Neither the peptide hormones or the metabotropic neurotransmitters such as the catecholamines, can enter their target cells. They bind to surface receptors, which then activate second-messenger systems in the cell. Peptides hormones are the hormones that consist of amino acids; they can be among the intracellular signalling peptides and proteins, and can include insect hormones, gastrointestinal hormones and thyroid hormones, but their actions are not limited to individual cells and they can act systemically.
