(a), Experimental design for studies in Type 2 diabetic mice. (b), Experimental design for studies in Type 1 diabetic rats. Adrenalectomized animals received corticosterone replacement via the drinking water (25μg ml^–1 in 0.9% saline). (c), Sham–operated db/db mice were unable to learn the location of the hidden platform in the Morris water maze. In contrast, db/db mice that received adrenalectomy and corticosterone replacement learned the location of the platform as effectively as non–diabetic mice. Shorter escape latencies on the first day of training in adrenalectomized db/db mice were attributable to performance on successive trials, as escape latencies were similar during the first trial (see Results). (d), db/db mice with intact adrenal glands exhibit impaired object recognition, while db/db mice with levels of corticosterone ‘clamped’ through adrenalectomy and corticosterone replacement show preference for the novel object that is similar to wildtype controls. (e), Learning is impaired in insulin–deficient diabetic rats experiencing elevated corticosterone levels, but not in diabetic rats that received adrenalectomy and corticosterone replacement. (f), Novel object preference is reduced in sham–operated diabetic rats, but preserved in diabetic rats with normal levels of corticosterone. Abbreviations: Adx + cort = adrenalectomized with 25μg ml^–1 corticosterone replacement; STZ = streptozocin–treated (70mg kg^–1). Error bars = s.e.m.

(a–b), Following a single injection of 300mg kg^–1 BrdU with a 24 hour survival period, we observed a significant reduction in the number of labeled cells in wildtype mice that had been infused with the antimitotic drug AraC for ten to fourteen days. The micrograph in (a) shows the dentate gyrus of a mouse infused with vehicle, and the micrograph in (b) shows the dentate gyrus of a mouse infused with AraC. Arrows indicate labeled cells. (c), We also quantified synaptic marker expression in the inner third of the dentate molecular layer, where the medial perforant path synapses are located. This confocal micrograph shows the anatomical region (outlined) where scans were taken for analysis of synaptophysin labeling, and where electrodes were positioned for electrophysiological recordings in slices. (d), Micrograph taken at the resolution and scale used for analysis of synaptophysin labeling (see Supplementary Methods). We observed no differences in the area or intensity of staining for the synaptic marker synaptophysin, suggesting that AraC treatment does not result in loss of synapses among the larger population of dentate granule neurons.

(a), Design for studies in Type 2 diabetic mice. (b), Design for studies in Type 1 diabetic rats. Adrenalectomized animals received corticosterone replacement via the drinking water (25μg ml^–1 in 0.9% saline). (c), Sham–operated db/db mice exhibit reduced BrdU labeling in the dentate gyrus, while db/db mice that received adrenalectomy and corticosterone replacement were not different from non–diabetic mice. (d), Labeling for the endogenous proliferation marker Ki67 is reduced in sham–operated Type 2 diabetic mice, while Type 2 diabetic mice that had been adrenalectomized and given corticosterone replacement were not different from non–diabetic animals. Legend in (c) applies to (d). (e), Type 1 diabetic rats with intact adrenal glands have fewer BrdU–labeled cells in the dentate gyrus – this reduction is not observed in Type 1 diabetic rats with normal levels of corticosterone. (f), Labeling for the endogenous proliferation marker, Ki67, follows a similar pattern – diabetic rats with intact adrenal glands have significantly fewer Ki67–labeled cells than controls, while diabetic animals that received adrenalectomy and low–dose corticosterone replacement were not significantly different from non–diabetic rats. Legend in (e) applies to (f). Error bars = s.e.m.

(a), Experimental design – db/db mice were sham–operated or adrenalectomized; adrenalectomized mice received corticosterone replacement at a dose of 25 or 250μg ml^–1 in 0.9% saline. One month after surgery, the mice were tested in the Morris water maze and object recognition tasks. (b), Lowering corticosterone levels restores hippocampal learning in insulin resistant mice, while a high replacement dose of corticosterone was associated with learning impairments comparable to sham–operated diabetic mice. Differences in performance on the first day of training in adrenalectomized db/db mice receiving low–dose corticosterone replacement were due to improvements over successive trials, as no differences were observed during the first trial (see Results). (c), db/db mice that had been adrenalectomized and given a low replacement dose of corticosterone spent more time exploring the novel object, relative to sham–operated db/db mice, and to db/db mice administered a high dose of corticosterone. Asterisk (*) reflects significance at p<0.05 following one–way repeated measures ANOVA. Error bars = s.e.m.

(a), Design for studies in Type 2 diabetic mice. (b), Design for studies in Type 1 diabetic rats. Adrenalectomized animals received corticosterone replacement via the drinking water (25μg ml^–1 in 0.9% saline). (c), Sham–operated db/db mice exhibit reduced dentate gyrus LTP, but db/db mice with normal physiological levels of corticosterone are not impaired. (d), Insulin–resistant mice that had been sham–operated also exhibit impaired LTP in the presence of picrotoxin, which decreases local inhibition and also blocks GABAergic excitation on new neurons34, 35. In contrast, insulin–resistant mice with normal physiological levels of corticosterone show control levels of LTP under these conditions. (e), Sham–operated insulin–deficient rats exhibit reduced LTP; preventing elevation of corticosterone levels prior to induction of experimental diabetes restores LTP. (f), STZ–diabetic rats with intact adrenal glands demonstrate reduced LTP in the presence of picrotoxin. Lowering corticosterone levels also reverses the effect of diabetes on LTP under these conditions. (g–h), Comparison of the amount of potentiation in slices from diabetic and non–diabetic mice (g) and rats (h) with different levels of corticosterone, recorded in ACSF and in ACSF with picrotoxin. Error bars = s.e.m.

(a), Experimental design for studies using minipump delivery of antimitotic drugs. (b), Comparison of the amount of LTP in vehicle– and AraC–infused mice, when recordings were made in the presence or absence of the GABAA antagonist picrotoxin (100μM). Asterisk (*) indicates significance at p < 0.05 following 2 × 2 ANOVA. (c), LTP at medial perforant path synapses in the dentate gyrus is impaired in AraC–infused mice. (d), LTP recordings in the presence of picrotoxin were not influenced by AraC infusion. (e), The relationship between the slope of the dendritic field potential and the amplitude of the axonal fiber volley is influenced by picrotoxin, but not by AraC treatment. (f), Presynaptic paired–pulse depression, measured in plain ACSF, was not altered by treatment with AraC. Error bars = s.e.m.

(a), BrdU labeled cells in the proliferative dentate subgranular zone of a wildtype mouse at 2 hours post–injection. Arrows indicate labeled cells. (b), BrdU labeled cells in the subgranular zone of a db/db homozygous mouse. (c), Progenitor cells expressing the endogenous proliferation marker Ki67 in the dentate gyrus of a wildtype mouse. (d), Cells labeled with antibodies to Ki67 in the dentate gyrus of a db/db mouse. Scalebar = 20 μm. (e), Cells co–expressing the proliferative marker BrdU (red, left) and the mature neuronal marker NeuN (green, middle) at 3 weeks post–injection (merged image shown at right). (f), Double labeling with BrdU (red) and Tuj1 (green), also at 3 weeks post–injection (merged image shown at right). (g), Cells double–labeled with antibodies to BrdU (red) and the astroglial marker GFAP (green) at 3 weeks post– injection (merged image shown at right). For (e–g), the far right panel shows the merged image on the Z-axis.