Biology Reference
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yield a pH of 7.4. The osmolality should be in the range of 290-
300 mOsm. Concentrations of K
+
([K
+
]), Ca
2+
([Ca
2+
]), and Mg
2+
([Mg
2+
]) may vary from laboratory to laboratory but should be
used consistently in one set of experiments since even minor
changes alter cell and network excitability and synaptic transmis-
sion (
13-15
). The brain is then glued onto a support block of a
vibrating-blade tissue slicer (e.g., Vibroslice, Campden Instruments,
Loughborough, UK; Leica VT1000S, Germany) using cyanoacry-
late glue. Acute slices of 300-400
m thickness should be prepared
(
16
). For our SD experiments, we obtain coronal neocortical slices
from the somatosensory cortex. Slices can be transferred to a
temperature-controlled interface-type recording chamber or a sub-
merged system and perfused with (bathed in) prewarmed (~35°C)
carbogenated aCSF in a humifi ed atmosphere of 95% O
2
/5% CO
2
.
We prefer the interface-type (
17
) chamber over the submerged-
type chamber (Oslo type) because it provides more mechanical sta-
bility. In our laboratory, brain slices rest on slips of lens paper
allowing aCSF fl ow beneath the slices. Before starting the record-
ings, the tissue is given time to recover from the preparation pro-
cedure for approximately 1 or 2 h which was shown to reduce
neuronal and mitochondrial swelling (
18
) and promote recovery
of synaptic transmission (
19, 20
). To test the viability of the slices,
we record fi eld potentials in neocortical layers II/III after stimula-
tion at the boundary of layer VI to the subjacent white matter
using a bipolar platinum or tungsten stimulation electrode (single-
pulse duration 100
μ
s, pulse amplitude of 5-15 mV). Slices are
accepted for further investigation if the fi eld potential is at least
1.5 mV in amplitude.
μ
3. Electro-
physiological
Recordings
and Measurement
of Ions
Extracellular recordings are frequently used to study properties of
hippocampal and neocortical cell populations, including neuronal
fi ring and synaptic responses. Combined ion-selective/reference
microelectrodes allow for measurements of extracellular ion con-
centrations. Extracellular recordings can be employed to study the
“slow” large DC potential shift of up to −30 mV associated with
SD or “fast” fi eld potentials from groups of nerve cells in response
to synaptic or antidromic stimulation. The negative DC shift is
often preceded by burst discharges, representing synchronized
neuronal fi ring (
21
), particularly apparent when recorded in the
hippocampal formation (
22
). Notably, the direction of current
fl ow during SD is opposite to that observed during seizure-like
events (
23
). Extracellular measurements of ions and pH during SD
reveal changes of unparalleled magnitude: [K
+
]
o
increases from 3
up to 60 mM accompanied by a precipitous drop in [Na
+
]
o
from
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