Electrophysiology and Morris Water Maze to Assess Hippocampal Function After Experimental Subarachnoid Hemorrhage - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 395

species ( 6 ). This belief is further strengthened by observing that
memory is impaired as a result of hippocampal damage ( 7 ).
Subarachnoid hemorrhage (SAH) causes high mortality and
signifi cant cognitive defects in human survivors. Different types of
memory are affected by SAH, including visual and spatial aspects
( 8 ). Few studies have, however, assessed hippocampal function,
either electrophysiologically or behaviorally in animal models of
SAH. This chapter focuses on some of the many methods of
assessment of hippocampal function after SAH in rodent models.

2. Assessment
of Hippocampal
Function at the
Neuronal Level
Using Electro-
physiology

Here, we mainly describe the method of fi eld potential recording
of synaptic transmission and plasticity in the hippocampus follow-
ing SAH. After the pioneering use of microelectrodes in the 1940s,
Cragg and Hamlyn were among the fi rst to record fi eld potentials
from the hippocampus in 1955 ( 9 ). Long-term potentiation (LTP)
in the synaptic transmission of the mammalian central nervous sys-
tem was described in 1973 by Bliss and colleagues ( 10, 11 ). LTP
in the hippocampus is a cellular mechanism that is believed to
underlie memory and learning because its induction, duration, and
mechanism closely mimic many properties associated with learning
and memory ( 12, 13 ). LTP is a long-lasting enhancement in the
synaptic strength between neurons that can be demonstrated electro-
physiologically after tetanic stimulation. Changes in the ability of
neurons to sustain the enhanced transmission may lead to loss
of LTP. Loss of LTP was observed in rats after SAH ( 14 ). This
loss of LTP might be an experimental equivalent of learning and
memory impairment in patients with SAH.
Extracellular fi eld recordings are an effi cient way to assess
hippocampal function by examining LTP in the hippocampal
Schaffer collateral (discussed in this chapter) or mossy fi ber and
perforant fi ber pathways. The main advantage of using extracellular
potential recordings on a laminated structure, such as the hippocam-
pus, are that the dense packing of neuronal cell bodies, roughly
parallel positioning of their apical dendrites and their synchronous
activation after stimulation gives an accurate index of cellular
activity with regards to amplitude, time, and polarity ( 15 ). In addi-
tion, it is a sensitive method to study excitatory and inhibitory
synaptic effects and to analyze pre- and postsynaptic components
of neurotransmission. The disadvantage of using extracellular
recordings is the low signal-to-noise ratio and therefore the use is
limited to fi eld action potentials and facilitation of excitatory post-
synaptic potential (EPSP) analysis. Single excitatory or inhibitory
subthreshold potentials or membrane potential oscillations cannot
be detected using this method ( 16 ).