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

D. J. Chandler, W.-J. Gao, B. D. Waterhouse
2014 Proceedings of the National Academy of Sciences of the United States of America  
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
more » ... 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. To test this hypothesis, we paired injections of fluorescently labeled retrograde tracers in M1 with injections in OFC, mPFC, or ACC. After confirming the existence of distinct populations of LC neurons with nonoverlapping projections to M1 and each prefrontal cortical subregion, several approaches were used to probe for potential functional differences among populations. First, we combined retrograde tract tracing with laser-capture microdissection and RT-PCR to quantify differences in expression of various mRNAs among populations. We then performed whole-cell patch-clamp recordings on retrogradely labeled cells in vitro to measure passive and active membrane properties within each population. Through this combination of techniques, we have determined that LC cells innervating specific subregions of the prefrontal cortex (PFC) are phenotypically and electrophysiologically distinct from those terminating in M1. Specifically, OFC and mPFC projection cells contain enriched mRNA transcripts coding for several markers of excitability and transmitter release relative to the M1 projection group. These same populations also differed in their spontaneous firing rates and several action potential and membrane properties, as well as size and frequency of glutamatemediated excitatory postsynaptic currents. Taken together, these findings suggest that LC cells projecting to subregions of PFC are more excitable than those projecting to cortical circuitries involved in the execution of motor acts, which may be indicative of a greater demand for NE by PFC. Deviations from this functional organization may have consequences for the sequencing of operations required to execute normal behaviors and may manifest in the form of hyperactivity, attentional impairments, and impulsivity that are related to various neuropsychiatric disorders. Overall, these data argue for a more specific organization of the LC with respect to the functions of its efferent targets and consequently more subtle, asynchronous control of NE release within and across its efferent domain. Target -Specific Projections. Retrograde tract-tracing experiments revealed largely segregated populations of cells projecting to OFC vs. M1, mPFC vs. M1, and ACC vs. M1. For Results LC Cells Exhibit Significance The locus coeruleus projection system in the brain is thought to exert uniform and synchronous modulatory effects on cells and circuits throughout the CNS by widespread release of its transmitter, norepinephrine. We challenge this notion by demonstrating that neurons in the locus coeruleus maintain segregated connections to brain regions with distinctly different functions. Specifically, cells that communicate with the prefrontal cortex, a region involved in cognition and executive function, are characterized by properties that allow for independent and asynchronous modulation of operations in this area, compared with those that project to the motor cortex and regulate movement generation. These findings have important implications for understanding the role of this system in normal brain physiology and pathologic neuropsychiatric conditions.
doi:10.1073/pnas.1320827111 pmid:24753596 pmcid:PMC4020069 fatcat:tmq6pleabranth6r3jvvcer6tm