Abstract The proper maturation of emotional and sensory circuits requires a fine tuning of serotonin (5-HT) level during early postnatal development. Consistently, dysfunctions of the serotonergic system have been associated with neurodevelopmental psychiatric diseases, including autism spectrum disorders (ASD). However, the mechanisms underlying the developmental effects of 5-HT remain partially unknown, one obstacle being the action of 5-HT on different cell types. Here, we focused on microglia, which play a role in brain wiring refinement, and we investigated whether the control of these cells by 5-HT is relevant for neurodevelopment and spontaneous behaviors. Since the main 5-HT sensor in microglia is the 5-HT 2B receptor subtype, we prevented 5-HT signaling specifically in microglia by conditionally invalidating Htr2b gene in these cells. We observed that abrogating the serotonergic control of microglia during postnatal development impacts the phagolysosomal compartment of these cells and their proximity to dendritic spines, and perturbs neuronal circuits maturation. Furthermore, this early ablation of microglial 5-HT 2B receptors leads to adult hyperactivity in a novel environment and behavioral defects in sociability and flexibility. Importantly, we show that these behavioral alterations result from a developmental effect, since they are not observed when microglial Htr2b invalidation is induced later, at P30 onward. Thus, a primary alteration of 5-HT sensing in microglia, during a critical time window between birth and P30, is sufficient to impair social and flexibility skills. This link between 5-HT and microglia may explain the association between serotonergic dysfunctions and behavioral traits like impaired sociability and inadaptability to novelty, which are prominent in psychiatric disorders such as ASD.
Many psychiatric diseases have been associated with serotonin (5-HT) neuron dysfunction. The firing of 5-HT neurons is known to be under 5-HT1A receptor-mediated autoinhibition, but functional consequences of coexpressed receptors are unknown. Using co-immunoprecipitation, BRET, confocal, and super-resolution microscopy in hippocampal and 5-HT neurons, we present evidence that 5-HT1A and 5-HT2B receptors can form heterodimers and co-cluster at the plasma membrane of dendrites. Selective agonist stimulation of coexpressed 5-HT1A and 5-HT2B receptors prevents 5-HT1A receptor internalization and increases 5-HT2B receptor membrane clustering. Current clamp recordings of 5-HT neurons revealed that 5-HT1A receptor stimulation of acute slices from mice lacking 5-HT2B receptors in 5-HT neurons increased their firing activity trough Ca2+-activated potassium channel inhibition compared to 5-HT neurons from control mice. This work supports the hypothesis that the relative expression of 5-HT1A and 5-HT2B receptors tunes the neuronal excitability of serotonergic neurons through potassium channel regulation.
Vezatin is an integral membrane protein associated with cell-cell adhesion complex and actin cytoskeleton. It is expressed in the developing and mature mammalian brain, but its neuronal function is unknown. Here, we show that Vezatin localizes in spines in mature mouse hippocampal neurons and codistributes with PSD95, a major scaffolding protein of the excitatory postsynaptic density. Forebrain-specific conditional ablation of Vezatin induced anxiety-like behavior and impaired cued fear-conditioning memory response. Vezatin knock-down in cultured hippocampal neurons and Vezatin conditional knock-out in mice led to a significantly increased proportion of stubby spines and a reduced proportion of mature dendritic spines. PSD95 remained tethered to presynaptic terminals in Vezatin-deficient hippocampal neurons, suggesting that the reduced expression of Vezatin does not compromise the maintenance of synaptic connections. Accordingly, neither the amplitude nor the frequency of miniature EPSCs was affected in Vezatin-deficient hippocampal neurons. However, the AMPA/NMDA ratio of evoked EPSCs was reduced, suggesting impaired functional maturation of excitatory synapses. These results suggest a role of Vezatin in dendritic spine morphogenesis and functional synaptic maturation.
Abstract Many psychiatric diseases including depression, schizophrenia and anxiety have been associated with serotonin (5-HT) neuron dysfunction. Pacemaker-like firing of raphe 5-HT neurons was proposed to be under unique 5-HT 1A receptor-mediated autoinhibition. We previously showed that 5-HT 2B receptors were expressed by 5-HT neurons together with 5-HT 1A receptors. However, functional consequences on 5-HT neurons of putative interaction between these receptors are unknown. Using co-immunoprecipitation, BRET, confocal and super-resolution microscopy in hippocampal and 5-HT neurons, we present converging evidence that 5-HT 1A and 5-HT 2B receptors can form heterodimers and co-cluster at the surface of dendrites. 5-HT 2B receptor clusters were redistributed upon 5-HT 1A receptor expression supporting functional interactions between the two receptors. Furthermore, 5-HT 2B receptor expression prevented agonist-induced internalization of 5-HT 1A receptors, whereas 5-HT 1A receptors mimicked the clustering effect of 5-HT 2B receptor stimulation on its surface expression. The functional impact of this interaction in-vivo was assessed by recording 5-HT neuron excitability from mice lacking 5-HT 2B receptors in 5-HT neurons. Upon 5-HT 1A receptor stimulation, the firing activity of 5-HT neurons was increased in the absence of 5-HT 2B receptors and decreased in their presence through regulation of SK channels, thus demonstrating functional output of this interaction in controlling 5-HT neuron firing activity.
