blood flow and an inhibitor of some sensations of pain; in recent years, its intriguing variety of effects on our mental life have also come under study. Serotonin is of great importance in regulating sleep; the old folk remedy for sleeplessness, a glass of warm milk before bedtime, may work because of the presence in milk of tryptophan, an amino acid that the brain uses to make serotonin.
This transmitter can affect many parts of the brain at once through the long-reaching axons of serotonergic (serotonin-using) neurons, which underlie the transmitter's role in such global phenomena as sleep and mood. The drugs that raise levels of available norepinephrine to alleviate depression also work on serotonin, by the same mechanisms. (Interestingly, the axons that carry serotonin are not myelinated. Without the electrical insulation afforded by the myelin sheath, impulses travel at less than the lightning speed achieved by, say, signals to the motor neurons, but this seems appropriate to the more global and subjective areas of life regulated by serotonin.) The remarkable effects of lysergic acid diethylamide, or LSD, in even the tiniest quantities, are based on its strong chemical resemblance to serotonin; it is as if a full system of preexisting receptor sites lies ready for the drug's use.
One further aspect of this versatile transmitter is that serotonin is featured in biochemical accounts of “sensitization,” the enhanced response to a stimulus as a result of training. Scientists have studied sensitization in extraordinary detail in simple animals such as the marine snail as a model for more complex processes of learning in the human brain (see Chapter 7 ).
Dopamine is chemically similar to serotonin and norepinephrine, and it overlaps with them in several biological functions. Formed, like serotonin, from an amino acid, dopamine is actually a precursor to norepinephrine—the same compound except for one different chemical bond—and a wide-ranging neurotransmitter in its own right. In many systems, dopamine acts as an “off” switch: it halts the release of prolactin (which is responsible for the function of the mammary glands), inhibits some cells of the olfactory tract, and also shuts off some of the action of autonomic nerve cells (although this function is not well understood).
Elsewhere in the nervous system, dopamine is important