Retrograde intraaxonal transport of horseradish peroxidase in retinal ganglion cells of the chick

Activity-Dependent Degradation of Synaptic Vesicle Proteins Requires Rab35 and the ESCRT Pathway

Synaptic vesicle (SV) pools must maintain a functional repertoire of proteins to efficiently release neurotransmitter. The accumulation of old or damaged proteins on SV membranes is linked to synaptic dysfunction and neurodegeneration. However, despite the importance of SV protein turnover for neuronal health, the molecular mechanisms underlying this process are largely unknown. Here, we have used dissociated rat hippocampal neurons to investigate the pathway for SV protein degradation. We find that neuronal activity drives the degradation of a subset of SV proteins and that the endosomal sorting complex required for transport (ESCRT) machinery and SV-associated GTPase Rab35 are key elements of this use-dependent degradative pathway. Specifically, neuronal activity induces Rab35 activation and binding to the ESCRT-0 protein Hrs, which we have identified as a novel Rab35 effector. These actions recruit the downstream ESCRT machinery to SV pools, thereby initiating SV protein degradation via the ESCRT pathway. Our findings show that the Rab35/ESCRT pathway facilitates the activity-dependent removal of specific proteins from SV pools, thereby maintaining presynaptic protein homeostasis.

SIGNIFICANCE STATEMENT

Synaptic transmission is mediated by the release of chemical neurotransmitters from synaptic vesicles (SVs). This tightly regulated process requires a functional pool of SVs, necessitating cellular mechanisms for removing old or damaged proteins that could impair SV cycling. Here, we show that a subset of SV proteins is degraded in an activity-dependent manner and that key steps in this degradative pathway are the activation of the small GTPase Rab35 and the subsequent recruitment of the endosomal sorting complex required for transport (ESCRT) machinery to SV pools. Further, we demonstrate that ESCRT-0 component Hrs is an effector of Rab35, thus providing novel mechanistic insight into the coupling of neuronal activity with SV protein degradation and the maintenance of functional SV pools.

The mammalian olivocochlear system--a legacy of non-cerebellar research in the Mugnaini lab

Although the major emphasis of Enrico Mugnaini's research has been on investigations of the cerebellum, a significant amount of work over a relatively short span of time was also done in his lab on a number of other brain systems. These centered on sensory systems. One of these extra-cerebellar systems that he embraced was the auditory system. Portions of the cochlear nucleus, the first synaptic relay station along the central auditory pathways, possess a cerebellar-like circuitry and neurochemistry, and this no doubt lured Enrico into the auditory field. As new tools became available to pursue neuroanatomical research in general, which included a novel antibody to glutamic acid decarboxylase (GAD), Enrico's lab soon branched out into investigating many other brain structures beyond the cerebellum, with an overall goal of producing a map illustrating GAD expression in the brain. In collaboration with long-term colleagues, one of these many non-cerebellar regions he took an interest in was an efferent pathway originating in the superior olive and projecting to the cochlea, the peripheral end organ for hearing. There was a need for a more complete neurochemical map of this olivocochlear efferent system, and armed with new antibodies and well-established tract tracing tools, together we set out to further explore this system. This short review describes the work done with Enrico on the olivocochlear system of rodents, and also continues the story beyond Enrico's lab to reveal how the work done in his lab fits into the larger scheme of current, ongoing research into the olivocochlear system.

Relating the neuron doctrine to the cell theory. Should contemporary knowledge change our view of the neuron doctrine?

The neuron doctrine, formulated in 1891, attacked in 1906 by Golgi and fiercely defended by Cajal, provided a powerful tool for analyzing the pathways of the brain. It has often been described as though it were merely the cell theory applied to nervous systems. In this essay I show that the neuron doctrine claims more than does the cell theory, and that in many instances, where it goes beyond the cell theory, it can no longer be defended on the basis of contemporary evidence. The neuron doctrine should be seen as a practical tool that is particularly useful for understanding the long pathways of the brain; it cannot be regarded as providing an accurate account of what nerve cells in general are really like.

Efferent neurons of the lateral line system and their innervation of lateral line branches in a euteleost and an osteoglossomorph

The efferent neurons of the lateral line system of the euteleost Aplocheilus lineatus and the osteoglossomorph Pantodon buchholzi, both surface feeding fish, were examined by neuronal tract tracing. Besides horse-radish peroxidase, fluorescent dextrans were used as tracers to allow simultaneus visualization of projections from different lateral line branches. Labeled efferent neurons were found in nuclei situated in the medulla ventral of ventricle IV. This position resembles the octa-volateralis efferent nucleus of previous studies. The number of labeled cells in the efferent nucleus is low in both species. Most neurons were found ipsilaterally to the application site, some along the midline and only very few contralaterally. The size of efferent cells differs distinctly between Aplocheilus, possessing small cell-bodies (length 16.5 microm), and Pantodon, which has very large efferent cells (length 47.0 micron). Efferent axon bundles course rostrally in both species, leaving the brain at the level of the anterior lateral line nerve. Only Aplocheilus has in addition lateral axon bundles leaving the brain at the level of the posterior lateral line nerve. After application of one fluorescent tracer to the lateral ramus and a different fluorescent tracer to the superficial ophtalmic ramus in a given animal, double-labeling of efferent cells hardly ever occurs. If the neuromasts I and IV of the dorsal skull of Pantodon are applied with one fluorescent tracer each, approximately 10% of centrally labeled cells are double-labeled. Considering the results of double-labeling, the concept of a differential innervation of lateral line branches is supported and discussed.

Ultraviolet irradiation of the skin attenuates calcitonin gene-related peptide mRNA expression in rat dorsal root ganglion cells

Calcitonin gene-related peptide (CGRP) mRNA expression in rat dorsal root ganglion cells was measured by polymerase chain reaction after ultraviolet (UV) irradiation of the skin. The aim was to investigate whether a noninvasive peripheral lesion set by UV irradiation influences neuropeptide gene expression thus possibly contributing to the pathophysiology of the UV erythema. Forty-eight hours after irradiation, when the inflammatory response of the skin was at its maximum, there was a decrease in CGRP mRNA levels to 50% of the control values. The results demonstrate that exposure of the peripheral receptive field to noxious UV radiation affects neuropeptide gene expression in primary sensory neurons.

Endocytosis of cationized ferritin by rat photoreceptors

Endocytosis by rods of the rat retina was studied at the ultrastructural level with cationized ferritin. Endocytosed ferritin was found within all photoreceptor subdivisions except the outer segment. Within the cell, synaptic vesicles, lysosomes and a variety of other membrane-bound organelles including the axonal agranular reticulum contained ferritin. These findings indicate that surface membrane and materials bound to surface membrane are recycled to form synaptic vesicles, that a portion of the captured membrane and extracellular material enters a lysosomal pathway, and that endocytosed materials can be retrogradely transported to the myoid region of the photoreceptor.

Regulatory mechanisms for substance P in the dorsal horn during a nociceptive stimulus: axoplasmic transport vs electrical activity

Substance P (SP) is believed to be a neuromediator of nociception in the dorsal horn of the spinal cord. SP precursor is synthesized in the dorsal root ganglia (DRG) and transported via axoplasmic transport to the nerve terminal where it is stored and released as SP. The chemical nociceptive stimulus, formalin, when injected into the hindpaw causes an increase in the level of SP in the dorsal horn. This increase in SP may be the result of increased electrical activity due to activation of free nerve endings or the transport of some chemical or trophic signal to the DRG or to the central terminal. This study investigates the mechanism of the SP increase during the formalin stimulus. Rats were anesthetized and a laminectomy performed. In some experiments the sciatic nerve was exposed. Agar gel pads containing either colchicine or tetrodotoxin (TTX) were applied to the dorsal root or sciatic nerve prior to the injection of 5% formalin or saline into the hindpaw. Electrical activity across the dorsal root distal to the gel pad was monitored to determine the effects of colchicine and TTX on the nerve. Sixty min after the injection into the hindpaw, the animal was perfused and the lumbar spinal cord removed. Ten-micron frozen sections were stained for SP. It was found that the formalin-evoked increase in SP could be partially blocked by either colchicine or TTX applied to the dorsal root and completely blocked by the application of both agents together. TTX or colchicine applied to the sciatic nerve completely blocked the formalin-evoked increase in SP.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous peroxidase in the visceral endoderm of early mouse embryos

Endogenous peroxidase activity was demonstrated in early mouse embryos by means of the diaminobenzidine staining reaction. This enzyme was observed in visceral endoderm on the seventh to eighth day of gestation in vivo, but was no longer detected on the ninth day of development. In cell layers developing from blastocysts or isolated inner cell masses cultured for 96-144 h (developmental stage equivalent to 6-7.5-day-old embryos), diaminobenzidine product was also observed in visceral endodermal cells. Most of the endogenous peroxidase was localized inside or close to the numerous apical vacuoles in the endoderm. Ectoderm, mesoderm, ectoplacental cone, and trophoblast cells did not contain endogenous peroxidase.

Axonal transport: each major rate component reflects the movement of distinct macromolecular complexes

The proteins of the three major rate components of axonal transport in guinea pig retinal ganglion cells were analyzed by one- and two-dimensional gel electrophoresis. Each rate component consisted of a different set of proteins that remained associated with each other during transport. This suggests that each rate component represents a distinct macromolecular complex and that these complexes may be definable organelles such as microtubules, microfilaments, and smooth endoplasmic reticulum. Thus, the transport of radiolabeled proteins in the axon reflects the movement of complete subcellular rather than the movement of individual proteins.

The movement of membranous organelles in axons. Electron microscopic identification of anterogradely and retrogradely transported organelles

To identify the structures to be rapidly transported through the axons, we developed a new method to permit local cooling of mouse saphenous nerves in situ without exposing them. By this method, both anterograde and retrograde transport were successfully interrupted, while the structural integrity of the nerves was well preserved. Using radioactive tracers, anterogradely transported proteins were shown to accumulate just proximal to the cooled site, and retrogradely transported proteins just distal to the cooled site. Where the anterogradely transported proteins accumulated, the vesiculotubular membranous structures increased in amount inside both myelinated and unmyelinated axons. Such accumulated membranous structures showed a relatively uniform diameter of 50--80 nm, and some of them seemed to be continuous with the axonal smooth endoplasmic reticulum (SER). Thick sections of nerves selectively stained for the axonal membranous structures revealed that the network of the axonal SER was also packed inside axons proximal to the cooled site. In contrast, large membranous bodies of varying sizes accumulated inside axons just distal to the cooled site, where the retrogradely transported proteins accumulated. These bodies were composed mainly of multivesicular bodies and lamellated membranous structures. When horseradish peroxidase was administered in the distal end of the nerve, membranous bodies showing this activity accumulated, together with unstained membranous bodies. Hence, we are led to propose that, besides mitochondria, the membranous components in the axon can be classified into two systems from the viewpoint of axonal transport: "axonal SER and vesiculotubular structures" in the anterograde direction and "large membranous bodies" in the retrograde direction.

Fine structure of initial outgrowth of processes induced in a pheochromocytoma cell line (PC12) by nerve growth factor

Cells of the PC12 line (which is derived froma rat pheochromocytoma) develop neuron-like processes upon exposure to nerve growth factor (NGF), and thus provide an opportunity to study this phenomenon de novo. We have used the transmission electron microscope to analyse the early stages of process outgrowth (1, 2, 3 and 7 days) to determine what organelles are involved and in what sequence they appear during development. Despite the non-synchronous response to NGF, we can derive three main stages in early process formation. (1) NGF-treated cells develop conical extensions similar to, but larger and more numerous than those of controls. Extensions terminate in bulbous expansions that contain large number of chromaffin-like granules and bear microspikes filled with microfilaments. (2) The extensions of NGF-treated cells then acquire membranous organelles indicative of transmitter packaging and/or recycling of cytoplasmic membranes, for example, tubular reticulum, clear and dense-cored vesicles, multivesicular bodies, and lysosomes. (3) As processes elongate, they develop a shaft that contains an array of microtubules and fine tubular reticulum dispersed in a filamentous matrix, and varicosities that exhibit the same organelles seen in stage 2. The discussion stresses the similarities in the outgrowth of processes in PC12 cells and neurons, and speculates that NGF causes a change in organization and/or quantity or organelles that already exist in non-treated control cells.

Features of foreign proteins affecting their retrograde transport in axons of the visual system

The retrograde axoplasmic transport of foreign proteins in the rat visual system shows certain specificities. The molecular features of these proteins which may underlie their entry into the retrograde phase have been examined using biochemical and morphologic techniques. Of the isoenzymes of horseradish peroxidase (HRP), the basic isoenzyme C is strongly transported, while the acidic isoenzymes Abeta and Aalpha are transported weakly and not at all, respectively. Decreasing the isoelectric point (pI) of isoenzyme C from 8.2 to 4.4 decreases its transport, but a basic pI is not the sole requisite for transportability since two other basic peroxidases (turnip isoenzyme P7 and lactoperoxidase) are not transported in retrograde. The sugar component as a whole of isoenzyme C does not appear to be required for determining transport. Isoenzyme C and the other proteins which are transported enter multivesicular bodies in axons and axon terminals, as well as synaptic and coated vesicles and fine tubules in axon terminals. The non-transported proteins enter only the vesicular organelles thought to be involved in neurotransmitter recycling in axon terminals and do not enter multivesicular bodies. Thus the two systems of axonal membraneous compartments involved in local synaptic recycling versus the retrograde phase of transport do not show the same specificity of uptake of extracellular tracers and can be dissociated by the experimental use of these peroxidases.

Cerebellar afferents from the paramedian reticular nucleus studied with retrograde transport of horseradish peroxidase

Details in the cerebellar projections from the paramedian reticular nucleus (PRN) were studied in cats and monkeys by means of retrograde axonal transport of horseradish peroxidase (HRP). In the cat the majority of the fibres projects to the anterior lobe and to the vermis of the posterior lobe (with the exception of lobules VIIB and VIIIA). A less conspicuous projection was found to the lobulus simplex, the crura and the flocculus. The cerebellar nuclei, the paramedian lobule and the paraflocculus appear to be weakly connected with the PRN. A similar distribution of the cerebellar afferent fibres was found in the monkey material. The three subgroups of the PRN in the cat are not equal in their projection. The dorsal group appears to be connected with the greater part of the cerebellar cortex and with all nuclei. The ventral group lacks a connection with lobulus IX, the flocculus and the paraflocculus, and the accessory group appears to have its strongest connection with lobulus I (lingula), the flocculus and the vermal lobules VII-X. The findings are discussed in relation to other studies on the efferent and afferent connections of the nucleus.

Association and commissural fiber systems of the olfactory cortex of the rat

The association and commissural fiber systems arising in the olfactory cortical areas caudal to the olfactory peduncle (the piriform cortex, nucleus of the lateral olfactory tract, anterior cortical nucleus of the amygdala, periamygdaloid cortex and entorhinal cortex) have been studied utilizing horseradish peroxidase as both an anterograde and a retrograde axonal tracer. In the piriform cortex two sublaminae within layer II (IIa and IIb) layer III have been found to give rise to distinctly different projections. Retrograde cell labeling experiments indicate that the association fiber projection from layer IIb is predominatnly caudally directed, while the projection from layer III is predominantly rostrally directed. Cells in layer IIa project heavily to areas both caudal and rostral to the piriform cortex. The commissural fibers from the piriform cortex are largely restricted in their origin to layer IIb of the anterior part of the piriform cortex and in their termination on the contralteral side to the posterior part of the piriform cortex and adjacent olfactory cortical areas. A projection to the olfactory bulb has also been found to arise from cells in layers IIb and III of the ipsilateral piriform cortex, but not in layer IIa. In addition to those from the piriform cortex, association projections have also been found from other olfactory cortical areas. The nucleus of the lateral olfactory tract has a heavy bilateral projection to the medial part of the anterior piriform cortex and the lateral part of the olfactory tubercle (as well as a lighter projection to the olfactory bulb); both the anterior cortical nucleus of the amygdala and the periamygdaloid cortex project ipsilaterally to several olfactory cortical areas. The entorhinal cortex has been found to project to the medial parts of the olfactory tubercle and the olfactory peduncle. The olfactory tubercle is the only olfactory cortical area from which no association fiber systems (instrinsic or extrinsic) have been found to originate. A broad topographic organization exists in the distribution of the fibers from several of the olfactory areas. This is most obvious in the anterior part of the olfactory cortex, in which fibers from the more rostral areas (the anterior olfactory nucleus and the anterior piriform cortex) terminate in regions near the lateral olfactory tract, while those from more caudal areas (the posterior piriform cortex and the entorhinal cortex) terminate in areas further removed, both laterally and medially, from the tract. Projection to olfactory areas from the hypothalamus, thalamus, diagonal band, and biogenic amine cell groups have been briefly described.

Selective binding, uptake, and retrograde transport of tetanus toxin by nerve terminals in the rat iris. An electron microscope study using colloidal gold as a tracer

A series of specific macromolecules (tetanus toxin, cholera toxin, nerve growth factor [NGF], and several lectins) have been shown to be transported retrogradely with high selectivity from terminals to cell bodies in various types of neurons. Under identical experimental conditions (low protein concentrations injected), most other macromolecules, e.g. horseradish peroxidase (HRP), albumin, ferritin, are not transported in detectable amounts. In the present EM study, we demonstrate selective binding of tetanus toxin to the surface membrane of nerve terminals, followed by uptake and subsequent retorgrade axonal transport. Tetanus toxin or albumin was adsorbed to colloidal gold particles (diam 200 A). The complex was shown to be stable and well suited as an EM tracer. 1-4 h after injection into the anterior eye chamber of adult rats, tetanus toxin-gold particles were found to be selectively associated with membranes of nerve terminals and preterminal axons. Inside terminals and axons, the tracer was localized mainly in smooth endoplasmic reticulum (SER)-like membrane compartments. In contrast, association of albumin-gold complexes with nervous structures was never observed, in spite of extensive uptake into fibroblasts. Electron microscope and biochemical experiments showed selective retrograde transport of tetanus toxin-gold complexes to the superior cervical ganglion. Specific binding to membrane components at nerve terminals and subsequent internalization and retrograde transport may represent an important pathway for macromolecules carrying information from target organs to the perikarya of their innervating neurons.

Cerebellar afferent projections from the perihypoglossal nuclei: an experimental study with the method of retrograde axonal transport of horseradish peroxidase

Details of cerebellar afferent projections from the perihypoglossal nuclei were studied in the cat by means of retrograde axonal transport of horseradish peroxidase (HRP). Labeled cells were observed bilaterally (with a preponderance ipsilaterally) in nuclei intercalatus and praepositus hypoglossi following injections in various folia of the entire vermis, paraflocculus, flocculus, fastigial nucleus, and the nucleus interpositus anterior and posterior. Relatively high densities of labeled cells were found in nucleus intercalatus following injections in the anterior part of the vermis, whereas labeled cells in nucleus praepositus hypoglossi were found more frequently following injections in the posterior part of the vermis. Labeled cells in the nucleus of Roller were found only following injections in the anterior lobe vermis, posterior vermal lobules VI and VII, in the flocculus and in the nucleus interpositus anterior. No labeled cells could be detected in the three subdivisions of the perihypoglossal nuclei following HRP injections in crus I, crus II, paramedian lobule, and lateral cerebellar nucleus. The distribution of the HRP positive cells indicated the presence of a topographically organized projection from certain regions of the perihypoglossal nuclei to different parts of the cerebellum. The afferent and efferent connections of the perihypoglossal nuclei in relation to a functional role in eye and head movements are discussed.

Brain stem afferents to the fastigial nucleus in the cat demonstrated by transport of horseradish peroxidase

Although retrograde and anterograde degeneration studies have provided important information concerning brain stem afferents to the fastigal nucleus (FN), these data may be incomplete and should be confirmed by axonal transport methods. Attempts were made to inject horseradish peroxidase (HRP) unilaterally into the FN in a series of adult cats. Animals were perfused with dextran and a fixative solution of paraformaldehyde and glutaraldehyde in 0.1 M phospate buffer. Representative sections were treated by the Graham and Karnovsky ('66) method. Selective HRP injections in one FN resulted in retrograde transport of the marker to Purkinje cells of the ipsilateral vermis and distinctive appendages of the contralateral medial accessory olivary (MAO) nucleus (nucleus beta and the dorso-medial cell column). Retrograde transport of the label was found bilaterally in cells of the medial (MVN) and inferior (IVN) vestibular nuclei, in cell group x and in the nucleus prepositus (PP). Labeled vestibular neurons, most numerous in MVN, were identified in dorsal, caudal and lateral regions, with a slight ipsilateral preponderance. Only a few neurons in caudal, dorsal and lateral regions of the IVN were labeled and none of these included cells of group f. Labeled cells in the caudal third of PP were greatest ipsilaterally. Rostral and caudal injections of FN labeled smaller numbers of cells in MVN, IVN, cell group x and PP. HRP injections of FN and portions of lobules VIII and IX resulted in bilateral retrograde labeling of larger numbers of cells in MVN, IVN and cell group x, and ipsilateral labeling of cells in group y and the interstitial nucleus of the vestibular nerve. Injections of HRP into basal folia of lobules V and VI resulted in retrograde transport of the marker to cells of the medial and dorsal accessory olivary nuclei contralaterally, and to cells of the ipsilateral accessory cuneate nucleus. Transport of label injected into portions of the pyramis was detected in parts of the contralateral MAO and bilaterally in parts of the pontine and reticulotegmental nuclei. This study suggests that the principal afferents of the fastigial nucleus arise from: (1) Purkinje cells of the ipsilateral vermis, (2) restricted portions of the contralateral MAO (nucleus beta and dorsomedial cell column), (3) portions of the MVN and IVN (bilaterally) and (4) caudal parts of the PP. Secondary vestibular inputs to the fastigial nucleus probably are relayed mainly by Purkinje cells in the cerebellar cortex.

Cytochemical studies of lectin binding sites in smooth membrane cisternae of rat brain

The cytochemical localization of concanavalin A (con A) binding sites has been studied in Purkinje cell axons and presynaptic terminals of rat cerebellum. Smooth membrane cisternae just beneath the axolemma contain con A binding sites on the side of the membrane facing the cisternal space. At certain regions, such as the node of Ranvier, these cisternae lie in virtual apposition to the axolemma. Such a specialized system of cisternae could serve as a channel through which some of the materials synthesized in the Purkinje somata are moved to the axon terminals. The close association of the cisternae and axolemma at certain regions could be a site at which some of the transported materials contribute to renewal of the axolemma. The con A binding sites on intracellular membranes of the Purkinje cell are removed by prior glycosidic and proteolytic enzyme digestions. The results suggest that at least some of the carbohydrates lining membrane cisternae are glycoprotein in nature.

Endogenous perioxidatic activity in brain stem neurons as demonstrated by their staining with diaminobenzidine in normal squirrel monkeys

In order to test for the presence or absence of endogenous peroxidatic activities within the brains of normal squirrel monkeys, sections from the brain stems were incubated in buffered media of diaminobenzidine (DAB) and hydrogen peroxide, according to the methods of Graham and Karnovsky20 or Novikoff and Goldfischer39. It was found that neurons within certain regions were consistently stained by both mediamthese reactive neurons belong to the extrapyramidal motor system (globus pallidus, substantia nigra (pars diffusa), red nucleus, pontine and mesencephalic reticular formation), to certain cranial nerve nuclei (oculomotor, trochlear, abducens, mesencephalic trigeminal, vestibular) and to relay nuclei of the auditory pathway (superior olivary nucleus and nucleus of the trapezoid body). Neurons within other regions of the brain stem, such as thalamus, hypothalamus and corpus striatum were not stained. On the other hand, the neuropil, excluding the neurons, in other motor areas (such as the subthalamic nucleus, inferior olivary nucleus and corpus striatum) and other auditory relay nuclei (inferior colliculus, nucleus of lateral lemniscus) were also significantly stained. Processes around the Purkinje cells of the cerebellum were particularly reactive. Under the electron microscope, reaction products were localized prominently along the cristae and limiting membranes of the mitochondria and within peroxisome-like bodies of the stained neurons. Non-reactive neurons have little or no reaction products in their organelles. The staining was absent when DAB was withheld, and it was weak or absent if H2O2 was omitted from the media. The addition of inhibitors of catalase, 3-amino-1,2,4-triazole and 2,6-dichlorophenolindophenol did not affect the staining intensity, whereas the addition of potassium cyanide, a strong inhibitor of metalloenzymes, especially heme-enzymes, almost completely abolished the reactions. In addition, a 3-day 'bleaching treatment' with 30% H2O2 did not affect the reactions. These results suggest that the reactivity in the above regions was due not to neuromelanin or other pigments, but rather to heme-enzymes and/or other hemoproteins. It was postulated that a higher concentration of the reactive agents was characteristic of certain neuronal types in the brain stem of the squirrel monkeys, and could be related to the cells' functional needs, energy requirements and perhaps to the metabolism of their specific neurotransmitters.

The smooth endoplasmic reticulum: structure and role in the renewal of axonal membrane and synaptic vesicles by fast axonal transport

The spatial arrangement of the smooth endoplasmic reticulum (SER) was studied in 0.5-2 mum thick sections of rat spinal and chick ciliary ganglia previously impregnated with heavy metal salts. Electron microscopy at low (10-5 V) or high (10-6 V) voltage showed the impregnated SER as a continuous system extending probably from the perikaryon to the axon terminal. Tubules of the SER, which were running in a parallel direction with the axon, were occasionally seen in close apposition with the axonal membrane. Moreover in the preterminal region, anastomosed tubules of the SER formed a subsurface 'primary network' and gave rise to a deeper 'secondary network' made of thinner tubules; synaptic vesicles bulging at the tip of thin tubules of the SER were frequently observed. To specify the role played by the SER in the fast axonal transport, chicken ciliary ganglia were slighty compressed and radioautographed 3 h after the intracerebral injection of [3-H]lysine. Quantitative analysis of the silver grain distribution indicated that labeled proteins, rapidly conveyed down the axon, piled up in regions containing an accumulation of SER profiles. On the basis of these results, it is concluded that: (1) the SER appears as a continuous intraaxonal pathway bridging the perikaryon and the axon terminal; (2) the SER conveys macromolecular components with the fast axonal transport; (3) the conveyed macromolecules, which are delivered to the axonal membrane and to the synaptic vesicles, are probably transferred by means of connections with the SER.