The regional distribution and the pharmacology of the binding sites labelled with the novel 5-hydroxytryptamine (serotonin) 5-HT1B/1D selective radioligand serotonin-O-carboxy-methyl-glycyl-[I-125]tyrosinamide (abbreviated [I-125]GTI for the sake of simplicity) was determined using quantitative autoradiography in rat brain. The distribution of [I-125]GTI binding sites was largely comparable to that of [I-125]iodocyanopindolol ([I-125] ICYP) which labels 5-HT1B binding sites (in the presence of 8-OH-DPAT (8-hydroxy-[2N-dipropylamino]tetralin) and isoprenaline, to prevent binding to 5-HT1A and beta-adrenoceptor binding sites), although a detailed analysis revealed differences. The pharmacology of the [I-125]GTI binding sites was analysed using compounds known to display high affinity for and/or distinguish between 5-HT1B and 5-HT1D sites: 5-carboxamidotryptamine (5-CT), sumatriptan, CP 93129 (5-hydroxy-3(4-1,2,5,6-tetrahydropyridyl)-4-azaindole), (-)pindolol, PAPP (4[2-[4-[3-(trifluoromethyl) phenyl]-1-piperazinyl]ethyl]benzeneamine), rauwolscine, and 8-OH-DPAT. The displacement of [I-125]GTI by 5-CT was monophasic. By contrast, the selective 5-HT1B compound CP 93129 and (-)pindolol produced biphasic curves showing a majority of high affinity sites in the globus pallidus and the substantia nigra, whereas PAPP and sumatriptan (which are somewhat 5-HT1D selective) produced biphasic curves indicating a minority of high affinity sites in these areas. In addition, by blocking the 5-HT1B sites with 100 nM CP 93129, the remaining population of [I-125]GTI binding sites could be studied and was found to have high affinity for PAPP, rauwolscine and 8-OH-DPAT. The pharmacological profile of the major binding component was typical of the 5-HT1B type: 5-CT>CP 93129 greater-than-or-equal-to (-)pindolol>sumatriptan greater-than-or-equal-to PAPP >rauwolscine. The profile of the minor component of [I-125]GTI binding is best characterised as that of a 5-HT1D site: 5-CT>PAPP greater-than-or-equal-to sumatriptan>rauwolscine > (-)pindolol greater-than-or-equal-to CP 93129. The localisation of the non 5-HT1B [I-125]GTI binding sites was characterised by blocking the 5-HT1B receptors with 100 nM CP 93129. Low densities of the 5-HT1D recognition sites were found to be present in globus pallidus, ventral pallidum, caudate-putamen, subthalamic nucleus, entopeduncular nucleus, substantia nigra (reticular part), nuclei of the (normal and accessory) optic tract, different nuclei of the geniculate body and frontoparietal cortex, although higher densities of 5-HT1B sites were always observed in the same structures. Thus, in agreement with the recent cloning of a rat 5-HT1Dalpha receptor cDNA, the presence and the distribution of 5-HT1D sites could be documented in rat brain. However, when compared to 5-HT1B sites, 5-HT1D sites represent only a minor component of the [I-125]GTI binding in the rat brain structures studied.
