Traumatic brain injury (TBI) poses a significant socioeconomic burden in the world.
The long lasting consequences in cognitive impairments are often underreported and
its mechanisms are unclear. In this perspective, cholinergic dysfunction and therapeutic
strategy targeting this will be reviewed. Novel agents that can target specific subtype
of acetylcholine receptors have been developed over the recent years and are at various
stages of development, which include AR-R 17779, GTS-21, SSR-180711A, AR-R17779, and
PNU-282987. A detailed review on this topic has been previously published (Shin and
Dixon, 2015).
Cholinergic system is regarded as an important modulator of cognitive function. It
has an important role in learning, memory formation, and attention. Thus, in pathologic
neurodegenerative diseases such as Alzheimer's disease (AD), loss of cholinergic functions
are believed to be an important contributor to cognitive deficits. Similarly, TBI
induces dysregulation of the cholinergic system, and this is believed to be one of
the significant underlying mechanisms of cognitive deficits (Zafonte et al., 2012;
Shin and Dixon, 2015). With recent advancements in pharmacological science, novel
agents that target specific receptor subtypes of the cholinergic system have been
developed (Sun et al., 2013; Barbier et al., 2015; Dineley et al., 2015). Specifically,
α7 nicotinic acetylcholine receptors (nAChRs) have been shown to have a major role
in both the neuronal injury as well as cognitive dysfunction after TBI. Agents that
target these specific receptors are promising potential future targets in both animal
studies and clinical trials.
Acetylcholine transduces signals by muscarinic and nicotinic acetylcholine receptors.
Whereas muscarinic receptors are G-protein coupled receptors, nicotinic receptors
are ligand gated ion channels composed of five subunits. Binding of a ligand will
induce conformational change to an open state, allowing an outflux of K+, and influx
of Na+ and Ca2+ to a minor extent. Various subunits of nAChRs have been characterized
over the years. They form heteromeric combinations of α2–10 and α2–4 subunits and
α7 homopentamers. As previously reviewed (Dineley et al., 2015), α4β2 subtype is a
major nAChR subtype in the brain, whereas α3β4 is a major subtype found in the peripheral
nervous system. Specifically, α4β2 and α7 nAChRs in the basal forebrain acetylcholine
neurotransmission have major roles in cognitive performance.
Nicotinic receptors are found widely throughout the brain, and their contribution
to cognitive function has been tested in several important regions. Specifically,
nAChRs in the hippocampus are involved in the regulation of working memory: blockade
of nAChRs in ventral or dorsal hippocampus leads to decreased performance in radial-arm
maze task (Bancroft and Levin, 2000; Levin et al., 2002). Infusions of nAChR antagonists
to habenula (Sanders et al., 2010) or brainstem (Cannady et al., 2009) also lead to
memory deficits. As shown in
Figure 1
, activation of nAChR leads to complex intracellular signaling pathways leading to
transcription of genes important for memory formation. The activation of nAChRs also
indirectly enhances dopaminergic signaling, as nAChR activation on dopaminergic neurons
would lead to dopamine release in the frontal cortex or striatum (Shin et al., 2012)
which are also important components of cognitive function. Recovery from dopaminergic
deficit after TBI can be indirectly enhanced by nAChR activation as previously shown
(Shin et al., 2012).
Figure 1
Postsynaptic functions of α7 nAChR in the hippocampus.
Intracellular signaling cascades leading to CREB activation for long term memory formation
are depicted. Presynaptic glutamate release as well as α7 nAChR activation will increase
Ca2+ influx. This increase in Ca2+ leads to activation of ERK, CaMKII/IV, and PKA
which all leads to CREB activity. Though activation of ERK can be triggered by the
activity of growth factor receptor tyrosine kinase (RTK), PKA can activate ERK via
RAP1. Intracellular Ca2+ can also activate protein kinase C (PKC) which in turn activates
Ras/Raf-1 cascade. PKA activity is also known to activate Raf-1 in SH-SY5Y cells.
The activation of α7 nAChR subsequently may contribute to enhancement of learning
and memory, among numerous other biochemical pathways that modulates it.
α7 nAChR: α7 nicotinic acetylcholine receptors; CaMKII/IV: calmodulin regulated kinases
II/IV; CREB: cAMP response element-binding protein; ERK: extracellular signal-regulated
kinase; MEK: Methyl ethyl ketone; NMDA: N-methyl-D-aspartate; PKA: protein kinase
A; RAF: rapidly accelerated fibrosarcoma; Rap1: Ras-proximate-1; p90RSK: p90 ribosomal
S6 kinase.
Recent studies have focused on the role of each nAChR subtype in cognitive function.
The α7 receptors are of particular interest due to many studies showing their important
contribution to learning and memory. These α7 receptors are found in the central nervous
system as well as peripheral organs and immune cells. They have been found in various
cell types such as astroglia, microglia, oligodendricytes, and endothelial cells.
Their distribution in the hippocampus as well as high permeability to calcium are
consistent with the fact that animal studies show their important role in enhancing
cognitive performance (Shin and Dixon, 2015). Activation of α7 receptors also modulates
production of inflammatory cytokines (Han et al., 2014).
In vitro studies have shown that cell lines expressing α7 receptors have activation
of signal transduction pathways important for learning and memory. Activation of α7
receptors induces the activation of extracellular signal-regulated kinase ½ (ERK ½)
(Dickinson et al., 2008). ERK is activated by phosphorylation, leading to the activation
of its downstream kinase p90 ribosomal S6 kinase (p90RSK) (
Figure 1
). This subsequently leads to activation of cAMP response element-binding protein
(CREB), which modulates the transcription of downstream genes that formulate learning
and memory. The high permeability of calcium by α7 receptors can also lead to calcium
regulated cascades which activate calmodulin and calmodulin regulated kinases II and
IV (CaMKII, CaMKIV) leading to the activation of CREB. This enhancement of learning
and memory has been demonstrated for AR-R 17779, an α7 receptor agonist as previously
reviewed (Shin and Dixon, 2015). Other α7 receptor activating agents such as GTS-21,
SSR-180711A, AR-R17779, and PNU-282987 have been developed and will be useful future
candidates in TBI research as previously reviewed (Shin and Dixon, 2015). Animal models
of TBI using controlled cortical impact (Dixon et al., 1991), fluid percussion injury
(McIntosh et al., 1987), and weight drop model (Marmarou et al., 1994) are well characterized
methods to study the effects of these agents in the setting of TBI. These models often
used reliable and reproducible tests of working memory such as Morris Water Maze and
tests of motor function such as balance walking, beam walking, and rotarod test (Hamm,
2001). Future studies for these new agents using various models of injury are warranted
to characterize the contribution of each nAChR subtypes.
Compared to other subtypes of nAChRs, α7 subunits are unique entities in the setting
of TBI. In TBI rats studied using quantitative autoradiography, there is a reduction
of α7 receptor binding at wide range of areas, whereas α3 or α4 subtypes showed lower
magnitude of reduction and in fewer regions (Verbois et al., 2002). Asides from their
role in cognitive function, α7 nAChRs are recently noted as possible targets that
can lead to neuroprotective effects in the setting of various acute injuries. Several
studies using agents that activate α7 nAChRs show improvement in memory and survival
of neurons, as well as reduction of inflammatory response to injury (Shin and Dixon,
2015). Microglia expresses α7 nAChRs, and activation of these receptors leads to reduction
of inflammatory cytokine release. The neuroprotective effect of α7 receptor activation
was also shown in the peripheral nervous system. In the setting of sciatic nerve crush
injury, activation of α7 nAChR by PNU-282987 lead to decreased TNF-α level and increased
axonal regeneration (Wang et al., 2015).
Another group of agents, known as positive allosteric modulators (PAM) for nAChRs,
have been developed in the recent years. Unlike agonists that directly bind to and
activate the receptors, PAMs enhance the amplitude of response or increase the duration
of activity when there is a preexisting cholinergic signaling. A newly developed PAM
agent, PNU-120596 was shown to protect against ischemic brain injury and improve motor
function (Sun et al., 2013; Shin and Dixon, 2015). In subarachnoid hemorrhage model
of rats, PNU-282987 improves motor function by reducing inflammation and neuronal
loss whereas α4β2 agonist RJR-2403 does not have this effect. However, this neuroprotective
effect may be injury specific and cell type specific, as SH-SY5Y cells and rat hippocampal
cultures provided with PNU-120596 had decreased viability due to overloading of intracellular
Ca2+ leading to cell death (Guerra-Alvarez et al., 2015).
Another major agent of interest is CDP-Choline, otherwise known as the effect of Citicholine.
It has both α7 nAChRs agonist effect as well as the effect of enhancing neuronal membrane
synthesis. When taken as dietary intake, it is metabolized to cytidine and choline
before resynthesizing into the CDP-choline in liver and other tissues. However, direct
injection of this agent into the local neural tissue can activate α7 nAChRs. It also
reduces apoptosis in AD models of animals and improves cognitive performance in schizophrenia,
such as working memory, verbal learning, verbal memory, and executive function (Knott
et al., 2015). These neuroprotective effects may lead one to expect therapeutic benefit
when applied in TBI. Disappointingly, COBRIT, a multicenter, randomized, double-blind,
place-controlled trial showed no beneficial effect of this agent (Zafonte et al.,
2012). This lack of improvement in cognitive function was attributed to the possibility
that different levels of injury severity may have different responses to this agent.
However, it should also be noted that this agent was taken enterally, and the degree
of direct activation of α7 nAChRs in the central nervous system by this route of administration
is unclear. Clinical trials of α7 enhancing agents DMXB-A, UCI-4083, and TC-5619 are
ongoing in Schizophrenia patients and showed some degree of success in improving cognitive
outcome (Freedman, 2014). However, these agents have not been used in human TBI trials.
Among the many agents mentioned in this article, clinical trials for some of these
agents in TBI will likely take place in the near future.
Asides the agents previously mentioned, pharmacological research still continues in
developing various new drugs that are α7 nAChR specific. These even more novel ligands
that are α7 receptor specific are currently under research investigation to be used
as therapeutic agents or radioligands. Examples are a radioligand [18F]NS10743 (Teodoro
et al., 2014) and α7 nAChR partial agonist Encenicline (Barbier et al., 2015) which
are currently at development stages. Further validation on dosing as well as their
applicability in TBI must be clarified in the next few years to come. However, with
such variety of α7 targeting agents already available for animal studies, some of
these agents may become key players in the long awaited pharmacological treatment
regimen for TBI.
Although there are many beneficial effects of nAChR activation, the possibility of
using nAChR modulating agents must be approached with caution. Prior studies have
shown various side effects of nAChR activation. High doses of nicotine administration
can increase symptoms of anxiety and depression (Newhouse et al., 1988) as well as
increased heart rate and blood pressure. This occurs via nonspecific activation of
nAChRs throughout the body, since nAChRs are found widely in autonomic nervous system
as well as adrenal medulla. Activation of adrenal medulla leads to increase in corticosterone,
epinephrine, and norepinephrine levels in blood (Cryer et al., 1976). Higher doses
can even induce convulsions (Okamoto et al., 1992) and can be lethal (Okamoto et al.,
1994). The synthetic nAChR agonists discussed in this article were designed to have
reduced side effect profiles, with less effect on cardiovascular parameters. Commonly
studied nAChR agonists in neurodegenerative disease and injury models are central
nervous system specific, such as mecamylamine (Shytle et al., 2002).
Moreover, there is a limitation of looking at the functional deficits after TBI only
from the perspective of nicotinic receptor dysfunction. The mechanism of TBI is much
more complex, involving dysfunction of other neurotransmitters, as well as oxidative
stress and inflammation. Dopamine synthesis and release deficit has been characterized
after TBI (Shin et al., 2011), as well as alterations in the postsynaptic regulators
of dopamine neurotransmission (Bales et al., 2009, 2010, 2011). Therapeutic effects
of enhancing other neurotransmitter systems such as the serotonin system, have also
been shown to be beneficial in animal models (Kline et al., 2012; Yelleswarapu et
al., 2012). Also, therapeutic strategies targeting oxidative stress, such as antioxidant
glutathione administration, reduce cell death and inflammation in animals (Roth et
al., 2014) and improve post TBI symptoms (Hoffer et al., 2013). In the midst of complex
interplay of various pathological cascades, targeting only α7 nAChRs may not be effective.
Combination therapies that target multiple pathological mechanisms of TBI have recently
gained interest (Margulies et al., 2015), and the use of α7 nAChR activating agent
in combination with other novel agents targeting multiple pathways may be the most
effective future treatment regimen for TBI.