Leuner and Gould’s article shows this to be the case. They compared brain tissue slices of mothers post-birth days 20-25 to those of control (virgin) rats. Specifically, the researchers looked at pyramidal neurons in the 1) anterior cingulate area of the medial prefrontal cortex, 2) orbitofrontal cortex, and 3) CA1 / dendate gyrus of the hippocampus. They then used Golgi-Cox staining and light microscopy to examine the differences in the number of dendritic spines between the groups. Here are their main anatomical results:
They also found that cognitive ability was better in the mothers in an attention-based set-shifting task, indicative of improved medial prefrontal cortex performance. So it seems plausible that the dendritic spine density changes are related to the improvements in cognitive ability.
As the authors point out in the discussion, living in an enriched environment could also be a factor here. That is, control rats were housed individually, so one could make the argument that the social contact is what is driving these changes, as opposed to parenting per se. Further research might attempt to differentiate these, and to determine what types of dendritic remodeling are specific to parenting.
Leuner B, et al. 2010 Dendritic Growth in Medial Prefrontal Cortex and Cognitive Flexibility Are Enhanced during the Postpartum Period. J Neuro, doi: 10.1523/JNEUROSCI.3388-10.2010 .