Ultrastructural organization of the retino‐pretecto‐olivary pathway in the rabbit: A combined WGA‐HRP tracing and gaba immunocytochemical study

Abstract

The ultrastructural organization of the pretecto-olivary projection neurons within the nucleus of the optic tract and dorsal terminal accessory optic nucleus of rabbits was studied by using anti-GABA immunolabelling and retrograde transport of WGA-HRP. GABA-like immunoreactivity was determined with a postembedding colloidal gold technique. WGA-HRP was injected in the dorsal cap of the inferior olive. The WGA-HRP-labelled neurons were incubated with gold-substituted silver peroxidase. Neurons projecting to the inferior olive had large to medium-sized cell bodies and were GABA negative. In the nucleus of the optic tract, projection neurons are found in the rostral parts, while the majority of the local GABAergic interneurons are mainly found in the caudal parts. In the dorsal terminal nucleus both types of neurons are intermingled. The projection neurons were frequently in synaptic contact by GABAergic terminals. These neurons also receive retinal afferents indicating the existence of a two-step synaptic connection from the retina to the inferior olive. It is suggested that this class of projection neurons forms the "direction-selective" neurons that can be antidromically stimulated from the inferior olive. The GABAergic terminals on the identified projection neurons are of axonal origin (F-terminals), whereas presynaptic dendrites of interneurons (P-terminals) were seldom observed to be in synaptic contact with retrogradely labelled profiles. The strong input of GABA on direction-selective neurons indicates that GABA is directly involved in modulating retinal signals to the inferior olive.

The Nucleus of the Optic Tract (NOT) and the Dorsal Terminal Nucleus (DTN) of Opossums <i>(Didelphis marsupialis aurita)</i>

“…In rabbits, Maekawa et al [1984] observed with recording techniques that the olivary cells were situated more rostral than the NRTP projecting cells in the NOT. Finally, a rostrocaudal segregation between the olivary and the GABAergic populations in the NOT-DTN was reported by Nunes Cardozo and Van der Want [1990] in this species. The latter has also been observed in the Didelphis ntarsupialis aurita [Vargas et al, 1992],…”

“…These cells could not be assigned to neighbouring retinal terminal fields, i.e., the tail of the PO and the rostral end of the PPN, in view of their distance from the terminal foci. Even though this region is likely part of the PPN, fur ther study should determine if designation of a sub-brachial NOT, receiving bilateral retinal and ipsilateral striate pro jections [Linden and Rocha-Miranda, 1981] and distinct from the PPN, is warranted.In rabbits, the olivary cells were also described as im mersed in the brachium of the SC [Nunes Cardozo and Van der Want, 1990]. In rats, simultaneous injections in the infe rior olive region and in the MTN revealed that although a majority of the olivary cells and MTN labelled terminals are superimposed in the brachium of the SC, a small popu lation of olivary cells and MTN terminals can be found ventral to the brachium [ Van der Togt and Van der Want, 1992].…”

“…In many mammalian species, the NOT has been described as hardly distinguishable from the other pretectal nuclei when conventional histological techniques are employed [Holstege and Collewijn, 1982;Simpson et al, 1988], The dorsal terminal nucleus (DTN), a small nucleus located in the optic tract at the boundary between the medial geniculate nucleus (¡VIGN) and the caudolateral aspect of the SC, is often considered as part of a complex with the NOT, based on anatomical and functional similarities [Simpson. 1984; Hoffmann ct al., 1988;Nunes Cardozo and Van der Want. 1990].…”

“…Later, Linden and Rocha-Miranda [1981] redefined the NOT in Didelphis marsupialis aurita, proposing the existence of a 'sub-brachial' portion characterized by bilat eral retinal inputs and ipsilateral striate cortical afferents. The 'intrabrachial' portion, which corresponded to the NOT defined by Lent et al [1976], received exclusively crossed retinal afferents and was shown to be devoid of inputs com ing from the striate cortex.The NOT and the DTN project to the dorsal cap and the nucleus beta of the inferior olive in many mammalian species [cats -Hoffmann et al, 1976;Berman, 1977; SaintCyr and Courville, 1981; rats -Terasawa et al, 1979;Robertson, 1983; tree shrews -Weber and Harting, 1980; rabbits -Takeda and Maekawa, 1976; Holstege and Collewijn, 1982;Nunes Cardozo and Van der Want, 1990; macaque monkeys -Mustari et al, 1994; fuscata monkeys -Watanabe et al, 1991; review in Simpson et al, 1988]. In fact, the distribution of neurons in the optic tract that pro ject to the inferior olive has been employed to define the NOT-DTN boundaries in the pretectal region [cat -Balias and Hoffmann, 1985; rabbit -Nunes Cardozo and Van dcr Want, 1990; rat -Van der Togt and Van der Want, 1992; monkey -Hoffmann et al, 1988;Cohen et al, 1990].…”

Cerebellum and Cerebellar Connections

“…These olivary subnuclei receive their main excitatory input from the nucleus of the optic tract (to the IOK) (Nunes Cardozo and van der Want, 1990) and from the accessory optic nuclei such as the medial terminal nucleus (to the IOVL). Additional IOVL projections stem from the periaquaductal central gray and the visual tegmental relay zone (VTRZ) (Giolli et al, 1984(Giolli et al, , 1985.…”