Kappa opioid partial agonist activity of the enkephalin-like pentapeptide BW942C based on urination and in vitro studies in humans and animals

A diuretic effect of the pentapeptide BW942C [Tyr-D-Met(O)-Gly-pNO2-Phe-Pro-NH2 HCl] was demonstrated in humans and rats; it was characterized pharmacologically using whole animal, isolated tissue and in vitro binding studies. A single 2-mg dose of BW942C increased urine output 5-fold over control values in humans. In Long-Evans rats, BW942C produced a biphasic dose-response curve for urine output with lower doses increasing and higher doses suppressing output. Low doses of naltrexone antagonized the antidiuresis, and high doses antagonized the diuresis produced by BW942C. BW942C was less efficacious in producing diuresis than the full kappa agonists bremazocine and U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide methanesulfonate, hydrate). Furthermore, BW942C antagonized the diuretic effects of bremazocine and U50,488H. Rats tolerant to U50,488H-induced diuresis were cross-tolerant to BW942C. In Brattleboro rats, which are unable to synthesize vasopressin, BW942C failed to produce a diuretic effect, demonstrating the necessity of vasopressin for its diuretic response. In the kappa-selective rabbit vas deferens bioassay, BW942C was less efficacious than a full agonist, it was antagonized by naloxone and BW942C in nondepressant doses antagonized a full agonist. In binding studies, BW942C had the highest affinity for mu and delta opioid receptors and an intermediate affinity for kappa opioid receptors. The data suggest that BW942C has the property of a partial kappa opioid agonist in addition to being a mu agonist.     

Evaluation of the receptor selectivities of opioid drugs by investigating the block of their effect on urine output by beta-funaltrexamine

The effects of a series of opioid drugs on urine output in the water-loaded rat were studied and also the block of those effects by the irreversible opioid receptor antagonist, beta-funaltrexamine (beta-FNA). Fentanyl, d-propoxyphene, profadol, bromadoline, buprenorphine and nalbuphine produced only a decrease in urine output, which was antagonised by pretreatment with beta-FNA, 40 mg/kg s.c., 24 hr beforehand. These drugs were thus characterized as selective mu receptor agonists. U-50,488, tifluadom, Mr2034, proxorphan, ethylketocyclazocine and butorphanol all produced an initial decrease in urine output, which was antagonized by beta-FNA, and therefore probably mu receptor mediated, followed by a beta-FNA insensitive diuretic effect, which was probably kappa receptor mediated. For U-50,488, tifluadom, Mr2034 and proxorphan the threshold dose for increasing urine output was lower than that for decreasing it, suggesting that these four compounds are kappa-selective agonists. For ethylketocyclazocine and butorphanol, the threshold doses for producing both effects were similar, suggesting that these two drugs are non-selective agonists. SKF 10,047 produced a diuretic effect at low dose-levels, which may be kappa receptor mediated, and a beta-FNA insensitive decrease in urine output at higher dose-levels, which may suggest a sigma receptor mediated effect.     

Agonist and antagonist activity of kappa opioids in the squirrel monkey: I. Antinociception and urine output.

The present study was conducted to evaluate the agonist and antagonist properties of kappa opioids in the squirrel monkey shock titration procedure. The opioid antagonist naltrexone, the kappa agonists U50,488, bremazocine, ethylketazocine and tifluadom, and the mu agonist l-methadone were administered alone and in combination with a single dose of the mu agonist morphine. When administered alone, all opioids except naltrexone produced dose-dependent increases in median shock level (the intensity below which monkeys maintained shock 50% of the time). In addition, all kappa agonists produced increases in urine output, whereas naltrexone and l-methadone did not. When combined with morphine, naltrexone and all kappa agonists antagonized, at least partially, morphine-induced increases in median shock level, whereas l-methadone did not. Naltrexone and the four kappa agonists also shifted an l-methadone dose-effect curve rightward in a parallel manner; however, the shifts produced by naltrexone were greater in magnitude than those produced by the kappa agonists. These studies demonstrate that a variety of kappa agonists can act as mu antagonists in a primate model of analgesia, although antagonist activity of kappa opioids appears to be limited by their agonist activity in this procedure. Order of potency among the kappa agonists for analgesic, diuretic and antagonist effects was very similar (bremazocine greater than ethylketazocine greater than tifluadom greater than or equal to U50,488), as was the dose range for peak diuretic and antagonist effects, suggesting that mu antagonism among kappa agonists may be kappa-mediated in the squirrel monkey.     

Selective tolerance at mu and kappa opioid receptors modulating norepinephrine release in guinea pig cortex

The development of selective tolerance, that is, a loss in the ability of an agonist to exert an effect without concomitant loss in the ability of an agonist which acts through another receptor type to similarly lose its effectiveness, has provided supporting evidence for the existence of multiple opioid receptor types in brain and peripheral tissues. In brain, this phenomenon has generally been demonstrated for agonists which produce different physiological effects. In this study, we describe selective tolerance at two opioid receptor types which converge upon a single function. The effects of chronic treatment for 6 days with the mu agonist morphine (1.7 mg/kg/hr) and the kappa agonist U50, 488H (300 micrograms/kg/hr) on the ability of mu and kappa opioid agonists to inhibit the stimulated release of [3H]norepinephrine from slices of guinea pig cortex were investigated. Mu, delta and kappa selective agonists have been shown previously to be capable of regulating the stimulated release of norepinephrine. Chronic administration of morphine resulted in a diminution in the ability of the mu agonists Tyr-D-Ala-Gly-N(Me)Phe-Gly-ol, morphine and etorphine, but not of U50, 488H, to inhibit the stimulated release of [3H]norepinephrine. Conversely, chronic U50, 488H infusion decreased the ability of U50, 488H to inhibit release, with no change in the effectiveness of Tyr-D-Ala-Gly-N(Me)Phe-Gly-ol. The degree of tolerance observed for mu agonists in tissue from morphinized animals is discussed with regard to their efficacy and selectivity.     

Kappa agonist-induced diuresis: evidence for stereoselectivity, strain differences, independence of hydration variables and a result of decreased plasma vasopressin levels

Marked diuresis has previously been reported after administration of kappa opioid agonists. The present study shows that this effect is stereospecific; MR-2034 markedly increased urinary output over the dose range 0.08 to 1.25 mg/kg, whereas the opposite isomer, MR-2035, was markedly less potent. Bremazocine increased urinary output in Long-Evans hooded and Sprague-Dawley albino rats as well as lean and fatty Zucker rats. In the lean Zucker and the albino rats, bremazocine produced an inverted U-shaped diuretic dose-effect curve, an effect characteristic of kappa agonists with mu agonist activity. This pattern was not seen with the fatty Zucker rats or the Long-Evans hooded rats. The full kappa agonists bremazocine, ethylketazocine and U-50,488 increased urinary output under three different conditions of hydration: water loaded, normal hydration and water deprived. In contrast, the partial kappa agonists reliably only increase urinary output under the normal hydration condition. The diuretic effects of full and partial kappa agonists correlated with plasma vasopressin levels in water-deprived rats. The full kappa agonists (ethylketazocine, U-50,488, tifluadom and MR-2034) suppressed plasma vasopressin levels below the threshold of detectability of the radioimmunoassay, whereas the partial kappa agonists (nalorphine and butorphanol) suppressed vasopressin levels compared with control values but did not have the efficacy of the full kappa agonists. All these results support the hypothesis that kappa agonists produce their diuretic effect by suppression of plasma vasopressin levels.     

Diuresis and suppression of vasopressin by kappa opioids: comparison with mu and delta opioids and clonidine

The effects of agonists at kappa, mu and delta opioid receptors were determined on vasopressin levels in water-deprived rats. Bremazocine and U-50, 488 (kappa agonists) markedly suppressed vasopressin levels, whereas morphine and methadone (mu agonists) and metkephamid (a mixed mu and delta agonist) did not suppress vasopressin levels. Likewise, clonidine, a potent diuretic in normally hydrated rats, did not clearly suppress vasopressin levels. Metkephamid was shown to have a modest diuretic effect in normally hydrated rats but not in water-deprived rats. The diuretic effects of bremazocine were blocked completely by simultaneous treatment with desmopressin, a synthetic vasopressin-like analog. Desmopressin also blocked the diuretic effect of a waterload, but only partially attenuated the diuretic effect of clonidine. These results support the hypothesis that kappa opioid agonists produce a diuretic effect by suppressing plasma levels of vasopressin, and at higher doses produce a pattern of urination similar to animals lacking vasopressin. By comparison, mu and delta opioid agonists have little effect on vasopressin levels in water-deprived rats.     

The effect of opioid antagonism on food intake behavior and body weight in a biobehavioral model of obese binge eating

The obese Zucker rat (OZR) exhibits a hyperphagic eating pattern similar to the obese binge eater. Dynorphin, an endogenous agonist of the kappa receptor, is associated with regulation offood intake. Lessened sensitivity to opioid antagonists and/or increased central dynorphin levels may contribute to the hyperphagic eating pattern observed in the OZR. This study examined the temporal effect of a single intracerebroventricular (ICV) dose of nor-binaltorphimine (NBNI), a specific and long-lasting kappa opioid antagonist, on food intake, body weight, and satiety measures (meal size and the shape of the cumulative food intake curve [CFIC]) in adult male OZRs. Analysis of individual subjects revealed a differential response to opioid antagonism with respect to weight loss, reduction in food intake, and change in the slope of the CFIC, with some responding and others responding poorly. Repeated-measures analysis of variance showed a significant decrease in body weight (P = 0.001) and food intake (P = 0.03) in responders compared to poor responders and controls. Satiation was influenced to a greater extent in responders, who showed a significant reduction in meal size and a greater change in the CFICfor the largest meal of the day toward a pattern of satiation. These data suggest that a differential response to chronic opioid antagonism may exist in the OZR.     

Differential cross-tolerance to opioids in squirrel monkeys responding under a shock titration schedule

The opioid agonists morphine, l-methadone, ethylketazocine, U50,488 and bremazocine were examined in squirrel monkeys responding under a schedule of shock titration before, during and after chronic morphine administration. Before chronic morphine administration, all drugs produced dose-dependent increases in median shock level (the intensity below which monkeys maintained shock 50% of the time). After approximately 6 weeks of daily morphine administration, dose-effect curves for the mu agonist morphine shifted to the right approximately 3/4 log unit. Dose-effect curves for the mu agonist l-methadone also shifted to the right, approximately 1/4 log unit. In contrast, dose-effect curves for the kappa agonist ethylketazocine did not shift during chronic morphine administration, whereas those for the kappa agonists U50,488 and bremazocine shifted to the left approximately 1/2 log unit. After termination of daily morphine administration, dose-effect curves returned toward their prechronic positions. The present study demonstrates that repeated administration of morphine produces tolerance to its antinociceptive effects, as well as cross-tolerance selective to other opioids possessing mu agonist properties. Morphine tolerance may be a useful procedure not only for distinguishing mu from kappa opioid antinociceptive activity in the squirrel monkey, but also for differentiating among kappa opioids that have varying degrees of mu agonist and antagonist activity.     

Kappa opioids in rhesus monkeys. I. Diuresis, sedation, analgesia and discriminative stimulus effects

The effects of the kappa agonists bremazocine, ethylketazocine, tifluadom and U-50,488 were examined in rhesus monkeys with four experimental procedures: urinary output was measured in normally hydrated monkeys; muscle relaxation and stupor were observed; and the time it took monkeys to withdraw their tails from warm water was evaluated. Lastly, the kappa agonists were examined in monkeys trained to respond differentially in the presence or absence of ethylketazocine. For comparison, morphine and another kappa agonist, Mr 2033, were examined under some of the procedures. Kappa agonists produced dose-dependent increases in urine output whereas morphine did not. Morphine, as well as each of the kappa agonists examined, produced dose-dependent increases in tail-withdrawal latencies from both 50 and 55 degrees C water; analgesic doses of kappa agonists produced marked stupor and muscle relaxation. Kappa agonists produced ethylketazocine-appropriate responding in the drug discrimination procedure. Doses of kappa agonists that produced ethylketazocine-appropriate responding were similar to doses that increased urine output and smaller than doses that increased tail-withdrawal latencies. The opioid antagonist quadazocine (WIN 44,441) antagonized the effects of bremazocine, ethylketazocine, tifluadom and U-50,488.     

Effect of chronic administration of U-50,488H on tolerance to its pharmacological actions and on multiple opioid receptors in rat brain regions and spinal cord

The effects of chronic administration of U-50,488H (trans-3,4-dichloro-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeacetamide ), a selective kappa opioid agonist, on the development of tolerance to its analgesic and hypothermic effects and on mu, delta and kappa opioid receptors in brain regions and spinal cord of male Sprague-Dawley rats were determined. Rats were injected i.p. twice daily with 25 mg/kg of U-50,488H for 4 days. The development of tolerance to the analgesic and hypothermic effects of U-50,488H was almost complete after 4 days of treatment. [3H]Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO), [3H]Tyr-D-Ser-Gly-Phe-Leu-Thr (DSTLE) and [3H]ethylketocyclazocine (EK) were used as ligands for mu, delta and kappa opioid receptors, respectively. The binding of [3H]DAMGO to membranes prepared from various brain regions (pons + medulla, midbrain, hypothalamus, corpus striatum and cortex) and spinal cord was unaffected by chronic administration of U-50,488H. The binding of [3H]DSTLE in U-50,488H-treated rats was decreased in spinal cord and increased in corpus striatum. The binding of [3H]EK to membranes prepared from pons + medulla, midbrain, cortex and spinal cord was decreased whereas it was increased in the corpus striatum. The changes in the binding of [3H]DSTLE and [3H]EK after chronic treatment with U-50,488H were due to changes in the maximum binding values and not in the Kd values. The results indicate that, in the rat, chronic administration of U-50,488H results in the development of tolerance to its analgesic and hypothermic effects and down-regulation of kappa and delta opioid receptors in the spinal cord and an up-regulation in the corpus striatum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies on the nature and mechanism of the diuretic activity of the opioid analgesic ethylketocyclazocine

Subcutaneous injection (0.1 - 3.0 mg/kg) of ethylketocyclazocine (EKC; a prototype kappa agonist) resulted in a dose-dependent increase in urine formation in conscious rats. The increase in urine volume was unaccompanied by a corresponding increase in electrolyte excretion; thus, EKC behaved like a "water diuretic." The diuretic activity was completely abolished by naltrexone, an opiate antagonist. Water loading (10 ml/kg) and EKC (0.5 mg/kg) diminished plasma vasopressin levels equally 60 min after treatment. However, urine formation during the 1 st hr was greater in EKC-treated rats than in water-loaded rats. These results suggested that more than one component was responsible for the diuretic activity of EKC. A central effect of EKC on plasma vasopressin and urine volume was not evident. EKC (10 micrograms/rat) when injected s.c. caused diuresis, but was ineffective as a diuretic when injected into the lateral ventricle. EKC was effective in blocking stimulation of vasopressin secretion caused by volume contraction. EKC also blocked vasopressin-stimulated water flow in the toad bladder, a model of the renal distal tubule and collecting duct. We propose that EKC is diuretic by virtue of inhibition of vasopressin secretion and attenuation of the ADH response in the kidney. Both of these actions may be mediated via opioid receptors responsive to kappa agonists and inaccessible from the cerebroventricle.     

In vivo binding of benzomorphans to mu, delta and kappa opioid receptors: comparison with urine output in the rat

In vivo binding affinities of three benzomorphans, Win 44,441-3, bremazocine and MR 2266, were determined at the mu, delta and kappa types of opioid binding sites in rat brain, using an ex vivo labeling technique. The receptor occupancy of the benzomorphans and of previously tested diprenorphine were compared with their activities in increasing urine output (agonist ED50: bremazocine) or inhibiting bremazocine-induced diuresis (antagonist ID50: Win 44,441-3, MR 2266 and diprenorphine). The agonist, bremazocine, bound (in order of decreasing affinity) to the kappa approximately equal to mu greater than delta binding sites, and it's pharmacological effects appeared in the dosage range of kappa and mu binding. In order to positively identify which receptor type is responsible, the potency of the three antagonists to block the effects of bremazocine were compared to their ability to occupy the individual sites in vivo. A fractional occupancy of 0.5 would be expected at the ID50 if one assumes a linear relationship between receptor occupancy by the antagonist and the antagonistic effect. Such a linear relationship was observed for the three antagonists only at the kappa site, whereas variable occupancies were observed at the mu and delta sites. These results support the previously proposed hypothesis that kappa receptors mediate the effects of benzomorphan opioid drugs on urine flow.     

Kappa opioid receptor activity in spontaneously hypertensive rats

Earlier studies from this laboratory had indicated that there is a selective increase in the density of brain kappa opioid receptors labeled with [3H]ethylketocyclazocine in spontaneously hypertensive (SHR) rats in comparison to normotensive Wistar-Kyoto rats. The binding of a mu-ligand, [3H]naltrexone, and a delta-ligand, [3H]Tyr-D-Ser-Gly-Phe-Leu-Thr, to brain membranes of hypertensive and normotensive rats did not differ. The present studies were undertaken to determine further the role of kappa opioid receptors in hypertension. The binding of [3H]ethylketocyclazocine to brain membranes of hypertensive rats was much greater than those of normotensive rats. The density of kappa receptors was significantly higher in hypothalamic membranes of hypertensive rats as compared to normotensive rats. In order to determine the functional significance of the increased density of brain kappa opioid receptors in SHR rats, the effect of the kappa receptor agonists, tifluadom, U-50,488H and bremazocine, on two known actions associated with kappa receptors, namely analgesia and diuresis, were determined in SHR and normotensive rats. All three kappa agonists produced dose-dependent analgesia as measured by the tail-flick test. The intensity of the analgesic responses at each dose of the drugs in SHR rats was much greater than in normotensive Wistar-Kyoto rats. The kappa drugs also produced dose-dependent diuretic effects when the rats were loaded with 5% saline intragastrically. The increases in the volumes of urine produced by kappa drugs were much greater in SHR rats in comparison to normotensive rats. The basal tail-flick reaction time or urinary output in the two strains did not differ.(ABSTRACT TRUNCATED AT 250 WORDS)     

Agonist and antagonist activity of kappa opioids in the squirrel monkey: II. Effect of chronic morphine treatment.

The kappa opioid agonists U50,488, bremazocine, ethylketazocine and tifluadom and the opioid antagonist naltrexone were examined alone and in combination with morphine in a squirrel monkey shock titration procedure, before and during chronic morphine administration. Before chronic morphine administration (prechronic phase), all opioids except naltrexone produced dose-dependent increases in median shock level when administered alone. When combined with a dose of morphine that increased median shock level to 90% of maximum (ED90), naltrexone, U50,488 and bremazocine completely antagonized the effects of morphine in most monkeys, whereas ethylketazocine and tifluadom partially antagonized the effects of this dose of morphine. After 10 weeks of daily morphine administration (chronic phase), the average ED90 for morphine was increased 1 log unit. In contrast, average ED50 values for U50,488, bremazocine and tifluadom were decreaed 1/4 to 1/2 log unit, whereas the average ED50 for ethylketazocine did not change from the prechronic to chronic phases. When combined with morphine during the chronic phase, naltrexone completely antagonized the effects of the morphine ED90 at approximately the same doses as during the prechronic phase. In contrast, antagonist activity decreased for U50,488 and bremazocine, increased for ethylketazocine and did not change consistently for tifluadom, compared with the prechronic phase. The present study demonstrates that chronic morphine administration alters both the agonist and antagonist activity of kappa opioids. Changes in antagonist activity of kappa opioids after chronic morphine administration may be explained by concurrent changes in their agonist potency and the extent to which their agonist effects are mu-mediated.     

Naloxone increases water and electrolyte excretion after water loading in patients with cirrhosis and ascites

Endogenous opioids may be involved in the pathogenesis of ascites and edema in patients with liver cirrhosis. We administered the opioid antagonist naloxone (5 mg bolus followed by a 0.06 mg/min infusion) to eight male patients with alcoholic cirrhosis and ascites and to five healthy age- and sex-matched control subjects and determined the effects of naloxone on water and electrolyte excretion after a nonsustained water load (20 ml/kg). In comparison with saline vehicle infusion carried out in the same subjects, naloxone administration resulted in a 50% increase in urine output and creatinine clearance and twofold increases in sodium and potassium excretion in patients with cirrhosis. Fractional sodium and potassium excretion, minimal urinary osmolality, plasma vasopressin and aldosterone levels, arterial blood pressure, and heart rate were not affected by naloxone treatment. The diuretic effect of naloxone was not observed in control subjects. Plasma naloxone levels were about six times higher in patients with cirrhosis than in control subjects (probably because of impaired metabolism of the drug) but only a weak correlation was found between drug levels and the degree of diuresis observed. The diuretic effect of naloxone may be related to an increase in glomerular filtration rate, possibly in conjunction with altered tubular reabsorption.     

Diuretic induced change in lung water assessed by electrical impedance tomography

Monitoring patients with left ventricular failure can be difficult. Electrical impedance tomography (EIT) produces cross-sectional images of changes in the impedance of the thorax. We measured changes in the electrical impedance of the lung in nine volunteers following a diuretic challenge. The hypothesis was that lung impedance would increase with diuretic induced fluid loss. Heart rate, blood pressure and urine output were also recorded. After diuretic the mean urine output was 1220 ml compared with 187 ml after placebo. Following diuretic administration, mean thoracic impedance increased by 13.6% (p < 0.01) and lung impedance increased by 7.8% (p < 0.05). Taken as a group there was a correlation between overall impedance change and total urine output. However, for each individual, the time course of change in impedance and urine output did not correlate significantly. Our findings show that EIT may offer a better guide to the response of the lung to diuretic treatment than simply measuring urine output. The urine output is neither specific nor sensitive in the assessment of lung water. Mean lung impedance, however, is largely determined by lung water. The study showed that lung impedance can be recorded at supra-normal values. EIT may help in the management of patients with excess lung water.     

Effects of specific mu and kappa opiate tolerance and abstinence on hypothalamo-pituitary-adrenal axis secretion in the rat

Chronic administration of opiates to rats results in HPA axis tolerance and abstinence-induced hypersecretion. The effects of specific mu and kappa tolerance and withdrawal on the functional secretion of the HPA axis were evaluated in this study. Adult male rats were injected s.c. twice daily with saline, morphine or U50,488 for 5 days. Serum adrenocorticotrophic hormone (ACTH) or corticosterone (CS) were determined by radioimmunoassay as measures of HPA axis function. Tolerance to morphine (10 mg/kg) and U50,488 (1 mg/kg), but no cross-tolerance, was observed suggesting the development of mu- or kappa-specific tolerance, respectively. Tolerance does not occur at the pituitary or adrenal levels after these paradigms because ACTH and CS responses to exogenous corticotropin-releasing factor and ACTH, respectively, were not attenuated. CS secretion in response to novelty stress was not affected by either chronic opiate treatment, but the circadian variation of CS levels was slightly blunted after chronic morphine. In contrast, the elevation of CS secretion by quipazine (0.5 mg/kg) and physostigmine (0.1 mg/kg) was attenuated after chronic U50,488, but not morphine administration. Both spontaneous and antagonist-precipitated withdrawal from morphine, but not U50,488, resulted in elevation of CS levels. Low doses of morphine suppressed morphine abstinence-induced CS hypersecretion, whereas, U50,488 and clonidine had no effect. In conclusion, alterations of HPA axis function occur during chronic mu or kappa opiate administration that are receptor-specific and involve multiple neural controls of the HPA axis.     

Activation of kappa opioid receptors decreases synaptic transmission and inhibits long‐term potentiation in the basolateral amygdala of the mouse

Background: The amygdala plays an important role in the processing of chronic pain and pain memory formation. Particularly, it is involved in the emotional and affective components of the pain circuitry. The role of kappa opioid receptors in these pain conditions is only partly known. The present study investigates the effect of kappa receptor activation on synaptic transmission and synaptic plasticity in the amygdala. Methods: Electrophysiological in vitro experiments were carried out in brain slices of male C57BL/6JOlaHsd mice. The effect of the kappa opioid receptor agonist U50,488H (5μM) and the selective kappa opioid receptor antagonist nor‐BNI (3μM) on field potential (FP) amplitude and the induction of long‐term potentiation (LTP) in the basolateral amygdala (BLA) was examined. Results: High frequency stimulation (HFS) of afferents in the lateral amygdala with two trains of 100 pulses at 50Hz increased the FP amplitudes to 119±2% (mean±SEM; n=6) in the BLA. U50,488H decreased synaptic transmission (baseline: 100±0.5%; U50,488H: 86.3±2.4%; n=6) and blocked the induction of LTP (U50,488H: 100±4.1%; HFS: 102.6±7%; n=6). The effect on synaptic transmission and on LTP was completely reversed or prevented by application of nor‐BNI, which itself had no effect on synaptic transmission or the induction of LTP. Conclusion: Kappa opioid receptor activation decreases synaptic transmission and inhibits the induction of LTP in the BLA of the mouse. These findings may be associated with the effects of kappa opioid agonists in chronic pain and pain memory formation.     

Effects of the highly selective kappa opioid, U-50, 488, on renal function in the anesthetized dog

Intravenous administration of U-50,488 (a chemically novel and highly selective kappa agonist) resulted in dose-dependent increases in urine formation in anesthetized dogs. The increase in urine formation was not accompanied by an increase in sodium or potassium excretion; thus, U-50,488 behaved like a water diuretic. In addition, a dose-dependent fall in blood pressure was noted after U-50,488 administration. Despite the effect on blood pressure, no changes in renal plasma flow (p-aminohippurate clearance) or glomerular filtration rate (inulin clearance) were detected at any dose tested. Apparently, the diuresis was primarily due to tubular effects and not to changes in renal hemodynamics. Intravenous infusion of antidiuretic hormone (2 pmol/min/kg) abolished completely the diuretic response to U-50,488 (5 mg/kg). This implied that the diuresis was mediated by an effect of the drug on antidiuretic hormone.     

Acute tolerance to furosemide diuresis in humans. Pharmacokinetic-pharmacodynamic modeling

Furosemide, 40 mg, was given to eight healthy volunteers as an i.v. dose and as oral doses (tablet and solution) with and without food intake. The urine and plasma were sampled frequently and analyzed on their content of furosemide (high-performance liquid chromatography). The urine flow and chloride excretion rate were used as measures of the effect. In spite of a 3-fold difference (28 vs. 9 mg/8 hr, P less than .001) in the cumulative urinary excretion of furosemide between i.v. and postprandial oral administration, no significant difference in the diuretic effect was found (2-2.2 liters/8 hr). The drug excretion-response curves showed parallel shifts depending on mode of administration of furosemide. Clockwise hysteresis, indicating acute tolerance development to the diuretic effect, was seen after the oral doses after food intake. This within-dose development of tolerance was modeled with an extended Hill equation. The tolerance development seems to have a near relationship to acute volume depletion (inadequate substitution of urine losses), probably activating some intrarenal mechanism for extracellular fluid volume preservation. Thus, the time course of furosemide excretion, as well as the degree of renal compensation, determine the renal sensitivity to furosemide. This has important implications for the proper design and interpretation of studies of the excretion-response relationship of diuretics.