Connectivity changes in an isolated molluscan ganglion during in vivo culture

Serotonin regulates electrical coupling via modulation of extrajunctional conductance: H-current

Synaptic strength can be highly variable from animal to animal within a species or over time within an individual. The process of synaptic plasticity induced by neuromodulatory agents might be unpredictable when the underlying circuits subject to modulation are themselves inherently variable. Serotonin (5-hydroxytryptomine; 5HT) and serotonergic signaling pathways are important regulators of animal behavior and are pharmacological targets in a wide range of neurological disorders. We have examined the effect of 5HT on electrical synapses possessing variable coupling strengths. While 5HT decreased electrical coupling at synapses with weak electrical connectivity, synapses with strong electrical coupling were less affected by 5HT treatment, as follows from the equations used for calculating coupling coefficients. The fact that the modulatory effect of 5HT on electrical connections was negatively correlated with the strength of electrical coupling suggests that the degree of electrical coupling within a neural network impacts subsequent neuromodulation of those synapses. Biophysical studies indicated that these effects were primarily due to 5HT-induced modulation of membrane currents that indirectly affect junctional coupling at synaptic contacts. In support of these experimental analyses, we created a simple model of coupled neurons to demonstrate that modulation of electrical coupling could be due solely to 5HT effects on H-channel conductance. Therefore, variability in the strength of electrical coupling in neural circuits can determine the pharmacological effect of this neuromodulatory agent.

Electrical synapse formation disrupts calcium-dependent exocytosis, but not vesicle mobilization

Electrical coupling exists prior to the onset of chemical connectivity at many developing and regenerating synapses. At cholinergic synapses in vitro, trophic factors facilitated the formation of electrical synapses and interfered with functional neurotransmitter release in response to photolytic elevations of intracellular calcium. In contrast, neurons lacking trophic factor induction and electrical coupling possessed flash-evoked transmitter release. Changes in cytosolic calcium and postsynaptic responsiveness to acetylcholine were not affected by electrical coupling. These data indicate that transient electrical synapse formation delayed chemical synaptic transmission by imposing a functional block between the accumulation of presynaptic calcium and synchronized, vesicular release. Despite the inability to release neurotransmitter, neurons that had possessed strong electrical coupling recruited secretory vesicles to sites of synaptic contact. These results suggest that the mechanism by which neurotransmission is disrupted during electrical synapse formation is downstream of both calcium influx and synaptic vesicle mobilization. Therefore, electrical synaptogenesis may inhibit synaptic vesicles from acquiring a readily releasable state. We hypothesize that gap junctions might negatively interact with exocytotic processes, thereby diminishing chemical neurotransmission.

Axonal regeneration of an identified Helisoma neuron depends on the site of axotomy

Axotomy of molluscan neurons usually results in axonal regeneration. In this study, we tested the axotomy response of an identified neuron of the pond snail Helisoma trivolvis (buccal neuron 4, B4). This neuron has two primary axonal branches, the ipsilateral axon and the contralateral axon, each innervating one of the paired salivary glands. The ipsilateral axon projects via the ipsilateral esophageal nerve trunk whereas the contralateral axon crosses both buccal ganglia and projects via the contralateral esophageal nerve trunk. We tested various procedures of axotomy: injury to one axon or both axons, close to the ganglion or more distal. Surprisingly, we found that proximal axotomy of the ipsilateral axon was not usually followed by axonal regeneration. By contrast, all other procedures of axotomy (e.g., distal ipsilateral, or proximal bilateral) resulted in robust axonal regeneration and target reinnervation. Thus, in this preparation, axotomy may or may not result in axonal regeneration, depending on the site(s) of axotomy. To the best of our knowledge, such a differential result has not yet been found in any other preparation. We conclude that axotomy is not always a sufficient condition for axonal regeneration of molluscan neurons. We hypothesize that a damaged axonal stump may be a necessary condition for the initiation of regeneration. An alternative hypothesis is that neurite outgrowth is inhibited in normal mature neurons by a target-derived factor. This hypothetical factor would be transported retrogradely, but not anterogradely, along axons.

Neuron-specific modulation by serotonin of regenerative outgrowth and intracellular calcium within the CNS of Helisoma trivolvis

We have investigated the cell-specific effect of serotonin (5-HT) on regenerating neurons within the adult central nervous system of the pond snail, Helisoma trivolvis. In culture, 5-HT arrests outgrowth of buccal neurons B19 but not neurons B5 (Haydon, McCobb, and Kater, 1984). After axotomy, neurons within the Helisoma nervous system typically exhibit profuse regenerative outgrowth. This study, on neurons within the CNS, shows that 5-HT selectively inhibits the outgrowth of specific identified neurons, and also causes significant elevations in intracellular calcium concentrations as measured by the calcium indicator dye, Fura-2. The outgrowth of neurons B19 and C1 was selectively inhibited when ganglia were incubated in 5 X 10(-5) M 5-HT. The outgrowth of buccal neurons B5, however, was not affected. Moreover, 5-HT caused significant transient elevations of calcium concentrations in neurons B19 over 30 minutes, but neurons B5 did not show any increases in calcium concentrations with the addition of 5-HT. These results suggest that the effect of 5-HT upon outgrowth of regenerating neurons may be due to an increase in the intracellular calcium concentration.

Synaptic efficacy of inhibitory synapses and repetitive firing in the reinnervating trigeminal and hypoglossal motoneurons

The synaptic efficacy and repetitive firing in masseteric motoneurons after the self-union operation and in tongue protruder motoneurons after their cut axons were reunited to tongue retractor muscles, the styloglossus muscle, were studied in cats. To ensure the correct identification of reinnervating motoneurons, the muscle response produce by an induced spike in a motoneuron by intracellularly injected depolarizing current was recorded. In both masseteric and tongue protruder motoneurons there were no differences on the patterns of postsynaptic potentials produced in reinnervating and non-reinnervating motoneurons by peripheral nerve stimulation, suggesting that the recovery of the synaptic efficacy of inhibitory synapses is time-dependent rather than muscle reinnervation. However, the present study demonstrated that the recovery of processes that control rhythmical firing of motoneurons is probably dependent on muscle reinnervation.

Pitfalls in the use of brain slices

In vitro brain slices are the preparation of choice for the detailed examination of local circuit properties in mammalian brain. However it is the investigator's responsibility to verify that the circuits under investigation are indeed confined within the boundaries of the functional region of the slice used. The medium in which the slice is maintained is under the full control of the investigator. This places the burden on the investigator to ensure that: (1) the properties of the medium are fully under control; (2) the effects of the medium on the slice are known; (3) the conditions under which the slice is being maintained bear some reasonable relation to those it enjoys (or endures) in vivo. Generalizations to in vivo conditions must be made with caution. If at all possible, similar studies (perhaps less extensive, due to the greater technical difficulties) should be done in vivo to provide a basis for comparison. Investigators using drugs should be aware of, and respect, the basic pharmacological principles cited in the text. In particular, the substantial freedom the investigator has in defining the extracellular medium should not be abused.

Formation, maintenance, and functional uncoupling of connections between identified Helisoma neurons in situ

Previous work with identified Helisoma neurons has characterized an array of neuroplastic responses to axotomy that include the generation of new neuritic outgrowth, the reinnervation of target organs, and the formation of new electrical synapses. These responses are not random, but rather occur in a precise, predictable manner under a variety of culture conditions. The present investigation demonstrates that specific identified neurons display similar neuroplastic "behavior" within the living animal. In response to in situ nerve crushes, neurons B4 and B5 generate new neuritic outgrowth, neuron B4 functionally reinnervates the salivary glands, and new electrical synapses form between the left and right neurons B5. The in situ paradigm employed in the present experiments made it possible to examine responses to axotomy over longer periods than in earlier studies with organ cultures. New B5R-B5L connections, previously found to be stable over the short term in culture, gradually decreased in strength in situ, and the cells effectively uncoupled by 8 weeks after axotomy. This uncoupling did not depend upon target reinnervation and occurred in the continued presence of neurites in the buccal commissure. It is suggested that the stability of new connections is related to whether the connection previously existed in the unperturbed nervous system. The similarities between the ability of identified neurons to grow and to form synaptic connections in situ and in culture suggests that neurons are endowed with a specific program of regenerative responses that can be expressed reliably in a wide variety of environmental conditions.

On the status of the study of invertebrate neurons in tissue culture--phyla Mollusca and Annelida

The utilization of tissue culture in neurobiological studies is discussed for all phyla phylogenetically preceding Phylum Arthropoda. Only two phyla, Mollusca and Annelida, are represented in such studies. The members of Phylum Mollusca which have been so investigated are Aplysia, Helisoma and Lymnaea. The mollusc Aplysia has been used to investigate several processes, including neurosecretion, synaptic transmission and synaptogenesis. Helisoma was employed to study factors regulating neurite growth and the specificity of synapse formation; mechanisms of neurite growth were investigated in the snail Lymnaea. The only member of Phylum Annelida involved in appropriate studies has been the leech Hirudo. This organism was used to investigate axonal regeneration and synaptic mechanisms.

Modified cobalt staining and silver intensification techniques for use with whole-mount gastropod ganglion preparations

While silver intensification of cobalt-filled cells is a common procedure for use with many arthropod preparations, it has not been routinely applied to gastropods. Several modifications in cobalt-staining techniques currently used with gastropods along with adaptations of silver intensification techniques used in insects are described. Cobalt was introduced into cells through axonal filling of cut nerve trunks or by either pressure injection or iontophoresis from intracellular, micropipette electrodes that had previously had their tips etched in dilute hydrofluoric acid. Etching produced consistent, sharp tips with large lumina. Further procedural modifications allowed complete, even intensification of neurons in large gastropod ganglia. These techniques have proved to be reliable and apparently broadly applicable, having been successfully used on three diverse gastropod species.

Recovery of damage in a molluscan nervous system is impaired with age

In the mollusc Lymnaea stagnalis recovery of the tentacle withdrawal reflex after peripheral nerve damage is delayed with age. Indications were obtained that this delay is due to impairment of recovery of central synaptic connections.

Differential ability of two adult molluscan neurons to regenerate electrical synapses

The ability of two electrical synapses (neuron L4-R4 and neuron L19-R19) to regenerate in the adult Helisoma nervous system was examined. The L4-R4 electrical connection exhibited rapid restoration to 50% of normal strength, whereas L19-R19 reconnection was weak or absent. This disparity is attributable to the inability of neuron 19 to sprout effectively across a crush site in the buccal commissure, although peripheral sprouting was pronounced. The factors which underlie the inability of neuron 19 (but not neuron 4) to sprout into a central environment are unknown, but dictate differential synapse restoration in this adult nervous system.

Incidence of neuronal dye-coupling in neocortical slices depends on the plane of section

The fluorescent dye Lucifer Yellow CH was intracellularly injected into neurons in slices of guinea-pig visual neocortex which had been prepared by sectioning either in a plane normal to the pial surface (radial slices) or in a plane parallel to the pial surface (tangential slices). In radial slices 44.3% of the injections resulted in dye-coupling and the number of cells coupled to the impaled neuron per injection followed a Poisson distribution. In contrast dye-coupling was not observed in tangential slices. Incidence of dye-coupling in slices that had been sectioned in both the radial and tangential planes was the same as in intact radial slices, indicating that slicing in the radial plane induced the formation of pathways for dye movement between neurons. The results suggest that formation and/or strengthening of direct intercellular junctions between neocortical neurons may occur as a specific neuronal response to partial dendrotomy.

Behavioral recovery associated with central nervous system regeneration in the snail Melampus

The pulmonate snail Melampus bidentatus regenerates central nervous tracts following commissurotomy, connective transection, and cerebral ganglion ablation. Our goal was to determine whether or not neural regrowth within the central nervous system restored behaviors disrupted by lesions. One behavior that is disrupted by commissurotomy is retraction of facial structures that are contralateral to a stimulated facial region, a response that normally accompanies the ipsilateral retraction. Tentacle withdrawal on the side contralateral to stimulation reappeared on a timescale that was correlated with growth of a commissural link (8-19 days post-lesion). Electrophysiological recordings from a labial nerve pathway that has a contralateral component similar to the contralateral tentacle response showed that development or strengthening of an alternative pathway could also mediate contralateral responses. Thus, a major conclusion of this study was that both tract regeneration and changes in existing CNS pathways can underlie recovery. The percentage (approx. 75%) of snails that regenerate the cerebral commissure and show behavioral recovery is established early in the period following commissure transection. Behavioral recovery and anatomical evidence of regeneration were also correlated in the other two operations: single cerebral ganglion removal and unilateral cerebropleural and cerebropedal connective transection. We conclude that Melampus is able to regenerate neuronal connectivity that can restore normal behavior.

Nerve growth-promoting factor produced in culture media conditioned by specific CNS tissues of the snail Helisoma

Medium conditioned by tissue from the CNS of the snail, Helisoma, is capable of promoting neurite outgrowth in isolated neurons from adult central ganglia. The conditioning factor(s) (CF), contained in conditioned medium (CM), is produced only by central ganglionic rings and buccal ganglia and not by other tissues, including hemolymph. CF requires a minimum of 24 h to be produced or released into the medium. At 12 h growth-promoting activity was not detectable. CF binds tightly to the polylysine substratum and its activity is not mimicked by addition of various sera, NGF or fibronectin. CF activity is abolished by chymotrypsin, trypsin or heating to 100 degrees C, but is stable to DNase and RNase treatment. The percentage of cells exhibiting neurite outgrowth is approximately linear with the amount of neural tissue used to condition the medium up to 2 ganglionic rings/ml. Addition of more ganglia fails to stimulate a greater response. This apparent plateau of CM activity appears to be a function of production and/or release of CF, rather than a saturation effect on plated cells, since dose-response curves for dilutions of CM are approximately linear regardless of the number of ganglia used for conditioning. In addition, anisomycin inhibits 35% of CF appearance under conditions of over 90% protein synthesis inhibition in the ganglia used to produce the CM. Under these conditions anisomycin has no apparent effect on the maintenance of electrical excitability. The inhibitor data suggest that 65% of CF is derived from a pre-existing storage pool and that the remainder is synthesized during the 72 h conditioning period.

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)

Generation of new cerebral ganglion neurons in the snail Melampus: an ultrastructural study

Reports in the literature have established that reconnection of central neural tracts occurs following commissurotomy and cerebral ganglion excision in the primitive pulmonate snail Melampus bidentatus and have suggested the possibility that long-term regeneration might result in the appearance of new neurons in the ganglion bud. We have used electron microscopy to examine the ganglion buds that form by reconnection of cerebral nerves, commissure, and connectives following cerebral ganglion excision in adult Melampus. The buds were examined from 2.5 to 12 months postoperatively. By 2.5 months, ganglion buds consist of a mixture of axon tracts that travel through the bud region and some dendritic processes; a few synaptic contacts can be identified at this stage, scattered throughout the bud. By 5--6 months, some of the most advanced ganglia have undifferentiated cells that are distinct from glia. By 7 months, differentiated neurons with clear, small dense-core or neurosecretory vesicles are present, although these cells are not all concentrated in a rind on the ganglion surface. Another cell type, the pigment-sheath cell, is present by this stage. By 11--12 months, the most advanced regenerating ganglia have neurons which form a cell rind on the ganglion surface. The gross appearance of a regenerated ganglion at this stage is similar to that of the intact contralateral cerebral ganglion, although the regenerated ganglion is smaller. One 12-month ganglion was found to possess fairly normal intraganglionic morphology, with lobes and cell types that were recognizable. Hence, nerve cell regeneration can occur in the absence of body part regeneration in adult members of one species of pulmonate snail.

Formation of novel central and peripheral connections between molluscan central neurons in organ cultured ganglia

An in vitro organ culture system for buccal ganglia of the adult snail, Helisoma, is described. The system supports: (1) maintenance of characteristic electrophysiological parameters of identified neurons over seven days of culture; (2) choline metabolism including uptake and synthesis over the same duration; (3) sprouting and growth of neurons in response to axotomy; (4) the formation of novel central electrotonic connections between identified neurons as a result of sprouting and growth. These observations on neuronal growth and the formation of connections are similar to those made with in vivo culture. The use of in vitro culture allows precise manipulations not previously possible. When buccal ganglia are cultured in vitro with the cut distal ends of peripheral nerve trunks held closely apposed, axons of neurons 5R and 5L in the nerve trunks are capable of forming electrotonic connections similar to central connections. The capability of these neurons to form electrotonic connections via their peripheral axons implies that special structures (i.e., central neurites) are not required for the formation of connections; and neither are special environments (i.e., the central neuropile) required for these connections.

Dye-coupling between pyramidal cells of rat hippocampus in vivo

Lucifer Yellow was intracellularly injected into pyramidal cells of rat hippocampus in vivo. In 10 successful staining experiments there were 5 cases of dye-coupling, in which 2-4 cells were stained though only one was injected. These results indicate that dye-coupling and, therefore, presumably electrotonic coupling, occurs between neurons of the hippocampus in the whole animal and confirms previous findings in the hippocampal slice in vitro.

Neuritic growth and neuroma formation by an isolated molluscan ganglion

The isolated, paired buccal ganglia of the snail Helisoma trivolvis were cultured in vivo (i.e., in host snails) for periods up to 377 days. Such ganglia typically become enveloped in a white mass of fibrous tissue which reaches a maximum plateau of size within four weeks. The scanning electron microscope revealed the newly formed tissue to be composed of fine fibers which also pervaded the neuropil. Injection of individual neurons with Lucifer Yellow CH filled many new neurites within the tissue mass. It is concluded that these masses are largely of neuronal origin and they are therefore designated as neuromas. The maximum size of neuromas may be limited by intrinsic mechanisms which limit neuronal volume.

Separate factors produced by the CNS of the snail Helisoma stimulate neurite outgrowth and choline metabolism in cultured neurons

Neurons from the snail Helisoma require a brain-derived factor(s) for neurite outgrowth in both organ and isolated cell culture. This factor is released from the CNS of Helisoma when brains are incubated in defined medium, producing a conditioned medium (CM). In addition to its growth-promoting activity, CM also enhances total uptake of 3H-choline and the incorporation of 3H-choline into specific metabolites: acetylcholine, phosphorylcholine and lipid. This choline metabolism-enhancing factor(s) is distinct and separable from neurite growth-promoting factor: 1. Over 95% of neurite growth-promoting activity can be removed from CM by adsorption to a polylysine surface while there is no loss of choline metabolism-enhancing activity. 2. When central ganglia were treated with anisomycin, a potent inhibitor of molluscan protein synthesis, the choline metabolism-enhancing activity was completely absent from the resulting CM, while the growth promoting activity was reduced by only 35%. These results suggest that the Helisoma CNS produces a variety of trophic factors that are involved in regulating the interaction between neuronal growth and metabolism.

Physiological basis of feeding behavior in Tritonia diomedea. III. Role of depolarizing afterpotentials

The nature and role of the depolarizing afterpotentials (DAPs) of buccal motoneurons of Tritonia diomedea were examined. Neuron B5 exhibits a DAP whose ionic dependence and modifiability by TEA and 4-AP suggest a similarity to the DAP previously described in pleural pacemaker neurons. Reduction of the DAP severely reduces the ability of these neurons to generate bursts of action potentials. Certain other motoneurons (B1 and b6) are reexcited by a slow DAP (SDAP) which appears to be of synaptic origin. It is concluded that DAPs, which are dependent upon motoneuron activity, contribute to the synthesis of motor output by the buccal ganglion.

Selection of a novel connection by adult molluscan neurons

Predictable change in neuronal connectivity can be induced in the buccal ganglia of adult Helisoma snails when neuritic growth is evoked by axotomy. Both transient and stable novel electrical connections are established between identified neurons. The breaking of inappropriate, normally transient connections is contingent on the formation of an appropriate connection.