Serotonergic similarities in neurons and platelets

In the Central Nervous System (SNC) serotonin acts mainly as an inhibitory neurotransmitter. The precursor for its synthesis is the aminoacid tryptophan. This is taken from the blood to the cerebral interstice, where it is taken up by the nervous terminals and converted into 5-hidroxytryptophan (5-HTP) by the enzyme tryptophan hydroxilase. The conversion to 5-HTP is a key regulatory step in serotonin synthesis, and is converted quickly in 5-HT by the action of the aromatic L-acid descarboxilase. However, platelets do not synthesize 5-HT, since they do not possess tryptophan hydroxilase. Thus they only display uptake, storage, and serotonin release functions.

Serotonin actions

The neurotransmitter functions of neuronal serotonin, generally inhibitory, depend on the serotonergic receptor characteristics it interacts with. Its action mechanism can be mediated through second messengers (metabotrophic receptors) or through a direct action over ionic channels (ionotrophic receptors). In the platelets, serotonin is stored in a slow replacement depot, where it can be released from by exocythotic mechanisms. Serotonin participates in the platelet activation that allows for their aggregation to each other for blood clotting process.

Serotonin uptake

To stop the serotonin neurotransmitter function, neuronal serotonin is taken up from the synaptic cleft by transporter proteins. The serotonin neuronal uptake is impelled by a proton gradient that requires ATP. The 5-HT uptake can follow two paths: the 5-HT can be metabolized by the MAO into 5-hydroxy-indolacetic acid, or it can be reintroduced into release vesicles in order to be reutilized as a neurotransmitter.

The serotonin uptake by platelets occurs either by passive diffusion or by active transport mechanisms. Under physiological conditions, the active uptake mechanism is the most effective. This uptake is mediated by proteins similar to the ones required for the neuronal serotonin uptake in the brain. It requires energy and the presence of Na+ and Cl-. The platelet uptake system has a relatively high affinity (Kd) for 5-HT, being similar in magnitude from platelets to neurons. The platelet storage of 5-HT is located mainly in the dense bodies and in the storage granules.

Serotonin transporters in platelets and synaptic terminals

The main form of ending a serotonergic transmission pulse is by taking up 5-HT molecules from the synaptic cleft directed to reduce the serotonin concentration, which then stops the serotonergic neurotransmission.

The uptake process involves a molecular recognition of 5-HT by the transporter, its binding, and passing through the membrane to be released within the cellular. Serotonin molecules bound to its transporter protein cross through the membrane using Na+ as a driving force. The return ofthe transporter to its original position requires K+ as the driving force to step this protein toward its original position. When a selective serotonin reuptake inhibitor is administered, the 5-HT concentration increases in the synaptic cleft, which enhances serotonin neurotransmission. This increase induces a down regulation cascade of both: serotonin autoreceptors (presynaptic) and postsynaptic receptors, that may finally reestablish the resting state of the neuron.

It has been confirmed that the protein for neuronal as well as platelet serotonin uptake transport are synthesized by the same gene. Experimental evidence has shown that the platelet transporter presents the same functional and pharmacological characteristics than the neuronal transporter.

Serotonergicreceptors

Seven types of pre and post synaptic serotonin receptors, which have also several subtypes, have been characterized.

Pre and post synaptic 5-HT 1 receptors

. The 5-HT1 receptors are involved in both pre and post synaptic serotonergic neurotransmission. The presynaptic 5-HT1A receptors are autoreceptors. Due to their localization in the cellular body and in the dendrites, they have been named somatodendritic autoreceptors, which control the serotonin release. The postsynaptic receptors may play a role in hypothalamic thermoregulation. The presynaptic 5-HT1D receptors are autoreceptors that perform a regulation by blocking the 5-HT release. These receptors are not synthesized in platelets.

Postsynaptic 5-HT 2 receptors

. The 5-HT2 receptor subtypes are 5-HT2A,BandC. When postsynaptic 5-HT2Areceptors are bound to serotonin, they drive the transduction of neuronal impulses through the production of second messengers within the postsynaptic neuron. These second messengers induce the synthesis of intracellular proteins denominated transcription factors, which may regulate the expression of several neuronal genes. Platelet 5-HT2A receptors correspond to the neuronal 5-HT2A metabothropic receptors and induce alterations in platelet density and affinity.

5-HT 3 receptors

. These receptors were originally described in the periphery, specifically as part of the enteric nervous system. In the CNS 5-HT3 receptors are densely present in the solitary tract nucleus and in the area postrema. These receptors are the onlymonoaminergic receptors consistingofionic channels operated by aminergic neurotransmitters. The stimulation of 5-HT3 receptors is responsible of several secondary effects of the selective inhibitors of serotonin reuptake (SISR). These effects are not mediated only in the CNS, but also in sites outside the brain, such as the intestine, which possess this type of receptors also. These receptors are not located in the platelets.

5HT 4-7 serotonergic receptors

. These receptors are distributed throughout the body, where they stimulate the alimentary tract secretions and facilitate peristaltic reflexes. Their localization in serotonergic areas in the brain and platelets has not been established.

Notwhithstanding their limitations, the characteristics reviewed support the conclusion that platelets can be used as partial models to study the neuronal serotonin 5-HT2 binding and uptake functions. As Alfred Pletscher stated: "although the incomplete of the pattern demands care in its application, they could have the advantage of the relative simplicity".

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