The effects of dizocilpine maleate (MK-801), an antagonist of the N-methyl-D-aspartate receptor, on neurologic recovery and histopathology following complete cerebral ischemia in primates.

The present study was designed to determine if the noncompetitive excitatory amino acid antagonist, dizocilpine maleate, when administered after a 17 min period of complete cerebral ischemia in primates, would improve postischemic neurologic function and hippocampal histopathologic outcome when compared to placebo-treated animals. Ten pigtail monkeys were anesthetized and subjected to complete cerebral ischemia using an established neck tourniquet model. Five minutes postischemia, five monkeys received dizocilpine 300 micrograms/kg i.v. over 5 min, followed by an infusion of 150 micrograms/kg/h for 10 h. This produced plasma levels of the drug in excess of 30 ng/ml for the duration of the infusion. An additional five monkeys were treated with an identical volume of saline placebo. All monkeys received intensive care for the initial 24 to 48 h postischemia. At 96 h postischemia, there was no significant difference in neurologic function between the two groups (p = 0.53, with the placebo group having the numerically better outcome). There also was no significant difference between hippocampal histopathology scores between dizocilpine and placebo-treated monkeys. The authors conclude that dizocilpine is not an efficacious therapy in the treatment of neurologic injury that occurs following complete cerebral ischemia in this primate model.

MK-801, an excitatory amino acid antagonist, does not improve neurologic outcome following cardiac arrest in cats.

The excitatory amino antagonist MK-801 was administered to cats following resuscitation from cardiac arrest to evaluate its effect on neurologic and neuropathologic outcome in a clinically relevant model of complete cerebral ischemia. In 29 cats studied, cardiac arrest (ventricular fibrillation) was maintained for 18 min and resuscitation was successfully performed in 21 cats. Four animals underwent a sham arrest. MK-801 or placebo was administered in a blinded, randomized manner. Beginning at 5 min post resuscitation (PR), MK-801 330 micrograms/kg over 2 min followed by 73 micrograms/kg/h for 10 h or the same volume of placebo was administered. Resuscitated animals remained paralyzed and sedated in an intensive care setting for 24-30 h PR. Neurologic examinations were performed at 2, 4, and 7 days PR by observers blinded to the treatment groups. Seventeen cats were entered into data analysis (nine MK-801-treated and eight placebo-treated). MK-801-treated animals had a significantly greater neurologic deficit score (NDS) rank (0 = normal, 100 = brain death) 2 days PR (mean rank 12.1 vs. 5.6; p = 0.008). This difference is most likely due to ongoing sedative actions of MK-801. There were no significant differences in NDS rank at 4 (10.3, MK-801 vs. 7.5, placebo) and 7 (9.6, MK-801 vs. 8.3, placebo) days PR. There were no significant differences in frontal cortex, hippocampus, occipital cortex, or cerebellar neuropathology between groups. Sham-arrested cats had normal neurologic and neuropathologic evaluations. In the circumstance of complete cerebral ischemia as employed in the current study, MK-801 had no beneficial effect upon neurologic or neuropathologic outcome.

Tifluadom-induced diuresis in rats. Evidence for an opioid receptor-mediated central action.

Tifluadom dose-dependently induced diuresis in rats after subcutaneous injection and oral application. (+)Tifluadom was at least 100-fold more potent than the (-)enantiomer in inducing diuresis. The diuretic action of tifluadom was dose-relatedly reduced by the opioid receptor antagonists naloxone and MR 2266. Naloxone methobromide did not antagonize the diuretic effect of tifluadom nor did the benzodiazepine receptor blocker Ro 15-1788. These data demonstrate that the diuretic effect of tifluadom is mediated centrally via an agonistic interaction between the drug and opioid receptors.

The noncompetitive N-methyl-D-aspartate receptor antagonist, MK-801 profoundly reduces volatile anesthetic requirements in rabbits.

Rabbits anesthetized with volatile anesthetics were given bolus doses of the n-methyl-D-aspartate (NMDA) receptor antagonist MK-801. Following observation and recording of the hemodynamic and electroencephalographic effects of MK-801, the animals were tested for requirements of volatile anesthetic to prevent movement to a noxious stimulus. It was demonstrated that MK-801 significantly reduced anesthetic requirements in a dose-dependent manner, while also affecting hemodynamics and the electroencephalogram in a manner consistent with the production of a deeper plane of anesthesia.

Effect of the selective 5-HT3 receptor antagonists ICS 205-930 and MDL 72222 on 5-HTP-induced head shaking and behavioral symptoms induced by 5-methoxy-N,N,dimethyltryptamine in rats: comparison with some other 5-HT receptor antagonists.

The effect of the selective 5-HT3 receptor antagonists ICS 205-930 and MDL 72222 on head shaking behavior induced by L-5-HTP and behavioral symptoms induced with 5-methoxy-N,N,-dimethyltryptamine (5-MeODMT) in rats was evaluated. Both drugs dose-dependently reduced L-5-HTP-induced head shaking but were at least 600 times less potent than pirenperone and ketanserin and at least 50 times less potent than methysergide. ICS 205-930 and MDL 72222 were more than 1000 times less potent than pirenperone or methysergide and 100 times less potent than ketanserin in blocking 5-MeODMT-induced forepaw treading and tremor. Since it appears that head shakes induced by L-5-HTP are mediated by 5-HT2 receptors, these data suggest that ICS 205-930 and MDL 72222 do not significantly interact with 5-HT2 receptors in the brain. Furthermore, the data suggest that ICS 205-930 and MDL 72222 lack appreciable antagonistic activity at the 5-HT receptor(s) mediating those behavioral effects induced by 5-MeODMT.

Ethanol and tetrahydroisoquinoline alkaloids do not produce narcotic discriminative stimulus effects.

Male Sprague-Dawley rats were trained in a two lever food-reinforced procedure to discriminate between the effects of saline and the synthetic narcotic analgesic fentanyl (0.04 mg/kg). After acquisition of this discrimination, generalization tests with morphine, ethanol and some tetrahydroisoquinoline alkaloids were conducted. The rats dose-dependently generalized the effect of morphine but did not generalize the effects of either ethanol, tetrahydropapaveroline, salsolinol or 3-carboxysalsolinol to the fentanyl discriminative stimulus. Thus, these date do not support a biochemical link between ethanol and opiates.

Discriminative stimulus properties of narcotic and non-narcotic drugs in rats trained to discriminate opiate kappa-receptor agonists.

The purpose of this study was to evaluate the discriminative stimulus properties of some narcotic and non-narcotic drugs in rats trained to discriminate the effect of the proposed opiate kappa-receptor agonists ethylketocyclazocine and bremazocine. Male Sprague-Dawley rats were trained in a two-lever food-reinforced paradigm to discriminate between the effect of ethylketocyclazocine (0.32 mg/kg) or bremazocine (0.04 mg/kg) and saline. Both groups of trained rats showed dose-dependent generalization to the effect of the proposed kappa-agonist MRZ-2033 and some animals generalized the effect of nalorphine and pentazocine. Some ethylketocyclazocine - but no bremazocine - trained rats generalized the effect of buprenorphine. The effect of dextrorphan, phencyclidine, and ketamine was generalized by some bremazocine -, but no ethylketocyclazocine-trained rats. Neither group of rats generalized the effect of etorphine, haloperidol, diazepam, or pentobarbital. These data suggest the usefulness of this procedure to evaluate the kappa-like properties of opioid drugs.

Lack of evidence for a role of endorphinergic mechanisms in mediating a discriminative stimulus produced by diazepam in rats.

Male Sprague-Dawley rats were trained in a two-lever food-reinforced procedure to discriminate between the effect of saline and diazepam (2.5 mg/kg). After acquisition of this discrimination, the ability of morphine to generalize, and naloxone to antagonize the diazepam discriminative stimulus was tested. The rats did not generalize the effect of morphine, and naloxone did not antagonize the diazepam discriminative stimulus whether it was given prior or subsequent to diazepam. These data suggest a lack of involvement of endorphins in mediating the discriminative stimulus property of diazepam.

Discriminative stimulus properties of beta-carbolines characterized as agonists and inverse agonists at central benzodiazepine receptors.

The discriminative stimulus properties of three beta-carboline derivatives were studied in three groups of rats trained, respectively, to discriminate diazepam (2.5 mg/kg IP), chlordiazepoxide (CDP, 5 mg/kg IP) or pentylenetetrazol (PTZ, 15 mg/kg IP) from saline in standard procedures employing two-lever operant chambers. Two beta-carbolines, ZK 91296 and ZK 93423, substituted for the benzodiazepines in both CDP- and diazepam-trained rats. The neutral benzodiazepine antagonists Ro 15-1788 blocked the diazepam discriminative stimulus and the ability of ZK 91296 to substitute for diazepam. A third beta-carboline, FG 7142, was not identified as benzodiazepine-like in generalization tests in either diazepam- or CDP-trained rats, but when administered together with CDP antagonized the benzodiazepine discriminative stimulus. In rats trained to discriminate PTZ from saline (a discrimination which is thought to depend on the anxiogenic properties of PTZ) the PTZ cue was antagonized by diazepam and ZK 93423, and partially antagonized by ZK 91296. The PTZ cue generalized to FG 7142 and this generalization was partially antagonized by Ro 15-1788. These results suggest that the three beta-carbolines provide more than one kind of discriminative stimulus, consistent with the classification of ZK 93423 as an agonist at central benzodiazepine receptors, with ZK 91296 as a partial agonist, and with FG 7142 as an inverse agonist. Pharmacologically, ZK 93423 and ZK 91296 may exhibit anxiolytic qualities, whereas FG 7142 produces anxiogenic effects.

Discriminative stimulus properties of muscarinic agonists.

In a two-lever, food-reinforced drug-discrimination paradigm separate groups of rats were trained to discriminate either arecoline, pilocarpine or oxotremorine from saline. The discriminative cues of all three agonists were potently blocked by scopolamine, but only by 30-60 fold higher doses of methylscopolamine. The three agonists all suppressed overall response rate. These rate-suppressant effects were not blocked by scopolamine in doses which blocked the discriminative cues. In generalization tests, arecoline elicited selection of the drug-appropriate lever in all groups of trained animals. Pilocarpine was discriminated as drug by all pilocarpine-trained animals and by a majority of oxotremorine-trained animals, but was not significantly discriminated by the arecoline-trained group. Oxotremorine was discriminated by all oxotremorine-trained animals but only by some pilocarpine-trained animals, and was not significantly discriminated by the arecoline-trained group. Morphine, haloperidol, chlordiazepoxide, pentobarbital and nicotine were not generalized to any of the training drugs. The discriminative stimuli produced by the training drugs are therefore specific and exhibit properties indicative of an origin at central muscarinic receptors but may not be identical.

Generalization tests with intraventricularly applied pro-enkephalin B-derived peptides in rats trained to discriminate the opioid kappa receptor agonist ethylketocyclazocine.

Rats were trained in a two-lever food-reinforced procedure to discriminate between the effects of saline and the opioid kappa receptor agonist ethylketocyclazocine. After acquisition of this discrimination, generalization tests with opioid peptides such as beta-endorphin, alpha-neoendorphin, dynorphin A and some dynorphin-derived peptides were conducted. The rats dose-dependently generalized the effects of intracerebroventricularly injected ethylketocyclazocine but not beta-endorphin, alpha-neoendorphin, dynorphin A1-8, dynorphin A1-13, D-Cys2-L-Cys5-dynorphin A1-13 or dynorphin A. D-Cys2-L-Cys5-dynorphin A1-13, in contrast to dynorphin A itself, dose-dependently caused analgesia and catatonia that was reversible with naloxone. Studies into the receptor preference of this derivative, using the technique of "selective tolerance", revealed that this dynorphin derivative is almost devoid of kappa-receptor activity.

Nonnarcotic antidiarrheal action of clonidine and lofexidine in the rat.

Clonidine (0.01 to 0.16 mg/kg) and lofexidine (0.01 to 0.64 mg/kg) produced a dose-dependent inhibition of diarrhea induced by castor oil treatment in the rat. Both drugs were more potent and longer acting than diphenoxylate. Pre- and posttreatment with naloxone (5 mg/kg) failed to prevent or antagonize the antidiarrheal effect of clonidine and lofexidine. These data suggest that clonidine and lofexidine may provide potent antidiarrheal activity of a nonnarcotic nature.

Effectiveness of nantradol in blocking narcotic withdrawal signs through nonnarcotic mechanisms.

Male rats were made narcotic dependent through continuous intravenous infusion of morphine. During withdrawal, nantradol, clonidine, and morphine were found to block withdrawal signs in a dose-dependent manner. Almost complete alleviation of withdrawal occurred with nantradol or clonidine at 0.16 mg/kg and with morphine at 40 mg/kg. The effectiveness of morphine, but not of nantradol or clonidine, was reversed by naloxone. Likewise, in naive rats given castor oil, naloxone did not block the antidiarrheal effect of nantradol or clonidine. In order to compare possible subjective effects of nantradol with other antiwithdrawal drugs, naive rats were trained to discriminate either morphine, cyclazocine, or clonidine from vehicle by selecting different levers for reinforcement. Nantradol failed to produce any generalization to morphine, cycloazocine, or clonidine, suggesting that this drug does not produce central subjective effects like those of the training drugs. In additional testing in behavioral experiments, nantradol failed to produce any sign of anxiogenic activity.

Mixed opioid/nonopioid effects of dynorphin and dynorphin related peptides after their intrathecal injection in rats.

Dynorphin dose-dependently increased the tail flick latency of rats to radiant heat following its intrathecal injection. This effect was accompanied by an alteration in motor function that was characterized by a flaccid extension of the hindlimbs and flaccidity of the tail. Naloxone (10 but not 1 mg/kg) blocked the antinociceptive effect and motor disturbance produced by dynorphin. The non-opioid analogue des-Tyr1-dynorphin(1-13) also increased tail flick latency and produced paralysis. Dynorphin(1-8) significantly elevated tail flick latency without affecting motor function. Furthermore, the effect of dynorphin(1-8) was blocked by 1 mg/kg naloxone. These data suggest a possible physiological role of dynorphin in influencing motor function in the spinal cord and a role of dynorphin(1-8) in modulating pain transmission. Another finding of the present study was that dynorphin was approximately ten times more potent in producing its effects when injected one day after surgery compared to when it when it was injected one week or more after surgery.

Evaluation of the glutamate antagonist dizocilipine maleate (MK-801) on neurologic outcome in a canine model of complete cerebral ischemia: correlation with hippocampal histopathology.

This study was designed to determine if dizocilipine maleate (MK-801), administered following 11 min of complete ischemia in dogs, could favorably alter neurologic outcome and hippocampal damage. Eighteen dogs were anesthetized and subjected to complete cerebral ischemia by temporary occlusion of the ascending aorta and the venae cavae via a thoracotomy. Five min postischemia, 9 dogs were given dizocilipine 150 micrograms/kg, followed by an infusion of 1.25 microgram/kg/min for 8 h. Control dogs were given equal volumes of placebo. Dogs were evaluated neurologically at 24, 48, and 72 h; thereafter, the brains were perfused, fixed and harvested. There was no significant difference in outcome between dizocilipine- and placebo-treated dogs: 5 of 9 given dizocilipine were normal, 1 was mildly injured and 3 were severely injured or dead. In the control animals given placebo, 3 of 9 were normal, 2 were mildly injured and 4 were moderately to severely injured. Histopathologic examination was limited to the hippocampus. CA1 and CA2,3,4 pyramidal neurons were graded according to degree of injury on a 5-point scale. There were no differences in histopathologic grades between the two groups. However, in both groups combined there was a significant correlation between neurologic outcome grade and histopathologic grade. The only notable systemic effect of dizocilipine appeared to be prolonged sedation which extended beyond 24 h postischemia but was not evident at 48 h postischemia. The authors conclude that more outcome studies in more sensitive models are needed.

Non-opioid psychotomimetic-like discriminative stimulus properties of N-allylnormetazocine (SKF 10,047) in the rat.

The purpose of this study was to characterize the discriminative stimulus property of dl-SKF 10,047 in the rat and to investigate if this property is the result of an interaction with opiate receptors or dopaminergic mechanisms. Male Sprague Dawley rats were trained in a two-lever food-reinforced procedure to discriminate between the effect of saline and of dl-SKF 10,047 (5 mg/kg). The trained rats dose dependently generalized the effect of dl-, d- and l-SKF 10,047, as well as cyclazocine, phencyclidine and ketamine but did not generalize the effect of either fentanyl, morphine, bremazocine, ethylketocyclazocine, pentazocine, nalorphine, naloxone, MR 2266, d-lysergic acid diethylamide, apomorphine or d-amphetamine. Neither naloxone, MR 2266, fentanyl, ethylketocyclazocine nor haloperidol antagonized the discriminative stimulus produced by dl-SKF 10,047. These data suggest that the discriminative stimulus property of dl-SKF 10,047 in the rat is not due to its interaction with either opiate receptors or dopaminergic mechanisms but may be related to its psychotomimetic effect in man.

Effect of acute and chronic pentylenetetrazol treatment on benzodiazepine and cholinergic receptor binding in rat brain.

The binding of [3H] diazepam and [3H] flunitrazepam in rat cerebral cortex was not altered by either acute or chronic administration of pentylenetetrazol except in rats made to convulse 30 min before sacrifice. Rats treated for up to 6 months with doses of pentylenetetrazol which are below seizure threshold in naive rats, became increasingly sensitive to the CNS stimulant effect of pentylenetetrazol as demonstrated by the development of myoclonus and convulsions during treatment periods. These effects were not correlated with any changes in benzodiazepine binding in cerebral cortex or cerebellum and [3H] quinuclidinyl benzilate binding in cerebral cortex. Acute convulsant doses of pentylenetetrazol increased benzodiazepine binding in cerebral cortex, but only in those rats which actively convulsed. Benzodiazepine and cholinergic receptors of the cortex, and benzodiazepine receptors of the cerebellum, therefore, do not appear to change with either the acute or chronic subconvulsive administration of pentylenetetrazol.

The kappa opioid receptor and food intake.

Many studies have suggested a role of opioid receptors in the modulation of food intake. Several distinct classes of opioid receptors have been postulated. In an attempt to establish which opioid receptor(s) modulate feeding we studied the effect of the kappa agonist, bremazocine, on feeding and compared its effects to the preferential mu agonist, morphine, and the mixed kappa-sigma agonist, butorphanol and the kappa agonist, ethylketocyclazocine. Bremazocine increased feeding to the same extent as morphine and was less potent than the mixed agonist/antagonists. The bremazocine effect demonstrated a bell-shaped dose response curve. Daily administration of bremazocine or morphine enhances the effect on increasing food intake. However, this effect of daily injections on enhancing food intake is not present when animals receiving morphine are crossed over to bremazocine and vice versa. The bremazocine effect is enhanced by diprenorphine and not inhibited by naloxone. Low doses of the dopamine antagonist, haloperidol, enhance the bremazocine effect and higher doses inhibit it. Finally, using another kappa agonist, tifluadom, we showed that the effect on food intake is stereospecific. Our studies provided further evidence for a role for the kappa opioid receptor in feeding. However, they also suggest that more than one subpopulation of opioid receptors is involved in feeding modulation.

Differential effects of SKF 10,047 (N-allyl-normetazocine) on peristalsis and longitudinal muscle contractions of the isolated guinea-pig ileum.

The purpose of this study was to investigate the differential involvement of distinct types of opioid receptors in the modulation of intestinal peristalsis compared to electrically induced longitudinal muscle contractions. Like naloxone, the proposed sigma-agonist and mu-antagonist SKF 10,047 (N-allyl-normetazocine) dose-dependently enhanced peristaltic circular muscle contractions in the isolated guinea-pig ileum. Pre-application of SKF 10,047 at a concentration which itself enhanced peristalsis by 20% on average strongly attenuated the inhibition of peristalsis produced by opioids previously proposed to act via mu-opioid-receptors in the guinea-pig ileum, i.e. normorphine, beta-endorphin, D-Ala2-D-Leu5-enkephalin and d-Ser2-L-Leu5-enkephalyl-Thr, but less strongly attenuated the inhibition produced by compounds suggested to act via kappa-opioid-receptors in this tissue, i.e. ethylketazocine and dynorphin (1-13). In contrast to its effect on peristalsis, SKF 10,047 inhibited the electrically induced contractions of the myenteric plexus-longitudinal muscle preparation in a naloxone-reversible fashion. It may be concluded that mu-and kappa-opioid receptors are of a greater functional significance than sigma-receptors in the control of peristalsis. sigma-Receptors might participate predominantly in modulating the release of acetylcholine which underlies the electrically induced longitudinal muscle contraction.

Opioid antinociception and positive reinforcement are mediated by different types of opioid receptors.

Fentanyl (FEN) and diprenorphine's (DIPR) potentials for analgesia and reinforcement were assayed using rats. Analgesia was measured by the classic tail-flick test. The test germane to opioid reinforcement involved measuring pressing rates for direct electrical stimulation of the lateral hypothalamus and ventral tegmental area. FEN, as does morphine and heroin, produced strong analgesia and enhanced pressing rates for brain stimulation. DIPR produced no analgesia and antagonized FEN's analgesia. DIPR, at doses antagonizing FEN's analgesia, enhanced pressing for brain stimulation. DIPR's enhancement of pressing was antagonized by naloxone (100 micrograms/kg). When FEN and DIPR were given concurrently, pressing for brain stimulation was not reduced and was greater than after FEN alone was given. These data support a conclusion that different types of receptors are associated with opioid analgesia and reinforcement.