Hippocampal CA 1 pyramidal
neurons in brain slices provide an ideal model system for studying drug
effects on synaptic transmission. Intact synaptic pathways and interneuron
circuits can be preserved in thin slices (0.4 to 0.5 mm) of brain tissue
which remain viable and physiologically stable.
Brain slice circuitry for the CA1 area is shown below, note the
trisynaptic pathway (pp to DG, mf to CA3 and sc to CA1) and synaptic circuit
loop (pp to CA1 directly, and indirectly via DG and CA3):
Synaptic circuits in the
hippocampus are known to process signals related to sensory/motor integration
and planning - Bland, learning
and memory formation - Buzsaki, and form an important hub within the
Brain's Limbic System.
In the illustration above,
projection neurons and main synaptic pathways are shown in black, inhibitory
interneuron colors represent cells that function in concert (red) vs cells that belong to different
inhibitory classes. By activating the appropriate interneuron class,
an input can selectively control large groups of projection cells.
Thus interneurons can gate or index groups of projection neurons, like CA1
pyramidal cells, so that subpopulations of CA1 cells can be synchronously
active during theta and/or gamma frequency EEG oscillations. EEG synchronization
provides timing specificity for circuit level memory formation, such as LTP
and spike timing-dependent plasticity - Poo.
Abbreviations: CA1 - cornu ammonis region # 1 of the hippocampal formation;
STIM - stimulating electrode used to activate excitatory and inhibitory inputs
to CA1 neurons; RECORD - recording electrode to measure pyramidal neuron
response to stimulation; a/c - association/commissural input pathways from
septum, hypothalamus and contralateral hippocampus;
alv - alveus, CA1 output axons to cortex; sc - Schaffer collateral pathway
from CA3 neurons; mf - mossy fiber pathway from dentate gyrus (DG) granule
neurons; pp - perforant path axons from entorhinal cortex; fim - fimbria pathway
to and from midbrain and other regions.