Sexual dimorphism in locus coeruleus dendritic morphology: a structural basis for sex differences in emotional arousal - PubMed (original) (raw)

Sexual dimorphism in locus coeruleus dendritic morphology: a structural basis for sex differences in emotional arousal

Debra A Bangasser et al. Physiol Behav. 2011.

Abstract

Stress-related psychiatric disorders, such as depression and anxiety, affect a disproportionate number of women. We previously demonstrated that the major brain norepinephrine (NE)-containing nucleus, locus coeruleus (LC) is more sensitive to stressors and to the stress-related neuropeptide, corticotropin-releasing factor (CRF) in female compared to male rats. Because the LC-NE system is a stress-responsive system that is thought to be dysregulated in affective disorders, sex differences in LC structure or function could play a role in female vulnerability to these diseases. The present study used different approaches to compare LC dendritic characteristics between male and female rats. Immunofluorescence labeling of tyrosine hydroxylase, the norepinephrine synthetic enzyme, revealed that LC dendrites of female rats extend further into the peri-LC region, covering a significantly greater area than those of males. Optical density measurements of dendrites in the peri-LC revealed increased dendritic density in females compared to their male counterparts. Additionally, immunoreactivity for synaptophysin, a synaptic vesicle protein, was significantly greater in the LC in female rats, suggesting an increased number of synaptic contacts onto LC processes. Individual LC neurons were juxtacellularly labeled with neurobiotin in vivo for morphological analysis. LC dendritic trees of females were longer and had more branch points and ends. Consistent with this, Sholl analysis determined that, compared to males, LC dendrites of females had a more complex pattern of branching. The greater dendritic extension and complexity seen in females predicts a higher probability of communication with diverse afferents that terminate in the peri-LC. This may be a structural basis for heightened arousal in females, an effect which may, in part, account for the sex bias in incidence of stress-related psychiatric disorders.

PubMed Disclaimer

Figures

Figure 1

Diagrams depict endpoints analyzed for densitometry and morphology. (A) Schematics represent the regions sampled in a level of the rostral LC (left) and a middle level of the rostrocaudal LC (right). The rostral and mid LC regions are AP −9.30 and −9.80 relative to Bregma, respectively [62]. The nuclear core (i.e., LC cell bodies) is defined by the dashed line. The double line demarcates the area of the ventromedial dendrites analyzed. The shaded triangle represents the region of interest used to assess the density of the ventromedial dendrites. The striped triangle represents the region of interested used to assess the density of the dorsolateral dendrites. (B) Shown on the representative trace are examples of an end, a node, and segments. This neuron also had primary (1st), secondary (2nd), and tertiary dendrites (3rd). Abbreviations: IVth, fourth ventricle; D, dorsal; DL, dorsolateral; M, medial; VM, ventromedial

Figure 2

Comparison of the TH-immunofluorescence in the LC and peri-LC of a male and female rat. Photomicrographs of coronal sections at the level of the rostral (top) and mid LC (bottom) of a male (left) and female (right) rat. Grayscale images of TH-immunofluorescence were inverted for quantification. Scale bars = 200 µm.

Figure 3

The LC dendritic field is more extensive and LC dendrites are denser in female rats. (A) Bars show the mean total area (µm2) covered by dendrites in the ventromedial peri-LC region. This area is more extensive in females than in males (n=7–8). (B) Bars show the mean optical integrated density of LC dendrites the ventromedial region. Compared to males, the density of dendrites in this area was greater in female rats. (C) Bars show the mean dendritic density in the dorsolateral peri-LC region, which was greater in females than in males. (D) Bars show the mean integrated density of TH expression in the cell body region of the mid LC (n=6–7). There was no sex difference in TH expression in LC cell bodies. (E) Representative Western blot of TH (top bands, MW = 58) and tubulin (bottom bands, MW = 51) from a representative male (left) and female (right) rat. Blots have been inverted and converted to grayscale for presentation. (F) Bars represent the mean amount of TH protein normalized to a tubulin loading control from LC punches taken from male and female rats (n=12). There was no sex difference in the level of TH protein. Asterisks indicate a significant main effect of sex (

<0.05). Number signs indicate a significant main effect of region (

<0.05). Data are represented as the mean (±s.e.m.).

Figure 4

Comparison of synaptophysin immunoreactivity in the LC of male and female rats. Photomicrographs of coronal mid LC sections showing synaptophysin immunoreactivity (red, middle) and the merged image of synaptophysin and TH immunoreactivity (green, right). A representative male is shown in the top row and a representative female is shown on the bottom row. Scale bars = 200 µm.

Figure 5

Quantification of synpatophysin labeling reveals increased density in the LC and peri-LC of female rats. (A–C) Bars represent the mean integrated density of synaptophysin immunoreactivity in the ventromedial peri-LC (A), dorsolateral peri-LC (B), and core (C) (n=6–8). Asterisks indicate a significant main effect of sex (

<0.05). Number signs indicate a significant main effect of region (

<0.05). Delta indicates a trend for a sex difference (

=0.06). Data are represented as the mean (±s.e.m.).

Figure 6

Comparison of individual neurobiotin labeled LC neurons in a male and female rat. Fluorescent photomicrograph (left) of a neurobiotin labeled LC cell (green) merged with TH (red), and the corresponding cell tracing (right). Branches on the tracing are colored according to branch order (1st is yellow, 2nd is red, 3rd is green, 4th is blue, and 5th is pink). A representative male is shown in the top row, while a representative female is shown on the bottom row.

Figure 7

Morphological analysis revealed that dendrites of females had more branches than those of males. (A) The left bars represent the surface area (µm2) of the somas, while the right bars represent soma volume (µm3). Both measurements indicate that males (black bars, n=19–20 cells from 9 male rats) had larger somas than females (gray bars, n=32 cells from 12 female rats). (B) The left bars represent the number of nodes (i.e., branch points) on the dendrites, while the right bars represent the number of ends. Dendrites of females had more nodes and ends, indicative of increased branching. (C) The left bar shows the length of the average dendritic tree, while the right bars show the length of the longest dendritic tree. The dendritic trees of females were significantly longer than those of males. (D) The left bars display the length of the average segment, while the right bars display the length of the longest segment. Segment length was comparable between male and female dendrites. (E) The graph represents the number of branches broken down by branch order (i.e., 1st, 2nd, 3rd, 4th, and 5th). Dendrites of females had significantly more branches, regardless of branch order, than males. (F) The graph represents the length of branches broken down by branch order. Dendrites of females were significantly longer than those of males, regardless of branch order. Data are represented as the mean (±s.e.m.). Asterisks indicate a significant sex difference (

<0.05).

Figure 8

Sholl analysis revealed increased complexity of LC dendrites in females. (A) This is a diagram of a Sholl analysis of a representative tracing from a male (left) and female rat (right). Concentric Sholls were placed so that they radiated in 20 µm increments from around the cell body. Intersections with these Sholls were counted. (B) This graph displays the number of intersections with the Sholls at increasing distances from the cell body. Dendrites of females (n=32 cells from 12 female rats) intersected with more Sholls than those of males (n=20 cells from 9 male rats), indicating that the female dendritic tree was more complex.

References

1. Kessler RC, McGonagle KA, Swartz M, Blazer DG, Nelson CB. Sex and depression in the National Comorbidity Survey. I: Lifetime prevalence, chronicity and recurrence. J Affect Disord. 1993;29:85–96. - PubMed
1. Kessler RC, Sonnega A, Bromet E, Hughes M, Nelson CB. Posttraumatic stress disorder in the National Comorbidity Survey. Arch Gen Psychiatry. 1995;52:1048–1060. - PubMed
1. Marcus SM, Young EA, Kerber KB, Kornstein S, Farabaugh AH, Mitchell J, et al. Gender differences in depression: findings from the STAR*D study. J Affect Disord. 2005;87:141–150. - PubMed
1. Ter Horst GJ, Wichmann R, Gerrits M, Westenbroek C, Lin Y. Sex differences in stress responses: focus on ovarian hormones. Physiol Behav. 2009;97:239–249. - PubMed
1. Young EA, Altemus M. Puberty, ovarian steroids, and stress. Ann N Y Acad Sci. 2004;1021:124–133. - PubMed

Sexual dimorphism in locus coeruleus dendritic morphology: a structural basis for sex differences in emotional arousal - PubMed (original) (raw)