ACTIVATION OF MUSCARINIC RECEPTORS DURING BLOCKADE OF GABA(A)-MEDIATED INHIBITION INDUCES SYNCHRONOUS EPILEPTIFORM ACTIVITY IN IMMATURE RATHIPPOCAMPUS

We investigated the effects of the cholinergic agonist carbachol (25 m u M) on the synaptic potentials recorded extracellularly and intracell ularly from the CA3 area of immature hippocampal slices of the rat (po stnatal days 10-20). In control conditions, carbachol reduced the ampl itude of evoked synaptic responses (n=8) and did not induce any sponta neous synchronous activity (n=12); the depressant effect of carbachol was mimicked by acetylcholine (100 mu M, in eserine 10 mu M, n=5) and was reversed by the muscarinic antagonist atropine (1 mu M, n=2). The GABA(A)-receptor antagonist bicuculline (10 mu M) enhanced the amplitu de and duration of the evoked synaptic responses and induced infrequen t (0.016-0.045 Hz) spontaneous synchronous discharges in 23/37 of the slices. Application of carbachol in the presence of bicuculline reduce d the amplitude of the evoked synaptic responses (n=21) and in additio n induced synchronous discharges with rates of occurrence 0.075-0.225 Hz, in 64/68 slices. Both effects were mimicked by acetylcholine and e serine, and antagonized by atropine. The specific muscarinic antagonis ts pirenzepine (MI-type), tripitramine (M2-type), 4-diphenylacetoxy-N- methylpiperidine methiodide (M3-type) and tropicamide (M4-type) (all t ested at 0.1-1 mu M) reversibly reduced the frequency of synchronous c arbachol-induced discharges. In addition, these discharges were revers ibly blocked by high Ca2+ perfusion medium (7 mM CaCl2, n=4) and by th e glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione ( 10 mu M, n=7). Synchronous epileptiform discharges were recorded from both CAI and CA3 areas in intact slices (n=3), but only from CA3 follo wing disruption of the CA1-CA3 synaptic connections (n=3). These exper iments suggest that activation of muscarinic receptors during blockade of GABA,mediated potentials, may enhance synchronous epileptiform act ivity in immature (postnatal days 10-20) hippocampus, through activati on of local excitatory circuits and that endogenous acetylcholine may be sufficient to play this role. (C) 1997 IBRO. Published by Elsevier Science Ltd.