5-HT₁ receptor binding sites in the guinea pig superior colliculus are predominantly of the 5-HT_1D class and are presynaptically located on primary retinal afferents

The superficial layers of the guinea pig superior colliculus are characterized by high densities of [³H]5-HT binding sites. We have chosen receptor autoradiography to establish drug binding profile and the localization of these sites. The binding of [³H]5-HT to guinea pig superior colliculus was nearly completely blocked by drugs such as 5-carboxamido-tryptamine and yohimbine, but only slightly sensitive to the 5-HT₁ receptor agonist 8-hydroxy-dipropylamino-tetralin. 5-HT_1C antagonists such as mesulergine or the β-adrenergic receptor blocking compound SDZ 21-009 did not show any effect. The profile of the majority of these sites corresponds to that of 5-HT_1D sites. Unilateral enucleation resulted in a nearly complete depletion of [³H]5-HT binding in the contralateral superior colliculus, whereas [¹²⁵I]Bolton-Hunter-8-methoxy-N-propylaminotetralin binding sites, corresponding to 5-HT_1A receptors, were preserved. These results indicate that 5-HT_1D (and not 5-HT_1A) receptors might be presynaptically localized on non-serotoninergic neuronal pathways. The guinea pig visual system may be a useful model for the study of the properties of these presynaptic 5-HT heteroreceptors.