Degenerating Axon Terminals Research Articles

A glutamate-sensitive neuronal system originating from the area postrema terminates in and transports acetylcholinesterase to the nucleus of the solitary tract

The possible cellular mechanism of action of systemically administered monosodium-L-glutamate and the projections of glutamate-sensitive area postrema neurons have been studied in rats. Parenteral administration of monosodium-L-glutamate induced a selective degeneration of a particular population of AChE-containing area postrema neurons. Electron microscopic cytochemistry and X-ray microanalysis revealed the presence of calcium-containing electron-dense deposits in the mitochondria of degenerating area postrema neurons indicating the possible pathogenetic role of an enhanced intracellular calcium level in the mechanism of monosodium-L-glutamate-induced nerve cell degeneration. Degeneration of area postrema neurons was followed by the appearance of degenerating axon terminals in a well-defined region of the nucleus of the solitary tract, the area subpostrema. Degenerating area postrema neurons and axon terminals were rapidly engulfed by phagocytes predominantly of microglial character. AChE activity, localized to the basal lamina of the capillaries of the area subpostrema under normal conditions, could no longer be detected in rats treated with monosodium-L-glutamate 3-4 weeks previously. These findings provide evidence for the existence of a particular population of glutamate-sensitive, AChE-containing area postrema neurons which project and transport AChE to the nucleus of the solitary tract. This specific neuronal pathway connecting the area postrema with the nucleus of the solitary tract may play an important role in some of the functions attributed to the area postrema. The results also strengthen the hypothesis that brain capillary AChE activity may be of neuronal origin.

Selective degeneration by capsaicin of a subpopulation of primary sensory neurons in the adult rat

The morphological effects of systemic capsaicin treatment have been studied in adult rats. Light and electron microscopy revealed that a subpopulation of small-to-medium sized B-type primary sensory neurons, representing about 17% of the total neuronal population in the 4th lumbar spinal ganglion, underwent rapid degeneration after the administration of capsaicin. Quantitative electron microscopy demonstrated a decrease of about 45% in the number of unmyelinated axons in the saphenous nerve. Light microscopy showed extensive axon terminal degeneration in the brainstem and spinal cord confined to the central projection areas of capsaicin-sensitive afferent fibers, as has already been revealed in the newborn rat. The present results furnish evidence for a hitherto unrecognized selective neurodegenerative action of capsaicin in the adult rat.

Synaptic degeneration in rat visual cortex after neonatal administration of methylmercury

Neonatal rats received subcutaneous injections of methylmercuric chloride in physiological saline (5 mg Hg/kg/day) beginning on postnatal day 5 and continuing until a given animal demonstrated a persistent loss of body weight during 48 h. Clinical signs of visual impairment were observed in all methylmercury-treated animals within 2 to 3 days of the initial weight loss (postnatal days 19 to 23). Four animals exhibited myoclonic jerking of the hind limbs, and one of these animals demonstrated a generalized motor seizure. Electron microscopy of visual cortex in the nonconvulsive animals revealed a selective degeneration of axon terminals forming symmetric synapses. Degenerating neurons were concentrated in layer IV with a more diffuse distribution in other cortical laminae. Dendritic spines were not found on degenerating dendritic profiles, although apparently normal axon terminals were observed to form synapses on degenerating dendritic shafts. These morphologic criteria suggest that aspinous or sparsely-spinous stellate neurons, the inhibitory GABAergic interneurons of the neocortex, are selectively impaired in methylmercury-induced lesions of the developing visual cortex.

Ultrastrukturelle veränderungen des supraoptico-neurohypophysären systems nach läsion des hypophysenstiels bei der ratte

Fine structural changes occurring in the supraoptic nucleus and the neural lobe have been studied in rats following the electrolytic lesion of the hypophysial stalk. Supraoptic neurosecretory neurones undergo a typical chromatolytic reaction. In addition, proliferating microglial cells disconnect axosomatic synapses and phagocytose degenerating neurones. Surviving neurones show signs of structural restitution 3 to 4 weeks after surgery. In the neurohypophysis degeneration of secretory nerve endings started with the disintegration of secretory vesicles 3 to 4 d postoperatively. Degenerated axon terminals were engulfed by pituicytes. Signs indicative of axonal regeneration were not observed in the neural lobe up to the end of the 4th postoperative week.

GABAergic intrinsic interneurons in the red nucleus of the cat demonstrated with combined immunocytochemistry and anterograde degeneration methods.

The presence of glutamic acid decarboxylase (GAD), the enzyme synthesizing gamma-aminobutyric acid (GABA), was investigated in the red nucleus by an immunocytochemical method. The ipsilateral sensorimotor cortex was ablated prior to the immunocytochemical procedures to examine whether cortical neurons make synaptic contacts with GAD-immunoreactive neurons. Small GAD-immunoreactive neurons with a major diameter of 16.1 +/- 3.2 micron (mean +/- S.D.) were observed in the red nucleus under both light and electron microscopy. They were uniformly distributed throughout the nucleus. Degenerating axon terminals were found making synaptic contact with GAD-immunoreactive neurons in the red nucleus, which suggests that there is an input from the ipsilateral sensorimotor cortex to these neurons. This observation, along with our previous findings that GABAergic axon terminals make synaptic contact with the rubrospinal neurons, provides anatomical evidence for the presence of intrinsic GABAergic interneurons which mediate cortical inhibition in cat rubrospinal neurons.

Intrinsic neuromuscular defects in the neurogenic bladder. 1. Short‐term ultrastructural changes in muscular innervation of the decentralized feline bladder base following unilateral sacral ventral rhizotomy

AbstractThe morphology of cholinergic and adrenergic muscular nerves of the feline bladder base was studied ultrastructurally 2, 3, and 4 weeks following its decentralization by unilateral sacral ventral rhizotomy. In all samples, there was widespread transsynaptic degeneration of cholinergic axon terminals and varicosities, with loss of neuroeffector junctions. This was associated with preservation of most adrenergic axons, and — especially in the 4‐week samples—with concurrent early regeneration or sprouting in many cholinergic and adrenergic axons. The findings are discussed in relation to neural ultrastructure in other known models of axonal degeneration, and to the pathogenesis of neurogenic dysfunction of the parasympathetic‐decentralized bladder. The concurrent degenerative and early regenerative axonal changes herein observed in such a bladder (1) suggest that the degeneration of cholinergic axons occurring shortly after rhizotomy is reversible, and probably is compensated for by developing adrenergic hyperinnervation; (2) support the existence of mutual peripheral cholinergic/adrenergic neurotrophism in the vesical muscularis; and (3) provide a probable structural basis for the altered motor and autonomic drug responses of the parasympathetic‐decentralized bladder.

Male-female differences in the intra-amygdaloid input to the medial amygdala.

Following lesion of the posterior cortical nucleus of the amygdala (PCAN), the number of degenerating axon terminals and alterations of synaptic pattern were studied in the molecular layer (ML) of the medial nucleus of the amygdala (MAN) of male and female rats. Semiquantitative analyses by electron microscopy indicated that, 1 and 2 days after the lesion, the number of degenerating terminals in the ventral ML of male rats was statistically greater than that of female rats. Ten days after the operation, intact synapses remaining on dendritic shafts of the medial ML and those on dendritic spines of the ventral ML of male rats significantly decreased in number, compared with unoperated controls. On the other hand, no significant reduction was noted in synapses of the lesioned female rats killed 10 days after the operation. Thus, the number of axon terminals in the male ML originating from the lesioned area was greater than that of the female ML. The number of synapses in the ML of unoperated male rats was statistically greater than that of unoperated females. However, these sex differences in synaptic number became undetectable 10 days after the operation. These findings provide morphological evidence indicating that the fibers from and/or through the PCAN participate in emergence of synaptic sexual dimorphism in the ML of the MAN.

The termination of callosal fibres in the auditory cortex of the rat. A combined Golgi--electron microscope and degeneration study.

When the corpus callosum of the rat is sectioned, the callosal fibres in the cerebral cortex undergo degeneration. In the auditory cortex (area 41) the degenerating axon terminals form asymmetric synapses, and the vast majority of them synapse with dendritic spines. Some other synapse with the shafts of both spiny and smooth dendrites, and a few with the perikarya of non-pyramidal cells. The degenerating axon terminals are contained principally within layer II/III, in which they aggregate in patches. Using a technique in which neurons within the cortex are Golgi-impregnated, then gold-toned and examined in the electron microscope, it has been shown that the dendritic spines of pyramidal neurons with cell bodies in different layers receive the degenerating callosal afferents. The spines arise from the main apical dendritic shafts and their branches, from the dendrites of the apical tufts, and in some cases from the basal dendrites of the pyramidal neurons. The shafts of some pyramidal cell apical dendrites also form asymmetric synapses with callosal afferents. Since we have encountered no spiny non-pyramidal neurons in Golgi preparations of rat auditory cortex, and because other types of non-pyramidal cells have few dendritic spines, it is concluded that practically all of the dendritic spines synapsing with callosal afferents originate from pyramidal neurons.

Cortico-Darkschewitsch-olivary projection in the cat: an electron microscope study with the aid of horseradish peroxidase tracing technique

The nucleus of Darkschewitsch (ND) of the cat was observed electron microscopically after surgical ablation of the motor cortex and horseradish peroxidase (HRP) injection into the inferior olive of the same animals. Degenerated axon terminals containing pleomorphic synaptic vesicles were observed to synapse chiefly with medium-sized or small dendritic processes, some of which were labeled with HRP retrogradely. Therefore, a cortico-olivary projection which was relayed at the ND was revealed at an ultrastructural level.

Terminal degeneration in supraoptic nucleus following subfornical organ lesions: ultrastructural observations in the rat

A projection from the subfornical organ (SFO) to the supraoptic nucleus, recently identified in light microscopic studies, was examined at the ultrastructural level following lesions in SFO. After 18–36 h, axon terminal degeneration was identified in axosomatic contacts with supraoptic neurosecretory neurons, and in axodendritic contacts within and around the supraoptic nucleus. These observations confirm a monosynaptic pathway from SFO to supraoptic neurosecretory neurons that may participate in the release of vasopressin following activation of angiotensin II receptors in SFO.

Terminations of olfactory afferents on layer II and III neurons in the entorhinal area: Degeneration-golgi-electron microscopic study in the rat

The degeneration-Golgi-electron microscopy technique was used to study the termination of olfactory afferents in the entorhinal area (EA) in the rat. Following resection of the olfactory bulb, degenerating axon terminals can be observed in a superficial lamina of layer of I of both medial (MEA) and lateral (LEA) part of EA. Twenty-one Golgi-impregnated cells of 7 neuronal types located in LEA were investigated with the electron microscope. Of these cells, two layer II spinous multipolar neurons and one layer III spinous pyramidal cell, all located in the ventral part of LEA (VLEA), and one layer II sparsely spined multipolar cell, located in the dorsal part of LEA (DLEA), were observed to form synaptic contact with degenerating axon terminals. Based on these findings the possibility of a 3-member neuronal chain is discussed: mitral cell of the olfactory bulb – layer II or III neuron of EA–CA3 cell or granule cell in the hippocampus.

Caudal neurosecretory system synaptic morphology following deafferentation: An electron microscopic degeneration study

The caudal neurosecretory system of Poecilia sphenops (molly) is an isolated population of neurosecretory cells located in the caudal most aspect of the teleost spinal cord. The structure of this neuroendocrine system is favorable for studies on the synaptic control of neurosecretory mechanisms. Little is known about the detailed synaptology of the system. Morphological and electrophysiological reports have shown that the caudal neurosecretory system is linked to higher brain centers by descending spinal projections. To examine the synaptology of the descending synaptic input, surgical deafferentation was performed by microsuction removal of a segment of spinal cord rostral to the caudal system. The degeneration of axon terminals was studied at various times following deafferentation and compared to control synaptology. Based on vesicle content and morphology, three axon terminal types were found in the caudal neurosecretory system. These terminals formed axosomatic, axodendritic, and axoaxonic synaptic contacts. Following deafferentation, axon terminals with dense-cored vesicles and boutons with round clear vesicles degenerated as evidenced by the electron dense dark reaction and the electron lucent reaction respectively. This suggested that at least two different types of axon terminals arise from the descending projection to the caudal neurosecretory system and that two different neurotransmitters may be influencing the neurosecretory activity of these cells.

Projections of the caudate nucleus in cats and kittens.

Small electrolytic lesions were placed on the head of the caudate nucleus of kittens between 8 and 21 days postnatally and in adult cats. Coronal sections of the whole brain were studied under the light microscope with the Fink and Heimer staining method. Degenerating axon terminals could be recognized after a survival period of 30 h for kittens and 8 days for adults, as dark spots and broken fibers. The results indicate extensive degenerative lesions at both diencephalic and brain stem levels. In general the degenerative lesions were denser in kittens when compared with adult animals. The results of this study suggest that the caudate nucleus of immature animals could be more sensitive to the lesions occurring during the time of early synaptogenesis. Moreover, the caudate nucleus could be an important point of convergence of descending pathways exerting a diffuse modulatory action upon several brain structures related to movement.

Toxic effects of adriamycin on the central nervous system. Ultrastructural changes in some circumventricular organs of the mouse after intravenous administration of the drug.

Recent experimental studies have shown that the cytotoxic antibiotic adriamycin (doxorubicin) after systemic administration can enter the so-called circumventricular organs (CVO) of the brain of the mouse. The present experiments were performed to find out whether such penetration of the brain is associated with signs of neurotoxic injury. For this purpose, light- and electron-microscopic observations were carried out on three of these organs: the neurohypophysis (NH), median eminence (ME), and postremal area (PA). Pronounced widening of the extracellular space indicating the presence of edema was present in all the regions, particularly in animals examined within 3 days of injection of the drug. Many degenerated axon terminals were observed in the NH and ME. The glial cells within these regions showed rarefaction of the nuclear chromatin, nucleolar segregation, and also cytoplasmic changes. The PA presented marked cellular changes resulting in degeneration of neurons, which was most evident 30 days after the injection. Hence, regions of the CNS outside the blood-brain barrier can be reached by adriamycin after systemic administration, and the drug can induce morphological changes there. The doses of the drug used in the present experiments were comparable to those given to patients for the treatment of malignant tumors.

Distribution patterns and individual variations of callosal connections in the albino rat.

After complete callosotomy the distribution of degeneration products was re-investigated in adult albino rats. Three to seven days post operation, coronal, horizontal and "flattened" sections were impregnated according to the new methods of Gallyas et al. (1980) which stain degenerating axons and terminals, respectively. The regional distribution patterns of callosal terminals were directly visualized with dark field illumination at low magnification. With this technique the distribution pattern of axons and terminals could be compared between different cortical regions and individuals. Callosal terminals tend to accumulate in patches or bands along the borders of cortical regions and areas. The concentration of callosal terminals was especially high at the common corners of more than two cortical areas. The callosal system shows a rather constant distribution pattern which is composed of column shaped subunits. Considerable individual variations were recognized concerning the number, position, shape, density and contiguity of the columnar units either occupied by callosal connections or empty. Although the laminar distribution of callosal terminals shows some similarities in different areas of the cortex, there is no common laminar pattern characteristic either for the whole neocortex or for any cortical region. The comparison between consecutive sections stained either for degenerating fibers or degenerating axon terminals revealed that the callosal axons do not determine directly the arrangement and packing density of callosal synapses. Whatever determines the position and amount of callosal synapses this influence seems to be exerted via translation into the columnar organization.

Area postrema and the mesencephalic trigeminal nucleus

The ultrastructural morphology of the area postrema (AP) was investigated in the guinea pig: no peculiar features differentiate it from the AP of other vomiting and nonvomiting species. After discrete chronic lesion of the AP, electron micrographs showed clear signs of axonal terminal degeneration on some cells of the ipsilateral mesencephalic trigeminal nucleus (MTN). A direct AP-MTN connection was supported also by electrophysiological investigations. Electrical stimulation of the AP, using liminal or just supraliminal stimuli (4 to 10 μA),—to avoid a simultaneous antidromic activation of Probst's tract—elicited evoked potentials in the ipsilateral MTN with a latency as short as 0.2 to 0.3 ms in both the guinea pig and the rabbit. The same microstimulation of the AP inhibited about the 50% of the ipsilateral MTN units and decreased the excitability of the ipsilateral MTN in the curarized rabbit. In agreement with such findings, AP microstimulation inhibited the jaw movements elicited by MTN stimulation in the noncurarized rabbit. We conclude that the AP exerts an inhibitory influence on the afferent limb of the monosynaptic masseteric reflex.

Terminal degeneration in the mediodorsal cerebral cortex of the lizard Agama agama: Light and electron microscopy.

The degeneration of axon terminals in the small-celled part of the mediodorsal cortex (sMDC) of the lizard Agama agama has been studied after lesions in the dorsal cortex at various survival periods. The Fink-Heimer stain was used to map and demonstrate terminal degeneration with the light and electron microscope. Electron microscopy was used to identify and describe degenerating boutons ultrastructurally. One sham-operated and three unoperated animals served as controls. Between 6 and 21 days postsurgically, degenerating terminals can be seen through 80% of the superficial plexiform layer, the zone adjacent to the cellular layer remaining free of degeneration. Swelling of dendrites in the outer part of the superficial plexiform layer and increased numbers of vacuolar invaginations, both present at short (24 hr-6 days; peak at 48-54 hr) survival periods, can be regarded as reaction to the surgical trauma. Degeneration of axon terminals takes three forms, all of the electron-dense type: gray boutons, degenerating bouton-dendritic spine complexes surrounded or engulfed by glia, and degeneration debris inside glial processes. Several forms of terminal degeneration occur concomitantly at any short (3-12 days) survival time. At longer survival times (15-21 days) only debris is present. From 6 days on, considerable numbers of degenerating structures are present, but the majority of degenerating boutons and debris are associated with reactive glia rather than with dendrites. From these observations it is concluded that in this lizard application of the combined degeneration-Golgi-EM technique would probably lead to little success. Electron microscopy of Fink-Heimer-stained sections suggests that degenerating bouton-dendritic spine complexes and degeneration debris accumulate silver particles, whereas gray boutons do not.

Noradrenergic axon terminals in the substantia gelatinosa of the rat spinal cord: an electron-microscopic study using glyoxylic acid-potassium permanganate fixation.

The noradrenergic terminals in the substantia gelatinosa of the dorsal horn of the cervical spinal cord of the rat were investigated by means of the histofluorescence technique and electron-microscopic cytochemistry using the glyoxylic acid-KMnO4 fixation technique. In accordance with the topographical distribution of fluorescent catecholaminergic fibers, noradrenergic terminals containing small granular vesicles were frequently observed electron microscopically in the outer layer of the substantial gelatinosa. These terminals were most frequently found to appose (without showing typical synaptic features, small-caliber dendrites, spine apparatus, and rarely, large caliber dendrites. Only in a few cases, the noradrenergic terminals exhibited typical synaptic contacts with dendritic elements of small size. In addition, noradrenergic terminals apposed non-noradrenergic terminals containing small agranular vesicles. In rats bearing surgical lesions of the dorsal roots, no noradrenergic terminal were found in contact with the degenerated axon terminals in the substantia gelatinosa. These findings suggest that the noradrenergic afferents to the substantia gelatinosa may exert their influence on sensory transmission via dorsal horn cells.

Synaptic termination of thalamic and callosal afferents in cingulate cortex of the rat.

The distribution of degenerating thalamic and callosal afferents to cingulate cortex in the rat is analyzed. Both light microscopic silver impregnation and quantitative electron microscopic techniques demonstrate differences in the form, number, and laminar distribution of these two afferents in anterior and posterior cingulate cortices. Afferents from the mediodorsal thalamic nucleus terminate in area 24. Most terminals are in layer IIIb, fewer in layer Ia-b, and least in layers V and VI. In contrast, callosal afferents terminate mainly in layers Ib-c, II, IIIa, V, and VI. Thus, thalamic and callosal afferents terminate in a complementary pattern except in layers Ib and IIIb where they overlap. Quantitative analysis of degenerating axon terminals in area 24 indicates that there may be as many as seven times more callosal than mediodorsal thalamic terminals in this cortex. Projections of the anterior thalamic nuclei terminate in areas 29b and 29c, primarily in layer Ia, with fewer in layers Ib-IV and least in layers V and VI. Callosal afferents end mainly in layers V and VI and less densely in layers I-IV, which results in some overlap of thalamic and callosal afferents in layers Ic, IV, and V. In addition, patterns of termination of callosal afferents in posterior cingulate cortex change at borders between previously defined cytoarchitectural areas. Anterior thalamic terminals in area 29c differ from other thalamocortical afferents described previously in that they form two types of terminals. One is large (2-4 micrometer in diameter) and occurs mainly in layer Ia, whereas the second type is smaller and is present in layers Ib-V. Both types of terminals form asymmetric synapses mainly with dendritic spines.

Functional details of the basal ganglia as related to the spatial organization of their fibers in cortex and thalamus

This study compared the results of experimental-morphological and physiological studies in which facts were obtained regarding the functional and morphological heterogeneity of the basal ganglia. Experiments were done on dogs in order to study delayed responses and Hunterian delayed choice responses. Electrical and chemical stimulation of striopallidal structures and recording of spike activity in neurons were done in cats during the formation of conditioned reflexes. Morphological studies were done on cats and dogs by the technique of axon-terminal degeneration. On the basis of an analysis of our own data and data in the literature we have advanced the hypothesis that: 1)There is a correlation between the general features of the disturbance in higher nervous activity and the overlap of striopallidal projections onto cortical and thalamic structures; 2) in a series of cases the behavioral disturbance after shut-off or stimulation of the thalamic structures; 2) in a series of cases the behavioral disturbance after shut-off or stimulation of the basal ganglia can be correlated with the topical details of their projection onto cortical fields and thalamic nuclei.