Gestational poly(I:C) attenuates, not exacerbates, the behavioral, cytokine and mTOR changes caused by isolation rearing in a rat 'dual-hit' model for neurodevelopmental disorders

Many psychiatric illnesses have a multifactorial etiology involving genetic and environmental risk factors that trigger persistent neurodevelopmental impairments. Several risk factors have been individually replicated in rodents, to understand disease mechanisms and evaluate novel treatments, particularly for poorly-managed negative and cognitive symptoms. However, the complex interplay between various factors remains unclear. Rodent dual-hit neurodevelopmental models offer vital opportunities to examine this and explore new strategies for early therapeutic intervention. This study combined gestational administration of polyinosinic:polycytidylic acid (poly(I:C); PIC, to mimic viral infection during pregnancy) with post-weaning isolation of resulting offspring (to mirror adolescent social adversity). After in vitro and in vivo studies required for laboratory-specific PIC characterization and optimization, we administered 10 mg/kg i.p. PIC potassium salt to time-mated Lister hooded dams on gestational day 15. This induced transient hypothermia, sickness behavior and weight loss in the dams, and led to locomotor hyperactivity, elevated striatal cytokine levels, and increased frontal cortical JNK phosphorylation in the offspring at adulthood. Remarkably, instead of exacerbating the well-characterized isolation syndrome, gestational PIC exposure actually protected against a spectrum of isolation-induced behavioral and brain regional changes. Thus isolation reared rats exhibited locomotor hyperactivity, impaired associative memory and reversal learning, elevated hippocampal and frontal cortical cytokine levels, and increased mammalian target of rapamycin (mTOR) activation in the frontal cortex - which were not evident in isolates previously exposed to gestational PIC. Brains from adolescent littermates suggest little contribution of cytokines, mTOR or JNK to early development of the isolation syndrome, or resilience conferred by PIC. But notably hippocampal oxytocin, which can protect against stress, was higher in adolescent PIC-exposed isolates so might contribute to a more favorable outcome. These findings have implications for identifying individuals at risk for disorders like schizophrenia who may benefit from early therapeutic intervention, and justify preclinical assessment of whether adolescent oxytocin manipulations can modulate disease onset or progression.

Dual-acting agents for improving cognition and real-world function in Alzheimer's disease: Focus on 5-HT6 and D3 receptors as hubs

To date, there are no interventions that impede the inexorable progression of Alzheimer's disease (AD), and currently-available drugs cholinesterase (AChE) inhibitors and the N-Methyl-d-Aspartate receptor antagonist, memantine, offer only modest symptomatic benefit. Moreover, a range of mechanistically-diverse agents (glutamatergic, histaminergic, monoaminergic, cholinergic) have disappointed in clinical trials, alone and/or in association with AChE inhibitors. This includes serotonin (5-HT) receptor-6 antagonists, despite compelling preclinical observations in rodents and primates suggesting a positive influence on cognition. The emphasis has so far been on high selectivity. However, for a multi-factorial disorder like idiopathic AD, 5-HT6 antagonists possessing additional pharmacological actions might be more effective, by analogy to "multi-target" antipsychotics. Based on this notion, drug discovery programmes have coupled 5-HT6 blockade to 5-HT4 agonism and inhibition of AchE. Further, combined 5-HT6/dopamine D3 receptor (D3) antagonists are of especial interest since D3 blockade mirrors 5-HT6 antagonism in exerting broad-based pro-cognitive properties in animals. Moreover, 5-HT6 and dopamine D3 antagonists promote neurocognition and social cognition via both distinctive and convergent actions expressed mainly in frontal cortex, including suppression of mTOR over-activation and reinforcement of cholinergic and glutamatergic transmission. In addition, 5-HT6 blockade affords potential anti-anxiety, anti-depressive and anti-epileptic properties, and antagonising 5-HT6 receptors may be associated with neuroprotective ("disease-modifying") properties. Finally D3 antagonism may counter psychotic episodes and D3 receptors themselves offer a promising hub for multi-target agents. The present article reviews the status of "R and D" into multi-target 5-HT6 and D3 ligands for improved treatment of AD and other neurodegenerative disorders of aging. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Age-related differences in appetitive trace conditioning and novel object recognition procedures

•

Longitudinal study of middle age in the rat with matched younger control cohort.

•

Appetitive trace conditioning and novel object recognition tests of working memory.

•

Transient between-groups working memory impairments aged 12 compared with 2 months.

•

Object exploration reduced with age but working memory recovered.

•

Object exploration and ITI nosepoking showed some correlation with 5-HIAA/5-HT.

Appetitive trace conditioning (TC) was examined over 6 months in younger-adult (2–8 months) and middle-aged (12–18 months) male Wistar RccHan rats, to test for early age-related impairment in working memory. Novel object recognition (NOR) was included as a comparison task, to provide a positive control in the event that the expected impairment in TC was not demonstrated. The results showed that TC improved at both ages at the 2 s but not at the 10 s trace interval. There was, however, evidence for reduced improvement from one day to the next in the middle-aged cohort tested with the 2 s trace conditioned stimulus. Moreover, within the 10 s trace, responding progressively distributed later in the trace interval, in the younger-adult but not the middle-aged cohort. Middle-aged rats showed NOR discriminative impairment at a 24 h but not at a 10 min retention interval. Object exploration was overall reduced in middle-aged rats and further reduced longitudinally. At the end of the study, assessing neurochemistry by HPLC-ED showed reduced 5-HIAA/5-HT in the dorsal striatum of the middle-aged rats and some correlations between striatal 5-HIAA/5-HT and activity parameters. Overall the results suggest that, taken in isolation, age-related impairments may be overcome by experience. This recovery in performance was seen despite the drop in activity levels in older animals, which might be expected to contribute to cognitive decline.

Post-weaning social isolation of rats leads to long-term disruption of the gut microbiota-immune-brain axis

Early-life stress is an established risk for the development of psychiatric disorders. Post-weaning isolation rearing of rats produces lasting developmental changes in behavior and brain function that may have translational pathophysiological relevance to alterations seen in schizophrenia, but the underlying mechanisms are unclear. Accumulating evidence supports the premise that gut microbiota influence brain development and function by affecting inflammatory mediators, the hypothalamic-pituitaryadrenal axis and neurotransmission, but there is little knowledge of whether the microbiota-gut-brain axis might contribute to the development of schizophrenia-related behaviors. To this end the effects of social isolation (SI; a well-validated animal model for schizophrenia)-induced changes in rat behavior were correlated with alterations in gut microbiota, hippocampal neurogenesis and brain cytokine levels. Twenty-four male Lister hooded rats were housed in social groups (group-housed, GH, 3 littermates per cage) or alone (SI) from weaning (post-natal day 24) for four weeks before recording open field exploration, locomotor activity/novel object discrimination (NOD), elevated plus maze, conditioned freezing response (CFR) and restraint stress at one week intervals. Post-mortem caecal microbiota composition, cortical and hippocampal cytokines and neurogenesis were correlated to indices of behavioral changes. SI rats were hyperactive in the open field and locomotor activity chambers traveling further than GH controls in the less aversive peripheral zone. While SI rats showed few alterations in plus maze or NOD they froze for significantly less time than GH following conditioning in the CFR paradigm, consistent with impaired associative learning and memory. SI rats had significantly fewer BrdU/NeuN positive cells in the dentate gyrus than GH controls. SI rats had altered microbiota composition with increases in Actinobacteria and decreases in the class Clostridia compared to GH controls. Differences were also noted at genus level. Positive correlations were seen between microbiota, hippocampal IL-6 and IL-10, conditioned freezing and open field exploration. Adverse early-life stress resulting from continuous SI increased several indices of 'anxiety-like' behavior and impaired associative learning and memory accompanied by changes to gut microbiota, reduced hippocampal IL-6, IL-10 and neurogenesis. This study suggests that early-life stress may produce long-lasting changes in gut microbiota contributing to development of abnormal neuronal and endocrine function and behavior which could play a pivotal role in the aetiology of psychiatric illness.

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-HT_1A receptor antagonist WAY-100635 (0.5 mg/kg), 5-HT_1B 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-HT_1B receptor antagonism, whereas the hypothermia was completely abolished by 5-HT depletion and lessened by 5-HT_1A 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-HT_1B and 5-HT_1A receptors.

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.

Dopamine D1 receptor stimulation modulates the formation and retrieval of novel object recognition memory: Role of the prelimbic cortex

Previous studies have shown that dopamine D1 receptor antagonists impair novel object recognition memory but the effects of dopamine D1 receptor stimulation remain to be determined. This study investigated the effects of the selective dopamine D1 receptor agonist SKF81297 on acquisition and retrieval in the novel object recognition task in male Wistar rats. SKF81297 (0.4 and 0.8 mg/kg s.c.) given 15 min before the sampling phase impaired novel object recognition evaluated 10 min or 24 h later. The same treatments also reduced novel object recognition memory tested 24 h after the sampling phase and when given 15 min before the choice session. These data indicate that D1 receptor stimulation modulates both the encoding and retrieval of object recognition memory. Microinfusion of SKF81297 (0.025 or 0.05 μg/side) into the prelimbic sub-region of the medial prefrontal cortex (mPFC) in this case 10 min before the sampling phase also impaired novel object recognition memory, suggesting that the mPFC is one important site mediating the effects of D1 receptor stimulation on visual recognition memory.

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.

Neonatal phencyclidine administration and post-weaning social isolation as a dual-hit model of 'schizophrenia-like' behaviour in the rat

RATIONALE

Schizophrenia is a debilitating disorder comprising positive, negative and cognitive deficits with a poorly defined neurobiological aetiology; therefore, animal models with greater translational reliability are essential to develop improved therapies.

OBJECTIVES

This study combines two developmental challenges in rats, neonatal phencyclidine (PCP) injection and subsequent rearing in social isolation from weaning, to attempt to produce more robust behavioural deficits with greater translational relevance to schizophrenia than either challenge alone.

METHODS

Forty-two male Lister-hooded rat pups received the N-methyl-D-aspartate (NMDA) receptor antagonist, phencyclidine (PCP, 10 mg/kg, s.c.), or vehicle on post-natal day (PND) 7, 9 and 11 and were weaned on PND 23 into group housing (saline-treated n = 11 or PCP-treated n = 10) or isolation (saline n = 10 or PCP n = 11). Six weeks post-weaning, novelty- and PCP-induced (3.2 mg/kg) locomotor activity, novel object discrimination, prepulse inhibition of acoustic startle and contextual memory in a conditioned emotion response (CER) were recorded.

RESULTS

Isolation rearing alone significantly elevated baseline locomotor activity and induced visual recognition memory impairment in novel object discrimination. Neonatal PCP treatment did not induce locomotor sensitisation to a subsequent acute PCP injection, but it impaired prepulse inhibition when combined with isolation rearing. CER freezing behaviour was significantly reduced by isolation rearing but an even greater effect occurred when combined with neonatal PCP treatment.

CONCLUSIONS

Neonatal PCP and isolation rearing both produce behavioural deficits in adult rats, but combined treatment caused a wider range of more severe cognitive impairments, providing a more comprehensive preclinical model to determine the neurobiological aetiology of schizophrenia than either treatment alone.

In vivo neurometabolic profiling to characterize the effects of social isolation and ketamine-induced NMDA antagonism: a rodent study at 7.0 T

Continued efforts are undertaken to develop animal models of schizophrenia with translational value in the quest for much needed novel drugs. Existing models mimic specific neurobiological aspects of schizophrenia, but not its full complexity. Here, we used proton magnetic resonance spectroscopy ((1)H-MRS) to assess the metabolic profile in the prefrontal cortex (PFC) of two established models, rearing in social isolation and acute N-methyl-D-aspartate receptor (NMDA-R) antagonism and their combination. Rats reared in social isolation or group housed underwent (1)H-MRS at baseline and dynamically after ketamine challenge (25mg/kg, intraperitoneal) under isoflurane anesthesia. A 7 T animal scanner was used to perform spectra acquisition from the anterior cingulate/medial PFC. LCModel was used for metabolite quantification and effects of rearing and ketamine injection were analyzed. Social isolation did not lead to significant differences in the metabolic profile of the PFC at baseline. Ketamine induced a significant increase in glutamine in both groups with significance specifically reached by the group-housed animals alone. Only rats reared in social isolation showed a significant 11% γ-aminobutyric acid (GABA) decrease. This study provides preliminary evidence that social interactions in early life predict the glutamatergic and GABAergic response to acute NMDA-R blockade. The similarity between the prefrontal GABA reduction in patients with schizophrenia and in rats reared as social isolates after challenge with ketamine suggests good potential translational value of this combined animal model for drug development.

The antimalarial drug quinine interferes with serotonin biosynthesis and action

The major antimalarial drug quinine perturbs uptake of the essential amino acid tryptophan, and patients with low plasma tryptophan are predisposed to adverse quinine reactions; symptoms of which are similar to indications of tryptophan depletion. As tryptophan is a precursor of the neurotransmitter serotonin (5-HT), here we test the hypothesis that quinine disrupts serotonin function. Quinine inhibited serotonin-induced proliferation of yeast as well as human (SHSY5Y) cells. One possible cause of this effect is through inhibition of 5-HT receptor activation by quinine, as we observed here. Furthermore, cells exhibited marked decreases in serotonin production during incubation with quinine. By assaying activity and kinetics of the rate-limiting enzyme for serotonin biosynthesis, tryptophan hydroxylase (TPH2), we showed that quinine competitively inhibits TPH2 in the presence of the substrate tryptophan. The study shows that quinine disrupts both serotonin biosynthesis and function, giving important new insight to the action of quinine on mammalian cells.

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.

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

BACKGROUND AND PURPOSE

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.

EXPERIMENTAL APPROACH

The acute effects of cathinone, methcathinone and mephedrone on rectal and tail temperature were examined in individually housed rats, with MDMA included for comparison. Rats were killed 2 h post-injection and brain regions were collected for quantification of 5-HT, dopamine and major metabolites. Further studies examined the impact of selected α-adrenoceptor and dopamine receptor antagonists on mephedrone-induced changes in rectal temperature and plasma catecholamines.

KEY RESULTS

At normal room temperature, MDMA caused sustained decreases in rectal and tail temperature. Mephedrone caused a transient decrease in rectal temperature, which was enhanced by α(1) -adrenoceptor and dopamine D(1) receptor blockade, and a prolonged decrease in tail temperature. Cathinone and methcathinone caused sustained increases in rectal temperature. MDMA decreased 5-HT and/or 5-hydroxyindoleacetic acid (5-HIAA) content in several brain regions and reduced striatal homovanillic acid (HVA) levels, whereas cathinone and methcathinone increased striatal HVA and 5-HIAA. Cathinone elevated striatal and hypothalamic 5-HT. Mephedrone elevated plasma noradrenaline levels, an effect prevented by α-adrenoceptor and dopamine receptor antagonists.

CONCLUSIONS AND IMPLICATIONS

MDMA and cathinones have different effects on thermoregulation, and their acute effects on brain monoamines also differ. These findings suggest that the adverse effects of cathinones in humans cannot be extrapolated from previous observations on MDMA.

The atypical antipsychotic risperidone reverses the recognition memory deficits induced by post-weaning social isolation in rats

RATIONALE

Rearing rats in isolation from weaning is an established preclinical neurodevelopmental model which induces behavioural deficits with apparent translational relevance to some core symptoms of schizophrenia.

OBJECTIVE

This study evaluated the ability of the atypical antipsychotic risperidone to reverse behavioural deficits induced by post-weaning social isolation of rat pups and to further characterise the predictive validity of this model.

METHOD

Forty-five male Lister hooded rats were housed in groups of 3-4 (n = 16) or singly (n = 29) for 4 weeks immediately after weaning on postnatal day (PND) 22-24. On PND 51, novel cage-induced locomotor activity (LMA) was assessed to subdivide rats into groups balanced for behavioural response. On PNDs 58, 59, 65 and 72, rats received either vehicle (1 ml/kg; i.p.) or risperidone (0.2 or 0.5 mg/kg; i.p.) 30 min prior to testing in LMA, novel object discrimination (NOD), prepulse inhibition (PPI) of acoustic startle and conditioned emotional response (CER) learning paradigms, respectively.

RESULTS

Isolation rearing had no effect on PPI, but produced LMA hyperactivity and impaired NOD and CER compared to group-housed controls. Risperidone caused a dose-dependent reduction in LMA, irrespective of rearing condition, but selectively reversed the NOD deficit in isolation-reared rats. Risperidone did not reverse the isolation rearing-induced CER deficit.

CONCLUSIONS

Similar to its clinical profile, risperidone only partially reverses the schizophrenic symptomology; since it reversed some, but not all, of the learning and memory deficits induced by post-weaning isolation, the isolation rearing model may be useful to predict antipsychotic activity of novel therapeutic agents.

5-HT(6) receptor recruitment of mTOR as a mechanism for perturbed cognition in schizophrenia

Cognitive deficits in schizophrenia severely compromise quality of life and are poorly controlled by current antipsychotics. While 5-HT₆ receptor blockade holds special promise, molecular substrates underlying their control of cognition remain unclear. Using a proteomic strategy, we show that 5-HT₆ receptors physically interact with several proteins of the mammalian target of rapamycin (mTOR) pathway, including mTOR. Further, 5-HT₆ receptor activation increased mTOR signalling in rodent prefrontal cortex (PFC). Linking this signalling event to cognitive impairment, the mTOR inhibitor rapamycin prevented deficits in social cognition and novel object discrimination induced by 5-HT₆ agonists. In two developmental models of schizophrenia, specifically neonatal phencyclidine treatment and post-weaning isolation rearing, the activity of mTOR was enhanced in the PFC, and rapamycin, like 5-HT₆ antagonists, reversed these cognitive deficits. These observations suggest that recruitment of mTOR by prefrontal 5-HT₆ receptors contributes to the perturbed cognition in schizophrenia, offering new vistas for its therapeutic control.

5-HT(6) receptor agonists and antagonists enhance learning and memory in a conditioned emotion response paradigm by modulation of cholinergic and glutamatergic mechanisms

BACKGROUND AND PURPOSE

5-HT(6) receptors are abundant in the hippocampus, nucleus accumbens and striatum, supporting their role in learning and memory. Selective 5-HT(6) receptor antagonists produce pro-cognitive effects in several learning and memory paradigms while 5-HT(6) receptor agonists have been found to enhance and impair memory.

EXPERIMENTAL APPROACH

The conditioned emotion response (CER) paradigm was validated in rats. Then we examined the effect of the 5-HT(6) receptor antagonist, EMD 386088 (10 mg·kg(-1) , i.p.), and agonists, E-6801 (2.5 mg·kg(-1) , i.p.) and EMD 386088 (5 mg·kg(-1) , i.p.) on CER-induced behaviour either alone or after induction of memory impairment by the muscarinic receptor antagonist, scopolamine (0.3 mg·kg(-1) , i.p) or the NMDA receptor antagonist, MK-801 (0.1 mg·kg(-1) , i.p).

KEY RESULTS

Pairing unavoidable foot shocks with a light and tone cue during CER training induced a robust freezing response, providing a quantitative index of contextual memory when the rat was returned to the shock chamber 24 h later. Pretreatment (-20 min pre-training) with scopolamine or MK-801 reduced contextual freezing 24 h after CER training, showing production of memory impairment. Immediate post-training administration of 5-HT(6) receptor antagonist, SB-270146, and agonists, EMD 386088 and E-6801, had little effect on CER freezing when given alone, but all significantly reversed scopolamine- and MK-801-induced reduction in freezing. CONCLUSION AND IMPLICATIONS Both the 5-HT(6) receptor agonists and antagonist reversed cholinergic- and glutamatergic-induced deficits in associative learning. These findings support the therapeutic potential of 5-HT(6) receptor compounds in the treatment of cognitive dysfunction, such as seen in Alzheimer's disease and schizophrenia.

Fos expression in the prefrontal cortex and ventral striatum after exposure to a free-operant timing schedule

It has been proposed that cortico-striato-thalamo-cortical circuits that incorporate the prefrontal cortex and corpus striatum regulate interval timing behaviour. In the present experiment regional Fos expression was compared between rats trained under an immediate timing schedule, the free-operant psychophysical procedure (FOPP), which entails temporally regulated switching between two operanda, and a yoked variable-interval (VI) schedule matched to the timing task for food deprivation level, reinforcement rate and overall response rate. The density of Fos-positive neurones (counts mm⁻²) in the orbital prefrontal cortex (OPFC) and the shell of the nucleus accumbens (AcbS) was greater in rats exposed to the FOPP than in rats exposed to the VI schedule, suggesting a greater activation of these areas during the performance of the former task. The enhancement of Fos expression in the OPFC is consistent with previous findings with both immediate and retrospective timing schedules. Enhanced Fos expression in the AcbS was previously found in retrospective timing schedules based on conditional discrimination tasks, but not in a single-operandum immediate timing schedule, the fixed-interval peak procedure. It is suggested that the ventral striatum may be engaged during performance on timing schedules that entail operant choice, irrespective of whether they belong to the immediate or retrospective categories.

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.

Translational neuropharmacology and the appropriate and effective use of animal models

This issue of the British Journal of Pharmacology is dedicated to reviews of the major animal models used in neuropharmacology to examine drugs for both neurological and psychiatric conditions. Almost all major conditions are reviewed. In general, regulatory authorities require evidence for the efficacy of novel compounds in appropriate animal models. However, the failure of many compounds in clinical trials following clear demonstration of efficacy in animal models has called into question both the value of the models and the discovery process in general. These matters are expertly reviewed in this issue and proposals for better models outlined. In this editorial, we further suggest that more attention be made to incorporate pharmacokinetic knowledge into the studies (quantitative pharmacology). We also suggest that more attention be made to ensure that full methodological details are published and recommend that journals should be more amenable to publishing negative data. Finally, we propose that new approaches must be used in drug discovery so that preclinical studies become more reflective of the clinical situation, and studies using animal models mimic the anticipated design of studies to be performed in humans, as closely as possible.

Fos expression in the prefrontal cortex and nucleus accumbens following exposure to retrospective timing tasks

The dorsal striatum and prefrontal cortex have been implicated in interval timing. We examined whether performance of temporal discrimination tasks is associated with increased neuronal activation in these areas, as revealed by Fos expression, a marker for neuronal activation. In Experiment 1, rats were trained on a discrete-trials temporal discrimination task in which a light (22 cd/m²) was presented for a variable time, t (2.5-47.5 s), after which levers A and B were presented. A response on lever A was reinforced if t < 25 s, and a response on lever B was reinforced if t > 25 s. A second group was trained on a light-intensity discrimination procedure, in which a light of variable intensity, i (3.6-128.5 cd/m²) was presented for 25 s. A response on lever A was reinforced if i < 22 cd/m², and a response on lever B was reinforced if i > 22 cd/m². In Experiment 2, bisection procedures were used to assess temporal (200-800 ms, 22 cd/m²) and light-intensity (3.6-128.5 cd/m², 400 ms) discrimination. The increase in proportional choice of lever B as a function of stimulus duration or intensity conformed to a two-parameter logistic equation. Fos expression in the prefrontal cortex and nucleus accumbens was higher in rats performing temporal discrimination tasks than in those performing light-intensity discrimination tasks, indicating greater neuronal activation in these areas during temporal discrimination tasks. Fos expression in the dorsal striatum did not differ between rats performing temporal and light-intensity discrimination tasks. These results suggest that the prefrontal cortex and nucleus accumbens are involved in temporal discrimination.

Animal models of schizophrenia

Developing reliable, predictive animal models for complex psychiatric disorders, such as schizophrenia, is essential to increase our understanding of the neurobiological basis of the disorder and for the development of novel drugs with improved therapeutic efficacy. All available animal models of schizophrenia fit into four different induction categories: developmental, drug-induced, lesion or genetic manipulation, and the best characterized examples of each type are reviewed herein. Most rodent models have behavioural phenotype changes that resemble 'positive-like' symptoms of schizophrenia, probably reflecting altered mesolimbic dopamine function, but fewer models also show altered social interaction, and learning and memory impairment, analogous to negative and cognitive symptoms of schizophrenia respectively. The negative and cognitive impairments in schizophrenia are resistant to treatment with current antipsychotics, even after remission of the psychosis, which limits their therapeutic efficacy. The MATRICS initiative developed a consensus on the core cognitive deficits of schizophrenic patients, and recommended a standardized test battery to evaluate them. More recently, work has begun to identify specific rodent behavioural tasks with translational relevance to specific cognitive domains affected in schizophrenia, and where available this review focuses on reporting the effect of current and potential antipsychotics on these tasks. The review also highlights the need to develop more comprehensive animal models that more adequately replicate deficits in negative and cognitive symptoms. Increasing information on the neurochemical and structural CNS changes accompanying each model will also help assess treatments that prevent the development of schizophrenia rather than treating the symptoms, another pivotal change required to enable new more effective therapeutic strategies to be developed.

The mGluR2/3 agonist LY379268 reverses post-weaning social isolation-induced recognition memory deficits in the rat

RATIONALE

Current antipsychotics are ineffective at treating the negative and cognitive symptoms of schizophrenia, so there is a substantial need to develop more effective therapeutics for this debilitating disorder. The type II metabotropic glutamate receptor (mGluR2/3) is a novel, potential therapeutic target requiring evaluation in appropriate preclinical models of schizophrenia.

OBJECTIVE

This study evaluated the potent, selective mGluR2/3 agonist, LY379268, on the behavioural deficits induced by rearing rat pups in social isolation from weaning, a neurodevelopmental model of schizophrenia, to investigate its antipsychotic potential.

METHODS

Male Lister Hooded rats were weaned on post-natal day 23-25 and either group-housed (3-4 per cage) or isolation-reared for 6 weeks. At subsequent weekly intervals, animals received acute systemic injection of either vehicle or LY379268 (1 mg/kg; i.p.) 30 min prior to recording locomotor activity in a novel arena, novel object recognition, pre-pulse inhibition of acoustic startle and conditioned emotional response paradigms.

RESULTS

Isolation rearing induced locomotor hyperactivity, deficits in novel object recognition, conditioned emotional behaviour and attenuated the magnitude of the initial acoustic startle response in the PPI paradigm compared to that of group-housed controls. LY379268 reversed the isolation-induced locomotor hyperactivity, the object recognition deficit, and restored startle responses in isolated animals, whilst having no effect on conditioned emotional response impairments.

CONCLUSIONS

These data show that LY379268 can reverse some, but not all, post-weaning social isolation-induced changes which have translational relevance to core symptom defects in schizophrenia and support a potential therapeutic role of mGluR2/3 agonists in its treatment.

Acute concomitant effects of MDMA binge dosing on extracellular 5-HT, locomotion and body temperature and the long-term effect on novel object discrimination in rats

RATIONALE

3,4-methylenedioxymethamphetamine (MDMA, ecstasy) produces an acute release of 5-HT in the brain, together with increased locomotion and hyperthermia.

OBJECTIVE

This study examined whether the acute functional changes of locomotor activity and body temperature are related to enhanced 5-HT release induced by MDMA.

METHODS

We concomitantly measured changes in extraneuronal 5-HT by in vivo brain microdialysis and used radiotelemetry to measure locomotion and body temperature to establish whether any positive correlations occur between these three parameters. 'Binge-type' repeated administration of low doses of MDMA (3 and 6 mg/kg given at 2-h intervals three times) were given to provide drug exposure similar to that experienced by recreational drug users.

RESULTS

MDMA induced acute hyperactivity, changes in core body temperature (both hypothermia and hyperthermia) and elevation of hippocampal 5-HT overflow, all of which were dependent on the dose of MDMA administered. The change in locomotor activity and the magnitude of the hyperthermia appeared to be unrelated both to each other and to the magnitude of MDMA-induced 5-HT release. The study also found evidence of long-term disruption of novel object discrimination 2 weeks following "binge-type" repeated MDMA administration.

CONCLUSIONS

MDMA-induced 5-HT release in the brain was not responsible for either the hyperthermia or increased locomotor activity that occurred. Since neither dose schedule of MDMA induced a neurotoxic loss of brain 5-HT 2 weeks after its administration, the impairment of recognition memory found in novel object discrimination probably results from other long-term changes yet to be established.

Effect of acute brain tyrosine depletion on MDMA-induced changes in brain 5-HT

The mechanism by which 3,4-methylenedioxymethamphetamine (MDMA) produces 5-hydroxytryptamine (5-HT, serotonin) neurotoxicity has been suggested to involve an acute release of tyrosine and its non-enzymatic conversion to dopamine. To determine whether brain tyrosine availability is important in MDMA-induced neurotoxicity, brain tyrosine was acutely depleted with a tyrosine-free amino acid mixture (1 g/kg intraperitoneal; twice 1 h apart) which was administered prior to an injection of MDMA (12.5 mg/kg intraperitoneal). A small increase in both the hippocampal and striatal tyrosine concentration occurred in control rats treated with MDMA. The tyrosine-free amino acid mixture significantly decreased tyrosine levels by more than 50% in both brain regions 2 h after injection of either MDMA or saline. MDMA significantly reduced brain 5-HT content 2 h later, but this was of a similar magnitude in control and tyrosine-depleted groups. The long-term neurotoxic 5-HT loss in the hippocampus induced two weeks after MDMA administration was unaltered by the tyrosine-free amino acid mixture. Striatal dopamine content was unaffected by acute MDMA in all groups, while the tyrosine-free amino acid mixture given with MDMA significantly decreased striatal dopamine content 2 weeks later. The tyrosine-free amino acid mixture given alone had no affect on rectal body temperature but attenuated the duration of MDMA-induced hyperthermia. The results confirmed the ability of systemic MDMA to acutely increase brain tyrosine content, but also indicated that a marked acute reduction of brain tyrosine does not directly affect either immediate 5-HT release (as measured by tissue depletion) or long-term hippocampal serotonergic neurotoxicity produced by MDMA.

Chronic fluoxetine differentially modulates the hippocampal microtubular and serotonergic system in grouped and isolation reared rats

Social isolation from weaning in rats produces behavioural and hippocampal structural changes at adulthood. Here, rats were group or isolation reared for eight-weeks. Following the initial four-week period of rearing, fluoxetine (10 mg/kg i.p.) was administered for 28 days. Changes in recognition memory, hippocampal monoamines, and cytoskeletal microtubules were investigated. Isolation-rearing for four- or eight-weeks produced recognition memory deficits that were not reversed by fluoxetine. Eight-weeks of isolation decreased alpha-tubulin acetylation (Acet-Tub) and the tyrosinated/detyrosinated alpha-tubulin ratio (Tyr/Glu-Tub), suggesting major alterations in microtubule dynamics and neuronal plasticity. In grouped rats, fluoxetine decreased Acet-Tub without changes in Tyr/Glu-Tub. In isolates, fluoxetine did not affect Acet-Tub but increased Tyr/Glu-Tub. Finally, fluoxetine altered serotonin metabolism in grouped, but not in isolated animals. Therefore, isolation-rearing changes the hippocampal responses of the serotonergic and microtubular system to fluoxetine. These findings show that early-life experience induces behavioural changes paralleled by alterations in cytoskeletal and neurochemical functions.

Mapping the central effects of methylphenidate in the rat using pharmacological MRI BOLD contrast

Methylphenidate (Ritalin) is a selective dopamine reuptake inhibitor and an effective treatment for attention deficit hyperactivity disorder (ADHD) however the anatomical foci and neuronal circuits involved in these therapeutic benefits are unclear. This study determines the temporal pattern of brain regional activity change produced by systemic administration of a therapeutically relevant dose of methylphenidate in anaesthetised Sprague-Dawley rats using BOLD MRI and a 2.35T Bruker magnet. Following 60 min basal recording separate rats received saline (n = 9) or +/- methylphenidate hydrochloride (2 mg/kg, i.p., n = 9) and BOLD changes were recorded for 90 min using statistical parametric maps. Methylphenidate produced significant positive random BOLD effects in the nucleus accumbens, substantia nigra, entorhinal cortex and medial orbital cortex. Negative random BOLD effects were more widespread and intense, occurring in the motor and somatosensory cortices, caudate putamen, lateral globus pallidus and bed nucleus of the stria terminalis, without accompanying changes in blood pressure or respiratory rate. Methylphenidate-induced negative BOLD in the striatum, and other dopamine terminal areas, may reflect post-synaptic changes produced by blockade of the neuronal dopamine reuptake transporter. While increased positive BOLD in the medial orbital cortex may reflect altered dopamine and/or noradrenaline release indirectly altering striatal activity. The overall pattern of BOLD changes is comparable to that seen in previous studies using guanfacine, amphetamine and atomoxetine, and suggests that although these compounds operate through distinct pharmacological mechanisms the BOLD changes may represent a 'fingerprint pattern' predictive of therapeutic benefit in ADHD.

Behavioural characterisation of the effects of acute and repeated administration of GBR 12909 in rats: further evaluation of a potential model of ADHD

Attention deficit hyperactivity disorder (ADHD) is a common neuropsychiatric disorder characterised by excessive levels of hyperactivity, inattentiveness and impulsivity. Stimulant drugs which increase dopamine neurotransmission are treatments for ADHD. Hypodopaminergic fronto-striatal function with associated overactivity of the dopamine transporter (DAT) represents one possible neurobiological mechanism underlying ADHD. Few, if any, of the existing animal models of ADHD mimic the underlying neurobiology of the disorder. In this study we have further characterised the behavioural profile of a model of a hyperactive inattentive animal through manipulation of the DAT. The behavioural effects of acute treatment and following withdrawal from sub-chronic treatment with GBR 12909 (30 mg/kg i.p.), a potent and highly selective DAT inhibitor, were examined in juvenile rats. GBR 12909 treatment was used to produce a compensatory up regulation following withdrawal. Acute treatment with GBR 12909 (30 mg/kg i.p.) resulted in a marked increase in locomotor and rearing behaviours on the first and fourth days during a 4 consecutive bi-daily drug treatment regime in postnatal weaned rats. Adolescent rats after 10, 20 and 30 days withdrawal from GBR 12909 pre-treatment maintained mild increases in locomotor activity and failed to discriminate a familiar over a novel object in the novel object discrimination task (using both 1 min and 3 h inter-trial intervals) indicating impaired learning and memory. Prepulse inhibition of acoustic startle was unaltered following withdrawal from GBR 12909 treatment. These data reinforce the potential role of the DAT in the underlying neurobiology of ADHD. They also add further evidence to suggest that postnatal changes in the DAT following withdrawal from treatment with the DAT inhibitor, GBR 12909, may prove to be a useful animal model of ADHD with potential for examining the effectiveness of novel ADHD treatments.

Increased dopamine D2High 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(2) (High)), 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(2) (High) determined by competition between dopamine and 2.27 nM [(3)H]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(2) (High), equivalent to a 3.3-fold increase (group 15.2% +/- 1.4%, isolate 49.8% +/- 4.8%; P < 0.0001, Student's unpaired t-test). These findings support both the hypothesis that elevated D(2) (High) 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.

Fluoxetine administration modulates the cytoskeletal microtubular system in the rat hippocampus

A number of studies suggest that stressful conditions can induce structural alterations in the hippocampus and that antidepressant drugs may prevent such deficits. In particular, the selective serotonin reuptake inhibitor (SSRI) fluoxetine was more effective in modulating different neuronal plasticity phenomena and related molecules in rat hippocampus. Cytoskeletal microtubule dynamics are fundamental to dendrites and axons remodeling, leading to the hypothesis that fluoxetine may affect the microtubular system. However, despite reports of stress-induced alterations in microtubule dynamics by different stressors, only few studies investigated the in vivo effects of antidepressants on microtubules in specific rat brain regions. The present study investigated the dose-related (1, 5, or 10 mg/kg i.p.) effects of acute and chronic (21 days) treatments with fluoxetine on the ratio of hippocampal alpha-tubulin isoforms which is thought to reflect microtubule dynamics. Western Blot analysis was used to quantify alpha-tubulin isoforms, high-performance liquid chromatography and fluorescence detection was used to measure ex vivo monoamine metabolism. The results showed that acute fluoxetine increased the stable forms acetylated and detyrosinated alpha-tubulin. Conversely, chronic fluoxetine decreased acetylated alpha-tubulin, indicative of increased microtubule dynamics. The neuron-specific Delta2-Tubulin was increased by chronic fluoxetine indicating neuronal involvement in the observed cytoskeletal changes. Although acute and chronic fluoxetine similarly altered serotonin metabolism by inhibition of serotonin reuptake, this showed no apparent correlation to the cytoskeletal perturbations. Our findings demonstrate that fluoxetine administration modulates microtubule dynamics in rat hippocampus. The cytoskeletal effect exerted by fluoxetine may eventually culminate in promoting events of structural neuronal remodeling.

Impact of regional 5-HT depletion on the cognitive enhancing effects of a typical 5-ht(6) receptor antagonist, Ro 04-6790, in the Novel Object Discrimination task

RATIONALE

Selective 5-ht(6) receptor antagonists like Ro 04-6790 prolong memory in many rodent preclinical paradigms, possibly by blocking tonic 5-HT-evoked GABA release and allowing disinhibition of cortico-limbic glutamatergic and cholinergic neurones. If this is the case, behavioural responses to Ro 04-6790 should be abolished by depletion of endogenous 5-HT, and selective lesions of dorsal raphé (DR) or median raphé (MR) 5-HT pathways would allow the neuroanatomical substrates underlying the cognitive effects of 5-ht(6) receptor antagonists to be elucidated.

OBJECTIVES

This study compared the effect of Ro 04-6790 on novel object discrimination (NOD) before and after sham or 5,7-dihydroxytryptamine (5,7-DHT)-induced lesions produced by injection into the lateral ventricles (LV), DR or MR.

MATERIALS AND METHODS

NOD tests used a 4 h inter-trial interval (ITI) and Ro 04-6790 (10 mg kg(-1) i.p.) was administered 20 min before the familiarization trial. Brain region-specific 5-HT depletion was assessed by high performance liquid chromatography with electrochemical detection (HPLC-ED).

RESULTS

Widespread LV or selective MR, but not DR lesions, abolished the ability of Ro 04-6790 to delay natural forgetting. Successful performance of all lesioned rats in subsequent 'drug-free' NOD tests using a 1 h ITI excluded the possibility of any confounding effects on visual acuity or motivation.

CONCLUSIONS

The ability of Ro 04-6790 to prolong object recognition memory requires blockade of MR 5-HT function. Because DR lesions did not produce the expected depletion of striatal 5-HT an additional contribution of DR inputs to this region cannot be completely excluded.

Effects of social isolation rearing on the limbic brain: a combined behavioral and magnetic resonance imaging volumetry study in rats

Rearing rats in social isolation from weaning induces robust behavioral and neurobiological alterations resembling some of the core symptoms of schizophrenia, such as reduction in prepulse inhibition of acoustic startle (PPI) and locomotor hyperactivity in a novel arena. The aim of this study was to investigate whether social isolation rearing induces volumetric remodeling of the limbic system, and to probe for anatomical structure-behavioral interrelations. Isolation- (n=8) and group-reared (n=8) rats were examined by magnetic resonance (MR) volumetry using high-resolution T2-weighted imaging at 7 T. Volumes of medial prefrontal cortex (mPFC), anterior cingulate cortex (ACC), retrosplenial cortex (RSC) and hippocampal formation were compared between groups and with behavioral measures, i.e. PPI and locomotor activity in a novel arena. Isolation rearing induced locomotor hyperactivity and impaired PPI compared with group-housed rats. The right mPFC was significantly reduced (5.4%) in isolation-reared compared with group-reared rats, with a similar trend on the left side (5.2%). mPFC volumes changes were unrelated to behavioral abnormalities. No significant volume changes were observed in ACC, RSC or hippocampal formation. Hippocampal volumes were associated with the magnitude of PPI response in control but not in isolation-reared rats. Rearing rats in social isolation induced remodeling of the limbic brain with selective prefrontal cortex volume loss. In addition, a dissociation of the interrelation between hippocampal volume and PPI was noted in the isolation-reared rats. Taken together, limbic morphometry is sensitive to the effects of social isolation rearing but did not reveal direct brain-behavior interrelations, calling for more detailed circuitry analysis.

Behavioural and neurochemical effects of post-weaning social isolation in rodents-relevance to developmental neuropsychiatric disorders

Exposing mammals to early-life adverse events, including maternal separation or social isolation, profoundly affects brain development and adult behaviour and may contribute to the occurrence of psychiatric disorders, such as depression and schizophrenia in genetically predisposed humans. The molecular mechanisms underlying these environmentally induced developmental adaptations are unclear and best evaluated in animal paradigms with translational salience. Rearing rat pups from weaning in isolation, to prevent social contact with conspecifics, produces reproducible, long-term changes including; neophobia, impaired sensorimotor gating, aggression, cognitive rigidity, reduced prefrontal cortical volume and decreased cortical and hippocampal synaptic plasticity. These alterations are associated with hyperfunction of mesolimbic dopaminergic systems, enhanced presynaptic dopamine (DA) and serotonergic (5-HT) function in the nucleus accumbens (NAcc), hypofunction of mesocortical DA and attenuated 5-HT function in the prefrontal cortex and hippocampus. These behavioural, morphological and neurochemical abnormalities, as reviewed herein, strongly resemble core features of schizophrenia. Therefore unravelling the mechanisms that trigger these sequelae will improve our knowledge of the aetiology of neurodevelopmental psychiatric disorders, enable identification of longitudinal biomarkers of dysfunction and permit predictive screening for novel compounds with potential antipsychotic efficacy.

Evidence for the sensitivity of operant timing behaviour to stimulation of D1 dopamine receptors

RATIONALE

Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Previous studies using the fixed-interval peak procedure implicated D(2)-like dopamine receptors in these effects. However, recent findings suggest that d-amphetamine alters timing performance on the free-operant psychophysical procedure via D(1)-like receptors. It is not known whether this effect of d-amphetamine is mimicked by direct D(1)-like receptor stimulation.

OBJECTIVE

The effects of a D(1)-like receptor agonist 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine (SKF-81297) on performance on the free-operant psychophysical procedure and the interaction between SKF-81297 and a D(1)-like receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566) and a D(2)-like receptor antagonist haloperidol, were examined.

MATERIALS AND METHODS

Rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement was provided intermittently for responding on A during the first half and on B during the second half of 50-s trials. Logistic psychometric functions were fitted to the relative response rate data (percent responding on B [%B] vs time from trial onset [t]) under each treatment condition, and quantitative indices of timing (T(50) [value of t corresponding to %B = 50] and the Weber fraction [(T(75)-T(25))/2T(50); T(25) and T(75) are values of t corresponding to %B = 25 and %B = 75] were compared among treatments.

RESULTS

SKF-81297 (0.8 mg kg(-1)) reduced T(50); this effect was antagonized by SKF-83566 (0.03 mg kg(-1)) but not by haloperidol (0.05, 0.1 mg kg(-1)).

CONCLUSIONS

Stimulation of D(1)-like dopamine receptors affects performance in the free-operant psychophysical procedure.

Effect of quinpirole on timing behaviour in the free-operant psychophysical procedure: evidence for the involvement of D2 dopamine receptors

RATIONALE

Operant timing behaviour is sensitive to dopaminergic manipulations. It has been proposed that this effect is mediated principally by D(2)-like dopamine receptors. However, we recently found that the effect of d-amphetamine on timing in the free-operant psychophysical procedure was mediated by D(1)-like dopamine receptors. It has not been established whether stimulation of D(2)-like receptors affects timing in this schedule.

OBJECTIVE

To examine the effects of a D(2)-like receptor agonist quinpirole on second-range timing and the ability of dopamine receptor antagonists to reverse quinpirole's effects.

MATERIALS AND METHODS

Rats responded on two levers (A and B) under a free-operant psychophysical schedule in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rates [percent responding on B (%B) vs time (t)] under each treatment; quantitative timing indices [T (50) (value of t when %B = 50) and Weber fraction] were compared among treatments.

RESULTS

Quinpirole (0.04, 0.08 mg kg(-1)) reduced T (50). This effect was attenuated by D(2)-like receptor antagonists haloperidol (0.05, 0.1 mg kg(-1)), eticlopride (0.04, 0.08 mg kg(-1)) and sulpiride (30, 60 mg kg(-1)), but not by the D(3) receptor-preferring antagonist nafadotride (0.5, 1 mg kg(-1)), the D(4) receptor antagonist L-745870 (1, 3 mg kg(-1)) or the D(1)-like receptor antagonist SKF-83566 (0.015 mg kg(-1)).

CONCLUSIONS

Results suggest that quinpirole reduced T (50) via an action at D(2) receptors. D(1)-like and D(2)-like receptors may mediate behaviourally similar but pharmacologically distinct effects on timing behaviour.

Tolerance to the effect of 2,5-dimethoxy-4-iodoamphetamine (DOI) on free-operant timing behaviour: interaction between behavioural and pharmacological mechanisms

RATIONALE

The psychostimulant d-amphetamine, the D(2/3) dopamine receptor agonist quinpirole and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behaviour. There is evidence that tolerance develops to the effects of psychostimulants on timing performance during chronic treatment; this tolerance is generally attributed to behavioural adaptation rather than to pharmacological desensitisation. There have been no previous investigations of tolerance to the effect of DOI on free-operant timing behaviour.

OBJECTIVE

To demonstrate tolerance to DOI's effect on timing performance and to examine the nature of this tolerance.

MATERIALS AND METHODS

Rats were trained under the free-operant psychophysical procedure to press two levers (A and B) in 80-s trials in which reinforcement was provided intermittently for responding on A in the first half and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 8-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices (T (50) [time corresponding to %B = 50]; Weber fraction).

RESULTS

In experiment 1, DOI (0.25 mg kg(-1)) reduced T (50) compared to vehicle; tolerance to this effect was seen after repeated daily treatments with DOI if the rats were exposed to behavioural training during the period of treatment but not if the repeated treatments took place during a 'holiday' from behavioural training. In experiment 2, repeated treatment with DOI resulted in tolerance to the effect of DOI on T (50) and cross-tolerance to the effect of d-amphetamine (0.4 mg kg(-1)), but no cross-tolerance was seen to the effect of quinpirole (0.08 mg kg(-1)).

CONCLUSIONS

The results indicate that behavioural adaptation is involved in the development of tolerance to DOI's effect on timing. The finding of cross-tolerance to d-amphetamine but not to quinpirole suggests that the reduction of T (50) in the free-operant psychophysical procedure may be brought about by two distinct pharmacological mechanisms, one activated by DOI and d-amphetamine, and the other by quinpirole.

Differential effects of the D- and L- isomers of amphetamine on pharmacological MRI BOLD contrast in the rat

RATIONALE

The D - and L-amphetamine sulphate isomers are used in the formulation of Adderall XR(R), which is effective in the treatment of attention-deficit hyperactivity disorder (ADHD). The effects of these isomers on brain activity has not been examined using neuroimaging.

OBJECTIVES

This study determines the pharmacological magnetic resonance imaging blood-oxygenation-level-dependent (BOLD) response in rat brain regions after administration of each isomer.

MATERIALS AND METHODS

Rats were individually placed into a 2.35 T Bruker magnet for 60 min to achieve basal recording of variation in signal intensity. Either saline (n = 9), D-amphetamine sulphate (2 mg/kg, i.p.; n = 9) or L: -amphetamine sulphate (4 mg/kg, i.p.; n = 9) were administered, and recording continued for a further 90 min. Data were analysed for BOLD effects using statistical parametric maps. Blood pressure, blood gases and respiratory rate were monitored during scanning.

RESULTS

The isomers show overlapping effects on the BOLD responses in areas including nucleus accumbens, medial entorhinal cortex, colliculi, field CA1 of hippocampus and thalamic nuclei. The L-isomer produced greater global changes in the positive BOLD response than the D-isomer, including the somatosensory and motor cortices and frontal brain regions such as the orbitofrontal cortices, prelimbic and infralimbic cortex which were not observed with the D-isomer.

CONCLUSIONS

The amphetamine isomers produce different BOLD responses in brain areas related to cognition, pleasure, pain processing and motor control probably because of variations on brain amine systems such as dopamine and noradrenaline. The isomers may, therefore, have distinct actions on brain regions affected in ADHD patients.

The effects of pharmacological blockade of the 5-HT(6) receptor on formalin-evoked nociceptive behaviour, locomotor activity and hypothalamo-pituitary-adrenal axis activity in rats

5-hydroxytryptamine (5-HT) mediates behavioural and neuroendocrine responses to noxious or stressful stimuli. 5-HT(6) receptors are expressed in brain regions involved in nociceptive processing, however, their role in nociception is unknown. Here we demonstrate that acute, systemic administration of the 5-HT(6) receptor antagonist, 5-chloro-N-(4-methoxy-3-benzothio-phenesulfonamide (SB-271046), reduces formalin-evoked nociceptive behaviour and increases plasma corticosterone. SB-271046 dose-dependently reduced pre-formalin distance moved, rearing, grooming and defecation. These data provide the first evidence for 5-HT(6) receptor-mediated regulation of nociception and hypothalamo-pituitary-adrenal axis activity in a model of persistent pain although effects on locomotor activity demand that the putative antinociceptive effect of SB-271046 be interpreted with some caution.

Effects of d-amphetamine and DOI (2,5-dimethoxy-4-iodoamphetamine) on timing behavior: interaction between D1 and 5-HT2A receptors

RATIONALE

The dopamine-releasing agent d-amphetamine and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behavior. The selective D(1) dopamine receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566), but not the D(2) dopamine receptor antagonist haloperidol, can antagonize the effect of d-amphetamine, and the selective 5-HT(2A) receptor antagonist (+/-)2,3-dimethoxyphenyl-1-(2-(4-piperidine)-methanol (MDL-100907) can antagonize the effect of DOI. However, it is not known whether the effect of d-amphetamine can be reversed by MDL-100907 and the effect of DOI by dopamine receptor antagonists.

OBJECTIVE

The objective of this work is to examine the interactions of d-amphetamine and DOI with MDL-100907, SKF-83566, and haloperidol on timing performance.

MATERIALS AND METHODS

Rats (n = 12-15 per experiment) were trained under the free-operant psychophysical procedure to press two levers (A and B) in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices [T (50) (time corresponding to %B = 50); Weber fraction]. Rats were treated systemically with d-amphetamine or DOI, alone and in combination with haloperidol, SKF-83566, or MDL-100907.

RESULTS

d-Amphetamine (0.4 mg kg(-1)) reduced T (50) compared to vehicle; this effect was antagonized by SKF-83566 (0.03 mg kg(-1)) and MDL-100907 (0.5 mg kg(-1)), but not by haloperidol (0.05, 0.1 mg kg(-1)). DOI (0.25 mg kg(-1)) also reduced T (50); this effect was reversed by MDL-100907 (0.5 mg kg(-1)), but not by SKF-83566 (0.03 mg kg(-1)) or haloperidol (0.05 mg kg(-1)).

CONCLUSIONS

The results suggest that both 5-HT(2A) and D(1) receptors, but not D(2) receptors, are involved in d-amphetamine's effect on timing behavior in the free-operant psychophysical procedure. DOI's effect on timing is mediated by 5-HT(2A) receptors, but neither D(1) nor D(2) receptors are involved in this effect.

Effects of 5-HT2A receptor stimulation on the discrimination of durations by rats

We recently found that rats' ability to discriminate durations of exteroceptive stimuli is disrupted by the non-selective 5-HT receptor agonist quipazine. Ketanserin reversed this effect, suggesting that the effect may be mediated by 5-HT2A receptors. Here, we report that the 5-HT2A/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) also disrupts temporal discrimination, and that this effect can be reversed by ketanserin and the highly selective 5-HT2A receptor antagonist (+/-)2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol] (MDL-100907). Twenty rats were trained to discriminate durations in a discrete-trials psychophysical procedure. In each 50-s trial, a light was presented for t seconds, following which two levers (A and B) were presented. A response on A was reinforced if t < 25 s, and a response on B if t > 25 s. Logistic psychometric curves were fitted to the proportional choice of B (%B) for derivation of timing indices [T50: time corresponding to %B = 50; Weber fraction: (T75-T25)/2T50, where T75 and T25 are times corresponding to %B = 75 and 25, respectively]. DOI 0.25 mg kg (subcutaneous) significantly increased the Weber fraction and tended to increase T50. Ketanserin 2 mg kg (subcutaneous) did not alter either parameter, but completely antagonized the effects of DOI. Similarly, MDL-100907 0.5 and 1 mg kg (intraperitoneal) did not affect performance, but completely antagonized the effects of DOI. The results indicate that the mixed 5-HT2A/2C receptor agonist DOI disrupts temporal discrimination via stimulation of 5-HT2A receptors.

Long-lasting changes in behavioural and neuroendocrine indices in the rat following neonatal maternal separation: Gender-dependent effects

Neonatal maternal separation (MS) has been used to model long-term changes in neurochemistry and behaviour associated with exposure to early-life stress. This study characterises changes in behavioural and neuroendocrine parameters following MS. On postnatal days (PND) 3-15, male and female Long-Evans rats underwent 3 h daily MS. Non-handled (NH) control offspring remained with the dams. Starting at PND 90, behaviour was assessed at weekly intervals in the elevated plus-maze, elevated T-maze, and locomotor activity boxes, and body weight monitored throughout. At the end of the study, adrenals were weighed and blood collected for analysis of plasma corticosterone and adrenocorticotropic hormone (ACTH) under basal conditions and following restraint stress. As adults, MS weighed more than NH animals. Activity on the open arms of the plus-maze was similar between MS and NH animals. In the T-maze, MS males had shorter emergence latencies than their NH counterparts. Spontaneous ambulation in a novel environment was significantly higher in MS than in NH animals, and males exhibited overall lower activity than females. Basal plasma corticosterone was lower in MS than in NH females, but no rearing condition difference was observed following restraint stress. Females had higher corticosterone and ACTH levels than males, whereas adrenal glands of MS animals weighed less than those of NH controls. The MS paradigm caused long-term gender dependent effects on behaviour and HPA axis status. The consistent gender differences confirm and expand existing results showing altered anxiety and stress reactivity in male and female rats.

Evidence for a role of D1 dopamine receptors in d-amphetamine's effect on timing behaviour in the free-operant psychophysical procedure

RATIONALE

Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Studies using the fixed-interval peak procedure implicated D2 dopamine receptors in these effects. Less is known about the effects of dopaminergic manipulations on temporal differentiation in other timing schedules.

OBJECTIVE

To examine the effects of a D1 antagonist,8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566), and a D2 antagonist, haloperidol, on performance on the free-operant psychophysical procedure, and the ability of these antagonists to reverse the effects of the catecholamine-releasing agent, d-amphetamine on performance. The antagonists' ability to reverse d-amphetamine-induced hyperlocomotion was also examined.

MATERIALS AND METHODS

Rats responded on two levers (A and B) under a free-operant psychophysical schedule, in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rate data (percent responding on B [%B] vs time [t]) in each treatment condition, and quantitative timing indices [T50 (value of t corresponding to %B=50) and Weber fraction] were compared among treatments. Effects of the treatments on locomotion were measured in a separate experiment.

RESULTS

SKF-83566 (0.015, 0.03, 0.06 mg kg(-1)) did not affect timing performance. Haloperidol (0.025, 0.05 mg kg(-1)) had no effect; a higher dose (0.1 mg kg(-1)) reduced T (50). d-Amphetamine (0.4 mg kg(-1)) reduced T50; this effect was antagonised by SKF-83566 but not by haloperidol. Both antagonists reduced d-amphetamine-induced hyperlocomotion.

CONCLUSIONS

The results suggest that d-amphetamine's effect on performance in the free-operant psychophysical procedure is mediated by D1 rather than D2 receptors.

Pre-treatment with 3,4-methylenedioxymethamphetamine (MDMA) causes long-lasting changes in 5-HT2A receptor-mediated glucose utilization in the rat brain

The current study examined the long-term effect of brief exposure to 3,4-methylenedioxymethamphetamine (MDMA) on local cerebral glucose utilization (LCGU) in specific brain regions immediately following administration of the 5-HT2A/2C receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Wistar rats (post-natal day (PND) 28, n = 24) were administered MDMA (5 mg/kg, i.p.) or saline (1 ml/kg, i.p.) four times daily for 2 consecutive days and core body temperature was recorded. Fifty-five days later and 10 min following injection of DOI (1 mg/kg, i.p.) or saline, LCGU was measured using the [14C]2-deoxyglucose (2-DG) technique. In the 4 hours following the initial injection (PND 28), MDMA-treated rats exhibited significant hyperthermia compared with saline-treated controls (p < 0.05-0.01). Eight weeks later, immediately following DOI challenge, LCGU was significantly elevated (an increase of 47%, p < 0.05) in the nucleus accumbens of MDMA/DOI pretreated rats, compared with that in MDMA/saline pre-treated controls. A similar trend was observed in other areas such as the lateral habenula, somatosensory cortex and hippocampal regions (percentage changes of 27-41%), but these did not reach significance. Blood glucose levels were significantly elevated in both groups of DOI-treated rats (p < 0.05-0.01). Thus, brief exposure of young rats to an MDMA regimen previously shown to cause anxiety-like behaviour and modest serotonergic neurotoxicity (Bull et al., 2004) increased DOI-induced energy metabolism in the nucleus accumbens and tended to increase metabolism in other brain regions, including the hippocampus, consistent with the induction of long-term brain region specific changes in synaptic plasticity.

Effects of 5-HT1A and 5-HT2A receptor stimulation on temporal differentiation performance in the fixed-interval peak procedure

We examined the effects of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-HT(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) on performance on the fixed-interval peak procedure, and the sensitivity of these effects to 5-HT1A and 5-HT2A receptor antagonists (N-[2-(4-[2-methoxyphenyl]-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide [WAY-100635] and ketanserin). Rats were trained to press a lever for food reinforcement in 50 min sessions consisting of 32 trials in which the lever was continuously available, separated by 10 s inter-trial intervals. In 16 trials, reinforcement was delivered following the first response after 30 s had elapsed since trial onset (fixed-interval 30 s). In 16 randomly interposed (peak/probe) trials, reinforcement was omitted, and the lever remained in the operant chamber for 120 s. Response rate in probe trials was plotted against time from trial onset. Time to peak response rate (t(peak)) and the Weber fraction were derived from modified Gaussian curves fitted to each rat's data. 8-OH-DPAT (0.05 mg kg(-1)) reduced t(peak) and increased the Weber fraction; the effect on t(peak) was antagonized by WAY-100635 (0.1 mg kg(-1)). DOI (0.25 mg kg(-1)) also reduced t(peak) and increased the Weber fraction; the reduction of t(peak) was antagonized by ketanserin (2 mg kg(-1)). Stimulation of 5-HT1A and 5-HT2A receptors alters temporal differentiation in qualitatively similar ways.

Effects of quipazine and m-chlorophenylbiguanide (m-CPBG) on temporal differentiation: evidence for the involvement of 5-HT2A but not 5-HT3 receptors in interval timing behaviour

RATIONALE

Temporal differentiation refers to animals' ability to regulate their behaviour during an ongoing interval. Striatal dopaminergic mechanisms are purported to be involved in temporal differentiation, and recent evidence also implicates 5-hydroxytryptaminergic (5-HTergic) mechanisms, possibly mediated by 5-HT(2A) receptors. There is evidence that 5-HT(3) receptors contribute to the regulation of dopamine release in the basal ganglia; however, it is not known whether 5-HT(3) receptor stimulation can influence temporal differentiation.

OBJECTIVE

We examined the effects of a selective 5-HT(3) receptor agonist m-CPBG, a mixed 5-HT(2A/3) receptor agonist quipazine, and selective 5-HT(3) and 5-HT(2A) receptor antagonists (MDL-72222 and ketanserin, respectively) on temporal differentiation in a free-operant psychophysical procedure.

METHODS

Twenty-four rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement (0.6 M: , 50 microl) was provided intermittently for responding on A during the first half and on B during the second half of 50-s trials. Logistic psychometric functions were fitted to the relative response rate data [percent responding on B (%B) vs time from trial onset (t)], and quantitative indices of timing performance [T (50) (value of t corresponding to %B=50), Weber fraction, and mean time of switching from A to B, S (50)] were derived.

RESULTS

Quipazine (0.5, 1, and 2 mg kg(-1)) altered timing performance, dose-dependently reducing T (50) and S (50); m-CPBG (2.5, 5, and 10 mg kg(-1)) had no significant effect. The effect of quipazine was antagonized by ketanserin (2 mg kg(-1)), but not by MDL-72222 (1 mg kg(-1)).

CONCLUSIONS

The present results provide no evidence for the involvement of 5-HT(3) receptors in temporal differentiation and indicate that the effect of quipazine on performance was mediated by 5-HT(2A) receptor stimulation. The results are consistent with previous evidence for the involvement of 5-HT(2A) receptors in interval timing behaviour.

Behavioural and pharmacological magnetic resonance imaging assessment of the effects of methylphenidate in a potential new rat model of attention deficit hyperactivity disorder

RATIONALE

The psychomotor stimulant methylphenidate is used in the treatment of attention deficit hyperactivity disorder (ADHD). Whereas the mechanism is not fully understood it is suggested to involve restoration of impaired dopamine function found in ADHD.

OBJECTIVES

The aim of this study was to determine the effects of methylphenidate on brain region activation in vivo using pharmacological magnetic resonance imaging (phMRI) in a potential rat model of ADHD.

METHODS

Rats were treated bi-daily [from postnatal day (PND) 24] for 4 days with the dopamine re-uptake inhibitor GBR 12909 (30 mg/kg i.p) or vehicle (control). On PND 57 rats were administered methylphenidate (4 mg/kg i.p) and locomotor activity measured. In a separate group of animals, blood oxygen level dependent (BOLD) response was measured using phMRI to determine changes in brain region activation produced by methylphenidate (4 mg/kg i.p.) in GBR 12909-pretreated or control rats.

RESULTS

Methylphenidate produced a greater locomotor-stimulant response in controls compared with GBR 12909 rats. Pretreatment with GBR 12909 reduced the BOLD response produced by methylphenidate compared with that in control animals. The main effects of methylphenidate on the BOLD response were seen in the caudate, frontal cortex, hippocampus and hypothalamus.

CONCLUSIONS

Short-term treatment with GBR 12909 in young rats causes long-term changes in dopaminergic systems, altering the methylphenidate-induced behavioural response and brain region activation compared with that in vehicle-pretreated rats. The results further support the view that altered dopaminergic function may be an important factor in ADHD and the value of animal models with this functional neurochemical deficit.

Stimulants: use and abuse in the treatment of attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is the most prevalent childhood developmental disorder and is also of unclear neurobiological aetiology. However, recent advances in molecular genetics and brain imaging implicate dopaminergic hypofunction in the frontal lobes and basal ganglia in ADHD. Psychostimulants (e.g. methylphenidate and amphetamine, which are potent inhibitors of the dopamine transporter) are the first choice medication for ADHD and have a good acute efficacy and safety profile when used for this disorder. Whether long-term psychostimulant administration to adolescents alters neural development and behaviour or increases the risk of substance abuse is less certain. The precise molecular mechanism of action of psychostimulants is beginning to be established. Furthermore, preclinical studies have begun to use lower clinically relevant doses and oral administration of psychostimulants to determine their long-term effect on development, behaviour and neurochemistry, which is an important public health issue associated with chronic medication of adolescents with ADHD.

Involvement of 5-HT receptors in the regulation of food intake in Siberian hamsters

The Siberian hamster provides a physiological model for understanding the hypothalamic control of energy metabolism as it undergoes annual photoperiod-regulated cycles of body weight (i.e. fattening in summer, and catabolism of fat stores in winter). As a first step to investigate whether enhanced serotonergic (5-HT) tone might underlie the catabolic processes in short days, we investigated whether serotonergic stimulation can produce catabolic actions in fat hamsters housed in long days. Acute treatment with the serotonin reuptake inhibitor (+/-) fenfluramine (8 mg/kg, i.p.) produced a prolonged, dose-dependent reduction in food intake in both photoperiods. Behavioural observations and radiotelemetry analyses revealed that this anorectic effect of fenfluramine was associated with short-term increases in locomotor activity and in core body temperature. In a subsequent series of studies, hamsters were pretreated with the 5-HT2C receptor antagonist SB242084 (4 mg/kg, i.p.). This 5-HT2C receptor antagonist completely blocked the anorectic actions of fenfluramine, but did not decrease the hyperthermia or hyperlocomotion induced by fenfluramine; thus, the anorectic actions of fenfluramine probably reflect actions via the 5-HT2C receptor. Consistent with these observations, treatment of hamsters with the 5-HT2C receptor agonist VER 3323 (10 mg/kg, i.p.) or the 5-HT1B/2C receptor agonist mCPP (3 mg/kg, i.p.) reduced food intake. The response to manipulation of serotonergic pathways was not affected by the ambient photoperiod in any of these studies. We conclude that the anorectic actions of fenfluramine are not an indirect consequence of serotonergic actions on arousal pathways, and that its actions on feeding in the Siberian hamster are most likely to be mediated by the 5-HT2C receptor.