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Prior dopamine-beta-hydroxylase immunohistochemistry suggested that the projections from the locus coeruleus (LC) to the orbital and medial prefrontal cortex (PFC) are heterogeneous (Lewis & Morrison, J Comp Neurol, 1989, 282:317-30). A tract-tracing corroboration of this heterogeneity is still lacking. In addition, whether areas of PFC that receive direct projections from LC are the same that provide modulatory feedback to LC remains unclear. Here, we examined the distribution of retrograde and anterograde labeling in LC with injections of multiple, differently-colored neuronal tracers in distinct architectonic areas in orbital and medial PFC. On the basis of its connectivity, the orbital and medial PFC was divided into orbital (OPFC) and medial (MPFC) ‘networks’ (Price, ANYAS, 2007, 1121:54-71). Injections of retrograde tracers in PFC produced dense to sparse labeling in the LC core. The distribution of this labeling varied with the location of the injection site, supporting the prior immunohistochemical evidence. In the MPFC network, injections in areas 24, 25, 32, 10, 14c, and the intermediate agranular insula (Iai) produced a moderate to dense labeling in LC, with injections in areas 24, 25 and 32 producing the densest labeling, and with injection in area 10m producing more labeling than injection in area 10o. In the OPFC network, injections in area 13b, 12l, and the posterior median agranular insula (Iapm) produced dense labeling in LC whereas injections in area 11l produced only sparse labeling. The distribution of retrograde labeling in LC revealed a conspicuous overlap of cells labeled from distinct areas, with no obvious internal topography. Despite this conspicuous overlap, no double labeled cells could be observed in cases with injections of differently-colored tracers in distinct areas. This absence of co-localization is consistent with similar recent evidence obtained in rats (Chandler & Waterhouse, Front Behav Neurosci, 2012, 6:1-9) and suggests a complex spatial segregation of LC projecting neurons. The injection of anterograde tracers in PFC produced dense to sparse labeling predominantly in the direct periphery of the LC core. The examination of the distribution of this labeling indicated that the connections between LC and the different areas of OPFC and MPFC are rather reciprocal. In the MPFC network, injections in areas 24, 25, 32, 11m and Iai produced dense labeling whereas injections in 10m and 10o produced moderate and no labeling, respectively. In the OPFC network, injections in area 13l, Iapm, and the medial agranular insula (Iam) produced dense labeling whereas injections in areas 11l produced sparse labeling.
