Ceruletide, a cholecystokinin-like decapeptide, differentially reduces the stimulant effect of MK-801 and ketamine: evaluation by discrete shuttle avoidance in mice

The non-competitive NMDA antagonist, MK-801 (dizocilpine: 0.03-0.3 mg/kg i.p.), a dissociative anesthetic, ketamine (1-20 mg/kg s.c.) and a CNS stimulant, methamphetamine (0.1-1 mg/kg s.c.), dose dependently increased the response rate in mice trained to discrete shuttle avoidance. MK-801 (0.1 mg/kg), ketamine (10 mg/kg) and methamphetamine (0.3 mg/kg) were almost equipotent. The cholecystokinin-like decapeptide, ceruletide, 0.1 micrograms/kg s.c., completely attenuated the response-increasing effect of MK-801 (0.1 mg/kg), although at this dose ceruletide alone did not produce any significant change in the avoidance response. Up to 10 micrograms/kg of ceruletide, which itself decreased the response rate, was required to reduce significantly the response-increasing effect of ketamine (10 mg/kg) and methamphetamine (0.3 mg/kg). On the other hand, haloperidol (0.01-0.1 mg/kg s.c.) decreased both the response rate and the % avoidance dose dependently, and inhibited the response-increasing effects of MK-801, ketamine and methamphetamine to almost the same degree. The present results suggest that comparatively lower doses of ceruletide inhibit selectively the stimulant effect of MK-801. Inhibition of dopamine release from the stored site in the nucleus accumbens may be mainly involved in this interaction.

Immobilization of mink (Mustela vison) with medetomidine-ketamine and remobilization with atipamezole

Four groups of mink were immobilized with medetomidine-HCl (MED) 0.1 mg/kg + ketamine (KET) 5 or 7.5 mg/kg at different ambient temperatures. The induction time, degree of immobilization and analgesia, rectal temperature, heart and respiration rates were recorded at intervals throughout the immobilization period. The animals were then given atipamezole-HCl (ATI) 0.5 mg/kg for reversal at different times after injection of MED/KET and the effects of the antagonist were evaluated. Subcutaneous administration of MED/KET induced complete immobilization in all 20 animals, and the highest dose was considered suitable for major surgery. Prolonged immobilization at low ambient temperatures (-10 to +5 degrees C) caused severe hypothermia in all animals. The mean rectal temperature had dropped to 37.8 degrees C and 32.1 degrees C at 15 and 85 min, respectively, after injection of MED/KET, significantly lower than the corresponding values for animals immobilized at room temperature. Intramuscular administration of ATI 20 or 40 min after injection of MED/KET rapidly remobilized the animals without apparent side-effects. Administration of ATI to animals recovering spontaneously 90 min after injection of MED/KET induced thermogenesis (shivering) in animals immobilized at a low ambient temperature, while no such effect was seen in animals immobilized at room temperature. One hour after injection of ATI, the rectal temperatures of all treated animals had returned to normal and there were no signs of abnormal behaviour.

The cardiovascular response to ketamine: the effects of clonidine and lignocaine

The effectiveness of clonidine or lignocaine in reducing the cardiostimulatory effects of ketamine was studied. A double-blind, randomized design was used in three groups of 20 patients each, of ASA grade 1 or 2 presenting for minor elective surgery. The clonidine group received 0.3 mg clonidine orally with the premedication while the lignocaine group received 1.5 mg.kg-1 lignocaine prior to induction. The third group formed the controls. All patients were induced with 2 mg.kg-1 ketamine intravenously. The systolic blood pressure in the clonidine group was significantly lower than that of both the control and lignocaine groups throughout the study (P < 0.05). It was concluded that clonidine was effective in reducing the hypertensive response to ketamine, whereas lignocaine had no effect.

Ketamine-induced anesthesia involves the N-methyl-D-aspartate receptor-channel complex in mice

The role of the N-methyl-D-aspartate (NMDA) receptor-channel complex in ketamine-induced anesthesia was examined in mice. General anesthetic potencies were evaluated on a rating scale, which provided the data for anesthetic scores, loss of righting reflex, sleeping time and recovery time. All drugs were administered intraperitoneally. NMDA (60-300 mg/kg), an NMDA receptor agonist, dose-dependently antagonized the general anesthetic potencies of ketamine at a dose of 100 mg/kg which produced loss of righting reflex in more than 90% of the mice. On the other hand, a high dose of N-methyl-L-aspartate (400 mg/kg), a stereoisomer of NMDA, did not. A dose of 300 mg/kg of NMDA significantly shifted the dose-response curve of ketamine for loss of righting reflex to the right. A high dose of D-cycloserine (200 mg/kg), an agonist at the glycine site on the NMDA receptor complex, slightly but significantly shortened the sleeping time caused by ketamine (100 mg/kg). However, neither a critical subconvulsive dose of kainate (15 mg/kg), a kainate receptor agonist, nor a subconvulsive dose of quisqualate (120 mg/kg), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor agonist, reversed general anesthesia induced by 100 mg/kg of ketamine.(ABSTRACT TRUNCATED AT 250 WORDS)

[Ketamine racemate or S-(+)-ketamine and midazolam. The effect on vigilance, efficacy and subjective findings]

Ketamine is a racemic mixture containing equal amounts of optical isomers that have almost identical pharmacokinetic properties but different pharmacodynamic effects. The S-(+)-isomer of ketamine has about twice the anaesthetic and analgesic potency of the racemic ketamine preparation and is judged to induce less psychic emergence reactions and to be followed by a more rapid recovery of vigilance. The present study was designed to assess whether the S-(+)-isomer of ketamine is superior to the racemic mixture in cardiovascular characteristics, emergence reactions and cognitive functions, and whether side effects may be reduced or prevented by administration of midazolam prior to injection of S-(+)-ketamine. METHODS. Following ethics committee approval and informed consent, 30 volunteers were randomly allocated in this double-blind study to three groups of 10 each. Group 1 received 2 mg/kg bw racemic ketamine, group 2, 1 mg/kg bw S-(+)-ketamine and group 3, 1 mg/kg bw S-(+)-ketamine after premedication with 0.1 mg/kg midazolam i.v. Cardiovascular changes, state of vigilance, cognitive performance, subjective mood and acceptance of anaesthesia were assessed by means of haemodynamic routine monitoring, electroencephalography (EEG), psychometric tests and interview. RESULTS. The increases in mean arterial pressure and heart rate following the injection of racemic ketamine and S-(+)-ketamine were identical and the differences from baseline values significant after both. Premedication with midazolam ensured stable haemodynamics after injection of S-(+)-ketamine. EEG analysis displayed the characteristic changes well known from ketamine anaesthesia for both racemic and S-(+)-ketamine. The vigilosomnoscript showed an identical profile of vigilance up to 30 min after injection of both drugs. The vigilance status after 125 min was less impaired by S-(+)-ketamine than by racemic ketamine. Psychological assessment showed a prompter recovery of visual attentiveness and sensorimotor performance in the S-(+)-ketamine group. Subjective mood was judged by the volunteers to be significantly better after S-(+)-ketamine, and volunteers found S-(+)-ketamine to be more acceptable than racemic ketamine. The frequency of dreams was the same after both drugs. No unpleasant dreams were reported after S-(+)-ketamine, but one of the volunteers who received racemic ketamine had uncomfortable dreams. Midazolam prevented any unpleasant emergence sequelae. On the other hand, the cognitive performance could not be restored to the baseline values until at least 240 min after injection of S-(+)-ketamine, because of the sedative effects of midazolam. DISCUSSION. These results suggest that S-(+)-ketamine offers the advantages of faster recovery of cognitive performance, greater acceptance by the volunteers and identical depth of anaesthesia after injection of half the dose compared with racemic ketamine. The clinical use of S-(+)-ketamine therefore seems to be justified. Premedication with benzodiazepines, e.g. midazolam, is essential. The dose to be administered, however, should be carefully selected in order not to abolish the positive effect of S-(+)-ketamine on vigilance by the sedative effects of the benzodiazepine.

A balanced anesthesia with a combination of xylazine, ketamine and butorphanol and its antagonism by yohimbine in pigs

The effects of intramuscular injections of xylazine (2 mg/kg)-ketamine (15 mg/kg) [X-K15], and xylazine (2 mg/kg)-ketamine (5 mg/kg)-butorphanol (0.22 mg/kg) [X-K5-B] were compared in atropinized (0.05 mg/kg) miniature pigs (pigs). Both combinations induced the anesthesia for more than 1 hr, however X-K5-B induced the more potent and well balanced anesthesia as compared with X-K15, although the amount of ketamine was reduced to one third. The duration of loss of pedal reflex, an indicator of surgical anesthesia, in X-K5-B (62 +/- 13 min) was significantly (P less than 0.05) longer than in X-K15 (28 +/- 19 min). In addition, X-K5-B was accompanied by loss of laryngeal reflex in all pigs. Recovery from anesthesia in X-K5-B was much smoother than in X-K15, and the administration of yohimbine (0.05 mg/kg) could rapidly and smoothly reverse the anesthesia induced by X-K5-B, although it was accompanied by a transient fall in blood pressure and tachycardia. The combination of xylazine, ketamine and butorphanol appears to be a relatively safe and widely available anesthesia for the period of one hour in pigs.

Central cholinergic action produces antagonism to ketamine anesthesia

Ketamine sometimes produces posthypnotic emergency reactions, such as prolonged hallucination or delirium. In a previous paper, we showed that physostigmine, an anticholinesterase agent, counteracts the manifestation of effects of ketamine at some doses. In the present study, we investigated the mechanism of the antagonistic effect of physostigmine on ketamine anesthesia. At first, rats were given ketamine 75 mg/kg. Immediately after the loss of righting reflex, the four groups of rats were given one of the three central cholinergic agents, physostigmine 0.1 mg/kg, oxotremorine 0.05 mg/kg, 4-aminopyridine 3 mg/kg, or saline as the control. The sleeping times were 10.7 +/- 1.0, 12.3 +/- 0.9, 11.4 +/- 1.3 and 21.2 +/- 0.7 min, respectively. The three cholinergic agents antagonized ketamine anesthesia. In the other groups of rats, the central anticholinergic agent, l-hyoscyamine, 0.5 mg/kg, was given subcutaneously for premedication before the above-mentioned procedure. The sleeping times were 16.3 +/- 1.2 min in the physostigmine group, 18.7 +/- 1.0 min in the oxotremorine group and 18.6 +/- 0.8 min in the 4-aminopyridine group. The sleeping time was significantly longer in the premedicated group than in the non-premedicated group, in the case of the three central cholinergic agents. The sleeping time in the saline group, 20.0 +/- 0.4 min, was not significantly different from that of the control in the non-premedicated case. It is, therefore, considered that the central cholinergic action produces antagonism to ketamine anesthesia.

Effects of yohimbine as a reversing agent for ketamine-xylazine anesthesia in budgerigars

Fourteen adult budgerigars (Melopsittacus undulatus) were anesthetized with a combination of ketamine hydrochloride (40 mg/kg) and xylazine hydrochloride (10 mg/kg) intramuscularly. Forty-five minutes after ketamine-xylazine injection, one of four yohimbine hydrochloride doses (0.0, 0.11, 0.275, or 0.44 mg/kg, IM) was administered in a 0.7% saline vehicle. Latencies were recorded in minutes from yohimbine injection until subjects' behavior indicated three different points of recovery: 1) lifting the head, 2) standing unaided without ataxia, and 3) perching. Means for all three recovery point latencies were significantly reduced by 0.275 mg/kg of yohimbine compared with saline vehicle alone. Mean latencies among treatment groups for each of the three recovery points were not significantly different, other than control versus treated groups. Based on these results, we recommend a yohimbine dose of 0.275 mg/kg as an effective reversing agent for ketamine-xylazine anesthesia in budgerigars.

Desmethylimipramine potentiates NMDA responses in a mouse cortical slice preparation

Desmethylimipramine (DMI) has been shown to interact with the N-methyl-D-aspartate (NMDA) receptor complex. Its probable action is through blockade of the cationic channel at the phencyclidine site and as a result it has potential anticonvulsant action. In this present study we have investigated the effects of DMI and ketamine on both NMDA-induced and spontaneous depolarizing shifts in cortical wedges prepared from genetically epilepsy-prone mice (DBA/2). Contrary to published reports, DMI potentiated the effects of NMDA and increased the frequency of spontaneous depolarizations. The actions of ketamine were inhibitory and these were reversed by DMI. Presynaptic mechanisms may be involved in the DMI-induced potentiation and this may explain the lowering of convulsive thresholds seen clinically with tricyclic antidepressants.

Mechanism of the positive inotropic effect of ketamine in isolated ferret ventricular papillary muscle

Ketamine is a cardiovascular stimulant through its sympathomimetic effects; however, its direct inotropic effect has been reported as positive in rat and negative in rabbit ventricular myocardium. This study reexamines the effect of ketamine on the contractile properties of mammalian ventricular myocardium. In isolated, electrically stimulated ferret right ventricular papillary muscles, the authors assessed the inotropic effect of ketamine (10(-6) M to 3 x 10(-4) M in 0.5 log M increments) alone and in various pharmacologic conditions designed to delineate ketamine's site(s) of action. Ketamine exerted a positive inotropic effect that was maximal at 10(-4) M. Bupranolol (10(-7) M) abolished this positive inotropic effect, whereas phentolamine (10(-6) M) did not. Depletion of norepinephrine stores by reserpine also eliminated ketamine's positive inotropic effect, indicating that ketamine caused indirect activation of the beta-adrenoceptor. Ketamine did not exert a positive inotropic effect in the presence of simultaneous inhibition of neuronal norepinephrine uptake with desmethylimipramine (DMI) (5 x 10(-6) M) and extraneuronal uptake with corticosterone (5 x 10(-5) M). It is likely that ketamine's action is to inhibit norepinephrine uptake at the neuroeffector junction rather than to augment norepinephrine release. In the presence of corticosterone, ketamine exerted a smaller positive inotropic effect than that seen with ketamine alone. Ketamine produced a small increase in force development in the presence of DMI, but this did not reach statistical significance. Inhibition of neuronal catecholamine uptake appears to be the predominant mechanism of ketamine's positive inotropic effect.

Antagonism of ketamine-xylazine anesthesia in rats by administration of yohimbine, tolazoline, or 4-aminopyridine

Antagonism of ketamine-xylazine (85 mg of ketamine/kg of body weight and 15 mg of xylazine/kg, IM) anesthesia in rats by yohimbine (YOH; 1, 5, 10, and 20 mg/kg, IP), tolazoline (TOL; 10, 20, or 50 mg/kg, IP), 4-aminopyridine (4-AP; 1 or 5 mg/kg, IP), or a combination of yohimbine and 4-aminopyridine (YOH:4-AP, 1 mg/kg:1 mg/kg or 5 mg/kg:1 mg/kg, IP) was studied. All dosages of YOH, TOL, 4-AP, and YOH:4-AP reduced the time to appearance of corneal and pedal reflexes. Only TOL was effective in reducing time to appearance of the crawl reflex and recovery time. Yohimbine, 4-AP, YOH:4-AP, and TOL were effective in reversing respiratory depression caused by ketamine-xylazine anesthesia, but anesthetic-induced hypothermia was not antagonized. When given to non-anesthetized rats, the antagonists had little influence on respiratory rate, but all antagonists caused significant (P less than 0.05) reduction in core body temperature for at least 90 minutes. When YOH was used as an anesthetic antagonist at dosage of 20 mg/kg, 20% mortality was observed and was attributable to acute respiratory arrest. The use of 4-AP and YOH:4-AP at the dosages studied induced moderate to severe muscular tremors. In conclusion, TOL at dosage of 20 mg/kg given IP, appears to be an appropriate antagonist for ketamine-xylazine anesthesia in rats.

[Acceleration of the restoration of conditioned reflexes following anesthesia using aminostigmine]

The effect of anesthesia with ketamine (80 mg/kg, intraperitoneally) combined with phenazepam (0.5 mg/kg, subcutaneously) on the reproduction of an elaborated active avoidance reflex in automated reflexometer has been studied on random bred white rats. In has been shown that during 3 hours after the animal is taken out of the lateral position the degree of its learning is significantly lowered and does not return to baseline even a day later. Intramuscular administration of a new cholinesterase inhibitor aminostigmine right after the end of anesthesia restores the disrupted reflex in 3 hours. The optimal antistigmine dose is 0.015 mg/kg, which corresponds to a therapeutic human dose.

[Does flumazenil antagonize the anesthetic effect of ketamine, etomidate or thiopental?]

The effect of flumazenil, a benzodiazepine antagonist, was assessed in a random, double-blind clinical study in which each of the four groups of surgical outpatients comprising 20 in each was given either ketamine 100 mg (K), etomidate 20 mg (E), thiopental 300 mg (T) or flunitrazepam 4 mg (F) for induction of anesthesia. On emergence, patients in each group were randomly given 2cc of either 2 coded solutions, one of which contained 0.2 mg flumazenil and the other of which was normal saline. Following injection of coded solution, all patients were assessed at 0, 5, 15, 30, 60 and 120 min for wakefulness. All 10 patients of group F who received flumazenil were alert and able to recall at 5 min, whereas in group T this was noted from 15 to 30 min. Patients of group E and K responded alike in a manner as of those who received normal saline placebo with onset of wakefulness at 30 and 60 min respectively. These results confirm that flumazenil antagonizes flunitrazepam (within 5 min) and also indicate that the antagonizing effect occurs 30 min following injection for thiopental, suggestive of some cross-reactivity between these two drugs.

The reversal of xylazine/ketamine immobilisation of fallow deer with yohimbine

Fallow deer were immobilised using a combination of xylazine and ketamine. Adult males (n = 10) and adult females (n = 10) received 4 mg/kg of each drug intramuscularly. Juveniles (n = 11) received 2 mg/kg of each drug, intravenously. Times to recumbency were as follows: adult males 4.9 +/- 2.9 min, adult females 4.1 +/- 1.9 min, juveniles 2.3 +/- 1.1 min. After 30 min each deer received 0.2 mg/kg of yohimbine, or an equal volume of sterile diluent intravenously. Yohimbine substantially reduced the recovery times of treated deer. Adults males were releasable 7.2 +/- 4.3 min after yohimbine administration, whereas control males were not releasable until 165 +/- 18 min. Treated adult females were releasable after 6.6 +/- 4.3 min, while control females were not releasable until 84 +/- 29 min. Juveniles were releasable 2.1 +/- 0.8 min after administration of yohimbine but control juveniles were not releasable until 62 +/- 16 min. Xylazine/ketamine administration produced statistically significant changes in packed cell volume, total plasma protein, albumin, sodium, glucose, creatine phosphokinase and inorganic phosphate values after 30 min. Yohimbine administration had no effect on these changes.

Antagonistic effect of physostigmine on ketamine-induced anesthesia

Effects of physostigmine on ketamine-induced anesthesia and analgesia were studied in male Sprague-Dawley rats using behavioral tests. Rats were divided into six groups. Immediately after loss of the righting reflex following an intraperitoneal injection of ketamine 75 mg/kg, each group of rats was given an intraperitoneal injection of either physostigmine 0.05, 0.1, 0.2, 0.4, 0.6 mg/kg or saline as the control, respectively. Physostigmine 0.1 mg/kg caused the greatest antagonistic effect on ketamine anesthesia as indicated by sleeping time, duration of ataxia and motor coordination. The antagonistic effects of physostigmine were reduced by a dose of physostigmine of greater than 0.1 mg/kg. However, at no dose did physostigmine antagonize ketamine analgesia as indicated by the tail-flick latency. Physostigmine (0.4 and 0.6 mg/kg) itself had analgesic and motor-suppressive actions. It can therefore be presumed that there is a limited threshold of the dose of physostigmine which develops an antagonistic effect on ketamine anesthesia due to the motor-suppressive action. It is also confirmed that physostigmine itself produces analgesia, and does not antagonize ketamine-induced analgesia.

Opioid effects of racemic ketamine on the excitability of sciatic nerve and skeletal muscle fibers of the frog

The effects of +/- ketamine were tested on the excitability of frog sciatic nerves using a sucrose gap apparatus and skeletal muscle fibers using intracellular microelectrodes. When applied extracellularly by perfusion, ketamine depressed the action potential of sciatic nerves in a dose-dependent manner. This depression was partially antagonized by the simultaneous treatment with a small concentration of naloxone. However, when the ketamine was applied intracellularly by placing it in a compartment with a cut end of the nerve, only very small and inconsistent decreases were produced. Ketamine also blocked excitability in skeletal muscle by depressing the sodium conductance (gNa). This also could be partly antagonized by the addition of a small concentration of naloxone to the solution bathing the muscle. These results support previous findings by other workers that ketamine has a stereospecific opioid agonist effect in addition to its other actions.

Dexamethasone-inhibitable stimulation of plasma prorenin by ketamine in cats

Plasma prorenin in humans is derived from both renal and extrarenal sources, but in the cat most plasma prorenin is normally of renal origin. Sodium depletion and beta-adrenergic blockade can increase plasma prorenin in cats, but the effects of sedatives and glucocorticoids are unknown. We examined the effect of ketamine, a centrally acting nonbarbiturate anesthetic, and of the glucocorticoid dexamethasone on plasma prorenin and renin. Intramuscular injection of ketamine (20 mg/kg, three times a day) for 4 days increased plasma prorenin slowly and consistently, from 7.4 +/- 1.4 (+/- SEM) to 11.4 +/- 2.2, 17.1 +/- 2.5, 20.2 +/- 3.0, and 29.2 +/- 3.4 ng/ml.h (P less than 0.001) on days 1, 2, 3, and 4, respectively, without any effect on plasma active renin. Plasma renin substrate and cortisol were also unchanged. Bilateral nephrectomy reduced both baseline and stimulated plasma prorenin to undetectable levels. Treatment with alpha 1-blocker, beta 1-blocker, angiotensin-converting enzyme inhibitor, or Ca2+ antagonist did not affect the rise in prorenin induced by ketamine, but dexamethasone completely blocked the response. In contrast, dexamethasone alone had little effect on plasma prorenin. These results demonstrate that repetitive ketamine administration selectively increases plasma prorenin, suggesting that renal prorenin secretion may be regulated independently of active renin. Blockade of stimulated, but not baseline, plasma prorenin by dexamethasone is consistent with a negative effect of glucocorticoids on the regulatory elements of the renin gene.

Reversal by tolazoline hydrochloride of xylazine hydrochloride-ketamine hydrochloride immobilizations in free-ranging desert mule deer

We captured 10 free-ranging desert mule deer (Odocoileus hemionus crooki) (five males and five females) by net-gun from a helicopter and immobilized them with xylazine hydrochloride (HCl) (100 mg) and ketamine HCl (300 to 400 mg) injected intramuscularly. Arousal and ambulation times were 13.9 +/- 4.2 and 14.3 +/- 4.2 min in eight deer injected intravenously with tolazoline HCl (3.0 mg/kg). We observed a curvilinear relationship (R = 0.50, P less than 0.01) between rectal temperature and time after induction of anesthesia. Mean peak temperature (41.4 C) occurred at 23.7 +/- 3.2 min postinduction and was greater (P less than 0.01) than the mean temperature measured initially (40.8 C). Heart and respiratory rates (108 beats/min and 75 breaths/min) were elevated prior to immobilization. Mean heart rate increased (P less than 0.05) from 90 +/- 9 beats/min in anesthetized deer to 120 +/- 13 beats/min after tolazoline HCl injection. A 20% capture-related mortality rate suggests this combination of physical and chemical capture has serious limitations. Captive deer permitted to recover from xylazine HCl-ketamine HCl immobilization without a reversal agent were able to walk in 290 +/- 79 min.

[The antagonism of ketamine/xylazine anesthesia ("Hellabrunn mixture") in wild zoo ruminants]

The ability of the antagonists tolazoline, yohimbine and the combination of yohimbine with 4-aminopyridine to reverse the effects of the xylazine-component of the "Hellabrunn mixture" (125 mg/ml xylazine and 100 mg/ml ketamine) on nondomestic zoo ruminants is discussed. Arousal time, recovery time and changes in the parameter of circulatory and respiratory functions after antagonization are shown. Tolazoline is able to antagonize the xylazine effect completely within a short time. Using a dosage of 3-5 mg/kg there is a marked negative effect on the cardio-vascular system. Yohimbine in the used dosage of 0.25-0.3 mg/kg in non-domestic ruminants did not approve in its effects. Combining yohimbine (0.25-0.3 mg) with 4-aminopyridine (0.5 mg/kg) recovery time is about 30 minutes. The negative effect on the cardio-vascular system is less pronounced compared with tolazoline.

Yohimbine reversal of ketamine-xylazine immobilization of raccoons (Procyon lotor)

Six adult raccoons (Procyon lotor) were sedated with a combination of ketamine hydrochloride (KH) at 10 mg/kg body weight and xylazine hydrochloride (XH) at 2 mg/kg body weight intramuscularly (i.m.). Twenty min after the KH-XH combination was given, yohimbine hydrochloride (YH) at either 0.1 mg/kg (Trial 1) or 0.2 mg/kg (Trial 2) body weight or a saline control (Trial 3) was administered intravenously (i.v.). The time to arousal, time to sternal recumbency and time to walking were recorded. These times were significantly shortened after YH administration [e.g., mean time to walking (MTW) at 0.2 mg/kg YH = 23.7 min] as compared to the saline controls (MTW = 108.8 min). Heart and respiratory rates both increased after YH administration, while body temperature remained constant. A fourth trial was performed using a higher ratio of KH to XH (45:1 rather than 5:1) to mimic sedation as performed in the field. The mean time to arousal (MTA) and MTW in this trial (1.3 and 23.7 min, respectively) were significantly shorter than controls and similar to YH trials performed after immobilization with 5:1 KH-XH. Yohimbine hydrochloride may be useful in field studies that require sedation of raccoons using KH-XH combinations.

Ketamine-xylazine anesthesia in red-tailed hawks with antagonism by yohimbine

Five red-tailed hawks (Buteo jamaicensis) were anesthetized at weekly intervals with intravenous ketamine hydrochloride (KET, 4.4 mg/kg) and xylazine hydrochloride (XYL, 2.2 mg/kg). Twenty min after anesthesia, yohimbine hydrochloride (YOH, 0.05, 0.10, 0.20 and 0.40 mg/kg) or a control was administered. All doses of YOH significantly reduced the head-up times (F = 20.84, df = 1,24, P less than 0.0001) and the standing times (F = 12.30, df = 1,24, P less than 0.0001), compared to the control group. The heart and respiratory rates following YOH (all doses) were significantly greater (P less than 0.01) than the anesthetized rates, but were comparable to the rates observed in restrained, unanesthetized hawks. Yohimbine did not appear to have any significant effect on body temperature. Based upon administration of 4.4 mg/kg KET and 2.2 mg/kg XYL, a dose of 0.10 mg/kg YOH was recommended to achieve antagonism without causing profound cardiovascular or respiratory changes.

An electrographic characterization of ketamine-induced linguopharyngeal motor activity

Ketamine-induced buccolinguopharyngeal motor activity was studied in rats visually and by means of force displacement transduction of tongue retrusions, electromyogram (EMG) of motor units of tongue muscles, and pressure transduction of swallowing acts. Each animal was anesthetized with an intramuscular injection of ketamine hydrochloride (100 mg/kg body weight). Through a tracheotomy the airway was intubated and the animal was mounted on a stereotaxic frame in a supine position for monitoring of the above parameters. Four varieties of events were demonstrated: (a) swallowing acts followed by tongue retrusion, (b) tongue retrusions in isolation, (c) tongue retrusions followed by swallowing events, and (d) swallowing events in isolation. All four types of events were vulnerable to intramuscular injection of haloperidol 0.75-2.5 mg/kg within 5 to 10 min and the suppression endured for at least several hours. We conclude that there is a parallel between ketamine-induced oral motor activity and neuroleptic-induced dyskinesia in that both are temporarily suppressed by neuroleptic drugs.

[Antagonism of caerulein, a CCK-8 receptor agonist, to the behavioral effects of ketamine in mice and rats]

It has been established in experiments on male mice and rats that caerulein antagonized the behavioural effects of ketamine, an agonist of phencyclidine receptors. Caerulein (75-375 micrograms/kg) and haloperidol (0.1-1.5 mg/kg) suppressed the stereotyped behaviour and motor excitation induced by ketamine (30 mg/kg) in mice. Caerulein and haloperidol failed to affect ketamine-induced ataxia. Caerulein (10 micrograms/kg) and the opioid antagonist naloxone (5 mg/kg) completely blocked the amnestic action of ketamine (30 mg/kg) in passive avoidance experiments on rats. It seems likely that the suppression of the behavioural effects of ketamine by caerulein is related to its functional antagonism with dopamine and opioid receptors.