Amino Acids

1. i-Glutamate

This common amino acid is synthesized from a-ketoglutarate by transamination or by the hydrolysis of glutamine. At least half of all of the synapses in the brain are glutaminergic, and most of these are excitatory. Five types of excitatory glutamate receptors have been identified. These are all ionotropic and have been classified by their sensitivity to various glutamate agonists. An important example is the NMDA (N-methyl-D-aspartate) receptor. This receptor permits the passage of Ca2+ as well as Na+ and K + ions only when the neuron is significantly depolarized. The NMDA receptor is thought to play a key role in learning and memory. Other receptors for glutamate are metabotrophic and mediate modulatory synaptic actions.

Under certain pathological conditions such as stroke or injury, large amounts of glutamate are released into the brain, which activate these glutamate receptors and cause neuronal death. This process is called excitotoxicity and is a significant way in which brain tissue is damaged during a stroke. Many neurons that are excited by glutamate are also excited by i -aspartate when it is applied. i -Aspartate fulfills all of the criteria of a neurotransmitter except one: it is not released from presynaptic neurons.

Released glutamate is removed from the synaptic cleft by a specific uptake mechanism in the membrane of glutaminergic neurons and surrounding glial cells. These transporters pump the transmitter molecules in against a concentration gradient and, therefore, require the energy of ATP. In glial cells, the glutamate is converted to glutamine by the enzyme glutamine synthase. The glutamine is then transported into neurons to be hydrolyzed back to glutamate.

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