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McNall SJ, Mansour TE. 
“H-LSD Binding To A Serotonin Receptor Coupled To Adenylate Cyclase Activity In The Liver Fluke Fasciola Hepatica”. 
Federation Proceedings. 1981;40(3, Pt. 1):653.
The fluke contains an adenylate cyclaso which can be activated to 30-fold over basal levels by serotonin3(5-HT). The 5-HT receptor was examined by binding studies using 3H-LSD as a radioligand for the receptor. The binding to fluke plasma membranes is saturable and stereospecific. D-LSD has greater than 100 times; the affinity of L-LSD for the receptor. Scatchard anaylsis gives) a concave curve with the high affinity site having an apparent Ed value = lx10 M. Binding shows a marked temperature effect. At 37 degrees binding is very rapid, reaching a maximum within 2 min.- and decreasing thereafter in a manner analogous to that seen with cyclase activation. Displacement of H-LSD by various agonists and antagonists at this temperature occurs at concentrations which correlate closely to concentrations found to affect cyclase activity. At 0 however, 3H-LSD binding takes 20-30 min. to reach maximum and remains nearly constant for up to 60 min. Antagonists at 0 degrees displace M-LSD In a similar manner to that seen at 37 degrees. The agonist 5-HT, however, displaces 3H-LSD at only at concentratrons 10-fold greater than that required at 37 degrees. The evidence suggests that the adenylate cyclase coupled 5-HT receptor can exist in at least two conformations and that temperature can affect the rate of conversion between these two states.
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