Pharmacological blockade of gap junctions induces repetitive surging of extracellular potassium within the locust CNS

The maintenance of cellular ion homeostasis is crucial for optimal neural function and thus it is of great importance to understand its regulation. Glial cells are extensively coupled by gap junctions forming a network that is suggested to serve as a spatial buffer for potassium (K+) ions. We have investigated the role of glial spatial buffering in the regulation of extracellular K+ concentration ([K+]o) within the locust metathoracic ganglion by pharmacologically inhibiting gap junctions. Using K+-sensitive microelectrodes, we measured [K+]o near the ventilatory neuropile while simultaneously recording the ventilatory rhythm as a model of neural circuit function. We found that blockade of gap junctions with either carbenoxolone (CBX), 18β-glycyrrhetinic acid (18β-GA) or meclofenamic acid (MFA) reliably induced repetitive [K+]o surges and caused a progressive impairment in the ability to maintain baseline [K+]o levels throughout the treatment period. We also show that a low dose of CBX that did not induce surging activity increased the vulnerability of locust neural tissue to spreading depression (SD) induced by Na+/K+-ATPase inhibition with ouabain. CBX pre-treatment increased the number of SD events induced by ouabain and hindered the recovery of [K+]o back to baseline levels between events. Our results suggest that glial spatial buffering through gap junctions plays an essential role in the regulation of [K+]o under normal conditions and also contributes to a component of [K+]o clearance following physiologically elevated levels of [K+]o.

Highlights
- We measured [K+]o within the locust metathoracic ganglion.
- Gap junctions were inhibited to reduce K+ spatial buffering by glial cells.
- Gap junction blockade induced repetitive surges in [K+]o and raised baseline levels.
- Gap junction blockade increased severity of ouabain-induced spreading depression.
- We propose glial spatial buffering to be a primary mechanism in [K+]o regulation.