Receptor Downregulation
Receptor downregulation is a sustained reduction in the number of receptors a cell presents, produced by prolonged agonist exposure and reversed only by resynthesis over hours to days.
Downregulation is a change in receptor inventory rather than receptor behaviour. Prolonged agonist exposure internalises receptors and biases their endosomal sorting toward lysosomal degradation instead of recycling, and sustained signalling can additionally suppress transcription of the receptor gene. Fewer binding sites per cell means the tissue responds less to any concentration of agonist, including its own ligand. Recovery needs new synthesis, so the timescale is hours to days.
The clearest therapeutic use is the GnRH agonist. Continuous occupancy of pituitary GnRH receptors first triggers a flare of LH and FSH, then downregulates the receptor population enough to suppress gonadotropins profoundly — the mechanism behind leuprolide in prostate cancer, endometriosis and central precocious puberty. Chronic hyperinsulinaemia is likewise associated with reduced insulin receptor number.
Distinguishing downregulation from desensitisation tells you how long a washout must be. Uncoupling reverses in minutes to hours once agonist clears; a depleted receptor population does not return until the cell rebuilds it, which is why axes suppressed for months recover over weeks. It also explains why a constant dose does not hold the effect constant, and why response after a break can exceed response before it.
Two claims misuse the term. The first is grey-market copy promising receptor upregulation, which would require measuring increased receptor number in a relevant tissue and essentially never is. The second is inferring lost efficacy from receptor counts: because of receptor reserve, many tissues lose a substantial fraction of their receptors before the maximal response falls at all.