Confluence of antianalgesic action of diverse agents through brain interleukin(1beta) in mice.

Spinal dynorphin A(1-17) (Dyn) has been shown previously to produce an antianalgesic action against intrathecal morphine in the tail-flick test in CD-1 mice. This action is known to be mediated indirectly from the spinal cord through an afferent pathway that activates flumazenil-sensitive benzodiazepine receptors in the brain and a descending circuit back down to the spinal cord sequentially involving cholecystokinin, leu-enkephalin, and N-methyl-D-aspartate receptors to produce antianalgesia. Interleukin (IL)-1beta is also known to act on peripheral afferent nerves to the brain to activate a descending circuit to release spinal cholecystokinin. The present investigation determined whether IL1beta is a supraspinal mediator for intrathecal Dyn-induced antianalgesia in CD-1 mice. Intracerebroventricular Lys193-D-Pro-Thr195, an IL1beta antagonist, or pretreatment with IL1beta antiserum eliminated intrathecal dynorphin antianalgesia, implicating brain IL1beta; 10 ng of IL(1beta) given intracerebroventricularly produced antianalgesia. Fittingly, Dyn was not antianalgesic in C3H/HeJ mice, which are genetically deficient in release of IL1beta. Activation of central benzodiazepine receptors preceded the IL1beta step because flumazenil inhibited Dyn but not IL1beta antianalgesia. On the other hand, [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide], an antagonist for peripheral benzodiazepine receptors that have also recently been detected in brain tissue, inhibited IL1beta antianalgesia; these latter benzodiazepine receptors formed a separate step after the flumazenil-sensitive benzodiazepine receptor step. IL1beta action in the brain was linked to the linear steps in the spinal cord (cholecystokinin/N-methyl-D-aspartate receptors) as shown by inhibition with appropriate antagonists. Thus, IL1beta is a central physiological mediator in the antianalgesic action evoked by spinal dynorphin.

Inverse agonist action of Leu-enkephalin at delta(2)-opioid receptors mediates spinal antianalgesia.

Dynorphin A(1-17) given intrathecally releases spinal cholecystokinin to produce an antianalgesic action against spinal morphine in the tail-flick test in CD-1 mice. The present study showed that following the cholecystokinin step, a delta(2)-opioid inverse agonist action of Leu-enkephalin (LE), was involved. Pretreatment with intrathecal LE antiserum eliminated dynorphin and cholecystokinin-8s antianalgesia. A small dose of LE intrathecally produced antianalgesia that like that from dynorphin A(1-17) and cholecystokinin was eliminated by naltriben but not 7-benzylidenenaltrexone (delta(2)- and delta(1)-opioid receptor antagonist, respectively). This LE step was followed by N-methyl-D-aspartate (NMDA) receptor activation. MK801, an NMDA receptor antagonist, eliminated the antianalgesia from dynorphin A(1-17), cholecystokinin-8s, and LE. Furthermore, none of the three were effective against morphine analgesia in 129S6/SvEv mice possibly because of their deficiency in NMDA receptor response. In 129S6/SvEv mice, [D-Ser(2)]-Leu-enkephalin-Thr analgesia was not attenuated by LE; thus, this delta(2)-analgesic agonist and LE inverse agonist action did not occur through competition at the same delta(2)-receptor in CD-1 mice. In CD-1 mice, a linear sequence of dynorphin A(1-17) --> cholecystokinin --> LE --> NMDA receptors was indicated: cholecystokinin antiserum inhibited cholecystokinin but not LE; naltriben inhibited LE but not NMDA. The uniqueness of LE in linking dynorphin A(1-17), cholecystokinin, delta(2)-opioid, and NMDA receptor activation may unify the separate known mechanisms involved in the antiopioid actions of these components against morphine.

Antianalgesic action of nociceptin originating in the brain is mediated by spinal prostaglandin E(2) in mice.

An antianalgesic action of intracerebroventricularly administered nociceptin was elicited against intrathecal morphine-induced antinociception in the tail-flick test in mice and investigated as a descending neuronal system for the spinal mediator involved. The nociceptin-induced antianalgesia originating in the brain was inhibited by intrathecally administered indomethacin and suggested the mediation of spinal prostaglandin. The antianalgesic action of intracerebroventricular nociceptin was closely matched by intrathecal prostaglandin (PG) E(2). Both shifted the dose-response curve of morphine to the right and these actions were eliminated by intrathecal PGD(2.) Desensitization of the antianalgesic action of PGE(2) by intrathecal PGE(2) pretreatment also produced cross-desensitization to the antianalgesic action of intracerebroventricular nociceptin. Neither intracerebroventricular nociceptin nor intrathecal PGE(2) produced antianalgesia against the delta-receptor agonists given intrathecally. Thus, the antianalgesic action of nociceptin originating in the brain is coupled to a descending neuronal pathway mediated by spinal PGE(2).

Acute cross-tolerance to opioids in heroin delta-opioid-responding Swiss Webster mice.

It is generally thought that the mu receptor actions of metabolites, 6-monoacetylmorphine (6MAM) and morphine, account for the pharmacological actions of heroin. However, upon intracerebroventricular (i.c.v.) administration in Swiss Webster mice, heroin and 6MAM act on delta receptors while morphine acts on mu receptors. Swiss Webster mice made tolerant to subcutaneous (s.c.) morphine by morphine pellet were not cross-tolerant to s.c. heroin (at 20 min in the tail flick test). Now, opioids were given in combination, s.c. (6.5 h) and i.c.v. (3 h) preceding testing the challenging agonist i.c.v. (at 10 min in the tail flick test). The combination (s.c. + i.c.v.) morphine pretreatment induced tolerance to the mu action of morphine but no cross-tolerance to the delta action of heroin, 6MAM and DPDPE and explained why morphine pelleting did not produce cross-tolerance to s.c. heroin above. Heroin plus heroin produced tolerance to delta agonists but not to mu agonists. Surprisingly, all combinations of morphine with the delta agonists produced tolerance to morphine which now acted through delta receptors (inhibited by i.c.v. naltrindole), an unusual change in receptor selectivity for morphine.

Morphine tolerance in mice changes response of heroin from mu to delta opioid receptors.

Heroin produced antinociception in the tail flick test through mu receptors in the brain of ICR and CD-1 mice, a response inhibited by 3-O-methylnaltrexone. Tolerance to morphine was produced by subcutaneous morphine pellet implantation. By the third day, the heroin response was produced through delta opioid receptors. The response was inhibited by simultaneous intracerebroventricular (i.c. v.) administration of naltrindole, a delta opioid receptor antagonist. More specifically, delta1 rather than delta2 receptors were involved because 7-benzylidenenaltrexone, a delta1 receptor antagonist, inhibited but naltriben, a delta2 antagonist, did not. Also, antinociception produced by i.c.v. heroin was inhibited by intrathecal administration of bicuculline and picrotoxin consistent with the concept that delta1 receptors in the brain mediated the antinociceptive response through descending neuronal pathways to the spinal cord to activate GABAA and GABAB receptors rather than spinal alpha2-adrenergic and serotonergic receptors activated originally by the mu agonist action in naive mice. The mu response of 6-monoacetylmorphine, a metabolite of heroin, was changed by morphine pellet implantation to a delta2 response (inhibited by naltriben but not 7-benzylidenenaltrexone). The agonist action of morphine in these morphine-tolerant mice remained mu. Thus, the opioid receptor selectivity of heroin and 6-monoacetylmorphine in the brain is changed by production of tolerance to morphine. Such a change explains how morphine tolerant mice are not cross-tolerant to heroin.

Antianalgesic action of dynorphin A mediated by spinal cholecystokinin.

Previous work indicates that the antianalgesic action of pentobarbital and neurotensin administered intracerebroventricularly in mice arises from activation of a descending system to release cholecystokinin (CCK) in the spinal cord where CCK is known to antagonize morphine analgesia. Spinal dynorphin, like CCK, has an antianalgesic action against intrathecally administered morphine. This dynorphin action is indirect; even though it is initiated in the spinal cord, it requires the involvement of an ascending pathway to the brain and a descending pathway to the spinal cord where an antianalgesic mediator works. The aim of the present investigation was to determine if the antianalgesic action of intrathecal dynorphin A involved spinal CCK. All drugs were administered intrathecally to mice in the tail flick test. Morphine analgesia was inhibited by dynorphin as shown by a rightward shift of the morphine dose-response curve. The effect of dynorphin was eliminated by administration of the CCK receptor antagonists lorglumide and PD135 158. One hour pretreatment with CCK antiserum also eliminated the action of dynorphin. On the other hand, the antianalgesic action of CCK was not affected by dynorphin antiserum. Thus, CCK did not release dynorphin. Both CCK and dynorphin were antianalgesic against DSLET but not DPDPE, delta 2 and delta 1 opioid receptor peptide agonists, respectively. The results suggest that the antianalgesic action of dynorphin occurred through an indirect mechanism ultimately dependent on the action of spinal CCK.

Supraspinal neurotensin-induced antianalgesia in mice is mediated by spinal cholecystokinin.

Intracerebral injection of neurotensin into specific brain loci in rats produces hyperalgesia due to the release of cholecystokinin (CCK) in the spinal cord. The present purpose was to show in another species that neurotensin can antagonize the antinociceptive action of morphine through the spinal CCK mechanism in mice. Neurotensin given intracerebroventricularly (i.c.v.) at doses higher than 100 ng produced antinociception in the tail flick test. However, at lower doses between 1 pg to 25 ng, neurotensin antagonized the antinociceptive action of morphine given intrathecally (i.t.), thus demonstrating the antianalgesic activity of neurotensin. The rightward shift in the morphine dose-response curve produced by i.c.v. neurotensin was eliminated by an i.t. pretreatment with CCK8 antibody (5 microl of antiserum solution diluted 1:1000). I.t. administration of lorglumide, a CCK(A)-receptor antagonist (10-1000 ng), and PD135,158, a CCK(B)-receptor antagonist (250-500 ng), also eliminated the antianalgesic action of neurotensin. Thus, the mechanism of the antianalgesic action of neurotensin given i.c.v. involved spinal CCK. This mode of action is similar to that for the antianalgesic action of supraspinal pentobarbital which also involves spinal CCK.

Opioid receptor selectivity of heroin given intracerebroventricularly differs in six strains of inbred mice.

Heroin administered i.c.v. acts on supraspinal mu opioid receptors in ICR mice but on delta receptors in Swiss Webster mice. The purpose of this study was to determine the degree to which genotype plays a role in the opioid receptor selectivity of heroin across a range of fully inbred strains of mice. Six inbred strains were given heroin i.c.v. 10 min before the tail-flick test. Differences in the descending neurotransmitter systems involved in supraspinal opioid-induced analgesia were evaluated as the first step. Antagonism by bicuculline given intrathecally indicated the involvement of supraspinal delta receptors in activating spinal gamma-aminobutyric acid (GABA) receptors; antagonism by intrathecal methysergide indicated either mu or kappa receptor involvement. Antagonism by intrathecal yohimbine implicated mu and eliminated kappa receptor involvement. Intracerbroventricular opioid antagonists (beta-funaltrexamine, 7-benzylidenenaltrexone, naltriben, or nor-binaltorphimine) provided further differentiation. Based on these initial results, receptor selectivity was determined by more extensive ED50 experiments with i.c.v. administration of heroin with opioid antagonists, beta-funaltrexamine (for mu), naltrindole (for delta), and nor-binaltorphimine (for kappa). The combined results indicated that heroin analgesia was predominantly mediated in C57BL/6J by delta, in DBA/2J and CBA/J by mu, and in BALB/cByJ and AKR/J by kappa receptors. The response in C3H/HeJ appeared to involve mu receptors. The results indicate that the opioid receptor selectivity of heroin is genotype-dependent. Because these genotypes are fully inbred, the genetically determined molecular and neurochemical substrate mediating the different opioid receptor selectivities of heroin can be studied further.

Heroin acts on delta opioid receptors in the brain of streptozocin-induced diabetic rats.

Heroin, like morphine, given intracerebroventricularly produces analgesia by acting on mu opioid receptors in most mice. In contrast, in Swiss Webster mice, heroin has the unusual property of acting on brain delta opioid receptors whereas morphine still acts on mu receptors. The literature indicates that in diabetic mice and rats, the mu agonist potency of morphine is diminished while that to a delta receptor agonist is enhanced. The purpose of the present study was to determine if the response to heroin occurred through a delta receptor in the brain of streptozotocin-induced diabetic Sprague-Dawley rats. One week after a cannula was surgically implanted in the lateral ventricle, diabetes was induced by intravenous administration of 55 mg/kg of streptozotocin. Three days later the receptor selectivity of intraventricular heroin in the tail flick test was determined by coadministration of opioid antagonists. In nondiabetic rats, a rightward shift in the dose response curve for heroin was produced by naloxone. D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-ThrNH2, a more mu receptor selective antagonist given in a single dose experiment, also inhibited heroin action. Thus, heroin acted on mu receptors. In diabetic rats, intracerebroventricular naltrindole, but not naloxone nor CTOP, inhibited the heroin response and indicated a delta agonist action for heroin. Inhibition by intrathecal yohimbine of the mu (nondiabetic) and bicuculline of the delta response (diabetic) suggested spinal alpha2-adrenergic and GABA(A) receptor mediation, respectively, for the descending systems. In conclusion, the response to heroin was changed from mu in nondiabetic rats to a delta receptor action in diabetic rats. Understanding the basis for this change in receptor selectivity of heroin could provide an important avenue for investigating determinants of opioid receptor function.

Supraspinal flumazenil inhibits the antianalgesic action of spinal dynorphin A (1-17).

DynorphinA (Dyn) administered intrathecally or released spinally in mice produces antianalgesia, that is, antagonizes morphine analgesia (tail-flick test). Spinal transection eliminates this Dyn antianalgesia. Present results in mice show that intracerebroventricular administration of flumazenil, a benzodiazepine receptor antagonist, also eliminated the antianalgesic action of Dyn; flumazenil in the brain eliminated the suppressant effect of intrathecal Dyn on intrathecal and intracerebroventricular morphine-induced antinociception. Intracerebroventricular clonidine, naloxone, and norbinaltorphimine release spinal Dyn. The latent antinociceptive actions of these compounds were uncovered by intracerebroventricular flumazenil. Thus, Dyn, given intrathecally or released spinally, activates a pathway that is inhibited by intracerebroventricular flumazenil. Dyn antianalgesia is not significantly altered by intracerebroventricular administration of bicuculline and picrotoxin, suggesting that activation of the gamma-aminobutyric acid receptor has little if any involvement in the antianalgesic action of Dyn. The antagonistic effect of Dyn seems to be mimicked by benzodiazepine agonists. Furthermore, administration of a benzodiazepine receptor inverse agonist (methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) inhibited Dyn antianalgesia as did flumazenil. Thus, flumazenil, through a benzodiazepine antagonist or inverse agonist action, interrupts, as does spinal transection, the neuronal circuit (cord/brain/cord) necessary for the antianalgesic action of spinal Dyn. Because Dyn antianalgesia is an indirect action, activation of the neuronal circuit must lead to the release of a direct-acting antianalgesic mediator in the spinal cord.

Pentobarbital antagonism of morphine analgesia mediated by spinal cholecystokinin.

Pentobarbital administered intracerebroventricularly to mice has been shown previously to inhibit the analgesic action of morphine given intrathecally. The purpose of the present study was to examine the proposal that this antianalgesic action was mediated spinally by cholecystokinin. First, intrathecal coadministration of cholecystokinin-B sulfate (CCK8s) with morphine inhibited the analgesic action of morphine in the mouse tail-flick test. This rightward shift of the morphine dose-response curve was reversed by the intrathecal administration of either the CCKA receptor antagonist, lorglumide, or the CCKB receptor antagonist, PD135, 158. Second, lorglumide and PD135, 158 given intrathecally also eliminated the antianalgesic effect of intracerebroventricularly administered pentobarbital against intrathecal morphine. Third, intrathecal pretreatment with CCKB antiserum eliminated the effect of pentobarbital. Thus, the results indicated that pentobarbital antianalgesia was obtained through activation of a descending system to the spinal cord where cholecystokinin inhibited the spinal analgesic action of morphine.

Heroin antinociception changed from mu to delta receptor in streptozotocin-treated mice.

CD-1 mice were treated intravenously with streptozotocin, 200 mg/kg, and tested 2 weeks later or treated with 60 mg/kg and tested 3 days later. Both treatments changed the tail flick response of heroin and 6-monoacetylmorphine (6 MAM) given intracerebroventricularly from a mu- to delta-opioid receptor-mediated action as determined by differential effects of opioid receptor antagonists. The response to morphine remained mu. Heroin and 6 MAM responses involved delta1 (inhibited by 7-benzylidenenaltrexone) and delta2 (inhibited by naltriben) receptors, respectively. These delta-agonist actions did not synergize with the mu-agonist action of morphine in the diabetic mice. The expected synergism between the delta agonist, [D-Pen2-D-Pen5]enkephalin (DPDPE), and morphine was not obtained in diabetic mice. Thus, diabetes disrupted the purported mu/delta-coupled response. In nondiabetic CD-1 mice, heroin and 6 MAM produced a different mu-receptor response (not inhibited by naloxonazine) from that of morphine (inhibited by naloxonazine). Also, these mu actions, unlike that of morphine, did not synergize with DPDPE. The unique receptor actions and changes produced by streptozotocin suggest that extrinsic in addition to genetic factors influence the opioid receptor selectivity of heroin and 6 MAM.

Spinal delta opioid receptor subtype activity of 6-monoacetylmorphine in Swiss Webster mice.

Heroin and 6-monoacetylmorphine (6MAM) given intracerebroventricularly in Swiss Webster mice, act on supraspinal delta (delta) opioid receptors to produce antinociception in the tail flick test. More specifically, this action of heroin involves delta 1 and 6MAM involves delta 2 opioid receptors. Even though 6MAM given intrathecally (IT) in Swiss Webster mice also activates delta receptors to produce antinociception, the subtype of delta receptor in the spinal cord is not known. The present study addressed this question. First, in order to confirm the subtype selectivity of the delta opioid receptor antagonists in the spinal cord, 7-benzylidenenaltrexone (BNTX, a selective delta 1 receptor antagonist) and naltriben (a selective delta 2 receptor antagonist) were administered IT against the prototypic delta 1 and delta 2 peptide agonists [D-Pen2,5]enkephalin (DPDPE) and [D-Ser2,Leu5]enkephalin-Thr (DSLET), respectively. DPDPE-induced antinociception was inhibited by BNTX, but not naltriben. The opposite selectivity occurred for DSLET; naltriben, but not BNTX, administered IT inhibited IT DSLET-induced antinociception. Therefore, the antagonists differentiated between spinal delta 1 and delta 2 opioid receptor subtype agonist actions. This differentiation was further demonstrated by administration of the antagonists IT against the antinociceptive action of beta-endorphin given intracerebroventricularly. The antinociceptive action of beta-endorphin is due to spinal release of met-enkephalin which results in spinal delta 2 receptor activation. This antinociception was reduced by IT naltriben, but not BNTX, administration. The antagonists were then administered against IT 6MAM-induced antinociception. Neither BNTX nor naltriben given alone, each at twice the usual dose, altered IT 6MAM-induced antinociception. When the antagonists were administered together, each at the usual dose, the antinociceptive action of 6MAM was inhibited. Thus, even though a differentiation between spinal delta 1 and delta 2 opioid receptor activity can be obtained with naltriben and BNTX, blockade of the individual delta receptor subtypes does not appear to alter IT 6MAM antinociception. Therefore, these results suggest that 6MAM, given IT, is acting on a delta opioid receptor but this receptor in the spinal cord appears to be different from the delta 2 receptor on which 6MAM acts in the brain.

Supraspinal delta 2 opioid agonist analgesia in Swiss-Webster mice involves spinal GABAA receptors.

The tail-flick response is a spinal reflex that can be modulated by administration of antinociceptive agents supraspinally through activation of descending systems and involvement of the action of neurotransmitters in the spinal cord. Descending noradrenergic and serotonergic systems are involved in morphine (and other mu opioid receptor agonists)-induced antinociception. These descending systems, however, are not involved in supraspinal delta opioid receptor agonist-induced antinociception. Recently, a descending system mediated by spinal gamma-aminobutyric acid (GABA) A and B receptors has been demonstrated to be involved in the antinociceptive action of delta 1 opioid receptor agonists ([D-Pen2,5]enkephalin in ICR mice and [D-Pen2,5]enkephalin and heroin in Swiss-Webster mice). In the present study, the involvement of spinal GABAA receptors in the antinociceptive action of supraspinal delta 2 opioid receptor agonists, [D-Ser2]-Leu-enkephalin-Thr and 6-monoacetylmorphine, action was demonstrated. The intrathecal administration of GABAA receptor antagonists, bicuculline and picrotoxin, inhibited the antinociceptive action of both [D-Ser2]-Leu-enkephalin-Thr and 6-monoacetylmorphine given intracerebroventricularly. The intrathecal administration of 2-hydroxysaclofen, a GABAB receptor antagonist, had no effect. These studies suggest that supraspinal delta 2, like delta 1, opioid receptor action involves spinal GABAA receptors, but delta 2, unlike delta 1, action does not involve GABAB receptors. Thus, the supraspinal delta 1 agonist action (heroin, DPDPE) and the delta 2 agonist action (6MAM, DSLET) can be further differentiated by the selectivity of the spinal GABA receptors involved in Swiss-Webster mice.

Spinal GABA receptors mediate brain delta opioid analgesia in Swiss Webster mice.

Morphine and heroin act on supraspinal mu-opioid receptors in ICR mice to activate descending noradrenergic and serotonergic systems to inhibit the tail flick response. Antinociception induced by supraspinal [D-Pen2,5]-enkephalin (DPDPE, delta agonist) involves a descending system mediated by spinal gamma-aminobutyric acid, GABAA and GABAB, receptors. Because in Swiss Webster mice the receptor selectivity of heroin changes to delta whereas morphine remains mu, the purpose of the present study was to determine whether this delta action of heroin was mediated spinally by GABAA and GABAB receptors. Bicuculline (GABAA receptor antagonist) and picrotoxin (chloride ion channel blocker) given intrathecally produced rightward shifts in the dose-response curves of DPDPE and heroin given intracerebroventricularly. Thus, spinal GABAA receptors were involved. Intrathecal administration of 2-hydroxysaclofen (GABAB receptor antagonist) also shifted the dose-response curves to the right. Thus, the antinociception produced by heroin, like DPDPE, by activation of delta receptors in the brain of Swiss Webster mice involved both GABAA and the GABAB receptors in the spinal cord.

The heroin metabolite, 6-monoacetylmorphine, activates delta opioid receptors to produce antinociception in Swiss-Webster mice.

Heroin activates delta receptors, whereas morphine activates mu receptors, in the brain of Swiss-Webster mice, to produce antinociception. The present study determined the type of opioid receptor activated by 6-monoacetylmorphine (MAM), a metabolite of heroin. Intracerebroventricular MAM-induced inhibition of the tail-flick response was reduced by coadministration of naltrindole (a delta opioid receptor antagonist), suggesting that i.c.v. MAM, like i.c.v. heroin, acted on delta receptors. This delta receptor-mediated response was not affected by intrathecal (i.t.) administration of yohimbine and methysergide. Thus, the descending noradrenergic and serotonergic neuronal pathways, which are activated by i.c.v. morphine, were not involved in MAM antinociception. In the spinal cord, coadministration of naltrindole with MAM, i.t., decreased antinociception suggesting that MAM acted on spinal delta receptors. This finding is in contrast to i.t. heroin which acts on mu receptors in the spinal cord. These receptor selectivities were also demonstrated for systemically administered MAM and heroin. Thus, i.c.v. naltrindole inhibited both MAM- and heroin-induced antinociception, but i.t. naltrindole only inhibited the MAM response. The following results in ICR mice contrast with those above. Antinociception induced by i.c.v. MAM was decreased by coadministration of naloxone, but not naltrindole, suggesting that MAM, like morphine and heroin, acted on supraspinal mu receptors. Also, this MAM response was inhibited by i.t. administration of methysergide which is consistent with supraspinal mu receptor activation. Furthermore, i.t. MAM acted on spinal mu receptors because i.t. administration of naloxone, but not naltrindole, produced inhibition. In conclusion, the ability to ascribe delta receptor selectivity to the action of heroin and MAM in Swiss-Webster mice served to reinforce the concept that heroin and MAM act primarily on their own and not through formation of morphine. Further elucidation of the difference in heroin and MAM receptor selectivities between Swiss-Webster and ICR mice might contribute to a better understanding of opioid receptor mechanisms.

Elimination of the antianalgesic action of dynorphin A by spinal transsection in barbital-anesthetized mice.

Previous studies in mice demonstrate that, when dynorphin A (1-17) (Dyn A) is administered intrathecally (i.t.) or released spinally (by administration of clonidine or midazolam i.c.v.), i.t. morphine-induced analgesia was reduced. The present aim was to determine whether this antianalgesic action of Dyn A was the result of a spinal or supraspinal site of action by performing studies in spinally transsected mice. The approach was to use anesthetized, acute spinally transsected mice rather than chronic spinally transsected animals to avoid the need for long-term special animal care. The first part of the study evaluated four nonvolatile general anesthetic agents in an attempt to obtain one that did not affect the antianalgesic action of Dyn A, the release of Dyn A, the analgesic action of i.t. morphine (inhibition of the tail-flick response) or the tail-flick latency by itself. alpha-Chloralose (120 mg/kg), urethane (1 g/kg) and pentobarbital (20 or 40 mg/kg) given i.p. did not meet one or more of these requirements. Barbital 400 mg/kg i.p. met the requirements. In the second part of the study, barbital combined with halothane anesthesia was used to perform acute spinal transsection experiments. As in sham controls, the analgesic action of i.t. morphine was undiminished in spinally transsected animals, which indicated that the inhibition of the tail-flick response produced by i.t. morphine was on a spinal reflex response. On the other hand, spinal transsection produced a loss of the action of i.t. Dyn A to antagonize i.t. morphine-induced analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Supraspinal delta receptor subtype activity of heroin and 6-monoacetylmorphine in Swiss Webster mice.

The purpose of this study was to determine which delta (delta) opioid receptor subtype, delta 1 or delta 2, was involved in producing the antinociceptive action of heroin and 6-monacetylmorphine (MAM) in Swiss Webster mice. Previous work from this laboratory established that heroin and MAM, given intracerebroventricularly (i.c.v.) in Swiss Webster mice, produce antinociception through activation of supraspinal delta receptors. Naltrindole, but not naloxone or nor-binaltorphimine, antagonizes the inhibitory action of heroin and MAM in the tail-flick test. Recent literature documents the occurrence of subtypes of the delta opioid receptor and the availability of selective antagonists. 7-Benzylidenenaltrexone (BNTX) antagonizes the antinociception induced by delta 1 receptor agonists without affecting that induced by delta 2 receptor agonists. Naltriben (NTB) selectively inhibits delta 2- but not delta 1-induced antinociception. In the present study BNTX and NTB were administered i.c.v. with heroin and MAM to determine the delta receptor subtype responsible for inhibition of the tail-flick response in Swiss Webster mice. The ED50 for heroin-induced antinociception was increased 19-fold by BNTX and was not altered by NTB administration. On the other hand, the ED50 value of MAM was increased 3-fold by NTB and was not altered by BNTX administration. These results suggest that heroin activated supraspinal delta 1 receptors and MAM acted on supraspinal delta 2 receptors to produce antinociception in Swiss Webster mice.

Dynorphin A(1-17) mediates midazolam antagonism of morphine antinociception in mice.

Studies have shown that midazolam acts in the brain to antagonize the antinociception produced by morphine. The purpose of this study was to determine if spinal dynorphin A(1-17) (Dyn) was involved in the antagonistic effects of midazolam. A number of drugs when administered intracerebroventricularly (ICV) to mice release Dyn in the spinal cord to antagonize morphine-induced antinociception. In the present study using the mouse tail-flick test, midazolam administered ICV produced a dose related reduction of the antinociception induced by morphine given intrathecally (IT). The antagonistic action of midazolam against morphine-induced antinociception involved the release of Dyn in the spinal cord, as evidenced by the following results. 1) Administration of naloxone, nor-binaltorphimine and dynorphin antiserum, IT, eliminated the antagonistic effect of midazolam, given ICV, against morphine. Treatment with these opioid antagonists and dynorphin antiserum is known to inhibit the action of spinally released Dyn. 2) Production of desensitization to the effect of spinal Dyn by pretreating with morphine, 10 mg/kg subcutaneously 3 h before the tail-flick test, abolished the antagonistic action of midazolam given ICV. A 3-h pretreatment with midazolam, ICV, also produced desensitization to the antianalgesic action of Dyn given IT. 3) Elimination of the Dyn component of action of midazolam by administration of naloxone, nor-binaltorphimine and dynorphin antiserum, IT, uncovered slight antinociceptive activity of midazolam, given ICV. Coadministration of flumazenil (a benzodiazepine antagonist), bicuculline (a GABA antagonist) and picrotoxin (a chloride ion channel blocker) inhibited the midazolam effect.(ABSTRACT TRUNCATED AT 250 WORDS)