It is important to appreciate the nature of the information being sent during synaptic transmission. Release of a neurotransmitter is triggered when a neuron is depolarized with the consequent influx of Ca2+ ion. The particular transmitter substance released does not determine the type of action in the target cell. That is determined entirely by the post-synaptic receptor. Thus, whereas the release of transmitter is permissive, the receptor is determining. All transmitters have more than one type of receptor. As we have seen, ACh activates a variety of ionotropic receptors, all ofwhich are excitatory. Its metabotropic receptors can be inhibitory, like those that Loewi studied in the heart, or excitatory, as those in the gut, bladder, bronchi, and iris. Thus, the actions of ACh can be excitatory or inhibitory. Another example is glutamate. In the hippocampus, glutamate activates two types of ionotropic receptors, the NMDA and the non-NMDA [AMPA (a-amino-3-hydroxy-5-methyl-isoxazoleproprionic acid), quisqualate, and kainate] receptors, and a metabotropic receptor. At normal resting potentials, the dendritic spines of a hippocam-pal neuron are briefly depolarized by a pulse of glutamate, because non-NMDA receptors respond but NMDA receptors do not. When the dendrites are depolarized for a longer time, the NMDA receptors are activated, causing prolonged Ca2+ entry and inducing a series of molecular changes within the dendrite. These changes enhance the responsiveness of the synapse, which can persist for many hours. This phenomenon is called long-term potentiation (LTP).
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