Activation Of Κ-opioid Receptors Research Articles

The selective κ-opioid receptor agonist U50,488H attenuates voluntary ethanol intake in the rat

Non-selective opioid receptor antagonists are increasingly used in the treatment of alcohol dependence. The clinical effects are significant but the effect size is rather small and unpleasant side effects may limit the benefits of the compounds. Ligands acting at μ- and/or δ- receptors can alter the voluntary intake of ethanol in various animal models. Therefore, the attenuating effects of selective opioid receptor ligands on ethanol intake may be of clinical interest in the treatment of alcoholism. The objective of this study was to examine the effects of a selective κ-receptor agonist, U50,488H on voluntary ethanol intake in the rat. We used a restricted access model with a free choice between an ethanol solution (10% v/v) and water. During the 3-days baseline period, the rats received a daily saline injection (1 ml/kg, i.p.) 15 min before the 2 h access to ethanol. The animals had free access to water at all times. The control group received a daily saline injection during the 4-days treatment-period, whereas the treatment groups received a daily dose of U50,488H (2.5, 5.0 or 10 mg/kg per day). Animals treated with U50,488H dose-dependently decreased their ethanol intake. The effect of the highest dose of U50,488H was reduced by pre-treatment with the selective κ-antagonist nor-binaltorphimine (nor-BNI). These results demonstrate that activation of κ-opioid receptors can attenuate voluntary ethanol intake in the rat, and the data suggest that the brain dynorphin/κ-receptor systems may represent a novel target for pharmacotherapy in the treatment of alcohol dependence.

Cyclic AMP regulates the calcium transients released from IP3-sensitive stores by activation of rat κ-opioid receptors expressed in CHO cells

We analyzed intracellular Ca2+and cAMP levels in Chinese hamster ovary cells expressing a cloned rat kappa opioid receptor (CHO-κ cells). Although expression of kappa (κ)-opioid receptors was confirmed with a fluorescent dynorphin analog in almost all CHO-κ cells, the κ-specific agonists, U50488H or U69593, induced a Ca2+transient only in 35% of the cells. The Ca2+response occurred in all-or-none fashion and the half-maximal dosage of U50488H (812.1nM) was higher than that (3.2nM) to inhibit forskolin-stimulated cAMP. The κ-receptors coupled to Gi/oproteins since pertussis toxin significantly reduced the U50488H actions on intracellular Ca2+and cAMP. The Ca2+transient originates from IP3-sensitive internal stores since the Ca2+response was blocked by a PLC inhibitor (U73122) or by thapsigargin depletion of internal stores while removal of extracellular Ca2+had no effect. Interestingly, application of dibutyryl cAMP (+ 56.2%) or 8-bromo-cAMP (+ 174.7%) significantly increased the occurrence of U50488H-induced Ca2+mobilization while protein kinase A (PKA) inhibitors, Rp-cAMP (−32.3%) or myr-ψ PKA (−73.9%) significantly reduced the response. Therefore, it was concluded that cAMP and PKA activity can regulate the Ca2+mobilization. These results suggest that the κ receptor-linked cAMP cascade regulates the occurrence of κ-opioid-mediated Ca2+mobilization.

κ-Opioid Receptor Activation Modifies Dopamine Uptake in the Nucleus Accumbens and Opposes the Effects of Cocaine

Coadministration of kappa-opioid receptor agonists (kappa-agonists) with cocaine prevents alterations in dialysate dopamine (DA) concentration in the nucleus accumbens (Acb) that occur during abstinence from repeated cocaine treatment. Quantitative microdialysis was used to determine the mechanism producing these effects. Rats were injected with cocaine (20 mg/kg, i.p.), or saline, and the selective kappa-agonist U-69593 (0.32 mg/kg, s.c.), or vehicle, once daily for 5 d. Extracellular DA concentration (DA(ext)) and extraction fraction (E(d)), an indirect measure of DA uptake, were determined 3 d later. Repeated cocaine treatment increased E(d), whereas repeated U-69593 treatment decreased E(d), relative to controls. Coadministration of both drugs yielded intermediate E(d) values not different from controls. In vitro DA uptake assays confirmed that repeated U-69593 treatment produces a dose-related, region-specific decrease in DA uptake and showed that acute U-69593 administration increases DA uptake in a nor-binaltorphimine reversible manner. Repeated U-69593 also led to a decrease in [(125)I]RTI-55 binding to the DA transporter (DAT), but did not decrease total DAT protein. These results demonstrate that kappa-opioid receptor activation modulates DA uptake in the Acb in a manner opposite to that of cocaine: repeated U-69593 administration decreases the basal rate of DA uptake, and acute U-69593 administration transiently increases DA uptake. kappa-agonist treatment also alters DAT function. The action of kappa-agonists on DA uptake or DAT binding, or both, may be the mechanism(s) mediating the previously reported "cocaine-antagonist" effect of kappa-opioid receptor agonists.

Kappa-opioid receptor modulation of nicotine-induced behaviour

The ability of κ-opioid receptor ligands to modulate dependence-related behavioural effects of drugs like morphine and cocaine is well documented. The present study examined the effects of κ-opioid agonists on nicotine-induced locomotor stimulation in rats chronically pre-exposed to nicotine (0.4 mg/kg/day). U50,488 [0.5–3 mg/kg subcutaneously (s.c.)], U69,593 [0.08–0.32 mg/kg intraperitoneally (i.p.)] and CI-977 (0.005–0.02 mg/kg s.c.) administered 30 min prior to nicotine (0.06, 0.2 and 0.4 mg/kg s.c.) dose-dependently antagonised its acute locomotor-activating effect, which was completely prevented by the highest tested dose of each agonist. Baseline activity was unaffected by the largest doses of U50,488 and U69,593, but it was reduced by 0.01 and 0.02 mg/kg of CI-977. The selective κ-opioid receptor antagonist nor-BNI [30 μg intracerebroventricularly (i.c.v.)] blocked the effects of U69,593 on nicotine-induced behaviour, thus supporting the involvement of κ-opioid receptors in this effect. In conclusion, the activation of κ-opioid receptors clearly prevented nicotine-induced locomotor stimulation. The effects of at least two of the κ-opioid agonists were not due to a general motor suppression. It is suggested that the mechanism entails a depression of nicotine-induced increases in accumbal dopamine by these compounds. The results should encourage further research on the role of the κ-opioid system in the behavioural and neurochemical effects of nicotine, including those related to nicotine dependence.

Blockade of μ- and activation of κ-opioid receptors in the dorsal periaqueductal gray matter produce defensive behavior in rats tested in the elevated plus-maze

We studied the effects of morphine injected into the dorsal periaqueductal gray using conventional and novel ethological measures of the behavior of rats submitted to the elevated plus-maze test. Morphine (20 and 40 nmol) applied into the dorsal periaqueductal gray produced dose dependent aversive effects with reduced entries and time spent in the open arms. Freezing behavior was the most prominent novel ethological measure produced by microinjections of these doses of morphine. These pro-aversive effects were not inhibited by previous dorsal periaqueductal gray microinjection of [ d-Phe-Cys-Tyr- d-Trp-Orn-Thr-Pen-Thr-NH 2](CTOP) (1 nmol), a selective peptide antagonist for μ-opioid receptors. On one hand, microinjection of CTOP produced a dose dependent increase in scanning and stretched attended postures, by its own. On the other hand, the aversive effects of morphine into the dorsal periaqueductal gray microinjections were significantly reduced by systemic administration of nor-binaltorphimine, an opioid receptor antagonist with a tardive and selective action at κ-opioid receptors. These findings suggest that mechanisms mediated by μ-opioid receptors in the dorsal periaqueductal gray may be involved in the control of risk assessment behavior. On the other hand, the pro-aversive effects produced by microinjections of morphine into the dorsal periaqueductal gray are probably mediated by κ-opioid receptors.

Blockade of U50488H on sodium currents in acutely isolated mice hippocampal CA3 pyramidal neurons

The effect of opioid agonist U50488H on Na + currents was examined in freshly dissociated hippocampal neurons of mice using the whole-cell patch clamp technique. U50488H (1–100 μM) caused a concentration dependent reversible inhibition of the voltage-activated sodium currents. IC50 of 15.5 μM and Hill constant of 1.4 were calculated respectively. The inhibitory actions of U50488H on I Na were still observed in the presence of 30 μM naloxone. Moreover, under the action of U50488H, repetitive stimulation induced further inhibition which was frequency-dependent. The activation curve did not change before and after application of 10 μM U50488H. However, after exposure to 10 μM U50488H and repetitive depolarizing at 10 Hz, frequency-dependent inhibition occurred, and a mean shift of half-activation membrane potential by +20 mV could be induced. The inactivation curve was significantly changed toward negative membrane potential with 10 μM U50488H, and further negative shift was observed after repetitive depolarizing at 10 Hz. Our results indicate that U50488H could directly inhibit neuronal Na + currents without involvement in the activation of κ-opioid receptors.

Cellular Sites for Dynorphin Activation of κ-Opioid Receptors in the Rat Nucleus Accumbens Shell

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.

Serotonergic and GABAergic neurons in the medial rostral ventral medulla express κ-opioid receptor immunoreactivity

Activation of κ-opioid receptors in the rostral ventral medulla has been reported to attenuate analgesia induced by activation of μ-opioid receptors in the periaqueductal gray matter. Previous studies have suggested that the cells associated with this effect might contain serotonin. In the present study, we investigated the relationship of the cloned κ-opioid receptor to spinally projecting neurons immunoreactive for serotonin or GABA. This was done by employing two-color immunofluorescence in combination with retrograde tract-tracing using Fluoro-Gold. In the rostral ventral medulla, neurons triple-labeled for the cloned κ-opioid receptor, serotonin and Fluoro-Gold were observed; neurons double-labeled for the cloned κ-opioid receptor and serotonin, or single-labeled for the cloned κ-opioid receptor or for serotonin were also observed. In addition, cloned κ-opioid receptor immunoreactivity was expressed in some cell profiles immunoreactive for GABA. The expression of the cloned κ-opioid receptor in the spinal cord dorsal horn was not associated with processes immunoreactive for serotonin. Our findings suggest that κ-opioid receptors in the rostral ventral medulla are positioned to directly control the activity of at least some serotonergic neurons projecting to the dorsal spinal cord. Thus, it appears possible that the anti-analgesic action resulting from microinjection of κ-opioid agonists into the rostral ventral medulla is mediated, at least in part, by these neurons.

Mechanism of the dynorphin-induced dualistic effect on free intracellular Ca 2+ concentration in cultured rat spinal neurons

In order to study the different mechanisms of dynorphin spinal analgesia and neurotoxicity at low and high doses, the effects of various concentrations of dynorphin A-(1–17) on the free intracellular Ca 2+ concentration ([Ca 2+] i) in the cultured rat spinal neurons were studied using single cell microspectrofluorimetry. While dynorphin A-(1–17) 0.1–100 μM had no significant effect on basal [Ca 2+] i, dynorphin A-(1–17) 0.1 and 1 μM significantly decreased the high KCl-evoked peak [Ca 2+] i by 94% and 83% respectively. Dynorphin A-(1–17) 10 and 100 μM did not affect the peak [Ca 2+] i following K + depolarization, but in all these neurons there was a sustained and irreversible rise in [Ca 2+] i following high-K + challenge. Pretreatment with the specific κ-opioid receptor antagonist nor-binaltorphimine 10 μM, but not the competitive NMDA receptor antagonist, dl-2-amino-5-phosphonovalerate (APV) 10 μM, significantly blocked the inhibitory effect of dynorphin A-(1–17) 0.1 μM on peak [Ca 2+] i. However, APV 10 μM and nor-binaltorphimine 10 μM significantly antagonized the sustained rise in [Ca 2+] i induced by a high concentration of dynorphin A-(1–17) 10 μM. Furthermore, in the presence, and following the addition, of increasing concentrations of dynorphin A-(1–17) (0.1, 1, 10 and 100 μM), the high concentrations of dynorphin A-(1–17) failed to produce a sustained rise in peak [Ca 2+] i. These results suggested that dynorphin exerted a dualistic modulatory effect on [Ca 2+] i in cultured rat spinal neurons, inducing a sustained and irreversible intracellular Ca 2+ overload via activation of both NMDA and κ-opioid receptors at higher concentrations, but inhibiting depolarization-evoked Ca 2+ influx via κ-opioid but not NMDA receptors at lower concentrations. Serial addition of graded concentrations of dynorphin A-(1–17) prevented the effect of high concentrations of dynorphin A-(1–17) on [Ca 2+] i.

Antinociceptive effects of (+)-matrine in mice

The antinociceptive potency of matridin-15-one ((+)-matrine) was examined using the acetic acid-induced abdominal contraction test and the tail-flick test in mice. (+)-Matrine, at doses of 1 to 10 mg/kg, s.c., produced a marked and dose-dependent inhibition of the number of acetic acid-induced abdominal contractions in mice. The antinociceptive effect of (+)-matrine in the acetic acid-induced abdominal contraction test in mice was identical to that of pentazocine. Indeed, there was no significant difference in the ED 50 (mg/kg with 95% confidence limits) values for the inhibition of acetic acid-induced abdominal contractions between (+)-matrine (4.7 (4.1–5.3)) and pentazocine (3.3 (2.2–5.0)). Furthermore, in the tail-flick assay, (+)-matrine at doses of 10 and 30 mg/kg, s.c., again produced a dose-dependent antinociceptive effect. When nor-binaltorphimine (20 mg/kg, s.c.), a selective κ-opioid receptor antagonist, was administered 3 h before treatment with (+)-matrine, the antinociceptive effect of (+)-matrine was markedly antagonized. Furthermore, the antinociceptive effect of (+)-matrine was partially antagonized by pretreatment with β-funaltrexamine, a selective μ-opioid receptor antagonist. Naltrindole, a selective δ-opioid receptor antagonist, had no effect on the antinociceptive effect of (+)-matrine. In conclusion, (+)-matrine produced an antinociceptive effect mainly through the activation of κ-opioid receptors and partially through μ-opioid receptors.

Effects of chronic U-50,488H treatment on the isolated right atrium of the rat

The aim of the present investigation was to determine if chronic activation of κ-opioid receptor induces development of tolerance and dependence to κ-opioid agonists on the isolated right atrium of the rat. Tolerance to the κ-agonist was induced by chronic administration of U-50,488H, a selective κ-agonist (15 mg/kg i.p. twice a day for 4 days). The rats were killed on day 5. Tolerance to U-50,488H was observed after its chronic administration and was revealed as a rightward shift of the concentration-response curve; it was accompanied by a decrease in the maximum response and in the slope. Withdrawal to the κ-agonist was induced by administration of Mr-2266 (preferentially κ-antagonist) or nor-binaltorphimine (nor-bni; selective κ-antagonist) to the organ bath. The administration of the κ-antagonists Mr-2266 or nor-bni to preparations from tolerant rats in the organ bath induced an increase in auricular contraction frequency. In contrast, the administration of the κ-antagonists to preparations from vehicle-treated rats induced a decrease in auricular contraction frequency. These findings demonstrate that the hearts of rats that had received chronic U-50488H treatment develop tolerance to the cardiac effects of U-50,488H and exhibit excitatory reactions to κ-antagonist's precipitated withdrawal.

κ-Opioid Receptor Activation Modulates Ca2+Currents and Secretion in Isolated Neuroendocrine Nerve Terminals

Whole-cell patch-clamp recordings were performed together with time-resolved measurements of membrane capacitance (Cm) in nerve terminals acutely dissociated from neurohypophysis of adult rats to investigate modulation of Ca2+ currents and secretion by activation of opioid receptors. Bath superfusion of the kappa-opioid agonists U69,593 (0.3-1 microM), dynorphin A (1 microM), or U50,488H (1-3 microM) reversibly suppressed the peak amplitude of Ca2+ currents 32. 7 +/- 2.7% (in 41 of 56 terminals), 37.4 +/- 5.3% (in 5 of 8 terminals), and 33.5 +/- 8.1% (in 5 of 10 terminals), respectively. In contrast, tests in 11 terminals revealed no effect of the mu-opioid agonist [D-Pen2,5]-enkephalin (1-3 microM; n = 7) or of the delta-agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (1 microM; n = 4) on Ca2+ currents. Three components of high-threshold current were distinguished on the basis of their sensitivity to blockade by omega-conotoxin GVIA, nicardipine, and omega-conotoxin MVIIC: N-, L-, and P/Q-type current, respectively. Administration of U69,593 inhibited N-type current in these nerve terminals on average 32%, whereas L-type current was reduced 64%, and P/Q-type current was inhibited 28%. Monitoring of changes in Cm in response to brief depolarizing steps revealed that the kappa-opioid-induced reductions in N-, L-, or P/Q-type currents were accompanied by attenuations in two kinetically distinct components of Ca2+-dependent exocytotic release. These data provide strong evidence of a functional linkage between blockade of Ca2+ influx through voltage-dependent Ca2+ channels and inhibitory modulation of release by presynaptic opioid receptors in mammalian central nerve endings.

κ-Opioid agonist U50488 inhibits P-type Ca 2+ channels by two mechanisms

The effects of U50488, κ-opioid agonist on P-type Ca 2+ channels, were studied. U50488 inhibited depolarization-induced Ca 2+ uptake into rat brain synaptosomes, which was sensitive to ω-Agatoxin IVA (ω-AgaIVA; P-type Ca 2+ channel blocker) and inhibited P-type Ca 2+ channel currents recorded from rat cerebellar Purkinje neurons by the whole-cell patch clamp method. Dynorphin A also inhibited P-type Ca 2+ channel currents. The inhibition by U50488 was biphasic; high affinity component (21%, ICS, = 8.9 × 10 −8 M) and low affinity component (79%, IC 50 = 1.1 × 10 −5 M). At low concentrations of U50488 (10 −6 M), P-type Ca 2+ channel current inhibition was attenuated by norbinartorphimine (nor-BNI), κ-opioid antagonist, and by dialysis of cells with a pipette solution containing guanosine 5′- O-(2-thiodiphosphate) (GDP-βS). At high concentrations of U50488 (10 −5 M), P-type Ca 2+ channel current inhibition was frequency-dependent. Thus U50488-induced current inhibition is mediated by two mechanisms. Its high affinity component is produced by activation of κ-opioid receptors, whereas the low affinity component is due to its direct action on the P-type Ca 2+ channel.

Design and synthesis of 10-oxo derivative of N-cyclopropylmethyl (−)-6β-acetylthiodihydro-normorphine, a potentially κ-selective opioid receptor ligand

The design and synthesis of 10-oxo derivative of N-cyclopropylmethyl-(−)-6β-acetylthio dihydronormorphine ( 4) are described. The result of pharmacological assays indicate that compound 4 acts as a μ-opioid receptor antagonist and κ-opioid receptor agonist which is suggested to be more potent analgesic action than that of 1 and morphine through an activation of κ-opioid receptors.

κ-Opioid receptor agonist protects against ischemic reduction of 2-deoxyglucose uptake in morphine-tolerant rats

We examined the effects of μ-opioid receptor agonist and antagonists, and κ-opioid receptor agonist on the hypoxia/hypoglycemia-induced reduction in 2-deoxyglucose uptake of rat hippocampal slices. Naloxone, a μ-opioid receptor antagonist and (5,7,8)-(+)-3,4-dichloro- N-methyl- N-(7,8,1-pyrrolidinyl)-1-oxaspirol 4,5 dec-8-yl)-benzeneacetamide methanesulfonate, U-62,066E, a κ-opioid receptor agonist, showed neuroprotective actions against the hypoxia/hypoglycemia-induced deficit in glucose uptake. In contrast, morphine exhibited an exacerbating action. These results suggest that blockade of μ-opioid receptor-and stimulation of κ-opioid receptor-mediated functions has a protective role against the hypoxia/hypoglycemia-induced decreases in glucose metabolism in hippocampal slices. Chronic administration of morphine (10 mg/kg) for 9 days affected neither the basal nor the hypoxia/hypoglycemia-induced reduction in 2-deoxyglucose uptake. Rats treated with morphine chronically exhibited not only tolerance to the analgesic effect but also tolerance to the exacerbating action. However, chronic morphine did not modify U-62,066E-induced neuroprotection. These findings indicate that the receptor mechanisms of neuroprotection produced by the activation of κ-opioid receptors may not be involved in μ-opioid receptor function.

Blockade of the antinociceptive activity of centrally administered ketorolac by nor-binaltorphimine

Antinociceptive activity of intracerebroventricularly administered ketorolac tromethamine was evaluated in mice by measuring inhibition of abdominal stretching induced by p-phenylquinone. Ketorolac tromethamine produced dose-dependent antinociception with an ED 50 of 7.34 μg/mouse (4.97–10.82) and a maximal effect at 30 μg. Selective antagonists of opioid receptors were used to determine ketorolac's mechanism of action. The ketorolac tromethamine-induced antinociception was not blocked by the μ- and δ-opioid receptor antagonists, naloxone and ICI-174,864 ( N,N-diallyl-Tyr-Aib-Aib-Phe-Leu), respectively; however, the κ-opioid receptor antagonist nor-binaltorphimine dihydrochloride significantly blocked this effect. These findings suggest that activation of κ-opioid receptors appears to play a role in the mechanism of the antinociceptive effect of ketorolac tromethamine. Ketorolac tromethamine may induce the release of endogenous κ-opioids to produce central nervous system antinociception.

Involvement of mesolimbic κ-opioid systems in the discriminative stimulus effects of morphine

The neuroanatomical basis of opiate addiction has been studied using a variety of behavioural techniques. The aim of the present study was to investigate the role of mesolimbic opioid systems, in particular κ-opioid systems, in the expression of the discriminative stimulus effects of abused drugs. Rats were trained to discriminate morphine (3.0 mg/kg s.c.) from saline under a fixed ratio schedule of food reinforcement. Once rats had acquired the discrimination, a randomized sequence of different doses of the highly selective κ-opioid receptor agonist U69593 (0.02–0.16 mg/kg s.c.) was given 20 min prior to a systemic morphine injection. U69593 dose-dependently blocked the morphine discrimination. It is important to note that U69593 at these doses failed to generalize to the systemic morphine cue. The site of action by U69593 (0.02–0.16 μg) was examined by microinjecting discrete amounts into target brain regions. Intra-nucleus accumbens injections of U69593 dose-dependently blocked the systemic morphine cue, whereas, U69593 failed to generalize to the discriminative stimulus. The same doses did not affect morphine discrimination after intra-ventral tegmental area or striatum injections. Besides the rewarding effects of drugs of abuse, the discriminative stimulus properties of these agents are seen as a major factor in drug seeking behaviours. The present study shows that the discriminative effects of morphine, a measure of the subjective effects of this drug can be blocked by the activation of κ-opioid receptors located in the nucleus accumbens. In view of these findings which show that the activity of endogenous κ-opioid systems (dynorphin) may serve as physiological antagonists to counteract the effects of morphine, κ-agonists therefore may be useful in the treatment of opioid addictions.

Glucocorticoid and mineralocorticoid receptors differentially modulate cultured dopaminergic neurons of rat ventral mesencephalon

Activation of κ-opioid receptors by U69,593 in a concentration of 1 μM maximally inhibited dopamine release. Withdrawal from chronic (4 days) treatment with U69,593 (1 μM) induced a long-lasting 9at least 7 days) increase (30–50%) in [ 3H]dopamine release induced by 25 mM K + from cultured rat mesencephalic neurons without causing functional κ-opioid receptor desensitization. Incubation of these cultured neurons with the glucocorticoid agonist, dexamethasone (3 nM), had no effect on basal or 25 mM K +-induced dopamine release, whereas the steroid completely blocked the development of opiate-induced neuronal supersensitivity to depolarization. This blockade was found to be concentration-dependent, with an EC 50 of about 0.3 nM and was not associated with changes in corticotropin-releasing factor (CRF) receptor, dopamine D 2 autoreceptor or κ-opioid receptor functioning. Therefore, opioid and glucocorticoid receptors appear to be co-localized in dopaminergic neurons of rat ventral mesencephalon and interact in a functionally antagonistic manner. Interestingly, the mineralocorticoid agonist, aldosterone (5 nM), itself increased K +-stimulated [ 3H]dopamine release by about 25%, consistent with the putative role of mineralocorticoid receptors in maintaining neuronal excitability. Moreover, whereas 1 nM corticosterone appeared to mimic the dopamine release-facilitating effect of aldosterone, 30 nM corticosterone displayed a modulatory effect similar to that of dexamethasone. Thus, whereas mineralocorticoid receptors that are already activated at low plasma corticosterone levels may uphold neuronal sensitivity to depolarizing stimuli, activation of glucocorticoid receptors by higher plasma corticosterone levels may prevent the development of neuronal supersensitivity, e.g. following chronic activation of κ-opioid receptors. It is suggested that these corticosteroid receptor-mediated effects play an important role in long-lasting behavioral sensitization caused by stress and drugs of abuse.

Antitussive effect of dihydroetorphine in mice

The present study examined the opioid receptors involved in the antitussive effect of dihydroetorphine in mice. Dihydroetorphine suppressed coughs dose dependently at doses between 0.1–1 μg/kg i.p. Blockade of μ-opioid receptors by pretreatment with β-funaltrexamine significantly reduced the antitussive effect of dihydroetorphine. Furthermore, the antitussive effect of dihydroetorphine was also antagonized by nor-binaltorphimine, a κ-opioid receptor antagonist. However, pretreatment with naltrindole, a δ-opioid receptor antagonist, did not affect the antitussive effect of dihydroetorphine. These results indicate that the antitussive effect of dihydroetorphine is mediated by the activation of μ-opioid receptors and κ-opioid receptors, but not δ-opioid receptors.

Blockade of morphine-induced place preference through the activation of κ-opioid receptors in mice

The effects of pretreatment with the κ-agonists U-50,488H (U-50) and E-2078 on the morphine (MRP)-induced conditioned place preference (CPP) were examined in mice. Both U-50 (1 mg/kg, s.c.) and E-2078 (0.1 mg/kg, s.c.) induced a slight, non-significant place aversion. Pretreatment with U-50 (1 mg/kg) or E-2078 (0.1 mg/kg) abolished the MRP (3 mg/kg, s.c.)-induced CPP. In addition, MRP (3 mg/kg, s.c.) significantly increased the levels of the DA metabolites, DOPAC and HVA in the limbic forebrain (nucleus accumbens and olfactory tubercle). These effects were reduced by pretreatment with U-50. These results suggest that κ-agonists suppress the MRP-induced CPP, and that activation of κ-receptors could suppress the reinforcing effects of MRP which may be induced by enhanced DA transmission in the mesolimbic DA system.