Enhancement of axonal regeneration in the brain of the rat by corticotrophin and triiodothyronine

Increased Adrenocorticotropic Hormone Levels Predict Recovery of Consciousness in Patients With Disorders of Consciousness

The potential influence of pituitary-related hormones (including both pituitary gland and target gland hormones) on functional recovery after traumatic brain injury has been observed. However, the relationship between these hormones and the recovery of consciousness in patients with disorders of consciousness (DOC) remains unclear. In this retrospective and observational study, 208 patients with DOC were recruited. According to the Glasgow Outcome Scale (GOS) scores after 6 months, patients with DOC were categorized into two subgroups: a favorable prognosis subgroup (n = 38) comprising those who regained consciousness (GOS score ≥3), and a poor prognosis subgroup (n = 156) comprising those who remained in DOC (GOS score <3). Comparative analyses of pituitary-related hormone levels between the two subgroups were conducted. Further, a binary logistic regression analysis was conducted to assess the predictive value of pituitary-related hormones for the patients' prognosis. The favorable prognosis subgroup showed a significant increase in adrenocorticotropic hormone (ACTH) levels (p = 0.036). Moreover, higher ACTH levels and shorter days since injury were significantly associated with a better prognosis, with odds ratios (ORs) of 0.928 (95% confidence interval [CI] = 0.873-0.985, p = 0.014) and 1.015 (95% CI = 1.005-1.026, p = 0.005), respectively. A subsequent receiver operating characteristic (ROC) analysis demonstrated the potential to predict patients' prognosis with an area under the curve value of 0.78, an overall accuracy of 75.5%, a sensitivity of 77.5%, and a specificity of 66.7%. Our findings indicate that ACTH levels could serve as a clinically valuable and convenient predictor for patients' prognosis.

Induction of type 2 iodothyronine deiodinase in astrocytes after transient focal cerebral ischemia in the rat

This study investigated the expression of deiodinases of thyroid hormones in the rat brain after transient occlusion of the middle cerebral artery. The activity of type 2 deiodinase (D2), which catalyzes the deiodination of thyroxine into the more active thyroid hormone 3,5,3'-triiodothyronine, was strongly increased by cerebral ischemia at 6 and 24 hours in the striatum and at 24 hours in the cerebral cortex. The activity of type 3 deiodinase, which catalyzes the inactivation of thyroid hormones, was not affected by ischemia. In situ hybridization showed, as soon as 6 hours, an upregulation of the expression of D2 mRNA in the ipsilateral striatum, which disappeared at 24 hours. In the ipsilateral cortex, the induction of D2 mRNA started at 6 hours, was increased at 24 hours and finally declined at 72 hours. These results were confirmed by reverse transcription-PCR for D2 mRNA in the striatum and cerebral cortex. The upregulation of D2 mRNA after ischemia was mainly localized in astrocytic cell bodies. These results show that D2 is rapidly induced in astrocytes after ischemic stroke. Future work will include the exploration of the role of the upregulation of this enzyme, responsible for local 3,5,3'-triiodothyronine production as a neuroprotective mechanism in the brain.

The influences of fragments and analogs of ACTH/MSH upon recovery from nervous system injury

Post-injury treatment with some fragments and analogs of the adrenocorticotropic hormone (ACTH) can influence recovery after nervous system injury. This review considers both the successful and unsuccessful attempts to facilitate neural and behavioral recovery from nervous system damage via post-injury administration of these compounds. To date no single unifying explanation for the mixed results observed in animals prepared with forebrain injuries has been achieved. Several possible explanations for the variety of observations reported and several potentially productive avenues for future research are suggested.

Changes in nuclear 3,5,3'-triiodothyronine receptor expression in rat dorsal root ganglia and sciatic nerve during development: comparison with regeneration

The action of the thyroid hormones on responsive cells in the peripheral nervous system requires the presence of nuclear triiodothyronine receptors (NT3R). These nuclear receptors, including both the alpha and beta subtypes of NT3R, were visualized by immunocytochemistry with the specific 2B3 monoclonal antibody. In the dorsal root ganglia (DRG) of rat embryos, NT3R immunoreactivity was first discretely revealed in a few neurons at embryonic day 14 (E14), then strongly expressed by all neurons at E17 and during the first postnatal week; all DRG neurons continued to possess clear NT3R immunostaining, which faded slightly with age. The peripheral glial cells in the DRG displayed a short-lived NT3R immunoreaction, starting at E17 and disappearing from the satellite and Schwann cells by postnatal days 3 and 7 respectively. In the developing sciatic nerve, Schwann cells also exhibited transient NT3R immunoreactivity restricted to a short period ranging from E17 to postnatal day 10; the NT3R immunostaining of the Schwann cells vanished proximodistally along the sciatic nerve, so that the Schwann cells rapidly became free of detectable NT3R immunostaining. However, after the transection or crushing of an adult sciatic nerve, the NT3R immunoreactivity reappeared in the Schwann cells adjacent to the lesion by 2 days, then along the distal segment in which the axons were degenerating, and finally disappeared by 45 days, when the regenerating axons were allowed to re-occupy the distal segment.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of peripheral nerve regeneration

Numerous factors external to the nerve cell can support and enhance nerve regeneration after injury. The definition of these factors and the elucidation of their mechanisms of action are the central goals of much contemporary neurobiologic research. This research will hopefully lead to the discovery of factors that will prove to be therapeutically beneficial for patients with either peripheral nervous system (PNS) injury or central nervous system (CNS) injury. This article reviews the biology of the regeneration response of the nerve to injury and discusses many of the factors that enhance nerve growth. Finally, the nerve guide or nerve regeneration chamber model for the evaluation of putative nerve regeneration enhancing agents in vivo is also discussed.

ACTH modulation of nerve development and regeneration

(1) The availability of short amino acid sequences of the naturally occurring ACTH 1-39 molecule has made it possible to separate the corticotropic characteristics of the parent molecule from its neurotrophic effects. Potent neurotrophic fragments are ACTH 4-10, an analog of ACTH 4-9 (Org 2766), and alpha-MSH (ACTH 1-13), peptide fragments that do not evoke corticosteroid secretion, yet clearly affect both the development and regeneration of peripheral nerve. (2) Early postnatal administration of either ACTH 4-10 or Org 2766 accelerates the neuromuscular development of the immature rat, increasing the contractile strength of the EDL muscle and inducing more rapid muscle contractions. Grasping strength and motor activity are increased; these are all changes indicative of more rapid neuromuscular maturation. Prenatal peptide treatment elicits a more complex pattern of response since administration early in gestation (GD 3-12) accelerates neuromuscular development whereas later administration (GD 13-21) decelerates maturation. (3) ACTH peptides have a similar accelerating effect on the morphology of the developing neuromuscular junction. At two weeks of age, nerve arborization is conspicuously increased by postnatal administration of either ACTH 4-10 or Org 2766, as is nerve terminal branching within the endplate itself. However, this is preceded by an initial depression of nerve branching in the 7-day-old rat pup. We conclude that while the developing neuromuscular system is sensitive to ACTH peptides, this susceptibility is age-related. The crucial role of these peptides may be limited to very brief, defined periods during which the peptides may interact with trophic or growth-associated substances, each of which may have its own decisive, circumscribed time frame of influence. (4) Perinatal administration of ACTH peptides affects CNS development. One measurable indication of this is an acceleration of eye opening. Early exposure to ACTH peptides has long-lasting effects on behavior, apparent when these animals are tested as adults. Increased spontaneous motor activity, heightened states of arousal and agitation, and changes in social behavior have been reported. Certain avoidance responses and tests of visual discrimination in male rats are improved by neonatal treatment with alpha-MSH. Overall motor activity is increased and the normal period of hyperactivity is initiated earlier. Male sexual behavior is decreased and sexually dimorphic behaviors in males are eliminated. alpha-MSH may alter the development of its own dopaminergic feedback circuitry while ACTH affects serotonin levels in the preoptic nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Perspectives in anatomy and pathology of paraplegia in experimental animals

Three models of inducing spinal trauma in experimental animals--weight-dropping model, severance-by-knife model, and laceration-type-lesions model--are reviewed critically. Contributions by these models in understanding paraplegia in anatomical and pathological terms are brought out. Important distinctions between subthreshold traumas vs. threshold and suprathreshold traumas, transient and permanent paraplegic syndrome, and regeneration of served axonal fibers vs. prevention of development of permanent paraplegia, are stressed while evaluating each model of spinal trauma. Conceptual contributions by these three models and their bearing on the potential clinical applications are discussed.

ACTH 4-9 analog (Org 2766) improves qualitative and quantitative aspects of motor nerve regeneration

ACTH 1-39 and many of its fragments enhance regeneration of rat peripheral nerve and improve motor unit reorganization. The analog of ACTH 4-9 (Org 2766), with longer-lasting activity and greater biological potency for many parameters, was evaluated for its restorative powers on neuromuscular function following denervation. Org 2766 (10 micrograms/kg/48 hr IP) was administered to rats starting 3 hr after crush denervation of the extensor digitorum longus (EDL) muscle. Contractile strength, motor unit performance under low, optimum and high frequency stimulation, and number and size distribution of motor units of the EDL stimulated through the regenerating peroneal nerve were compared to saline-treated controls. Muscles of parallel animal groups were stimulated directly in vitro. Contractile strength and motor unit performance during high frequency stimulation were significantly improved by Org 2766 treatment at both 7 and 11 days after crush denervation. Org 2766 improves motor unit reformation both qualitatively (more small motor units that are highly resistant to fatigue) and quantitatively (stronger motor units). This peptide does not affect muscle contractile components. Prolonged treatment (21 days) with Org 2766 has a negative effect on motor unit performance, indicating that it exerts its favorable effects early in regeneration and exposure to this peptide should be limited to this period.

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.

Effects of triiodothyronine and propylthiouracil on regeneration of catecholaminergic nerve terminals in the paraventricular hypothalamic nucleus of the adult rat

Effects of triiodothyronine and the antithyroid drug, propylthiouracil, on regeneration of catecholaminergic nerve terminals in the paraventricular hypothalamic nucleus of adult rats were studied. Lesions were produced by 6-hydroxydopamine neurotoxin and then the animals were treated with triiodothyronine or propylthiouracil inducing hyperthyroidism or hypothyroidism, respectively, as determined by radioimmunoassay. Although catecholaminergic varicosities increased with time in the paraventricular hypothalamic nucleus of rats after lesion, fluorescent microscopic quantitation showed no statistical difference in their number between rats treated with triiodothyronine and the vehicle for as long as 56 days. Furthermore, electron microscopic quantitation at 56 days postlesion showed no significant difference between the triiodothyronine-treated and control rats in terms of the density, proportion, size, types of synapses, and synaptic frequency of catecholaminergic nerve terminals. There were growth cones in the paraventricular hypothalamic nucleus, suggesting growth activity after lesion. However, we found that exogenous administration of large doses of triiodothyronine at 25 micrograms/kg had little effect on the enhancement of regeneration of central catecholaminergic terminals after their destruction by 6-hydroxydopamine.

The effect of adrenalectomy and corticosterone on homotypic collateral sprouting of serotonergic fibers in hippocampus

Lesioning of serotonergic (5-HT) fibers using 5,7-dihydroxytryptamine (5,7-DHT) in one of the two median raphe-hippocampal pathways induced homotypic sprouting from the other. We have utilized this model with the horseradish peroxidase (HRP) tracing technique to test the effects of an adrenal corticosteroid-corticosterone on homotypic sprouting in female adult rats. Removal of the adrenal glands does not interfere with the 5,7-DHT destruction of 5-HT fibers observed at 3 days. However, the animals without circulating adrenal steroids do not show the increase of the HRP-labeled cells normally seen 21 days after the 5,7-DHT lesions. Furthermore, s.c. implantation of corticosterone pellets was able to completely restore homotypic sprouting of 5-HT fibers in the hippocampus.

Prenatal and postnatal hypothyroidism abolishes lesion-induced noradrenergic sprouting in the adult rat

The effects of altered thyroid states on regeneration in the central nervous system are equivocal. This work was undertaken to examine the influence of propylthiouracil-induced (PTU-induced) pre- and postnatal hypothyroidism on collateral sprouting of noradrenergic (NA) axons in the habenula (Hb), following lesions in the stria medullaris (SM) of the adult rat. Ten pregnant dams were divided into control and PTU-treated groups. Control rats had free access to food and water and the hypothyroid group received, in addition, 0.05% (w/v) of PTU in their drinking water, beginning on day 12 of gestation and continuing post partum through lactation until the offspring were weaned at age 22 days. The pregnant dams received, in addition, daily injections of thyroxine (25 micrograms/kg, ip) and only male pups were used in the study. At weaning the pups were removed from the dams and placed in cages with free access to food and water. The hypothyroid offspring received 0.05% (w/v) PTU in the drinking water until sacrificed at 10 weeks of age. At 6 weeks, some rats in each group received bilateral lesions in the SM and the remainder were sham-operated. All rats were sacrificed 4 weeks after operation. Thus, four groups were formed: 1) control/sham, 2) control/lesion, 3) PTU/sham, 4) PTU/lesion. Sprouting of NA fibers in discrete regions of the Hb was identified by norepinephrine (NE) levels and by fluorescence histochemistry. Blood levels of thyroid stimulating hormone (TSH) were determined. The results show that pre- and postnatal induced chronic hypothyroidism abolished NA sprouting in the partially deafferented Hb. Furthermore, lesions of the SM resulted in a marked decrease in serum TSH levels.

Effects of reducing sensory-motor feedback on the appearance of crossed nigro-thalamic projections and recovery from turning induced by unilateral substantia nigra lesions

This experiment addressed the question of whether the increase in interhemispheric nigrothalamic projections, found after unilateral lesions of the substantia nigra (SN), is related to recovery from the lesion-induced sensory-motor asymmetry. We examined the effects of diminished information feedback about the animals' own behavior on recovery from turning induced by unilateral substantia nigra lesions, and the appearance of an increase in number of crossed nigro-thalamic projections. The animals received unilateral 6-hydroxydopamine lesions and were placed in a hammock for the next 7 days. Thus, they were prevented from engaging in turning behavior. They were then implanted with horseradish peroxidase (HRP) in the thalamus ipsilateral to the lesion. No HRP-labeled cells were found in the SN contralateral to the HRP implantation in these animals. In rats, which were allowed 7 days normal interaction with the environment subsequent to unilateral SN damage, labeled cells were found in the undamaged SN when HRP was implanted in the thalamus ipsilateral to the lesion. Animals which were kept in the hammock and subsequently allowed a week of normal interaction with the environment showed the same pattern of recovery from turning behavior as the animals which were allowed to recover normally immediately after the unilateral SN lesion. It was concluded that the relationship between the appearance of the nigro-thalamic projections and the cessation of turning behavior is not fortuitous. Also, it would appear that some form of sensory-motor learning is involved in the recovery of function after unilateral SN lesions. Thus, the results substantiate the idea that these cross-projections form part of a physical basis of recovery from unilateral lesion-induced asymmetries.

The effect of human chorionic gonadotropin (HCG) on functional recovery of spinal cord sectioned rats

The effect of human chorionic gonadotropin (HCG) on functional recovery after complete spinal cord transection in the rat was studied. Twelve rats (6 male and 6 female) received HCG daily for the first week and then every alternate day for three weeks. Ten rats (5 male and 5 female) served as controls. Return of bladder function, Tarlov's grading and measuring the maximum angle at which the rats were able to climb up an inclined plane were used as criteria in assessing the functional recovery. Improvement was noted in the HCG-treated rats by the end of two weeks. At six weeks the recovery of HCG-treated rats as compared to the control group was very significant (p less than 0.001). No recovery whatsoever was noted in the control group even after six weeks. Histological evidence of bridging of the gap between the cut ends of the spinal cord by nerve fibres containing tissue was noted in the HCG-treated rats only. The possible mode of action of HCG on spinal cord injury is briefly discussed.

An ACTH4-9 analog (ORG 2766) speeds recovery from septal hyperemotionality in the rat

The amount of hyperemotionality initially demonstrated after septal area lesions was reduced, and the rate at which the hyperemotionality attenuated over repeated testing, was enhanced by the administration of an ACTH4-9 analog, ORG 2766. This ACTH fragment was given for 4 consecutive days after surgery but terminated before testing began. Two weeks after the daily tests of emotionality, the animals were trained in a two-way active avoidance task. The typical increase in avoidance behavior seen in animals with septal lesions was observed in the lesioned animals tested with ORG 2766, but the usual high number of intertrial responses was greatly reduced in these animals. The results indicate that even after a brief series of ORG 2766 administration, there are changes in emotionality that may last for an extended period of time after the cessation of treatment.

Inhibition of non-neuronal cell proliferation in the goldfish visual pathway affects the regenerative capacity of the retina

Proliferating cells associated with the visual pathway were found in the present study to affect the regenerative capacity of the goldfish retina following optic nerve injury. The contribution of these cells to the process of regeneration was investigated in the goldfish visual system by reducing their proliferation in the optic tract and tecta, using X-irradiation. The regenerative ability of the retina was then evaluated by the following parameters: sprouting from retinal explants, protein synthesis in the retina and accumulation of radiolabeled transported components in the tectum. X-irradiation of the visual system at an early stage of the regeneration process had a promoting effect whereas irradiation at a later stage resulted in a reduced capacity to regenerate. The results are discussed with respect to the possibility that proliferating cells, possibly glia, exert two contradictory contributions: an inhibitory effect at the site of injury, whereas distal to it, a supportive, perhaps trophic effect.

Infantile spasms: some new theoretical aspects

Infantile spasms begin in a very active phase of brain development. The new neuropathological techniques have shed light on changes in the fine structure of brain in children with infantile spasms. An understanding of the action of adrenocorticotropic hormone on the biochemistry and development of the brain provides, in an indirect way, a partial explanation of the biochemical events in this syndrome; most of these actions are well documented in animal experiments. Some anticonvulsants are also effective in treating infantile spasms, and modern research has explained many actions of these agents as well. The principal mechanism causing retardation in most children is unknown. Some future aspects of the research are reviewed.

Effect of triiodo-L-thyronine on axonal regeneration in the rat spinal cord after acute compression injury

Studies were performed on the effect of triiodo-L-thyronine (T3) on clinical recovery and axonal counts in the pyramidal tract of 56 rats subjected to an acute spinal cord compression injury at T-7. The T3 was given at a daily dose of 5 micrograms/kg for 4 weeks to 28 rats in the treatment group. The treatment and control animals were tested weekly for clinical recovery, and cord function as determined by the inclined-plane technique. Groups of animals were killed at 4 weeks and 12 weeks, and the axons in the pyramidal tract cephalad and caudad to the injury site were counted in sections prepared with Holmes' silver stain. There was no difference in clinical recovery between the treatment and control groups. This negative result contrasts with other studies which showed improved recovery of cord-injured animals treated with thyroid hormones. The possible explanations for this discrepancy are discussed. Similarly, there was no difference in the axon counts between the treated and control groups. Thus, T3 did not improve recovery or axonal regeneration in the pyramidal tract of rats after acute spinal cord compression injury. Between 4 and 12 weeks, there was a marked reduction in the cephalad axon counts in the pyramidal tract in both groups, indicating that approximately 50% of the axons in the pyramidal tract had undergone retrograde degeneration or dying back by 12 weeks after this degree of injury. The T3 did not affect the degree of retrograde degeneration.

The effect of acute spinal cord compression injury on thyroid function in the rat

The effect of acute spinal cord injury on thyroid function was studied in rats subjected to severe spinal cord compression at T1. Serum thyroxine (T4), effective thyroxine index (ETI), and thyroid stimulating hormone (TSH) were measured at 1 and 100 minutes at one, three, and seven days after laminectomy and spinal cord injury. Control animals were subjected to laminectomy only. T4 was decreased at 1 minute after laminectomy with or without spinal cord injury, though the animals with cord injury had a much more profound reduction. The effects on TSH at 1 minute were dramatically different: laminectomy alone caused an elevation of TSH, while spinal cord injury produced a marked decline. At the later time intervals both groups showed gradual normalization of T4 and TSH levels, and at seven days there were no significant differences between the groups. Thus, acute spinal cord compression injury produced a major alteration in thyroid function during the acute phase.

Influence of triiodothyronine on the amino acid uptake of brain and spinal cord in normal and spinal hemisected adult rats

The thyroid hormones triiodothyronine (T3) and L-thyroxine appear to enhance regeneration in the peripheral and central nervous system (CNS). The following experiments examine possible metabolic substrates for the action of T3 on the adult rat CNS after spinal hemisection. The protein incorporation of (3H)lysine after a left spinal hemisection (T2) or control operations was examined 1, 3, 7, and 14 days postoperation. Triiodothyronine (1 microgram/kg body weight in a bicarbonate buffer) was injected daily for the postoperation or equivalent time period. One hour prior to decapitation, animals were given a subcutaneous injection of 200 microCi of (3H)lysine. Samples of brain and spinal cord were dissolved, and the radioactivity of acid-precipitable protein and acid-soluble fractions were determined by scintillation counting. T3 treatment influenced the general levels of incorporation of all treated groups over all days postoperation. Specific effects were observed in spinal hemisected T3-treated animals. A significant hemispheric (P less than 0.05) asymmetry was present at 3 days postoperation with the right somatomotor cortex higher in protein radioactivity than the left. In spinal cord, the area of the lesion and areas just caudal to the lesion were higher in (3H)lysine incorporation in T3-treated rats relative to controls. T3 effects appear to involve an increased sensitivity of the cells of the injured nervous system to the hormone.

The effect of exogenous thyroid hormones on functional recovery of the rat after acute spinal cord compression injury

The effect of triiodothyronine (T3) or thyroxine (T4) on functional recovery after acute spinal cord compression injury in the rat was assessed. Rats treated with T3 for 14 consecutive days after injury showed significantly improved recovery at 12 and 16 weeks, and rats treated with T4 for 4 days after injury showed significantly improved recovery at 12 weeks as compared with control animals. The possible modes of action of these two hormones on the injured spinal cord are briefly discussed.

ACTH accelerates recovery of neuromuscular function following crushing of peripheral nerve

Adrenocorticotropin (ACTH)-treated adrenalectomized rats subjected to crush denervation recover sensation and functional movement sooner than saline-treated rats. Axonal regeneration is accelerated, the number of large endplates and the frequency of preterminal branching are increased. ACTH has no effect on either intact or denervated muscles. The ameliorative action of ACTH during regeneration is apparently neurogenic and independent of corticoids.

Vascular permeability and axonal regeneration in tissues autotransplanted into the brain

Pieces of skin, peripheral nerve, muscle, tendon, thyroid gland, and submandibular gland were autotransplanted into the brains of mice. The animals were killed after 5-week periods. Fluorescently labelled albumin was injected i.v. 1 h prior to death. Silver-impregnated sections were examined under the light microscope for the regeneration of axons from the brains into the implanted tissues. Unstained sections were studied by fluorescence microscopy for the presence of the labelled tracer in the extracellular spaces of the grafts. The muscle and submandibular gland received the fewest regenerating axons, skin and tendon received an intermediate amount of reinnervation, and the thyroid gland and vagus nerve were the most richly innervated. The amount of reinnervation could be roughly correlated with the presence of extravascular protein within the tissues. These data support the hypothesis that regeneration of axons may be dependent upon a source of protein in the extracellular fluid surrounding their growing tips.

Thyroxine increases nerve growth factor concentration in adult mouse brain

The effects of thyroxine and propylthiouracil on nerve growth factor concentrations in cerebral cortex, cerebellum, and brainstem of adult male mice were assessed by using a sensitive radioimmunoassay for the beta-subunit of mouse nerve growth factor. Thyroxine administration significantly increased the concentration of nerve growth factor in all three brain areas compared to control values, whereas propylthiouracil was without effect. These results suggest that thyroid hormones stimulate nerve growth factor synthesis in the mature central nervous system, and raise the possibility that the influence of thyroid hormones on central nervous system development might be mediated or influenced by nerve growth factor.

An explanation of axonal regeneration in peripheral nerves and its failure in the central nervous system

Nerve fibres severed within peripheral nerves are able to regenerate and reinnervate the structures they formerly supplied. Most axons severed within the mammalian central nervous system (CNS) do not regenerate in this way. Regenerative axonal growth begins to occur in the CNS but ceases about two weeks after injury. Five earlier theories purporting to explain this difference are reviewed and found not to account satisfactorily for many experimental observations. A new hypothesis is advanced in which it is held that in order for regeneration to take place, the growing tips of the axons must be surrounded by extracellular fluid containing proteins (of specified identity) derived from the blood plasma. Such proteins are thought to be imbibed by the tips of the fibres and transported retrogradely to the neuronal cell-bodies. With this hypothesis it is possible to explain the success of axonal regeneration in peripheral nerves and its failure in the CNS. It is also possible to account for the exceptional circumstances in which axons do regenerate in the CNS. Various experiments are suggested for testing the validity of the new hypothesis.

Influence of exogenous triiodothyronine on axonal regeneration and wound healing in the brain of the rat

Rats with parasagittal incised wounds of the telencephalon were treated for 1,7, 21 or 56 days with triiodothyronine (T3), 0.5 mug/100 g body weight, injected once daily. Controls were injected with the aqueous solution in which the T3 was dissolved. The brains were examined histologically and quantitative assessments were made of the regeneration of axons into and across the lesions and of the healing of the wounds. T3, when administered over an 8 week period, stimulated axonal regeneration in the dorsal cortex and corpus callosum and promoted healing of the wound in the corpus callosum. The results of this investigation suggest that the use of T3 in the clinical treatment of injury to the central nervous system may be of less value than the work of earlier authors had indicated.

Alterations by estrogen and hypothyroidism in the effects of septal lesions on lordosis behavior of male rats

Earlier experiments indicated that chronic exposure to estradiol benzoate (EB) following septal lesions can increase the subsequent levels of female sexual behavior in male rats tested several months later following priming doses of EB. However, the present study demonstrates that a single injection of a large dose of EB (50 microng) two days after septal destruction did not modify subsequent responsiveness to EB priming in male rats relative to sham operated controls. Yet male rats given 10 daily injections of 5.0 microng EB/day immediately following a septal lesion were more responsive to EB than oil treated controls when tested later for lordosis behavior. Therefore, the capacity for EB to alter the behavioral effects of septal lesions on lordosis behavior in male rats is related to both chronic administration and a period of susceptibility to some action of EB during the immediate post-lesion period. In two additional experiments, chronic hypothyroidism induced by propylthiouracil or thyroidectomy was also found to modify the effects of septal lesions on female sexual behavior of male rats. These latter results indicate that EB is not unique in its capacity to alter the behavioral effects of septal lesions in male rats, and are consistent with the view that both EB and hypothyroidism may interact with some dynamic process associated with recovery from brain lesions.

Increases in collateral axonal growth rostral to a thoracic hemisection in neonatal and weanling rat

The spinal cords of newborn and weanling rats were hemisected at the mid-thoracic level. Control studies revealed that Fink-Heimer positive debris was absent in the gray matter at three months postoperative. The remaining animals were given a second lesion, a high cervical spinal hemisection, at five to seven months after the original thoracic hemisection. The pattern of degeneration rostal to the thoracic lesion was compared with similar regions of the spinal cord from animals receiving only a cervical hemisection at the adult stage. In neither experimental group of doubly hemisected rats was there any degeneration observed below the thoracic lesion site, even though no glial or connective tissue scar had formed in animals originally operated at birth. Thus no regeneration had occurred. At least one segment above the initial hemisection: 1. the majority of degenerating axons were localized toward the lateral edge of the spinal cord, especially in the doubly lesioned neonatal group; 2. the erae of ipsilateral white matter was reduced more in the neonatal than the weanling operates; 3. there was an upward shift in axonal diameter of ipsilateral fibers in both the region of the rubrospinal tract and the ventrolateral portion of the lateral funiculus of the doubly hemisected rats when compared with the cervically lesioned controls; 4. a significantly greater amount of degeneration was present in lamina VII of Rexed in both the neonatal and weanling experimental operates (p less than 0.05 weanling; p less than 0.001 neonate); 5. no mean difference in area was seen between the ipsilateral and contralateral gray matter in any group for the segments of the spinal cord in which the judgements and measurements were taken. These data suggest that there has been sprouting of axons from descending nerve tracts rostral to the thoracic lesion in both the neonatal and weanling experimental groups. The question remains whether the sprouting of descending nerve tracts is from collateral of axons which normally project rostral to the thoracic hemisection and are not cut by the thoracic lesion (collateral sprouting) or from collaterals of lesioned axons (regenerative sprouting). Present evidence favors collateral sprouting, expecially in the neonatal operate where much retrograde cell death appears to have taken place.

Central nervous system regeneration and ophthalmology

Various adult lower vertebrates are capable of optic nerve, and even retinal, regeneration with functional recovery of vision. Possible factors responsible for regenerative failure in mammals are discussed. It is suggested that potentially neuroregenerative agents be tested in the mammalian retina in an attempt to induce visual pathway regeneration.

Nerve regeneration and thyroid hormone treatment

On the basis of 3 reports that thyroid hormone treatment stimulates axon outgrowth in rats with nervous system injuries, a 43-year-old patient with an ulnar nerve laceration was given desiccated thyroid (up to 360 mg/day) following nerve suture. The Hoffmann-Tinel sign of sensory axon outgrowth advanced at a rate of 5.0 mm/day in the forearm, approximately 200% faster than the anticipated rate. A good functional result was obtained in 7 months. On the maximum dosage of desiccated thyroid, the patient developed mild weight loss associated with an increased appetite; there was no other sign or symptom of hyperthyroidism. Further investigation of the role of thyroid hormone during nerve regeneration seems warranted.