Synaptic transmission, i.e. the signal transfer from one neuron to the next, is a 2-step process. Electrical activity in the sending neuron releases a chemical message, the so-called neurotransmitter, which is stored in vesicles in the nerve terminal. The neurotransmitter then diffuses to the ‘receiving neuron‘, where it elicits electrical signal. It has been known since the work of Katz and collaborators in the early 50s, that an increase in intracellular Ca++ concentration ([Ca++]) in the ‘sending‘ nerve terminal is the immediate trigger for the release of the neurotransmitter. Later work has shown that next to Ca++ many other signaling pathways, particularly via cAMP, modulate the release of the neurotransmitter. These ‘modulatory‘ effects are central to the process of ‘synaptic plasticity‘. Similar molecular mechanisms are also responsible for the release of hormones from neuroendocrine cells.
Regulated release or secretion is a multistep process; the signaling pathways involved act at many stages and the question arises, which particular step is affected by one or the other of the common second messengers. Biochemical and traditional electrophysiological techniques very often cannot dissociate between signaling actions on ion channels, vesicle trafficking, and the secretory process itself. We have made an effort to dissect the stimulus secretion pathway by developing assays in chromaffin cells (for adrenaline release) and at a glutamatergic central nervous synapse (the Calyx of Held), which allow us to study secretion in single cells under voltage clamp conditions. This enables us to clearly distinguish between influences on electrical signaling from those on the processes of recruitment of vesicles and on the process of exocytosis. Our approach confirms that an increase in [Ca++] triggers neurotransmitter release and provides quantitative information about the spatial and temporal aspects of this Ca++-signal. Surprisingly, other modulatory signals, such as those mediated by cAMP do not influence this step, but rather enhance the ‘recruitment‘ of transmitter-filled vesicles, making them ready for release.