Excessive use of a chemical messenger, such as a hormone or neurotransmitter, can lead to a phenomenon known as down-regulation, which ultimately declines its effectiveness in communicating with target cells.
When a chemical messenger is released in high amounts over an extended period, target cells can respond by decreasing the number of receptors on their surface that are available to bind that messenger. This process is known as down-regulation. As a result, even though there are high levels of the chemical messenger in circulation, the target cells become less sensitive to it due to the reduced number of receptors.
Consequently, with fewer receptors available to initiate the desired cellular response, the effectiveness of the chemical messenger diminishes over time. This decline can lead to a need for higher concentrations of the chemical messenger to achieve the same level of response that was initially observed when the receptor numbers were optimal. Thus, the excessive presence of a chemical messenger may lead to a situation where target cells become less responsive, necessitating alterations in the system to restore homeostasis and sensitivity to the messenger.