Is schizophrenia a dopamine supersensitivity psychotic reaction?

MAM (E17) rodent developmental model of neuropsychiatric disease: disruptions in learning and dysregulation of nucleus accumbens dopamine release, but spared executive function

RATIONALE

Gestational day 17 methylazoxymethanol (MAM) treatment has been shown to reproduce, in rodents, some of the alterations in cortical and mesolimbic circuitries thought to contribute to schizophrenia.

OBJECTIVE

We characterized the behavior of MAM animals in tasks dependent on these circuitries to see what behavioral aspects of schizophrenia the model captures. We then characterized the integrity of mesolimbic dopamine neurotransmission in a subset of animals used in the behavioral experiments.

METHODS

MAM animals' capacity for working memory, attention, and resilience to distraction was tested with two different paradigms. Cue-reward learning and motivation were assayed with Pavlovian conditioned approach. Measurements of electrically stimulated phasic and tonic DA release in the nucleus accumbens with fast-scan cyclic voltammetry were obtained from the same animals used in the Pavlovian task.

RESULTS

MAM animals' basic attentional capacities were intact. MAM animals took longer to acquire the working memory task, but once learned, performed at the same level as shams. MAM animals were also slower to develop a Pavlovian conditioned response, but otherwise no different from controls. These same animals showed alterations in terminal DA release that were unmasked by an amphetamine challenge.

CONCLUSIONS

The predominant behavioral-cognitive feature of the MAM model is a learning impairment that is evident in acquisition of executive function tasks as well as basic Pavlovian associations. MAM animals also have dysregulated terminal DA release, and this may contribute to observed behavioral differences. The MAM model captures some functional impairments of schizophrenia, particularly those related to acquisition of goal-directed behavior.

Aptamers as promising molecular recognition elements for diagnostics and therapeutics in the central nervous system

Oligonucleotide aptamers are short, synthetic, single-stranded DNA or RNA able to recognize and bind to a multitude of targets ranging from small molecules to cells. Aptamers have emerged as valuable tools for fundamental research, clinical diagnosis, and therapy. Due to their small size, strong target affinity, lack of immunogenicity, and ease of chemical modification, aptamers are an attractive alternative to other molecular recognition elements, such as antibodies. Although it is a challenging environment, the central nervous system and related molecular targets present an exciting potential area for aptamer research. Aptamers hold promise for targeted drug delivery, diagnostics, and therapeutics. Here we review recent advances in aptamer research for neurotransmitter and neurotoxin targets, demyelinating disease and spinal cord injury, cerebrovascular disorders, pathologies related to protein aggregation (Alzheimer's, Parkinson's, and prions), brain cancer (glioblastomas and gliomas), and regulation of receptor function. Challenges and limitations posed by the blood brain barrier are described. Future perspectives for the application of aptamers to the central nervous system are also discussed.

Age-dependent changes in cocaine sensitivity across early ontogeny in male and female rats: possible role of dorsal striatal D2(High) receptors

RATIONALE

Responsiveness to acute psychostimulant administration varies across ontogeny.

OBJECTIVE

The purpose of the present study was to determine if age-dependent changes in D2(High) receptors may be responsible for the ontogeny of cocaine sensitivity in preweanling, adolescent, and adult rats.

METHODS

[(3)H]-Domperidone/dopamine competition assays were used to determine ontogenetic changes in the proportion of D2(High) receptors in male and female preweanling [postnatal day (PD) 5, 10, 15, and 20], adolescent (PD 40), and adult (PD 80) rats. In the behavioral experiment, responsiveness to cocaine (2.5, 5, 10, or 20 mg/kg) was assessed on PD 20, PD 40, and PD 80 for 60 min. Male and female rats were habituated to the apparatus on the 2 days prior to testing. Distance traveled data were presented both untransformed and as percent of saline controls.

RESULTS

Male and female preweanling rats (PD 5-PD 20) had a significantly greater percentage of dorsal striatal D2(High) receptors than adolescent or adult rats. Likewise, preweanling rats (PD 20) were more sensitive to the behavioral effects of cocaine than the two older age groups. Adolescent and adult rats responded in a generally similar manner; however, analysis of the untransformed locomotor activity data suggested that adolescent rats were hyporesponsive to 2.5 and 20 mg/kg cocaine when compared to adults.

CONCLUSIONS

Data from the present study are consistent with the hypothesis that ontogenetic changes in D2(High) receptors are responsible for age-dependent differences in psychostimulant sensitivity.

Biological factors underlying sex differences in neurological disorders

The prevalence, age of onset, pathophysiology, and symptomatology of many neurological and neuropsychiatric conditions differ significantly between males and females. Females suffer more from mood disorders such as depression and anxiety, whereas males are more susceptible to deficits in the dopamine system including Parkinson's disease (PD), attention-deficit hyperactivity disorder (ADHD), schizophrenia, and autism spectrum disorders (ASD). Until recently, these sex differences have been explained solely by the neuroprotective actions of sex hormones in females. Emerging evidence however indicates that the sex chromosome genes (i.e. X- and Y-linked genes) also contribute to brain sex differences. In particular, the Y-chromosome gene, SRY (Sex-determining Region on the Y chromosome) is an interesting candidate as it is expressed in dopamine-abundant brain regions, where it regulates dopamine biosynthesis and dopamine-mediated functions such as voluntary movement in males. Furthermore, SRY expression is dysregulated in a toxin-induced model of PD, suggesting a role for SRY in the pathogenesis of dopamine cells. Taken together, these studies highlight the importance of understanding the interplay between sex-specific hormones and sex-specific genes in healthy and diseased brain. In particular, better understanding of regulation and function of SRY in the male brain could provide entirely novel and important insights into genetic factors involved in the susceptibility of men to neurological disorders, as well as development of novel sex-specific therapies.

Chronic Toxoplasma gondii in Nurr1-null heterozygous mice exacerbates elevated open field activity

Latent infection with Toxoplasma gondii is common in humans (approximately 30% of the global population) and is a significant risk factor for schizophrenia. Since prevalence of T. gondii infection is far greater than prevalence of schizophrenia (0.5-1%), genetic risk factors are likely also necessary to contribute to schizophrenia. To test this concept in an animal model, Nurr1-null heterozygous (+/-) mice and wild-type (+/+) mice were evaluate using an emergence test, activity in an open field and with a novel object, response to bobcat urine and prepulse inhibition of the acoustic startle response (PPI) prior to and 6 weeks after infection with T. gondii. In the emergence test, T. gondii infection significantly decreased the amount of time spent in the cylinder. Toxoplasma gondii infection significantly elevated open field activity in both +/+ and +/- mice but this increase was significantly exacerbated in +/- mice. T. gondii infection reduced PPI in male +/- mice but this was not statistically significant. Aversion to bobcat urine was abolished by T. gondii infection in +/+ mice. In female +/- mice, aversion to bobcat urine remained after T. gondii infection while the male +/- mice showed no aversion to bobcat urine. Antibody titers of infected mice were a critical variable associated with changes in open field activity, such that an inverted U shaped relationship existed between antibody titers and the percent change in open field activity with a significant increase in activity at low and medium antibody titers but no effect at high antibody titers. These data demonstrate that the Nurr1 +/- genotype predisposes mice to T. gondii-induced alterations in behaviors that involve dopamine neurotransmission and are associated with symptoms of schizophrenia. We propose that these alterations in murine behavior were due to further exacerbation of the altered dopamine neurotransmission in Nurr1 +/- mice.

Pedunculopontine arousal system physiology-Implications for schizophrenia

Schizophrenia is characterized by major sleep/wake disturbances including increased vigilance and arousal, decreased slow wave sleep, and increased REM sleep drive. Other arousal-related symptoms include sensory gating deficits as exemplified by decreased habituation of the blink reflex. There is also dysregulation of gamma band activity, suggestive of disturbances in a host of arousal-related mechanisms. This review examines the role of the reticular activating system, especially the pedunculopontine nucleus, in the symptoms of the disease. Recent discoveries on the physiology of the pedunculopontine nucleus help explain many of these disorders of arousal in, and point to novel therapeutic avenues for, schizophrenia.

Update on the Mechanism of Action of Aripiprazole: Translational Insights into Antipsychotic Strategies Beyond Dopamine Receptor Antagonism

Dopamine partial agonism and functional selectivity have been innovative strategies in the pharmacological treatment of schizophrenia and mood disorders and have shifted the concept of dopamine modulation beyond the established approach of dopamine D2 receptor (D2R) antagonism. Despite the fact that aripiprazole was introduced in therapy more than 12 years ago, many questions are still unresolved regarding the complexity of the effects of this agent on signal transduction and intracellular pathways, in part linked to its pleiotropic receptor profile. The complexity of the mechanism of action has progressively shifted the conceptualization of this agent from partial agonism to functional selectivity. From the induction of early genes to modulation of scaffolding proteins and activation of transcription factors, aripiprazole has been shown to affect multiple cellular pathways and several cortical and subcortical neurotransmitter circuitries. Growing evidence shows that, beyond the consequences of D2R occupancy, aripiprazole has a unique neurobiology among available antipsychotics. The effect of chronic administration of aripiprazole on D2R affinity state and number has been especially highlighted, with relevant translational implications for long-term treatment of psychosis. The hypothesized effects of aripiprazole on cell-protective mechanisms and neurite growth, as well as the differential effects on intracellular pathways [i.e. extracellular signal-regulated kinase (ERK)] compared with full D2R antagonists, suggest further exploration of these targets by novel and future biased ligand compounds. This review aims to recapitulate the main neurobiological effects of aripiprazole and discuss the potential implications for upcoming improvements in schizophrenia therapy based on dopamine modulation beyond D2R antagonism.

Does treatment of schizophrenia with antipsychotic medications eliminate or reduce psychosis? A 20-year multi-follow-up study

BACKGROUND

This research assesses whether multi-year treatment with antipsychotic medications reduces or eliminates psychosis in schizophrenia. It provides 20 years of longitudinal data on the frequency and severity of psychotic activity in samples of schizophrenia patients (SZ) treated versus those not treated with antipsychotic medications.

METHOD

A total of 139 early young schizophrenia and mood-disordered patients were assessed at index hospitalization and then reassessed six times over 20 years for psychosis and other major variables.

RESULTS

At each follow-up assessment over the 20 years, a surprisingly high percentage of SZ treated with antipsychotics longitudinally had psychotic activity. More than 70% of SZ continuously prescribed antipsychotics experienced psychotic activity at four or more of six follow-up assessments over 20 years. Longitudinally, SZ not prescribed antipsychotics showed significantly less psychotic activity than those prescribed antipsychotics (p < 0.05).

CONCLUSIONS

The 20-year data indicate that, longitudinally, after the first few years, antipsychotic medications do not eliminate or reduce the frequency of psychosis in schizophrenia, or reduce the severity of post-acute psychosis, although it is difficult to reach unambiguous conclusions about the efficacy of treatment in purely naturalistic or observational research. Longitudinally, on the basis of their psychotic activity and the disruption of functioning, the condition of the majority of SZ prescribed antipsychotics for multiple years would raise questions as to how many of them are truly in remission.

Dopamine and glucose, obesity, and reward deficiency syndrome

Obesity as a result of overeating as well as a number of well described eating disorders has been accurately considered to be a world-wide epidemic. Recently a number of theories backed by a plethora of scientifically sound neurochemical and genetic studies provide strong evidence that food addiction is similar to psychoactive drug addiction. Our laboratory has published on the concept known as Reward Deficiency Syndrome (RDS) which is a genetic and epigenetic phenomena leading to impairment of the brain reward circuitry resulting in a hypo-dopaminergic function. RDS involves the interactions of powerful neurotransmitters and results in abnormal craving behavior. A number of important facts which could help translate to potential therapeutic targets espoused in this focused review include: (1) consumption of alcohol in large quantities or carbohydrates binging stimulates the brain’s production of and utilization of dopamine; (2) in the meso-limbic system the enkephalinergic neurons are in close proximity, to glucose receptors; (3) highly concentrated glucose activates the calcium channel to stimulate dopamine release from P12 cells; (4) a significant correlation between blood glucose and cerebrospinal fluid concentrations of homovanillic acid the dopamine metabolite; (5) 2-deoxyglucose (2DG), the glucose analog, in pharmacological doses is associated with enhanced dopamine turnover and causes acute glucoprivation. Evidence from animal studies and fMRI in humans support the hypothesis that multiple, but similar brain circuits are disrupted in obesity and drug dependence and for the most part, implicate the involvement of DA-modulated reward circuits in pathologic eating behaviors. Based on a consensus of neuroscience research treatment of both glucose and drug like cocaine, opiates should incorporate dopamine agonist therapy in contrast to current theories and practices that utilizes dopamine antagonistic therapy. Considering that up until now clinical utilization of powerful dopamine D2 agonists have failed due to chronic down regulation of D2 receptors newer targets based on novel less powerful D2 agonists that up-regulate D2 receptors seems prudent. We encourage new strategies targeted at improving DA function in the treatment and prevention of obesity a subtype of reward deficiency.

A novel aminotetralin-type serotonin (5-HT) 2C receptor-specific agonist and 5-HT2A competitive antagonist/5-HT2B inverse agonist with preclinical efficacy for psychoses

Development of 5-HT2C agonists for treatment of neuropsychiatric disorders, including psychoses, substance abuse, and obesity, has been fraught with difficulties, because the vast majority of reported 5-HT2C selective agonists also activate 5-HT2A and/or 5-HT2B receptors, potentially causing hallucinations and/or cardiac valvulopathy. Herein is described a novel, potent, and efficacious human 5-HT2C receptor agonist, (-)-trans-(2S,4R)-4-(3'[meta]-bromophenyl)-N,N-dimethyl-1,2,3,4-tetrahydronaphthalen-2-amine (-)-MBP), that is a competitive antagonist and inverse agonist at human 5-HT2A and 5-HT2B receptors, respectively. (-)-MBP has efficacy comparable to the prototypical second-generation antipsychotic drug clozapine in three C57Bl/6 mouse models of drug-induced psychoses: the head-twitch response elicited by [2,5]-dimethoxy-4-iodoamphetamine; hyperlocomotion induced by MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (dizocilpine maleate)]; and hyperlocomotion induced by amphetamine. (-)-MBP, however, does not alter locomotion when administered alone, distinguishing it from clozapine, which suppresses locomotion. Finally, consumption of highly palatable food by mice was not increased by (-)-MBP at a dose that produced at least 50% maximal efficacy in the psychoses models. Compared with (-)-MBP, the enantiomer (+)-MBP was much less active across in vitro affinity and functional assays using mouse and human receptors and also translated in vivo with comparably lower potency and efficacy. Results indicate a 5-HT2C receptor-specific agonist, such as (-)-MBP, may be pharmacotherapeutic for psychoses, without liability for obesity, hallucinations, heart disease, sedation, or motoric disorders.