The indolaminergic innervation of the inferior olive. 1. Convergence with the direct spinal afferents in the areas projecting to the cerebellar anterior lobe

The distribution of indolaminergic innervation has been mapped in the inferior olivary complex using the Falck-Hillarp formaddehyde histofluorescence method. The results demonstrate that olivary neurones projecting to different sagittal zones of the cerebellar cortex receive widely different densities of indolamine-containing (probably serotoninergic) terminals. In the cat the densest innervation was found in the caudal part of the medial accessory nucleus and the caudal and caudolateral parts of the dorsal accessory nucleus, known to project to the medial A zone and the lateral B zone, respectively, of the anterior lobe vermis. Interestingly, these olivary areas seem to coincide with those previously shown to receive direct afferent connections from the spinal cord. In contrast, the rostromedial part of the dorsal accessory nucleus, projecting to the C 1 and C 3 zones of the cerebellar pars intermedia, showed only sparse innervation. In the rostral part of the medial accessory nucleus, projecting to the C 2 zone of pars intermedia, virtually no indolaminergic terminals were found. Indolaminergic innervation of medium density occurred in olivary areas that have been described to project further caudally in the cerebellar vermis, such as the dorsomedial cell column, the β-subnucleus and the dorsal cap of Kooy. In the rat inferior olive, the indolaminergic innervation was confined to the lateral part of the dorsal accessory nucleus, all other regions exhibiting no or only sparse terminal supplies. A comparison between the patterns of indolaminergic and catecholaminergic innervations has also been made, showing that the distributions of the two types of monoaminergic olivary afferents are very different in both species. A particularly interesting situation was observed in the dorsal accessory nucleus where the two innervations seemed to be complementary. Thus, the catecholaminergic innervation was confined to that part of the nucleus having only sparse indolaminergic fibre supply.