Abstracts

Neurons in the cortex fire with short, high frequency bursts or trains during interictal epileptiform discharges and with longer, trains during ictal activity. The nature of the responses in neurons that are synaptically coupled to these neurons, however, is not well characterized. Our laboratory has been analyzing short term synaptic plasticity in both excitatory and inhibitory circuits in response to repetitive firing of the presynaptic neuron in order to understand the mechanisms by which seizures may develop and spread throughout the brain.
Isolated pairs of synaptically connected hippocampal neurons in cell culture were studied with patch clamp electrodes, in the WCVC mode, and in current clamp to more closely mimic conditions that exist physiologically. Extracellular medium was similar to rat CSF (145 mM NaCl, 3 mM KCl, 2.0 mM CaCl[sub]2[/sub], 1 mM MgCl[sub]2[/sub], 10 mM glucose, 10 mM Hepes, pH 7.3). Action potentials (1, 2, or trains at 5, 20 and 100 hz) were evoked in the presynaptic neurons. Excitatory postsynaptic potentials (or currents) were recorded in first the absence, then presence of blockers of GABA[sub]A[/sub] and NMDA receptors.
The majority of excitatory synapses (137/154, 84%) showed paired pulse depression (PPD) in the WCVC mode at interstimulus intervals of 200 msec. Only 8% showed facilitation. By contrast, repetitive activation of 58 monosynaptic excitatory connections, in the WCVC mode and with NMDA receptors blocked, demonstrated three kinds of short term plasticity (STP): (1) 32 pairs (55%) showed tetanic depression, with the average EPSC declining to [lt]50% of its control value after 50 stimuli at even 5 hz. (2) 17 pairs (29%) showed tetanic facilitation, with the EPSCs increasing up to [gt]200% of control value and (3) 9 pairs (16%) showed first depression and then facilitation or a variable response within the train. Each of these patterns was a result of changes in presynaptic release of neurotransmitter. When the neurons were studied in a more physiological condition, repetitive activation at 5, 20 and 100 hz produced marked progressive neuronal depolarization, usually reaching threshold for activation of the postsynaptic neuron and often well outlasting the stimulus train, especially at 20 and 100 hz. At 100 hz, simply stimulating one presynaptic neuron with 10 APs frequently induced repetitive [ldquo]seizure-like[rdquo] discharges in the follower cell that lasted for seconds at least. Blocking NMDA receptors reduced but did not eliminate the large underlying postsynaptic depolarizations.
Despite reduction in neurotransmitter release with repetitive activation of excitatory synapses, small excitatory circuits demonstrate dramatic of synaptic transmission. This is mediated by both AMPA and NMDA receptor-mediated currents. Such powerful short term synaptic facilitation can clearly play an important role in the development and spread of seizures.
[Supported by: MAD is supported by NS 24260.]