2 TYPES OF CHOLINERGIC PROJECTIONS TO THE RAT AMYGDALA

The cholinergic innervation of the rat amygdala was studied immunohist ochemically with antibodies against choline acetyltransferase and the low affinity p75 nerve growth factor receptor in normal rats and in ra ts lesioned with an immunotoxin, 192 IgG-saporin, directed against the p75 nerve growth factor receptor. The density of choline acetyltransf erase-positive fibers was high in the nucleus of the lateral olfactory tract, the basolateral nucleus, and the amygdalohippocampal area; med ium in the lateral nucleus, the cortical nucleus, the accessory basal nucleus, the periamygdaloid cortex, and the anterior amygdaloid area; and low in the medial and central nuclei. Nerve growth factor receptor -positive fibers were of medium density in the lateral nucleus, the ac cessory basal nucleus, the cortical nucleus, the anterior amygdaloid a rea, the periamygdaloid cortex, and the amygdalohippocampal area. The medial nucleus and the central nucleus displayed a low density of nerv e growth factor receptor-positive fibers. The basolateral nucleus and the nucleus of the lateral olfactory tract also contained a low densit y of nerve growth factor receptor-positive fibers even though the two nuclei displayed the highest density of choline acetyltransferase-posi tive fibers in the amygdala. Injections of 192 IgG-saporin induced a c omplete loss of cholinergic nerve growth factor receptor-positive neur ons in the basal forebrain but spared a subpopulation of nerve growth factor receptor-negative cholinergic neurons in the nucleus basalis-su bstantia innominata complex. Following 192 IgG-saporin injections, cho line acetyltransferase-positive and acetylcholinesterase-positive fibe rs were essentially unchanged in the nucleus of the lateral olfactory tract and the basolateral nucleus and showed a partial reduction in th e remaining nuclei of the amygdaloid complex. Cholinergic fibers emana ting from cholinergic cell group 4 neurons reached the amygdala via th e stria terminalis and the ventral amygdalofugal pathway. These observ ations indicate that two amygdaloid nuclei, the nucleus of the lateral olfactory tract and the basolateral nucleus, receive their cholinergi c projections predominantly, if not exclusively, from nerve growth fac tor receptor-negative cholinergic neurons whereas all remaining amygda loid regions receive fibers from nerve growth factor receptor-negative as well as nerve growth factor receptor-positive cholinergic neurons.