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. 2017 Sep;95:94-102.
doi: 10.1016/j.yhbeh.2017.08.003. Epub 2017 Aug 30.

Cryptic Sexual Dimorphism in Spatial Memory and Hippocampal Oxytocin Receptors in Prairie Voles (Microtus Ochrogaster)

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Free PMC article

Cryptic Sexual Dimorphism in Spatial Memory and Hippocampal Oxytocin Receptors in Prairie Voles (Microtus Ochrogaster)

Marissa A Rice et al. Horm Behav. .
Free PMC article

Abstract

Sex differences are well documented and are conventionally associated with intense sex-specific selection. For example, spatial memory is frequently better in males, presumably due to males' tendency to navigate large spaces to find mates. Alternatively, monogamy (in which sex-specific selection is relatively relaxed) should diminish or eliminate differences in spatial ability and the mechanisms associated with this behavior. Nevertheless, phenotypic differences between monogamous males and females persist, sometimes cryptically. We hypothesize that sex-specific cognitive demands are present in monogamous species that will influence neural and behavioral phenotypes. The effects of these demands should be observable in spatial learning performance and neural structures associated with spatial learning and memory. We analyzed spatial memory performance, hippocampal volume and cell density, and hippocampal oxytocin receptor (OTR) expression in the socially monogamous prairie vole. Compared to females, males performed better in a spatial memory and spatial learning test. Although we found no sex difference in hippocampal volume or cell density, male OTR density was significantly lower than females, suggesting that performance may be regulated by sub-cellular mechanisms within the hippocampus that are less obvious than classic neuroanatomical features. Our results suggest an expanded role for oxytocin beyond facilitating social interactions, which may function in part to integrate social and spatial information.

Keywords: Cognitive ecology; Hippocampus; Neuromodulators; Sex-specific selection; Spatial cognition.

Conflict of interest statement

Competing interests

We declare that we have no competing interests.

Figures

Figure 1
Figure 1
(a) The apparatus of the Morris water maze test consisted of a 1000L tank, a submerged platform (red), and room dividers surrounding the tank. (b) Quadrants and zones within the Morris water maze used to analyze swimming performance. The platform area was divided into two zones: the entire platform area (red), and only the centermost point of the platform (blue). (c) Track visualization in red depicts the swim path of a single participant.
Figure 2
Figure 2
(a) Coronal section of brain tissue showing hippocampal oxytocin receptor autoradiogram (arrow, left) and atlas (right). Hippocampal sub-regions, CA1, CA3 and dentate gurus (DG) are labeled. The box indicates magnified section for panel b. (b) Cresyl violet stained bright-field image of hippocampus with an overlaid grid for calculating cell density. (c) Cartoon example demonstrates our method of volume estimation (see methods). The actual values provided in the figure (totaling 380μm) do not accurately represent the total distance from the beginning of the hippocampus to the point at which we stopped collecting data (i.e., the level of the periaqueductal grey).
Figure 3
Figure 3
Marginal mean (±SE) latency in seconds (s) to reach the platform throughout all nine learning trials for males (gray circles) and females (white squares) in the Morris water maze.
Figure 4
Figure 4
Mean (±SE) time in seconds (s) subjects spent swimming (a) in the platform-containing quadrant of the water maze, and (b) in the area specifically where the hidden platform was located. Panels c–e present the mean (±SE) number of times subjects swam in the platform-containing quadrant of the water maze (c), in the area where the hidden platform was located (d), or in the area where the center of the hidden platform was located (e). For panels c–e, mean frequencies are presented, however data were analyzed using Generalized Linear Models with a poisson distribution and a log link. Males are represented in gray; females are represented by white. The text ns indicates p > 0.05.
Figure 5
Figure 5
(a) Mean (±SE) hippocampus (HPC) volume in mm3. (b) Marginal mean (±SE) relative hippocampus volume (mm3). (c) Mean (±SE) hippocampus oxytocin receptor (OTR) density measured in disintegrations per minute in tissue equivalence (TE) of 1 mg of rat brain tissue (dpm/mg TE). (d) Mean (±SE) number of cells per area of hippocampus in regions that express OTR. Males are represented in gray; females are represented by white. The text ns indicates p > 0.05.

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