Intracellular effector substance increasing or decreasing as a response to the stimulation of a receptor by an agonist, considered as the 'first messenger'.
Source:
PAC, 2004, 76, 1033. 'Glossary of terms used in toxicokinetics (IUPAC Recommendations 2003)' on page 1070 (https://doi.org/10.1351/pac200476051033)