Mechanisms of GABA release in hippocampal circuits

Synapses are key sites of communication in the brain. Upon stimulation, presynaptic terminals release a chemical substance called transmitter, into the synaptic cleft, and activate specific receptors in postsynaptic target cells. Thus, at the synapse an electrical waveform is converted into a chemical signal, and reconverted into an electrical event. Roughly speaking, the mammalian brain contains two types of synapses: excitatory synapses that use glutamate as transmitter and inhibitory synapses which use gamma-aminobutyric acid (GABA) for synaptic signaling. However, detailed analysis revealed that GABAergic synapses are highly diverse in their functional properties, particularly the time course of transmitter release. The molecular, subcellular, and cellular mechanisms underlying this functional diversity are largely unknown. In this project, we will compare the properties of the output synapses of two highly abundant types of GABAergic neuron: interneurons expressing the calcium-binding protein parvalbumin (PV) and interneurons expressing the neuropeptide cholecystokinin (CCK). The output synapses of the two types of interneuron differ substantially in the timing of transmitter release, which is fast and synchronous in PV+ interneurons and slow and asynchronous in CCK+ interneurons. Specifically, we want to address three major questions: a. What are the biophysical mechanisms underlying differential signaling at the two types of inhibitory synapse? To tackle this question, we will use cutting-edge paired and multi-cell recording in acute mouse brain slices to study GABAergic synaptic signaling in the most precise way. b. What is the molecular machinery of synaptic vesicle fusion at the two types of inhibitory synapse? In particular, we want to study the role of Rab3-interacting molecules (RIMs), central organizers of the active zone, and synaptotagmins, putative calcium sensors of vesicle fusion. c. What are the structural changes associated with GABA release? To answer this question, we will use Flash and Freeze functional electron microscopy. The key idea is to stimulate GABAergic neurons and, after precisely defined time intervals, produce an ultrastructural snapshot by high-pressure freezing. Applying this innovative approach to GABAergic synapses, we will address the structural correlates of synchronous versus asynchronous release, different forms of endocytosis, and exocytosis of different populations of synaptic vesicles. The results of this project will provide key information about the mechanisms of inhibitory synaptic transmission in the mammalian brain. Furthermore, the new data will provide an important basis for quantitatively incorporating inhibitory synapses into neuronal network models. Finally, they will help in the understanding of the pathomechanisms of neurological and psychiatric diseases, such as schizophrenia, autism, or neurodegenerative diseases, in which GABAergic synaptic transmission is often perturbed.