Selectivity of dynorphin for kappa opioid receptors

The life and times of endogenous opioid peptides: Updated understanding of synthesis, spatiotemporal dynamics, and the clinical impact in alcohol use disorder

The opioid G-protein coupled receptors (GPCRs) strongly modulate many of the central nervous system structures that contribute to neurological and psychiatric disorders including pain, major depressive disorder, and substance use disorders. To better treat these and related diseases, it is essential to understand the signaling of their endogenous ligands. In this review, we focus on what is known and unknown about the regulation of the over two dozen endogenous peptides with high affinity for one or more of the opioid receptors. We briefly describe which peptides are produced, with a particular focus on the recently proposed possible synthesis pathways for the endomorphins. Next, we describe examples of endogenous opioid peptide expression organization in several neural circuits and how they appear to be released from specific neural compartments that vary across brain regions. We discuss current knowledge regarding the strength of neural activity required to drive endogenous opioid peptide release, clues about how far peptides diffuse from release sites, and their extracellular lifetime after release. Finally, as a translational example, we discuss the mechanisms of action of naltrexone (NTX), which is used clinically to treat alcohol use disorder. NTX is a synthetic morphine analog that non-specifically antagonizes the action of most endogenous opioid peptides developed in the 1960s and FDA approved in the 1980s. We review recent studies clarifying the precise endogenous activity that NTX prevents. Together, the works described here highlight the challenges and opportunities the complex opioid system presents as a therapeutic target.

Turning the 'Tides on Neuropsychiatric Diseases: The Role of Peptides in the Prefrontal Cortex

Recent advancements in technology have enabled researchers to probe the brain with the greater region, cell, and receptor specificity. These developments have allowed for a more thorough understanding of how regulation of the neurophysiology within a region is essential for maintaining healthy brain function. Stress has been shown to alter the prefrontal cortex (PFC) functioning, and evidence links functional impairments in PFC brain activity with neuropsychiatric disorders. Moreover, a growing body of literature highlights the importance of neuropeptides in the PFC to modulate neural signaling and to influence behavior. The converging evidence outlined in this review indicates that neuropeptides in the PFC are specifically impacted by stress, and are found to be dysregulated in numerous stress-related neuropsychiatric disorders including substance use disorder, major depressive disorder (MDD), posttraumatic stress disorder, and schizophrenia. This review explores how neuropeptides in the PFC function to regulate the neural activity, and how genetic and environmental factors, such as stress, lead to dysregulation in neuropeptide systems, which may ultimately contribute to the pathology of neuropsychiatric diseases.

Incisional Injury Modulates Morphine Reward and Morphine-Primed Reinstatement: A Role of Kappa Opioid Receptor Activation

BACKGROUND

Persistent use of prescription opioids beyond the period of surgical recovery is a large part of a public health problem linked to the current opioid crisis in the United States. However, few studies have been conducted to examine whether morphine reward is influenced by acute pain and injury.

METHODS

In a mouse model of incisional injury and minor trauma, animals underwent conditioning, extinction, and drug-primed reinstatement with morphine to examine the rewarding properties of morphine in the presence of acute incisional injury and drug-induced relapse, respectively. In addition, we sought to determine whether these behaviors were influenced by kappa opioid receptor signaling and measured expression of prodynorphin messenger RNA in the nucleus accumbens and medial prefrontal cortex after conditioning and before reinstatement with morphine and incisional injury.

RESULTS

In the presence of incisional injury, we observed enhancement of morphine reward with morphine-conditioned place preference but attenuated morphine-primed reinstatement to reward. This adaptation was not present in animals conditioned 12 days after incisional injury when nociceptive sensitization had resolved; however, they showed enhancement of morphine-primed reinstatement. Prodynorphin expression was greatly enhanced in the nucleus accumbens and medial prefrontal cortex of mice with incisional injury and morphine conditioning and remained elevated up to drug-primed reinstatement. These changes were not observed in mice conditioned 12 days after incisional injury. Further, kappa opioid receptor blockade with norbinaltorphimine before reinstatement reversed the attenuation induced by injury.

CONCLUSIONS

These findings suggest enhancement of morphine reward as a result of incisional injury but paradoxically a protective adaptation with incisional injury from drug-induced relapse resulting from kappa opioid receptor activation in the reward circuitry. Remote injury conferred no such protection and appeared to enhance reinstatement.

General Pathways of Pain Sensation and the Major Neurotransmitters Involved in Pain Regulation

Pain has been considered as a concept of sensation that we feel as a reaction to the stimulus of our surrounding, putting us in harm's way and acting as a form of defense mechanism that our body has permanently installed into its system. However, pain leads to a huge chunk of finances within the healthcare system with continuous rehabilitation of patients with adverse pain sensations, which might reduce not only their quality of life but also their productivity at work setting back the pace of our economy. It may not look like a huge deal but factor in pain as an issue for majority of us, it becomes an economical burden. Although pain has been researched into and understood by numerous researches, from its definition, mechanism of action to its inhibition in hopes of finding an absolute solution for victims of pain, the pathways of pain sensation, neurotransmitters involved in producing such a sensation are not comprehensively reviewed. Therefore, this review article aims to put in place a thorough understanding of major pain conditions that we experience-nociceptive, inflammatory and physiologically dysfunction, such as neuropathic pain and its modulation and feedback systems. Moreover, the complete mechanism of conduction is compiled within this article, elucidating understandings from various researches and breakthroughs.

Is the experience of thermal pain genetics dependent?

It is suggested that genetic variations explain a significant portion of the variability in pain perception; therefore, increased understanding of pain-related genetic influences may identify new targets for therapies and treatments. The relative contribution of the different genes to the variance in clinical and experimental pain responses remains unknown. It is suggested that the genetic contributions to pain perception vary across pain modalities. For example, it has been suggested that more than 60% of the variance in cold pressor responses can be explained by genetic factors; in comparison, only 26% of the variance in heat pain responses is explained by these variations. Thus, the selection of pain model might markedly influence the magnitude of the association between the pain phenotype and genetic variability. Thermal pain sensation is complex with multiple molecular and cellular mechanisms operating alone and in combination within the peripheral and central nervous system. It is thus highly probable that the thermal pain experience is affected by genetic variants in one or more of the pathways involved in the thermal pain signaling. This review aims to present and discuss some of the genetic variations that have previously been associated with different experimental thermal pain models.

From local to global-fifty years of research on Salvia divinorum

ETHNOPHARMACOLOGICAL RELEVANCE

In 1962 ethnopharmacologists, Hofmann and Wasson, undertook an expedition to Oaxaca, Mexico. These two researchers were the first scientists to collect a flowering specimen of Salvia divinorum allowing the identification of this species. While the species' traditional use is confined to a very small region of Mexico, since Hofmann and Wasson's expedition 50 years ago, Salvia divinorum has become globally recognized for its main active constituent, the diterpene salvinorin A, which has a unique effect on human physiology. Salvinorin A is a kappa-opioid agonist and the first reported psychoactive diterpene.

METHODS

This review concentrates on the investigation of Salvia divinorum over the last 50 years including ethnobotany, ethnopharmacology, taxonomy, systematics, genetics, chemistry and pharmacodynamic and pharmacokinetic research. For the purpose of this review, online search engines were used to find relevant research. Searches were conducted between October 2011 and September 2013 using the search term "Salvia divinorum". Papers were excluded if they described synthetic chemical synthesis of salvinorin A or analogues.

RESULTS

Ethnobotanically there is a comprehensive body of research describing the traditional Mazatec use of the plant, however, the modern ethnobotanical use of this plant is not well documented. There are a limited number of botanical investigations into this plant and there are still several aspects of the botany of Salvia divinorum which need further investigation. One study has investigated the phylogenetic relationship of Salvia divinorum to other species in the genus. To date the main focus of chemistry research on Salvia divinorum has been salvinorin A, the main active compound in Salvia divinorum, and other related diterpenoids. Finally, the effects of salvinorin A, a KOR agonist, have primarily been investigated using animal models.

CONCLUSIONS

As Salvia divinorum use increases worldwide, the emerging cultural use patterns will warrant more research. More botanical information is also needed to better understand this species, including germination, pollination vector and a better understanding of the endemic environment of Salvia divinorum. As well there is a gap in the genetic knowledge of this species and very little is known about its intra-species genetics. The terpenes in Salvia divinorum are very well documented, however, other classes of constituents in this species warrant further investigation and identification. To date, the majority of the pharmacology research on Salvia divinorum has focused on the effects of salvinorin A using animal models. Published human studies have not reported any harmful effects when salvinorin A is administered within the dose range of 0.375-21µg/kg but what are the implications when applied to a larger population? More data on the toxicology and safety of Salvia divinorum are needed before larger scale clinical trials of the potential therapeutic effects of Salvia divinorum and salvinorin A are undertaken.

Involvement of the opioid system in the orexigenic and hedonic effects of melanin-concentrating hormone

Melanin-concentrating hormone (MCH) exerts an orexigenic effect that resembles that of opioids, suggesting that the MCH and opioid systems could interact in controlling the food intake behavior. Three series of experiments were conducted in male Wistar rats: 1) to test the ability of the κ-, μ-, and δ-opioid receptor antagonists binaltorphimine (nor-BNI-κ), β-funaltrexamine (β-FNA-μ), and naltrindole (NTI-δ), respectively, to block the stimulating effects of MCH on food intake; 2) to verify the ability of MCH to induce a positive hedonic response to a sweet stimulus when injected into the nucleus accumbens shell (NAcSh) or right lateral ventricle (LV) of the brain; and 3) to assess the ability of nor-BNI, β-FNA, and NTI to block the effects of MCH on the hedonic response to a sweet stimulus. Nor-BNI, NTI (0, 10 and 40 nmol), and β-FNA (0, 10 and 50 nmol) were administered into the LV prior to injecting MCH (2.0 nmol). To assess the hedonic response, rats were implanted with an intraoral cannula allowing for the infusion of a sweet solution into the oral cavity. Food intake was assessed in sated rats during the first 3 h following the MCH or vehicle (i.e., artificial cerebrospinal fluid) injection. The hedonic response to a sweet stimulus was assessed by examining facial mimics, following the intraoral administration of a sucrose solution. Blockade of each of the three opioid receptors by selective antagonists prevented MCH-induced feeding. Furthermore, MCH-injections into the NAcSh and right LV resulted in enhanced hedonic responses. Finally, antagonism of the three opioid receptors blunted the LV-injected, MCH-induced, facial-liking expressions in response to an intraoral sweet stimulus. Overall, the present study provides evidence to link the MCH and opioid systems in the food intake behavior.

Big dynorphin as a putative endogenous ligand for the kappa-opioid receptor

The diversity of peptide ligands for a particular receptor may provide a greater dynamic range of functional responses, while maintaining selectivity in receptor activation. Dynorphin A (Dyn A), and dynorphin B (Dyn B) are endogenous opioid peptides that activate the kappa-opioid receptor (KOR). Here, we characterized interactions of big dynorphin (Big Dyn), a 32-amino acid prodynorphin-derived peptide consisting of Dyn A and Dyn B, with human KOR, mu- (hMOR) and delta- (hDOR) opioid receptors and opioid receptor-like receptor 1 (hORL1) expressed in cells transfected with respective cDNA. Big Dyn and Dyn A demonstrated roughly similar affinity for binding to hKOR that was higher than that of Dyn B. Dyn A was more selective for hKOR over hMOR, hDOR and hORL1 than Big Dyn, while Dyn B demonstrated low selectivity. In contrast, Big Dyn activated G proteins through KOR with much greater potency, efficacy and selectivity than other dynorphins. There was no correlation between the rank order of the potency for the KOR-mediated activation of G proteins and the binding affinity of dynorphins for KOR. The rank of the selectivity for the activation of G proteins through hKOR and of the binding to this receptor also differed. Immunoreactive Big Dyn was detected using the combination of radioimmunoassay (RIA) and HPLC in the human nucleus accumbens, caudate nucleus, hippocampus and cerebrospinal fluid (CSF) with the ratio of Big Dyn and Dyn B being approximately 1:3. The presence in the brain implies that Big Dyn, along with other dynorphins, is processed from prodynorphin and secreted from neurons. Collectively, the high potency and efficacy and the relative abundance suggest that Big Dyn may play a role in the KOR-mediated activation of G proteins.

Inhibition of oestradiol-induced DNA synthesis by opioid peptides in the rat uterus

Opioid drugs and peptides were investigated for their effect on uterine DNA synthesis induced by a single injection of 17 beta-oestradiol given to ovariectomized rats 24 h prior to decapitation. [D-Met2,Pro5]-enkephalinamide administered 12, 2 or 1 h before killing resulted in a significant (approximately 50%) inhibition of in vitro [3H]-thymidine incorporation into DNA, while injections given 24 or 6 h before decapitation were ineffective. Non-linear regression of the dose-effect curves resulted in an ED50 of approximately 0.26 and approximately 0.45 microgram/100 g b.wt. for the opioid treatments given 12 or 2 h before killing, respectively. These effects could be completely reversed by the opioid antagonist naloxone injected 30 min prior to the agonist treatment, while naloxone itself had no effect. Morphine and [D-Ala2,D-Leu5]-enkephalin administered 12 h, as well as dynorphin A fragment 1-13 given 2 h before decapitation also inhibited oestradiol-induced uterine DNA synthesis. In ovariectomized animals without 17 beta-oestradiol priming no significant effect of [D-Met2,Pro5]-enkephalinamide or naloxone on [3H]TdR incorporation was found.

Lithium and haloperidol differentially alter the dynorphin A (1-8) and enkephalin levels in the neurointermediate lobe of rat pituitary

Repeated administration of the antimanic drug lithium (4 mEq/kg/day for 2, 4 or 6 days, i.p.) to rats produced a progressive decline and eventual depletion of dynorphin-A (1-8) (DYN) concentration whereas Met5-enkephalin (ENK) was only marginally decreased in the neurointermediate lobe of the pituitary (NIL). Administration of a neuroleptic haloperidol neither affected ENK and DYN levels nor influenced lithium-induced changes. The study reveals that lithium produces a preferential perturbation in the dynorphin system relative to the enkephalin system. These results taken together with other evidence, indicate that dynorphin is possibly coreleased with vasopressin following lithium administration and provide a pharmacological support to the previously described colocalization and corelease of these endogenous peptides in the NIL.

Characterization of kappa opioid receptors in neurosecretosomes from bovine posterior pituitary

The binding properties of opioid receptors on isolated nerve terminals (neurosecretosomes) from bovine posterior pituitaries were characterized. Both [3H]etorphine and [3H]ethylketocyclazocine ([3H]EKC) showed high-affinity binding with complex binding isotherms, consistent with the presence of multiple classes of binding sites. [D-Ala2,D-Leu5]enkephalin showed no specific binding and failed to displace [3H]etorphine at high concentrations, indicating the absence of mu, delta, or benzomorphan (kappa 2) sites. Mathematical modelling of the data suggested the presence of three classes of binding sites. The first was of high affinity with Kd values of 0.9 and 2.0 nM for etorphine and EKC, respectively. The second class of sites appeared to bind etorphine with a KD of 150 nM, and EKC with extremely low affinity (unmeasurable binding). The third class of sites was characterized by KD values of 7 and 2 microM for etorphine and EKC, respectively. These results indicate that the nerve terminals of bovine posterior pituitary contain opioid binding sites of the kappa type. Furthermore, these binding sites appear heterogeneous, consisting of at least two and possibly more subtypes or states.

Morphine- and ketocyclazocine-induced analgesia in the developing rat: differences due to type of noxious stimulus and body topography

Patterns of morphine- and ketocyclazocine-induced analgesia in limb withdrawal and tail-flick tests of thermal and mechanical nociception were examined in the preweanling rat. In the forepaw test, morphine was more effective than ketocyclazocine with both thermal and mechanical stimuli. Both drugs first induced analgesia between 3 and 5 days of age. In the tail-flick test, ketocyclazocine-induced analgesia preceded morphine's effects against both thermal and mechanical stimuli by several days. Ketocyclazocine produced robust analgesia between 7 and 10 days of age, while the effects of morphine did not peak until day 14. In the hindpaw, morphine was more effective than ketocyclazocine against a higher intensity mechanical stimulus, while ketocyclazocine was more effective against a lower intensity mechanical stimulus. Morphine-induced analgesia was reversed by lower doses of naloxone than was ketocyclazocine-induced analgesia, regardless of body part tested, against all noxious stimuli. These findings demonstrate differences in morphine- and ketocyclazocine-induced analgesia that are dependent upon age, body topography, stimulus type and intensity and imply different physiologic roles of mu- and chi-opioid receptors in analgesia.

Effects of dynorphin on the PHA-induced lymphocyte proliferation in vitro

The effect of the opioid peptide dynorphin (DYN) on PHA-induced lymphocyte proliferation has been evaluated in the present study. A significant increase in PHA-induced lymphocyte activation was observed when DYN was added to cultures 48 hr after the mitogenic stimulation. This effect occurred at suboptimal (3.12 and 6.25 micrograms/ml) PHA concentrations and at DYN doses ranging from 10(-9) to 10(-12) M. Conversely, DYN did not affect the lymphocyte blastogenic process either when added before, or simultaneously or after intense PHA (12.5 micrograms/ml) stimulation. Naloxone preincubation did not modify the above described effects. Results suggest that DYN might play a role in neuroendocrine regulation of the lymphocyte blastogenic process.

Dynorphin(1-8) immunoreactivity in brainstem and hypothalamic nuclei of normotensive and age-matched hypertensive rat strains

The concentration of dynorphin (1-8) immunoreactivity [ir-dyn(1-8)] was measured in 10 hypothalamic and 11 brainstem nuclei of Sprague-Dawley (SD) and 6- and 14-week old Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats. The highest concentrations of ir-dyn(1-8) were found in the lateral preoptic and lateral hypothalamic areas of the hypothalamus and the solitary tract nucleus of the brainstem. Levels of the peptide were low in other brainstem nuclei compared to hypothalamic areas. There was a significant reduction in ir-dyn(1-8) concentrations at 14 weeks of age compared to 6 weeks of age in all 9 nuclei examined in SH and WKY rats. However, there were no differences between the strains at either age. These changes may be related to the increase in blood pressure that occurs in both SH and WKY rats over this age range although other factors must also be involved to produce the higher blood pressure levels of the SH rat.

Intracerebroventricular morphine produces diuresis 24 h after previous dynorphin/morphine treatment

The effect of dynorphin A-(1-13) on morphine-induced urine output was studied in the rat. The previous simultaneous intracerebroventricular (i.c.v.) injection of dynorphin (20 micrograms/rat) and morphine (20 micrograms/rat) altered the response of rat given morphine (20 micrograms/rat) 24 h later, producing a 3-fold increase in urine output. In contrast, previous injection of dynorphin or morphine alone had no effect.

Kappa opioid receptor-mediated analgesia in the developing rat

The prototypic kappa opiate ketocyclazocine produced robust analgesia in 10-day-old rats in the tail-flick nociceptive test. The kappa-opiate behavioral response coincided with the onset of a rapid rise to adult levels in brain kappa receptor site density. In contrast, morphine (prototypic mu opiate) was without marked effect until 14 days of age. The period of rapid mu receptor increase did not take place until days 14-16, which was after kappa receptor levels had already plateaued. Further, there was no or incomplete cross-tolerance between ketocyclazocine and morphine at 14 days of age. The present study, therefore, establishes a role for the kappa binding site in thermal analgesia in the tail flick test and differentiates its ontogenetic pattern from that of the mu receptor.

Centrally-administered opioid selective agonists inhibit drinking in the rat

The effects of intracerebroventricular injection of mu (morphine), kappa (dynorphin-(1-13), ethylketocyclazocine, and U50,488H), and delta ([D-Pen2, D-Pen5]enkephalin) opioid agonists on water intake of 14 hr water deprived rats was studied. All agonists caused a dose related decrease in time spent drinking, with a rank order potency of dynorphin-(1-13) greater than morphine greater than ethylketocyclazocine greater than [D-Pen2, D-Pen5]enkephalin = U50, 488H. With the exception of morphine, all of the compounds increased the latency to begin drinking, but only at the highest doses tested. The rank order potency for this endpoint was dynorphin-(1-13) = ethylketocyclazocine greater than [D-Pen2, D-Pen5]enkephalin greater than U50, 488H. The potent inhibition of drinking following centrally-given dynorphin-(1-13), at doses that did not affect the latency to begin drinking, supports a role for endogenous dynorphin in the homeostatic control of water balance. This function may not be primarily mediated through activation of a kappa opioid receptor since dynorphin-(1-13) was 80-230 times more potent than the selective kappa agonist, U50,488H or ethylketocyclazocine.

Interaction of [3H](-)-SKF-10,047 with brain sigma receptors: characterization and autoradiographic visualization

The sigma opiates differ from other opiates in their stimulatory and psychotomimetic actions. The sigma opiate [3H](-)-SKF-10,047 has been used to characterize sigma receptors in rat nervous tissue. Binding of [3H](-)-SKF-10,047 to rat brain membranes was of high affinity, saturable, and reversible. Scatchard analysis revealed the apparent interaction of this drug with two distinct binding sites characterized by affinities of 0.03 and 75 nM (5 mM Tris-HCl buffer, pH 7.4, at 4 degrees C). Competition analyses involving rank order determinations for a series of opiates and other drugs indicate that the high-affinity binding site is the mu opiate receptor. The lower-affinity site (revealed after suppression of mu and delta receptor binding) has been identified as the sigma opiate/phencyclidine receptor. In vitro autoradiography has been used to visualize neuroanatomical patterns of receptors labeled using [3H](-)-SKF-10,047 in the presence of normorphine and [D-Ala2,D-Leu5]enkephalin to block mu and delta interactions, respectively. Labeling patterns differ markedly from those for mu, delta, or kappa receptors. The highest densities (determined by quantitative autoradiography) are found in the medial portion of the nucleus accumbens, amygdaloid nucleus, hippocampal formation, central gray, locus coeruleus, and the parabrachial nuclei. Receptors in these structures could account for the stimulatory, mood-altering, and analgesic properties of the sigma opiates. Although not the most selective sigma opiate ligand, [3H](-)-SKF-10,047 binds to sigma opiate receptors in brain, and this interaction can be readily distinguished from its interactions with other classes of brain opiate receptors.

Selected opioids, ethanol and intake of ethanol

Rats were given an opportunity to drink tap water or a sweetened ethanol solution once a day. Across initial days of opportunity, rats increased their intake of the ethanol solution. Prior to some days' sessions with presented fluids, rats received either an injection of placebo (the carrier of drugs) or doses of ethylketocyclozocine, diprenorphine, or ethanol. Diprenorphine increased rats' intake of the ethanol solution compared to placebo. The other agents did not reliably modify intakes. These findings support a conclusion that selected activity in opioid systems of brain increase the propensity to drink alcoholic beverages.

Self-administration of dynorphin-[1-13] and D-ala2-dynorphin-[1-11] (kappa opioid agonists) in morphine (mu opioid agonist)-dependent rats

Adult female Sprague-Dawley rats were prepared with permanent cortical EEG and temporalis EMG electrodes and i.v. cannulae. They were made tolerant to and physically dependent on morphine by automatic, hourly injections. These physically dependent rats were then trained to lever press for 10 mg/kg injections of morphine on a fixed ratio (FR) schedule of reinforcement. Upon stabilization of morphine self-administration at a FR-10, dynorphin-[1-13] (DYN) or D-ala2-dynorphin-[1-11] (D-ala2-DYN) at doses of 125 or 250 micrograms/kg/inj was substituted for morphine. Rats self-administered these opioid-like peptides at both dose levels. As expected, self-injections were more numerous at the lower dose. No signs of morphine withdrawal were seen during the peptide substitutions. Following DYN or D-ala2-DYN abstinence, no withdrawal symptoms were noted. The question is raised as to whether DYN or D-ala2-DYN and morphine are producing their reinforcing effects in sustaining self-administration via the same receptor populations. Since morphine abstinence is associated with severe withdrawal symptoms and the peptides studied are not, the involvement of separate receptor populations in the process of dependence on morphine and these opioid-like peptides is indicated. In conclusion, both a mu and two kappa agonists exhibited an analogous reinforcing property in the rat. However, the degree of physical dependence and the intensity of withdrawal differed; being higher with the mu agonist and lower with the kappa agonists.

Preparation of brain membranes containing a single type of opioid receptor highly selective for dynorphin

Opioid receptors on guinea pig brain membranes were alkylated by the naltrexone analogue beta-chlornaltrexamine. Binding of the prototypical mu and kappa ligands, [3H]dihydromorphine and [3H]ethylketocyclazocine, was more readily affected by the reagent than was binding of the delta ligand, 3H-labeled [D-Ala2, D-Leu5]enkephalin. Treatment of membranes with beta-chlornaltrexamine in the presence of dynorphin resulted in significant protection of [3H]ethylketocyclazocine binding sites, without protection of [3H]dihydromorphine or 3H-labeled [D-Ala2, D-Leu5]enkephalin sites. Similarly, [D-Ala2, D-Leu5]enkephalin and sufentanil selectively protected binding sites for 3H-labeled [D-Ala2, D-Leu5]enkephalin and [3H]dihydromorphine, respectively. Scatchard analysis of [3H]ethylketocyclazocine binding to untreated membranes suggested two types of binding site with 40-fold difference in affinities. Membranes treated with beta-chlornaltrexamine in the presence of dynorphin retained about 40% of the high-affinity sites and lost the low-affinity sites. Selective protection of sites with high affinity for dynorphin and ethylketocyclazocine was confirmed in competition binding assays. These results strongly suggest that the three types of opioid receptor are not interconvertible and provide further evidence that the endogenous peptide dynorphin is a highly selective ligand of the kappa opioid receptor.