Morphology of striatal neurons containing VIP-like immunoreactivity.

We have used light and electron microscopic immunocytochemical techniques to study the distribution and morphology of neurons that contain vasoactive intestinal polypeptide-like immunoreactivity (VIP-Ir) in the adult rat striatum. VIP-Ir cells were sparsely distributed throughout all rostrocaudal levels of the striatum. Cell bodies were of medium size (12-17 microns) and gave rise to three to five primary dendrites, which branched close to the soma and became varicose. These dendrites appeared aspiny at the light microscope level and could be traced up to 250 microns in length. Dendrites frequently traversed axonal bundles in the striatum, a pattern not exhibited by neurons containing somatostatin-like or substance P-like immunoreactivity. In several instances, very fine varicose processes arborized extensively within 40 microns of the VIP-Ir soma; these may represent axons. In thin-sectioned preparations, examined under the electron microscope, the nucleus of VIP-Ir neurons was eccentrically located and showed several deep invaginations. Immunoreactive dendrites of VIP-Ir cells appeared virtually spine-free. Synapses with asymmetric or symmetric junctional specializations were present on the dendritic surface. Several VIP-Ir varicosities were found to terminate on the VIP-Ir cell body, forming synaptic junctions with symmetric specializations; these synapses may derive from recurrent collaterals. VIP-Ir cells thus resemble other aspiny striatal neurons considered likely to be local circuit neurons.