Mean synaptic and mitochondrial density by treatment group: multiple and prolonged exposure to anesthesia was associated with greater reductions in both synaptic and mitochondrial density. Synaptic and mitochondrial density are expressed as mean ± SEM. §Treatment group (n = 5/group): 1 × 2 h = single 2-h exposure postnatal day 7 (P7); 1 × 6 h = single 6-h exposure (P7); 3 × 2 h = three 2-h exposures (P7, P10, and P13). a, b, c indicates groups with differing superscripts (a, b, or c) are significantly different from one another (statistical significance defined as P < 0.05). For example, for synaptic density: the control and 1 × 2-h groups do not differ from each other, but both differ from the 1 × 6-h and 3 × 2-h groups, which also differ from one another.