New Hippocampal Neurons Mature Rapidly in Response to Ketamine But Are Not Required for Its Acute Antidepressant Effects on Neophagia in Rats

Examples of immunohistochemical markers. A, After kainate injection, all mature granule neurons and some BrdU-labeled 16-d-old NeuN⁺ neurons expressed zif268, indicating synaptic activation. GCL, granule cell layer B, Dividing cells (arrows) were identified using PCNA immunohistochemistry. C, Cells surviving 2-3 weeks (arrows) were identified with BrdU immunohistochemistry (gray-brown); immunonegative cells were stained with blue-purple counterstain.

Rapid effects of ketamine on granule cell maturation and proliferation. A, S-ketamine (ket) increased the proportion of 16-d-old BrdU⁺ cells colabeled with NeuN and zif268 (zif) 16 h later, relative to saline-treated controls (sal) (*t test, t₍₄₎ = 3.065, p =0.0375). All bars represent mean ± SEM. B, S-ketamine increased the number of PCNA⁺ (dividing) cells in the subgranular zone 16 h later (*t test, t₍₁₀₎ = 2.42, p = 0.0359). C, Animal treatment time course for short-term effects; ketamine injection was 10 mg/kg, i.p., in each experiment. D, The maturation effect was not seen in 7-d-old cells (t test, t₍₉₎ = 0.98, p = 0.35). E, Animal treatment time course for short-term effects in young cells. F, G, Increased zif/NeuN coexpression and strong NeuN expression were seen in 14-d-old cells within 2 h of ketamine treatment (zif: *t test, t₍₉₎ = 2.33, p = 0.0450; strong NeuN: *t test, t₍₉₎ = 3.55, p = 0.0062). H, Animal treatment time course for very rapid effects on maturation.

Effects of long-term ketamine treatment. A, B, Animal treatment time courses; all ketamine injections were 10 mg/kg, i.p. C, D, Long-term daily ketamine treatment for 14 d (C) or 21 d (D) increased the proportion of zif/NeuN⁺ BrdU⁺ granule cells (*14 d: t test, t₍₉₎ = 3.20, p = 0.0108; *21 d: t test, t₍₁₁₎ = 2.773, p = 0.0181). E, F, S-ketamine had no effect on cell proliferation when administered daily for 14 or 21 d (14 d: t₍₁₀₎ = 0.035, p = 0.973; 21 d: t₍₉₎ = 0.955, p = 0.365). G, H, BrdU⁺ cell survival was unaffected by 14 d of daily treatment with S-ketamine (t test, t₍₁₀₎ = 1.03, p = 0.33) but was decreased after 21 d (*t test, t₍₁₂₎ = 2.71, p = 0.0191). All bars represent the mean ± SEM (n = 6-7 per group).

Sustained effects of S-ketamine in a depression model. A, Ketamine given 32 d earlier increased zif/NeuN expression in 16-d-old cells regardless of long-term corticosterone exposure (main effect of CORT: F_(1,17) = 0.00, p = 0.996; *main effect of ketamine: F_(1,17) = 14.99, p = 0.0017; CORT × ketamine interaction: F_(1,17) = 0.0005, p = 0.9821 by two-way ANOVA). B, S-ketamine increased the number of PCNA⁺ cells 32 d later and prevented the inhibition of proliferation by long-term corticosterone treatment (*main effect of ketamine: F_{(1, 23)} = 7.44, p = 0.013; *main effect of CORT: F_(1,23) = 8.35, p = 0.009; CORT × ketamine interaction: F_(1,23) = 0.00, p = 0.988 by two-way ANOVA). C, Animal treatment time course for maturation and proliferation effects. D, A single S-ketamine injection prior to long-term CORT treatment prevented a decrease in sucrose preference (one-way ANOVA, F_(2,15) = 6.98, p = 0.0072; *p < 0.05 vs saline in post hoc test). Values represent the mean ± SEM (n = 4-6 per group). E, Neither long-term exposure to CORT nor short-term ketamine exposure prior to CORT significantly altered new cell survival (F_(2,15) = 1.12, p = 0.35 by one-way ANOVA). Values represent the mean ± SEM (n = 6-7 per group. F, Animal treatment time course for sucrose preference and survival effects.