Receptors and ligand binding.
Receptor subtypes;
neuronal autoreceptors, heteroreceptors
Mechanisms of drug action;
non-specific, specific.
*
Alexandra Šulcová, M.D., PhD., Professor of Pharmacology
CEITEC (Central European Institute of Technology) Masaryk University
Mechanisms of drug effects
SPECIFIC - RECEPTOR MEDIATED NONSPECIFIC – non-receptor
- physical
(e.g. osmotic diuretics)
- chemical
(e.g. antacids)
DRUG RECEPTOR
afinity
&
intrinsic activity = efficacy
drug = ligand of the receptor
- impact on function
of the receptor system
functioning
( beyond the receptor)
- direct
- indirect
Mechanisms of drug effects
SPECIFIC NONSPECIFIC
- physical
- chemicalDRUG RECEPTOR
affinity,
intrisic activity
- RECEPTOR MEDIATED - NONRECEPTOR
• DIRECT
• INDIRECT
- binding to macromolecules
of the organism which do not serve
physiologically as receptors
(e.g. influence on ion channel,
proton pump,
modification of DNA,
substrate inhibition of enzyme,
binding to cell components,
.....)
Non-receptor Mediated Drug Effects
• Physical
• Chemical
• Metabolic pathways ( beyond the receptor)
- changes of ion channel permeability
- changes of proton pump funtioning,
- DNA modification,
- substrate enzyme inhibition,
- binding to cell components
Biological Effect ∝ [DR]
The term “receptor” specifically refers to proteins
that participate in intracellular communication via chemical signals
Upon recognition of an appropriate chemical signaling molecule
(“ligand“), receptor proteins transmit the signal into a biochemical
change in the target cell
Ligands include
drugs as well as endogenous signaling molecules
such as e.g. hormones and neurotransmitters
What is a Receptor?
Drug binding in the organism }
receptor – modulation of activity
ion channel – modulation of permeability
enzyme – false substrate ( abnormal metabolite;
inhibition of the function;
activation of a "pro-drug" )
transporter –false substrate; inhibition
p r o t e i n s
endogenous ligand = agonist
endogenous ligand = agonist
drug = agonist
drug = antagonist
TYPES of RECEPTOR LIGANDS
agonist
has
affinity (ligand)
has
intrinsic activity
antagonist
has
affinity (ligand)
has no
intrinsic activity
Major Receptor Families
 Ligand-gated ion channels (ionotropic r.)
 G protein–coupled receptors (metabotropic r.)
 Enzyme-linked receptors
 Intracellular receptors
Examples:
~Nitric oxide (NO)
~Steroid (e.g., estradiol, progesterone, testosterone)
Receptor type Receptor Drug Effects, treatment
_____________________________________________________________________
MEMBRANE:
1. G-protein coupled β2 adrenergic salbutamol asthma bronch.
( metabotropic )
2. enzyme coupled insulin insulin diabetes mell.
3. ion channel GABAA muscimol hallucinogen
( ionotropic )
CYTOSOLIC :
4. Cytosol DNA intercalator cancer
(intracellular r.) (inserts itself into the DNA structure)
RECEPTOR SIGNAL TRANSDUCTION ( from intrinsic activity
to cell response )
E
hyperpolarization
or
depolarization
ion
protein
phosphorylation
gen
transcription
protein
synthesis
cytoblast
gene
transcription
protein
synthesis
3 2 1
4
G
second messenger
production
(cGMP; cAMP; IP3. .)
protein
phosphorylation
ion
change of
excitability
cell response
1. within seconds
2. within hours
3. within milliseconds
4. within hours
TYPES OF RECEPTOR LIGANDS
agonist
partial agonist (competitive dualist)
antagonist ▬ competitive
▬ noncompetitive
▪ specific
▪ nonspecific
R R`R
E E
A AB B
A
B
= agonist
= antagonist
ANTAGONISM
competitive
and
noncompetitive specific
ANTAGONISM
noncompetitive
nonspecific
allosteric
receptor
modulation
TYPES OF RECEPTOR LIGANDS
agonist
partial agonist (competitive dualist)
antagonist ▬ competitive
▬ noncompetitive
▪ specific
▪ nonspecific
inverse agonist
partial inverse agonist
R R ´ response
resting state activated state
In some of receptors systems , even in the absence
of an endogenous ligand or an exogenously administered agonist,
there is intrinsic activity ("tone"]
- there is an inherent stability of constitutively activated receptors.
Quantitative model of receptor influence
= the “two-state model“
+ agonist
R R ´ response
Quantitative model of receptor influence
= the “two-state model“
resting state activated state
agonistinverse agonist
antagonist
acts by abrogating the intrinsic activity
of the free (unoccupied) receptors
agonists ... stabilize R*
partial agonists ... stabilize R + R*
inverse agonists ... stabilize R
competitive antagonist ... prevent full, partial,
and inverse agonists
from binding to the receptor
AGONISTIC LIGANDS ACTION
agonist . . . . . . . . . . . . . . . . . . . maximal receptor activation
partial agonist . . . . . . . . . . . . . . receptor activation but not maximal
inverse agonist . . . . . . . . . . . . . . inactivation of constitutively
active receptors
ANTAGONISTS ACTION
competitive . . . . . . . . . . . . . . . . reversible receptor blockade
noncompetitive . . . . . . . . . . . . . irreversible receptor blockade
(specific)
noncompetitive . . . . . . . . . . . . . . reversible or irreversible binding
(nonspecific, to site other than active site of
allosteric) receptor
SYNAPSE
electric
(“gap junction”) chemic
receptors
neurotransmitter
secundary neuron
terminal of the primary neuronterminal of the
descendent modulatory
neuron
M
Na
Ach
2
2
β2
M
N
(+)
(+)
( - )
( - )( - )
postsynaptic receptor
postsynaptic receptor
presynaptic autoreceptor
heteroreceptor
presynaptic autoreceptor
Na
2
2
β2(+)
(-)
( - )
(+ )( - )
postsynaptic receptor
postsynaptic receptor
presynaptic autoreceptor
homoreceptor
presynaptic autoreceptor
Na
β2
2
receptor
“up-regulation”
receptor
“down-regulation”
chronic antagonist influence
chronic agonist influence
Mechanisms of drug effects
SPECIFIC NONSPECIFIC
- physical
- chemicalDRUG RECEPTOR
affinity,
intrisic activity
- RECEPTOR MEDIATED - NONRECEPTOR
• DIRECT
• INDIRECT
- binding to macromolecules
of the organism which do not serve
physiologically as receptors
(e.g. influence on ion channel,
proton pump,
modification of DNA,
substrate inhibition of enzyme,
binding to cell components,
.....)
MAO
synthesis
CH - blockade of reuptake
( antidepressants )
F- inhibition
( antidepressants - IMAO )
G - depletion of transmitter
D - precursorsE - false precursors
A – agonists,
invers.agon.
B - antagonists
C - agonists/antagonists
Drug effects on synapse function DIRECT
H – impact on release
INDIRECT- D, E, F, G, H, CH
- A, B, C
[R] + [A] [RA] effector
k+1
k-1
signal
R = receptor
A = drug "A"
RA = complex receptor/drug
k+1 = association constant
k-1 = dissociation constant
effectors = molecules of transduction of the drug/receptor
interaction into changes of cell activity
(e.g. adenylylcyclase)
B - Fraction of available receptors
bound
Bmax -Maximal binding of receptors
(=1)
[D] - Concentration of drug
Kd - Equilibrium Dissociation
Constant
- Drug concentration at which
1/2 of available receptors are
bound
- Measure of affinity of
drug/receptor interaction
Relationship of Drug Concentration and Receptor Binding
Spare Receptors
 In some systems, full agonists are capable of eliciting 50% response
with less than 50% of the receptors bound (receptor occupancy)
 Pool of available receptors exceeds the number required for a full
response
 Common for receptors that bind hormones and neurotransmitters
 if [R] is increased, the same [DR] can be achieved with a smaller [D]
 a similar physiological response is achieved with a smaller [D]
Without spare receptors:
• 50% response = 50% occupancy
• Biological effect is proportional to
[DR] at all drug concentrations
With spare receptors:
• 50% response = 10% occupancy
• Biological effect is proportional to
[DR] only at low drug concentrations
Receptor Occupancy versus Biological Response
- Semi-logarithmic transformation (Common representation of pharmacological data)
- Expands concentration scale at low concentration
(where binding is changing rapidly)
- Compresses concentration scale at high concentrations
(where binding is changing slowly)
- Does not change value of Bmax and Kd
Emax - the maximum response achieved by an agonist -also referred to as
drug efficacy
ED50 - the drug concentration (or dose) at which 50% of Emax is achieved
- also referred to as drug potency
Agonist Types: Its All Relative
A: -agonist + no antagonist
- agonist has maximum potency,
maximum efficacy
B: -agonist + competitive
antagonist
- agonist has reduced potency,
but maximum efficacy
Competitive Antagonists - Effect on Dose Response Curves
A: agonist + no antagonist
- agonist has maximum potency,
maximum efficacy
B: agonist + non-competitive
antagonist
- agonist has maximum potency,
but reduced efficacy
Non-Competitive Antagonists - Effect on Dose Response Curves
Quantal Dose-Response Curves
Quantal Phenomena: - all-or-none
describe population rather than single individual responses to drugs
based on plotting cumulative frequency distribution of responders
against the log drug dose