The smoking cessation drug varenicline improves deficient P20-N40 inhibition in DBA/2 mice

Procognitive effects of varenicline in the animal model of schizophrenia depend on α4β2- and α 7-nicotinic acetylcholine receptors

BACKGROUND

Varenicline, a partial agonist of the α4β2 nicotinic acetylcholine receptor (α4β2-nAChR), is currently used to facilitate smoking cessation. Preclinical and clinical studies have suggested that this compound may also be effective in treating cognitive impairments in schizophrenia. However, it is unclear which nicotinic acetylcholine receptor subtypes may be involved because varenicline is not only a partial agonist for α4β2-nAChRs but also a full agonist for α7 nicotinic acetylcholine receptors (α7-nAChRs).

AIM

We investigated the effects of varenicline, compared to the α4β2-nAChR partial agonist TC-2403 and the α7-nAChR full agonist PNU-282987, in a ketamine-based model of schizophrenia-like cognitive deficits on the attentional set-shifting task in rats. The second goal was to elucidate whether the procognitive efficacy of varenicline was due to the compound's action on α4β2-nAChRs or α7-nAChRs.

METHODS

Ketamine was administered to rats for 10 consecutive days and the test was performed 14 days following the last injection. The tested compounds were administered 30 min prior to the attentional set-shifting task.

RESULTS

Varenicline, TC-2403 and PNU-282987 ameliorated ketamine-evoked set-shifting deficits. While the α4β2-nAChR antagonist dihydro-β-erythroidine and the α7-nAChR antagonist methyllycaconitine completely prevented the procognitive actions of TC-2403 and PNU-282987, respectively, varenicline's effect was only partially blocked by any given antagonist. Moreover, the combined treatment with TC-2403 and PNU-282987 more effectively facilitated rats' set-shifting ability than activation of either type of nicotinic acetylcholine receptor alone.

CONCLUSION

The present findings demonstrated that varenicline's actions on both α7-nAChRs and α4β2-nAChRs may be necessary to produce its full procognitive effect in the present experimental setting.

Effects of a ketogenic diet on auditory gating in DBA/2 mice: A proof-of-concept study

Although the ketogenic diet has shown promise in a pilot study and case report in schizophrenia, its effects in animal models of hypothesized disease mechanisms are unknown. This study examined effects of treatment with the ketogenic diet on hippocampal P20/N40 gating in DBA/2 mice, a translational endophenotype that mirrors inhibitory deficits in P50 sensory gating in schizophrenia patients. As expected, the diet increased blood ketone levels. Animals with the highest ketone levels showed the lowest P20/N40 gating ratios. These preliminary results suggest that the ketogenic diet may effectively target sensory gating deficits and is a promising area for additional research in schizophrenia.

Therapeutic Potential of α7 Nicotinic Acetylcholine Receptors

Progress in the fields of neuroscience and molecular biology has identified the forebrain cholinergic system as being important in many higher order brain functions. Further analysis of the genes encoding the nicotinic acetylcholine receptors (nAChRs) has highlighted, in particular, the role of α7 nAChRs in these higher order brain functions as evidenced by their peculiar physiologic and pharmacological properties. As this receptor has gained the attention of scientists from academia and industry, our knowledge of its roles in various brain and bodily functions has increased immensely. We have also seen the development of small molecules that have further refined our understanding of the roles of α7 nAChRs, and these molecules have begun to be tested in clinical trials for several indications. Although a large body of data has confirmed a role of α7 nAChRs in cognition, the translation of small molecules affecting α7 nAChRs into therapeutics has to date only progressed to the stage of testing in clinical trials. Notably, however, most recent human genetic and biochemical studies are further underscoring the crucial role of α7 nAChRs and associated genes in multiple organ systems and disease states. The aim of this review is to discuss our current knowledge of α7 nAChRs and their relevance as a target in specific functional systems and disease states.

The Role of Nicotine in Schizophrenia

Schizophrenia is associated with by severe disruptions in thought, cognition, emotion, and behavior. Patients show a marked increase in rates of smoking and nicotine dependence relative to nonaffected individuals, a finding commonly ascribed to the potential ameliorative effects of nicotine on symptom severity and cognitive impairment. Indeed, many studies have demonstrated improvement in patients following the administration of nicotine. Such findings have led to an increased emphasis on the development of therapeutic agents to target the nicotinic system as well as increasing the impetus to understand the genetic basis for nicotinic dysfunction in schizophrenia. The goal of this review article is to provide a critical summary of evidence for the role of the nicotinic system in schizophrenia. The first part will review the role of nicotine in normalization of primary dysfunctions and endophenotypical changes found in schizophrenia. The second part will provide a summary of genetic evidence linking polymorphisms in nicotinic receptor genes to smoking and schizophrenia. The third part will summarize attempts to treat schizophrenia using agents specifically targeting nicotinic and nicotinic receptor subtypes. Although currently available antipsychotic treatments are generally able to manage some aspects of schizophrenia (e.g., positive symptoms) they fail to address several other critically effected aspects of the disease. As such, the search for novel mechanisms to treat this disease is necessary.

Translational utility of rodent hippocampal auditory gating in schizophrenia research: a review and evaluation

Impaired gating of the auditory evoked P50 potential is one of the most pharmacologically well-characterized features of schizophrenia. This deficit is most commonly modeled in rodents by implanted electrode recordings from the hippocampus of the rodent analog of the P50, the P20-N40. The validity and effectiveness of this tool, however, has not been systematically reviewed. Here, we summarize findings from studies that have examined the effects of pharmacologic modulation on gating of the rodent hippocampal P20-N40 and the human P50. We show that drug effects on the P20-N40 are highly predictive of human effects across similar dose ranges. Furthermore, mental status (for example, anesthetized vs alert) does not appear to diminish the predictive capacity of these recordings. We then discuss hypothesized neuropharmacologic mechanisms that may underlie gating effects for each drug studied. Overall, this review supports continued use of hippocampal P20-N40 gating as a translational tool for schizophrenia research.

Maximizing the effect of an α7 nicotinic receptor PAM in a mouse model of schizophrenia-like sensory inhibition deficits

Positive allosteric modulators (PAMs) for the α7 nicotinic receptor hold promise for the treatment of sensory inhibition deficits observed in schizophrenia patients. Studies of these compounds in the DBA/2 mouse, which models the schizophrenia-related deficit in sensory inhibition, have shown PAMs to be effective in improving the deficit. However, the first published clinical trial of a PAM for both sensory inhibition deficits and related cognitive difficulties failed, casting a shadow on this therapeutic approach. The present study used both DBA/2 mice, and C3H Chrna7 heterozygote mice to assess the ability of the α7 PAM, PNU-120596, to improve sensory inhibition. Both of these strains of mice have reduced hippocampal α7 nicotinic receptor numbers and deficient sensory inhibition similar to schizophrenia patients. Low doses of PNU-120596 (1 or 3.33mg/kg) were effective in the DBA/2 mouse but not the C3H Chrna7 heterozygote mouse. Moderate doses of the selective α7 nicotinic receptor agonist, choline chloride (10 or 33mg/kg), were also ineffective in improving sensory inhibition in the C3H Chrna7 heterozygote mouse. However, combining the lowest doses of both PNU-120596 and choline chloride in this mouse model did improve sensory inhibition. We propose here that the difference in efficacy of PNU-120596 between the 2 mouse strains is driven by differences in hippocampal α7 nicotinic receptor numbers, such that C3H Chrna7 heterozygote mice require additional direct stimulation of the α7 receptors. These data may have implications for further clinical testing of putative α7 nicotinic receptor PAMs.

Evaluating the role of the alpha-7 nicotinic acetylcholine receptor in the pathophysiology and treatment of schizophrenia

The group of schizophrenia disorders affects approximately 1% of the population and has both genetic and environmental etiologies. Sufferers report various behavioral abnormalities including hallucinations and delusions (positive symptoms), reduced joy and amotivation (negative symptoms), plus inattention and poor learning (cognitive deficits). Despite the heterogeneous symptoms experienced, most patients smoke. The self-medication hypothesis posits that patients smoke to alleviate symptoms, consistent with evidence for nicotine-induced enhancement of cognition. While nicotine acts on multiple nicotinic acetylcholine receptors (nAChRs), the primary target of research is often the homomeric α7 nAChR. Given genetic linkages between schizophrenia and this receptor, its association with P50 sensory gating deficits, and its reduced expression in post-mortem brains, many have attempted to develop α7 nAChR ligands for treating schizophrenia. Recent evidence that ligands can be orthosteric agonists or positive allosteric modulators (PAMs) has revitalized the hope for treatment discovery. Herein, we present evidence regarding: (1) pathophysiological alterations of α7 nAChRs that might occur in patients; (2) mechanistic evidence for the normal action of α7 nAChRs; (3) preclinical studies using α7 nAChR orthosteric agonists and type I/II PAMs; and (4) where successful translational testing has occurred for particular compounds, detailing what is still required. We report that the accumulating evidence is positive, but that greater work is required using positron emission tomography to understand current alterations in α7 nAChR expression and their relationship to symptoms. Finally, cross-species behavioral tasks should be used more regularly to determine the predictive efficacy of treatments.