Research PaperGABAergic interneuronal loss and reduced inhibitory synaptic transmission in the hippocampal CA1 region after mild traumatic brain injury
Introduction
Mild traumatic brain injury (mTBI) is a brain trauma that results in the disruption of brain function. mTBI is characterized by one or more of the following: a) loss of consciousness up to 30 min; b) posttraumatic amnesia up to 24 h; c) alterations in consciousness (i.e. dazed feeling, disorientation up to 24 h); d) transient neurological dysfunction such as a seizure and an intracranial lesion that does not require surgical intervention; and e) a Glasgow coma scale of 13–15 when performed at least 30 min after initial injury (Borg et al., 2004a, Borg et al., 2004b, Carroll et al., 2004, Cassidy et al., 2004, Peloso et al., 2004). Occurring in more than 80% of all head trauma cases (Moore et al., 2006), mTBI patients commonly report acute memory and concentration problems, and other deficits in learning and memory, all of which can have devastating effects on patients and families (Gasquoine, 1997, Miotto et al., 2010, Moore et al., 2006, Rimel et al., 1981, Stuss et al., 1985).
Recent evidence from human and animal studies suggests that a single brain injury may cause acute changes in learning and cognition, though the mechanism by which this occurs remains poorly understood. Moreover, functional changes that occur after a mTBI may be exacerbated by multiple head injuries (Aungst et al., 2014, Prins et al., 2013, Shultz et al., 2012, Tavazzi et al., 2007, Vagnozzi et al., 2007, Vagnozzi et al., 2008). Indeed, compared to athletes that have experienced only a single brain injury, athletes experiencing three or more concussions display more severe symptoms, long-term cognitive deficits, and an increase in mood disorders, including depression (Guskiewicz et al., 2003, Guskiewicz et al., 2005, Guskiewicz et al., 2007). Thus, to understand why cognitive and neurological deficits are more severe after a repeated trauma, it is essential to first identify the functional and morphological deficits that take place after the initial injury in brain regions essential to learning and memory.
The hippocampus plays a major role in learning and memory, and is particularly susceptible to mTBI-related injury due to its anatomic location (Hicks et al., 1993, Kotapka et al., 1991, Umile et al., 2002). Following mTBI there is rarely overt morphological damage in the hippocampus (Bigler and Maxwell, 2012, Vos et al., 2012); stereological analysis of animals exposed to mTBI do not reveal a significant loss of hippocampal neurons (Almeida-Suhett et al., 2014a, Eakin and Miller, 2012). However, functional impairments to the hippocampus after mTBI may be associated with learning and memory deficits (McDonald et al., 2012). Indeed, studies suggest that up to 75% of patients suffering from mTBI may have functional abnormalities in the medial temporal lobe, including the hippocampus (Umile et al., 2002), and that alterations in hippocampal function and excitability after mild brain injury may be present in the absence of clear morphological damage (Eakin and Miller, 2012, Greer et al., 2012, Griesemer and Mautes, 2007, Reeves et al., 1995, Reeves et al., 2000).
Changes in hippocampal excitability and memory deficits observed after TBI may result from reductions in GABAergic inhibitory synaptic transmission (Gupta et al., 2012, Mtchedlishvili et al., 2010, Pavlov et al., 2011, Raible et al., 2012, Witgen et al., 2005) or alterations in N-methyl-d-aspartate (NMDA) receptor function (Schwarzbach et al., 2006). Indeed, fluid percussion injury, which leads to mild to moderate injury, reduces NMDA receptor mediated currents (Schwarzbach et al., 2006), while severe TBI leads to reductions in GABAA-mediated inhibitory synaptic transmission in dentate granule cells (DGCs) in rats (Mtchedlishvili et al., 2010, Pavlov et al., 2011) and mice (Witgen et al., 2005). Thus, while moderate to severe TBI is known to reduce GABAergic inhibition and impair NMDA receptor mediated currents, it remains unknown whether functional alterations to the GABAergic and glutamatergic systems occur in animals that receive a mild brain injury in the absence of overt morphological deficits. More specifically, it remains unknown how mTBI, using the controlled cortical impact (CCI) model, alters the activity of GABAergic inhibitory synaptic transmission and NMDA receptor function in the CA1 region of the hippocampus.
The purpose of the current study is to investigate the functional and morphological alterations in the CA1 hippocampal region that lead to acute cognitive deficits after mTBI. Our studies focus on the CA1 region as it is the major output within the hippocampal trisynaptic circuit and alterations in this region may have an impact in memory function (Daumas et al., 2005, Ji and Maren, 2008, Karasawa et al., 1994, Stubley-Weatherly et al., 1996, Vago et al., 2007, Zola-Morgan et al., 1986). We found that within 7 days of receiving a mild brain injury induced by a CCI, animals displayed significant cognitive deficits and an inability to induce long-term potentiation (LTP) of synaptic transmission. It is likely that the inability to induce LTP was a result of impaired activation of NMDA but not AMPA receptors. In addition, GABAA receptor mediated inhibitory synaptic transmission was also impaired 7 days after CCI. Alterations in inhibitory synaptic transmission were due to a delayed loss of GABAergic interneurons and alterations in the surface expression of the α1, β2, or γ2 GABAA receptor subunits, which comprise the majority of GABAA receptors (Möhler, 2006).
Section snippets
Animals
Experiments were performed on male Sprague–Dawley rats (Taconic Farms, Rockville, MD, USA), 5–6 weeks old, weighing 150–200 g at the start of the experiments. Animals were housed paired until the day of the surgery and then housed individually in an environmentally controlled room (20–23 °C, 12-h light/dark cycle, lights on at 6:00 AM), with food (Harlan Teklad Global Diet 2018, 18% protein rodent diet; Harlan Laboratories; Indianapolis, IN) and water ad libitum. Cages were cleaned weekly and animal
mTBI induces memory deficits in the passive avoidance test
Rats were exposed to mild TBI and then tested for memory impairments in the passive avoidance context. Prior to surgery all animals were trained in the passive avoidance apparatus and their latency to cross to the dark chamber was recorded. There were no significant differences in latency to cross to the dark chamber between the sham (21.73 ± 3.7 s, n = 16) and CCI groups (20.05 ± 4.7 s, n = 16; P = 0.78) during the training session. However, CCI rats showed a significantly lower latency to cross to the
Discussion
Mild traumatic brain injury can lead to cognitive and neuropsychiatric impairments in humans despite the absence of clear structural damage. We show, for the first time, that a single, mild brain injury leads to functional deficits in GABAergic and glutamatergic synaptic transmission in the CA1 hippocampal region, which may contribute to the cognitive deficits observed after a mild TBI. In addition, a cellular mechanism underlying learning and memory, LTP of synaptic transmission, is also
Conclusions
The results from this study demonstrate that a single mild brain injury leads to cognitive deficits including impaired mechanisms associated with learning (LTP of synaptic transmission). Our data confirm that there is no significant difference in AMPA receptor mediated excitatory synaptic transmission after mTBI, but that within 7 days of mTBI, there is a significant decrease in NMDA receptor mediated currents. Moreover, we found significant deficits in GABAA receptor mediated inhibitory
Disclosures
No conflicts of interest, financial or otherwise, are declared by the author(s).
Acknowledgments
We gratefully acknowledge Dr. Cara Olsen for statistical assistance.
Grants
The authors acknowledge the Department of Defense in the Center for Neuroscience and Regenerative Medicine for financially supporting the present work. Grant# G1702Z. URL of funder's website: http://www.usuhs.mil/cnrm/. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Author contributions
A.M.M., Z.L., L.E.E., and M.F.M.B. acquired funding for the
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Authors contributed equally to this work.