How Peptides Work

Most peptides bind a receptor on the outside of a cell. That binding changes the receptor's shape, which triggers a cascade of events inside the cell — often involving second messengers such as cAMP or calcium.

Because the peptide never enters the cell, the effect depends entirely on which receptors are present and where.

Selectivity, and its absence

Melanotan II and bremelanotide both act on melanocortin receptors. Bremelanotide is relatively more selective; Melanotan II is not, and its broad off-target effects follow directly from that.

When you read that a peptide is selective, ask: selective for which receptor, and demonstrated in what species?