The pharmacology of the orexin-like peptides, hypocretin-1 and hypocretin-2
, was studied in Chinese hamster ovary (CHO) cells stably expressing orexin
-1 (OX2) or orexin-2 (OX1) receptors by measuring intracellular calcium ([C
a2+](i)) using Fluo-3AM. Orexin-A and orexin-B increased [Ca2+](i) in CHO-O
X1 (pEC(50) = 7.99 +/- 0.05 and 7.00 +/- 0.10 respectively, n = 8) and CHO-
OX2 (pEC(50) = 8.30 +/- 0.05 and 8.21 +/- 0.07 respectively, n = 5). Howeve
r, hypocretin-1 and hypocretin-2 were markedly less potent, with pEC(50) va
lues of 5.31 +/- 0.04 and 5.41 +/- 0.04 respectively in CHO-OX2 cells (n =
5). In CHO-OX1 cells 10 mu M, hypocretin-1 only elicited a 37.5 +/- 3.4% re
sponse whilst 10 mu M hypocretin-2 elicited a 18.0 +/- 2.1% response (n = 8
). Desensitisation of OX1 or OX2 with orexin-A (100 nM) abolished the respo
nse to orexin-A (10 nM) and the hypocretins (10 mu M), but not to UTP (3 mu
M). In conclusion, the hypocretins are only weak agonists at the orexin re
ceptors.