Sex and estrous cycle affect experience-dependent plasticity in mouse primary visual cortex.

Sex hormones can affect cellular physiology and modulate synaptic plasticity, but it is not always clear whether or how sex-dependent differences identified in vitro express themselves as functional dimorphisms in the brain. Historically, most experimental neuroscience has been conducted using only male animals and the literature is largely mute about whether including female mice in will introduce variability due to inherent sex differences or endogenous estrous cycles. Though this is beginning to change following an NIH directive that sex should be included as a factor in vertebrate research, the lack of information raises practical issues around how to design experimental controls and apply existing knowledge to more heterogeneous populations. Various lines of research suggest that visual processing can be affected by sex and estrous cycle stage. For these reasons, we performed a series of in vivo electrophysiological experiments to characterize baseline visual function and experience-dependent plasticity in the primary visual cortex (V1) of male and female mice. We find that sex and estrous stage have no statistically significant effect on baseline acuity measurements, but that both sex and estrous stage have can modulate two mechanistically distinct forms of experience dependent cortical plasticity. We also demonstrate that resulting variability can be largely controlled with appropriate normalizations. These findings suggest that V1 plasticity can be used for mechanistic studies focusing on how sex hormones effect experience dependent plasticity in the mammalian cortex

Estrous cycling in example animals.

Proestrus (P), estrus (E), metestrus (M), and diestrus (D).</p

Experience dependent cortical plasticity is affected by estrous stage.

A. Five-day SRP experiments were conducted with animals grouped by estrous cycle stage on day 1. B. Average traces over SRP induction and expression. Normalized, C., and raw, D., VEP magnitudes for mice in diestrus (gray) and estrus (white) stage on day 1. E. Two-day SRP experiments were conducted in animals that were in the same estrus stage on both days 1 and 2. F. Average VEPs and G. normalized quantifications or animals in diestrus or estrus during induction and expression. There was a significant effect of sex and familiar responses were significantly larger than novel on day 2. H-J. Two-day sequence learning experiment, average VEPs, and quantification as in E-G. See main text for statistical reporting.</p

Visual acuity as measured by VEPs is comparable in female and male mice.

A. Local field potentials are recorded from head-fixed mice (left) while they view phase-reversing sinusoidal gratings with spatial frequencies varying from 0.05–0.7 (right). B. Violin plots (dashed lines mark quartile boundaries and solid horizontal lines show the mean) showing the peak-to-peak magnitude of VEPs recorded in female (shaded) and male (white) mice as a function of spatial frequency. The red line marks the approximate noise-level recorded absent visual stimulation. C. The same as in B, but for female mice in either estrus (shaded) or diestrus (white). There is no effect of either sex or stage (see main text for statistical reporting).</p

Example images of estrous stages from unstained vaginal cytology.

Estrous cycle stage is determined by the presence of specific cell types. Nucleated epithelium, rounded with visible nucleus (black boxes). Cornified epithelium, flat, irregularly shaped with no visible nucleus (white boxes). Leukocytes, small and spherical (black circles). A. Proestrus, majority nucleated epithelium. B. Estrus, majority cornified epithelium. C. Metestrus, mix of cornified epithelium, nucleated epithelium, and leukocytes. D. Diestrus, majority leukocytes.</p

Sex can affect experience dependent plasticity.

A. SRP experiment protocol. Animals were trained with repeated presentation of a 45° stimulus over five days. On the 5th day, a novel stimulus (red, 135°) was interleaved with the familiar stimulus (blue). B. Average VEP traces during SRP induction and expression in female and male mice, color coded by day as in A. C. Violin plots showing the distribution of VEP magnitudes, normalized to group averages on day 1, of female (shaded) and male (white) mice as a function of experimental day. On day 5, novel responses are significantly smaller than familiar. D. In Sequence Learning experiments, mice were shown 200 presentations of the sequence ABCD every day for five days. On the fifth day, a novel sequence DCBA (red) was interleaved with ABCD (blue). E. Group averaged sequence responses during sequence learning (black triangles indicate sequence element onset times, the scale bar is 100 μV by 100 ms, color code as in D). F. Average quantified responses to elements B and C (indicated by the gray bars in E) of female (shaded) and male (white) animals normalized to day 1 group averages. G. The same as E but with raw voltages. The familiar sequence drives a significantly larger response than a novel sequence in both normalized and raw groups, but there is a significant effect of sex only in the raw data. See main text for detailed statistical reporting.</p