The Imbalanced Plasticity Hypothesis of Schizophrenia-Related Psychosis: A Predictive Perspective

Modulation of mGlu5 reduces rewarding associative properties of nicotine via changes in mesolimbic plasticity: Relevance to comorbid cigarette smoking in psychosis

RATIONALE

Antipsychotic medications that are used to treat psychosis are often limited in their efficacy by high rates of severe side effects. Treatment success in schizophrenia is further complicated by high rates of comorbid nicotine use. Dopamine D2 heteroreceptor complexes have recently emerged as targets for the development of more efficacious pharmaceutical treatments for schizophrenia.

OBJECTIVE

The current study sought to explore the use of the positive allosteric modulator of the mGlu5 receptor 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) as a treatment to reduce symptoms related to psychosis and comorbid nicotine use.

METHODS

Neonatal treatment of animals with the dopamine D2-like receptor agonist quinpirole (NQ) from postnatal day (P)1-21 produces a lifelong increase in D2 receptor sensitivity, showing relevance to psychosis and comorbid tobacco use disorder. Following an 8-day conditioning paradigm, brain tissue in the mesolimbic pathway was analyzed for several plasticity markers, including brain derived neurotrophic factor (BDNF), phosphorylated p70 ribosomal S6 kinase (phospho-p70S6K), and cadherin-13 (Cdh13).

RESULTS

Pretreatment with CDPPB was effective to block enhanced nicotine conditioned place preference observed in NQ-treated animals. Pretreatment was additionally effective to block the nicotine-induced increase in BDNF and sex-dependent increases in cadherin-13 in the ventral tegmental area (VTA), as well as increased phospho-p70S6K in the nucleus accumbens (NAcc) shell found in NQ-treated animals.

CONCLUSION

In conjunction with prior work, the current study suggests positive allosteric modulation of the mGlu5 receptor, an emerging target for schizophrenia therapeutics, may be effective for the treatment of comorbid nicotine abuse in psychosis.

Reinstatement of nicotine conditioned place preference in a transgenerational model of drug abuse vulnerability in psychosis: Impact of BDNF on the saliency of drug associations

RATIONALE

Psychotic disorders such as schizophrenia are often accompanied by high rates of cigarette smoking, reduced quit success, and high relapse rates, negatively affecting patient outcomes. However, the mechanisms underlying altered relapse-like behaviors in psychosis are poorly understood.

OBJECTIVES

The present study analyzed changes in extinction and reinstatement of nicotine conditioned place preference (CPP) and resulting changes in brain-derived neurotrophic factor (BDNF) in a novel heritable rodent model of psychosis, demonstrating increased dopamine D₂ receptor sensitivity, to explore mechanisms contributing to changes in relapse-like behaviors.

METHODS

Male and female offspring of two neonatal quinpirole-treated (1 mg/kg quinpirole from postnatal day (P)1-21; QQ) and two neonatal saline-treated (SS) Sprague-Dawley rats (F1 generation) were tested on an extended CPP paradigm to analyze extinction and nicotine-primed reinstatement. Brain tissue was analyzed 60 min after the last nicotine injection for BDNF response in the ventral tegmental area (VTA), the infralimbic (IfL) and prelimbic (PrL) cortices.

RESULTS

F1 generation QQ offspring demonstrated delayed extinction and more robust reinstatement compared to SS control animals. In addition, QQ animals demonstrated an enhanced BDNF response to nicotine in the VTA, IfL and Prl cortices compared to SS offspring.

CONCLUSIONS

This study is the first to demonstrate altered relapse-like behavior in a heritable rodent model with relevance to comorbid drug abuse and psychosis. This altered pattern of behavior is hypothesized to be related to elevated activity-dependent BDNF in brain areas associated with drug reinforcement during conditioning that persists through the extinction phase, rendering aberrantly salient drug associations resistant to extinction and enhancing relapse vulnerability.

Biological hypotheses, risk factors, and biomarkers of schizophrenia

Both the discovery of biomarkers of schizophrenia and the verification of biological hypotheses of schizophrenia are an essential part of the process of understanding the etiology of this mental disorder. Schizophrenia has long been considered a neurodevelopmental disease whose symptoms are caused by impaired synaptic signal transduction and brain neuroplasticity. Both the onset and chronic course of schizophrenia are associated with risk factors-induced disruption of brain function and the establishment of a new homeostatic setpoint characterized by biomarkers. Different risk factors and biomarkers can converge to the same symptoms of schizophrenia, suggesting that the primary cause of the disease can be highly individual. Schizophrenia-related biomarkers include measurable biochemical changes induced by stress (elevated allostatic load), mitochondrial dysfunction, neuroinflammation, oxidative and nitrosative stress, and circadian rhythm disturbances. Here is a summary of selected valid biological hypotheses of schizophrenia formulated based on risk factors and biomarkers, neurodevelopment, neuroplasticity, brain chemistry, and antipsychotic medication. The integrative neurodevelopmental-vulnerability-neurochemical model is based on current knowledge of the neurobiology of the onset and progression of the disease and the effects of antipsychotics and psychotomimetics and reflects the complex and multifactorial nature of schizophrenia.