Sally Martin

Neuregulin 1 hypomorphic mutant mice: enhanced baseline locomotor activity but normal psychotropic drug-induced hyperlocomotion and prepulse inhibition regulation

Neuregulin 1 (Nrg1) has been widely recognized as a candidate gene for schizophrenia. This study therefore investigated mice heterozygous for a mutation in the transmembrane domain of this trophic factor (Nrg1+/- mice) in a number of behavioural test systems with relevance to schizophrenia, including psychotropic drug-induced locomotor hyperactivity and prepulse inhibition (PPI) of startle. Baseline locomotor activity in the open field or in photocell cages was slightly, but significantly enhanced in Nrg1+/- mice compared to wild-type littermate controls at age 12-16 wk, but not at age 6 months. The ability of amphetamine, phencyclidine (PCP) or MK-801 to induce locomotor hyperactivity was not significantly different between the genotypes. There was no difference in baseline PPI, startle or startle habituation and there was no difference in the effect of apomorphine, amphetamine or MK-801 on any of these parameters. Only treatment with the 5-HT1A receptor agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) showed a differential effect between genotypes, with a disruption of PPI occurring in Nrg1+/- mice compared to no effect in wild-type controls. This treatment also induced a significant reduction of startle which could have influenced the result. The density of dopamine D2 receptors in the forebrain and of 5-HT1A receptors in the hippocampus and raphe nuclei was not different between Nrg1+/- mice and controls. These studies add to the knowledge about behavioural effects in this mouse model of impaired Nrg1 function and suggest that a number of the behavioural tests with relevance to schizophrenia are normal in these mice.

Neurodevelopmental and neuropsychiatric behaviour defects arise from 14-3-3ζ deficiency.

Complex neuropsychiatric disorders are believed to arise from multiple synergistic deficiencies within connected biological networks controlling neuronal migration, axonal pathfinding and synapse formation. Here, we show that deletion of 14-3-3ζ causes neurodevelopmental anomalies similar to those seen in neuropsychiatric disorders such as schizophrenia, autism spectrum disorder and bipolar disorder. 14-3-3ζ-Deficient mice displayed striking behavioural and cognitive deficiencies including a reduced capacity to learn and remember, hyperactivity and disrupted sensorimotor gating. These deficits are accompanied by subtle developmental abnormalities of the hippocampus that are underpinned by aberrant neuronal migration. Significantly, 14-3-3ζ-deficient mice exhibited abnormal mossy fibre navigation and glutamatergic synapse formation. The molecular basis of these defects involves the schizophrenia risk factor, DISC1, which interacts isoform specifically with 14-3-3ζ. Our data provide the first evidence of a direct role for 14-3-3ζ deficiency in the aetiology of neurodevelopmental disorders and identifies 14-3-3ζ as a central risk factor in the schizophrenia protein interaction network.

A genetic epilepsy rat model displays endophenotypes of psychosis

The incidence of psychosis is increased in people with epilepsy, including idiopathic generalized epilepsies. To study the biological basis for this co-morbidity, we compared GAERS, a genetic rat model of absence epilepsy, to non-epileptic control rats (NEC). Mature, 14-week old GAERS showed enhanced amphetamine-induced locomotor hyperactivity - a feature also present in young (6-week old) GAERS prior to epilepsy onset. Prepulse inhibition and its disruption by psychotropic drugs did not differ between strains, although GAERS displayed elevated startle responses at both epileptic and pre-epileptic ages. The frontoparietal cortex of GAERS displayed a twofold increase in the power of gamma (30-80 Hz) oscillations, a proposed neurophysiological correlate of psychosis. Radioligand binding autoradiography demonstrated reduced densities of dopamine transporters in the caudate nucleus and nucleus accumbens core and of dopamine D2 receptors in the caudate nucleus. GAERS provide an opportunity to study the neurodevelopmental, genetic and therapeutic aspects of psychiatric comorbidities associated with epilepsy.

Effect of adrenalectomy and corticosterone replacement on prepulse inhibition and locomotor activity in mice.

Stress is a risk factor in psychiatric illnesses such as schizophrenia. The aim of the present study was to investigate the effect of different circulating levels of the adrenal steroid corticosterone (CORT) on locomotor hyperactivity and prepulse inhibition of acoustic startle, two behavioural animal models of aspects of schizophrenia. Male C57BL/6J mice (n=10 per group) were anaesthetised with isoflurane and sham‐operated or adrenalectomised (ADX). ADX mice were implanted with 50 mg pellets consisting of 100% cholesterol, or 2, 10 or 50 mg of CORT mixed with cholesterol. CORT pellet implantation dose dependently increased plasma CORT levels 3 weeks after surgery. Starting 1 week after surgery, mice were tested for prepulse inhibition after injection of saline or 5 mg kg−1 of haloperidol. In intact mice and in mice implanted with 10 mg of CORT, haloperidol treatment significantly increased prepulse inhibition (average values from 38 – 42 to 52%). Similar results were observed when testing the mice for amphetamine‐induced locomotor hyperactivity (5 mg kg−1). In contrast, there was no significant effect of haloperidol in mice implanted either with cholesterol or 2 or 50 mg of CORT. These results in behavioural animal models of schizophrenia suggest an important role of the stress hormone CORT in modulating dopaminergic activity in this illness.

Brain-derived neurotrophic factor expression is increased in the hippocampus of 5-HT2C receptor knockout mice

Several studies have suggested a close interaction between serotonin (5‐HT) and BDNF; however, little is known of the specific relationship between BDNF and the 5‐HT2C receptor. Therefore, in this study we investigated BDNF expression in 5‐HT2C receptor knockout mice (5‐HT2CKO). We also assessed functional consequences of any changes in BDNF using a behavioral test battery. Western blot analysis demonstrated a significant 2.2‐fold increase in the expression of the mature form of BDNF in 5‐HT2CKO mice when compared with wild‐type controls (WT) in the hippocampus (P = 0.008), but not frontal cortex or striatum. No differences in the expression of the pro‐BDNF isoform were found, and the ratio of mature/pro BDNF was significantly increased in 5‐HT2CKO (P = 0.003). BDNF mRNA expression in the hippocampus was not different between the genotypes. Hence, increased mature BDNF levels in 5‐HT2CKO hippocampus are most likely due to increased extracellular cleavage rates of pro‐BDNF to its mature form. Protein expression of the BDNF receptor, tropomycin‐related receptor B (TrkB), was also unchanged in the hippocampus, frontal cortex and striatum. With repeated training in a 10‐day win‐shift radial arm maze task, 5‐HT2CKO and WT showed similar decreases of the number of working memory and reference memory errors. In addition, no genotype specific differences were observed for passive or active avoidance learning. 5‐HT2CKO showed modest locomotor hyperactivity but no differences in tests for anxiety, sensorimotor gating, or depressive‐like behaviors; however, in the tail suspension test 5‐HT2CKO showed significantly reduced climbing (P < 0.05). In conclusion, loss of 5‐HT2C receptor expression leads to a marked and selective increase in levels of the mature form of BDNF in the hippocampus. Despite this marked increase, 5‐HT2CKO show only subtle behavioral changes.

Enhanced effect of dopaminergic stimulation on prepulse inhibition in mice deficient in the alpha subunit of G z

RationaleGz is a member of the Gi G protein family associated with dopamine D2-like receptors; however, its functions remain relatively unknown. The aim of the present study was to investigate prepulse inhibition (PPI) of acoustic startle, locomotor hyperactivity and dopamine D2 receptor binding in mice deficient in the α subunit of Gz.MethodsWe used automated startle boxes to assess startle and PPI after treatment with saline, amphetamine, apomorphine or MK-801. We used photocell cages to quantitate locomotor activity after amphetamine treatment. Dopamine D2 receptor density was determined by autoradiography.ResultsStartle responses and baseline PPI were not different between the Gαz knockout mice and wild-type controls (average PPI 46±4 vs 49±3%, respectively). Amphetamine treatment caused a marked disruption of PPI in Gαz knockouts (average PPI 22±2%), but less so in controls (average PPI 42±3%). Similar genotype-dependent responses were seen after apomorphine treatment (average PPI 23±3% vs 40±3%), but not after MK-801 treatment (average PPI 29±5 vs 33±2%). Amphetamine-induced locomotor hyperactivity was greater in Gαz knockouts than in controls. There was no difference in the density of dopamine D2 receptors in nucleus accumbens.ConclusionsMice deficient in the α subunit of Gz show enhanced sensitivity to the disruption of PPI and locomotor hyperactivity caused by dopaminergic stimulation. These results suggest a possible role for Gz in neuropsychiatric illnesses with presumed dopaminergic hyperactivity, such as schizophrenia.

Altered N-methyl-d-aspartate receptor function in reelin heterozygous mice: Male–female differences and comparison with dopaminergic activity

The aim of this study was to investigate the in vivo relationship between reelin and NMDA receptor function in schizophrenia. We assessed the effect of reelin deficiency in behavioral models of aspects of this illness, NMDA receptor subunit levels, and NMDA receptor, dopamine D₂ receptor, and dopamine transporter density. Male, but not female, reelin heterozygous mice showed significantly enhanced MK-801-induced locomotor hyperactivity compared to wildtype controls (7.4-fold vs. 5.2-fold effect of MK-801 over saline, respectively) but there were no genotype differences in the response to amphetamine. Both male and female reelin heterozygous mice showed enhanced effects of MK-801 on startle, but not prepulse inhibition (PPI) of startle. There were no group differences in the effect of apomorphine on startle or PPI. The levels of NMDA receptor subunits were not altered in the striatum. In the frontal cortex, male and female reelin heterozygous mice showed significant up-regulation of NR1 subunits, but down-regulation of NR2C subunits, which was associated with significantly elevated NR1/NR2A and NR1/NR2C ratios. However, there were no differences in [³H]MK-801 binding density in the nucleus accumbens or caudate nucleus, nor in the density of [³H]YM-09151 or [³H]GBR12935 in these brain regions. The enhanced effects of MK-801 in reelin heterozygous mice in this study could be reflective of the role of reelin deficiency in schizophrenia. This genotype effect was male-specific for locomotor hyperactivity, a model of psychosis, but was seen in male and female mice for startle, which could be an indication of changes in anxiety. Changes in NMDA receptor subunit levels and ratios were also seen in both male and female mice. These results suggest that the role of reelin deficiency in schizophrenia may be particularly mediated by altered NMDA receptor responses, with some of these effects being strictly sex-specific.

Enhanced effects of amphetamine but reduced effects of the hallucinogen, 5-MeO-DMT, on locomotor activity in 5-HT1A receptor knockout mice: Implications for schizophrenia

Serotonin-1A (5-HT(1A)) receptors may play a role in schizophrenia and the effects of certain antipsychotic drugs. However, the mechanism of interaction of 5-HT(1A) receptors with brain systems involved in schizophrenia, remains unclear. Here we show that 5-HT(1A) receptor knockout mice display enhanced locomotor hyperactivity to acute treatment with amphetamine, a widely used animal model of hyperdopaminergic mechanisms in psychosis. In contrast, the effect of MK-801 on locomotor activity, modeling NMDA receptor hypoactivity, was unchanged in the knockouts. The effect of the hallucinogen 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) was markedly reduced in 5-HT(1A) receptor knockout mice. There were no changes in apomorphine-induced disruption of PPI, a model of sensory gating deficits seen in schizophrenia. Similarly, there were no major changes in density of dopamine transporters (DAT) or dopamine D(1) or D(2) receptors which could explain the behavioural changes observed in 5-HT(1A) receptor knockout mice. These results extend our insight into the possible role of these receptors in aspects of schizophrenia. As also suggested by previous studies using agonist and antagonist drugs, 5-HT(1A) receptors may play an important role in hallucinations and to modulate dopaminergic activity in the brain.

Mice deficient in the alpha subunit of Gz show changes in pre-pulse inhibition, anxiety and responses to 5-HT1A receptor stimulation, which are strongly dependent on the genetic background

RationaleGz, a member of the Gi G protein family, is involved in the coupling of dopaminergic and serotonergic receptors. In the present study, we investigated behaviour of mice deficient in the α subunit of Gz and focused on pre-pulse inhibition (PPI) and anxiety-like responses and the role of serotonin-1A (5-HT1A) receptors.Materials and methodsWe compared male and female wild-type and knock-out mice on either a C57Bl/6 or Balb/c background. We used automated startle boxes to assess startle and PPI and elevated plus maze to assess anxiety-like behaviours.ResultsBalb/c mice showed higher baseline PPI than C57Bl/6 mice, and there was no difference between the genotypes. The 5-HT1A receptor agonist, 8-hydroxy-di-propylaminotetralin (8-OH-DPAT), had no effect on PPI in C57Bl/6 mice but markedly increased PPI in Balb/c mice, with the effect being attenuated in Gαz knock-outs. On the elevated plus maze, there was little effect of the knock-out or 8-OH-DPAT in C57Bl/6 mice, whereas in Balb/c mice, Gαz knock-outs showed a phenotype of high levels of anxiety-like behaviour. 8-OH-DPAT was anxiogenic in Balb/c mice, but this effect was attenuated in Gαz knock-outs.Conclusions5-HT1A receptors couple to Gz. In a strictly background strain-dependent manner, Gαz knock-out mice display high levels of anxiety-like behaviour and are less sensitive to the action of 8-OH-DPAT. Balb/c mice show much more clear effects of the Gαz knock-out than C57Bl/6 mice, which are often considered the standard background strain for genetic modifications. Therefore, our results suggest caution when studying the behavioural effects of genetic modifications only in C57Bl/6 mice.

Interaction of corticosterone and nicotine in regulation of prepulse inhibition in mice

The aim of the present study was to investigate if different levels of circulating corticosterone (CORT) modulate the effect of nicotine on prepulse inhibition (PPI), a measure of sensorimotor gating that is disrupted in schizophrenia and other mental illnesses. Four groups of mice were investigated: sham-operated, adrenalectomized (ADX) and implanted with a cholesterol pellet, ADX and implanted with a 10 mg CORT pellet, or ADX and 50 mg of CORT. Different CORT levels or doses of nicotine did not significantly affect startle responses. Baseline PPI was significantly reduced in mice implanted with the highest dose of CORT. In ADX mice implanted with cholesterol, nicotine treatment influenced PPI depending on the prepulse intensity. In ADX mice implanted with 50 mg of CORT, treatment with 10 mg/kg of nicotine caused a significant increase in PPI at all prepulse intensities. Binding studies showed that corticosterone treatment had significantly affected nicotinic acetylcholine receptor (nAChR) density in the mouse brain. Treatment with 50 mg CORT decreased 125I-epibatidine binding in the globus pallidus and 125I-alpha-bungarotoxin binding in the claustrum. These results suggest a possible interaction of corticosterone and nicotine at the level of the alpha4- and alpha7-type nAChR in the regulation of PPI. In situations of high circulating levels of corticosterone, nicotine may be beneficial to restore disruption of PPI.