The distribution of somatostatin-immunoreactive neurons and fibers in the rat cerebral cortex: light and electron microscopic studies.

The distribution of somatostatin (SOM)-immunoreactive neurons in rat motor and somatosensory cortices was studied by immunohistochemistry at both light and electron microscopic levels. Three types of SOM-positive neurons were described. Type A cells are large to medium in size, multipolar in shape and have 3-5 long primary cell processes. Type B cells are medium to small, fusiform in shape and have two primary processes. These two subtypes show abundant subcellular organelles and are heavily stained throughout their cytoplasm. Type C cells are small, ovoid or fusiform in shape and are lightly stained. These previously undescribed cells are the largest SOM-immunoreactive population in the cortex. Ultrastructurally they have few subcellular organelles and only a patchy immunostaining in the cytoplasm. SOM-immunoreactive neurons occur in all cortical layers except I, and are most numerous in layer V of the somatosensory and motor cortex. In SOM-positive dendrites, electron-dense immunoreactive/peroxidase end product is primarily associated with microtubules. Fine deposits also occur deep to the postsynaptic membrane asymmetric synapses making contact with such dendrites.