Thalamocortical synapses of pyramidal cells which project from SmI to MsI cortex in the mouse

Electrolytic lesions were made of the nucleus ventralis posterior pars lateralis thalami and of the nucleus posterior thalami in the male CD/1 mice to label thalamocortical axon terminals in mouse SmI cortex. Pyramidal cells in layers II through V of SmI cortex were labeled by the retrograde transport of horseradish peroxidase injected into ipsilateral MsI. Numerous pyramidal cells, particularly in the more superficial layers of SmI cortex, were filled with reaction product so that even their dendritic spines and local axon collaterals were clearly visible. Six well-filled pyramidal cells with somata in layers III and IV were serial thin-sectioned, and portions of their dendrites in layer IV were examined with the electron microscope to determine the distribution of their thalamocortical and other synapses. It was found that, in general, different dendrites of a single pyramidal cell formed similar proportions of thalamocortical synapses and that the six pyramidal cells, as a group, also formed similar proportions of thalamocortical synapses with their dendrites in layer IV. In contrast, when the thalamocortical connectivity of the six cells was considered as a function of their depths within the cortex, a clear trend was seen for the proportion of thalamocortical synapses to increase with increasing proximity of the cell body to layer IV. A hypothesis based on the timing of developmental events is proposed to account for this observation.