NKCC1 Chloride Importer Antagonists Attenuate Many Neurological and Psychiatric Disorders

In physiological conditions, adult neurons have low intracellular Cl⁻ [(Cl⁻)_I] levels underlying the γ-aminobutyric acid (GABA)ergic inhibitory drive. In contrast, neurons have high (Cl⁻)_I levels and excitatory GABA actions in a wide range of pathological conditions including spinal cord lesions, chronic pain, brain trauma, cerebrovascular infarcts, autism, Rett and Down syndrome, various types of epilepsies, and other genetic or environmental insults. The diuretic highly specific NKCC1 chloride importer antagonist bumetanide (PubChem CID: 2461) efficiently restores low (Cl⁻)_I levels and attenuates many disorders in experimental conditions and in some clinical trials. Here, I review the mechanisms of action, therapeutic effects, promises, and pitfalls of bumetanide.

Introduction

GABA provides most of the inhibitory tone of the adult nervous system and by means of a heterogeneous population of interneurons orchestrates the generation of behaviorally relevant oscillations. Reducing GABAergic inhibition produces neuronal hyperexcitability and the generation of aberrant oscillations leading to neurological and psychiatric deleterious sequels including seizures and epilepsy. A large number of GABAergic reinforcing drugs have been developed and used as sedatives, anesthetics, anxiolytics, or pain-reducing agents. Yet, many disorders either do not respond to GABA-acting drugs or are aggravated by these agents suggesting that novel conceptual tools are needed to restore the failure of inhibition. The regulation of (Cl⁻)_I levels is such a promising novel target.

GABAergic (and glycinergic) currents are unique in being able to actively and dependently reverse their polarity of action shifting from hyperpolarization/inhibition to depolarization/excitation. The actions of GABA – and glycine – are dependent on (Cl⁻)_I levels: high levels of activity can increase these levels with often a shift of the polarity of action from inhibition to excitation depending on the density of sodium channels, the timing of GABAergic versus glutamatergic synaptic currents, and intrinsic properties. The importance of (Cl⁻)_I regulation was first illustrated by the demonstration of a developmental sequence of GABAergic currents. Immature neurons have high (Cl⁻)_I levels and depolarizing actions, whereas adult ones are endowed with usually low (Cl⁻)_I levels and hyperpolarizing actions of GABA 1, 2, 3. The developmental reduction of (Cl⁻)_I levels is mediated primarily by two chloride cotransporters – the NKCC1 importer (Box 1) and the KCC2 chloride exporter 1, 4, 5. The depolarization produced by GABA activates voltage-dependent calcium currents and N-methyl-d-aspartate (NMDA) receptors, leading to sodium spikes and large calcium influx endowed with trophic actions on neuronal growth, synapse formation, and network construction (Figure 1) 1, 6. There is subsequently a reduction of (Cl⁻)_I levels that is dependent on animal species, neuronal age, and brain structure. The biological relevance of this sequence is illustrated by the abrupt neuroprotective oxytocin-mediated dramatic reduction of (Cl⁻)_I levels occurring during delivery [7]. In addition, oxytocin exerts an analgesic action during delivery and this is mediated by a reduction of (Cl⁻)_I levels in pain pathways [8]; bumetanide exerts the same effects [9]. Blocking NKCC1 in utero and in early postnatal life can produce long-lasting adverse sequels stressing the importance of high (Cl⁻)_I levels during development [10]. Therefore, in addition to triggering labor, the hormone exerts neuroprotective and analgesic actions via a reduction of (Cl⁻)_I levels and reinforcement of GABAergic efficacy. These observations provide a vivid illustration of the biological importance of (Cl⁻)_I levels and the roles of critical periods including delivery and birth [11].

High (Cl⁻)_I levels and excitatory GABA actions are also observed in a wide range of insults and disorders including neonatal and adult epilepsies, autism spectrum disorders (ASDs), chronic pain, spinal-cord lesions, brain traumas, cerebral edema, stress conditions, neurogenic hypertension, cerebral artery occlusion, and diabetic ketoacidosis (Figure 1). In many of these conditions, there is downregulation of the chloride exporter KCC2 and upregulation of the NKCC1 chloride importer. I have suggested elsewhere the “neuroarcheology” concept, according to which intrauterine insults deviate developmental sequences, leading to the persistence of immature features in misplaced or misconnected neurons [12]. The expression of “immature features” such as high (Cl⁻)_I levels (Figure 1) paves the way to novel treatments based on the use of drugs that selectively block immature currents in the adult brain, thereby alleviating the perturbing activity that these neurons generate [12] (Box 2).

To restore physiological (Cl⁻)_I levels, it is possible to enhance KCC2 exporter activity or reduce NKCC1 importer. As reviews on KCC2 have been published recently 13, 14, I discuss here the therapeutic promises of blocking NKCC1 activity and in particular the actions of the highly selective antagonist of NKCC1 bumetanide, which at low doses restores physiological (Cl⁻)_I levels and enhances GABAergic inhibition (Figure 1 and Box 1). Bumetanide has no effects on GABA actions in NKCC1 knock outs [15], suggesting that antagonizing NKCC1 is the most likely underlying mechanism of action. Identified and synthesized almost four decades ago [16], bumetanide has been used to treat hypertension and brain edema with side effects limited to diuresis, hypokalemia, and dehydration [17]. As its side effects are well documented, clinical trials can be readily performed in parallel with experimental investigations.