Adult medial habenula neurons require GDNF receptor GFRalpha1 for synaptic stability and function

The medial habenula (mHb) is an understudied small brain nucleus linking forebrain and midbrain structures controlling anxiety and fear behaviors. The mechanisms that maintain the structural and functional integrity of mHb neurons and their synapses remain unknown. Using spatio-temporally controlled Cre-mediated recombination in adult mice, we found that the GDNF receptor alpha 1 (GFR1) is required in adult mHb neurons for synaptic stability and function. mHb neurons express some of the highest levels of GFR1 in the mouse brain, and acute ablation of GFR1 results in loss of septo-habenular and habenulo-interpeduncular glutamatergic synapses, with the remaining synapses displaying reduced numbers of presynaptic vesicles. Chemo- and opto-genetic studies in mice lacking GFR1 revealed impaired circuit connectivity, reduced AMPA receptor postsynaptic currents, and abnormally low rectification index of AMPARs, suggesting reduced Ca2+-permeability. Further biochemical and proximity ligation assay studies defined the presence of GluA1/GluA2 (Ca2+-impermeable) as well as GluA1/GluA4 (Ca2+-permeable) AMPAR complexes in mHb neurons, as well as clear differences in the levels and association of AMPAR subunits in mHb neurons lacking GFR1. Finally, acute loss of GFR1 in adult mHb neurons reduced anxiety-like behavior and potentiated context-based fear responses, phenocopying the effects of lesions to septal projections to the mHb. These results uncover an unexpected function for GFR1 in the maintenance and function of adult glutamatergic synapses, and reveal a potential new mechanism for regulating synaptic plasticity in the septo-habenulo-interpeduncular pathway and attuning of anxiety and fear behaviors.