Substance P receptor activation induces downregulation of the AMPA receptor functionality in cortical neurons from a genetic model of Amyotrophic Lateral Sclerosis

Substance P (SP), a neuropeptide member of the tachykinin (TK) family, has a functional role both in physiological and pathological conditions, including Amyotrophic Lateral Sclerosis disease. One hypothesis of the selective motor neuron death in ALS involves the excitatory neurotransmitter glutamate, because these neurons are extremely susceptible to excessive stimulation of AMPA receptors. It has been reported that SP exerts its action against a variety of insults including excitotoxicity, and that altered levels of SP have been observed in the cerebrospinal fluid (CSF) of patients with ALS. Here we have analyzed the interaction between SP and AMPA receptor functionality, both in Control cortical neurons in culture and in those obtained from a genetic mouse model of ALS (G93A). Our studies demonstrate that SP reduces the kainate-activated currents in Control and G93A neurons and that this reduction is significantly higher in the mutated neurons. SP effect is mediated by its receptor NK1 because GR 82334 (5μM), a NK1 competitive antagonist, is able to suppress the current reduction. Analysis of miniature excitatory postsynaptic currents (mEPSCs) in Control and G93A neurons indicates that SP (200nM) is able to significantly decrease the mEPSC amplitudes in G93A neurons, whereas it is ineffective on Control mEPSCs. Western blotting experiments in cultures and cortical tissues show a higher NK1 expression level in G93A mice compared to that of Control. This is also confirmed by immunocytochemistry experiments in cultured neurons. In addition, the amount of GluR1 subunit AMPA receptors is not modified following SP exposure, indicating a non internalization of the AMPA receptors. Finally, toxicity experiments have revealed that SP is able to rescue G93A cortical cells whereas it is ineffective on those of Control. These findings provide the first evidence of SP having a physiological and protective role in the G93A mouse model of ALS, and may suggest the possible use of SP as a clinical therapeutic treatment.