Agonists
binds to a receptor and activates the receptor to produce a biological response.
Receptors can be activated by either endogenous agonists (like hormones or neurotransmitters) or exogenous agonists (like drugs).
Agonists can bedivided into following sub-categories:
• Full agonists
They bind to and activate a receptor with the maximum response that an agonist can elicit at the receptor.
• Co-agonists
A co-agonist works with other co-agonists to produce the desired effect together.
• Selective agonists
A selective agonist is selective for a specific type of receptor only.
• Partial agonists
Partial agonists like buprinorpine also bind and activate a given receptor but have only partial efficacy at the receptor relative to a full agonist, even at maximal receptor occupancy.
• Inverse agonists
An inverse agonist is an agent which binds to the same receptor binding-site as an agonist and inhibits the constitutive activity of the receptor. exert the opposite pharmacological effect to that of an agonist.
• Super agonists
It is a term used to identify a compound which is capable of producing a greater response than the endogenous agonists for the target receptor.
• Irreversible agonists
An irreversible agonist is a type of agonist which binds permanently to a receptor through formation of covalent bonds.
Antagonists
Competitive Antagonists
Competitive antagonists bind to receptors at the same binding site (active site) as the endogenous ligand or agonist, but without activating the receptor.
Agonists and antagonists compete for the same binding site on the receptor. Once bound, an antagonist will block binding of agonist. Sufficient concentration of an antagonist will displace the agonist from the binding sites, resulting in a lower frequency of receptor activation. The level of activity of
the receptor will depend on relative affinity of each molecule for the site and their relative concentrations. Competitive antagonists are used to prevent the activity of drugs and to
reverse the effects of drugs that have already been consumed.
non-competitive antagonist
A non-competitive antagonist is a act in one of two ways:
by binding to the active site of receptor or by binding to an allosteric site of the receptor. If it binds to the allosteric site, it is called as allosteric antagonist. In both the
cases, end-results are functionally similar. Unlike competitive antagonists, which affect the
amount of agonists necessary to achieve a maximal response but do not affect the magnitude of that maximal response, non-competitive antagonists reduce the magnitude of
the maximum response that can be attained by any amount of agonists.
Spare Receptors
Spare receptors are defined as those receptors without combining with which maximal response can be obtained.
Addiction
Addiction is a brain disorder characterised by compulsive engagement in rewarding
stimuli despite adverse consequences. It is related to addictive behaviour which is both
rewarding and reinforcing.
Tolerance
Drug tolerance is defined as, "the diminishing effect of drug resulting from repeated administration at a given dose". Drug tolerance is a pharmacological concept describing
subjects’ reduced reaction to a drug following its repeated use.
Pharmacodynamic tolerance begins when the cellular response to a substance is reduced with repeated use.
Pharmacokinetic tolerance occurs because of a decreased quantity of the substance reaching the site it affects, this may be caused by
increase in induction of enzymes required for degradation of drug.
Behavioural tolerance occurs with the use of some psycho-active
drugs, where tolerance to a behavioural effect of a drug occurs with repeated use of the drug. Amphetamine causes behavioural tolerance.
Dependence
It is defined as, "an adaptive state associated with a withdrawal syndrome upon cessation of repeated exposure to a stimulus (e.g. drug intake)".
Tachyphylaxis
Tachyphylaxis is a sub-category of drug tolerance referring to cases of sudden, shortterm onset of tolerance following the administration of drug. It is a rapid and short term onset of drug tolerance. It can occur after an initial dose or after a series of small doses.
Idiosyncrasy
Idiosyncratic drug reactions occur rarely and unpredictably amongst the population. They frequently occur with exposure to new drugs. They are listed as rare ADR. They do not appear to be concentration dependent.
Allergy
Allergic reaction to a drug will not occur on the first exposure to a substance. The first exposure allows the body to create antibodies and memory lymphocyte cells for the antigen. Subsequently antibodies or lymphocytes interact with the antigen causing what we understand as allergic reactions.