HCN channels in the medial entorhinal cortex are crucial for the consolidation of single-trial spatial learning.
1 German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany
2 Department of Cellular Neuroscience, Leibniz Institute for Neurobiology, Magdeburg, Germany
3 Center for Molecular Neurobiology Hamburg (ZMNH), Hamburg, Germany
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels conduct the h-current (Ih) which is an important regulator of neuronal excitability as it contributes to the resting membrane potential, dendritic integration, and subthreshold resonance in the theta-frequency range. To investigate the functional role of Ih in the medial entorhinal cortex (MEC), we generated a transgenic mouse line expressing a dominant-negative HCN subunit (HCN-DN) specifically in MEC LII stellate cells (MEC-SCs) under the control of the Neuropsin (Nop) promoter. The HCN-DN subunit, upon assembly with endogenous HCN1-4 alpha subunits, renders the channel tetramer non-functional. We found that mice expressing HCN-DN in MEC-SCs exhibited a specific impairment in short/single-trial but not in multi-trial long-term spatial learning. Furthermore, Nop-HCN-DN mice showed significantly fewer c-fos positive labeled neurons in the hippocampal CA1, CA3 and Dentate Gyrus (DG) subregions following single-trial fear conditioning, and showed a suppression of delta (<4 Hz) and high gamma (>90 Hz) oscillations in the DG of awake head-fixed mice. Although HCN-DN expression abolished the theta resonance and the “sag” potential in acute MEC slice recordings, Ih ablation in MEC-SCs showed no effect on theta oscillations in the MEC or hippocampus. Finally, single-unit recordings showed that putative excitatory but not inhibitory neurons in MEC LII were suppressed in Ih–deficient mice. In order to test whether the effect of Ih loss can be replicated or rescued by altering MEC-SC excitability, we co-expressed our transgene with an excitatory or inhibitory (hM3Dq or hM4Di, respectively) DREADD receptor (designer receptor exclusively activated by designer drug), which we activated by administrating compound 21 (C21). We found that the administration of C21 prior to memory acquisition rescues behavioral performance, restores the c-fos expression levels, and normalizes DG delta and high gamma oscillations. Moreover, C21 administration immediately after behavioral acquisition also restored behavioral performance, indicating that loss of Ih in MEC-SCs disturbed memory consolidation processes. In addition, mice expressing only hM3Dq in MEC-SCs showed impaired behavioral performance, increased c-fos baseline expression levels, and augmented high gamma activity following C21 administration; whereas, conversely, hM4Di-induced inhibition reproduced the behavioral and physiological effects of Ih ablation. Finally, the increased excitation due to hM3Dq activation restored the average population firing rate of putative MEC excitatory units to control levels. Together, these results indicate that Ih in MEC-SCs is crucial for maintaining an optimal excitability balance necessary for rapid acquisition of spatial information following brief exposure to the environment. Funding: DFG SFB 1089 – Project A05