Experimental design for recording activity-dependent synaptic changes at the CA3-CA1 synapse during classical eyeblink conditioning.
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(A) Location of recording (left) and stimulating (right) electrodes implanted chronically in the CA1 and CA3 areas respectively. (B) Animals were implanted with electromyographic (EMG) recording electrodes in the orbicularis oculi (O.O.) muscle, and with stimulating electrodes on the supraorbital nerve for presentation of unconditioned (US) stimuli. Conditioned stimulus (CS) consisted of a tone preceding the US by 500 ms. Animals were also implanted with recording (Rec.) electrodes in the CA1 area and with stimulating (St.) electrodes at the ipsilateral Schaffer collaterals. (C) Example of fEPSP evoked at the CA3-CA1 synapse in an 18-month-old Polµ−/− mouse (1) and eyeblink evoked in the O.O. muscle by the electrical stimulation of the supraorbital nerve (2). (D) The top two traces illustrate the trace conditioning paradigm, and the moment at which a single pulse was presented to Schaffer collaterals (arrow, St. Hipp.). Samples of EMG activity of the O.O. muscle and hippocampal extracellular activity collected from the 9th conditioning session from an animal of each experimental group (1–4). Calibrations in (1) are for all of the records. Note fEPSPs evoked by the pulse presented to Schaffer collaterals (bend arrows).