Table 2.

The GPCRs of the Rhodopsin family and their transmitters

Ligand type	Phylogenetic cluster	Ligands	Number of paired receptors	Number of orphan receptors	Names of orphan receptors
Light	Opsin	Photon	9	3	GPR21, GPR45, GPR52
Amine	Biogenic amine	Acetylcholine, serotonin, adrenaline, noradrenaline, dopamine, histamine, tyramine	37	12	GPR88, GPR152, GPR153, GPR160, GPR162, and seven TAARs
	Melatonin	Melatonin	2	4	GPR50, GPR61, GPR62, GPR135
Peptide	Peptide	Endothelins, prokineticins, orexins, TRH, CCK neurokinins, neuromedins, NPY, NPAF-FF, GRP, PrRP, vasopressin, oxytocin, GnRH, neurotensin, NPS, motilin, ghrelin, apelin, QRFP	37	5	BRS-3, GPR37, GPR37L1, GPR150, GPR175
	SOG	Galanin, kisspeptin, opioids, somatostatins, urotensins, MCH, NPB-NPW	18	1	GPR151
	MECA	MSH, AgRP	5
	MAS-related	Various peptides	4	8	GPR19, GPR101, GPR161, MAS1L, MRGPRF, MRGPRE, MRGPRG, MRGPRX3
	Chemokine	Angiotensin, bradykinin, formylpeptide, XCL relaxin, anaphylatoxin, CCL, CXCL, CXCL3	34	7	CCRL2, GPR1, GPR15, GPR20, GPR25, GPR32, GPR182
	Glycoprotein	LH, FSH, TSH, relaxin, R-spondins	8	2	GPR26, GPR78
	Purine	Thrombin	4
Lipid	MECA	Cannabinoids, LPA, S1P, estrogen	17	3	GPR3, GPR6, GPR12
	Purine	Leukotrienes, LPI, LPS, NA-glycine, free fatty acids, PAF	15	1	GPR31
	Prostaglandin	Prostaglandins	9
	SOG	Leukotrienes	2
Nucleotide/nucleoside	Purine	ADP, ATP, UDP, UTP, UDP-glucose	12	5	GPR27, GPR85, GPR171, GPR173, GPR174
	MECA	Adenosine	4
Amino acid	MAS-related	β-alanine, D-amino acids	2
Proton	Purine	H+	4
Olfactory	Olfactory	Olfactants	few	~380