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[99,100]. The time course of onset and expression of LTD e,nNOS deficient mutants. However, there is a progressive
21
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
SNP alone decreases the amplitude of the field potential beyond 30 min, suggesting that the late effects of the drug may be due to damage to the
The results reported above have shown that refinement
cell. All recordings taken from the SC of the isolated brainstem preparation.
of the ipsilateral retinocollicular pathway depends in part
R .R. Mize, F.-S. Lo Brain Research 886 2000 15 –32
23
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Fig. 6. Presence of NMDA and GABAa receptors and L-type Ca channels in rat neonatal SC. A NMDA receptor response. A moderate stimulus
applied to OT of P1 rat elicits an excitatory post-synaptic potential EPSP, consisting of a short and long duration component trace 1. Application of
D
-aminophosphonovalerate APV, an NMDA antagonist, blocks the late component trace 2. B GABAa receptor response. Application of bicuculline, a GABAa receptor antagonist, in P3 rat can prolong the EPSP trace 2 which is masked by a GABAa receptor mediated IPSP trace 1. C Strong
stimulation of OT can also induce a long-lasting plateau potential in neonatal SC neurons trace 1. This plateau potential is blocked by application of 10
21
m M nitrendipine, an L-type Ca
channel blocker trace 2. C modified from Lo and Mize [99].
upon NO because the refinement is delayed in e,nNOS nels. We have now shown that this in fact occurs. The
double knockout mutants [159,160]. LTD can also be refinement of both the ipsilateral retinocollicular and
recorded in rodent SC during this period of postnatal retinogeniculate pathways are delayed in mice which lack
development, and we have pilot data showing that it may the gene for the voltage gated calcium channel b3 subunit
also be mediated by NO. It is also dependent upon the CC-KO mice. L-type calcium channel currents are re-
21
L-type Ca channel, because the magnitude of LTD is
duced 60 in these homozygous knockouts [115].
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significantly reduced by an L-type Ca channel blocker
Fig. 7 shows the distribution of retinal fibers in the SC [100]. The possible link between pathway refinement and
of both wild-type and CC-KO knockout mice at P15. In LTD would be further established if it could be shown that
these examples the ipsilateral pathway is much more the refinement of the ipsilateral retinocollicular pathway is
extensive in distribution in the CC-KO knockout than in
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also delayed by down-regulating or blocking Ca chan-
the wild type mouse Fig. 7A,B. The differences in area
24 R
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Fig. 7. Development of the retinocollicular pathway in normal and Ca channel b3 subunit knockout mice. A,B Stacked sections of SC showing the
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rostral bottom to caudal top distribution of the ipsilateral right and contralateral left retinocollicular pathways in wild-type A and Ca channel b3
subunit knockout mice B at P15. Note the more extensive distribution in caudal sections in the knockout animal. Modified from Cork et al. [41]. C
2 21
Histogram showing the area mm occupied by the ipsilateral retinocollicular pathway in wild type and Ca channel b3 subunit knockout mice. Note that
there is a dramatic difference in the amount of label in all 10 100 mm intervals caudal to the rostral pole of SC. D Coronal section of mouse SC at age
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P15 showing the distribution of neurons within the superficial gray layer SGL labeled by an antibody to the L-type Ca channel. These neurons are
located within the retinorecipient zone of SC.
of tissue occupied by the label are considerable, even 6. Neurotrophins and their interaction with nitric
within the rostral SC Fig. 7C. This result is consistent oxide in pathway refinement
21
with findings that L-type Ca channel immunoreactivity
is expressed in retinorecipient neurons in mouse SC during One reason that NO is such an appealing candidate as a
the period of pathway refinement Fig. 7D. There is also retrograde signal in synaptic plasticity is that its diffusion
less segregation and greater expansion of the ipsilateral properties as a gas allow it to act in a time-frame of
retinogeniculate pathway in CC-KO mice. This expansion milliseconds to seconds and to spread beyond a single
includes a larger patch of label within the ipsilateral synapse in order to influence a volume of surrounding
domain of the LGN with many ipsilateral fibers extending tissue [57,94,156]. NO can thus influence multiple
beyond the domain Fig. 8A. The contralateral re- synapses on a time scale in which synaptic transmission
tinogeniculate pathway also invades the ipsilateral re- operates. However, other candidates with properties that
tinogeniculate fiber domain to a greater extent in CC-KO likely operate on other time scales have also been proposed
compared to wild type mice Fig. 8B,D. Our data thus as retrograde messengers in synaptic plasticity, notably the
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show that down-regulation of voltage gated Ca channels
neurotrophins [17,106,150, for reviews]. The neurotrophins in the CC-KO mutants can affect pathway refinement in
include nerve growth factor NGF, brain-deprived neuro- the SC to a degree similar to or greater than that seen in
trophic factor BDNF, and the neurotrophins NT-3 and e,nNOS knockouts; and it can also delay refinement of
NT4 5, each of which binds to specific tyrosine kinase both the contralateral and ipsilateral retinogeniculate path-
receptors TrkA, TrkB, TrkC [18]. Some of these are ways in the LGN.
well-established as target-derived retrograde trophic factors
R .R. Mize, F.-S. Lo Brain Research 886 2000 15 –32
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Fig. 8. Distribution of ipsilateral retinocollicular pathway label in the lateral geniculate nucleus LGN of wild type and Ca channel b3 knockout mice at
P15. A,C Ipsilateral LGN in knockout A and wild type C mice. Note that the ipsilateral ‘patch’ of label in the knockout is expanded relative to that of the normal mice. B,D Contralateral LGN in knockout B and wild type D mice. The contralateral fibers invade the ipsilateral ‘patch’ to a much greater
extent in the knockout. Modified from Cork et al. [41].
in the peripheral nervous system [18,118,145]. More been shown to be developmentally regulated. Thus, there is
recently, the neurotrophins have also been implicated as an increase in expression of NGF, BDNF and NT-3
retrograde signals in visual system plasticity that occurs mRNAs in the occipital cortex of rats during the first three
during development. weeks after birth [31,135]. The TrkB receptor is also
The most extensive work to date has been in visual expressed during postnatal development in ferret visual
cortex. Most of the neurotrophins and their associated Trk cortex [27] where TrkB antibody immunoreactivity in-
receptors are expressed in visual cortex and some have creases in layers III and V during the critical period of
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ocular dominance plasticity [27]. BDNF antibody immuno- identifies two factors which can interact in the process of
reactivity also peaks during the critical period of cortical stabilizing or eliminating axonal connections. In this
plasticity in rats where it is preferentially distributed in schema, BDNF promotes axonal growth and growth cone
layers II–III and V–VI [128]. Levels of neurotrophins are extension while NO promotes axon retraction and elimina-
also regulated by visual input. Thus, monocular depriva- tion. The two when acting together can stabilize synapses
tion decreases both BDNF mRNA and BDNF immuno- by preventing further growth or retraction. It remains to be
reactivity in rat visual cortex [20,128] while dark rearing determined whether this mechanism is mediated by im-
decreases BDNF but increases NGF [135]. Thus, many of pulse activity.
the neurotrophins and their associated receptors are ex- The actions of the neurotrophins in regulating pathway
pressed at the proper sites and at the appropriate times in refinement are clearly activity dependent in other visual
order to be able to mediate synaptic plasticity in visual system structures. A series of pioneering studies by Maffei
cortex, and they can be regulated by visual stimulation, et al. showed that intraventricular infusion of NGF 1
which is further evidence that they play a role in activity prevents the shift in ocular dominance that occurs physio-
dependent refinement. logically in single cells of rat visual cortex after monocular
Application of exogenous neurotrophins and or their deprivation MD [12,101]; 2 prevents loss of acuity in
receptors also influence neuronal and axonal growth in the deprived eye [52]; and 3 reduces the anatomical
developing visual cortex. As an example, McAllister et al. shrinkage of neurons that normally occurs in the deprived
[104–106] have demonstrated that several classes of laminae of the LGN [53]. A similar effect has been found
neurotrophins can modulate dendritic length and branching in kitten visual cortex where intraventricular administration
in pyramidal neurons of ferret striate cortex [106, for of NGF also reduces the shift in cortical ocular dominance
review]. The effect is different for different neurotrophins. and cell shrinkage in deprived LGN laminae [30]. By
BDNF increases the length of the basal dendrites of contrast, implanting anti-NGF producing cells into the
pyramidal neurons in layer IV while NT-4 effects those in ventricles of rats prolongs the period of sensitivity of
layers V–VI. Endogenous neurotrophins have also been visual cortex cells to MD [13,54] and produces a further
shown to regulate cortical dendritic growth, as demon- loss of visual acuity and shrinkage of cells in the LGN
strated by application of Trk receptor bodies Trk-IgGs [13]. These results have been interpreted to show that NGF
which down-regulate these substances. Downregulation of promotes synaptic consolidation in afferents which are
endogenous BDNF disrupts growth of dendritic arbors of driven by visual stimulation when they are competing with
neurons in lamina IV while downregulation of endogenous those which are deprived by MD.
NT-3 disrupts dendritic arbor growth of neurons in lamina More recently, the neurotrophins BDNF, NT-3, and
VI [105]. The two neurotrophins also have antagonistic NT-4 5, have also been shown to influence the formation
actions, in that NT-3 can reduce dendritic growth produced of ocular dominance. Thus, in ferret visual cortex local
by BDNF in layer IV and BDNF can inhibit dendritic application of BDNF or NT-4 5 delays the segregation of
growth in layer VI that is produced by NT-3 [105]. The ipsilateral and contralateral geniculocortical afferents that
two neurotrophins acting together thus provide a mecha- result in eye-specific ocular dominance columns [26].
nism by which dendritic growth can be stimulated or BDNF also blocks the shift in ocular dominance that
inhibited. occurs in kitten visual cortex neurons after MD [64].
The neurotrophins also mediate growth of axons during Interestingly, reduction of BDNF and NT-4 5 in ferret
development of the retinotectal pathway. Thus, BDNF and visual cortex by application of Trk-B fusion proteins also
NT-3,NT-4 5 promote axon arbor growth in chick re- delays segregation of inputs from the two eyes. This result
tinotectal co-cultures [83]; and BDNF promotes retinal suggests that either up or down-regulation of neurotrophins
ganglion cell axon branching while BDNF antibodies reduces the ability of the fibers to compete for postsynaptic
block this branching in frog optic tectum in vivo [37]. space [28].
Recently, BDNF has been shown to interact with NO in Over-expression of BDNF in transgenic mice has a
controlling retinal axon growth in chick [59]. Application somewhat different effect. Excess BDNF expressed in the
of BDNF in culture, but not NGF or NT-3, protects retinal visual cortex of these mice does not block ocular domi-
growth cones and their axons from the collapse and nance plasticity since similar shifts in OD occur in both
retraction produced by application of NO. Simultaneous transgenic and wild type mice in response to MD. How-
application of both BDNF and NO ‘stabilizes’ these ever, BDNF overexpression does shift the critical period
growth cones and axons, possibly related to the appearance for this plasticity to an earlier age [72]. In contrast to these
of cytochalasin D-resistant actin filaments that occurs results, Hata et al. [74] have found that infusion of
shortly after exposure. Nitric oxide stimulated ADP- exogenous BDNF into visual cortex in kittens desegregates
ribosylation of actin has been demonstrated by others [34]. OD columns in both normal and MD kittens, suggesting
Finally, individual growth cones are protected from the that an oversupply of this trophic factor promotes exuber-
effects of NO when in contact with BDNF coated-latex ant growth of both active and deprived afferents [74].
beads [59]. This study is particularly instructive because it Accelerated expression of BDNF in transgenic mice also
R .R. Mize, F.-S. Lo Brain Research 886 2000 15 –32
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promotes an earlier termination of the critical period and facilitated by presynaptic depolarization at the neuro-
precocious development of visual acuity [80]. muscular synapse. BDNF and presynaptic depolarization
NT-4, but not BDNF, NT-3, and NGF, also affects the interact because application of one or the other alone does
ferret LGN. Thus, focal application of fluorescent latex not produce the effect. This evidence is an important link
beads coated with the TrkB ligand NT-4 blocks atrophy of between synaptic activity and the neurotrophins because it
LGN cells in layers connected to the deprived eye demonstrates at the synaptic level that BDNF can selec-
[125,126]. This selective effect of NT-4 is surprising given tively strengthen synapses with high levels of impulse
the other neurotrophins affect ocular dominance plasticity activity [19].
in the visual cortex of ferret [26,28], rat [101], and kitten [64,74]. In summary, the influences of neurotrophins on
visual cortical plasticity are complex and considerably
7. Conclusions and summary
