Phaseolus vulgaris leucoagglutinin (PHA‐L): A neuroanatomical tracer for electron microscopic analysis of synaptic circuitry in the cat's dorsal lateral geniculate nucleus

Abstract

Phaseolus vulgaris leucoagglutinin (PHA‐L) is a plant lectin that is anterogradely transported by neurons in the central nervous system. PHA‐L is selectively taken up by cells at iontophoretic injection sites and, when immunohistochemically demonstrated, labels individual neurons completely, including their dendrites, axons, and terminal boutons. PHA‐L is generally not taken up by fibers passing through the injection site and, because it produces a Golgi‐like staining of even very fine axons over long distances, it is sometimes possible to light microscopically reconstruct individual neurons and their entire axon terminal arbors. When prepared for electron microscopy, the PHA‐L‐labeled terminals are densely and completely stained, allowing their synaptic relationships to be defined. These properties make PHA‐L advantageous for studying the patterns of projection and the modes of termination of select groups of neurons in their target nuclei. We used PHA‐L to study the extraretinal innervation of the cat's dorsal lateral geniculate nucleus, a thalamic visual center. Although much is known about the retinal contribution to geniculate synaptic circuitry, relatively little is known about other sources of innervation, even though these provide the majority of synaptic terminals in the nucleus (Guillery: Z. Zellforsch., 96:1–38, 39–48, 1969; Wilson et al.: Proc. R. Soc. Lond. [Biol.], 221:441–436, 1984). We used both light and electron microscopy to describe synaptic circuitry from three extraretinal sources of projections to the lateral geniculate nucleus: the visual cortex, the perigeniculate nucleus, and the parabrachial region of the brainstem. Cortical terminals labeled with PHA‐L were small and formed asymmetrical synaptic contacts onto small‐caliber dendrites of geniculate neurons. Peri‐geniculate terminals formed symmetrical synaptic contacts primarily onto small‐caliber dendrites, but some synapses were also formed onto the proximal, retinorecipient portions of geniculate dendrites. Parabrachial terminals synaptically contacted the retinorecipient portions of dendritic appendages and shafts, small‐caliber dendrites, and the specialized dendritic (F2) terminals of geniculate interneurons. The symmetry of the parabrachial synaptic contacts was variable and was related to the postsynaptic target. Contacts onto dendritic appendages were asymmetrical while those onto dendritic shafts and F2 terminals were symmetrical. Our data suggest that in unlabeled material these brainstem terminals would be difficult to distinguish from cortical or perigeniculate profiles. The positioning of the parabrachial input onto the retinorecipient portions of geniculate dendrites indicates that this projection is well situated to control primary retinal transmission through the nucleus, while the location of most cortical and perigeniculate innervations implicates them in secondary feedback interactions or other aspects of geniculate function.