Thesis defense – Viviana Villicaña-Muñoz

Venue : Centre Broca

Viviana Villicaña-Muñoz

Teams:
Dopamine and neuronal assemblies – IMN

Title

Basis of nanoscale function of neuromodulatory GPCRs inhibitory pathways in NMDAR-dependent LTD.

Abstract

Long-term potentiation (LTP) and long-term depression (LTD) are fundamental processes of synaptic plasticity that underlie long-lasting changes in synaptic strength. Together, they form the cellular basis of learning and memory. The balance between these two forms of plasticity is essential during neuronal development, as it refines synaptic connections and enables the emergence of efficient neuronal networks. During critical developmental periods, LTD has been proposed to act as a signal-to-noise filter that enhances circuit efficiency through mechanisms such as synaptic pruning. One of the most prominent forms of LTD at excitatory synapses is mediated by N-methyl-D-aspartate receptors (NMDARs), which recruit intracellular signaling cascades leading to the downregulation of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors. Alongside these glutamatergic receptors, excitatory synapses express a variety of highly sensitive G-protein-coupled receptors (GPCRs) that are activated by neuromodulatory inputs in key brain regions such as the hippocampus, where they play a critical role in modulating synaptic plasticity. Understanding how these receptors interact is crucial for elucidating the mechanisms that tune synaptic physiology.

In my thesis, I focused on the inhibitory Gi/o-coupled pathway (iGPCRs)—specifically dopaminergic, serotonergic, and histaminergic receptors—to explore the interplay between their intracellular signaling pathways and their potential synergistic interactions with NMDARs in inducing LTD in the hippocampus. First, we established a causal relationship between NMDAR-dependent LTD and synaptic pruning through the chemical induction of LTD, evidenced by electrophysiological and synaptic protein content analyses. Additionally, we characterized the synaptic environments that render connections more susceptible to prune. I then examined whether subthreshold activation of NMDARs, insufficient on its own to induce LTD, could be potentiated through the synergistic activation of inhibitory GPCRs such as the dopaminergic D2 receptor (D2R), serotonergic 5-HT1A receptor, and histaminergic H3 receptor. I found that D2R consistently synergized with NMDARs to restore LTD and promote synaptic pruning, whereas this effect was not observed for the other GPCRs tested. Interestingly, activation of D2R, 5-HT1A, and H3 receptors altered AMPA receptor current kinetics and induced synaptic pruning—effects that were partially reversed by subthreshold NMDAR activation in the serotonergic and histaminergic systems. These findings suggest that non-canonical pathways mediated by GPCR βγ subunits may interact intracellularly with NMDAR signaling.

The nanoscale organization of receptors has been shown to be a relevant marker for understanding synaptic transmission. Therefore, in the second part of my research, I employed direct stochastic optical reconstruction microscopy (dSTORM) to investigate the nanoscale organization of D2R. Using a Fab fragment targeting the extracellular loop of D2R, I revealed that the receptors cluster within dendritic spine nanodomains and colocalize with postsynaptic markers.

Altogether, these results advance our understanding of the fine neuromodulatory control of LTD and structural plasticity. They highlight dopaminergic receptors as key modulators under conditions of NMDAR hypofunction during neurodevelopment, suggesting that they could represent potential therapeutic targets in neurodevelopmental disorders such as certain forms of autism, where these processes are disrupted.

Key Words: Long-term-depression, GCPR, neuromodulation, dopamine, super resolution, dopamine, electrophysiology.

Jury

Sabine Levi, Directrice de recherche Rapporteure
Marianne Renner, Professeur Rapporteure
Marianne Benoit-Marand, Pofesseur (associé) Examinatrice
Pierre Trifilieff, Directeur de recherche Examinateur
Eric Hosy, Chercheur Directeur de thèse
Jérôme Baufreton, Directeur de recherche Co-directeur de thèse

• Place
  Centre Broca
  
• Dates
  On 18 December 2025 from 11:00 to 16:00