<i>Fmr1</i>  knockout disrupts multiple intrinsic properties via reduced HCN channel activity in mediodorsal thalamocortical neurons

The neurodevelopmental disorder fragile X syndrome (FXS) results from hypermethylation of the FMR1 gene, which prevents production of the FMRP protein. FMRP modulates the expression and function of a variety of proteins, including voltage-gated ion channels, such as hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels, which are integral to rhythmic activity in thalamic structures. Thalamocortical pathology, particularly involving the mediodorsal thalamus (MD), has been implicated in neurodevelopmental disorders such as FXS. MD connectivity with the medial prefrontal cortex (mPFC) is integral to executive functions such as working memory and social behaviours that are disrupted in FXS. We used a combination of retrograde labelling and ex vivo brain slice whole-cell electrophysiology in 40 wild-type and 42 Fmr1 knockout male mice to investigate how a lack of Fmr1 affects intrinsic cellular properties in lateral (MD-L) and medial (MD-M) MD neurons that project to the mPFC (MD→mPFC neurons). In MD-L neurons, Fmr1 knockout decreased the HCN-mediated membrane properties voltage sag and membrane after-hyperpolarization. We also identified a delay in rebound spike timing in both complex bursts and low-threshold spikes. In Fmr1 knockout mice, reduced HCN channel activity in MD-L→mPFC neurons impaired both the timing and the magnitude of HCN-mediated membrane potential regulation. Changes in response timing might adversely affect rhythm propagation in Fmr1 KO thalamocortical circuitry. MD thalamic neurons are crucial for maintaining rhythmic activity involved in cognitive and affective functions. Understanding specific mechanisms of thalamocortical circuit activity might lead to therapeutic interventions for individuals with FXS and other conditions characterized by thalamic dysrhythmia.