Modulation of muscarinic facilitation of epileptiform discharges in immature rat neocortex.

We examined the cholinergic effects on epileptiform discharge generation in immature (postnatal days 10-20) rat neocortex. Evoked and spontaneous field potentials were recorded from the deep layers of neocortical slices during GABA(A) receptor blockade by bicuculline methiodide (BMI, 50 microM). The anticholinesterase eserine (10 microM) as well as the ACh-analog carbamylcholine chloride (CCh, 25 microM) decreased the amplitude and duration of evoked field potentials and in parallel, increased significantly the rate of occurrence of spontaneous discharges. These effects were reversed by the muscarinic antagonist atropine (2.5 microM, n = 20), but not by the nicotinic receptor antagonist hexamethonium (50 microM, n = 3). The M1 subtype-selective muscarinic antagonist pirenzepine (1 microM, n = 12) blocked spontaneous discharges in 8/12 slices, while muscarinic antagonists of the M2 (AFDX 116 n = 4), M3 (4-DAMP n = 4) and M4 (gallamine n = 5, tropicamide n = 6) type, all at 1 microM, only reduced their frequency. CCh-induced spontaneous discharges were blocked by the combination of the glutamate receptor antagonists AP5 and CNQX (both at 10 microM; n = 11). Gap junction blockers abolished them (halothane, n = 7) or reduced their frequency by 65% (carbenoxolone, n = 8). Inhibiting Ca2+ release from intracellular stores by dantrolene (100 microM, n = 5) or thapsigargin (1 microM, n = 5) also depressed their frequencies by 55-65%. By contrast, their rates were not altered by perfusion with high Ca2+ (7 mM; n = 6) medium, a manipulation suppressing polysynaptic connections. These findings demonstrate that activation of muscarinic receptors, notably of the M1 type, in immature rat neocortex facilitates the generation of glutamatergic epileptiform discharges. These discharges are strongly inhibited by gap junction blockers, and are also partly mediated by the, presumably muscarinic receptor-dependent, mobilization of intracellular calcium.