52 CHAPTER 2
memories, and low levels of ACh have been associated with Alzheimer ’s disease, the
most common type of dementia. to Learning Objective 6.13. We will focus more
on agonists and antagonists later in the chapter.
Dopamine (DA) is a neurotransmitter found in the brain, and like some of the other
neurotransmitters, it can have different effects depending on the exact location of its
activity. For example, if too little DA is released in a certain area of the brain, the result
is Parkinson’s disease—the disease that is currently being battled by actor Michael J.
Fox, and that affected the late former boxing champ Muhammad Ali (Almasay, 2016;
Ahlskog, 2003). If too much DA is released in other areas, the result is a cluster of symp-
toms that may be part of schizophrenia (Akil et al., 2003). to Learning Objec-
tive 14.13.
Serotonin (5-HT) is a neurotransmitter originating in the lower part of the brain
that can have either an excitatory or inhibitory effect, depending on the particular syn-
apses being affected. It is associated with sleep, mood, anxiety, and appetite. For exam-
ple, low levels of 5-HT activity have been linked to depression. to Learning
Objective 14.9.
Although ACh was the first neurotransmitter found to have an excitatory effect
at the synapse, the nervous system’s major excitatory neurotransmitter is glutamate.
Like ACh, glutamate plays an important role in learning and memory and may also
be involved in the development of the nervous system and in synaptic plasticity (the
ability of the brain to change connections among its neurons). However, an excess
of glutamate results in overactivation and neuronal damage and may be associated
with the cell death that occurs after stroke or head injury or in degenerative diseases
like Alzheimer’s disease and Huntington disease (Julien et al., 2011; Siegelbaum et al.,
2013).
Another neurotransmitter is gamma-aminobutyric acid or GABA. Whereas glutamate
is the major neurotransmitter with an excitatory effect, GABA is the most common neu-
rotransmitter producing inhibition in the brain. GABA can help calm anxiety, for exam-
ple, by binding to the same receptor sites that are affected by tranquilizing drugs and
alcohol. In fact, the effect of alcohol is to enhance the effect of GABA, which causes the
general inhibition of the nervous system associated with getting drunk. This makes alco-
hol an agonist for GABA. to Learning Objective 4.13. Table 2.1 below lists some
neurotransmitters and their functions.
A group of substances known as neuropeptides can serve as neurotransmitters or
hormones or influence the action of other neurotransmitters (Schwartz & Javitch, 2013).Table 2.1 Some Neurotransmitters and Their Functions
Neurotransmitters Functions
Acetylcholine (ACh) Excitatory or inhibitory; involved in arousal, attention, memory,
and controls muscle contractions
Norepinephrine (NE) Mainly excitatory; involved in arousal and mood
Dopamine (DA) Excitatory or inhibitory; involved in control of movement and
sensations of pleasure
Serotonin (5-HT) Excitatory or inhibitory; involved in sleep, mood, anxiety,
and appetite
Gamma-aminobutyric acid
(GABA)Major inhibitory neurotransmitter; involved in sleep and inhibits
movement
Glutamate Major excitatory neurotransmitter; involved in learning, memory
formation, nervous system development, and synaptic plasticity
Endorphins Inhibitory neural regulators; involved in pain reliefThe venom of the black widow spider
causes a flood of acetylcholine to be
released into the body’s muscle system,
causing convulsions.