Biomedical subjects

C H Bailey

Publications and source records attributed to C H Bailey.

12 recordsLinked to original sources

PubMed1992

Serotonin-mediated endocytosis of apCAM: an early step of learning-related synaptic growth in Aplysia.

The long-term facilitation of synaptic efficacy that is induced by serotonin in dissociated cell cultures of sensory and motor neurons of Aplysia is accompanied by the growth of new synaptic connections. This growth is associated with a down-regulation in the sensory neuron of Aplysia cell adhesion molecules (apCAMs). To examine the mechanisms of this down-regulation, thin-section electron microscopy was combined with immunolabeling by gold-conjugated monoclonal antibodies specific to apCAM. Within 1 hour, serotonin led to a 50% decrease in the density of gold-labeled complexes at the surface membrane of the sensory neuron. This down-regulation was achieved by a heterologous, protein synthesis-dependent activation of the endosomal pathway, which leads to internalization and apparent degradation of apCAM. The internalization is particularly prominent at sites where the processes of the sensory neurons contact one another and may act there to destabilize process-to-process contacts that normally inhibit growth. In turn, the endocytic activation may lead to a redistribution of membrane components to sites where new synapses form.

Animals↗

PubMed1992

Inhibitors of protein and RNA synthesis block structural changes that accompany long-term heterosynaptic plasticity in Aplysia.

Synaptic connections between the sensory and motor neurons of Aplysia in culture undergo long-term facilitation in response to serotonin (5-HT) and long-term depression in response to FMRFamide. These long-term functional changes are dependent on the synthesis of macromolecules during the period in which the transmitter is applied and are accompanied by structural changes. There is an increase and a decrease, respectively, in the number of sensory neuron varicosities in response to 5-HT and FMRFamide. To determine whether macromolecular synthesis is also required for the structural changes, we examined in parallel the effects of inhibitors of protein (anisomycin) or RNA (actinomycin D) synthesis on the structural and functional changes. We have found that anisomycin and actinomycin D block both the enduring alterations in varicosity number and the long-lasting changes in synaptic potential. These results indicate that macromolecular synthesis is required for expression of the long-lasting structural changes in the sensory cells and that this synthesis is correlated with the long-term functional modulation of sensorimotor synapses.

Animals↗

PubMed1991

Morphological aspects of synaptic plasticity in Aplysia. An anatomical substrate for long-term memory.

The morphological basis of long-term sensitization of the gill-and-siphon withdrawal reflex in Aplysia was explored by examining the structure of identified sensory neuron synapses in control and behaviorally modified animals. Following long-term training, sensitized animals displayed an increase in the number of sensory neuron synapses compared to control animals. The relative permanence of these structural changes and their similarity in time course to the behavioral duration of sensitization suggest a role for synapse number changes during long-term memory.

Animals↗

PubMed1979

Two different and compatible intraneuronal labels for ultrastructural study of synaptically related cells.

To examine the structural interactions between synaptically connected neurons in Aplysia, we have developed a method for simultaneously labeling two identified cell with different and compatible intraneuronal marking agents (horseradish peroxidase and [3H]N-acetyl-D-galactosamine) visible in both the light and electron microscopes. Combining these two agents within a single cell yields a third label.

Acetylgalactosamine↗

PubMed1979

Indented synapses in Aplysia.

A new type of synaptic contact has been found in Aplysia californica, in which a post-synaptic spine extensively invaginates the pre-synaptic element. The post-synaptic spine, usually less than 0.25 micrometer in diameter, may protrude up to 2 micrometer into the pre-synaptic element. In some instances a larger post-synaptic element indents and forms multiple thin projections into the pre-synaptic varicosity. Along or at the end of these projections a zone occurs at which the surface membranes of the two apposed synaptic elements are rigidly parallel, and the extracellular gap is approximately 60% greater than normal and contains a small amount of electron-dense material. Synaptic vesicles are concentrated against the pre-synaptic membrane in these regions. There are twice as many vesicles per unit area positioned against the membrane at these zones than at similar active zones occurring in the alternative type of synapse, which has a flat, rather than indented, geometry. Single pre-synaptic varicosities have been found to form both flat and indented synapses. These findings raise the possibility that these two forms of synapse may be dynamic transformations of each other, having differing synaptic effectiveness.

Animals↗

PubMed1979

Ultrastructure of the synapses of sensory neurons that mediate the gill-withdrawal reflex in Aplysia.

We have identified the processes of mechanoreceptor sensory neurons by intracellular injection of horseradish peroxidase in order to study the structure of synapses which exhibit profound, behaviourally-relevant plasticity. These synapses are located at small, varicose expansions along or at the end of the fine, microtubule-containing neurites, and they are crowded with vesicles some of which are associated with the varicosity membrane at regions of membrane specialization morphologically equivalent to active zones described in other species. These active zones occur between pre- and postsynaptic processes at two varieties of apposition: a conventional flat one, and a more elaborate indented one. At indented appositions, the presynaptic varicosity is invaginated by a thin (less than 0.25 micrometer diameter) spine of variable length. The active zones of indented synapses have approximately twice the vesicle frequency of flat synapses, suggesting that indented synapses are more effective. Sensory neuron terminals are relatively uniform in their structure, having similar concentrations of vesicles and numbers of active zones, and the majority of the processes postsynaptic to them are less than 0.5 micrometer in diameter. These regularities, and the presence of two strikingly-different types of synaptic apposition, flat and indented, should facilitate structural comparisons of neurons from naive and behaviourally-modified animals. The possible dynamic interconversion of indented and flat appositions at the synaptic terminals of sensory neurons and its behavioural relevance are discussed.

Animals↗

PubMed1979

Cellular studies of peripheral neurons in siphon skin of Aplysia californica.

1. To account for the similarity in the kinetics of habituation between the central and peripheral components of siphon withdrawal, we have tested the idea (52) that each centrally located mechanoreceptor sensory neuron sends two branches to siphon motor neurons; one to centrally located siphon motor neurons and a collateral branch that remains in the periphery and innervates the peripheral siphon motor neurons. 2. We have found a group of peripheral siphon motor neurons and tested the connection onto these cells by central mechanoreceptors. In addition, we have defined by various electrophysiological and morphological criteria two general classes of peripheral neurons that lie along the course of the siphon nerve. 3. One class (type I) consists of only a single cell in each animal. This peripheral neuron typically has the largest cell body found lying along the siphon nerve and is the only peripheral nerve cell that appears white when viewed under epi-illumination. The type I neuron often has a highly regular firing pattern, which occurs in the absence of spontaneous synaptic input. The three-dimensional morphology of this neuron suggests a paucity of fine processes, most of which do not arborize and may terminate in the connective tissue sheath. Fine structural observations of the peripheral white cell have revealed the presence of large densecore granules. The peripheral type I neuron is similar in most of its electrophysiological and morphological properties to central neurons postulated to be neurosecretory. The peripheral white cell is, at present, the only peripheral neuron we can identify with certainty as a unique individual. 4. The second class (type II) of peripheral neurons are siphon motor neurons for the peripheral component of the siphon-withdrawal reflex. In contrast to the type I neurons, members of the second class of peripheral neurons possess smaller, more spherical cell bodies that have varying amounts of orange pigmentation and which give rise to a relatively well-developed and arborized dendritic tree. Type II neurons feature an irregular spontaneous firing pattern that is occasionally modulated by a rich spontaneous synaptic input. Peripheral siphon motor neurons have restricted motor fields that produce contraction of the mantle floor and the base of the siphon. Most of the type II neurons were found to be electrically coupled to one another. 5. The peripheral siphon motor neurons resemble the central siphon motor neurons in that they receive a collateral synapse from centrally located mechanoreceptor sensory neurons. This peripheral sensory-to-motor synapse exhibits the same kinetics of decrement as its central counterpart, both of which parallel behavioral habituation. 6. The rich mechanoreceptor input onto the relatively isolated dendritic trees of the peripheral siphon motor neurons provide a uniquely restricted neuropil to study the sensory-to-motor synapse. The peripheral motor neurons may, therefore, be a useful simple preparation for the cellular study of behavioral plasticity.

Animals↗

PubMed1977