The Chemogenetic Receptor Ligand Clozapine N-Oxide Induces<em> in vivo</em> Neuroreceptor Occupancy and Reduces Striatal Glutamate Levels

Chemogenetic studies with the ligand clozapine N-oxide (CNO) are predicated upon the assumption that CNO is devoid of actions at natural neuroreceptors. However, recent evidence shows that CNO may be converted back to clozapine (CLZ) in vivo, which could yield plasma concentrations that may be sufficient to occupy inter alia dopamine D2=3 and serotonin 5HT2A receptors in living brain. To test this phenomenon, we measured striatal dopamine D2=3 receptor occupancy with [18F]fallypride PET and serotonin 5HT2A occupancy ex vivo using [18F]MH.MZ. We found a CNO dosedependent effect on the availability of both neuroreceptor sites. In parallel MR spectroscopy experiments, we found that CNO reduced creatine C phosphcreatine (CrCPCr) and increased N-acetylaspartate C N-acetylaspartylglutamate (NAACNAAG) signals in the prefrontal cortex, and also reduced the glutamate signal in dorsal striatum, with peak effect at 2 mg/kg. Thus, our findings suggest that conversion of CNO to CLZ in living rats imparts significant occupancy at endogenous neuroreceptors and significant changes to neurometabolite levels

A pilot study of cerebral metabolism and serotonin 5-HT2A receptor occupancy in rats treated with the psychedelic tryptamine DMT in conjunction with the MAO inhibitor harmine

Rationale: The psychedelic effects of the traditional Amazonian botanical decoction known as ayahuasca are often attributed to agonism at brain serotonin 5-HT2A receptors by N,N-dimethyltryptamine (DMT). To reduce first pass metabolism of oral DMT, ayahuasca preparations additionally contain reversible monoamine oxidase A (MAO-A) inhibitors, namely β-carboline alkaloids such as harmine. However, there is lacking biochemical evidence to substantiate this pharmacokinetic potentiation of DMT in brain via systemic MAO-A inhibition.Objectives: We measured the pharmacokinetic profile of harmine and/or DMT in rat brain, and tested for pharmacodynamic effects on brain glucose metabolism and DMT occupancy at brain serotonin 5-HT2A receptors.Methods: We first measured brain concentrations of harmine and DMT after treatment with harmine and/or DMT at low sub-cutaneous doses (1 mg/kg each) or harmine plus DMT at moderate doses (3 mg/kg each). In the same groups of rats, we also measured ex vivo the effects of these treatments on the availability of serotonin 5-HT2A receptors in frontal cortex. Finally, we explored effects of DMT and/or harmine (1 mg/kg each) on brain glucose metabolism with [18F]FDG-PET.Results: Results confirmed that co-administration of harmine inhibited the formation of the DMT metabolite indole-3-acetic acid (3-IAA) in brain, while correspondingly increasing the cerebral availability of DMT. However, we were unable to detect any significant occupancy by DMT at 5-HT2A receptors measured ex vivo, despite brain DMT concentrations as high as 11.3 µM. We did not observe significant effects of low dose DMT and/or harmine on cerebral [18F]FDG-PET uptake.Conclusion: These preliminary results call for further experiments to establish the dose-dependent effects of harmine/DMT on serotonin receptor occupancy and cerebral metabolism

The Chemogenetic Receptor Ligand Clozapine N-Oxide Induces in vivo Neuroreceptor Occupancy and Reduces Striatal Glutamate Levels

Chemogenetic studies with the ligand clozapine N-oxide (CNO) are predicated upon the assumption that CNO is devoid of actions at natural neuroreceptors. However, recent evidence shows that CNO may be converted back to clozapine (CLZ) in vivo, which could yield plasma concentrations that may be sufficient to occupy inter alia dopamine D2/3 and serotonin 5HT2A receptors in living brain. To test this phenomenon, we measured striatal dopamine D2/3 receptor occupancy with [18F]fallypride PET and serotonin 5HT2A occupancy ex vivo using [18F]MH.MZ. We found a CNO dose-dependent effect on the availability of both neuroreceptor sites. In parallel MR spectroscopy experiments, we found that CNO reduced creatine + phosphcreatine (Cr+PCr) and increased N-acetylaspartate + N-acetylaspartylglutamate (NAA+NAAG) signals in the prefrontal cortex, and also reduced the glutamate signal in dorsal striatum, with peak effect at 2 mg/kg. Thus, our findings suggest that conversion of CNO to CLZ in living rats imparts significant occupancy at endogenous neuroreceptors and significant changes to neurometabolite levels

A single dose of psilocybin induces lasting changes in metabolic connectivity within biologically informed rat brain networks related to compulsions and anxiety.

Pharmacotherapy with psilocybin, a serotonergic psychedelic, shows promising benefits in a range of neuropsychiatric disorders. While its psychedelic effects last for a few hours, therapeutic effects of psilocybin can persist long after a single administration in clinical and preclinical studies. We tested the hypothesis that these therapeutic effects entail remodeling of biologically informed rat brain networks related to compulsions and anxiety. Specifically, the cortico-striato-thalamo-cortical (CSTC) network and the cortico-amygdalo-hippocampal-hypothalamus (CAHH) networks. We used serial positron emission tomography (PET) to assess the acute and persistent effects of a single psilocybin dose on glucose metabolism and metabolic connectivity in the CSTC and CAHH networks in rat brain, in relation to the in vivo occupancy at serotonin 5HT2A receptors after acute psilocybin challenge. Notably, we found metabolic changes in two of the main hubs in the CSTC network i.e. thalamus and dorsal striatum accompanied by a persistent change in metabolic connectivity between thalamus and medial prefrontal cortex which may indicate a shift in thalamocortical gating after acute administration of psilocybin. Within the CAHH network, we found an acute metabolic increase in insula after psilocybin administration and a persistent enhancement of metabolic connectivity between the insula and the medial prefrontal cortex, and the ventral hippocampus. Our findings of acute and persistent changes in metabolic activity and connectivity within these networks provide new insights towards understanding the mechanism of the therapeutic effects of psilocybin in a range of neuropsychiatric disorders