Psychotropic Drug Classification
-Drugs with almost identical molecular structures may induce entirely different effect,
while other drugs whose molecular structures are quite different may induce almost
-A drug may have multiple psychological effects in a certain dose range, and depending
on the population taking them, some of these effects may be viewed as desirable in one
person and undesirable in another.
Molecular Structure—Activity Relationships
-One classification scheme that appears to be intuitively reasonable involves grouping
drugs according to their chemical structure.
-In many cases what appear to be relatively minor changes in the molecular structure of a
drug can considerably change its basic activity in the body.
-The first is simply the structure of one of the basic neurotransmitters, dopamine.
-The second structure is that of amphetamine, the actions of certain doses of it are very
similar to the actions of dopamine
-This similarity is not surprising, because amphetamine has several biochemical actions
that essentially amplify dopamine’s actions.
-One of the amphetamine’s biochemical actions is to temporarily inhibit the enzyme
monoamine oxidase (Mao)
-The third structure is that of tranylcypromine, a drug with antidepressant properties.
www.notesolution.com -While its structure is quite similar to that of amphetamine, it is several thousand times
more potent in its ability to inhibit MAO.
-The fourth structure is that of mescaline, Both mescaline and amphetamine have
structural similarities, elevate mood in low doses, and are capable of inducing psychosis-
mimicking effects, such distortion in perception and in some case hallucinations
-The psychosis associated with mescaline is readily distinguishable from naturally
occurring psychoses, whereas the psychosis induced by amphetamine is not.
-The fifth structure is that of a drug with mescaline-like effects, but it is 10 to 100 times
more potent than mescaline.
-the structure of that is lysergic acid diethlamine (LSD), although LSD is considerably
more potent thatn mescaline, it is capable of inducing effects that are practically
indistinguishable from those of mescaline.
-There are many cases in which molecules that are identical except for their being mirror
images of each often have very different effects on the body.
-Such molecules are called optical isomers (or more technically enantiomers) which are
commonly differentiated by the prefixes levo (l-dopa) and dextro (d-amphetamine)
because the two forms in solution rotate plane-polarized light in different direction—to
the left with levo isomer and to the right with dextro isomer.
-When the two isomers display differential receptor-binding properties and therefore
induce different effects are said to be stereospecific.
-Parts (a) and (b) show skeleton molecular structures of the levo (active) and dextro
(inactive) isomers of morphine.
-They are mirror images of the morphine; one produces analgesia, and the other do not.
www.notesolution.com -A slight modification of the active molecule produces the molecular structure shown in
part c, nalorphine, an antagonist of morphine.
-Nalorphine still has some weak narcotic-like effects.
-It has mized agonist—antagonist properties.
-It is thought that when the chain attachment to the nitrogen atom is in the “down”
position, it acts as an antagonist, whereas when it swings around and “up”—indicated by
the dashed lines—it acts as an agonist.
-Another modification of the active morphine molecule produces naloxone, shown in part
(d), a narcotic antagonist without any narcotic actions at all. It only has antagonist
properties as the OH present the chain to swing to agonist conformation.
-Slight modifications in a molecule can change it from an active molecule at its receptor
into an inactive molecule, a mixed agonist—antagonist, or a “pure antagonist”.
-Despite the dissimilarity of their molecular structures, studies have indicated that they do
possess analogous regions that probably allow them to both bind and activate opioid
-Biologists, using sophisticated molecular computer modeling programs and visualization
software, can now easily create proteins on a computer screen and experiment with them
—bringing other molecules up close to them and determining which drugs will fit best
into their active sites.
-this capability is particularly valuable for pharmaceutical companies, whose drug
designers can test thousands of potential drugs on a computer to find the most promising
candidate for producing a specified effect.
Depressant versus Stimulant Classifications
-Another classification – Activation dimension
-Classifying drugs as stimulant or depressant can often be misleading.
-The terms depressant and stimulant do not mean much unless one specifies at which
biological level the depression or stimulation is occurring, or the dose of the drug.
-A drug may stimulate the receptors of a particular set of neurons, but if this receptor
allow the influx of negative ions and hyperpolarize the cells, the excitability of the
neurons will decrease.
-Alternatively, the drug may increase the firing rate of one set of neurons or stimulate the
release of particular type of neurotransmitter.
-If the neurotransmitter released happens to be at inhibitory synapses, the drug in effect
would increase the excitability of the receiving cell, decrease their firing rate, and
decrease the neurotransmitter they release.
Conversely, a drug may depress a particular set of neurons in terms of their excitability
and their rate of firing, if these neurons play an inhibitory role with respect to other
neuron, the later neurons excitability may increase.
-Although a drug may decrease the relative frequency of one set of behaviours, it may
increase the relative frequency of other sets of behaviours. – one dose elevate mood
another dose depress mood. Eg Alcohol and Barbiturates
www.notesolution.com -Amphetamine (commonly-Stimulant) increase the rate of firing of some neuron while
simultaneously decreasing the rate of firing of others.- increases the production of some
behaviours and reduces others.
-The failure to specify the dimension referred to with respect to a “stimulant” drug’s
actions or effects i.e., receptor, neuronal, behavioural, affective, or cognitive, leads to the
common misconception that pychostimulants