Partners in plasticity: serotonergic glial interactions in brain circuit remodeling

Experience-dependent brain circuit optimization choreographed by environmental sensory input activity involves synapse formation, pruning, and remodeling to sculpt appropriate connectivity. The serotonin (5-HT) neuromodulator acts as a core regulator of this circuit plasticity. Classically, serotonergic control has been understood solely through neuronal mechanisms, however new evidence reveals glial 5-HT signaling roles. This review focuses on recent studies in Drosophila with reference to foundational mammalian work to discuss 5-HT functions in both neurons and glia, particularly experience-dependent extracellular matrix remodeling, glial infiltration, and synapse elimination in early-life critical periods. Disruption of serotonergic regulation is proposed to contribute to a spectrum of neurodevelopmental disorders, including Fragile X syndrome, in which failure to prune and persistence of immature connectivity cause severe life-long behavioral impairments. Recent discoveries further reveal targeted induction of glial serotonergic signaling can re-open “critical period-like” synapse pruning at maturity. Enabling large-scale connectivity changes has broad potential therapeutic applications for disease, injury, trauma, and cognitive dysfunction. A key advance is the emerging evidence that glia—not just neurons—are serotonergic mediators of synaptic remodeling: glial 5-HT biosynthesis, 5-HT 2A receptor activation, and matrix metalloprotease-mediated function together allow access for experience-driven synapse elimination. We propose glia-to-glia class serotonergic signaling—linking sensory experience to synapse pruning—may represent a conserved plasticity gating mechanism that determines whether circuitry is permissive or resistant to synaptic connectivity modification. Harnessing glial class-specific serotonergic control of experience-dependent brain circuit remodeling may enable new targeted therapies to correct brain function while avoiding the negative side effects of global serotonin elevation.