Ultrastructural dynamics of exocytosis in the ovulation-neurohormone producing caudo-dorsal cells of the freshwater snail Lymnaea stagnalis (L.)

Exocytosis by synaptic terminals innervating the adrenal gland of the goldfish reveals multiple domains within the plasmalemma

The adrenal chromaffin gland of the goldfish has typical synaptic terminals embedded in its surface which are homologues of the cholinergic fibres innervating the mammalian adrenal medulla. The terminals contain both lucent synaptic vesicles and larger secretory granules with dense cores, known to be storage sites for transmitters and peptides, respectively. Three domains are present within the terminal plasmalemma. Exocytosis of vesicles is thought to be associated with a 'synaptic domain' marked by synaptic thickenings around which the vesicles cluster. Exocytosis of granules, stimulated by high K+ and visualized with the aid of tannic acid, is almost exclusively associated with areas of the membrane adjacent to chromaffin cells, and in particular with unspecialized regions which constitute the 'parasynaptic domain', creating a pattern of targeted secretory discharge. Sites of release within the 'non-synaptic domain', which is sheathed in glial cell lamellae, are extremely rare, despite the expansive character of this domain and the close association of granules with the plasmalemma within it. The pattern of secretory release described may be correlated with the position of the terminals at the surface of the innervated organ.

Voltage gated calcium channels in molluscs: classification, Ca2+ dependent inactivation, modulation and functional roles

Molluscan neurons and muscle cells express transient (T-type like) and sustained LVA calcium channels, as well as transient and sustained HVA channels. In addition weakly voltage sensitive calcium channels are observed. In a number of cases toxin or dihydropyridine sensitivity justifies classification of the HVA currents in L, N or P-type categories. In many cases, however, pharmacological characterization is still preliminary. Characterization of novel toxins from molluscivorous Conus snails may facilitate classification of molluscan calcium channels. Molluscan preparations have been very useful to study calcium dependent inactivation of calcium channels. Proposed mechanisms explain calcium dependent inactivation through direct interaction of Ca2+ with the channel, through dephosphorylation by calcium dependent phosphatases or through calcium dependent disruption of connections with the cytoskeleton. Transmitter modulation operating through various second messenger mediated pathways is well documented. In general, phosphorylation through PKA, cGMP dependent PK or PKC facilitates the calcium channels, while putative direct G-protein action inhibits the channels. Ca2+ and cGMP may inhibit the channels through activation of phosphodiesterases or phosphatases. Detailed evidence has been provided on the role of sustained LVA channels in pacemaking and the generation of firing patterns, and on the role of HVA channels in the dynamic changes in action potentials during spiking, the regulation of the release of transmitters and hormones, and the regulation of growth cone behavior and neurite outgrowth. The accessibility of molluscan preparations (e.g. the squid giant synapse for excitation release studies, Helisoma B5 neuron for neurite and synapse formation) and the large body of knowledge on electrophysiological properties and functional connections of identified molluscan neurons (e.g. sensory neurons, R15, egg laying hormone producing cells, etc.) creates valuable opportunities to increase the insight into the functional roles of calcium channels.

Multiple second messenger routes enhance two high-voltage-activated calcium currents in molluscan neuroendocrine cells

Two types of high-voltage-activated calcium currents were identified in whole-cell voltage-clamp recordings of the neuroendocrine caudodorsal cells, which control egg-laying in the freshwater snail Lymnaea stagnalis. The currents were: (i) a rapidly inactivating high-voltage-activated current, with an activation threshold of -40 mV and maximal amplitude at +10 mV; and (ii) a slowly inactivating high-voltage-activated current, with a threshold of -10 mV and a peak at +30 mV. Both currents were reduced by nifedipine and verapamil, but not by omega-conotoxin GVIA, suggesting that they belong to the L-type family of calcium currents. The voltage-dependence of inactivation of the rapidly inactivating high-voltage-activated current was bell-shaped. Time-constants of inactivation ranged from 10 to 25 ms. Steady-state inactivation was characterized by a potential of half maximal inactivation of -21.7 +/- 3.4 mV and a slope factor of 8.1 +/- 1.7 mV. The voltage-dependence of inactivation of the slowly inactivating high-voltage-activated current was S-shaped. Time-constants of inactivation increased with depolarization up to a maximum of 300 ms. The steady-state inactivation parameters were a potential of half maximal inactivation of +6.8 +/- 2.2 mV and a slope factor of 6.0 +/- 1.1 mV. The membrane-permeable analog of cAMP, 8-chlorophenylthio-cyclic AMP, predominantly increased the slowly inactivating high-voltage-activated current, and shifted its voltage-dependence of activation and inactivation 10 mV to the left. The rapidly inactivating high-voltage-activated current was slightly increased by 8-chlorophenylthio-cyclic AMP. 8-Bromo-cyclic GMP and the phorbol ester, 12-O-tetradecanoyl-13-phorbol acetate, had qualitatively similar effects. Both agents enhanced the rapidly inactivating current and, to a lesser degree, the slowly inactivating current, without affecting their voltage-dependence. The cyclic AMP-dependent protein kinase inhibitor, Walsh inhibitor peptide, antagonized the stimulating effect of 8-chlorophenylthio-cyclic AMP. The broad-spectrum protein kinase inhibitor 1-(5-isoquino-linylsulfonyl)-2-methyl-piperazine (H-7) strongly attenuated the effects of 8-chlorophenylthio-cyclic AMP, 8-bromo-cyclic GMP and 12-O-tetradecanoyl-13-phorbol acetate, suggesting that all treatments increase both types of high-voltage-activated calcium currents through phosphorylation of the channel-complex.

Discharge induction in molluscan peptidergic cells requires a specific set of autoexcitatory neuropeptides

The peptidergic caudodorsal cells of the pond snail Lymnaea stagnalis generate long lasting discharges of synchronous spiking activity to release their products. During caudodorsal cell discharges a peptide factor is released which induces similar discharges in silent caudodorsal cells [Ter Maat A. et al. (1988) Brain Res. 438, 77-82]. To identify this factor, the electrophysiological effects of putative caudodorsal cell gene products, calfluxin, caudodorsal cell hormone, four alpha caudodorsal cell peptides and three beta caudodorsal cell peptides, were tested individually and in various combinations. Calfluxin, alpha caudodorsal cell peptide and beta 1 caudodorsal cell peptide each had no effect on membrane potential or excitability of the caudodorsal cells. All other caudodorsal cell peptides caused excitatory responses, but did not induce discharges. Instead, only a specific combination of four caudodorsal cell peptides, caudodorsal cell hormone and alpha caudodorsal cell peptide (1-11, 3-11 and 3-10), evoked caudodorsal cell discharges with similar characteristics to electrically evoked discharges. Incomplete versions of this combination failed to cause a discharge. In addition, antibodies to caudodorsal cell hormone or alpha caudodorsal cell peptide reduced caudodorsal cell excitability and prevented the generation of discharges by electrical stimulation. These results suggest that excitatory autotransmission caused by four caudodorsal cell peptides provides a means to amplify excitatory inputs, thus leading to the generation of the all-or-nothing caudodorsal cell discharge.

Structural aspects, potassium stimulation and calcium dependence of nonsynaptic neuropeptide release by the egg laying controlling caudodorsal cells of Lymnaea stagnalis

The cerebral peptidergic caudodorsal cells of the freshwater snail Lymnaea stagnalis control egg laying and egg-laying behaviour by releasing peptides into (1) the haemolymph, from neurohaemal axon terminals in the periphery of the cerebral commissure and (2) the intercellular space of the central nervous system, from collaterals in the inner compartment of this commissure. Recently, it was shown that collateral release occurs from nonsynaptic release sites, which lack the morphological specializations that are characteristic of classical synapses. Probably, these sites enable the caudodorsal cells to communicate with central neurons in a nonsynaptic ("paracrine", "diffuse", "hormone-like") fashion. The structural and ionic bases of nonsynaptic release were studied using the tannic acid-Ringer incubation-method for the detection of exocytotic release of secretory granule contents in vitro. Elevation of the extracellular potassium concentration strongly stimulates exocytotic activity in the collaterals. No stimulation was found in the absence of extracellular calcium ions. Similar results have been obtained for the neurohaemal axon terminals. Electron-dense material occurs apposed at the cytoplasmic side of the axolemma of collaterals (ethanolic phosphotungstic acid method). This material appears homologous with the presynaptic dense projections forming the "vesicular grid" in classical synapses. Such projections are also present in the neurohaemal axon terminals. It is concluded that secretion from nonsynaptic release sites in caudodorsal cell collaterals shares fundamental characteristics with secretion from conventional neuronal release sites (neurohaemal axon terminals and classical synapses); release occurs by exocytosis of secretory granules, is associated with a vesicular grid, is stimulated by membrane depolarization, and depends on the presence of extracellular calcium ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Tannic acid enhances staining of microtubule associated proteins, but impairs neuronal physiology

Exposure of the buccal ganglion of the mollusc, Helisoma trivolvis, to tannic acid ringer (TARI), pH 6.8, permits the ultrastructural detection of exocytotically released material within the neuropil. At pH 4.0, tannic acid penetrated cell membranes and resulted in a marked enhancement in the electron density of regularly speed microtubule associated proteins. Electrophysiological analysis of neurons 19, 4 and 5 exposed to TARI, pH 6.8 and 4.0, demonstrated that the TARI procedure altered the neuronal firing patterns in an irreversible manner. We conclude that the neurotoxic effects of TARI preclude its usefulness in quantitative ultrastructural analysis of exocytotic events, but that at pH 4.0 it is useful for ultrastructural investigations of microtubules and their associated proteins.

Morphology of neurosecretory cells in basommatophoran snails homologous with egg-laying and growth hormone-producing cells of Lymnaea stagnalis

In a light and electron microscope study, neurosecretory cells morphologically homologous with the egg-laying hormone-producing caudodorsal cells (CDC) and growth hormone-producing dorsal cells (DC, light green cells) of the freshwater basommatophoran snail Lymnaea stagnalis have been found in five genera (seven species) of Basommatophora, viz. in Lymnaea palustris and Lymnaea ovata, in Planorbis planorbis and Planorbis vortex, in Planorbarius corneus, in Bulinus truncatus, and in Biomphalaria glabrata. It is concluded that the functions of these cells are homologous as well. The homologies of the respective neuron types regard their locations in the cerebral ganglia, their clustering in groups, location of their neurohemal area (CDC: cerebral commissure; DC: median lip nerves), and ultrastructural characteristics (e.g., abundance of rough endoplasmic reticulum, well-developed Golgi apparatus, presence of two types of neuron-specific secretory granules, and release of granule contents by exocytosis into the hemolymph). In addition, CDC show large electron-dense granules and DC reveal infoldings of the plasma membrane at the abaxonal side of the soma as well as synaptic input. On the other hand, each neuron type shows species-specific characteristics, particularly with regard to the number of cells and the structure of the neurohemal area. Furthermore, the CDC show marked differences between genera in the morphology (especially the mean diameter) of type 2 and, particularly, type 1 secretory granules. The morphology of the two types of secretory granules in the DC differs strongly between species. The possible relation between the morphology and the chemical contents of secretory granules has been discussed.

Role of cAMP in electrical and secretory activity of the neuroendocrine caudo-dorsal cells of Lymnaea stagnalis

The peptidergic neuroendocrine caudo-dorsal cells (CDC) in the cerebral ganglia of the freshwater snail Lymnaea stagnalis L. produce an ovulation-stimulating neurohormone (CDCH). Release occurs by exocytosis in a calcium-dependent way from axon terminals in the periphery of the intercerebral commissure, particularly during a period of electrical activity (the 'discharge'). An important factor in electrical and, hence, secretory activity of the CDC appears to be cyclic adenosine 3', 5'-monophosphate (cAMP). Incubation of cerebral ganglia in snail Ringer with the cAMP-analogue 8-(4-chlorophenylthio)-cAMP (cpt-cAMP) or with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) leads to activation of the CDC: electrophysiological, quantitative electron microscopic and bioassay studies show that incubation results in the onset of intense electrical activity, a marked reduction in the number of secretory granules in the axon terminals, an enormous increase in the number of exocytosis phenomena and a strong stimulation of CDCH-release. It is assumed that treatment with IBMX or with cpt-cAMP mimics a rise in the cytoplasmic level of cAMP when the CDC become activated by a physiological stimulus. This rise most likely effectuates a permeability change of the axolemma for ions involved in the discharge. As a consequence of the depolarization of the axolemma during the discharge, calcium ions would enter the axon terminal and induce exocytotic release of CDCH.

Heterogeneity of peptides released by electrically active neuroendocrine caudodorsal cells of Lymnaea stagnalis

The peptidergic caudodorsal cells (CDC) are located in the cerebral ganglia (CG) of the snail Lymnaea stagnalis. They have the periphery of the intercerebral commissure (COM) as the neurohaemal area. Release of the ovulation hormone (CDCH) and other presumed peptides by electrically active CDC was studied in vitro using CG/COM preparations which were preincubated with radioactive amino acids. Electrical activity of the CDC was induced by electrical stimulation of these cells, and by adenosine 3',5'-cyclic phosphate (cAMP) or potassium depolarization. Four classes of radioactively-labelled peptides are released by active CDC. They have molecular weights of greater than or equal to 6000, approximately 5500, approximately 4500 and approximately 2500 daltons, respectively. Released CDCH bioactivity co-eluted with the approximately 4500 dalton radioactive peptides. Selective removal of CDC demonstrated that the CDC release these peptides. cAMP and potassium depolarization also induce release of these peptides. The release after potassium depolarization is dependent upon the presence of calcium ions. CG/COMs from juvenile snails are not capable of releasing the peptides. With Bio-Gel P-6 chromatography, sodium dodecyl sulphate urea polyacrylamide electrophoresis, isoelectric focusing and proteolytic enzymes it was found that released CDCH is a peptide with the same molecular weight and pI as CDCH stored in the COM (molecular weight approximately 4700; pI approximately 9.3).

Direct effects of the hyperglycemic factor of the freshwater snail Lymnaea stagnalis on isolated glycogen cells

In the freshwater pulmonate snail Lymnaea stagnalis special cells, the glycogen cells (GC) are present for the storage of glycogen reserves. These cells occur in large numbers in the anterior mantle region. In a previous paper in vitro experiments with intact anterior mantle tissue indicated that a hyperglycemic factor is released from the central nervous system (CNS) which stimulates glycogen mobilization in mantle tissue (M. A. Hemminga, J. J. Maaskant, W. Koomen, and J. Joosse (1985). Neuroendocrine control of glycogen mobilization in the freshwater snail Lymnaea stagnalis. Gen. Comp. Endocrinol. 57, 117-123). In the present study the question of whether this factor affects glycogen metabolism of GC isolated from mantle tissue was investigated. It is reported that in high-K+ Ringers the CNS is stimulated to release a factor which, in a dose-dependent way, inhibits glycogen synthesis in isolated GC (measured as a decreased incorporation of [14C]glucose into glycogen). Simultaneous with this glycogen synthesis-inhibiting effect, stimulation of glycogen degradation is found (measured as a decreased retention of prelabeled glycogen). It is concluded that (1) the factor released by the CNS having these effects on isolated GC is the same as the hyperglycemic factor which was reported to stimulate glycogen mobilization in intact mantle tissue, and (2) GC after isolation from mantle tissue have retained their ability to respond to this factor.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuroendocrine control of glycogen mobilization in the freshwater snail Lymnaea stagnalis

In the freshwater snail Lymnaea stagnalis the anterior mantle region, which mainly consists of large numbers of special glycogen-storing cells, is an important depot for the energy reserves of this snail. In organ culture experiments the central nervous system (CNS), in contrast to other tissues, both inhibits glycogen synthesis (measured as incorporation of [14C]glucose into glycogen) and stimulates glycogen breakdown in anterior mantle tissue (measured as a decreased retention of prelabeled glycogen). These effects are dose dependent, with saturation at the highest doses tested. High-potassium Ringer solution stimulates the secretion of a CNS factor which induces glucose release by anterior mantle tissue. This glucose-release-stimulating effect of the CNS is also dose dependent, but saturation of the response was not achieved. It is concluded that inhibition of glycogen synthesis and stimulation of glycogen breakdown and glucose release are probably effects of a single neurohormone which controls glycogen mobilization from the storage cells in the mantle. Like similar factors in other animal phyla, this putative neurohormone is referred to as a hyperglycemic factor.

The effect of temperature on spontaneous, and ovulation hormone-induced female reproduction in Lymnaea stagnalis

Spontaneous oviposition of the freshwater pulmonate snail Lymnaea stagnalis stopped after transfer from 20 degrees to low temperatures (8 and 4 degrees). This was due to a reduction in the activities of the neurosecretory caudodorsal cells (CDC), which produce the ovulation hormone (CDCH). Oviposition latencies (time interval between CDCH-injection and start of oviposition) increased with decreasing temperature. The ovotestis and the female accessory sex organs of snails placed at 8 degrees became less sensitive, and those at 4 degrees became completely insensitive to injected CDCH. This was probably caused by a reduction in activities of the endocrine dorsal bodies (DB), which control vitellogenesis and the activities of the female accessory sex organs. After a change from 4 to 20 degrees, CDCH injections rapidly became effective in inducing egg mass production, and spontaneous oviposition also restarted quickly, suggesting a rapid increase in DB and CDC activities.

Ovulation hormone, nutritive state, and female reproductive activity in Lymnaea stagnalis

A study of the relation between nutritive state and female reproductive activity as affected by the ovulation hormone (CDCH) has been made in the freshwater pulmonate snail Lymnaea stagnalis. CDCH is produced by the neuroendocrine caudodorsal cells (CDC) in the cerebral ganglia. Spontaneous oviposition ceased within 6 days of the beginning of a starvation period. This is most probably partially due to a reduction in the CDC activities because (1) quantitative electron microscopy showed a nearly 80% decrease in the number of release phenomena in the CDC-axon terminals in the neurohemal area in the intercerebral commissure, and (2) a bioassay showed a considerable reduction in the amount of CDCH in this area. During starvation the ovotestis and the female accessory sex organs became progressively less sensitive and, after 25 days, were completely insensitive to injected CDCH. This was indicated by a decrease in the responses to CDCH injection and, correspondingly, by a gradual increase in the CDCH thresholds for ovulation and egg formation. It is argued that the insensitivity may be caused by a reduction in the activities of the endocrine dorsal bodies. During refeeding, CDCH injections again become effective in inducing egg mass production, followed by resumption of spontaneous oviposition. This suggests a rapid restoration of DB and CDC activities following refeeding.

Ultrastructural demonstration of exocytosis of neural, neuroendocrine and endocrine secretions with an in vitro tannic acid (TARI-) method

The release of neural, neuroendocrine, and endocrine secretory products by exocytosis was ultrastructurally studied by means of tissue incubation in Ringer containing tannic acid (Tannic Acid Ringer Incubation-method; TARI -method), followed by conventional fixation. Tannic acid strongly enhances the electron density of extracellular (secretory) substances. During TARI -treatment of tissues exocytosis proceeds, but the exteriorized contents of the secretory granules are immediately fixed by tannic acid and do not diffuse away into the extracellular space. In this way detection of exocytosis is markedly facilitated since the number of exocytosis phenomena visible at the ultrastructural level considerably increases with progressing incubation time. Studies of the central nervous system of the mollusc Lymnaea stagnalis show that the occurrence of exocytosis during TARI -treatment is calcium-dependent. With the TARI -method exocytosis has been clearly demonstrated in a variety of structures (endocrine cells, neurohaemal axon terminals, synapses) of L. stagnalis, the insect Locusta migratoria, and the rat, including cell types where exocytotic release had not been shown before.

Effects of two gonadotropic hormones on polysaccharide synthesis in the albumen gland of Lymnaea stagnalis, studied with the organ culture technique

A medium for culturing organs of the basommatophoran freshwater snail Lymnaea stagnalis is described. One of the accessory sex glands, the albumen gland, which produces proteins and galactogen as nutritive substances for the embryos, was cultured along with parts of the central nervous system (CNS), including the female gonadotropic hormone-producing dorsal bodies (DB). Culture time was 4 days. The cerebral ganglia and the DB appeared to have a strong stimulating influence on polysaccharide synthesis in the albumen gland. The remaining part of the CNS has a weak stimulating effect. The use of extracts (culture time 6-26 hr) of parts of the cerebral ganglia and of the DB showed that not only the dorsal body hormone (DBH), but also the ovulation hormone (CDCH)--a neurohormone produced by the caudodorsal cells located in the cerebral ganglia--stimulates the synthetic activity of the albumen gland. The action of these hormones is direct; i.e., it is not exerted via the gonad. Dose- and time-response relations for the DBH and the CDCH are very similar. No additional rise in stimulation occurred when the two hormones were administered together. Experiments with albumen glands of adult snails demonstrated that the presence of large quantities of secretory material in the gland inhibits the response to DBH and CDCH.

Ultrastructure of two opposite types of abdominal perisympathetic organs in the heteropteran, Roscius elongatus Stål (Pyrrhocoridae). Abundant release of neurosecretory product by intraganglionic organs

The ultrastructural study of perisympathetic organs (POs) in Roscius elongatus revealed the existence of two opposite neurohemal structures. Distal POs formed a network at the end of each of the first pro-, meso-, metathoracic, and abdominal nerves. They display a thin branched structure favoring contact between axon endings and hemolymph. The distal metathoracic POs contained two types of neurosecretory ending, and the distal abdominal POs three types. On the contrary, "subjacent" POs, strangely located laterally in the abdominal area of the ganglionic nerve mass, display a dense compact structure, containing only one type of neurosecretory ending. These characters demonstrate that the "subjacent" POs, earlier classified as transverse POs because they are symmetrically located, are in fact median POs. In transverse POs, signs of extrusion by exocytosis were observed only in one type of neurosecretory ending. In "subjacent" POs unusual pictures were found indicating abundant release by single or compound exocytosis. Such pictures prove that "subjacent" POs have a neurohemal function despite their dense structure and internal localization. Physiological implications of these opposite structures are discussed.

Adenylate cyclase activity in axon terminals of ovulation-hormone producing neuroendocrine cells of Lymnaea stagnalis (L.)

Adenylate cyclase activity was demonstrated ultracytochemically in the neurohaemal area of the ovulation-neurohormone producing Caudo-Dorsal Cells (CDC) of the freshwater snail L. stagnalis. During electrical activity the axon terminals of the CDC release the hormone by exocytosis and show high adenylate cyclase activity on their plasma membranes. Electrically inactive terminals hardly show exocytosis and exhibit only little reaction product of adenylate cyclase. This result indicates that cAMP plays a role in the control of release of the CDC hormone.