Introduction to Psychobiology - Part 6 (A Neurotransmitter known as Acetylcholine)

For more information about psychobiology, please check out "Physiology of Behavior," by Neil Carson

In this post we will cover a neurotransmitter known as Acetylcholine (ACh)



Acetylcholine (ACh)



This is thought to be the first discovered neurotransmitter by Otto Loewi (1).  To the right a drawing of a rat's brain and its acetylcholine pathway can be seen. ACh can be found in the peripheral nervous system(PNS) and the central nervous system (CNS). In the PNS it is involved with neuromuscular junction (2). This is where an axon reaches a muscle (3). Here the ACh causes EPSPs. This is excitatory postsynaptic potential and it refers to the action that increases the probability of an action potential occuring. On the other hand, IPSPs inhibit postsynaptic potentials (4). In the CNS, ACh in the basal forebrain is involved in perceptual learning and memory. In the Medial Septum, specifically the Hippocampus, ACH is also involved in learning and memory. In the Dorsolateral Pons, it can be found when individuals are passing through their REM sleep (Rapid eye movement). Finally, in the Putamen, Nucleus Accumbens, and the Caudate Nucleus Acetylcholine is involved in motor functions.



Biosynthesis of ACh

We will now look at the synthesis of acetylcholine (synthesis is the construction of a complex chemical (5)). As you can see the synthesis starts with two substances: Acetyl coenzyme A (acetyl-CoA) and Choline. Then, Choline acetyltransferase (ChAT), which is an enzyme (remember that enzymes are catalysts that either help break or unite two substances) transfers the acetate ion to Choline, thus, ending up with acetylcholine (6).



Two Subtypes of Receptors

Both of the subtypes are found in the CNS. They are nicotinic and muscarinic. The former is ionotropic, which is a receptor that is linked to an ion channels (7), is only found on muscle fibers (8). The latter is metabotropic, which obtained this name because the "movement of ions through a channel depends on one or more metabolic steps" (7).

Six Cholinergic Drugs

  1. Black Widow Spider Venom: This venom triggers the release of acetylcholine.
  2. Hemicholinium: This prevents the uptake of choline
  3. Atropine: This is a direct muscarinic antagonist. It produces dilated eyes.
  4. Curare: This comes from a plant and it prevents muscle contraction. It is a nicotinic antagonist that is direct.
  5. Neostigmine: This is an indirect agonist that blocks AChe
  6. Botulinum Toxin: Also known as Botox, it prevents release the release  of acetylcholine (8).







References
1. http://neuroscience.uth.tmc.edu/s1/chapter11.html

2. http://neuroscience.uth.tmc.edu/s1/chapter04.html

3. http://education.ucf.edu/litsymposium/resources2013/crittenden_handout1.pdf

4.http://www.ncbi.nlm.nih.gov/books/NBK11117/

5. http://www.britannica.com/science/chemical-synthesis

6. http://www.ncbi.nlm.nih.gov/pubmed/10594838

7.http://www.ncbi.nlm.nih.gov/books/NBK10855/

8. "Physiology of Behavior," by Neil Carson.