Benjamin K. Yee

Adenosine hypothesis of schizophrenia –opportunities for pharmacotherapy

Pharmacotherapy of schizophrenia based on the dopamine hypothesis remains unsatisfactory for the negative and cognitive symptoms of the disease. Enhancing N-methyl-D-aspartate receptors (NMDAR) function is expected to alleviate such persistent symptoms, but successful development of novel clinically effective compounds remains challenging. Adenosine is a homeostatic bioenergetic network modulator that is able to affect complex networks synergistically at different levels (receptor-dependent pathways, biochemistry, bioenergetics, and epigenetics). By affecting brain dopamine and glutamate activities, it represents a promising candidate for reversing the functional imbalance in these neurotransmitter systems believed to underlie the genesis of schizophrenia symptoms, as well as restoring homeostasis of bioenergetics. Suggestion of an adenosine hypothesis of schizophrenia further posits that adenosinergic dysfunction might contribute to the emergence of multiple neurotransmitter dysfunctions characteristic of schizophrenia via diverse mechanisms. Given the importance of adenosine in early brain development and regulation of brain immune response, it also bears direct relevance to the aetiology of schizophrenia. Here, we provide an overview of the rationale and evidence in support of the therapeutic potential of multiple adenosinergic targets, including the high-affinity adenosine receptors (A(1)R and A(2A)R), and the regulatory enzyme adenosine kinase (ADK). Key preliminary clinical data and preclinical findings are reviewed.

Evaluating Early Preventive Antipsychotic and Antidepressant Drug Treatment in an Infection-Based Neurodevelopmental Mouse Model of Schizophrenia

Current pharmacotherapy of schizophrenia remains unsatisfactory with little hope for complete functional restoration in patients once the disease has developed. A preventive approach based on intervention in the prodromal stage of the disease aiming to preserve functional integrity by halting the progress of the disease is therefore extremely attractive. Here, we investigated the effects of preventive antipsychotic or antidepressant drug treatment in a well-established neurodevelopmental mouse model of multiple schizophrenia-related abnormalities. Pregnant mice on gestation day 9 were exposed to the viral mimic polyriboinosinic-polyribocytidylic acid (2 mg/kg, intravenously) or corresponding vehicle treatment, and the resulting offspring from both prenatal treatment conditions were subjected to chronic antipsychotic (haloperidol or clozapine), antidepressant (fluoxetine), or placebo treatment during the periadolescent stage of development. The effects of the preventive pharmacotherapy on behavioral and pharmacological functions were then investigated in adulthood using paradigms relevant to schizophrenia, namely prepulse inhibition, latent inhibition, and sensitivity to psychostimulant drugs. We show that periadolescent treatment with the reference antipsychotic and antidepressant drugs can successfully block the emergence of multiple psychosis-related behavioral and pharmacological abnormalities in subjects predisposed to adult brain pathology by exposure to prenatal immune challenge. At the same time, however, our study reveals numerous negative influences of the early pharmacological intervention on normal behavioral development in control subjects. Hence, even though preventive pharmacotherapy may be beneficial in individuals with predisposition to psychosis-related brain dysfunctions, chronic antipsychotic or antidepressant drug treatment in false-positive subjects is associated with substantial risk for long-term behavioral disturbances in adulthood.

The glycine transporter 1 inhibitor SSR504734 enhances working memory performance in a continuous delayed alternation task in C57BL/6 mice

RationaleInhibition of the glycine transporter 1 (GlyT1) activity increases extra-cellular glycine availability in the CNS. At glutamatergic synapses, increased binding to the glycine-B site located in the N-methyl-d-aspartate receptor (NMDAR) can enhance neurotransmission via NMDARs. Systemic treatment of 2-chloro-N-[(S)-phenyl [(2S)-piperidin-2-yl] methyl]-3-trifluoromethyl benzamide, monohydrochloride (SSR504734), a selective GlyT1 inhibitor, is effective against social recognition impairment induced by neonatal phencyclidine treatment and enhances pre-pulse inhibition in a mouse strain (DBA/2) with intrinsic sensorimotor gating deficiency, suggesting that SSR504734 may be an effective cognitive enhancer.ObjectiveThe objective of the study was to examine if SSR504734 exhibits a promnesic effect on working memory function in wild-type C57BL/6 mice using an automatic continuous alternation task.Materials and methodsHungry mice were trained to alternate their nose pokes between two food magazines across successive discrete trials in an operant chamber in order to obtain food reward. Correct choice on a given trial thus followed a non-matching or win-shift rule in relation to the preceding trial, with manipulation of the demand on memory retention, by varying the delay between successive trials.ResultsPre-treatment with SSR504734 (30xa0mg/kg, i.p.) improved choice accuracy when the delay from the previous trial was extended to 12–16xa0s. Furthermore, a dose–response analysis (3, 10, 30xa0mg/kg) revealed a clear dose-dependent efficacy of the drug: 3xa0mg/kg was without effect, whilst 10xa0mg/kg led to an intermediate enhancement in performance.ConclusionThe present findings represent the first demonstration of the promnesic effects of SSR504734 under normal physiological conditions, lending further support to the suggestion of its potential as a cognitive enhancer.

Disruption of hippocampus-regulated behavioural and cognitive processes by heterozygous constitutive deletion of SynGAP

The brain‐specific Ras/Rap‐GTPase activating protein (SynGAP) is a prime candidate linking N‐methyl‐d‐aspartate receptors to the regulation of the ERK/MAP kinase signalling cascade, suggested to be essential for experience‐dependent synaptic plasticity. Here, we evaluated the behavioural phenotype of SynGAP heterozygous knockout mice (SG+/−), expressing roughly half the normal levels of SynGAP. In the cognitive domain, SG+/− mice demonstrated severe working and reference memory deficits in the radial arm maze task, a mild impairment early in the transfer test of the water maze task, and a deficiency in spontaneous alternation in an elevated T‐maze. In the non‐cognitive domain, SG+/− mice were hyperactive in the open field and appeared less anxious in the elevated plus maze test. In contrast, object recognition memory performance was not impaired in SG+/− mice. The reduction in SynGAP thus resulted in multiple behavioural traits suggestive of aberrant cognitive and non‐cognitive processes normally mediated by the hippocampus. Immunohistochemical evaluation further revealed a significant reduction in calbindin‐positive interneurons in the hippocampus and doublecortin‐positive neurons in the dentate gyrus of adult SG+/− mice. Heterozygous constitutive deletion of SynGAP is therefore associated with notable behavioural as well as morphological phenotypes indicative of hippocampal dysfunction. Any suggestion of a possible causal link between them however remains a matter for further investigation.

The acquisition, retention and reversal of spatial learning in the morris water maze task following withdrawal from an escalating dosage schedule of amphetamine in wistar rats

Two experiments were carried out to evaluate the effects of amphetamine withdrawal in rats on spatial learning in the water maze. A schedule of repeated d-amphetamine administration lasting for 6 days, with three injections per day (1-5 mg/kg, i.p.), was employed. Experiment 1 demonstrated that amphetamine withdrawal did not impair the acquisition of the water maze task (third to fourth withdrawal days), but amphetamine-withdrawn rats made more target-zone visits and reached the former location of the platform quicker than controls during the probe test (fifth withdrawal day). In experiment 2, retention of the location of the escape platform was assessed in animals having been pre-trained on the water maze task before treatment. On the third withdrawal day, retention of the former platform location was assessed in a probe test. Retention was only clearly seen in the measure of target zone visits, and performance did not differ between groups. Next, the animals were trained to escape to a new location in the water maze on withdrawal days 4-5. A reversal effect could be discerned across the first four trials, as evident by the animals tendency to search in the former target quadrant. This interfered with the new learning, but amphetamine-withdrawn animals appeared to overcome it more rapidly than saline-treated controls. This finding is consistent with the view that amphetamine withdrawal can enhance behavioural switching, which could be expressed as a reduction of proactive interference during learning; and, it is in line with our previous finding that latent inhibition is also attenuated during amphetamine withdrawal.

Latent inhibition is spared by N-methyl-D-aspartate (NMDA)-induced ventral hippocampal lesions, but is attenuated following local activation of the ventral hippocampus by intracerebral NMDA infusion.

Repeated non-reinforced exposures of a neutral stimulus retard the development of a conditioned response to that stimulus when it is subsequently paired with a significant event. This stimulus pre-exposure effect is known as latent inhibition (LI). Early lesion studies have initially suggested an important role for the hippocampus in the normal development and expression of LI. This view has since been modified with the emergence of data derived from selective cell body lesions of the hippocampus and of the entorhinal cortex, with an abolition of LI only seen after lesions of the latter. This suggests that the significance of the hippocampus might have been overestimated in the past, possibly due to interruption of fibres en passage. However, intact behavioural expression of LI following hippocampal damage does not preclude the suggestion that the hippocampus participates in the control and regulation of LI expression in intact animals. The present study demonstrated that whilst cell body lesions of the ventral hippocampus spared LI (as expected), chemical activation of the ventral hippocampus by local N-methyl-D-aspartate infusion disrupted LI. These results parallel our earlier observations on prepulse inhibition (PPI) with similar manipulations [Neuroreport 10 (1999) 2533]. Thus, although the ventral hippocampus is itself not responsible for the behavioural manifestation of LI and PPI, it exerts at least a modulatory control over the form and/or magnitude of their expression. Our results should prompt a re-evaluation of the relative roles of the hippocampus and retrohippocampus in the development and expression of LI.

Apomorphine-induced disruption of prepulse inhibition that can be normalised by systemic haloperidol is insensitive to clozapine pretreatment

RationalePrepulse inhibition (PPI) of startle refers to the phenomenon in which a weak prepulse attenuates the startle response to a succeeding intense stimulus. PPI can be disrupted by systemic apomorphine in animals, and reduced PPI has been consistently reported in schizophrenia patients. The ability of the atypical antipsychotic clozapine to reverse apomorphine-induced PPI deficit has been demonstrated in the rat, but has not yet been tested in the mouse. The present study was designed to fill this gap.Objective and resultsWe investigated the efficacy of clozapine in reversing apomorphine-induced (2.0 or 2.5xa0mg/kg, SC) PPI deficit in C57BL6 mice. Clozapine failed to restore PPI disruption in apomorphine-treated mice in two independent laboratories across two dose ranges (1–3xa0mg/kg, IP, or 3–30xa0mg/kg, PO), whereas the typical antipsychotic haloperidol (1xa0mg/kg,IP) completely normalised PPI performance.ConclusionsUnlike the rat, apomorphine-induced PPI disruption in mice might be instrumental in distinguishing between typical and atypical antipsychotic drugs. This also lends further support to the suggestion that the neuropharmacology of PPI is not identical in the two rodent species.

Working memory and the homeostatic control of brain adenosine by adenosine kinase

The neuromodulator adenosine maintains brain homeostasis and regulates complex behaviour via activation of inhibitory and excitatory adenosine receptors (ARs) in a brain region-specific manner. AR antagonists such as caffeine have been shown to ameliorate cognitive impairments in animal disease models but their effects on learning and memory in normal animals are equivocal. An alternative approach to reduce AR activation is to lower the extracellular tone of adenosine, which can be achieved by up-regulating adenosine kinase (ADK), the key enzyme of metabolic adenosine clearance. However, mice that globally over-express an Adk transgene (Adk-tg mice) were devoid of a caffeine-like pro-cognitive profile; they instead exhibited severe spatial memory deficits. This may be mechanistically linked to cortical/hippocampal N-methyl-d-aspartate receptor (NMDAR) hypofunction because the motor response to acute MK-801 was also potentiated in Adk-tg mice. Here, we evaluated the extent to which the behavioural phenotypes of Adk-tg mice might be modifiable by up-regulating adenosine levels in the cortex/hippocampus. To this end, we investigated mutant fb-Adk-def mice in which ADK expression was specifically reduced in the telencephalon leading to a selective increase in cortical/hippocampal adenosine, while the rest of the brain remained as adenosine-deficient as in Adk-tg mice. The fb-Adk-def mice showed an even greater impairment in spatial working memory and a more pronounced motor response to NMDAR blockade than Adk-tg mice. These outcomes suggest that maintenance of cortical/hippocampal adenosine homeostasis is essential for effective spatial memory and deviation in either direction is detrimental with increased expression seemingly more disruptive than decreased expression.

Appetitively motivated instrumental learning in SynGAP heterozygous knockout mice.

The synaptic Ras/Rap-GTPase-activating protein (SynGAP) regulates specific intracellular events following N-methyl-d-aspartate receptor (NMDAR) activation. Here, the impact of SynGAP heterozygous knockout (SG+/-) on NMDAR-dependent functions was assessed using different positive reinforcement schedules in instrumental conditioning. The knockout did not affect the temporal control of operant responding under a fixed interval (FI) schedule, but led to a putative enhancement in response vigor and/or disinhibition. When examined on differential reinforcement of low rates of response (DRL) schedules, SG+/- mice showed increased responding under DRL-4s and DRL-8s, without impairing the response efficiency (total rewards/total lever presses) because both rewarded and nonrewarded presses were elevated. Motivation was unaffected as evaluated using a progressive ratio (PR) schedule. Yet, SG+/- mice persisted in responding during extinction at the end of PR training, although an equivalent phenotype was not evident in extinction learning following FI-20s training. This extinction phenotype is therefore schedule-specific and cannot be generalized to Pavlovian conditioning. In conclusion, constitutive SynGAP reduction increases vigor in the execution of learned operant behavior without compromising its temporal control, yielding effects readily distinguishable from NMDAR blockade.

Startle and prepulse inhibition as a function of background noise: A computational and experimental analysis

Schmajuk and Larrauri [Schmajuk NA, Larrauri JA. Neural network model of prepulse inhibition. Behav Neurosci 2005;119:1546-62.] introduced a real-time model of acoustic startle, prepulse inhibition (PPI) and facilitation (PPF) in animals and humans. The model assumes that (1) positive values of changes in noise level activate an excitatory and a facilitatory pathway, and (2) absolute values of changes in noise level activate an inhibitory pathway. The model describes many known properties of the phenomena and the effect of brain lesions on startle, PPI, and PPF. The purpose of the present study is to (a) establish the magnitude of startle and PPI as a function of pulse, prepulse, and background intensity, and (b) test the model predictions regarding an inverted-U function that relates startle to the intensity of the background noise.