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McNall SJ, Mansour TE. 
“Desensitization Of Serotonin-stimulated Adenylate Cyclase In The Liver Fluke Fasciola Hepatica”. 
Biochemical Pharmacology. 1984;33(17):2799-2805.
Incubation of the intact liver fluke Fasciola heparica with serotonin (5-HT) resulted in a time dependent decrease in both 5-HT-stimulated adenylate cyclase activity and specific [3H]LSD binding in the subsequently prepared cell-free fluke particles. In control fluke particles, the activation of adenylate cyclase by 5-HT was biphasic, indicating a high and low affinity form of the 5-HT receptor with half maximal activation constants (KA) of 0.35 and 2.8 mcM respectively. In contrast, 5-HT activation of desensitized particles occurred through a single set of low affinity sites having a KA value of 6.3 mcM. The maximum level of 5-HT activation of adenylate cyclase was also reduced in the desensitized particles. Lysergic acid diethylamide (LSD)-stimulated adenylate cyclase activity was also less in the desensitized particles. However, unlike with 5-HT, activation by LSD occurred through a single set of sites for both control and desensitized particles. [3H]LSD binding studies showed that the affinity of LSD for the 5 HT receptors in the control and desensitized particles was similar. Thus, the decrease in [3H]LSD binding and serotonin-stimulated adenylate cyclase activity in the desensitized particles appeared to be due to a preferential loss or inactivation of the high affinity form of the 5-HT receptor. A similar time dependent loss in 5-HT-stimulated adenylate cyclase and in [3H]LSD bmding occurred in cell-free fluke particles upon the addition of 5-HT or LSD. These effects were not due to protein denaluration or metabolism of the ligand during the incubation procedure. This cell-free desensitization was reversible and temperature dependent, and was not affected by ATP or other nucleotides.
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