Somatostatin and vasoactive intestinal polypeptide-like immunoreactive cells and terminals in the retrohippocampal region of the rat brain.

The retrohippocampal region of the rat brain was analyzed by using immunohistochemistry with specific antibodies against somatostatin (SOM) and vasoactive intestinal polypeptide (VIP). Specifically immunoreactive neurons and terminal processes were labeled with either the anti-SOM or anti-VIP antiserum and they were referred to as SOM-like immunoreactive (SOM-LI) or VIP-like immunoreactive (VIP-LI) neurons and processes, respectively. The retrohippocampal region was rich in neuronal cell bodies and terminal processes showing immunoreactivity for SOM and VIP. In the entorhinal area SOM-LI neurons were located mainly in layers IV through VI and the VIP-LI neurons were found mainly in layers I through III. Thick (70-120 microns) sections treated with the immunoperoxidase method to achieve a Golgi-like staining pattern showed that cytological differences existed between SOM- and VIP-positive neurons. SOM-LI neurons were usually multipolar, fusiform, or occasionally pyramidal while VIP-LI neurons were usually bipolar, stellate, or fusiform. SOM-LI and VIP-LI axons and preterminal processes were differentially distributed within the laminae of the retrohippocampal region. VIP-LI terminals were found throughout all layers except layer I. SOM-LI terminals were found primarily in the molecular layers of all areas, layer IV of the medical and lateral entorhinal areas, and in the angular bundle. Thus, SOM-LI and VIP-LI neurons are distinguished by their morphology and their different distribution within the cortical layers and areas of the retrohippocampal region.