| The dorsal thalamic nuclei can be classified into first order/relay and higher order/association nuclei based on their primary inputs. The first order nuclei receive their major inputs from peripheral ascending pathways, while the higher order nuclei receive their major inputs from cortical layer V pyramidal cells (Guillery, 1995; Sherman & Guillery, 1996; Guillery et al, 1998). Distinguishing these two types of dorsal thalamic nuclei is very helpful to begin to understand the function of each nucleus and of the thalamus as a whole. Recently, a theoretical division of thalamic inputs into drivers or modulators attracted much attention. In each thalamic nucleus, the driver inputs define the receptive field properties of the thalamocortical cells, while modulator inputs only subtly adjust the activities of the relay cells.; There are four terminal types in the visual thalamus: two GABA-negative (RS and RL) and two GABA-positive (F1 and F2). RS profiles are small, contain densely packed round vesicles, and form short, highly asymmetric contacts. RL profiles are large, contain moderately packed round vesicles, and form multiple, slightly asymmetric contacts. F1 profiles contain densely packed, pleomorphic vesicles, and form symmetric contacts. F2 profiles contain more scattered, pleomorphic vesicles and form symmetric contacts. It has been proposed that all RL terminals are driving inputs, while RS and F profiles are modulating inputs.; To provide a quantitative comparison of the synaptic organization of a relay nucleus, the dorsal lateral geniculate nucleus (dLGN) and an association visual nucleus, the pulvinar nucleus (PUL), we used sampling methods identical to those used by Van Horn et al. (2000) to examine the distribution of terminal types within the cat PUL. We observed the following distribution of synaptic contacts: RL profiles 3.5%, F1 profiles 7.3%, F2 profiles 5.0%, and RS profiles 84.2%.; Next, we compared the synaptic organization of some inputs thought to modulate thalamic activity. A major inhibitory input to the dorsal thalamus arises from the thalamic reticular nucleus (TRN).; Besides the GABAergic TRN projections, previous studies have revealed that GABAergic cells in the pretectum (PT) project to the dLGN. We examined the projections from the PT to dLGN and the PUL at the electron microscopic level. The results provide evidence that PT-dLGN terminals are GABAergic and they mostly contact GABAergic interneurons. In order to understand the role of PT-PUL projection, we investigated this pathway with both anterograde and retrograde tracing methods. Our results showed that the majority (over 70%) of the PT-PUL terminals are not GABAergic. The targets of PT-PUL terminals are mainly thalamocortical cell dendrites (over 80%). Our results suggest that while the PT may modulate geniculate activity, it likely provides at least some of the driving input to the PUL. (Abstract shortened by UMI.)... |
