Synchronous potentials and elevations in [K+]o in the adult rat entorhinal cortex maintained in vitro

Extracellular field-potential recordings and measurements of the extracellular concentration of potassium ([K+]o) were made in layers II and III of the adult rat entorhinal cortex in a slice preparation. Two types of spontaneous, synchronous potentials were induced by the convulsant drug 4-aminopyridine (4AP, 50 microM). The first type was seen in all slices (n = 19) and consisted of a negative-going field potential that lasted 0.5-3.5 s and occurred at rates of 0.013-0.13 Hz. This event was accompanied by an elevation in [K+]o that attained peak values of 4.0-7.6 mM. The second type was reminiscent of ictal epileptiform discharges and was recorded in 6 of 19 slices; it lasted 21-190 s, recurred at 0.001-0.003 Hz and was associated with [K+]o increases that had peak values of 14-17 mM. Whenever such an ictal discharge occurred, it was closely preceded and thus appeared to be initiated by the first type of field potential. Perfusion with N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)-3-(2-carboxy-piperazine-4-yl)propyl-1-phosphonate (CPP; 10 microM) abolished the ictal discharge (n = 4). This pharmacological procedure did not abolish the negative-going potentials that continued to occur during further application of the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM; n = 4). These glutamatergic-independent potentials were, however, blocked by the GABBAA-receptor antagonist bicuculline methiodide (10 microM, n= 3). Thus, as in hippocampus, 4AP can induce in the entorhinal cortex a synchronous GABA-mediated potential that is resistant to excitatory amino acid antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)