Abstract
Field potential oscillations reflect repetitive firing and synaptic activity by ensembles of neurons in certain areas of the brain. They can be distinguished as slow (e.g., alpha, delta, and theta), fast (e.g., beta and gamma), and high frequency oscillations (HFOs). Neuronal oscillations are involved in a variety of physiological and pathophysiological states such as cognition, consciousness, and seizures. The laminar structure of rat hippocampus affords a way to study these oscillations in hippocampal slices. Rat ventral hippocampal brain slices were cut and maintained in a recording chamber that permitted 64 simultaneous extracellular recordings in the presence of artificial CSF. Brief single stimulus pulses were applied with bipolar electrodes to the CA3 or CA1 regions of hippocampus. Single pulses triggered epileptiform population events that included HFOs in the 150–250 Hz range in the presence of GABAA receptor blockade or kainic acid. HFOs also occurred spontaneously in the presence of kainic acid. The oscillations had the largest amplitude in the CA3c cell layer, regardless of the drug, and were synchronized throughout the cell layer. AMPA receptor blockade stopped these HFOs, whereas NMDA receptor blockade did not. Gap junction activation did not restore HFOs in the presence of AMPA receptor blockade. Our findings suggest that a population of excitatory neurons in CA3c may be a primary focus of seizure-like activity in Ammon's Horn. We suggest that the interconnection of CA3c is different from the rest of CA3.
Highlights
Field potential oscillations reflect synchronized rhythmic synaptic potentials and/or firing by populations of neurons
Using the laminar characteristic of the rat hippocampus, the rat hippocampal slice model is ideal for studying High frequency oscillations (HFOs) in hippocampus
We found HFOs to occur either after direct electrical stimulation in the presence of GABAA receptor blockade or kainic acid, or spontaneously in the presence of kainic acid
Summary
Field potential oscillations reflect synchronized rhythmic synaptic potentials and/or firing by populations of neurons. High frequency oscillations (HFOs), in some studies referred to as “ripples,” exist in the 80–600 Hz range. It has been proposed that this broad frequency range reflects different kinds of activity, and recent reviews have outlined the possibilities for HFO generation involving synaptic and non-synaptic mechanisms as well as the challenges associated with identification of mechanism in brain [1, 2]. HFOs can be observed in limbic structures and all over neocortex [3,4,5,6] in both pathologic contexts like seizures [7, 8] and in normal contexts such as cognition and sleep [9, 10]. The oscillatory periods tend to be of shorter duration and amplitude on account of the neuronal synchrony necessary to achieve them [11].
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