Estructura sináptica de los circuitos tálamo-corticales: análisis cuantitativo 3D de los botones sinápticos de los núcleos ventral posteromedial y posterior del ratón adulto

Resumo

Thalamocortical projections are the main input relying sensory information from the periphery to the cortex. In addition, they are part of a corticothalamocortical loop between areas with a modulatory control. Far from being similar, thalamic neurons in different thalamic nuclei have different axonal architectures, which determine its function. In rodents, synapses from the �specific� or �lemniscal� dorsomedial portion of the ventral posteromedial nucleus (VPMdm) drive the primary somatosensory cortex layer 4 neurons, while the 'paralemniscal' or 'multispecific' posterior nucleus (Po), connects with layer 5a pyramidal neurons. Despite the relative low number of thalamocortical boutons, their synapses are highly effective, which may depend on the structural synaptic features of these boutons. Through an electron microscopy approach, we 3D-reconstructed and quantitatively analyzed anterogradely labeled axons from both thalamic nuclei in their corresponding cortical layers. Virtually all synapses are located in boutons, which have shown differences in their size, number and mitochondrial size and number of synaptic vesicles. In addition, 80% of synapses are on dendritic spines, some of which form large dendritic protrusions included by the presynaptic bouton membrane. Both populations has been showed a postsynaptic density of similar size, however, there is a higher percentage of multisynaptic boutons in VPMdm axons. Taken together, the structural features of different pre- and postsynaptic elements contribute to the different functional phenotype observed for VPMdm axons as a 'first order' thalamic nucleus, and for Po axons as a 'higher order' thalamic nucleus involved in the modulation of synaptic transmission. Keywords: Thalamus, Somatosensory cortex, Synaptic diversity, Transmission electron microscopy, Dendritic spines.