GLUTAMATE AND ASPARTATE IMMUNOREACTIVE NEURONS OF THE RAT BASOLATERALAMYGDALA - COLOCALIZATION OF EXCITATORY AMINO-ACIDS AND PROJECTIONS TO THE LIMBIC CIRCUIT

The basolateral amygdala has projections to several structures that ta ke part in the limbic cortico-striato-pallido-thalamic circuit, includ ing the prefrontal cortex, ventral striatum, and mediodorsal thalamic nucleus. The present investigation used a technique that combines retr ograde tract tracing with immunohistochemistry for glutamate and aspar tate to determine if amygdaloid neurons projecting to different target s in the limbic circuit can be distinguished on the basis of their con tent of excitatory amino acids. Cell counts revealed that at least 85- 95% of the neurons in the basolateral nucleus projecting to the prefro ntal cortex or ventral striatum were pyramidal cells that exhibited gl utamate or aspartate immunoreactivity. Colocalization studies indicate d that 94-100% of aspartate-immunoreactive neurons in the basolateral nucleus were also glutamate positive and that 99-94% of glutamate-immu noreactive neurons were also aspartate positive. A small number of glu tamate-positive pyramidal neurons in the anterior subdivision of the c ortical nucleus were found to project to the mediodorsal thalamic nucl eus. However, the great majority of amygdaloid neurons with projection s to the mediodorsal nucleus did not exhibit glutamate or aspartate im munoreactivity. The absence of glutamate and aspartate immunoreactivit y in these cells suggests that these neurons do not use excitatory ami no acids as neurotransmitters. The finding of high levels of glutamate and aspartate in basolateral amygdaloid neurons projecting to the pre frontal cortex and ventral striatum is consistent with previous report s indicating that these neurons may use excitatory amino acids as neur otransmitters, but is not a definitive criterion for this determinatio n. (C) 1996 Wiley-Liss, Inc.