Abstracts

RATIONALE: Neocortex, a structure with a dense cholinergic innervation which develops early in postnatal life, is involved in the generation and the propagation of epileptiform activity. Because of our previous findings in immature hippocampus, we were interested to study the effects of endogenous acetylcholine (ACh) on excitatory (disinhibited) field potentials in immature rat neocortex and to compare these effects with those of the exogenous agonist carbachol (CCh).
METHODS: We recorded evoked and spontaneous field potentials from the deep layers of parasagittal slices of immature (P10-P20) rat neocortex in the presence of 50[mu]M of the GABA[sub] A [/sub] antagonist BMI. We compared the effects of the anticholinesterase eserine (10[mu]M), an indicator of the activity of endogenous ACh, with those of the non-hydrolysable ACh-analog carbachol (CCh, 25[mu]M).
RESULTS: Eserine increased the rate of occurrence of ictal-like (6-16s) and/or interictal-like (~1s) spontaneous discharges and at the same time it decreased the amplitude and duration of the evoked field potentials. CCh mimicked the effects of eserine; however CCh induced spontaneous discharges with significantly higher frequency than those induced by eserine. All cholinergic effects were reversed by the muscarinic antagonist atropine (2.5[mu]M) suggesting they were mediated by the activation of muscarinic receptors. By contrast, the nicotinic antagonist hexamethonium (50[mu]M) had no effect on the CCh-induced activity (n=3), indicating no significant implication of nicotinic receptors. The NMDA and non-NMDA glutamate receptor antagonists APV (10[mu]M) and CNQX (10[mu]M) depressed the evoked field potentials and their combination blocked them (n=14 slices). Spontaneous discharges were blocked only by CNQX, while their frequency was reduced by either APV or CNQX. Spontaneous cholinergic epileptiform discharges were blocked by the gap-juntion blockers carbenoxolone (500[mu]M, 7/8 slices) and/or halothane (2.5%, 7/7 slices).
CONCLUSIONS: Activation of muscarinic receptors by endogenous ACh during disinhibition in immature neocortex, provokes the generation of epileptiform discharges with frequency dependent on the availability of the agonist. This type of network activity is mediated by both NMDA and non-NMDA glutamate receptors and in addition, gap-junctions play an essential role in its generation.
Support: NSERC, Savoy Foundation for Epilepsy, FRSQ (CP), Ste-Justine Hospital Foundation (SP).