Gamma oscillations induced by kainate receptor activation in the entorhinal cortex in vitro Mark O. Cunningham1, Ceri H. Davies2, Eberhard H. Buhl1 & Miles A. Whittington1 1 School of Biomedical Sciences, Worsley Building, University of Leeds, Leeds LS2 9NQ, UK2 GlaxoSmithkline, Harlow, Essex CM19 5AW, UK

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Networks of neurones in mammalian cortex have the propensity to adopt synchronous oscillations in the gamma frequency range (~20-80 Hz) due to definite stimuli. The appearance of these oscillations during sensory stimulation has led to the suggestion that they are involved in cognitive functions such as memory formation and recollection. The medial entorhinal cortex (mEC) appears to be crucial in the formation and consolidation of long term memories. Previously, in vivo experiments have demonstrated that the entorhinal cortex can generate gamma frequency oscillations in response to various stimuli (Chrobak et al., 2000) We can now report that application of the specific AMPA/kainate receptor agonist kainic acid induces gamma activity in the mEC.

Combined entorhinal-hippocampal slices (~450 µm), were taken from adult (200-250 g) Wistar rats after terminal anaesthesia using ketamine-xylazine (administered intramuscularly), and intracardial perfusion with artificial cerebrospinal fluid (ACSF) in which NaCl was replaced with sucrose. Slices were maintained at an interface of oxygenated ACSF and humidified 95%0₂:5%CO₂gas at 36°C. Extracellular field recordings were made in the mEC, using glass microelectrodes containing ACSF (resistance <3 M). Intracellular recordings were made neurones, using glass microelectrodes (resistance 100-120 M) containing 1.5 M KCH₃SO₄.

Gamma activity was evoked by the bath perfusion of the AMPA/kainate receptor agonist kainic acid (200-400 nM) alone. Once initiated the oscillations were stable for several hours. Gamma oscillations were observed across all layers of the mEC, however there was little or no gamma activity in the lateral EC and/or perirhinal cortex. Whilst activity within layer (<1mm) was highly synchronous and with no apparent phase lag, activity recorded across superficial and deep layers exhibited a 180° phase angle shift. A laminar profile of the mEC demonstrated a marked phase shift occurring between layer II and layer III. Across the mEC, the power of the gamma activity peaked at layer III (74.7±23.9 µV²/Hz), whilst the frequency (42.3±2.1Hz) was consistent. Bath application of the GABA_a antagonist bicuculline (2µM), consistently and reversible blocked all gamma activity, suggesting a crucial role of GABA_A receptors in generating this activity. Gamma activity was also concurrently in the hippocampus and the subiculum. In order to ascertain if gamma activity in the mEC was dependent on activity propagating from these areas, a number of lesions were carried out. Lesions separating the mEC from first, the hippocampus and then the subiculum had no significant effect (P>0.01, Student's t test) on the activity seen in the mEC.

Thus, the mEC can initiate and maintain gamma activity independent of input from other limbic structures. Therefore, we can conclude that a modest increase in tonic excitatory drive causes the emergence of gamma oscillations in the mEC in vitro.

Chrobak J.J., Lorincz A. & Buzsaki G. Hippocampus 10, (4): 457-65 (2000).