Loss of Fmr1 causes subnucleus-specific decreases in dendritic complexity in mediodorsal thalamus

In the mediodorsal thalamus (MD), neurons of the medial (MD-M) and lateral (MD-L) subnuclei project to the medial prefrontal cortex (mPFC) to control complementary aspects of behavior, with MD-M supporting more of the social/emotional domain and MD-L promoting cognitive functions. Our recent work shows that MD-M and MD-L have differences in hyperpolarization-activated, cyclic nucleotide-gated (HCN) pacemaker channels that may enable their distinct roles in behavior. Low HCN channel activity in MD-M neurons makes them relatively sensitive and responsive to inputs, while higher HCN channel activity makes MD-L neurons more specific and selective for which inputs it transforms into outputs. Knockout (KO) of the fragile X syndrome-related gene Fmr1 decreased the activity of HCN channels in MD-L but not MD-M neurons, making MD-L neurons more like MD-M cells physiologically. We sought to understand if these Fmr1-related changes in MD function were accompanied by changes in subcellular structure. Here, we assessed the dendritic morphology of MD neurons in adult Fmr1 KO and wildtype (WT) littermate mice. In acute ex vivo brain slices, we filled MD neurons with biocytin via whole cell recording pipettes. We used Neurolucida to trace the dendrites, performed quantitative analysis of dendritic architecture, compared MD-M and MD-L neurons in wildtype mice, and compared WT to KO mice for each subnucleus. In WT mice, MD-M neurons showed more distal dendritic branching compared with MD-L neurons. In Fmr1 KO mice, MD-M neurons lost their distal dendritic branching and exhibited significant reductions in multiple measures of dendritic complexity compared to WT controls (total dendritic length, number of nodes, and number of terminals), while MD-L neurons showed no morphological differences between WT and KO animals. Notably, these morphological changes show a double dissociation with our previous physiological findings: MD-M neurons exhibit structural but not functional changes, while MD-L neurons show functional but not structural changes in Fmr1 KO mice. Together, these findings suggest that FMRP regulates structure and function through independent, circuit-specific mechanisms in thalamocortical neurons, with implications for understanding the diverse cellular and network consequences of Fmr1 loss in fragile X syndrome.