It’s Professor Dave, let’s check out your
brain on drugs.
A drug is defined as any substance that has
a physiological effect when ingested.
Drugs can therefore range from something as
common as aspirin or caffeine to all the illicit
or hallucinogenic substances that typically
come to mind during any discussion of drugs.
A thorough analysis of specific drugs and
how they interact with the body to produce
a particular physiological effect will have
to wait for the upcoming pharmacology series.
But in the context of biopsychology, the phenomenon
of drug addiction will be of significant interest to us.
What does it mean to get addicted to a drug?
How does this happen, and what does the corresponding
brain activity look like?
Let’s take a closer look at the drug-addicted
brain now.
Drugs enter the body in a few different ways.
They can be ingested orally, like a pill,
injected into the bloodstream, inhaled into
the lungs, or absorbed through any of the
external mucous membranes of the body.
The path taken will impact the severity of
the effect, as well as the rate of its onset.
The bloodstream is the most direct, and thus
the fastest and most predictable, while the
other methods eventually make it to the bloodstream
after being absorbed into blood vessels from
wherever they were administered.
Some drugs are able to penetrate the blood-brain
barrier that we discussed earlier, thus making
their way into the brain, while others are not.
Of the ones that do, most can be referred
to as psychoactive drugs, which generally
means any drug that affects the mind.
This is typically achieved in one of several ways.
Some drugs bind to certain synaptic receptors,
acting as inhibitors, also called antagonists,
while others bind and behave as agonists,
meaning that they mimic the role of the native ligand.
Some drugs influence the synthesis, transport,
release, or deactivation of specific neurotransmitters.
Whatever the case may be, the drug will continue
to have its particular effect until it is
metabolized by enzymes, which essentially
chop them up until they can no longer perform
any function.
The body will respond to the presence or absence
of a drug differently over time.
If exposed to a particular drug regularly,
a tolerance can be developed.
This is a decreased sensitivity to the drug,
either in the way of metabolic tolerance,
where less and less of the drug makes it to
its destination, or functional tolerance,
where the drug makes it to where its going,
but its efficacy diminishes, often because
receptors undergo endocytosis.
If the body grows accustomed to a drug, its
sudden elimination can trigger symptoms of
withdrawal.
These tend to be the opposite of the effects
of the drug, and if withdrawal is experienced,
it means a physical dependency has developed.
This is a big part of what we refer to as
drug addiction.
A drug addict will use a particular drug habitually,
despite the adverse effects on the health
or social life of the individual.
This goes beyond a mere physical dependence
that can develop with certain substances,
as addiction can also be a psychological condition,
as is evidenced by addictions to activities
like gambling, which has nothing to do with
any substance, but works quite similarly from
a neural standpoint.
While physical addiction can arise with a
wide variety of substances, a few common ones
are tobacco, alcohol, cocaine, and opiates.
We can examine these to introduce a few concepts.
With tobacco, there are many compounds that
are ingested, and many of these are harmful
to one’s health, but the one that causes
addiction is nicotine.
This acts on nicotinic cholinergic receptors
in the brain.
These normally respond to acetylcholine, but
nicotine is an agonist for these receptors
as well.
This causes the receptors to open, allowing
ions to enter, which eventually results in
the release of neurotransmitters like dopamine,
which generates a pleasurable sensation.
The brain responds through neuroadaptation,
affecting the binding sites for nicotine,
which produces withdrawal symptoms, thus establishing
tolerance and dependence.
Nicotine addiction can arise very quickly,
even after just a few weeks of regular use.
In alcoholic beverages, the active agent is ethanol.
This interacts with the brain in a variety of ways.
In the cerebral cortex, behavioral inhibitory
centers are depressed, which lowers behavioral
inhibition, and processing of information
slows down.
It affects the center of movement and balance
in the cerebellum, as well as the medulla,
which impacts breathing and consciousness.
Long-term alcohol exposure causes neurological
changes resulting in tolerance, which then
causes excitation of certain neurotransmitter
systems as well as withdrawal symptoms in
absence of the drug.
This is alcohol addiction.
There is also a major genetic component to
alcohol addiction, or a predisposition that
has about a fifty percent probability of being
passed on to offspring.
Cocaine, on the other hand, is a stimulant,
meaning it increases neural activity.
It acts by inhibiting the reuptake of dopamine
from the synaptic space, thus keeping their
levels quite high.
And finally, opiates like heroin and morphine
bind to opioid receptors that normally bind
to endogenous neurotransmitters like endorphins,
so they mimic innate mechanisms of pain reduction,
causing euphoria.
Heroin is widely regarded as the most addictive
substance we are aware of.
As we mentioned, we will get a much closer
look at specific drugs and their mechanisms
of action during the upcoming pharmacology
series.
For now, let’s mention just a few more things
about the brain and addiction in a general sense.
As we have begun to discuss, an understanding
of addiction must combine the notion of physical
dependence with positive incentive.
Sometimes drug use involves a direct effort
to alleviate withdrawal symptoms.
However, a very high percentage of drug users
that go through rehabilitation and completely
rid themselves of physical dependence will
nevertheless return to the drug, illustrating
that the craving for the pleasurable properties
of the drug is a huge factor with addiction.
Countless experiments done with rats in isolation
show that they will self-administer electrical
stimulation to pleasure-producing areas of
the brain, foregoing essentially all other
activity in favor of maintaining this stimulation.
Dopaminergic neurons project from the midbrain
into a number of regions of the telencephalon,
including the prefrontal cortex, limbic cortex,
amygdala, and more, and these structures are
heavily involved with this self-stimulating behavior.
We can therefore identify dopamine as a crucial
component of drug addiction, or addiction
in general.
There is much more to discuss regarding drugs,
in both a recreational and medicinal context,
but right now let’s move on to some other
topics in biopsychology.
