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Journal of Neuroscience, Vol 7, 894-903, Copyright © 1987 by Society for Neuroscience
Characterization of a novel 3H-5-hydroxytryptamine binding site subtype in bovine brain membranes
RE Heuring and SJ Peroutka
3H-5-Hydroxytryptamine (5-HT) binding sites were analyzed in bovine brain
membranes. The addition of either the 5-HT1A-selective drug 8-OH- DPAT (100
nM) or the 5-HT1C-selective drug mesulergine (100 nM) to the assay resulted
in a 5-10% decrease in specific 3H-5-HT binding. Scatchard analysis
revealed that the simultaneous addition of both drugs decreased the Bmax of
3H-5-HT binding by 10-15% without affecting the KD value (1.8 +/- 0.3 nM).
Competition studies using a series of pharmacologic agents revealed that
the sites labeled by 3H-5-HT in bovine caudate in the presence of 100 nM
8-OH-DPAT and 100 nM mesulergine appear to be homogeneous. 5-HT1A selective
agents such as 8- OH-DPAT, ipsapirone, and buspirone display micromolar
affinities for these sites. RU 24969 and (-)pindolol are approximately 2
orders of magnitude less potent at these sites than at 5-HT1B sites which
have been identified in rat brain. Agents displaying nanomolar potencies
for 5-HT1C sites such as mianserin and mesulergine are 2-3 orders of
magnitude less potent at the 3H-5-HT binding sites in bovine caudate. In
addition, both 5-HT2- and 5-HT3-selective agents are essentially inactive
at these binding sites. These 3H-5-HT sites display nanomolar affinity for
5-carboxyamidotryptamine, 5-methoxytryptamine, metergoline, and 5-HT.
Apparent Ki values of 10-100 nM are obtained for d-LSD, RU 24969,
methiothepin, tryptamine, methysergide, and yohimbine, whereas I-LSD and
corynanthine are significantly less potent. In addition, these 3H-5-HT
labeled sites are regulated by guanine nucleotides and calcium. Regional
studies indicate that this class of sites is most dense in the basal
ganglia but exists in all regions of bovine brain. These data therefore
demonstrate the presence of a homogeneous class of 5-HT1 binding sites in
bovine caudate that is pharmacologically distinct from previously defined
5-HT1A, 5-HT1B, 5- HT1C, 5-HT2, and 5-HT3 receptor subtypes. We therefore
suggest that this class of sites be designated the 5-HT1D subtype of
binding sites labeled by 3H-5-HT.
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