Differential Intervention Roles Of Neurokinin Peptide NK1 And NK3 In The Excitotoxic Injuries Of The Basal Forebrain Neurons

Alzheimer's disease (AD) is one common and severe neurodegenerative disease in human beings, but so far the pathogenesis of AD has been ambiguous. Previous studies have shown that massive loss of cholinergic neurons in the basal forebrain are obvious in the AD pathogenesis, and the glutamate-driving excitotoxicity consititutes one of main reasons resulting in the death of these neurons. Intervention ways of excitotoxicity has thus raised much research interests, which may promte to set up new therapy against AD.Mammalian neurokinins(NKs)are a family of tachykinin peptides that include substance P (neurokinin-1,NK1), neurokinin A (neurokinin-2, NK2) and neurokinin B (neurokinin-3, NK3). Their biological functions are mediated by three distinct neurokinin receptors, namely NK1 receptor, NK2 receptor and NK3 receptor. Neurokinin peptides and neurokinin receptors are abundantly distributed in the basal forebrain regions, but their function is unclear. In the previous observations we have found that neurokinins are involved in regulating excitotoxicity in the striatal and substantia nigra neurons. Further studies should be devoted to elucidate whether neurokinins are involved in modulating exicitotoxicity or exicitotoxic death of basal forebrain neurons. Therefore, the intervention roles of neurokinins NK1, NK3 in regulation of excitotoxicity of cholinergic neurons in the basal forebrain and possible interaction substrate were studied by using animal models induced by kainic acid (KA) leison, administration of neuronkinin receptor specific agonists and antagonist, Fluoro-Jade B (FJB) staining for visualizing neuronal degeneration, choline acetyltransferase (ChAT)-immunohistochemistry, double immuno- fluorescence and laser scanning confocal microscopy.Firstly, colocalization of AMPA and NMDA glutamate receptor subunits were found in both NK1R-containing neurons and NK3R-containing neurons in the basal forebrain of mice. Semiquantitative analysis revealed that majority (57-95%) of NK1R-positive neurons express AMPA receptor subunits GluR1-4, and NK3R-positive neurons (14%-77%) express AMPA receptor subunits GluR1-3 and NMDA receptor subunit 1.Secondly, the differential interventions roles of neurokinins NK1 and NK3 were found in regulating KA-induced excitotoxic neuronal injuries of basal forebrain in animal models. It revealed that NK1 exhibited neuronal protection, while the NK3 potentated neuronal excitotoxicity. In compared with single KA injection group, pretreatment with NK1 receptor agonist septide decreased FJB-positive neurons and increased ChAT-postive ones, while pretreatment with NK3 receptor agonist senktide increased FJB-positive neurons and decreased ChAT-positive ones. Experiment with NK3 receptor antagonist SB218795 confirmed effect of NK3 on excitotoxicity.Thirdly, double labeling experiments were further applied to identify cell types of FJB stained cells and possible involvement of D-serine or phosphorylation of NMDA receptor 1 (pNMDAR1) in the death of basal forebrain neurons. It revealed that nearly all (100%) of FJB-positive cells were neurons, and they overlapped with that of pNMDAR1 or D-serine-positive neurons in the injured basal forebrain.This study presents evidence on neuronal colocalization of NK1/NK3 and AMPA/NMDA receptors, which provided substrate for functional interaction between neurokinins and glutamate signals in the basal forebrain. More interestingly, differential intervention roles of neurokinins NK1 and NK3 have been found in regulation of excitotoxic injuries of neurons in the basal forebrain, in which NK1 has neuroprotection effect, while NK3 exhibits synergistic effect on neurotoxicity in the basal forebrain. This study suggests that neurokinin peptides may implicate in pathogenesis of neurodegenerative diseases and neurokinin receptors may act as potential intervention targets in the neuroprotection treatment of AD in human beings.