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Essentially, behavior occurs as a result of neurons releasing
neurotransmitters to activate or signal other neurons. Neurons receive
this chemical data at their receptor sites. If enough receptor sites are
stimulated by the chemical data, behavior occurs.

To help understand the biology of behavior including opioid addiction,
the “lock and key” model can sometimes assist in describing how
neurotransmitters and receptors work. In this model, the
neurotransmitter acts like a chemical “key” that fits into and binds to a
receptor site “lock”. If the correct neurotransmitter keys fit into and
unlock the receptor site, communication between neurons takes place
and a behavioral effect results. Exactly what type of behavior is
determined by where in the brain this activity occurs and with what
type of neurotransmitters, and at which receptor sites. The discovery
of specific neurotransmitters and receptors in the brain and body for
opiate drugs was central to the discovery of opioid neurotransmitters
and then, to the discovery of specific opioid receptor sites.
Endorphins are the chemical “keys” for the opiate receptor sites.
Endorphins and any type of opiate drug will bind at opiate receptors
and signal a response. Receptors are located along the outer
membrane of the neuron. (figure 2)

figure 2