Biology Reference
In-Depth Information
neurons scale down NMDA currents in response to enhanced activity, this may make it more
difficult to evoke LTP and easier to induce LTD.
Figure 1. Models of synaptic plasticity induction. A. Frequency dependent synaptic plasticity. Diagram
showing the dependence of synaptic plasticity, represented on the x-axis by the change of the measured
neuronal response in mV, on the frequency (Hz) of homosynaptic interaction. A lower frequency, of ca.
1Hz, results in LTD, whereas higher frequencies will tend to evoke LTP. B. Spike-timing dependent
plasticity. The diagram illustrates the importance of the sequence in pre- and postsynaptic activity as
well as the significance of their temporal order. The timing is expressed in milliseconds on the x-axis,
with the y-axis indicating the moment of coincident activity. If a presynaptic spike follows the
postsynaptic response, LTD is induced and if presynaptic activity precedes the postsynaptic one, LTP is
induced.
In an effort to more closely approximate endogenous conditions for synaptic plasticity
many researchers have studied the relationship between hippocampal LTP and neuronal
oscillations during exploratory behaviour. Long-term synaptic potentiation is optimal when
the time interval between stimuli is approximately 200ms due to activation of NMDA
receptor-mediated inward current or removal of the inactivation of T-type of Ca
2+
channels
followed by a rebound depolarization after 100-200ms. The timing corresponds ideally with
the synchronized depolarization during theta oscillation, which considers the theta cycle as an
information quantum. The specifically-expressed amplitude of the theta rhythm in the limbic
system implies its involvement in memory formation.
One of the more convincing links
between learning and hippocampal LTP involves the use of theta-frequency stimulation,
establishing a connection between theta rhythm and LTP (Larson & Lynch, 1986; Rose &
Dunwiddie, 1986; Buzsáki
et al.
, 1987; Larson & Lynch, 1989). Patterned after the
endogenous theta rhythm, one could effectively induce LTP extracellularly with short 100 Hz
bursts delivered at 5 to 8 cycles per second (about 50 pulses total). LTP is more effectively
induced in the dentate gyrus when tetanus was delivered on positive phases of theta in
urethane anesthetized rats (Pavlides
et al.
, 1988). Similar results have been found in freely-
moving animals with stimulation of the perforant path (Orr
et al.
, 2001). Thus the induction
of synaptic plasticity in hippocampal regions after high-frequency firing activity
coheres with
naturally occurring spiking patterns. For the experience-dependent alterations of hippocampal
place cells frequency-dependent plasticity is mediated by the complex-spike bursts (Muller
et
al.
, 1987; Gothard
et al.
, 2001).
