.c.v. has been demonstrated to be mediated by the stimulation of epsilon- but not mu-, delta- or kappa-opioid receptors. beta-Endorphin given i.t. also inhibited the tail-flick response. The present studies were designed to determine what types of opioid receptors in the spinal cord were involved in i.t. beta-endorphin-induced tail-flick inhibition. Blockade of kappa-opioid receptors by coadministration of nor-binaltorphimine or Win 44,441-3 with beta-endorphin given i.t. dose dependently inhibited i.t. beta-endorphin-induced inhibition of the tail-flick response. Blockade of mu-opioid receptors by i.t. coadministration of D-Phe-Cys-Tyr-D-Try-Orn-Thr-Pen-Thr-NH2 with beta-endorphin blocked i.t. beta-endorphin-induced inhibition of the tail-flick response. I.t. injection of delta-opioid receptors antagonists, ICI 174,864 and naltrindole, or epsilon-opioid receptor antagonist, beta-endorphin-(1-27), did not affect inhibition of the tail-flick response induced by beta-endorphin given i.t. Blockade of alpha 2-adrenoceptors and 5-HT receptors by i.t. injection of yohimbine and methysergide, respectively, also did not affect inhibition of the tail-flick response induced by beta-endorphin given i.t. The results indicate that the inhibition of the tail-flick response induced by beta-endorphin given i.t. is mediated by the stimulation of kappa- and mu-opioid receptors but not delta- and epsilon-opioid receptors, alpha 2-adrenoceptors or 5-HT receptors.

Intrathecal cholecystokinin octapeptide attenuates the antinociception and release of immunoreactive Met-enkephalin induced by intraventricular beta-endorphin in the rat

The effects of cholecystokinin octapeptide (CCK8s) given intrathecally (i.t.) on antinociception and the release of immunoreactive Met-enkephalin in the spinal perfusate induced by intraventricular (i.vt.) injection of beta-endorphin were studied in anesthetized rats. beta-Endorphin (5 micrograms) given i.vt. inhibited the tail-flick response. The inhibition of the tail-flick response induced by beta-endorphin was blocked dose-dependently by CCK8s (0.1-7 micrograms) given i.t. The antagonistic effect of CCK8s on beta-endorphin-induced inhibition was blocked dose dependently by co-intrathecal injection of proglumide (3 and 10 micrograms), a CCK8s receptor antagonist. beta-Endorphin (5 micrograms) given i.vt. elicited a release of immunoreactive Met-enkephalin in the spinal perfusate. Repeated injections of the same dose of beta-endorphin released about the same amount of the immunoreactive Met-enkephalin in the spinal perfusate. CCK8s at concentrations from 1 x 10(-9) to 1 x 10(-6) M added into the spinal perfusate decreased the release of Met-enkephalin induced by beta-endorphin given i.vt. in a dose-dependent manner. The results suggest that CCK8s may attenuate beta-endorphin-induced inhibition of the tail-flick response by inhibiting the release of Met-enkephalin from the spinal cord.

Stereoselective effect of morphine on antinociception and endogenous opioid peptide levels in plasma but not cerebrospinal fluid of dogs

Morphine releases endogenous opioids into the circulation of dogs. To test the stereospecificity of this effect, as well as to determine whether morphine also releases endogenous opioids centrally, which might be involved in its antinociceptive action, the effects of (-)-morphine sulfate (10 mg/kg, sc) or (+)-morphine hydrobromide on antinociception in a dog tail-flick test, on semi-quantified morphine-induced signs of salivation, emesis, defecation and ataxia, and on the plasma and cerebrospinal fluid (CSF) levels of endogenous opioid peptides were studied. Plasma and CSF levels of immunoreactive beta-endorphin (i-BE), met-enkephalin (i-ME), leu-enkephalin (i-LE), and dynorphin (i-DY) were quantified by radioimmunoassay in octadecylsilyl-silica cartridge extracts. Immunoreactive morphine (i-M) levels were measured in unextracted samples. (-)-Morphine treatment significantly increased antinociception, morphine-induced signs, i-M levels in plasma and CSF, and i-BE, i-ME, and i-LE levels in plasma, but not CSF. Levels of i-DY remained constant in plasma and CSF. (+)-Morphine treatment did not alter any of these parameters, indicating that the effects of morphine on nociception, behavioral signs, and plasma endogenous opioids in dogs were stereoselective. It is concluded that morphine does not cause an increase in immunoreactive endogenous opioid peptides in the CSF at the time of its peak antinociceptive effect.

Intrathecal administration of thiorphan and bestatin enhances the antinociception and release of Met-enkephalin induced by beta-endorphin intraventricularly in anesthetized rats

Effects of bestatin and thiorphan administered intrathecally, on inhibition of the tail-flick response and the release of Met-enkephalin induced by beta-endorphin administered intraventricularly were studied in anesthetized rats. Intrathecal pretreatment with 100 micrograms of thiorphan or bestatin potentiated the inhibition of the tail-flick response induced by beta-endorphin injected intraventricularly in pentobarbital anesthetized rats; the ED50 values for beta-endorphin were decreased 5- and 7-fold by thiorphan and bestatin, respectively. To determine if the potentiating effect was due to the inhibition of the degradation of Met-enkephalin released by intraventricular beta-endorphin, the effects of intrathecal perfusion with thiorphan or bestatin on the release of immunoreactive Met-enkephalin from the spinal cord by intraventricular injection of beta-endorphin were studied. beta-Endorphin injected into the 4th ventricle at a dose of 5 micrograms increased immunoreactive Met-enkephalin in the spinal perfusate in urethane-anesthetized rats. Thiorphan or bestatin (1 x 10(-7) to 1 x 10(-4) M each) increased the amount of immunoreactive Met-enkephalin released by intraventricular beta-endorphin in a dose-dependent manner. The results provide additional evidence for the hypothesis that antinociception induced by beta-endorphin is mediated by release of Met-enkephalin.

Differential effects of sulfated cholecystokinin octapeptide and proglumide injected intrathecally on antinociception induced by beta-endorphin and morphine administered intracerebroventricularly in mice

The effects of sulfated cholecystokinin octapeptide (CCK-8s) and CCK-8s antagonist, proglumide, given intrathecally (i.t.) on inhibition of the tail-flick and hot-plate paw-licking responses induced by beta-endorphin and morphine given intracerebroventricularly (i.c.v.) were studied in male ICR mice. Both CCK-8s (up to 0.5 ng) and proglumide (up to 10 micrograms) injected alone did not affect significantly the control latencies of the tail-flick and paw-licking responses. I.t. injection of CCK-8s as doses from 0.125 to 0.5 ng dose dependently attenuated inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin. The antagonistic effect of CCK-8s on beta-endorphin-induced inhibition was blocked by the co-i.t. injection of proglumide (0.1-1 micrograms) in a dose-dependent manner. High doses (2.5-10 micrograms) of proglumide given i.t. dose dependently enhanced inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin. However, i.t. injection of CCK-8s and proglumide did not affect inhibition of the paw-licking response induced by i.c.v. administered beta-endorphin. The inhibitions of the tail-flick and paw-licking responses induced by i.c.v. administered morphine were not affected by i.t. injection of CCK-8s or proglumide. Our results suggest that CCK-8s in the spinal cord may play an important modulatory role in attenuating the descending pain inhibition induced by i.c.v. administered beta-endorphin but not morphine.

Tolerance to delta- but not mu-opioid receptors in the spinal cord attenuates inhibition of the tail-flick response induced by beta-endorphin administered intracerebroventricularly in mice

Male ICR mice were rendered tolerant by intrathecal (IT) injection once a day with either mu-agonist, D-Ala2-NMePhe4-Gly-ol-enkephalin (DAMGO) or delta-agonist, D-Pen2-D-Pen5-enkephalin (DPDPE) (toleragen) by doubling the dose each day starting from 0.125 and 1 microgram for DAMGO and DPDPE, respectively, for 6 days. On day 6, the magnitude of tolerance was assessed by establishing IT dose-response lines for the effect of the chronic drug given as bolus injections (probe). The antinociception was assessed by the tail-flick and hot-plate test. Repeated IT injections of DPDPE reduced inhibition of the tail-flick and hot-plate response induced by DPDPE (ED50 values for DPDPE increase 10-fold) but not DAMGO. Repeated IT injections of DAMGO reduced inhibition of the tail-flick and hot-plate response induced by DAMGO (ED50 value for DAMGO increase 7- to 10-fold) but not DPDPE. The effects of the tolerance to mu- and delta-opioid receptor activity in the spinal cord on inhibition of the tail-flick and hot-plate response induced by intracerebroventricularly (ICV) administered beta-endorphin and morphine were then studied. beta-Endorphin or morphine at different doses were injected ICV 4 hr after the last IT injection of DPDPE or DAMGO. Repeated IT bolus injections of DPDPE reduced inhibition of the tail-flick response but not the hot-plate response induced by beta-endorphin. On the other hand, repeated IT bolus injections of DAMGO did not affect inhibition of the tail-flick and hot-plate response induced by beta-endorphin.(ABSTRACT TRUNCATED AT 250 WORDS)