Biology Reference
In-Depth Information
III.
P
ROPERTIES AND
M
ECHANISMS OF
LTP
AND
LTD
I
NDUCTION IN
LA
P
ROJECTION
N
EURONS
In recent years, important progress has been made in delineating the cellular and
molecular mechanisms of LA-LTP. These data show that there are great similarities in the
induction, maintenance, and expression mechanisms of LTP in the LA to LTP recorded in the
hippocampal area CA1, which has become the prototypical site of mammalian LTP studies.
LTP in the amygdala has been characterized in-vivo using tetanic stimulation of a
number of afferents, including the piriform cortex (Racine et al. 1983), the medial geniculate
body (Clugnet and LeDoux 1990), and the hippocampus (Maren and Fanselow 1995). In
anesthetized rats LA-LTP can be induced by theta burst stimulation (TBS) of thalamic
afferents (Yaniv et al. 2001). LA-LTP can be also induced in freely moving animals (Doyere
et al. 2003), where it has been shown that LA-LTP at cortical inputs exhibited the largest
change at early time points (24 h) but faded within 3 days. In contrast, LTP at thalamic inputs,
though smaller initially than cortical LTP, remained stable until at least six days. Similarly, in
freely behaving Wistar rats the BLA is able to sustain entorhinal cortex-induced LTP for
seven days (Vouimba et al. 2004).
A. Afferent pathways that support LTP and LTD in the lateral amygdala
Although cellular mechanisms of LA-LTP have been nearly exclusively investigated in
coronal brain slices (Figure 1B), LA-LTP was first characterized in-vitro in horizontal brain
slices (Chapman et al. 1990) by stimulating fibers running through the external capsule (EC)
(Figure 1A). In horizontal brain slices, EC stimulation activates excitatory afferents from
cortical structures, including the lateral entorhinal and perirhinal cortices, that course through
the EC and synapse in the LA and the BLA (von Bohlen und Halbach and Albrecht 2002). In
contrast to EC stimulation, stimulation within the LA (intranuclear stimulation site) not only
causes stable LA-LTP but also reliable LA-LTD (Drephal et al. 2006; Kaschel et al. 2004). It
can be suggested that, in addition to activation of cortical fibers, the intranuclear stimulation
also activates local connections within the LA and afferents from other amygdaloid nuclei.
These connections are preserved in horizontal brain slices (von Bohlen und Halbach and
Albrecht 1998c).
In coronal brain slices synaptic responses are either elicited by stimulation of thalamic
fibers (Fendt and Schmid 2002; Lee et al. 2002; Schafe et al. 2000; Weisskopf et al. 1999) or
by stimulation of EC fibers (Abe et al. 1996; Lin et al. 2003a; Schroeder and Shinnick-
Gallagher 2004; Tsvetkov et al. 2002), which in coronal slices contains amygdala afferents
from higher-order sensory cortices (deOlmos et al. 1985). Much of the work in coronal slices
has involved the auditory modality (Medina et al. 2002), whereas findings concerning the
somatosensory and visual afferents are rare.
We have learned that LA-LTP exhibits the main properties, initially described for
hippocampal neurons: rapid induction, input specificity, cooperativity and associativity. The
characteristics of LTP, cooperativity, associativity, input specificity
as well as the durability
or persistence of LTP have been identified as solid arguments that support the hypothesis that
LTP may be a biological substrate for at least some forms of memory (Lynch 2004). Once
