Regeneration of central cholinergic neurones in the adult rat brain

The Role of Tissue Geometry in Spinal Cord Regeneration

Unlike peripheral nerves, axonal regeneration is limited following injury to the spinal cord. While there may be reduced regenerative potential of injured neurons, the central nervous system (CNS) white matter environment appears to be more significant in limiting regrowth. Several factors may inhibit regeneration, and their neutralization can modestly enhance regrowth. However, most investigations have not considered the cytoarchitecture of spinal cord white matter. Several lines of investigation demonstrate that axonal regeneration is enhanced by maintaining, repairing, or reconstituting the parallel geometry of the spinal cord white matter. In this review, we focus on environmental factors that have been implicated as putative inhibitors of axonal regeneration and the evidence that their organization may be an important determinant in whether they inhibit or promote regeneration. Consideration of tissue geometry may be important for developing successful strategies to promote spinal cord regeneration.

Global expression of NGF promotes sympathetic axonal growth in CNS white matter but does not alter its parallel orientation

Axonal regeneration is normally limited after injuries to CNS white matter. Infusion of neurotrophins has been successful in promoting regenerative growth through injured white matter but this growth generally fails to extend beyond the infusion site. These observations are consistent with a chemotropic effect of these factors on axonal growth and support the prevailing view that neurotrophin-induced axonal regeneration requires the use of gradients, i.e., gradually increasing neurotrophin levels along the target fiber tract. To examine the potential of global overexpression of neurotrophins to promote, and/or modify the orientation of, regenerative axonal growth within white matter, we grafted nerve growth factor (NGF) responsive neurons into the corpus callosum of transgenic mice overexpressing NGF throughout the CNS under control of the promoter for glial fibrillary acidic protein. One week later, glial fibrillary acidic protein and chondroitin sulfate proteoglycan immunoreactivity increased within injured white matter around the grafts. NGF levels were significantly higher in the brains of transgenic compared with non-transgenic mice and further elevated within injury sites compared with the homotypic region of the non-injured side. Although there was minimal outgrowth from neurons grafted into non-transgenic mice, extensive parallel axonal regeneration had occurred within the corpus callosum up to 1.5 mm beyond the astrogliotic scar (the site of maximum NGF expression) in transgenic mice. These results demonstrate that global overexpression of neurotrophins does not override the constraints limiting regenerative growth to parallel orientations and suggest that such factors need not be presented as positive gradients to promote axonal regeneration within white matter.

Fine structure of rat septohippocampal neurons: II. A time course analysis following axotomy

Previous light microscopic immunocytochemical studies with antibodies against transmitter-synthesizing enzymes have suggested that septohippocampal neurons undergo retrograde degeneration following transection of their axons by cutting the fimbria-fornix. However, a fine-structural analysis of the degeneration process in these cells is lacking so far. Here we have identified septohippocampal neurons by retrograde tracing with Fluoro-Gold. Thereafter, the fimbria-fornix was transected bilaterally. Fine-structural changes in prelabeled septohippocampal neurons were then studied after varying survival times up to 10 weeks. Examination under the fluorescence microscope of Vibratome sections through the septal region revealed numerous retrogradely labeled cells after all survival times following axotomy. These neurons were then intracellularly injected with the fluorescent dye Lucifer Yellow in order to stain their dendritic arbor. Many cells were found after each survival time that displayed characteristics of septohippocampal neurons in control rats (see Naumann et al., J Comp Neurol 325:207-218, 1992). In addition, increasing with survival time, there were many shrunken neurons with a reduced dendritic arbor. Representative examples of both normal appearing and shrunken neurons were photoconverted for subsequent electron microscopic analysis. Relatively few signs of neuronal degeneration were found at each survival time analyzed. The majority of cells, including the heavily shrunken ones, displayed fine-structural characteristics of normal neurons. However, a few degenerating neurons and reactive glial cells were present in all survival stages. We conclude that axotomized septohippocampal projection neurons cease the expression of transmitter-synthesizing enzymes and shrink, but many more cells survive for extended periods of time without target-derived neurotrophic factor than was assumed in previous light microscopic studies.

Nerve growth factor promotes collateral sprouting of cholinergic fibers in the septohippocampal cholinergic system of aged rats with fimbria transection

Nerve growth factor (NGF) was injected intraventricularly into aged (24 months) rats with unilateral fimbria transection. Controls received intraventricular injections of cytochrome c. A quantitative analysis of acetylcholinesterase (AChE)-positive fibers was used to evaluate whether the NGF treatment can stimulate regeneration and reinnervation of the cholinergic axons in the septohippocampal system of aged rats with fimbria transection. A marked increase in the density of AChE-positive fibers was observed in the lateral septum, the dorsal fornix and the dorsal hippocampus of the NGF-treated animals, as compared to the controls. In the lateral septum, the increase was observed in the 2-month NGF-treated animals but not in the 15-day NGF-treated animals. In the dorsal fornix at the level of the dorsal hippocampus, the increase was observed on both the lesioned and unlesioned sides of both the 15-day and 2-month NGF-treated animals. In the denervated (lesioned side) hippocampus, the increase took place in the dorsal hippocampus but not in the ventral hippocampus of both the 15-day and 2-month NGF-treated animals. There was no recovery of AChE-positive fibers on the lesioned side of the fimbria distal to the lesion site even in the 2-month NGF-treated animals. These results demonstrate that intraventricular injections of NGF can stimulate collateral sprouting of intact cholinergic axons in the septohippocampal system and promote cholinergic reinnervation of the denervated hippocampus of aged rats with fimbria transection.

Neural transplantation: an historical perspective

Literature on transplantation of neural and nonneural tissues into the brains of host animals is reviewed in the perspective of various issues. The two dominant issues determining this research were elucidation of embryological processes underlying the development of the nervous system and regeneration in the host brain. A comprehensive review of studies on regeneration in the central nervous system (CNS), using this technique of transplantation, indicates that regeneration of axonal fibers is small in magnitude and extent, and that it is more directly related to the trauma caused to the brain than to any other variable. This literature review attempts to provide a perspective to the contemporary research on neural transplantation and on regeneration in the CNS.

Long-term administration of mouse nerve growth factor to adult rats with partial lesions of the cholinergic septohippocampal pathway

Nerve growth factor (NGF), a neurotrophic factor acting on cholinergic neurons of the basal forebrain, has been proposed as a treatment for Alzheimer's disease. Experimental support for its pharmacological use is derived from short-term studies showing that intraventricular administration of NGF during 2-4 weeks protects cholinergic cell bodies from lesion-induced degeneration, stimulates synthesis of choline acetyltransferase, and improves various behavioral impairments. To investigate the consequences of long-term NGF administration, we tested whether cholinergic cell bodies are protected from lesion-induced degeneration and whether cholinergic axons are stimulated to regrow into the denervated hippocampus following fimbrial transections. We found that intraventricular injections of NGF twice a week for 5 months to adult rats resulted in extended protection of cholinergic cell bodies from lesion-induced degeneration and did not produce obvious detrimental effects on the animals. NGF treatment mildly stimulated growth of cholinergic neurites within the 2-mm area directly adjacent to the fimbrial lesion but it failed to induce significant homotypic growth of cholinergic neurites into the deafferented hippocampus.

Symptomatic hemidystonia of delayed onset. Magnetic resonance demonstration of pathology in the putamen and the caudate nucleus

We present a case of symptomatic hemidystonia of delayed onset. The primary disease was a perinatal, presumed cerebrovascular infarction brought about by febrile illness with convulsions 12 weeks after partus. After many years without neurological symptoms, the hemidystonia started in adolescence, and became stationary after 4 years of mild progression. Magnetic resonance imaging revealed atrophy of the right striatum including the caudate nucleus and putamen. The symptoms responded moderately to treatment with benzhexol.

Examination of autologous and embryonic cortical brain tissue transplantation to adult brain cortex in rats

Autologous and embryonic cortical brain tissue was transplanted to adult rats in order to reconstruct experimentally degenerated cortical brain tissue. Rats were decapitated within 6 or 12 weeks. Viability of the graft tissues was studied by light and electron microscopy. Embryonic cortical brain tissue grafts became enlarged but adult cortical brain tissue grafts were found to be unaltered. Electron-microscopically observed mitochondria and other cell organellae and the newly vascularized areas clearly showed that the graft tissues were alive.

Comparison of growth and reinnervation properties of cholinergic neurons from different brain regions grafted to the hippocampus

Grafts of five different types of central cholinergic neurons, from the septal-diagonal band region, the nucleus basalis magnocellularis region (NBM), the striatum, the pontomesencephalic tegmentum of the brainstem, and the spinal cord, were compared with respect to their ability to grow and to reinnervate the cholinergically denervated hippocampal formation of adult rats. The areas were dissected from 14 to 15-day-old rat fetuses, and the same number of viable cells (35 X 10(4) from each of the different regions were stereotaxically injected as cell suspensions into the hippocampus of rats subjected to a transection of the intrinsic septo-hippocampal cholinergic pathways. At 17-19 weeks after transplantation, the various graft types differed considerably in their volume, the total amount of acetylcholinesterase (AChE)-positive fiber outgrowth, and the innervation pattern and morphology of the AChE-positive fibers growing into the host hippocampus. On average the NBM and spinal cord grafts had grown to become three to four times larger than the septal and the brainstem grafts, and 15-20 times larger than the striatal grafts. By contrast, the total ingrowth score of AChE-positive fibers in the host hippocampus from the septal grafts was about twice that of the NBM and brainstem grafts, about five times greater than the striatal grafts, and about six times greater than that of the spinal cord grafts. The large NBM grafts thus exhibited similar fiber outgrowth to the much smaller brainstem grafts, and the AChE-positive neurons of the grafted spinal cord grew very poorly into the hippocampus despite the fact that they survived very well. The innervation pattern and morphological features of the ingrowing AChE-positive fibers in the host hippocampus proper and in the dentate gyrus resembled those of normal rats in animals with grafts from any of the three forebrain regions (i.e., septum, NBM, or striatum), whereas ingrowth from the brainstem and spinal cord grafts were markedly abnormal with respect to both innervation pattern and fiber morphology. These results provide further evidence that the overall survival, growth, and fiber outgrowth of intracerebral neural grafts depend on interactions with the surrounding host tissue. Since the ability to reinnervate the previously denervated host target was greatest for the neuron type normally innervating that area, i.e., the septal-diagonal band neurons, we conclude that neuronal properties beyond the transmitter type are essential for the optimal performance of implanted neurons in intracerebral grafting experiments.

The avian pecten provides a potent substrate for growth and development of dissociated embryonic neural implants

A cell suspension of the optic tecta of 3-day-old chick embryos was injected into the vitreal chamber of 2-day-old posthatch chicks. After a 14-21-day survival period, examination of eyeballs showed that all implants survived and, in 50% of cases, were attached to the pecten. The implants had proliferated and showed a laminated pattern of organization, with small cells in the superficial regions and large cells in the deep regions of the implant. The implants also contained a well-developed neuropil with mature synapses. The host retina was not affected by the presence of the implant. We suggest that the avian pecten represents a highly amenable structure for studies involving the response(s) by damaged retinae to neural implants.

Effects of ganglioside treatment in rats with a lesion of the cholinergic forebrain nuclei

The effects of GM1 ganglioside (30 mg/kg i.p.) administration for 22 days on choline acetyltransferase (ChAT) activity and noradrenaline (NA) levels in the cerebral cortex and on the acquisition of active and passive avoidance-conditioned responses were investigated in both sham-operated rats and in rats with a unilateral electrolytic lesion of the magnocellular forebrain nuclei (MFN). A statistically significant ChAT decrease in cortical areas ipsilateral to the lesion was found in saline-treated lesioned rats. In the lesioned GM1-treated rats, ChAT activity was only reduced in the frontoparietal areas and was significantly increased in the ipsilateral parietooccipital areas as well as in both contralateral regions. NA levels in the cortex were neither significantly affected by the lesion nor by GM1 treatment. The lesion impaired the acquisition of active and passive conditioned avoidance responses. GM1 treatment improved acquisition of the active avoidance response in the lesioned rats as indicated by a larger number of avoidances and a smaller number of escape failures during training in comparison with saline treatment. Ganglioside had no effect on the passive avoidance responses. These results demonstrate that GM1 administration facilitates the recovery of the cortical cholinergic system and of behavioral responses impaired by an electrolytic lesion of the cholinergic forebrain nuclei.

Antiserum-induced growth of axons across lesions of the adult rat brain

Growth of axons across lesions of the adult rat brain occurred when the lesions were treated with a heterologous antiserum developed against lesioned areas of the brain. Rabbits were immunized with blocked dissected damaged rat brain plus adjuvant. The antiserum was prepared by ammonium sulfate precipitation of the immune rabbit sera followed by pepsin digestion to prevent complement-mediated damage to the recipient rats. Rats treated with the antiserum had dense cellular bridges crossing the brain lesions which contained axons. The axons within the dense cellular bridges were newly formed, since they were observed to pass through the center of paper rings which were implanted into the lesion. Individual axons were traced from one side of the lesion, through a dense cellular bridge, and into the tissue on the opposite side of the lesion. Rats lesioned in a similar manner, but treated with either phosphate-buffered saline or normal rabbit serum displayed no such growth. In addition, the limited axonal growth observed was enhanced by increasing the concentration of the antiserum administered. Thus, the antiserum induced the formation of dense cellular bridges and the growth of axons across lesions of the mammalian central nervous system.

Replacement of damaged cortical projections by homotypic transplants of entorhinal cortex

The extent to which transplants of embryonic cortical tissue can be used to replace damaged cortical projections has been examined. Embryonic entorhinal cortex was implanted into the entorhinal region of young adult rats that had previously received a lesion through the angular bundle. Projections between transplant and host were examined by using WGA-HRP and the fluorescent dye Fast Blue. Implants selectively innervated areas of the host hippocampus and amygdala which normally receive entorhinal afferents. Implants were innervated by cells in the host diagonal band and, in one case, by cells in the contralateral entorhinal and/or presubicular cortex. In most cases, host fibers were differentially distributed within transplants, possibly reflecting an ability of host fibers to recognize and selectively innervate their appropriate targets even though the cellular organization of the implant is different from that present during normal development. These data suggest that homotypic implants of embryonic entorhinal cortex can, in some ways, replace severed cortical projections and may eventually be able to reconstitute normal cortical circuitry.

The injury response of nerve fibres in the anterior medullary velum of the adult rat

The injury response of myelinated central nervous system (CNS) axons was documented in the anterior medullary velum (AMV) of the adult rat. Study of silver-stained AMV whole-mounts revealed sprouting of injured axons as early as 14 h post-lesion (hpl), with a complex network of fibres formed by 48 hpl. Signs of fibre degeneration were also apparent from 48 hpl, increasing in extent until 15 days post-lesion (dpl). Fragmentation was largely confined to specific fibre bundles, constituted by the distal portions of severed axons. Although some degeneration of regenerated axons was evident from 15-20 dpl, many remained intact beyond this time, particularly in the area adjacent to the exit of the trochlear nerve, where most regenerated fibres penetrated the ipsilateral trochlear nerve. Counts of HRP filled neurons in the trochlear nucleus after injection of the superior oblique muscle showed that axons entering the IVth nerve rootlet were exclusively ipsilateral trochlear fibres. Less than 50% regenerated; most other severed axons degenerated. The few axons remaining in the AMV may have been fibres, undamaged by the original lesion, which normally course longitudinally through the ipsilateral AMV. These results show that IVth nerve fibres preferentially enter IVth nerve rootlets and, in so doing, survive the effects of injury. Most other CNS axons in the AMV which do not enter the trochlear root probably degenerate.

Neuronal and oligodendrocytic response to cortical injury: ultrastructural and cytochemical changes

A needle wound was made in the adult rat cerebral cortex. Responses of neurons and oligodendrocytes at the site of injury were followed over a period of 450 days and correlations made between morphological and enzyme cytochemical changes to clarify some phenomena previously unresolved. Evidence from acid phosphatase activity in degenerating neurons showed no increase in the number of cytochemically stained lysosomal profiles nor changes in the subcellular localization of the acid phosphatase reaction product. Our observations indicated that the majority of dying neurons were not digested by their own acid phosphatase 'autodigestion' but by the process of heterodigestion. The time-course study revealed that not all the traumatized neurons were eliminated but some persisted permanently in an attenuated 'atrophic' state. The atrophic neurons were small in size with low cytoplasmic-nuclear ratios and exhibited low levels of glucose-6-phosphatase and cytochrome oxidase activities. The acid phosphatase activity was slightly increased as evidenced by cytochemically stained hypertrophic Golgi cisternae and a slight increase in the number of lysosomes. The low level of enzyme activities concerned with carbohydrate metabolism reflected the low metabolic activity in atrophic neurons whilst an increase in Golgi-lysosomal enzyme activity suggested some anabolic process necessary for their survival. Oligodendrocytes displayed only minor changes in morphology, and their glucose-6-phosphatase and cytochrome oxidase activities were normal, suggesting that these cells have little or no involvement in the repair of a cerebral wound. The absence of significant changes in lysosomal acid phosphatase activity indicated a minimal role, if any, of oligodendrocytes in the process of phagocytosis.

Regeneration of central catecholamine fibers in young and aged rat brain

Catecholamine histofluorescence patterns were examined in brains of young and aged rats, 1 to 14 days following neurosurgical transection of the medial forebrain bundle. At all ages examined, two phenomena were observed: degeneration of nerve fibers and vigorous regrowth of catecholamine-containing fibers in the lesion site. Regenerated catecholamine fibers invaded the area of scarred tissue. This invasion of the scarred area implies that the robust plasticity of catecholaminergic pathways, known to exist in young animals, persists in aged brain.

Mossy fibre projections into and out of hippocampal transplants

Hippocampal primordia taken from one day old postnatal rats and grown for 1 month in the hippocampus of adult rat hosts developed hippocampal pyramids, dentate granule cells, and specific patterns of mossy fibres. In 26 cases where certain boundary conditions were precisely met, the transplant mossy fibres crossed into the host and ramified for a distance of from 0.5 mm to no more than 1 mm in the stratum oriens of the host field CA1. They formed a layer 35 micron thick adjacent to the host CA1 pyramidal cell layer. The necessary boundary conditions were: (1) direct (and "unscarred") contact between the neuropil of the transplant and the host field CA1, (2) that the hilar (and not the molecular) aspect of the transplanted dentate granule cell lamina faced the host CA1 pyramids, and (3) that some of the interface was devoid of transplant hilar cells or CA3-type pyramids interposed between the transplant dentate granules and the host CA1 pyramids. In 3 cases a converse connection was found--viz. the host mossy fibres entered the transplant. In these cases the transplants consisted entirely of pyramidal cells (with no dentate granule cells of their own), and the part of the transplant receiving the host mossy fibres was embedded directly in the host mossy fibre pathway. For the dentato-hippocampal mossy fibre system, therefore, it is shown that postsynaptic targets in the adult mammalian brain can receive specific patterns of innervation from growing axons derived from a transplant, and that cut central axons of the same type can grow and form target-specific terminal arborizations in a transplant.

Heart conditioned medium elicits post-lesion muscarinic receptor recovery in vivo

Intraseptal administration of heart conditioned medium (HCM) stimulates the growth of injured cholinergic fibers into iris implants placed in the anterodorsal hippocampus. The aims of the present report were to monitor muscarinic cholinergic receptor concentrations during denervation and central innervation of the peripheral tissue targets, and to evaluate the effect of HCM on these changes. For 0, 4, 8, 16 and 28 days after mechanical injury to septohippocampal axons, animals received either intraseptal injections of HCM or control vehicle. Binding of [3H]quinuclidinylbenzilate ([3H]QNB) within iris implants was used as an index of cholinergic muscarinic receptor concentration. The results indicate that within the iris implants: (1) a dramatic drop in the number of muscarinic receptors is observed 4 days after denervation; (2) under control conditions, central cholinergic innervation is not associated with muscarinic receptor recovery; and (3) after administration of HCM, muscarinic receptor levels begin to increase within two weeks and approach the pre-lesion endogenous concentration following 28 days of treatment. These results support the hypothesis that trophic factors may facilitate the restoration of effective, appropriate connections between nerve fibers and their targets.

Histochemical studies of sympathetic sprouting: fluorescence morphology of noradrenergic axons

The importance of distinguishing between central and peripheral noradrenergic axons is evident from recent observations that sympathetic fibers will invade the central nervous system following specific lesions. The present paper reviews the normal histofluorescence appearance of peripheral and central NE fibers in several species as well as their appearance following experimental manipulations. The most striking differences between these two types of NE neurons is their axonal fluorescence morphology which is apparently determined by the target tissue, and their responsiveness to nerve growth factor (NGF). The latter may account for the remarkable growth of sympathetic axons into regions of the central nervous system denervated of cholinergic fibers. The use of glyoxylic acid in studying such sprouting is also discussed.

Regeneration of ganglion cell axons in the adult mouse retina

The hypothesis that regenerative failure of axons in the adult mammalian CNS is due to release of a growth inhibitor from injured oligodendrocytes and/or myelin2, predicts that regeneration of injured fibers would proceed unchecked in unmyelinated CNS regions. This prediction was borne out by observations on the stratum opticarum of the mouse retina. Axonal sprouts, first seen 14-16 h post-lesion (pl), continued growing until at least 100 days pl, well beyond the time at which regeneration fails in myelinated CNS regions.

Heart conditioned medium promotes central cholinergic regeneration in vivo

The aim of the present study was to evaluate the effect of a proposed cholinergic growth factor, heart conditioned medium (HCM), on the regeneration of the cholinergic septo-hippocampal pathway in rats. At 0, 4, 8 and 16 days after mechanical injury to septo-hippocampal axons, animals received either repeated intraseptal injections of HCM or control vehicles. Recovery of choline acetyltransferase (CAT) activity in iris implants placed in the hippocampus was used as an index of cholinergic regeneration. The results indicate that CAT activity in iris implants from animals receiving HCM was significantly greater than in control implants. Thus, this is the first report of the in vivo activity of heart conditioned medium as a cholinergic growth factor.

Reformation in adult rats of functional septo-hippocampal connections by septal neurons regenerating across an embryonic hippocampal tissue bridge

Implants of embryonic hippocampus, placed into a cavity transecting the septo-hippocampal pathway in adult rats, have previously been shown to promote the regeneration of the lesional septo-hippocampal axons across the cavity. The present study provides electrophysiological evidence that these regenerating acetylcholinesterase-containing axons are able to re-establish atropine-sensitive excitatory synaptic connections in the grafted hippocampus as well as in the dorsal part of the initially denervated host hippocampus, thus demonstrating that intracerebral tissue implants can promote true, functional regeneration across a major tissue defect in the adult mammalian CNS.

Outgrowth of cholinergic nerves in the rat urinary bladder either partially denervated or partially denervated and decentralized

Unilateral excision of the pelvic ganglion caused a loss in the number of AChE-positive nerves in the rat urinary bladder both on the operated side and on the contralateral side, thus indicating a bilateral intramural distribution of cholinergic nerves derived from the pelvic nerve. In the course of the subsequent observation period (3-28 days) the AChE-positive nerves increased in number and in staining intensity and further, the nerves became ramified and twisted. Similar events were found to occur in the urinary bladder decentralized on one side and denervated on the other. The morphological findings indicate an outgrowth of cholinergic nerves by collateral sprouting. These findings are discussed in relation to previous physiological studies.

Regeneration of the septohippocampal pathways in adult rats is promoted by utilizing embryonic hippocampal implants as bridges

The ability of embryonic hippocampal tissue to promote regeneration of cholinergic axons in the septohippocampal system has been studied in adult rats. Strips of embryonic hippocampus, taken from 7-40 mm rat fetuses, were implanted into a 2-3 mm wide cavity which completely transected the septal cholinergic axons innervating the intrinsic hippocampus. The ingrowth of cholinergic fibres into the denervated host hippocampal formation was monitored by measuring the activity of the enzyme, choline acetyltransferase (ChAT), and by acetylcholine esterase (AChE) histochemistry. The results demonstrated a gradual, partial return of both ChAT enzyme activity and AChE-positive fibres in the initially denervated hippocampal formation of the adult recipient. Time-course studies indicated that this ingrowth progressed from the implant into the rostral tip of the host hippocampus, and continued caudally to cover the entire dorsal hippocampus by 3-6 months postoperative. Although the regenerating AChE-positive fibres reached the hippocampal target in the recipient along abnormal routes, they reinnervated selectively the appropriate terminal areas within the host hippocampus and dentate gyrus, suggesting the presence of quite specific mechanisms to guide the regenerating axons back to their original targets. Lesions of the medial septum-diagonal band area of the host and horseradish peroxidase (HRP) injections into the host hippocampus, caudal to the implant, indicated that the origin of the regenerating axons was predominately from the ipsilateral ventral medial septum and diagonal band area of the host. The results provide evidence that axonal regeneration and reinnervation of a denervated target zone can be promoted by utilizing implants of embryonic CNS tissue to bridge a tissue defect between the target and the lesioned axonal stumps.

Innervation of embryonic hippocampal implants by regene-rating axons of cholinergic septal neurons in the adult rat

The regeneration of the septal cholinergic system in adult rats has been studied in animals bearing transplants of hippocampus taken from 20-40 mm rat fetuses (approximately 17-21 days of gestation). The septal axons located within the fimbria and the dorsal fornix were lesioned and a cavity was prepared at the rostral end of the hippocampus. The embryonic tissue was placed adjacent to the severed end of the fornix-fimbria. The time-course of ingrowth of cholinergic fibers into the transplant was monitored by acetylcholine esterase (AChE) histochemistry and the determination of the levels of choline acetyltransferase (ChAT). Both methods indicate that there is a progressive ingrowth into the transplant of cholinergic fibers up to 3 months after transplantation. The newly-formed AChE-positive fibers in the transplant remain beyond one year after transplantation and are thus presumably permanent. Both horseradish peroxidase (HRP) injections into the implant and radiofrequency lesions of the septal-diagonal band area indicate that the principal source of these fibers is the AChE-positive neurons of the medial septum and the nucleus of the diagonal band which normally form the septohippocampal cholinergic projection. The results suggest: (1) that implants of a normal embryonic target tissue can promote axonal regeneration in mature neurons of the mammalian central nervous system; (2) that some neurons in the adult mammalian CNS retain at least part of their embryonic capacity to generate axons and recognize specific postsynaptic targets in developing CNS tissue; and (3) that this host-implant interaction can result in the formation of quite specific innervation patterns in the implanted target tissue.

Delayed-onset dystonia in patients with "static" encephalopathy

Eight cases of persistent dystonia appearing one to 14 years after non-progressive cerebral insults are described. Five were due to perinatal anoxia, one to trauma, and two to cerebral infarction. This phenomemon of delayed-onset dystonia has not been described previously, although review of earlier literature reveals several probable examples. Delayed-onset dystonia due to perinatal anoxia is an important diagnostic alternative to dystonia musculorum deformans for dystonia occurring in childhood.

Axons from CNS neurons regenerate into PNS grafts

Axons in the peripheral nervous system (PNS) and central nervous system (CNS) form sprouts after injury. Elongation of regenerating axonal sprouts has been observed as the exception within the adult mammalian CNS but is the rule in the PNS of mammals as well as in the CNS of some fish and amphibians. The relative importance of intrinsic neuronal properties and axonal environment in determining the extent of axonal regrowth is unknown. Neuroglial cells, nerve growth factor and target tissues such as smooth muscle are known to influence neuronal responses to injury. Here we have examined the capacity of transected axons originating in the CNS to regrow into nerve grafts containing Schwann cells.

Dopamine metabolism, spiperone binding and adenylate cyclase activity in the adult rat hippocampus after ingrowth of dopaminergic neurones from embryonic implants

The growth of dopamine (DA) neurones of rat embryonic mesencephalic brain tissue implanted close to the anterior part of the hippocampus of adult rats was studied by measuring the levels of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) in the host hippocampus. The hippocampal levels of DA and DOPAC reached maximal values 6 months after transplantation. The neuroleptic drug haloperidol evoked an increase in the levels of DOPAC. No evidence was found for the induction of DA-sensitive adenylate cyclase activity or of binding sites of [3H]spiperone. These results indicate that there is a substantial ingrowth of transplanted DA neurones into the host hippocampus, but no evidence was found for the development of functional contacts between the embryonic DA neurons and the host tissue.

Regeneration of the magnocellular system of the rhesus monkey following hypothalamic lesions

The hypothalamic magnocellular system of the rhesus monkey was studied with specific immunocytochemical techniques in animals that had undergone hypothalamic lesions. The results indicate that this system maintains a regenerative capacity even when its tracts are interrupted within the hypothalamus. New neurohemal units are reconstituted from newly formed vessels within the scar as well as from preexistent blood vessels, such as perforating and pial arterioles, and the vessels of the pars tuberalis of the pituitary gland, which normally do not contain neurosecretory terminals.

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.

Reinnervation, myelination and organization of iris tissue implanted into the rat midbrain--an ultrastructural study

The ultrastructure of autologous irides implanted into the midbrain of mature Sprague-Dawley rats was studied over a time period of 4-14 days. Most features of the normal iris still persisted throughout this time, including typical iris blood vessels and amelanotic melanocytes. At 5-4 days after implantation, the normal innervation of the implanted iris had degenerated, except for the presence of intact Schwann cells and some myelin debris. Reinnervation, beginning about the seventh day, proceeded rapidly. By the fourteenth day, extensive reinnervation of the implant was evidenced by the presence of numerous small (0.1-0.6 micrometer) and large (2-4 micrometer) non-myelinated axons ensheathed in Schwann cell cytoplasm. Axonal varicosities, filled either with dense core or clear core vesicles, formed junctions with axons. These junctions were characterized by an accentuation of areas of the axonal membrane and pre- or post-junctional thickenings; however, we did not observe typical synaptic complexes. Some large axons within the myelinated iris developed thick myelin sheaths of the peripheral nervous system (PNS) type; we believe this is the first reported instance in which myelination of central axons by Schwann cells within the brain parenchyma has been produced by the implantation of PNS elements.

Quantitative cytochemistry of RNA in axotomized feline rubral neurons

One-sided lateral funiculotomy at the C-2 segment induced axon reaction in the contralateral red nucleus of adult cats. Two to 60 days postoperatively the animals were sacrificed and the mesencephalon was fixed in ethanol-acetic acid, 3:1. Ten micrometer paraffin sections including both red nuclei were stained for RNA with azure B after incubation in DNAse. Cytophotometric measurements of RNA content of neurons from the caudal 600-1000 micrometer of each red nucleus were made with a Zeiss Cytoscan system using an automatic scanning stage. In contrast to the heightened RNA synthesis that has been reported for axotomized peripheral (extrinsic) neurons, the axotomized central (intrinsic) neurons of the red nucleus showed no evidence of accumulation of cytoplasmic or nucleolar RNA. Rather depletion of cellular RNA occurred. Further indication of the regressive nature of rubral axon reaction derived from morphometric measurements that showed cytoplasmic, nuclear and nucleolar atrophy of the neurons of the red nucleus contralateral to operation with the exception of a possible transient cytoplasmic enlargement 9 days postoperatively. From the cytophotometric and morphometric data here reported we are led to suggest that the frequently observed failure of axonal repair in mammalian CNS results from the innately regressive nature of the axon reaction of many mammalian central neurons.