Regulation of synaptic transmission at the photoreceptor terminal: a novel role for the cation–chloride co‐transporter NKCC1

Abstract

Key points• The Cl − uptake cotransporter, Na + -K + -2Cl − type 1 (NKCC1), is expressed in the distal synaptic layer of the vertebrate retina, where photoreceptor terminals contact the dendrites of horizontal and bipolar cells.• Light adaptation, dopamine and a D1 receptor agonist increased the expression levels of phosphorylated NKCC1, the active form of the transporter inserted in the cell membrane.• Pharmacological blockage of NKCC1 with bumetanide increased the rod-and cone-mediated excitatory postsynaptic currents in horizontal cells.• Inhibiting NKCC1 increased exocytotic membrane capacitance, intracellular Ca 2+ levels, and voltage-dependent Ca 2+ channel currents in both rod and cone terminals, all of which are associated with increased transmitter release.• This study describes a new function of NKCC1, specifically the suppression of transmitter release at the photoreceptor terminals, a process that serves to prevent the depletion of glutamate and protect retinal neurons from its putative cytotoxic effects.Abstract The Na + -K + -2Cl − co-transporter type 1 (NKCC1) is localized primarily throughout the outer plexiform layer (OPL) of the distal retina, a synaptic lamina that is comprised of the axon terminals of photoreceptors and the dendrites of horizontal and bipolar cells. Although known to play a key role in development, signal transmission and the gating of sensory signals in other regions of the retina and in the CNS, the contribution of NKCC1 to synaptic transmission within the OPL is largely unknown. In the present study, we investigated the function of NKCC1 at the photoreceptor-horizontal cell synapse by recording the electrical responses of photoreceptors and horizontal cells before and after blocking the activity of the transporter with bumetanide (BMN). Because NKCC1 co-transports 1 Na + , 1 K + and 2 Cl − , it is electroneutral and its activation had little effect on membrane conductance. However, recordings from postsynaptic horizontal cells revealed that inhibiting NKCC1 with BMN greatly increased glutamate release from both rod and cone terminals. In addition, we found that NKCC1 directly regulates Ca 2+ -dependent exocytosis at the photoreceptor synapse, raising the possibility that NKCC1 serves to suppress bulk release of glutamate vesicles from photoreceptor terminals in the dark and at light offset. Interestingly, NKCC1 gene and protein expressions were upregulated by light, which we attribute to the light-induced release of dopamine acting on D1-like receptors. In sum, our study reveals a new role for NKCC1 in the regulation of synaptic transmission in photoreceptors.

Postnatal Changes in K+/Cl− Cotransporter-2 Expression in the Forebrain of Mice Bearing a Mutant Nicotinic Subunit Linked to Sleep-Related Epilepsy

“…Physiological evidence indicates that presynaptic NKCC1 can stimulate (Bos et al, 2011) as well as inhibit (Shen et al, 2013) neurotransmitter release, making it difficult to provide a general interpretation of our results. In immature rat spinal cord, NKCC1 activity enhances transmitter release from afferent fibers by favoring a GABAdependent presynaptic depolarization (Bos et al, 2011).…”

“…We can hypothesize that a similar mechanism is operant in the developing TH, thus facilitating the usual depolarizing role of GABA in early stages. In contrast, NKCC1 has been found to inhibit glutamate release in photoreceptors (Shen et al, 2013), a mechanism worth further investigation in the PFC VGLUT1+ terminals. At variance with NKCC1, the cellular distribution of KCC2 was different in cortical and TH neurons.…”

Targeted Expression of Anoctamin Calcium-Activated Chloride Channels in Rod Photoreceptor Terminals of the Rodent Retina

“…An interesting finding is that the amount of phosphorylated NKCC1 in the OPL increases upon light adaptation and decreases in the light-adapted retina. 44 The significance of this light-controlled phosphorylation is not yet understood, mainly because its time course has not yet been established. But the NKCC1-phosphorylation data suggest that intraterminal [Cl À ] i is higher in light than in darkness.…”

“…22,43 Moreover, a recent study demonstrated that NKCC1 within the OPL is phosphorylated upon illumination of the retina. 44 The electroneutral Na þ -K þ -2Cl À cotransporter is dynamically regulated by a phosphorylation/ dephosphorylation equilibrium. 45 Three threonine residues in the C-terminal region of the NKCC1 protein serve as targets for phosphorylation by the kinases SPAK and OSR1.…”

Expanding Spectrum of Sodium Potassium Chloride Co-transporters in the Pathophysiology of Diseases

“…Within the cardiovascular system, extensive distribution of NKCC1 in cardiomyocytes, fibroblasts, vascular endothelial cells, smooth muscle cells and red blood cells has been described [ 1 , 41 ]. NKCC1 is also localized throughout the outer plexiform layer of the distal retina, a synaptic lamina that is comprised of the axon terminals of photoreceptors and the dendrites of horizontal and bipolar cells [ 42 ]. The NKCC2 isoform has been identified only in the medullary regions of the kidney, the loop of Henle and the juxtaglomerular apparatus [ 5 , 43 ].…”