Synaptic connections, axonal and dendritic patterns of neurons immunoreactive for cholecystokinin in the visual cortex of the cat

Abstract

A subpopulation of γ-aminobutyric acid (GABA) containing neurons was reported to contain cholecystokinin-immunoreactive material in the visual cortex of cat [Somogyi et al., J. Neurosci. (1984) 4, 2590–2603]. In the present study pre-embedding immunocytochemistry was used to identify which of the several types of presumed GABAergic nonpyramidal cells in areas 17 and 18 contain cholecystokinin immunoreactivity. Most of the cholecystokinin-immunoreactive somata were found in layers II-III, they were less frequent in layers I and VI, and relatively rare in layers IV and V. The distribution and density of the axon terminals resembled that of the cell bodies. Two well defined types of cholecystokinin-immunoreactive neuron were distinguished: (1) double bouquet cells in layers II–III with vertically projecting axons, and (2) small basket cells with local axons either restricted to layers II–III, or descending to layer V. Additional cholecystokinin-positive cells showed features of bitufted or multipolar neurons in layers II–VI and horizontal cells in layer I, but these cells could be defined less well due to partial staining. Cholecystokinin-immunoreactive dendrites were found to run horizontally in layer I for several hundred micrometers. Some of the cholecystokininimmunoreactive cells in layer VI had very long dendrites ascending radially up to layer III, as did their axons. A few cholecystokinin-immunoreactive cells appeared to have two axons and this was confirmed by electron microscopy. All cholecystokinin-immunoreactive neurons and terminals were separated from the basal lamina of blood vessels by glial endfeet. Random samples of boutons from each layer as well as identified terminals traced to their origin from local neurons were examined in the electron microscope. All of the boutons established symmetrical (type II) synaptic contacts with dendritic shafts, spines or somata. Quantitative electron microscopy of the postsynaptic targets of double bouquet cells and small basket cells demonstrated clear differences between these two types of neuron; basket cells having a higher proportion of their terminals in synaptic contact with somata. The findings that several distinct types of cortical neurons, as defined by their synaptic connections, contain cholecystokinin-immunoreactive material and that identified axons of all examined neurons form type II synaptic contacts suggests that the majority, if not all cholecystokinin-positive boutons forming type II contacts originate from local cortical cells. The distribution of targets postsynaptic to cholecystokinin-positive neurons is compared to those of cells labelled by other methods.