In pharmacology, an<unk>  for a particular receptor is a compound naturally produced by the body which binds to and activates that receptor. For example, the primary<unk>  for serotonin receptors is serotonin, and the primary<unk>  for dopamine receptors is dopamine. In general, receptors for small molecule neurotransmitters such as serotonin will have only one<unk> , but often have many different receptor subtypes (e.g. 13 different receptors for serotonin). On the other hand, neuropeptide receptors tend to have fewer subtypes, but may have several different<unk> s. This allows for a high degree of complexity in the body's signalling system, with different tissues often showing quite distinct responses to a particular ligand. Some<unk>  antagonists and inverse<unk> s are also known (e.g., kynurenic acid at the NMDA receptor), but these are much less common.
Endogenous agonist
Chaotropic agent
Natural neuroactive substance
Depressogen
Xenohormone
Stimulant
Norepinephrine releasing agent
Serotonin releasing agent