The GLP-1 Receptor Agonist Liraglutide Improves Memory Function and Increases Hippocampal CA1 Neuronal Numbers in a Senescence-Accelerated Mouse Model of Alzheimer's Disease

Abstract

Recent studies indicate that glucagon-like peptide 1 (GLP-1) receptor agonists, currently used in the management of type 2 diabetes, exhibit neurotrophic and neuroprotective effects in amyloid-β (Aβ) toxicity models of Alzheimer's disease (AD). We investigated the potential pro-cognitive and neuroprotective effects of the once-daily GLP-1 receptor agonist liraglutide in senescence-accelerated mouse prone 8 (SAMP8) mice, a model of age-related sporadic AD not dominated by amyloid plaques. Six-month-old SAMP8 mice received liraglutide (100 or 500 μg/kg/day, s.c.) or vehicle once daily for 4 months. Vehicle-dosed age-matched 50% back-crossed as well as untreated young (4-month-old) SAMP8 mice were used as control groups for normal memory function. Vehicle-dosed 10-month-old SAMP8 mice showed significant learning and memory retention deficits in an active-avoidance T-maze, as compared to both control groups. Also, 10-month-old SAMP8 mice displayed no immunohistological signatures of amyloid-β plaques or hyperphosphorylated tau, indicating the onset of cognitive deficits prior to deposition of amyloid plaques and neurofibrillary tangles in this AD model. Liraglutide significantly increased memory retention and total hippocampal CA1 pyramidal neuron numbers in SAMP8 mice, as compared to age-matched vehicle-dosed SAMP8 mice. In conclusion, liraglutide delayed or partially halted the progressive decline in memory function associated with hippocampal neuronal loss in a mouse model of pathological aging with characteristics of neurobehavioral and neuropathological impairments observed in early-stage sporadic AD.

Keywords: Alzheimer’s disease; GLP-1 receptor agonist; SAMP8 mouse; hippocampus; liraglutide; memory function; neuroprotection; stereology.

Fig.1

Memory acquisition and retention function in SAMP8 mice assessed in an active avoidance T-maze test. Memory acquisition (A) and retention (B) performance in vehicle-dosed or liraglutide-treated 10-month-old SAMP8 mice. Vehicle-dosed 50% backcrossed SAMP8 mice and untreated 4-month-old SAMP8 control mice, respectively, served as controls for normal memory function. Long-term liraglutide treatment restored memory retention in 10-month-old SAMP8 mice, as compared to age-matched vehicle-dosed SAMP8 control mice. The number of trials to make one active avoidance was a measure of acquisition. Retention was tested one week later by continuing training until the mice achieved the criterion of making five active avoidances in six consecutive trials. The number of trials needed to reach this criterion was the measure of memory retention.  ^**p <  0.01,  ^***p <  0.001 (one-way ANOVA, Dunnet’s post-hoc test).

Fig.2

Recognition memory function in SAMP8 mice assessed in a novel object recognition (NOR) test. NOR performance in 10-month-old vehicle-dosed or liraglutide-treated SAMP8 mice. Vehicle-dosed 50% backcrossed SAMP8 mice and untreated 4-month-old SAMP8 control mice, respectively, served as controls for normal memory function. Vehicle-dosed 10-month-old SAMP8 mice showed poor object recognition memory performance, as compared to untreated 4-month-old SAMP8 mice. Liraglutide-treated 10-month-old SAMP8 mice did not show significant improvement of memory performance in the NOR test. The discrimination index was defined as the amount of time exploring the familiar object or the novel object over the total time spent exploring both objects multiplied by 100.  ^*p <  0.05 (one-way ANOVA, Dunnet’s post-hoc test).

Fig.3

Brain weight of SAMP8 mice. Total brain wet weight was significantly higher in vehicle-dosed 10 month-old 50% back-crossed SAMP8 control mice, as compared to all other experimental groups.  ^***p <  0.001 (one-way ANOVA, Newman-Keuls post-hoc test).

Fig.4

Histological assessment of hippocampal CA1 pyramidal neuron numbers. Brains were sampled in the horizontal plane using systematic uniform random sampling to ensure representation of the entire hippocampus. The major subfields of the mouse hippocampus are indicated in panel A (2x magnification). Panel B shows the border between CA1 and CA2. The CA1 region (Giemsa stained) was delineated on average in 8 sections per animal using a 10x objective based on cell morphology (B). CA1 pyramidal neurons are smaller and more densely organized than CA2 neurons (C; white arrows = CA1 pyramidal neuron, black arrows = CA2 pyramidal neuron; 40x magnification).

Fig.5

Stereological quantification of total hippocampal CA1 pyramidal neuron numbers in SAMP8 mice. Total hippocampal CA1 pyramidal neuron numbers were assessed in vehicle-dosed or liraglutide-treated 10-month-old SAMP8 mice, vehicle-dosed 10-month-old 50% backcrossed SAMP8 control mice, and untreated 4-month-old (young) SAMP8 control mice, respectively. Liraglutide-treated 10-month-old SAMP8 mice exhibited higher total pyramidal neuron numbers and neuronal density in the hippocampal CA1 region, as compared to age-matched vehicle-dosed SAMP8 mice. (A) total CA1 pyramidal neuron number; (B) total CA1 volume (mm³); (C) CA1 pyramidal neuron density (neurons per mm³).  ^**p <  0.01 (one-way ANOVA, Dunnet’s post-hoc test).

Fig.6

Immunohistochemical analysis of Aβ and neurofibrillary tangles in SAMP8 mouse brains. 10-month-old SAMP8 mice showed no presence of amyloid plaques or neurofibrillary tangles. Representative photomicrographs of Aβ (A) and phospho-tau (B) immunohistochemical staining of 5- μm thick brain sections from 10-month-old SAMP8 mice. Immunohistochemical control samples for amyloid plaques (C, AβPP/PS1 mouse) and neurofibrillary tangles (D, hTau P301L mouse) are inserted. Enlarged photos represent corresponding framed area on each specimen (scale bar = 100 μm).