A comparative autoradiographic study of 5-HT1D binding sites in human and guinea-pig brain using different radioligands

Abstract

Quantitative receptor autoradiography was used to examine the 5-hydroxytryptamine (5-HT, serotonin) binding sites labelled with serotonin-5-O-carboxymethyl-glycyl-[125I]tyrosinamide ([125I]GTI) in human and guinea-pig brain. Competition experiments using 5-carboxamidotryptamine (5-CT), 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93129) and sumatriptan revealed monophasic displacement curves in various brain regions, suggesting that a homogeneous population of 5-HT1D binding sites was labelled. Displacement of [3H]5-HT (in the presence of 100 nM 8-hydroxy-2(N-dipropylamino)tetralin (8-OH-DPAT) and 100 nM mesulergine) with unlabelled GTI resulted in monophasic competition curves in substantia nigra, globus pallidus and central gray. In contrast, biphasic displacement was observed in hippocampus, nucleus accumbens, claustrum, caudate-putamen and frontal cortex. The distribution of [125I]GTI sites was compared to that of [3H]5-HT binding sites (under so-called '5-HT1D conditions', i.e. in the presence of 100 nM 8-OH-DPAT and 100 nM mesulergine, in order to block 5-HT1A and 5-HT1C sites, respectively) in human and guinea-pig brain. Qualitative analysis revealed differences in the distributions of [125I]GTI and [3H]5-HT binding sites. Regions such as CA3 and CA4 of the hippocampus, claustrum and putamen showed [3H]5-HT binding (under '5-HT1D conditions') but no [125I]GTI binding sites, indicating that [3H]5-HT labels besides a GTI sensitive (5-HT1D) receptor population, a non-5-HT1A/1B/1C/1D [3H]5-HT binding site in human and guinea-pig brain. The distribution of these non-5-HT1A/1B/1C/1D [3H]5-HT binding sites was studied with [3H]5-HT under conditions where 5-HT1A, 5-HT1C and 5-HT1D [3H]5-HT binding sites were saturated by the presence of 100 nM 8-OH-DPAT, 100 nM mesulergine and 1 microM GTI. Significant densities of these non-5-HT1A/1B/1C/1D sites were observed in cortical areas, hippocampal structures, nucleus accumbens, amygdala, caudate-putamen and claustrum. It is concluded that [125I]GTI does not label the 5-HT1E binding site, since all competition curves obtained with this radioligand were monophasic. By contrast, [3H]5-HT labels non-5-HT1A/1B/1C/1D [3H]5-HT binding sites, but it remains to be established whether these sites represent a single receptor population.