Synaptic and non-synaptic mechanisms underlying low calcium bursts in the in vitro hippocampal slice

1. The epileptiform activity generated by lowering extracellular [Ca++] was studied in the CA1 subfield of rat hippocampal slices maintained "in vitro" at 32 degrees C. Extracellular and intracellular recordings were performed with NaCl and KCl filled microelectrodes. 2. Synaptic potentials evoked by stimulation of the stratum radiatum and alveus were blocked upon perfusion with artificial cerebrospinal fluid (ACSF) containing 0.2 mM Ca++, 4 mM Mg++. Blockade of synaptic potentials was accompanied by the appearance of synchronous field bursts which either occurred spontaneously or could be induced by stimulation of the alveus. 3. Both spontaneous and stimulus-induced low Ca++ bursts recorded extracellularly in stratum pyramidale consisted of a negative potential shift with superimposed population spikes. This extracellular event was closely associated with intracellularly recorded action potentials rising from a prolonged depolarization shift. Steady hyperpolarization of the cell membrane potential decreased the amplitude of the depolarizing shift suggesting that synaptic conductance were not involved in the genesis of the low Ca++ burst. 4. Spontaneous depolarizing inhibitory potentials recorded in normal ACSF with KCl filled microelectrodes were reduced in size in low Ca++ ACSF. However, small amplitude potentials could still be observed at a time when low CA++ bursts were generated by hippocampal CA1 pyramidal neurons. 5. Bicuculline methiodide, an antagonist of gamma-aminobutyric acid (GABA), was capable of modifying the frequency of occurrence and the shape of synchronous field bursts. The effects evoked by bicuculline methiodide were, however, not observed when 81-100% of NaCl was replaced with Na-Methylsulphate.(ABSTRACT TRUNCATED AT 250 WORDS)