Analysis of chloride transport mechanisms that regulate inhibitory neurotransmission in the Caenorhabditis elegans nervous system

Activation of ligand-gated chloride channels is the primary mechanism by which neurotransmitters inhibit the electrical activity of postsynaptic neurons, and this function requires a chloride gradient across the plasma membrane. In order to understand the mechanisms by which the cellular chloride gradient is generated and maintained, we studied the function of the potassium chloride cotransporter KCC-2 and the sodium-dependent bicarbonate-chloride exchanger ABTS-1 in extruding chloride to support inhibitory neurotransmission in two simple behavioral circuits in the nematode C. elegans.;Mutants lacking KCC-2 and ABTS-1 function were obtained from a genetic screen for mutants that restore egg-laying behavior to a mutant that is unable to lay eggs due to a defect in neurotransmitter release from the HSN motor neuron that activates the egg-laying muscles. We demonstrated that both KCC-2 and ABTS-1 function in the HSN to support inhibition of egg-laying behavior by neurotransmitters such as GABA, suggesting that both transporters extrude chloride to generate the cellular chloride gradient used to inhibit the HSN via ligand-gated chloride channels. Expression of both transporters is also upregulated in the HSN during HSN maturation, suggesting that both transporters may play a role in the maturation of neuronal electrical properties.;KCC-2 and ABTS-1 also function in the body wall muscles of C. elegans, which receive input from GABAergic motor neurons that drive muscle relaxation. While the GABA receptor agonist muscimol causes muscle relaxation in wild-type animals, we observed muscle contraction in response to muscimol in animals lacking KCC-2 or ABTS-1 function, suggesting that loss of either transporter results in reversal of the cellular chloride gradient to produce an excitatory effect of GABA receptor activation. We also demonstrated that KCC-2 and ABTS-1 may act partially redundantly with each other to control the cellular chloride gradient, as mutants lacking both transporters display defects in muscle contraction that are much more severe than those observed in either single mutant.;This work demonstrates the presence of two chloride-extrusion mechanisms in the C. elegans nervous system and also suggests a novel role for sodium-dependent bicarbonate-chloride exchangers in supporting inhibitory neurotransmission.