Ischemic preconditioning protects the hippocampal CA1 neurons by inhibiting synaptic activity in rat

Introduction

Neurological disorders like global cerebral ischemia induce excitotoxicity, overactivation of NMDA receptors and high calcium influx leading to impaired synaptic function. Further, synaptic dysfunction is also accompanied subsequently by selective and delayed degeneration of pyramidal neurons in CA1 region of hippocampus. A previous report from our lab has shown that the ischemic preconditioning (IPC) improves neuronal survival in the CA1 region of hippocampus ¹ . Ischemic preconditioning (IPC) is an endogenous condition in which sublethal ischemic episodes render neurons resistant against subsequent lethal ischemic insults. The goal of present study was to determine whether IPC-induced neuroprotection also protected against synaptic dysfunction.

Methods

In male Wistar (275 ± 10 g) rats, IPC was produced by bilateral carotid occlusions and systemic hypotension (50 mm Hg) for 2 minutes. 48 h following IPC, hippocampal slices of 400-μm thickness were sectioned, oxygenated with 95% O₂–5% CO₂ in artificial cerebrospinal fluid, stored at 28 C for 1 h and transferred to the recording chamber. General population measurements of excitatory post-synaptic field potentials (fEPSP) were recorded from stratum radiatum of the CA1 hippocampal subfield, after electrical stimulation to the Schaffer collaterals (0.3 ms constant current pulses). After stable fEPSPs were recorded, input–output curves relating stimulus current intensity to fEPSP slope and amplitude were generated. To induce LTP tetanic stimulation (100 Hz) was applied for 1 s and test stimulation (1/30 sec) resumed for a period of 1 hr. To determine paired pulse facilitation/depression, paired stimulations at interval of 50 or 200 ms were applied through a constant-current stimulator with intensity sufficient to elicit 30% from maximum field potential. The initial negative-going slope of each fEPSP was measured as an index of synaptic strength and was expressed in mV/ms.

Results

Our results demonstrated inhibition of LTP induction following IPC. In slices harvested from control animals, tetanic stimulation of 100 Hz to the Schaffer collaterals potentiated the fEPSP amplitude from pre-tetanus values of 4.39 ± 0.54 mV/msec to 6.73 ± 0.64 (n = 4) at 60 min post-tetanus (p<0.05). In contrast, fEPSP amplitudes of pre-tetanus and post-tetanus were 3.49 ± 0.41 mV/msec and 4.62 ± 0.63 mV/msec, respectively, in IPC group. To further understand the influence of IPC on short-term synaptic plasticity we measured paired-pulse modulation. Impaired pre-synaptic release following IPC was evident from significant inhibition in paired–pulse facilitation in hippocampal slices harvested 48 h after IPC when compared to controls.

Conclusion

The present study clearly demonstrates inhibition of synaptic activity following IPC, which might be a possible mechanism for improved neuronal survival following lethal ischemic insult. Further, studies investigating the role of specific receptors or other cellular components will be essential to understand IPC mediated neuroprotection of synaptic function.