Amino acid concentrations in the Aplysia nervous system: neurons with high glycine concentrations

Decoding leukemia at the single-cell level: clonal architecture, classification, microenvironment, and drug resistance

Leukemias are refractory hematological malignancies, characterized by marked intrinsic heterogeneity which poses significant obstacles to effective treatment. However, traditional bulk sequencing techniques have not been able to effectively unravel the heterogeneity among individual tumor cells. With the emergence of single-cell sequencing technology, it has bestowed upon us an unprecedented resolution to comprehend the mechanisms underlying leukemogenesis and drug resistance across various levels, including the genome, epigenome, transcriptome and proteome. Here, we provide an overview of the currently prevalent single-cell sequencing technologies and a detailed summary of single-cell studies conducted on leukemia, with a specific focus on four key aspects: (1) leukemia's clonal architecture, (2) frameworks to determine leukemia subtypes, (3) tumor microenvironment (TME) and (4) the drug-resistant mechanisms of leukemia. This review provides a comprehensive summary of current single-cell studies on leukemia and highlights the markers and mechanisms that show promising clinical implications for the diagnosis and treatment of leukemia.

Single-cell mass spectrometry

Owing to recent advances in mass spectrometry (MS), tens to hundreds of proteins, lipids, and small molecules can be measured in single cells. The ability to characterize the molecular heterogeneity of individual cells is necessary to define the full assortment of cell subtypes and identify their function. We review single-cell MS including high-throughput, targeted, mass cytometry-based approaches and antibody-free methods for broad profiling of the proteome and metabolome of single cells. The advantages and disadvantages of different methods are discussed, as well as the challenges and opportunities for further improvements in single-cell MS. These methods is being used in biomedicine in several applications including revealing tumor heterogeneity and high-content drug screening.

Categorizing Cells on the Basis of their Chemical Profiles: Progress in Single-Cell Mass Spectrometry

The chemical differences between individual cells within large cellular populations provide unique information on organisms' homeostasis and the development of diseased states. Even genetically identical cell lineages diverge due to local microenvironments and stochastic processes. The minute sample volumes and low abundance of some constituents in cells hinder our understanding of cellular heterogeneity. Although amplification methods facilitate single-cell genomics and transcriptomics, the characterization of metabolites and proteins remains challenging both because of the lack of effective amplification approaches and the wide diversity in cellular constituents. Mass spectrometry has become an enabling technology for the investigation of individual cellular metabolite profiles with its exquisite sensitivity, large dynamic range, and ability to characterize hundreds to thousands of compounds. While advances in instrumentation have improved figures of merit, acquiring measurements at high throughput and sampling from large populations of cells are still not routine. In this Perspective, we highlight the current trends and progress in mass-spectrometry-based analysis of single cells, with a focus on the technologies that will enable the next generation of single-cell measurements.

Metabolic differentiation of neuronal phenotypes by single-cell capillary electrophoresis-electrospray ionization-mass spectrometry

Single-cell mass spectrometry (MS) is a rapidly emerging field in metabolic investigations. The inherent chemical complexity of most biological samples poses analytical challenges when using MS platforms to measure sample content without prior chemical separation. Here, a single-cell capillary electrophoresis (CE) system was coupled with electrospray ionization (ESI) MS to enable the simultaneous measurement of a vast array of endogenous compounds in over 50 identified and isolated large neurons from the Aplysia californica central nervous system. More than 300 distinct ion signals (m/z values) were detected from a single neuron in the positive ion mode, 140 of which were selected for chemometric data analysis. Metabolic features were evaluated among six different neuron types (B1, B2, left pleural 1 (LPl1), metacerebral cell (MCC), R2, and R15) chosen for their various physiological functions. The results indicated chemical similarities among some neuron types (B1 to B2 and LPl1 to R2) and distinctive features for others (MCC and R15 cells). The quantitative nature of the MS platform allowed the comparison of metabolite levels for specific neurons. The CE-ESI-MS approach for examination of individual nanoliter-volume cells as described herein is readily adaptable to other volume-limited samples.

Profiling metabolites and peptides in single cells

The intracellular levels and spatial localizations of metabolites and peptides reflect the state of a cell and its relationship to its surrounding environment. Moreover, the amounts and dynamics of metabolites and peptides are indicative of normal or pathological cellular conditions. Here we highlight established and evolving strategies for characterizing the metabolome and peptidome of single cells. Focused studies of the chemical composition of individual cells and functionally defined groups of cells promise to provide a greater understanding of cell fate, function and homeostatic balance. Single-cell bioanalytical microanalysis has also become increasingly valuable for examining cellular heterogeneity, particularly in the fields of neuroscience, stem cell biology and developmental biology.

Localization of glutamate and glutamate transporters in the sensory neurons of Aplysia

The sensorimotor synapse of Aplysia has been used extensively to study the cellular and molecular basis for learning and memory. Recent physiologic studies suggest that glutamate may be the excitatory neurotransmitter used by the sensory neurons (Dale and Kandel [1993] Proc Natl Acad Sci USA. 90:7163-7167; Armitage and Siegelbaum [1998] J Neurosci. 18:8770-8779). We further investigated the hypothesis that glutamate is the excitatory neurotransmitter at this synapse. The somata of sensory neurons in the pleural ganglia showed strong glutamate immunoreactivity. Very intense glutamate immunoreactivity was present in fibers within the neuropil and pleural-pedal connective. Localization of amino acids metabolically related to glutamate was also investigated. Moderate aspartate and glutamine immunoreactivity was present in somata of sensory neurons, but only weak labeling for aspartate and glutamine was present in the neuropil or pleural-pedal connective. In cultured sensory neurons, glutamate immunoreactivity was strong in the somata and processes and was very intense in varicosities; consistent with localization of glutamate in sensory neurons in the intact pleural-pedal ganglion. Cultured sensory neurons showed only weak labeling for aspartate and glutamine. Little or no gamma-aminobutyric acid or glycine immunoreactivity was observed in the pleural-pedal ganglia or in cultured sensory neurons. To further test the hypothesis that the sensory neurons use glutamate as a transmitter, in situ hybridization was performed by using a partial cDNA clone of a putative Aplysia high-affinity glutamate transporter. The sensory neurons, as well as a subset of glia, expressed this mRNA. Known glutamatergic motor neurons B3 and B6 of the buccal ganglion also appeared to express this mRNA. These results, in addition to previous physiological studies (Dale and Kandel [1993] Proc Natl Acad Sci USA. 90:7163-7167; Trudeau and Castellucci [1993] J Neurophysiol. 70:1221-1230; Armitage and Siegelbaum [1998] J Neurosci. 18:8770-8779)) establish glutamate as an excitatory neurotransmitter of the sensorimotor synapse.

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.

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.

Synthesis of gamma-glutamyldopamine and other peptidoamines in the nervous system of Aplysia californica

The synthesis of a series of gamma-glutamyl amines (gamma-Glu-amines), including gamma-Glu-dopamine, gamma-Glu-5-hydroxytryptamine, gamma-Glu-octopamine, gamma-Glu-tryptamine, gamma-Glu-tyramine, and gamma-Glu-phenylethylamine, by nervous tissue of the marine mollusc Aplysia californica is described. After ganglia were incubated in vitro with 14C-amines, the unchanged amine and a new 14C-labeled product, identified as the gamma-Glu conjugate of the amine, were isolated from the tissue extracts. Identification was made by comparing the chromatographic properties (HPLC, TLC, and LC) of the isolated conjugates with chemically synthesized gamma-Glu-amines before and after acid hydrolysis.

Identification and primary structural analysis of peptide II, an end-product of precursor processing in cells R3-R14 of Aplysia

Peptide II, which is encoded on a gene for a precursor protein in abdominal ganglion neurons R3-R14, was purified from extracts of abdominal ganglia of Aplysia californica. Native peptide II comigrates with synthetic standards on HPLC under isocratic conditions. Amino acid sequence and composition analyses indicate that the sequence of peptide II is Glu-Ala-Glu-Glu-Pro-Ser-Phe-Met-Thr-Arg-Leu, as predicted from the precursor. The molluscan cardioexcitatory peptide Phe-Met-Arg-Phe-amide was also identified in abdominal ganglion extracts by similar means. The large amount of peptide II recovered (100 ng/ganglion), and its location on the precursor between two pairs of basic residues, strongly suggest that the precursor is processed into peptide II and at least two other peptides. Although cells R3-R11 have been postulated to play a role in cardiovascular control, peptide II was without effect at less than or equal to 10(-4) M concentrations on identified abdominal ganglion neurons, the gastroesophageal artery or the heart. The physiological role of peptide II therefore remains to be elucidated.

Synthesis of the novel dipeptide beta-aspartylglycine by Aplysia ganglia

Isolated ganglia from Aplysia californica rapidly took up [14C]glycine or [14C]aspartate from a sea-water medium. Approximately 20% of the tissue radioactivity was recovered in the peptides beta-aspartylglycine and glutathione after incubation with [14C]glycine. Compared with other individual cells isolated from the abdominal ganglion, the glycine-containing white cells (R3-R14) incorporated less [14C]glycine into beta-aspartylglycine, but similar amounts into glutathione. In contrast, [14C]aspartate was metabolized primarily to nonamino dicarboxylic acids and relatively little radioactivity was incorporated into beta-aspartylglycine.

Excitatory effect of amino acids on identified neuron R14 of Aplysia. II. Neutral amino acids and structure-activity relationships

The effects of 20 amino acids on identified neuron R14 of Aplysia kurodai were studied by conventional intracellular recording and voltage-clamp techniques. Neutral alpha-amino acids caused marked dose-dependent depolarizations of the neuron. The effects of several amino acids applied simultaneously were additive. L-isomers were much more effective than the corresponding D-isomers. R14 was at most slightly depolarized by perfusion with 2-aminoisobutyric acid and the non-alpha-amino acids beta-alanine and beta- and gamma-aminobutyric acid. Putative neurotransmitters ACh, 5-HT, dopamine, GABA, and taurine were much less effective in depolarizing R14 than neutral amino acids. Values of the Hill coefficient (nH) and the apparent dissociation constant (KA) were approximately 0.64 and 80 mM, respectively, for glycine. These results suggest that R14 has a general sensitivity to neutral amino acids and that amino acids in the hemolymph may be able to influence the electrical activity of R14.

Excitatory effect of amino acids on identified neuron R14 of Aplysia. I. Glycine-induced depolarization and its ionic mechanism

The ionic mechanism of the membrane effect of glycine on identified neuron R14 of Aplysia was investigated with conventional intracellular recording and voltage-clamp techniques. Both localized and bath applications of glycine markedly depolarize R14. Bath-applied glycine induced an inward current that gradually reached a maximum and remained at that level until glycine was washed out. Displacement of the holding potential from -46 to -121 mV increased the inward current. The extrapolated reversal potential was +38.6 mV. Reduction of [Na+]o reversibly decreased the inward current. Alterations of [K+]O, [Cl-]O, and [Ca2+]O, as well as bath-applied ouabain and sodium cyanide, did not affect the inward current. These results suggest that glycine can induce an Na+ current and that the glycine-induced inward current does not reflect an active uptake by an Na+-coupled transport system of glycine into the neuron.

Identification of an acidic dipeptide, beta-aspartylglycine, in the CNS of Aplysia

A novel dipeptide, beta-aspartylglycine (beta-DG), has been isolated from tissues of the marine gastropod mollusc Aplysia californica. This compound was detected only in Aplysia and not in other molluscs, such as Helix or Mercenaria, or in lobster or frog. Among the Aplysia tissues, the highest levels of beta-DG were in nervous tissue and in the reproductive tract. beta-DG was assayed by HPLC as the o-phthaldialdehyde derivative and found to be present in all individual, identified neurons at a concentration of approximately 40 pmol/microgram protein. The peptide was identified as beta-DG by gas chromatography-mass spectrometry (GCMS) using trimethylsilyl derivatives prepared before and after acid hydrolysis. It was further characterized as the beta-isomer by TLC, including Rf, atypical blue-gray color with ninhydrin, and a violet color with Cu2+-ninhydrin. A fractionation scheme is described whereby acid-soluble tissue constituents can be divided into acidic, neutral, and basic components using mini ion-exchange columns. This partial purification prior to TLC analysis was necessary to remove compounds that interfered with the isolation of beta-DG.

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.

Determination of glutathione and ATP in ganglia and individual neurons of Aplysia californica

The levels of two gamma-glutamyl cycle substrates, glutathione and ATP, were determined in single identified nerve cell bodies from the CNS of Aplysia californica. The glutathione content of single cells averaged 30 +/- 4.9 mumol/g protein. Glutathione levels were similar in identified cholinergic, serotonergic, and histaminergic cells, as well as in neurons whose transmitters are not yet identified. The abdominal rostral white cells, which are enriched in glycine, a component amino acid of glutathione, did not possess distinctively higher glutathione concentrations. The ATP content of single Aplysia nerve cell bodies averaged 15.0 +/- 1.5 mumol/g protein. Despite the vast chemical, anatomical, and functional heterogeneity between Aplysia central neurons, no cells were found that contained unusually high or low ATP levels.

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effects of amino acids on the period of the circadian rhythm from the Aplysia eye

The effect of continuous treatments of single L-amino acids (0.1mM) on the free running rhythm from the isolated Aplysia eye was examined. A variation in the change in free running period produced by different amino acids was observed. Two well-known precursors of neurotransmitters (tyrosine, tryptophan) had the largest effects. These amino acids lengthened the period ca. 1.7 h. Another group of amino acids (alanine, threonine, proline) lengthened the period by about 1 h. Smaller effects were produced by aspartic acid and leucine and no effects were caused by lysine, glycine, valine, and serine. Phenylalanine may shorten the period a small amount. Glucose (5mM) lengthens the period a small amount (0.4 h), decreases the effect of tyrosine somewhat, and has no effect on the lengthening of the period produced by tryptophan. Three amino acids not involved in protein synthesis (ornithine, beta-alanine, citrulline) had at most small effects on the free running period. Also, D-tryptophan lengthened the period by 0.6 h but the effect of D-tryptophan was considerably smaller than the effect of L-tryptophan. A few of the amino acids had small short-term effects on spike rate and longer-term effects on the amplitudes of the rhythms but these effects did not correlate with the effects of the amino acids on the free running period. Though continuous treatments of certain amino acids lengthened the periods, shorter treatments (tryptophan, 6 h) did not phase-shift the rhythm. Since eyes maintained in a commonly used culture medium have longer periods than eyes in a simple seawater medium, the amino acids of the culture medium must be responsible, at least in part, for the lengthening effect of the culture medium. The mechanism of action of the amino acids is unknown. The magnitude of the effects did not correlate with physical-chemical properties of the amino acids nor with whether the amino acids were "essential" or "nonessential." The effects of the amino acids may be mediated by their effects on neurotransmitter and/or protein synthesis.

Mass fragmentographic determination of endogenous glycine and glutamic acid released in vivo from the pigeon optic tectum. Effect of electric stimulation of a midbrain nucleus

Various investigations suggest glycine to be an inhibitory transmitter in the pigeon optic lobe in a pathway originating in the nucleus isthmi, pars parvocellularis (Ipc) terminating in the optic tectum. In order to obtain additional evidence for this hypothesis the in vivo release of endogenous glycine in the optic tectum upon electrical stimulation of Ipc was investigated. By perfusing the upper strata of the optic tectum with Ringer solution using a push-pull cannula endogenous amino acids released from the surrounding tissue were collected. Concentration of glycine and glutamic acid in the perfusates were determined by mass fragmentography of their N-pentafluoropropionyl hexafluoroisopropyl esters. Deuterium-labeled glycine and glutamic acid were used as internal standards for quantitative measurements. The resting release of glycine and glutamic acid was 2.9 pmol/min and 1.4 pmol/min, respectively. Electrical stimulation of Ipc was found to induce a 2--40-fold increase of the glycine efflux into the perfusate whereas the efflux of glutamic acid remained at a constant level. These findings strongly support the hypothesis that glycine is a transmitter in Ipc-tectal neurons.

Localization of axonally transported [3H]glycine in vesicles of identified neurons

A specific association of axonally transported, free [3H]glycine with vesicles in the identified neurons R3-R15 of Aplysia is demonstrated by high resolution autoradiography. The association of glycine with vesicles, the first such finding in any animal for a neuroactive amino acid, adds to evidence that glycine may be utilized as a neuro-chemical messenger by R3-R14.

Modulation of arterial muscle contraction in Aplysia by glycine and neuron R14

Glycine and electrical activity in neuron R14 both enhance the contractility of the anterior aorta of the gastropod Aplysia californica. Glycine and R14 do not seem to cause contraction directly, change membrane permeabilities or alter junctional potentials occurring in the muscle fibers, yet they increase the force of contractions induced by other means. Modulation of muscle contraction is a new function for glycine.

Primary afferent depolarization. Distribution of the gamma-aminobutyric acid system in frog spinal cord

In the frog spinal cord primary afferent depolarization (PAD) constitutes a powerful inhibitory control mechanism. It has been suggested that gamma-aminobutyric acid (GABA) is the transmitter substance involved in the genesis of PAD. In these studies we show that maximal glutamic acid decarboxylase activity is localized roughly 400-600 micrometers from the dorsal surface, and that correlates well with the intraspinal distribution of field potentials associated with PAD. Measurements of GABA in serial spinal cord sections cut in a dorsal--ventral direction shows that high levels of GABA are seen at 400--600 micrometers, with a peak at 800 micrometers from the dorsal surface. Stimulation at frequencies shown to produce PAD augments the release of endogenous GABA from a superfused frog hemicord preparation.

Effects of hypothalamic peptide hormones on the electrical activity of Aplysia neurons

The effect of luteinizing hormone-releasing hormone (LHRH) and thyrotropin-releasing hormone (TRH) on the electrical activity of neurons in the abdominal ganglion of Aplysia californica was studied. Where tested, TRH had no effect. The neurosecretory white-cell neurons were the most responsive of the neurons tested with LHRH. Bath applications of 1 micro M LHRH increased firing rates in 4 of 5 white cells for extended periods of time. The increased rates persisted in an LHRH-deficient bath. A similar result was obtained with bath applications of the LHRH agonist analog D-Ala6, des-Gly10-LHRH-ethylamide. The iontophoresis of LHRH onto white-cell somata either produced no change in electrical activity or initiated an increase in firing rate and bursting patterns which outlasted the application period. Two types of white cells are suggested by the white-cell responsiveness to LHRH. The white cells responsive to the decapeptide are candidate model neurons for studying the membrane actions of LHRH.

Anatomy and ultrastructure of the axons and terminals of neurons R3-R14 in Aplysia

Using light and electron microscopy and autoradiography, we have traced the axons of neurons R3-R14 in the parietovisceral ganglion (PVG) of Aplysia to terminal fields associated with vascular tissue. The axons are identified by their large size (15-30 micrometer diameter), extensive glial infolding, characteristic dense core vesicles (DCV; approximately 180 nm diameter), and specific, rapid uptake of 3H-glycine. Each neuron in this homogeneous group sends an axon via the branchial nerve to the pericardial region surrounding the junction of the efferent gill vein and the heart. R14 also sends axons to major arteries near the PVG. The R3-R14 axons branch extensively; we estimate that there are at least several hundred per cell. Branches along axons in the branchial nerve exit the nerve, subdivide, and end blindly in the sheath which is bathed by hemolymph. Similar blind endings from R3R14 occur in the sheath of the PVG (Coggeshall, '67). Axonal branches in the pericardial region and the special R14 axons in the arterial walls form both varicose endings near and terminals in contact with vasvular smooth muscle. All R3-R14 endings are free of glia, packed with DCV, show occasional omega-shaped profiles and rapidly take up 3H-glycine. R3-R14 manufacture specific low molecular weight peptides (Gainer and Wollberg, '74), and both the cell bodies (Iliffe et al., '77) and the germinals contain unusually high concentrations of glycine. The presence of peptides as putative neurohormones and sheath endings (neurohormonal release areas) are consistent with R3-R14 being neurosecretory (Coggeshall et al., '66). While glycine could not be a circulating hormone due to its high circulating levels (Iliffe et al., '77), glycine could act as a local chemical messenger between R3-R14 and smooth muscle. The terminal morphology of R3-R14 is consistent with these neurons having both synaptic-type and neurosecretory-type functions.

Bidirectional axonal transport of free glycine in identified neurons R3--R14 of Aplysia

The axonal transport of 3H-amino acids was studied in the axons of identified neurons R3--R14 in the parietovisceral ganglion (PVG) of the mollusc Aplysia. The PVG was incubated (3--24 hr) in media containing physiological concentrations of single 3H-amino acids while the isolated nerve was superfused with plain or chemically altered media. The nerve was then sliced into sequential segments for biochemical analyses or fixed for autoradiography. 3H-glucine was transported at 70 mm/day in 6X greater quantities than other amino acids which were transported at less than 40 mm/day. In the 3H-glycine experiments, greater than 80% of the label transported into the nerve remained as free glycine, comigrating with glycine in thin-layer chromatographs. In autoradiographs of sections 4 mm from the ganglion-nerve barrier, greater than 50% of the silver grains were over R3--R14 axons which occupy less than 10% of the nerve cross-sectional area. EM autoradiographs confirmed that grains were within R3--R14 and not in surrounding glia. The selective transport of glycine was inhibited by Hg2+, by vinblastine and Nocodazole, and by low Ca2+ media. Autoradiographs of vinblastine-treated nerves showed a drastic reduction in label over R3--R14 and other axons. Label was also transported retrogradely; this transport rate was similar to the orthograde rate, but 5--10 times less label moved retrogradely. Autoradiographs showed that the retrograde label was localized to R3--R14 axons. This report clearly demonstrates the rapid, selective, and bidirectional transport of a free amino acid and provides further evidence that glycine may be used as a neurochemical messenter by neurons R3--R14.

Specific glycine uptake by identified neurons of Aplysia californica. I. Autoradiography

The identified giant neurons R3-R14 in the Aplysia parietovisceral ganglion (PVG) have a rapid, Na+-dependent and Hg2+-sensitive uptake system for glycine not found in neighboring neurons. In autoradiographs of PVG incubated in [3H]glycine (glutaraldehyde fix), the cytoplasms of R3-R14 have 3--4 times more silver grains (No./100 sq.micrometer) than other neurons. The glycine uptake system in R3-R14 is selective (alanine, serine, leucine, and proline are taken up equally by all neurons) and is unaffected by reserpine and anisomycin. Neurons R3-R14 contain 2 times less label when ganglia are fixed in formaldehyde than when glutaraldehyde is used as a fixative. Because formaldehyde fixes free amino acids poorly, much of the glycine taken up by R3-R14 is, therefore, not incorporated into protein. In autoradiographs of PVG incubated in [3H]glycine, silver grains are distributed randomly throughout the cytoplasm and nucleus of R3-R14; no association of the grains with the dense core granules characteristic of these neurons7 or other cellular components was found. In contrast, grains in the neurosecretory "bag cells" of the PVG were clustered in numerous discrete areas of the cytoplasm (Golgi complex areas) and the nucleus was only sparsely labeled. The existence of a rapid and selective glycine uptake system in R3-R14, together with their high endogenous glycine concentrations17, suggests that glycine may be a neurotransmitter in these neurons.

Specific glycine uptake by identified neurons of Aplysia californica. II. Biochemistry

Glycine is taken up twice as rapidly by neurons R3-R14 as by other identified neurons in the Aplysia parietovisceral ganglion. Earlier studies had shown that R3-R14 have much higher glycine concentrations than other Aplysia neurons. Most of the glycine taken up by R3-R14 was biochemically untransformed for at least 1 h following its uptake. Glycine is actively transported into into R3-R14 and other Aplysia neurons by carrier-mediated processes. Glycine uptake by R3-R14 was markedly reduced in the absence of Na+ and in the presence of Hg2+, while these treatments had little effect on glycine uptake by other Aplysia neurons. There appears to be a special glycine uptake system present in R3-R14 and a general glycine uptake system common to all Aplysia neurons. The elevated glycine concentrations and special glycine uptake associated with R3-R14 may indicate that glycine is utilized as a neurotransmitter by those neurons.