Background

Hippocampus

Deep in the temporal lobe lies an ancient cortical structure called the hippocampus, which is Latin for "sea horse" due to its appearance in histological sections. The hippocampus plays a central role in memory formation and recall, and is implicated in a number of cortical pathologies including epilepsy and Alzheimer's disease. In this experiment, you will be recording electrical activity from the hippocampus of the mouse.

Oscillations

The brain runs on electricity. If we record the electrical activity of a large group of cells by sticking an electrode on (in which case we would be recording the EEG) or in (as we will do in this lab) the brain, we can often record rhythmic electrical signals that look like noisy sine waves. These rhythmic signals are referred to as oscillations and represent the activity of many cells doing the same thing at the same time, i.e. synchronous, repetitive activity. Depending on the mechanism generating the oscillations, the frequency of the sine wave can be fast or slow. These oscillations have been extensively studied over the years, and have been categorized based on their frequency. Two examples are "theta" (5-10 Hz) and "gamma" (40 Hz) oscillations. The reason brain rhythms are interesting is that their occurrence and their properties change depending on the behavioral state of the subject (e.g. attentive, bored, sleeping, aroused), and depending on the region of the brain being studied. Theta oscillations are especially prominent in the hippocampus, and will be the focus of these two labs. Although the function of brain oscillations is not known, these rhythms are thought to act as timing signals in the brain, sort of like a computer's clock cycle, organizing and coordinating cortical activity amongst different regions of the brain.

How is synchronous, repetitive activity generated in the brain? Consider the example of a sextet of musicians. The tempo of the music can either be imposed by the conductor, or can arise from musicians behaving as a collective, listening and watching others in the group. There are two analogous mechanisms for generating synchrony in the brain: pacemaker cells (conductor) and network monitoring (collective). Often, both mechanisms play a role: a network of cells may act as pacemakers to impose their self-generated rhythm on other cells.

In the hippocampus and neocortex, there are two broad classes of cells: pyramidal cells (PCs) and inhibitory cells (GABAergic interneurons). Pyramidal cells receive excitatory input onto their dendrites from other pyramidal cells, and generate the output of the cortical structure. Interneurons regulate the activity of PCs and other interneurons via synapses onto the postsynaptic cells. Particularly relevant to this lab, networks of inhibitory cells in the hippocampus are thought to act as pacemakers, imposing their self-generated rhythms on pyramidal cells.

When an interneuron fires an action potential, GABA is released at the synapse and binds to GABA_A receptors on the postsynaptic cell. This causes a hyperpolarizing inhibitory postsynaptic potential (IPSP) to occur in the pyramidal cell or interneuron that is the target of this inhibition. The duration and amplitude of the IPSP is determined to a large extent by the properties of the receptor proteins. Drugs that modulate these properties will change the duration and amplitude of the IPSPs.

If the postsynaptic cell (PC or interneuron) is firing action potentials at the time it gets inhibited, for example because of tonically active excitatory inputs, the IPSP silences the postsynaptic cell. When the postsynaptic cell gets hyperpolarized, its sodium channels get jazzed up and when it recovers from the IPSP it's primed to start firing as soon as it's able. Since each interneuron is connected to many PCs and other interneurons, all of these postsynaptic cells will be inhibited simultaneously, and if the ongoing excitation in each cell is similar, they all will tend to fire at the same time when they recover from the inhibition. (In the figure above, the interneuron fires at the time marked by the black triangles, and the three traces of action potentials are recorded from three cells that are inhibited simultaneously.

If the interneuron is firing repetitively, for example because it and all of its neighbors in the network are generating their own theta rhythm, then the postsynaptic cells will be silenced repetitively, i.e. the interneuron will impose on all of the cells it contacts a rhythmic inhibition, sculpting the output of the postsynaptic cells.  The duration and amplitude of the IPSP will determine the frequency of the rhythm (i.e. the bigger and/or longer the IPSP, the longer the postsynaptic cells will be silent and the slower the rhythm).  Note that the interneurons rhythmically inhibit each other, and in this way generate a rhythmicity within their network.  The interneuron network, firing synchronously and repetitively, acts as a pacemaker by imposing its rhythmicity on all of the pyramidal cells in the region.

Function of cortical oscillations

Oscillations serve to organize the firing activity of cortical cells, restricting the occurrence of action potentials to certain time periods. The interneurons generate a timing signal, and the pyramidal cells generate their output at a fixed time relative to the ongoing oscillation.  What's the point of organizing pyramidal cell activity in this way?  One possible function is based on the idea that the exact timing of the pyramidal cell action potentials will depend on how strong its excitation is relative to the inhibition.  If a pyramidal cell is only weakly excited, then it will only be able to fire when the inhibition has decayed back to zero.  As the pyramidal cell gets more and more excited, it will tend to fire earlier and earlier relative to the inhibition.  Thus the phase of the pyramidal cell activity depends on the strength of its excitatory inputs during that cycle of the rhythm.  All the pyramidal cells that are equally excited will tend to fire together, and cells that fire together are interpreted by the brain as being related in some way, for example when forming associative memories or when coding for different aspects of the same sensory stimulus. 

Behavioral effects on oscillations

Anesthetic effects on oscillations

If oscillations act as timing signals in the brain that organize conscious perception and cognition, anything that alters our level of consciousness should alter these timing signals.  General anesthetics, i.e. drugs that cause you to lose consciousness, are a diverse class of drugs that are likely to have many overlapping mechanisms of action.  (Incidentally, even though these drugs have been used for over 100 years and continue to be used routinely in every hospital in the world, their mechanism of action is still a mystery.)  One effect that is common to many of these drugs is that they slow and ultimately eliminate cortical oscillations.  Nearly all of these drugs also target GABA_A receptors, and it is through their modulation of GABA_A receptors that their effects on oscillations are likely to arise.