The Brain

Marijuana, Cocaine, And The Opiates Part 3

Localization Of Opiate Binding Sites

When a person injects heroin or morphine, it too, travels quickly to the brain. Point to the areas where opiates concentrate. The VTA, nucleus accumbens, caudate nucleus and thalamus are highlighted. The opiates bind to opiate receptors that are concentrated in areas within the reward system. The action of opiates in the thalamus contributes to their ability to produce analgesia.

A Picture Of Opiate Binding Sites

Opiates Binding To Opiate Receptors

Opiates activate the reward system using the nucleus accumbens as an example. The action is a little more complicated than cocaine's because more than two neurons are involved.

A Picture Of Opiates Binding To Opiate Receptors

3 neurons participate in opiate action; the dopamine terminal, another terminal (on the right) containing a different neurotransmitter (probably GABA for those that would like to know), and the post-synaptic cell containing dopamine receptors.

Opiates bind to opiate receptors (green) on the neighboring terminal and this sends a signal to the dopamine terminal to release more dopamine.

One theory is that opiate receptor activation decreases GABA release, which normally inhibits dopamine release - so dopamine release is increased.

Increased cAMP Produced In Post-Synaptic Cell

Since there is more dopamine released, there is increased activation of dopamine receptors, similar to the effect of cocaine. This causes increased production of cAMP inside the post-synaptic cell which alters the normal activity of the neuron.

As a result of opiate actions in the nucleus accumbens, there are increased impulses leaving the nucleus accumbens to activate the reward system. As with cocaine, continued use of opiates makes the body rely on the presence of the drug to maintain rewarding feelings and other normal behaviors. The person is no longer able to feel the benefits of natural rewards (food, water, sex) and can't function normally without the drug present.

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