| Project
1: [Top
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| Synaptic
Plasticity in Hippocampal Interneurons |
| Collaborators:
K. Franks, T. Sejnowski |
To test the hypothesis that Hippocampal
interneurons are able to exhibit both long-lasting increases (LTP or Long-Term
Potentiation), as well as decreases (LTD or Long-Term Depression), in their
response to incoming synaptic stimuli. Past research in the hippocampal
formation has mainly examined the excitatory neurons in the hippocampal
formation. The inhibitory neuronal population, while only composing approximately
one percent of the total neuronal pool, appears to play a crucial role
in regulating complex interactions of hippocampal excitatory cells, including
oscillatory activity, epileptic synchronization, and synaptic plasticity.
Although there is considerable morphological diversification of interneurons,
in general, each interneuron is capable of influencing the electrical activity
of hundreds of excitatory cells. These cells can thus be seen as being
in a pivotal position to regulate overall hippocampal excitability. To
date, there have been few investigations into interneuron synaptic plasticity,
and published reports leave the question open as to whether interneurons
can actually exhibit synaptic weight changes, or if this is a phenomenon
restricted to excitatory cells. To examine this issue I will use Differential
Interference Contrast (DIC) optics to visualize individual interneurons
in the stratum radiatum of the hippocampal formation. Using whole-cell
recording techniques, electrophysiological responses are recorded from
interneurons prior to and following the application of either LTP-inducing
(100 Hz, 1 sec) or LTD-inducing (3 Hz, 5 min) synaptic stimulation. Post-recording
histological verification of the morphology and position of each interneuron
in the stratum radiatum is required in all instances. To date I have been
able to induce an activity-dependent form of LTP and LTD in stratum radiatum
interneurons, however it remains unclear as to whether some relation exists
between the morphology of the cell and the degree to which it exhibits
synaptic weight changes. These relationships will be the target of future
investigations.
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| Project
2: [Top
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| Calcium
Channel Distribution in Interneurons. |
| Collaborators:
L. Schultz, T. Sejnowski |
Hippocampal interneurons, or GABAergic
non-principal cells, regulate processes as diverse as population oscillations,
synaptic plasticity, and epileptogenesis. The functional diversity of interneurons
is mirrored by their morphological diversity, raising the possibility that
subsets of interneurons serve unique functional roles. Given the ubiquitous
role of calcium in cellular physiology, this project is aimed at characterizing
the distribution of voltage-gated calcium channels on morphologically distinct
groups of interneurons. Fluorescence imaging, whole-cell patch recording
and anatomical reconstruction techniques will be used to characterize the
spatiotemporal distribution of voltage-gated calcium channel subtypes along
the somatodendritic axis of various subtypes of CA1 interneurons in the
rat hippocampal slice preparation. Given that there are separate lines
of evidence that dysfunctional interneurons and voltage-gated calcium channels
contribute to epileptic discharges, this research could provide additional
insight into the nature of epileptogenesis.
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| Project
3: [ Top
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| Effects
of Neuromodulators on the Reliability of Spike Timing. |
| Collaborator:
T. Sejnowski |
Acetylcholine, norepinephrine, serotonin
and histamine are neuromodulators that affect several potassium currents,
including the calcium-dependent potassium current. When applied to neocortical
neurons in slice preparations they increase the firing rates of pyramidal
neurons as well as alter the interspike intervals in response to square
pulse current injection by decreasing spike rate adaptation. I intend to
examine the effects of these neuromodulators on the responses of interneurons
in the rat Hippocampus, and visual and prefrontal cortices to fluctuating
inputs that resembled synaptic inputs in vivo. Compartmental models of
interneurons and a two-pulse stimulation paradigm will be used to isolate
putative factors underlying changes in spike timing due to neuromodulation.
Such factors may include a direct effect, such as changing the calcium-dependent
potassium current, or more indirect effects such as spike insertion with
an increased firing rate. These models will provide a framework for studying
the relationship between the effects of not only neuromodulation on spike
timing and the firing rate, but the ionic and morphological mechanisms
which may contribute to this phenomenon.
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| Project
4: [ Top
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| Neurogenesis
in the Adult Hippocampal Formation. |
| Collaborators:
H. Vanderprag, F. Gage |
Neurons can continue to proliferate
and differentiate well into adulthood in certain brain areas, such as the
olfactory bulb and hippocampus. However, it is unclear whether these "new"
cells manage to establish and maintain functional connections in an "old"
brain. The present experiments examine the electrophysiological and anatomical
characteristics of newly generated granule cells in the dentate gyrus of
mice and rats. Specifically, hippocampal progenitor cells will be infected
using a retroviral vector carrying the transgene for green fluorescent
protein (GFP). GFP can be visualized immediately with a fluorescent microscope
without any immunocytochemical processing, allowing "new" neurons derived
from the progenitor cells to be individually selected for electrophysiological
and histological analysis. Whole-cell recording techniques will be used
to examine intrinsic membrane and extrinsic network properties of these
neurons. Comparisons will be made with non-fluorescing neurons obtained
from the same slice, or from age matched controls. In addition to electrophysiological
analysis, all cells will be labeled with biocytin and reconstructed to
allow a more in-depth examination of the morphological characteristics
of neurons derived from progenitor cells.
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| Project
5: [ Top
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| Role
of Post-Synaptic Calcium in Determining the Threshold for the Induction
of Long-Term Depression (LTD) and Potentiation (LTP) in Hippocampal CA1
Pyramidal Cells. |
| Collaborators:
L. Schexnayder, D. Johnston |
In the present experiments we have
used whole-cell recording techniques in conjunction with high-speed fluorescence
imaging to investigate the changes in intracellular Ca2+ that
occur during the stimulation used to induce LTD or LTP. We applied subthreshold
synaptic stimuli paired with back-propagating action potentials and found
that low stimulus frequencies produced modest increases in post-synaptic
Ca2+ and LTD, while higher frequencies produced larger increases
in post-synaptic Ca2+ and LTP. Thus, we hypothesized that reducing
the level of intracellular Ca2+ during higher frequency stimulus
protocols would lead to the induction of LTD rather than LTP. Nimodipine
(10 mM) was applied to block L-type Ca2+ channels. In other
experiments, a low concentration of D,L-APV (10 mM) was applied to reduce
but not block the NMDA receptor mediated Ca2+ influx. The application
of each of these drugs reduced the stimulation-induced increases in post-synaptic
Ca2+. In the control condition, depression was observed at 3
and 10 Hz while potentiation was observed at 30, 50, 100 and 200 Hz. A
transition from LTD to LTP thus occurred between 10 and 30 Hz in control
conditions. In the presence of nimodipine or APV, however, this transition
occurred at a higher stimulus frequency. Under these conditions, there
was no plasticity between 3 and 10 Hz, while LTD was observed between 30
and 100 Hz. Frequencies up to 200 Hz were needed to induce LTP in the presence
of APV, but no LTP was induced by 200 Hz stimulation in the presence of
nimodipine. These data suggest important relationships among stimulus frequency,
the magnitude of post-synaptic Ca2+, and the magnitude and direction
of changes in synaptic strength. (Supported by NIH).
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