Parabrachial innervation of the cat's dorsal lateral geniculate nucleus: an electron microscopic study using the tracer Phaseolus vulgaris leucoagglutinin (PHA-L).
Ascending pathways from the brain stem play a key role, generally facilitatory, in controlling the transmission of retinal information through the lateral geniculate nucleus to the visual cortex (for reviews, see Singer, 1977; Burke and Cole, 1978; Sherman and Koch, 1986). In order to characterize the morphological basis of this brain-stem control, we used the electron microscope to study synaptic terminals labeled anterogradely from injections of the tracer Phaseolus vulgaris leucoagglutinin into the parabrachial region of the brain stem. The labeled axons, which are fine and unmyelinated in our material, form conventional synaptic contacts onto both relay cells and interneurons. These connections are surprisingly selective for certain postsynaptic elements such as the dendritic shafts and appendages of relay cells and the presynaptic dendritic terminals of interneurons. That is, the morphology of contacts made from parabrachial axons varies with the specific postsynaptic profile. Even a single axon can form symmetrical contacts onto F2 terminals, which are synaptic terminals deriving from dendrites of interneurons, and dendritic shafts of relay X cells, and form asymmetrical contacts onto dendritic appendages of the same relay X cells. Reconstructions of the dendritic segments postsynaptic to the labeled terminals show that the dendritic appendages receive retinal and parabrachial input in triadic relationships with F2 terminals: a retinal or parabrachial axon contacts the F2 terminal, and the F2 terminal plus the retinal or parabrachial axon contact the dendritic appendage. This positioning of the parabrachial innervation is well suited for control of retinal transmission. Finally, the dual morphology of the parabrachial synaptic contacts suggests that their actions may differ depending on the postsynaptic target.