The Structure, Function and Research Applications of the Hippocampus

Structure

Hippocampal neurons play a major role in the functioning of the human brain. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus belongs to the limbic system and plays an important role in the consolidation of information from short to long-term memory, and enables navigation via spatial memory. The hippocampus can be seen as a ridge of gray matter tissue, elevating from the floor of each lateral ventricle in the region of the inferior or temporal horn. The cortex thins from six layers to the three or four layers that make up the hippocampus. The term hippocampal formation is used to refer to the hippocampus proper and its related parts. The neural layout and pathways within the hippocampal formation are very similar in all mammals.¹

It can be distinguished as an area where the cortex narrows into a single layer of densely packed pyramidal neurons, which curl into a tight U shape. One edge of the "U," – CA4, is embedded into the backward-facing, flexed dentate gyrus.² The hippocampus is described as having an anterior and posterior part (in primates) or a ventral and dorsal part in other animals. Both parts are of similar composition but belong to different neural circuits.³ In rat, the two hippocampi resemble a pair of bananas, joined at the stems by the commissure of fornix (also called the hippocampal commissure). In primates, the part of the hippocampus at the bottom, near the base of the temporal lobe, is much broader than the part at the top. This means that in cross-section the hippocampus can show a number of different shapes, depending on the angle and location of the cut.

Function

Memory: Hippocampal neurons play a critical role in the formation of new memories and in the detection of new surroundings, occurrences and stimuli. It is involved in declarative memory; that is memories that can be stated verbally such as facts and figures. However, studies have shown that damage to the hippocampus does not affect a person's ability to learn a new skill such as playing a musical instrument or solving certain types of puzzles.⁴

Spatial navigation and spatial memory: There are several navigational cell types within the brain that are either present in the hippocampus itself or strongly connected to it. One such example are speed cells present in the medial entorhinal cortex.⁵ Together these cells form a network that serves as spatial memory. When the hippocampus is dysfunctional, orientation is affected; people may have difficulty in remembering how they arrived at a location and how to proceed further.⁶ Getting lost is a common symptom associated with amnesia. Studies with animals have shown that an intact hippocampus is required for initial learning and long-term retention of some spatial memory tasks, in particular ones that require finding the way to a hidden goal.

Behavioral inhibition: Damage to the hippocampus causes hyperactivity and affects the ability to inhibit previously learned responses.

Anxiety and Depression: The involvement of the hippocampus in mood disorders is suggested by magnetic resonance imaging (MRI) studies demonstrating a small reduction in hippocampal volume in depressed patients.⁷

Stress Regulation: A less well known function of the hippocampus is its role as a negative feedback regulator of the Hypothalamic Pituitary Axis (HPA). The high concentration of adrenal steroid receptors in the hippocampus and the hippocampal projections to the hypothalamus provide an indirect link between the hippocampus and regulation of the stress response.^8,9

In vitro Neural Research Applications

In vitro neural cell culture systems (primary cell cultures and cell lines) are one of the most valuable research tools in areas like neuropathology, drug screening studies, gene-transfer technology, and three dimensional (3D) cell culture models. Lonza primary cells and media have been used by different research groups for a better understanding of these applications.

Neuropathology

Schizophrenia: The hippocampus is crucial for normal brain function, especially for the encoding and retrieval of multimodal sensory information. Neuropsychiatric disorders such as temporal lobe epilepsy, amnesia, and the dementias are associated with structural and functional abnormalities of specific hippocampal neurons. Various studies have used neuronal cells and media from Lonza in order to understand the role of hippocampus in the pathophysiology of schizophrenia.¹⁰ In contrast to neurodegenerative disorders, total hippocampal cell number is not markedly decreased in schizophrenia. However, the expression of several genes, including those related to the GABAergic system, neurodevelopment, and synaptic function, is decreased in schizophrenia. Taken together, recent studies of hippocampal cell number, protein expression, and gene regulation point towards an abnormality of hippocampal architecture in schizophrenia.¹¹

Epilepsy: The hippocampus is the most widely studied brain region in both human and experimental epilepsy. Sclerosis of the hippocampus in epilepsy was first noted in 1825.12,13 In the modern era of epilepsy surgical programs for the management of drug-resistant epilepsy, hippocampal sclerosis (HS) is one of the common pathologies.¹⁴

Alzheimer’s Disease: It is believed that in Alzheimer's disease (AD) some areas of the brain are particularly vulnerable to specific degenerative processes and that they could exhibit neuronal dysfunction in the earliest stage of the disease. The implications of the hippocampus in memory processes are very well known and it is likely that the hippocampus would be among the first areas of the brain affected by the pathogenic mechanisms occurring in AD. (Figure 2) A study demonstrated and confirmed a significant neuronal loss of hippocampus in AD, as compared to an age-matched control group.¹⁵ Additionally, it seems that this decrease of hippocampal neuronal density was more prominent at the CA1 and CA3 hippocampal areas. This could have important implications in the design of therapeutic and investigative strategies of AD. Rat Hippocampal Neurons and Primary Neuron Growth Medium (PNGM^TM) from Lonza have been used in order to study the significant degeneration of hippocampal neurons in AD.¹⁶

Three Dimensional (3D) Models

Two-dimensional substrates have a limitation in the replication of 3D cellular environments and the absence of 3D factors in cellular developments could limit the biological outcome in the culture. Moreover, cells cultured on the 2D substrate have to adapt to rigid and flat surfaces while natural in vivo environments have large influences from microenvironments derived from extracellular matrix (ECM) and cell-cell interactions.¹⁷

Biofidelic 3D culture: The purpose of this model is to provide an in vivo-like environment to investigate the behavior of brain cellular systems outside of the body while maintaining the highest possible relevance to the system’s natural environment.¹⁸ The model consists of cells that are freshly isolated from rat brain tissue. A few studies have used E18 cells from Lonza cultured in PNGM^TM  Medium to study the 3D models. The model not only kept the cells alive and functional but it also demonstrated the fundamental biological/electrical/anatomical features of a developing brain at a multicellular systems level over a long period of time.^19,20 Silk biomaterial was chosen for its compatibility with versatile uses in neurobiological studies.²¹

Gene Transfer Technology

Efficient gene transfer is an important tool for studying the biology of neuronal function. The Nucleofector^TM Technology, based on electroporation in a celltype–specific solution, enables direct delivery of DNA and small interfering (si) RNA oligonucleotides into the cell nucleus.²³ This strategy results in reproducible, rapid, and efficient transfection of a broad range of cells, including primary neurons. Lonza has developed protocols for successful transfection of post-mitotic neurons. Primary hippocampal neurons were transfected with pmaxGFP^® Vector (Lonza), or double transfected with pSyn-GFP (eGFP under the control of the neuron-specifi c synapsin promoter, and pDsRed monomer-C1 RFP (CMV promoter, BD Pharmingen) with 30-50% efficiency and normal neural development. Lonza Nucleofector^TM Kits for Mouse and Rat Hippocampal cells have been used and reported by various other groups.²⁴

Cancer Therapy Neurotoxicity

The overall aim of current treatments for cancer (such as radiotherapy and chemotherapy) is to prevent aberrant cell division of cell populations associated with malignancy.²⁵ There is accumulating clinical evidence that chemotherapeutic agents induce neurological side effects, including memory deficits and mood disorders, in cancer patients who have undergone chemotherapeutic treatments. Chemotherapy-induced neurodegeneration & hippocampal dysfunctions are measured by in vivo and in vitro approaches. These investigations are helpful in determining how best to further explore the causal mechanisms of chemotherapy-induced neurological side effects and in providing direction for the future development of novel optimized chemotherapeutic agents.²⁶

Drug Screening

In recent years the need of efficient in vitro neuropharmacological and neurotoxicological testing is increasing, as there are new directives to restrict animal use for laboratory tests.²⁷ New experimental strategies based on alternative methods, in which the use of time, materials, and animals is reduced and refined or animal use is completely replaced, are required. Arrays of releasable polystyrene micro-rafts to generate thousands of uniform, mobile neuron mini-cultures have been fabricated. These mini-cultures sustain synaptically-active neurons which can be easily transferred, thus increasing screening throughput by >30-fold. Compared to conventional methods, micro-raft cultures exhibited significantly improved neuronal viability and sample-to-sample consistency. A study validated the screening utility of these mini-cultures for both mouse neurons and human induced pluripotent stem cell-derived neurons by successfully detecting disease-related defects in synaptic transmission and identifying candidate small molecule therapeutics.²⁸ This affordable high-throughput approach has the potential to transform drug discovery in neuroscience.

References

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