CHAPTER 7. NEUROTRANSMITTERS
Neurotransmitters are the chemicals ferriers of messages in the brain. The importance of neurotransmitters for the social sciences is that they serve to fight the anti-biological bias in the social sciences. The chemical imbalances in the brain involving neurotransmitters, along with other aspects such a faulty wiring, explain much of social deviancy, criminality, and even human personality.
To fully understand the nature of neurotransmitters we need to do a very brief review of some basic definitions in organic chemistry. Many of these words will reoccur in this chapter and, therefore, the reader should be familiar with them.
Organic compounds are those that involve the element carbon. These compounds were crucial in the development of life because four of the chemicals are necessary for the creation of life itself. The first group of organic compounds includes the carbohydrates, the most abundant organic compounds in nature. These are hydrocarbons joined with the element oxygen. Carbohydrates include sugars, starches, and related substances. The second group of organic compounds contains the lipids. This includes the fats, cutin, suberin, and waxes, and the phospholipids that are so important in cellular membranes. The third group are the nucleic acids. Each nucleic acid consists of a phosphate group, a five-carbon sugar (pentose), and a nitrogenous base. The important genetic materials of DNA and RNA are made of nucleic acids. The last group of important organic compounds are the proteins. Some twenty different amino acids are the building blocks of proteins. Chains of amino acids are called polypeptides. Some amino acids can be synthesized in the animal body. Those which cannot are known as the essential amino acids. For man, these are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
A functional group is a reactive group of atoms in a molecule that has very noticeable specific properties or characteristics that make it easily identifiable. One of these functional groups is the amines, which we will learn more about shortly.
Types of Neurotransmitters
The Biogenic Amines
As we have seen an amine is one of the functional groups. In fact, it is one of the most common functional groups found in medicinal agents. An amine is an organic substance containing the radical group NH2 and that acts as a binding site that holds a drug to a specific site in the body to produce its biological result.
There are two groups of neurotransmitters that are biogenic amines: the catecholamines and indoleamines. The catecholamines have a ring structure called a catechol (which is a member of the phenol functional group). The catecholamine neurotransmitters are dopamine, norepinephrine, and epinephrine. The catecholamines are synthesized from tyrosine in turn taken from the essential amino acid phenylalanine.
The three catecholamines are intimately related chemically. Phenylalanine is converted to tyrosine. Further changes to tyrosine produces dopa which is, in turn, changed to dopamine -- a compound with many of the physiological properties of norepinephrine to which it is converted. In the brain two important enzymes, catechol-O-methyl transferase (COMT) and monoamine oxidase (MAO), initiate the conversion of norepinephrine to its metabolites. Outside the central nervous system, changes to norepinephrine produce epinephrine (Chalmers 1971:166).
There are about a dozen nuclei in the brain stem that produce catecholamines. Norepinephrine is contained in six nuclei. Some axons from these cell groups pour into the spinal cord as well as to higher parts of the brain. The neurons of the locus ceruleus contain large quantities of norepinephrine, which they use as a transmitter substance. There are only four prominent groups of dopaminergic neurons: the hypothalamus, pituitary gland, basal ganglia, and other parts of the limbic system. Dopamine is the transmitter also used by the substantia nigra and ventral tegmental area, and by certain nuclei of the hypothalamus. The sympathetic division activates the adrenal glands to release epinephrine (also called adrenalin) into the bloodstream, which produces racing heart and other effects.
Now that we have discussed the catecholamines, we can discuss the indoleamines. The neurotransmitter serotonin is an indoleamine. It is a biogenic amine that is synthesized from the essential amino acid tryptophan, which contains a ring structure called an indole. The vast majority of neurons containing serotonin are located in the raphe nuclei, a group of nine nuclei. The neurons in these nuclei send axonal projections to widespread regions of the brain and spinal cord.
Some of the twenty common amino acids function as neurotransmitters as well as being universal cellular constituents. There are two main functional groups of amino acids: those that excite and those that inhibit. Amino acids that play an excitatory role are: glutamic acid, aspartic acid, cysteic acid, and homocysteic acid. Glutamate may serve as the universal excitatory transmitter used by afferent inputs and interneurons in the central nervous system.
Inhibitory amino acids are gamma-aminobutyric acid (GABA), glycine, taurine, and beta-alanine. Derived from the amino acid precursor glutamate is GABA. Blocking the effects of this neurotransmitter in the brain may produce convulsions. Indeed, GABA, as well as norepinephrine, dopamine, and serotonin, are believed to protect the brain against seizures. Glycine acts as an inhibitory neurotransmitter in a number of brain regions, including pathways from the cortex to the hypothalamus. The administration of certain drugs, such as strychnine, will block the action of glycine, thereby producing convulsions.
Neuropeptides are combinations of amino acids that serve as neurotransmitters. The study of neuroactive peptides has become popular because of its connection to external drugs, such as the opiates. A group known as the opiate-like peptides are localized in regions of the brain thought to be involved in the perception of pain, pleasure, feeling, and emotion.
In recent years, about twenty-five short peptides have been found, which can cause inhibition, excitation, or both. There are two families of peptides that possess opiate-like actions, the endorphins and the enkephalins (Kandel and Schwartz, 1981:111). They play an important role involved with pain and analgesia (relief from pain) and in the etiology of drug addiction. Different types of endorphins have been isolated from the brain and pituitary gland. Enkephalins are released at synapses in the periaqueductal gray matter, the raphe nuclei, and the substantia gelatinosa of the dorsal horn of spinal gray matter.
A neuromodulator regulates the synthesis, degradation, and reuptake of neurotransmitters. One of the most important neuromodulators is the enzyme monoamine oxidase (MAO). MAO, contained in the mitochondria of neurons, catalyzes the oxidative deamination of catecholamines and indoleamines. If there is too much MAO, the enzyme breaks down too much of the neurotransmitter causing a deficit of the chemical substance. If there is too little MAO, then there will an excess of the neurotransmitter (Zuckerman, 1979:340).
Neurotransmitters can be produced in certain places and affect those places but they may also effect more distant locations. Neurotransmitters have their effects wherever there are structures known as receptors. The receptors are sites designed to accept the neurotransmitters. So far, receptors have been identified for all the proven neurotransmitters (Cooper, et. al. 1991:88). An example are the enkephalins that have analgesic actions similar to those of morphine and related opiate drugs largely because they bind to the same postsynaptic receptor molecules.
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