There Are More Than 100 Different Neurotransmitters

The classification of transmitters into neuropeptides and small molecules is based on how the transmitter is made. Typically neuropeptides are synthesized on polyribosomes that attach to the endoplasmic reticu-lum during translation. Characteristically, a polyprotein, while it is translated from messenger RNA, is extruded into the lumen of the endoplasmic reticulum and is then transported to the Golgi apparatus. During its passage through the major membrane systems of the cell, the polyprotein is cut into peptides, which ultimately are packaged into secretory vesicles. Some of these peptides had been identified earlier as hormones in other tissues, notably the gut (for example, gastrin), or as neurosecretory products (for example, oxytocin, vasopressin, somatostatin, and thyrotropin-releasing hormone). A partial list of neuropeptides is presented in Table I.

In contrast, small-molecule neurotransmitters are synthesized in the neuron's cytoplasm, usually in short enzymatic pathways much like intermediates of metabolism (Table II). There are three types of small-molecule neurotransmitters. The first is amine transmitters, which are amines or molecules derived from amines in a few enzymatic steps. This group includes (1) ACh, (2) biogenic amines, and (3) various amino acids. Because these molecules were discovered first historically, they are often referred to as classical neurotransmitters. Loewi's experiment with acetylcholine (1921) has already been mentioned; the pharmacology of norepinephrine (NE) and related amines was being studied contemporaneously by

Table I

Some Families of Peptide Neurotransmitters

Table II

Small-Molecule Neurotransmitters and Their Neuron-Specific Enzymes

Transmitter

Table I

Some Families of Peptide Neurotransmitters

Family

Neurotransmitter

Opioid

Opiocortins, enkephalins,

Conquering Fear In The 21th Century

Conquering Fear In The 21th Century

The Ultimate Guide To Overcoming Fear And Getting Breakthroughs. Fear is without doubt among the strongest and most influential emotional responses we have, and it may act as both a protective and destructive force depending upon the situation.

Get My Free Ebook


Post a comment