2.2 Extracellular recording of synaptic transmission and
plasticity
A slice was transferred to the recording chamber, submerged in aCSF and
perfused at 4-6 ml/min (32°C). The slice was placed on a grid allowing
perfusion above and below the tissue and all tubing (Tygon) was gas
tight (to prevent loss of oxygen). To record field excitatory
postsynaptic potentials (fEPSPs), an aCSF-filled microelectrode was
placed on the surface of stratum radiatum in CA1. A bipolar concentric
stimulating electrode (FHC) controlled by an isolated pulse stimulator
model 2100 (AM Systems, WA) was used to evoke fEPSPs at the Schaffer
collateral–commissural pathway. Field (f)EPSPs were evoked every 30 s
(0.03 Hz). Stimulus input/output curves for fEPSPs were generated using
stimulus strength of 1-5 V for all slices (stimulus duration 200 µs).
For the synaptic plasticity experiments, the stimulus strength was set
to produce a fEPSP slope ~ 40 % of the maximum response
and a 20-minute baseline was recorded before plasticity induction.
Paired-pulse facilitation was measured over an interval range of 20 to
500 ms. Recordings of mGluR-LTD were made in the presence of 50 µM
picrotoxin to block GABAA receptors (Sigma) and the NMDA
receptor antagonist L-689,560
(trans-2-carboxy-5,7-dichloro-4-phenylaminocarbonylamino-1,2,3,4-
tetrahydroquinoline; 5 µM; Tocris) with area CA3 removed from the
slices. To induce mGluR-LTD, a paired-pulse low frequency protocol was
used (PP-LFS) consisting of 900 paired pulses (50 ms interval) delivered
at 1 Hz (Oliet et al. , 1997).
Long-term potentiation was induced by two protocols: high frequency
stimulation (HFS 100 stimuli in 1s, 100 Hz) and theta burst stimulation
(TBS, 4 trains separated by 20s with train consisting of 10 bursts of 4
pulses at 100 Hz, separated by 200 ms as in Kyrke-Smith et al., 2021).
For LTP priming the CA3 region was removed from slices and 20 µM
(RS)-3,5-DHPG (3,5-dihydroxyphenylglycine, Tocris) was applied for 10
minutes and then washed for 20 minutes after which LTP was induced by
HFS. Signals were filtered at 3 kHz and digitised on-line (10 kHz) with
a Micro CED (Mark 2) interface controlled by Spike software (Vs 6.1,
Cambridge Electronic Design, Cambridge UK). The fEPSP slope was measured
from a 1 ms linear region following the fibre volley.