Mechanism of diuretic action of spiradoline (U-62066E)--a kappa opioid receptor agonist in the human

Exploring the neurobiology of the premonitory phase of migraine preclinically - a role for hypothalamic kappa opioid receptors?

Background

The migraine premonitory phase is characterized in part by increased thirst, urination and yawning. Imaging studies show that the hypothalamus is activated in the premonitory phase. Stress is a well know migraine initiation factor which was demonstrated to engage dynorphin/kappa opioid receptors (KOR) signaling in several brain regions, including the hypothalamus. This study proposes the exploration of the possible link between hypothalamic KOR and migraine premonitory symptoms in rodent models.

Methods

Rats were treated systemically with the KOR agonist U-69,593 followed by yawning and urination monitoring. Apomorphine, a dopamine D1/2 agonist, was used as a positive control for yawning behaviors. Urination and water consumption following systemic administration of U-69,593 was also assessed. To examine if KOR activation specifically in the hypothalamus can promote premonitory symptoms, AAV8-hSyn-DIO-hM4Di (Gi-DREADD)-mCherry viral vector was microinjected into the right arcuate nucleus (ARC) of female and male KOR^CRE or KOR^WT mice. Four weeks after the injection, clozapine N-oxide (CNO) was administered systemically followed by the assessment of urination, water consumption and tactile sensory response.

Results

Systemic administration of U-69,593 increased urination but did not produce yawning in rats. Systemic KOR agonist also increased urination in mice as well as water consumption. Cell specific Gi-DREADD activation (i.e., inhibition through Gi-coupled signaling) of KOR^CRE neurons in the ARC also increased water consumption and the total volume of urine in mice but did not affect tactile sensory responses.

Conclusion

Our studies in rodents identified the KOR in a hypothalamic region as a mechanism that promotes behaviors consistent with clinically-observed premonitory symptoms of migraine, including increased thirst and urination but not yawning. Importantly, these behaviors occurred in the absence of pain responses, consistent with the emergence of the premonitory phase before the headache phase. Early intervention for preventive treatment even before the headache phase may be achievable by targeting the hypothalamic KOR.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01497-7.

A systematic review on the kappa opioid receptor and its ligands: New directions for the treatment of pain, anxiety, depression, and drug abuse

Kappa opioid receptor (KOR) is a member of the opioid receptor system, the G protein-coupled receptors that are expressed throughout the peripheral and central nervous systems and play crucial roles in the modulation of antinociception and a variety of behavioral states like anxiety, depression, and drug abuse. KOR agonists are known to produce potent analgesic effects and have been used clinically for the treatment of pain, while KOR antagonists have shown efficacy in the treatment of anxiety and depression. This review summarizes the history, design strategy, discovery, and development of KOR ligands. KOR agonists are classified as non-biased, G protein-biased, and β-arrestin recruitment-biased, according to their degrees of bias. The mechanisms and associated effects of the G protein signaling pathway and β-arrestin recruitment signaling pathway are also discussed. Meanwhile, KOR antagonists are classified as long-acting and short-acting, based on their half-lives. In addition, we have special sections for mixed KOR agonists and selective peripheral KOR agonists. The mechanisms of action and pharmacokinetic, pharmacodynamic, and behavioral studies for each of these categories are also discussed in this review.

Kappa Opioid Agonist-Induced Diuresis: Characteristics, Mechanisms, and Beyond

Activation of the kappa opioid receptor (KOR) induces antinociception, anti-pruritic activity, diuresis, sedation, and dysphoria. KOR agonist-induced diuresis is characterized as water diuresis, in which water excretion with urine is increased without altering electrolyte excretion. Both centrally and peripherally acting KOR agonists promote diuresis. KOR antagonists block KOR agonist-evoked diuresis suggesting that the diuretic effect is through activation of the KOR. Studies in different experimental animal species and in humans indicate that KOR agonists decrease antidiuretic hormone (ADH) secretion and release from the hypothalamus and posterior pituitary; decrease response to ADH in kidneys; increase renal sympathetic nerve activity; and increase adrenaline, noradrenaline, and dopamine release from the adrenal medulla. The therapeutic potentials of KOR agonists as water diuretics have been studied in animal models of cerebral edema due to ischemia and intracranial mass, hypertension, and cirrhosis. This chapter reviews characteristics, possible mechanisms, as well as therapeutic potentials of KOR agonist-induced diuresis.

Opioid-induced endocrinopathies

The use of opioids is becoming a global epidemic, leading to a rise in the occurrence and recognition of the effects of opioid drugs on the endocrine system. Nonetheless, opioid-induced endocrinopathies still remain underdiagnosed, mainly because of symptom under-reporting by patients and poor clinician awareness. Hypogonadism is the most well recognised consequence of opioid use, but the inhibitory effects of opioid drugs on the hypothalamo-pituitary-adrenal axis and their negative effects on bone health also require attention. Hyperprolactinaemia might be detected in opioid users, but clinically relevant thyroid dysfunction has not been identified. The effects of opioids on other hormones have not been clearly defined. Assessment of gonadal and adrenal function (particularly if high index of clinical suspicion of hypogonadism or hypoadrenalism) and evaluation of bone health are advised in people that use opiods. Discontinuation or reduction of opioid dose and appropriate hormone replacement are the management approaches that should be considered for hypogonadism and hypoadrenalism. Further research is needed to facilitate the development of evidence-based guidelines on the diagnosis and optimal management of opioid-induced endocrinopathies.

Asimadoline and its potential for the treatment of diarrhea-predominant irritable bowel syndrome: a review

Irritable bowel syndrome (IBS) is a multifactorial condition with principal symptoms of pain and altered bowel function. The kappa-opioid agonist asimadoline is being evaluated in Phase III as a potential treatment for IBS. Asimadoline, to date, has shown a good safety profile and the target Phase III population - diarrhea-predominant IBS patients with at least moderate pain - was iteratively determined in a prospective manner from a Phase II dose-ranging study. The clinical data in support of this population are reviewed in this article. Furthermore, the scientific rationale for the use of asimadoline in the treatment of IBS is reviewed. Considering the high patient and societal burdens of IBS, new treatments for IBS represent therapeutic advances.

Quantitative PK-PD model-based translational pharmacology of a novel kappa opioid receptor antagonist between rats and humans

Pharmacokinetic-pharmacodynamic (PK-PD) modeling greatly enables quantitative implementation of the "learn and confirm" paradigm across different stages of drug discovery and development. This work describes the successful prospective application of this concept in the discovery and early development of a novel κ-opioid receptor (KOR) antagonist, PF-04455242, where PK-PD understanding from preclinical biomarker responses enabled successful prediction of the clinical response in a proof of mechanism study. Preclinical data obtained in rats included time course measures of the KOR antagonist (PF-04455242), a KOR agonist (spiradoline), and a KOR-mediated biomarker response (prolactin secretion) in plasma. Clinical data included time course measures of PF-04455242 and prolactin in 24 healthy volunteers following a spiradoline challenge and single oral doses of PF-04455242 (18 and 30 mg). In both species, PF-04455242 successfully reversed spiradoline-induced prolactin response. A competitive antagonism model was developed and implemented within NONMEM to describe the effect of PF-04455242 on spiradoline-induced prolactin elevation in rats and humans. The PK-PD model-based estimate of K(i) for PF-04455242 in rats was 414 ng/mL. Accounting for species differences in unbound fraction, in vitro K(i) and brain penetration provided a predicted human K(i) of 44.4 ng/mL. This prediction was in good agreement with that estimated via the application of the proposed PK-PD model to the clinical data (i.e., 39.2 ng/mL). These results illustrate the utility of the proposed PK-PD model in supporting the quantitative translation of preclinical studies into an accurate clinical expectation. As such, the proposed PK-PD model is useful for supporting the design, selection, and early development of novel KOR antagonists.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

Evaluation of the mu and kappa opioid actions of butorphanol in humans through differential naltrexone blockade

RATIONALE

Butorphanol exerts activity at mu, kappa, and delta opiate receptors in rats and monkeys but produces predominant mu-like effects in humans.

OBJECTIVES

The aim of this study was to determine if the kappa receptor-mediated actions of butorphanol could be unmasked or enhanced by giving it in combination with naltrexone, an opioid antagonist with higher affinity for mu vs kappa receptors.

MATERIALS AND METHODS

Ten healthy adult inpatient volunteers (eight men, two women), with opioid abuse histories, completed this double-blind, randomized, placebo-controlled study. Naltrexone (0, 1, 3, 10, or 30 mg, p.o.) was administered 1 h before butorphanol (0, 6, or 12 mg/70 kg, i.m.) during 15 test sessions. An array of physiological (e.g., vital signs, urine output, and subject- and observer-rated) measures was collected before and for 4 h after drug administration.

RESULTS

Naltrexone alone produced no direct effects. Butorphanol alone produced typical mu-, but not kappa-, related physiological effects (e.g., miosis, respiratory depression) and produced mood and drug effects considered typical of both mu (e.g., "liking," "good drug effects") and kappa agonists (e.g., increases in perceptual disturbances). Naltrexone pretreatment led to significant butorphanol-induced diuresis (i.e., increased urine output and decreased urine osmolality). Naltrexone generally produced a dose-dependent blockade of subjective responses.

CONCLUSION

These data suggest that naltrexone antagonism unveiled the kappaergic activity of butorphanol as measured by diuresis, while subjective responses generally attributed to mu vs kappa receptors were not dissociable. Moreover, these data demonstrate that butorphanol exerts physiologically relevant kappa agonist activity at these supraanalgesic doses in humans.

Sex differences in the potency of kappa opioids and mixed-action opioids administered systemically and at the site of inflammation against capsaicin-induced hyperalgesia in rats

RATIONALE

Sex differences in the potency of the antinociceptive effects of kappa opioids have been reported in various acute pain models with evidence suggesting that these sex differences are mediated by activity in the N-methyl-D: -aspartate (NMDA) system.

OBJECTIVES

The purpose of the present study was to evaluate sex differences in the antihyperalgesic actions of selected kappa and mixed-action opioids in a persistent pain model and determine if the NMDA system modulates these effects in a sexually dimorphic manner.

METHODS

Using gonadally intact male and female F344 rats, hyperalgesia was induced by local administration of capsaicin in the tail, after which the tail was immersed in a mildly noxious thermal stimulus (45 degrees C water), and tail-withdrawal latency measured. Opioids were then administered systemically (s.c.) and locally (in the tail) alone, and in selected combinations with the noncompetitive NMDA antagonist dextromethorphan.

RESULTS

When administered systemically and locally, the kappa opioids spiradoline, U69,593 and U50,488, and the mixed-action opioids butorphanol and nalbuphine, produced dose-dependent antihyperalgesic effects. Whereas the kappa opioids were generally more potent in males, sex differences were not observed with the mixed-action opioids. Peripheral receptor activity was confirmed for local administration of kappa opioids by the antagonism observed after local, but not intracerebroventricular (i.c.v.), administration of the kappa antagonist nor-binaltorphamine (nor-BNI). Dextromethorphan was equally potent in attenuating the antihyperalgesia induced by kappa opioids in both males and females.

CONCLUSIONS

These findings demonstrate sex differences in kappa opioid activity in a persistent pain model. Although an NMDA antagonist blocked the effects of kappa opioids in this model, these effects were not sexually dimorphic as reported in most acute pain models.

Endogenous central κ-opioid systems augment renal sympathetic nerve activity to maximally retain urinary sodium during hypotonic saline volume expansion

Intracerebroventricular injection of κ-opioid agonists produces diuresis, antinatriuresis, and a concurrent increase in renal sympathetic nerve activity (RSNA). The present study examined whether endogenous central κ-opioid systems contribute to the renal excretory responses produced by the stress of an acute hypotonic saline volume expansion (HSVE). Cardiovascular, renal excretory, and RSNA responses were measured during control, acute HSVE (5% body weight, 0.45 M saline over 30 min), and recovery (70 min) in conscious rats pretreated intracerebroventricularly with vehicle or the κ-opioid receptor antagonist nor-binaltorphimine (nor-BNI). In vehicle-pretreated rats, HSVE produced a marked increase in urine flow rate but only a low-magnitude and delayed natriuresis. RSNA was not significantly suppressed during the HSVE or recovery periods. In nor-BNI-treated rats, HSVE produced a pattern of diuresis similar to that observed in vehicle-treated rats. However, during the HSVE and recovery periods, RSNA was significantly decreased, and urinary sodium excretion increased in nor-BNI-treated animals. In other studies performed in chronic bilateral renal denervated rats, HSVE produced similar diuretic and blunted natriuretic responses in animals pretreated intracerebroventricularly with vehicle or nor-BNI. Thus removal of the renal nerves prevented nor-BNI from enhancing urinary sodium excretion during HSVE. These findings indicate that in conscious rats, endogenous central κ-opioid systems are activated during hypotonic saline volume expansion to maximize urinary sodium retention by a renal sympathoexcitatory pathway that requires intact renal nerves.

Novel developments with selective, non-peptidic kappa-opioid receptor agonists

Despite the recent introduction of a number of new compounds, there has of late been a cooling of interest by pharmaceutical companies in the development of centrally-active, selective kappa opioid agonists for therapeutic purposes. This is reflected in the discontinuation of a number of clinical trials, for reasons that are often not completely clear to outside observers. Spiradoline and enadoline have apparently been abandoned as potential analgesics because they induce dose-limiting central side-effects (i.e., dysphoria) in models of post-surgical pain. The development of niravoline as an aquaretic for the treatment of cirrhosis with ascites and other hyponatraemic disorders has also been halted. Enadoline may yet find some application against ischaemic stroke and severe head injury, presumably in comatose patients in whom psychiatric side-effects are taken to be immaterial, while apadoline and TRK 820 remain in Phase II clinical testing against cancer pain. The peripherally-selective kappa agonists, asimadoline, and the atypical compound, fedotozine, are well-tolerated in man. Results of Phase III trials of fedotozine against irritable bowel syndrome and dyspepsia have, however, ultimately been disappointing, whereas asimadoline is currently in Phase II clinical trials against pain of rheumatic and osteoarthritic origin. The results of these trials are eagerly awaited.

Differential Cardiovascular and Renal Responses Produced by Microinjection of the κ-Opioid U-50488H [(trans-3,4-Dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene-acetamide) Methane Sulfonate] into Subregions of the Paraventricular Nucleus

kappa-Opioids produce a centrally mediated diuresis, antinatriuresis, and renal sympathoexcitation in vivo; however, the specific brain sites mediating these responses are unknown. This study examined the role of the hypothalamic paraventricular nucleus (PVN) and the renal sympathetic nerves in mediating the cardiovascular and renal responses to central kappa-opioid receptor activation. In ketamine/xylazine-anesthetized rats, bilateral microinjection of the selective kappa-agonist U-50488H [(trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene-acetamide) methane sulfonate; 100 ng] into the posterior magnocellular division of the PVN significantly increased urine flow rate (control, 47 +/- 9 microl/min; 40 min, 108 +/- 10 microl/min) without changing urinary sodium excretion or cardiovascular function. In other animals, microinjection of U-50488H into the same site elicited a similar water diuresis without a change in renal sympathetic nerve activity. In contrast, microinjection of U-50488H (100 ng) into the parvocellular PVN produced an immediate pressor response (Delta 16 +/- 3 mm Hg) that occurred with a potential baroreflex evoked bradycardia (Delta -26 +/- 8 beats per minute), renal sympathoinhibition (Delta -18 +/- 4%), natriuresis (Delta 38 +/- 1%), and delayed (30-min) antidiuresis (Delta -22 +/- 9%). These results were prevented by pretreatment with the kappa-receptor antagonist nor-binaltorphimine and were not obtained when U-50488H was injected outside the PVN, or when vehicle was injected into the PVN. Together, these results demonstrate that the posterior magnocellular PVN is a brain site where central kappa-opioids act to produce diuresis, presumably by inhibiting the secretion of arginine vasopressin. Alternatively, central kappa-opioids evoke antinatriuresis via augmenting renal sympathetic nerve activity and/or other neurohumoral sodium retaining pathways at brain sites other than the hypothalamic PVN.

Opioids and renal function

Opioids, both endogenous and exogenous, have a strong influence on the renal function through different mechanisms, producing changes in the renal excretion of water and sodium. Several studies have demonstrated that opioids influence renal function, according to the agonist profile used. Mu, kappa, and delta agonists produce different renal effects, although the mechanisms remain unclear. Experimental data have given the input for a possible therapeutic role of kappa agonists for some specific conditions, for example, in treating water retention or hyponatremia occurring in patients who have hepatic cirrhosis with ascites. On the other hand, changes in renal function might strongly condition the use of opioids in the clinical setting, and the knowledge of the relationship between opioids and renal function is mandatory for a tailored approach to accommodate the individual responses in terms of pain intensity, tolerance, and adverse effects experienced by these groups of patients. The influence of renal function when using different opioids in the clinical setting is reviewed, as well as problems related to transplantation, renal damage induced by opioid addiction, and problems related to the use of opioid antagonists in such conditions.

PERSPECTIVE

Endogenous opioids exert physiologic effects on renal function, and the use of opioids may have an influence on renal activity. Renal impairment has a serious impact on the clearance of most opioids used in the clinical setting. Biochemical and clinical monitoring is mandatory to prevent serious complications.

Opioid binding and in vitro profiles of a series of 4-hdroxy-3-methoxyindolomorphinans. Transformation of a delta-selective ligand into a high affinity kappa-selective ligand by introduction of a 5,14-substituted bridge

In investigation of the effects of 14-substitution in the indolomorphinan series of delta-selective opioid ligands, 5,14-bridged indolomorphinans (4) were prepared from the equivalent dihydrothebainone acid-catalyzed rearrangement products of the dihydrothevinols. Though the new ligands generally had low affinity for opioid receptors and no delta-selectivity, 4b had high kappa-affinity and substantial selectivity which was also seen in the precursor morphinanone (3b). This indicates that the methylbenzylidene-substituted bridge in these compounds is a dominant kappa-opioid receptor binding motif.

A review of the properties of spiradoline: a potent and selective kappa-opioid receptor agonist

The selective kappa-opioid receptor agonist spiradoline mesylate (U62,066E), an arylacetamide, was synthesized with the intention of creating an analgesic that, while still retaining its analgesic properties, would be devoid of the, mainly mu receptor mediated, side effects such as physical dependence and respiratory depression associated with morphine. Spiradoline is highly selective for the kappa receptor with K(i) of 8.6 nM in guinea pig. Examination of the enantiomers of spiradoline, showed the (-)enantiomer to be responsible for the kappa agonist properties. Spiradoline easily penetrates the blood brain barrier, and does not seem to have any significant active metabolites. In preclinical studies, spiradoline has a short duration of action with a peak at around 30 min after administration. The analgesic properties of spiradoline are well documented in mice and rats. Antitussive properties have also been reported in rats. Furthermore, spiradoline was reported to display effects suggestive of neuroprotective properties in animal models of ischemia. In humans, spiradoline is a potent diuretic. It also produces significant sedation presumably due to its antihistamine properties. Preclinical studies have shown that spiradoline reduces blood pressure and heart rate, and has possible antiarrhythmic properties. Clinical studies did not confirm these findings. kappa Receptors inhibit dopaminergic neurotransmission. Spiradoline, given systematically to rats, produces a significant and long lasting decrease in dopamine release, and in locomotor activity. It has also antipsychotic-like effect in animal behavioral tests. At low doses spiradoline was reported to decrease tics in patients with Tourette's syndrome. Although spiradoline had promising effects in animal tests of analgesia, and a reasonably good safety profile in preliminary studies, it did not replace morphine as an analgesic. The available clinical data suggest that spiradoline produces disturbing adverse effects such as diuresis, sedation, and dysphoria at doses lower than those needed for analgesic effects. Thus, future development of spiradoline-like analgesic compounds should preferably focus on reduction of unwanted effects on the central nervous system. Spiradoline, which currently is commercially available for preclinical research, might prove useful in some psychiatric conditions and possibly as a neuroprotective agent.

The diverse molecular mechanisms responsible for the actions of opioids on the cardiovascular system

The actions of opioid agonist and antagonist drugs have not been well characterized in the heart and cardiovascular system. This stems from the limited role opioid receptors have been perceived to have in the regulation of the cardiovascular system. Instead, the focus of opioid receptor research, for many years, relates to the characterization of the actions of opioid drugs in analgesia associated with receptor activation in the CNS. However, recent studies suggest that opioid receptors have a role in the heart and cardiovascular system. While some of these actions may be mediated by activation of peripheral opioid receptors, others are not, and may result from direct or receptor-independent actions on cardiac tissue and the peripheral vascular system. This review will outline some of the diverse molecular mechanisms that may be responsible for the cardiovascular actions of opioids, and will characterize the role opioid receptors have in several cardiovascular pathophysiological disease states, including hypertension, heart failure, and ischaemic arrhythmogenesis. In many instances, it would appear that the effects of opioid agonists (and antagonists) in cardiovascular disease models may be mediated by opioid receptor-independent actions of these drugs.

Influence of asimadoline, a new kappa-opioid receptor agonist, on tubular water absorption and vasopressin secretion in man

AIMS

The purpose of the study was to investigate the effects of asimadoline, a new kappa-opioid agonist, on renal function and on hormones related to body fluid balance as well as its tolerability in healthy subjects.

METHODS

In a placebo-controlled, randomised, double-blind crossover design we studied the effects of single oral doses of 1, 5, and 10 mg of asimadoline, in 24 healthy volunteers. Two hour control urine collections were followed by 2 h postdose urine collections and subsequently 2.5% saline was given i.v. at a rate of 0.3 ml min(-1) kg(-1) during another 2 h urine collection. Blood was obtained hourly. Arginine-vasopressin (AVP), atrial natriuretic peptide (alpha-hANP), endothelin (ET-1) and cAMP were determined by r.i.a. or ELISA.

RESULTS

GC-MS measurements revealed Cmax values of asimadoline in plasma ranging from 18 ng ml(-1) at the 1 mg dose, 91 ng ml(-1) at the 5 mg dose, to 214 ng ml(-1) at the 10 mg dose after an average of 1.1-1.4 h. Without effects on blood pressure, heart rate, GFR or urine electrolyte excretion, urine volume increased after 1-2 h after administration of 5 and 10 mg asimadoline from 3.3+/-1.3 to 5.6+/-1.4 (P<0.05) and from 3.2 +/-1.6 to 5.5+/-2.2 ml min(-1) (P<0.01), respectively. CH2O rose from 0.2+/-1.5 to 2.0+/-1.6 (P<0.05) and from 0.6+/-1.6 to 3.0+/-1.6 ml min(-1) (P<0.01). Urinary excretion of AVP was suppressed only with the 10 mg dose from 46+/-23 to 25+/-15 fmol min(-1) (P<0.05) without and from 410+/-206 to 181+/-125 fmol min(-1) (P<0.05) with stimulation by 2.5% saline. Plasma AVP was suppressed only by the 10 mg dose of asimadoline in six of eight subjects during the 2.5% saline infusion. Changes in the alpha-hANP or ET-1 systems were not affected by asimadoline.

CONCLUSIONS

Asimadoline is diuretic in man after single doses of 5 or 10 mg probably through a direct effect at the renal tubular level. Suppression of AVP secretion was observed only at the highest dose level of 10 mg of asimadoline.

Interactions between kappa opioid agonists and cocaine. Preclinical studies

Kappa opioid agonists inhibit dopamine release from mesolimbic dopaminergic neurons and attenuate some behavioral effects of cocaine in rodents. Evidence that kappa opioid agonists may act as functional antagonists of cocaine led us to examine their interactions with cocaine's abuse-related effects in rhesus monkeys. In cocaine self-administration studies, four arylacetamides (U50,488, enadoline, (-) spiradoline and PD117302) and four benzomorphans (ethylketocyclazocine [EKC], bremazocine, Mr2033 and cyclazozine) each were administered as continuous infusions over 10 days. EKC, Mr2033, bremazocine, U50,488 and enadoline produced significant dose-dependent and sustained decreases in cocaine self-administration and also decreased food-maintained responding at some doses. Emesis and sedation were occasionally observed during the first two days of kappa agonist treatment, but tolerance developed rapidly to these effects. Cyclazocine, PD117302 and spiradoline did not significantly alter cocaine self-administration. The behavioral effects of EKC and U50,488 were antagonized by both the kappa opioid antagonist nor-binaltorphimine and the non-selective opioid antagonist naloxone. In general, compounds with mixed activity at both kappa and mu opioid receptors (e.g. EKC, Mr2033) decreased cocaine self-administration more consistently and with fewer or less severe undesirable side effects than more selective kappa agonists (e.g. U50,488, spiradoline). Although several kappa agonists decreased cocaine self-administration, EKC and U50,488 did not consistently block the discriminative stimulus effects of cocaine in monkeys trained to discriminate cocaine from saline. The extent to which kappa agonist-induced decreases in cocaine self-administration reflect an antagonism of cocaine's abuse-related effect remains to be determined.

Neurohumoral effects of orphanin FQ/nociceptin: relevance to cardiovascular and renal function

Orphanin FQ/Nociceptin (OFQ/N) is a peptide whose structure resembles that of the endogenous opioid peptides (endorphins). OFQ/N and its receptor are distributed in neural tissue and brain regions involved in the regulation of pituitary hormone release. Functional studies have shown that this peptide evokes a unique pattern of cardiovascular and renal excretory responses. This review will focus on the neural and humoral effects of OFQ/N and how this peptide may participate in the regulation of cardiovascular and renal function.

Aquaretic effects of niravoline, a kappa-opioid agonist, in patients with cirrhosis

BACKGROUND/AIMS

In patients with cirrhosis, decreased renal water excretion is a common complication. Niravoline (RU51599), a kappa-opioid receptor agonist, has been shown to induce an aquaretic response. The aim of this study was to evaluate the aquaretic effect and tolerance of niravoline in patients with cirrhosis.

METHODS

Biochemical tests and hemodynamic values were determined before and 1, 2, 3 and 24 h after niravoline administration at doses ranging from 0.5 to 2 mg iv in 18 patients with cirrhosis.

RESULTS

Diuresis significantly increased in the first hour from 64+/-9 to 146+/-31 ml/h, and returned to basal values after 3 h. Free water clearance also significantly increased, reaching the positive range at 1 h. Plasma osmolality significantly decreased at 2 h (from 290+/-4 to 286+/-4 mOsm/kg). Plasma sodium concentrations increased significantly at 3 h (from 133+/-1 to 134+/-1 mEq/l). Heart rate and arterial pressure did not change. The highest doses (1.5 mg or 2 mg) induced personality disorders and mild confusion within 2 h. These effects reversed completely within 8 h.

CONCLUSION

This study shows that niravoline administration induces an aquaretic response and is well tolerated, at moderate doses, in patients with cirrhosis. Thus, moderate doses of niravoline may be useful for treating patients with cirrhosis and water retention.

Comparative hypotensive actions of three nonpeptide kappa opioid agonists on hippocampus of SHRs and normotensive WKY rats

Comparative centrally mediated hypotensive effects of three nonpeptide kappa opioid agonist drugs (bremazocine, spiradoline, and U-50,488H) were evaluated in chloralose-anesthetized male spontaneously hypertensive rats (SHRs) and in normotensive Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats. The drugs were administered unilaterally into previously established active hypotensive sites in the dorsal hippocampus at doses of 12, 24, and 48 nmol. Each drug produced dose-related decreases in mean arterial pressure, ranging from -5 to -40% of predrug control values, with bremazocine being somewhat more effective than spiradoline, which was in turn slightly more active than U-50,488H. The effects were only marginally greater in SHRs than in normotensive controls. Each drug caused a modest decrease in heart rate, but except for the highest dose of bremazocine, the effects were not statistically significant. The onset of hypotension after intrahippocampal injection of each agent was approximately 2 min and lasted approximately 30 min with U-50,488H and spiradoline and >60 min with bremazocine. The responses to all three drugs were completely blocked by prior injection of the active hippocampal sites with nor-binaltorphimine (nor-BNI), a selective kappa-receptor antagonist. Because bremazocine is more selective for kappa-2 opioid receptors, whereas U-50,488H and spiradoline favor the kappa-1 subtype, the results suggest that drugs active on each of these subtypes should be investigated as potential antihypertensives.

The effects of spiradoline (U-62066E), a kappa-opioid receptor agonist, on neuroendocrine function in man

1. Opioid drugs act on specific receptors which are principally classified into mu, delta and kappa subtypes. Spiradoline (U-62066E) is a kappa-selective agent which has been shown to possess potent anti-nociceptive effects but does not show cross tolerance with morphine. 2. We have assessed the neuroendocrine effects of spiradoline in healthy volunteers with two doses (1.6 and 4.0 micrograms kg-1, i.m.) of the compound. Six male non-smokers aged 19-27 years were studied by use of a randomized, double-blind three-limb placebo-controlled cross-over design. Blood was taken from an in-dwelling venous cannula basally and at 15 min intervals for 2 h for determination of serum cortisol, prolactin, growth hormone (GH) and catecholamines. 3. Psychological function was assessed by the Stanford Sleepiness Scale (SSS) and the Addiction Research Centre Inventory (ARCI) administered before the medication and at 35 min, 1 h 25 min and 2 h afterwards. Cardiovascular variables were recorded at 10 min intervals. Results were analysed by analysis of variance. 4. Spiradoline showed a significant (P < 0.05) dose-dependent increase in free water clearance, as predicted for a kappa-opioid agonist. It also caused a dose-dependent stimulation of prolactin, (increment over baseline for higher dose 214%), GH (433%) and cortisol (215%) release (P < 0.05). There were no significant drug-related changes in plasma catecholamines, blood pressure, pulse or psychological variables. 5. We have therefore confirmed that kappa-opioids increase free-water clearance and may participate in the stimulation of prolactin and GH release. In contrast to mu and delta-opioid agonists, this novel kappa-agonist stimulates cortisol release in man.

Aquaretic agents: a new potential treatment of dilutional hyponatremia in cirrhosis

An impairment in the renal capacity to excrete water is a common finding in patients with cirrhosis and ascites. In some patients this abnormality is minor since it is only detectable by measuring urine volume or free water clearance after a water load and is not associated with changes in plasma osmolality and serum sodium concentration. In other patients the intensity of the disorder is such that they are not able to eliminate their regular water intake, and develop dilutional hyponatremia and hypoosmolality. The renal capacity to excrete water is one of the most useful prognostic indicators in patients with cirrhosis and ascites. The main pathogenic factors of the impaired water excretion in human cirrhosis are an increased plasma concentration of AVP, a reduced renal synthesis of prostaglandins and a reduced delivery of filtrate to the ascending limb of the loop of Henle. At present, no effective therapy exists for the management of this complication. Two types of drugs have recently been reported that selectively increase renal water excretion, antagonists of the AVP V2 receptors and kappa-opioid agonists. Experimental studies have shown that both substances improve water excretion in rats with cirrhosis and ascites. Therefore, these drugs may represent a novel therapeutic tool in the management of spontaneous hyponatremia in cirrhosis and in the treatment or prevention of diuretic-induced hyponatremia in these patients.

kappa-Opioid agonist inhibition of osmotically induced AVP release: preferential action at hypothalamic sites

Although several studies indicate that kappa-opioid agonists induce a water diuresis by inhibiting vasopressin (AVP) secretion, the locus of the kappa-receptors (neurohypophysial vs. hypothalamic) responsible for this effect remains unclear. We have ascertained the effect of the selective kappa-agonist BRL-52656 (BRL) on AVP secretion by using compartmentalized rat hypothalamoneurohypophysial explants in culture. When applied to the hypothalamus, nanomolar concentrations of BRL inhibited osmotically stimulated AVP secretion. This response was blocked by the highly selective kappa-opioid antagonist nor-binaltorphimine (BNI). However, osmotically stimulated AVP release was suppressed at the neurohypophysial site only by 100 nM BRL and was not reversed by BNI but only by naloxone. This dose of BRL, administered to the posterior pituitary compartment, did not appear to act by the agonist gaining access to hypothalamic kappa-opiate receptors, because BNI added to the hypothalamus failed to prevent the inhibition of AVP secretion. The data demonstrate that BRL is a potent inhibitor of osmotically stimulated AVP secretion via activation of kappa-opiate receptors within the hypothalamus, but that higher concentrations of the drug may also stimulate non-kappa-neurohypophysial opiate receptors that suppress AVP release.

Opioid mechanisms controlling renal function

1. Over the past 50 years considerable evidence has been reported suggesting that endogenous opioids participate in the control of renal function. 2. Exogenous administration of opioids produces profound changes in the renal excretion of water and sodium. 3. Opioids produce changes in urine output and urine sodium excretion by multiple integrated neural and hormonal mechanisms within the periphery, central nervous system and kidneys. 4. Although opioid antagonist administration does not consistently reveal an action of endogenous opioid systems on renal function, this may result from the quiescent nature of the endogenous opioid system under basal conditions. 5. Manipulations that activate endogenous opioid systems have begun to reveal important, previously unrecognized mechanisms that control kidney function and can enhance renal tubular sodium reabsorption in normal and potentially pathological states.

Diuretic effects, pharmacokinetics, and safety of a new centrally acting kappa-opioid agonist (CI-977) in humans

The diuretic effects, pharmacokinetics, and safety of CI-977, a new centrally acting selective kappa-opioid agonist, were determined in 16 healthy subjects. Subjects received single intramuscular doses of CI-977 (5, 15, or 25 micrograms) or placebo 1 week apart according to a randomized, double-blind, placebo-controlled, four-period, crossover design. Serial blood and urine specimens were collected after each dose. Significant dose-related decreases in negative free water clearance and urine osmolality and increases in urine volume were observed after administration of 15- and 25-micrograms doses of CI-977. CI-977 had no effect on urine electrolyte excretion or serum antidiuretic hormone. Absorption of CI-977 was rapid with individual tmax values ranging from 0.17 to 1.5 hours. Cmax and AUC(0-infinity) increased proportionally with dose. Individual elimination half-life values ranged from 0.6 to 3.3 hours and were independent of dose. Changes in free water clearance were related to CI-977 Cmax (r2 = 0.29, P = 0.0001) and AUC(0-4 hr) (r2 = 0.32, P = 0.0001) values. The most frequently reported adverse events after CI-977 administration were dizziness, fatigue, paresthesia, headache, vasodilatation (facial flushing), emotional lability, high feeling, and abnormal thinking. The frequency and intensity of adverse events increased with increasing CI-977 dose. In conclusion, CI-977 Cmax and AUC(0-infinity) increased in proportion to dose over the range of 5 to 25 micrograms; decreases in negative free water clearance were related to CI-977 dose and Cmax and AUC(0-4 hr) values; and the frequency and intensity of adverse events increased with increasing CI-977 dose.

The cardiovascular and central nervous system effects in the human of U-62066E. A selective opioid receptor agonist

The cardiovascular and central nervous system effects of the kappa opioid receptor agonist U-62066E were investigated in ten normal male subjects who received U-62066E or placebo with low or high dose naloxone in a randomized, double blind study. Blood pressure and heart rate in the supine and standing position, plasma adrenaline and noradrenaline, regional Doppler blood velocity indices and psychometric assessments were recorded for 1.25 h before and 6 h following injection. U-62066E caused sedation and dysphoria but no euphoria. Plasma noradrenaline was increased by U62066E when compared with basal levels. This action of U62066E was prevented by high but not low dose naloxone. U-62066E had no significant effect on blood pressure, heart rate or regional blood flow indices in the vessels studied and no effect on plasma adrenaline levels. Since U62066E at a dose known to have marked kappa effects was not found to influence cardiovascular indices our results do not support a major role for kappa opioids in the control of the circulation. However, U62066E may influence noradrenaline release or clearance and cause sedation and psychotomimetic effects.

Antagonism of metergoline on the diuretic effect of cyclazocine and U-50488 drugs with a kappa agonist activity

In rats receiving a normal saline load of 2.5 ml/100 g, sc, (moderately hydrated rats), injections of the serotonin (5-HT) antagonist, metergoline (0.25-1-4 mg/kg), resulted in a dose-dependent decrease in the urine output induced by a dose of 8 mg/kg of cyclazocine (a benzomorphan derivative, mixed kappa and sigma agonist) at the 2-h time period. The antagonist effect of metergoline (1 mg/kg) on cyclazocine doses ranging from 0.25 to 8 mg/kg, was observed only at 2 mg/kg higher doses. Other 5-HT receptor blockers, methysergide, pizotifen, cyproheptadine, caused a significant degree of antagonism. In rats receiving a saline load and a water load of 5.5 ml/100 g, ip (hyperhydrated rats), metergoline (1 mg/kg) completely antagonized the diuretic effect of cyclazocine (8 mg/kg) at the 4-h and 5-h time periods. Similarly, metergoline (1 and 4 mg/kg) administered in moderately hydrated rats, markedly decreased at the 2-h time period, the urine output produced by 5 mg/kg of U-50488 (a non benzomorphan derivative, highly selective kappa agonist), and in hyperhydrated rats, completely suppressed, at the 4-h and 5-h time periods the drug-induced diuresis. Metergoline administered alone had no effect on urine output in moderately hydrated rats or in hyperhydrated rats. These results suggest the hypothesis that 5-HT may be involved in the complex mechanisms of kappa agonist-induced diuresis in rats.