The role of N-methyl-d-aspartate receptors and metabotropic glutamate receptor 5 in the prepulse inhibition paradigms for studying schizophrenia: pharmacology, neurodevelopment, and genetics

Understanding the role of the NMDA receptor subunit, GluN2D, in mediating NMDA receptor antagonist-induced behavioral disruptions in male and female mice

Noncompetitive NMDA receptor (NMDAR) antagonists like phencyclidine (PCP) and ketamine cause psychosis-like symptoms in healthy humans, exacerbate schizophrenia symptoms in people with the disorder, and disrupt a range of schizophrenia-relevant behaviors in rodents, including hyperlocomotion. This is negated in mice lacking the GluN2D subunit of the NMDAR, suggesting the GluN2D subunit mediates the hyperlocomotor effects of these drugs. However, the role of GluN2D in mediating other schizophrenia-relevant NMDAR antagonist-induced behavioral disturbances, and in both sexes, is unclear. This study aimed to investigate the role of the GluN2D subunit in mediating schizophrenia-relevant behaviors induced by a range of NMDA receptor antagonists. Using both male and female GluN2D knockout (KO) mice, we examined the effects of the NMDAR antagonist's PCP, the S-ketamine enantiomer (S-ket), and the ketamine metabolite R-norketamine (R-norket) on locomotor activity, anxiety-related behavior, and recognition and short-term spatial memory. GluN2D-KO mice showed a blunted locomotor response to R-norket, S-ket, and PCP, a phenotype present in both sexes. GluN2D-KO mice of both sexes showed an anxious phenotype and S-ket, R-norket, and PCP showed anxiolytic effects that were dependent on sex and genotype. S-ket disrupted spatial recognition memory in females and novel object recognition memory in both sexes, independent of genotype. This datum identifies a role for the GluN2D subunit in sex-specific effects of NMDAR antagonists and on the differential effects of the R- and S-ket enantiomers.

Different maturation patterns for sensorimotor gating and startle habituation deficits in male and female RHA vs RLA rats

Neurodevelopmental anomalies are thought to play a crucial role in the emergence of schizophrenia. The Roman high-avoidance (RHA) rats exhibit impaired prepulse inhibition (PPI), as well as other behavioral and cognitive singularities related to schizophrenia syndromes compared to the Roman low-avoidance (RLA) rats. In the present study, we aimed at elucidating whether PPI deficits in the RHA rats take place during prepubescence, adolescence, or adulthood. Thus, we evaluated the levels of PPI of both strains and both sexes during these three developmental phases. Additionally, we also investigated the onset of startle habituation deficits in the same groups. The results showed that male RHA rats exhibit a clear-cut PPI reduction compared to their RLA counterparts in adulthood. In female RHA rats, we observed lower levels of PPI since adolescence and through adulthood. We also found no differences between PPI percentages among the three ages in RHA male rats. Contrarily, in male RLA rats, PPI levels were increased in adults compared to their adolescent and prepubescent counterparts. Finally, a deficit in startle habituation was observed in adulthood of both male and female RHA rats, although in the latter case the disturbance in startle habituation was more profound. These results further the description of the maturational trajectory of cognitive markers relevant to schizophrenia prodrome and they add face validity to the RHA rats as a model of schizophrenia-relevant phenotypes.

Looming Effects on Attentional Modulation of Prepulse Inhibition Paradigm

In a hazardous environment, it is fundamentally important to successfully evaluate the motion of sounds. Previous studies demonstrated "auditory looming bias" in both macaques and humans, as looming sounds that increased in intensity were processed preferentially by the brain. In this study on rats, we used a prepulse inhibition (PPI) of the acoustic startle response paradigm to investigate whether auditory looming sound with intrinsic warning value could draw attention of the animals and dampen the startle reflex caused by the startling noise. We showed looming sound with a duration of 120 ms enhanced PPI compared with receding sound with the same duration; however, when both sound types were at shorter duration/higher change rate (i.e., 30 ms) or longer duration/lower rate (i.e., more than 160 ms), there was no PPI difference. This indicates that looming sound-induced PPI enhancement was duration dependent. We further showed that isolation rearing impaired the abilities of animals to differentiate looming and receding prepulse stimuli, although it did not abolish their discrimination between looming and stationary prepulse stimuli. This suggests that isolation rearing compromised their assessment of potential threats from approaching objects and receding objects.

The glutamatergic synapse: a complex machinery for information processing

Being the most abundant synaptic type, the glutamatergic synapse is responsible for the larger part of the brain's information processing. Despite the conceptual simplicity of the basic mechanism of synaptic transmission, the glutamatergic synapse shows a large variation in the response to the presynaptic release of the neurotransmitter. This variability is observed not only among different synapses but also in the same single synapse. The synaptic response variability is due to several mechanisms of control of the information transferred among the neurons and suggests that the glutamatergic synapse is not a simple bridge for the transfer of information but plays an important role in its elaboration and management. The control of the synaptic information is operated at pre, post, and extrasynaptic sites in a sort of cooperation between the pre and postsynaptic neurons which also involves the activity of other neurons. The interaction between the different mechanisms of control is extremely complicated and its complete functionality is far from being fully understood. The present review, although not exhaustively, is intended to outline the most important of these mechanisms and their complexity, the understanding of which will be among the most intriguing challenges of future neuroscience.

Transgenerational evidence of increases in dopamine D2 receptor sensitivity in rodents: Impact on sensorimotor gating, the behavioral response to nicotine and BDNF

BACKGROUND/AIMS

Neonatal quinpirole (NQ) treatment to rats increases dopamine D₂ (DAD₂) receptor sensitivity in adult animals. We investigated if increased DAD₂ sensitivity would be passed to the next (F1) generation, and if these animals demonstrated sensorimotor gating deficits and enhanced behavioral responses to nicotine.

METHODS

Male and female rats were intraperitoneal (IP) administered quinpirole (1 mg/kg) or saline (NS) from postnatal day (P)1-21. Animals were either behaviorally tested (F0) or raised to P60 and mated, creating F1 offspring.

RESULTS

Experiment 1 revealed that F1 generation animals that were the offspring of at least one NQ-treated founder increased yawning behavior, a DAD₂-mediated behavioral event, in response to acute quinpirole (0.1 mg/kg). F1 generation rats also demonstrated increased striatal β arrestin-2 and decreased phospho-AKT signaling, consistent with increased G-protein independent DAD₂ signaling, which was equal to F0 NQ-treated founders, although this was not observed in all groups. RNA-Seq analysis revealed significant gene expression changes in the F1 generation that were offspring of both NQ-treated founders compared to F0 NQ founders and controls, with enrichment in sensitivity to stress hormones and cell signaling pathways. In Experiment 2, all F1 generation offspring demonstrated sensorimotor gating deficits compared to controls, which were equivalent to F0 NQ-treated founders. In Experiment 3, all F1 generation animals demonstrated enhanced nicotine behavioral sensitization and nucleus accumbens (NAcc) brain-derived neurotrophic factor (BDNF) protein. Further, F1 generation rats demonstrated enhanced adolescent nicotine conditioned place preference equivalent to NQ-treated founders conditioned with nicotine.

CONCLUSIONS

This represents the first demonstration of transgenerational effects of increased DAD₂ sensitivity in a rodent model.

Prepulse inhibition based grouping of rats and assessing differences in response to pharmacological agents

Schizophrenia modeling by disrupting prepulse inhibition (PPI) is one of the most frequently used psycho-pharmacological methods by administering pharmacological agents to stimulate disruption. However, since PPI is also a biological indicator of schizophrenia, it is possible to classify subjects based on their basal PPI values and group them as "low inhibition" and "high inhibition without taking any pharmacological agent. Therefore this study was conducted to show that rats can be divided into groups in terms of susceptibility to schizophrenia according to basal PPI values. It was also observed that these groups might give different responses to different pharmacological agents (apomorphine, amphetamine, MK-801, scopolamine, nicotine, caffeine). Male Sprague Dawley rats (250-350 g) were used in the study. To examine the effects of different pharmacological agents on the groups, apomorphine (0.5 mg/kg and 1 mg/kg), amphetamine (4 mg/kg), MK-801 (0.05 mg/kg and 0.15 mg/kg), scopolamine (0.4 mg/kg), nicotine (1 mg/kg) and caffeine (10 mg/kg and 30 mg/kg) were used. Amphetamine showed a disruptive effect on PPI in both low and high inhibitory groups, while apomorphine, MK-801, scopolamine, and nicotine showed PPI decrease only in the high inhibitory group. Besides, caffeine decreased PPI levels at two doses in the high inhibitory group; however, 10 mg/kg dose caffeine was increased only in the low inhibitory group. According to the data obtained from this study, rats can be grouped with baseline inhibition values by using PPI, and response differences of pharmacological agents to groups may vary.

Behavioral Tasks Evaluating Schizophrenia-like Symptoms in Animal Models: A Recent Update

BACKGROUND

Schizophrenia is a serious mental illness that affects more than 21 million people worldwide. Both genetics and the environment play a role in its etiology and pathogenesis. Symptoms of schizophrenia are mainly categorized into positive, negative, and cognitive. One major approach to identify and understand these diverse symptoms in humans has been to study behavioral phenotypes in a range of animal models of schizophrenia.

OBJECTIVE

We aimed to provide a comprehensive review of the behavioral tasks commonly used for measuring schizophrenia-like behaviors in rodents together with an update of the recent study findings.

METHODS

Articles describing phenotypes of schizophrenia-like behaviors in various animal models were collected through a literature search in Google Scholar, PubMed, Web of Science, and Scopus, with a focus on advances over the last 10 years.

RESULTS

Numerous studies have used a range of animal models and behavioral paradigms of schizophrenia to develop antipsychotic drugs for improved therapeutics. In establishing animal models of schizophrenia, the candidate models were evaluated for schizophrenia-like behaviors using several behavioral tasks for positive, negative, and cognitive symptoms designed to verify human symptoms of schizophrenia. Such validated animal models were provided as rapid preclinical avenues for drug testing and mechanistic studies.

CONCLUSION

Based on the most recent advances in the field, it is apparent that a myriad of behavior tests are needed to confirm and evaluate the congruency of animal models with the numerous behaviors and clinical signs exhibited by patients with schizophrenia.

Characterization of a New Positive Allosteric Modulator of AMPA Receptors - PAM-43: Specific Binding of the Ligand and its Ability to Potentiate AMPAR Currents

BACKGROUND

Currently, the most dynamic areas in the glutamate receptor system neurobiology are the identification and development of positive allosteric modulators (PAMs) of glutamate ionotropic receptors. PAM-based drugs are of great interest as promising candidates for the treatment of neurological diseases, such as epilepsy, Alzheimer's disease, schizophrenia, etc. Understanding the molecular mechanisms underlying the biological action of natural and synthetic PAMs is a key point for modifying the original chemical compounds as well as for new drug design.

OBJECTIVE

We are trying to elaborate a system of molecular functional screening of ionotropic glutamate receptor probable PAMs.

METHODS

The system will be based on the radioligand - receptor method of analysis and will allow rapid quantification of new AMPAR probable PAMs molecular activity. We plan to use a tritiumlabeled analogue of recently elaborated ionotropic GluR probable PAM ([3H]PAM-43) as the main radioligand.

RESULTS

Here, we characterized the specific binding of the ligand and its ability to potentiate ionotropic GluR currents. The existence of at least two different sites of [3H]PAM-43 specific binding has been shown. One of the above sites is glutamate-dependent and is characterized by higher affinity. "Patchclamp" technique showed the ability of PAM-43 to potentiate ionotropic GluR currents in rat cerebellar Purkinje neurons in a concentration-dependent manner.

CONCLUSION

The possibility of using PAM-43 as a model compound to study different allosteric effects of potential regulatory drugs (AMPAR allosteric regulators) was shown. [3H]PAM-43 based screening system will allow rapid selection of new AMPAR probable PAM structures and quantification of their molecular activity.

Comparative psychopharmacology of autism and psychotic-affective disorders suggests new targets for treatment

The first treatments showing effectiveness for some psychiatric disorders, such as lithium for bipolar disorder and chlorpromazine for schizophrenia, were discovered by accident. Currently, psychiatric drug design is seen as a scientific enterprise, limited though it remains by the complexity of brain development and function. Relatively few novel and effective drugs have, however, been developed for many years. The purpose of this article is to demonstrate how evolutionary biology can provide a useful framework for psychiatric drug development. The framework is based on a diametrical nature of autism, compared with psychotic-affective disorders (mainly schizophrenia, bipolar disorder and depression). This paradigm follows from two inferences: (i) risks and phenotypes of human psychiatric disorders derive from phenotypes that have evolved along the human lineage and (ii) biological variation is bidirectional (e.g. higher vs lower, faster vs slower, etc.), such that dysregulation of psychological traits varies in two opposite ways. In this context, the author review the evidence salient to the hypothesis that autism and psychotic-affective disorders represent diametrical disorders in terms of current, proposed and potential psychopharmacological treatments. Studies of brain-derived neurotrophic factor, the PI3K pathway, the NMDA receptor, kynurenic acid metabolism, agmatine metabolism, levels of the endocannabinoid anandamide, antidepressants, anticonvulsants, antipsychotics, and other treatments, demonstrate evidence of diametric effects in autism spectrum disorders and phenotypes compared with psychotic-affective disorders and phenotypes. These findings yield insights into treatment mechanisms and the development of new pharmacological therapies, as well as providing an explanation for the longstanding puzzle of antagonism between epilepsy and psychosis. Lay Summary: Consideration of autism and schizophrenia as caused by opposite alterations to brain development and function leads to novel suggestions for pharmacological treatments.

Both Val158Met Polymorphism of Catechol-O-Methyltransferase Gene and Menstrual Cycle Affect Prepulse Inhibition but Not Attentional Modulation of Prepulse Inhibition in Younger-Adult Females

Prepulse inhibition (PPI) can be modulated by both the Val158Met (rs4680) polymorphism of the Catechol-O-Methyltransferase (COMT) gene and the menstrual-cycle-related hormone fluctuations, each of which affects the subcortical/cortical dopamine metabolism. PPI can also be modulated by attention. The attentional modulation of PPI (AMPPI) is sensitive to psychoses. Whether the Val158Met polymorphism affects the AMPPI in female adults at different menstrual-cycle phases is unknown. This study examined whether AMPPI and/or PPI are affected by the Val158Met polymorphism in 177 younger-adult females whose menstrual cycles were mutually different across the menstruation, proliferative, or secretory phases. The AMPPI was evaluated by comparing PPI under the condition of the auditory precedence-effect-induced perceptual spatial separation between the prepulse stimulus and a masking noise (PPI_PSS) against that under the condition of the precedence-effect-induced perceptual spatial co-location (PPI_PSC). The results showed that both the menstrual cycle and the COMT Val158Met polymorphism affected both PPI_PSC and PPI_PSS, but not the AMPPI (difference between PPI_PSS and PPI_PSC). Moreover, throughout the menstrual cycle, both PPI_PSC and PPI_PSS decreased monotonously in Val/Val-carrier participants. However, the decreasing pattern was not overserved in either Met/Met-carrier or Met/Val-carrier participants. Thus, in healthy younger-adult females, PPI_PSC and PPI_PSS, but not the AMPPI, is vulnerable to changes of ovarian hormones, and the COMT Val158Met polymorphism also has a modulating effect on this menstrual-cycle-dependent PPI variation. In contrast, the AMPPI seems to be more steadily trait-based, less vulnerable to ovarian hormone fluctuations, and may be useful in assisting the diagnosis of schizophrenia in female adults.