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

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

Effect of repeated administration of TRK-820, a kappa-opioid receptor agonist, on tolerance to its antinociceptive and sedative actions

Repeated administration of micro-opioid receptor agonist, morphine induces tolerance not only to the antinociceptive effect but also to other pharmacological effects, resulting in shortened working duration and decreased efficacy. But less is known about kappa-opioid agonist-induced tolerance. The tolerance-development potency of kappa-opioid receptor agonists with a focus on TRK-820 was characterized. After five administrations of kappa-opioid receptor agonists, TRK-820 (0.1-0.8 mg/kg), U-50,488H (10-80 mg/kg) and ICI-199,441 (0.025-0.2 mg/kg) subcutaneously over 3 days, tolerance to the antinociceptive effects, assessed by an acetic acid-induced abdominal constriction test, developed in a repeated dose-dependent manner. The tolerance-development potency of TRK-820 was the least among these kappa-opioid receptor agonists. Similarly, TRK-820 and U-50,488H induced tolerance to their sedative effects as judged by a wheel-running test in mice. Greater tolerance was developed to the sedative effect than to the antinociceptive effect in both compounds. After repeated administration, the number of kappa-opioid receptors in the mouse brain was reduced by U-50,488H (80 mg/kg) but not by TRK-820 (0.4 mg/kg). There was no change of the affinity by the treatment with both compounds. These results demonstrated that the kappa-opioid receptor agonists developed tolerance both to the antinociceptive and the sedative effects, though the tolerance to the sedative effect developed more readily than tolerance to the antinociceptive effect. The difference in the potency for down-regulating the kappa-opioid receptors in the brain may account for the tolerance-development potency of the compounds.     

Streptozotocin-induced diabetes selectively alters the potency of analgesia produced by mu-opioid agonists, but not by delta- and kappa-opioid agonists.

To investigate the possible mechanisms of the alterations in morphine-induced analgesia observed in diabetic mice, we examined the influence of streptozotocin-induced (STZ-induced) diabetes on analgesia mediated by the different opioid receptors. The antinociceptive potency of morphine (10 mg/kg), administered s.c., as determined by both the tail-pinch and the tail-flick test, was significantly reduced in diabetic mice as compared to that in controls. Mice with STZ-induced diabetes had significantly decreased sensitivity to intracerebroventricularly (i.c.v.) administered mu-opioid agonists, such as morphine (10 micrograms) and [D-Ala2,N-Me Phe4,Gly-ol5]enkephalin (DAMGO, 0.5 micrograms). However, i.c.v. administration of [D-Pen2,5]enkephalin (DPDPE, 5 micrograms), a delta-opioid agonist, and U-50,488H (50 micrograms), a kappa-opioid agonist, produced pronounced antinociception in both control and diabetic mice. Furthermore, there were no significant differences in antinociceptive potency between diabetic and control mice when morphine (1 microgram), DAMGO (10 micrograms), DPDPE (0.5 micrograms) or U-50,488H (50 micrograms) was administered intrathecally. In conclusion, mice with STZ-induced diabetes are selectively hyporesponsive to supraspinal mu-opioid receptor-mediated antinociception, but they are normally responsive to activation of delta- and kappa-opioid receptors.     

kappa-Opioid receptors in the substantia nigra pars reticulata mediate the U-50,488-induced locomotor activity of preweanling rats

The purpose of the present study was to determine the neuroanatomical location where kappa-opioid receptor stimulation induces locomotor activity in the preweanling rat. To confirm that the U-50,488-induced locomotor activity of preweanling rats is mediated by kappa-opioid receptors, 18-day-old rats were initially injected with vehicle or the kappa-opioid receptor agonist U-50,488 (5 mg/kg, s.c.) followed, 15 min later, by an injection of the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI; 0, 2, 4, 8, or 12 mg/kg, s.c.). In subsequent experiments, 18-day-old rats were injected with vehicle or U-50,488 (5 mg/kg, s.c.) 15 min prior to bilateral administration (0.25 or 0.5 microl per side) of nor-BNI (0, 5, 10, or 20 microg) into the substantia nigra pars reticulata (SNR) or medial dorsal striatum (MDS). In the final experiment, 18-day-old rats received bilateral administration (0.25 microl per side) of vehicle or U-50,488 (0.0, 0.8, 1.6, or 3.2 microg) into the SNR. Results showed that systemically administered nor-BNI (0-12 mg/kg, s.c.) produced a dose-dependent reduction in the U-50, 488-induced locomotor activity of preweanling rats. The site of action for U-50,488's locomotor-activating effects appeared to be the SNR, because (a) bilateral administration of nor-BNI (5, 10, or 20 microg) into the SNR caused a complete attenuation of U-50, 488-induced locomotion, and (b) bilateral administration of U-50,488 into the SNR caused a dose-dependent increase in the locomotor activity of preweanling rats. Striatal injections of nor-BNI did not affect U-50,488-induced locomotor activity. When these findings are considered together it is apparent that stimulation of kappa-opioid receptors in the SNR is both necessary and sufficient for the occurrence of U-50,488-induced locomotor activity in the preweanling rat.     

Comparison of G-protein activation in the brain by mu-, delta-, and kappa-opioid receptor agonists in mu-opioid receptor knockout mice

Mice lacking the mu-opioid receptor gene have been developed by a gene knockout procedure. In this study, the activity of opioid receptor coupled G-proteins was examined to investigate whether there is a change in the extent of coupling for mu, delta-, and kappa-opioid receptors in mu-opioid receptor knockout mice. Selective agonists of mu- (DAMGO), delta- (DPDPE), and kappa- (U-69,593) opioid receptors stimulated [(35)S]GTPgammaS binding in the caudate putamen and cortex of wild-type mice. In contrast, only U-69,593 stimulated [(35)S]GTPgammaS binding in these regions of mu-opioid receptor knockout mice. These results confirmed the absence of G-protein activation by a mu-opioid receptor agonist in mu-opioid receptor knockout mice, and demonstrated that coupling of the kappa-opioid receptor to G-proteins is preserved in these mice. However, G-protein activation by the delta-opioid receptor agonist, DPDPE, was reduced in the mu-opioid receptor knockout mice, at least in the brain regions studied using autoradiography.     

Characterization of mechanical withdrawal responses and effects of mu-, delta- and kappa-opioid agonists in normal and mu-opioid receptor knockout mice

Clinical and experimental observations suggest that opiates can exert different influences on the perception of stimuli from distinct sensory modalities. Thermally-induced nociception is classically responsive to opiate agonists. mu-Opioid receptor-deficient transgenic mice are more sensitive to thermal nociceptive stimuli and morphine fails to attenuate the nociceptive responses to thermal stimuli in these animals. To enhance our understanding of opiate influences on mechanical sensitivity, we have examined withdrawal responses to a sequence of ascending forces of mechanical stimuli in mice with normal (wild type), half-normal (heterozygous) and absent (homozygous) mu-opioid receptor levels. We report data from mice examined without drug pretreatment or following pretreatment with morphine, the selective kappa-opioid agonist, U50488H, and the selective delta-opioid agonist, DPDPE. Saline-pretreated mice of each genotype displayed similar, monotonically increasing frequency of withdrawal responses to the graded stimuli. Subcutaneously administered morphine produced a dose-dependent reduction in withdrawal responses in wild type and heterozygous mice, but had no significant effect in homozygous mice. Intraventricular administration of DPDPE also reduced the frequency of paw withdrawal (FPW) in wild type mice, but not in homozygous mice. In contrast, systemic U50488H produced a dose-dependent attenuation of paw withdrawal in both wild type and homozygous mice. These findings suggest that (1) interactions of endogenous peptides with mu-opioid receptors may not play a significant role in the response to mechanical stimuli in drug-free animals, and (2) deficiency of mu-opioid receptors has no functional consequence on the response to the prototypical kappa-opioid receptor agonist, but decreases responses to the prototypical mu- and delta-opioid receptor agonists.     

Direct evidence for the up-regulation of spinal micro-opioid receptor function after repeated stimulation of kappa-opioid receptors in the mouse

The present study was designed to investigate the possible change in spinal micro -opioid receptor function after repeated administration of a selective kappa-opioid receptor agonist (1S-trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzeneacetamide hydrochloride [(-)U-50,488H] in the ICR mouse. A single s.c. or i.t. injection of (-)U-50,488H produced a dose-dependent antinociception. Repeated s.c. or i.t. administration of (-)U-50,488H resulted in the development of tolerance to (-)U-50,488H-induced antinociception. Under these conditions, we demonstrated here that repeated s.c. injection of (-)U-50,488H significantly enhanced the antinociceptive effect induced by the i.t. administration of a selective micro -opioid receptor agonist [d-Ala2,N-Me-Phe4,Gly5-ol] enkephalin (DAMGO). Using the guanosine-5'-o-(3-[35S]thio) triphosphate ([35S]GTPgammaS) binding assay, we found that (-)U-50,488H was able to produce a dose-dependent increase in [35S]GTPgammaS binding to membranes of the mouse spinal cord. Repeated administration of (-)U-50,488H caused a significant reduction in the (-)U-50,488H-stimulated [35S]GTPgammaS binding in this region, whereas repeated treatment with (-)U-50,488H exhibited an increase in the DAMGO-stimulated [35S]GTPgammaS binding in membranes of the spinal cord. Using a receptor binding assay, repeated treatment with (-)U-50,488H significantly increased the density of [3H]DAMGO binding sites in membranes of the mouse spinal cord. In contrast, the expression of micro -opioid receptor was not affected after repeated treatment with (-)U-50,488H. These results suggest that repeated stimulation of kappa-opioid receptors leads to the up-regulation of micro -opioid receptor functions in the spinal cord, which may be associated with an increase in the number of functional micro -opioid receptors in the mouse spinal cord.     

Involvement of kappa-opioid receptors and sigma receptors in memory function demonstrated using an antisense strategy

Although antinociceptive effects of U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] cyclohexyl) benzeneacetamide methanesulfonate and (-)-pentazocine have been reported to influence kappa-opioid receptors, the involvement of kappa-opioid receptors in learning and/or memory is still controversial. We have recently reported that the memory improving effect of (-)-pentazocine was antagonized by sigma1 receptor antagonist. In this study, we examined the effects of several antisense oligodeoxynucleotides (antisenses) to kappa1-opioid receptors and sigma1 receptor on memory and nociceptive function. Male ddY mice were treated subcutaneously (s.c.) with scopolamine (1.65 mumol/kg) and/or test drugs 30 min before a Y-maze test. U-50,488H significantly improved the scopolamine-induced impairment of spontaneous alternation behavior. Twenty micrograms of antisense targeting exons 2 and 3 of the kappa1-opioid receptor significantly reversed the effects of U-50,488H, but antisense targeting exon 1 and mismatch sense did not. The antisense targeting exon 3 was most effective. These antisenses themselves did not affect normal mice, indicating that kappa1-opioid receptors do not tonically regulate memory function. All three antisenses equally prevented U-50,488H-induced antinociceptive effects in the acetic-acid-induced writhing test. Pretreatment with antisense targeting sigma1 receptors (AS-sigma1) completely prevented the memory-improving effects of (-)- and (+)-pentazocine, although U-50,488H ameliorated the scopolamine-induced impairment of spontaneous alternation behavior in AS-sigma1-treated mice. These results suggest that kappa1-opioid receptors containing different exons have a distinct function in memory and nociceptive functions. Furthermore, kappa-opioid receptors agonist showing analgesic effects act on kappa-opioid receptors or sigma receptors and play important roles only when memory function is impaired, but the two neuronal systems regulate memory function independently.     

Central effect of mu-opioid agonists on antral motility in conscious rats

Centrally applied opioids delay gastric emptying and inhibit intestinal transit. However, the mechanism of inhibitory effects of central opioids on gastric motility still remains unclear. It also remains unclear which opioid receptor (mu, delta, and kappa) stimulation affects gastric motility. We studied the central effect of opioids on antral motility in conscious rats. A strain gauge transducer was implanted on the gastric antrum to record the circular muscle contractions. The area under the curve of the antral motility, calculated as a motility index, was evaluated before and after the intracerebroventricular (icv) injection of various opioid agonists in each rat. [D-Ala2, N-Me-Phe4, Gly5-ol] enkephalin (DAMGO, 0.1-10 nmol), a mu-opioid selective agonist, significantly inhibited antral motility in a dose-dependent manner (n=4). The motility index was significantly decreased to 47.3+/-10.8% (n=4) of controls at 20 min after icv injection of DAMGO (1.0 nmol). In contrast, [D-pen2, L-Pen5] enkephalin (DADLE, 1.0 nmol), a delta-opioid selective agonist, and U50,488 (1.0 nmol), a kappa-opioid selective agonist, had no significant effects on antral motility. Pretreatment with subcutaneous guanethidine (5 mg/kg) and propranolol (1 mg/kg), but not phentolamine (1 mg/kg), significantly antagonized the inhibitory effect of DAMGO (1.0 nmol). Reduced motility index induced by DAMGO (1.0 nmol) was restored from 48.7+/-3.5% to 88.6+/-10.9% (n=5) and 80.4+/-2.2% (n=5) by guanethidine and propranolol, respectively. Our findings suggest that central mu-opioid receptor has major inhibitory effects on antral motility in conscious rats. The inhibitory effects of mu-opioid receptors are mediated via sympathetic pathways and beta-adrenoceptors.     

Mu and delta opioid receptor regulation of pro-opiomelanocortin peptide secretion from the rat neurointermediate pituitary in vitro

We investigated the ability of selective opioid agonists and antagonists to influence pro-opiomelanocortin peptide secretion from the rat neurointermediate lobe in vitro. The mu-opioid agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]enkephalin) significantly stimulated beta-endorphin and alpha-melanocyte-stimulating hormone release relative to controls early (30 min) in the incubation period. Similar effects on beta-endorphin secretion were observed with the selective mu-opioid agonist dermorphin. The delta-opioid receptor agonist DPDPE ([D-Pen(2,5)]enkephalin) weakly inhibited beta-endorphin secretion relative to controls while the kappa-opioid receptor agonist U50488 had no effect. The mu-opioid selective antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2)) inhibited basal beta-endorphin secretion while kappa- and delta-opioid receptor antagonists had no effect. Our data support a role for local mu-opioid receptor control of intermediate lobe pro-opiomelanocortin peptide secretion. Peptide secretion from melanotropes appears to be tonically stimulated by activation of mu-opioid receptors in the absence of intact neuronal innervation to the intermediate lobe.     

Kappa- and delta-opioids block sympathetically dependent hyperalgesia

Direct hyperalgesia induced by prostaglandin E2 (PGE2) can be blocked by mu- but not delta- or kappa-opioids. However, there is evidence that kappa- and delta-opioid receptors are located on sympathetic postganglionic neuron (SPGN) terminals, which mediate bradykinin (BK) hyperalgesia via SPGN-terminal-dependent production of PGE2. Therefore, we evaluated the antinociceptive effect of delta- and kappa-opioids on BK hyperalgesia. We demonstrate that the mechanical hyperalgesia induced by intradermal injection of BK can be blocked by the kappa-opioid agonist trans-3,4-dichloro-N-methyl-N[2-(-pyrrolidinyl)cyclo-hexyl] benzeneacetamide (U50,488H) and by the delta-opioid agonist (D-Pen2,5)-enkephalin (DPDPE), as well as the mu-opioid agonist Tyr-D-Ala-Gly-NMe-Phe-Gly-ol(DAMGO). Pertussis toxin prevented the inhibition of BK-induced hyperalgesia by U50,488H, DPDPE, or DAMGO. We conclude that the observed peripheral analgesic effects of kappa- and delta-opioid agonists result from actions upon SPGN terminals and that these effects are mediated by inhibitory G-proteins.     

The anxiolytic effect of acute ethanol or diazepam exposure is unaltered in mu-opioid receptor knockout mice

Previous researchers demonstrate an opioidergic involvement in the anxiolytic and rewarding actions of ethanol and diazepam. Therefore, to further characterize the role of the opioid system in the anxiolytic action of ethanol and diazepam, normal (C57BL/6J), hybrid (B6129F1) and mu-opioid receptor knockout mice were given i.p. ethanol (0, 1.0 or 1.6 g/kg) or diazepam (1.5 mg/kg). The anxiolytic properties of these agents were then tested in the elevated plus-maze. Additional ethanol-treated mu-opioid receptor knockout mice (1 g/kg) were pretreated with the kappa-opioid receptor antagonist nor-BNI (0 or 3 mg/kg) to assess the involvement of kappa-opioid activity in ethanol's anxiolytic actions. The anxiolytic action of ethanol and diazepam in the mu-opioid receptor knockout mouse did not differ from the effects obtained in normal mice and pretreatment with nor-BNI did not significantly attenuate ethanol's actions in mu-opioid receptor knockout mice. Thus, the anxiolytic actions of ethanol and diazepam appear to be independent of opioid system activity in the mu-opioid receptor knockout mouse.     

Multi-dimensional analyses of behavior in mice treated with U-50,488H, a purported kappa (non-mu) opioid agonist

The effects of U-50, 488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl ]-benzeneacetamide, methane sulfonate, hydrate), a purported selective kappa (non-mu) opioid agonist on spontaneous locomotor activity were investigated using a multi-dimensional behavioral analyzer (Animex II). U-50,488H (1 mg/kg) failed to affect behavior in mice, however, 3 mg/kg significantly reduced rearing and grooming. In addition, 10 mg/kg markedly reduced linear locomotion, rearing and grooming. The behavioral depression induced by U-50,488H (10 mg/kg) was reversible by the opioid antagonist Mr2266 (10 mg/kg). These results suggest that the selective activation of the kappa (non-mu) opioid receptor by U-50,488H decreases linear locomotion, rearing and grooming in mice.     

Involvement of endogenous opioid systems in nociceptin-induced spinal antinociception in rats

The present study investigates the involvement of opioid receptors in the antinociceptive effects of nociceptin in the spinal cord of the rat. Intrathecal administrations of 5 and 10 nmol of nociceptin significantly increase the withdraw response latencies to noxious thermal and mechanical stimulations. This nociceptin-induced antinociceptive effect is significantly attenuated by intrathecal injection of (Nphe(1))nociceptin(1-13)-NH(2), a selective antagonist of the nociceptin receptor (opioid receptor-like receptor ORL1), indicating an ORL1 receptor-mediated mechanism. This antinociceptive effect is also significantly attenuated by intrathecal injections of naloxone (a nonselective opioid receptor antagonist), naltrindole (a selective delta-opioid receptor antagonist), and beta-funaltrexamine (a selective mu-opioid receptor antagonist) in a dose-dependent manner, but not by the selective kappa-opioid receptor antagonist norbinaltorphimine. Since it is unlikely that nociceptin acts by direct binding to opioid receptors, these results suggest a possible interaction between the nociceptin/ORL1 and opioid systems in the dorsal horn of the rat spinal cord.     

Inhibition of spinal opioid analgesia by supraspinal administration of selective opioid antagonists

The effect of intracerebroventricular administration of a selective mu- (CTOP) or delta- (ICI 174,864) opioid receptor antagonist on the antinociceptive effects produced by intrathecal administration of selective mu- (DAMGO), delta- (DPDPE) and kappa- (U50-488H) opioid receptor agonists was evaluated using the Randall-Selitto paw-withdrawal test, in the rat. While the intracerebroventricular administration of CTOP or ICI 174,864, alone, had no effect on nociceptive thresholds, intracerebroventricular administration of CTOP and ICI 174,864 produced marked antagonism of the antinociceptive effects of intrathecal DAMGO. The antinociceptive effects of intrathecal administration of DPDPE or U50,488H were not antagonized by intracerebroventricular administration of CTOP or ICI 174,864. These data suggest that, in the rat, along with the established descending antinociceptive pathways, there is an ascending antinociceptive control mechanism projecting from the spinal cord to the brainstem. The ascending antinociceptive control involves mu- and delta-opioid agonism at supraspinal sites and appears to be mediated selectively by mu-, but not by delta- or kappa-opioid agonism at the spinal level.     

Place preference conditioning reveals the involvement of D1-dopamine receptors in the motivational properties of mu- and kappa-opioid agonists

The role of D1 dopamine receptors in mediating the motivational properties of opioid agonists was investigated by use of place preference conditioning. Administration of the D1 receptor antagonist SCH 23390 (0.001-0.5 mg/kg) or the kappa-opioid receptor agonist U-69593 (0.16 mg/kg) produced conditioned place aversions. In contrast, the mu-opioid agonist, morphine (3.0 mg/kg) was appetitively reinforcing. Chronic infusion of SCH 23390 (1.0 mg/kg/day) during conditioning abolished the effects of both opioid agonists. These data demonstrate the specific involvement of D1 receptors in the motivational properties of mu- and kappa-opioid agonists and suggest that dopaminergic systems are crucial for the expression of both reinforcing and aversive motivational states.     

Neuroprotective effects of hibernation-regulating substances against low-temperature-induced cell death in cultured hamster hippocampal neurons

The neuroprotective effects of hibernation-regulating substances (HRS) such as adenosine (ADO), opioids, histamine and thyrotropin-releasing hormone (TRH) on low-temperature-induced cell death (LTCD) were examined using primary cultured hamster hippocampal neurons. LTCD was induced when cultures were maintained at <22 degrees C for 7 days. ADO (10-100 microM) protected cultured neurons from LTCD in a dose-dependent manner. The neuroprotective effects of ADO were reversed by both 8-cyclopenthyltheophilline (CPT; A(1) receptor antagonist) and 3,7-dimethyl-1-propargylxanthine (DMPX; A(2) receptor antagonist). Morphine (a non-selective opioid receptor agonist) was also effective in attenuating LTCD at an in vitro dose range of 10-100 muM. The neuroprotective effects of morphine were antagonized by naloxone (a non-selective opioid receptor antagonist). In addition, although [D-Ala(2), N-Me-Phe(4), Gly-ol(5)]-enkephalin (DAMGO; mu-opioid receptor agonist), [D-Pen(2,5)]-enkephalin (DPDPE; delta-opioid receptor agonist) and U-69593 (kappa-opioid receptor agonist) were also effective, LTCD of cultured hippocampal neurons was not affected by TRH. Furthermore, histamine produced hypothermia in Syrian hamsters and protected hippocampal neurons in vitro at 100 microM. The neuroprotective effect of histamine was reversed by pyrilamine (H(1) receptor antagonist). Apoptosis was probably involved in LTCD. These results suggest that ADO protected hippocampal neurons in vitro via its agonistic actions on both A(1) and A(2) receptors, whereas morphine probably elicited its neuroprotective effects via agonistic effects on the mu-, delta- and kappa-opioid receptors. In addition, histamine also protected hippocampal neurons via its agonistic action on the H(1) receptor. Thus, HRS-like adenosine-, opioid- and histamine-like hypothermic actions would most likely induce neuroprotective effects against LTCD in vitro.     

Effect of dizocilpine (MK-801) on analgesia and tolerance induced by U-50,488H, a κ-opioid receptor agonist, in the mouse

The effect of dizocilpine (MK-801), anN-methyl-D-aspartate (NMDA) receptor antagonist, on the analgesic response to U-50,488H, a κ-opioid receptor agonist, and tolerance to the analgesic effect of U-50,488H was determined in mice. The doses of MK-801 used were 0.03–0.30 mg/kg, whereas U-50,488H was administered at a dose of 25 mg/kg. Intraperitoneal (i.p.) administration of U-50,488H (25 mg/kg) produced analgesia as evidenced by the delay in the tail-flick latency in the mouse and lasted for a period of 240 min. MK-801 (0.03–0.30 mg/kg, i.p.) given 30 min prior to the injection of U-50,488H did not modify U-50,488H-induced analgesia. Twice daily administration of U-50,488H (25 mg/kg) for 9 days produced tolerance to its analgesic action. Administration of MK-801 (0.03 and 0.10 mg/kg) injected 30 min before each injection of U-50,488H prevented the development of tolerance to its analgesic effect. The higher dose, 0.3 mg/kg, of MK-801 had a minimal effect on U-50,488H tolerance. It is concluded that MK-801 in doses which do not affect U-50,488H-induced analgesia blocks the development of tolerance to its analgesic action in mice. These studies suggest that NMDA receptors play a crucial role in the development of tolerance to κ-opioid agonist in mice.     

Opioids induce convulsions and wet dog shakes in rats: mediation by hippocampal mu, but not delta or kappa opioid receptors

The opioid receptor subtypes and brain regions involved in eliciting convulsions and wet dog shakes (WDS) were studied by testing different opioid receptor selective agonists in unanesthetized rats. Selective mu agonists, [NMe-Phe3-D-Pro4]-morphiceptin (PL017) and [D-Ala2-N-methyl-Phe3-Gly5-ol]-enkephalin, induced convulsions and WDS when unilaterally injected into the ventral hippocampus. [D-Ala2,D-Leu5]-enkephalin (DADLE), a mixed mu and delta agonist, also elicited such behavioral changes, but its effect was less potent than the selective mu agonists. DADLE-induced WDS were dose dependent, and both convulsions and WDS were antagonized by the irreversible mu receptor antagonist, beta-funaltrexamine, but not by the selective delta receptor antagonist, ICI-174,864. Treatment with the selective delta agonist [D-Pen2,5]-enkephalin or the selective kappa agonists U-50,488H, dynorphin-A amide, or dynorphin-A(1-8) did not produce convulsions or WDS. The injection of a high dose of PL017 intraventricularly or into other brain regions such as the dorsal hippocampus, frontal cortex, striatum, and amygdala did not produce convulsions or WDS, therefore suggesting the ventral hippocampus is an important site for the expression of opioid-induced convulsions and WDS. These results suggest that opioid-induced convulsions and WDS are mediated exclusively by mu but not delta or kappa opioid receptors in the ventral hippocampus.     

Nonopioid receptor-mediated effects of U-50,488H on [Ca(2+)]i and extracellular dopamine in PC12 cells

The present studies were carried out to determine the effects of a kappa-opioid receptor agonist on cytosolic Ca(2+) concentration, [Ca(2+)](i), and extracellular dopamine in undifferentiated PC12 cells. The kappa-opioid receptor agonist U-50,488H caused concentration-dependent increases in [Ca(2+)](i) and extracellular dopamine. Neither effect was blocked by the selective kappa-opioid receptor antagonist nor-binaltorphimine. Increases in extracellular dopamine content and [Ca(2+)](i) caused by U-50,488H were correlated positively in the presence of extracellular Ca(2+); however, reduction of extracellular Ca(2+) abolished the increase in [Ca(2+)](i), but not that in dopamine. The latter observation suggests that stimulation of exocytotic release is not the primary mechanism involved in the increase in extracellular dopamine caused by U-50,488H. Effects on dopamine synthesis or catabolism also seem unlikely because the enhancement of extracellular dopamine occurred rapidly, and the amount of a major metabolite of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), was not affected. In any event, neither the increase in [Ca(2+)](i) nor the increase in extracellular dopamine caused by U-50,488H is mediated by the kappa-opioid receptor.