Receptors - A brief note

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What is a receptor?

Receptors are proteins interacting with extracellular physiological signals and converting them into intracellular effects

This definition is by no means perfect, but seems far more useful than the blurry definition you find in so many pharmacology textbooks, along the lines of 'a receptor is any functional macromolecular component of an organism to which a drug binds'. We should perhaps avoid semi-religious bickering about the 'essential nature' of receptors, and look at how well the definition works for us!

The most important concept is that the receptor:

1. Receives a signal; and
2. transduces the signal to
3. an effector mechanism.

Types of Receptor

If we accept Hucho's definition, we can identify at least four groups of receptors:

• Receptors as enzymes: These receptors usually span the cell membrane just once, and in response to binding of a ligand, usually increase the phosphorylation of intracellular proteins, for example the tyrosine residues on vital regulatory components (Occasionally, serine or threonine residues are phosphorylated).

• Receptors that activate transmembrane ion channels. These channels are called 'ligand-gated ion channels'.

• Receptors that use as their transducer the 'G-protein'. Often these receptors can couple promiscuously with several different G-proteins, who are in turn not particularly faithful to a single receptor. The end results are effected by the G protein stimulating one of a variety of pathways, for example adenyl cyclases and phospholipases, or even ion channels that permit certain ions to enter or leave the cell.

• Perhaps the most interesting of the lot - receptors located within the cell (transcription factors) that once bound to the ligand, either increase or decrease DNA transcription, either by binding DNA or by modulating the effects of histones.

Once we start looking at receptors in detail, and especially how they respond when we expose them to different ligands that bind to them, strange things start to happen! Exposing receptors to different molecules, singly or in combination, can give rise to a vast array of effector responses.

1. Invariably, exposing the receptor to the endogenous ligand(s) normally found in the body gives rise to an agonist response - the normal 'positive' effect of the endogenous ligand is duplicated. In fact, we can often identify substances not normally found in the body which have either a more specific effect on a particular receptor than does the normal ligand, a greater affinity for a particular receptor, or even a greater biological effect (greater intrinsic activity).

2. It is usually possible to identify specific antagonists that antagonise the effect of agonists. Such antagonism is commonly reversible by increasing the concentration of agonist (competitive antagonism) but sometimes we get lucky and find an antagonist that blocks the receptor by binding at a separate site, and this is not overcome by increasing the concentration of agonist. We call the latter non-competitive antagonism.

3. A partial agonist is somewhere betwixt and between! In the absence of an agonist, the partial agonist may do a very good job of stimulating the receptor - however, if you add one of these to a preparation where the receptor is already exposed to a decent concentration of a full agonist, then the partial agonist will sit on the receptor and interfere, causing less agonist activity. As with an antagonist, this effect will be reversed by increasing the concentration of the agonist. We say that the effects of the full and partial agonist are subadditive.

4. Even more peculiar is the inverse agonist. It's difficult (but not impossible) to explain how this works - if you take a preparation and add an agonist, you see a particular effect. Add an inverse agonist instead, and you see the opposite of the effect - for example benzodiazepines like diazepam cause sedation and drowsiness through their agonist effects at the GABA_A receptor, while inverse agonists at this receptor cause hyper-excitability and even seizures!

Pharmacodynamics

There are several theories about how, for example, agonists "bind to and activate" a receptor. Such theories have to explain the various effects of not only agonists, but also antagonists, partial agonists, and even inverse agonists. One such theory that seems to fit the facts rather well is as follows:

1. The receptor exists in two forms ('conformations') which we can call 'active' and 'inactive'. The active receptor is normally in the minority, and gets the effector process cracking. The inactive receptor just sits there, biding its time.

2. There is a dynamic equilibrium between the active and inactive forms of the receptor. Most of the receptors are inactive, but from time to time they flip over into the active state, this being counterbalanced by active receptors becoming inactive.

3. Agonists shifts the equilibrium in the direction of the active form - by binding the active form more avidly, they drive things in the direction of activation.

4. In contrast, competitive antagonists have equal affinity for both conformations, and all they do is get in the way of an agonist trying to bind to the active form. Similarly, partial agonists have only a slightly higher affinity for the active receptor than for the inactive form, so they display some agonism, but interfere with the function of a 'full' agonist.

5. The attractive feature of this model is that it explains inverse agonists rather well. If there is a basal tendency of the receptor to be in the active state (as seems to pertain with GABA_A receptors) then these active receptors will demonstrate a tonic, basal effect even in the total absence of agonist ligands binding to them! Add an inverse agonist with a preference for the inactive receptor, the equilibrium shifts to the left, and the tonic 'agonist' effect is reversed!

The above model is well-described in Goodman and Gilman.

Receptor Characteristics

• The ligand-binding site must be saturable, and half of this maximal saturation should be at a concentration similar to the physiological concentration of the ligand;
• Specific binding of the physiological ligand is desirable, although structurally similar ligands may also bind. The 'wrong' stereoisomer must not bind with a similar affinity;
• The organism must contain the binding site for the ligand (Hucho gives examples of high-affinity saturable binding of insulin to glass beads, and so on)!

Note that if we accept a more fuzzy and broad definition of a receptor, then these criteria are difficult to apply!

Down and up- regulation

Such changes are not necessarily just alterations in numbers of receptors - types and activities of receptors may also change. Over- or under-stimulation of one receptor can also have profound effects on other receptors (so called "heterologous desensitization").

Potency versus Efficacy

The term commonly used is efficacy - in other words, if two drugs occupy the same number of receptors on a tissue, and one drug elicits a greater biological response than the other drug (by its action on the receptor) then that drug has greater efficacy. {The term intrinsic activity is sometimes used to express more or less the same concept}.

A little maths

the more A we have, the faster we form B. If we express this mathematically, we say:

the corresponding differential equation is:

We apply the law of mass action to the forward reaction, and from Equation 2 we find:

but we also know that because we have equilibrium, this rate of consumption of L is equal to the rate of dissociation of LR into its components, given by Equation 1:

so therefore, equating the two:

We can rearrange this to:

Which is another way of saying:

We know from its definition that K_D is given by:

So if we rearrange to get an equation describing [LR] and then substitute this in Equation 3, we get:

Why are pharmacologists so interested in the fractional occupancy? Well, it makes sense that the effect of a drug should depend on the fraction of receptors that are actually occupied. So the measured effect should depend on the fractional occupancy, which in turn should depend on [L] / ( K_D + [L] ). Also, if we have measured the maximum effect (where almost 100% of the receptors are occupied) and we know the fractional occupancy, then we can determine the effect for any given concentration of a ligand. This effect will be given by:

If you sit down and plot a graph of effect versus [L] you'll soon see that we get a simple rectangular hyperbola:

Use graphs to compare drugs

The advantage is that we can now compare different ligands, and even examine the case where we add (for example) a partial agonist to a preparation containing a full agonist.

Let's first compare three drugs, A, B and C:

You can see that drug A is more potent than the other two, and that the maximal response for drug C is less than that for the other two - drug C is a partial agonist when compared with drug A or drug B.

Next, let's look at the effect on drug A of adding a competitive antagonist (X) and then a non-competitive antagonist (Y). See how increasing concentrations of drug A can overcome the effect of the competitive antagonist, but not that of the non-competitive one!

Conclusion

References

1. Ross EM. Pharmacodynamics in Hardman JG, Limbird LE et al. [ed] Goodman & Gilman's The pharmacological basis of therapeutics. 9ed. McGraw-Hill 1996. ISBN 007 0262667.

2. Hucho F in. Hucho F [ed] Neurotransmitter Receptors. Elsevier 1993 pp 3-14 ISBN 0444 899030.

3. There's a good web introduction to receptor kinetics here. Go visit it!