Monoamine oxidase–B (MAO-B) has recently emerged as a potential therapeutic target for Alzheimer’s disease (AD) because of its association with aberrant γ-aminobutyric acid (GABA) production in reactive astrocytes. Although short-term treatment with irreversible MAO-B inhibitors, such as selegiline, improves cognitive deficits in AD patients, long-term treatments have shown disappointing results. We show that prolonged treatment with selegiline fails to reduce aberrant astrocytic GABA levels and rescue memory impairment in APP/PS1 mice, an animal model of AD, because of increased activity in compensatory genes for a GABA-synthesizing enzyme, diamine oxidase (DAO). We have developed a potent, highly selective, and reversible MAO-B inhibitor, KDS2010 (IC₅₀ = 7.6 nM; 12,500-fold selectivity over MAO-A), which overcomes the disadvantages of the irreversible MAO-B inhibitor. Long-term treatment with KDS2010 does not induce compensatory mechanisms, thereby significantly attenuating increased astrocytic GABA levels and astrogliosis, enhancing synaptic transmission, and rescuing learning and memory impairments in APP/PS1 mice.

Key points: Here we show that glial gamma aminobutyric acid (GABA) is produced by monoamine oxidase B (MAOB), utilizing a polyamine, putrescine. The concentration of GABA in Bergmann glial cells is estimated to be around 5–10 mM. General gene silencing of MAOB resulted in elimination of tonic GABA currents recorded from granule cells in the cerebellum and medium spiny neurons (MSN) in the striatum. Glial‐specific rescue of MAOB resulted in complete restoration of tonic GABA currents. Our results identify MAOB as a synthesizing enzyme of glial GABA, which is released to mediate tonic inhibition in the cerebellum and striatum. GABA is the major inhibitory transmitter in the brain and is released not only from a subset of neurons but also from glia. Although neuronal GABA is well known to be synthesized by glutamic acid decarboxylase (GAD), the source of glial GABA is unknown. After estimating the concentration of GABA in Bergmann glia to be around 5–10 mm by immunogold electron microscopy, we demonstrate that GABA production in glia requires MAOB, a key enzyme in the putrescine degradation pathway. In cultured cerebellar glia, both Ca²⁺‐induced and tonic GABA release are significantly reduced by both gene silencing of MAOB and the MAOB inhibitor selegiline. In the cerebellum and striatum of adult mice, general gene silencing, knock out of MAOB or selegiline treatment resulted in elimination of tonic GABA currents recorded from granule neurons and medium spiny neurons. Glial‐specific rescue of MAOB resulted in complete rescue of tonic GABA currents. Our results identify MAOB as a key synthesizing enzyme of glial GABA, which is released via bestrophin 1 (Best1) channel to mediate tonic inhibition in the brain.

Tonic inhibition in the brain is mediated through an activation of extrasynaptic GABA_A receptors by the tonically released GABA, resulting in a persistent GABAergic inhibitory action. It is one of the key regulators for neuronal excitability, exerting a powerful action on excitation/inhibition balance. We have previously reported that astrocytic GABA, synthesized by monoamine oxidase B (MAOB), mediates tonic inhibition via GABA-permeable bestrophin 1 (Best1) channel in the cerebellum. However, the role of astrocytic GABA in regulating neuronal excitability, synaptic transmission, and cerebellar brain function has remained elusive. Here, we report that a reduction of tonic GABA release by genetic removal or pharmacological inhibition of Best1 or MAOB caused an enhanced neuronal excitability in cerebellar granule cells (GCs), synaptic transmission at the parallel fiber-Purkinje cell (PF-PC) synapses, and motor performance on the rotarod test, whereas an augmentation of tonic GABA release by astrocyte-specific overexpression of MAOB resulted in a reduced neuronal excitability, synaptic transmission, and motor performance. The bidirectional modulation of astrocytic GABA by genetic alteration of Best1 or MAOB was confirmed by immunostaining and in vivo microdialysis. These findings indicate that astrocytes are the key player in motor coordination through tonic GABA release by modulating neuronal excitability and could be a good therapeutic target for various movement and psychiatric disorders, which show a disturbed excitation/inhibition balance. Significance Tonic inhibition plays critical roles in cognitive functions under physiological and pathological conditions by controlling neuronal excitability. Although we have previously reported that cerebellar tonic inhibition is critically dependent on both the synthesis of GABA through monoamine oxidase B (MAOB) enzyme and the release via the bestrophin 1 (Best1) channel, the role of astrocytic GABA in cerebellar function in vivo has remained elusive. Here, we report that a reduction of tonic GABA release by genetic or pharmacological removal of Best1 or MAOB caused enhanced neuronal excitability, synaptic transmission, and motor performance on the rotarod test, whereas an augmentation of tonic GABA release by astrocyte-specific overexpression of MAOB caused opposite results. Our findings suggest the actions of astrocytic GABA in excitation/inhibition balance and motor coordination.