Serotonin modulates axo-axonal coupling between neurons critical for learning in the leech

Purinergic modulation of neuronal gap junction circuits in the CNS of the leech

Gap junctions (GJs) are widely distributed in brains across the animal kingdom. To visualize the GJ- coupled networks of two major mechanosensory neurons in the ganglia of medicinal leeches, we injected these cells with the GJ-permeable tracer Neurobiotin. When diffusion time was limited to only 30 min, tracer coupling was highly variable for both cells, suggesting a possible modulation of GJ permeability. In invertebrates the innexins (homologs of vertebrate pannexins) form the GJs. Because extracellular adenosine triphosphate (ATP) modulates pannexin and leech innexin hemichannel permeability and is released by leech glial cells following injury, we tested the effects of bath application of ATP after the injection of Neurobiotin and observed a significant increase in the number of neurons tracer coupled to the sensory neurons. This effect required the elevation of intracellular Ca²⁺ and could be produced by bath application of caffeine. Conversely, scavenging endogenous extracellular ATP with the ATPase apyrase decreased the number of coupled cells. ATP also increased electrical conductance and tracer permeability between the bilateral Retzius neurons. This modulatory effect of ATP on GJ coupling was blocked by siRNA knockdown of a P1-like adenosine receptor. Finally, exposure of leech ganglia to extracellular ATP induced a characteristic low frequency (<0.3 Hz) rhythmic bursting activity that was roughly synchronous among multiple neurons, a behavior that was significantly attenuated by the GJ blocker octanol. These findings highlight the mediation by ATP of a robust physiological mechanism for modifying neuronal circuits by rapidly recruiting neurons into active networks and entraining synchronized bursting activity.

Studies on regeneration of central nervous system and social ability of the earthworm Eudrilus eugeniae

Earthworms are segmented invertebrates that belong to the phylum Annelida. The segments can be divided into the anterior, clitellar and posterior parts. If the anterior part of the earthworm, which includes the brain, is amputated, the worm would essentially survive even in the absence of the brain. In these brain amputee-derived worms, the nerve cord serves as the primary control center for neurological function. In this current work, we studied changes in the expression levels of anti-acetylated tubulin and serotonin as the indicators of neuro-regenerative processes. The data reveal that the blastemal tissues express the acetylated tubulin and serotonin from day four and that the worm amputated at the 7th segment takes 30 days to complete the regeneration of brain. The ability of self-assemblage is one of the specific functions of the earthworm's brain. The brain amputee restored the ability of self-assemblage on the eighth day.

Gap junctions facilitate propagation of synchronous firing in the cortical neural population: a numerical simulation study

This study investigates the effect of gap junctions on firing propagation in a feedforward neural network by a numerical simulation with biologically plausible parameters. Gap junctions are electrical couplings between two cells connected by a binding protein, connexin. Recent electrophysiological studies have reported that a large number of inhibitory neurons in the mammalian cortex are mutually connected by gap junctions, and synchronization of gap junctions, spread over several hundred microns, suggests that these have a strong effect on the dynamics of the cortical network. However, the effect of gap junctions on firing propagation in cortical circuits has not been examined systematically. In this study, we perform numerical simulations using biologically plausible parameters to clarify this effect on population firing in a feedforward neural network. The results suggest that gap junctions switch the temporally uniform firing in a layer to temporally clustered firing in subsequent layers, resulting in an enhancement in the propagation of population firing in the feedforward network. Because gap junctions are often modulated in physiological conditions, we speculate that gap junctions could be related to a gating function of population firing in the brain.

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.

Fluoxetine treatment abolishes the in vitro respiratory response to acidosis in neonatal mice

Background

To secure pH homeostasis, the central respiratory network must permanently adapt its rhythmic motor drive to environment and behaviour. In neonates, it is commonly admitted that the retrotrapezoid/parafacial respiratory group of neurons of the ventral medulla plays the primary role in the respiratory response to acidosis, although the serotonergic system may also contribute to this response.

Methodology/Principal Findings

Using en bloc medullary preparations from neonatal mice, we have shown for the first time that the respiratory response to acidosis is abolished after pre-treatment with the serotonin-transporter blocker fluoxetine (25–50 µM, 20 min), a commonly used antidepressant. Using mRNA in situ hybridization and immunohistology, we have also shown the expression of the serotonin transporter mRNA and serotonin-containing neurons in the vicinity of the RTN/pFRG of neonatal mice.

Conclusions

These results reveal that the serotonergic system plays a pivotal role in pH homeostasis. Although obtained in vitro in neonatal mice, they suggest that drugs targeting the serotonergic system should be used with caution in infants, pregnant women and breastfeeding mothers.

Properties of cannabinoid-dependent long-term depression in the leech

Previously, a cannabinoid-dependent form of long-term depression (LTD) was discovered at the polysynaptic connection between the touch mechanosensory neuron and the S interneuron (Li and Burrell in J Comp Physiol A 195:831-841, 2009). In the present study, the physiological properties of this cannabinoid-dependent LTD were examined. Increases in intracellular calcium in the S interneuron are necessary for this form of LTD in this circuit. Calcium signals contributing to cannabinoid-dependent LTD are mediated by voltage-dependent calcium channel and release of calcium from intracellular stores. Inositol triphosphate receptors, but not ryanodine receptors, appear to mediate this store-released calcium signal. Cannabinoid-dependent LTD also requires activation of metabotropic serotonin receptors, possibly a serotonin type 2-like receptor. Finally, this form of LTD involves the stimulation of nitric oxide synthase and a decrease in cyclic adenosine monophosphate signaling, both of which appeared to be downstream of cannabinoid receptor activation. Based on these findings, the cellular signaling mechanisms of cannabinoid-dependent LTD in the leech are remarkably similar to vertebrate forms of cannabinoid-dependent synaptic plasticity.

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 coupling synchronises spinal motoneuron activity during swimming in hatchling Xenopus tadpoles

The role of electrical coupling between neurons in the swimming rhythm generator of Xenopus embryos has been studied using pharmacological blockade of gap junctions. A conspicuous effect of 18beta-glycyrrhetinic acid (18beta-GA) and carbenoxolone, which have been shown to block electrical coupling in this preparation, was to increase the duration of ventral root bursts throughout the spinal cord during swimming. The left-right coordination, the swimming frequency and the duration of swimming episodes were not affected by concentrations of 18beta-GA which significantly increased burst durations. However, the longitudinal coupling was affected such that 18beta-GA led to a significant correlation between rostrocaudal delays and cycle periods, which is usually only present in older larval animals. Patch clamp recordings from spinal motoneurons tested whether gap junction blockers affect the spike timing and/or firing pattern of motoneurons during fictive swimming. In the presence of 18beta-GA motoneurons continued to fire a single, but broader action potential in each cycle of swimming, and the timing of their spikes relative to the ventral root burst became more variable. 18beta-GA had no detectable effect on the resting membrane potential of motoneurons, but led to a significant increase in input resistance, consistent with the block of gap junctions. This effect did not result in increased firing during swimming, despite the fact that multiple spikes can occur in response to current injection. Applications of 18beta-GA at larval stage 42 had no discernible effect on locomotion. The results, which suggest that electrical coupling primarily functions to synchronize activity in synergistic motoneurons during embryo swimming, are discussed in the context of motor system development.

Summation of excitatory postsynaptic potentials in electrically-coupled neurones

Dendritic electrical coupling increases the number of effective synaptic inputs onto neurones by allowing the direct spread of synaptic potentials from one neurone to another. Here we studied the summation of excitatory postsynaptic potentials (EPSPs) produced locally and arriving from the coupled neurone (transjunctional) in pairs of electrically-coupled Retzius neurones of the leech. We combined paired recordings of EPSPs, the production of artificial excitatory postsynaptic potentials (APSPs) in neurone pairs with different coupling coefficients and simulations of EPSPs produced in the coupled dendrites. Summation of the EPSPs produced in the dendrites was always linear, suggesting that synchronous EPSPs are produced at two or more different pairs of coupled dendrites and not in both sides of any one gap junction. The different spatio-temporal relationships explored between pairs of EPSPs or APSPs produced three main effects. (1) Synchronous pairs of EPSPs or APSPs exhibited an elongation of their decay phase compared to single EPSPs. (2) Asymmetries in the amplitudes between the pair of EPSPs added a "hump" to the smallest EPSP. (3) Modelling the inputs near the electrical synapse or anticipating the production of the transjunctional APSP increased the amplitude of the compound EPSP. The magnitude of all these changes depended on the coupling coefficient of the neurones. We also show that the hump improves the passive conduction of EPSPs by adding low frequency components. The diverse effects of summation of local and alien EPSPs shown here endow electrically-coupled neurones with a wider repertoire of adjustable integrative possibilities.

CNQX and AMPA inhibit electrical synaptic transmission: a potential interaction between electrical and glutamatergic synapses

Electrical synapses play an important role in signaling between neurons and the synaptic connections between many neurons possess both electrical and chemical components. Although modulation of electrical synapses is frequently observed, the cellular processes that mediate such changes have not been studied as thoroughly as plasticity in chemical synapses. In the leech (Hirudo sp), the competitive AMPA receptor antagonist CNQX inhibited transmission at the rectifying electrical synapse of a mixed glutamatergic/electrical synaptic connection. This CNQX-mediated inhibition of the electrical synapse was blocked by concanavalin A (Con A) and dynamin inhibitory peptide (DIP), both of which are known to inhibit endocytosis of neurotransmitter receptors. CNQX-mediated inhibition was also blocked by pep2-SVKI (SVKI), a synthetic peptide that prevents internalization of AMPA-type glutamate receptor. AMPA itself also inhibited electrical synaptic transmission and this AMPA-mediated inhibition was partially blocked by Con A, DIP and SVKI. Low frequency stimulation induced long-term depression (LTD) in both the electrical and glutamatergic components of these synapses and this LTD was blocked by SVKI. GYKI 52466, a selective non-competitive antagonist of AMPA receptors, did not affect the electrical EPSP, although it did block the glutamatergic component of these synapses. CNQX did not affect non-rectifying electrical synapses in two different pairs of neurons. These results suggest an interaction between AMPA-type glutamate receptors and the gap junction proteins that mediate electrical synaptic transmission. This putative interaction between glutamate receptors and gap junction proteins represents a novel mechanism for regulating the strength of synaptic transmission.

Widespread inhibition proportional to excitation controls the gain of a leech behavioral circuit

Changing gain in a neuronal system has important functional consequences, but the underlying mechanisms have been elusive. Models have suggested a variety of neuronal and systems properties to accomplish gain control. Here, we show that the gain of the neuronal network underlying local bending behavior in leeches depends on widespread inhibition. Using behavioral analysis, intracellular recordings, and voltage-sensitive dye imaging, we compared the effects of blocking just the known lateral inhibition with blocking all GABAergic inhibition. This revealed an additional source of inhibition, which was widespread and increased in proportion to increasing stimulus intensity. In a model of the input/output functions of the three-layered local bending network, we showed that inhibiting all interneurons in proportion to the stimulus strength produces the experimentally observed change in gain. This relatively simple mechanism for controlling behavioral gain could be prevalent in vertebrate as well as invertebrate nervous systems.

Sustained rhythmic activity in gap-junctionally coupled networks of model neurons depends on the diameter of coupled dendrites

Gap junctions are known to be important for many network functions such as synchronization of activity and the generation of waves and oscillations. Gap junctions have also been proposed to be essential for the generation of early embryonic activity. We have previously shown that the amplitude of electrical signals propagating across gap-junctionally coupled passive cables is maximized at a unique diameter. This suggests that threshold-dependent signals may propagate through gap junctions for a finite range of diameters around this optimal value. Here we examine the diameter dependence of action potential propagation across model networks of dendro-dendritically coupled neurons. The neurons in these models have passive soma and dendrites and an action potential-generating axon. We show that propagation of action potentials across gap junctions occurs only over a finite range of dendritic diameters and that propagation delay depends on this diameter. Additionally, in networks of gap-junctionally coupled neurons, rhythmic activity can emerge when closed loops (re-entrant paths) occur but again only for a finite range of dendrite diameters. The frequency of such rhythmic activity depends on the length of the path and the dendrite diameter. For large networks of randomly coupled neurons, we find that the re-entrant paths that underlie rhythmic activity also depend on dendrite diameter. These results underline the potential importance of dendrite diameter as a determinant of network activity in gap-junctionally coupled networks, such as network rhythms that are observed during early nervous system development.

Mechanisms of serotonergic facilitation of a command neuron

The lateral giant (LG) command neuron of crayfish responds to an attack directed at the abdomen by triggering a single highly stereotyped escape tail flip. Experimentally applied serotonin (5-hydroxytrptamine, 5-HT) can increase or decrease LG's excitability, depending on the concentration, rate, and duration of 5-HT application. Here we describe three physiological mechanisms that mediate serotonergic facilitation of LG. Two processes strengthen electrical coupling between the primary mechanosensory afferent neurons and LG: first, an early increase in the conductance of electrical synapses between primary afferent neurons and LG dendrites and second, an early increase in the membrane resistance of LG dendrites. The increased coupling facilitates LG's synaptic response and it promotes recruitment of weakly excited afferent neurons to contribute to the response. Third, a delayed increase in the membrane resistance of proximal regions of LG increases the cell's input resistance near the initial segment. Together these mechanisms contribute to serotonergic facilitation of LG's response.

Neuronal competition for action potential initiation sites in a circuit controlling simple learning

The spatial and temporal patterns of action potential initiations were studied in a behaving leech preparation to determine the basis of increased firing that accompanies sensitization, a form of non-associative learning requiring the S-interneurons. Little is known at the network level about mechanisms of behavioral sensitization. The S-interneurons, one in each ganglion and linked by electrical synapses with both neighbors to form a chain, are interposed between sensory and motor neurons. In sensitized preparations the strength of shortening is related to S-cell firing, which itself is the result of impulses initiating in several S-cells. Because the S-cells, as independent initiation sites, all contribute to activity in the chain, it was hypothesized that during sensitization, increased multi-site activity increased the chain's firing rate. However, it was found that during sensitization, the single site with the largest initiation rate, the S-cell in the stimulated segment, suppressed initiations in adjacent ganglia. Experiments showed this was both because (1) it received the earliest, greatest input and (2) the delayed synaptic input to the adjacent S-cells coincided with the action potential refractory period. A compartmental model of the S-cell and its inputs showed that a simple, intrinsic mechanism of inexcitability after each action potential may account for suppression of impulse initiations. Thus, a non-synaptic competition between neurons alters synaptic integration in the chain. In one mode, inputs to different sites sum independently, whereas in another, synaptic input to a single site precisely specifies the overall pattern of activity.

Simulator for neural networks and action potentials

A key challenge for neuroinformatics is to devise methods for representing, accessing, and integrating vast amounts of diverse and complex data. A useful approach to represent and integrate complex data sets is to develop mathematical models [Arbib (The Handbook of Brain Theory and Neural Networks, pp. 741-745, 2003); Arbib and Grethe (Computing the Brain: A Guide to Neuroinformatics, 2001); Ascoli (Computational Neuroanatomy: Principles and Methods, 2002); Bower and Bolouri (Computational Modeling of Genetic and Biochemical Networks, 2001); Hines et al. (J. Comput. Neurosci. 17, 7-11, 2004); Shepherd et al. (Trends Neurosci. 21, 460-468, 1998); Sivakumaran et al. (Bioinformatics 19, 408-415, 2003); Smolen et al. (Neuron 26, 567-580, 2000); Vadigepalli et al. (OMICS 7, 235-252, 2003)]. Models of neural systems provide quantitative and modifiable frameworks for representing data and analyzing neural function. These models can be developed and solved using neurosimulators. One such neurosimulator is simulator for neural networks and action potentials (SNNAP) [Ziv (J. Neurophysiol. 71, 294-308, 1994)]. SNNAP is a versatile and user-friendly tool for developing and simulating models of neurons and neural networks. SNNAP simulates many features of neuronal function, including ionic currents and their modulation by intracellular ions and/or second messengers, and synaptic transmission and synaptic plasticity. SNNAP is written in Java and runs on most computers. Moreover, SNNAP provides a graphical user interface (GUI) and does not require programming skills. This chapter describes several capabilities of SNNAP and illustrates methods for simulating neurons and neural networks. SNNAP is available at http://snnap.uth.tmc.edu .

Serotonin mediates learning-induced potentiation of excitability

Sensitization potentiates excitability in an interneuron, the S-cell, that is critical for this form of learning in the whole-body shortening reflex of the medicinal leech. Serotonin (5-HT) also increases S-cell excitability, and serotonergic modulation is known to be critical for sensitization of whole-body shortening, suggesting that 5-HT mediates learning-induced enhancement of S-cell excitability. In this paper, the role of 5-HT in mediating sensitization-induced potentiation of S-cell excitability was examined. Potentiation of S-cell excitability by 5-HT was blocked by the 5-HT receptor antagonist methysergide and by intracellular injection of the G-protein inhibitor GDP-beta-S, indicating that a metabotropic 5-HT receptor was involved. Bath application of Rp-cAMP, an inhibitor of protein kinase A (PKA), blocked 5-HT-induced potentiation of excitability, whereas db-cAMP, a cAMP analogue that activates PKA, mimicked the potentiating effects of 5-HT on the S-cell. During sensitization of the shortening reflex in semi-intact preparations, methysergide and Rp-cAMP prevented learning-induced potentiation of S-cell excitability, as well as the increase in S-cell activity that normally occurs during sensitization. Furthermore, sensitization-induced increases in the shortening reflex did not occur in preparations treated with methysergide or Rp-cAMP. These results demonstrate that sensitization-induced enhancement of S-cell excitability is mediated by 5-HT and suggests that these changes may contribute to this form of learning.