Binaltorphimine and nor-binaltorphimine, potent and selective kappa-opioid receptor antagonists

Effect of mu-, kappa-, and delta-selective opioid agonists on thermoregulation in the rat

The effect of selective mu-, kappa-, and delta-agonists on brain surface temperature (Tb), oxygen consumption (Vo2), and heat exchange (Q) was studied in unrestrained, male Sprague-Dawley rats using whole-body calorimetry. Hyperthermia, produced by PL-017 (1.86 nM) given ICV, resulted from increased Vo2 and reduced Q during the first 15-45 min postinjection. Tb returned to control levels due to a combination of increased Q and reduced Vo2. PL-017-induced hyperthermia was abolished by the mu-selective antagonist CTAP (0.75 nM). Dynorphin A1-17 (4.65 nM), a kappa-selective agonist, reduced both Vo2 and Q, resulting in hypothermia that was blocked by the kappa-selective antagonist nor-binaltorphimine (25 nM). The delta-selective agonist DPDPE (4.64 nM) caused no significant changes in Tb, Vo2, or Q. The data indicate that central stimulation of the mu- and kappa-opioid receptors affects both oxidative metabolism and heat exchange, which result in a change in Tb. These alterations can be prevented with selective opioid antagonist pretreatment.

Opioid receptor-mediated inhibition of evoked catecholamine release from cultured neurons of rat ventral mesencephalon and locus coeruleus

The effects of selective opioid agonists on the evoked release of [3H]dopamine and [3H]noradrenaline were studied in cultured dopaminergic neurons of the ventral mesencephalon (containing the substantia nigra and ventral tegmental area) and in cultured neurons of the noradrenergic locus coeruleus, respectively. The cultures were prepared from embryonic day 15 rat brains. After 9 days in culture, the calcium-dependent release of [3H]dopamine from dopaminergic substantia nigra/ventral tegmental area neurons induced by 23 mM k+ appeared to be inhibited exclusively by activation of kappa-opioid receptors, as [3H]dopamine release was inhibited selectively by the kappa agonists U69,593 and dynorphin-(1-13) (EC50 8 and 5 nM, respectively), and this inhibitory effect was antagonized by the kappa-selective antagonist nor-binaltorphimine (Ki 0.07 nM). In contrast, cultured noradrenergic locus coeruleus neurons appeared to contain release-inhibitory mu-opioid receptors only, as evoked [3H]noradrenaline release was inhibited selectively by the mu agonist [D-Ala2, MePhe4, Gly-ol5]enkephalin (EC50 45 nM), a response that was antagonized by the preferential mu antagonist naloxone (Ki = 0.7 nM). The delta-opioid receptor agonist [D-Ser2(O-butyl), Leu5]enkephalyl-Thr6 did not affect catecholamine release. Dopamine release from cultured ventral mesencephalic neurons, induced by 100 microM N-Methyl-D-Aspartate (NMDA), also appeared to be subject to kappa receptor-mediated inhibition, whereas NMDA-induced noradrenaline release from cultured locus coeruleus neurons was under the inhibitor control of mu receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Norbinaltorphimine blocks the feeding but not the reinforcing effect of lateral hypothalamic electrical stimulation

The role of central kappa opioid receptors in the regulation of feeding and reward was evaluated using electrical brain stimulation paradigms in combination with the selective kappa antagonist, norbinaltorphimine (nor-BNI). Lateral ventricular injection of 10.0 and 50.0 nmol doses of nor-BNI increased the lateral hypothalamic stimulation frequency threshold for eliciting feeding behavior but had no effect on threshold for self-stimulation in the absence of food. This result is identical to those previously reported for naloxone and antibodies to dynorphin A and suggests that opioid activity is associated with feeding behavior rather than the eliciting brain stimulation. A further similarity between naloxone, dynorphin antiserum, and nor-BNI is their preferential effect on feeding threshold values obtained later, rather than initially, in a post-injection test session. This pattern of threshold elevation is shown to differ from that of the appetite suppressants, amphetamine and phenylpropanolamine, which elevate threshold uniformly throughout a post-injection test. The signature pattern of threshold elevation produced by opioid antagonism is consistent with the hypothesis that opioid activity is involved in the maintenance rather than the initiation of feeding. Specifically, it is hypothesized that a dynorphin A/kappa receptor mechanism is triggered by food taste and sustains feeding behavior by facilitating incentive reward.

The mixed antinociceptive agonist-antagonist activity of beta-endorphin(1-27) in mice

beta-Endorphin(1-27) (i.c.v.) has been reported to inhibit the antinociceptive activity of i.c.v. administered beta-endorphin in mice. In this study the antagonist activity of beta-endorphin(1-27) has been confirmed and the antagonism appears to be mediated at delta 1 opioid receptors. At higher doses than that used for antagonism, i.c.v. administered beta-endorphin(1-27) was a full antinociceptive agonist. The antinociceptive activity of beta-endorphin is attributed to the release of met-enkephalin in the spinal cord and is antagonized by the selective delta 2 opioid receptor antagonist, naltriben (NTB) but not by the selective delta 1 opioid receptor antagonist, 7-benzylidenenaltrexone (BNTX). In contrast, the antinociceptive activity of i.c.v. administered beta-endorphin(1-27) was not affected by either NTB or BNTX administered i.c.v. or i.t. Also, the antinociceptive activity of beta-endorphin(1-27) was unaffected by the selective mu opioid receptor antagonist, beta-funaltrexamine (beta-FNA) or the selective kappa opioid receptor antagonist, norbinaltorphimine (norBNI). Thus, beta-endorphin(1-27) appears to mediate antinociception supraspinally through the interaction of a unique receptor, i.e. a receptor that is different from mu, kappa, delta 1 or delta 2 opioid receptors. Alternatively, a non-opioid mechanism may be considered.

Kappa opioid receptor-mediated regulation of prolactin and alpha-melanocyte-stimulating hormone secretion in male and female rats

The purpose of this study was to examine the effects of the kappa opioid receptor agonist U-50,488 and antagonist nor-binaltorphimine (NOR-BNI) on the secretion of prolactin and alpha-melanocyte-stimulating hormone (alpha MSH) in male and female rats. Activation of kappa opioid receptors with U-50,488 increased plasma prolactin concentrations in both male and female rats, whereas blockade of kappa opioid receptors with NOR-BNI decreased plasma prolactin concentrations in male, but not in female rats. U-50,488 also increased and NOR-BNI decreased plasma concentrations of alpha MSH; similar effects were observed in both male and female rats. These results reveal a sexual difference in kappa opioid receptor-mediated regulation of prolactin, but not alpha MSH secretion.

Precipitation of morphine withdrawal syndrome in rats by administration of mu-, delta- and kappa-selective opioid antagonists

The acute effects of opioid drugs are generally hypothesized to be mediated by multiple receptors, for which three types of binding sites have been established. In order to evaluate the selective participation of each type of opioid receptor in opiate withdrawal, the opiate withdrawal syndrome, precipitated by the intraventricular acute administration of mu-, delta- and kappa-selective opioid antagonists was investigated. After implantation of the cannula into the lateral ventricle, rats were made physically dependent by subcutaneous insertion of two 75-mg pellets of morphine (base). D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) (5-5000 ng), a mu-selective opioid antagonist, naltrindole (62-2000 ng), a delta-selective antagonist or nor-binaltorphimine (nor-BNI) (600-20,000 ng), a kappa-selective antagonist, were administered 72 hr after implantation of the pellets. All three drugs elicited some signs of morphine withdrawal but they differed in both their potency and their efficacy. The most efficacious and the most potent was CTAP, eliciting 8 of the 14 withdrawal signs at doses of 5-5000 ng. Nor-BNI was less efficacious and less potent, eliciting a significant increase in 5 of the 14 withdrawal signs in a dose range of 600-20,000 ng. Naltrindole was the least potent and least efficacious of the three drugs, eliciting a significant increase of only 2 withdrawal signs after intraventricular administration of 2000 ng. In a second experiment, the withdrawal syndrome was precipitated by the combined administration of CTAP+naltrindole or CTAP+nor-BNI. The severity of withdrawal, obtained with these two combinations, was similar to that observed with CTAP alone. These results support the importance of the mu receptor in the expression of central opiate dependence and suggest a minor role for delta and kappa receptors.

kappa-Opioid binding sites on a murine lymphoma cell line

As a first step in determining whether any subset of lymphocytes expresses opioid receptors, membranes prepared from mouse lymphoma cell lines were screened for [3H]naloxone binding sites. Membranes from the R1.1 cell line specifically bound [3H]naloxone. The Hill coefficient for [3H]naloxone binding was 0.93 +/- 0.18, and nonlinear regression analysis indicated that a one-site model was the best fit of the [3H[naloxone saturation binding data. Low concentrations of kappa-selective opioids, but neither mu nor delta opioids, inhibited [3H]naloxone binding. Saturation binding studies with the kappa-selective compound [3H]U69,593 revealed a single binding site with a KD value of 0.204 +/- 0.039 nM and a Bmax value of 31.7 +/- 3.1 fmol/mg of membrane protein. The Hill coefficient for [3H]U69,593 binding was 1.03 +/- 0.11, indicative of a single site. Time courses for the association and dissociation of [3H]U69,593 binding at 25 degrees C exhibited properties consistent with a single class of binding sites. Low concentrations of kappa-selective opioids, including dynorphin peptides, inhibited [3H]U69,593 binding, while high concentrations of mu opioids were needed to inhibit binding, and the delta-selective ligands were ineffective at concentrations up to 10 microM. Stereoselectivity of the binding site was demonstrated by the finding that the Ki value for (-)-pentazocine in inhibiting [3H]U69,593 binding was 25 times less than for the (+)-isomer. Based on its high affinity for U69,593, alpha-neo-endorphin, and dynorphin B, the kappa opioid binding site on R1.1 cell membranes belongs to the kappa 1b subtype. As observed with brain kappa opioid binding sites, sodium inhibited [3H]U69,593 binding to R1.1 cell membranes in a concentration-dependent manner. These data demonstrate that the murine lymphoma cell line R1.1 expresses kappa opioid binding sites that are very similar to brain kappa opioid binding sites.

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.

Tyr-MIF-1 and hemorphin can act as opiate agonists as well as antagonists in the guinea pig ileum

The brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) was tested for its effects on electrically stimulated contractions in the guinea pig ileum assay. Tyr-MIF-1 acted as an opiate agonist in reducing these contractions. Its IC50 was about 9 microM, and its effects were reversed by naloxone and CTOP. The ability of Tyr-MIF-1 also to antagonize the inhibitory effects of opiates on electrically stimulated contractions was more evident in the ileum removed from a guinea pig tolerant to morphine or after partial inactivation of opiate receptors with beta-CNA. Similar results were observed with hemorphin. The endogenous peptide Tyr-MIF-1 and the blood-derived peptide hemorphin, therefore, can act as agonists as well as antagonists in the guinea pig ileum. The effects as antagonists are best observed in preparations of ileum with reduced receptor reserve (tolerant or beta-CNA treated) and are consistent with the idea that properties of endogenous peptides as opiate antagonists are enhanced in the tolerant state.

Pharmacological profiles of fentanyl analogs at mu, delta and kappa opiate receptors

Receptor binding assays using [3H]DAGO ([D-Ala2,MePhe4-Gly5-ol]enkephalin) (mu), [3H]DPDPE ([D-Pen2,D-Pen5]enkephalin) (delta) and [3H]U-69593 (kappa) were done in guinea pig whole brain membranes. Agonist activity was determined in norbinaltorphimine or beta-funaltrexamine (beta-FNA) treated guinea pig ileum (mu and kappa, respectively) and beta-FNA-treated mouse vas deferens (delta). The compounds with highest affinity were the most potent at the mu-receptor. The selectivity observed in the binding affinities was also found in in vitro activity. No correlation was found between mu-affinity and selectivity; the highest affinity analog, lofentanil, was found to be among the least selective, while another high affinity analog, R30490, was the most mu-selective. The results show that not all fentanyls are highly mu-selective, and could produce actions through delta- and kappa-opiate receptors.

Manifestations of acute opiate withdrawal contracture in rabbit jejunum after mu-, kappa- and delta-receptor agonist exposure

1. Following a 5 min in vitro exposure to morphine (1.3 x 10(-7) M), U-50,488H (2.5 x 10(-8) M) and deltorphin (1.6 x 10(-8)-6.5 x 10(-9) M), the rabbit isolated jejunum exhibited a precipitated contracture after the addition of naloxone (2.75 x 10(-7) M). 2. The precipitated responses to U-50,488H and deltorphin but not to morphine were reproducible in the same tissue. 3. The precipitated contractures were blocked completely by tetrodotoxin (3 x 10(-7) M), partially by atropine (1.5 x 10(-7) M) and not affected by hexamethonium (1.4 x 10(-5) M). 4. Naloxone administration (2.75 x 10(-7) M) before the agonist prevented the development of the adaptive response to morphine and U-50,488H but not to deltorphin. 5. The selective antagonists norbinaltorphimine (2.7 x 10(-8)-2.7 x 10(-9) M) and naltrindole (1.1 x 10(-7) M) prevented the adaptive response development only to the respective agonists. 6. The opioid agonists partially inhibited the spontaneous activity of the tissue. This study has shown that independent activation of mu-, kappa- and delta-opioid receptors can induce dependence in this isolated tissue. Rabbit jejunum is a suitable tissue for studying the acute effects of opioids on the adaptative processes determined by their administration.

Activation of kappa-opioid receptors depresses electrically evoked excitatory postsynaptic potentials on 5-HT-sensitive neurones in the rat dorsal raphé nucleus in vitro

Intracellular recordings from dorsal raphé neurones in slices from rat brains were used to study the actions of kappa-opioid receptor agonists on an excitatory postsynaptic potential (epsp) evoked by local electrical stimulation of afferent terminals. The epsp was observed on all 5-HT-sensitive neurones and was blocked by 1 microM TTX. The epsp was reduced in a dose-dependent manner by the specific kappa-opioid receptor agonist [5R-(5 alpha,7 alpha,8 beta)]-N-methyl-N-[7-(1-pyrrolidinyl)-1- oxaspiro[4.5]dec-8-yl]-4-benzofuranacetamide monohydrochloride (CI-977) (1-100 nM). The effects of CI-977 were blocked by the specific kappa-opioid receptor antagonist norbinaltorphimine (NorBNI) (0.1-1 microM). In the presence of the GABAA receptor antagonists picrotoxin and bicuculline (30 microM), CI-977 still had its depressant action on the epsp. Application of the excitatory amino acid receptor antagonists either kynurenic acid (0.5-1 mM) or 6-cyano-2,3-dihydro-7-nitro-quinoxaline-2,3-dione (CNQX) (30 microM) and DL-2-amino-5-phosphonovaleric acid (APV) reduced both the peak and area of the epsp suggesting that the main component of the epsp evoked by electrical stimulation was largely due to release of excitatory amino acids from afferent terminals. Using potassium chloride-filled recording electrodes an epsp which was only partially occluded by kynurenic acid or CNQX and APV was seen on some neurones, this residual epsp was insensitive to CI-977 but was blocked by 30 microM picrotoxin and bicuculline. The specific mu-opioid receptor agonist, DAGOL, had no consistent effect on the fast epsp. Longer duration electrical stimuli produced a slow inhibitory postsynaptic potential (ipsp) and a long duration increase in firing. CI-977 did not affect either the slow 5-HT-mediated ipsp which was blocked by spiperone or the slow noradrenaline-mediated increase in firing which was sensitive to prazosin. CI-977 did not change the depolarizing response to brief applications of either glutamic acid or N-methyl-D-aspartic acid (NMDA). CI-977, NorBNI, naloxone, DAGOL, picrotoxin, bicuculline and kynurenic acid had no consistent effects on the resting postsynaptic membrane potential or conductance. Under voltage-clamp conditions CI-977 had no effect on a membrane current resembling IA. These results suggest that kappa-opioid receptors are present on the terminals of afferents which release excitatory amino acids onto the 5-HT-sensitive neurones in the raphé.

Central opioid receptor subtype antagonists differentially alter sucrose and deprivation-induced water intake in rats

The present study compared the effectiveness of centrally-administered opioid receptor subtype antagonists to inhibit intake of either a 10% sucrose solution under ad libitum conditions, or water following 24 h of water deprivation. Full dose-response functions were evaluated over a 1 h period for the following antagonists: naltrexone (general: 1-50 micrograms), nor-binaltorphamine (Nor-BNI, kappa: 1-20 micrograms), beta-funaltrexamine (beta-FNA, mu: 1-20 micrograms), naltrindole (delta 2: 1-20 micrograms), [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, delta 1: 10-40 micrograms) and naloxonazine (mu 1: 10-50 micrograms). Naltrexone significantly and dose-dependently inhibited both sucrose intake (64-67%) and deprivation-induced water intake (53-67%). Nor-BNI significantly and dose-dependently inhibited sucrose intake (53-55%), but failed to significantly affect (28%) deprivation-induced water intake. beta-FNA significantly and dose-dependently inhibited both sucrose intake (31-34%) and deprivation-induced water intake (36-50%). Naltrindole failed to significantly alter either sucrose intake (24%) or deprivation-induced water intake (16%). Whereas DALCE significantly, but transiently (15-20 min) inhibited sucrose intake (28%), it failed to significantly alter deprivation-induced water intake (14%). Naloxonazine significantly, but transiently (5-10 min) stimulated sucrose intake at low doses (26%), but non-significantly reduced sucrose intake at higher doses (20%). Naloxonazine failed to significantly alter deprivation-induced water intake (16% reduction). These data indicate that whereas the kappa and mu 2 binding sites participate in the opioid modulation of sucrose intake, the mu 2 binding site participates in the opioid modulation of deprivation-induced water intake.

Effects of immunoneutralization of dynorphin1–17 and dynorphin1–18 on the activity of central dopaminergic neurons in the male rat

The effects of administration of antibodies against dynorphin1-17 (DYN1-17-AB) and dynorphin1-8 (DYN1-8-AB) were examined on the activity of dopaminergic (DA) neurons comprising the nigrostriatal, mesolimbic, tuberoinfundibular and periventricular-hypophysial systems in the male rat brain. DA neuronal activity was estimated by measuring the concentration of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in brain (striatum, nucleus accumbens, median eminence) and pituitary regions (intermediate lobe) containing terminals of these neurons. The intracerebroventricular administration of either DYN1-17-AB or DYN1-8-AB produced a time-related increase in the activity of tuberoinfundibular and periventricular-hypophysial DA neurons, but failed to alter the activity of nigrostriatal or mesolimbic DA neurons. The ability of both DYN1-17-AB and DYN1-8-AB to enhance the activity of tuberoinfundibular and periventricular-hypophysial DA neurons was reversed by the kappa opioid agonist U-50,488. These results indicate that DYN1-17-AB and DYN1-8-AB, presumably by binding endogenous dynorphins, remove a tonic inhibitory action of these opioid peptides on tuberoinfundibular and periventricular-hypophysial DA neurons.

Endogenous opioids may be involved in idazoxan-induced food intake

In this study it has been shown that the unexpected increase in food consumption, produced by the alpha 2-adrenoceptor antagonist idazoxan (10 mg/kg, i.p.) in rats, was significantly attenuated by small doses of the opioid antagonist (-)-naloxone (0.1, 1 mg/kg, i.p.) and totally inhibited by a small dose of naltrexone (1 mg/kg, i.p.). On the other hand, idazoxan-induced feeding was not affected by (+)-naloxone (0.1, 1 mg/kg, i.p.), which is inactive at opioid receptors. In addition, idazoxan-induced food consumption was not blocked by the delta-opioid antagonist, naltrindole (0.1, 1 mg/kg, i.p.) nor by the mu/delta-antagonist, RX8008M (16-methyl cyprenorphine; 0.1, 1 mg/kg, i.p.), which clearly discriminates between mu/delta- and kappa-opioid receptor function in vivo. These findings suggest that idazoxan may lead to the release of endogenous opioid peptides, which subsequently stimulate feeding by activation of kappa-, as opposed to mu- or delta-opioid receptors. This response is unlikely to be due to alpha 2-adrenoceptor blockade, since other highly selective alpha 2-adrenoceptor antagonists do not increase food intake and, instead may reflect the high affinity of idazoxan for non-adrenoceptor idazoxan binding sites.

Peripheral analgesic activities of peptides related to alpha-melanocyte stimulating hormone and interleukin-1 beta 193-195

1. The hyperalgesic effects of interleukin-1 beta (IL-1 beta) and prostaglandin E2 (PGE2) were measured in rats. 2. Hyperalgesic responses to IL-1 beta were inhibited in a dose-dependent manner by alpha-melanocyte stimulating hormone (alpha-MSH)-related peptides with the following order of potency: [N1(4),D-Phe7]alpha-MSH greater than alpha-MSH greater than Lys-D-Pro-Val greater than Lys-Pro-Val greater than Lys-D-Pro-Thr greater than D-Lys-Pro-Thr. 3. Hyperalgesic responses to PGE2 were not inhibited by Lys-D-Pro-Thr and D-Lys-Pro-Thr but were inhibited in a dose-dependent manner by the other peptides with the same order of potency as against IL-1 beta. 4. The potencies of [N1(4), D-Phe7]alpha-MSH and alpha-MSH were greatly diminished by deletion of their C-terminal tripeptide, Lys11-Pro-Val13. 5. Nor-binaltorphimine (Nor-BNI) largely reversed the analgesic effects of alpha-MSH, [N1(4), D-Phe7]alpha-MSH, Lys-Pro-Val and Lys-D-Pro-Val indicating that kappa-opioid receptors mediated the analgesic activity of these peptides. 6. Nor-BNI did not antagonize the inhibition by Lys-D-Pro-Thr and D-Lys-Pro-Thr of IL-1 beta evoked hyperalgesia indicating that these peptides were not acting via kappa-opioid receptors.

Naloxone benzoylhydrazone, a kappa 3 opioid agonist, stimulates food intake in rats

Naloxone benzoylhydrazone (NalBzoH) is a selective, short-acting agonist at the kappa 3 opioid receptor and a slowly dissociating potent antagonist at the mu opioid receptor. Given the important role of kappa receptors in the opioid control of food intake, the present study examined the central and peripheral effects of NalBzoH upon food intake. Central administration of NalBzoH (1-20 micrograms, i.c.v.) significantly increased food intake for up to 12 h, but failed to alter intake or body weight after 24 or 48 h. The 12 h duration of NalBzoH-mediated effects may be due to either persistent kappa 3 receptor occupancy, and/or activation of an ingestive system which maintains its activity. Peripheral administration of NalBzoH (20 mg/kg, s.c.) significantly increased food intake for up to 1 h. To distinguish kappa 1 (U50,488H) and kappa 3 (NalBzoH) hyperphagic effects, these agonist effects were compared following pretreatment with either naltrexone or the kappa 1 antagonist, nor-binaltorphamine (Nor-BNI). Whereas naltrexone significantly reduced both U50,488H and NalBzoH hyperphagia, Nor-BNI blocked U50,448H, but not NalBzoH hyperphagia. These data indicate a distinct role for the kappa 3 receptor in ingestive behavior separable from that of kappa 1 effects.

N-methyl-D-aspartate (NMDA) and opioid receptors mediate dynorphin-induced spinal cord injury: behavioral and histological studies

Both N-methyl-D-aspartate (NMDA) and opioid receptors have been implicated in the pathophysiology of traumatic spinal cord injury and dynorphin-induced paralysis. The present studies compared the effects of the non-competitive NMDA antagonist dextrorphan (Dex) and the kappa-selective opioid antagonist nor-binaltorphimine (nor-BNI) on the acute motor deficits and chronic neuropathological alterations caused by intrathecally administered dynorphin A-(1-17) (Dyn A). Infusion of Dyn A into the rat lower thoracic spinal subarachnoid space produced acute, reversible hindlimb paresis. Histological evaluations of spinal cord sections from these animals at 2 weeks post-infusion revealed ventral grey matter necrosis, neuronal loss and gliosis as well as axonal loss in adjacent white matter; however, there was minimal alteration in serotonin immunocytochemistry caudal to the injury zone. Dex or non-BNI pretreatment each significantly (P less than 0.05) reduced, and to a similar degree, the acute motor deficits and certain histological changes associated with Dyn A administration. These findings further support the hypothesis that dynorphin-induced spinal cord injury involves both NMDA receptors and opioid receptors.

Long term kappa-opioid receptor blockade following nor-binaltorphimine

This study determined the influence of nor-binaltorphimine (nor-BNI, 10 micrograms intracisternally), 1, 7 and 21 days post-administration, upon spiradoline-induced antinociception measured in the hot-plate test. On each test day, nor-BNI prevented the spiradoline-induced increase in the latency of rats to paw-lick without affecting the antinociceptive response to morphine. These data suggest that a single administration of nor-BNI can cause a long term blockade of kappa-opioid receptors in rats.

Opposing tonically active endogenous opioid systems modulate the mesolimbic dopaminergic pathway

The mesolimbic dopaminergic system has been implicated in mediating the motivational effects of opioids and other drugs of abuse. The site of action of opioids within this system and the role of endogenous opioid peptides in modulating dopamine activity therein remain unknown. Employing the technique of in vivo microdialysis and the administration of highly selective opioid ligands, the present study demonstrates the existence of tonically active and functionally opposing mu and kappa opioid systems that regulate dopamine release in the nucleus accumbens, the major terminal area of A10 dopaminergic neurons. Thus, stimulation of mu-type receptors in the ventral tegmental area, the site of origin of A10 dopaminergic neurons, increases dopamine release whereas the selective blockade of this opioid receptor type results in a significant decrease in basal dopamine release. In contrast, stimulation of kappa-type receptors within the nucleus accumbens decreases dopamine release whereas their selective blockade markedly increases basal dopamine release. These data show that tonic activation of mu and kappa receptors is required for the maintenance of basal dopamine release in the nucleus accumbens. In view of the postulated role of the mesolimbic system in the mediation of drug-induced alterations in mood and affect, such findings may have implications for the treatment of opiate dependence and affective disorders.

The mu-opioid activity of kappa-opioid receptor agonist compounds in the guinea pig ileum

On the basis of their in vivo activity and binding affinity, nalorphine and (-)SKF 10,047 were classified as mixed agonist/antagonist compounds. However, in isolated tissue preparations without a selective antagonist to block their agonist effect, the characterization of these compounds and the determination of their antagonist activity were very difficult. Nor-binaltorphimine, a selective kappa-opioid receptor antagonist, was used in the longitudinal muscle preparations of the guinea pig ileum to block the kappa-agonist activity of nalorphine and (-)SKF 10,047. In the absence of their kappa-agonist activity, we were able to determine the mu-antagonist activity using the mu-selective agonist DAMGO ([D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin). The pA2 values for nalorphine and (-)SKF 10,047 were 7.50 and 7.69, respectively.

A highly selective kappa-opioid receptor agonist, CI-977, reduces excitatory synaptic potentials in the rat locus coeruleus in vitro

Intracellular recordings were made from neurons in a rat locus coeruleus slice preparation in vitro. A postsynaptic potential was evoked by electrical stimulation of afferents to the neurons. CI-977 ([5R-(5a,7a,8b)]-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec -8-yl[-4-benzofuranacetamide monohydrochloride) caused a depression of the evoked postsynaptic potential on locus coeruleus neurons. This action was reversed on washout. Bremazocine had a similar action on less than 50% of locus coeruleus neurons. Concentrations of CI-977 which depressed the postsynaptic potential did not affect either passive membrane conductance or a voltage-sensitive potassium current resembling IA. The depression of the excitatory postsynaptic potential caused by CI-977 remained in the presence of either 30 microM bicuculline and picrotoxin or when potassium acetate-filled recording electrodes were used. Using potassium chloride-filled recording electrodes and in the presence of 30 microM 6-cyano-2,3-dihydro-7-nitroquinoxaline-2,3-dione and either 30 microM DL-2-amino-5-phosphonovaleric acid or 500 microM kynurenic acid, CI-977 had no effect on the postsynaptic potential. The effects of CI-977 were reversed by 30-100 nM naloxone and 1-10 nM norbinaltorphimine but not by 1-10 nM naloxone. The hyperpolarizing response to the mu-opioid receptor-selective agonist D-Ala2,Nme Phe4,Gly-ol5 (DAGOL) was blocked by 1-10 nM naloxone but not by 1-100 nM norbinaltorphimine. The hyperpolarizing response to DAGOL was not affected by high doses of CI-977.(ABSTRACT TRUNCATED AT 250 WORDS)

All three types of opioid receptors in the spinal cord are important for 2/15 Hz electroacupuncture analgesia

The analgesic effect induced by 2/15 Hz electroacupuncture as shown by the increase in tail flick latency decreased steadily as electroacupuncture stimulation was given continuously for 6 h, showing the development of tolerance to electroacupuncture analgesia. These rats were then given an intrathecal (i.t.) injection of one of the following opioid agonists: the mu agonist, ohmefentanyl 7.5, 15 and 30 pmol, 10 min apart; the delta agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE) 5, 10 and 20 nmol, 10 min apart and the kappa agonist, dynorphin-(1-13) 2.5, 5 and 10 nmol, 10 min apart, respectively. The analgesic effect induced by ohmefentanyl, DPDPE or dynorphin was dramatically reduced in rats rendered tolerant to 2/15 Hz electroacupuncture analgesia. Rats were injected i.t. with one of the three specific opioid antagonists: the mu antagonist, beta-funaltrexamine (beta-FNA) (5, 10 and 20 nmol), the delta antagonist, ICI174,864 (1, 2 and 4 nmol) and the kappa antagonist, nor-binaltorphimine (nor-BNI) (3.125, 6.25 and 12.5 nmol). It was found that analgesia induced by 2/15 Hz electroacupuncture stimulation was significantly and almost totally blocked by any one of the three opioid antagonists. These results suggest that all three types of opioid receptors, the mu, delta and kappa receptors in the spinal cord of the rat play important roles in mediating analgesia induced by electroacupuncture of 2/15 Hz.

Maintenance of acute morphine tolerance in mice by selective blockage of kappa opioid receptors with norbinaltorphimine

In this study we investigated the effect of the highly selective kappa opioid antagonist, norbinaltorphimine (norBNI) on the development of tolerance to a single dose of morphine. Mice were pretreated with 100 mg/kg of morphine sulfate (morphine), s.c. and 2 h later, norBNI (20 mg/kg s.c.) was administered and various times after this pretreatment, antinociceptive ED50 value of morphine was determined in the tail-flick assay. Twenty-four and 72 h after morphine injection, ED50 values of morphine were significantly increased by about 2.5-fold from those of their control mice that received saline instead of the tolerance-inducing dose of morphine. In a second set of experiments, animals were pretreated similarly with morphine and norBNI and 72 h after morphine injection, various opioid agonists were applied by the i.c.v. or i.t. route to see whether or not any cross-tolerance had developed to these agonists. The ED50 of i.c.v.-administered morphine was significantly greater than that of the non-pretreated controls. A small degree of cross-tolerance was observed with U-50,488H but not with DPDPE [D-Pen2,D-Pen5]enkephalin (DPDPE) at the supraspinal site. At the spinal site, tolerance to morphine was not observed. These results suggest that antagonism at kappa opioid sites after morphine administration, modulates positively the development of opioid tolerance.

Purification of opioid receptor in the presence of sodium ions

Opioid receptor was solubilized from rat brain membranes with a mixture of the detergents CHAPS and digitonin in the presence of protease inhibitors and 1 M NaCl. The solubilized receptor bound mu-opioid agonists and antagonists with affinities similar to those of native membrane receptor. The affinity of solubilized receptor for the agonist PL017 was greatly reduced by GTP gamma S, suggesting the receptor is still associated with G-protein. The solubilized material was passed through an opioid antagonist (10cd) affinity column and a wheat germ agglutinin column, set up in series, to obtain a partially purified receptor preparation. This partially purified material bound mu-agonist with low affinity and the binding affinity was no longer affected by GTP gamma S. The partially purified receptor was further purified by repeating the affinity and lectin chromatography with smaller size column. Binding of opioid antagonist [3H]diprenorphine to the partially or purified receptors was dependent upon the presence of sodium ions. The purified receptor showed saturable and stereospecific binding for opioid ligands, was predominantly of the mu-type, and exhibited as a diffuse band with a medium molecular mass of 62 kD upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The average specific binding activity of the purified receptor was 18.8 +/- 2.3 pmol/micrograms protein, a value close to the theoretical estimation.

Inhibition of suckling-induced prolactin release by mu- and kappa-opioid antagonists

Evidence suggests that endogenous opioid peptides (EOP) are involved in the hyperprolactinemia and suppression of luteinizing hormone (LH) release associated with lactation. To address this hypothesis, we investigated the effects of various opioid receptor antagonists on suckling-induced prolactin (PRL) and LH responses in primiparous, lactating rats. All animals were fitted with indwelling jugular catheters to allow serial blood sampling, and some rats received intracerebroventricular (i.c.v.) cannulae for central drug injection. Naloxone (2.0 mg/kg, i.v.) was employed as a broad spectrum opioid antagonist, whereas beta-funaltrexamine (beta-FNA, 1.0-5.0 micrograms, i.c.v.), naloxonazine (NAZ, 20 mg/kg, i.v.) and nor-binaltorphimine (nor-BNI, 4.0-16.0 micrograms, i.c.v.) were used to block mu, mu 1 and kappa receptor sites, respectively. In vehicle-treated rats, pup suckling evoked a dramatic increase in plasma PRL and a concurrent decrease in circulating LH. Naloxone caused a modest, though significant, attenuation of the PRL surge during nursing. beta-FNA and nor-BNI inhibited suckling-induced PRL release in a dose-related fashion, and at sufficient doses, both antagonists abolished the PRL response. Conversely, the suckling-induced rise in plasma PRL was not affected by NAZ. Naloxone, beta-FNA, and NAZ did not alter the profile of circulating LH in suckled rats, but the highest dose nor-BNI (16 micrograms, i.c.v.) produced a significant elevation in plasma LH. However, even in rats treated with 16.0 micrograms of nor-BNI, plasma LH levels declined in response to the nursing stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal administration of dynorphin A and its fragments increase heart rate and arterial pressure in the urethane anesthetized rat: mediation by a nonopioid mechanism

Intrathecal administration of 6.50 nmol of dynorphin A (dyn A) (1-13) and (1-17) to the ninth thoracic (T9) spinal segment provoked a transient (5-10 min) increased in heart rate (40-60 beats per minute (bpm] and arterial pressure (20-25 mmHg). Intravenous administration and administration to the second thoracic (T2) segment failed to mimic the effect of T9 administration, suggesting that the cardiovascular effects of T9 administration did not occur via diffusion to the periphery or to the brainstem. The cardioacceleratory and hypertensive responses to T9 dyn A (1-13) administration were prevented by pretreatment with the nicotinic ganglion blocker hexamethonium (10 mg/kg), but were unaffected by bilateral adrenalectomy. These results suggest that the cardiovascular effects of dyn A were mediated predominantly via a sympathetic pathway that does not innervate the adrenal glands. The effects were not antagonized by pretreatment with the opiate receptor antagonist naloxone or by the specific kappa opiate receptor antagonist nor-binaltorphimine, suggesting that they were not mediated via activation of kappa opiate receptors. Further support for this conclusion was provided by experiments demonstrating that dyn A (3-13) (30 nmol), a dynorphin fragment which is devoid of kappa activity, mimicked the effect of dyn A (1-13), whereas administration of the synthetic kappa agonist U50, 488H (100 nmol), failed to elicit effects similar to those provoked by dyn A (1-13). It is concluded that the cardiovascular effects of intrathecal dyn A administration are mediated via a nonopioid mechanism.

E-2078, a potent, selective and stable dynorphin analog with preferential activity for the kappa-opioid receptor subtype on the mouse vas deferens neuroeffector junction

The profile of opioid activity of E-2078, a synthetic stable dynorphin analog, was examined in the mouse vas deferens bioassay and compared to that of methionine enkephalin and nonpeptide kappa agonists in the absence and in the presence of selective antagonists for the mu-, kappa- and delta-opioid receptor subtypes. The inhibitory action of E-2078 and related kappa agonists was specifically and potently antagonized only by norbinaltorphimine, revealing the presence of kappa receptors in this tissue and the predominant kappa activity of E-2078.

Beta-funaltrexamine (beta-FNA) decreases deprivation and opioid-induced feeding

We studied the effect of the mu antagonist, beta-funaltrexamine (beta-FNA) on deprivation and opioid-induced feeding. Intracerebroventricular pre-treatment of 20 h deprived rats with 0.1, 1, 10 and 20 nmol of beta-FNA decreased feeding by 24%, 50%, 50% and 38% during the first hour. Central administration of beta-FNA (0.1, 1 and 10 nmol) also decreased feeding induced by the mu opioid agonist, DAMGO by 57%, 60% and 71%. Feeding induced by the delta agonist, DSLET, was decreased by pre-treatment with beta-FNA; but only during the 1-2 h time points, a time when relatively little food was ingested. Intraventricular injection of beta-FNA failed to alter feeding stimulated by the kappa opioid agonist, U-50,488H. These data further substantiate a role for the opioid receptor in deprivation and opioid-induced feeding.

Opioid activity of dermenkephalin analogues in the guinea-pig myenteric plexus and the hamster vas deferens

1. To elucidate the structural features required for selective and potent action of dermenkephalin at the delta-opioid receptor, a series of analogues of dermenkephalin and dermorphin were tested for their effectiveness in depressing electrically-evoked contractions of the vas deferens of the hamster (delta-opioid receptors) and the guinea-pig myenteric plexus-longitudinal muscle preparation (mu- and kappa-opioid receptors). 2. Dermenkephalin was more selective and more potent at delta-receptors than the delta-ligand [D-Pen2, D-Pen5]-enkephalin. The responses to dermenkephalin in the hamster vas deferens were increased by addition of peptidase inhibitors; the maximum effect was obtained with 3 microM thiorphan. 3. [L-Met2]-dermenkephalin had 0.2% and [L-Ala2]-dermorphin 0.01% of the agonist activity of the corresponding endogenous peptides which have D-amino acids in position 2. The pharmacological activity of these analogues was unaffected by inhibition of peptidases. This emphasizes the role that the D-configuration plays in determining the bioactive folding of these highly active peptides. 4. Dermenkephalin-(1-6)-NH2 was more potent at delta-receptors than at mu-receptors whereas, dermenkephalin-(1-4)-NH2 is a selective mu-agonist, having no activity at delta-receptors. 5. Substitution of the C-terminal tripeptide of dermorphin with the C-terminal tripeptide of dermenkephalin abolished the mu-receptor preference of dermorphin. The resulting hybrid peptide, Tyr-D-Ala-Phe-Gly-Leu-Met-Asp-NH2 was as potent as dermenkephalin at delta-receptors. A shift towards a preference for delta-receptors was obtained when the C-terminal tetrapeptide of dermorphin was replaced by the C-terminal tetrapeptide of dermenkephalin. 6. Substitution of Asp by Asn in position 7 of dermenkephalin caused an increase in mu-receptor potency and a decrease in delta-receptor potency, resulting in a 20 fold decrease in mu-receptor selectivity. Dermenkephalin-(1-6)-NH2 and [Asn7]-dermenkephalin have almost identical delta-receptor agonist potencies and ratios of IC50 in the myenteric plexus to IC50 in the hamster vas deferens. 7. The results obtained emphasise the importance of a negative charge at the C-terminus of dermenkephalin for selectivity at the delta-opioid receptor. Furthermore, the hydrophobic residues Leu5 and Met6 may be critical in ensuring tight binding to the receptor which results in high agonist potency.

Investigation of the different types of opioid receptor involved in electroconvulsive shock-induced antinociception and catalepsy in the rat

The effects of novel opioid antagonists on the behavioural syndrome induced by electroconvulsive shock (ECS) in rats have been examined and compared with those of the established agent naloxone. A single ECS produced catalepsy and significantly increased tail immersion response times during the 15 min following the seizure. These responses were inhibited by a low dose of naloxone (1 mg kg-1, i.p.) and also by RX8008M (16-methylcyprenorphine; 1 mg kg-1, i.p.) which blocks mu- and delta- but not kappa-opioid receptor function. In comparison, the antinociception and catalepsy induced by ECS was not attenuated by the selective delta-receptor antagonist naltrindole (1 mg kg-1, i.p.). These results suggest that ECS-induced antinociception and catalepsy may be mediated by endogenous opioids acting at mu-opioid receptors and are consistent with biochemical studies showing the release of beta-endorphin in both animals and man following this procedure.

Differential effects of selective opioid peptide antagonists on the acquisition of pavlovian fear conditioning

Pretreatment with opioid antagonists enhances acquisition of Pavlovian fear conditioning. The present experiments attempted to characterize the type of opioid receptor responsible for this effect using a procedure that assessed the fear of rats to a chamber previously associated with electric shock (1 mA, 0.75 s). Freezing, a species-typical immobility, was employed as an index of fear. Two mu opioid antagonists, CTOP (40 ng) and naloxonazine (10 micrograms), enhanced conditioning. On the other hand, the kappa antagonist nor-binaltorphimine reduced conditioning. Two delta antagonist treatments (16-methyl cyprenorphine and naltrindole) had no reliable effect on acquisition. Thus the enhancement of conditioning appears to be mediated by mu receptors. Previous research has shown that the conditional fear produced by these procedures caused an analgesia that is also mediated by mu receptors. It is argued that the enhancement effect occurs because of an antagonism of this analgesia and that the analgesia normally acts to regulate the level of fear conditioning.

Role of spinal kappa opioid receptors in the blockade of the development of antinociceptive tolerance to morphine

The site of action of the kappa opioid receptor agonist, U-50,488H in suppressing the development of tolerance to morphine antinociception was examined by local application, either intrathecal (i.t., spinal) or intracerebroventricular (i.c.v., supraspinal) in mice. Mice given morphine s.c., i.c.v. or i.t. daily developed tolerance regardless of the route. Co-administration of U-50,488H i.p. at a subanalgesic dose suppressed the development of tolerance to s.c. and i.t. administered morphine without affecting the antinociceptive effect of morphine. U-50,488H did not influence the development of tolerance to i.c.v. administered morphine. The antinociceptive effect of s.c. administered morphine was not affected by co-administration of U-50,488H given i.t. or i.c.v.; however, the development of tolerance to morphine was suppressed by i.t. but not i.c.v. administered U-50,488H. The suppressive effect of U-50,488H on the development of tolerance to morphine was abolished by pretreatment with nor-binaltorphimine (nor-BNI) given i.p. or i.t. Intracerebroventricularly administered nor-BNI failed to abolish the effect of U-50,488H. We suggest that U-50,488H suppresses the development of tolerance to morphine at the spinal level by interacting with kappa opioid receptors in this area.

Pharmacological characterization of the inhibitory activity of beta h-endorphin (beta h-EP), [Arg9,19,24,28,29]-beta h-EP, [Gln8,Gly31]-beta h-EP-Gly-Gly-NH2, in the neuroeffector junction of the mouse vas deferens

The inhibitory opioid activities of beta h-endorphin (beta h-EP), its structurally related peptide analogues [Gln8,Gly31]-beta h-EP-Gly-Gly-NH2 (Gly-Gly-beta h-EP), [Arg9,19,24,28,29]-beta h-EP (Arg-beta h-EP) and methionine enkephalin have been examined in the electrically stimulated mouse vas deferens bioassay. All four peptides behaved as full agonists; methionine enkephalin was the most potent followed by Arg-beta h-EP, beta h-EP and Gly-Gly-beta h-EP. Neither Gly-Gly-beta h-EP nor Arg-beta h-EP antagonized the inhibitory action of beta h-EP or methionine enkephalin. An hour of tissue exposure to 30 nM beta-funaltrexamine followed by thorough washing, displaced to the right, in a parallel fashion, the concentration-response curves of beta h-EP and analogues. Whereas the displacement of the concentration response curves was 8 to 10-fold for beta h-EP and Arg-beta h-EP, it was only about 3-fold for Gly-Gly-beta h-EP and methionine enkephalin. Naltrindole was the most potent antagonist of methionine enkephalin with an apparent pA2 of 9.4; its potency as an antagonist of beta h-EP and related analogues was approximately one-tenth of this with pA2 values approximately 8.5. Norbinaltorphimine also antagonized the action of the opioid peptides with pA2 values close to 7.8.

Inhibition of carbachol-stimulated phosphoinositide turnover by U-50,488H in rat hippocampus--involvement of GTP-binding protein

The effect of U-50,488H, a selective kappa-opioid agonist, on carbachol-stimulated phosphoinositide (PI) turnover response in rat hippocampal slices was examined. U-50,488H which stimulates PI turnover response in this preparation (Periyasamy and Hoss, 1990, Life Sci. 47, 219), inhibited carbachol-stimulated PI turnover in a concentration-dependent manner with an IC50 value of 33 +/- 9.0 microM. The inhibitory effect of U-50,488H was not blocked by the kappa-selective antagonists, e.g., nor-binaltorphimine (10 microM), and MR2266 (10 microM), or tetrodotoxin (1 microM) suggesting that the effect of U-50,488H was mediated neither through the kappa-receptors nor through the release of an endogenous neurotransmitter(s). A Lineweaver-Burke plot of the stimulation of PI turnover by carbachol in the presence and absence of U-50,488H showed that the Km was not changed (11.4 +/- 3.4 and 11.5 +/- 2.6 microM) whereas the Vmax was reduced from 3849 +/- 460 to 1534 +/- 31 cpm indicating that the inhibition was non-competitive. U-50,488H also inhibited guanosine 5'-[beta, gamma-imido]triphosphate (Gpp[NH]p)-stimulated PI turnover in rat hippocampal membranes in a concentration-dependent manner with an IC50 value of 33 +/- 12 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Ingestive behavior following central [D-Ala2, Leu5, Cys6]-enkephalin (DALCE), a short-acting agonist and long-acting antagonist at the delta opioid receptor

DALCE (1-40 micrograms, ICV), a short-acting agonist and long-acting antagonist at the delta opioid receptor, was examined for its effects upon food intake in rats under spontaneous, deprivation, glucoprivic and palatable conditions. DALCE (10 micrograms) significantly stimulated free feeding for up to 10 h but only minimally decreased (40 micrograms) food intake and body weight after 24-72 h. DALCE, administered prior to food deprivation (24 h), failed to affect subsequent 24-h intake and sporadically decreased intake and body weight change after 48-72 h. 2-Deoxy-D-glucose (650 mg/kg, IP) hyperphagia was transiently (2 h) decreased by long-term DALCE (10 micrograms) pretreatment. Hyperphagia following exposure to a high-fat diet was significantly potentiated by long-term DALCE (1 microgram) pretreatment. DALCE (10 micrograms) hyperphagia (2-10 h) was eliminated by central pretreatment with either naltrexone (20 micrograms) or the kappa antagonist, nor-binaltorphamine (20 micrograms) but was minimally affected by central pretreatment with the mu antagonist, beta-funaltrexamine (20 micrograms) or long-term DALCE (40 micrograms). The general inability of the antagonist actions of DALCE to alter these forms of feeding argues against a role for the delta opioid receptor in these responses.

Mediation of insulin hyperphagia by specific central opiate receptor antagonists

The hyperphagic properties of insulin (10 U/kg, s.c.) were transiently (2h) and dose-dependently inhibited (30%) by central pretreatment with naltrexone (20-50 micrograms, i.c.v.). The irreversible mu opioid antagonist, beta-funaltrexamine (B-FNA, 20 micrograms, i.c.v.) significantly inhibited insulin hyperphagia by 28-54% over the 6-h time course. In contrast, insulin hyperphagia was only transiently (2 h) inhibited (27-30%) by either the irreversible mu 1 antagonist, naloxonazine (50 micrograms, i.c.v.) or the selective kappa antagonist, nor-binaltorphamine (NorBNI, 20 micrograms, i.c.v.). The delta-antagonistic actions of [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, 40 micrograms, i.c.v.) failed to affect insulin hyperphagia. These data suggest that the mu 2 opioid receptor subtype modulates insulin hyperphagia.

Regulation of proopiomelanocortin messenger RNA concentrations by opioid peptides in primary cell cultures of rat hypothalamus

The effects of opioid peptides on a 1.1-kb long proopiomelanocortin messenger RNA (POMC mRNA) have been investigated in rat hypothalamic cells maintained in culture. Most opioid peptides exerted an inhibitory control on POMC mRNA steady-state concentrations. beta-Endorphin caused a 65% maximal inhibitory effect (IC50 = 6.1 x 10(-9) M) while slightly less inhibition was caused by Met- and Leu-enkephalin, dynorphin A and DADLE ([D-Ala2,D-Leu5] enkephalin). The effects of beta-endorphin and of Met-enkephalin were completely reversed by the delta opioid antagonist ICI 174,864 while the kappa-receptor specific antagonist binaltorphimine or the sigma-receptor specific antagonist DTG (1,3-di(2-tolyl) guanidine) respectively blocked the inhibitory actions of dynorphin A and of DADLE. The mu-receptor specific agonist DAGO ([D-Ala2,N-Me-Phe4,Gly5-OL]enkephalin) did not affect POMC mRNA levels. The failure of the dopaminergic D2 antagonist haloperidol to modify the inhibitory effects of opioid peptides argues for a direct inhibitory opioid peptide modulation of hypothalamic POMC mRNA levels mediated by the delta-, kappa- and sigma- (but not mu-) receptors in vivo.

Kappa-opioid antagonist strongly attenuates drinking of genetically polydipsic mice

Effects of opioid antagonists on the genetic polydipsia of the STR/N strain of mice were investigated. Naltrexone (0.5-5.0 mg/kg) injected subcutaneously before dark period attenuated spontaneous drinking for the first 3 h after injection only in the inbred polydipsic mice (STR/N), whose water intake was 5 times that of controls (non-polydipsic mutant, STR/1N, and Swiss/Webster mice). The highest dose (5 mg/kg) of naltrexone administration reduced drinking also during the next 3-6 h period and overnight feeding. Cerebroventricular (i.c.v.) injection of naltrexone, 1.0 and 2.5 micrograms (per mouse), suppressed drinking only in the polydipsic mice, while the higher dose (5.0 micrograms) attenuated drinking and feeding of both the polydipsic mice and their controls. However, i.c.v. injection of specific kappa-receptor antagonist, nor-binaltorphimine (nor-BNI, 0.5-2.5 micrograms), suppressed drinking only in the polydipsic strain of mice at one-half dose of that needed for naltrexone. Furthermore, even a higher dose of nor-BNI administration was without effect on food intake in all strains. These findings suggest that the central opioid system plays an important role in causing the polydipsia in the STR/N mice, probably through the kappa-opioid receptor.

Effects of parabrachial opioid antagonism on stimulation-induced feeding

The pontine parabrachial nucleus (PBN) contains gustatory relay neurons and a high concentration of opioid receptors. To investigate the involvement of PBN opioid activity in feeding behavior, antagonists were infused into the PBN bilaterally and effects on stimulation-induced feeding were determined. Naloxone, a mu-preferring antagonist, increased the lateral hypothalamic stimulation threshold for eliciting feeding behavior while nor-binaltorphimine, a kappa-selective antagonist, did not. Neither antagonist increased threshold when infused into dorsal pontine sites outside of the PBN or the fourth ventricle. In as much as PBN contains mu and kappa but no detectable delta receptors, the present results suggest that mu opioid activity within the PBN is involved in the mediation of feeding behavior.

Endogenous opioids tonically inhibit the depressor neurones in the caudal ventrolateral medulla of rabbits: mediation through delta- and kappa-receptors

In the present studies, an attempt was made to elucidate the role of endogenous opioid inputs to the depressor region of the caudal ventrolateral medulla in the tonic regulation of arterial pressure and to examine the subtype(s) of receptor underlying any observed effects by use of receptor-specific antagonists. The depressor region of the caudal ventrolateral medulla in chloralose-anesthetized, artificially ventilated rabbits was functionally identified by injection of l-glutamate (5 nmol). Bilateral injection of the non-selective opioid antagonist naloxone (0.3, 5 and 20 nmol) into the caudal ventrolateral medulla produced a dose-dependent depressor response, accompanied by a bradycardia, suggesting a tonically active inhibitory opioid input to this region. Bilateral injection of the selective delta-receptor antagonist ICI 174,864 (0.3 nmol) or of the kappa-receptor antagonist nor-binaltorphimine (1 nmol), also markedly reduced both arterial pressure and heart rate. In contrast, injection of the mu-selective antagonist beta-funaltrexamine (0.3-0.6 nmol) produced no effect on arterial pressure or heart rate. These data support the hypothesis that tonically active endogenous opioid inputs, possibly enkephalinergic and/or dynorphinergic, inhibit the depressor neurones of the caudal ventrolateral medulla in the rabbit through activation of delta- and kappa-receptors. Surprisingly, injection of the opioid agonists leu-enkephalin (1 nmol) or dynorphin 1-13 (0.1 nmol), but not the selective mu-receptor agonist DAGO (1 nmol), in the depressor area of the caudal ventrolateral medulla also induced naloxone-sensitive (5 mg/kg, i.v.) decreases in both arterial pressure and heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

The dopamine D2 agonists RU 24213 and RU 24926 are also kappa-opioid receptor antagonists

The di-(phenethyl)-amine derivatives, RU 24213 and RU 24926 are widely used as selective dopamine D2-receptor agonists. Binding studies now show that they also have affinity for the kappa-opioid receptor. Their affinity is not greatly reduced in the presence of NaCl/GTP, suggesting an antagonist action. This is confirmed for RU 24926, the more active of the two, using the field-stimulated rabbit vas-deferens. With respect to the mu-receptor, RU 24926 shows low binding affinity and also an antagonist effect. These results demonstrate kappa- as well as mu-antagonist activity in a novel chemical series. Moreover, since the kappa-receptor may regulate dopamine release, the results of experiments using these compounds as dopamine D2 agonists should be interpreted with caution.

Bivalent ACTH antagonists: influence of peptide and spacer components on potency enhancement

The antagonist potency of a series of bivalent adrenocorticotropic hormone (ACTH) peptides was examined using suspensions of inner zone rat adrenocortical cells. Bivalent antagonists were prepared by bis(maleimide) covalent cross-linking of carboxyl terminal cysteine sulfhydryl groups of synthetic ACTH peptides, Cys25 ACTH(7-25) and Cys39 ACTH(7-39). Antagonist potency enhancement was defined by shifts in ACTH(1-39) concentration-steroidogenic response curves relative to monovalent antagonist analogs. The EC50 values measured in the presence of 0.5 microM monovalent antagonist were 110 +/- 28 pM for Cys25 ACTH(7-25)-S-N-ethylsuccinimide and 44 +/- 9 pM for Cys39 ACTH(7-39)-S-N-ethylsuccinimide. Some bivalent ACTH antagonists displayed much greater antagonist potency than their monovalent analogs, which supports the findings of Stolz and Fauchere (Helv Chim Acta 71: 1421-1428, 1988). The level of potency enhancement, however, was found to be dependent upon the spacer used to link receptor binding domains and the length of the ACTH peptide used in bivalent antagonist synthesis. The most potent inhibitor, bis(Cys25 ACTH(7-25)-S-succinimidopropionyl)2-hydroxy-1,3-propanediamine, was 28 times more potent than its monovalent analog, Cys25 ACTH(7-25)-S-N-ethylsuccinimide. However, a bivalent Cys25 ACTH(7-25) peptide containing two bis(succinimidopropionyl)2-hydroxy-1,3-propanediamine spacers that had been linked end-to-end via dithioerythritol showed no potency enhancement. Cys25 ACTH(7-25) based peptides containing one receptor binding domain and having the structure peptide-spacer-cysteine displayed no enhancement in antagonist potency. Bivalent Cys39 ACTH(7-39) linked by bis(succinimidopropionyl)2-hydroxy-1,3-propanediamine spacer exhibited only 4-fold enhancement in antagonist potency relative to Cys39 ACTH(7-39)-S-N-ethylsuccinimide. We therefore conclude that the potency enhancement observed with bivalent ACTH peptides: (1) is optimal with spacers less than approximately 40 A in length, (2) is not due to direct interactions between the spacer and cell surface, and (3) is dependent on the length of the ACTH peptide component. In addition, these results indicate that electrostatic interaction between bivalent ACTH peptides and plasma membrane lipids does not adequately account for the potency enhancements observed.

Effects of selective opiate antagonists on morphine-induced hyperalgesia in domestic fowl

Although morphine typically produces analgesia in a variety of species, recent research has identified a biological model in which morphine produces a naloxone-reversible, paradoxical hyperalgesic response to a noxious thermal stimulus in young domestic fowl. The present study examined opioid receptor-mediation of this atypical opiate effect. Patterns of morphine hyperalgesia (1.25 to 5.0 mg/kg IM) were examined on a standard hot-plate test following administration (10 micrograms/5 microliters ICV) of the mu antagonist beta-funaltrexamine, the delta antagonist naltrindole, or the kappa antagonist nor-binaltorphimine in 15-day-old White Leghorn cockerels. Respiration measures were also recorded because they are indicative of opiate effects. Morphine produced a dose-dependent decrease in mean jump latencies (i.e., hyperalgesic effect). Mu receptor antagonism attenuated this morphine-induced hyperalgesic effect. Kappa receptor antagonism attenuated morphine-induced hyperalgesia only at the highest morphine dose (i.e., 5.0 mg/kg) and delta receptor antagonism failed to attenuate morphine-induced hyperalgesia. These results suggest that morphine-induced hyperalgesia, like morphine-induced analgesia, is mediated primarily by mu receptor activation.

Opioids regulate the release of human chorionic gonadotropin hormone from trophoblast tissue

Opioid ligands were investigated for their effect on hCG release from trophoblast tissue obtained from term human placenta. Data obtained indicate that opiate agonists stimulate in vitro basal hCG release from trophoblast tissue. The potency of these opioid agonists correspond to their kappa receptor selectivity, i.e., the greater the selectivity the lower is the effective concentration causing maximum stimulation. Opioid antagonists inhibit the release of hCG due to their reversal of the stimulation caused by endogenous opioid peptides. Potency of the antagonists correspond also to their kappa receptor selectivity. Antagonists reverse the stimulation of hCG release caused by agonists indicating that the ligand's action is mediated by the placental kappa opioid receptors. The bell shaped response curves for agonists and antagonists suggest that opioids play a role in the regulation of hCG release from trophoblast tissue, but other mechanism(s) may also exist.

Opioid innervation of the caudal ventrolateral medulla is not critical for the expression of the aortic depressor nerve response in the rabbit

We investigated the influence of endogenous opioids in the caudal ventrolateral medulla (CVLM) on the expression of the baroreflex response induced by the electrical stimulation (50 Hz, 0.2 ms, 11 V, 10 s) of the aortic depressor nerve. We used microinjection of selective opioid antagonists into the functionally identified depressor area of the CVLM in chloralose-anesthetized rabbits. Injection of vehicles or the mu-antagonist beta-funaltrexamine (0.3 nmol) into the CVLM had no effects, while naloxone (20 nmol), ICI 174,864 (delta-antagonist, 0.3 nmol) or nor-binaltorphimine (kappa-antagonist, 1 nmol) abolished the depressor response, but themselves all elicited a tonic depressor effect as well. In contrast, intravenous naloxone (5 mg/kg) induced a small but significant increase in arterial pressure and did not alter the depressor response. Hypotensive hemorrhage induced a decrease in arterial pressure similar to that seen with local injection of naloxone into the CVLM, but did not change the reflex, suggesting that the reflex abolition was not due to the decrease in basal arterial pressure per se. CVLM injection of glutamate (10 nmol) or the GABA-antagonist bicuculline (0.1 nmol), non-opioid agents which activate CVLM and induce a tonic depressor effect, also abolished the depressor response suggesting that the reflex abolition was secondary to general activation or disinhibition of the CVLM. Thus, although the CVLM is tonically inhibited by endogenous opioid inputs acting via delta- and kappa-receptors, our data provide no evidence that opioid neurons which provide input to this region constitute a specific and integral component in mediating the aortic depressor response. However, the more general role that opioids play in tonically influencing the resting level of activity in the CVLM, is nevertheless very important in enabling the normal expression of this baroreflex.