Chandelier cells in rat visual cortex.

Golgi-impregnated chandelier cells in rat visual cortex have been examined by both light and electron microscopy. All of the chandelier cells impregnated have their cell bodies within layer II/III and although they occur throughout area 17, there are increased numbers at the area 17/18a border and to a lesser extent at the area 17/18 border. Most of the chandelier cells are bitufted neurons, with groups of dendrites extending from the upper and lower poles of an elongate cell body, but some cells have a more multipolar configuration. The perikaryal cytoplasm is rich in rough endoplasmic reticulum and both the cell body and the sparsely spinous dendrites receive axon terminals forming symmetric and asymmetric synapses. The axons of these neurons arise from either the lower pole of the cell body or the base of one of the dendrites in the lower tuft, and the axons form laterally spread plexuses which terminate in vertical strings of boutons. The boutons in each string synapse with axon initial segments of layer II/III pyramidal cells, the uppermost bouton in each string being 7 to 14 micrometers distant from the pyramidal cell body. Some layer II/III pyramidal cells seem to receive boutons from more than one chandelier cell, others from a single chandelier cell, and still other appear to receive no chandelier cell terminals. The axon terminals of the chandelier cells are irregular in shape, contain pleomorphic synaptic vesicles, and form symmetric synapses. Evidence is presented to show that axon terminals exhibiting the same morphological features and site of synaptic termination as those of the chandelier cells contain glutamic acid decarboxylase (GAD), the enzyme which synthesizes GABA. Hence the chandelier cells are probably GABAergic, inhibitory neurons. Other GAD-positive axon terminals synapse with the cell bodies, axon hillocks, and proximal portions of the axon initial segments of the layer II/III pyramidal cells, and these terminals are probably derived from the smooth and sparsely spinous stellate cells.