ON THE SYNCHRONOUS ACTIVITY-INDUCED BY 4-AMINOPYRIDINE IN THE CA3 SUBFIELD OF JUVENILE RAT HIPPOCAMPUS

1. Extracellular field potential and intracellular recordings were made in the CA3 subfield of hippocampal slices obtained from 10- to 24-day-old rats during perfusion with artificial cerebrospinal fluid (ACSF) containing the convulsant 4-aminopyridine (4-AP, 50 muM). 2. Three types of spontaneous, synchronous activity were recorded in the presence of 4-AP by employing extracellular microelectrodes positioned in the CA3 stratum (s.) radiatum: first, interictal-like discharges that lasted 0.2-1.2 s and had an occurrence rate of 0.3-1.3 Hz; second, ictal-like events (duration: 3-40 s) that occurred at 4-38 . 10(-3) Hz; and third, large-amplitude (up to 8 mV) negative-going potentials that preceded the onset of the ictal-like events and thus appeared to initiate them. 3. None of these synchronous activities was consistently modified by addition of antagonists of the N-methyl-D-aspartate (NMDA) receptor to the ACSF. In contrast, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 2-10 muM) reversibly blocked interictal- and ictal-like discharges. The only synchronous, spontaneous activity recorded in this type of medium consisted of the negative-going potentials that were abolished by the GABA, receptor antagonists bicuculline methiodide (5-20 muM) or picrotoxin (50 muM). Hence they were mediated through the activation of the GABA(A) receptor. 4. Profile analysis of the 4-AP-induced synchronous activity revealed that the gamma-aminobutyric acid (GABA)-mediated field potential had maximal negative amplitude in s. lacunosum-moleculare, attained equipotentiality at the border between s. radiatum and s. pyramidale, and became positive-going in s. oriens. These findings indicated that the GABA-mediated field potential presumably represented a depolarization occurring in the dendrites of CA3 pyramidal cells. 5. This conclusion was supported by intracellular analysis of the 4-AP-induced activity. The GABA-mediated potential was reflected by a depolarization of the membrane of CA3 pyramidal cells that triggered a few variable-amplitude, fractionated spikes or fast action potentials. By contrast, the ictal-like discharge was associated with a prolonged depolarization during which repetitive bursts of action potentials occurred. Short-lasting depolarizations with bursts of action potentials occurred during each interictal-like discharge. 6. The GABA-mediated potential recorded intracellularly in the presence of CNQX consisted of a prolonged depolarization (up to 12 s) that was still capable of triggering a few fast action potentials and/or fractionated spikes. This depolarization lacked the initial hyperpolarization that often precedes a similar potential generated by adult (greater-than-or-equal-to 40-day-old) hippocampal cells perfused with ACSF containing 4-AP and CNQX. 7. Presumed glial cells recorded intracellularly in CA3 generated depolarizations that correlated in time with the three types of spontaneous field potentials induced by 4-AP. These depolarizations attained an initial peak (amplitude, 11-36 mV) within 200 ms from the onset of the GABA-mediated potential, grew further up to 20-48 mV during the initial phase of the ictal-like discharge, and returned toward baseline over several tens of seconds. Glial depolarizations were thought to reflect mainly increases in [K+]o. 8. After blockade of excitatory synaptic transmission, glial cells generated a prolonged depolarization during each GABA-mediated field potential. Segregation of some characteristics of these depolarizations according to age, revealed that their amplitude and duration were greater in the 10- to 14-day-old group than in the 18- to 23-day-old group. Moreover, values obtained from either juvenile group were significantly larger than those seen in adult rats. 9. It is concluded that both interictal- and ictal-like epileptiform discharges induced by 4-AP in the CA3 subfield of juvenile rat hippocampal slices are caused by an excitatory synaptic mechanism mediated through the activation of non-NMDA receptors. In addition, our findings demonstrate that as in the adult rat hippocampus, 4-AP discloses a synchronous potential that is mediated through GABA(A) receptors, presumably located on the dendrites of pyramidal cells. 10. Our study also indicates that in the juvenile hippocampus the GABA-mediated potential can synchronize neurons and thus initiate ictal-like epileptiform discharges. In the light of our glial recordings, we propose that this phenomenon is mediated through an augmentation of [K+]o that is caused by the activation of GABA(A) receptors located on the dendrites. Such an increase in [K+]o is more pronounced in the juvenile hippocampus because of a decreased ability to regulate [K+]o at this stage of brain maturation.