Inhibition of phosphodiesterase 2 reverses impaired cognition and neuronal remodeling caused by chronic stress

Abstract

Chronic stress and neuronal vulnerability have recently been recognized as factors contributing to cognitive disorders. One way to modify neuronal vulnerability is through mediation of phosphodiesterase 2 (PDE2), an enzyme that exerts its action on cognitive processes via the control of intracellular second messengers, cGMP and, to a lesser extent, cAMP. This study explored the effects of a PDE2 inhibitor, Bay 60-7550, on stress-induced learning and memory dysfunction in terms of its ramification on behavioral, morphologic, and molecular changes. Bay 60-7550 reversed stress-induced cognitive impairment in the Morris water maze, novel object recognition, and location tasks (object recognition test and/or object location test), effects prevented by treatment with 7-NI, a selective inhibitor of neuronal nitric oxide synthase; MK801, a glutamate receptor (NMDAR) inhibitor; myr-AIP, a CaMKII inhibitor; and KT5823, a protein kinase G inhibitor. Bay 60-7550 also ameliorated stress-induced structural remodeling in the CA1 of the hippocampus, leading to increases in dendritic branching, length, and spine density. However, the neuroplasticity initiated by Bay 60-7550 was not seen in the presence of 7-NI, MK801, myr-AIP, or KT5823. PDE2 inhibition reduced stress-induced extracellular-regulated protein kinase activation and attenuated stress-induced decreases in transcription factors (e.g., Elk-1, TORC1, and CREB phosphorylation) and plasticity-related proteins (e.g., Egr-1 and brain-derived neurotrophic factor). Pretreatment with inhibitors of NMDA, CaMKII, neuronal nitric oxide synthase, and protein kinase G (or protein kinase A) blocked the effects of Bay 60-7550 on cGMP or cAMP signaling. These findings indicate that the effect of PDE2 inhibition on stress-induced memory impairment is potentially mediated via modulation of neuroplasticity-related NMDAR-CaMKII-cGMP/cAMP signaling.

Keywords: Bay 60-7550; Cognition; Neuroplasticity; PDE2; Stress.

Fig. 1

Learning curve in Morris water maze of control mice and stressed mice treated with vehicle, Bay 60-7550, MK801, myr-AIP, L-NAME, 7-NI, KT5823 and H89. Each point shows the average time taken for 10 mice. ^*p < 0.05, ^**p < 0.01 vs. non-stressed control group. ^##p < 0.01, vs. vehicle-treated stressed group. ^$p < 0.05, ^$$p < 0.01 vs. BAY (3) (3 mg/kg Bay 60-7550) treated-group (n=10).

Fig. 2

Chronic treatment with Bay 60-7550 (14 days) improves learning and memory behaviors on the Morris water maze task during the 1 h (A, B) or 24 h (C, D) test trials. ^**p < 0.01, ^***p < 0.001 vs. non-stressed control group. ^#p < 0.05, ^##p < 0.01, vs. vehicle-treated stressed group. ^$p < 0.05, ^$$p < 0.01 vs. BAY(3) treated-group (n=10).

Fig. 3

The effects of Bay 60-7550 on ORT (A, B) and OLT (C, D) memory performance between T1 and T2. Mean value of the discrimination index (DI) in the task with 1h (A, C) and 24h (B, D) retention delays. ^**p < 0.01 vs. non-stressed control group. ^#p < 0.05, ^##p < 0.01 vs. vehicle-treated stressed group. ^$p < 0.05, ^$$p < 0.01 ^$$$p < 0.001 vs. BAY (3)-treated group (n=10).

Fig. 4

Photomicrographs of representative Golgi stained hippocampal CA1 pyramidal neurons from each group. A: The CA1 region in hippocampus of the mice. B: High magnification of the marked area in A. C: Spines on the dendrites. D: vehicle-treated stressed group. E: stress + BAY (1). F: stress + BAY (3). G: stress + BAY (3) + MK801 (10 µM). H: stress + BAY (3) + myr-AIP (20 µM). I: stress + BAY (3) + L-NAME (20 mg/kg). J: stress + BAY (3) +7-NI (20 mg/kg). K: stress + BAY (3) + KT5823 (20 µM). L: stress + BAY (3) + H89 (5 µM). Magnification: 10× in A, 40× in B, D, E, F, G, H, I, J, K, L, M and 100× in C. Scale bars = 50 µm (B, D, E, F, G, H, I, J, K, L, M) and 5 µm (C, d, e, f, g, h, i, j, k, l, m) (n=10).

Fig. 5

Subtle effects of stress and Bay 60-7550 on apical dendritic points. Analysis the number of branch points for 50-400 µm segments from the soma using Sholl analysis. Each point represents the mean ± SEM of 10 mice (50 neurons/ group). ** p < 0.01, *** p < 0.001 vs. non-stressed control group. ^##p < 0.01, ^###p < 0.001 vs. the vehicle-treated stressed group. ^$$$p < 0.001 vs. the Bay 60-7550-treated group.

Fig. 6

Subtle effects of stress and Bay 60-7550 on apical dendritic length. Analysis the dendritic length for 50-400 µm segments from the soma using Sholl analysis. Each point represents the mean ± SEM of 10 mice (50 neurons/ group). *** p < 0.001 vs. non-stressed control group. ^###p < 0.001 vs. the vehicle-treated stressed group. ^$$$p < 0.001 vs. the Bay 60-7550-treated group.

Fig. 7

Subtle effects of stress and Bay 60-7550 on basal dendritic points. Analysis the number of branch points for 50-400 µm segments from the soma using Sholl analysis. Each point represents the mean ± SEM of 10 mice (50 neurons/ group). *** p < 0.001 vs. non-stressed control group. ^# p < 0.05, ^###p < 0.001 vs. the vehicle-treated stressed group. ^$$p < 0.01, ^$$$p < 0.001 vs. the Bay 60-7550-treated group.

Fig. 8

Subtle effects of stress and Bay 60-7550 on basal dendritic length. Analysis the dendritic length for 50-400 µm segments from the soma using Sholl analysis. Each point represents the mean ± SEM of 10 mice (50 neurons/ group). *** p < 0.001 vs. non-stressed control group. ^###p < 0.001 vs. the vehicle-treated stressed group. ^$$$p < 0.001 vs. the Bay 60-7550-treated group.

Fig. 9

The effects of Bay 60-7550 on spine density (A) and the ratio of pERK1/2 and ERK1/2 (B) in chronically stressed mice. Results are expressed as mean ± SEM from 10 mice (for spine density) and 10 mice (for the ratio of pERK1/2 and ERK1/2). ** p < 0.01 vs. non-stressed control group; ^##p < 0.01 vs. vehicle-treated stressed group; ^$ p < 0.05, ^$$p <0.01 vs. Bay 60-7550-treated group.

Fig. 10

The effects of Bay 60-7550 on pCREB, TORC1 and pElk expression in chronically stressed mice. Results are expressed as mean ± SEM from 10 mice. ** p < 0.01, *** p < 0.001 vs. non-stressed control group. ^###p < 0.001 vs. vehicle-treated stressed group. ^$ p < 0.05, ^$$ p < 0.01, ^$$$p < 0.001 vs. Bay 60-7550-treated group.

Fig. 11

The effects of Bay 60-7550 on c-fos, Egr-1, Arc and BDNF expression in chronically stressed mice. Results are expressed as mean ± SEM from 10 mice. * p< 0.05, **p< 0.01 vs. non-stressed control group; ^##p< 0.01 vs. vehicle-treated stressed group; ^$ p< 0.05, ^$$p< 0.01 vs. Bay 60-7550-treated group.

Fig. 12

Representation of the involvement of PDE2 in NMDA-CaMKII-NO-cAMP/cGMP signaling. Ca²⁺ influx through NMDA receptor on the membrane activates nNOS and cAMP/cGMP, which are required for ERK activation and for the full expression of plasticity-related proteins, i.e. CREB, TORC1, Elk. This effect ignites the immediate early genes (IEG) and brain derived neurotrophic factor (BDNF) expression. And finally, all these changes regulate the synaptic plasticity and behaviors (n=10).