Neuron–glial trafficking of NH₄⁺ and K⁺: separate routes of uptake into glial cells of bee retina

Abstract

Ammonium (NH4+ and/or NH3) and K+ are released from active neurons and taken up by glial cells, and can modify glial cell behaviour. Study of these fluxes is most advanced in the retina of the honeybee drone, which consists essentially of identical neurons (photoreceptors) and identical glial cells (outer pigment cells). In isolated bee retinal glial cells, ammonium crosses the membrane as NH4+ on a Cl- cotransporter. We have now investigated, in the more physiological conditions of a retinal slice, whether the NH4+-Cl- cotransporter can transport K+ and whether the major K+ conductance can transport NH4+. We increased [NH4+] or [K+] in the superfusate and monitored uptake by recording from the glial cell syncytium or from interstitial space with microelectrodes selective for H+ or K+. In normal superfusate solution, ammonium acidified the glial cells but, after 6 min superfusion in low [Cl-] solution, ammonium alkalinized them. In the same low [Cl-] conditions, the rise in intraglial [K+] induced by an increase in superfusate [K+] was unchanged, i.e. no K+ flux on the Cl- cotransporter was detected. Ba2+ (5 mm) abolished the glial depolarization induced by K+ released from photoreceptors but did not reduce NH4+uptake. We estimate that when extracellular [NH4+] is increased, 62-100% is taken up by the NH4+-Cl- cotransporter and that when K+ is increased, 77-100% is taken up by routes selective for K+. This separation makes it possible that the glial uptake of NH4+ and of K+, and hence their signalling roles, might be regulated separately.

GABA and Glutamate Uptake and Metabolism in Retinal Glial (Müller) Cells

“…The mechanism of ammonia uptake by Müller cells is unclear. Retinal glial cells of the bee take up ammonia via a chloride cotransporter (Marcaggi et al, 2004). …”

Ammonia triggers neuronal disinhibition and seizures by impairing astrocyte potassium buffering

“…Ammonia homeostasis is uniquely important in the brain as equimolar NH 4 + is released with glutamate during neuronal firing 6 . Interestingly, the enzyme that removes ammonia and glutamate, glutamine synthetase, has a higher affinity for ammonia than this excitotoxic neurotransmitter 2,7 .…”

“…Since the widespread increase in astrocyte calcium signaling could not be due to swelling 15 , we next asked whether it might be linked to the interference of ammonia with potassium transport previously described in cell culture and kidney 4,6,17,18 . Using NH 4 + and K + ion-sensitive microelectrodes (ISM) 19 in awake Otc spf-ash animals we found that systemic ammonia intoxication increased extracellular potassium ([K + ] o ) by 1.93 ± 0.19 mM (subtracted for interference 17 , Fig.…”

“…2g), indicating that NH 4 + competes with K + for transport. Notably, NH 4 + and K + ions have a comparable hydrated radius and charge 6 . To test whether the [K + ] o increase alone is sufficient to reproduce the phenotype of ammonia neurotoxicity in vivo , we superfused KCl (12.5 mM) across the cortex of awake wild-type mice.…”

Ammonium increases Ca2+ signalling and upregulates expression of Cav1.2 gene in astrocytes in primary cultures and in the in vivo brain

“…In the central nervous system (CNS), ammonium, released in the extracellular space in the course of glutamatergic neurotransmission (Marcaggi et al . ), is neutralized by glutamine synthetase localized exclusively in astrocytes (Martinez‐Hernandez et al . , Rose et al .…”