Autoradiographic characterisation and localisation of 5-HT1D compared to 5-HT1B binding sites in rat brain

Summary The regional distribution and the pharmacology of the binding sites labelled with the novel 5-hydroxytryptamine (serotonin) 5-HT_1B/1D selective radioligand serotonin-O-carboxy-methyl-glycyl-¹²⁵I]tyrosinamide (abbreviated ¹²⁵I]GTI for the sake of simplicity) was determined using quantitative autoradiography in rat brain. The distribution of ¹²⁵I]GTI binding sites was largely comparable to that of ¹²⁵I] iodocyanopindolol (¹²⁵I] ICYP) which labels 5-HT_1B binding sites (in the presence of 8-OH-DPAT (8-hydroxy-2N-dipropylamino]tetralin) and isoprenaline, to prevent binding to 5-HT_1A and -adrenoceptor binding sites), although a detailed analysis revealed differences.The pharmacology of the ¹²⁵I]GTI binding sites was analysed using compounds known to display high affinity for and/or distinguish between 5-HT_1B and 5-HT_1D sites: 5-carboxamidotryptamine (5-CT), sumatriptan, CP 93129 (5-hydroxy-3(4-1,2,5,6-tetrahydropyridyl)-4-azaindole), (–)pindolol, PAPP (42-4-3-(trifluoromethyl)phenyl]-1-piperazinyl]ethyl] benzeneamine), rauwolscine, and 8-OH-DPAT. The displacement of ¹²⁵I]GTI by 5-CT was monophasic. By contrast, the selective 5-HT_1B 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-HT_1D selective) produced biphasic curves indicating a minority of high affinity sites in these areas. In addition, by blocking the 5-HT_1B sites with 100 nM CP 93129, the remaining population of ¹²⁵I]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-HT_1B type: 5-CT > CP 93129 (–)pindolol > sumatriptan >/ PAPP > rauwolscine. The profile of the minor component of ¹²⁵I] GTI binding is best characterised as that of a 5-HTID site: 5-CT > PAPP sumatriptan > rauwolscine > (–)pindolol CP 93129.The localisation of the non 5-HT_1B ¹²⁵I]GTI binding sites was characterised by blocking the 5-HT_1B receptors with 100 nM CP 93129. Low densities of the 5-HT_1D 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-HT_1B sites were always observed in the same structures. Thus, in agreement with the recent cloning of a rat 5-HT_1D receptor cDNA, the presence and the distribution of 5-HT_1D sites could be documented in rat brain. However, when compared to 5-HT_1B sites, 5-HT_1D sites represent only a minor component of the ¹²⁵I]GTI binding in the rat brain structures studied.Correspondence to: D. Hoyer at the above address