Cats were prepared under general anesthesia with heavy-duty dental cement crowns for later head restraint during the conscious state. After adaptation to head restraint and acquisition of tone-shock conditioning, the frontal cortex was exposed and implanted with platinum black transcortical electrodes (0.06 mm) and potassium ion-selective micropipettes (2-5 μm). The extracelllular potassium activity (aK(o)) was measured with double-barrel micropipettes, one filled with a K+ ion exchanger (Corning 477317) and the other with NaCl for measuring the local field potential (FP). Two types of event-related aK(o) responses were observed following presentation of either a conditioned stimulus, an unexpected cutaneous shock, or a sardine food reinforcer. Type I was a large aK(o) increase that persisted throughout the duration of the event-related slow potential recorded by the transcortical electrodes. Type II was an even larger initial aK(o) increase (10-100 mM) followed immediately by a large aK(o) decrease below the 3 mM base-line level (to 2.0-1.0 mM). The leading edges of the type II aK(o) increases were composed of discrete events that summated and preceded the onset of the negative FPs. The type I aK(o) increases all had smooth onsets that exactly paralleled the onset of the negative FPs. Small primary undershoot responses to 2.9 mM could be evked by conditioned stimuli; these were recorded from the same electrode locations in which unexpected shocks or sardines could evoke large type II shifts in aK(o). Careful consideration of limb movements as a source of artifact precluded the interpretation that the aK· Shifts were movement related. It is concluded that the type II responses may best represent aK(o) changes because discrete aK(o) summations and undershoots suggest that a small dead space surrounds the electrode tips. It is interpreted that although the early aK(o) undershoots may be produced by membrane pumps, the later part is best explained by inactivation of the resting potassium ion conductance in the neurons, according to a theory by Libet.
ASJC Scopus subject areas