Synaptic targets of cholinergic terminals in the cat lateral posterior nucleus

We examined profiles in the neuropil of the lateral division of the lateral posterior (LP) nucleus of the cat stained with antibodies against choline acetyl transferase (ChAT) or gamma-aminobutyric acid (GABA), and several di fferences in the synaptic circuitry of the lateral LP nucleus compared with the pulvinar nucleus and lateral geniculate nucleus (LGN) were identified. In the lateral LP nucleus, there are fewer glomerular arrangements, fewer GABAergic terminals, and fewer cholinergic terminals. Correspondingly, the neuropil of the lateral LP nucleus appears to be composed of a higher perce ntage of small type I cortical terminals (RS profiles). Similar to the pulv inar nucleus and the LGN, the cholinergic terminals present in the lateral LP nucleus contact both GABA-negative profiles (thalamocortical cells; 74%) and GABA-positive profiles (interneurons; 26%). However, in contrast to th e pulvinar nucleus and the LGN, the majority of cholinergic terminals in th e lateral LP nucleus contact small-caliber dendritic shafts outside of glom eruli (60 of 82; 73%). Consequently, most cholinergic terminals are in clos e proximity to RS profiles. Therefore, whereas the cholinergic input to the LGN and pulvinar nucleus appears to be positioned to selectively influence the response of thalamocortical cells to terminals that innervate glomerul i (retinal terminals or large type II cortical terminals), the cholinergic input to the lateral LP nucleus may function primarily in the modulation of responses to terminals that innervate distal dendrites (small type I corti cal terminals). (C) 1999 Wiley-Liss, Inc.