The novel ketamine analog methoxetamine produces dissociative-like behavioral effects in rodents

Understanding the role of the NMDA receptor subunit, GluN2D, in mediating NMDA receptor antagonist-induced behavioral disruptions in male and female mice

Noncompetitive NMDA receptor (NMDAR) antagonists like phencyclidine (PCP) and ketamine cause psychosis-like symptoms in healthy humans, exacerbate schizophrenia symptoms in people with the disorder, and disrupt a range of schizophrenia-relevant behaviors in rodents, including hyperlocomotion. This is negated in mice lacking the GluN2D subunit of the NMDAR, suggesting the GluN2D subunit mediates the hyperlocomotor effects of these drugs. However, the role of GluN2D in mediating other schizophrenia-relevant NMDAR antagonist-induced behavioral disturbances, and in both sexes, is unclear. This study aimed to investigate the role of the GluN2D subunit in mediating schizophrenia-relevant behaviors induced by a range of NMDA receptor antagonists. Using both male and female GluN2D knockout (KO) mice, we examined the effects of the NMDAR antagonist's PCP, the S-ketamine enantiomer (S-ket), and the ketamine metabolite R-norketamine (R-norket) on locomotor activity, anxiety-related behavior, and recognition and short-term spatial memory. GluN2D-KO mice showed a blunted locomotor response to R-norket, S-ket, and PCP, a phenotype present in both sexes. GluN2D-KO mice of both sexes showed an anxious phenotype and S-ket, R-norket, and PCP showed anxiolytic effects that were dependent on sex and genotype. S-ket disrupted spatial recognition memory in females and novel object recognition memory in both sexes, independent of genotype. This datum identifies a role for the GluN2D subunit in sex-specific effects of NMDAR antagonists and on the differential effects of the R- and S-ket enantiomers.

Prepulse inhibition of the acoustic startle reflex impairment by 5-HT2A receptor activation in the inferior colliculusis prevented by GABAA receptor blockade in the pedunculopontine tegmental nucleus

The relationship between serotonin dysfunction and schizophrenia commenced with the discovery of the effects of lysergic acid diethylamide (LSD) that has high affinity for 5-HT_2A receptors. Activation of these receptors produces perceptual and behavioural changes such as illusions, visual hallucinations and locomotor hyperactivity. Using prepulse inhibition (PPI) of the acoustic startle, which is impaired in schizophrenia,we aimed to investigate:i) the existence of a direct and potentially inhibitory neural pathway between the inferior colliculus (IC) and the pedunculopontine tegmental nucleus (PPTg) involved in the mediation of PPI responses by a neural tract tracing procedure;ii) if the microinjection of the 5-HT_2A receptors agonist DOI in IC would activate neurons in this structure and in the PPTg by a c-Fos protein immunohistochemistry study;iii) whether the deficits in PPI responses, observed after the administration of DOI in the IC, could be prevented by the concomitant microinjection of the GABA_A receptor antagonist bicuculline in the PPTg.Male Wistar rats were used in this study. An IC-PPTg reciprocated neuronal pathway was identified by neurotracing. The number of c-Fos labelled cells was lower in the DOI group in IC and PPTg, suggesting that this decrease could be due to the high levels of GABA in both structures. The concomitant microinjections of bicuculline in PPTg and DOI in IC prevented the PPI deficit observed after the IC microinjection of DOI. Ourfindings suggest that IC 5-HT_2A receptors may be at least partially involved in the regulation of inhibitory pathways mediating PPI response in IC and PPTg structures.

Prenatal ketamine exposure impairs prepulse inhibition via arginine vasopressin receptor 1A-mediated GABAergic neuronal dysfunction in the striatum

Ketamine is a widely used anesthetic with N-methyl-D-aspartate (NMDA) receptor antagonism. Exposure to ketamine and NMDA receptor antagonists may induce psychosis. However, the mechanism underlying the effects of ketamine on the immature brain remains unclear. In this study, NMDA receptor antagonists, ketamine and methoxetamine, were administered to pregnant F344 rats (E17). These regimens induce psychosis-like behaviors in the offspring, such as hyperlocomotion induced by MK-801, a non-competitive NMDA receptor antagonist. We also observed that prepulse inhibition (PPI) was significantly reduced. Interestingly, ketamine administration increased the arginine vasopressin receptor 1A (Avpr1a) expression levels in the striatum of offspring with abnormal behaviors. Methoxetamine, another NMDA receptor antagonist, also showed similar results. In addition, we demonstrated a viral vector-induced Avpr1a overexpression in the striatum-inhibited PPI. In the striatum of offspring, ketamine or methoxetamine treatment increased glutamate decarboxylase 67 (GAD67) and δ-aminobutyric acid (GABA) levels. These results show that prenatal NMDA receptor antagonist treatment induces GABAergic neuronal dysfunction and abnormalities in sensorimotor gating via regulating Avpr1a expression in the striatum.

Evaluation of neurotoxicity and long-term function and behavior following intrathecal 1 % 2-chloroprocaine in juvenile rats

Spinally-administered local anesthetics provide effective perioperative anesthesia and/or analgesia for children of all ages. New preparations and drugs require preclinical safety testing in developmental models. We evaluated age-dependent efficacy and safety following 1 % preservative-free 2-chloroprocaine (2-CP) in juvenile Sprague-Dawley rats. Percutaneous lumbar intrathecal 2-CP was administered at postnatal day (P)7, 14 or 21. Mechanical withdrawal threshold pre- and post-injection evaluated the degree and duration of sensory block, compared to intrathecal saline and naive controls. Tissue analyses one- or seven-days following injection included histopathology of spinal cord, cauda equina and brain sections, and quantification of neuronal apoptosis and glial reactivity in lumbar spinal cord. Following intrathecal 2-CP or saline at P7, outcomes assessed between P30 and P72 included: spinal reflex sensitivity (hindlimb thermal latency, mechanical threshold); social approach (novel rat versus object); locomotor activity and anxiety (open field with brightly-lit center); exploratory behavior (rearings, holepoking); sensorimotor gating (acoustic startle, prepulse inhibition); and learning (Morris Water Maze). Maximum tolerated doses of intrathecal 2-CP varied with age (1.0 μL/g at P7, 0.75 μL/g at P14, 0.5 μL/g at P21) and produced motor and sensory block for 10-15 min. Tissue analyses found no significant differences across intrathecal 2-CP, saline or naïve groups. Adult behavioral measures showed expected sex-dependent differences, that did not differ between 2-CP and saline groups. Single maximum tolerated in vivo doses of intrathecal 2-CP produced reversible spinal anesthesia in juvenile rodents without detectable evidence of developmental neurotoxicity. Current results cannot be extrapolated to repeated dosing or prolonged infusion.

New insights into methoxetamine mechanisms of action: Focus on serotonergic 5-HT2 receptors in pharmacological and behavioral effects in the rat

Methoxetamine (MXE) is a dissociative substance of the arylcyclohexylamine class that has been present on the designer drug market as a ketamine-substitute since 2010. We have previously shown that MXE (i) possesses ketamine-like discriminative and positive rewarding effects in rats, (ii) affects brain processing involved in cognition and emotional responses, (iii) causes long-lasting behavioral abnormalities and neurotoxicity in rats and (iv) induces neurological, sensorimotor and cardiorespiratory alterations in mice. To shed light on the mechanisms through which MXE exerts its effects, we conducted a multidisciplinary study to evaluate the various neurotransmitter systems presumably involved in its actions on the brain. In vivo microdialysis study first showed that a single administration of MXE (0.25 and 0.5 mg/kg, i.v.) is able to significantly alter serotonin levels in the rat medial prefrontal cortex (mPFC) and nucleus accumbens. Then, we observed that blockade of the serotonin 5-HT₂ receptors through two selective antagonists, ketanserin (0.1 mg/kg, i.p.) and MDL 100907 (0.03 mg/kg, i.p.), at doses not affecting animals behavior per se, attenuated the facilitatory motor effect and the inhibition on visual sensory responses induced by MXE (3 mg/kg, i.p.) and ketamine (3 mg/kg, i.p.), and prevented MXE-induced reduction of the prepulse inhibition in rats, pointing to the 5-HT₂ receptors as a key target for the recently described MXE-induced sensorimotor effects. Finally, in-vitro electrophysiological studies revealed that the GABAergic and glutamatergic systems are also likely involved in the mechanisms through which MXE exerts its central effects since MXE inhibits, in a concentration-dependent manner, NMDA-mediated field postsynaptic potentials and GABA-mediated spontaneous currents. Conversely, MXE failed to alter both the AMPA component of field potentials and presynaptic glutamate release, and seems not to interfere with the endocannabinoid-mediated effects on mPFC GABAergic synapses. Altogether, our results support the notion of MXE as a NMDA receptor antagonist and shed further lights into the central mechanisms of action of this ketamine-substitute by pointing to serotonin 5-HT₂ receptors as crucial players in the expression of its sensorimotor altering effects and to the NMDA and GABA receptors as potential further important targets of action.

The dopaminergic alterations induced by 4-F-PCP and 4-Keto-PCP may enhance their drug-induced rewarding and reinforcing effects: Implications for abuse

Novel psychoactive substances remain the popular recreational drugs of use over the years. They continue to bypass government restrictions due to their synthesis and modifications. Recent additions to the lists are the 4-F-PCP and 4-Keto-PCP, analogs of the drug phencyclidine (PCP) known to induce adverse effects and abuse potential. However, studies on the abuse potential of 4-F-PCP and 4-Keto-PCP remain scarce. The rewarding and reinforcing effects of the drugs were assessed using conditioned place preference (CPP), self-administration, and locomotor sensitization tests. Dopamine (DA) receptor antagonists (SCH23390 and haloperidol) were administered during CPP to evaluate the involvement of the mesolimbic dopaminergic system. DA-related protein expression in the nucleus accumbens (NAcc) and ventral tegmental area (VTA) was measured. Additionally, phosphorylated cyclic-adenosine monophosphate-activated protein (AMP) response element-binding (p-CREB) protein, deltaFosB (∆FosB), and brain-derived neurotrophic factor (BDNF) protein levels in the NAcc were measured to assess the addiction neural plasticity effect of the drugs. Both 4-F-PCP and 4-Keto-PCP-induced CPP and self-administration; however, only 4-F-PCP elicited locomotor sensitization. Treatment with DA receptor antagonists (SH23390 and haloperidol) inhibited the 4-F- and 4-Keto-induced CPP. Both substances altered the levels of DA receptor D1 (DRD1), thyroxine hydroxylase (TH), DA receptor D2 (DRD2), p-CREB, ∆FosB, and BDNF. The results suggest that 4-F-PCP and 4-Keto-PCP may induce abuse potential in rodents via alterations in dopaminergic system accompanied by addiction neural plasticity.

Increasing Prevalence of Ketamine in Drivers in New York City Including the Identification of 2-Fluoro-Deschloroketamine

Ketamine is a dissociative anesthetic used in veterinary and human medicine since the 1970s. Its clinical use has expanded to control of seizures, pre-hospital emergency medical services (EMS), and is finding new purpose as an analgesic alternative and antidepressant. Ketamine brings hope for effective management of chronic pain in the absence of opioids, and decreasing suicidal ideations, however, its persistence as a recreational drug for its hallucinogenic properties remains. In the wake of expanding medicinal purposes, the diversity of New York City's population was explored to better understand its misuse. This retrospective study looks at the prevalence of ketamine in driver fatalities over a period of 18 years (2003-2020) and cases involving suspected driving under the influence of drugs (DUID) over a period of 6 years (2015-2020). Ketamine was identified in 6 driver fatalities and in 47 DUID cases. None of the driver fatalities were suspected of ketamine misuse, due to administration either in hospital or EMS administration. In the DUID cases, an increasing trend was observed over the 6-year study period with 100% (N = 47) of the cases confirmed as non-hospital/non-EMS administered ketamine. Of the DUID cases, 94% were male, with the majority between the age of 21-39 years (85%) and were predominantly Hispanic (36%) and Asian (34%). Blood concentrations of ketamine ranged from 27 to > 2000 ng/mL with polydrug use prevalent. The most common drug classes detected in addition to ketamine were cannabinoids (38%), ethanol (32%), benzodiazepines (26%), cocaine (19%), and amphetamines/MDMA (15%). In 2019, 2-fluoro-deschloroketamine (2F-DCK) was identified in two cases for the first time. Despite its increased acceptance for mental health disorders, ketamine's persistence and misuse as a recreational drug remains and should continue to be monitored by relevant toxicological, clinical, and law enforcement communities along with emerging illicit ketamine analogs.

1-Phenylcyclohexan-1-amine hydrochloride (PCA HCl) alters mesolimbic dopamine system accompanied by neuroplastic changes: A neuropsychopharmacological evaluation in rodents

The recreational use of N-methyl-D-aspartate (NMDA) antagonist phencyclidine (PCP) and ketamine have grown rapidly due to their psychotomimetic properties. These compounds induce both non-fatal and fatal adverse effects and despite the enhanced regulation, they are continuously synthesized and are being sold in the illegal drug market, including 1-phenylcyclohexan-1-amine hydrochloride (PCA). Therefore, we evaluated its abuse potential through the conditioned-place preference (CPP), self-administration, and locomotor sensitization paradigms. Pretreatment with SCH 2 3390 and haloperidol was also performed during a CPP test. We used ELISA to measure dopamine (DA) levels and western blotting to determine effects on the DA-related proteins as well as on phosphorylated CREB, deltaFosB, and brain-derived neurotrophic factor (BDNF) in the ventral tegmental area (VTA) and nucleus accumbens (NAc). Finally, we examined the effects on brain wave activity using electroencephalography (EEG). PCA induced CPP in mice and was self-administered by rats, suggesting that PCA has rewarding and reinforcing properties. PCA increased locomotor of mice on the first treatment and challenge days. SCH 23390 and haloperidol blocked the CPP. PCA altered the DA, tyrosine hydroxylase, dopamine D1 and D2 receptors as well as p-CREB and deltaFosB. Also, PCA altered the delta and gamma waves in the brain, which were then normalized by SCH 2 3390 and haloperidol. The present findings indicate that PCA may induce abuse potential through the dopaminergic system and probably accompanied with alterations in brain wave activity which is similar to that of other psychotomimetic NMDA antagonists. We advocate thorough monitoring of PCP analogs as they pose potential harm to public health.

Potential Of High-Resolution Mass Spectrometry For The Detection Of Drugs And Metabolites In Hair: Methoxetamine In A Real Forensic Case

The analysis of drugs of abuse in hair and other biological matrices of forensic interest requires great selectivity and sensitivity. This is done traditionally through target analysis, with one or more analytical methods, or with different and specific preanalytical phases, and complex procedures performed by the toxicological laboratories, and there is no exception with ketamine-like compounds, such as methoxetamine, a new psychoactive substance (NPS) whose use has increased in the last decades, and continues to grow quickly year by year. More validated methods of analysis are needed to detect these substances in low concentrations selectively. Reanalyzing the samples of a former case of a polydrug consumer accused of a crime against public health in Spain, five metabolites of methoxetamine (normethoxetamine, O-desmethylmethoxetamine, dehydromethoxetamine, dihydronormethoxetamine and hydroxynormethoxetamine) were tentatively detected using a high-resolution technique that is liquid chromatography coupled to high-resolution mass spectrometry (LC-HR-MS/MS). The most selective analytical LC-HR-MS/MS method together a universal and simpler pretreatment stages has demonstrated to allow faster analysis and more sensitivity than the one performed traditionally at the INTCF laboratories, which was gas chromatography coupled to mass spectrometry (GC-MS).

The Abuse Potential of Novel Synthetic Phencyclidine Derivative 1-(1-(4-Fluorophenyl)Cyclohexyl)Piperidine (4'-F-PCP) in Rodents

The dissociative anesthetic phencyclidine (PCP) and PCP derivatives, including 4′-F-PCP, are illegally sold and abused worldwide for recreational and non-medical uses. The psychopharmacological properties and abuse potential of 4′-F-PCP have not been fully characterized. In this study, we evaluated the psychomotor, rewarding, and reinforcing properties of 4′-F-PCP using the open-field test, conditioned place preference (CPP), and self-administration paradigms in rodents. Using Western immunoblotting, we also investigated the expression of dopamine (DA)-related proteins and DA-receptor-mediated downstream signaling cascades in the nucleus accumbens (NAc) of 4′-F-PCP-self-administering rats. Intraperitoneal administration of 10 mg/kg 4′-F-PCP significantly increased locomotor and rearing activities and increased CPP in mice. Intravenous administration of 1.0 mg/kg/infusion of 4′-F-PCP significantly enhanced self-administration during a 2 h session under fixed ratio schedules, showed a higher breakpoint during a 6 h session under progressive ratio schedules of reinforcement, and significantly altered the expression of DA transporter and DA D1 receptor in the NAc of rats self-administering 1.0 mg/kg 4′-F-PCP. Additionally, the expression of phosphorylated (p) ERK, pCREB, c-Fos, and FosB/ΔFosB in the NAc was significantly enhanced by 1.0 mg/kg 4′-F-PCP self-administration. Taken together, these findings suggest that 4′-F-PCP has a high potential for abuse, given its robust psychomotor, rewarding, and reinforcing properties via activation of DAergic neurotransmission and the downstream signaling pathways in the NAc.

4-MeO-PCP and 3-MeO-PCMo, new dissociative drugs, produce rewarding and reinforcing effects through activation of mesolimbic dopamine pathway and alteration of accumbal CREB, deltaFosB, and BDNF levels

RATIONALE

A high number of synthetic dissociative drugs continue to be available through online stores, leading to their misuse. Recent inclusions in this category are 4-MeO-PCP and 3-MeO-PCMo, analogs of phencyclidine. Although the dissociative effects of these drugs and their recreational use have been reported, no studies have investigated their abuse potential.

OBJECTIVES

To examine their rewarding and reinforcing effects and explore the mechanistic correlations.

METHODS

We used conditioned place preference (CPP), self-administration, and locomotor sensitization tests to assess the rewarding and reinforcing effects of the drugs. We explored their mechanism of action by pretreating dopamine receptor (DR) D1 antagonist SCH23390 and DRD2 antagonist haloperidol during CPP test and investigated the effects of 4-MeO-PCP and 3-MeO-PCMo on dopamine-related proteins in the ventral tegmental area and nucleus accumbens. We also measured the levels of dopamine, phosphorylated cyclic-AMP response element-binding (p-CREB) protein, deltaFosB, and brain-derived neurotrophic factor (BDNF) in the nucleus accumbens. Additionally, we examined the effects of both drugs on brain wave activity using electroencephalography.

RESULTS

While both 4-MeO-PCP and 3-MeO-PCMo induced CPP and self-administration, only 4-MeO-PCP elicited locomotor sensitization. SCH23390 and haloperidol inhibited the acquisition of drug CPP. 4-MeO-PCP and 3-MeO-PCMo altered the levels of tyrosine hydroxylase, DRD1, DRD2, and dopamine, as well as that of p-CREB, deltaFosB, and BDNF. All drugs increased the delta and gamma wave activity, whereas pretreatment with SCH23390 and haloperidol inhibited it.

CONCLUSION

Our results indicate that 4-MeO-PCP and 3-MeO-PCMo induce rewarding and reinforcing effects that are probably mediated by the mesolimbic dopamine system, suggesting an abuse liability in humans.

Ketamine analogues: Comparative toxicokinetic in vitro-in vivo extrapolation and quantification of 2-fluorodeschloroketamine in forensic blood and hair samples

Recently, the new psychoactive substance (NPS) ketamine analogue 2-fluoro-deschloroketamine (2FDCK) was observed in driving-under-the-influence-of-drugs whole blood samples and in a forensic hair investigation case in Denmark. The molecular structure variations among the NPS subgroups may alter the metabolic fate and drug potency, thereby posing a threat for drug users. This study reports quantification of 2FDCK in whole blood samples and forensic hair and compares the following toxicokinetic parameters: unbound fraction (f_u) and in vitro-in vivo-extrapolation (IVIVE) of hepatic clearance for ketamine, norketamine, 2FDCK, methoxetamine and deschloroketamine. The f_u was investigated with ultrafiltration, while clearance studies were conducted at 1 μM with pooled human liver microsomes. Samples were analysed by liquid chromatography and mass spectrometry. For the first time, 2FDCK was determined in a concentration range between 0.005 and 0.48 mg/kg in blood samples. Segmental hair analysis demonstrated 2FDCK at concentrations from 0.007 to 0.034 ng/mg throughout the three investigated segments. Toxicokinetic comparison of the five compounds gave a f_u between 0.54 and 0.84, with ketamine being the most bound and deschloroketamine being the least bound, in accordance with the logP of the compounds. Conversely, a negative correlation was observed between the molecular weight of the halogen in the ortho-position and IVIVE hepatic clearance. The IVIVE of hepatic clearance, CL_{parallel-tube}, gave values from 18.1 to 5.44 mL/min/kg for ketamine and methoxetamine, respectively. The deschloroketamine IVIVE was disregarded due to low drug elimination under the experimental conditions used. This study provides a basis for toxicokinetic understanding of ketamine analogues.

Rapid-acting antidepressant ketamine, its metabolites and other candidates: A historical overview and future perspective

Major depressive disorder (MDD) is one of the most disabling psychiatric disorders. Approximately one-third of the patients with MDD are treatment resistant to the current antidepressants. There is also a significant therapeutic time lag of weeks to months. Furthermore, depression in patients with bipolar disorder (BD) is typically poorly responsive to antidepressants. Therefore, there exists an unmet medical need for rapidly acting antidepressants with beneficial effects in treatment-resistant patients with MDD or BD. Accumulating evidence suggests that the N-methyl-D-aspartate receptor (NMDAR) antagonist ketamine produces rapid and sustained antidepressant effects in treatment-resistant patients with MDD or BD. Ketamine is a racemic mixture comprising equal parts of (R)-ketamine (or arketamine) and (S)-ketamine (or esketamine). Because (S)-ketamine has higher affinity for NMDAR than (R)-ketamine, esketamine was developed as an antidepressant. On 5 March 2019, esketamine nasal spray was approved by the US Food and Drug Administration. However, preclinical data suggest that (R)-ketamine exerts greater potency and longer-lasting antidepressant effects than (S)-ketamine in animal models of depression and that (R)-ketamine has less detrimental side-effects than (R,S)-ketamine or (S)-ketamine. In this article, the author reviews the historical overview of the antidepressant actions of enantiomers of ketamine and its major metabolites norketamine and hydroxynorketamine. Furthermore, the author discusses the other potential rapid-acting antidepressant candidates (i.e., NMDAR antagonists and modulators, low-voltage-sensitive T-type calcium channel inhibitor, potassium channel Kir4.1 inhibitor, negative modulators of γ-aminobutyric acid, and type A [GABA_A ] receptors) to compare them with ketamine. Moreover, the molecular and cellular mechanisms of ketamine's antidepressant effects are discussed.

Pharmacological characterizations of the 'legal high' fluorolintane and isomers

1,2-Diarylethylamines represent a class of molecules that have shown potential in the treatment of pain, epilepsy, neurodegenerative disease and depression. Examples include lefetamine, remacemide, and lanicemine. Recently, several 1,2-diarylethylamines including the dissociatives diphenidine, methoxphenidine and ephenidine as well as the opioid MT-45, have appeared as 'research chemicals' or 'legal highs'. Due to their recent emergence little is known about their pharmacology. One of these, 1-[1-(2-fluorophenyl)-2-phenylethyl]pyrrolidine (fluorolintane, 2-F-DPPy), is available for purchase with purported dissociative effects intended to resemble phencyclidine (PCP) and ketamine. To better understand this emerging class, pharmacological investigations were undertaken for the first time on fluorolintane and its five aryl-fluorine-substituted isomers. In vitro binding studies revealed high affinity for N-methyl-D-aspartate (NMDA) receptors with fluorolintane (K_i = 87.92 nM) with lesser affinities for related compounds. Additional affinities were seen for all compounds at several sites including norepinephrine (NET), serotonin (SERT) and dopamine (DAT) transporters, and sigma receptors. Notably high affinities at DAT were observed, which were in most cases greater than NMDA receptor affinities. Additional functional and behavioral experiments show fluorolintane inhibited NMDA receptor-induced field excitatory postsynaptic potentials in rat hippocampal slices and inhibited long-term potentiation induced by theta-burst stimulation in rat hippocampal slices with potencies consistent with its NMDA receptor antagonism. Finally fluorolintane inhibited prepulse inhibition in rats, a measure of sensorimotor gating, with a median effective dose (ED₅₀) of 13.3 mg/kg. These findings are consistent with anecdotal reports of dissociative effects of fluorolintane in humans.

(R)-Ketamine exerts antidepressant actions partly via conversion to (2R,6R)-hydroxynorketamine, while causing adverse effects at sub-anaesthetic doses

BACKGROUND AND PURPOSE

(R)-Ketamine (arketamine) may have utility as a rapidly acting antidepressant. While (R)-ketamine has lower potency than (R,S)-ketamine to inhibit NMDA receptors in vitro, the extent to which (R)-ketamine shares the NMDA receptor-mediated adverse effects of (R,S)-ketamine in vivo has not been fully characterised. Furthermore, (R)-ketamine is metabolised to (2R,6R)-hydroxynorketamine (HNK), which may contribute to its antidepressant-relevant actions.

EXPERIMENTAL APPROACH

Using mice, we compared (R)-ketamine with a deuterated form of the drug (6,6-dideutero-(R)-ketamine, (R)-d2 -ketamine), which hinders its metabolism to (2R,6R)-HNK, in behavioural tests predicting antidepressant responses. We also examined the actions of intracerebroventricularly infused (2R,6R)-HNK. Further, we quantified putative NMDA receptor inhibition-mediated adverse effects of (R)-ketamine.

KEY RESULTS

(R)-d2 -Ketamine was identical to (R)-ketamine in binding to and functionally inhibiting NMDA receptors but hindered (R)-ketamine's metabolism to (2R,6R)-HNK. (R)-Ketamine exerted greater potency than (R)-d2 -ketamine in several antidepressant-sensitive behavioural measures, consistent with a role of (2R,6R)-HNK in the actions of (R)-ketamine. There were dose-dependent sustained antidepressant-relevant actions of (2R,6R)-HNK following intracerebroventricular administration. (R)-Ketamine exerted NMDA receptor inhibition-mediated behaviours similar to (R,S)-ketamine, including locomotor stimulation, conditioned-place preference, prepulse inhibition deficits, and motor incoordination, with approximately half the potency of the racemic drug.

CONCLUSIONS AND IMPLICATIONS

Metabolism of (R)-ketamine to (2R,6R)-HNK increases the potency of (R)-ketamine to exert antidepressant-relevant actions in mice. Adverse effects of (R)-ketamine require higher doses than those necessary for antidepressant-sensitive behavioural changes in mice. However, our data revealing that (R)-ketamine's adverse effects are elicited at sub-anaesthetic doses indicate a potential risk for sensory dissociation and abuse liability.

Novel ketamine analogues cause a false positive phencyclidine immunoassay

Background

Immunoassays are commonly used to test for drugs of abuse in patients in a variety of settings. The increasing prevalence of ‘designer’ drugs causes difficulties for the toxicology laboratory and may result in unexpected false positives and identification of unfamiliar compounds. Within the past decade, there have been a variety of ketamine and phencyclidine analogues identified, particularly as drugs of abuse.

Method

We present a case of intoxication with a novel ketamine analogue, deschloro-N-ethyl-ketamine, causing a false positive phencyclidine immunoassay. Additionally, we performed spiking studies and 2D molecular similarity calculations for deschloro-N-ethyl-ketamine, ketamine and three other analogues on the Siemens Viva-E EMIT-II phencyclidine assay to assess their cross-reactivity.

Results

Four of the tested compounds (deschloro-N-ethyl-ketamine, 3-methoxy-phencyclidine, 3-methoxy-eticyclidine and methoxetamine) cause false positive phencyclidine immunoassay results, while ketamine gives a negative result. The cross-reactivity data are in accord with the similarity calculations of these molecules, further validating the ability of 2D molecular similarity analysis to predict the molecular cross-reactivity in immunoassays.

Conclusions

The cross-reactivity data of phencyclidine and ketamine analogues presented in this study could help toxicology laboratories and clinicians in evaluating unexpected results, particularly when novel PCP and ketamine analogues are being considered.

Comparison of antidepressant and side effects in mice after intranasal administration of (R,S)-ketamine, (R)-ketamine, and (S)-ketamine

The N-methyl-d-aspartate receptor (NMDAR) antagonist (R,S)-ketamine produces rapid and sustained antidepressant effects in treatment-resistant patients with depression although intranasal use of (R,S)-ketamine in ketamine abusers is popular. In March 5, 2019, nasal spray of (S)-ketamine for treatment-resistant depression was approved as a new antidepressant by the US Food Drug Administration. Clinical study of (R)-ketamine is underway. In a chronic social defeat stress (CSDS) model, we compared the antidepressant effects of (R,S)-ketamine, (R)-ketamine, and (S)-ketamine after a single intranasal administration. Furthermore, we also compared the side effects (i.e., locomotion, prepulse inhibition (PPI), abuse liability) of these three compounds in mice. The order of potency of antidepressant effects after a single intranasal administration was (R)-ketamine > (R,S)-ketamine > (S)-ketamine. In contrast, the order of locomotor activity and prepulse inhibition (PPI) deficits after a single intranasal administration was (S)-ketamine > (R,S)-ketamine > (R)-ketamine. In the conditioned place preference (CPP) test, both (S)-ketamine and (R,S)-ketamine increased CPP scores in mice after repeated intranasal administration, in a dose dependent manner. In contrast, (R)-ketamine did not increase CPP scores in mice. These findings suggest that intranasal administration of (R)-ketamine would be a safer antidepressant than (R,S)-ketamine and (S)-ketamine.

Methoxetamine: A foe or friend?

Methoxetamine (MXE) is an N-methyl-D-aspartate (NMDA) receptor antagonist that is chemically and pharmacologically similar to other dissociative substances, such as ketamine and phencyclidine. There are reports on the misuse of MXE, which sometimes resulted in adverse consequences and death. Studies have also shown that MXE has abuse liability and stimulates dopamine neurotransmission in the mesolimbic reward pathway in the brain. These findings have contributed to the negative impression on MXE. However, recent preclinical studies have identified the antidepressant properties of MXE, which are attributed to its ability to affect the glutamatergic and serotonergic systems. MXE is also reported to have analgesic effects. These findings show some of the "redeeming qualities" of MXE and indicate its possible therapeutic uses. In this paper, we have reviewed the findings that provide insights into the adverse and potential therapeutic effects of MXE. We compiled studies on the toxicity, psychotomimetic effects, and abuse liability of MXE, as well as its promising antidepressant and analgesic properties. We also have discussed the mechanism of action that might mediate the somewhat paradoxical effects observed. Importantly, this review provides valuable information on MXE for future research and will enable a better understanding of its psychopharmacological properties and the mechanisms responsible for its unique effects.

Neurological, sensorimotor and cardiorespiratory alterations induced by methoxetamine, ketamine and phencyclidine in mice

Novel psychoactive substances are intoxicating compounds developed to mimic the effects of well-established drugs of abuse. They are not controlled by the United Nations drug convention and pose serious health concerns worldwide. Among them, the dissociative drug methoxetamine (MXE) is structurally similar to ketamine (KET) and phencyclidine (PCP) and was created to purposely mimic the psychotropic effects of its "parent" compounds. Recent animal studies show that MXE is able to stimulate the mesolimbic dopaminergic transmission and to induce KET-like discriminative and rewarding effects. In light of the renewed interest in KET and PCP analogs, we decided to deepen the investigation of MXE-induced effects by a battery of behavioral tests widely used in studies of "safety-pharmacology" for the preclinical characterization of new molecules. To this purpose, the acute effects of MXE on neurological and sensorimotor functions in mice, including visual, acoustic and tactile responses, thermal and mechanical pain, motor activity and acoustic startle reactivity were evaluated in comparisons with KET and PCP to better appreciate its specificity of action. Cardiorespiratory parameters and blood pressure were also monitored in awake and freely moving animals. Acute systemic administrations of MXE, KET and PCP (0.01-30 mg/kg i.p.) differentially alter neurological and sensorimotor functions in mice depending in a dose-dependent manner specific for each parameter examined. MXE and KET (1 and 30 mg/kg i.p.) and PCP (1 and 10 mg/kg i.p.) also affect significantly cardiorespiratory parameters, systolic and diastolic blood pressure in mice.

Effects of N-Methyl-D-aspartate (NMDA) antagonists ketamine, methoxetamine, and phencyclidine on the odor span test of working memory in rats

The glutamate hypothesis proposes that N-Methyl-D-aspartate (NMDA) receptor hypofunction underlies cognitive and perhaps other schizophrenic symptoms. The present study used the odor span task to assess the effects of NMDA antagonists on remembering multiple stimuli in rodents. This task uses an incrementing nonmatching-to-sample procedure in which responses to a new olfactory stimulus are reinforced on each trial, whereas responses to previously presented stimuli are not. NMDA antagonists have been associated with memory impairments in a variety of animal models; however, there are inconsistencies across different NMDA antagonists and tasks used. The current study compared the acute effects of phencyclidine (PCP), ketamine (KET), and the novel NMDA antagonist methoxetamine (MXE) on responding in the odor span task and a simple discrimination control task. PCP and MXE impaired odor span accuracy at doses that did not impair simple discrimination in most rats; however, the effects of KET were less selective. Within-session analyses indicated that the effects of PCP and MXE depended on the number of stimuli to remember, that is, impairment only occurred when the memory load was relatively high. These effects of PCP and MXE were consistent with the hypothesis that NMDA antagonists may interfere with working memory, but the basis for less selective results with KET are unclear. (PsycINFO Database Record

1,2-Diarylethylamine- and Ketamine-Based New Psychoactive Substances

While phencyclidine (PCP) and ketamine remain the most well-studied and widely known dissociative drugs, a number of other agents have appeared since the late 1950s and early 1960s, when the pharmacological potential of this class was first realized. For example, hundreds of compounds have been pursued as part of legitimate research efforts to explore these agents. Some of these found their way out of the research labs and onto illicit markets of the 1960s and following decades as PCP analogs. Other "illicit analogs" apparently never appeared in the scientific literature prior to their existence on clandestine markets, thus originating as novel innovations in the minds of clandestine chemists and their colleagues. Like so much else in this world, new technologies changed this dynamic. In the 1990s individuals separated by vast geographical distances could now communicate nearly instantaneously with ease through the Internet. Some individuals used this newly found opportunity to discuss the chemistry and psychoactive effects of dissociative drugs as well as to collaborate on the design and development of novel dissociative compounds. Similar to modern pharmaceutical companies and academic researchers, these seekers tinkered with the structure of their leads pursuing goals such as improved duration of action, analgesic effects, and reduced toxicity. Whether all these goals were achieved for any individual compound remains to be seen, but their creations have been let out of the bag and are now materialized as defined compositions of matter. Moreover, these creations now exist not only in and of themselves but live on further as permutations into various novel analogs and derivatives. In some cases these compounds have made their way to academic labs where potential clinical applications have been identified. These compounds reached wider distribution when other individuals picked up on these discussions and began to market them as "research chemicals" or "legal highs". The result is a continuously evolving game that is being played between legislatures, law enforcement, and research chemical market players. Two structurally distinct classes that have appeared as dissociative-based new psychoactive substances (NPS) are the 1,2-diarylethylamines and β-keto-arylcyclohexylamines. Examples of the former include diphenidine and various analogs such as fluorolintane and N-ethyl-lanicemine, and examples of the latter are analogs of ketamine such as methoxetamine, deschloroketamine, and 2-fluoro-2-deschloroketamine. The subject of this chapter is the introduction to some of the dissociative NPS from these classes and their known pharmacology that have emerged on the market in recent years.

Effect of Hallucinogens on Unconditioned Behavior

Because of the ethical and regulatory hurdles associated with human studies, much of what is known about the psychopharmacology of hallucinogens has been derived from animal models. However, developing reliable animal models has proven to be a challenging task due to the complexity and variability of hallucinogen effects in humans. This chapter focuses on three animal models that are frequently used to test the effects of hallucinogens on unconditioned behavior: head twitch response (HTR), prepulse inhibition of startle (PPI), and exploratory behavior. The HTR has demonstrated considerable utility in the neurochemical actions of hallucinogens. However, the latter two models have clearer conceptual bridges to human phenomenology. Consistent with the known mechanism of action of hallucinogens in humans, the behavioral effects of hallucinogens in rodents are mediated primarily by activation of 5-HT2A receptors. There is evidence, however, that other receptors may play secondary roles. The structure-activity relationships (SAR) of hallucinogens are reviewed in relation to each model, with a focus on the HTR in rats and mice.

Methoxetamine affects brain processing involved in emotional response in rats

BACKGROUND AND PURPOSE

Methoxetamine (MXE) is a novel psychoactive substance that is emerging on the Internet and induces dissociative effects and acute toxicity. Its pharmacological effects have not yet been adequately investigated.

EXPERIMENTAL APPROACH

We examined a range of behavioural effects induced by acute administration of MXE (0.5-5 mg·kg^-1 ; i.p.) in rats and whether it causes rapid neuroadaptive molecular changes.

KEY RESULTS

MXE (0.5-5 mg·kg^-1 ) affected motor activity in a dose- and time-dependent manner, inducing hypermotility and hypomotility at low and high doses respectively. At low and intermediate doses (0.5 and 1 mg·kg^-1 ), MXE induced anxious and/or obsessive-compulsive traits (marble burying test), did not significantly increase sociability (social interaction test) or induce spatial anxiety (elevated plus maze test). At a high dose (5 mg·kg^-1 ), MXE induced transient analgesia (tail-flick and hot-plate test), decreased social interaction time (social interaction test) and reduced immobility time while increasing swimming activity (forced swim test), suggesting an antidepressant effect. Acute MXE administration did not affect self-grooming behaviour at any dose tested. Immunohistochemical analysis showed that behaviourally active doses of MXE (1 and 5 mg·kg^-1 ) increased phosphorylation of ribosomal protein S6 in the medial prefrontal cortex and hippocampus.

CONCLUSIONS AND IMPLICATIONS

MXE differentially affected motor activity, behaviour and emotional states in rats, depending on the dose tested. As reported for ketamine, phosphorylation of the ribosomal protein S6 was increased in MXE-treated animals, thus providing a 'molecular snapshot' of rapid neuroadaptive molecular changes induced by behaviourally active doses of MXE.

Phencyclidine-like in vivo effects of methoxetamine in mice and rats

Methoxetamine (MXE) is a novel drug of abuse that is structurally similar to phencyclidine (PCP). In the present study, rats were trained to discriminate PCP from saline and substitution tests were performed with arylcyclohexylamines PCP, eticyclidine (PCE), tenocyclidine (TCP), and MXE. PCP and PCE engendered PCP-lever selection in all subjects, whereas MXE and TCP produced PCP-lever selection in animals that did not display behavioral disruption. Last, the substituted tryptamine dipropyltryptamine (DPT) produced moderate PCP-lever selection and elicited behavioral disruption in all subjects at the highest dose tested. Immediately following the final substitution test in the drug discrimination experiment, the same rats and a separate group of experimentally-naïve rats were implanted with osmotic mini-pumps delivering continuous PCP infusions for 11 days. Consistent with PCP withdrawal, disruption of food-maintained operant responding was observed when the pumps were removed, but cumulative MXE administration dose-dependently reversed this effect. A third group of rats self-administered several unit doses of PCP and MXE. Results of the self-administration tests revealed that MXE was a less effective reinforcer than PCP. Lastly, mice were implanted with radiotelemetry probes to simultaneously monitor thermoregulatory and locomotor responses following injections of PCP, PCE, or MXE. All three arylcyclohexylamines elicited dose-dependent hypothermic effects, but only PCP produced increases in locomotor activity. Together, these findings indicate that MXE elicits PCP-like interoceptive effects, but reduced reinforcing and locomotor stimulant effects in vivo. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Effects of the psychotomimetic benzomorphan N-allylnormetazocine (SKF 10,047) on prepulse inhibition of startle in mice

N-allylnormetazocine (NANM; SKF 10,047) is a benzomorphan opioid that produces psychotomimetic effects. (+)-NANM is the prototypical agonist for the sigma-1 (σ1) receptor, and there is a widespread belief that the hallucinogenic effects of NANM and other benzomorphan derivatives are mediated by interactions with σ1 sites. However, NANM is also an agonist at the κ opioid receptor (KOR) and binds to the PCP site located within the channel pore of the NMDA receptor, interactions that could potentially contribute to the effects of NANM. NMDA receptor antagonists such as phencyclidine (PCP) and ketamine are known to disrupt prepulse inhibition (PPI) of acoustic startle, a measure of sensorimotor gating, in rodents. We recently found that racemic NANM disrupts PPI in rats, but it is not clear whether the effect is mediated by blockade of the NMDA receptor, or alternatively whether interactions with KOR and σ1 receptors are involved. The present studies examined whether NANM and its stereoisomers alter PPI in C57BL/6J mice, and tested whether the effects on PPI are mediated by KOR or σ1 receptors. Racemic NANM produced a dose-dependent disruption of PPI (3-30mg/kg SC). (+)-NANM also disrupted PPI, whereas (-)-NANM was ineffective. Pretreatment with the selective KOR antagonist nor-binaltorphimine (10mg/kg SC) or the selective σ1 antagonist NE-100 (1mg/kg IP) failed to attenuate the reduction in PPI produced by racemic NANM. We also found that the selective KOR agonist (-)-U-50,488H (10-40mg/kg SC) had no effect on PPI. These findings confirm that NANM reduces sensorimotor gating in rodents, and indicate that the effect is mediated by interactions with the PCP receptor and not by activation of KOR or σ1 receptors. This observation is consistent with evidence indicating that the σ1 receptor is not linked to hallucinogenic or psychotomimetic effects.

Pharmacological Investigations of the Dissociative 'Legal Highs' Diphenidine, Methoxphenidine and Analogues

1,2-Diarylethylamines including lanicemine, lefetamine, and remacemide have clinical relevance in a range of therapeutic areas including pain management, epilepsy, neurodegenerative disease and depression. More recently 1,2-diarylethylamines have been sold as ‘legal highs’ in a number of different forms including powders and tablets. These compounds are sold to circumvent governmental legislation regulating psychoactive drugs. Examples include the opioid MT-45 and the dissociative agents diphenidine (DPH) and 2-methoxy-diphenidine (2-MXP). A number of fatal and non-fatal overdoses have been linked to abuse of these compounds. As with many ‘legal highs’, little is known about their pharmacology. To obtain a better understanding, the effects of DPH, 2-MXP and its 3- and 4-MeO- isomers, and 2-Cl-diphenidine (2-Cl-DPH) were investigated using binding studies at 46 central nervous system receptors including the N-methyl-D-aspartate receptor (NMDAR), serotonin, dopamine, norepinephrine, histamine, and sigma receptors as well as the reuptake transporters for serotonin, dopamine and norepinephrine. Reuptake inhibition potencies were measured at serotonin, norepinephrine and dopamine transporters. NMDAR antagonism was established in vitro using NMDAR-induced field excitatory postsynaptic potential (fEPSP) experiments. Finally, DPH and 2-MXP were investigated using tests of pre-pulse inhibition of startle (PPI) in rats to determine whether they reduce sensorimotor gating, an effect observed with known dissociative drugs such as phencyclidine (PCP) and ketamine. The results suggest that these 1,2-diarylethylamines are relatively selective NMDAR antagonists with weak off-target inhibitory effects on dopamine and norepinephrine reuptake. DPH and 2-MXP significantly inhibited PPI. DPH showed greater potency than 2-MXP, acting with a median effective dose (ED₅₀) of 9.5 mg/kg, which is less potent than values reported for other commonly abused dissociative drugs such as PCP and ketamine.