Endocytic Cycle - Synapse Function

Synapse Function

Impulses between neurons are transmitted by the release of neurotransmitters at the junction between the two cells, a region called a synapse. This release is effected by exocytosis at the presynaptic terminal. A vesicle full of transmitter, acetylcholine (for example), in the presynaptic terminal fuses with its neighbouring plasma membrane and thereby releases a burst of acetylcholine into the synaptic space. The acetylcholine is rapidly degraded here, but before this happens it activates acetylcholine receptors on the postsynaptic terminal and triggers an electrical impulse in that cell. The membrane added to the presynaptic terminal is recovered by endocytosis and recycled to form fresh vesicles full of neurotransmitter, ready for another cycle of postsynaptic excitation.

Thus, the function of the nervous system is dependent on this endocytic cycle. An example of this dependence is found in fruit flies. A key protein required for endocytosis is dynamin: It assists in budding a coated pit into a cell to form a coated vesicle. Mutations in the dynamin gene in which the activity of the dynamin protein is lost at above-normal temperatures (for the fly) exist: These are called temperature-sensitive mutations. Such mutant flies have the property that, when the fly is brought from its normal 22°C to 30°C, the dynamin function is lost. However, when the flies are cooled to 22°C, it is regained. In other words, in these mutant flies, the endocytic cycle can be turned off at 30°C, and turned back on by cooling. What one observes is that, within seconds of warming to 30°C, the fruit flies become paralysed: They drop out of the air and appear near-dead. On cooling, they slowly get up, flutter their wings and fly away. The endocytic cycle has been temporarily suspended with drastic effects.