This series o f experiments reveals two important aspects o f the consequences o f neonatal peripheral nerve damage. Firstly, the somatosensory nervous system responds in a far more profound way to injury than similar damage to the adult nervous system. The magnitude o f the response is regulated by the degree o f maturation o f the nervous system. Studies o f the visual and trigeminal system development have revealed a critical period o f postnatal plasticity during which synaptic connectivity is ‘fine-tuned’ by physiological stimuli in an activity-dependent manner. In the visual system, monocular deprivation during the critical period induces changes such that the cortical territory devoted to the non-deprived eye is increased, while that for the other, deprived, eye is reduced (LeVay et al 1980). Axotomy o f the sciatic nerve during the immediate postnatal period has an analagous effect on the spinal cord territories devoted to the axotomised sciatic (‘deprived’) and intact adjacent saphenous ( ‘non-deprived’) nerves. The representation o f the periphery innervated by the sciatic nerve within the spinal cord shrinks, and a concommitant expansion by collateral sprouting o f intact adjacent nerves occurs. As with the visual system, this effect only occurs if axotomy is performed within a critical period o f postnatal plasticity. The time course o f denervation in the two systems is comparable: deprived afferents in the visual system lose half o f their arbors in less than a week (Antonini and Stryker 1996), and sciatic central denervation has a similar temporal course (Bondok and Sansone 1984; Fitzgerald 1985). This rapid restructuring o f connectivity underlies the second aspect o f the CNS response to early axotomy. Whereas the consequences o f axotomy in the adult are generally detrimental to the animal; sciatic axotomy is an established model o f neuropathic pain in rats, and has its own pattern o f cell loss, denervation and associated sprouting, the response o f the neonate appears more compensatory. The sprouting o f intact adjacent afferents into the denervated central territory o f the axotomised nerve is a mechanism that ensures a complete, if topographically distorted, representation o f the periphery persists within the cord. This conservation is also seen in the neurochemical composition o f the axotomised DRG. Despite extensive cell death, the overall phenotypic distribution o f the ganglia remains remarkably conserved, due to compensatory changes within intact populations o f DRG neurons (Reynolds and Fitzgerald 1991, Nothias et al 1993 and chapter 1).
produced some interesting possibilities. The analagous nature o f the ontogeny o f connectivity within these systems, and the activity-dependent nature o f postnatal development and critical period plasticity make it possible to speculate that common mechanisms underlie normal and pathological changes within them.
In the normal animal, the representation o f the periphery within the spinal cord is somatotopically established from initial innervation. Presumably local cues are responsible for this organisation. The collateral sprouting that follows neonatal axotomy requires that the restrictive guidance cues that determine this strict delineation are either removed or overcome. The nature o f these cues is not known (but see chapter 2) but it is possible that the axotomy-induced removal o f this action is a prerequisite for the initiation o f sprouting, and may involve a totally separate mechanism. A Nogo type molecule may be the source o f the restrictive boundary that underlies the topographic nature o f peripheral nerve innervation in the dorsal horn. Further investigation may reveal if there are neuronal or glial markers fulfilling this role, and whether axotomy removes their central inhibitory influence. The evidence for BDNF as a mediator o f sprouting is strong. W hether this is an instructive role is not clear, further analysis o f BDNF transgenic mice may hold the key. Alternatively, BDNF may simply be permissive. The interactions o f BDNF, GAB A and ionotropic glutamate receptors provide several postulates. Synaptic stability and elimination is an important concept in the postnatal nature o f spinal cord connectivity. Activity-dependent excitatory activity underlies these phenomena. During the first postnatal week, GABA acts as an excitatory transmitter. The comprehensive downregulation o f GABA in the spinal cord following neonatal axotomy therefore removes this source o f excitation. This may have two consequences; firstly, if the hypothesised role o f GABAergic intemeurons as signpost cells in the hippocampus has a spinal correlate, then this action would be removed, possibly facilitating afferent sprouting. Secondly, the excitatory nature o f GABA may have a role in stabilising appropriate synapses. Removal o f this may destabilise connectivity within the dorsal horn, again facilitating sprouting.
Is sprouting epiphenomenal?
One intriguing possibility is that intact afferent sprouting into denervated areas o f cord is an epiphenomenon associated with the consequences o f denervation. The pattern o f BDNF immunoreactivity could perhaps derive from excitotoxic glutamate levels.
Axotomy at PO leads to rapid denervation within the dorsal horn, afferent terminals break up, releasing glutamate into the dorsal horn. This is excitotoxic and a proportion o f dorsal horn cells undergoes excitotoxic cell death. This is supported by the appearance o f numerous apoptotic profiles in the dorsal horn following axotomy (Oliveira et al 1997; Whiteside et al 1998). However, as described, a certain level o f postsynaptic activation can be neuroprotective against glutamate toxicity involving an autocrine BDNF mechanism. Are the BDNF positive cells simply surviving the excitotoxic challenge by BDNF upregulation? The implication o f this would be that, if the BDNF is secreted, it may provide a trophic source for intact adjacent afferents. BDNF released within the sciatic territory o f the dorsal horn provides the necessary trophic source in the correct place for sprouting to occur. Furthermore, it may act to stabilise the synaptic connections that are subsequently made by a postsynaptic AMPA insertion mechanism, accounting for the permanency o f the effect. This speculation provides a mechanism to account for the sprouting phenomenon although evidence is scant. Analysis o f the trkfi expression o f sprouting afferents, for example, is necessary.
N orm al developm ental plasticity in the postnatal dorsal horn
The normal pattern o f competition between A and C-fibres in SG, whatever the source o f that competition, is disrupted by chronic NMDA blockade such that normal synaptic elimination and withdrawal is halted resulting in A-fibres remaining in SG into adulthood. This could represent a failure to induce long-term depression at A-fibre synapses on SG neurons. Silent synapse theories may not completely explain the phenomenon. Synaptic elimination requires activation/strengthening o f other synapses to initiate it (cf NMJ). However, the spinal cord scenario may represent a situation whereby all synapses remain active, as they are in the normal situation for a short time. This is the key to the mechanism. The situation in the MK801 ELVAX treated animals is not necessarily abnormal, but simply that the normal developmental process has been halted at a particular point. Behavioural/electrophysiological experiments will answer this question but provisional results suggest this is the case. Therefore the MK801 treatment does not silence synapses, but removes the capacity for synapse elimination. Other developmental processes presumably continue as normal, eg 5HT innervation as described. This has been postulated as a mechanism for silent synapse activation within the dorsal horn. 5HT
whereas in the naïve animal, the A-fibres would already have withdrawn. The results presented here therefore have confirmed a critical period o f synaptic plasticity regulates the sensory nervous system responses to peripheral injury. One consequence, collateral sprouting within the denervated dorsal horn, is associated with an upregulation o f BDNF. Initial studies in BDNF transgenic mice suggest that the BDNF has an instructive role in the sprouting response. The normal postnatal changes that occur vyithin the dorsal horn are activity-dependent, requiring NMDA receptor activation. The intimate link between BDNF and NMDA receptors in regulating synaptic transmission, long-term potentiation and depression provide a rich vein o f future research in further elucidating the mechanisms underlying critical period plasticity within the spinal cord.
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