Biomedical subjects

A M Kavookjian

Publications and source records attributed to A M Kavookjian.

8 recordsLinked to original sources

PubMed1999

Ultrastructural analysis of ectopic synaptic boutons arising from peripherally regenerated primary afferent fibers.

The central axons of peripherally regenerated Abeta primary sensory neurons were impaled in the dorsal columns of alpha-chloralose-anesthetized cats 9-12 mo after axotomy. The adequate peripheral stimulus was determined, and the afferent fibers intracellularly stimulated while simultaneously recording the resulting cord dorsum potentials (CDPs). Fibers that successfully had reinnervated the skin responded to light tactile stimulation, and evoked CDPs that suggested dorsally located boutons were stained intracellularly with horseradish peroxidase (HRP). Two HRP-stained regenerated Abeta afferent fibers were recovered that supported large numbers of axon collaterals and swellings in laminae I, IIo, and IIi. Sections containing the ectopic collateral fibers and terminals in the superficial dorsal horn were embedded in plastic. Analyses of serial ultrathin sections revealed that ectopic projections from both regenerated fibers supported numerous synaptic boutons filled with clear round vesicles, a few large dense core vesicles (LDCVs) and several mitochondria (>3). All profiles examined in serial sections (19) formed one to three asymmetric axo-dendritic contacts. Unmyelinated portions of ectopic fibers giving rise to en passant and terminal boutons often contained numerous clear round vesicles. Several boutons (47%) received asymmetric contacts from axon terminals containing pleomorphic vesicles. These results strongly suggest that regenerated Abeta fibers activated by light tactile stimuli support functional connections in the superficial dorsal horn that have distinct ultrastructural features. In addition, the appearance of LDCVs suggests that primary sensory neurons are capable of changing their neurochemical phenotype.

Animals↗

PubMed1995

Synaptic connectivity of local circuit neurons in laminae III and IV of hamster spinal cord.

The present study was undertaken to examine the morphological bases of local synaptic interactions between dorsal horn interneurons. Seven interneurons responding to innocuous mechanical stimuli were intracellularly recorded in lamina III/IV of an isolated preparation of hamster spinal cord with partially intact innervation from an excised patch of hairy skin. Axonal arborizations were stained with horseradish peroxidase (HRP) and examined with an electron microscope. Five cells had extensive synaptic terminations (375-1,785 boutons/axon) with localized distributions (rostrocaudal distance, 425-1,251 microns) overlapping the dendritic trees. Two cells gave rise to deep stem axons that bifurcated into rostrocaudal daughter branches with collaterals ventral to the parent cell bodies (79-661 boutons/axon). Axons of local interneurons were thinly myelinated and formed terminal and en passant enlargements (mean [+/- S.D.] diameter = 0.88 +/- 0.24 microns, n = 157) containing clear, round vesicles 20-60 nm in diameter. Collateral branches of deep axon cells produced round, vesicle-containing boutons comparable in diameter (0.93 +/- 0.22 microns, n = 31) to local axon cells. Both types of interneurons formed asymmetric synaptic contacts with dendritic profiles, but not with cell bodies or axon terminals. Postsynaptic profiles contained sparse ribosomes and had a mean diameter of 1.0 +/- 0.5 microns (n = 49), significantly smaller than a population of identified proximal dendrites (2.3 +/- 0.9 microns, n = 47). HRP-labeled boutons were rarely (5/45 or 11%) in synaptic contact with more than one profile. We conclude that lamina III/IV interneurons make axodendritic synapses predominantly with distal dendrites. Thus, terminations of deep dorsal horn interneurons appear to have a postsynaptic distribution overlapping with axodendritic contacts formed by several functional classes of cutaneous sensory fibers signaling innocuous mechanical stimuli. Such overlap suggests that local spinal networks selectively and strongly influence afferent signals at initial stages of somatosensory integration.

Adaptation, Physiological↗

PubMed1993

Ultrastructural morphology, synaptic relationships, and CGRP immunoreactivity of physiologically identified C-fiber terminals in the monkey spinal cord.

The spinal cord terminations of two electrophysiologically identified single C-fibers (one identified as a C-nociceptor) were intra-axonally labeled with horseradish peroxidase and analyzed with both light and electron microscopy. Serial section ultrastructural analysis and postembedding immunocytochemical techniques for calcitonin gene-related peptide (CGRP), substance P (SP), and GABA were used to study the synaptology, and neuropeptide content. All C-terminal synapses were in laminae I and II. The terminals sampled (n = 73) from these two C-fibers rarely established glomerular synaptic complexes, but rather, simple terminals, usually measuring 1-4 microns in length and 1-3 microns in diameter. They most often established 1 or 2 (range 1 to 5) quite large asymmetric axodendritic synaptic contacts. Postsynaptic structures included dendritic spines and shafts with and without vesicles. C-terminals were filled with small round synaptic vesicles (45-60 nm) and also contained variable numbers of large dense-core vesicles (LDCVs, 80-110 nm). LDCVs inside identified C-terminals frequently displayed CGRP immunoreactivity. We were unable to detect SP immunoreactivity inside our sample of C-fiber LDCVs. C-terminals were never found postsynaptic to other profiles. Thus, the C-fiber terminals sampled in this study have simple synaptology, do not receive presynaptic control and contain CGRP immunoreactivity. They differ greatly from the terminals of A delta nociceptors studied previously by our group that had glomerular endings, often received presynaptic input and did not contain CGRP immunoreactivity. This suggests the existence of different processing mechanisms, at the level of the first synapse, for nociceptive inputs arriving to lamina I and II through different types of primary afferents.

Animals↗

PubMed1992

Synaptic interactions between GABA-immunoreactive profiles and the terminals of functionally defined myelinated nociceptors in the monkey and cat spinal cord.

This study analyzes the synaptic interactions between the central terminals of A delta high threshold mechanoreceptors (A delta HTMs) and GABA-immunoreactive profiles. A delta HTM primary afferents from three monkeys and one cat were electrophysiologically identified and intracellularly labeled with HRP, and their terminal arborizations in laminae I and II of the sacrocaudal spinal cord were studied at the ultrastructural level. GABA-immunoreactive profiles in relation to A delta HTM terminals were demonstrated using postembedding colloidal gold techniques. Monkey A delta HTM terminals (n = 131) usually constituted the central element of synaptic glomeruli; they established large asymmetric synaptic contacts with 1-13 dendrites (modal value 2-4) and were surrounded by 0-6 peripheral axon terminals (modal value 2-3). The large majority (around 85%) of the peripheral axon terminals were GABA immunoreactive. They were found presynaptic to the A delta HTM terminal and/or to dendrites postsynaptic to the primary afferent terminal. Furthermore, all peripheral axon terminals found presynaptic to the A delta HTM terminals showed GABA immunoreactivity. Within a single A delta HTM fiber, this synaptic arrangement was found in 20-60% of its boutons. In addition, 28% of the postsynaptic dendritic profiles displayed weak GABA immunoreactivity. Some of them contained vesicles; however, only in a few cases did we observe synapses between a GABA-immunoreactive vesicle-containing dendrite and a dendritic profile postsynaptic to an A delta HTM terminal. Similar synaptology and interactions with GABA-immunoreactive profiles were displayed by the terminals of the single cat A delta HTM fiber studied. Our data support the hypothesis that GABA-containing neurons use both presynaptic and/or postsynaptic mechanisms to exert a powerful control, presumably inhibitory, over the transmission of nociceptive information between A delta HTM afferents and second-order neurons in monkey and cat spinal cord. Our results also imply that GABA may be released within the synaptic glomeruli formed by A delta HTM terminals either by local dendrites or by axon terminals. We discuss the possibility that these GABAergic synapses can be driven by inputs from both primary afferents and/or descending systems to modulate the transmission of nociceptive sensory information.

Animals↗

PubMed1988

Morphology and ultrastructure of physiologically identified substantia gelatinosa (lamina II) neurons with axons that terminate in deeper dorsal horn laminae (III-V).

In order to determine their local circuit function, we have examined physiologically characterized, intracellularly labeled neurons in laminae I and II with light and electron microscopes. Single neurons in the spinal substantia gelatinosa (lamina II) of the cat and monkey were recorded intracellularly and characterized physiologically. Following characterization, the neurons were labeled with horseradish peroxidase that was iontophoretically ejected from the recording micropipette. After fixation and sectioning, histochemical reaction allowed visualization of the neuron soma, dendrites, and axon. The four nociceptive neurons reported here (three from cats and one from a monkey) had axons that distributed terminal collaterals to deeper laminae of the spinal cord, including laminae III, IV, and V. Electron microscopy of the axons demonstrated that the parent axons were myelinated and that the terminal collaterals established synaptic contact with neurons in the deeper laminae. These results suggest that some substantia gelatinosa neurons relay nociceptive information to neurons in deeper regions of the spinal dorsal horn via myelinated axons.

Animals↗

PubMed1985

The ultrastructure and synaptic connections of the spinal terminations from single, physiologically characterized axons descending in the dorsolateral funiculus from the midline, pontomedullary region.

Single axons descending in the spinal, dorsolateral funiculus which were directly activated by stimulating in or near nucleus raphe magnus (nRM) in the rostral medulla were iontophoretically injected with horseradish peroxidase (HRP). Labeled axons and terminal arborizations in the spinal cord were demonstrated by diaminobenzidine histochemistry following fixation. Sections were processed for electron microscopy and embedded between coverslips. Following examination with the light microscope, selected boutons were sectioned in ultrathin series and examined with the electron microscope. Two arborization patterns were observed with the light microscope. One terminated dorsally in laminae I, II, V, and X, while the other terminated ventrally in laminae V, VII, and X. At the ultrastructural level, boutons predominantly contacted dendrites and occasionally contacted cell somas. Ventrally projecting axons had boutons characterizable as Gray's type I. Some dorsally projecting axons also had Gray's type I boutons, while other axons had Gray's type II boutons. Thus, these descending axons may be heterogeneous in function. No differences in physiological characteristics were found between axons with Gray's type I boutons versus axons with Gray's type II boutons.

Animals↗

PubMed1983

The ultrastructure and synaptic connections of serotonin-immunoreactive terminals in spinal laminae I and II.

In order to study the synaptic relationships of serotonin (5-HT)-containing axons, boutons in laminae I and II of the cat spinal cord were labeled for serotonin with peroxidase-antiperoxidase immunocytochemistry. Labeled boutons were examined with the light microscope and recut into serial ultrathin sections for examination with the electron microscope. Labeled axons exhibiting boutons were sagittally oriented, and were most numerous in lamina I and outer lamina II (IIo) and least numerous in inner lamina II (IIi). Two types of labeled boutons were observed ultrastructurally. A relatively rare, large, scalloped or egg-shaped bouton, which contained many mitochondria and dense core vesicles, was found in laminae I and IIo. A smaller dome-shaped bouton, which contained fewer dense core vesicles and round or pleomorphic, clear vesicles, was found throughout laminae I and II. Both types commonly established symmetrical synaptic contacts with the distal portion of a dendritic tree, rarely with proximal portions or cell somas, and never with axon terminals. The results suggest that there are heterogeneous serotonergic systems that may selectively modify different inputs postsynaptically to functionally different types of neurons in the superficial dorsal horn of the spinal cord.

Animals↗

PubMed1978

Antigenic properties of the Golgi derived scales of Pleurochrysis scherffelii.

The cell wall of the marine alga Pleurochrysis scherffelii is composed of Golgi-derived scales with four distinct subcomponents: amorphous coating material, radial microfibrils, spiral coating substances and spiral microfibrils. The antigenic properties of the Pleurochrysis scales were studied. Antibodies were produced in rabbits to the amorphour to the amorphous coating material and to a heretofore undescribed peripheral subcompoent, known as the amorphoous rim modification substance. The antigenic subcomponents were detected by agglutination/precipitation, negative staining, and shadowing procedures. Scales of the Cricosphaera phase produced negative results when tested with Pleurochrysis scale antisera. A possible function of the amorphous rim modification substance in maintaining the integrity of spiral microfibrillar organization is introduced.

Animals↗