Glutamate and aspartate immunoreactive neurons of the rat basolateral amygdala: colocalization of excitatory amino acids and projections to the limbic circuit.

The basolateral amygdala has projections to several structures that take part in the limbic cortico-striato-pallido-thalamic circuit, including the prefrontal cortex, ventral striatum, and mediodorsal thalamic nucleus. The present investigation used a technique that combines retrograde tract tracing with immunohistochemistry for glutamate and aspartate to determine if amygdaloid neurons projecting to different targets in the limbic circuit can be distinguished on the basis of their content of excitatory amino acids. Cell counts revealed that at least 85-95% of the neurons in the basolateral nucleus projecting to the prefrontal cortex or ventral striatum were pyramidal cells that exhibited glutamate or aspartate immunoreactivity. Colocalization studies indicated that 94-100% of aspartate-immunoreactive neurons in the basolateral nucleus were also glutamate positive and that 92-94% of glutamate-immunoreactive neurons were also aspartate positive. A small number of glutamate-positive pyramidal neurons in the anterior subdivision of the cortical nucleus were found to project to the mediodorsal thalamic nucleus. However, the great majority of amygdaloid neurons with projections to the mediodorsal nucleus did not exhibit glutamate or aspartate immunoreactivity. The absence of glutamate and aspartate immunoreactivity in these cells suggests that these neurons do not use excitatory amino acids as neurotransmitters. The finding of high levels of glutamate and aspartate in basolateral amygdaloid neurons projecting to the prefrontal cortex and ventral striatum is consistent with previous reports indicating that these neurons may use excitatory amino acids as neurotransmitters, but is not a definitive criterion for this determination.