Changes in neuronal excitability and synaptic transmission in nucleus accumbens in a transgenic Alzheimer's disease mouse model.

Several previous studies showed that hippocampus and cortex are affected in Alzheimer's disease (AD). However, other brain regions have also been found to be affected and could contribute with new critical information to the pathophysiological basis of AD. For example, volumetric studies in humans have shown a significant atrophy of the striatum, particularly in the nucleus Accumbens (nAc). The nAc is a key component of the limbic reward system and it is involved in cognition and emotional behaviors such as pleasure, fear, aggression and motivations, all of which are affected in neurodegenerative diseases such as AD. However, its role in AD has not been extensively studied. Therefore, using an AD mouse model, we investigated if the nAc was affected in 6 months old transgenic 2xTg (APP/PS1) mice. Immunohistochemistry (IHC) analysis in 2xTg mice showed increased intraneuronal Aß accumulation, as well as occasional extracellular amyloid deposits detected through Thioflavin-S staining. Interestingly, the intracellular Aß pathology was associated to an increase in membrane excitability in dissociated medium spiny neurons (MSNs) of the nAc. IHC and western blot analyses showed a decrease in glycine receptors (GlyR) together with a reduction in the pre- and post-synaptic markers SV2 and gephyrin, respectively, which correlated with a decrease in glycinergic miniature synaptic currents in nAc brain slices. Additionally, voltage-clamp recordings in dissociated MSNs showed a decrease in AMPA- and Gly-evoked currents. Overall, these results showed intracellular Aß accumulation together with an increase in excitability and synaptic alterations in this mouse model. These findings provide new information that might help to explain changes in motivation, anhedonia, and learning in the onset of AD pathogenesis.

Role of membrane GM1 on early neuronal membrane actions of Aß during onset of Alzheimer's disease.

The ability of beta-amyloid peptide (Aß) to disrupt the plasma membrane through formation of pores and membrane breakage has been previously described. However, the molecular determinants for these effects are largely unknown. In this study, we examined if the association and subsequent membrane perforation induced by Aß was dependent on GM1 levels. Pretreatment of hippocampal neurons with D-PDMP decreased GM1 and Aß clustering at the membrane (Aß fluorescent-punctas/20µm, control=16.2±1.1 vs. D-PDMP=6.4±0.4, p<0.001). Interestingly, membrane perforation with Aß occurred with a slower time course when the GM1 content was diminished (time to establish perforated configuration (TEPC) (min): control=7.8±2 vs. low GM1=12.1±0.5, p<0.01), suggesting that the presence of GM1 in the membrane can modulate the distribution and the membrane perforation by Aß. On the other hand, increasing GM1 facilitated the membrane perforation (TEPC: control=7.8±2 vs. GM1=6.2±1min, p<0.05). Additionally, using Cholera Toxin Subunit-B (CTB) to block the interaction of Aß with GM1 attenuated membrane perforation significantly. Furthermore, pretreatment with CTB decreased the membrane association of Aß (fluorescent-punctas/20µm, Aß: control=14.8±2.5 vs. CTB=8±1.4, p<0.05), suggesting that GM1 also plays a role in both association of Aß with the membrane and in perforation. In addition, blockade of the Aß association with CTB inhibited synaptotoxicity. Taken together, our results strongly suggest that membrane lipid composition can affect the ability of Aß to associate and subsequently perforate the plasma membrane thereby modulating its neurotoxicity in hippocampal neurons.