Autoradiographic mapping of 5-HT1 receptors in the guinea-pig brain with particular reference to the 5-HT1D receptor sites.

The anatomical distribution of 5-HT1 receptors in the guinea-pig brain was studied by means of in vitro quantitative autoradiography using [3H]-5-HT as ligand. The relative presence of the subtypes of the 5-HT1 binding site was investigated by adding selective concentrations of 8-OH-DPAT, (-)21,009, mesulergine and 5-CT. In addition, differentiation of 5-HT1D receptors was achieved by incubation of the tissues with [3H]-5-HT in the presence of 100 nmol/l 8-OH-DPAT together with 100 nmol/l mesulergine. Areas presenting high densities of 5-HT1A receptors included the neocortex (internal layers), hippocampal formation (dentate gyrus, CA1 field), septum and raphe nuclei, while 5-HT1C sites accounted for most of the [3H]-5-HT binding to the choroid plexus. Non 5-HT1A-non 5-HT1C sites (mainly 5-HT1D and, also probably, 5-HT1E receptors) were clearly predominant in the guinea-pig brain. These sites were mainly present in the neocortex (external layers), basal ganglia, hypothalamus and midbrain (substantia nigra, superior colliculus). As previously described, sites with the properties of 5-HT1B receptors could not be clearly identified in the guinea-pig brain. The present results, in addition to providing a detailed map of the 5-HT1 receptors in the guinea-pig brain, indicate that the guinea-pig is a useful laboratory animal for the study of 5-HT1D receptors.