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Neurons in the lateral and basolateral nuclei of the rat amygdala were studied using Golgi-Kopsch and rapid Golgi techniques. According to differences in perikaryal, dendritic, and axonal morphology, three main neuronal classes are recognized. Class I neurons, the predominant cell type in both nuclei, are large, spiny neurons that vary in size in different subdivisions of the lateral and basolateral nuclei. These neurons often have a pyramidal shape, exhibiting one or two thick "apical" dendrites and several thinner "basal" dendrites. Axons of class I neurons, which appear to pass out of the nucleus of origin, usually give off several collaterals that arborize modestly in the vicinity of the cell. Class II neurons are smaller, ovoid cells that comprise approximately 5% of impregnated neurons. These neurons are characterized by spine-sparse dendrites and fairly dense local axonal arborizations. Class II neurons may be classified as multipolar, bitufted, or bipolar, depending on dendritic branching pattern. Another type of class II neuron, the amygdaloid chandelier cell, is recognized by virtue of its distinctive axon. The chandelier cell axon gives off numerous collaterals that form nestlike entanglements exhibiting clusters of axonal varicosities. Isolated chandelierlike axons of undetermined origin were observed forming multiple contacts with initial segments of class I axons. Several small, spherical class III neurons with short, varicose dendrites were observed. Axons branch profusely to form a dense tangle of collaterals in the vicinity of the cell. Both axons and dendrites establish numerous contacts with class I dendrites. This investigation, the first detailed Golgi study of the basolateral amygdala of the rat, reveals that the cytoarchitecture of this brain region in the rat is basically similar to that of the opossum and other mammals. Morphologic details described in this report should be useful in the interpretation of ultrastructural, immunocytochemical, and electrophysiological studies of the basolateral amygdala.

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The generation of emotional responses by the basolateral amygdala is largely determined by the balance of excitatory and inhibitory inputs to its principal neurons, the pyramidal cells. The activity of these neurons is tightly controlled by GABAergic interneurons, especially a parvalbumin-positive (PV+) subpopulation that constitutes almost half of all interneurons in the basolateral amygdala. In the present semi-quantitative investigation we studied the incidence of synaptic inputs of PV+ axon terminals onto pyramidal neurons in the rat basolateral nucleus (BLa). Pyramidal cells were identified using calcium/calmodulin-dependent protein kinase II (CaMK) immunoreactivity as a marker. In order to appreciate the relative abundance of PV+ inputs compared to excitatory inputs and other non-PV+ inhibitory inputs, we also analyzed the proportions of asymmetrical (presumed excitatory) synapses and symmetrical (presumed inhibitory) synapses formed by unlabeled axon terminals targeting pyramidal neurons. The results indicate that the perisomatic region of pyramidal cells is innervated almost entirely by symmetrical synapses, whereas the density of asymmetrical synapses increases as one proceeds from thicker proximal dendritic shafts to thinner distal dendritic shafts. The great majority of synapses with dendritic spines are asymmetrical. PV+ axon terminals mainly form symmetrical synapses. These PV+ synapses constitute slightly more than half of the symmetrical synapses formed with each postsynaptic compartment of BLa pyramidal cells. These data indicate that the synaptology of basolateral amygdalar pyramidal cells is remarkably similar to that of cortical pyramidal cells, and that PV+ interneurons provide a robust inhibition of both the perisomatic and distal dendritic domains of these principal neurons. Keywordsimmunocytochemistry; electron microscopy; inhibition, calcium/calmodulin-protein kinase II The basolateral amygdala (ABL), which consists of the lateral, basolateral, and basomedial amygdalar nuclei, is one of the most important brain regions for the generation of emotional behavior and the formation of emotional memories (Aggleton, 1992;Aggleton, 2000;Shinnick-Gallagher et al., 2003). It receives sensory information from the thalamus and cerebral cortex (McDonald, 1998) and produces appropriate emotional responses by activating a variety of subcortical regions including the central amygdalar nucleus, bed nucleus of the stria terminalis, and striatum. The outputs of the ABL arise from pyramidal cells (McDonald, 1992b), which resemble their counterparts in the cerebral cortex. These neurons, which constitute about 85% of the neurons in the ABL, are characterized by a pyramidal or piriform cell body, and spiny dendrites (Hall, 1972;McDonald, 1982McDonald, , 1984 1992a,b;Millhouse and DeOlmos, 1983). Some ABL pyramidal cells have a marked pyramidal morphology, with a clear differentiation of thicker "apical" dendrites from thinner "basal" dendrites, whereas others have a semipyramidal or even stellate appe...