The biogenic amines are neurotransmitters that are synthesized from amino acids and contain an amino group (R-NH2). The most common biogenic amines are dopamine, norepinephrine, serotonin, and histamine. Epinephrine, another biogenic amine, is not a common
PART TWO Biological Control Systems neurotransmitter in the central nervous system but is the major hormone secreted by the adrenal medulla. Norepinephrine is an important neurotransmitter in both the central and peripheral components of the nervous system.
Catecholamines Dopamine, norepinephrine (NE), and epinephrine all contain a catechol ring (a six-carbon ring with two adjacent hydroxyl groups) and an amine group; thus they are called catecholamines. The catecholamines are formed from the amino acid tyrosine and share the same basic synthetic pathway (Figure 8-34), which begins with the uptake of tyro-sine by the axon terminals. Depending on the enzymes present in the terminals, any one of the three cate-cholamines may be ultimately released. Synthesis and release of the catecholamines from the presynaptic terminals are strongly modulated by autoreceptors on the presynaptic terminals.
After activation of the receptors on the postsyn-aptic cell, the catecholamine concentration in the synaptic cleft declines, mainly because the cate-cholamine is actively transported back into the axon terminal. The catecholamine neurotransmitters are also broken down in both the extracellular fluid and the axon terminal by enzymes such as monoamine oxidase. Monoamine oxidase inhibitors, which increase the brain extracellular concentration of the cate-cholamine neurotransmitters, are used in the treatment of diseases such as depression, as will be discussed in Chapter 13.
Within the central nervous system, the cell bodies of the catecholamine-releasing neurons lie in parts of the brain called the brainstem and hypothalamus, and although relatively few in number, their axons branch greatly and may go to virtually all parts of the brain and spinal cord. The catecholamines exert a much greater influence in the central nervous system than the number of neurons alone would suggest, possibly because of their neuromodulator-like effects on post-synaptic neurons. These neurotransmitters play essential roles in states of consciousness, mood, motivation, directed attention, movement, blood-pressure regulation, and hormone release, all functions that will be covered in later chapters.
During the early experiments on norepinephrine and epinephrine, norepinephrine was mistakenly taken to be epinephrine, and epinephrine was called by its British name "adrenaline." Consequently, nerve fibers that release epinephrine or norepinephrine came to be called adrenergic fibers. Norepinephrine-releasing fibers are also called noradrenergic.
There are two major classes of receptors for nor-epinephrine and epinephrine: alpha-adrenergic receptors and beta-adrenergic receptors (these are also called alpha-adrenoceptors and beta-adrenoceptors). The major way of distinguishing between the two classes of receptors is that they are influenced by different drugs. Both alpha- and beta-adrenergic receptors can be subdivided still further (alpha! and alpha2, for example), again according to the drugs that influence them and their second-messenger systems.
Catecholamine biosynthetic pathway. The red asterisk indicates the site of action of tyrosine hydroxylase, the rate-limiting enzyme; the colored screen indicates the more common CNS catecholamine neurotransmitters.
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Neural Control Mechanisms CHAPTER EIGHT
Serotonin While not a catecholamine, serotonin (5-hydroxytryptamine, or 5-HT) is an important biogenic amine. It is produced from tryptophan, an essential amino acid. Its effects generally have a slow onset, indicating that it works as a neuromodulator. Serotonin-releasing neurons innervate virtually every structure in the brain and spinal cord and operate via at least 16 different receptor types.
In general, serotonin has an excitatory effect on pathways that are involved in the control of muscles, and an inhibitory effect on pathways that mediate sensations. The activity of serotonergic neurons is lowest or absent during sleep and highest during states of alert wakefulness. In addition to their contributions to motor activity and sleep, serotonergic pathways also function in the regulation of food intake, reproductive behavior, and emotional states such as mood and anxiety.
Serotonin is also present in many nonneural cells (for example, blood platelets and certain cells of the immune system and digestive tract). In fact, the brain contains only 1 to 2 percent of the body's serotonin.
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