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Soblosky JS, Jeng I. 
“Influence on 5-3H]Hydroxytryptamine Binding Site Development in Chick Embryo by Serotonergic Compounds”. 
J.Neurochem.. 1985;44:544-551.
Saturable and specific binding sites for 5- [3H]hydroxytryptamine (5-HT, serotonin) characterized by a KD of 3.5-4.5 nM were detected in the chick embryo brain and were shown to develop linearly as a function of age, weight, and protein content. Saturation and displacement studies using unlabeled 5-HT as the displacing ligand suggested a single population of binding sites. However, displacement studies using 5-methoxytryptamine, lysergic acid diethylamidc (LSD), 2-bromo-lysergic acid diethylamide (BOL), methysergide, and spiperone as competing ligands suggested the existence of subclasses of [3H]5-HT binding sites because the Hill coefficients were less than unity. When compared with the reported [3H]5-HT binding sites (5-HT') in the rat forebrain. the IC50, values of the competing ligands were similar. However, the Hill coefficients for LSD and methysergide were less than unity which suggested that the [3H]5-HT binding sites in the chick embryo brain may be more similar to those found in rat spinal cord than rat forebrain. To study [3H]5-HT-'binding site regulation and development. various serotonergic compounds were injected into the chorioallantoic fluid of the eggs at different times during embryonic development, Multiple pretreatments with d,l-5-hydroxytryptophan, 5-HT, or BOL were found to have no significant effects on either the affinity (KD) or number (BmaX) of specific [3H]5-HT binding sites. Multiple pretreatments with p-chlorophenylalanine were found to increase the BmaX of specific [3H]5-HT binding by 23% (p < 0.01) whereas multiple pretreatments with LSD were found to decrease the Bmn of specific binding by 45% (p < 0.01). Neither of these pretreatments affected KD values which indicated that only the number of specific [3H]5-HT binding sites was altered. Evidence was presented suggesting that these effects were probably not due to excess endogenous 5-HT or LSD remaining in the tissue preparation. The overall evidence indicated that the chick embryo htnin may have a functioning serotonergic system and that the chick embryo may be an ideal system for the study of [3H]5-HT binding site regulation and development.
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