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

Stimulation and release from neurons via a dual capillary collection device interfaced to mass spectrometry

Neuropeptides are cell to cell signaling molecules that modulate a wide range of physiological processes. Neuropeptide release has been studied in sample sizes ranging from single cells and neuronal clusters, to defined brain nuclei and large brain regions. We have developed and optimized cell stimulation and collection approaches for the efficient measurement of neuropeptide release from neuronal samples using a dual capillary system. The defining feature is a capillary that contains octadecyl-modified silica nanoparticles on its inner wall to capture and extract releasates. This collection capillary is inserted into another capillary used to deliver solutions that chemically stimulate the cells, with solution flowing up the inner capillary to facilitate peptide collection. The efficiency of peptide collection was evaluated using six peptide standards mixed in physiological saline. The extracted peptides eluted from these capillaries were characterized via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) with low femtomole detection limits. Using the capillary collection system in small custom-fabricated culturing chambers, individual cultured neurons and neuronal clusters from the model animal Aplysia californica were stimulated with distinct neuronal secretagogues and the releasates were collected and characterized using MALDI-TOF MS.

Distribution of the Aplysia cardioexcitatory peptide, NdWFamide, in the central and peripheral nervous systems of Aplysia

NdWFamide is an Aplysia cardioexcitatory tri-peptide containing D-tryptophan. To investigate the roles of this peptide, we examined the immunohistochemical distribution of NdWFamide-positive neurons in Aplysia tissues. All the ganglia of the central nervous system (CNS) contained NdWFamide-positive neurons. In particular, two left upper quadrant cells in the abdominal ganglion, and the anterior cells in the pleural ganglion showed extensive positive signals. NdWFamide-positive processes were observed in peripheral tissues, such as those of the cardio-vascular system, digestive tract, and sex-accessory organs, and in the connectives or neuropils in the CNS. NdWFamide-positive neurons were abundant in peripheral plexuses, such as the stomatogastric ring. To examine the NdWFamide contents of tissues, we fractionated peptidic extracts from the respective tissues by reversed-phase high-pressure liquid chromatography and then assayed the fractions by competitive enzyme-linked immunosorbent assay. A fraction corresponding to the retention time of synthetic NdWFamide contained the most immunoreactivity, indicating that the tissues contained NdWFamide. The prevalence of the NdWFamide content was roughly in the order: abdominal ganglion >heart >gill >blood vessels >digestive tract. In most of the tissues containing NdWFamide-positive nerves, NdWFamide modulated the motile activities of the tissues. Thus, NdWFamide seems to be a versatile neurotransmitter/modulator of Aplysia and probably regulates the physiological activities of this animal.

NdWFamide: a novel excitatory peptide involved in cardiovascular regulation of Aplysia

Although diverse peptides are known to affect invertebrate cardiac activity, the peptidergic regulation of the cardiovascular system of Aplysia is still poorly understood. Asn-D-Trp-Phe-NH(2) (NdWFamide) is a recently purified cardioactive peptide in Aplysia. Pharmacological experiments showed that NdWFamide was one of the most potent cardioexcitatory peptides among the known endogenous cardioactive peptides in Aplysia. NdWFamide-immunopositive neuronal processes were abundant in the cardiovascular region of Aplysia, and many of them originated from neurosecretory cells in the abdominal ganglion (R3-R13 cells). The data suggest that NdWFamide is a cardioexcitatory peptide utilized by R3-R13 cells of Aplysia.

Functional morphology of the neuropeptidergic light-yellow-cell system in pulmonate snails

The light yellow neuropeptidergic cell system of the basommatophoran snail Lymnaea stagnalis is homologous to the R3-R14 system of the opisthobranch Aplysia californica, and produces three different neuropeptides. Systems homologous to the light yellow cells of Lymnaea stagnalis have been investigated morphologically in two Basommatophora (Lymnaea ovata, Bulinus truncatus) and three Stylommatophora (Helix aspersa, Cepaea nemoralis, Deroceras reticulatum). To this end, an antibody to synthetic light-yellow-cell peptide-II and oligonucleotides to mRNAs encoding parts of peptide-I and peptide-III, were used. The in situ hybridization probes gave negative results. On the other hand, neuronal cell clusters were observed in the central nervous system of all species studied by immunocytochemistry. These clusters were located in the ganglia of the visceral complex. The neurons project axons into all nerves of these ganglia, especially into the pallial nerves, into the connective tissue of the central nervous system, and into the neuropile of various ganglia. The morphology of the systems is similar to that of the light-yellow-cell system of Lymnaea stagnalis. In all species, the wall of the aorta was innervated by immunoreactive axons. Peripheral innervation by the light-yellow-cell system was investigated in Helix aspersa and Deroceras reticulatum. Serial and alternate sections of whole snails were studied. Reconstructions were made of the heart-kidney-lung complex of these animals. In both species, the muscular vessels of the pulmonary system at the right side of the body were strongly innervated by immunoreactive axons.(ABSTRACT TRUNCATED AT 250 WORDS)

The isolation of a cDNA encoding a neuropeptide prohormone from the light yellow cells of Lymnaea stagnalis

1. The central nervous system (CNS) of the freshwater snail Lymnaea stagnalis contains several clusters of neuroendocrine cells, which synthesize neuropeptides that act as neurotransmitters, neurohormones, and/or neuromodulators, controlling a broad range of physiological processes. Using a protein chemical approach, we have previously characterized a peptide [named LYCP-A (Hoek et al., 1992], which is produced by the neuroendocrine light yellow cells (LYC), which are present as two clusters of endogenously bursting neurons in the visceral and right parietal ganglion, respectively. 2. A differential screening technique was used to isolate the cDNA that encodes the prohormone of LYCP-A. The prohormone appeared to contain three or four putative neuropeptides, one of which is LYCP-A. The organization of the identified prohormone resembles that of the histidine-rich basic peptide precursor previously identified in the R3-14 neurons of the marine snail Aplysia californica (Campanelli and Scheller, 1987). 3. In situ hybridization analysis indicates that the gene encoding the LYC prohormone is expressed in a subset of the LYC. The LYC release their peptides into the hemolymph from a neurohemal area, which is located around the CNS. In addition, the peptides are released from axonal branches in the aorta of the heart, suggesting a role in the regulation of cardiovascular functions.

Mechanisms of circulatory homeostasis and response in Aplysia

This review concerns the organization and function of arterial vasculature in Aplysia californica, especially the vasomotor reflexes that support circulatory homeostasis, and fixed patterns of response that may reroute blood flow during changes in behavioral state. The observations presented here raise three hypotheses for further study: 1) Arterial vasculature is functionally organized with precisely structured, independently regulated subdivisions; these are most evident for arterial systems serving digestive and reproductive processes; 2) arterial musculature is inherently responsive to local pressure changes, having both static and dynamic reflexes that promote efficient, evenly-distributed flow of blood; and 3) complex, long-lasting behaviors like egg laying have, as part of their makeup, equally prolonged and stereotypical changes in the pattern of circulation. Taken together, these observations support the view that maintenance and adjustment of blood flow in gastropod molluscs is an unexpectedly complex and highly integrated component of behavior.

Characterization of protein-linked glycoconjugates produced by identified neurons of Aplysia californica

The biosynthetic capabilities of individual neurons of the abdominal ganglion of the marine mollusc Aplysia californica have been analyzed after intrasomatic injection of 3H-monosaccharides. Glycopeptides prepared from the metabolically labeled cells were fractionated using serial lectin affinity and gel filtration chromatography. The fractionation procedure yielded eight populations of glycopeptides, and comparison of two different neurons (R2 and R14) showed that the quantity of the individual species produced is cell-dependent. Structural analysis indicated that the glycoconjugates produced by the Aplysia neuron constitute both O- and N-linked structures as well as an unusual class of oligosaccharide whose linkage to protein is unknown. The O-linked units are small and consist only of N-acetylglucosamine or N-acetylgalactosamine attached to protein. High-mannose-type asparagine-linked units are produced by the neurons, and some of these appear to be processed to biantennary complex-type units that bind to lentil lectin-agarose. Overall, although the Aplysia neurons produce oligosaccharides of a nature similar to that produced by higher eucaryotes, the N- and O-linked structures produced by the neurons do not achieve the complexity of the comparable structures produced by mammalian cells. The results provide a basis for further studies aimed at understanding the role of glycoconjugates in the development of the nervous system.

Identification of protein-bound oligosaccharides on the surface of growth cones that bind to muscle cells

In the accompanying paper (Gabel, Den, and Ambron, in press) it was shown that eight populations of glycopeptides are synthesized by single neurons of Aplysia californica. To see which glycopeptides might mediate interactions with target cells, we first identified glycopeptides that are transported selectively to synapses and growth cones. The giant neuron R2 was injected intrasomatically with 3H-glucosamine. Twenty-four hours later, 3H-glycopeptides in the axon and cell body were isolated and resolved by serial lectin affinity chromatography. Of the eight populations, the biantennary-type glycopeptides (GPbi) and those that bind to WGA (GPwga) were preferentially associated with rapidly transported glycoproteins. In contrast, the glycopeptide that consists of N-acetylglucosamine O-linked to ser/thr was mostly retained in the cell body. GPbi and GPwga were also preferentially transported to growth cones. Analyses of RUQ cells, exposed to 3H-glucosamine in vitro for 36 h showed an enrichment of GPbi and GPwga at the growth cone relative to the cell body. The disposition of the various glycopeptides in growing neurons was also examined using FITC lectins. FITC-coupled WGA, Vicia vellosa, and lentil lectin showed extensive staining of the cell body, but only WGA stained the growth cones. To investigate if GPwga interacts specifically with target cells, these glycopeptides were isolated from the neurons of 180 abdominal ganglia. GPwga, other Aplysia glycopeptides, and glycopeptides prepared from ovalbumin were coupled separately to fluorescent spheres. The spheres were then added to muscle cells isolated from the auricle of the heart, which is innervated by many neurons from the ganglion. While spheres coupled to GPwga bound to the muscle cell surface, the other glycopeptides did not. These results indicate that glycopeptides class GPwga, found among rapidly transported glycoproteins and on the growth cone surface, is able to bind to muscle cells and may therefore play some role in neuron-target interactions.

Aplysia brasiliana neurons R3-R14: primary structure of the myoactive histidine-rich basic peptide and its prohormone

Neurons R3-R14 of the marine mollusc Aplysia are model neuroendocrine cells thought to regulate cardiovascular activity in vivo. The cells express a gene encoding three peptides--peptides I, II and the histidine-rich basic peptide (HRBP)--each of which has been chemically characterized in Aplysia californica. In the studies presented here, HRBP and its prohormone (proHRBP) were purified from A. brasiliana abdominal ganglion extracts by reversed-phase high-performance liquid chromatography and characterized by amino acid compositional and sequence analyses. ProHRBP was an 85-residue peptide whose sequence was: NH2-Glu-Glu-Val-Phe-Asp-Asp-Thr-Asp-Val-Gly-Asp-Glu-Leu-Thr-Asn-Ala-Leu- Glu-Ser - Val-Leu-Thr-Asp-Leu-Lys-Asp-Lys-Arg-Asp-Ala-Glu-Glu-Pro-Ser-Ala-Phe-Met- Thr-Arg - Leu-Arg-Arg-Gln-Val-Ala-Gln-Met-His-Ile-Trp-Arg-Ala-Asn-His-Asp-Arg-His- His-Ser - Thr-Gly-Ser-Gly-Arg-His-Ser-Arg-Phe-Leu-Thr-Arg-Asn-Arg-Tyr-Gly-Gly-Gly- His-Leu - Ser-Asp-Ala-COOG. It differed from A. californica pro-HRBP at seven of the 85 positions. Compositional and sequence analyses demonstrated that A. brasiliana HRBP was a 43-residue peptide corresponding to residues 43 through 85 of proHRBP, and that a significant proportion of the isolated peptide possessed a blocked NH2 terminus. Although this sequence differed from that of A. californica HRBP at five of 43 residues, the two peptides were approximately equipotent in inducing contractions of A. californica crop muscle in vitro, suggesting that the substituted residues may not be critical for biological activity.

Aplysia californica neurons R3-R14: primary structure of the myoactive histidine-rich basic peptide and peptide I

The R3-R14 neurons of the marine mollusc Aplysia are neuroendocrine cells that express a gene encoding peptides I, II and histidine-rich basic peptide (HRBP), a myoactive peptide that excites Aplysia heart and enhances gut motility in vitro. Peptide II has been chemically characterized (35), but the complete primary structures of peptide I and HRBP have not been established by amino acid sequence analysis. HRBP, peptide I, and the prohormone (proHRBP) were therefore purified from acid extracts of Aplysia californica neural tissue using sequential gel filtration and reverse-phase high-performance liquid chromatography and chemically characterized. Amino acid sequence analysis demonstrated that HRBP was a 43-residue peptide whose sequence was: less than Glu-Val-Ala-Gln-Met-His-Val-Trp-Arg-Ala-Val-Asn-His-Asp-Arg-Asn-His-Gly- Thr-Gly - Ser-Gly-Arg-His-Gly-Arg-Phe-Leu-Ile-Arg-Asn-Arg-Tyr-Arg-Tyr-Gly-Gly-Gly- His-Leu - Ser-Asp-Ala-COOH. Compositional and sequence analyses of peptide I and proHRBP demonstrated that peptide I was a 26-residue peptide with the following sequence: NH2-Glu-Glu-Val-Phe-Asp-Asp-Thr-Asp-Val-Gly-Asp-Glu-Leu-Thr-Asn-Ala- Leu-Glu-Ser-Val-Leu-Thr-Asp-Phe-Lys-Asp-COOH. These results demonstrated that the pro-HRBP sequence predicted by nucleotide sequence analysis of a cDNA clone (24) was in fact synthesized in R3-R14 neurons. Hydrophilicity and hydrophobicity profiles of preproHRBP, combined with charge distribution profiles and predictive secondary structural analysis, showed that cleavage at dibasic sequences was strongly associated with peaks of hydrophilicity in alpha-helical regions of the preprohormone.

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.

Ganglionic circulation and its effects on neurons controlling cardiovascular functions in Aplysia californica

The abdominal ganglion of the mollusk Aplysia californica receives most of its blood supply through a small caudal artery that branches off the anterior aorta near its junction with the heart. Injection of an ink/gelatin mixture into the caudal artery revealed a consistent pattern of arterial branching within the ganglion and a general proximity of larger vessels to identified neurons controlling circulation in this animal. This morphological arrangement was particularly evident for the heart excitor interneuron, cell L10, which lies next to the caudal artery near its entry into the ganglion. In electrophysiological experiments, L10 was excited when blood flow or oxygen tension within the ganglion was reduced. This effect was expressed as a gradual increase in impulse frequency of L10 and conversion from tonic to bursting mode of spike discharge. L10 follower cells in the RB and LD neuron clusters were affected synaptically by the changes in L10 activity, while other follower cells (L3 and RD neurons) responded independently of L10's synaptic influence. The neurosecretory white cells (R3 to R14) that innervate the major arteries and pericardial tissues were also excited when ganglionic circulation was interrupted. In innervated preparations of the heart and respiratory organs, decreased circulation through the abdominal ganglion stimulated a transient increase in the rate and amplitude of respiratory (gill) pumping and pericardial contractions and caused a sustained increase in activity of the heart. Both responses increase cardiac output and both appear to involve a direct influence of ganglionic circulation on interneurons controlling the gill and heart. These results indicate that the cell-specific patterns of excitation and inhibition caused by fluctuations in ganglionic circulation may be important factors for maintaining circulatory homeostasis in this animal.

Growth cones isolated from identified Aplysia neurons in vitro: biochemical and morphological characterization

The right upper quadrant (RUQ) cells (R3-R13) of Aplysia regenerating in dissociated cell culture form unusually large growth cones. The movement of these growth cones was observed by time-lapse phase microscopy and their ultrastructure was examined by transmission electron microscopy. Their behavior and ultrastructure have features that are typical of growth cones in vitro. Additionally, they contain neurosecretory granules similar to those found in these cells in vivo. Because RUQ growth cones are large, they can be isolated by manual dissection. RUQ cells were grown in the presence of [35S]methionine and the labeled proteins transported to the growth cones were analyzed by SDS-PAGE. These proteins were compared to those in RUQ cell bodies, RUQ neurites, and to those in the neurites and cell bodies of other identified neurons grown in vitro. Most proteins synthesized by RUQ cells in vitro are transported to their growth cones, including several glycoproteins and the precursor to the R3-R14 neuropeptide. Neuropeptides are also synthesized by a number of other Aplysia neurons growing in vitro. We examined R2, LPL1, R15, and left upper quadrant neurons and found that their precursor peptides, like those of R3-R14, are readily recognized as major cell-specific radiolabeled bands on SDS gels. The presence in regenerating growth cones of neuropeptides, neurosecretory granules, and glycoproteins known to be rapidly transported toward synapses in vivo supports the emerging view that the growth cone in vitro contains not only a motility apparatus but also a macromolecular assembly capable of forming an active synapse immediately upon or shortly after contacting targets.

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.

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.

Localization of Aplysia neurosecretory peptides to multiple populations of dense core vesicles

Many neurons in the mollusc Aplysia are identifiable and provide a useful model system for investigating the cellular mechanisms used by the neuroendocrine system to mediate simple behaviors. In this study we determined the subcellular localization of eight Aplysia neuropeptides using immunogold labeling techniques, and analyzed the size distribution of dense core and granular vesicles in peptidergic neurons. Recent observations demonstrate that many neurons use multiple chemical messengers. Thus, an understanding of the functional significance of cotransmitters requires an analysis of their relative subcellular distributions. The peptides are expressed in a subset of neurons, or the exocrine atrial gland, and are primarily localized to dense core vesicles. Multiple regions of precursors which are cleaved into several components are co-localized. Each neuron has a distinct size distribution of peptide-containing dense core vesicles ranging in size from 65 to 600 nm. The atrial gland contains very large (up to 2 micron) peptide-containing granules. Single neurons have multiple populations of granules whose quantal sizes agree with predictions based on physical constraints. Some cells contain very large peptide-containing granules which are found in the cell soma and not in processes. Thus, the genetic determination of neuronal cell type includes not only transmitter choices but also multiple modes of packaging the intercellular messengers.

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.

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.

Morphology of two pairs of identified peptidergic neurons in the buccal ganglia of the mollusc Tritonia diomedea

The morphology of two pairs of identified peptidergic neurons (B11 and B12) located in the buccal ganglia of Tritonia diomedea was described. Both pairs of neurons contained a large quantity of a small cardioactive peptide (SCP) in their somata. One of the pairs (B11), the large dorsal white cells, contained ACh in their somata along with SCP. Both pairs of cells appeared white in live preparations under epi-illumination. Each B11 and B12 was a unipolar neuron and sent its major axonal branch through the ipsilateral gastro-esophageal nerve to the gut. In addition, B12 sent a small branch to the contralateral buccal ganglion. A characteristic feature of both neuron pairs was their vesicular content. Three types of vesicles were observed in B11. Vesicles with electron-lucent core (LCV) and electron-dense core (DCV) were found in the somata. The axon hillock and the beginning of axon contained vesicles with variable electron dense core (VDCV) in addition to LCV and DCV. The ratio of DCV: LCV: VDCV changed from 5:95:0 for the perinuclear cytoplasm to 8:55:37 for the beginning of axon. The average maximum diameters were 97 +/- 23 nm for DCV, 103 +/- 32 nm for LCV and 106 +/- 29 nm for VDCV. B12 somata contained DCV (average maximum diameter 100 +/- 26 nm), LCV (109 +/- 23 nm) and elliptical vesicles with eccentric electron-opaque core (115 +/- 20 nm).

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.

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

The fine structure of the cellular layers and innervation of smooth muscle in the anterior aorta of Aplysia were examined. The inner layer of circular muscle is not innervated but its fibers may be electrically coupled. In contrast, longitudinal fibers in the outer layer are well separated and richly innervated by highly specialized neuromuscular junctions (NMJ). Three distinct types of NMJ are present on this smooth muscle, each identifiable by a set of quantitatively described morphological features including size, degree of contact with sarcolemma, density of active zones, number of mitochondria and vesicular content. The three types of NMJ are likely to arise from the identified serotonergic (RDAAE), cholinergic (RDAAI), and glycinergic (R14) neurons that provide the major known excitatory, inhibitory, and modulatory inputs to this vessel. Each longitudinal muscle fiber is separately innervated by one or more NMJ of each type. Since there are no intercellular junctions between longitudinal fibers, coordination of contractility is clearly a function of the pattern of neural activity. This report further characterizes the rapid and fine control of the vasculature in Aplysia and demonstrates the utility of this preparation for cellular-level studies on the neural control of smooth muscle and neurochemical messengers mediating its activity.

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)

Gene isolation with cDNA probes from identified Aplysia neurons: neuropeptide modulators of cardiovascular physiology

The Aplysia abdominal ganglion neurons, R3-R14, modulate cardiovascular activity. In vitro translations of poly(A)+ RNA from these cells suggest that they contain a prevalent mRNA encoding a 14 kd protein. Utilizing differential screening techniques with 32P-labeled cDNA synthesized from the poly(A)+ RNA of identified neurons, we isolated the corresponding gene. The Aplysia haploid genome contains a single copy of this sequence, which is interrupted by two large introns and spans approximately 7 kb of genomic DNA. The R3-R14 neurons specifically express this gene, resulting in the synthesis of a 1.25 kb mRNA not found in other abdominal ganglion cells or in the head ganglia. The gene was shown to encode a 13.5 kd precursor, which is proteolytically cleaved into at least three peptides with molecular weights of 5.0, 3.3, and 1.3 kd. These peptides and glycine are thought to act as chemical messengers in the central nervous system and peripherally.

A light and electron microscopic investigation of the neurosecretory bag cells of Aplysia

The two bilateral clusters of neurosecretory bag cells of Aplysia were studied with both light and electron microscopy. Autoradiography revealed that the bag cells rapidly accumulate 3H-labelled amino acids and that after 1-2 h, heavy concentrations of silver grains appear over Golgi complexes and in the proximal axons. Intrasomatic injections of CoCl2 or lucifer yellow showed clear branch points and numerous varicosities along individual axons. Many of the bag cells are multipolar. Electron-microscopic observations confirmed that individual fibres branch and showed that the varicosities are packed with dense-cored vesicles similar in size (180 nm diameter) and electron density to those found in the somata. The axons of several cells are usually associated into bundles that travel (within the connective tissue sheath) either rostrally up the pleurovisceral connective or toward the contralateral bag cell cluster. Bundled in groups of tens to hundreds, a total of many thousands of axons fill the sheath around each cell cluster and around the proximal 2-5 mm of the pleurovisceral connective; the number of axon bundles in the sheath decreases rapidly with distance from the cluster. Individual axons reaching the outer edges of bundles from neurosecretory endings near blood sinuses in the sheath, creating an extensive neurohemal release area. Dense-cored vesicles are packed into the endings, often in very close apposition to the plasma membrane. Possible release profiles (omega-shaped) and smaller clear vesicles (85 nm diameter) were observed in the axon endings. A number of axons also enter and travel among the conventional (non-neurosecretory) axons in the core of the pleurovisceral connective nerve. These 'core' bag cell axons project for several millimetres beyond the terminations of the bundled axons of the sheath. The findings support the hypothesis proposed in physiological studies that the distribution and branching of the axonal tree are the basis for the extracellularly recorded wave forms and of the potentiation of electrical signals during bag-cell activity. Additional evidence indicates that exocytosis is the means by which bag-cell hormone is released during afterdischarges.

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.

Cytoarchitecture of primitive brains: Golgi studies in flatworms

Notoplana acticola, a free-living polyclad flatworm, has a small but well-defined brain that controls behavior of peripherally based motoneurones. This is the most primitive brain currently being studied electrophysiologicaly, but little is known of its cytoarchitecture. Using a modified rapid Golgi method, we have investigated neurone configurations within this brain. Superficially the brain resembles those of other invertebrates, but its cells also possess many vertebrate features. There is a surprising diversity of cell types with complicated branching patterns. Multipolar neurones appear to be the most common type. A few typical invertebrate monopolar cells were also stained. Bipolar cells occur in the rind. Processes resembling dendritic spines were observed. Measurements of these indicate that they fall within the range found in other vertebrates and invertebrates. Small multipolar cells that could either be glial or interneurones were found scattered through the brain.