Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease, involving the selective degeneration of cortical upper synapses in the primary motor cortex (M1). Excitotoxicity in ALS occurs due to an imbalance between excitation and inhibition, closely linked to the loss/gain of astrocytic function. Using the ALS SOD1G93A mice, we investigated the astrocytic contribution for the electrophysiological alterations observed in the M1 (layer 2/3) of SOD1G93A mice, throughout disease progression. Astrocytic metabolism was selectively reduced using fluorocitrate (FC). Synaptic plasticity/transmission was assessed by extracellular field recordings and whole-cell patch clamp recording was used to identify differences in M1 neurons’ intrinsic properties and firing behavior, as well as astrocytic glutamate transport currents. Proteomic analysis was used to further investigate synaptic alterations in the SOD1G93A mice. Results showed that SOD1G93A mice present deficits in synaptic plasticity, together with reduced protein levels of GluN2B and GluA1 subunits, even before the onset of symptoms. Moreover, results showed that astrocytes are involved in the synaptic dysfunction observed in presymptomatic SOD1G93A mice, since astrocytic glutamate transport currents are diminished and pharmacological inhibition of astrocytes only impaired long-term potentiation and basal transmission in wild-type mice. Proteomic analysis revealed major differences in neuronal transmission, metabolism, and immune system in upper synapses, confirming early communication deficits between neurons and astroglia. These results provide valuable insights into the early impact of upper synapses in ALS and the lack of supportive functions of cortical astrocytes, highlighting the possibility of manipulating astrocytes to improve synaptic function.