Voltage-gated ion channels are important determinants of neuronal excitability. The Hyperpolarization-activated Cation Non-selective (HCN) channels are voltage-gated ion channels that open with hyperpolarization at subthreshold potentials. Four subtypes (HCN1-4) have been cloned. There is considerable interest in HCN1 channels as they have been shown to have roles in physiological processes such as learning and memory as well as pathophysiological conditions such as epilepsy. Many studies have shown that they are highly localised to hippocampal and cortical cell pyramidal cell dendrites. Interestingly, emerging immunohistochemical evidence suggests that they may also be present in axons and synaptic terminals. In support of this, our recent work shows that excitatory synaptic transmission is significantly enhanced in the entorhinal cortex (EC) in HCN1 null mice, suggesting that HCN1 channels may play a role in regulating neurotransmitter release. In the proposed work, we wish to test the hypothesis that HCN1 channels exist at glutamatergic synaptic terminals in the EC, where they regulate synaptic transmission. We also wish to investigate the cellular mechanisms by which pre-synaptic HCN1 channels may influence synaptic release. Finally, we wish to explore the physiological function of these pre-synaptic HCN1 channels. To address these questions, we will use a multi-disciplinary approach involving conventional electrophysiology, state-of-the art multi-photon microscopy imaging and electron microscopy coupled with transgenic mice and pharmacology. The results produced will have significant ramifications not only in the field of ion channels, particularly HCN channels, and neuron excitability but also for understanding the factors involved in controlling neurotransmission, synaptic strength and ultimately neural network excitability.