AXONAL MIGRATION OF PROTEINS IN THE CENTRAL NERVOUS SYSTEM AND PERIPHERAL NERVES AS SHOWN BY RADIOAUTOGRAPHY

Slow axonal protein transport and axoplasmic organization

A recently formulated structural theory of axonal transport suggests that each group of transported material is a distinct class of functionally related structures and that the proteins making up those structures, with few exceptions, are found in only one rate component, thus maintaining a close noncovalent association during transport. If such a relationship exists, one would postulate that the labelled proteins at the leading edge and trailing edge of a rate component following a pulse of radioactive amino acid would be present in similar proportions. After retinal ganglion cell proteins were labelled by intraocular injection of radioactive amino acid in the guinea pig eye, the optic nerve and tract were analyzed at several post-injection intervals. The data thus derived support the existence of a close relationship among the "soluble" proteins of slow component b and are consistent with the structural concept of axonal transport. If diffusion were the mechanism of transport, the larger proteins would be expected to move slower than the smaller proteins. Such insights into the process of axonal transport should help to identify the variables critical to survival of neurons following acute trauma and degenerative diseases.

Afferent influences on brain stem auditory nuclei of the chicken: changes in succinate dehydrogenase activity following cochlea removal

We have examined one of the metabolic consequences of unilateral cochlea (basilar papilla) removal in the chick brain stem auditory system. We assessed changes in succinate dehydrogenase (SDH), a mitochondrial enzyme involved in energy metabolism, in neurons of second-order n. magnocellularis (NM) and third-order n. laminaris (NL). Chickens undergoing surgery at 10 days of age were perfused 4 hours to 35 days postlesion. Chickens 6 or 66 weeks of age at cochlea removal were examined 1 or 8 days after surgery. In all groups, cryostat sections were prepared for SDH histochemistry or Nissl staining. In normal chickens, NM cell bodies and NL neuropil contain SDH reaction product. In young birds, the density of SDH reaction product in NM shows a rapid biphasic response to cochlea removal. From 8 to 60 hours postlesion, density increases ipsilateral to cochlea removal; for survival times of 3-35 days, SDH density decreases in ipsilateral NM. In NL, no changes were observed until 3 days after cochlea removal. Then we observed a long-lasting decrease in density of SDH reaction product in the neuropil regions receiving input from the deafferented NM. All of these changes are age-dependent in that they were observed only following cochlea removal on or before 6 weeks of age.

Afferent influences on brain stem auditory nuclei of the chicken: cessation of amino acid incorporation as an antecedent to age-dependent transneuronal degeneration

Previous studies of the avian auditory system have revealed that removal of the peripheral receptor (the cochlea) leads to a transneuronal degeneration of auditory relay neurons in nucleus magnocellularis (NM) of the brain stem. An early manifestation of the degeneration which can be observed within 12 hours is a decrease of histochemical staining for RNA (Nissl staining); such a decrease could reflect an alteration in protein synthetic activity within the NM neurons. The present study evaluates this possibility by determining whether the cochlea removal led to an alteration incorporation of protein precursors in the target neurons which exhibit transneuronal degeneration and if so, how early the changes appeared. The cochlea was removed unilaterally in seventeen 10-day-old chicks and two 66-week-old mature chickens, and incorporation of protein precursors was evaluated in the neurons of NM at 0.5, 1.5, 3, 6, 12, and 24 hours following the cochlea removal. Each chick received an intravenous injection of 3H leucine, and was allowed to survive for 30 minutes after the injection of precursor. The brains were then prepared for autoradiography. The extent of incorporation by neurons in NM was determined by counting grains overlying each cell body and determining grain density/micrometers2 of neuron cross-sectional area. We found that auditory relay neurons whose synaptic inputs have been silenced exhibit dramatic decreases in protein synthesis within 30 minutes after removal of the cochlea; leucine incorporation was reduced by about 50%. In chicks sacrificed 3 to 24 hours after removal of the cochlea, some neurons (about 1/3) were entirely unlabeled despite heavy labeling of their neighbors and heavy labeling of all NM neurons on the opposite side of the brain. The remaining neurons exhibited about a 15% reduction in incorporation in comparison with the cells in the contralateral (control) NM. While the decreases in incorporation were apparent at all survival intervals, there was no consistent decrease in Nissl staining until 6 hours after cochlea removal. There were no changes in protein precursor incorporation following removal of the cochlea in adult birds, a result which is in keeping with the relative absence of transneuronal degeneration following removal of the cochlea at maturity. The results suggest a very rapid transneuronal regulation of protein metabolism within target neurons in young animals, perhaps by activity-related events.

Axon reaction in hypoglossal and dorsal motor vagal neurons of adult rat: incorporation of [3H]leucine

Pairs of adult rats received [3H]leucine (i.p., 5 microCi/g body weight) 0.25, 1, and 16 h before killing and zero (unoperated control animals) and 1 to 164 days after unilateral cervical vagotomy and hypoglossal neurotomy. Grain counts and morphometric measurements were made on axotomized and uninjured neurons in histoautoradiographs of the medullary nuclei. Axotomized hypoglossal neurons, which largely survive the injury, both enlarged and incorporated increased amounts of tritiated leucine at each labeling interval, 3 through 28 days postoperatively. In the vagal dorsal motor nucleus (DMN), axotomized cells, which frequently die after neurotomy, enlarged slightly through 28 days postoperatively, then atrophied; DMN neurons increased amino acid uptake for a shorter period (days 7 through 14) than hypoglossal neurons. This increase achieved statistical significance only when the labeling intervals were 0.25 or 1.0 h. Neurons of the DMN contralateral to vagotomy also enlarged. Axotomized DMN neurons did not sustain increased protein synthesis as long as their hypoglossal counterparts and seemed to fail to increase synthesis of structural proteins with long half-lives (16-h labeling interval). The frequently necrobiotic response of axotomized DMN neurons may relate to these phenomena. From these and earlier results, we conclude that axon reaction appears to differ fundamentally in peripheral and central neurons. This difference may have significance for research on regeneration in the central nervous system.

Cellular composition of a cerebral hemisphere primary culture

In this overview attention is given to available markers and methods for characterizing cell elements in a culture system. Primary cultures from newborn rat cerebral hemispheres were grown for 14 days. The population of cells was dominated by astrocytic glial cells (60-70%), but cells with properties of macrophages, endothelial-like cells, mesenchymal-like cells, ependymal-like cells, and oligoblasts were also found. Neither mature neurons nor oligodendroglial cells were observed. The enrichment in astroglial-like cells makes the cultures a satisfactory astroglial-cell model, at least for some purposes.

Neurofilament accumulation induced in synapses by leupeptin

The hypothesis that the usual absence of neurofilaments in synaptic terminals is due to their degradation by the calcium-activated protease present in axoplasm was tested by injecting leupeptin, which inhibits the protease, into the optic tectum of goldfish kept at 15 degrees and at 25 degrees C. The resulting accumulation of neurofilaments in synaptic terminals provides in vivo evidence in support of the hypothesis. The significance of these results and the potential uses of this pharmacological tool are discussed.

Neuronal and transneuronal tracing in the trigeminal system of the rat using the herpes virus suis

The herpes virus suis has been used as a tracer for the pathways in the central nervous system of the rat. The viruses have been inoculated in various peripheral structures innervated by the trigeminal nerve, namely in the cornea by instillation or scarification, in the anterior chamber of the eye by injection and also by subconjunctival injection, nasal instillation and injection in the masseter muscle. The herpes virus suis is easy to detect by immunofluorescence or electron microscopy, the tracing is precise because it does not diffuse, as some other tracers. The virus is replicated at the site of inoculation and at each neuronal relay, thus 'fresh' tracer is continuously brought into the system. The herpes virus suis is transported by retrograde axonal flow. It has been observed in the motor sensory, sympathetic and parasympathetic pathways up to the central nuclei, which demonstrates transneuronal transport. The selectivity of this tracer, applied to the trigeminal pathways, has allowed us to understand the function of the 3 types of neurons present in the trigeminal ganglion, namely to confirm their somatotopy and establish their central projections in the trigeminal system.

Increases in protein-precursor incorporation in the denervated neuropil of the dentate gyrus during reinnervation

Cellular metabolic events accompanying postlesion synaptogenesis in the hippocampus were studied by analyzing incorporation of protein precursor ([3H]leucine) in the dentate gyrus. Adult rats were injected intravenously with [3H]leucine at periods from 2 to 60 days following unilateral destruction of the entorhinal cortex, and were killed 30 min later. Precursor incorporation was quantified autoradiographically by counting silver grains over the cell bodies and dendrites of dentate granule cells ipsi- and contralateral to the lesion. The relative grain density was increased over the denervated portion of the neuropil at 6-12 days postlesion, corresponding to the early phase of terminal proliferation and reactive synaptogenesis. Whereas incorporation was increased over the denervated neuropil, the availability of [3H]leucine was decreased relative to the contralateral side in autoradiographic preparations designed to reveal the concentration of the unincorporated 3H-labeled precursor and its diffusible degradation products. Silver grains were not selectively associated with glial cells bodies or vascular elements, but rather were distributed diffusely throughout the neuropil. Increases in grain density over the denervated zone were observed when animals were killed 8 min after the leucine injection, suggesting that the increases were not due solely to rapid transport from granule cell bodies to dendrites. We propose that an increased incorporation of protein precursor occurs primarily within the denervated dendrites of granule cells during the early phase of reinnervation, and that protein synthetic activity in these cells might be involved in the process of reinnervation.

Axonal transport in neuropathy

Recent studies on the distribution of labeled endogenous proteins in the experimental neuropathies induced by streoptozotocin diabetes, galactose feeding, zinc pyridinethione, 2,5-hexanedione, acrylamide, and p-bromophenylacetylurea (BPAU) have demonstrated an impaired build up of retrogradely transported material derived from the more distal parts of peripheral nerve. In BPAU neuropathy the transport abnormality is strongly related to the degree of muscle weakness; following treatment with acrylamide the extent of the impairment is dose related. In both toxic conditions the transport abnormality is present well in advance of the first clinical signs of neuropathy. We therefore suggest that changes in retrograde axonal transport play an initial and important role in the development of many axonopathies.

Metabolic variation among rat lumbosacral alpha-motoneurons

Despite the wealth of literature concerned with muscle fiber biochemical, ultrastructural and physiological characteristics, little information is available regarding the metabolic enzyme activities of alpha-motoneurons. The present study examines the metabolism of alpha-motoneurons located in the lateral cell column of the rat lumbosacral enlargment with a quantitative histochemical technique. Variation in the activities of alpha-glucan phosphorylase, NADH-diaphorase, succinic dehydrogenase and acid phosphatase were detectable with the photographic densitometry and atomic absorption spectrophotometry technique. No difference in the glycolytic enzyme activity (mitochondrial alpha-glycerophosphate dehydrogenase) was observed. Analysis of lactate dehydrogenase isoenzymes demonstrated the existence of H type isoenzyme in alpha-motoneurons, consistent with other observations indicating predominance of aerobic metabolism within these neurons. The activities of the former enzymes in alpha-motoneurons formed a complete spectrum of activities, distributed unimodally. Smaller motoneurons exhibited the greatest NADH-D and acid phosphatase activities; phosphorylase activity was greatest in larger motoneurons. Significant variation in the enzyme activity of similar-sized motoneurons suggests that the metabolism of the motoneuron is regulated by factors other than cell size. Relationships between motoneuron metabolic enzyme activity and motor unit type are under current investigation.

Axonal transport of glycerophospholipids in regenerating sciatic nerve of the rat during aging

The effect of age upon the axoplasmic transport of glycerophospholipids has been studied using as a model the regenerating sciatic nerve of young (2-month-old), young adult (6-month-old), middle-aged (16-month-old), and aged (20-month-old) male rats. The right sciatic nerve was crushed 0.5 mm down the incisura ischiadica. Four and nine days after the lesion, a mixture of [2-3H] glycerol and [methyl-14C] choline was bilaterally injected into the spinal cord, at a level of the L4-L5 vertebrae. The animals were killed 18 hr after the isotope injection. Proximal and distal portions of crushed nerve and of contralateral sham-operated ones were dissected and consecutive 5-mm segments were subjected to lipid extraction and analysis. The findings of the present study are summarized as follows: (1) The accumulation of labeled lipid material axonally transported four days after nerve injury was mainly located at the crush site in young, young adult, middle-aged, and aged rats. The accumulation of both 3H-glycerolipids and 14C-choline phospholipids in postcrush segments was markedly higher for young and young adult than for aged rats, four and nine days after crush; (2) the average rate of axonal regeneration, determined between days 4 and 9 following crush injury was 3.6 and 4.2 mm/day for 2-month-old and 6-month-old rats, respectively; it decreased to the value of 2.5 mm/day for 16-20-month-old rats.

Focal seizures disrupt protein synthesis in seizure pathways: an autoradiographic study using [1-14C]leucine

We have used a new autoradiographic technique developed by Smith et al.22,33 for visualizing rates of incorporation of [1-14C]leucine into protein in brain. Focal seizures caused by topical convulsants resulted in a marked decrease in autoradiographic density. This was primarily confined to the seizure focus, especially marked in pyramidal cell layers, and to subcortical seizure pathways. There were no distinct changes in cortico-cortical pathways beyond the seizure focus. Pure orthodromic pathways through basal ganglia showed an 18% inhibition of leucine incorporation in caudate nucleus and substantia nigra, pars compacta (P less than 0.05). By contrast, thalamic nuclei connected both ortho- and antidromically to the focus showed a 30-63% inhibition (P less than 0.01). The topographic pattern and intensity of the thalamic changes were related to the site, size and intensity of the seizure focus. As seizures became severe there was a more generalized depression of metabolism beyond seizure pathways, especially in the ipsilateral hemisphere. The results suggest that seizures block incorporation of leucine into protein either by an increase oxidation of the precursor, and/or an inhibition of protein synthesis per se. The effect is most severe in neurons undergoing epileptic burst discharge in the focus and in thalamic neuronal beds connected reciprocally with the focus.

Cross-linker system between neurofilaments, microtubules, and membranous organelles in frog axons revealed by the quick-freeze, deep-etching method

The elaborate cross-connections among membranous organelles (MO), microtubules (MT), and neurofilaments (NF) were demonstrated in unifixed axons by the quick-freeze, deep-etch, and rotary-shadowing method. They were categorized into three groups: NF-associated cross-linker, MT-associated cross-bridges, and long cross-links in the subaxolemmal space. Other methods were also employed to make sure that the observed cross-connections in the unfixed axons were not a result of artifactual condensation or precipitation of soluble components or salt during deep-etching. Axolemma were permeablized either chemically (0.1% saponin) or physically (gentle homogenization), to allow egress of their soluble components from the axon; or else the axons were washed with distilled water after fixation. After physical rupture of the axolemma or saponin treatment, most of the MO remained intact. MT were stabilized by adding taxol in the incubation medium. Axons prepared by these methods contained many longitudinally oriented NF connected to each other by numerous fine cross-linkers (4-6 nm in diameter, 20-50 nm in length). Two specialized regions were apparent within the axons: one composed of fascicles of MT linked with each other by fine cross-bridges; the other was in the subaxolemmal space and consisted of actinlike filaments and a network of long cross-links (50-150 nm) which connected axolemma and actinlike filaments with NF and MT. F-actin was localized to the subaxolemmal space by the nitrobenzooxadiazol phallacidin method. MO were located mainly in these two specialized regions and were intimately associated with MT via fine short (10-20 nm in length) cross-bridges. Cross-links from NF to MO and MT were also common. All these cross-connections were observed after chemical extraction or physical rupture of the axon; however, these procedures removed granular materials which were attached to the filaments in the fresh unextracted axons. The cross-connections were also found in the axons washed with distilled water after fixation. I conclude that the cross- connections are real structures while the granular material is composed of soluble material, probably protein in nature.

A method of dark-field and simultaneous light--dark-field illumination for photomacrography of autoradiographic preparations

A technique is described for dark-field and light--dark-field illumination of autoradiographic preparations for photomacrogra. A right-angle prism is employed as a substrate through which illumination is introduced to the specimen. The technique permits uniform illumination of large fields of view, and also minimizes the image-degrading effects of chromatic dispersion and excessive noncoherent scatter which are often associated with conventional dark-field illumination.

Inhibition of axoplasmic transport in the developing visual system of the rat-L Structural changes in the retina and optic nerve with graded doses of intraocular colchicine

In order to study the role of axonal transport in the mediation of transneuronal metabolic stimulations upon a population of differentiating neurons, colchicine, a potent inhibitor of rapid and slow phases of axonal transport, was injected into the eye of albino rats at 1, 3, 5, 10, 15 and 20 days postnatal in concentrations ranging from 10-5 M to 2 X 10-2 M and in quantities of 0.3 to 0.5 microliter. Quantitative light and electron microscopy were subsequently employed to assess reactive alterations in the developing retina and optic nerve. Application of colchicine severely retarded the development of the sensory elements, with disappearance of synaptic ribbons of sensory cell axons, a significant reduction in the thickness of the inner plexiform layer, due to the presence of numerous shruken synaptic elements and the appearance of rosettes of sensory cells displaced to the inner nuclear layer. These alterations were found to be dose-dependent. Counts of ganglion cell populations at various times after application of colchicine demonstrated optimal concentrations which could be injected at each postnatal age without causing ganglion cell degeneration. Ultrastructural examination of such cells revealed varying degrees of disorganization and dissolution of the endoplasmic reticulum with the formation of occasional small cytoplasmic vacuoles. Higher concentrations of colchicine caused extensive vacuole formation in all classes of retinal neurons, scattered hyperchromic cells and widespread degeneration and autolysis. The diameter of the optic nerve was reduced to 60-95% of normal following intraocular colchicine, depending on the concentration employed, but electron microscopy revealed normal patterns of distribution of axoplasmic microtubules and filaments in control and experimental animals and quantitative analysis revealed no significant loss of axons. While no reactive changes took place in individual elements, the periphery of the nerve was often indented by a highly-folded glia limitans. Maximal doses of intraocular colchicine for each age level were established by this study. These were: 1 day. 10-3 M: 5 days, 5 x 10-3 M; 10 days, 5 X 10-3 M; 15 and 20 days, 10-2 M. The information derived from this morphological analysis provides the foundation for subsequent measurements of axonal transport inhibition in the developing visual system to be reported in the second article of this series.

Protein synthesis by astrocytes in primary cultures

Protein synthesis, measured as leucine incorporation into acid-precipitable proteins, was determined in astrocytes in primary cultures obtained from the cerebral hemispheres of newborn mice. As can be expected for eucaryotic, ribosomal protein synthesis, the incorporation was almost completely inhibited by cycloheximide (0.01 mM), but unaffected by chloramphenicol (0.03 mM). The rate of synthesis, measured during exposure to a high (0.8 mM) concentration of leucine was 5.4 nmol/hr/mg protein in mature (i.e., at least 4-week-old) cultures. This value is at least twice as high as the protein synthesis rates reported for the adult brain in vivo, suggesting that a very considerable part of the protein synthesis in the adult brain may take place in astrocytes. The molecular weight distribution of the synthesized proteins was determined by polyacrylamide gel electrophoresis, demonstrating synthesis of at least 50 different polypeptides, ranging in molecular weight between 190,000 and 27,000 daltons. The pattern of the synthesized proteins underwent considerable alteration with age in young cultures in which the total content of protein was still increasing, but it was remarkably stable after the age of two weeks. Exposure to dibutyryl cyclic AMP, which is known to alter morphology, content of glial fibrillary acidic protein (GFA), and activities of certain enzymes in the cultures in the cultured astrocytes, caused marked alterations in the pattern of the synthesized proteins.

Regional protein synthesis in rat brain following acute hemispheric ischemia

Regional protein synthesis was measured in rat brain at intervals up to 48 h following occlusion of the four major arteries to the brain for either 10 or 30 min. Four-vessel occlusions produces ischemia in the cerebral hemispheres and oligemia in the midbrain-diencephalon and brainstem. During the hour following 10 min of ischemia, protein synthesis, measured by incorporation of [14C]valine into protein, was inhibited in the cerebral cortex by 67%. Normal rates of protein synthesis were attained within 4 h of recirculation. In rats subjected to 30 min of ischemia, protein synthesis was inhibited by 83% during the first hour of recirculation in the cortex, caudate-putamen, and hippocampus. Recovery of protein synthesis in these regions was slow (25-48 h). The midbrain-diencephalon showed less inhibition, 67%, and faster recovery (by 12 h). Protein synthesis was unaffected in the brainstem. [14C]Autoradiography revealed that the pyramidal neurons of the hippocampus and areas of the caudate and cortex failed to recover normal rates of protein synthesis even after 48 h. The accumulation of TCA-soluble [14C]valine was enhanced (55-65%) in the cortex, caudate, and hippocampus after 30 min of ischemia; the increase persisted for 12 h. A smaller rise in [14C]valine content (30%) and more rapid normalization of valine accumulation (by 7 h) were observed in the midbrain-diencephalon; no changes were found in the brainstem. In the cortex, recovery was more rapid when the duration of ischemia was reduced. Thus, the degree of inhibition of protein synthesis, the accumulation of valine in the tissue, and the length of time required to reestablish normal values for these processes were dependent on both the severity and the duration of the ischemic insult. Restoration of normal rates of protein synthesis after ischemia was slow compared with the normalization of cerebral energy metabolites.

Axoplasmic transport in vagal afferent fibres in normal and alloxan-diabetic rabbits

Fast and slow components of anterograde axoplasmic transport have been studied in the sensory fibres of the vagus nerve of alloxan-diabetic rabbits and age-matched controls by incorporation of tritiated leucine into nodose ganglion cells. The diabetic rabbits were maintained for 2 months with blood glucose levels in the range 20--40 mmol/l. They showed growth retardation and one third developed cataracts. No alteration of either fast or slow axoplasmic transport was detected in the diabetic animals. These results are discussed in the light of the present understanding of the role of axoplasmic transport, of the findings in other axoplasmic transport studies, and of other data available on the pathogenesis of human and experimental diabetic neuropathy.

Impaired retrograde axonal transport from a nerve crush in streptozotocin diabetic rats

The axonal transport of proteins in crushed nerves of streptozotocin (40 mg/kg) diabetic rats was investigated 4 weeks after induction of diabetes. 35S-methionine was used as a marker for protein and 3H-fucose as a marker for glycoprotein. The precursors were injected into the fifth lumbar spinal ganglion and the accumulation of TCA-insoluble activity proximal and distal to a sciatic nerve ligature was measured at different time intervals after application of a crush. The start of accumulation distal to the ligature was delayed by 1 hour for proteins as well as for glycoproteins. Furthermore, the total amount of accumulated protein after 19 h was decreased by 18% while the decrease was 21% for glycoprotein. By insulin treatment the differences could both be prevented and reversed after 3 days of normoglycaemia. These findings demonstrate an impaired response to a nerve crush and might be the explanation for the regenerative abnormalities of peripheral nerves in diabetes.

Decreased axonal transport of structural proteins in streptozotocin diabetic rats

We have examined the various axonal transport rates in sciatic nerve of streptozotocin diabetic rats 3 h and 10,25, and 50 d after the injection of tritiated proline into the fifth lumbar dorsal root ganglion. Proline-labeled proteins conveyed by the slow transport system were advanced more slowly in diabetic rats. No compensation for this delay took place in terms of protein synthesis, half-life, or transported amount. The decreased deliverance of slowly transported proteins (structural proteins) to the axons may well account for the reduced axon calibre shown in earlier reports. A hypothesis is proposed suggesting that the primary event in the development of neurological abnormalities in diabetes is an impairment of the retrograde axonal transport, secondarily leading to the abnormality of the anterograde transport of structural proteins.

Pattern of development of ascending and descending fibers in embryonic spinal cord of chick: I. Role of position information

Previous studies on the rigid time and position pattern of embryonic spinal cord development are extended to include the pattern of growth of axonal processes. This study was designed to analyze the position of axons in the marginal zone as they course (fasciculate) in the ascending and descending direction from a single spinal segment. Injections of 3H-proline were made into L1 or T6 of chick embryos of stages 21--29 (3.5--6 days of incubation). The location of radioactive label in the marginal zone was analyzed autoradiographically to show the location of labeled axons originating from the injected segment. In the early stages of development (stage 21), the labeled descending axons were located in the most ventromedial aspect of the marginal zone. The labeled ascending axons were located more laterally and to the perimeter of this zone. In stages 27--29, the labeled axons near the site of injection were located in the inner two-thirds of the marginal zone, and in the progressively more rostral levels the label shifted laterally and to the perimeter in a laminar pattern. It was concluded that the labeled ascending axons from an individual spinal segment layer upon the axons which originate from the more rostral segments. How this rigid time and position pattern of axonal growth can impart a topographic order into fiber tracts of the spinal cord was discussed.

Pattern of development of ascending and descending fibers in embryonic spinal cord of chick: II. A correlation with behavioral studies

Studies on the embryonic spinal cord were extended to include the development of the ascending and descending fibers from the brachial level. This study was designed to correlate morphological development of the intersegmental system with physiological and behavioral observations of early embryonic behavior. Chick embryos were analyzed prior to day 6 of incubation, which is before closure of the reflex arc and arrival of supraspinal inputs to the embryonic spinal cord. Injections of 3H-proline were made in the upper brachial level of chick embryos of stages 27--29 (5--6 days). The location of radioactive label in the marginal zone was analyzed autoradiographically to show the projection of labeled axonal flow from the site of injection. The majority of the labeled axons in the marginal zone was in the first 2--3 segments in both ascending and descending directions from the site of injection. By stage 27 (day 5), ascending labeled axons extended to the upper cervical level and labeled descending axons extended only about 4--5 segments to the upper thoracic level. By stage 29 (day 6) labeled ascending axons extended into the medulla oblongata, and labeled descending axons still extended only 4--5 segments caudally. These studies (Parts I and II) show that at stage 27 (day 5) there was considerable axoplasmic flow of radioactive label in the marginal zone from the lumbar to the brachial plexus but not in the opposite direction from the brachial to the lumbar plexus. Also, axoplasmic flow of label initially extended from the brachial plexus to the medulla oblongata sometime between stages 27--29. It was concluded that the intersegmental system of the embryonic spinal cord is well developed by day 6 of incubation and provides the neuro-anatomical substrate for early embryonic behavior.

The orthograde labeling of postganglionic sympathetic cardiac nerves in the dog and cat as demonstrated by autoradiography

Previous functional and anatomical techniques have characterized the cardiac nerves and plexuses. They cannot, however, determine the course of fibers arising from a specific ganglion. This study has found that the intraaxonal orthograde labeling of axons can be used to determine the course of postganglionic sympathetic cardiac fibers in the dog and cat. The canine left caudal cervical ganglion and the feline right stellate ganglion were exposed through appropriate thoracotomies. Each ganglion received multiple injections of tritiated leucine (500 muCi/animal). Following a 3--14-day survival period the anesthetized animals were sacrificed by vascular perfusion. The injected ganglia, extracardiac nerves and selected portions of the heart were processed for autoradiography. Autoradiographs from the dog demonstrated labeled postganglionic sympathetic nerves in the extracardiac plexus, left atrial epicardium and the epicardium beneath the coronary sulcus. Labeled nerves in the cat heart were found within the epicardial layers and associated with blood vessels in the left ventricular myocardium. Neither myelinated fibers traveling through the injection site nor intrinsic cardiac ganglion cells were labeled, although the latter were often closely approximated to heavily labeled fibers.

Taurine in the developing rabbit visual system: changes in concentration and axonal transport including a comparison with axonally transported proteins

[35S]Taurine injected intravitreally into rabbits was transported axonally to the optic nerve terminals. Considerably more [35S]taurine was transported in young rabbits than in mature rabbits. The time course of taurine transport did not parallel that of proteins labeled with [3H]proline in the same system. The concentration of taurine in all components of the visual system, except retina, was greater in young animals than in mature animals, and was especially high in optic nerve. The possible functions of the high concentrations of taurine and the greater amount of axonally transported taurine in developing mammalian CNS are discussed.

Axonal transport of [3H]proteins in a noradrenergic system of the rat brain

Proteins synthesized from [3H]leucine injected into the rat nucleus locus coeruleus (LC) were transported through the hypothalamus in 5 successive waves at rates of 72-192, 24-48, 13-20, 3-4 and 1.4-2.9 mm/day (waves I through V, respectively). Waves I through IV began axoplasmic transport in the LC within the first few hours after [3H]protein synthesis began in the LC. Wave V was delayed in onset for 1.7-3.7 days and was also probably transported through the contralateral hypothalamus. Wave IV was not transported within the LC-hypothalamic axons ascending through the dorsal noradrenergic bundle since its transport was not blocked by 6-OHDA lesions in this bundle, as was transport of the other 4 waves. Unilateral dorsal bundle lesions caused a well defined caudal backup of dopamine-beta-hydroxylase immunofluorescence and a fall in dopamine-beta-hydroxylase activity in the ipsilateral frontal cortex and hypothalamus of 55% and 9%, respectively. Bilateral lesions caused only a significantly further reduction in hypothalamic levels indicating crossed innervation of the hypothalamus by the LC of 27%. Waves I and II have been classified as rapid transport and contained 33% of the transported [3H]protein. Wave V was slowly transported and contained 51% of the transported [3H]protein, while wave III was intermediate in rate and contained 16% of transported [3H]proteins.

Fluids in the anterior part of the optic nerve in health and disease

New techniques have recently made it possible to study the flow of fluids (blood, axoplasm, and interstitial fluid) in the anterior part of the optic nerve. Blood flow has been reviewed previously; axoplasm and interstitial fluid are considered in this review. General concepts of axoplasmic transport (anterograde and retrograde) are outlined, and the role of axoplasmic transport in the pathogeneses of optic disc edema of various types, in glaucoma, and in ischemic and toxic optic neuropathies is discussed. The probable sources of interstitial fluid in the anterior part of the optic nerve are capillaries in the nerve itself, peripapillary choroid, vitreous, cerebrospinal fluid and possibly axoplasm in the local axons; the flow is defined by various barrier systems. The role of the interstitial fluid in the pathogeneses of optic edema (and associated phenomena) and in serous retinal detachment in the macular region associated with optic disc pit is discussed. Its involvement in the process of diffusion of retrobulbar medication into the optic nerve and vitreous is also considered.

Intraaxonal transport of Herpes simplex virus in the rat central nervous system

Light and electron microscopic observation 3--4 days after microinjection of Herpes simplex virus (HSV) into the left neostriatum of rat demonstrated the following results. (1) Virus labeled nerve cells were found in the ipsilateral substantia nigra; a large number of infected neurons were in the zona compacta and some were in the zona reticulata. No virus infection was evident in the contralateral side. (2) Virus labeled neurons were found in the cortex, a greater number ipsilaterally than contralaterally, and in the dorsal raphé nuclei. Cortical microinjection of HSV led to infection of some cortical cells but no neostriatal cells. We conclude, therefore, that spread of the virus to the cortex, the substantia nigra and the dorsal raphé following neostriatal injection was by retrograde axonal transport. (3) The left neostriatum, where HSV was injected, showed a surprisingly small number of virus infected neurons. The infected neurons were mostly the large neurons; the majority of medium sized neurons were well preserved. There was massive degeneration of nerve terminals throughout the neuropil. Most of these degenerating nerve terminals are considered to be afferent fibers.

Ascending projections of the locus coeruleus in the rat. I. Axonal transport in central noradrenaline neurons

Axonal transport of protein and metabolites of L-[3H(G)]3, 4-dihydroxyphenylalanine ([3H]DOPA) was studied in the central noradrenaline neurons of the pontine nucleus locus coeruleus and was correlated with regional alterations of noradrenaline content following destruction of the nucleus. Unilateral lesions of the locus coeruleus produce a partial depletion of noradrenaline in the ipsilateral hypothalamus and telencephalon, indicating that these neurons project widely to the ipsilateral forebrain. Twenty-four to 48 h following local injections of 50 micronCi [3H]proline, locus coeruleus neurons take up labeled material and transport it, presumably as protein, to ipsilateral structures in the midbrain, diencephalon and telencephalon including the neocortex. Similarly 8 h after injection of 25 micronCi [3H]DOPA into the locus coeruleus, transport of material including catecholamines occurs to ipsilateral diencephalon and telencephalon. Axonal transport of proteins to telencephalic structures is greatly diminished by selective lesions of catecholamine terminals with 6-hydroxydopamine (6-OHDA) and following destruction of the medial forebrain bundle. These results provide further support for the view that noradrenaline neurons of the locus coeruleus nucleus project widely within the neuraxis to ipsilateral structures of the brain stem, diencephalon and telencephalon, including all cortical areas. In addition, evidence is presented for a contralateral projection with a similar distribution. The rate of axonal transport of labeled protein and metabolites of [3H]DOPA including [3H]catecholamines in central noradrenaline neurons is estimated to be 3-4 mm/h and is accordingly similar to that reported for noradrenaline neurons of the peripheral sympathetic nervous system.

Columnar distribution of cortico-cortical fibers in the frontal association, limbic, and motor cortex of the developing rhesus monkey

The terminal distribution of cortico-cortical connections was examined by autoradiography 7-8 days following injections of tritium labeled amino acids into the dorsal bank of the principal sulcus, the posterior part of the medial orbital gyrus, or the hand and arm area of the primary motor cortex in monkeys ranging in age from 4 days to 5.5 months. Labeled axons originating in these various regions of the frontal lobe have topographically diverse ipsilateral and contralateral destinations but virtually all of these projections share a common mode of distribution: they terminate in distinct vertically oriented columns, 200-500 mum wide, that extend across all layers of cortex and alternate in regular sequence with columns of comparable width in which grains do not exceed background. Spatial periodicity in the pattern of transported label in such regions as the prefrontal association cortex, the retrosplenial limbic cortex and the motor cortex indicates that columination in the intracortical distribution of afferent fibers is not unique to sensory specific cortex but is instead a general feature of neocortical organization. A columnar mode of distribution of cortico-cortical projections is present in monkeys at all ages investigated but is especially well delineated in the youngest of them. Thus, grain concentrations within columns are very high in monkeys injected at 4 days of age, somewhat lower in monkeys injected at 39-45 days of age, and least dense in those injected at 5.5 months. The distinctness of the spatially segregated pattern of innervation in the cortex of neonates indicates that the columnar organization of association-fiber systems in the frontal and limbic cortex is achieved before or shortly after birth.