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.     

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.     

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.     

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.     

Effects of differential modulation of mu-, delta- and kappa-opioid systems on bicuculline-induced convulsions in the mouse

The present study investigated the effects of micro-, delta- and kappa-opioid receptor agonists on seizures produced by blockade of gamma-aminobutyric acid (GABA)-mediated synaptic transmission in the mouse. The selective GABA(A) receptor antagonist bicuculline (1.25-3 mg/kg) given subcutaneously caused dose-dependent clonic-tonic convulsions. These convulsions were potentiated by the prototypic mu-opioid receptor agonist morphine given subcutaneously 20 min prior to a subconvulsive dose of bicuculline. The potentiation by morphine was completely reversed by pretreatment intraventricularly with the selective mu-opioid receptor antagonist beta-funaltrexamine (0.5 microgram/mouse). Pretreatment intraventricularly with the selective delta-opioid receptor agonists 2-methyl-4aalpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12, 12abeta-octahydro-quinolino[2,3,3-g]isoquinoline ((-)TAN-67) or [D-Pen(2,5)]-enkephalin (DPDPE) showed a dose-dependent increase in the incidence of convulsions. Pretreatment with naltrindole (2 mg/kg, s.c.), a selective delta-opioid receptor antagonist, abolished the enhancement of the bicuculline-induced convulsions by DPDPE. In contrast, pretreatment with the selective kappa-opioid receptor agonist U-50,488H (0.6-80 mg/kg, subcutaneously or 25-100 microgram/mouse, intraventricularly) produced a dose-dependent suppression of the bicuculline-induced convulsions. The inhibitory effect of U-50,488H was completely blocked by pretreatment subcutaneously with nor-binaltorphimine (5 mg/kg), a selective kappa-opioid receptor antagonist. This study demonstrates that activation of both mu- and delta-opioid receptors increases the incidence of convulsions produced by blockade of GABA-mediated synaptic transmission, while stimulation of kappa-opioid receptors has an anticonvulsive effect.     

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.     

Effects of kappa opiate agonists on palatable food consumption in non-deprived rats,with and without food preloads

There is increasing evidence to suggest that kappa opiate receptors may be importantly involved in the mediation of feeding responses in the rat. A series of experiments is reported in which the effects of four kappa receptor agonists (ethylketocyclazocine, U-50,488H, tifluadom, bremazocine) on the consumption of a highly palatable diet were investigated. Under one condition, non-deprived male rats were administered drug treatments before a 30 min feeding test. Bremazocine (0.1 mg/kg) and ethylketocyclazocine (3.0 mg/kg) both significantly decreased the level of food consumption. In contrast, U-50,488H and tifluadom each produced significant increases in food intake. In a second condition, non-deprived male rats were first allowed to consume some of the palatable diet to achieve partial satiation, prior to the administration of the drug treatments. In this case, evidence for hyperphagic effects of all four kappa agonists was obtained, within the first 30 min access to the palatable diet. Thus, hyperphagia occurred with 0.01 mg/kg bremazocine and 0.1 mg/kg ethylketocyclazocine. We conclude that some kappa agonists have mixed stimulant/inhibitory effects on food intake, whereas others are more consistent in producing hyperphagia. In neither condition did morphine (0.3–10.0 mg/kg) show any hyperphagic effect. Our data support an involvement of kappa opiate receptors in mechanisms which control palatable food consumption in non-deprived rats.     

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.     

The kappa opioid-related anticonvulsants U-50488H and U-54494A attenuate N-methyl-D-aspartate induced brain injury in the neonatal rat.

The neuroprotective effects of the kappa opioid-related anticonvulsants U-50488H and U-54494A were tested in a model of N-methyl-D-aspartate (NMDA)-induced brain injury in the neonatal rat. Seven-day-old rat pups were injected intracerebrally with 7.5 nmol NMDA. Five days later, the ensuing unilateral hemisphere weight reduction was measured and used to assess the severity of insult. Control animals (n = 85) exhibited a 21.7 +/- 0.5% hemisphere weight reduction. Animals treated with U-54494A in split doses before and after NMDA administration showed significant neuroprotection at 10, 15, and 20 mg/kg, with the maximum effect observed at 15 mg/kg (33.8% protection). Animals treated with U-50488H on a similar dosing schedule showed significant neuroprotection at all doses tested, with peak protection observed at 30 mg/kg (51.8% protection). Both compounds exhibited a neuroprotective effect when hemisphere cross-sectional area and hippocampal histology were assessed. Treatment with U-54494A after NMDA administration also afforded neuroprotection at various doses, as measured by hemisphere weight disparity, with peak protection occurring at a dose of 20 mg/kg (32.4% protection). These data show that both U-50488H and U-54494A afford neuroprotection against NMDA-induced neuronal injury in the neonatal rat brain.     

The stimulation of central kappa opioid receptors decreases male sexual behavior and locomotor activity.

Systemic injections of the kappa (kappa) opioid receptor agonist U-50,488H decreased male sexual behavior, locomotor activity, body temperature and bodily grooming, and induced body flattening. The U-50,488H-induced inhibitions of male sexual behavior were prevented by systemic injections of naloxone and by intra-cranial injections of the kappa opioid antagonist nor-binaltorphimine (NBNI). Injections of NBNI to either the ventral tegmental area (VTA) or the nucleus accumbens septi (NAS) increased female-directed behavior, and prevented the U-50,488H-induced decreases in female-directed behavior. Intra-VTA NBNI prevented U-50,488H-induced decreases in the mean number of ejaculations, intra-NAS NBNI prevented U-50,488H-induced increases in copulation latencies. Intra-medial preoptic area (mPOA) injections of NBNI increased female-directed behavior, and attenuated U-50,488H-induced decreases in female-directed behavior as well as U-50,488H-induced increases in both copulation and ejaculation latencies. Injections of NBNI dorsal to the mPOA were ineffective. Two of 26 days following the central injection of NBNI, systemic injections of U-50,488H remained behaviorally ineffective, leaving both sexual behavior and locomotor activity undiminished. These results suggest that the stimulation of central kappa opioid receptors inhibits sexual behavior in the male rat; perhaps endogenous kappa opioid agonists induce sexual refractory periods.     

U-50,488H inhibits dynorphin and glutamate release from guinea pig hippocampal mossy fiber terminals.

The selective kappa opioid agonist U-50,488H was tested for its ability to modulate the potassium-induced rise of cytosolic Ca2+ in, and transmitter release from, guinea pig hippocampal mossy fiber synaptosomes. U-50,488H dose dependently inhibited the potassium-induced rise in synaptosomal free Ca2+ levels. This inhibition was attenuated by the selective kappa opioid antagonist nor-binaltorphimine, but was insensitive to naloxone and the sigma opioid antagonist ICI 174,864. U-50,488H also dose dependently depressed the potassium-induced release of L-glutamate and dynorphin B-like immunoreactivity from mossy fiber synaptosomes in a nor-binaltorphimine-sensitive manner. This is the first report to confirm the presence of a presynaptic kappa opioid receptor in the hippocampal mossy fiber-CA3 synapse and the nature of its influence on neurotransmitter release. The present results may be used to suggest that endogenous dynorphin peptides interact with this kappa opioid receptor to autoregulate the excitatory mossy fiber synaptic input.     

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.     

The interaction between kappa-opioid agonist, U-50, 488H, and kainic acid: behavioral and histological assessments

The effects of a selective kappa-agonist, U-50,488H, on systemic kainic acid-induced behavioral and histological changes were studied in rats. U-50,488H inhibited kainic acid-induced wet dog shakes in a naloxone reversible manner; however, U-50,488H did not protect rats against kainic acid-evoked behavioral seizures. As revealed by histological analysis, kainic acid caused edema and severe neuronal damage in several brain regions, notably in CA1 but also in the CA3 fields of both hippocampi. Pretreatment of rats with U-50,488H markedly protected hippocampal neurons, especially those in CA1, against kainic acid-induced neurotoxicity. Naloxone by itself had little effect on kainic acid-induced seizures or hippocampal neuron loss. Naloxone plus U-50,488H resulted in less severe seizures and, consequently, less hippocampal cell loss than after kainic acid alone. These data indicate that U-50,488H can markedly attenuate the neurotoxic and behavioral consequences of systemic kainic acid administration. However, the mechanism of these effects requires further study with more specific opioid antagonists.     

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.     

Pituitary-adrenocortical response to acute and chronic administration of U-50,488H in the rat.

K-opioid substances have been shown to stimulate and/or depress the HPA activity. The objective of this study was to determine the effects of the acute and chronic administration of U-50,488H, a k-opioid receptor agonist, on the pituitary-adrenocortical activity in the rat. The acute administration of U-50,488H (25 mg/kg i.p.) produced a hypothermic effect and an increase in plasma levels of B-END-LI and cortisol, effects which were prevented by naloxone (3 mg/kg s.c.). Chronic administration of U-50,488H twice a day for 4 days resulted in a decrease in basal plasma levels of B-END-LI and cortisol and in the development of tolerance to its neuroendocrine and hypothermic effects. In rats made tolerant to U-50,488H, naloxone precipitated hypothermia (which is an index of opiate dependence in rats), whereas no changes in plasma B-END-LI and cortisol levels were seen. These data suggest that k receptors may be involved in the regulation of pituitary-adrenocortical activity in physiological conditions and during opiate abuse. On the other hand, U-50,488H induced only negligible dependence in rats, which was not morphine-like.     

The kappa-opioid receptor is involved in the stimulating effect of nicotine on adrenocortical activity but not in nicotine induced anxiety

The kappa (kappa) opioid system appears to interact with nicotine in the modulation of locomotion and addiction related processes. In this study we have investigated the possible implication of the kappa-opioid system in the effects of nicotine on anxiety and adrenocortical activity. In two different experiments, we analysed the possible interaction between nicotine (0.5 mg/kg i.p.) and either the kappa-opioid receptor antagonist nor-binaltorphimine (5 mg/kg i.p.) or the kappa-opioid receptor agonist U50,488H (1 mg/kg s.c.). Behavioural and endocrine experiments were performed in different groups of animals. Animals were exposed to the holeboard immediately followed by the plus-maze. Serum corticosterone levels were determined by radioimmunoassay. Nicotine induced an anxiogenic-like effect in the plus-maze and a significant decrease of holeboard activity. The anxiogenic-like effect in the plus-maze was not modified by any of the kappa-opioid receptor ligands. Nicotine also induced a significant increase in the corticosterone levels, and the kappa antagonist, which did not exert any effect per se, antagonised this effect. The kappa-agonist U50,488H induced a significant increase in corticosterone concentration when administered alone. We provide the first evidence for the involvement of the kappa-opioid receptor in the stimulatory effect of nicotine on adrenocortical activity.     

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.     

Opioid receptor subtypes associated with ventral tegmental facilitation of lateral hypothalamic brain stimulation reward

Rats were trained to lever-press for lateral hypothalamic electrical stimulation, and tested following ventral tegmental microinjections of morphine, delta ([D-Pen2,D-Pen5]enkephalin: DPDPE), or kappa (U-50, 488H) receptor agonists or saline. Across the range of effective stimulation frequencies, morphine (8 nmol) and DPDPE (8 nmol) reduced the current necessary to sustain responding, while U-50,488H (8 nmol) had no effect. Naloxone (1 mg/kg) blocked the effects of morphine and DPDPE. Since each of these opioids facilitates eating induced by stimulation of the lateral hypothalamus, but only the mu and delta agonists facilitate brain stimulation reward, it would appear that ventral tegmental kappa receptors are linked to circuit elements which play a role in eating but not lateral hypothalamic brain stimulation reward. Ventral tegmental mu and delta receptors, on the other hand, appear to be linked to circuit elements involved in both behaviors.     

The interaction between κ-opioid agonist, U-50,488H, and kainic acid: behavioral and histological assessments

The effects of a selective κ-agonist, U-50,488H, on systemic kainic acid-induced behavioral and histological changes were studied in rats. U-50,488H inhibited kainic acid-induced wet dog shakes in a naloxone reversible manner; however, U-50,448H did not protect rats against kainic acid-evoked behavioral seizures. As revealed by histological analysis, kainic acid caused edema and severe neuronal damage in several brain regions, notably in CA1 but also in the CA3 fields of both hippocampi. Pretreatment of rats with U-50,488H markedly protected hippocampal neurons, especially those in CA1, against kainic acid-induced neurotoxicity. Naloxone by itself had a little effect on kainic acid-induced seizures or hippocampal neuron loss. Naloxone plus U-50,448H resulted in less severe seizures and, consequently, less hippocampal cell loss than after kainic acid alone. These data indicate that U-50,448H can markedly attenuate the neurotoxic and behavioral consequences of systemic kainic acid administration. However, the mechanism of these effects requires further study with more specific opioid antagonists.     

Regulation of G-PROTEIN mRNA abundancy and cAMP accumulation after long-term opioid incubation in primary cultures of astroglia from the rat cerebral cortex.

Primary astroglial cultures were incubated with delta (10(-6) M DPDPE) or kappa (10(-5) M U-50,488H) receptor agonists for 5 days. Thereafter, the acute inhibitory actions of delta or kappa receptor agonists on forskolin stimulated cAMP accumulation were assayed. The G alpha s, G alpha i-1 and G alpha i-2 mRNA levels were quantified after 5 days of either delta or kappa receptor agonist treatment using a solution hybridization, RNase protection assay. Pronounced effects were observed after 5 days of kappa receptor agonist [10(-5) M U-50,488H] incubation. This treatment resulted in an attenuation in the acute inhibitory action of delta and kappa receptor agonists. Furthermore, a decreased stimulatory action of forskolin was seen. Similar effects were also seen after delta receptor stimulation. We also investigated the effects after 24 h and 3 days of incubation with the kappa receptor agonist (10(-5) M) U-50,488H. The 24 h incubation resulted in a decreased sensitivity to the acute inhibitory action of delta and kappa receptor agonists in the astroglial cultures. This effect was further accentuated after the 3 days of incubation with 10(-5) M U-50,488H. No significant change was seen in the basal accumulation of cAMP after incubation with the kappa agonist U-50,488H. However, after 5 days of incubation with the delta agonist DPDPE, a significantly increased basal accumulation of cAMP was seen in the astroglial cultures. After 5 days of delta or kappa agonist incubation, an increase in G alpha s mRNA level and a decrease in G alpha i-2 mRNA level was seen compared with controls. No statistically significant alterations in the amount of G alpha i-1 mRNA were seen. The data obtained in the present study indicate that the effects of long-term opioid treatment alters the sensitivity of glial cell opioid receptors. Furthermore, long term opioid treatment induces alterations in glial G-protein mRNA levels.