Anatomy and innervation of the anterior aorta of Aplysia and the ultrastructure of specialized neuromuscular junctions on vascular smooth muscle

Post-synaptic specialization of the neuromuscular junction: junctional folds formation, function, and disorders

Post-synaptic specialization is critical to the neurotransmitter release and action potential conduction. The neuromuscular junctions (NMJs) are the synapses between the motor neurons and muscle cells and have a more specialized post-synaptic membrane than synapses in the central nervous system (CNS). The sarcolemma within NMJ folded to form some invagination portions called junctional folds (JFs), and they have important roles in maintaining the post-synaptic membrane structure. The NMJ formation and the acetylcholine receptor (AChR) clustering signal pathway have been extensively studied and reviewed. Although it has been suggested that JFs are related to maintaining the safety factor of neurotransmitter release, the formation mechanism and function of JFs are still unclear. This review will focus on the JFs about evolution, formation, function, and disorders. Anticipate understanding of where they are coming from and where we will study in the future.

Control of the cardiovascular system ofAplysia by identified neurons

The neural network that controls the cardiovascular system of Aplysia adapts cardiovascular function to a variety of different physiological and behavioral situations. It (1) coordinates the cardiovascular system with the renal and respiratory systems; (2) modifies both systemic and regional blood flow during food-elicited arousal and feeding; and (3) changes the tension of longitudinal vascular muscle to adapt the arterial tree to changes in body shape. Indirect evidence suggests that the cardiovascular control circuit may also play a role in maintaining homeostasis during egg laying. Several putative neurotransmitters, including acetylcholine, serotonin, R15 alpha 1 and R15 alpha 2 peptides, have been localized to identified neurons in this circuit.

Cytochrome oxidase activity as a measure of synaptogenesis by multifunctional neuron L7 of Aplysia

Neuron L7 of the marine mollusc, Aplysia californica, is unique in that it innervates five different target tissues in the animal. We show that when L7 is grown in vitro with two of these targets, that is, muscle cells isolated from the auricle or the gill vein, newly formed L7 neurites contact the muscle cells. Chemical synapses are formed since intracellular stimulation of L7 elicits contraction of individual muscle cells. Interestingly, auricle muscles are also innervated by neuron RBhe and co-cultures of RBhe and auricle muscle cells also exhibit synapse formation. To explore the molecular basis for synaptogenesis between L7 and its targets, it would be useful to quantify the extent of synapse formation in vitro, that is, to determine how many muscle cells can be innervated by a single L7. We show that this can be attained by staining for cytochrome oxidase activity. Cultures of auricle and gill vein muscles were exposed to the appropriate neurotransmitter in order to elicit contraction. The cells were then fixed and stained. In both cases, only cells that contracted were stained and electron microscopy showed reaction product associated with the cristae of mitochondria. When this procedure was applied to cultures of L7 and muscle cells, 38 +/- 2.8% (S.E.M.; n = 7) of the cells on the neurites were stained and therefore responded to L7 stimulation. Thus, part of the L7-RBhe circuit can be assembled in vitro and the extent of synaptogenesis can be accurately quantitated.

Amino acid uptake and incorporation not cell-specific peptides and evidence for intracellular peptide pools in Aplysia neurons R3-R14

1. Relationships between intracellular amino acid concentrations and uptake rates and their utilization in synthesis of cell-specific peptides in neurons R3-R14 in the Aplysia parietovisceral ganglion are explored. 2. The uptake rates and intracellular concentrations of most amino acids are positively correlated and inversely related to their degree of incorporation into the peptides. 3. The bulk cellular pool of arginine is probably utilized in the synthesis of R3-R14 peptides, but much of the glycine taken up appears not to be readily available for protein synthesis. 4. There are rapidly and slowly turning over pools of the peptides, and portions of the peptides stay in the cell bodies for days.

Cellular and synaptic morphology of a feeding motor circuit in Aplysia californica

The cellular and synaptic morphology of a component of the feeding motor circuit in Aplysia californica was examined with light and electron microscopic techniques. The circuit consists of a pair of inhibitory premotor interneurons, B4 and B5, as well as two motoneurons, B15 and B16, which innervate the accessory radula closer muscle. The neurons have wide, varicose arborizations in the buccal ganglion neuropil. All four of these neurons are cholinergic, and in addition, B15 contains immunoreactivity to sera raised against small cardioactive peptide B. Varicose processes in the accessory radula closer muscle are immunoreactive with antisera against several neuropeptides. We identified specific neuromuscular junctions by visualizing horseradish peroxidase uptake in recycled synaptic vesicles. Direct innervation of the accessory radula closer muscle by B15 and B16 is demonstrated by experiments in which horseradish peroxidase is transported from motoneuronal soma to the terminals on muscle fibers. In addition, specific synaptic contacts between B4 and B5 and each of the motoneurons are observed in the buccal ganglion neuropil. Finally, multiple contacts consistent with peptidergic, serotoninergic, and cholinergic synapses are made onto the neurons, suggesting that a variety of transmitters modulate motor output at each level of the hierarchical circuit. These results support the physiological evidence suggesting the involvement of neuropeptides as well as "classical" transmitters in the modulation of circuitry governing feeding behavior in Aplysia.

Anatomical and electrophysiological study of multitransmitter neuron R14 of Aplysia

This study provides detailed information on the Aplysia neuron R14, including its endogenous electrical activity and extensive axonal projections to a variety of vascular and vascular-related tissues. With the aid of intracellular recording techniques, R14 was found to display in vitro variable spontaneous patterns of silent, beating, or bursting activity. Electrophysiological tracing and intracellular cobalt staining revealed the peripheral processes and target tissues of R14. The white-colored axons of R14 exit the parietovisceral ganglion in the genito-pericardial, spermathecal, branchial, and vulvar nerves. These processes extended 20 mm or more into peripheral tissues: the pericardial wall and lumen, digestive gland sheath, aortae, arteries, and veins. R14 axons also project to the right bag cell cluster. Its extensive axonal projections to tissues associated with the cardiovascular system verify physiological studies that show that R14 plays a role in cardiovascular regulation. This neuron appears to have a wide influence over several aspects of circulation in contrast to individual neurons of the R3-13 group, each of which projects to limited numbers of vascular and vascular-related tissues. R14 also uniquely innervates digestive tissues, thus suggesting that it may act as a nexus between influences on digestive and renal physiology such as ion/water regulation, in addition to modulating cardiovascular homeostasis.

Properties of muscle cells and remodeling of neuromuscular junctions as related to age in Aplysia

Age-related morphologic properties of muscle cells and of their innervation in the gill were sought to explain reduced contractility in old Aplysia. As a consequence of previous physiological findings, properties of two muscle groups were examined: one group, MPn, innervated by motor neuron, L7, whose ability to elicit muscle contraction was reduced during aging, and the other group, LEV, innervated by motor neuron, LDG1, whose ability was not. In neither group did muscle properties, such as cell diameter and density and thick filament diameter and density, and resting potential, change with age. In contrast, age-related remodeling of nmjs did occur. The results show that remodeling is expressed differently in the two types of junctions: In L7-nmjs contact between terminals and muscle cells significantly increased with age; in LDG1-nmjs the terminal perimeter was enlarged significantly and not the contact between terminals and muscle cells. With increased age, the proportion of the terminal perimeter in contact with muscle in L7-nmjs increased significantly, and in LDG1-nmjs it remained essentially the same. Accompanying the remodeling of LDG1 terminals was a significant increase of vesicles in them; no significant change in vesicle number was measured in L7 terminals. The effect on transmission of the age-related presynaptic changes in the two types of junctions is discussed.

Biochemical and morphological correlates of transmitter type in C2, an identified histaminergic neuron in Aplysia

There is compelling evidence that histamine serves as a neurotransmitter in C2, a pair of symmetrical neurons in the cerebral ganglion of Aplysia californica. These cells had previously been shown to contain high concentrations both of histamine and of its biosynthetic enzyme, histidine decarboxylase; in addition, 3H-histamine injected intrasomatically was found to move along C2's axons by fast transport. Furthermore, several actions of C2 on identified follower cells were simulated by the application of histamine. We have now characterized this identified neuron further. C2 converts 3H-histidine to histamine: 16% of the labeled precursor was converted to histamine 1 hour after intrasomatic injection. Synthesis of 3H-histamine is specific, since no conversion occurred after injection of other identified Aplysia neurons that are known to use other neurotransmitter substances. We also examined the fine structure of C2's cell body, axons, and axon terminals within the cerebral ganglion and in the nerves that carry its three peripheral branches, identified after injection of Lucifer Yellow, 3H-histamine, or horseradish peroxidase. Characteristic dense-core vesicles are present in all regions of the neuron, and are labeled after intrasomatic injection of 3H-histamine. These 100-nm vesicles together with 60-nm electron-lucent vesicles fill the varicose extensions of C2's neurites that are widely distributed within the ganglion, but only the smaller vesicles cluster at the membrane specializations presumed to be active zones that make contact with many neurons. The widespread distribution of axon terminals and varicosities is consistent with the idea that C2 is modulatory in function; 3H-histamine is taken up selectively by the cell body and axons of C2 and of several other putative histaminergic neurons in a Na+ -dependent manner. Characterization of these biochemical and morphological features of C2 adds to the large amount of information already available to make this identified cell a standard for identifying other neurons that use histamine as a transmitter.

Peripheral axons of the parabolic burster neuron R15

In a combined electrophysiological and anatomical study, the parabolic burster neuron R15 was found to project axons through the genito-pericardial nerve onto the pericardial wall and digestive gland sheath and, more variably, into the heart and pericardial coelom. Projection into these tissues is consistent with the hypothesis that R15 is neurosecretory and may play a role in circulation and/or ion-water regulation in Aplysia.

Selective retrograde axonal transport of free glycine in identified neurons of Aplysia

The specific retrograde axonal transport of free glycine within the identified neurons R3-14 of Aplysia californica was studied. The soma of the R3-14 neurons are located in the parietovisceral ganglion and their axons project down the branchial nerve to end in a large peripheral field. Using a double-chambered apparatus, the peripheral tissue was incubated in medium containing a 3H-amino acid for 4-48 hr, while the nerve and ganglion were isolated and perfused with plain or chemically altered medium. The nerve and ganglion were then either rapidly frozen for scintillation counting or fixed for autoradiography. When 3H-glycine was used, radioactivity entered the nerve rapidly, reached the ganglion in 3 hr, and was transported largely (greater than 80%) in the free amino acid form [trichloroacetic acid (TCA) soluble]. The right parietovisceral hemiganglion accumulated up to nine times more radioactivity than the left hemiganglion, reflecting the presence of the R3-14 axons and soma. Two phases of radioactivity were observed, a fast component moving at about 3 mm/hr and a slower (but larger) component moving at about 0.4 mm/hr. Light microscope autoradiography on nerves containing 3H-glycine revealed that the R3-14 axons accounted for more than 30% of the total label in the nerve but occupied less than 7% of the total cross-sectional area of the axonal core. Electron microscope autoradiography showed a close association of silver grains and dense core vesicles in the R3-14 axons. Retrograde axonal transport of free glycine was inhibited by (in decreasing order of effectiveness) mercuric chloride, vinblastine, colchicine, Nocodazole, and 2,4-dinitrophenol (2,4-DNP). Comparative studies of other amino acids [3H-leucine, 3H-serine, 3H-glutamic acid, 3H-gamma-aminobutyric acid (3H-GABA), and 3H-alanine] showed that 3H-glycine is the only amino acid that is rapidly axonally transported in large quantities within the R3-14 axons. This work demonstrates, for the first time, that a free amino acid, glycine, is transported in the retrograde direction within a select group of axons. The significance of this transport of glycine is discussed in relation to its use as a neural messenger by neurons R3-14.