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) showed vasoactive intestinal polypeptide immunoreactive (VIP-ir) neurons localized at the lateral and ventral walls of the aqueduct. Some varicose VIP-ir elements were detected closely associated with the ependyma. While several 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 ependymal cells lining the aqueduct. Electron microscopy revealed that most of these 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 axo-somatic synapses established by VIP-negative 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 contacts. VIP-ir axon terminals formed asymmetric synapses with immunonegative dendritic shafts often in glomerulus-like assemblies. The postsynaptic immunonegative dendrites were of the aspinous, beaded type. We suggest that VIP-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 instrumental in the coordination of responses evoked by the stimulation of PAG columns.