Figure 10.

Neonatal seizures alter the effect of synaptic activation of GABA_A receptors. a–c, Effects on spontaneous neuronal firing rate of suppressing synaptic excitation and inhibition in control and during seizures. In control (a), KYNA (2 mm) coapplied with CGP 55845 (CGP; 1 μm) reduced the mean frequency of MUA in the CA3 pyramidal cell layer in the intact neonatal hippocampi at P4–P5. Subsequent application of the GABA_A-R antagonist SR 95531 (SR), in the presence of KYNA and CGP 55845, increased MUA frequency, demonstrating the net inhibitory effect of GABA_A-R when activated by endogenously released GABA. b, Spontaneous seizures were induced by brief (2–3 min) application of SR 95531. After 30 min of spontaneous seizure activity after washout of SR 95531, application of KYNA and CGP 55845 stopped recurrent seizures and reduced MUA frequency. Subsequent application of SR 95531 did not change the mean frequency of MUA. Red plots represent the averaged mean frequency of MUA. SE, Status epilepticus. c, Mean ratio of MUA frequency [SR 95531/(KYNA + CGP 55845)] reversed as a function of the number of seizures. Solid line represents corresponding linear regression fit (r = −0.98; p = 0.02). d, e, Synchronous extracellular field potentials and two-photon fluorescence [Ca²⁺]_i imaging in the CA3 pyramidal cell layer at P5 in the intact hippocampal preparation. Neurons were labeled by the calcium indicator dye OGB. Field EPSPs and intracellular calcium signals were induced by electrical stimulation in the stratum radiatum proximal of the CA3 area. In control (d), bath application of KYNA (2 mm) and CGP 55845 (1 μm) depressed calcium signals (ΔF/F) in the majority of CA3 neurons. Subsequent application of SR 95531 (10 μm), in the presence of KYNA and CGP 55845, increased ΔF/F in the majority of the cells. The increase in Ca²⁺ in the presence of ionotropic glutamate and GABA receptor antagonists likely represents metabotropic receptor activation and direct electrical stimulation. e, After spontaneous seizures, the effect of SR 95531 on ΔF/F was reversed in ∼50% of neurons. f, g, Mean population response (ΔF/F) and fraction of cells with a stimulus-induced increase in calcium responses during applications of drugs in control and after spontaneous seizures. *p < 0.05. h, After recurrent seizure activity, nearly half of neurons lose their inhibitory response to endogenous GABA or switch their inhibitory response from inhibition to excitation.