Glutamate (Glu)-mediated excitotoxicity is a major cause of amyotrophic lateral sclerosis (ALS) and our previous work highlighted that abnormal Glu release may represent a leading mechanism for excessive synaptic Glu. We demonstrated that group I metabotropic Glu receptors (mGluR1, mGluR5) produced abnormal Glu release in SOD1^G93A mouse spinal cord at a late disease stage (120 days). Here, we studied this phenomenon in pre-symptomatic (30 and 60 days) and early-symptomatic (90 days) SOD1^G93A mice. The mGluR1/5 agonist (S)-3,5-Dihydroxyphenylglycine (3,5-DHPG) concentration dependently stimulated the release of [³H]d-Aspartate ([³H]d-Asp), which was comparable in 30- and 60-day-old wild type mice and SOD1^G93A mice. At variance, [³H]d-Asp release was significantly augmented in 90-day-old SOD1^G93A mice and both mGluR1 and mGluR5 were involved. The 3,5-DHPG-induced [³H]d-Asp release was exocytotic, being of vesicular origin and mediated by intra-terminal Ca²⁺ release. mGluR1 and mGluR5 expression was increased in Glu spinal cord axon terminals of 90-day-old SOD1^G93A mice, but not in the whole axon terminal population. Interestingly, mGluR1 and mGluR5 were significantly augmented in total spinal cord tissue already at 60 days. Thus, function and expression of group I mGluRs are enhanced in the early-symptomatic SOD1^G93A mouse spinal cord, possibly participating in excessive Glu transmission and supporting their implication in ALS. Please define all abbreviations the first time they appear in the abstract, the main text, and the first figure or table caption.

Glycine plays a pivotal role in the Central Nervous System (CNS), being a major inhibitory neurotransmitter as well as a co-agonist of Glutamate at excitatory NMDA receptors. Interactions involving Glycine and other neurotransmitters are the subject of different studies. Functional interactions among neurotransmitters include the modulation of release through release-regulating receptors but also through transporter-mediated mechanisms. Many transporter-mediated interactions involve the amino acid transmitters Glycine, Glutamate, and GABA. Different studies published during the last two decades investigated a number of transporter-mediated interactions in depth involving amino acid transmitters at the nerve terminal level in different CNS areas, providing details of mechanisms involved and suggesting pathophysiological significances. Here, this evidence is reviewed also considering additional recent information available in the literature, with a special (but not exclusive) focus on glycinergic neurotransmission and Glycine–Glutamate interactions. Some possible pharmacological implications, although partly speculative, are also discussed. Dysregulations in glycinergic and glutamatergic transmission are involved in relevant CNS pathologies. Pharmacological interventions on glycinergic targets (including receptors and transporters) are under study to develop novel therapies against serious CNS pathological states including pain, schizophrenia, epilepsy, and neurodegenerative diseases. Although with limitations, it is hoped to possibly contribute to a better understanding of the complex interactions between glycine-mediated neurotransmission and other major amino acid transmitters, also in view of the current interest in potential drugs acting on “glycinergic” targets.

Evidence supports the pathophysiological relevance of crosstalk between the neurotransmitters Glycine and Glutamate and their close interactions; some reports even support the possibility of Glycine–Glutamate cotransmission in central nervous system (CNS) areas, including the hippocampus. Functional studies with isolated nerve terminals (synaptosomes) permit us to study transporter-mediated interactions between neurotransmitters that lead to the regulation of transmitter release. Our main aims here were: (i) to investigate release-regulating, transporter-mediated interactions between Glycine and Glutamate in hippocampal nerve terminals and (ii) to determine the coexistence of transporters for Glycine and Glutamate in these terminals. Purified synaptosomes, analyzed at the ultrastructural level via electron microscopy, were used as the experimental model. Mouse hippocampal synaptosomes were prelabeled with [³H]D-Aspartate or [³H]Glycine; the release of radiolabeled tracers was monitored with the superfusion technique. The main findings were that (i) exogenous Glycine stimulated [³H]D-Aspartate release, partly by activation of GlyT1 and in part, unusually, through GlyT2 transporters and that (ii) D-Aspartate stimulated [³H]glycine release by a process that was sensitive to Glutamate transporter blockers. Based on the features of the experimental model used, it is suggested that functional transporters for Glutamate and Glycine coexist in a small subset of hippocampal nerve terminals, a condition that may also be compatible with cotransmission; glycinergic and glutamatergic transporters exhibit different functions and mediate interactions between the neurotransmitters. It is hoped that increased information on Glutamate–Glycine interactions in different areas, including the hippocampus, will contribute to a better knowledge of drugs acting at “glycinergic” targets, currently under study in relation with different CNS pathologies.

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.

Glycine (Gly) is a peculiar neurotransmitter (NT) in the Central Nervous System (CNS) exhibiting dual functions: it is mostly inhibitory, in different CNS areas, when it activates the ionotropic Gly receptors (GlyRs)

Background/Objectives: The neurotransmitter glycine is involved in several physiological and pathological conditions in the Central Nervous System. Different biological structures, including glycine receptors and transporters, are under study as targets for potential drugs acting against serious neurological and psychiatric disorders. The regulation of glycine release from nerve terminals is only partially understood. We report here preliminary evidence of the modulation of glycine release through presynaptic metabotropic glutamate receptors 1 (mGlu1) from glycinergic nerve terminals in mouse hippocampi. Methods: Purified mouse hippocampal synaptosomes labeled with [³H]glycine were used to study glycine release under superfusion conditions. Results: The group I metabotropic glutamate receptor agonist 3,5-DHPG potentiated depolarization-evoked [³H]glycine release from hippocampal synaptosomes, an effect strongly counteracted by the selective mGlu1 antagonist LY 367385. 3,5-DHPG failed to increase [³H]glycine release in Grm1^crv4/crv4 mice, a mouse model lacking mGlu1. Although further research is needed to clarify these mechanisms, data suggest that glycine-releasing hippocampal nerve terminals are endowed with presynaptic mGlu1 receptors whose activation potentiates glycine release. Conclusions: Considering that in the hippocampus, glycine is relevant as a co-agonist of glutamate at NMDA receptors and that mGlu1 receptor ligands are under study as potential drugs, we propose that the possible effects of these agents on the release of glycine should be considered when studying these compounds.