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2. BRAIN MECHANISMS: NEUROBIOLOGY AND NEUROANATOMY

When the neurotransmitter binds to its receptors on the postsynaptic cell,
the postsynaptic cell can either become more or less excitable, and thus more
or less likely to fire an action potential. These are known as excitatory and
inhibitory postsynaptic potentials, respectively.

Receptors
Receptors are protein complexes that are located in distinct regions of the
cell membrane, and that neurotransmitters bind with to initiate the
communication of a signal between neurons. There are specific receptors
for each specific neurotransmitter found in the brain. Psychoactive
substances are able to bind to these receptors, interfering with normal
transmitter function. Different classes of substances bind with distinct
receptors, thus giving the characteristic effects of each substance class – e.g.
opioids such as heroin and morphine bind to opioid receptors, cannabinoids
bind to cannabinoid receptors, and nicotine binds to nicotinic receptors in
the brain – and have powerful effects on behaviour. These and other
mechanisms will be discussed in more detail in Chapter 4.
There are two basic mechanisms of signal transduction that are important
when considering the actions of psychoactive substances. Binding of
neurotransmitters to receptors can cause the opening of ion channels directly,
through ligand-gated ion channels (Fig. 2.12). Binding of a ligand to the
receptor opens the ion channel, allowing rapid changes in the postsynaptic
membrane. An example of this type of channel is the γ-aminobutyric acid
(GABA)-A receptor, to which benzodiazepines and barbiturates can bind to
increase the opening of this channel. Alternatively, binding of the ligand can

Fig. 2.12 Two types of chemical synapses
The first diagram shows binding to and opening of a ligand-gated ion channel.
The second diagram demonstrates activation of a G protein-coupled receptor
resulting in the opening of an ion channel via a second messenger.

Neurotransmitter Neurotransmitter
Ions binds binds
Neurotransmitter
receptor
Neuro-
transmitter
Outside cell
Ion
channel
γ opens.
β
Inside cell α α
G protein
Ions
Channel opens. Ions flow across G protein is G protein subunits or Ions flow across
membrane. activated intracellular messengers membrane
modulate ion channels
Source: Reproduced from Rosenzweig, Leiman, & Breedlove, 1999, with permission from the
publishers.
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