Evidence for Broad Versus Segregated Projections from Cholinergic and Noradrenergic Nuclei to Functionally and Anatomically Discrete Subregions of Prefrontal Cortex

Abstract

The prefrontal cortex (PFC) is implicated in a variety of cognitive and executive operations. However, this region is not a single functional unit; rather, it is composed of several functionally and anatomically distinct networks, including anterior cingulate cortex (ACC), medial prefrontal cortex (mPFC), and orbitofrontal cortex (OFC). These prefrontal subregions serve dissociable behavioral functions, and are unique in their afferent and efferent connections. Each of these subregions is innervated by ascending cholinergic and noradrenergic systems, each of which likewise has a distinct role in cognitive function; yet the distribution and projection patterns of cells in the source nuclei for these pathways have not been examined in great detail. In this study, fluorescent retrograde tracers were injected into ACC, mPFC, and OFC, and labeled cells were identified in the cholinergic nucleus basalis of Meynert (NBM) and noradrenergic nucleus locus coeruleus (LC). Injections into all three cortical regions consistently labeled cells primarily ipsilateral to the injection site with a minimal contralateral component. In NBM, retrogradely labeled neurons were scattered throughout the rostral half of the nucleus, whereas those in LC tended to cluster in the core of the nucleus, and were rarely localized within the rostral or caudal poles. In NBM, more than half of all retrogradely labeled cells possessed axon collaterals projecting two or more PFC subregions. In LC, however, only 4.3% of retrogradely labeled neurons possessed collaterals targeting any two prefrontal subregions simultaneously, and no cells were identified that projected to all three regions. Of all labeled LC neurons, 49.3% projected only to mPFC, 28.5% projected only to OFC, and 18.0% projected only to ACC. These findings suggest that subsets of LC neurons may be capable of modulating neuronal activity in individual prefrontal subregions independently, whereas assemblies of NBM cells may exert a more unified influence on the three areas, simultaneously. This work emphasizes unique aspects of the cholinergic and noradrenergic projections to functionally and anatomically distinct subregions of PFC and provides insights regarding global versus segregated regulation of prefrontal operations by these neuromodulatory pathways.

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

“…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).…”

“…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.…”

“…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.…”

Central relaxin-3 receptor (RXFP3) activation reduces elevated, but not basal, anxiety-like behaviour in C57BL/6J mice

“…The largest population of relaxin-3 expressing neurons is located within the tegmental area known as the nucleus incertus (NI), and these neurons project broadly throughout the brain [15][16][17][18][19]. The neuroanatomy of the relaxin-3/RXFP3 system suggests a broad role as an ascending neuromodulatory network [20,21], akin to the monoamine systems including serotonin, and noradrenaline [22][23][24][25]. Anatomical and functional data [15][16][17][18] suggest that relaxin-3/RXFP3 systems may interact directly with monoamine [19,26] and other peptide systems [27][28][29], and/or act at shared downstream limbic and hypothalamic target areas to modulate 'anxiety' and other stress-related responses [30][31][32][33][34].…”

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

“…Using this type of approach, several studies found a high degree of heterogeneity in LC neurons with respect to their efferent connectivity. One study injected three different fluorescent retrograde tracers into the orbitofrontal cortex (OFC), mPFC, and anterior cingulate cortex (ACC) (Chandler and Waterhouse 2012). Interestingly, largely nonoverlapping cell populations projecting to these three regions were detected in the LC.…”