Reuptake is a required aspect of neurological activity which regulates the amount of neurotransmitters present in a synapse after the transmission of a neural signal. Neurotransmission occurs by transporting information across neurons by an electrical impulse called an action potential. When an action potential reaches the synapse between two neurons, the pre-synaptic neuron releases neurotransmitters to transport the chemical signal across the synapse by binding to receptors on the post-synaptic neuron. Reuptake is achieved by transporter proteins which reabsorb the extracellular neurotransmitter back into the pre-synaptic neuron for reuse. Reuptake can determine the extent, duration, and spatial domain of receptor activation.
Most known reuptake inhibitors affect the monoamine neurotransmitters serotonin, dopamine, noradrenaline, and adrenaline. However, there are also a number of pharmaceuticals and research chemicals that act as reuptake inhibitors for other neurotransmitters such as glutamate, GABA, glycine, adenosine, choline (the precursor of acetylcholine), and the endocannabinoids.
Mechanism of action
Standard reuptake inhibitors are believed to function by binding directly to the transporter protein of the neurotransmitter in question. By occupying the transporter, a reuptake inhibitor competitively blocks its respective neurotransmitter from binding to the transporter protein and thus prevents it from being transported from the synapse into the presynaptic neuron. Reuptake inhibitors, like agonists and antagonists, are an example of ligand-receptor binding.
Alternatively, some reuptake inhibitors bind to allosteric sites and inhibit reuptake indirectly and non-competitively. Allosteric binding may cause a conformational change in the transporter protein that prevents it from reuptaking its respective neurotransmitter. Several dissociative drugs have been shown to work this way, including PCP and related drugs ketamine and dizocilpine (MK-801). They appear to exert their reuptake inhibition by binding to allosteric sites on each of the respective monoamine transporters. In addition to their high affinity for the main site of the monoamine transporters, several competitive transporter substrates, such as cocaine, have lower affinity for these allosteric sites as well.
Types of reuptake inhibitors
- A selective reuptake inhibitor will inhibit the reuptake of a specific neurotransmitter with negligible or no effects on the reuptake of other neurotransmitters. An example of this is the class of antidepressants known as selective serotonin reuptake inhibitors, or SSRIs. SSRIs solely target serotonin transporters (SERTs), blocking the reuptake of serotonin and increasing extracellular concentrations of the neurotransmitter.
- A non-selective reuptake inhibitor will inhibit the reuptake of more than one type of neurotransmitter. An example of this is cocaine, which acts as a serotonin-noradrenaline-dopamine reuptake inhibitor, or SNDRI. Ketamine is an example of a weak SNDRI; it is non-selective because it affects the reuptake of multiple neurotransmitters.
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