Protein synthesis and rapid axonal transport during regrowth of dorsal root axons

The intriguing nature of dorsal root ganglion neurons: Linking structure with polarity and function

Dorsal root ganglion (DRG) neurons are the first neurons of the sensory pathway. They are activated by a variety of sensory stimuli that are then transmitted to the central nervous system. An important feature of DRG neurons is their unique morphology where a single process -the stem axon- bifurcates into a peripheral and a central axonal branch, with different functions and cellular properties. Distinctive structural aspects of the two DRG neuron branches may have important implications for their function in health and disease. However, the link between DRG axonal branch structure, polarity and function has been largely neglected in the field, and relevant information is rather scattered across the literature. In particular, ultrastructural differences between the two axonal branches are likely to account for the higher transport and regenerative ability of the peripheral DRG neuron axon when compared to the central one. Nevertheless, the cell intrinsic factors contributing to this central-peripheral asymmetry are still unknown. Here we critically review the factors that may underlie the functional asymmetry between the peripheral and central DRG axonal branches. Also, we discuss the hypothesis that DRG neurons may assemble a structure resembling the axon initial segment that may be responsible, at least in part, for their polarity and electrophysiological features. Ultimately, we suggest that the clarification of the axonal ultrastructure of DRG neurons using state-of-the-art techniques will be crucial to understand the physiology of this peculiar cell type.

Gene regulation and genetics in neurochemistry, past to future

Ask any neuroscientist to name the most profound discoveries in the field in the past 60 years, and at or near the top of the list will be a phenomenon or technique related to genes and their expression. Indeed, our understanding of genetics and gene regulation has ushered in whole new systems of knowledge and new empirical approaches, many of which could not have even been imagined prior to the molecular biology boon of recent decades. Neurochemistry, in the classic sense, intersects with these concepts in the manifestation of neuropeptides, obviously dependent upon the central dogma (the established rules by which DNA sequence is eventually converted into protein primary structure) not only for their conformation but also for their levels and locales of expression. But, expanding these considerations to non-peptide neurotransmitters illustrates how gene regulatory events impact neurochemistry in a much broader sense, extending beyond the neurochemicals that translate electrical signals into chemical ones in the synapse, to also include every aspect of neural development, structure, function, and pathology. From the beginning, the mutability - yet relative stability - of genes and their expression patterns were recognized as potential substrates for some of the most intriguing phenomena in neurobiology - those instances of plasticity required for learning and memory. Near-heretical speculation was offered in the idea that perhaps the very sequence of the genome was altered to encode memories. A fascinating component of the intervening progress includes evidence that the central dogma is not nearly as rigid and consistent as we once thought. And this mutability extends to the potential to manipulate that code for both experimental and clinical purposes. Astonishing progress has been made in the molecular biology of neurochemistry during the 60 years since this journal debuted. Many of the gains in conceptual understanding have been driven by methodological progress, from automated high-throughput sequencing instruments to recombinant-DNA vectors that can convey color-coded genetic modifications in the chromosomes of live adult animals. This review covers the highlights of these advances, both theoretical and technological, along with a brief window into the promising science ahead. This article is part of the 60th Anniversary special issue.

Constitutive expression of GAP-43 correlates with rapid, but not slow regrowth of injured dorsal root axons in the adult rat

It has been postulated that the neuronal growth-associated protein GAP-43 plays an essential role in axon elongation. Although termination of developmental axon growth is generally accompanied by a decline in expression of GAP-43, a subpopulation of dorsal root ganglion (DRG) neurons retains constitutive expression of GAP-43 throughout adulthood. Peripheral nerve regeneration occurring subsequent to injury of the peripheral axon branches of adult DRG neurons is accompanied by renewed elevation of GAP-43 expression. Lesions of DRG central axon branches in the dorsal roots are also followed by some regenerative growth, but little or no increase in GAP-43 expression above the constitutive level is observed. To determine whether dorsal root axon regeneration occurs only from neurons which constitutively express GAP-43, we have used retrograde fluorescent labeling to identify those DRG neurons which extend axons beyond a crush lesion of the dorsal root. Only GAP-43 immunoreactive neurons supported axon regrowth of 7 mm or greater within the first week. At later times, axon regrowth is seen to occur from neurons both with and without GAP-43 immunoreactivity. We conclude that regeneration of injured axons within the dorsal root is not absolutely dependent on the presence of GAP-43, but that expression of GAP-43 is correlated with a capacity for rapid growth.

The cellular and molecular basis of peripheral nerve regeneration

Functional recovery from peripheral nerve injury and repair depends on a multitude of factors, both intrinsic and extrinsic to neurons. Neuronal survival after axotomy is a prerequisite for regeneration and is facilitated by an array of trophic factors from multiple sources, including neurotrophins, neuropoietic cytokines, insulin-like growth factors (IGFs), and glial-cell-line-derived neurotrophic factors (GDNFs). Axotomized neurons must switch from a transmitting mode to a growth mode and express growth-associated proteins, such as GAP-43, tubulin, and actin, as well as an array of novel neuropeptides and cytokines, all of which have the potential to promote axonal regeneration. Axonal sprouts must reach the distal nerve stump at a time when its growth support is optimal. Schwann cells in the distal stump undergo proliferation and phenotypical changes to prepare the local environment to be favorable for axonal regeneration. Schwann cells play an indispensable role in promoting regeneration by increasing their synthesis of surface cell adhesion molecules (CAMs), such as N-CAM, Ng-CAM/L1, N-cadherin, and L2/HNK-1, by elaborating basement membrane that contains many extracellular matrix proteins, such as laminin, fibronectin, and tenascin, and by producing many neurotrophic factors and their receptors. However, the growth support provided by the distal nerve stump and the capacity of the axotomized neurons to regenerate axons may not be sustained indefinitely. Axonal regenerations may be facilitated by new strategies that enhance the growth potential of neurons and optimize the growth support of the distal nerve stump in combination with prompt nerve repair.

Studies of axonal regeneration in C57BL/6J and A/J mice

The genetic and biological nature of a deficiency in axonal regeneration in C57BL/6J mice was investigated. From analysis of recombinant inbred strains, the genetic basis for the deficient regeneration behaviours of C57BL/6J and A/J mice is deduced to involve multiple loci. The defect in axonal regeneration in C57BL/6J mice appears to be a delay rather than permanent impairment and appears to involve sensory more than motor axons.

Satellite cell proliferation in the adult rat trigeminal ganglion results from the release of a mitogenic protein from explanted sensory neurons

Explant of trigeminal ganglia neurons in adult rats induces perineuronal glial proliferation of primarily satellite cells as opposed to Schwann cells. This proliferation begins at 15 h after explant culture and by 27 h there is a significant increase in glial proliferation as measured by scintillation counts of [3H]thymidine. Blocking protein synthesis between 0 and 3.5 h after explant culture (early) results in an enhanced proliferative response, while blocking protein synthesis between 3.5 and 7 h (late) causes a complete block of the proliferative response assessed at 27 h. Conditioned media experiments demonstrate that both the mitogenic and inhibitory signals are diffusible and heat labile. Finally, the addition of neurotrophic factors to rescue injured ganglionic neurons attenuates the proliferative glial response suggesting that injured neurons produce and release signals that induce glial proliferation.

Absence of early destructive changes of cytoskeletal proteins after transient ischemia in the rat

We examined the changes in immunoreactivity of microtubuli-associated protein (MAP) 2 in dendrites by immunohistochemical analysis following 20 min of cerebral ischemia in the rat. A decrease of immunoreactivity of MAP 2 in dendrites in the CA1 subfield of the hippocampus was observed on days 3 and 7 but not on day 1 after ischemia. Early destructive changes of this protein were not observed, a finding which was confirmed by polyacrylamide gel electrophoresis analysis. We elucidated one factor which indicated that destruction of the dendrites of CA1 pyramidal neurons would not take place any earlier than the destruction of the neurons themselves after ischemia in rats.

Comparison of dorsal and ventral spinal root regeneration through semipermeable guidance channels

Semipermeable guidance channels have been shown to support nerve regeneration in the peripheral nervous system (PNS) possibly through interactions with the wound healing environment. This study quantitatively assesses the ability of such channels to support regeneration in the PNS segment of the spinal roots across a 4 mm gap and compares the resultant dorsal and ventral root regeneration. Acrylic copolymer guidance channels with a molecular weight (Mw) cutoff of 50,000 Da were used in a transected rat spinal root model. Cohorts of 23 animals (11 ventral, 12 dorsal) were examined at four weeks; 6 animals (3 ventral, 3 dorsal) at ten weeks; and 10 animals (5 ventral, 5 dorsal) at twenty-four weeks post-implantation. Both the dorsal and ventral roots were able to regenerate across the gap within the semipermeable channel. At all time periods, the regenerated dorsal roots contained fewer myelinated axons than found in the contralateral control root and consisted of an abundance of collagenous tissue. In contrast, by ten weeks the regenerated ventral roots contained twice the contralateral control number of myelinated axons and were composed predominantly of large, myelinated axons. At twenty-four weeks the number of unmyelinated axons was also quantified, with the regenerated dorsal root containing only one-fifth of the control number and the regenerated ventral root containing more than four times the control. Due to the proximity of the dorsal root lesion to the axonal cell bodies, the dorsal root ganglion (DRG) neuronal cell loss was investigated at four weeks post-implantation.(ABSTRACT TRUNCATED AT 250 WORDS)

Premature onset of fast axonal transport in bromophenylacetylurea neuropathy: an electrophoretic analysis of proteins exported into motor nerve

To test whether abnormal processing of proteins for fast axonal transport is involved in the neuropathy induced by BPAU (p-bromophenylacetylurea) we examined transport onset. [35S]Methionine was injected into the lumbar ventral horn of rats 2 weeks after BPAU, 400 mg/kg (i.p.) or vehicle. At intervals of 30-90 min consecutive 3-mm segments of the L4 and L5 ventral roots were digested for polyacrylamide gel electrophoresis and fluorography. Fast transported proteins were identified by comparison with samples from mid-thigh sciatic nerve ligated for 16 h after radiolabeling. A prominent 26 kDa band represented the earliest exported protein. It was usually absent at 30 min, but it entered the roots by 45 min. This band was consistently displaced further in BPAU nerve (n = 11) than in controls (n = 11). The mean difference was 5 +/- 0.6 mm (P less than 0.001). However, there was no difference in the apparent velocity of transport. These results imply premature onset of transport in BPAU neuropathy.

Expression of beta-preprotachykinin mRNA and tachykinins in rat dorsal root ganglion cells following peripheral or central axotomy

The changes in gene expression and protein synthesis induced in neurons by axotomy usually lead to increased production of axon constituents and decreased production of molecules related to neurotransmission. Exceptions to this generalization occur, however, and it is unclear whether the injury itself changes the pattern of synthesis or whether individual mechanisms regulate the synthesis of the various axonal components. We used in situ hybridization histochemistry and immunocytochemistry to compare the changes in L4 and L5 rat dorsal root ganglion neuron levels of preprotachykinin mRNA and tachykinin peptides caused by sciatic nerve injury with those caused by dorsal root injury. Both lesions elicit regeneration, although only the axotomized peripheral processes re-establish functional contact with their targets. In the contralateral, intact dorsal root ganglia approximately 17% of neurons contained detectable levels of both mRNAs and peptides. Sciatic nerve section decreased by 70% the number of neurons labeled for preprotachykinin mRNA at three days post-operatively. Not all cells in the ganglion are axotomized by the sciatic nerve lesion; grain counts over the cells spared by the lesion showed an increased level of labeling, possibly a result of collateral sprouting by these spared cells. By two weeks, the number of cells labeled for preprotachykinin mRNA had decreased to 80% of control levels. The numbers of neurons labeled for tachykinin peptides decreased more slowly and reached approximately 50% of control numbers at two weeks. By six months post-operatively, when regeneration is largely complete, the number of neurons containing both mRNAs and peptides returned to normal. In contrast, dorsal root section did not elicit a decrease in the number of neurons labeled either for the mRNAs or the peptides at any of the post-operative intervals examined. These results indicate that axotomy is not the stimulus that elicits changes in the expression of genes coding for tachykinins. Evidence is considered indicating that interruption of the supply of peripherally derived nerve growth factor may be responsible for the changes in gene expression for tachykinins after axotomy.

Effects of a conditioning lesion on bullfrog sciatic nerve regeneration: analysis of fast axonally transported proteins

We have shown that bullfrog sciatic nerves respond to a conditioning lesion similarly to goldfish optic nerve and rat or mouse sciatic nerve; that is, following a crush the rate of regeneration is faster in nerves that have received a conditioning lesion compared to nerves that have not. Also, damaged nerve fibres show initial growth or sprouting earlier in a previously conditioned nerve compared to nerves that have not received a prior conditioning lesion. We have not detected changes in the transport of fast axonally transported proteins with the conditioning lesion paradigm, other than those changes seen in regenerating nerves after receiving a single lesion. However, more label was present in a few fast axonally transported proteins at the lesion site in conditioned nerves compared to non-conditioned nerves, and this difference is not apparently due to increased transport. It seems that changes in fast axonally transported proteins probably do not contribute directly to the mechanism underlying the conditioning lesion effect of higher out growth rates, although some of the fast transported proteins may be involved in functions, possibly at the growing tip of damaged fibres, which promote or result from the conditioning effect.

Axonal regeneration in dorsal spinal roots is accelerated by peripheral axonal transection

Regeneration of crushed axons in rat dorsal spinal roots was measured to investigate the transganglionic influence of an additional peripheral axonal injury. The right sciatic nerve was cut at the hip and the left sciatic nerve was left intact. One week later, both fifth lumbar dorsal roots were crushed and subsequently, regeneration in the two roots was assessed with one of two anatomical techniques. By anterograde tracing with horseradish peroxidase, the maximal rate of axonal regrowth towards the spinal cord was estimated to be 1.0 mm/day on the left and 3.1 mm/day on the right. Eighteen days after crush injury, new, thinly myelinated fibers in the root between crush site and spinal cord were 5-10 times more abundant ipsilateral to the sciatic nerve transection. The central axons of primary sensory neurons regenerate more quickly if the corresponding peripheral axons are also injured.

Subsets of axonally transported and periaxonal polypeptides are released from regenerating nerve

Using two-dimensional polyacrylamide gel electrophoresis to analyze proteins, we have found subsets of periaxonal and fast-transported axoplasmic proteins that are released in vitro from regenerating sciatic nerve into a surrounding bath. Of the fast-transported proteins that are released from nerve, there is a subset of at least five polypeptides that appears in greater relative abundance in the bath than in the nerve. Some of these released, fast-transported proteins are glycosylated. Several periaxonally synthesized polypeptides are released in significantly greater amounts from regenerating nerve, and of these polypeptides, two are released in greater amounts from nerve only at regions of regeneration or distal to regeneration. These released polypeptides do not represent the most abundant of the locally synthesized proteins. The released, fast-transported and periaxonal proteins may play a role in intercellular signaling or in modulation of the extracellular environment during nerve regeneration.

Protein synthesis in postischemic rat brain: a two-dimensional electrophoretic analysis

This study examined the pattern of protein synthesis in the neocortex, caudate-putamen, and the hippocampus following transient forebrain ischemia in rats. The animal model of temporary ischemia used in this study causes permanent damage to vulnerable neurons with a time course of injury that varies from hours (caudate nucleus) to days (hippocampus). To examine the spectrum of proteins synthesized in these regions at 3 and 18 h after recirculation, cerebral proteins were pulse-labeled in vivo by an intravenous injection of [35S]methionine. Newly synthesized (35S-labeled) and constitutive (unlabeled) proteins were analyzed by two-dimensional gel electrophoresis and fluorography. In all three brain regions, specific proteins underwent preferential synthesis (Mr approximately 27,000, approximately 65,000, approximately 70,000, approximately 110,000), while others showed decreased synthesis (neuron-specific enolase, alpha- and beta-tubulin). There was an early (3 h post ischemia) induction of the Mr approximately 70,000 mammalian "stress" protein; at 18 h post ischemia, its synthesis remained high in the hippocampus but was diminished in the neocortex and had largely subsided in the caudate-putamen. All regions at 18 h showed increased synthesis of an Mr approximately 50,000 protein, tentatively identified as glial fibrillary acidic protein. The results show that temporary forebrain ischemia induces changes in protein synthesis that include features similar to those observed in other eukaryotic cells subjected to injurious stress. These postischemic changes in protein synthesis are qualitatively similar in all brain regions examined despite regional differences in the severity of subsequent neuronal damage. The persistent synthesis of the Mr approximately 70,000 stress protein in the hippocampus, however, may reflect continued metabolic injury long after the ischemic episode has passed.

The induction of a regenerative propensity in sensory neurons following peripheral axonal injury

Injury of the peripheral axons of primary sensory neurons has been previously shown to increase the probability that the corresponding central axons would grow from the injured spinal cord into a peripheral nerve graft. This phenomenon has been used to investigate the nature of extrinsic cues from injured nerves that enhanced regenerative propensity within sensory neurons. In 13 groups of rats, a segment of the right sciatic nerve was grafted to the dorsal columns of the spinal cord and the left sciatic nerve was subjected to mechanical injury, injection of colchicine or infusion of nerve growth factor. Subsequently, neurons in lumbar dorsal root ganglia with axons growing from the spinal cord into a graft were identified by retrograde perikaryal labelling and compared for the two sides. The aim was to mimic or modify the inductive effect of nerve transaction by alternative or additional manipulation of the nerve. Growth of central axons was less enhanced by peripheral axonal interruption if the length of the proximal stump was increased or if a distal stump was present to permit rapid regeneration. However, the regenerative response following nerve transection was altered little by crushing the proximal stump or injecting it with colchicine or nerve growth factor. It is suggested that sensory neurons are stimulated to regenerate by peripheral axonal injuries that reduce some normal retrograde regulatory influence of Schwann cells.

Ornithine decarboxylase activity in dorsal root ganglia of regenerating frog sciatic nerve

Ornithine decarboxylase (ODC) activity was studied in dorsal root ganglia (DRG) of regenerating frog sciatic nerve. There was a significant increase in activity two days after a crush lesion of the nerve 2.5 cm distal to the DRG. The increase reached a maximum after 7 days, then declined but remained above control levels for at least 9 days. An endoneural injection of vinblastine, a potent inhibitor of retrograde and orthograde axonal transport, between the DRG and the crush inhibited the increase in ODC. In contrast, injections of vinblastine into undamaged nerves failed to affect ODC. The increase in ODC and also the regenerative properties of the nerve could be prevented by daily i.p. injections of alpha-difluoromethyl ornithine. We suggest that a signal is formed at the site of injury in the sciatic nerve. This signal is conveyed to the DRG by retrograde axonal transport where it initiates the events leading to an increase in ODC. This increase appears to be necessary for the regeneration of sensory fibers in the frog sciatic nerve.

Synthesis of heat shock proteins in rat brain cortex after transient ischemia

Cell-free protein synthesis and two-dimensional gel autoradiography were used to characterize early postischemic protein synthesis in rat neocortex. Severe forebrain ischemia was induced for 30 min (four-vessel occlusion model) and followed by 3 h of recirculation. Polysomes were isolated from the cerebral cortex, translated in vitro in a reticulocyte system, and analyzed by two-dimensional gel electrophoresis. The translation products of postischemic polysomes included a major new protein family (70 kDa) with multiple isoelectric variants that was found to comigrate with the 68- to 70-kDa "heat shock" protein synthesized from polysomes of hyperthermic rats. Two other stress proteins (93 and 110 kDa) also appeared to be synthesized in increased amounts after ischemia. A complement of proteins that was indistinguishable from that of controls was also synthesized after ischemia, indicating that messenger ribonucleic acid coding for most brain proteins is preserved after ischemia and is bound to polysomes.

Selective gene expression in focal cerebral ischemia

Regional patterns of protein synthesis were examined in rat cortex made ischemic by the occlusion of the right common carotid and middle cerebral arteries. At 2 h of ischemia, proteins were pulse labeled with intracortical injections of a mixture of [3H]leucine, [3H]isoleucine, and [3H]proline. Newly synthesized proteins were analyzed by two-dimensional gel fluorography, and the results correlated with local CBF, measured with [14C]iodoantipyrine as tracer. Small blood flow reductions (CBF = 50-80 ml 100 g-1 min-1) were accompanied by a modest inhibition in synthesis of many proteins and a marked increase in one protein (Mr 27,000). With further reduction in blood flow (CBF = 40 ml 100 g-1 min-1), synthesis became limited to a small group of proteins (Mr 27,000, 34,000, 73,000, 79,000, and actin) including two new polypeptides (Mr 55,000 and 70,000). Severe ischemia (CBF = 15-25 ml 100 g-1 min-1) caused the isoelectric modification of several proteins (Mr 44,000, 55,000, and 70,000) and induced synthesis of another protein (Mr 40,000). Two polypeptides (Mr 27,000 and 70,000) dominated residual protein synthesis in severe ischemia. The changes in protein synthesis induced by different grades of ischemia most likely comprise a variation of the so-called "heat shock" or "stress" response found in all eukaryotic cells subjected to adverse conditions. Since heat shock genes are known to confer partial protection against anoxia and a variety of other noxious insults, their induction may be a factor in limiting the extent of ischemic tissue damage.

Comparison of the effects of sciatic nerve crush or resection on the proteins of fast axonal transport in rat dorsal root ganglion cell axons

Proteins of fast axonal transport in rat sciatic nerve axons were separated and characterized by SDS-polyacrylamide gel electrophoresis and fluorography, after injection of L-[35S]methionine into the dorsal root ganglion. The effects of crushing or resecting the sciatic nerve on the relative labeling of specific polypeptide bands were compared. Initially, both types of axon injury produced the same response, but after 3 weeks there was a partial return to normal composition in crushed nerves. In resected nerves, the changes characteristic of axon injury persisted beyond 7 weeks. Behavioral testing showed that crushed nerves reinnervated foot skin, whereas no reinnervation was detected after resection. We conclude that in sensory neurons, as in several other neuronal types, the restoration in normal composition of fast-transported protein after axon injury depends on reinnervation of target tissues. This aspect of the cell body reaction to injury seems to be regulated by a retrograde trophic interaction with the target.

Changes in protein content of goldfish optic nerve during degeneration and regeneration following nerve crush

After the goldfish optic nerve was crushed, the total amount of protein in the nerve decreased by about 45% within 1 week as the axons degenerated, began to recover between 2 and 5 weeks as axonal regeneration occurred, and had returned to nearly normal by 12 weeks. Corresponding changes in the relative amounts of some individual proteins were investigated by separating the proteins by two-dimensional gel electrophoresis and performing a quantitative analysis of the Coomassie Brilliant Blue staining patterns of the gels. In addition, labelling patterns showing incorporation of [3H]proline into individual proteins were examined to differentiate between locally synthesized proteins (presumably produced mainly by the glial cells) and axonal proteins carried by fast or slow axonal transport. Some prominent nerve proteins, ON1 and ON2 (50-55 kD, pI approximately 6), decreased to almost undetectable levels and then reappeared with a time course corresponding to the changes in total protein content of the nerve. Similar changes were seen in a protein we have designated NF (approximately 130 kD, pI approximately 5.2). These three proteins, which were labelled in association with slow axonal transport, may be neurofilament constituents. Large decreases following optic nerve crush were also seen in the relative amounts of alpha- and beta-tubulin, which suggests that they are localized mainly in the optic axons rather than the glial cells. Another group of proteins, W2, W3, and W4 (35-45 kD, pI 6.5-7.0), which showed a somewhat slower time course of disappearance and were intensely labelled in the local synthesis pattern, may be associated with myelin. A small number of proteins increased in relative amount following nerve crush. These included some, P1 and P2 (35-40 kD, pIs 6.1-6.2) and NT (approximately 50 kD, pI approximately 5.5), that appeared to be synthesized by the glial cells. Increases were also seen in one axonal protein, B (approximately 45 kD, pI approximately 4.5), that is carried by fast axonal transport, as well as in two axonal proteins, HA1 and HA2 (approximately 60 and 65 kD respectively, pIs 4.5-5.0), that are carried mainly by slow axonal transport. Other proteins, including actin, that showed no net changes in relative amount (but presumably changed in absolute amount in direct proportion to the changes in total protein content of the nerve), are apparently distributed in both the neuronal and nonneuronal compartments of the nerve.

Control of axonal caliber by neurofilament transport

The role of neurofilaments, the intermediate filaments of nerve cells, has been conjectural. Previous morphological studies have suggested a close relationship between neurofilament content and axonal caliber. In this study, the regenerating neuron was used as a model system for testing the hypotheses that neurofilaments are intrinsic determinants of axonal caliber, and that neurofilament content is controlled by the axonal transport of neurofilaments. This system was chosen because previous studies had shown that, after axotomy, axonal caliber was reduced within the proximal stump of the regenerating nerve and, because the relative amount of neurofilament protein undergoing axonal transport in regenerating axons was selectively reduced. The relationship between axonal caliber and neurofilament number was examined in a systematic fashion in both regenerating and control motor axons in rat L5 ventral root. Reconstruction of the spatial and temporal sequences of axonal atrophy in the proximal stump after axotomy showed that reductions in axonal caliber were first detected in the most proximal region of the root and subsequently progressed in a proximal-to-distal direction at a rate of 1.7 mm/day, which is identical to the rate of neurofilament transport in these neurons. Quantitative ultrastructural studies showed that these reductions in caliber correlated with a proportional decrease in the number of axonal neurofilaments but not microtubules. These results support the hypotheses that neurofilament content is a major intrinsic determinant of axonal caliber and that neurofilament content is controlled by the axonal transport of neurofilaments. On this basis, we suggest a role for neurofilaments in the control of axonal volume.

Peripheral injury enhances central regeneration of primary sensory neurones

The success of peripheral and fetal neural tissue in promoting outgrowth of axons from the adult mammalian central nervous system has tended to focus attention on local interactions between extending axons and their environment. The contribution to axon regeneration of biochemical and morphological changes in injured neurones is more difficult to evaluate. We report here that long spinal axons of primary sensory neurones are 100 times more likely to regenerate into peripheral nerve grafts if their peripheral axons are also cut. Regenerative behaviour at the axon tip seems to be strongly influenced by inducible events in the nerve cell.