Characterizing the Hippocampal Dentate Gyrus during Recent and Remote Trace Eyeblink Conditioning
In order to develop effective treatments for amnesia associated with dementia and Alzheimer's disease, the neural mechanisms underlying long term memory storage need to be better understood. The hippocampus is a brain region that has been widely studied for its role in memory processes given that it is often the area of the brain that is damaged in the amnesic condition. Currently, the role of the hippocampus during the consolidation of declarative memories is highly contested. One view is that the hippocampus plays a time-limited role in memory consolidation; it is primarily active during initial stages of memory acquisition, but these memories later become independent of the hippocampus as they are transferred to areas of the neocortex for long-term storage. In contrast, other theories contend that the hippocampus plays an active role in both memory acquisition and remote recall. Most of the evidence supporting these contested theories has been based on lesion studies that do not provide insight into the normal physiological activity of the intact brain. Further, these theories view the functional role of the hippocampus as one homogenous structure, while in reality, multiple subregions and independent signaling pathways exist in the hippocampus. Thus, how the activity of neurons within distinct subregions of the hippocampus changes over successive stages of memory remains unclear. This study used a trace eyeblink conditioning associative learning paradigm to characterize single neuron and local field potential activity of neurons within the dentate gyrus subregion of the hippocampus during both acquisition and remote recall of a consolidated memory. Results suggest that dentate gyrus activation is most prominent during periods of memory acquisition as opposed to remote memory recall. These findings are compared with other literature in the field to discuss a more holistic view of the hippocampus during memory acquisition and retrieval.
Introduction and Literature Survey
A. Types of Memory
While memory is often thought of merely as the recall of an experience or process, it actually is the amalgamation of several entities (Squire 2004). Memory encompasses the processes by which information is encoded, consolidated, and later retrieved. The process of encoding involves the acquisition of sensory information from the external world and conversion of those signals into a form that can be handled by the brain. Memory consolidation is the process by which acquired memories are then stabilized as long-term memory traces so that they can be later retrieved. Memories are divided into two general categories: nondeclarative and declarative (Figure 1). Nondeclaractive memories are memories that can be recalled without conscious awareness. A prime example of nondeclarative memory is procedural memory, for example the muscle memory for knowing how to ride a bike. In contrast, declarative memories refer to memories that can be consciously recalled, such as episodes of experiences and facts (Eichenbaum 1997, Squire 2004). Declarative memories are further classified as either episodic memories, which entail the recall of events or specific experiences (e.g. what, when, where), or semantic memories, which is the recall of general facts and knowledge (Eichenbaum 1997).
Many studies have examined the mechanisms underlying systems level memory consolidation, which involves the gradual stabilization of neural signals within distinct brain regions as new memories are consolidated into long term ones, and have found that the medial temporal lobe and cortical brain regions are involved in the formation, storage, and recall of declarative memories (Eichenbaum 1997, Frankland and Bontempi 2005). However, how exactly these regions work together and the sequence in which they are activated remain to be fully understood.
B. Standard Consolidation Theory
One prominent theory of memory acquisition and retention is the standard consolidation theory (SCT) which contends that different regions of the brain are preferentially active at certain stages of memory acquisition and consolidation (Frankland and Bontempi 2005). According to SCT, memory acquisition initially depends on networks in the medial temporal lobe, such as the hippocampus. However, hippocampal networks serve only as a temporary store for new information. As new memories are reorganized and consolidated into long term ones, hippocampal activation leads to changes in synaptic activity that distribute memories to areas within the neocortex, such as the medial prefrontal cortex (Figure 2). Eventually, the retrieval of stored memories becomes a process dependent only on neocortical networks and independent of hippocampal networks (Frankland and Bontempi 2005, Winocur et al. 2010).
C. Multiple Trace Theory
Alternate theories of memory consolidation have emerged in which the hippocampus is posited to not have a time limited role, but rather is activated during both memory acquisition and consolidation. Multiple trace theory (MTT) is one model that argues for the continued role of the hippocampus during the retrieval of episodic memory details (McKenzie and Eichenbaum 2011, Moscovitch and Nadel 1998). MTT proposes that episodic memories are initially stored in hippocampal traces. Reactivations of these memories lead to the development of multiple traces that extend to cortical areas. However, these neocortical traces primarily contain semantic memory content. Thus, the recall of remote memories rich in episodic detail continues to remain dependent on the hippocampus, a phenomena discordant with the tenets of SCT (McKenzie and Eichenbaum 2011, Winocur et al. 2010).
D. Evidence for Standard Consolidation Theory and Trace Eyeblink Conditioning
Animal models using lesion studies and pharmacological manipulations as well as human case studies have provided support for SCT. In the notable case of human patient H.M., bilateral removal of his hippocampus in an effort to treat epilepsy led to symptoms of anterograde and temporally graded retrograde amnesia, which are the inabilities to form new memories and to recall remotely acquired, but not recently acquired memories (Frankland and Bontempi 2005). This is consistent with the idea that hippocampal networks serve as temporary stores of information during acquisition of new memories, but have limited roles in memory retrieval.
Observations from trace eyeblink conditioning (EBC) studies have also supported SCT. Trace EBC is an associative learning paradigm which consists of noncontiguous paired presentations of a conditioned stimulus (CS), such as an auditory tone or whisker vibration, with an unconditioned stimulus (US), such as an eyeblink inducing corneal airpuff. After repeated CS-US presentations, subjects learn that the CS precedes the US and will eventually elicit a conditioned response (CR), for example a learned blink of the eye, in response to the CS alone (Kronforst-Collins and Disterhoft 1998). Kim et al. (1995) and Takehara et al. (2003) showed that animals with hippocampal lesions were impaired from acquiring a trace EBC paradigm, but remote retention of the paradigm was not significantly impaired. Further, lesions in the medial prefrontal cortex disrupted retention, but not acquisition of the paradigm, observations consistent with SCT (Takehara et al. 2003).
However, most of the current evidence for theories of memory acquisition and consolidation has originated from lesion studies, which are unable to describe the normal physiological activity occurring inside an intact brain. No study to date has examined in vivo hippocampal activity during both acquisition and remote retrieval of memory. Moreover, it remains unclear whether individual regions of the hippocampal formation, including the dentate gyrus (DG), CA1, and CA3 are uniformly or differentially active during memory acquisition and consolidation.
E. Anatomy of the Hippocampus
Anatomical connections within the hippocampus are arranged such that signals are transferred to the DG region of the hippocampus from the entorhinal cortex via the perforant pathway, positioning the DG as an input station into the hippocampus. From the DG, signals are then transferred to the CA3 via mossy fibers and subsequently to the CA1 via Schaffer collaterals, composing the trisynaptic, or perforant, pathway (Neves et al. 2008). From there, the CA1 acts as the output of the hippocampus and projects to widespread areas of the brain, including the neocortex, to mediate memory processes. However, anatomical tracings have shown that direct connections also exist from the entorhinal cortex to the CA1 and the entorhinal cortex to the CA3 (Figure 3; Amaral et al. 2007). Thus, it remains unknown whether signals for memory acquisition and memory consolidation are transferred exclusively through the trisynaptic pathway or if signals are transferred through other direct connections as well.
Recently, our laboratory has undertaken efforts to record from the CA1 region of the hippocampus during trace EBC to determine how neural activity changes across acquisition and remote retrieval of memory (Hattori et al. 2013). However, activity within the DG during trace EBC has never before been recorded. Therefore, using a trace EBC paradigm, this study sought to record and analyze the neuronal activity of the DG at the single-neuron and local field potential levels during stages of memory acquisition and consolidation and to examine whether this activity aligns with SCT or with alternate theories of memory consolidation, such as MTT.
Lillian Chen (’14) is the recipient of numerous grants and awards such as the Constance Campbell Prize in Basic Research (2014), Fletcher Award for Outstanding Research (2013), Summer URG (2013), and URAP (2012). During her time at Northwestern, Chen developed an interest in reproductive epidemiology and adolescent health that led her to pursue an MPH in epidemiology after her graduation in 2014. She is currently a graduate student at Columbia University's Mailman School of Public Health and hopes to pursue a future career that integrates biology and medicine with the public health field. Outside of academics, she enjoys traveling, cooking, and Bikram yoga.
Amaral, D., Scharfman, H.E., and P. Lavenex. “The dentate gryus: fundamental neuroantaomical organization (dentate gyrus for dummies).” Prog Brain Res 163: 3-22.
Deuker, L., Doeller, C., Fell, J., and N, Axmacher (2014). “Human neuroimaging studies on the hippocampal CA3 region- integrating evidence for pattern separation and completion.” Front Cell Neurosci 8 (64): 1-9.
Eichenbaum, H. (1997). "Declarative memory: insights from cognitive neurobiology." Annu Rev Psychol 48: 547-572.
Frankland, P. W. and B. Bontempi (2005). "The organization of recent and remote memories." Nat Rev Neurosci 6(2): 119-130.
Green, J.D. and A. Arduini (1954). “Hippocampal activity in arousal.” J Neurophysiol 17(6): 533-557.
Hasselmo, M. (2005). “The role of the hippocampal regions CA3 and CA1 in matching entorhinal input with retrieval of associations between objects and context: theoretical comment on Lee et al.” Behavioral Neuroscience 119(1): 342-345.
Hasselmo, M. (2005). “What is the function of hippocampal theta rhythm? Linking behavioral data to phasic properties of field potential and unit recording data.” Hippocampus 15: 936-949.
Hasselmo, M. and H. Eichenbaum (2005). “Hippocampal mechanisms for the context-dependent retrieval of episodes.” Neural Networks 18: 1172-1190.
Hattori, S., Chen, L., Disterhoft J.F., and C. Weiss. Activity of Dorsal Hippocampal CA1Neurons During Acquisition and Retrieval of Consolidated Trace Eyeblink Conditioning. Poster session presented at: Society for Neuroscience 43rd Annual Meeting. 2013 Nov. 9-13; San Diego, CA.
Kim, J.J., Clarke, R.E, and R.F. Thompson (1995). “Hippocampectomy imapires the memory of recently, but not remotely, acquired trace eyeblink conditioning responses. Behav Neurosci 109: 195-203.
Kronforst-Collins, M. A. and J. F. Disterhoft (1998). "Lesions of the caudal area of rabbit medial prefrontal cortex impair trace eyeblink conditioning." Neurobiol Learn Mem 69(2): 147-162.
Leutgeb, J., Leutgeb, S., Moser, M., and E. Moser (2007). “Pattern separation in the dentate gyrus and CA3 of the hippocampus.” Science 315: 961-966.
McKenzie, S. and H Eichenbaum (2011). “Consolidation and reconsolidation: Two lives of memories?” Neuron 71(2): 224-233.
Moscovitch, M. and L. Nadel (1998). “Consolidation and the hippocampal complex revisited: in defense of the multiple-trace model.” Cur Opin Neurobiol 8: 297-300.
Neves, G., S. F. Cooke and T. V. Bliss (2008). "Synaptic plasticity, memory and the hippocampus: a neural network approach to causality." Nat Rev Neurosci 9(1): 65-75.
Parasum, H., Nair, B., Naldi, G., D’Angelo, E., and S. Diwakar (2011). “A modeling based study on the origin and nature of evoked post-synaptic local field potentials in granular layer.” J Physiol Paris 105: 71-82.
Squire, L. R. (2004). "Memory systems of the brain: a brief history and current perspective." Neurobiol Learn Mem 82(3): 171-177.
Takehara, K., Kawahara, S., and Y. Kirino (2003). “Time-dependent reorganization of the brain components underlying memory retention in trace eyeblink conditioning.” J Neurosci 23: 9897-9905.
Winocur, G., M. Moscovitch and B. Bontempi (2010). "Memory formation and long-term retention in humans and animals: convergence towards a transformation account of hippocampal-neocortical interactions." Neuropsychologia 48(8): 2339-2356.