Distribution and synaptology of vasoactive intestinal polypeptide (VIP) immunoreactive structures in the rat periaqueductal grey

Light microscopic analysis of the rat midbrain periaqueductal grey (PAG) sh owed vasoactive intestinal polypeptide immunoreactive (VIP-ir) neurons loca lized at the lateral and ventral walls of the aqueduct. Some varicose VIP-i r elements were detected closely associated with the ependyma. While severa l VIP-ir elements were encountered immediately under the ependyma, in a few cases, VIP-ir cell bodies were seen on the luminal surface of the ependyma l cells lining the aqueduct. Electron microscopy revealed that most of thes e cells possessed the characteristics of a local circuit neuron. All VIP-ir cells had indented nuclei. Two types were distinguished: one with rounded cell body receiving numerous axe-somatic synapses established by VIP-negati ve axons. The other cell type was fusiform and its surface was almost fully isolated from axonal contacts by a glial sheath. The VIP-ir processes were interconnected with other periaqueductal cells by a variety of synaptic co ntacts. VIP-ir axon terminals formed asymmetric synapses with immunonegativ e dendritic shafts often in glomerulus-like assemblies. The postsynaptic im munonegative dendrites were of the aspinous, beaded type. We suggest that V IP-ir cells and processes in the midbrain PAG Establish connections between the longitudinal functional columns of this region. On the basis of their morphology, VIP-ir cells in the PAG appear to be excitatory, terminating on inhibitory interneurons. Thus, a VIP-stimulated inhibition may be instrume ntal in the coordination of responses evoked by the stimulation of PAG colu mns.