Noradrenergic innervation of pyramidal cells in the rat basolateral amygdala

The basolateral nuclear complex of the amygdala (BLC) receives dense noradrenergic/norepinephrine (NE) inputs from the locus coeruleus that play a key role in modulating emotional memory consolidation. Knowledge of the extent of synapse formation by NE inputs to the BLC, as well as the cell types innervated, would contribute to an understanding of how NE modulates the activity of the BLC. To gain a better understanding of NE circuits in the BLC, dual-label immunohistochemistry was used at the light and electron microscopic levels in the present study to analyze NE axons and their innervation of pyramidal cells in the anterior subdivision of the basolateral amygdalar nucleus (BLa). NE axons and BLa pyramidal cells were labeled using antibodies to the norepinephrine transporter (NET) and Ca(2+)/calmodulin-dependent protein kinase (CaMK), respectively. Dual localization studies using antibodies to NET and dopamine-beta-hydroxylase (DBH) revealed that virtually all NE axons and varicosities expressed both proteins. The BLa exhibited a medium density of NET+ fibers. Ultrastructural analysis of serial section reconstructions of NET+ axons revealed that only about half of NET+ terminals formed synapses. The main postsynaptic targets were small-caliber CAMK+ dendritic shafts and spines of pyramidal cells. A smaller number of NET+ terminals formed synapses with unlabeled cell bodies and dendrites. These findings indicate that the distal dendritic domain of BLa pyramidal cells is the major target of NE terminals in the BLa, and the relatively low synaptic incidence suggests that diffusion from non-synaptic terminals may be important for noradrenergic modulation of the BLa.