Imaging the induction and spread of seizure activity in the isolated brain of the guinea pig: the roles of GABA and glutamate receptors.

1. The induction and spread of seizure activity was studied using imaging and electrophysiological techniques in the isolated whole brain of the guinea pig. We examined the role of GABA and glutamate receptor subtypes in controlling the spread of seizure activity across the olfactory cortex from a focus in the entorhinal cortex. Seizure spread was monitored by video imaging of intrinsic optical signals (reflectance changes) combined with multiple extracellular recordings. Both the unilateral and bilateral spread of seizure activity was monitored in different experiments. 2. Electrical stimulation of the lateral entorhinal cortex (10-15 V, 5 Hz, 5-10 s) evoked seizure activity that originated in the entorhinal cortex/hippocampus and later spread preferentially toward the posteromedial cortical amygdaloid nucleus ipsilaterally and bilaterally. The pattern of seizure spread in a given brain was highly reproducible. 3. The influence of gamma-aminobutyric acid (GABA) receptors on the spread of seizure activity was monitored at higher resolution on one side of the brain. Perfusion of a low concentration of the GABAA antagonist bicuculline methiodide (20 microM) resulted in spontaneous seizures that spread to the posteromedial cortical amygdaloid nucleus more rapidly than electrically evoked seizures [spread times: 5.5 +/- 3.7 s vs. 15.5 +/- 2.7 s, respectively (means +/- SE)]. Seizure spread was also more extensive in the presence of bicuculline involving the posterior perirhinal cortex and larger areas over the medial amygdala. Higher concentrations of bicuculline (100 microM) resulted in even more widespread propagation of spontaneous seizure activity throughout the olfactory cortex as well as to the perirhinal, insular, and occipital cortices. This concentration of bicuculline also further reduced the time required for seizure activity to spread from the entorhinal cortex to the posteromedial cortical amygdaloid nucleus (spread time = 2.3 +/- 1.7 s). The GABAB antagonist, CGP 35348 (200 microM), in contrast, had no significant effect of seizure induction or propagation. 4. The role of glutamate receptor subtypes in seizure propagation was studied by examining the bilateral spread of seizures. Perfusion of the kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (K/A) receptor antagonist (6-cyano-7-nitroquinoxaline-2,3-dione, CNQX, 20 microM) completely and reversibly suppressed stimulus-evoked seizure activity as detected electrophysiologically and optically. CNQX also reduced the magnitudes of field potentials recorded in the isolated brain in a reversible manner by an average of 70.8 +/- 2.21% of control. The N-methyl-D-aspartate (NMDA) receptor antagonist dibenzocyclohepteneimine (MK-801) did not significantly alter the magnitudes or shapes of field potentials recorded in the isolated brain nor did it significantly alter seizure activity measured optically or electrophysiologically. 5. Perfusion of the metabotropic glutamate receptor agonist [trans-1-amino-(IS,3R)-cyclopentanedicarboxylic acid (trans-ACPD), 150 microM] completely and reversibly suppressed stimulus-evoked seizure activity as detected electrophysiologically and optically. The magnitudes of field potentials recorded in the isolated brain also were reduced by trans-ACPD an average of 75.4 +/- 5.39% of control values. 6. These results demonstrate that GABAA-mediated transmission is functionally present and may play an important role in epileptic tissue in limiting the spread of seizure activity from the entorhinal cortex to the posteromedial cortical amygdaloid nucleus and in creating functional pathways or preferential routes of seizure spread. GABAB-mediated postsynaptic inhibition played no significant role in the induction or spread of seizure activity in this study. K/A receptors but not NMDA receptors are necessary for the induction and subsequent spread of seizure activity originating in the entorhinal cortex/hippocampus.