Partial hippocampal kindling decreases efficacy of presynaptic GABAB autoreceptors in CA1

Abstract

The effect of partial hippocampal kindling, a model of temporal lobe seizure, on monosynaptic inhibition mediated by GABA was studied. Kindled rats were given 15 nonconvulsive hippocampal afterdischarges, and control rats were given low frequency or no stimulations. At 1-2 d after kindling, paired-pulse depression (PPD) of the IPSCs recorded in CA1 neurons in vitro was significantly smaller in kindled as compared with control rats. The difference in PPD persisted for at least 21 d after kindling. The decrease in PPD of the IPSCs after partial hippocampal kindling was likely caused by a reduced GABA autoinhibition after downregulation of presynaptic GABAB receptors. The GABAB antagonist CGP35348 (1 mM) suppressed PPD of the IPSCs more strongly in control than in kindled rats. Direct activation of the presynaptic GABAB receptors by baclofen suppressed the monosynaptic IPSCs significantly more in control than in kindled rats. The decay rate of a single-pulse IPSC was faster in kindled than in control rats on day 1 or day 21 after partial kindling. The difference in IPSC decay between kindled and control rats was found with or without a GABAB receptor antagonist. The low efficacy of the presynaptic GABAB receptors in kindled rats may provide compensatory stabilization of the postsynaptic membrane against further seizures or plasticity.

Fig. 2.

Ensemble averages of IPSCs from neurons recorded in CNQX and d-AP5 medium. For each pair of traces, thetop trace is the ensemble mean + 1 SEM and thebottom trace is the ensemble mean − 1 SEM. The kindled group is blue, and the control group isred. Day 1 (A) and day 21 (B) averages of kindled and control groups, without GABA_B antagonist CGP35348. C, Day 1 groups recorded with CGP35348. All IPSCs were recorded using 2 × threshold stimulus intensity, 100 msec interpulse interval, and −50 mV holding potential. For each neuron, the recorded trace was normalized by the first-pulse IPSC peak, and then all neurons in the same group were combined in the ensemble average (see Materials and Methods). Number of neurons in kindled (K) and control (C) groups:A, C = 14, K = 16; B, C = 11, K = 18; C, C = 17, K = 17. Note faster decay of the first-pulse IPSC in the kindled as compared with control group under all conditions.

Fig. 1.

Kindled seizures reduced PPD of GABA_Amediated IPSCs recorded at a holding potential of −50 mV.A, Examples of paired-pulse IPSCs in CA1 neurons from control and kindled rat, evoked by stratum radiatum stimuli of 2 × threshold intensity in the presence of CNQX and d-AP5, before and after perfusion with a CGP35348 (1 mm) medium. Peaks of IPSC1 and IPSC2 indicated by peak1 andpeak2, respectively. Resting membrane potential of control and “kindled” neuron was −59 and −60 mV, respectively.Filled circles indicate shock artifacts.B, Plot of the ratio of the peak IPSC evoked by the second pulse (peak IPSC2) to that evoked by the first pulse (peak IPSC1), in the absence of CGP35348, as a function of interpulse interval. Error bars are one SEM. Control group (C) = 8 neurons and kindled group (K) = 10 neurons, except for 500 msec interpulse interval data, where C = 6 and K = 4 neurons. ** (p < 0.01) indicates significant difference between control and kindled groups of neurons as assessed by the nonparametric Wilcoxon test.

Fig. 3.

Average IPSC (n = 4 sweeps) versus voltage relation, in a medium with CNQX and d-AP5 but without CGP35348, at −50 to −110 mV holding potentials for a neuron from (A) day 1 control and (B) day 1 kindled rat. Outward rectification was seen in both neurons. A depolarizing shift of the reversal potential for IPSC2 as compared with IPSC1 is shown in A. At all holding potentials, PPD of the IPSCs was observed in Abut not in B.

Fig. 4.

Partial hippocampal kindling reduced the effect of GABA_B agonist baclofen in suppressing IPSCs.A, Examples of paired-pulse IPSCs in CA1 neurons from control and kindled rat, recorded at −50 mV holding potential, before and after the perfusion of 10 μm baclofen (added to the CNQX and d-AP5 medium). Resting membrane potential for control and “kindled” neuron was −60 and −63 mV, respectively.Filled circles indicate shock artifacts.B, Baclofen reduced the first IPSC in neurons from both kindled and control rats, but the effect was significantly larger in control as compared with kindled rats. C, The difference in the PPD (ratio of IPSC peaks) between kindled and control rats was abolished after 5 min of baclofen perfusion. * (p < 0.05), ** (p < 0.01) indicate significant difference between kindled and control groups (Wilcoxon) at a fixed time after baclofen.