Amygdaloid projections to the frontal cortex and the striatum in the rat

Projections from the basolateral nucleus of the amygdala (BLA) to the frontal cortex and the striatum were studied by using Phaseolus vulgaris‐leucoagglutinin (PHA‐L) anterograde tracing technique in the rat. PHA‐L injections into the rostral part of the BLA resulted in a dense labeling of fibers with boutons in the dorsal bank of the rhinal fissure and in the lateral and the medial agranular cortex. PHA‐L injections into the caudal part of the BLA produced a dense labeling of fibers in the medial surface of the frontal cortex. In most of the cortical regions, labeled fibers were predominantly distributed in two bands: one in the deep part of layers I and II and the other, heavier band, in layers V and VI. PHA‐L injections into the rostral BLA resulted in a dense labeling of fibers with boutons in the olfactory tubercle, the rostral and caudolateral portion of the nucleus accumbens, and a large region of the caudate–putamen. The labeled area of the caudate–putamen included the rostroventral area, the central area, and the area caudal to the anterior commissure and dorsal and lateral to the globus pallidus. PHA‐L injections into the caudal BLA produced fiber labeling in the most rostromedial area of the caudate–putamen facing the lateral ventricle, the medial portion of the nucleus accumbens, and the lateral septum. In the rostroventral striatum, PHA‐L‐labeled fibers selectively innervated the matrix compartment that contains abundant somatostatin‐immunoreactive fibers. Compartmental segregation was less clear in the caudo‐dorsolateral caudate–putamen and in the nucleus accumbens. Electron microscopy revealed that PHA‐L‐labeled boutons in the striatum contained abundant, small, round vesicles. These boutons formed asymmetrical synapses with dendritic spines of striatal neurons.