An antibody to vasoactive intestinal polypeptide (VIP) was used to examine the forms of VIP-positive neurons and the synapses made by VIP-positive axon terminals. Vasoactive intestinal polypeptide-positive cells are most common in layers II and III and the majority of them are typical bipolar neurons, with two primary dendrites which emanate from the upper and lower poles of the cell body. Their somata, which have only a few symmetric and asymmetric synapses, generally have a fusiform or "tear-drop" shape and contain nuclei with a vertically oriented cleft. The dendritic trees are arranged vertically and often extend through five cortical layers. The axons are thin and extend either from the soma or from one of the primary dendrites. The axons also follow a vertical trajectory. Other VIP-positive neurons are modified bipolar cells and a few of them are multipolar cells. The synapses formed by the VIP-positive axon terminals in the neuropil are symmetric in form, and although the synaptic clefts are narrow, the junctions are usually long and continuous, rather like those described for asymmetric synapses. Most of the VIP-positive axon terminals synpase with small dendritic shafts, but a few synapse with neuronal cell bodies. Since the majority of the VIP-positive neurons are bipolar cells it is concluded that these are the source of most of the VIP-positive axon terminals. If this is so, then the VIP-positive bipolar cells form symmetric synapses. This is in contrast to the observations of Peters and Kimerer (1981. J. Neurocytol. 10, 921-946) for the bipolar cells they examined in a Golgi-electron microscopic study had axon terminals forming asymmetric synapses. It is suggested that this disparity can be reconciled if it is assumed that the bipolar cell population consists of subgroups which have different biochemical characteristics and different synaptic relationships.
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