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2. BRAIN MECHANISMS: NEUROBIOLOGY AND NEUROANATOMY

Serotonin
Serotonin, like dopamine and norepinephrine, is a monoamine. It is an
indoleamine that is derived from the amino acid tryptophan. It is involved in
regulation of mood, arousal, impulsivity, aggression, appetite and anxiety.
Serotonin-synthesizing cell bodies are found in the midbrain in a region called
the raphe nuclei. These neurons project to many areas of the brain such as
the cortex, hypothalamus and limbic system. There are many subtypes of
serotonin receptor. In the body, serotonin is found in the gastrointestinal tract,
platelets and spinal cord. Most antidepressant drugs work by increasing the
action of serotonin in the brain. Serotonin is also involved in the primary
actions of some psychoactive drugs such as lysergic acid diethylamide (LSD)
and ecstasy, and is also implicated in the effects of cocaine, amphetamine,
alcohol and nicotine.

Peptides
Peptides are chains of two or more amino acids linked by peptide bonds.
There are many peptides that are widely distributed throughout the nervous
system, and at least 200 identified neuropeptides to date. Some are hormones
that cause the release of other hormones, such as corticotrophin-releasing
hormone and growth hormone-releasing hormone. There are pituitary
peptides such as adrenocorticotropin, prolactin and growth hormone, and
there are a wide variety of peptides that were originally discovered in the gut,
but that also have actions in the brain, such as cholecystokinin, substance P
and vasoactive intestinal polypeptide. The endogenous opioids are also an
important class of peptide neurotransmitters. Substances such as heroin and
morphine bind to the receptors used by the endogenous opioids. Peptides
control a wide variety of functions in the body, from food intake and water
balance, to modulating anxiety, pain, reproduction and the pleasurable effects
of food and drugs. Although the opioids are widely recognized as being
involved in substance dependence, it has been shown that other peptides
also play a role (Kovacs, Sarnyai & Szabo, 1998; McLay, Pan & Kastin, 2001;
Sarnyai, Shaham & Heinrichs, 2001).

Genes
Inside the nucleus of the cell are the chromosomes, which are made up of
strands of DNA. The chromosomes are made up of distinct sets of
instructions, or genes, that “code” for proteins. Messenger ribonucleic acid
(mRNA) makes copies of sections of DNA, and transports it into the
cytoplasm. In the cytoplasm, the mRNA binds to ribosomes, which “read”
the genetic code and assemble the appropriate proteins from amino acids
in the cytoplasm (Fig. 2.9). These proteins are then used to carry out the
functions of the cell.

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