130 Dispatch

Cortical development: With an eye on neurotrophinsAnirvan Ghosh

Recent observations suggest that neurotrophins areinvolved in activity-dependent plasticity of thedeveloping cerebral cortex. What molecularmechanisms underlie activity-dependent competitionbetween axons for trophic factors?

Address: Department of Neuroscience, Johns Hopkins UniversitySchool of Medicine, 725 North Wolfe Street, Baltimore, Maryland21205, USA.

Current Biology 1996, Vol 6 No 2:130–133

© Current Biology Ltd ISSN 0960-9822

Sensory stimulation profoundly influences the formationof appropriate connections in the developing cerebralcortex. The most compelling evidence for this came fromthe pioneering work of David Hubel and Torsten Wiesel[1], who first described the effects of monocular depriva-tion on the organization of ‘ocular dominance’ columns inthe developing visual cortex (Fig. 1). (Ocular dominancecolumns are alternating regions of the primary visualcortex that receive input preferentially from one or theother eye.) The formation of ocular dominance columns,and their reorganization following monocular deprivation,can be prevented by blocking neuronal activity. Theseobservations led to the hypothesis that neuronal-activity-dependent mechanisms underlie the development andplasticity of geniculocortical connections (reviewed in [2]).There has been intense interest in identifying suchmechanisms, as they are likely to be involved both in theformation of appropriate cortical connections duringdevelopment and in the adaptive responses of the brainsuch as learning. A number of recent observations suggestthat neurotrophins, identified initially as regulators ofneuronal differentiation and survival, mediate importantaspects of activity-dependent cortical plasticity.

The notion that trophic factors are involved in sculptingpatterns of connections during development is itself notnew. The idea is that ingrowing axons compete for limitedamounts of target-derived trophic factors, and that thoseaxons which do not successfully compete are eliminatedby cell death. It is also thought that the more active inputsmight have a competitive advantage, and that such mecha-nisms may underlie the shift in ocular dominance seenafter monocular deprivation. Despite the simplicity of thismodel, direct supporting evidence has been difficult tocome by. The nature of the trophic factors in the visualcortex for which ingrowing thalamic axons compete hasnot been clear, and the mechanism by which neuronalactivity might confer a competitive advantage is not

known. In the past few years, however, a connection hasbeen made between neuronal activity and the action ofneuronal growth factors which may well turn out to becentral to mechanisms of activity-dependent plasticity.

Neurotrophins are regulated by neuronal activityNeurotrophins are a family of neuronal growth factors thatinclude nerve growth factor (NGF), brain-derived neu-rotrophic factor (BDNF), neurotrophin-3 (NT3), NT4/5and NT6 (reviewed in [3]). These factors have been mostextensively characterized with regard to their role in neu-ronal differentiation and survival. The possibility of a linkbetween neurotrophins and neuronal activity first becameapparent with the discovery that there was a markedincrease in the expression of NGF and BDNF in the hip-pocampus and cortex following experimentally inducedseizures (reviewed in [4]). Subsequent experiments onprimary cultures of hippocampal and cortical neurons indi-cated that excitatory and inhibitory neurotransmitters hadopposing effects on the expression of BDNF, and thatinduction of BDNF by excitatory amino-acid stimulationrequired calcium influx via either glutamate receptors ofthe N-methyl-D-aspartic acid (NMDA) receptor class orvoltage-sensitive calcium channels (VSCCs).

Figure 1

Diagrammatic representation of the development of ocular dominancecolumns and their reorganization following monocular deprivation.

Normal

Monoculardeprivation

Cortex

Retina

Neuronal atrophyin activity-deprived

layer

Segregation ofLGN axons

LGN

© 1996 Current Biology

The first evidence that the activity-dependent regulationof trophic factors is of physiologic consequence came fromexperiments exploring the mechanisms of activity-depen-dent survival of cortical neurons in vitro [5]. In this cellularmodel, VSCC activation led to an increase in the survivalof cortical neurons in culture and an increase in BDNFexpression, but this survival could be completely pre-vented by neutralizing BDNF antibodies. These experi-ments therefore suggested that one mechanism ofactivity-dependent cell survival involved the regulation ofBDNF expression by calcium channel activation.

Neurotrophins can influence the development ofthalamocortical connectionsThe discovery that neuronal activity can regulate theexpression of neurotrophins, together with the knowngrowth-promoting effects of neurotrophins in the peri-pheral nervous system, suggested that neurotrophinsparticipate in the activity-dependent rearrangement ofthalamocortical axons. Some of the first evidence thatneurotrophins may influence visual cortical plasticity camefrom the work of Lamberto Maffei and his colleagues [6],who demonstrated that the physiological shift in oculardominance distribution could be prevented by infusion ofNGF into the cortex during the critical period. The mech-anism of this effect, however, is unlikely to involve adirect action on thalamic neurons, as they do not expresshigh-affinity NGF receptors (Trk), and instead is morelikely to be mediated by the effects of NGF on the basalforebrain cholinergic projection.

Neurons in the lateral geniculate nucleus (LGN) doexpress receptors for the neurotrophins BDNF (Trk-B),NT-3 (Trk-C) and NT-4/5 (Trk-B), and evidence thatthese factors can influence patterns of connectivity in thecortex has come from Carla Shatz’s laboratory. Shatz andcolleagues reported last year that infusion of BDNF orNT-4/5 (but not NGF or NT-3) during the critical periodcould prevent the formation of ocular dominance columnsin the cat visual cortex [7]. This finding was of importance,not only because it indicated that neurotrophins can influ-ence the patterning of projections within the cortex, butalso because it suggested that thalamic axons normallycompete for limiting amounts of trophic factors. Theseexperiments did not, however, address the role of neuro-trophins in activity-dependent plasticity, as the observedeffects could also be explained by a growth-promotingaction of BDNF or NT-4/5 on thalamic axons within layer4, independent of neuronal activity.

The involvement of neurotrophins in activity-dependentevents has recently been tested more directly by examin-ing the influence of exogenously applied neurotrophins onthe development of LGN neurons in monocularly depriv-ed ferrets [8]. As in the cat, monocular deprivation in theferret causes not only a redistribution of geniculocortical

axons within layer 4, but also leads to an atrophy of theLGN cell bodies that receive input from the deprived eye.To test whether neurotrophins could overcome suchatrophy, Larry Katz and his colleagues [8] injected fluores-cent microspheres coated with neurotrophins into thevisual cortex of ferrets at the onset of monocular depriva-tion. These microspheres were taken up locally by theLGN axon terminals and transported back to the cellbodies, allowing for unambiguous detection of LGN cellsexposed to the neurotrophin-coated beads. An analysis ofsuch retrogradely labelled LGN neurons indicated that,while most neurotrophins had no detectable effect,NT-4/5 was effective in preventing, to a large extent, theatrophy of LGN neurons. These observations suggest thatLGN neurons may normally compete for cortex-derivedNT-4/5, and that the atrophy of LGN neurons followingmonocular deprivation may indeed be due to a competi-tive disadvantage that the silent inputs have in accessingor responding to NT-4/5.

Although these are exciting observations, suggesting thatcompetition for BDNF or NT-4/5 may be involved inactivity-dependent plasticity in the developing visualcortex and providing some of the first direct evidencelinking neurotrophins to plasticity in the cortex, there arestill many outstanding issues that need to be resolved.For example, it is implicitly assumed in these studies thatthe neurotrophins are acting directly on thalamic axonterminals. This may turn out to be the case, but it shouldbe noted that, at present, there is no evidence to favourthat interpretation over the possibility that the neu-rotrophins act on cortical neurons which in turn influencethe development of thalamic axons or cell bodies. Thispossibility is particularly important to consider, as it hasrecently been reported that neurotrophins, includingNT-4/5 and BDNF, can have marked effects on themorphology of cortical neurons [9]. These effects includean increase in dendritic complexity and dendritic spinedensity, which could provide additional synaptic sites forthalamic inputs and thereby lead to decreased competi-tion among thalamic axons. To rule out such indirecteffects of neurotrophins on the development of thalamicneurons, it will be important to demonstrate directtrophic effects of these factors on thalamic projectionneurons.

Even if it is convincingly demonstrated that neurotrophinscan act directly on thalamic axons to regulate their growthand survival, it needs to be determined whether BDNFand/or NT-4/5 are indeed the endogenous factors thatparticipate in activity-dependent plasticity in the cortex.With the availability of mice that carry targeted disrup-tions of the BDNF and/or NT-4/5 genes, experimentalevidence that addresses this issue should be forthcoming.In addition, it should be possible to inhibit the action ofendogenous neurotrophins (perhaps by using neutralizing

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132 Current Biology 1996, Vol 6 No 2

antibodies) in cats or ferrets, to evaluate their effects onthalamocortical development and plasticity.

Finally, the electrophysiologic consequences of modulatingneurotrophins on ocular dominance plasticity in the cortexneed to be examined. It will be particularly important toexamine the acute effects of neurotrophins on corticalelectrophysiology, in the light of recent reports that synap-tic physiology may be modulated by the brief application ofneurotrophic factors [10–12].

Mechanisms of activity-dependent competition fortrophic factorsIf thalamic axons compete for cortex-derived BDNF orNT-4/5 during development, then how does neuronalactivity confer a competitive advantage? One can imaginea number of molecular mechanisms by which neuronalactivity might influence how well a neuron competes for atarget-derived trophic factor. For example, neuronal activ-ity might modulate the binding and uptake of neuro-trophins by their receptors, active inputs may induce thelocalized release of neurotrophins at synaptic sites, or theability of an axonal input to respond to a target-derivedtrophic factor may be modulated by neuronal activity.

One mechanism by which the activity of a neuron mayinfluence its trophic response is by the activity-dependentregulation of neurotrophin receptor expression. In supportof this possibility, it has been shown that depolarizationleads to an increase in Trk receptor expression in thesympathoadrenal cell line MAH, and this level of induc-tion appears to be sufficient to confer an NGF response onthe cells [13]. It is not yet known whether stimulation ofthe visual pathways influences neurotrophin receptorexpression in the LGN, which would have immediateimplications for the mechanisms of activity-dependentcompetition for trophic factors.

A recent set of experiments on the survival of retinalganglion cells (RGCs) in vitro suggests an alternativemechanism. Barbara Barres and her colleagues [14] haveused purified cultures of RGCs to examine their trophicrequirements, and report that although various growthfactors, including BDNF, insulin-like growth factor 1(IGF1) and ciliary neurotrophic factor (CNTF), showlimited trophic action on RGCs, their influence isenhanced several-fold when the cells are simultaneouslytreated with forskolin or potassium chloride. Theincrease in trophic effects of a purified growth factorcaused by these agents appears to be mediated by anincrease in intracellular cAMP, as they are blocked byinhibitors of the cAMP-dependent protein kinase. Simi-larly, in PC12 cells, the ability of NGF to elicit neuriteoutgrowth is enhanced by simultaneous depolarization ofthe cells with increased extracellular potassium chloride(A.G. and M.E. Greenberg, unpublished observations).

This enhancement does not involve a change in thelevels of Trk receptor expression. Thus, depolarizingstimuli appear to enhance the response of certainneuronal populations to neurotrophic stimulation.

If such observations were to be generalized to geniculocor-tical development, one would predict that the response ofthalamic inputs to target-derived factors such as BDNFand NT-4/5 may be modulated by their levels of activity.Together with the evidence that neural activity can regu-late the expression of BDNF, one can postulate a mecha-nism for activity-dependent competition in the developingvisual cortex (Fig. 2). In this model, the activity of thalam-ocortical afferents regulates the expression of BDNF (andperhaps NT-4/5) in the cortex. (There is evidence thatvisual stimulation can regulate BDNF expression in theadult cortex [15], but the effects of visual activity ofBDNF expression during the critical period have not beenreported.) The BDNF thus produced and released locallymay then act to ensure survival of the post-synaptic cell,and would also be available to thalamic axons for uptake.The efficacy with which the thalamic axons respond to theneurotrophins may in turn be regulated by their levels ofactivity, thereby conferring a growth advantage to theactive inputs.

Figure 2

A possible mechanism by which neurotrophins mediate activity-dependent plasticity in the visual system. In this model, the productionof neurotrophins by the post-synaptic cell is regulated by trans-synaptic activity, and the response of a thalamic axon to the target-derived trophic factors is proportional to its level of activity.

Thalamicaxons

Ca2+

Corticalneuron

BDNFor NT4/5

Trophic effects

Low activity

High activity

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Although elements of such a model may well be correct,the mechanisms of cortical plasticity are unlikely to be thissimple. For example, just as evidence has been accumulat-ing that neuronal activity can influence the expression ofneurotrophins, it is becoming increasingly clear that neuro-trophins can modulate synaptic transmission [10–12]. Sucha role for neurotrophins complicates the interpretation ofthe neurotrophin infusion experiments [7,8]. Although it iseasier to interpret the results of those experiments asreflecting the direct effects of neurotrophins on the growthand survival of thalamic axons, the factors may act in partby altering levels of activity in the cortex. If so, are BDNFand NT-4/5 influencing thalamocortical development pri-marily by modulating neuronal activity (which, by defini-tion, regulates activity-dependent plasticity)?

Finally, one must be cautious not to over-interpret therole of activity-dependent competition for trophic factorsin cortical plasticity, as it does not explain certain centralobservations regarding synaptic plasticity in the develop-ing cortex. The behaviour of thalamocortical connectionsappears to follow the Hebbian principle that correlatedactivity between the pre- and post-synaptic cells leads tosynaptic strengthening. A striking demonstration of thisprinciple came from experiments in Mike Stryker’s labora-tory [16], which showed that pharmacologically suppress-ing the activity of post-synaptic cortical neuronsstrengthens inputs from the deprived eye. In this experi-ment, infusion of the gamma aminobutyric acid (GABA)agonist, muscimol, led to the effective strengthening ofconnections driven by the deprived eye and a relativeweakening of open eye inputs.

If active inputs have a competitive advantage in respondingto target-derived trophic factors, why would the inputsdriven by the open eye lose out to those from the deprivedeye in the experimental paradigm used by Stryker andcolleagues [16]? One could think of ways of elaborating onthe model to incorporate this observation, perhaps bydrawing a distinction between synaptic plasticity andanatomical plasticity. But such model-building is likely tobe much more fruitful once we have experimentalevidence that addresses some of the many unresolvedissues regarding the involvement of neurotrophins inthalamocortical development. Are we really on the vergeof formulating a molecular framework for activity-depen-dent cortical plasticity? With all the excitement that therecent observations have generated, we can hope that thenecessary evidence will not be long in coming.

AcknowledgementsI would like to thank Hollis Cline, Paul Worley and David Ginty for theircomments on the manuscript.

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