Profiling of behavioral effects evoked by ketamine and the role of 5HT2 and D2 receptors in ketamine-induced locomotor sensitization in mice

Ketamine has addictive potential, a troublesome fact due to its promising use as a therapeutic drug. An important phenomenon associated with drug addiction is behavioral sensitization, usually characterized as augmented locomotion. However, other behaviors may also be susceptible to sensitization, and/or interfere with locomotor activity. Thus, this study drew a comprehensive behavioral 'profiling' in an animal model of repeated administration of ketamine. Adult Swiss mice received single daily ketamine injections (30 or 50 mg/Kg, i.p.), which were followed by open field testing for 7 days (acquisition period, ACQ). A ketamine challenge (sensitization test, ST) was carried out after a 5-day withdrawal. Locomotion, rearing, grooming, rotation and falling were assessed during ACQ and ST. All behaviors were affected from the first ACQ day onwards, with no indication of competition between locomotion and the other behaviors. Only locomotion in response to 30 mg/Kg of ketamine both escalated during ACQ and expressed increased levels at ST, evidencing development and expression of locomotor sensitization. Considering the involvement of serotonin 5HT₍₂₎ and dopamine D₍₂₎ receptors on addiction mechanisms, we further tested the involvement of these receptors in ketamine-induced sensitization. Ketanserin (5HT₂ antagonist, 3 mg/Kg, s.c.) prevented ketamine-evoked development of locomotor sensitization. However, ketanserin pretreatment during ACQ failed to inhibit its expression during ST. Raclopride (D₂ antagonist, 0.5 mg/Kg, s.c.) evoked less robust reductions in locomotion but prevented the development of ketamine-evoked sensitization. Pretreatment during ACQ further inhibited the expression of sensitization during ST. These results indicate that a partial overlap in serotonergic and dopaminergic mechanisms underlies ketamine-induced locomotor sensitization.

Comparative effects of sertraline, haloperidol or olanzapine treatments on ketamine-induced changes in mouse behaviours

Effects of sertraline, haloperidol or olanzapine administration on ketamine-induced behaviours in mice were examined. The aim was to ascertain the degree of reversal of such behaviours by sertraline, and compare its effectiveness to haloperidol and olanzapine. Ten-week old mice (N = 120) were equally divided into main groups; 1 (open-field, radial-arm maze and elevated plus maze {EPM} tests), and 2 (social interaction test). Mice in each main group were assigned into six groups of ten (n = 10) each. Group 1 received intraperitoneal (i.p) injection of vehicle, while groups 2-6 received i.p ketamine at 15 mg/kg daily for 10 days. From day 11 to 24, mice in group 1 (vehicle) were given distilled water (i.p at 2 ml/kg and oral at 10 ml/kg), group 2 (ketamine control) received daily i.p ketamine and oral distilled water; while animals in groups 3-6 received daily i.p. ketamine and oral haloperidol (4 mg/kg), olanzapine (2 mg/kg), or one of two doses of sertraline (SERT) (2.5 or 5 mg/kg), respectively. Treatments were administered daily, and behaviours assessed on days 11 and 24. Results showed that repeated ketamine administration caused hyperlocomotion, increased self-grooming, memory loss and social withdrawal. Administration of sertraline (both doses), haloperidol, and olanzapine reversed ketamine-induced behavioural changes. However, in the EPM, sertraline and olanzapine were anxiolytic, while haloperidol was anxiogenic. Sertraline's effect on behaviours tested was comparable to olanzapine and better than haloperidol. In conclusion, this study shows that sertraline's ability to counteract ketamine-induced behavioural changes in mice is comparable to known antipsychotics.

Clozapine counteracts a ketamine-induced depression of hippocampal-prefrontal neuroplasticity and alters signaling pathway phosphorylation

Single sub-anesthetic doses of ketamine can exacerbate the symptoms of patients diagnosed with schizophrenia, yet similar ketamine treatments rapidly reduce depressive symptoms in major depression. Acute doses of the atypical antipsychotic drug clozapine have also been shown to counteract ketamine-induced psychotic effects. In the interest of understanding whether these drug effects could be modeled with alterations in neuroplasticity, we examined the impact of acutely-administered ketamine and clozapine on in vivo long-term potentiation (LTP) in the rat’s hippocampus-to-prefrontal cortex (H-PFC) pathway. We found that a low dose of ketamine depressed H-PFC LTP, whereas animals that were co-administrated the two drugs displayed LTP that was similar to a saline-treated control. To address which signaling molecules might mediate such effects, we also examined phosphorylation and total protein levels of GSK3β, GluA1, TrkB, ERK, and mTOR in prefrontal and hippocampal sub-regions. Among the statistically significant effects that were detected (a) both ketamine and clozapine increased the phosphorylation of Ser9-GSK3β throughout the prefrontal cortex and of Ser2481-mTOR in the dorsal hippocampus (DH), (b) clozapine increased the phosphorylation of Ser831-GluA1 throughout the prefrontal cortex and of Ser845-GluA1 in the ventral hippocampus, (c) ketamine treatment increased the phosphorylation of Thr202/Tyr204-ERK in the medial PFC (mPFC), and (d) clozapine treatment was associated with decreases in the phosphorylation of Tyr705-TrkB in the DH and of Try816-TrkB in the mPFC. Further analyses involving phosphorylation effect sizes also suggested Ser831-GluA1 in the PFC displayed the highest degree of clozapine-responsivity relative to ketamine. These results provide evidence for how ketamine and clozapine treatments affect neuroplasticity and signaling pathways in the stress-sensitive H-PFC network. They also demonstrate the potential relevance of H-PFC pathway neuroplasticity for modeling ketamine-clozapine interactions in regards to psychosis.

Neonatal inhibition of Na+-K+-2Cl--cotransporter prevents ketamine induced spatial learning and memory impairments

Prolonged ketamine exposure in neonates at anesthetic doses is known to cause long-term impairments of learning and memory. A current theoretical mechanism explains this phenomenon as being neuro-excitotoxicity mediated by compensatory upregulation of N-methyl-d-aspartate receptors (NMDARs), which then initiates widespread neuroapoptosis. Additionally, the excitatory behavior of GABAergic synaptic transmission mediated by GABA_A receptors (GABA_ARs), occurring during the early neuronal development period, is proposed as contributing to the susceptibility of neonatal neurons to ketamine-induced injury. This is due to differential developmental expression patterns of Na⁺-K⁺-2Cl^- co-transporter (NKCC1) and K⁺-Cl^- co-transporter. Studies have shown that bumetanide, an NKCC1 inhibitor, allows neurons to become inhibitory rather than excitatory early in development. We thus hypothesized that bumetanide co-administration during ketamine treatment would reduce over excitation and protect the neurons from excitotoxicity. In this initial study, the Morris Water Maze test was used to assess the effects of co-administration of ketamine and bumetanide to neonatal Sprague-Dawley rats on long-term learning and memory changes seen later in life. It was revealed that bumetanide, when co-treated with ketamine neonatally, significantly impeded behavioral deficits typically seen in animals exposed to ketamine alone. Therefore, these findings suggest a new mechanism by which neonatal ketamine induced learning impairments can be prevented.

Ceftriaxone reverses ketamine-induced lasting EEG and astrocyte alterations in juvenile mice

BACKGROUND

Ketamine, an N-methyl-d-aspartate receptor antagonist, is used as a pediatric anesthetic because of its favorable safety profile. It is also being investigated as an antidepressant. Unfortunately, ketamine causes adverse reactions including hallucinations and is associated with a high prevalence of abuse among adolescents. Although chronic ketamine use has been shown to produce cognitive impairments even years following cessation, little is known about its long-term consequences on adolescents. The beta-lactam ceftriaxone has been shown to attenuate alcohol withdrawal, and alleviate early brain injury and memory impairments following subarachnoid hemorrhage. However, its ability to reverse the effects of adolescent ketamine exposure is not known. Previous data indicate that ketamine causes a reduction in the number of Excitatory Amino Acid Transporter Type 2 (EAAT2)-containing astrocytes. Additionally, the beta lactam antibiotic ceftriaxone increased expression of EAAT2. As EAAT2 is a principal mechanism of glutamate clearance from the synapse, the current study tests the hypothesis that ceftriaxone may reverse functional consequences of ketamine exposure.

METHODS

We examined the effects of chronic ketamine in juvenile mice as well as reversal by ceftriaxone using electroencephalography (EEG). Subsequently, we assessed the effects of these treatments on markers of astrocyte proliferation, using Glial Fibrillary Acidic Protein (GFAP), and function, as evidenced by EAAT2.

RESULTS

Juvenile mice exposed to chronic ketamine showed lasting alterations in EEG measurements as well as markers of astrocyte number and function. These alterations were reversed by ceftriaxone.

CONCLUSIONS

Data suggest that ceftriaxone may be able to ameliorate ketamine-induced long-term disturbances in adolescent brains.

Reversible chemical restraint of free-range cattle with a concentrated combination of tiletamine-zolazepam, ketamine, and detomidine

The aim of this study was to determine the efficacy of a concentrated combination of tiletamine-zolazepam [TZ, 0.53 mg/kg body weight (BW)], ketamine (Ket, 0.53 mg/kg BW), and detomidine (Det, 0.04 mg/kg BW) in the immobilization of free-range cattle for clinical procedures. The combination was administered intramuscularly to 53 animals. Anesthesia was reversed with the α2-adrenoceptor antagonist atipamezole. Locoregional anesthesia was provided with lidocaine when required. The TZKD combination induced suitable immobilization for minor surgical procedures or medical treatments. Anesthetic onset was rapid, taking a mean of 6.1 min [standard deviation (SD) 2.8 min]. The duration of anesthesia depended on the time of administration of the antagonist; the animals recovered in the standing position in 12.9 ± 8.9 min after the administration of atipamezole. The quality of anesthesia and analgesia were satisfactory. In conclusion, this TZKD combination can be used for both immobilization and minor surgical procedures in free-range cattle.

Reversal of ketamine-induced working memory impairments by the GABAAalpha2/3 agonist TPA023

BACKGROUND

Ketamine has been used to model cognitive and behavioral symptoms of schizophrenia. Current hypotheses state that inadequate glutamatergic transmission in schizophrenia leads to a deficiency in gamma-aminobutyric acid (GABA)ergic inhibitory mechanisms and treatment with a GABA type A receptor subunits alpha2/alpha3 (GABA(Aalpha2/3)) modulator improved working memory performance in a preliminary study in patients. Here, we used ketamine to impair spatial working memory and disrupt behavior to examine the capacity for the GABA(Aalpha2/3) agonist 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (TPA023) to reverse these symptoms.

METHODS

Rhesus monkeys received TPA023 (.7, 2.0, and 5 mg/kg; by mouth) or vehicle 45 minutes before ketamine (1.0-1.7 mg/kg; intramuscular) or saline in a semirandomized Latin square design. Behavioral observations were acquired at approximately 5 minutes, and spatial delayed response performance was tested at 15 minutes postinjection.

RESULTS

Ketamine produced a profound impairment in spatial working memory in association with the emergence of hallucinatory-like behaviors. TPA023 at all doses blocked ketamine's cognitive-impairing ability but did not influence the behavioral symptoms.

CONCLUSIONS

Acute GABA(Aalpha2/3) agonist administration reverses the working memory deficits induced by ketamine in primates. This finding indicates that the consequences of N-methyl-D-aspartate deficiency on the function of prefrontal circuits involved in working memory can be completely overcome by acute enhancement of GABA signaling.

Antagonistic effects of atipamezole, flumazenil and 4-aminopyridine against anaesthesia with medetomidine, midazolam and ketamine combination in cats

Antagonistic effects of atipamezole (ATI), flumazenil (FLU) and 4-aminopyridine (4AP) alone and in various combinations after administration of medetomidine–midazolam–ketamine (MED–MID–KET) were evaluated in cats. Animals were anaesthetised with MED (50 μg/kg), MID (0.5 mg/kg) and KET (10 mg/kg) given intramuscularly. Twenty minutes later, physiological saline, ATI (200 μg/kg), FLU (0.1 mg/kg), 4AP (0.5 mg/kg), ATI–FLU, FLU–4AP, ATI–4AP or ATI–FLU–4AP was administered intravenously. FLU, 4AP alone, or FLU–4AP did not effectively antagonise the anaesthesia, hypothermia, bradycardia, and bradypnoea induced by MED–MID–KET. ATI alone was effective. ATI–FLU, ATI–4AP and ATI–FLU–4AP combinations produced an immediate and effective recovery from anaesthesia. The combination of ATI–FLU–4AP was the most effective in antagonising the anaesthetic effects, but was associated with tachycardia, tachypnoea, excitement, and muscle tremors. Combinations with ATI are more effective for antagonising anaesthesia, but ATI–FLU–4AP is not suitable.

Psychosis: atypical limbic epilepsy versus limbic hyperexcitability with onset at puberty?

Phencyclidine (PCP), ketamine (Special K), and MK-801 are noncompetitive N-methyl-d-aspartate (NMDA) antagonists that produce acute psychosis in humans. The psychosis produced by these psychomimetic drugs is indistinguishable from schizophrenia and includes both positive and negative symptoms. This drug-induced psychosis occurs after puberty in humans. On the basis of the MK-801-induced spike-and-wave activity in rats and increased blood flow and metabolism in brain of patients with psychosis caused by these psychomimetics, this brief review argues that this psychosis is an atypical form of limbic epilepsy. Moreover, there is a specific limbic thalamcortical psychosis circuit that mediates cell injury in limbic cortex of rodents and may mediate this PCP-induced psychosis in humans. It is proposed that this thalamocortical psychosis circuit develops at puberty and can mediate PCP and ketamine-mediated psychosis and possibly the psychosis of schizophrenia, bipolar disease and other disorders that have their onset at puberty. Finally, based on this developmentally regulated psychosis/epilepsy-related thalamocortical circuitry, it is proposed that antiepileptic drugs that promote GABAergic mechanisms may decrease the probability of episodic psychosis from any cause.

(+/-) Ketamine-induced prepulse inhibition deficits of an acoustic startle response in rats are not reversed by antipsychotics

Prepulse inhibition (PPI) is the reduction in the startle response caused by a low intensity non-startling stimulus (the prepulse) which is presented shortly before the startle stimulus and is an operational measure of sensorimotor gating. PPI is impaired in psychiatric disorders such as schizophrenia. Ketamine, a non-competitive N-methyl-D-aspartate antagonist has been shown to induce schizophrenia-like behavioural changes in humans and PPI deficits in rats, which can be reversed by antipsychotics. Thus, ketamine-induced PPI deficits in rats may provide a translational model of schizophrenia. The aim of this study was to investigate the effects of antipsychotic drugs and drugs known to alter the glutamate system upon ketamine-induced PPI deficits in rats. Rats were habituated to the PPI procedure [randomized trials of either pulse alone (110 dB/50 ms) or prepulse + pulse (80 dB/10 ms)]. Animals were assigned to pre-treatments based on the level of PPI on the last habituation test and balanced across startle chambers. Ketamine (1-10 mg/kg s.c; 15 min ptt) increased startle amplitude and induced PPI deficits at 6 and 10 mg/kg. PPI deficits induced by ketamine at 6 mg/kg were not attenuated by clozapine (2.5-10 mg/kg s.c.; 60 min ptt), risperidone (0.1-1 mg/kg i.p.; 60 min ptt), haloperidol (0.1-1 mg/kg i.p.; 60 min ptt), lamotrigine (3-30 mg/kg p.o.; 60 min ptt), or SB-271046-A (5-20 mg/kg p.o.; 2 hour ptt) nor potentiated by 2-methyl-6-(phenylethynyl)-pyridine (3-10 mg/kg i.p.; 30 min ptt). These results suggest that under these test conditions ketamine-induced PPI deficits in rats is relatively insensitive and does not represent a translational model for drug discovery in schizophrenia.

Ketamine, but not propofol, anaesthesia is regulated by metabotropic glutamate 5 receptors

BACKGROUND

Group I metabotropic glutamate receptors (mGluRs) have been reported to regulate N-methyl-d-aspartate (NMDA) receptor function in various brain regions. The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) can potentiate NMDA antagonists such as PCP and MK-801-induced behavioural responses. In the present study, the role of group I mGluRs on ketamine- and propofol-induced general anaesthesia was examined.

METHODS

Mice were pretreated with various doses of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG), selective mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), mGluR1 antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) and mGluR5 antagonist MPEP followed by administration of ketamine (120 mg kg(-1)) or propofol (140 mg kg(-1)) to induce anaesthesia. The duration of loss of righting reflex was recorded.

RESULTS

DHPG and CHPG antagonized and MPEP potentiated ketamine-induced anaesthesia in a dose-dependent manner. CPCCOEt was ineffective. However, propofol-induced anaesthesia was not affected after manipulating mGluR1 and mGluR5 receptors.

CONCLUSIONS

mGluR5 receptors play an important role in modulation of anaesthesia induced by ketamine, but not propofol.

Hypericum extract reverses S-ketamine-induced changes in auditory evoked potentials in humans - possible implications for the treatment of schizophrenia

BACKGROUND

Auditory evoked potentials (AEP) provide a correlate of cognitive dysfunction in schizophrenia. Both cognitive dysfunction and AEP-characteristics might be related to reduced glutamatergic neurotransmission as induced by glutamate-antagonist like ketamine. Hypericum extract LI160 has demonstrated a ketamine-antagonising effect. We examined whether LI160 reverses changes of a low dose ketamine on AEP in healthy subjects.

METHODS

We performed a double-blind randomized treatment with either 2 x 750 mg LI 160 or placebo given one week, using a crossover design, in 16 health subjects. A test-battery including AEPs, the oculodynamic test (ODT) and a cognitive test were performed before and after an infusion with 4 mg of S-ketamine over a period of 1 hour.

RESULTS

S-ketamine lead to a significant decrease in the N100-P200 peak to peak (ptp) amplitude after the placebo treatment, whereas ptp was significantly increased by S-ketamine infusion in the LI160 treated subjects. The ODT and the cognitive testing revealed no significant effect of ketamine-infusion and therefore no interaction between treatment groups.

CONCLUSIONS

AEP measures are sensitive means to assess the effect of low dose ketamine. Provided that ketamine mimics cognitive deficits in schizophrenia, LI160 might be effective to treat these symptoms.

Use of Medetomidine-Ketamine and Atipamezole for Reversible Immobilization of Free-ranging Hog Deer (Axis porcinus) Captured in Drive Nets

A combination of 0.05 mg/kg medetomidine and 1.5 mg/kg ketamine was used to immobilize nine adult free-ranging hog deer (Axis porcinus) captured in drive nets in the Royal Bardia National Park, Nepal, 22-23 February 2000. The drugs were administered intramuscularly from separate syringes and the mean time (+/-SD) to complete immobilization was 4.6+/-1.0 min. Muscle relaxation was good and no major clinical side effects were seen. Mean values for physiologic parameters, recorded at 10-12 and 18-20 min after drug administration, were 40.6+/-0.5 and 41.1+/-0.6 C, 87+/-5 and 84+/-4%, 107+/-16 and 113+/-16 beats/ min, and 46+/-9 and 40+/-8 breaths/min for rectal temperature, SpO2, pulse rate, and respiratory rate, respectively. All animals received 0.25 mg/ kg atipamezole intramuscularly 20-22 min after administration of medetomidine-ketamine and the mean time to coordinated running was 4.8+/-0.8 min. All animals survived for at least 5 mo post-capture. To reduce stress and to facilitate handling, medetomidine-ketamine and atipamezole are recommended for reversible immobilization of free-ranging hog deer captured in drive nets.

Reversal of medetomidine-ketamine combination anesthesia in rabbits by atipamezole

This study was performed to determine the optimal reversal dosage of atipamezole on medetomidine-ketamine combination anesthesia. The subject rabbits were divided into five groups (n=5/group), and all were anesthetized with intravenous medetomidine (0.35 mg/kg) and ketamine (5 mg/kg). Atipamezole was administered intravenously 35 min after administration of the medetomidine-ketamine mixture, at doses of a quarter, a half, equal, or two times higher than the preceding medetomidine -ketamine dose according to experimental group. Heart rate (HR), mean arterial pressure (MAP), respiratory rate (RR) and rectal temperature (RT) were measured every five minutes and the mean arousal time (MAT) was also recorded. This study revealed that the optimal atipamezole dosage to achieve reversal effects is equal to or double the dose of medetomidine. At these dosages, HR and MAP significantly recovered and MAT was significantly shortened with no side effects being observed (p<0.05).

Intramuscular anesthesia of bonito and Pacific mackerel with ketamine and medetomidine and reversal of anesthesia with atipamezole

OBJECTIVE

To determine anesthetic effects of ketamine and medetomidine in bonitos and mackerels and whether anesthesia could be reversed with atipamezole.

DESIGN

Clinical trial.

ANIMALS

43 bonitos (Sarda chiliensis) and 47 Pacific mackerels (Scomber japonica).

PROCEDURE

28 bonitos were given doses of ketamine ranging from 1 to 8 mg/kg (0.5 to 3.6 mg/lb), i.m., and doses of medetomidine ranging from 0.2 to 1.6 mg/kg (0.1 to 0.7 mg/lb), i.m. (ratio of ketamine to medetomidine, 2.5:1 to 20:1). Doses of atipamezole equal to 1 or 5 times the dose of medetomidine were used. The remaining 15 bonitos were used to determine the anesthetic effects of ketamine at a dose of 4 mg/kg (1.8 mg/lb) and medetomidine at a dose of 0.4 mg/kg (0.2 mg/lb). The mackerels were given ketamine at doses ranging from 11 to 533 mg/kg (5 to 242 mg/lb) and medetomidine at doses ranging from 0.3 to 9.1 mg/kg (0.1 to 4.1 mg/lb; ratio of ketamine to medetomidine, 3:1 to 800:1). Doses of atipamezole equal to 5 times the dose of medetomidine were used.

RESULTS

I.m. administration of ketamine at a dose of 4 mg/kg and medetomidine at a dose of 0.4 mg/kg in bonitos and ketamine at a dose of 53 to 228 mg/kg (24 to 104 mg/lb) and medetomidine at a dose of 0.6 to 4.2 mg/kg (0.3 to 1.9 mg/lb) in mackerels was safe and effective. For both species, administration of atipamezole at a dose 5 times the dose of medetomidine reversed the anesthetic effects.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that a combination of ketamine and medetomidine can safely be used for anesthesia of bonitos and mackerels and that anesthetic effects can be reversed with atipamezole.

Comparative Immobilization of Wild Felids in Thailand

We immobilized individuals of four free-ranging felid species, leopard cat (Prionailurus bengalensis), clouded leopard (Neofelis nebulosa), Asiatic golden cat (Catopuma temminckii), and marbled cat (Pardofelis marmorata) with ketamine hydrochloride and xylazine hydrochloride (KH-XH) and with tiletamine hydrochloride and zolazepam hydrochloride (TH-ZH) between March 1998 and July 2002. Mean (+/-SD) dose of KH and XH was 26.51+/-5.71 mg/kg and 1.89+/-0.43 mg/kg, respectively (n=25), and mean dose of TH-ZH was 11.61+/-3.39 mg/kg (n=28). Dose was significantly correlated with induction time (P<0.001) and duration of anesthesia (P<0.05), but not with recovery time. There were significant differences between the drug combinations in time to induction (P<0.03) and time to anesthesia (P<0.01); recovery times were not significantly different. We conclude that immobilization of these felids with TH-ZH and KH-XH is effective and safe, but TH-ZH is preferred because of the smaller volume of drug necessary for sedation, faster time to induction, and absence of prolonged muscle rigidity during anesthesia.

Lamotrigine prevents ketamine but not amphetamine-induced deficits in prepulse inhibition in mice

RATIONALE

Lamotrigine, a broad-spectrum anticonvulsant known to block brain sodium channels, is effective in the treatment of persons with bipolar disorder, perhaps by virtue of its ability to reduce glutamate release. Furthermore, lamotrigine decreases the perceptual abnormalities produced by the N-methyl- d-aspartate (NMDA) antagonist ketamine in humans, similar to the effects of the atypical antipsychotic clozapine. Acutely manic bipolar patients, like persons with schizophrenia, Tourette's, and obsessive compulsive disorder, exhibit decreases in sensorimotor gating, as measured by prepulse inhibition of the startle response (PPI).

OBJECTIVE

We assessed the ability of lamotrigine to reduce the PPI-disruptive effects of ketamine and the dopaminergic agent amphetamine in two inbred mouse strains, C57BL/6J and 129SvPasIco.

METHODS

Mice were tested in a standard PPI paradigm after administration of lamotrigine (0, 6.7, 13, or 27 mg/kg) or a combination of lamotrigine (27 mg/kg) and either d-amphetamine (10 mg/kg) or ketamine (100 mg/kg).

RESULTS

In the 129SvPasIco mice, lamotrigine reversed the ketamine-induced PPI deficit, without altering PPI in control mice. In C57BL/6J mice, however, 27 mg/kg lamotrigine generally increased PPI in both control and ketamine-treated mice. Lamotrigine did not ameliorate the amphetamine-induced PPI deficit in either strain.

CONCLUSIONS

In conclusion, lamotrigine can increase PPI on its own and prevent ketamine-induced, but not amphetamine-induced, disruptions of PPI. These results suggest that lamotrigine may exert its effects on PPI through the glutamatergic system.

Comparison of ketamine versus combination of ketamine and medetomidine in injectable anesthetic protocols: chemical immobilization in macaques and tissue reaction in rats

This study compared balanced anesthesia between ketamine alone and ketamine with medetomidine and assessed the repeated intramuscular use of ketamine and its potential for tissue damage. The combination of ketamine and medetomidine was tested in newly arrived macaques undergoing a period of quarantine in an animal facility. Results indicated that the medetomidine and ketamine combination induced a deeper, more level plane of anesthesia of longer duration than did ketamine alone. Furthermore, use of the medetomidine-reversing agent, atipamezole, permitted more rapid recovery. In addition, a preliminary study in adult rats was undertaken to assess tissue damage induced by intramuscular injection of ketamine versus the combination of ketamine and medetomidine. Histological evaluation of tissue inflammation and muscle necrosis in rats indicated that the lower dose of ketamine afforded by combination with medetomidine caused markedly less damage to muscle tissue at injection sites.

Immobilization of free-ranging European mink (Mustela lutreola) an polecat (Mustela putorius) with medetomidine-ketamine and reversal by atipamezole

From March 1996 to August 1999, 24 free-ranging European mink (Mustela lutreola) and 25 free-ranging polecats (Mustela putorius) were immobilized for clinical procedures and to place radio transmitters. Data were recorded during 14 and 12 trials, respectively. Animals received intramuscularly 10 mg/kg ketamine (KET) combined with 0.20 mg/kg medetomidine (MED), antagonized by 1.00 mg/kg atipamezole (ATI). Anesthesia times were similar between species. Induction was smooth and rapid (0.7-3.9 min); the degree of anesthesia and muscle relaxation was satisfactory in most animals. Two individuals showed signs of spontaneous recovery before injection of ATI. In other individuals, ATI was injected 28.1-54.0 min after the MED-KET injection and rapidly reversed the effects of the MED. Rectal temperature and heart and respiratory rates decreased significantly 5-25 min post MED-KET injection in both species. Rectal temperature successfully remained stable by placing animals on a warmed plastic table (37 C) during anesthesia. According to these results, this anesthetic protocol produces a safe and rapid immobilization in free-ranging European mink and polecats and is recommended for surgical procedures such as radio transmitter implantation. However caution is required as hypothermia can be severe. Body temperature must be monitored and means provided to maintain stability.

GABA(A) receptor blockade antagonizes the immobilizing action of propofol but not ketamine or isoflurane in a dose-related manner

The enhancing action of propofol on gamma-amino-n-butyric acid subtype A (GABA(A)) receptors purportedly underlies its anesthetic effects. However, a recent study found that a GABA(A) antagonist did not alter the capacity of propofol to depress the righting reflex. We examined whether the noncompetitive GABA(A) antagonist picrotoxin and the competitive GABA(A) antagonist gabazine affected a different anesthetic response, immobility in response to a noxious stimulus (a tail clamp in rats), produced by propofol. This effect was compared with that seen with ketamine and isoflurane. Picrotoxin increased the 50% effective dose (ED(50)) for propofol by approximately 379%; gabazine increased it by 362%, and both antagonists acted in a dose-related manner with no apparent ceiling effect (i.e., no limit). Picrotoxin maximally increased the ED(50) for ketamine by approximately 40%-50%, whereas gabazine increased it by 50%-60%. The isoflurane minimum alveolar anesthetic concentration increased by approximately 60% with the picrotoxin and 70% with the gabazine infusion. The ED(50) for propofol was also antagonized by strychnine, a non-GABAergic glycine receptor antagonist and convulsant, to determine whether excitation of the central nervous system by a non-GABAergic mechanism could account for the increases in propofol ED(50) observed. Because strychnine only increased the immobilizing ED(50) of propofol by approximately 50%, GABA(A) receptor antagonism accounted for the results seen with picrotoxin and gabazine. We conclude that GABA(A) antagonism can influence the ED(50) for immobility of propofol and the non-GABAergic anesthetic ketamine, although to a different degree, reflecting physiologic antagonism for ketamine (i.e., an indirect effect via a modulatory effect on the neural circuitry underlying immobility) versus physiologic and pharmacologic antagonism for propofol (i.e., a direct effect by antagonism of propofol's mechanism of action). This study also suggests that the immobilizing action of isoflurane probably does not involve the GABA(A) receptor because antagonism of GABA(A) receptors for animals anesthetized with isoflurane produces results quantitatively and qualitatively similar to ketamine and markedly different from propofol.

IMPLICATIONS

IV picrotoxin and gabazine antagonized the immobilizing action of propofol in a dose-related manner, whereas antagonism of the immobilizing action of ketamine and isoflurane was similar, smaller than for propofol, and not dose-related. These results are consistent with a role for gamma-amino-n-butyric acid subtype A receptors in mediating propofol anesthesia but not ketamine or isoflurane anesthesia.

Tiletamine-zolazepam, ketamine, and xylazine anesthesia of captive cheetah (Acinonyx jubatus)

Thirty-two anesthetic episodes used a combination of tiletamine-zolezepam (50 mg/ml each), ketamine (80 mg/ml), and xylazine (20 mg/ml) at various dosages for routine diagnostic and minor surgical procedures in 13 captive cheetahs (Acinonyx jubatus). The mean dosage (0.023 +/- 0.003 ml/kg) provided rapid induction with a single i.m. injection along with safe predictable working time, good muscle relaxation, and analgesia. Yohimbine administration subsequently accelerated smooth and rapid recovery.

Medetomidine-ketamine in reindeer (Rangifer tarandus tarandus): effective immobilization by hand- and dart-administered injection

Twelve reindeer (Rangifer tarandus tarandus) were immobilized by hand injection in indoor stalls with established optimal hand-injection doses of medetomidine-ketamine and then moved to outside paddocks where they were immobilized again with the same dose by dart. The reindeer in paddocks were immobilized a second time with a 50% higher dose, hereafter referred to as the optimal darting dose. Mean time to first sign of sedation was longer and mean induction time was significantly longer (55% and 79%, respectively) when the optimal hand-injection dose was dart injected versus hand injected. Mean time to first sign of sedation was not significantly shorter (although 21% shorter, numerically) but mean induction time was significantly shorter (30%) when animals were darted with the optimal darting dose versus darted with the optimal hand-injection dose. There were no significant differences in respiratory rate, rectal temperature, and relative arterial oxygen saturation in animals injected with different doses and by different routes. but there was a significantly lower heart rate in animals dart injected with the optimal darting dose versus dart injected with the optimal hand-injection dose. All animals responded at similar rates to atipamezole injection.

Tumor necrosis factor-alpha reduces ketamine- and propofol-induced anesthesia time in rats

Tumor necrosis factor-alpha (TNFalpha) is a crucial neuromodulator in the brain. TNFalpha is involved in many physiological events including pain response and sleep. However, the interactions between TNFalpha and anesthetics have not been elucidated yet. In the present study, we investigated the effects of four intracerebroventricular (ICV) doses (1, 10, and 100 pg, and 1 ng) and two intraperitoneal (IP) doses (10 and 100 ng) of TNFalpha on anesthesia time of ketamine (100 mg/kg IP) and propofol (80 mg/kg IP) in rats. All ICV doses of TNFalpha reduced anesthesia time of ketamine and propofol compared with the saline ICV group (ketamine control group, 45.4 +/- 6.5 min; propofol control group, 43.5 +/- 11.0 min). The maximum effect was obtained after the ICV injection of 10 pg of TNFalpha (76% and 54% of ketamine and propofol control groups, respectively). Anesthesia time of ketamine or propofol was also decreased by IP injection of TNFalpha in a dose-dependent manner. Injection of 100 ng of TNFalpha IP reduced anesthesia time of ketamine and propofol by 67% and 64% of each control group, respectively. These data show that TNFalpha can modulate the anesthesia time of IV anesthetics, suggesting that anesthetic requirements might be altered in the presence of cerebral or systemic inflammation.

IMPLICATIONS

Tumor necrosis factor alpha (TNFalpha) regulates many physiological events in the brain. We investigated the effects of TNFalpha on anesthesia time in rats. Both central and peripheral administration of TNFalpha decreased anesthesia time induced by ketamine and propofol.

The involvement of nitric oxide in the analgesic effects of ketamine

We investigated the contribution of NO-cyclic GMP (cGMP) pathway to the antinociceptive effects of ketamine in mice by using the nitric oxide synthase inhibitor, nitro(g)- L-arginine methyl ester (L-NAME). Intraperitoneal (i.p.) (1, 5 or 10 mg/kg) or intrathecal (i.th.) (10, 30 or 60 microg/mouse) administration of ketamine produced dose-dependent antinociceptive effects in the acetic acid-induced writhing and formalin tests but not in the tail-flick nor in hot-plate tests. Pretreatment of mice with L-NAME (10 mg/kg, i.p.) which produced no antinociception on its own, significantly inhibited the antinociceptive effect of ketamine (1, 5 or 10 mg/kg, i.p.). However, L-NAME (30 microg/mouse) was given intrathecally, it neither modified the antinociceptive effect of i.th. ketamine (10, 30 or 60 microg/mouse) nor did it produce an antinociceptive effect alone. These data suggest that the activation of the NO-cGMP pathway probably at the supraspinal level, but not spinal level, contributes to the antinociceptive effects of ketamine.

Cardiorespiratory effects of medetomidine-butorphanol, medetomidine-butorphanol-diazepam, and medetomidine-butorphanol-ketamine in captive red wolves (Canis rufus)

Safe, effective, and reversible immobilization protocols are essential for the management of free-ranging red wolves (Canis rufus). Combinations using an alpha2-adrenoceptor agonist and ketamine have been shown to be effective for immobilization but are not reversible and can produce severe hypertension and prolonged or rough recoveries. To minimize hypertension and provide reversibility, 24 red wolves were immobilized using three medetomidine-butorphanol (MB) combinations without the use of ketamine in the initial injection. All wolves were administered medetomidine (0.04 mg/kg i.m.) and butorphanol (0.4 mg/kg i.m.). Seven wolves received no other immobilization agents (MB wolves), nine received diazepam (0.2 mg/kg i.v.) at the time they were instrumented (MBD wolves), and eight received ketamine (1 mg/kg i.v.) 30 min after instrumentation (MBK30 wolves). Physiologic parameters were monitored during immobilization. The heart rate was similar among the three groups for the first 30 min, and marked bradycardia was noted in one wolf from each group. Hypertension was observed initially in all three groups but was resolved within 10-30 min. The MBK30 wolves had significant elevations in heart rate and transient hypertension after intravenous ketamine administration. Most wolves had mild to moderate metabolic acidemia. Immobilizing drugs were antagonized in all wolves with atipamezole (0.2 mg/kg i.m.) and naloxone (0.02 mg/kg i.m.). The medetomidine-butorphanol-diazepam wolves were also given flumazenil (0.04 mg/kg i.v.). All wolves were standing within 12 min and were fully recovered within 17 min. Medetomamine-butorphanol and MBD combinations provided effective and reversible immobilization of red wolves without the sustained hypertension associated with the use of alpha2-adrenoceptor agonist-ketamine combinations. Delaying the administration of ketamine reduced its hypertensive effects.

Evaluation of medetomidine-ketamine anesthesia with atipamezole reversal in American alligators (Alligator mississippiensis)

Sixteen captive and wild-caught American alligators (Alligator mississippiensis), seven juveniles (< or = 1 m total length [TL]; 6.75 +/- 1.02 kg), and nine adults (> or = 2 m TL; 36.65 +/- 38.85 kg), were successfully anesthetized multiple times (n = 33) with an intramuscular (i.m.) medetomidine-ketamine (MK) combination administered in either the triceps or masseter muscle. The juvenile animals required significantly larger doses of medetomidine (x = 220.1 +/- 76.9 microg/kg i.m.) and atipamezole (x = 1,188.5 -/+ 328.1 microg/kg i.m.) compared with the adults (medetomidine, x = 131.1 +/- 19.5 microg/kg i.m.; atipamezole, x = 694.0 +/- 101.0 microg/kg i.m.). Juvenile alligators also required higher (statistically insignificant) doses of ketamine (x = 10.0 +/- 4.9 mg/kg i.m.) compared with the adult animals (x = 7.5 +/- 4.2 mg/kg i.m.). The differences in anesthesia induction times (juveniles, x = 19.6 +/- 8.5 min; adults, x = 26.6 +/- 17.4 min) and recovery times (juveniles, x = 35.4 +/- 22.1 min; adults, x = 37.9 +/- 20.2 min) were also not statistically significant. Anesthesia depth was judged by the loss of the righting, biting, corneal and blink, and front or rear toe-pinch withdrawal reflexes. Recovery in the animals was measured by the return of reflexes, open-mouthed hissing, and attempts to high-walk to the opposite end of the pen. Baseline heart rates (HRs) were significantly higher in the juvenile animals (x = 37 +/- 4 beats/min) compared with the adults (x = 24 +/- 5 bpm). However, RRs (juveniles, x = 8 +/- 2 breaths/min; adults, x = 8 +/- 2 breaths/min) and body temperatures (juveniles, x = 24.1 +/- 1.1 degrees C; adults, x = 25.2 +/- 1.2 degrees C) did not differ between the age groups. In both groups, significant HR decreases were recorded within 30-60 min after MK administration. Cardiac arrhythmias (second degree atrio-ventricular block and premature ventricular contractions) were seen in two animals but were not considered life-threatening. Total anesthesia times ranged from 61-250 min after i.m. injection. Although dosages were significantly different between the age groups, MK and atipamezole provided safe, effective, completely reversible anesthesia in alligators. Drug-dosage differences appear to be related to metabolic differences between the two size-classes, requiring more research into metabolic scaling as a method of calculating anesthetic dosages.

Attenuation of ketamine effects by nimodipine pretreatment in recovering ethanol dependent men: psychopharmacologic implications of the interaction of NMDA and L-type calcium channel antagonists

Ketamine blocks the calcium channel associated with N-methyl-D-aspartate (NMDA) glutamate receptors. It has transient behavioral effects in healthy humans that resemble aspects of schizophrenia, dissociative disorders, and ethanol intoxication. Ethanol is an antagonist of both NMDA receptors and L-type voltage-sensitive calcium channels (VSCC) and it has minimal psychotogenic activity in humans. A double-blind placebo-controlled study was conducted that evaluated whether pretreatment with the L-type VSCC antagonist, nimodipine, 90 mg D, modulated ketamine response (bolus 0.26 mg/kg, infusion of 0.65 mg/kg/hr) in 26 ethanol-dependent inpatients who were sober for at least one month prior to testing. This study found that nimodipine reduced the capacity of ketamine to induce psychosis, negative symptoms, altered perception, dysphoria, verbal fluency impairment, and learning deficits. Nimodipine improved memory function, but had no other intrinsic behavioral activity in this patient group. Nimodipine pretreatment attenuated the perceived similarity of ketamine effects to ethanol as well as ketamine-induced euphoria and sedation. However, nimodipine did not reduce the stimulant effects of ketamine. These data suggest that antagonism of L-type VSCCs attenuates the behavioral effects of NMDA antagonists in humans. They support the continued evaluation of nimodipine in the treatment of neuropsychiatric disorders. They also suggest that drugs, such as ethanol, that combine NMDA and L-type VSCC antagonism may have enhanced tolerability without attenuation of their stimulant effects.

Effects of benztropine on ketamine-induced behaviors in Cebus monkeys

Ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, causes a schizophrenic-like psychosis in normal volunteers and exacerbates psychotic symptoms in patients with schizophrenia. Recent work has shown that ketamine and other NMDA antagonists affect a range of behaviors in nonhuman primates, particularly those associated with motor and mental function such as attention and perception. Several lines of study also suggest that NMDA antagonists interact with cholinergic mechanisms. The effects of benztropine, an anticholinergic agent, on ketamine-induced behaviors were evaluated in a double-blind randomized test design in 20 Cebus monkeys. Benztropine (0.05, 0.1 and 0.25 mg/kg, i.m.) was injected 1 hour before ketamine (2.5 and 5.0 mg/kg, i.m.) administration. Behaviors scored for 90 minutes after ketamine administration included salivation, dystonia and reactivity to external stimuli. Benztropine almost completely blocked ketamine-induced hypersalivation, and partially ameliorated the dystonia syndrome by 50%, but did not affect ketamine-induced decreased reactivity to external stimuli. These results suggest that cholinergic mechanisms only moderately influence ketamine-induced central nervous system effects of motor dysfunction, and may not play a substantive role in the ketamine-induced deficit of reactivity to external stimuli, which involves a complex interaction of mental functions such as attention and perception, as well as motor behavior.

Reversible immobilization of eurasian otters with a combination of ketamine and medetomidine

The efficacy and safety of the combination of medetomidine and ketamine was examined in order to establish an adequate chemical immobilization protocol in the Eurasian otter (Lutra lutra) for use during translocation projects in Spain. Thirty-eight Eurasian otters ranging in body mass from 3 to 8.7 kg (mean 5.3 kg) were successfully anesthetized on 82 occasions. The dosage of ketamine was 5.1+/-0.8 (3.4-6.6) mg/kg (mean +/- SD; range) combined with medetomidine at a dosage of 51+/-8 Rg/kg (34-66 microg/kg). In most cases anaesthetic effect occurred within 3 min and the mean induction time was 5.5+/-3.2 min. The mean pulse rate was 95 beats/min. The mean respiratory rate was 32 respirations/min while the relative oxyhemoglobin saturation was 93%. According to these results, this anesthetic protocol is considered safe and can be recommended in wild caught Eurasian otters for immobilization during translocation projects. It is safe, rapid and can be reversed when needed with atipamezole. However caution is required as heart depression resulting in bradychardia may occur.

Use of medetomidine and ketamine for immobilization of free-ranging giraffes

OBJECTIVE

To develop a dosage correlated with shoulder height (SH) in centimeters for effective immobilization of free-ranging giraffes, using a combination of medetomidine (MED) and ketamine (KET) and reversal with atipamezole (ATP).

DESIGN

Prospective study.

ANIMALS

23 free-ranging giraffes.

PROCEDURE

The drug combination (MED and KET) was administered by use of a projectile dart. Quality of induction, quality of immobilization, and time to recovery following injection of ATP were evaluated. Physiologic variables measured during immobilization included PaO2, PaCO2, oxygen saturation, end-tidal CO2, blood pH, indirect arterial blood pressure, heart and respiratory rates, and rectal temperature.

RESULTS

Sixteen giraffes became recumbent with a dosage (mean +/- SD) of 143 +/- 29 microg of MED and 2.7 +/- 0.6 mg of KET/cm of SH. Initially, giraffes were atactic and progressed to lateral recumbency. Three giraffes required casting with ropes for data collection, with dosages of 166 +/- 5 microg of MED and 3.2 +/- 0.6 mg of KET/cm of SH. Four giraffes required administration of etorphine (n = 2) or were cast with ropes (2) for capture but remained dangerous to personnel once recumbent, precluding data collection. In giraffes successfully immobilized, physiologic monitoring revealed hypoxia and increased respiratory rates. Values for PaCO2, end-tidal CO2, and heart rate remained within reference ranges. All giraffes were hypertensive and had a slight increase in rectal temperature. Atipamezole was administered at 340 +/- 20 microg/cm of SH, resulting in rapid and smooth recoveries.

CONCLUSIONS AND CLINICAL RELEVANCE

Medetomidine and KET was an effective immobilizing combination for free-ranging giraffes; however, at the dosages used, it does not induce adequate analgesia for major manipulative procedures. Quality of induction and immobilization were enhanced if the giraffe was calm. Reversal was rapid and complete following injection of ATP.

Capture and immobilisation of aardvark (Orycteropus afer) using different drug combinations

Nine aardvarks (Orycteropus afer) were captured in the southern Free State, South Africa, for the placement of abdominal radio transmitters. Five combinations of ketamine hydrochloride with xylazine hydrochloride, midazolam or medetomidine hydrochloride were used to induce anaesthesia. In some cases the level of anaesthesia was maintained with 1.5% halothane. A mixture of ketamine hydrochloride and medetomidine hydrochloride was found to be most effective. Atipamizole reversed the affects of medetomidine hydrochloride, resulting in a smooth and full recovery within 8 minutes. The immobilisation and subsequent anaesthesia of these animals on cold winter nights resulted in hypothermia, and keeping the animals warm was essential to the success of the procedures undertaken. Reversal of the sedative medetomidine hydrochloride proved to be important, because animals that were released before they were fully conscious took refuge in their burrows so that care was impossible.

Attenuation of the neuropsychiatric effects of ketamine with lamotrigine: support for hyperglutamatergic effects of N-methyl-D-aspartate receptor antagonists

BACKGROUND

The cognitive, behavioral, and mood effects of N-methyl-D-aspartate (NMDA) receptor antagonists, such as phencyclidine and ketamine, have been used to study the effects of NMDA receptor dysfunction. Pharmacological modulation of the effects of NMDA receptor antagonists, such as ketamine, may lead to development of novel therapeutic agents for psychiatric illnesses such as schizophrenia. Preclinical studies indicate that some ketamine effects may be mediated through increased glutamate release. In this study, we tested the hypothesis that lamotrigine, a drug reported to inhibit glutamate release, will reduce the neuropsychiatric effects of ketamine in humans.

METHOD

Healthy subjects (n = 16) completed 4 test days involving the administration of lamotrigine, 300 mg by mouth, or placebo 2 hours prior to administration of ketamine (0.26 mg/kg by intravenous bolus and 0.65 mg/kg per hour by intravenous infusion) or placebo in a randomized order under double-blind conditions. Behavioral and cognitive assessments were performed at baseline and after administration of the medications.

RESULTS

Lamotrigine significantly decreased ketamine-induced perceptual abnormalities as assessed by the Clinician-Administered Dissociative States Scale (P<.001); positive symptoms of schizophrenia as assessed by the Brief Psychiatric Rating Scale positive symptoms subscale (P<.001); negative symptoms as assessed by the Brief Psychiatric Rating Scale negative symptoms subscale (P<.05); and learning and memory impairment as assessed by the Hopkins Verbal Learning Test (P<.05). However, lamotrigine increased the immediate mood-elevating effects of ketamine (P<.05).

CONCLUSIONS

Glutamate release-inhibiting drugs may reduce the hyperglutamatergic consequences of NMDA receptor dysfunction implicated in the pathophysiologic processes of neuropsychiatric illnesses such as schizophrenia. Further study is needed.

Immobilization of sika deer with medetomidine and ketamine, and antagonism by atipamezole

Forty wild sika deer (Cervus nippon) were immobilized with medetomidine and ketamine and reversed by atipamezole in summer and fall captures from September 1994 to October 1995. For large yearling and older deer, mean +/- SD doses of 57.0+/-15.6 microg/kg medetomidine and 1.64+/-0.49 mg/kg (male) or 4.02+/-1.16 mg/kg (female) of ketamine were administered by intramuscular injection. For calves and small yearlings, 69.3+/-7.0 microg/kg medetomidine and 2.69+/-0.44 mg/kg ketamine were administered. While immobilized, deer were easy to handle, and muscles were well relaxed. After intramuscular administration of atipamezole (about 5 times the dose of medetomidine), deer recovered rapidly and smoothly.

Immobilization of wild kinkajous (Potos flavus) with medetomidine-ketamine and reversal by atipamezole

As part of a wildlife rescue during the filling of a lake created by a hydroelectric dam (Petit Saut, French Guiana), 10 wild kinkajous (Potos flavus) were immobilized with medetomidine and ketamine for clinical examination and collection of biological samples. A mean (+/-SD) i.m. dose of 0.11+/-0.01 mg/kg medetomidine and 5.5+/-0.6 mg/kg ketamine rapidly induced complete immobilization (3.0+/-0.9 min) with good muscle relaxation and loss of corneal and pedal withdrawal reflexes. The duration and the quality of the anesthesia allowed procedures including minor surgery. Rectal temperature, heart and respiration rates, and relative oxyhemoglobin saturation (SpO2) were monitored at 5 min, 15 min, and 30 min after the medetomidine ketamine injection. Rectal temperature and heart rate significantly decreased during this time (P < 0.05). Low values of SpO2 (<90%) were recorded shortly after the injection. Hypoxemia partially resolved with time, confirmed by an increase in most SpO2 values. Atipamezole given i.m. at 5 mg/mg of medetomidine reversed the effects of the medetomidine in kinkajous. No adverse effects were observed during recovery. In group I, the antagonist was injected at 40.6+/-3.9 min. In group II, the animals showed signs of spontaneous recovery 37.9+/-6.9 min before antagonist injection at 52.2+/-6.1 min. Time from antagonist injection to ambulatory state was significantly shorter (P < 0.05) in group II (2.8+/-1.1 min) than in group I (6.9+/-1.2 min).

Antagonism of ketamine-induced anesthesia by an inhibitor of nitric oxide synthesis: a pharmacokinetic explanation

Because ketamine is an antagonist of NMDA receptors, and because some NMDA receptors activate nitric oxide synthesis in brain, this study examined if nitric oxide synthase (NOS) inhibition by L-NAME altered the course of ketamine-induced behavioral impairment. Rats given progressive doses of L-NAME until NOS activity was inhibited at least 90% displayed reduced depth and duration of behavioral depression after i.m. ketamine. Blood and brain concentrations of ketamine, norketamine, and its dehydrogenated derivative were isolated from rats previously given saline or L-NAME as above, by ether extraction, HPLC separation, and ultraviolet quantitation. The same doses of L-NAME that altered ketamine behavior reduced blood and brain ketamine concentrations 15 min after administration to about three-fourths and one-third of control, respectively. The content of norketamine and its adventitial extraction product were similarly reduced relative to control but the ratio of metabolites to ketamine was not significantly altered (p > 0.05) in brain. The decreased delivery of ketamine into brain, perhaps due to L-NAME-induced alterations in blood flow, may explain the reduced behavioral response to ketamine in these rats.

Observations on the use of medetomidine/ketamine and its reversal with atipamezole for chemical restraint in the mouse

Ketamine and medetomidine produced chemical restraint for minor procedures in mice. Male mice required 50 mg/kg ketamine, 10 mg/kg medetomidine intraperitoneally (i.p.), and females a higher dose of ketamine (75 mg/kg i.p.). The onset of restraint effects, judged by loss of righting reflex, was more rapid in males than females. The effects were reversed using atipamezole (1-2.5 mg/kg). Recovery following administration of atipamezole was more rapid in males than females. We conclude that ketamine/medetomidine, followed by reversal with atipamezole, is an effective technique for chemical restraint in the mouse.

Halothane and diazepam inhibit ketamine-induced c-fos expression in the rat cingulate cortex

BACKGROUND

Ketamine, a noncompetitive N-methyl-D-aspartate antagonist, has psychotomimetic side effects. Recent studies have shown that noncompetitive N-methyl-D-aspartate antagonists cause morphologic damage to the cingulate and retrosplenial cortices and induce c-fos protein (c-Fos) in the same regions. Although benzodiazepines are effective in preventing these side effects, the neural basis of the drug interactions has not been established.

METHODS

The effects of diazepam and halothane on c-Fos expression induced by ketamine were studied. Diazepam (1 and 5 mg/kg) or vehicle were administered subcutaneously, followed 7 min later by 100 mg/kg ketamine given intraperitoneally. Halothane (1.0 and 1.8%), was administered continuously from 10 min before ketamine administration until brain fixation. Two hours after ketamine injection, rats were perfused and their brains fixed and extracted. Brain sections were prepared in a cryostat and c-Fos expression was detected using immunohistochemical methods.

RESULTS

Ketamine induced c-Fos-like immunoreactivity in the cingulate and retrosplenial cortices, thalamus, and neocortex. Diazepam suppressed the ketamine-induced c-Fos-like immunoreactivity in the cingulate and retrosplenial cortices in a dose-dependent manner, leaving the thalamus and neocortex less affected. Halothane suppressed the ketamine-induced c-Fos-like immunoreactivity in the cingulate and retrosplenial cortices and the neocortex in a dose-dependent manner, leaving the thalamus relatively unaffected.

CONCLUSION

Halothane and diazepam inhibited ketamine-induced c-Fos expression in the cingulate and retrosplenial cortices, leaving the thalamus relatively unaffected.

DNQX inhibits phencyclidine (PCP) and ketamine induction of the hsp70 heat shock gene in the rat cingulate and retrosplenial cortex

Phencyclidine (PCP) and ketamine are known to block NMDA receptor mediated excitotoxicity by non-competitively blocking the NMDA receptor calcium channel. PCP and ketamine have the paradoxical effect of also inducing the heat shock gene, hsp70, in the cingulate and retrosplenial cortex of the rat. The present study shows that DNQX, a specific AMPA receptor antagonist, given as either a 5 mg/kg or 10 mg/kg intraperitoneal dose or into the lateral cerebral ventricle (5 microliters of 0.5 mg/ml) significantly diminished PCP (40 mg/kg) and ketamine (80, 100, 120 mg/kg) hsp70 induction in the posterior cingulate and retrosplenial cortex. The most dramatic decrease of hsp70 induction was seen with the intraventricular dose of DNQX. Present findings show that the AMPA receptor has a role in PCP/ketamine induction of hsp70 in the cortex. DNQX inhibition of PCP/ketamine hsp70 induction was likely related to AMPA receptor antagonism which prevented excess calcium influx via voltage-gated calcium channels.

Antagonism of some cyclohexamine-based drug combinations used for chemical restraint of southern elephant seals (Mirounga leonina)

This study examined the use of 4 antagonists of chemical restraint in mature female southern elephant seals (Mirounga leonina) that were restrained with ketamine and diazepam, ketamine and xylazine, or tiletamine and zolazepam. The antagonists were: 4-aminopyridine, yohimbine, doxapram and sarmazenil. The effects of the antagonists on the animal's time to first movement forward and recovery, heart rate, respiratory rate and venous blood gas and pH values, and level of chemical restraint were recorded. Sarmazenil (1.0 mg/kg) and doxapram (5.0 mg/kg) partially antagonised 50:1 ketamine: diazepam (ketamine = 3.0 mg/kg, diazepam = 0.06 mg/kg) and tiletamine and zolazepam (tiletamine = 0.5 mg/kg, zolazepam = 0.5 mg/kg). However, the rapid recovery after low doses of anaesthetics means that antagonism is usually unnecessary, and it may increase the likelihood of shaking. Routine antagonism of ketamine and xylazine (ketamine = 3.0 mg/kg, xylazine = 0.5 mg/kg) is more useful given its usually delayed recovery time and potential for thermoregulatory problems. For this purpose yohimbine (0.06 mg/kg) offered advantages over doxapram in giving a smoother recovery with less aggression. 4-aminopyridine (0.2 mg/kg) prolonged chemical restraint by 100:1 ketamine:diazepam (ketamine = 3.0 mg/kg, diazepam = 0.03 mg/kg) and ketamine and xylazine, and should be contraindicated. Doxapram (5.0 mg/kg) was the most useful general antagonist for all groups of drugs but shaking was seen and a lower dose is recommended.

Physostigmine in recovery from anaesthesia

Intravenous ketamine anaesthesia has been used by the British army in the field for many years. A recognised problem has been the unpredictable recovery profile this produces. We anaesthetised 28 ASA 1 patients using a standard British military technique. At termination of the anaesthetic, half of the patients were given a physostigmine/glycopyrronium mixture and half were given the equivalent volume of saline 0.9%. There was a significant difference between the two groups with regard to recovery times (p < 0.001). There was no significant difference with regard to other variables. In trauma anaesthesia the improved recovery profile from the use of physostigmine following ketamine anaesthesia may lead to earlier evacuation of the patient.

[Interaction of S-(+)-ketamine with opiate receptors. Effects on EEG, evoked potentials and respiration in awake dogs]

To check for suspected opioid-receptor mediated hypnotic and antinociceptive effects of S(+)-ketamine, highly selective antagonists were used after the anaesthetic. METHODS. To determine the hypnotic effects of increasing doses of S(+)-ketamine (2-5-10-20 mg/kg given at 10-min intervals), EEG power spectra (delta, theta, alpha, beta) were derived (Lifescan), and antinociceptive potency was evaluated using the somatosensory evoked potential (SEP, Lifescan) in awake, trained dogs (n = 10). To check for an opioid-receptor-related interaction, an antagonist of the methoxymorphinane series (HS-275, 80 micrograms/kg i.v.) with higher selectivity than naloxone for the mu-receptor was given at the end. After washout the same animals were exposed to S(+)-ketamine. This time, however, the highly selective delta-antagonist naltrindole (160 micrograms/kg i.v.) was given. To show up any respiratory depression arterial blood gases were taken after each dose. RESULTS. S(+)-Ketamine induced a dose-related increase in power in the theta band (3-8 Hz), with a ceiling effect at 10 mg/kg. The changes were reversed by both antagonists. In the beta band (13-30 Hz) and in the delta domain, power decreased or increased, respectively, in a highly significant manner (P < 0.005) at 20 mg/kg. Both effects reversed after the antagonists with an overshoot in beta (+12% and +14%, respectively) and a decrease in delta (-45% and -62%, respectively) compared with control. S(+)-Ketamine induced a dose-dependent increase in peak latency and depression of the SEP amplitude by a maximum of over 50%. Latency changes were completely reversed only by HS-275. Amplitude height was only partly restored by both antagonists. A clinical relevant decrease in PaO2 and increase in PaCO2 increase were seen at 20 mg/kg. Hypoxia was reversed by both antagonists; hypercapnia was only partially reversed. CONCLUSION. The results confirm the suspicion that S(+)-ketamine induces an opioid theta- and delta-receptor-mediated deep hypnotic effect. Blockade of nociceptive impulses in afferent sensory nervous pathways suggests an efficient analgesic effect mediated partly by the opioid mu-receptor. Other mechanisms, such as an interaction with the NMDA receptor, have to be taken into consideration to account for the full antinociceptive effect. Respiratory depression may be of clinical importance when high dosages of S(+)-ketamine are given.

[Ketamine racemate versus S-(+)-ketamine with or without antagonism with physostigmine. A quantitative EEG study on volunteers]

The potency of S-(+)-ketamine is approximately double that of the racemic ketamine. This study was carried out to investigate the recovery of cerebral electrical function after a bolus of 1.3 mg/kg ketamine or 0.65 mg/kg S-(+)-ketamine and subsequent continuous application of 4 mg/kg h ketamine per h or 2 mg/kg S-(+)-ketamine, per h for 15 min. Furthermore, the centrally acting, cholinergic agonist physostigmine has been reported to antagonize ketamine and to shorten the recovery period. Therefore, after S-(+)-ketamine 0.012 mg/kg physostigmine was tested against saline placebo. METHODS. With their own informed consent and the approval of the ethics committee 12 healthy volunteers were enrolled in a double-blind cross-over study. All drugs were dissolved in identical volumes. On three dates with intervals of at least 1 week between, ketamine/NaCl, S-(+)-ketamine/physostigmine or S-(+)-ketamine/NaCl was administered (Table 1). The sequence was randomized. The EEG was recorded from 20 sites according to the 10/20 system and after Fast-Fourier transformation computed into amplitudes within the delta, theta, alpha, and beta bands and within the total spectrum. The median, the spectral edge frequency and the dominant frequency (dF) were also determined. Mean values of all electrodes before and at 10, 15, 30, 45 and 195 min after the bolus injection were compared using two-dimensional analysis of variance (ANOVA, significance level P < 0.05). RESULTS. The characteristic increase in theta-amplitude and decrease of alpha-amplitude were observed after ketamine and S-(+)-ketamine. Median and dF dropped from the alpha to the theta frequency range. Ketamine led to a greater increase in total, delta, theta and beta amplitude during anaesthesia. 3 hours after ketamine/S-(+)-ketamine anaesthesia a significant decrease in the median and dominant frequency and in total, delta, theta, alpha and beta amplitudes confirmed residual impairment of cerebral function after all study drugs. No differences were found between physostigmine and placebo. DISCUSSION. The EEG changes during ketamine/S-(+)-ketamine administration suggest a slightly deeper anaesthetic level after ketamine. The course of recovery was not different after ketamine and after S-(+)-ketamine. The spectral edge frequency did not differ between measurement points, and is therefore not suitable for assessment of the depth of anaesthesia reached with ketamine/S-(+)-ketamine. The dose of physostigmine tested was probably too low to produce antagonism of S-(+)-ketamine. An increased dosage of physostigmine has yet to be studied, but is likely to cause a higher rate of side effects, such as nausea, vomiting and bradycardia, and possibly even tonic-clonic seizures.

L-arginine attenuates ketamine-induced increase in renal sympathetic nerve activity

BACKGROUND

It has been reported that ketamine produces sympathoexcitation by directly stimulating the central nervous system. It also has been shown that nitric oxide (NO) may play a role in signal transduction of the nervous system. Therefore, we hypothesized that the sympathoexcitation of ketamine may be linked to central NO formation. To test this hypothesis, we examined the effects of L-arginine, a substrate of NO formation, on renal sympathetic nerve activity (RSNA) during ketamine anesthesia.

METHODS

Using 45 rabbits given basal anesthesia with alpha-chloralose, we measured changes in heart rate, mean arterial pressure, and RSNA in response to intravenous ketamine (1 mg/kg) and investigated the effect of intravenous L-arginine and D-arginine (bolus 30 mg/kg followed by continuous 30 mg.kg-1.min-1). The animals were divided into intact, sinoaortic- and vagal-deafferented, and spinal cord-transected groups.

RESULTS

Ketamine caused significant increases in RSNA (172 +/- 16%), heart rate (12 +/- 2 beats/min), and mean arterial pressure (8 +/- 1 mmHg) in the intact rabbits. Ketamine also increased RSNA in sinoaortic- and vagal-deafferented rabbits, but not in spinal cord-transected rabbits. L-Arginine attenuated the ketamine-induced increase in RSNA in intact and deafferented rabbits, whereas D-arginine had no effect on RSNA. In addition, NG-nitro-L-arginine methyl ester, a NO synthase inhibitor, increased RSNA and the increase was attenuated by L-arginine.

CONCLUSIONS

Ketamine may act centrally to increase sympathetic outflow, and the sympathoexcitation may be attenuated by increasing NO formation with L-arginine in the central nervous system.

Serotonin 1A-receptor activation suppresses respiratory apneusis in the cat

Malfunction of inhibitory synaptic processes in the brainstem result in abnormal prolonged inspiration (apneusis). Since we previously found that the serotonin (5-hydroxytryptamine; 5-HT) 5-HT1A receptor agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) shortens inspiratory discharges, we tested its ability to suppress apneusis. We recorded phrenic nerve activity and the membrane potential of medullary expiratory (E-2) and postinspiratory (PI) neurons in 14 anaesthetized, paralyzed, artificially ventilated cats. Systemic hypoxia or i.v. injection of pentobarbital sodium or the N-methyl-D-aspartate (NMDA) receptor blocker ketamine induced apneustic phrenic nerve discharges, delayed depolarization to threshold of E-2 neurons and prolonged hyperpolarization in PI neurons. 8-OH-DPAT (10-40 micrograms/kg i.v.) produced partial to complete restoration of normal phrenic nerve discharges and membrane potential.

Potentiation of the ambulation-increasing effect induced by combined administration of MK-801 with ethanol in mice

Although both MK-801 (dizocilpine: 0.1 mg/kg, IP) and ethanol (1.6 nd 2.4 g/kg, PO) only slightly increased ambulatory activity in mice, their combination produced a marked enhancement of the ambulation-increasing effect. The combination of MK-801 (0.03 mg/kg) with ethanol (1.6 and 2.4 g/kg) also elicited a significant increase in the mouse's ambulation. A significant enhancement of the effect was produced by the combination of ketamine (3 and 10 mg/kg) with ethanol only (2.4 g/kg). The ambulation increment induced by the combination of MK-801 (0.1 mg/kg) plus ethanol (1.6 g/kg) was dose-dependently inhibited by YM-09151-2 (0.0001-0.01 mg/kg IP), SCH 23390 (0.001-1 mg/kg IP), reserpine (0.1-1 mg/kg, IP) and ceruletide (0.00001-0.001 mg/kg, IP), and the highest dose of each drug was effective for complete inhibition of the ambulation. Naloxone (0.05-5 mg/kg IP), apomorphine (0.001-0.1 mg/kg IP) and alpha-methyl-p-tyrosine (50-200 mg/kg, IP) partially reduced the ambulatory activity induced by the combination of MK-801 with ethanol. These results suggest that the dopaminergic system, particularly via presynaptic changes in the release of stored dopamine, as well as the opioid system, are involved in the interaction of MK-801 with ethanol.

Postpartum immobilization of adult female moose using xylazine, ketamine and yohimbine hydrochlorides

Twenty-two free-ranging adult female moose (Alces alces) were immobilized with a 1:4 mixture of xylazine hydrochloride (XH) and ketamine hydrochloride (KH). Mean (SD) dosages/animal for XH and KH were 419 (148) and 1565 (433) mg, respectively. Mean (SD) induction time was 18.4 (9.7) minutes. Reversal with yohimbine hydrochloride using a mean dosage of 83 mg/animal resulted in a mean (SD) recovery time of 22.8 (28.5) minutes.

Low-dose midazolam antagonizes cerebral metabolic stimulation by ketamine in the pig

In order to test the hypothesis that low-dose midazolam reduces excitatory cerebral symptoms by attenuating ketamine-induced increases in the cerebral metabolic rate for oxygen (CMRO2), we compared the cerebral effects of a combination of an anesthetic dose of ketamine hydrochloride (10.0 mg.kg-1 i.v.) and a subanaesthetic dose of midazolam maleate (0.25 mg.kg-1 i.v., n = 6; or 0.10 mg.kg-1 i.v., n = 6) with results recently obtained with ketamine (10.0 mg.kg-1 i.v.) in normoventilated pigs anaesthetized with fentanyl, nitrous oxide and pancuronium. Cerebral blood flow (CBF) was measured with the intra-arterial 133Xe clearance technique, and CMRo2 was calculated from CBF and the cerebral arteriovenous oxygen content difference (CaVO2). The CMRO2 did not increase significantly. In contrast, the maximal increase in cerebral CaVo2 (by 56-59% at 10 min; P < 0.01) was similar to that induced by ketamine, since CBF was more depressed (by 35-45% at 1 min: P < 0.001) by ketamine-midazolam than by ketamine only. Midazolam was found to increase CVR (P < 0.01) and further depress CBF (P < 0.01), and to antagonize the ketamine-induced increase in CMRO2 (P < 0.05). Ketamine-induced effects on mean arterial pressure (MAP) and spectral electroencephalographic (EEG) voltage were not significantly altered by midazolam. The pharmacokinetics of ketamine, as measured during an 80-min period, were not affected by the concomitant administration of midazolam. We propose that a ketamine-midazolam combination comprising a low-dose fraction (1/100-1/40) of midazolam is superior to ketamine alone for anaesthetic use.

SCH 23390 equivalently, but YM-09151-2 differentially reduces the stimulant effects of methamphetamine, MK-801 and ketamine: assessment by discrete shuttle avoidance in mice

The inhibitory actions of the selective dopamine D1-and D2-antagonists SCH 23390 and YM-09151-2, respectively, on the mouse's discrete shuttle avoidance were almost equipotent at doses ranging from 0.01-0.1 mg/kg. SCH 23390 reduced the stimulant action of methamphetamine (0.5 mg/kg), MK-801 (0.1 mg/kg) and ketamine (10 mg/kg) with a similar potency. YM-09151-2 also antagonized the actions of these drugs, with the following order of effectiveness: ketamine > MK-801 > methamphetamine. The present results indicate that methamphetamine, MK-801 and ketamine have different characteristics of CNS stimulant action through dopamine D2-receptors.