Synaptic targets of cholinergic terminals in the pulvinar nucleus of the cat

Abstract

We compared the cholinergic innervation of the pulvinar nucleus, a thalamic association nucleus, to previous studies of the cholinergic innervation of the dorsal lateral geniculate nucleus (dLGN), a thalamic relay nucleus. Both nuclei receive a dense innervation from cholinergic cells of the brainstem parabrachial region (PBR). In the dLGN, PBR terminals are located in close proximity to retinal terminals. Our goal was to determine whether PBR terminals in the pulvinar nucleus are located in close proximity to corticothalamic terminals. We identified PBR terminals with a monoclonal antibody directed against choline acetyltransferase (ChAT). Cholinergic terminals contacted dendrites (142 of 160, or 89%) or vesicle-filled profiles (18 of 160, or 11%). A subset of 55 terminals was stained for gamma-aminobutyric acid (GABA) to determine whether profiles postsynaptic to cholinergic terminals originate from thalamocortical cells (GABA-) or interneurons (GABA+). The majority (44 of 55, or 80%) of postsynaptic profiles were GABA- dendrites. The minority (11 of 55, or 20%) were GABA+ dendrites with vesicles. This distribution of contacts is very similar to that seen in the dLGN. However, the most significant finding was that most cholinergic contacts (121 of 160, or 76%) were located within complex clusters identified as glomeruli. This is the primary site of contacts made by corticothalamic terminals originating from layer V cells. These results suggest that while the PBR enhances retinal signals in the dLGN, it may also enhance cortical signals in the pulvinar nucleus. Thus, activity in the PBR may stimulate both an increased flow of retinal information to visual cortex, as well as an increased flow of information between different visuomotor areas of cortex.