Antagonists tend to be small molecules that bind irreversibly to the receptor preventing its normal function, while agonists may be small or large molecules (e.g. peptide or protein hormones).

The brain cells containing these mRNA's can then be isolated and their mRNA's decoded to determine just what their protein products are and how closely the products resemble the true peptide hormones.

Cows, like humans, produce insulin-like growth factor in response to growth hormones and release it into the blood, and the protein makes its way into their milk.