Pharm 100 - Lesson B.5
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Psychoactive drugs are agents that can act on the central nervous system and alter sensation,perception, mood, behaviour or consciousness.
Possible Classification of Psychoactive Drugs
- 1. Mechanism of action: At the present time there is insufficient information available to make aclassification on this basis, but as research progresses such a classification may become possible.
- 2. Chemical structure: Classification based on chemical structure does not work since some drugswith similar chemical structures have different pharmacological activities, while other drugs withdissimilar structures have closely similar pharmacological activities.
- 3. Major behavioural effect or major clinical or non-medical use: Classification of agents basedon either of these is the most practical method.
- Psychoactive drugs do not create new behavioural or physiological responses, but act by modifying ongoing physiological and biochemical responses. This appreciation of the limitations ofpsychoactive drugs has been appropriately summarised as described below by the well known author,Koestler, in 1967.
- “. . . It is fundamentally wrong, and naive, to expect that drugs can present the mind with gratis gifts – put into it something which is not already there. Neither mystic insights, norphilosophic wisdom, nor creative power can be provided by the pill or injection. Thepsychopharmacologist cannot add to the faculties of the brain – but he can, at best, eliminateobstructions and blockages which impede their proper use. He cannot aggrandise us – but he can, within limits, normalize us he cannot put additional circuits into the brain, but he can,again within limits, improve the coordination between existing ones, alternate conflicts,prevent blowing of the fuses, and ensure a steady power supply. That is all the help we canask for -- but if we were able to obtain it, the benefits to mankind would be incalculable . . .”
- The following are examples of drugs that are psychomotor stimulants and their mechanism ofaction which involve actions on neurotransmission.
- (a) Cocaine: Acts by blocking dopamine reuptake into presynaptic nerve terminals.
- (b) Amphetamine and derivatives: Act by releasing dopamine from presynaptic nerve terminals.
- (c) Caffeine: Caffeine is a blocker (competitive antagonist) of adenosine at its receptors located oncell membranes in the central and peripheral nervous system. Its action as a psychostimulant resultsfrom antagonism of adenosine-induced neuronal inhibition.
- (d) Nicotine: Stimulates a selective subgroup of acetylcholine receptors in the central nervous systemknown as nicotinic receptors
general CNS depressants
- There are at least six categories,namely, barbiturates, non-barbiturate hypnotics, general anesthetics, ethyl alcohol,benzodiazepines, and inhalants of abuse.
- The effects of CNS depressants are dependent on dose. Asmall dose of barbiturate will cause relief of anxiety. As the dose increases, depression of inhibitoryneuronal pathways will result in disinhibition. Further increases in dosage will result in sedationfollowed by hypnosis (sleep). A further increase in dosage will result in general anesthesia, followed bycoma, and if the dosage is large enough, to death. Thus. barbiturates, commonly prescribed as hypnotics(sleeping tablets) before the advent of the benzodiazepines, were commonly used in suicide attempts.
- CNS depressants are marketed by the pharmaceutical industry as sedatives, anti-anxiety (Anxiolytic)agents, hypnotics, minor tranquillizers, and major tranquillizers. It should be appreciated that themedical use of a CNS depressant drug is usually a function of dose and that the above terms areconvenient means for marketing of drugs.
Romeo and Juliet, Act IV,Scene I,
- An interesting exercise for you is to read the following section from Romeo and Juliet, Act IV,Scene I, and try to deduce which drug available at the present time might be appropriate for the purpose. You might also consider what the elimination half-time (t1/2) of the drug should be. Remember that adrug would be eliminated completely after a period of 5 × t1/2.
- Friar Lawrence to Juliet:
- “. . . Take thou this vial, being then in bed,And this distilled liquor drink thou off:When presently, through all thy veins shall runA cold and drowsy humour; for no pulse Shall keep his native progress, but surcease: No warmth, no breath, shall testify thy liv’st;The roses in the lips and cheeks shall fadeTo paly ashes; thy eyes’ windows fall,Like death, when he shuts up the day of life;Each part depriv’d of supple government,Shall, stiff and stark and cold appear like death:And in this borrow’d likeness of shrunk deathThou shalt continue two-and forty-hours And then awake as from a pleasant sleep . . .
Four Principles of CNS Depressant Use
- 1. The effect of CNS depressants are additive, and sometimes supra-additive, with one another. Aparticular hazard is that which occurs when an individual has ingested a CNS depressant such as a benzodiazepine, an antihistamine, or a narcotic analgesic and then attends a cocktail party and hasseveral alcoholic drinks. Not surprisingly, the additive effect can cause the individual to havesevere CNS depression. The combined CNS depressant effect can be even more severe in anindividual who is physically tired or depressed.
- 2. Use of a behavioural stimulant such as caffeine in a patient severely depressed by a CNSdepressant drug may, through non-specific antagonism, cause a temporary arousal of the depressed individual. However, when the stimulant effect of the caffeine terminates the individual may be leftin an even more depressed state.
- 3. An individual consuming large doses of a CNS depressant for a prolonged period will usuallybecome physiologically dependent (addiction) upon the drug. When physical dependence to thedrug occurs, rebound excitability is observed upon withdrawal from the drug.
- 4. The use of any CNS depressant drug is associated with the risk of inducing psychologicaldependence and tolerance. Cross tolerance may be observed. In cross tolerance, individualstolerant to one CNS depressant drug may show a diminished response to a second CNS depressantdrug. In cross dependence, one drug (e.g. a benzodiazepine) may ameliorate the symptoms whichdevelop upon withdrawal from a second drug (e.g. ethanol) in a physiologically-dependentindividual.
Classification of Drugs that Alter Mood or Behaviouror that are Used to Treat CNS Disorders 8
- 1. CNS depressants (e.g. barbiturates, ethyl alcohol).
- 2. CNS stimulants (e.g. cocaine and amphetamine).
- 3. Antidepressants (e.g. tricyclic antidepressants) and mood stabilizers (e.g. lithium).
- 4. Antipsychotic drugs (e.g. phenothiazines, atypical antipsychotics.)
- 5. Narcotic analgesics (e.g. morphine).
- 6. Psychedelics and hallucinogens (e.g. lysergic acid diethylamide, LSD).
- 7. Cannabis
- 8. Neurological drugs (e.g. antiepileptic drugs such as phenytoin).
Medical and Non-Medical Use
Some drugs have medical uses and are used to treat individuals who are sick (medical use).Some drugs are used to alter the mental state of a normal person (non-medical use).
Immediate and Slowly Evolving Pharmacological Effects
Some drug effects can be immediate, e.g. amphetamines which can rapidly alter behaviour.Some drug effects can evolve slowly, e.g. antidepressant action can take weeks to develop fully
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