Gap junctions do not underlie changes in whole-cell conductance in anoxic turtle brain

An acute reduction in cell membrane permeability could provide an effective strategy to prolong anoxic survival. A previous study has shown that in the western painted turtle whole-cell neuronal conductance (G_w) decreases during anoxia, which may be mediated by the activation of adenosine A₁ receptors and calcium. Reduction in G_w is thought to be the result of ion channel closure, but closure of gap junctions could also be responsible for this phenomenon. In our study, antibody staining of connexin 32 and 43 (Cx32 and Cx43) suggested the presence of gap junctional components in the turtle cortex. To examine if gap junctions were involved in the previously measured anoxic decrease in G_w, neuronal connectivity was assessed through the measurement of whole-cell capacitance (C_w). Turtle cortical sheets were perfused with normoxic (95%O₂/5%CO₂), anoxic (95%N₂/5%CO₂), high calcium (4 mM) and adenosine (200 μm) artificial cerebral spinal fluid (aCSF). No significant change in C_w was observed under any of the above conditions. However, during hypo-osmotic aCSF perfusion C_w decreased significantly, with the lowest value of 50±10.4 pF (P<0.05) occurring at 30 min. To visualize changes in gap junction permeability lucifer yellow was loaded into turtle neurons during normoxic, anoxic, 0 calcium, hypo-osmotic, cold shock, (+)-isoproterenol, nitric oxide donor S-nitoso-acetyl penicillamine, and 8-bromo-guanosine 3′,5′-cyclic monophosphate aCSF perfusion. Dye propagation was only observed in 3 of 20 cold shock experiments (4°C). We conclude that gap junctions are not involved in the acute reduction in G_w previously observed during anoxia and that our results support the hypothesis that ion channel arrest is involved.