Receptor Desensitisation
Receptor desensitisation is the rapid, reversible loss of responsiveness that occurs during continued agonist exposure, as the activated receptor is uncoupled from its signalling machinery.
Desensitisation is the fastest adaptive response to sustained stimulation. For a G protein-coupled receptor the canonical sequence is phosphorylation of the agonist-occupied receptor by a GPCR kinase, then beta-arrestin binding that sterically blocks G protein coupling. The receptor is still on the surface and still binds ligand, but no longer transduces. Homologous desensitisation affects only occupied receptors; heterologous desensitisation, driven by PKA and PKC, blunts others on the same cell.
The beta2-adrenergic receptor is where this was worked out, with kinase phosphorylation detectable within seconds and functional uncoupling within minutes. The growth hormone secretagogue receptor behaves the same way: continuous or closely spaced stimulation attenuates the GH response, while the endogenous axis avoids the problem by releasing its hypothalamic drivers in separated bursts.
Desensitisation is worth separating from the slower adaptations it is lumped with. It occurs in seconds to minutes, needs no change in receptor number, and reverses on dephosphorylation once agonist clears, typically within hours. Downregulation means fewer receptors and requires resynthesis over days. That is why an intermittent exposure pattern is a genuine mitigation for one and only a partial one for the other.
The frequent error is calling any waning response desensitisation, when depletion of a releasable pool, counter-regulatory feedback or true downregulation may be responsible, each with a different recovery time. The error running the other way: recombinant systems overexpressing the receptor carry large reserve and can appear to desensitise far less than native tissue, so an in-vitro absence of desensitisation says little about a whole animal.