20 R
synaptic field potentials Fig. 3B,C, or extracellular action potentials Fig. 3D,E, or even intracellular post-synaptic
responses from individual cells in which the synaptic circuitry remains intact. Preliminary data shows that we
can elicit responses from both pathways Fig. 3B,C. Spontaneous activity can also be recorded from single cells
in SC Fig. 3F. This activity sometimes displays a rhythmicity suggesting that the retina produces sponta-
neous retinal waves in this preparation. We do not yet know how NO will modulate these responses, but NO does
affect LTD in a similar preparation see below.
4. NO Mediated pathway refinement and LTP LTD
It has been proposed that the cellular mechanisms which mediate weakening and strengthening of synapses during
development are long term depression LTD and long term potentiation LTP [8,10,14,110,136,144]. LTP is a
long-lasting potentiation of an evoked response that occurs following application of a tetanus or other event which
enhances the long term efficacy of synaptic transmission. LTD is a long lasting depression that reduces the long term
efficacy of synaptic transmission see [9,103] for recent review. LTP and LTD have been shown to be mechanisms
underlying associative learning in the adult brain, and there is a body of evidence showing that both LTP and LTD are
mediated by NO in some but not all experimental con- ditions [3,16,49,75,84,96–98,116,136,146]. Evidence that
LTP and LTD play a role in pathway refinement is more recent. Hypothetically, LTP in developing brain should
lead to strengthening and stabilization of synapses by potentiating synaptic transmission while LTD should lead
to weakening and eventual retraction of synapses by depressing the synapse [8,10,70]. Weakening of the
synapse via LTD would occur because activity in the depressed afferent is not correlated with that of the
postsynaptic neuron or occurs at a time when the post- synaptic neuron is at less than a threshold membrane
potential; and conversely, strengthening of the synapse via LTP would occur because activity in the potentiated
Fig. 3. Isolated brainstem preparation used for physiology experiments.
afferent is correlated with that of the postsynaptic neuron
A The rat isolated brainstem includes the inferior colliculus IC,
or occurs at a time when the postsynaptic neuron is at
superior colliculus SC, thalamus TH and associated diencephalic structures. The optic tracts, optic nerves ON, and eyes are also left
higher than a threshold membrane potential [10,14]. The
intact in some preparations. Either or both optic nerves can be stimulated.
level of calcium influx is likely to be one factor which
Field potential, extracellular, and intracellular recordings are made from
determines whether LTP or LTD will be induced [4,102].
the SC or LGN. B–F Recordings from the SC of this preparation. B,C
Despite the appeal of this idea, there is to date only
Field potentials generated by stimulation of the contralateral B or
limited evidence to support a relationship between LTP,
ipsilateral C ON. D,E Evoked extracellular action potentials can also be recorded from single cells in this preparation. F Spontaneous activity
LTD, and pathway refinement [39], for review. LTP has
is also present in SC neurons, confirming that the retina is functional.
been shown to be present in synapses during the period of barrel field plasticity in somatosensory cortex [43] and
and contralateral optic nerves can be stimulated separately during the period of ocular dominance plasticity in visual
or together in order to generate postsynaptic responses cortex [11,85,87,91,119]. LTD has also been shown to
within the SC Fig. 3B–F. The preparation is thus ideal occur in neonatal somatosensory [60] and visual cortex
for studying the interaction between synaptic inputs be- [56,90]. These forms of LTP and LTD are for the most
cause the inputs can be stimulated while recording post- part homosynaptic, NMDA mediated, and age dependent
R .R. Mize, F.-S. Lo Brain Research 886 2000 15 –32
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[43,56,60,89–92]. Thus, for example, LTP can be induced stimulation induces LTD, irrespective of frequency Fig.
in layer III neurons of visual cortex only at ages which 4A. LTD induced by both low and high frequency stimuli
roughly match the critical period for ocular dominance reduces the amplitude of both excitatory field potentials
formation, and it can be prolonged by dark-rearing and is and excitatory post-synaptic currents EPSCs by approxi-
therefore visually mediated [89,91]. This LTP is also mately 40. This LTD is developmentally regulated. It
NMDA dependent but is not affected by an antagonist to can occur as early as P0–P3 Fig. 4B and the magnitude
NOS [89]. LTD has also been induced in developing visual of the depression is maximal from P0–P9 and then
cortex [56,127,130,137]. This synaptic depression is also decreases in older animals Fig. 4B. This form of neonatal
sometimes NMDA dependent [130] and is also regulated LTD does not depend upon the NMDA receptor because
by GABA receptors [56]. D-APV does not block it Fig. 4C. Induction is also
We have now demonstrated that both long-term depres- independent of GABAa receptor function because bicucul-
sion LTD and long-term potentiation LTP can also be line also fails to block it. Perinatal LTD is also in-
induced in perinatal rodent SC [99,100]. Both low LF dependent of Group I, II mGLUr receptors because neither
and high frequency HF tetanic stimulation 1 Hz or 50 agonists nor antagonists of these receptors effect LTD
Hz of the optic tract can induce LTP and LTD in the SC amplitude [111]. It is, however, mediated by L-type
of an isolated brainstem Fig. 4A as early as P0. Low calcium channels because it can be partially blocked by
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intensity stimulation induces LTP while high intensity nitrendipine, an L-type Ca
channel blocker Fig. 4D
Fig. 4. LTD and LTP in developing rodent superior colliculus A Response of SC to high frequency tetanic stimulation. The field potential recorded from the SC of the isolated brainstem can be either potentiated open circles or depressed black circles, depending upon the intensity of stimulation. The effect
is frequency independent. The change in field potential induced by the tetanus is long term .90 min, showing that the change in synaptic transmission is like that of LTP and LTD. B Histogram showing that LTD is down-regulated during development. The magnitude of depression of the field potential
produced by a high frequency stimulus declines with age. The differences in amplitude in rats aged P0–P9 vs. those aged P10–P25 is statistically significant P,0.05. C LTD is NMDA independent. Application of D-APV 100 mM does not alter the depression of the amplitude of the field potential
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induced by an HF tetanus. D LTD is partially dependent upon activation of L-type Ca channels. The depression of the field potential amplitude induced
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by tetanus is less after application of 10 mM nitrendipine, an L-type Ca channel blocker, compared to that induced by tetanus in control isolated
brainstem. B–D modified from Lo and Mize [100].
22 R
[99,100]. The time course of onset and expression of LTD e,nNOS deficient mutants. However, there is a progressive
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during development, its Ca channel dependence, and its
decline in field potential amplitude beyond 30 min after NMDA independence are consistent with it playing a role
application of the drug Fig. 5B, which suggests that some in the induction of ipsilateral retinocollicular refinement
of the effect may be due to damage to the neuron. Thus, see below.
our preliminary evidence on the role of NO in LTD is We are as yet uncertain whether LTD in neonatal SC is
mixed, and further experiments must be performed. mediated by NO. LTD magnitude is depressed in some
Recent studies of HF LTD show that both nitric oxide
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e,nNOS double knockout mice when compared to C57 and the L-type Ca
channel also mediate this event in BL-6 controls Fig. 5A; however, the reduction in mag-
other brain structures. In hippocampus and striatum, HF nitude of LTD is not seen in all e,nNOS mutants at all
LTD, which is found only in developing tissue, is mediated
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ages, and variability in LTD amplitude from animal to by L-type Ca
channels but not by NMDA receptors animal may in part explain the variable results. SNP, an
[7,29,33,55]. In striatum, HF LTD can also be blocked by NO donor, greatly enhances LTD when applied to the
NO synthase inhibitors, and is probably expressed pre- isolated brainstem of e,nNOS double knockout mice Fig.
synaptically because there is a decrease in the probability 5B, which is evidence that LTD can be rescued in
of neurotransmitter release as measured by paired pulse facilitation [33]. HF LTD has also been reported in visual
cortex, where it is mGLUr but not NMDA dependent see [4], for reviews; [5,6]. Thus, there is growing evidence
that LTD is present in neonatal tissue and that its charac- teristics differ depending upon the structure involved.
We have also been able to induce LTP in the developing rodent SC Fig. 4A. This LTP can also be induced by high
frequency tetanus 50 Hz or low frequency 1 Hz stimulation, and it appears in SC tissue as early as P1. The
average LTP amplitude in rats aged P1–P13 is 46.560.7 mean6S.E. above baseline control Fig. 4A. We do not
yet know for certain whether this type of LTP is NMDA mediated. However, we do know that both NMDA and
non-NMDA glutamate receptors are functional during the first week after birth in rodent SC. Thus, OT stimulation at
moderate intensity between the ages of P1–P13 always evokes intracellular EPSPs which have both early and late
components. The late component is mediated by the NMDA receptor since application of APV blocks this
component Fig. 6A. The early component is likely mediated by non-NMDA glutamate receptors. GABAa
mediated IPSPs are also present in neonatal SC by P3 because application of bicuculline, a GABAa receptor
antagonist, prolongs the excitatory postsynaptic potential that is masked by an IPSP Fig. 6B. Strong stimulation of
OT also evokes a sustained depolarizing potential plateau
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potential, Fig. 6C, trace 1 that is mediated by L-type Ca channels Fig. 6C, trace 2. This may account for why high
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intensity tetanus induces L-type Ca channel mediated
LTD. LTD and LTP have also been recorded in the rodent lateral geniculate nucleus by our own and other lab-
Fig. 5. Nitric oxide dependent effects on LTD. A LTD magnitude is
oratories. As is the case in SC, LTD in the LGN is
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reduced in e,nNOS double knockout mice. The magnitude of LTD
mediated in part by the L-type Ca channel, is only
induced by an HF tetanus is less in the SC of mice in which the
partially NMDA dependent, and is present shortly after
endothelial and neuronal isoforms of NOS have been disrupted than in
birth [162].
normal C57 BL-6 mice. B SNP, an NO donor, markedly enhances LTD of e,nNOS double-knockout mice. Application of 0.5 mM SNP prior to
and during HF tetanus increases the magnitude of LTD approx. 40 of control compared to LTD amplitude induced by tetanus without drug.
5. Pathway refinement and calcium channel function
