Endogenous ACh effects on NMDA‐induced interictal‐like discharges along the septotemporal hippocampal axis of adult rats and their modulation by an early life generalized seizure

Abstract

Our earlier findings of the modulation of cholinergic neurotransmission by an early life generalized seizure and the reported interaction between muscarinic and N-methyl-d-aspartate (NMDA) receptors prompted us to investigate the effects of endogenous acetylcholine (ACh) on the frequency (Hz) of the epileptiform discharges following NMDA-receptor activation in the hippocampal slice. A sustained (>20 min) generalized convulsion was induced in Sprague-Dawley juvenile rats by intraperitoneal injection with pentylenetetrazole (PTZ, 70-90 mg/kg) at postnatal day (P) 20. Temporal and septal hippocampal slices were prepared of normal (N) and PTZ-treated (PTZ) adult (≥P60) rats, and CA3 field potentials were recorded during perfusion with Mg(2+) -free artificial cerebrospinal fluid (ACSF) or with ACSF containing 50 μm 4-aminopyridine (4-AP). In Mg(2+) -free ACSF, spontaneous interictal-like epileptiform discharges (IEDs) were recorded in all slices, with significantly higher frequencies in temporal (0.46 ± 0.03 Hz, n = 85) versus septal slices (0.20 ± 0.02 Hz, n = 47, p < 0.000001) but no consistent differences in any other group (i.e., male vs. female or N vs. PTZ). The anticholinesterase eserine (10 μm) increased their frequencies by 150-200% in N-septal and in all temporal slices and by 300% in PTZ-septal slices (p = 0.0028). In 60% of the slices the excitatory effect persisted throughout drug perfusion, whereas in the remaining ones it was distinguished in two phases: an early "transient" and a late "steady state." The steady-state frequencies resembled the predrug ones in N slices but remained significantly elevated in PTZ slices, especially in the septal group. The muscarinic antagonist atropine (1 μm) decreased IED frequency in all slices (n = 36, p = 0.005) and also fully reversed the eserine effect (n = 38, p < 0.0001). In 4-AP ACSF, eserine increased spontaneous IED frequency (n = 21) in N and PTZ slices alike; IEDs were subsequently abolished by addition of the NMDA-receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP5; 50 μm, n = 6). These results demonstrate an intrinsic tonic positive muscarinic acetylcholine receptor (mAChR) contribution to the frequency of NMDA receptor-dependent epileptiform discharges that is amplified following an elevation of endogenous ACh and is more pronounced in the septal hippocampus. Moreover, this positive mAChR contribution to the frequency of IEDs is even more pronounced and persistent in the septal extremity after an early life generalized sustained convulsion. This cholinergic enhancement of the excitatory septal hippocampal output may influence cognitive function and performance, and possibly the adult seizure threshold.