Developmental protein changes in normal and chromatolytic facial nerve nuclear regions

Microelectrophoresis and auxilary micromethods

In this retrospect of approximately 30 years of work with micromethods, some of them developed in our own laboratory, their principles and application to different separation problems are described, such as one- and two-dimensional microelectrophoresis in capillaries and microslab gels, isoelectric focusing in capillaries or microslab gels, microchromatography, microphotometry, and microfluorometry for qualitative and quantitative evaluation of separation patterns. In addition, some useful auxiliary methods are also described, e.g., a method for quantitative protein determination in a microliter volume when neither the volume nor the protein content in that volume are known, and methods for the determination of glycoproteins, amino acids, and sugars in the picomole range.

Axonal sprouting and frank regeneration in the lizard tail spinal cord: correlation between changes in synaptic circuitry and axonal growth

In our previous studies, we found that the number of supraspinal neurons projecting to the level of tail spinal cord increases by 74% during tail regeneration and that the number of local spinal neurons with descending projections increases 233%. However, only a small fraction of the supraspinal axons (less than 4%) and half of the local spinal axons actually enter the regenerated spinal cord. We suggested that this may be the result of "synaptic capture" in which regrowing axons make synapses on denervated targets rostral to the transection, aborting further regeneration. To examine this hypothesis, morphometric analysis of electron microscope (EM) photomontages was used to test for changes in synaptic distribution on ventral horn neurons rostral to regenerating tail spinal cord. In addition, 3H-thymidine and retrograde markers were used to determine whether the regenerate axons arose from cut axons, neurogenesis, or sprouting from uninjured neurons. 3H-thymidine injections during regeneration, combined with retrograde HRP pathway tracing, did not reveal the production of new neurons in the tail spinal cord. To test whether cut axons regenerate, fluorescein isothiocyanate conjugated latex beads were applied to the exposed end of the tail spinal cord. After tail regeneration, HRP was applied to the new spinal cord in the regenerated tail. Examination of local spinal neurons (the primary source of axons that enter the regenerated tail spinal cord) revealed that 28% of the neurons contained both labels. This indicated that cut axons successfully regrew into the new tail spinal cord. The regenerated axons that fail to enter the new tail spinal cord can be found in the normal spinal cord immediately rostral to the regenerated tail. To determine whether these axons were making synaptic contacts, lamina IX ventral horn neurons were examined. EM photomontages of the spinal cord rostral to the regenerate tail revealed the following properties: (1) neurons rostral to regenerated tails are larger in area compare to non-regenerates (mean increase = 112%); (2) axosomatic contacts cover a greater percentage of the neuronal soma following regeneration compared to normal (mean increase = 23%); and (3) this increased innervation is the result of an increase in the number of synaptic boutons rather than larger boutons. The number of synaptic contacts in regenerated lizards returned to normal following lumbar transection, indicating that supraspinal and/or long descending propriospinal afferents were the major source of the increased synaptic contacts.(ABSTRACT TRUNCATED AT 400 WORDS)

Lysosomal activity in developing cat alpha-motor axons under normal conditions and during retrograde axonal transport of horseradish peroxidase

The occurrence of acid phosphatase (AcPase)-positive bodies, i.e., lysosomes, in lumbosacral alpha-motor axons of kittens, 0-16 weeks of age, was analyzed by light and electron cytochemical methods under normal conditions and after intramuscular injection of horseradish peroxidase (HRP). Axonal lysosomes were rare early postnatally. In 3-week-old animals, a few AcPase-positive bodies appeared in the axoplasm at some nodes of Ranvier in the peripheral nervous system (PNS) and internodally in the intrafunicular motor axon parts within the central nervous system (CNS). From 6 weeks postnatally, a nodal concentration of AcPase-positive bodies was also noted in the CNS. The number of AcPase-positive bodies continued to increase gradually in the course of neuronal maturation. In 16-week-old animals, axonal AcPase activity was still at considerably lower levels than at adult stages. At all ages, acid hydrolase-containing organelles were most commonly found at ventral root nodes. After injection of HRP in the medial gastrocnemius muscle, accumulations of AcPase-positive bodies were seen in the axoplasm at some PNS nodes of the HRP-injected sides of kittens aged 8, 12, and 16 weeks. Incubation for demonstration of both HRP and AcPase activity showed that some organelles at HRP-transporting nodes contained both types of reaction product. The nodal AcPase activity in the intrafunicular, CNS parts of alpha-motor axons of the HRP-exposed sides did not differ from that of the contralateral, uninjected sides. In view of our previous observations in alpha-motor neurons of adult cats in which a lysosome-mediated degradation of axonally transported materials may take place at PNS nodes of Ranvier, the present study illuminates possible differences in the ability to interfere with axonal transport between developing and mature neurons. The infrequent presence of lysosomes in developing alpha-motor axons and the implied disability of their nodal regions to interfere with axonally transported constituents in a way similar to that seen in adult animals may be of significance in that trophic and chemical signals can pass unhindered between the periphery and perikaryon. However, this could also have negative consequences for the vulnerable immature neuron in that various materials retrieved at the axon terminals outside the CNS are permitted a more-or-less free access to the perikaryon.

A fluorescence study of changes in noradrenergic sympathetic fibres in experimental peripheral nerve neuromas

Experimental neuromas were produced in rats by sciatic nerve section and avulsion of the distal stumps. At intervals varying from 3 days to 8 weeks after nerve section, the developing neuromas were resected and processed for noradrenaline (NA) fluorescence microscopy by the sucrose-phosphate-glyoxylic acid (SPG) method. From serial longitudinal sections through the neuromas and the nerve proximally, counts of noradrenergic sympathetic axons were made, together with qualitative observations of axon sprouting and NA content. By 3 days after nerve section there was a massive sprouting of sympathetic axons, with increased NA content, particularly towards the distal tip of the neuroma. Axon counts remained high 1 week following section then fell to below normal levels at 2 weeks, returning towards normal 8 weeks after nerve section. These results are discussed in relation to the known pathophysiological interaction between sympathetic efferent and sensory afferent fibres, which develops in neuromas following nerve section.

Effects of advancing age on the central response of rat facial neurons to axotomy: light microscope morphometry

Following axotomy, the regrowth of peripheral axons takes longer in older individuals than in young ones. The present study compares central responses of facial motor neurons to a crush injury of the facial nerve in 3-month-old and 15-month-old male rats sampled through 28 days post-crush (dpc). Neuronal somata, nuclei, and nucleoli were measured in 30 microns brain stem sections within subdivisions of the facial nucleus that contain the cell bodies responsible for the movement of the vibrissae. The temporal patterns of change in the size of the three structures were interpreted with reference to the re-establishment of functional connections, i.e., the return of voluntary vibrissae activity, which is delayed by 4 days in the older animals relative to the younger ones. There was no age-related difference in the pattern of somal swelling and recovery, nor was there an age-related difference in the response of nuclei and nucleoli to axotomy through 4 dpc. Both nuclei and nucleoli increased in size in animals of both age groups, but after 4 dpc in the older animals nuclear enlargement was prolonged and the nucleolar increases were less robust compared to the younger animals. The greatest age difference appeared with the re-establishment of functional connections. In the 3-month-old animals, the resumption of whisker activity coincided with vigorous transient increases in the sizes of nuclei and nucleoli; in the 15-month-old animals, there was little nuclear response to functional recovery and a comparatively small increase in nuclear sizes.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased glucose use in the hypoglossal nucleus after hypoglossal nerve transection in aged rats

We used [2-14C]deoxyglucose as a marker of increased metabolism of the hypoglossal nucleus after transection of its nerve. We studied this metabolism in 3-, 12-, and 24-month-old rats. We found an increase in glucose uptake in the control nucleus of 24-month-old rats which was significant when compared to that of 3-month-old rats. We also found a twofold increase in the difference between glucose uptake on the side of nerve transection compared to the control side in old rats.

Ultrastructural changes in the nucleoplasm of hamster facial neurons during a postnatal maturation period

Changes in the characteristics of the nucleoplasm were examined in young hamster facial motoneurons of 15 and 20 days postnatal age and in the adult (100 days postnatal age) at both the light and electron microscope levels. In toluidine-blue stained 1-micron thick sections, a progressive increase in basophilic islands within the nucleoplasm occurred during maturation. Ultrastructural changes that were observed during final development included a transition from a homogeneous, 'filled-in' appearing nucleoplasm to a clumped-appearing nucleoplasm. This process principally involved the formation of distinct clusters of interchromatin granules that was associated with a loss of fine fibrils, an increase in clear spaces between intervening fibrillar and granular material, and an increase in small scattered clumps of heterochromatin. These changes in both ribonucleoprotein (RNP)-and DNA-containing nuclear constituents (interchromatin granules and heterochromatin, respectively) occurred during a postnatal maturational period previously demonstrated to involve other alterations in nuclear structures. It is interpreted that these cytomorphic changes in the nucleoplasm reflect an underlying metabolic shift at the transcriptional level during the transition from an actively growing neuron to an adult functioning neuron.

Electrophysiological and morphological correlates of axotomy-induced deafferentation of the goldfish Mauthner cell

Axotomy-induced changes in afferent synapses to the goldfish Mauthner cell have been analyzed with intracellular recordings and with electron microscopy. The studies encompassed 7-208 days after cervical spinal cord transection. The physiological findings suggest a persistent and specific reduction in excitatory chemical inputs to the soma and proximal lateral dendrite, with no changes in somatic inhibition or in the electrotonic and chemical inputs to the more distal regions of the lateral dendrite. Corroborative morphological evidence includes swelling of the M-cell soma, as indicated by a 35% increase in the length of its minor diameter, an increased spacing and a quantitatively lower density of terminals on the soma, and the appearance of astrocytic processes partially or completely engulfing the terminals in that region. Similar changes were observed on the inferior dendrites projecting from the ventral surface of the soma, although these dendrites do not exhibit the chromatolytic changes observed at the soma. In contrast, there are no noticeable changes in either the synaptic investment of the lateral dendrite or its ultrastructure. Quantitative and qualitative data support the conclusion that there is a restricted and specific reduction in the proximal excitatory inputs to the M-cell. The evidence also suggests that electrotonic junctions between afferents and the M-cell remain intact, functionally and structurally.

Alterations of [3H]actinomycin D binding to axotomized dorsal root ganglion cell nuclei: an autoradiographic method to detect changes in chromatin structure and RNA synthesis

An autoradiographic method was developed to quantify on a comparative basis the binding of [3H]actinomycin D (Act D) to the cell nuclei of frozen, unfixed sections of spinal sensory ganglia in rats. After a crush lesion of the sciatic nerve, alterations of [3H]Act D binding were found in L5 and L6 dorsal root ganglia which corresponded to changes in RNA synthesis observed in other studies. An increase in Act D binding was seen at 1 to 3 days postoperation, followed by a decrease at 5 to 7 days. By 9 to 11 days a second increase in binding occurred, followed by a decrease at 14 days. Contralateral ganglia exhibited an increase in Act D binding only at 5 days compared with unoperated controls. The timing of the response in axotomized ganglia differed with the distance of the lesion from the cell body. The observed patterns of Act D binding confirm that changes of chromatin structure are closely associated with the alterations of RNA and protein synthesis occurring after axon injury. The method may be useful as an indicator for alterations in RNA synthesis related to changes in chromatin structure in complex tissues.

Increased in vitro labeling of stable RNA within the rat nodose ganglion following abdominal vagotomy

An in vitro procedure for labeling of RNA in the excised rat nodose ganglion was used to evaluate the changes in incorporation of [3H]uridine into ganglionic RNA following transection of the abdominal vagus nerves. Significant increases in the incorporation into 28S, 18S and 4S RNA were observed at 1 day after injury, which were maximal at 4 days before returning to unoperated control level by 7 days. A second transient increase in the labelling of these RNA species occurred between 9 and 11 days after injury. Comparison of the time course of these increases with those seen previously following cervical vagus nerve crush injury indicate that the time of onset of the increase in incorporation in independent of the site of injury, but that the maximal response is delayed by 1 day with the more distal lesion. These data are consistent with the existence of separate signals for initiating and modulating the cell body response to axon injury, which are transported retrogradely from the site of injury at rates exceeding the slow component of axoplasmic transport.

Increased choline kinase activity in the rat superior cervical ganglion after axonal injury

The activity of choline kinase (CK) was examined in the rat superior cervical ganglion (SCG) during development and following postganglionic axotomy. The highest specific enzyme activity (nmol phosphorylcholine/mg protein/h) 52 +/- 8, is observed 5 d before birth, then it rapidly decreases by about 50%, reaching at the day of birth levels observed in the ganglion throughout life. During development the total enzyme activity per ganglion is increased steadily until it reaches a 5-fold increase which parallels the increase in protein content. Following axotomy the enzyme activity per ganglion is rapidly increased by about 2-fold between 1 and 5 d postoperative and then gradually decreases reaching control levels at 30 d. The transient increase in enzyme activity parallels the increase in protein content of the axotomized ganglia. The peak increase in enzyme activity coincides with the peak chromatolytic response of the axotomized ganglion. We conclude that choline kinase activity is transiently increased within neurons after axonal injury, and that this event represents an effort of the nerve cell body to enhance its phosphatidylcholine biosynthesis essential for new membrane synthesis during the regeneration of the cut axon.

Tritiated leucine incorporation in the developing hamster facial nucleus with injury: a liquid scintillation study

Tritiated leucine incorporation was examined after either crush or axotomy of the hamster facial nerve at specific stages in the maturation of the neuronal nucleolus. Changes in the neuronal metabolic response to injury in development were demonstrated with liquid scintillation examination of tritiated leucine incorporation into the trichloroacetic acid (TCA)-insoluble and TCA-soluble fractions derived from whole reactive and normal facial nuclear groups. Changes in incorporation seen in the developmental sequence were attributed to actual changes in neuronal protein metabolism, and not to changes in the amino acid pool, glial changes or hyperemic capillary changes. The ability to increase leucine incorporation over the normal as a result of injury in development coincided with the time of final nucleolar maturation in the facial motor neurons, beginning at approximately 20 days postnatal age. Thus, there is a correlation between a specific morphological event, the attainment of the mature nucleolar configuration, and the acquisition of the mature synthetic capacity as indicated by the ability to respond to injury in the mature manner.

Polysaccharide and cytoplasmic changes in motoneurons during "chromatolysis" in the opossum spinal cord

Following axotomy, motoneurons of the opossum spinal cord display an early "axon reaction" or "chromatolysis" characterized by a redistribution of ribosomes accounting for a widespread basophilia and an apparent reduction in the size of two distinct varieties of Nissl bodies. This alteration is accompanied by zones of increased extracellular glycocalyx demonstrable in light and electron microscopy. In addition, large intracellular periodic acid-Schiff-positive vacuolated zones in the neuron periphery possess numerous free ribosomes, glycogen, lipids, and huge vacuolated sacs containing a flocculent matrix material similar to that found within the sacs of granular endoplasmic reticulum. "Artifacts" in the neuronal periphery associated with chromatolysis seen in light microscopy are probably related to polysaccharide alterations and redistribution of granular endoplasmic reticulum.

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

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

Ontogeny of oral function in hamsters (Mesocricetus auratus)

The oral apparatus of neonatal and juvenile golden hamsters was investigated by clearing and staining of whole crania, videotaping of behavior, and electromyography of several jaw muscles. Chewing developed during the first postnatal week and matured in the second; however, suckling was still the primary mode of feeding. Micromovements of the jaws occurred early when the osseous skeleton and joints developed. Macromovements correlated well with EMG records and were limited to jaw opening at birth. Muscles of the oral floor generated large bursts of activity during jaw opening and tongue protrusion from 0 days postnatal (dpn), when simple and stereotyped gaping was induced, until 14 dpn, when movements were spontaneous and not stereotyped nor inducible. However, adductor muscle activity was brief, low in amplitude, and primarily involved with jaw stabilization until 4 dpn, when these muscles became active during closing the jaws; closing activity increased in frequency and amplitude until the end of the second week. Development of frequent, coordinated macromovements of chewing was associated with the refinement of joint structure and dental occlusion and with the growth of the craniofacial skeleton. Jaw movements and associated EMG's correlated better with available data on development of neural circuitry than with that for musculoskeletal development.

Uptake and retrograde axonal transport of horseradish peroxidase in normal and axotomized motor neurons during postnatal development

The axonal uptake and somatopetal transport of horseradish peroxidase (HRP) was studied during early postnatal development of facial neurons in mice and rats. HRP injected systemically or locally into the muscles of the vibrissae, diffused into the region of the immature neuromuscular junction and was incorporated into vesicles in the axon terminals on the first and third postnatal days, at a time when synaptic vesicles were already present. HRP later was found in the nerve cell bodies of the facial nucleus in the brain stem indicating a somatopetal transport of the tracer in axons. The response of facial neurons to nerve transection changed from rapid neuronal death to prolonged survival between the 6th and 10th postnatal day. HRP was transferred to nerve cell bodies after topical application to the proximal stump of transected facial nerves in rats 3 days-of-age. In the perikaryon it was localized to vesicles and vacuoles with no signs of leakage into the cytoplasm. In the light of our findings different hypotheses for the mechanism of the neuronal death in the immature animals are discussed.

Explant culture of adult goldfish retina: a model for the study of CNS regeneration

Conditions are described for culture of retinal explants of adult goldfish which favour outgrowth of neuritic processes onto a substratum. A growth index to quantitate the outgrowth was developed. If the optic nerve is crushed several days prior to explantation, a marked enhancement of neuritic outgrowth is seen relative to control retinas. Histological examination of the explants revealed that retinal ganglion cells in explants from unoperated eyes became hypertrophied in vitro with a time course similar to that observed in vivo following optic nerve crush. Experiments with hemiaxotomized retinas indicate that the perikaryal regenerative response is mediated intracellularly.

Changes in synthesis of specific proteins following axotomy: detection with two-dimensional gel electrophoresis

Changes in protein synthesis during development and following axotomy were analyzed by two-dimensional gel electrophoresis. The two major postganglionic nerves emerging from the superior cervical sympathetic ganglia (SCSG) of adult rats were either cut or crushed unilaterally. At intervals ranging from 1 to 112 days after surgery both SCSG were removed and incubated for 1 hr in the presence of 14C-leucine. Proteins were extracted and subjected to two-dimensional electrophoretic separation and autoradiography. With this technique, proteins are separated on the basis of isoelectric point and molecular weight. Also, intact SCSG from 1, 2, 7, and 14 day old rats were labeled and analyzed. It was found that a minority of the separated proteins exhibited some detectable change in relative rate of synthesis following axotomy. Actin exhibited a slight (less than 20%) increase in relative synthesis rate while tubulin did not change significantly. There were small but significant differences in the protein patterns following nerve crush, as opposed to nerve cut. Comparison of protein synthesis patterns from developing rat SCSG with those from intact and from axotomized adult SCSG failed to demonstrate any marked similarity between the developmental and the axotomy patterns.

Incorporation of tritiated leucine by axotomized rubral neurons

Fourteen kittens, 7--10 weeks of age, were injected with [3H]leucine 0.5--24 h before sacrifice 1--30 days after unilateral high cervical rubrospinal tractotomy. Histoautoradiographs of the red nuclei were prepared and counterstained with thionin. Axon reaction, evident histologically 24 h after surgery, was manifested by central chromatolysis or diffuse cytoplasmic chromophobia. Partial reversion toward a normal cytologic appearance was apparent 10--30 days postoperatively. Nucleolar and nuclear shrinkage and cytoplasmic atrophy were conspicuous accompaniments of axon reaction in rubral neurons. Expressed per cell the radioactivity of axotomized rubral nerve cells was consistently less than controls in animals surviving operation from 5 to 30 days. The data indicate that axon reaction in red nucleus is regressive in character and early associated with diminished protein synthesis. The frequently regressive nature of axon reaction in intrinsic neurons, such as those of red nucleus, probably is important in accounting for failure of regeneration of many mammalian CNS fiber tracts after injury.

Fast axonal transport in motor nerve regeneration

This report describes the fast transport of [3H]-leucine-labeled proteins in regenerating rat sciatic motor nerves. A normal rate of fast transport (383 +/- 33 mm/day) was present in the regenerating sprouts, as well as in the central stumps. The rapidly transported proteins passed the level of axotomy without impediment, and accumulated in the endings of the regenerating sprouts, as shown by electron microscope autoradiography. In addition, transported proteins accumulated in terminal neuromas. The relative amount of protein-incorporated radioactivity in the crest of transport in the regenerating nerves was increased compared to control nerves. These results are interpreted to suggest that the mechanism of fast transport is the same in regenerating nerves was increased compared to control nerves. These results are interpreted to suggest that the mechanism of fast transport is the same in regenerating sprouts as in normal axons; during regeneration fast transport appears to add newly synthesized materials to the growing tip.

Ultrastructural changes in the developing nucleolus following axotomy

A large, basophilic, Feulgen-negative structure has been observed within the nucleoli of golden hamster facial motor neurons. This 'intranucleolar body' was seen, at the electron microscopic level, to be composed of granules which are thought to be ribonucleoprotein particles. At 15 days postnatal age, this aggregation of granules had not yet be appeared. However, by 20--24 days, a small intranucleolar body had developed. At maturity, the intranucleolar body had a diameter of up to 2 mum. Facial nerve axotomy at 15 days did not appreciably alter the nucleolar morphology at 19 days; neither control (opposite side) nor experimental nucleoli contained intranucleolar bodies. Normal nucleoli developed intranucleolar bodies, however, between the ages of 20 and 24 days. After axotomy at 20 days and sacrifice at 24 days, a comparison of facial neuronal nucleoli demonstrated that while an intranucleolar body formed in the control, axotomy prevented this structure from forming at its normal time. Also, when the facial nerve of an adult hamster was axotomized and observed 4 days postoperatively, an intact intranucleolar body was not seen. Instead, the granular portions of the nucleolus appeared to have been dispersed into several small aggregates. We believe that the changes observed in granule distribution, both with development and after axotomy, are morphological indications of a qualitative and/or a quantitative change in rRNA synthesis.

Reversible changes in the accumulation and activities of tyrosine hydroxylase and dopamine-beta-hydroxylase in neurons of nucleus locus coeruleus during the retrograde reaction

To examine the biochemical events associated with the retrograde reaction in central noradrenergic neurons, changes in the activities of several enzymes subserving the metabolism of catecholamines, including tyrosine hydroxylase (TH),dopamine-beta-hydroxylase (DBH), DOPA decarboxylase (DDC), and monoamine oxidase (MAO), were measured in the nucleus locus coeruleus of rat brain following transection of the ascending axons from neurons in this nucleus by electrolytic lesions of the posterolateral hypothalamus. Such lesions produced a triphasic response in the activities of TH and DBH consisting of: (a) an increase to approximately 150 percent of control during the first 48 h followed by (b) a reduction reaching 60 percent of control by day 14, and (c) a full recovery of activity by day 21-28. In contrast, the activities of DDC and MAO, enzymes non-specific for catecholamine neurons, were unchanged. Immunochemical titration with specific antibodies to TH and DBH demonstrated that the fall in enzyme activity was entirely attributable to reduced accumulation of specific enzyme protein and not inhibition of pre-existing enzyme molecules. There was no reduction in the number of neurons in the nucleus locus coeruleus as a consequence of the lesion. We conclude that a reduction in the accumulation of specific enzymes subserving transmitter biosynthesis characterizes a reversible retrograde reaction of central noradrenergic neurons. The coincidence of the time course of reduced enzyme accumulation with regenerative sprouting from damaged noradrenergic axons and also the absence of classical signs of chromatolysis in locus coeruleus neurons following comparable lesions suggest that, first, during the retrograde reaction there may be a reordering of priorities governing accumulation of specific proteins favoring accumulation of those required for reconstitution of cellular processes by sprouting at the expense of proteins utilized in the synthesis of neurotransmitters, and second, some intrinsic neurons of the CNS may undergo reversible biochemical changes of a retrograde reaction in the absence of the classical morphological appearance of chromatolysis.