Jonny Daborg
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channel to open, it requires glutamate, which is released from the presynaptic
terminal, in conjunction with postsynaptic depolarisation which is needed to
relieve the channel from of a magnesium ion that blocks the channel at
resting membrane potential. Hence, if the NMDAR is located in the
postsynaptic membrane, the channel only opens when the two neurons that
are connected via the synapse, are simultaneously active. Moreover, the
NMDAR is permeable to calcium ions, and these work as intracellular second
messengers, signalling that the synapse should be reinforced. There has been
some debate as to whether the strengthening is of a pre- or postsynaptic
locus, and the answer seems to be both, mainly depending on which synapse
the investigator is examining. In the hippocampal CA3/CA1 synapse, most
researchers agree that LTP is expressed as an increased number of glutamate
receptors of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
receptor (AMPAR) type in the postsynaptic membrane.
LTP is balanced by the process of long-term depression (LTD). The function
and mechanisms of LTD is less well understood, but it appears as if at least
some forms of LTD are dependent on the NMDAR. This is a puzzling
feature, but it is generally believed that LTP requires a robust and temporally
precise activation of the NMDARs, whereas an intermediate or scanty
activation leads to LTD (Malenka and Bear 2004).
Other kinds of synaptic plasticity include various forms of short-term
plasticity and homeostatic plasticity. Short-term plasticity is usually of a
presynaptic nature, affecting release probability in various ways (Zucker and
Regehr 2002). Probably these phenomena aid in the processing of
information, rather than storage of it. Paired pulse plasticity is perhaps the
most familiar form of short-term plasticity; it is frequently used to investigate
release probability, and the common conception is that residual calcium after
the first pulse increases the release probability of the second pulse and
therefore giving rise to a greater postsynaptic response. This occurs in
synapses with a low release probability and is called facilitation; when the
second pulse generates a smaller postsynaptic response, the phenomenon is
called depression, and this occurs in synapses with a high initial release
probability.
The aforementioned forms of synaptic plasticity affects specific synapses to
enable processing and storage of information. Homeostatic plasticity on the
other hand, is thought to globally affect synaptic transmission, thereby
keeping the relative differences in synaptic efficacy intact (Turrigiano 2008;
Vitureira et al. 2011).
Synaptic elimination and the complement system in Alzheimer’s disease
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1.2.3
Synaptogenesis and synaptic elimination
The generation of functional neuronal networks during brain development is
based on an extensive iteration of synaptogenesis (the formation of new
synapses) and synaptic elimination (Hua and Smith 2004). Since generation
and elimination of synapses is relatively fast, occurring on a scale of minutes
to hours, and extensive generation/elimination of synapses is thought to
continue up until puberty (Hua and Smith 2004; Bourgeois et al. 1994;
Bourgeois and Rakic 1993; Zecevic and Rakic 1991), the evolving neuronal
networks are expected to have the possibility of testing the functionality of a
vast number of combinations. In this scenario, a disturbance of synaptic
elimination is expected to decrease the functionality of the synaptic networks,
since it would force the brain to keep the synaptic connections that form,
regardless of functionality. Accordingly, faulty regulation of synapse
numbers has been implicated in the aetiology of several psychiatric disorders
(Penzes et al. 2011). Schizophrenia and autism spectrum disorders probably
have aetiologies of a developmental origin, whereas AD could be the result of
a faulty reactivated developmental synaptic elimination (Penzes et al. 2011;
Wasling et al. 2009).
Synaptogenesis, which would be the positive regulation of n, has been quite
extensively studied (Christopherson et al. 2005; Eroglu et al. 2009;
Kucukdereli et al. 2011; Terauchi et al. 2010). However, as in most
biological systems, the parameter n is also under negative regulation.
Although this process is far less well understood, it has been shown that
synaptogenesis is limited by proteins in the Nogo receptor family (Wills et al.
2012), and recent efforts have begun to unravel the mechanisms of synaptic
elimination in the cerebrum.
There are good reasons to believe that LTP and LTD are involved in synaptic
elimination. Since the functional synapses are those that participate in
synchronous firing, and this circumstance also leads to potentiation of them,
it should be expected that LTP protects from elimination, perhaps indirectly,
by reducing the probability of LTD (Peineau et al. 2007) – which on the other
hand, is often the consequence of asynchronous firing, thus, affecting
synapses with little value in the network. In accordance, it has been shown
that LTD can induce elimination of synapses (Nagerl et al. 2004; Shinoda et
al. 2005; Kamikubo et al. 2006; Bastrikova et al. 2008; Becker et al. 2008).
Interestingly, it was recently shown that freezing behaviour after fear
conditioning correlated with spine elimination in the frontal association
cortex of mice (Lai et al. 2012). Excitatory synapses are often localized to