Stress-induced reinstatement of Fix-C and Esc-C memory

Vulnerability to relapse following prolonged periods of abstinence presents a
major challenge to combating drug addiction. Stress is an unavoidable part of life and a
major contributor to relapse to drug use. However, a thorough understanding of the
neural mechanisms that sub-serve stress-mediated relapse is lacking. In Chapter 4, the
contribution of different signaling molecules to stress-induced reinstatement of Fix-C and
Esc-C CPP was investigated. While antagonism of NMDAR and inhibition of nNOS
effectively attenuated forced swim-induced reinstatement of Fix-C CPP, these
manipulations had no effect on Esc-C CPP (Fig. 4.1 and 4.3). Thus, like the acquisition
and reconsolidation of Fix-C memory, stress-induced reinstatement of Fix-C memory is
NO-dependent while Esc-C memory is NO-independent. My studies add to the list of
signaling molecules that play a role in stress-induced reinstatement of Fix-C CPP.
However, none of the test drugs investigated that successfully attenuated stress-induced
reinstatement of Fix-C was effective against Esc-C CPP. Therefore, my studies point to
the existence of additional signaling molecules that contribute to stress-induced
reinstatement of Esc-C CPP.
Proposed model for the development of Fix-C and Esc-C memory
Figure 5.1 proposes a model for the contribution of different signaling pathways
to the formation of Fix-C and Esc-C memory. Fix-C and Esc-C memory results from
increased protein expression levels of NR2B subunit of the NMDAR. However, NR2B is
markedly elevated in mice conditioned by Esc-C compared to mice conditioned by Fix-C.
NR2B-containing NMDARs allow greater calcium entry thus elevated NR2B levels in
Esc-C conditioned mice allow for increased calcium influx upon NMDAR activation by
glutamate. My findings show that Fix-C memory acquisition, reconsolidation and stressinduced
reinstatement can be blocked by inhibiting nNOS but Esc-C memory remains
unperturbed. With respect to the Fix-C model, calcium influx activates calmodulin which
mediates nNOS-induced increases in NO levels. NO stimulates soluble guanylate cyclase
(sGC) which leads to cGMP-mediated activation of protein kinase G (PKG) which
subsequently contributes to the phosphorylation of ERK. With respect to Esc-C memory,
evidence suggests that the NR2B subunit of NMDAR has potential to carry greater
calcium current per unit charge (Sobcyk et al., 2005) which may confer a greater
influence on downstream signaling cascades that affect synaptic plasticity and learning
and memory such as the NMDAR-RasGRF1-MEK-ERK pathway (Krapivinsky et al.,
2003). Since RasGRF1 specifically binds the NR2B subunit of the NMDAR, it couples
the activity of ERK with NR2B-containing NMDARs (Krapvinisky et al., 2003). My
studies show that inhibition of MEK, the ERK kinase, disrupted reconsolidation of Esc-C
memory but had no effect on Fix-C memory (Fig. 2.5). Thus the MEK-ERK pathway
plays a role in Esc-C memory. While the nNOS signaling pathway may also be activated
in response to training by Esc-C, it appears that other signaling pathways including
NMDAR-MEK-ERK signaling plays a more behaviorally significant role in the
development of Esc-C CPP. Additionally, though both NO-cGMP-PKG and MEK
signaling pathways converge at the level of ERK (Ota et al., 2008) it is conceivable that
the contribution of each pathway to drug memory is dependent on cocaine conditioning
schedule. The differential activation of ERK could result in different degrees of
activation of molecules downstream of ERK including cAMP response element binding
protein (CREB). CREB is a known mediator of synaptic plasticity and the generation of
new synapses through upregulation of gene expression which subsequently contribute to
memory strength. Thus increased phosphorylation of CREB (pCREB) provides greater
propensity for rapid metaplasticity to strengthen drug-associated synapses associated with
‘strong’ Esc-C memory.

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