Distribution of nNOS-ir neurons within the mouse medial preoptic area (MPA). Comparison among females in different stages of the cycle (estrus and diestrus) and males. Scale bar: 150 μm. The histogram reports the quantitative differences in the number of nNOS-ir cells (mean ± standard error). * = p < 0.05 in comparison to males; °° = p < 0.01 in comparison to estrus female.