(A, B) The number of each type of dendritic spine in untrained and IA-trained rats during different periods of the estrous cycle. IA-learning tasks significantly increased the number of mushroom-shaped spines on CA1 pyramidal neurons of D2 rat dendrites (untrained, n = 27; IA-trained, n = 25) but not in ProE rat dendrites (untrained, n = 17; IA-trained, n = 15). (C, D) The number of each type of dendritic spine in untrained and IA-trained rats with or without E2 priming. IA-learning tasks significantly increased the number of mushroom-shaped spines on the dendrites of unprimed OVX rats (untrained, n = 31; IA-trained, n = 44), but not on those of E2-primed OVX rats (untrained, n = 18; IA-trained, n = 20). *P < 0.05, unpaired Student’s t test. Error bars represent SEM. (Bottom) Representative photomicrographs (×400) of tertiary apical dendrites of CA1 pyramidal neurons. Black arrowheads indicate mushroom-shaped spines; white arrowheads indicate filopodial spines. Scale bar: 3 μm.