23.7: There are many types of neurotransmitters.

The neurotransmitters released by neurons can be thought of as the chemicals that initiate or modify a wide variety of actions, moods, feelings, or other sensations. Some make us feel happy or sad, restless or sedate. Some stimulate muscles to contract. Others influence learning or memory. In all, several dozen neurotransmitters have been identified, each of which has a sort of “personality” based on the effects associated with its release. A detailed discussion of all the neurotransmitters is beyond the scope of this book. Here we discuss four that are particularly important.

Acetylcholine Acetylcholine is the neurotransmitter released by motor neurons at the point where they synapse with muscle cells. When enough acetylcholine binds to a muscle cell, the muscle contracts. Botulinum toxin—a protein produced by several species of bacteria—is the most toxic substance known: less than a microgram is lethal if injected into a person or inhaled. One type of botulinum toxin, known commercially as Botox, is an increasingly popular drug used in cosmetic procedures, with more than a billion dollars spent on it worldwide each year (FIGURE 23-14). What does this have to do with neurotransmitters? When injected into muscles, Botox blocks the fusion of acetylcholine-filled vesicles with the presynaptic neuron’s membrane, preventing release of acetylcholine into the synapse and thus paralyzing the muscle. A tiny amount of Botox can prevent a muscle from contracting for three or four months!

Why would a healthy person want to get near the most toxic substance known? Tiny injections of Botox can smooth lines in the forehead and can also reduce “crow’s-feet” wrinkles around the eyes. Because Botox essentially paralyzes muscles, however, it can lead to problems. At the extreme, if Botox spreads to muscles other than its intended targets, it can interfere with the muscles needed for swallowing and breathing. Most of the 180 reported cases of life-threatening problems resulting from Botox injections between 1997 and 2006 stemmed from problems of this nature. Less life-threatening, but disturbing to many, is the severe reduction in individuals’ ability to move parts of their face that can result from Botox injections, often causing droopy eyelids or a mask-like face and an inability to express emotions.

Glutamate Another neurotransmitter is glutamate. Although its mechanism of action isn’t well understood, it appears to be involved with learning and memory. Mice that were genetically engineered to have more sensitive glutamate receptors, for instance, learned tasks better and became much better at running mazes than normal mice.

Dopamine Dopamine is important in initiating and coordinating movement. Loss of dopamine neurons may be responsible for Parkinson’s disease, for which the symptoms include tremors of a hand or foot, stiffness or inflexibility of muscles, and impaired balance and coordination. Dopamine is also one of the body’s chief “happiness” neurotransmitters. Its release in certain parts of the brain is associated with feelings of intense pleasure. In studies of brain functioning, for example, when subjects are shown their favorite food items, the levels of dopamine released in the brain skyrocket. We explore dopamine in greater detail later in this chapter.

Serotonin Serotonin generally functions as an inhibitory neurotransmitter. It affects appetite, sleep, anxiety, and mood, and produces feelings of contentment and satiation when released. Women make serotonin only about two-thirds as quickly as men, an observation that may be related to why depression is twice as common among women as men. As we see later in the chapter, antidepressant medications such as Prozac increase the amount of serotonin in the synapses and are effective at elevating the mood of someone who is depressed.