1. Receptor desensitization refers to the gradual diminishing effect of a drug given continuously or repeatedly over time. It involves changes in receptors like phosphorylation, internalization, and loss of receptors from the cell surface. 2. G protein-coupled receptors (GPCRs) undergo desensitization mainly through receptor phosphorylation by G protein receptor kinases (GRKs) and arrestin binding, leading to uncoupling from G proteins and internalization. 3. Upon prolonged agonist stimulation, GRKs are activated and phosphorylate agonist-bound GPCRs, promoting arrestin binding and receptor internalization via clathrin-coated pits. This removes the receptors from the cell surface and inhibits G protein signaling

1. Receptor desensitisation
and tachyphylaxis

2. o Often, the effect of a drug gradually diminishes when it is given
continuously or repeatedly.
o Desensitisation and tachyphylaxis are synonymous terms used to
describe this phenomenon, which often develops in the course of a few
minutes.
o The term tolerance is conventionally used to describe a more gradual
decrease in responsiveness to a drug, taking days or weeks to develop,
but the distinction is not a sharp one.
o Drug resistance is a term used to describe the loss of effectiveness of
antimicrobial or antitumour drugs.
Desensitization

3. ➢Many different mechanisms can give rise to this type of
phenomenon. They include:
o Change in receptors
o loss of receptors
o Exhaustion of mediators
o Increased metabolic degradation of the drug
o Active extrusion of drug from cells (mainly relevant in cancer
chemotherapy;
Desensitization

4. Desensitization
• Receptor phosphorylation
• Phosphorylation by PKA and PKC
• Not very selective, receptors other than that for the desensitizing
agonist will also be affected
• Heterologous desensitization
• Phosphorylation by GRKs
• Receptor-specific to a greater or lesser degree
• Affects mainly receptors in their activated (i.e. agonist-bound) state
• Homologous desensitization

5. oDesensitisation is a feature of all GPCRs, two main processes are
involved:
oReceptor phosphorylation
oReceptor internalisation (endocytosis).
Desensitization
oThe sequence of GPCRs includes certain residues (serine and
threonine), mainly in the C-terminal cytoplasmic tail, which can be
phosphorylated by kinases such as:
o Protein kinase A (PKA),
oProtein kinase C (PKC) and
oSpecific membrane-bound GPCR kinases (GRKs).

6. oPhosphorylation by PKA and PKC, which are activated by many
GPCRs, generally leads to impaired coupling between the activated
receptor and the G-protein, so the agonist effect is reduced.
Receptor phosphorylation
oPhosphorylation of the receptor interferes with its ability to activate
second messenger cascades, although it can still bind the agonist
molecule.
oThis type of desensitisation usually takes a few minutes to develop,
and recovers at a similar rate when the agonist is removed.
oThese kinases are not very selective, so receptors other than that for
the desensitising agonist will also be affected. This effect, whereby
one agonist can desensitise other receptors, is known as
heterologous desensitisation, and is generally weak and short-lasting

7. Receptor internalisation (endocytosis):
GPCR and arrestins
o Following continued agonist binding to GPCR
o Cytosolic GRKs are induced/phosphorylated to translocate to GPCR
o This phosphorylation attracts -arrestins to the receptors
o Compete with G proteins for binding to the cytoplasmic site of the
receptor, arrestins uncouple GPCRs from G proteins causing
desensitization, internalization of GPCR
o Universal response to agonist activation and is critical for the inactivation
of GPCRs and the termination of neurotransmitter and hormone action
o Phosphorylation by GRKs is receptor-specific to a greater degree, and
affects mainly receptors in their activated (i.e. agonist-bound) state,
resulting in homologous desensitization.

8. oThe residues that GRKS phosphorylate are different from those
targeted by other kinases, and the phosphorylated receptor
serves as a binding site for arrestins, intracellular proteins that
block the interaction with G-proteins and also target the
receptor for endocytosis, producing a more profound and long-
lasting desensitisation.
GPCR and arrestins
oThe first GRK to be identified was the β-adrenoceptor kinase,
BARK, but several others have since been discovered, and this
type of desensitisation seems to occur with most GPCRs.

11. o Shown to have in vivo physiological roles in mediating the
functions of GPCRs
o Implicated in development of tolerance to and dependence on
drugs
o Safety mechanisms to prevent the over stimulation of GPCRs
o Could be important targets for the development of drugs to
prevent tolerance development to established drugs and prolong
the therapeutic activity
GPCR and arrestins

12. Luttrell LM and Lefkowitz RJ. 2002 J Cell Sci 115: 455.
Receptor desensitization:
1. Ligand binding promotes
G-protein activation, and
dissociation of Ga and Gbg
subunits
2. Gbg recruits G-protein
coupled receptor kinase
(GRK) leading to
phosphorylation of the
receptor.
3. Phosphorylation of the
receptor promotes
binding of b-Arrestin.
4. This promotes formation
of clathrin coated pits,
and formation of
endosomes containing
internalized receptor.
Receptor mediated endocytosis

13. Luttrell LM and Lefkowitz RJ. 2002 J Cell Sci 115: 455.
Receptor down regulation:
1. Receptors are
internalized in clathrin
coated vesicles
2. These endosomal
vesicles shed the clathrin
and become acidified.
3. Ligand dissociates and
the receptor is
dephosphorylated
4. Receptors are targeted
for degradation or
recycled to the
membrane.
GRK- family of six conserved ser/thr kinases that
phosphorylate ligand bound GPCRs
b-Arrestins are adapter proteins that form complexes with
most GPCRs promoting internalization BUT – are also
involved in promoting G-protein-independet signaling
mediated by GPCRs
Receptor mediated endocytosis

14. o b-arrestin-promoted sequestration of GPCRs via the clathrin-mediated internalization
pathway.
o Agonist-activated phosphorylated receptors are guided to clathrin-coated pits by the
recruitment of b-arrestin, which binds to AP-2 (adaptin promoting protein) and clathrin.
o The vesicles are subsequently pinched off by the GTPase dynamin, which leads to
receptor internalization into early endosomes.
Receptor mediated endocytosis

15. A, agonist; P, phosphate moiety;
b-ARR, b-arrestin; ARF6, G protein ADP-ribosylation factor 6;
ARNO, ARF nucleotidebinding site opener;
Mdm2, mouse double minute protein 2, an E3 ubiquitin ligase;
NSF, N-ethylmaleimide-sensitive fusion protein; Ub, Ubiquitin.

16. Vesicles
fuses with an
endosome
Receptor mediated endocytosis

17. o Among receptors directly coupled to ion channels, desensitisation is
often rapid and pronounced.
o At the neuromuscular junction the desensitised state is caused by a
conformational change in the receptor, resulting in tight binding of
the agonist molecule without the opening of the ionic channel.
o Phosphorylation of intracellular regions of the receptor protein is a
second, slower mechanism by which ion channels become
desensitised.
Change in receptors:

18. Loss of receptors:
oProlonged exposure to agonists often results in a gradual
decrease in the number of receptors expressed on the cell
surface, as a result of internalisation of the receptors.
oThis is shown for β-adrenoceptors and is a slower process than
the uncoupling described above.
oIn studies on cell cultures, the number of β- adrenoceptors can
fall to about 10% of normal in 8 hours in the presence of a low
concentration of isoprenaline, and recovery takes several days.
oThe internalised receptors are taken into the cell by
endocytosis of patches of the membrane, a process that also
depends on receptor phosphorylation.

19. o In some cases, desensitisation is associated with depletion of
an essential intermediate substance.
o Drugs such as amphetamine, which acts by releasing amines
from nerve terminals, show marked tachyphylaxis because
the amine stores become depleted.
Exhaustion of mediators:

20. Altered drug metabolism:
o Tolerance to some drugs, for example barbiturates and
ethanol, occurs partly because repeated administration of the
same dose produces a progressively lower plasma
concentration, because of increased metabolic degradation.
o On the other hand, the pronounced tolerance to
nitrovasodilators results mainly from decreased metabolism,
which reduces the release of the active mediator, nitric oxide.

21. Thank You