Abstract
In neuropharmacology, the recent concept of 'biased agonism' denotes the capacity of certain agonists to target-specific intracellular pathways of a given receptor in specific brain areas. In the context of serotonin pharmacotherapy, 5-HT1A receptor-biased agonists could be of great interest in several neuropsychiatric disorders. The aim of this study was to determine whether biased agonists could be differentiated in terms of regional targeting by use of simultaneous functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) brain imaging. We compared two 5-HT1A-biased agonists, NLX-112 and NLX-101, injected at three different doses in anaesthetised cats (n = 4). PET imaging was acquired for 90 min after bolus administration followed by constant infusion of the 5-HT1A radiotracer, [18F]MPPF. Drug occupancy was evaluated after injection at 50 min and BOLD fMRI was simultaneously acquired to evaluate subsequent brain activation patterns. 5-HT1A receptor occupancy was found to be dose-dependent for both agonists, but differed in magnitude and spatial distribution at equal doses with distinct BOLD patterns. Functional connectivity, as measured by BOLD signal temporal correlations between regions, was also differently modified by NLX-112 or NLX-101. Voxel-based correlation analyses between PET and fMRI suggested that NLX-112 stimulates both 5-HT1A autoreceptors and post-synaptic receptors, whereas NLX-101 preferentially stimulates post-synaptic cortical receptors. In cingulate cortex, the agonists induced opposite BOLD signal changes in response to receptor occupancy. These data constitute the first simultaneous exploration of 5-HT1A occupancy and its consequences in terms of brain activation, and demonstrates differential signalling by two 5-HT1A-biased agonists. Combined PET/fMRI represents a powerful tool in neuropharmacology, and opens new ways to address the concept of biased agonism by translational approaches.
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Acknowledgements
We thank Thomas Troalen (Siemens Healthiness France) for assistance with the PET/MR camera, Thibault Iecker and Christian Tourvielle for the production of [18F]MPPF and Véronique Gualda for the zootechnical assistance.
Funding:
This work was supported by the Fondation NEURODIS, the IHU CESAME and the French national programme ‘Investissement d’Avenir’ Programmes (LILI – Lyon Integrated Life Imaging: hybrid MR-PET ANR-11-EQPX-0026), and performed within the framework of the LABEX PRIMES (ANR-11-LABX-0063) of Université de Lyon, within the programme 'Investissements d’Avenir' (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR).
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Vidal, B., Fieux, S., Redouté, J. et al. In vivo biased agonism at 5-HT1A receptors: characterisation by simultaneous PET/MR imaging. Neuropsychopharmacol 43, 2310–2319 (2018). https://doi.org/10.1038/s41386-018-0145-2
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DOI: https://doi.org/10.1038/s41386-018-0145-2
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