Vasoactive intestinal polypeptide (VIP) immunoreactive elements in the caudal ventral striatum of the rat: a light...

Under the posterior limb of the anterior commissure, a brain region intercalated between the ventral striatum and the ventral pallidum was previously identified as the interstitial nucleus of the posterior limb of the anterior commissure by de Olmos. This region, referred here as the caudal ventral stratum (VSc), is characterized by a dense plexus of vasoactive intestinal peptide immunoreactive (VIP+) axons. Double-fluorescence immunocytochemical reactions reveal that the dense VIP+ plexus is found in a region also rich in dopaminergic (i.e., tyrosine-hydroxylase immunoreactive) fibers but poor in enkephalinergic terminals. The dense plexuses of VIP+ axons in VSc appear to be contiguous with those in the bed nucleus of stria terminalis (BNST) and the central nucleus of amygdala (CNA). These results support the notion that this VSc region is a part of the "extended amygdala" as proposed recently by Alheid and Heimer, and confirms that its anatomical properties are closer related to the ventral striatum than the ventral pallidum. Electron microscopic analysis reveals that the VIP+ boutons form asymmetrical synapses with dendrites and spines, and symmetrical synapses with somata of unlabeled VSc neurons. The few VIP+ neurons within this area form synapses with many unlabeled axon terminals on both their somata and dendrites. Some VIP+ neurons, however, also form axosomatic and axodendritic synapses with VIP+ boutons.