Title: Study of the regulation of synaptic transmission and long-term synaptic plasticity by the astrocyte in adult hippocampus.

Abstract : The functioning of the brain relies on communication between neurons at the level of specialized structures: the synapses. Information processing at synapses is plastic and finely modulated over time. In the hippocampus, this cellular plasticity is proposed as the cellular basis of learning and memory. It is important to note that neurons are not alone in the brain, glial cells perform many functions essential to its functioning. Among them, astrocytes, ideally positioned in between blood vessels and synapses, play a key role in modulating transmission and long-term synaptic plasticity. Indeed, works done in recent years have shown that astrocytes are able to detect neurotransmitters released in the synaptic cleft and in turn to regulate the efficiency with which neurons communicate. To do this, astrocytes, like neurons, release gliotransmitters at the synapse, as purines and D-serine. In the hippocampus, it has been shown in juvenile rats that astrocytes are able to regulate the efficiency of synaptic transmission at the level of a single synapse. In addition, each astrocyte occupies an exclusive territory allowing it to be in close apposition with more than 100,000 neighbouring synapses belonging to several neurons. Functionally, this may allow them to coordinate the regulation of transmission at the scale of 100,000 synapses.
The main goal of my thesis was to better understand how information is transmitted and regulated in the hippocampus in adulthood. To this end, I studied the role of astrocytes in regulating the efficiency of synaptic transmission at the single synapse and domain levels. Using electrophysiological techniques, combining pharmacological and viral approaches, the work I performed during my thesis revealed that:
(1) The astrocyte, in adulthood is still able to regulate the efficiency of basic synaptic transmission at the single synapse level. To do so, the astrocyte detects glutamate released in the synaptic cleft via the metabotropic glutamatergic receptor type 5 (mGluR5) and in return releases purines allowing it to increase the efficiency of synaptic transmission via their action on presynaptic adenosine receptors type A2A. Interestingly, this regulation seems to be activity-dependent since it is no longer present when the number of activated synapses increases.
(2) The mGluR5, even in the adult hippocampus, is a key receptor for the astrocyte since it is necessary for the mechanism of regulation of transmission by purines that I studied. Indeed, to better understand this regulatory loop, I used a new viral vector developed in collaboration with Dr. Carole Escartin's team. This vector allows to functionally block mGluR5 pathway specifically in astrocytes by the expression of a dominant negative peptide. This technique allowed me to demonstrate the importance of this astrocytic receptor in the regulation of basal synaptic transmission and long-term synaptic plasticity.
(3) In collaboration with Dr. Jean-Michel Revest's team, I study long-term synaptic plasticity and its alteration in the context of Down syndrome (DS). DS or trisomy 21, is the most common genetic cause of intellectual disability. In this study, I demonstrated that the mouse model of DS studied showed a deficit in long-term synaptic plasticity that was dependent on endocannabinoid receptor signalling (CB1) and correlated with a memory deficit. As CB1 signalling is important for the production of gliotransmitters necessary for the induction of plasticity, these results open new perspectives to understand the involvement of astrocytes in cognitive deficits of DS.

Keywords: Astrocytes ; Gliotransmission ; synaptic plasticity ; Electrophysiology ; mGluR5 ; Purines ; Down syndrome.
Date de la soutenance: 13/12/2021 - 14h30
Lieu: Neurocentre Magendie Seminar room

from Revest's lab will defend her thesis entitled 'Development and characterization of a new therapeutic approach of the Down Syndrome, systeme endocannabinoid)'

Summary
Down syndrome (DS) is the most common genetic cause of intellectual disability (ID) due to complete or partial triplication of human chromosome 21 (HSA21). Although many advances have improved the living conditions of patients, ID prevents them from leading a normal life. Despite several ongoing preclinical and clinical studies, no curative pharmacological solution to improve ID is available. The memory deficits associated with ID are due to a reduction in hippocampal volume (HPC) and in particular dysfunctions in GABAergic and glutamatergic transmissions. The lack of effective treatments for ID in Down's reflects the urgent need to identify new therapeutic targets and develop a translational test.
The endocannabinoid system (ECS) is involved in memory and learning processes. Recently, it has been suggested that alteration of the ECS, and in particular overexpression and hyperactivation of the cannabinoid type-1 (CB1) receptor, may play a major role in ID in DS mouse models. Indeed, aged Ts65Dn trisomic mice showed elevated levels of endocannabinoid 2-AG in the brain and CB1 in Ts65Dn and TgDyrk1A mice. Supporting this hypothesis, the CB1 antagonist rimonabant improved memory deficits in these mice. Therefore, these data in DS mouse models suggesting the involvement of ECS in cognitive deficits encouraged further exploration of the deregulation of CB1 activity.
Our team recently discovered an endogenous modulatory system involving the neurosteroid pregnenolone (Preg) that counteracts CB1 signalling activity blocking the toxic and addictive effects of the exogenous CB1 agonist, delta9-tetrahydrocannabinol (THC), thus providing a new therapeutic target for CB1-related pathologies. Although Preg has a suitable therapeutic pharmacological profile, it cannot be used as a drug, mainly due to its steroidal metabolism. Therefore, non-metabolizable synthetic analogues of Preg (AEFs) have been developed in collaboration with the biotech Aelis Farma and protected by patents (No. WO2019162328A1 and WO2020127468A1). AEFs retain the pharmacological properties of Preg against CB1 but with an improved and enhanced therapeutic profile. Indeed, AEFs have an increased half-life, are not metabolised into steroid hormones and have a very good bioavailability after oral administration. AEF0217 was investigated as a leading candidate for the treatment of DI in DS mice.
The objectives of this thesis were:
1) To identify the neurobiological basis underlying DI in DS;
2) To validate a translational test for the assessment of ID in Down's and Ts65Dn mice;
3) To propose a therapeutic solution for DI in DS.Our work confirmed the key role of ECS in DI in Ts65Dn mice. 1) We showed overexpression of CB1 in specific regions and cell populations of the dorsal HPC and decreased levels of Preg and the Cyp11a1 gene encoding the synthetic enzyme. Preg restored synaptic plasticity and memory deficits. CB1-induced inhibition of Erk1/2MAPK activity restored memory deficits in Ts65Dn mice. 2) We validated the radial maze procedure as a translational test to assess DI in Down's providing the preclinical and clinical basis for a new therapeutic approach. 3) AEF0217 restored cognitive deficits in several cognitive tests, including the translational radial maze test in Ts65Dn mice.In conclusion, the therapeutic profile of AEF0217 associated with the translational test constitutes promising assets towards a clinical application.

Jury
Silvia-Gines Padros, Professor; Neuroscience Institute, University of Barcelona, SP – Reviewer

Marie-Claude Poitier, PharmD, PhD; Institut du cerveau of Paris, FR – Reviewer

Yann Herault, PhD; IGBMC Institute, Strasbourg, FR – Examiner

Filippo Caraci, Professor, MD, PhD; Neuroscience Institute, University of Catania, IT – Examiner

Monique Vallée, PhD; Neurocentre Magendie, Bordeaux, FR – Guest

Jean-Michel Revest, PhD; Neurocentre Magendie, Bordeaux, FR – Supervisor
Date de la soutenance: 23/07/2021 - 10h00
Lieu: A huis clos

from Revest's lab (Thesis directed by Umberto Spampinato) will defend her thesis a presentation entitled 'Role of central serotonin receptors in the regulation of ascending dopaminergic pathways: implication in the effects of cocaine.'

Link to the Zoom meeting will be sent on the liste.u1215 mailing list.

Abstract
This thesis investigates the functional role of the central serotonin receptor2B (5-HT2B) in the regulation of ascending dopaminergic (DA) pathways. Recent results from the laboratory have shown that 5-HT2B receptors differentially modulate ascending DA pathways. Indeed, 5-HT2B receptor antagonists reduce DA release in the nucleus accumbens (NAc), increase DA release in the medial prefrontal cortex (mPFC), and do not alter striatal DA release. This differential control of the mesocorticolimbic DA system involves a functional interaction between 5-HT2B receptors in the dorsal raphe (DR) and 5-HT1A receptors expressed in the CPFm, and results from activation of 5-HT neurons in the DR projecting to the CPFm. These results identified the RD as the major site of action of 5-HT2B receptors in controlling the activity of 5-HT and DA neurons. Furthermore, it was shown that blocking 5-HT2B receptors controls the neurochemical and behavioural responses induced by psychostimulants such as amphetamine, 3,4-methylenedioxymethamphine and cocaine, one of the most widely used drugs in the world. Thus, blocking 5-HT2B receptors suppresses the hyperlocomotion caused by cocaine. This effect, which occurs independently of DA release in the NAc and striatum, where DA activity is closely linked to cocaine-induced behavioural effects, may involve a postsynaptic interaction at the level of DA neurotransmission in subcortical regions. However, (1) the involvement of the CPFm DA in this interaction remains to be determined, given that this brain region is known to have anatomical and functional relationships with the NAc and striatum, and to be involved in cocaine behavioural responses. Furthermore, (2) the cellular localisation of 5-HT2B receptors within the DR and the cellular mechanisms underlying their interactions between the DA and 5-HT networks are unknown at the start of this thesis work.

Thus, the objective of this thesis is to address both of the above points. To this end, we studied the effects of two potent and selective 5-HT2B receptor antagonists (RS 127445 and LY 266097) on 5-HT and DA activity, using neurochemical, cellular and behavioural approaches in rats.

A first group of experiments provided anatomical and functional evidence that 5-HT2B receptors on the DR exert a tonic inhibitory control of 5-HT neurons innervating the CPFm, dependent on GABA interneurons. This 5-HT control is the first step in a complex poly-synaptic regulation leading to the differential control of mesocorticolimbic DA pathways. A second group of experiments shows that blocking 5-HT2B receptors in the DR inhibits cocaine-induced hyperlocomotion by acting at the level of DA neurotransmission in the NAc; this effect results from the potentiation of cocaine-induced DA release in the CPFm.

In conclusion, the work accomplished over the past three years provides substantial information regarding the functional role of the 5-HT2B receptor in the regulation of ascending DA pathways. Furthermore, our results not only improve the understanding of the DA and 5-HT interaction, but also highlight the therapeutic potential of 5-HT2B receptor antagonists for the treatment of cocaine addiction.

Key words: Serotonin, serotonin2B receptors, dopamine, cocaine, rat

Jury
Sgambato Véronique: Research Fellow (Lyon) - Rapporteur

Maroteaux Luc: Research Director (Paris) - Rapporteur

Cota Daniela : Research Director (Bordeaux) - Examiner

Voisin Daniel: University Professor (Bordeaux) - Examiner

Artigas Francesc: University Professor (Barcelona) - Guest

Spampinato Umberto: University Professor (Bordeaux) - Supervisor

Date de la soutenance: 17/03/2021 - 14h00
Lieu: Visioconférence Zoom

Pour plus de détails: https://www.bordeaux-neurocampus.fr/event/soutenance-de-theseadeline-cathala/

The competition of the SVS doctoral school of the University of Bordeaux has given its verdict: the Neurocentre gleaned 4 of the 16 doctoral contracts. Congratulations to Mohamed-Lyès KACI (Eq. Abrous, Thesis Director Muriel Koehl) who ranks 5th, Emma MESGUICH (Eq. Marsicano, Thesis Director France Chaouloff) who ranks 7th, Pierre-Louis RAUX (Eq. Revest, Thesis Director Monique Vallée) who ranks 11th and Yifan WU (Eq. Beyeler) who ranks 17th. They will join the centre's teams from next school year.