ABSTRACT
1Dip. Scienze Biomediche, Università degli Studi di Foggia, Foggia
2Dip. Fisiologia e Farmacologia Vittorio Erspamer, Università La Sapienza, Roma
Soluble beta amyloid (βA) has been proposed as a key mediator of synaptic dysfunction, because of the stronger correlation between cortical levels of soluble AB species and synaptic loss in Alzheimer’s disease (AD) patients. Moreover, several lines of evidence suggest that early memory deficits may be explained by the presence of soluble forms of βA rather than by the form aggregated [1]. However, the mechanism through which soluble βA acts remains uncertain, but interactions have been reported with several receptors such as nicotine, insulinic and glutamatergic receptors [2]. The crucial role of the glutamatergic system in learning processes has been shown. In this respect, the predominant role the N-methyl-D-aspartate (NMDA) type is proven. Early research focused on the role of βA enhancement of glutamatergic excitotoxicity, including that mediated through NMDA receptors. Consistent with this, βA enhances NMDA receptor agonist-induced delayed cognitive dysfunction. Indeed, βA can directly inhibit Glu uptake by cultured astrocytes and neurons and synaptosomes. Soluble oligomers of βA were found to trigger Ca2+ influx and oxidative stress in cultured hippocampal neurons that was prevented by NMDA receptor antagonists.
The aim of the present study was to investigate whether an acute intracerebroventricular (icv, 4 μM) injection of soluble βA1-42 might induce alterations in cognitive domains as well as impairment of glutamatergic transmission in rat brain. The freshly dissolved βA1−42 was characterized by atomic force microscopy before investigation. Only a few small βA1-42 particles were detected in the initial state. After βA1-42 or vehicle treatment, behavioral reactivity to the novel object recognition, open field and passive avoidance test was assessed after icv injection. Moreover, we investigated whether, in rat prefrontal cortex and hippocampus, glutamatergic neurotransmission was affected by soluble βA1-42 treatment. Biochemical studies were also carried out, such as mRNA and protein glutamate transporter expression measurements in the prefrontal cortex and hippocampus. Finally, in order to investigate the direct involvement of NMDA receptors in mediating the effects of βA1-42, the NMDA receptor antagonist memantine was used, at a concentration that is reported to be in the NMDA receptor-blocking and therapeutic range.
βA1-42 –injected rats were unable to distinguish the new object, and the discrimination index was significantly lower than control rats. To investigate if disruptive effects of soluble βA1-42 on memory mechanisms were NMDA receptor-dependent, an acute dose of memantine was administered, immediately after the familiarization phase. Memantine restored long-term memory impairment induced by injections withβA1-42 and preserved memory recall. Toconfirm that implantation of cannula or βA1-42 injection did not adversely affect overt behavior or motor function, rats were placed in an open field arena. No alterations were observed in crossing, rearing and wall rearing behavior. In the passive avoidance task, βA1-42 induced retention impairment. Neurochemical experiments revealed an increased Glu levels in the prefrontal cortex, but not in the hippocampus, after βA1-42 injection. Since the vast majority of sinaptically released Glu is taken up by surrounding astroglia that typically express two Glu transporter subtypes, GLAST/EAAT1 and GLT-1/EAAT2, we measured mRNA and protein glutamate transporter expression. Other Glu transporter subtypes, EAAC1/EAAT3, EAAT4, and EAAT5 are expressed in neurons. In our experimental model, βA1-42-treated rats did not exhibit alteration of those glutamate transporters.
In conclusion, our results suggest that soluble βA1-42 is able to disrupt learned behavior as well as induces impairment of glutamatergic transmission in rat prefrontal cortex, through activation of NMDA receptors.
[1] Balducci et al., (2010) PNAS 107, 2295-2300.
[2] De Felice et al., (2009) PNAS 10;106(6):1971-6.