Madeleine V. King

5-HT6 receptor antagonists reverse delay-dependent deficits in novel object discrimination by enhancing consolidation—an effect sensitive to NMDA receptor antagonism

5-HT(6) receptors are expressed in brain regions associated with learning and memory, and blockade of their function increases central cholinergic and glutamatergic neurotransmission and enhances cognitive processes. This study examined the effects of acute systemic administration of two selective 5-HT(6) receptor antagonists Ro 04-6790 and SB-271046 (10 mg kg(-1) i.p.) on acquisition, consolidation, and retrieval in the novel object discrimination (NOD) task, a two-trial test of recognition memory in which rats exposed to two identical objects during a familiarisation trial can discriminate a novel from a familiar object during the subsequent choice trial, following inter-trial delays of up to 3 h. 5-HT(6) receptor antagonist administration 20 min prior to or immediately after the familiarisation trial, but not 20 min prior to the choice trial reversed the deficit in object discrimination produced by a 4 h inter-trial interval. The nootropic effects of the 5-HT(6) receptor antagonists in this task thus appear to involve enhanced consolidation. Pre-treatment with the non-competitive NMDA receptor antagonist MK-801 (0.05 mg kg(-1) i.p.) prevented the effect of Ro 04-6790 on delay-induced deficits in object discrimination. This suggests that the 5-HT(6) receptor antagonist-induced enhancement of consolidation involves increased central glutamatergic neurotransmission.

Increased dopamine D 2High receptors in rats reared in social isolation

Postweaning social isolation in the rat induces lasting alterations that parallel several of the core symptoms seen in human schizophrenics, including hyperreactivity to novel environments, cognitive impairment, and deficits in sensorimotor gating. The current study determined whether these changes are accompanied by any elevation in the proportion of striatal dopamine receptors in the functional high affinity state (D  2High ), as observed in other preclinical models of psychosis. Male Lister hooded rats (20–24 days) were housed in groups of three or alone. On Day 36 postweaning locomotor activity was monitored for 60 min in a novel arena, and on Day 37 novel object discrimination was assessed using a 2 h intertrial interval. Three days later striata were collected, homogenized, washed three times to remove endogenous dopamine, and the proportion of D  2High determined by competition between dopamine and 2.27 nM [3H]domperidone. Isolates were significantly more active than group housed controls for both ambulation and rears. Although both groups exhibited comparable levels of familiarization trial object exploration, group housed animals were able to discriminate between novel and familiar objects during the choice trial while isolates were not. Social isolation was associated with a highly significant elevation in the proportion of striatal D  2High , equivalent to a 3.3‐fold increase (group 15.2% ± 1.4%, isolate 49.8% ± 4.8%; P < 0.0001, Students unpaired t‐test). These findings support both the hypothesis that elevated D  2High is a common feature of multiple animal models of psychosis, and the validity of isolation rearing as a neurodevelopmental model of a “schizophrenia‐like” state. Synapse 63:476–483, 2009.

Influence of social isolation in the rat on serotonergic function and memory – Relevance to models of schizophrenia and the role of 5-HT6 receptors

There is increasing awareness of the importance that early environmental factors have on brain development and their role in the neurobiology of neurodevelopmental disorders including schizophrenia. The isolation reared rat attempts to model adverse effects that human social isolation (absence of social contact) can have on normal brain development. The isolation reared rat also models aspects of schizophrenia including the development of persistent learning and memory deficits. This short review concentrates on the effects of isolation rearing on cognition, including deficits in novel object discrimination, and the neural mechanisms that may underlie this impairment. There is evidence that a key effect of social isolation may be loss of neuronal plasticity combined with change in the functional state of various cortical and hippocampal neurotransmitters, including glutamate and serotonin. Reduced glutamate function may underlie the deficits in novel object discrimination, which can be reversed by administration of a 5-HT(6) receptor antagonist. This suggests that the 5-HT(6) antagonists may act by reducing 5-HT(6) receptor mediated activation of GABA, resulting in glutamate disinhibition. Thus drugs acting at 5-HT(6) receptors may offer a novel approach to treat neurodevelopmental cognitive symptoms, including those seen in schizophrenia.

Behavioural and neurochemical comparison of chronic intermittent cathinone, mephedrone and MDMA administration to the rat

The synthetic cathinone derivative, mephedrone, is a controlled substance across Europe. Its effects have been compared by users to 3,4-methylenedioxymethamphetamine (MDMA), but little data exist on its pharmacological properties. This study compared the behavioural and neurochemical effects of mephedrone with cathinone and MDMA in rats. Young-adult male Lister hooded rats received i.p. cathinone (1 or 4 mg/kg), mephedrone (1, 4 or 10mg/kg) or MDMA (10mg/kg) on two consecutive days weekly for 3 weeks or as a single acute injection (for neurochemical analysis). Locomotor activity (LMA), novel object discrimination (NOD), conditioned emotional response (CER) and prepulse inhibition of the acoustic startle response (PPI) were measured following intermittent drug administration. Dopamine, 5-hydroxytryptamine (5-HT) and their major metabolites were measured in striatum, frontal cortex and hippocampus by high performance liquid chromatography 7 days after intermittent dosing and 2h after acute injection. Cathinone (1, 4 mg/kg), mephedrone (10mg/kg) and MDMA (10mg/kg) induced hyperactivity following the first and sixth injections and sensitization to cathinone and mephedrone occurred with chronic dosing. All drugs impaired NOD and mephedrone (10mg/kg) reduced freezing in response to contextual re-exposure during the CER retention trial. Acute MDMA reduced hippocampal 5-HT and 5-HIAA but the only significant effect on dopamine, 5-HT and their metabolites following chronic dosing was altered hippocampal 3,4-dihydroxyphenylacetic acid (DOPAC), following mephedrone (4, 10mg/kg) and MDMA. At the doses examined, mephedrone, cathinone, and MDMA induced similar effects on behaviour and failed to induce neurotoxic damage when administered intermittently over 3 weeks.

Lost in translation: preclinical studies on 3,4-methylenedioxymethamphetamine provide information on mechanisms of action, but do not allow accurate prediction of adverse events in humans.

3,4‐Methylenedioxymethamphetamine (MDMA) induces both acute adverse effects and long‐term neurotoxic loss of brain 5‐HT neurones in laboratory animals. However, when choosing doses, most preclinical studies have paid little attention to the pharmacokinetics of the drug in humans or animals. The recreational use of MDMA and current clinical investigations of the drug for therapeutic purposes demand better translational pharmacology to allow accurate risk assessment of its ability to induce adverse events. Recent pharmacokinetic studies on MDMA in animals and humans are reviewed and indicate that the risks following MDMA ingestion should be re‐evaluated. Acute behavioural and body temperature changes result from rapid MDMA‐induced monoamine release, whereas long‐term neurotoxicity is primarily caused by metabolites of the drug. Therefore acute physiological changes in humans are fairly accurately mimicked in animals by appropriate dosing, although allometric dosing calculations have little value. Long‐term changes require MDMA to be metabolized in a similar manner in experimental animals and humans. However, the rate of metabolism of MDMA and its major metabolites is slower in humans than rats or monkeys, potentially allowing endogenous neuroprotective mechanisms to function in a species specific manner. Furthermore acute hyperthermia in humans probably limits the chance of recreational users ingesting sufficient MDMA to produce neurotoxicity, unlike in the rat. MDMA also inhibits the major enzyme responsible for its metabolism in humans thereby also assisting in preventing neurotoxicity. These observations question whether MDMA alone produces long‐term 5‐HT neurotoxicity in human brain, although when taken in combination with other recreational drugs it may induce neurotoxicity.

The preclinical pharmacology of mephedrone; not just MDMA by another name

The substituted β‐keto amphetamine mephedrone (4‐methylmethcathinone) was banned in the UK in April 2010 but continues to be used recreationally in the UK and elsewhere. Users have compared its psychoactive effects to those of 3,4‐methylenedioxymethamphetamine (MDMA, ‘ecstasy’). This review critically examines the preclinical data on mephedrone that have appeared over the last 2–3 years and, where relevant, compares the pharmacological effects of mephedrone in experimental animals with those obtained following MDMA administration. Both mephedrone and MDMA enhance locomotor activity and change rectal temperature in rodents. However, both of these responses are of short duration following mephedrone compared with MDMA probably because mephedrone has a short plasma half‐life and rapid metabolism. Mephedrone appears to have no pharmacologically active metabolites, unlike MDMA. There is also little evidence that mephedrone induces a neurotoxic decrease in monoamine concentration in rat or mouse brain, again in contrast to MDMA. Mephedrone and MDMA both induce release of dopamine and 5‐HT in the brain as shown by in vivo and in vitro studies. The effect on 5‐HT release in vivo is more marked with mephedrone even though both drugs have similar affinity for the dopamine and 5‐HT transporters in vitro. The profile of action of mephedrone on monoamine receptors and transporters suggests it could have a high abuse liability and several studies have found that mephedrone supports self‐administration at a higher rate than MDMA. Overall, current data suggest that mephedrone not only differs from MDMA in its pharmacological profile, behavioural and neurotoxic effects, but also differs from other cathinones.

Differential effects of cathinone compounds and MDMA on body temperature in the rat, and pharmacological characterization of mephedrone-induced hypothermia.

Recreational users report that mephedrone has similar psychoactive effects to 3,4‐methylenedioxymethamphetamine (MDMA). MDMA induces well‐characterized changes in body temperature due to complex monoaminergic effects on central thermoregulation, peripheral blood flow and thermogenesis, but there are little preclinical data on the acute effects of mephedrone or other synthetic cathinones.

The dopamine D3-preferring D2/D3 dopamine receptor partial agonist, cariprazine, reverses behavioural changes in a rat neurodevelopmental model for schizophrenia

Current antipsychotic medication is largely ineffective against the negative and cognitive symptoms of schizophrenia. One promising therapeutic development is to design new molecules that balance actions on dopamine D2 and D3 receptors to maximise benefits and limit adverse effects. This study used two rodent paradigms to investigate the action of the dopamine D3-preferring D3/D2 receptor partial agonist cariprazine. In adult male rats, cariprazine (0.03-0.3 mg/kg i.p.), and the atypical antipsychotic aripiprazole (1-3 mg/kg i.p.) caused dose-dependent reversal of a delay-induced impairment in novel object recognition (NOR). Treating neonatal rat pups with phencyclidine (PCP) and subsequent social isolation produced a syndrome of behavioural alterations in adulthood including hyperactivity in a novel arena, deficits in NOR and fear motivated learning and memory, and a reduction and change in pattern of social interaction accompanied by increased ultrasonic vocalisations (USVs). Acute administration of cariprazine (0.1 and 0.3 mg/kg) and aripiprazole (3 mg/kg) to resultant adult rats reduced neonatal PCP-social isolation induced locomotor hyperactivity and reversed NOR deficits. Cariprazine (0.3 mg/kg) caused a limited reversal of the social interaction deficit but neither drug affected the change in USVs or the deficit in fear motivated learning and memory. Results suggest that in the behavioural tests investigated cariprazine is at least as effective as aripiprazole and in some paradigms it showed additional beneficial features further supporting the advantage of combined dopamine D3/D2 receptor targeting. These findings support recent clinical studies demonstrating the efficacy of cariprazine in treatment of negative symptoms and functional impairment in schizophrenia patients.

Contribution of serotonin and dopamine to changes in core body temperature and locomotor activity in rats following repeated administration of mephedrone.

The psychoactive effects of mephedrone are commonly compared with those of 3,4‐methylenedioxymethamphetamine, but because of a shorter duration of action, users often employ repeated administration to maintain its psychoactive effects. This study examined the effects of repeated mephedrone administration on locomotor activity, body temperature and striatal dopamine and 5‐hydroxytryptamine (5‐HT) levels and the role of dopaminergic and serotonergic neurons in these responses. Adult male Lister hooded rats received three injections of vehicle (1 ml/kg, i.p.) or mephedrone HCl (10 mg/kg) at 2 h intervals for radiotelemetry (temperature and activity) or microdialysis (dopamine and 5‐HT) measurements. Intracerebroventricular pre‐treatment (21 to 28 days earlier) with 5,7‐dihydroxytryptamine (150 µg) or 6‐hydroxydopamine (300 µg) was used to examine the impact of 5‐HT or dopamine depletion on mephedrone‐induced changes in temperature and activity. A final study examined the influence of i.p. pre‐treatment (−30 min) with the 5‐HT1A receptor antagonist WAY‐100635 (0.5 mg/kg), 5‐HT1B receptor antagonist GR 127935 (3 mg/kg) or the 5‐HT7 receptor antagonist SB‐258719 (10 mg/kg) on mephedrone‐induced changes in locomotor activity and rectal temperature. Mephedrone caused rapid‐onset hyperactivity, hypothermia (attenuated on repeat dosing) and increased striatal dopamine and 5‐HT release following each injection. Mephedrone‐induced hyperactivity was attenuated by 5‐HT depletion and 5‐HT1B receptor antagonism, whereas the hypothermia was completely abolished by 5‐HT depletion and lessened by 5‐HT1A receptor antagonism. These findings suggest that stimulation of central 5‐HT release and/or inhibition of 5‐HT reuptake play a pivotal role in both the hyperlocomotor and hypothermic effects of mephedrone, which are mediated in part via 5‐HT1B and 5‐HT1A receptors.

Cathinone increases body temperature, enhances locomotor activity, and induces striatal c-fos expression in the Siberian hamster.

Cathinone is a β-keto alkaloid that is the major active constituent of khat, the leaf of the Catha edulis plant that is chewed recreationally in East Africa and the Middle East. Related compounds, such as methcathinone and mephedrone have been increasing in popularity as recreational drugs, resulting in the recent proposal to classify khat as a Class C drug in the UK. There is still limited knowledge of the pharmacological effects of cathinone. This study examined the acute effects of cathinone on core body temperature, locomotor and other behaviors, and neuronal activity in Siberian hamsters. Adult male hamsters, previously implanted with radio telemetry devices, were treated with cathinone (2 or 5mg/kg i.p.), the behavioral profile scored and core body temperature and locomotor activity recorded by radio telemetry. At the end of the study, hamsters received vehicle or cathinone (5mg/kg) and neuronal activation in the brain was determined using immunohistochemical evaluation of c-fos expression. Cathinone dose-dependently induced significant (p<0.0001) increases in both temperature and locomotor activity lasting 60-90min. Cathinone (2mg/kg) increased rearing (p<0.02), and 5mg/kg increased both rearing (p<0.001) and lateral head twitches (p<0.02). Both cathinone doses decreased the time spent at rest (p<0.001). The number of c-fos immunopositive cells were significantly increased in the striatum (p<0.0001) and suprachiasmatic nucleus (p<0.05) following cathinone, indicating increased neuronal activity. There was no effect of cathinone on food intake or body weight. It is concluded that systemic administration of cathinone induces significant behavioral changes and CNS activation in the hamster.