Csilla Lea Fazekas1,2,3,4, Bibiána Török1,2, Adrienn Szabó1,2,3, Pedro Correia1,2,3,Tiago Chaves1,2,3, Stéphanie Daumas4 and Dóra Zelena1,2
1 Institute of Experimental Medicine, Budapest, Hungary;
2 Centre for Neuroscience, Szentágothai Research Centre, Institute of Physiology, Medical School, University of Pécs, Pécs, Hungary
3 János Szentágothai Doctoral School of Neurosciences, Semmelweis University, Budapest, Hungary
4 Sorbonne Université, Neuroscience Paris Seine-Institut de Biologie Paris Seine (NPS-IBPS) INSERM, CNRS, Paris, France
Introduction: The median raphe region (MRR) is classically considered a serotonergic nucleus, even though a significant subpopulation of neurons is actually positive for the vesicular glutamate transporter type 3 (VGLUT3). Moreover, it is known that these cells project to the hippocampus and influence its oscillations. Our aim was to characterise the behavioural effects of MRR-VGLUT3+ neuronal activity during spatial learning and memory formation.
Methods: We utilised chemogenetics (DREADDs) and optogenetics (ChR2) in VGLUT3-Cre mice in the hippocampus dependent Morris water maze paradigm. VGLUT3-Cre male and female mice were injected with Cre-dependent adenoassociated viral vectors containing either control or excitatory sequences. During spatial reference memory (SRM) phase the ligand clozapine-N-oxide was injected daily to the chemogenetic groups. For optogenetics, excitation was at 20 Hz for 10 sec while on the platform. Short-term memory was assessed 10 mins after last learning trial, while long-term memory was studied at 72 hours.
Results: Manipulations of MRR-VGLUT3+ neurons did not impact learning in SRM task. Chemogenetic manipulation resulted in no differences between the groups in short-term memory. However, 72 hours later, the excitatory group performed similarly well as before and found the place of the platform significantly faster than controls. Previous optogenetic excitation did not affect memory retrieval during the probes. These results were confirmed by proper quadrant preference during short-term memory test for all groups, which was maintained only in the chemogenetically stimulated group during long-term memory probe.
Conclusion: Our data showed a role of MRR-VGLUT3+ neurons in long-term memory formation for which long, extended excitation is required.
Funding: This project was supported by Campus France, SE 250+ Excellence Scholarship, bilateral Balaton Program (2021-1.2.4-TÉT-2021-00010) and National Research Development and Innovation Office of Hungary (grant numbers K141934, K138763).