Synaptic connections of callosal projection neurons in the vibrissal region of mouse primary motor cortex: An electron microscopic/horseradish peroxidase study

Reciprocal axonal projections between homotypic areas of the vibrissal region of mouse primary motor cortex (MsI) (Porter and White: Neurosci. Lett. 47:37–40, '84) suggested the existence of reciprocal synaptic connections between callosal projection neurons and callosal afferents. In the present study, the retrograde transport of horseradish peroxidase (HRP) was combined with lesion‐induced degeneration to identify synapses between callosal afferents and callosal neurons in the corresponding region of the contralateral cortex. The procedure was as follows: MsI was injected with HRP and aspirated on the following day. After 4 days, the animals were perfused and motor cortex was processed for HRP according to a variation of the Adams (Brain Res. 176:33–47,'77) technique, and, postfixed in OsO4. The methods used consistently filled fine dendritic branches and spines with dense reaction product, thus allowing examination of synaptic contacts with these processes.

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