Ultrastructural analysis of prefrontal cortical inputs to the rat amygdala: spatial relationships to presumed dopamine axons and D1 and D2 receptors

Abstract

Projections from the prefrontal cortex (PFC) to the amygdala (AMG) regulate affective behaviors in a manner that is modulated by dopamine (DA). Although PFC and DA inputs overlap within the basolateral nucleus (BLA) and intercalated cell masses (ICMs), the spatial relationship between these afferents has not been investigated, nor is it known how DA D1 (D1R) and D2 (D2R) receptors are localized in relationship to PFC terminals. We therefore combined tract-tracing from the rat PFC to the AMG with immunocytochemical labeling of tyrosine hydroxylase (TH) to identify presumed DA axons or D1R and D2R. In both the ICMs and BLA, PFC terminals formed asymmetric synapses onto spines that typically did not receive secondary synaptic inputs. TH-immunoreactive (-ir) fibers in the adjacent neuropil typically contacted different structures. Although PFC and TH-ir axons were sometimes apposed to the same dendrites or to each other, PFC terminals only rarely synapsed onto dendrites that also received synapses from TH-ir axons. D1R-ir spines and dendrites were observed commonly within the ICMs but less frequently within the BLA, and PFC axons in the ICMs occasionally synapsed onto D1R-ir spines. Within both regions, D2R-ir spines, dendrites, and axons were observed. PFC terminals occasionally contained presynaptic labeling for D2R but were not observed to synapse onto D2R-ir targets. The infrequent observation of synaptic convergence between PFC and presumed DA terminals within the AMG suggests that DA modulates PFC inputs primarily via extrasynaptic mechanisms, a conclusion supported by the localization of D2R within and D1R postsynaptic to PFC terminals.