Neuropharmacology: full agonists, partial agonists and constitutively active receptors (Dr. Moss)

Question 1

Why does a conformational change occur when an agonist bind to a reeptor ?

Answer 1

Because on binding an agonist molecule the receptor is able to make bonds (e.g. H-bonds, charge-charge interactions, hydrophobic interactions etc.) that favor the receptor occupying the active conformation.

Question 2

Is the conformational change that pushes the receptor into the active conformation reversible ?
Why ?

Answer 2

Yes, because at some point the receptor-agonist complex will have enough thermal energy to break the bonds that hold them in the active conformation.

Question 3

What is the Del Castillo-Katz model of receptor activation ?

Answer 3

A + R = AR = AR*
AR –> binding
AR* –> conformational change to an active state
This was first model to separate agonist binding and receptor gating steps in the pathway towards receptor activation.

Question 4

What does the seperate step for receptor activation in the Del Castillo-Katz model suggest ?

Answer 4

It embodies the idea of agonist efficacy – the ability of the agonist to stabilise the active state of the receptor.

Question 5

What are full agonists ?

Answer 5

Drugs that elicit maximal tissue response.

Question 6

What are partial agonists ?

Answer 6

Drugs that cannot elicit maximal tissue response no matter how high their concentration.

Question 7

What are examples of full agonists/partial agonists acting at the beta-adrenoreceptors ?

Answer 7

Full agonists: adrenaline, isoprenaline

Partial agonists : prenalterol

Question 8

What are examples of full agonists/partial agonists acting at the histamine H2 receptors ?

Answer 8

Full agonists: histamine

Partial agonists: impromidine

Question 9

How can we rule out the idea that partial agonists are not able to combine with all the receptors (and therefore cannot elicit maximal response) ?

Answer 9

By testing the effect of increasing concentrations of a partial agonist on the response of a tissue to a fixed concentration of a full agonist. As the concentration of the partial agonist is raised, the response of the tissue gradually falls from the large value seen with the full agonist alone, and eventually reaches the maximum response to the partial agonist acting on its own.

Question 10

How can the Del Castillo-Katz model account for the difference between partial and full agonist ?

Answer 10

The Del Castillo-Katz model shows that the crucial difference between a partial agonist and a full agonist lies in their ability to activate (rather than bind to) the receptor, and stabilize that active conformation.

Question 11

How did the patch clamp technique, allowing single channel recordings, validate the Del Castillo-Katz model ?

Answer 11

Most of the time, the nACh channels is closed, waiting for an agonist molecule to arrive. When agonist and receptor form a complex the channel opens in a ‘burst’ of activity. The long shut times are periods where the channel is waiting to associate with another agonist molecule. The brief shut times however, are surrounded by openings and this ‘burst’ of activity corresponding rapid transitions between the AR and the AR* state during which time the ligand is always bound.

Question 12

What are Slow-Channel Congenital Myasthenic Syndromes (SSCMSs) ?

Answer 12

SCCMSs are disorders of NM transmission. They are characterized by muscle weakness and fatigability. Although a variety of mutations cause SCCMS, all SCCMS mutations produce prolonged endplate currents.

Question 13

What is the consequence of SSCMS at single channel level ?

Answer 13

The response of a channel carrying mutation epsilon L78P for example will stay open much longer. Detailed analysis reveals that such mutants differ in ACh efficacy and binding.

Question 14

How did the crystal structures of full and partial alpha 1 adrenoreceptor agonists reveal the crucial role of receptor stabilization (Warne et al., 2011, Nature 469, 241) ?

Answer 14

These showed that both full and partial agonists encourage the same conformational change in the receptor. However, the full agonist isoprenaline was able to stabilize that conformation by making 2 H-bonds with serine residues 211 and 215 of the receptor, while the partial agonist dobutamine could only make a H-bond w/ serine 211.

Question 15

How can a receptor be constitutively active ?

Answer 15

Sometimes, by chance, random collisions can provide the thermal energy that is required to for the receptor to enter the active state in absence of an agonist –> constitutively active receptor

Question 16

What is an inverse agonist/negative antagonist ?

Answer 16

A molecule that has a higher affinity for the receptor in its inactive form.

Question 17

What is a reversible competitive antagonist ?

Answer 17

A ligand that has an equal affinity for a receptor in its active and inactive state.

Question 18

What is desensitization ?

Answer 18

It is when the response declines despite the continued presence of the agonist.

Question 19

What factors can contribute to desensitization ?

Answer 19

• for “fast” receptor (l-gated ion channels), the receptor is able to isomerise into an inactive, desensitized state
• for GPCRs, it can result from phosphorylation of the receptor by protein kinases which become active following the application of agonist (this may be followed by loss of some receptors from the cell surface)

Question 20

What are “spare receptors” ?

How can we prove their existence ?

Answer 20

Extra receptors that do not need to be activated to elicit a maximal response in a tissue.
Maximal muscle contraction can still occur when a significant fraction of the nAChRs are blocked at the NM jct.
The AP is still propagated even when a significant proportion of the Na+ channels are blocked.

Question 21

Why do we have spare receptors ?

Answer 21

These can be necessary when the response of the tissue is limited not by the receptors but by some subsequent events occurring as a consequence of receptor activation e.g. a piece of smooth muscle may not be able to shorten further. This maximal shortening may occur in response to a rise in cytosolic calcium that is much less than can be elicited by activating all the receptors.

Question 22

What does the notion of “spare receptors” imply for partial agonists acting on different tissues ?

Answer 22

The nb of spare receptors partially determine the maximal response –> a partial agonist for one tissue could be a full agonist, at the same receptors, on another tissue if there is a big enough receptor reserve