Spreading depression (SD) waves in the brainstem can be elicited after blockade of potassium channels – evidence for the role of extracellular potassium ions as a driving force?

Abstract

SD waves can be elicited by KCl but not by pricking in the brainstem of rats younger than 13 days after acetate conditioning whereas this protocol failed to elicit SDs in the brainstem of older or adult rats (J. Neurophysiol. 90, 2003, 2163–70). In the past, we have shown that the blockade of neither N-, nor L- and T-, nor P/Q-type calcium channels interfered with the occurrence of brainstem SD waves. Even after blockade of NMDA receptors SD waves could still be elicited in the immature brainstem. These findings raised the issue whether potassium ions ([K+]) according to Grafsteins hypothesis could ignite and propagate SDs in the brainstem irrespectively from the above-mentioned channels and receptors. To test this we recorded SD-related DC deflections and changes in [K+]e with ion-selective microelectrodes based on valinomycine (FLUKA 60031) in the brainstem of urethane-anesthetized rats (1.5 g/kg, i.p.) aged 11–14 days, and in the brainstem of sodium-thiopental (100 mg/kg, i.p.) anesthetized adult rats. Two pipettes were glued together spanning horizontally over 400 m to observe propagation of SDs across the brainstem and were inserted into a region close to the trigeminal nucleus. For acetate conditioning the brainstem was superfused with warmed, carbogen-equilibrated artificial cerebrospinal fluid (ACSF) in which 75% of the chloride was replaced by sodium acetate. K+-channels were either blocked by 10 mM tetraethylammonium chloride (TEA) or by 1 mM barium chloride (BaCl2). After testing for SD by applying a KCl crystal to the surface of the non-conditioned brainstem, the brainstem was superfused for a period of 20 min with acetate-ACSF containing the K+-channel blocker. Then KCl was applied again to elicit SD, and after either a waiting time of 5 min or after a SD wave superfusion with the blocking solution was continued for another 20 min. In immature rats usually 3 sequences of acetate-ACSF containing TEA or BaCl2 were given, in adult rats up to 6 sequences. BaCl2 did not interfere with brainstem SDs in 11 to 12-day old rats. They were unchanged in amplitudes and shape (DC deflections up to 15 mV, increases in [K+]e up to 55 mM). TEA was also without immediate effect, but after 20 min superfusion a continuously increasing negative deflection of the DC potential was observed that was in some rats accompanied by spontaneously and repetitively occurring SDs. The concomitant elevations in [K+]e did not differ from those of SDs without TEA, but the [K+]e remained elevated after the SD (2–4 mM compared to previous values per 20 min superfusion) and did not return to initial values. Interestingly, we were able to elicit SD even in the brainstem of 14-day-old or adult rats with the same TEA-acetate preparation. We conclude that