Adrenergic and non-adrenergic neurons in the C1 and C3 areas project to locus coeruleus: a fluorescent double labeling study.

Following iontophoretic injections of the retrograde tracer Fluoro-gold into the rat locus coeruleus (LC), retrogradely labeled neurons were seen predominantly in the area of C1 adrenergic neurons in the ventrolateral medulla (nucleus paragigantocellularis; PGi) and in the area of C3 adrenergic neurons in the dorsomedial medulla (nucleus prepositus hypoglossi; PrH). Subsequent immunofluorescence for phenylethanolamine-N-methyltransferase indicated that adrenergic and non-adrenergic LC projecting neurons in both areas are interdigitated, and that 21% of LC afferent neurons in the PGi are adrenergic while only 4% of LC afferent neurons in the area of PrH are adrenergic.