Abstract
The locus coeruleus (LC) is a vertebrate-specific nucleus and the primary source of norepinephrine (NE) in the brain. This nucleus has conserved properties across species: highly homogeneous cell types, a small number of cells but extensive axonal projections, and potent influence on brain states. Comparative studies on LC benefit greatly from its homogeneity in cell types and modularity in projection patterns, and thoroughly understanding the LC-NE system could shed new light on the organization principles of other more complex modulatory systems. Although studies on LC are mainly focused on mammals, many of the fundamental properties and functions of LC are readily observable in other vertebrate models and could inform mammalian studies. Here, we summarize anatomical and functional studies of LC in non-mammalian vertebrate classes, fish, amphibians, reptiles, and birds, on topics including axonal projections, gene expressions, homeostatic control, and modulation of sensorimotor transformation. Thus, this review complements mammalian studies on the role of LC in the brain.
Highlights
The locus coeruleus (LC) is a vertebrate-specific norepinephrinergic (NE) nucleus [1,2,3,4]
In chicken Gallus domesticus, the LC forms region-specific projections to the cerebellum: the caudal LC forms a latero-caudal bundle in the inferior cerebellar peduncle, while the rostral LC forms a medio-rostral bundle in the superior peduncle [35,38]
The LC projects to sympathetic and parasympathetic preganglionic neurons located in the intermediolateral cell column (IML) of the spinal cord and exerts intricate effects involving both excitatory and inhibitory depending on the NE receptor types [55,56,57]
Summary
The locus coeruleus (LC) is a vertebrate-specific norepinephrinergic (NE) nucleus [1,2,3,4]. Remarkable conservation at multiple scales has been reported in LC, including a relatively homogeneous cell identity [12], broad axonal projections [13], and general roles in enhancing arousal and alertness [14]. Axonal Projections of the LC Extensive axonal projections of the LC enable neural modulation at the whole-brain scale In both mammals and non-mammalian vertebrates, conservations of anatomical features can be observed at multiple levels, such as LC axons bearing varicosities [13,16,17]. The LC neurons project to the dorsal medial cortex (DCm) in lizard Gekko gecko [24], and the secondary auditory cortex (the caudomedial nidopallium, NCM) and the wulst in zebra finch Taeniopygia guttata and pigeon Columba livia [33,34], indicating their involvement in species-specific functions such as generating bird songs
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