Spatial navigation in rodents is a widely used paradigm for studies of cognition, learning, and memory in the mammalian brain. The subiculum is an important output structure for circuits that underlie navigation, yet few studies have examined its physiology in behaving animals. This proposal aims to identify how an individual subiculum neuron integrates information during navigation from its pre-synaptic inputs in the CA1 region of the hippocampus. CA1 neurons form a spatial map of the environment that is sensitive to small changes;for example, moving from a circular chamber to a square chamber can drastically alter the representation in CA1. The subiculum, on the other hand, seems to form stable maps that do not undergo substantial changes in different environments, despite receiving a major input from CA1. How the subiculum maintains a stable representation remains an open question. To investigate these mechanisms in the context of navigation, the Tank lab has recently developed new technology for calcium imaging and electrophysiological recordings to be performed in the brain of awake mice during navigation in a virtual environment. This technology consists of a spherical treadmill that allows for motion in two dimensions, a virtual reality environment controlled by the motion of the animal on the spherical treadmill, and interchangeable equipment allowing for either electrophysiological recordings or optical imaging. A major advantage of this experimental setup over conventional methods for studying navigation is that the animal is head-fixed, yet allowed to navigate within a virtual environment, providing stability for calcium imaging or whole cell recordings. It is also advantageous because the virtual environment can be dynamically altered during navigation tasks, providing complete control over the animal's experience. This proposal takes advantage of these new technologies in order to understand how the subiculum forms a map of the environment.
In aim 1, optical imaging of calcium activity will be used to characterize what kinds of environmental alterations change the CA1 representation but preserve the map in the subiculum.
Aim 2 will address the anatomical organization of subiculum circuitry by tracing mono-synaptic inputs to a single subiculum neuron. These neurons include inputs from CA1 and recurrent connections within the subiculum.
In aim 3, these techniques will be combined in a single animal. The spatial representation in individual subiculum neurons will be compared to the representation of mono-synaptic inputs in CA1 and subiculum. This comparison will shed light on how subiculum neurons integrate incoming signals to form a stable representation. Unraveling this computation will help to understand the role of the hippocampal formation more generally.
The subiculum plays an important role in cognition, learning, and memory in the mammalian brain. Many health problems are associated with disruption of the subiculum, such as Alzheimer's disease, schizophrenia, epilepsy, and drug addiction. Understanding how microcircuits in the subiculum operate will provide a template for assaying when it is altered by disease states.