Professor Dave here, let's talk about drugs.
We have all heard of drugs before. Whether the word brings up images of aspirin bottles or
marijuana leaves, a drug is defined as any substance that has a physiological
effect when introduced into the body.
Let's discuss this concept in two parts. Later we will discuss drugs in the
pharmaceutical or medicinal context, but for now let's tackle some of the more
common ones everyone has heard of. For many people, the word drug makes them
think first of narcotics. A narcotic is generally a substance that produces a
numbing or dulling effect, and many of these substances are illegal. The first
narcotic discovered was opium, which is the dried sap of the poppy plant,
popularly used in opium dens of the past. In actuality, opium is comprised of a
number of alkaloids, which are named as such because of their basicity, that is
due to the presence of amino groups. Morphine was the first of these
alkaloids to be isolated and it was immediately observed to be useful as an
analgesic, which is another term for a painkiller. It was also observed to
produce various psychological responses like euphoria and the potential for
crippling addiction.
How does morphine work? Morphine acts as an opioid receptor agonist. That means
that it binds to opioid receptors found on cells in the central nervous system
and mimics the role of endorphins, which are a natural way that the body doles
out pain relief. So it's like a forced physiological response. Once morphine was
introduced into medicine at the beginning of the 20th century, heroin was
almost immediately derived from it.
Heroin is almost the same as morphine although instead of these two hydroxyls
there are two ester groups. Heroin is a more potent narcotic than morphine and
even more addictive. Another drug that was isolated from a plant is cocaine,
which was found in the leaves of the South American coca bush. Cocaine is
another alkaloid and it first found medical application
as an anesthetic. Its use was immediately popularized by Sigmund Freud, the founder
of psychoanalysis, who used it regularly. Cocaine essentially operates as a
reuptake inhibitor, which causes accumulation of the neurotransmitters dopamine,
serotonin, and norepinephrine in the synaptic cleft. This accumulation is
responsible for the feelings of euphoria associated with the drug. Cocaine does
not have physical withdrawal symptoms but it is nonetheless quite
psychologically addictive, due to the reward pathway that it activates.
Not all alkaloid narcotics are illegal.
Caffeine is also an alkaloid and it is the world's most widely consumed
psychoactive drug, found in coffee beans. It operates both by stimulating portions
of the nervous system and by inhibiting receptors that typically bind with
adenosine to produce drowsiness, so the inhibition of this mechanism helps
generate the alertness associated with caffeinated beverages. Nicotine is
another alkaloid, which is found in the tobacco plant. It works by binding to
acetylcholine receptors in the brain, increasing the levels of certain
neurotransmitters. However, nicotine is highly toxic, with a lethal dose for a
human being
about 50 milligrams. Tetrahydrocannabinol, or THC, is found in the marijuana plant
and is classified as a cannabinoid. This molecule operates as an agonist for
cannabinoid receptors located in the nervous system and immune system. Binding
to these receptors will in turn inhibit adenylate cyclase, which means a
reduction in the concentration of cAMP, which results in a variety of
psychological effects. Because psychological effects include the
alteration of cognition and perception, this drug is classified as a psychedelic
drug. Other psychedelic drugs include psilocybin, LSD, mescaline, and DMT,
which predominantly operate by activating serotonin receptors. A key
feature that is common to many of these molecules is the phenylethylamine
functional group.
This structural fragment appears to be crucial in modifying the way nerve cells
carry messages to the brain. Mixed opinions exist about all of the drugs we
have discussed, and there is some controversy over the benefits and harms
associated with each of them. In the end, much of that discussion goes beyond the
scope of biochemistry.
But what does not go beyond biochemistry is their mechanism of action. Every drug
exhibits a particular bioactivity because of its shape and where it fits
in the body. By now, after learning quite a bit about enzymes and receptors, we
should be able to view the cellular landscape within our bodies as a
collection of nooks and crannies where small molecules can fit, and certain
molecules do indeed fit into these nooks and crannies that elicit very specific
biochemical responses. It is important to understand drugs on this fundamental
level, because although an assessment of how they affect the human experience is
more in the domain of psychology or possibly even philosophy, this can always
be reduced to interactions between small molecules.
Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always, feel free to email me: