Before we continue... A look at another important neurotransmitter!

What is 5-HT?

While we have discussed how glutamate can affect certain processes involved in memory and learning, it is necessary to state the importance of another neurotransmitter which also plays a big part in these mechanisms: 5-HT or Serotonin, the name by which it is more universally recognized.

This image was obtained from: http://stickerish.com/products-page/science-and-math-decals/serotonin-molecule/
Yup. There it is. Admire it in all it's glory. This is literally among the only things that makes you happy... or not. 

Serotonin is created naturally by your body through the ingestion of foods that contain the amino acid L-Tryptophan. This molecule is later metabolized into L-5-Hydroxytryptophan (5-HTP), a Serotonin precursor, by the enzime Tryptophan Hydroxilase. The second step in the metabolism for the creation of Serotonin is through the action of Aromatic L-amino acid decarboxylase onto 5-HTP. After this step we now have Serotononin!

Image obatined from: http://medicineworld.org/cancer/lead/10-2006/a-rheostat-in-brain-for-emotions.html

Here is an image illustrating a serotonergic neuron. Serotonin, still in vesicles, makes it's way out of the pre-synaptic cell (the one on top), it later acts in a Serotonin receptor in the post-synaptic neuron and once it is done it is taken back to the pre-synaptic neuron, by a Serotonin transporter, to be re-packed into vesicle for later use.

Provided is a very comprehensive video on Tryptophan and Serotonin, and whether or not there is enough tryptophan in turkey to make you as sleepy as they say it does:

What does 5-HT affect?

There are various Serotonergic receptors distributed throughout the brain, mainly around the brainstem, associated with the raphe nuclei. Of these receptors, there are two main types: 5-HT1a receptors which are mainly concentrated in the hippocampus, septal area, amygdala and dorsal raphe nucleus, and 5-HT2a receptors which can be found in the cerebral cortex, striatum, nucleus accumbens, among other areas.
 Serotonin affects various aspects of behaviour including eating behavior (activation or inhibition of certain receptors affect food intake), regulation of anxiety, pain, learning and memory. Considering the main focus of our blog, most of the attention will be directed at the role of serotonin regarding learning and memory processes.
So then, what exactly is up with 5-HT in our learning and memory processes? 

Yeah... What's going on with that?

Image obtained from: http://jwalexander3.com/limbic/index.htm

Learning and memory are mechanisms that, as has been previously discussed, are affected by various receptor agonists and antagonists in the brain. Like with Glutamate, 5-HT receptors also play a part in learning and memory. Regarding these processes, some of the receptors which are involved are 5-HT1a, 5-HT4, and 5-HT6. Receptors 5-HT1a are found in high concentrations in the hippocampus, the brain area that controls memory. As for receptors 5-HT4, these are also found in the hippocampus, but more so in basal ganglia of the brain. Finally, receptors 5-HT6 are located in the striatum, nucleus accumbens and olfactory tubercules, but these are a bit more dispersed around different brain areas, since they are also present in the hippocampus. These receptors are mainly inhibitory, which means that they prevent cell firing when activation occurs due to binding of an agonist. You might be wondering, how does this aid memory and learning then?

Tell me already!

Well, some studies have found that agonists acting upon these receptors cause memory impairment (since they don't permit action by the post-synaptic cell to occur in the hippocampus = no message is relayed), so by blocking these memory impairing receptors, it may be possible to enhance memory and memory related tasks.  This article (Wallace, A., et al., 2014), talks a bit about learning, memory, and reversal memory, considering the presence of depression and 5-HT depletion. Now, all of this happens as a regular process in your body... what happens when drugs interfere with 5-HT release?