Heterogeneous organization of the locus coeruleus projections to prefrontal and motor cortices

Abstract

The brainstem nucleus locus coeruleus (LC) is the primary source of norepinephrine (NE) to the mammalian neocortex. It is believed to operate as a homogeneous syncytium of transmitter-specific cells that regulate brain function and behavior via an extensive network of axonal projections and global transmitter-mediated modulatory influences on a diverse assembly of neural targets within the CNS. The data presented here challenge this longstanding notion and argue instead for segregated operation of the LC-NE system with respect to the functions of the circuits within its efferent domain. Anatomical, molecular, and electrophysiological approaches were used in conjunction with a rat model to show that LC cells innervating discrete cortical regions are biochemically and electrophysiologically distinct from one another so as to elicit greater release of norepinephrine in prefrontal versus motor cortex. These findings challenge the consensus view of LC as a relatively homogeneous modulator of forebrain activity and have important implications for understanding the impact of the system on the generation and maintenance of adaptive and maladaptive behaviors.T he brainstem nucleus locus coeruleus (LC), the primary source of the catecholamine neurotransmitter norepinephrine (NE) to the forebrain, cerebellum, and spinal cord, is conserved across several taxa, including fish, birds, and mammals (1). In mammals, it is the largest noradrenergic nucleus in the brain and the main source of NE to the neocortex. This projection system modulates sensory processing, motor behavior, arousal, and executive functions (2-10) and is implicated in a number of neuropsychiatric disorders (4). The LC has long been considered a homogeneous assembly of NE-containing cells, each with highly divergent axons that innervate broad regions of the CNS. There is only limited evidence of functional or topographic order within the nucleus (11-16), leading to the generally accepted notion that activation of the LC leads to simultaneous release and uniform physiologic action of NE throughout the brain. However, using the rat as a model, we have recently demonstrated the existence of three minimally overlapping populations of LC neurons that project to orbitofrontal (OFC), medial prefrontal (mPFC), and anterior cingulate (ACC) cortices (17,18). This projection pattern suggests a more segregated mode of operation for this projection system.The goals of the present study were to determine whether this trend of minimal axonal collateralization extended to LC neurons innervating primary motor cortex (M1) and to identify the molecular and electrophysiological characteristics that distinguish the target-specific cell populations within the rat LC. Because these prefrontal subregions regulate higher-order executive operations (19,20) whereas M1 regulates the generation of motor behaviors downstream of prefrontal cortical processes, we hypothesized that M1 and each prefrontal cortical subregion receives input from functionally distinct populations of LC neurons. ...

Expanding the power of recombinase-based labeling to uncover cellular diversity

“…Our proof-of-principle application of RC::FLTG and RC::RFLTG to analysis of the central noradrenergic system confirms the value of key features of these alleles. There are no molecular markers known to uniquely label subpopulations of noradrenergic neurons, so investigation of noradrenergic neuron heterogeneity, particularly axon projection patterns, depends either on genetic intersectional labeling (Robertson et al, 2013) or strategies for retrograde labeling by injection of dyes or viruses at target sites (Chandler and Waterhouse, 2012;Chandler et al, 2013Chandler et al, , 2014Schwarz et al, 2015). The ability of RC::FLTG to label axons of the entire noradrenergic system, subdivided between an intersectional and a subtractive population, reveals diversity of noradrenergic projections at target sites.…”

“…Wide-ranging evidence indicates the importance of galanin in noradrenergic neurons and possible functional differences between galanergic and non-galanergic LC neurons (Sevcik et al, 1993;Miller et al, 1999;Sciolino et al, 2015). However, despite evidence that the LC contains neurons with distinct projection profiles (Chandler and Waterhouse, 2012;Chandler et al, 2013Chandler et al, , 2014, the subpopulations distinguished by Gal-Cre expression show no gross difference in their axonal targets. This surprising result is compatible with the recently reported finding that specifically activating the Gal-Creexpressing LC subpopulation has the same effect on aversive behavior as activating the whole LC (McCall et al, 2015).…”

“…As we have shown above, genetic access to the LC complex is achieved through a shared history of En1 and Dbh expression. However, it is well established that neurons of the locus coeruleus are heterogeneous with respect to their axon projection profile, electrophysiological characteristics and expression of neuropeptides (Chandler and Waterhouse, 2012;Chandler et al, 2013Chandler et al, , 2014Holets et al, 1988;Olpe and Steinmann, 1991;Xu et al, 1998). Until now, it has not been possible to selectively label subpopulations of the LC complex.…”

Projection specificity in heterogeneous locus coeruleus cell populations: implications for learning and memory

“…A related follow-up study found that these unique subpopulations of LC neurons projecting to OFC, mPFC, and ACC were also distinct from another population of LC neurons projecting to motor cortex (M1) ( Fig. 1C; Chandler et al 2014a). Importantly, OFC and mPFC projecting LC neurons displayed a different molecular profile from M1 neurons, expressing higher transcript levels of proteins associated with glutamatergic transmission and excitability.…”

“…Based on the evidence presented above including previous anatomical/physiological studies showing projection specificity in the LC system (Chandler and Waterhouse 2012;Chandler et al 2013Chandler et al , 2014a we propose a conceptual, hypothetical model of LC function in which distinct subpopulations of LC noradrenergic neurons modulate specific aspects of learning and memory based on their projection specificity. Specifically, we propose that anatomically distinct populations of LC neurons project to either the amygdala or mPFC (Fig.…”