Subcellular localization of 5-HT1B binding sites in the stratum griseum superficiale of the rat superior colliculus: An electron microscopic quantitative autoradiographic study.

Previous works have shown the potential location of 5-HT1B binding sites on the retinal afferents to the superior colliculus in the rat. In order to confirm this hypothesis, the distribution of 5-HT1B binding sites, labelled by S-CM-G[125I]TNH2, was analysed by quantitative autoradiography at both light and electron microscopic levels in the upper stratum griseum superficiale (SGS) of the superior colliculus, 5 days after unilateral eye ablation. At the light microscopic level, no 5-HT1B sites were found to be associated with capillaries, glial, or neuronal cell bodies, but the fine neuropile was specifically labelled. At the electron microscopic level, the quantitative analysis performed with 50% probability circles showed that classical dendritic processes, presynaptic dendritic processes, and processes containing flat synaptic vesicles were not labelled. In the SGS ipsilateral to the eye ablation, silver grains were specifically associated with processes containing round and pleomorphic vesicles and with non-synaptic contacts between those processes and dendritic or other non identified neuronal processes. In the deafferented contralateral SGS, 5-HT1B receptors were associated with degenerating retinal terminals, with processes containing round vesicles and with non-synaptic contacts between those two tissue compartments. This is the first direct demonstration of the existence of 5-HT1B receptors in non-synaptic contacts and in non-serotonergic terminals. The existence of 5-HT1B terminal heteroreceptors localised on primary visual afferents shows that serotonin might modulate the transmission of visual messages to the superior colliculus.