Synaptic organization of claustral and geniculate afferents to the visual cortex of the cat.

Claustral and geniculate afferents to area 17 were labeled by anterograde axonal transport of peroxidase-conjugated wheatgerm agglutinin, and examined in the electron microscope. A peroxidase reaction protocol that led to labeling in the form of minute holes in the EM sections was used. Both types of afferents formed type 1 (presumed excitatory) synapses exclusively. In agreement with previous reports the great majority of geniculate afferents to layers 4 and 6 contacted dendritic spines. The claustral afferents to layers 1 and 6 also predominantly contacted spines. In layer 4, however, claustral afferents contacted spines and dendritic shafts about equally. The results suggest a substantial claustral input to smooth-dendrite cells in layer 4, which are thought to be inhibitory in function. This may be the circuit by which the claustrum helps to generate end-stopped cortical receptive fields (Sherk and LeVay, 1983).