Untersuchung der synaptischen Neurotransmitterfreisetzung mit kombiniert elektrophysiologischen und bildgebenden Verfahren

Neuronal networks in the brain are comprised of a mixture of excitatory and inhibitory neurons. The excitatory and inhibitory cells play distinct roles in the functional network since the excitatory and inhibitory neurotransmitters naturally have opposite effects on the receiving neuron. It is thought that synapses that release neurotransmitters that are functionally distinct, can be distinguished based not only on their morphology but also based on different molecular mechanisms of synaptic transmission and plasticity. The goal of this project was to determine if excitatory and inhibitory synapses indeed display different properties of synaptic transmission. Neurons from mouse hippocampus and striatum tissue were grown in primary autaptic cell culture and functionally characterized with patch-clamp techniques in combination with FM1-43 dye imaging.With this approach, several parameters were determined. The number of synapses was counted for each neuron. On average, autaptic inhibitory neurons had only half as many synapses as autaptic excitatory neurons. The fraction of active synapses, however, was similar for both. Excitatory synapses nevertheless showed a three-fold higher frequency of spontaneous release of single vesicles, although the size of the readily releasable pool was smaller than in inhibitory cells. In contrast, the synaptic release probability was found to be about three times larger in inhibitory neurons compared with excitatory neurons. These findings underscore the distinct functional needs of inhibitory synapses in neuronal networks of the brain.Furthermore, neurons deficient for the synaptic proteins Munc13-1, Munc- 13-2, Bassoon, or VGLUT1 were examined with the combined electrophysiological and imaging techniques. The observed phenotypes led to the assignment of the proteins to their apparent function in the mechanism of neurotransmitter release of a neuronal synapse. These studies thereby contribute to the understanding of elementary mechanisms of signal processing in the brain at the molecular and cellular level.