Upregulation
An adaptive increase in the number, density, or sensitivity of receptors or cellular components in response to reduced stimulation or signal deprivation.
Upregulation is the homeostatic counterpart to desensitization — when a cell receives insufficient stimulation, it compensates by increasing receptor expression to become more sensitive to available signals. This is why periods of abstinence from certain compounds can restore and even exceed baseline responsiveness.
In peptide research protocols, cycling strategies deliberately leverage upregulation: taking breaks from GH secretagogue use allows pituitary receptors to upregulate, potentially restoring or amplifying GH pulse responses when the compound is reintroduced. Upregulation is also observed after antagonist treatment, which is why some receptor antagonists are used clinically to sensitize systems before introducing agonist therapy.