Transport

Most classical transmitters are inactivated by their elimination from the cleft through high affinity, saturable, and secondary active transport. The amino acid transmitters might be transported into both neurons and glia while amines are transported only into the neurons. Two families of the transporter have been specified that serve this function:

- Na⁺/K⁺- cotransporter family holds the glutamate (and aspartate) transporters. The Glutamate transport is an electrogenic, which is, it results in a modest potential difference being set up across the membrane, inside positive as shown in figure. The consequence of this is that the excessive depolarization of membrane can reverse the direction of the transport causing glutamate efflux into the cleft. This can outcome in excitotoxicity. The Glutamate transporters have been cloned and sequenced.

                Figure: Glutamate transport by a Na⁺/K⁺ cotransporter.
- Na⁺/Cl^- -cotransporter family is as shown in the figure below. This is a big family which involves transporters for GABA, serotonin, glycine, dopamine, noradrenaline/adrenaline (norepinephrine/epinephrine), and choline. The noradrenaline and serotonin transporters are the aims for the tricyclic antidepressant class of drugs. The serotonin transporters are aims of selective serotonin reuptake inhibitors (SSRIs), like fluoxetine (Prozac), however the question now being raised about their efficacy as antidepressants and their safety. The dopamine transporter is the aim for cocaine.

                     Figure: GABA transport by a Na⁺/Cl^- cotransporter.

By inhibit the dopamine reuptake; cocaine deranges dopamine transmission in reward pathways in the brain, accounting partially for its addictive properties. Most of the membe of this family have been seriated. They are big glycoproteins with 12 transmembrane segments, but have no homology with the vesicle transporters.