Focus on the excitatory and inhibitory neurotransmission imbalance in amyotrophic lateral sclerosis: a harmful disease player or a potential therapeutic opportunity?

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal neurodegenerative disease affecting both upper and lower motor neurons. Evidence indicates that ALS is a "multifactorial" and "multicellular" disease; however, the causes of ALS remain elusive, as the mechanisms underlying the disease have not yet been completely clarified. One major proposed mechanism, first described in 1990, is the glutamate excitotoxicity theory. This theory suggests that excessive glutamatergic neurotransmission, combined with impaired glutamate clearance, significantly contributes to motor neuron degeneration. Aberrant glutamate neurotransmission may lead to precocious motor neuron hyperexcitability in the brain cortex and spinal cord, which can be later followed by hypoexcitability phases. Accumulating evidence suggests that impairment in inhibitory neurotransmission is relevant for excitation/inhibition imbalance, leading to excitotoxicity, a critical feature of ALS. Gamma-aminobutyric acid (GABA) and glycine are the primary inhibitory neurotransmitters that modulate neuronal excitability, including that of motor neurons. In ALS, dysfunction of inhibitory processes and loss of cortical and spinal inhibitory interneurons are observed. Renshaw cells, which mediate recurrent inhibition in the spinal cord, seem particularly vulnerable. The interactions among neurotransmitters, including glutamate, GABA, and glycine, play pivotal roles in regulating the excitation/inhibition balance. Auto- or hetero-receptor-mediated interactions are crucial, but auto- or hetero-transporter-mediated neurotransmission control, as well as other molecular mechanisms that regulate neuronal interplay, are also relevant, as they can be altered in pathological conditions such as ALS. To facilitate the search for new effective therapies for ALS, attention toward the impairment of inhibitory neurotransmission is essential to determine the role of excitation/inhibition imbalance on excitotoxicity. Different pharmacological agents are being used to treat other pathologies in which the excitation/inhibition ratio is impaired. Among these, we highlighted the potential of novel glycine and GABA receptor ligands and transporter inhibitors, as stand-alone interventions or in combination with other treatments. The present review aims to elucidate the complex interplay between excitatory and inhibitory neurotransmission in ALS, exploring the potential to target this imbalance for therapeutic purposes.