Evidence for a regional specificity in the density and distribution of noradrenergic varicosities in rat cortex

Abstract

The brainstem nucleus locus coeruleus (LC) is the sole source of norepinephrine (NE)-containing fibers in the mammalian cortex. Previous studies suggest that the density of noradrenergic fibers in rat is relatively uniform across cortical regions and that cells in the nucleus discharge en masse. This implies that activation of the LC results in equivalent release of NE throughout the cortex. However, it is possible that there could be differences in the density of axonal varicosities across regions, and that these differences, rather than a difference in fiber density, may contribute to the regulation of NE efflux. Quantification of dopamine β-hydroxylase (DβH)-immunostained varicosities was performed on several cortical regions and in the ventral posterior medial (VPM) thalamus by using unbiased sampling methods. The density of DβH varicosities is greater in the prefrontal cortex than in motor, somatosensory, or piriform cortices, greater in superficial than in deep layers of cortex, and greater in the VPM than in the somatosensory cortex. Our results provide anatomical evidence for non-uniform release of NE across functionally discrete cortical regions. This morphology may account for a differential, region-specific, impact of LC output on different cortical areas.

Figure 1

Regional borders used to delineate cortical and subcortical areas of interest. Demarcation of the areas of interest. All anatomical borders were taken from Paxinos and Watson (4th edition, 1998). Distance of section from bregma indicated at lower right of each section. Abbreviations: ACA; anterior cingulate, BF; barrel field, ILA; anterior infralimbic, M1; primary motor, M2; secondary motor LO; lateral orbitofrontal, MO; medial orbitofrontal, VO; ventral orbitofrontal, DLO; dorsolateral orbitofrontal, PIR; piriforn, PL; prelimbic, S1; primary somatosensory, S2; secondary somatosensory, VPM; ventral posterior medial thalamic nucleus.

Figure 2

A sample coronal section paired with atlas diagram (Paxinos and Watson, 1998) at bregma −0.26.

Figure 3

Parcellation of pyramidal superficial (layer II–III) and deep (layer V–VI) cortical layers. Cortical layers were differentiated by cytoarchitectural boundaries with methyl green staining. (A) A sample coronal section from somatosensory cortex. DβH varicosities were observed in both superficial and deep layers (B).

Figure 4

Photomicrographs illustrating DβH-stained fibers with and without varicosities. A. DβH-stained fiber from prefrontal cortex. Arrows top to bottom indicate typical large, intermediate, and small varicosities along the fiber length. B. DβH-stained fibers from the dorsal noradrenergic bundle where varicosities are typically absent.

Figure 5

Photomicrographs illustrating DβH fibers and varicosities in the frontal, motor, sensory, and piriform cortices; and VPM thalamus at 40× (lower rows) and 100× (upper row) magnification. While the 40× images appear more uniform, differences in varicosity density can be observed at higher magnification. VPM: ventral posterior medial thalamus.

Figure 6

Density of noradrenergic varicosities in cortical regions associated with different modalities. Density is calculated by dividing the estimated total number of noradrenergic varicosities by the summed volume for each anatomical region. PIR: piriform. * significantly different from sensory, motor and piriform cortex (p<0.05).

Figure 7

Differences in varicosity density in primary and secondary cortices (A) and superficial and deep layers of cortex (B). No significant difference in density was found between primary and secondary regions. Superficial layers of cortex show varicosity density marginally greater than deep layers (see text; * p = 0.0507).

Figure 8

Density of noradrenergic varicosities within subdivisions of frontal cortex. There are no significant differences in the density of noradrenergic varicosities among the medial prefrontal, orbital, or anterior cingulate cortices. PL: prelimbic. ILA anterior infralimbic. MO: medial orbitofrontal. VO: ventral orbitofrontal. LO: lateral orbitofrontal. DLO: dorsolateral orbitofrontal. ACA: anterior cingulate.

Figure 9

Density of noradrenergic varicosities in somatosensory regions. VPM: ventral posterior medial thalamus. BF: barrel field cortex. S1: primary somatosensory cortex. S2: secondary somatosensory cortex. * significantly different from BF and S1 (p<0.05).

Figure 10

Density of noradrenergic varicosities in dorsolateral cortex arranged by distance from the midline. ACA: anterior cingulate. M2: secondary motor. M1: primary motor. S1: primary somatosensory. BF: barrel field. S2: secondary somatosensory.

Figure 11

Single fiber inter-varicosity distances vs varicosity density. Means ± SEM for both x and y values. Dashed line: predicted form of the relationship (distance × density = a constant) fit to pass through the coordinates for frontal cortex (constant = 6.19), which would apply if the density of fibers were constant across regions, and differences in varicosity density were explained by differences in inter-varicosity distance alone. Motor, sensory, frontal, piriform (PIR) cortices and VPM thalamus.