Biology Reference
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facilitation or depression can occur. Depression of synaptic responses is typical of a high
presynaptic release probability, while the converse is true for facilitation (Atzori et al. 2001;
Rozov et al. 2001). On a longer time scale, synaptic augmentation, which lasts up to tens of
milliseconds, and post-tetanic potentiation (PTP), which lasts up to several minutes, can
occur. Both of these are measured by evoking an EPSP after a delay period following a train
of stimuli, and both are thought to result from a build up of presynaptic calcium in the
afferent terminal (Zucker and Regehr 2002). Finally, short-term potentiation (STP) can occur.
This is an enhancement of synaptic responses that lasts up to ten minutes, and is due to
postsynaptic effects such as receptor saturation or desensitisation (Xu-Friedman and Regehr
2004).
Both synaptic depression and facilitation have been reported at layer 5 pyramidal neuron
synapses during trains of EPSPs in the rat (Hempel et al. 2000; Huang et al. 2004) and ferret
(Wang et al. 2006). At layer 3 inputs to layer 5 pyramidal neurons in the rat, synaptic
depression was observed in the majority of synapses (at frequencies of 1-50 Hz), with a
minority of synapses showing facilitation followed by depression (Hempel et al. 2000). At
layer 5-layer 5 synapses in the ferret mPFC, facilitation was observed in the majority of
neurons at low frequencies (approximately 10 Hz), while at higher frequencies (20-50 Hz)
these synapses displayed an initial facilitation followed by depression (Wang et al. 2006).
Depressing synapses and synapses that showed equal facilitation and depression were also
observed, but in lesser proportions (Wang et al. 2006). Pyramidal neurons with facilitating
synapses and with synapses where facilitation balanced depression tended to have a
bifurcated apical dendrite, while depressing synapses had a single apical dendrite, similar to
pyramidal neurons in other brain regions (Wang et al. 2006). It is unclear whether the
differences in proportions of depressing versus facilitating synapses are a feature of the
different inputs (layer 2/3 versus layer 5 inputs) or due to species differences. However since
pyramidal neurons with bifurcating dendrites have not been described in the rat, this may be a
specific feature of pyramidal neurons in the ferret mPFC.
Following a brief tetanus, inputs to layer 5 pyramidal neurons from layer 2/3 and layer 5
both show frequency-dependent synaptic augmentation, lasting up to several hundred
milliseconds (Hempel et al. 2000; Wang et al. 2006). Augmentation is unaffected by the
NMDA receptor antagonist, 2-amino-5-phosphonobutyric acid (AP5; Hempel et al. 2000),
consistent with the notion that synaptic augmentation is likely to be due to a build up of
residual calcium in the presynaptic terminal during the tetanic stimulation (Zucker and
Regehr 2002). Both short-term depression and synaptic augmentation have also been
observed during paired recordings from layer 5 pyramidal neurons, discounting the possibility
that activation of afferents containing neuromodulators or polysynaptic activity underlies
these processes.
PTP and STP have also been observed at synapses on layer 5 pyramidal neurons in the rat
prelimbic mPFC. Short-term plasticity at layer 3 and layer 5 inputs to layer 5 pyramidal
neurons is frequency- and layer-dependent, and determined by the activity of the neuron prior
to tetanic stimulation (15 stimuli at 50 Hz delivered twice; Young and Yang 2005). Following
baseline stimulation at 0.5 Hz, PTP was evoked that lasted for approximately 1 minute at
layer 3 but not layer 5 inputs. The same tetanus delivered following a baseline recording
frequency of 0.067 Hz evoked STP at both layer 3 and layer 5 inputs, although this was
longer lasting at layer 5 inputs (6 minutes versus 4 minutes). In contrast, in the ferret, PTP
lasting up to a few minutes was observed only at layer 5-layer 5 synapses following tetanic
