Lack of antidepressant effects of burst-suppressing isoflurane anesthesia in adult male Wistar outbred rats subjected to chronic mild stress

Pimavanserin treatment increases plasma brain-derived neurotrophic factor levels in rats

Background

Pimavanserin, a serotonin 5HT-2A receptor inverse agonist is the first-line, FDA-approved treatment of hallucinations and delusions associated with Parkinson's Disease psychosis (PDP), which occurs in up to 50% of PD patients. The neurobiological mechanism underlying the therapeutic effectiveness of Pimavanserin in PDP remains unknown. Several earlier studies have shown that treatment with 5HT-2A antagonists and other drugs acting on the serotonergic system such as SSRIs increase Brain derived neurotrophic factor (BDNF) levels in rodents. BDNF is synthesized as the precursor proBDNF, that undergoes cleavage intra or extracellularly to produce a mature BDNF (mBDNF) protein. mBDNF is believed to play a key role in neuroplasticity and neurogenesis. The present study tested the hypothesis that treatment with Pimavanserin is associated with higher and sustained elevations of mBDNF.

Methods

Adult Sprague-Dawley male rats were treated with Pimavanserin, Fluoxetine or vehicle for 4 weeks (chronic) or 2 h (acute). BDNF levels were determined by enzyme-linked Immunosorbent assay (ELISA).

Results

We found significant increases in plasma mBDNF levels in rats following chronic Pimavanserin treatment, but not in Fluoxetine-treated rats. No significant changes in mBDNF levels were found in the prefrontal cortex or hippocampus following Pimavanserin or Fluoxetine treatment.

Conclusion

These findings suggest that increase in mBDNF levels could be a contributing mechanism for the neuroprotective potential of Pimavanserin.

Rethinking data treatment: The sucrose preference threshold for anhedonia in stress-induced rat models of depression

BACKGROUND

Exposing rats to repeated unpredictable stressors is a popular method for modelling depression. The sucrose preference test is used to assess the validity of this method, as it measures a rat´s preference for a sweet solution as an indicator of its ability to experience pleasure. Typically, if stressed rats show a lower preference compared to unstressed rats, it is concluded they are experiencing stress-induced anhedonia.

METHODS

While conducting a systematic review, we identified 18 studies that used thresholds to define anhedonia and to distinguish "susceptible" from "resilient" individuals. Based on their definitions, researchers either excluded "resilient" animals from further analyses or treated them as a separate cohort. We performed a descriptive analysis to understand the rationale behind these criteria.

RESULTS

we found that the methods used for characterizing the stressed rats were largely unsupported. Many authors failed to justify their choices or relied exclusively on referencing previous studies. When tracing back the method to its origins, we converged on a pioneering article that, although employed as a universal evidence-based justification, cannot be regarded as such. What is more, through a simulation study, we provided evidence that removing or splitting data, based on an arbitrary threshold, introduces statistical bias by overestimating the effect of stress.

CONCLUSION

Caution must be exercised when implementing a predefined cut-off for anhedonia. Researchers should be aware of potential biases introduced by their data treatment strategies and strive for transparent reporting of methodological decisions.

Nitrous Oxide, a Rapid Antidepressant, Has Ketamine-like Effects on Excitatory Transmission in the Adult Hippocampus

BACKGROUND

Nitrous oxide (N2O) is a noncompetitive inhibitor of NMDA receptors that appears to have ketamine-like rapid antidepressant effects in patients with treatment-resistant major depression. In preclinical studies, ketamine enhances glutamate-mediated synaptic transmission in the hippocampus and prefrontal cortex. In this study, we examined the effects of N2O on glutamate transmission in the hippocampus and compared its effects to those of ketamine.

METHODS

Glutamate-mediated synaptic transmission was studied in the CA1 region of hippocampal slices from adult albino rats using standard extracellular recording methods. Effects of N2O and ketamine at subanesthetic concentrations were evaluated by acute administration.

RESULTS

Akin to 1 μM ketamine, 30% N2O administered for 15-20 minutes resulted in persistent enhancement of synaptic responses mediated by both AMPA receptors and NMDA receptors. Synaptic enhancement by both N2O and ketamine was blocked by co-administration of a competitive NMDA receptor antagonist at saturating concentration, but only ketamine was blocked by an AMPA receptor antagonist. Synaptic enhancement by both agents involved TrkB (tropomyosin receptor kinase B), mTOR (mechanistic target of rapamycin), and NOS (nitric oxide synthase) with some differences between N2O and ketamine. N2O potentiation occluded enhancement by ketamine, and in vivo N2O exposure occluded further potentiation by both N2O and ketamine.

CONCLUSIONS

These results indicate that N2O has ketamine-like effects on hippocampal synaptic function at a subanesthetic, but therapeutically relevant concentration. These 2 rapid antidepressants have similar, but not identical mechanisms that result in persisting synaptic enhancement, possibly contributing to psychotropic actions.

Isoflurane Rescue Schizophrenia-Related Deficits through Parvalbumin-Positive Neurons in the Dentate Gyrus

The therapeutic effects of volatile anesthetics on mental diseases, particularly schizophrenia, have gained considerable interest. Although isoflurane is a commonly used volatile anesthetic, there’s no more evidence that it could work on treating schizophrenia. Here, we discovered that inhaling isoflurane at low concentrations might reverse the behavioral phenotypes of schizophrenia caused by MK801, such as hyperlocomotion, pre-pulse inhibition impairment, and working memory loss. Isoflurane also helped recovering adult neurogenesis and synaptic plasticity impairments in the dentate gyrus (DG) induced by MK801. To better understand the mechanism, we discovered that isoflurane could reverse the reduction of parvalbumin (PV)-positive GABAergic interneuron (PVI) number and the aberration of NRG1-ErbB4 signaling in the DG; however, isoflurane could not reverse the schizophrenia-related phenotypes caused by PVI ablation, indicating that PVI are necessary for the therapeutic effect of isoflurane. Interestingly, isoflurane could reverse phenotypes caused by blocking PVIs GABA release in the DG, indicating the therapeutic impact is independent of PVI GABA release. Our research revealed that isoflurane might be used to treat schizophrenia, possibly through PVI in the DG.

Chronic mild stress paradigm as a rat model of depression: facts, artifacts, and future perspectives

RATIONALE

The chronic mild stress (CMS) paradigm was first described almost 40 years ago and has become a widely used model in the search for antidepressant drugs for major depression disorder (MDD). It has resulted in the publication of almost 1700 studies in rats alone. Under the original CMS procedure, the expression of an anhedonic response, a key symptom of depression, was seen as an essential feature of both the model and a depressive state. The prolonged exposure of rodents to unpredictable/uncontrollable mild stressors leads to a reduction in the intake of palatable liquids, behavioral despair, locomotor inhibition, anxiety-like changes, and vegetative (somatic) abnormalities. Many of the CMS studies do not report these patterns of behaviors, and they often fail to include consistent molecular, neuroanatomical, and physiological phenotypes of CMS-exposed animals.

OBJECTIVES

To critically review the CMS studies in rats so that conceptual and methodological flaws can be avoided in future studies.

RESULTS

Analysis of the literature supports the validity of the CMS model and its impact on the field. However, further improvements could be achieved by (i) the stratification of animals into 'resilient' and 'susceptible' cohorts within the CMS animals, (ii) the use of more refined protocols in the sucrose test to mitigate physiological and physical artifacts, and (iii) the systematic evaluation of the non-specific effects of CMS and implementation of appropriate adjustments within the behavioral tests.

CONCLUSIONS

We propose methodological revisions and the use of more advanced behavioral tests to refine the rat CMS paradigm, which offers a valuable tool for developing new antidepressant medications.