Effect of serotonergic afferents on quantal release at central inhibitory synapses

The Neural Basis of Escape Behavior in Vertebrates

Escape is one of the most studied animal behaviors, and there is a rich normative theory that links threat properties to evasive actions and their timing. The behavioral principles of escape are evolutionarily conserved and rely on elementary computational steps such as classifying sensory stimuli and executing appropriate movements. These are common building blocks of general adaptive behaviors. Here we consider the computational challenges required for escape behaviors to be implemented, discuss possible algorithmic solutions, and review some of the underlying neural circuits and mechanisms. We outline shared neural principles that can be implemented by evolutionarily ancient neural systems to generate escape behavior, to which cortical encephalization has been added to allow for increased sophistication and flexibility in responding to threat.

Development of a vibrational startle response assay for screening environmental pollutants and drugs impairing predator avoidance

The present paper describes the vibrational startle response assay (VSRA), a new robust, simple and automated in vivo medium- to high-throughput procedure for assessment of the escape response and its habituation in zebrafish larvae. Such behaviors enable fish larvae to escape from predator strikes in aquatic ecosystems. The assay is based on measuring the distance moved by each larva during the startle response evoked by repetitive vibrational stimuli. The iterative reduction observed in the response to a series of tapping stimulus in VSRA met the main criteria of habituation. Subsequently, the analysis of concordance using a battery of neuroactive compounds modulating different neurotransmitter systems demonstrated that the results of VSRA are highly predictive of the effects on other vertebrates. Finally, as a proof of concept, VSRA was used to test two relevant environmental pollutants at different concentrations. The results demonstrated that VSRA is suitable for concentration-response analysis of environmental pollutants, opening the possibility to determine the potency and the associated hazard of impaired escape response for the different compounds. Therefore, we suggest that VSRA could be a valuable tool for screening of chemical compounds capable of compromising predator avoidance behavior.

Serotonin enhances excitability and gamma frequency temporal integration in mouse prefrontal fast-spiking interneurons

The medial prefrontal cortex plays a key role in higher order cognitive functions like decision making and social cognition. These complex behaviors emerge from the coordinated firing of prefrontal neurons. Fast-spiking interneurons (FSIs) control the timing of excitatory neuron firing via somatic inhibition and generate gamma (30–100 Hz) oscillations. Therefore, factors that regulate how FSIs respond to gamma-frequency input could affect both prefrontal circuit activity and behavior. Here, we show that serotonin (5HT), which is known to regulate gamma power, acts via 5HT2A receptors to suppress an inward-rectifying potassium conductance in FSIs. This leads to depolarization, increased input resistance, enhanced spiking, and slowed decay of excitatory post-synaptic potentials (EPSPs). Notably, we found that slowed EPSP decay preferentially enhanced temporal summation and firing elicited by gamma frequency inputs. These findings show how changes in passive membrane properties can affect not only neuronal excitability but also the temporal filtering of synaptic inputs.

Serotonergic modulation of startle-escape plasticity in an African cichlid fish: a single-cell molecular and physiological analysis of a vital neural circuit

Social life affects brain function at all levels, including gene expression, neurochemical balance, and neural circuits. We have previously shown that in the cichlid fish Astatotilapia burtoni brightly colored, socially dominant (DOM) males face a trade-off between reproductive opportunities and increased predation risk. Compared with camouflaged subordinate (SUB) males, DOMs exposed to a loud sound pip display higher startle responsiveness and increased excitability of the Mauthner cell (M-cell) circuit that governs this behavior. Using behavioral tests, intracellular recordings, and single-cell molecular analysis, we show here that serotonin (5-HT) modulates this socially regulated plasticity via the 5-HT receptor subtype 2 (5-HTR2). Specifically, SUBs display increased sensitivity to pharmacological manipulation of 5-HTR2 compared with DOMs in both startle-escape behavior and electrophysiological properties of the M-cell. Immunohistochemistry showed serotonergic varicosities around the M-cells, further suggesting that 5-HT impinges directly onto the startle-escape circuitry. To determine whether the effects of 5-HTR2 are pre- or postsynaptic, and whether other 5-HTR subtypes are involved, we harvested the mRNA from single M-cells via cytoplasmic aspiration and found that 5-HTR subtypes 5A and 6 are expressed in the M-cell. 5-HTR2, however, was absent, suggesting that it affects M-cell excitability through a presynaptic mechanism. These results are consistent with a role for 5-HT in modulating startle plasticity and increase our understanding of the neural and molecular basis of a trade-off between reproduction and predation.

Social and Ecological Regulation of a Decision-Making Circuit

Ecological context, sensory inputs, and the internal physiological state are all factors that need to be integrated for an animal to make appropriate behavioral decisions. However, these factors have rarely been studied in the same system. In the African cichlid fish Astatotilapia burtoni, males alternate between two phenotypes based on position in a social hierarchy. When dominant (DOM), fish display bright body coloration and a wealth of aggressive and reproductive behavioral patterns that make them conspicuous to predators. Subordinate (SUB) males, on the other hand, decrease predation risk by adopting cryptic coloration and schooling behavior. We therefore hypothesized that DOMs would show enhanced startle-escape responsiveness to compensate for their increased predation risk. Indeed, behavioral responses to sound clicks of various intensities showed a significantly higher mean startle rate in DOMs compared with SUBs. Electrophysiological recordings from the Mauthner cells (M-cells), the neurons triggering startle, were performed in anesthetized animals and showed larger synaptic responses to sound clicks in DOMs, consistent with the behavioral results. In addition, the inhibitory drive mediated by interneurons (passive hyperpolarizing potential [PHP] cells) presynaptic to the M-cell was significantly reduced in DOMs. Taken together, the results suggest that the likelihood for an escape to occur for a given auditory stimulus is higher in DOMs because of a more excitable M-cell. More broadly, this study provides an integrative explanation of an ecological and social trade-off at the level of an identifiable decision-making neural circuit.

The Mauthner cell half a century later: a neurobiological model for decision-making?

The Mauthner (M) cell is a critical element in a vital escape "reflex" triggered by abrupt or threatening events. Its properties at the molecular and synaptic levels, their various forms of plasticity, and the design of its networks, are all well adapted for this survival function. They guarantee that this behavior is appropriately unilateral, variable, and unpredictable. The M cell sets the behavioral threshold, and, acting in concert with other elements of the brainstem escape network, determines when, where, and how the escape is executed.

Relationship of tyrosine hydroxylase and serotonin immunoreactivity to sensorimotor circuitry in larval zebrafish

Our previous study tracked the ontogeny of aminergic systems in zebrafish (Danio rerio). Here we use tyrosine hydroxylase (TH) and serotonin (5-hydroxytryptamine; 5-HT) immunoreactivity, in conjunction with retrograde and genetic labeling techniques, to provide a more refined examination of the potential synaptic contacts of aminergic systems. Our focus was on different levels of the sensorimotor circuit for escape, from sensory inputs, through identified descending pathways, to motor output. We observed 5-HT reactivity in close proximity to the collaterals of the Rohon-Beard sensory neurons in spinal cord. In the brainstem we found TH and 5-HT reactivity closely apposed to the dendritic processes of the nucleus of the medial longitudinal fascicle (nMLF), in addition to the ventral dendrites of the Mauthner neuron and its serial homologs MiD2cm and MiD3cm. Only TH reactivity was observed near the lateral dendrites of the Mauthner cell. TH and 5-HT reactivity were also positioned near the outputs of reticulospinal cells in spinal cord. Finally, both TH and 5-HT reactivity were detected close to the dendritic processes of primary and secondary spinal motor neurons. We also confirmed, using dual TH and 5-HT staining and retrograde labeling, that the sources of spinal aminergic reactivity include the posterior tuberculum (dopamine) and inferior raphe region (5-HT). Our data indicate that aminergic systems may interact at all levels of the sensorimotor pathways involved in escape. The identification of some of these likely sites of aminergic action will allow for directed studies of their functional roles using the powerful combination of techniques available in zebrafish.

Effects of L-thyrosyl - L-arginine (kyotorphin) on the behavior of rats and goldfish

The effects of kyotorphin (KTP), a dipeptide (L-Tyr-L-Arg), on the level of sensory attention to stimuli of different modalities in rats and the exploratory behavior in goldfish were investigated. In both cases KTP was found to suppress the exploratory activity. When 5-HTP, a precursor of serotonin synthesis, is activated the inhibitory effects of KTP increased. It is assumed that the regulatory effect of KTP on the exploratory behavior of animals is mediated by the monoaminergic (neurotransmitting) brain systems, as distinct from its analgetic effects, which are mediated by the opioid brain systems.

Comparison of the effects of serotonin in the hippocampus and the entorhinal cortex

Among the molecular, cellular, and systemic events that have been proposed to modulate the function of the hippocampus and the entorhinal cortex (EC), one of the most frequently cited possibilities is the activation of the serotonergic system. Neurons in the hippocampus and in the EC receive a strong serotonergic projection from the raphe nuclei and express serotonin (5-HT) receptors at high density. Here we review the various effects of 5-HT on intrinsic and synaptic properties of neurons in the hippocampus and the EC. Although similar membrane-potential changes following 5-HT application have been reported for neurons of the entorhinal cortex and the hippocampus, the effects of serotonin on synaptic transmission are contrary in both areas. Serotonin mainly depresses fast and slow inhibition of the principal output cells of the hippocampus, whereas it selectively suppresses the excitation in the entorhinal cortex. On the basis of these data, we discuss the possible role of serotonin under physiological and pathophysiological circumstances.

Aminergic modulation of glycine release in a spinal network controlling swimming in Xenopus laevis

1. Neuromodulators can effect changes in neural network function by strengthening or weakening synapses between neurons via presynaptic control of transmitter release. We have examined the effects of two biogenic amines on inhibitory connections of a spinal rhythm generator in Xenopus tad poles. 2. Glycinergic inhibitory potentials occurring mid-cycle in motoneurons during swimming activity are reduced by 5-hydroxytryptamine (5-HT; serotonin) and enhanced by noradrenaline (NA). These opposing effects on inhibitory synaptic strength are mediated presynaptically where 5-HT decreases and NA increases the probability of glycine release from inhibitory terminals. 3. The amines also have contrasting effects on swimming: 5-HT increased motor burst durations while NA reduced swimming frequency. Aminergic modulation of glycinergic transmission may thus control fundamental parameters of swimming and force the spinal network to generate opposite extremes of its spectrum of possible outputs.

Serotonergic axons in monkey prefrontal cerebral cortex synapse predominantly on interneurons as demonstrated by serial section electron microscopy

Anatomical approaches were used to describe the distribution, appearance, and synaptic interactions of serotonin (5-HT)-immunoreactive axons in monkey prefrontal cortex. A plexus of 5-HT axons was found throughout the gray matter, with an especially high density in layer I and a slight increase in layer IV. They were strikingly heterogeneous, with a gradient of morphologies ranging from fine and nonvaricose to highly varicose or thick and nonvaricose. Electron microscopy showed that both varicose and nonvaricose axons were typically filled with clear vesicles and less abundant dense core vesicles. A serial section analysis of 5-HT varicosities in layers I, III, and V showed consistent results across layers. Only about 23% of labeled varicosities formed identifiable synapses. These synapses were consistently asymmetric and were 2-5 serial sections (or 0.08-0.38 mu) in diameter. Targets of identified 5-HT synapses were dendritic shafts with the exception of one cell soma. Followed in serial sections, postsynaptic dendrites typically had morphological features of interneurons, i.e. they lacked spines, had a high density of synaptic inputs, and often had a varicose morphology. Only 8% of postsynaptic shafts were classified as pyramidal dendrites. This is in striking contrast to our previous study in this cortex of dopamine axons, which synapsed predominantly on pyramidal dendrites. These are the first results to indicate that interneurons are the major recipient of identifiable 5-HT synapses in the monkey prefrontal cortex.

Regulation of synaptic strength at mixed synapses: effects of dopamine receptor blockade and protein kinase C activation

Previous studies of the mixed excitatory synapses between eighth nerve afferents and the lateral dendrite of the goldfish Mauthner (M-) cell have shown that synaptic strength is enhanced for an hour or longer following either repeated brief tetanizations or local extracellular applications of dopamine. Both the initial electrotonic coupling potential, mediated via current flow through gap junctions, and the subsequent chemically mediated excitatory postsynaptic potentials (EPSPs) are potentiated. Different second messenger pathways are implicated in the postsynaptic induction of these potentiations, with a Ca2+ influx presumably triggering the activity dependent long-term potentiations (LTP) and dopamine acting via a cAMP dependent pathway. Experiments performed to determine whether the LTP involves a stimulus-induced release of dopamine or requires a background level of dopamine receptor activation suggest neither is the case, as tetanization in the presence of a D1 receptor antagonist, which blocks the dopamine effects, produced an LTP comparable to that in the absence of the blocker. The effects of Ca2+ are presumably not due to protein kinase C (PKC) activation, since phorbol esters had no effect on the mixed excitatory synaptic responses, although they did enhance the frequency of spontaneously occurring inhibitory PSPs.

Long-term potentiation of inhibitory circuits and synapses in the central nervous system

Glycinergic inhibition evoked disynaptically in the teleost Mauthner cell by stimulation of the contralateral eighth nerve exhibits long-term potentiation following classical tetanization of that pathway. This enhancement occurs at the synapses between primary afferents onto second-order interneurons and the connections between these inhibitory cells and the Mauthner neuron. The evidence for modifications of glycinergic transmission is that the slope of the relation between the presynaptic volley and the synaptic conductance can be greater after the tetanus. This increase in gain is still manifest after pharmacological block of potentiation at the excitatory synapse with glutamate antagonists. Inhibitory long-term potentiation is induced by tetani weaker than those required for enhancement of the monosynaptic excitation of the other (ipsilateral) Mauthner cell. Thus, in vivo learning can alter the balance between excitation and inhibition within a network by modifying one or both of them.

Quantal analysis and synaptic efficacy in the CNS

Quantal analysis of synaptic transmission at connections between neurons in the CNS has provided insights concerning the structural constraints on transmitter release and postsynaptic responsiveness. However, it has proven difficult in many cases to resolve the size and variability of a single quantum or to distinguish clear peaks in amplitude histograms of evoked responses, due in part to the superposition of background instrumental and biological noise. These limitations raise questions about recent attempts to use direct or indirect methods of quantal analysis in order to distinguish between pre- and postsynaptic loci of the modifications underlying long-term potentiation, particularly since the interpretations are model-dependent and the statistical treatments and experimental techniques employed incorporate simplifying assumptions not yet proven.

The synaptic organization of the prepacemaker nucleus in weakly electric knifefish, Eigenmannia: a quantitative ultrastructural study

Weakly electric knifefish (Eigenmannia sp.) produce continuous electric organ discharges at very constant frequencies. Modulations of the discharges occur during social interactions and are under control of the diencephalic prepacemaker nucleus. Abrupt frequency modulations, or 'chirps', which are observed predominantly during the breeding season, can be elicited by stimulation of neurons in a ventro-lateral portion of the prepacemaker nucleus, the so-called PPn-C. The PPn-C consists of approximately 100 loosely scattered large multipolar neurons which send dendrites into three territories, called 'dorso-medial', 'dorso-lateral', and 'ventral'. In the present ultrastructural investigation, the synaptic organization of these neurons, identified by retrograde labelling with horseradish peroxidase, was studied quantitatively. Somata and dendrites of the PPn-C receive input from two classes of chemical synapses. Class-1 boutons contain predominantly agranular, round vesicles and are believed to be excitatory. Class-2 boutons display predominantly flattened or pleiomorphic vesicles and are probably inhibitory. The action of the agranular vesicles in the synaptic boutons of these two classes may be modulated by the content of large dense-core vesicles. These comprise approximately 1% of the total vesicle population and are found predominantly in regions distant from the active zone of the synaptic bouton. The density of chemical synapses exhibits marked topographic differences. Class-1 boutons occur typically at densities of 3-12 synapses per 100 microns of profile length on dendrites and cell bodies. No significant differences in density of class-1 boutons could be found between distal dendrites of the three territories, proximal dendrites and cell bodies. The density of class-2 synapses, on the other hand, increases significantly from usually less than 1 synapse per 100 microns of profile length on distal dendrites to 2-3 synapses per 100 microns of profile length on proximal dendrites and cell bodies. Such a topographic organization could enable the proximal elements to 'veto' the depolarizing response of distal dendrites to excitatory inputs. The growth of dendrites in the dorso-medial territory during the breeding season, as shown in a previous study, and the concurrent doubling of excitatory input received by class-1 synapses, could overcome the inhibition caused on somata and proximal dendrites by class-2 synapses and thus account for the dramatic increase in the fish's propensity to chirp in the context of sexual maturity.

Differential distribution of GABA- and serotonin-containing afferents on an identified central neuron

The distributions of gamma-aminobutyric acid (GABA)- and serotonin (5-HT)-containing terminals impinging on the surface of the Mauthner (M-) cell were studied at the light microscopic level using double immunofluorescent labeling and were compared with that of the glycine receptor. The latter was visualized indirectly, using a monoclonal mouse antibody which recognizes its 93-kDa associated protein. This neuron has two large principal dendrites: one extending ventrorostrally (ventral dendrite) and the other dorsolaterally (lateral dendrite). There are also two other classes of smaller processes: one that projects ventrally (small ventral dendrites) and one penetrating in the axon cap (cap dendrites), a peculiar neuropil surrounding the initial segment of the M-cell axon. A cellular regionalization of these afferent systems was found: GABA boutons, labeled for glutamic acid decarboxylase (GAD), were localized preferentially on the lateral dendrite while 5-HT-filled endings predominated on the ventral one. The density of these two classes of inputs was comparable in the other areas of the M-cell: less of their terminals were in contact with the soma outside the axon cap, and more numerous boutons, which presented either GABA or 5-HT immunoreactivities, were apposed to the small ventral dendrites. This preferential pattern of innervation differed with the ubiquitous presence of glycine receptor clusters on the M-cell membrane. Finally no evidence of a colocalization of GABA and 5-HT in afferent endings was detected at any portion of the M-cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin facilitates GABAergic transmission in the CA1 region of rat hippocampus in vitro

1. The effect of serotonin on inhibitory synaptic transmission was examined in forty-one CA1 pyramidal neurones using intracellular voltage recordings in vitro. 2. Serotonin (20-50 microM) increased the synaptic noise of most (85%) neurones loaded with chloride (n = 33). The duration of this effect was enhanced with increasing concentrations of serotonin and was fully reversible within 5 min. When serotonin was applied at short intervals (less than 10 min), fading of the response was observed. 3. The effect of serotonin on synaptic noise persisted in the presence of the glutamate NMDA and non-NMDA antagonists, APV (100 microM) and CNQX (10 microM), but it was blocked (n = 5) by a GABAA antagonist, bicuculline (10 microM). 4. The increase in inhibitory synaptic events resulted from an enhanced frequency of unitary IPSPs from 4.6 +/- 3.8 Hz in control to 17.2 +/- 12.5 Hz (n = 5) in serotonin, especially of large events. Serotonin caused no change in the amplitude and frequency of miniature synaptic events recorded in the presence of TTX (n = 5). The mean amplitude of unitary inhibitory postsynaptic potentials (IPSPs) increased from 1.37 +/- 0.35 mV in control to 3.67 +/- 1.38 mV in serotonin. The coefficient of variation of unitary IPSPs increased from 0.40 +/- 0.11 in control to 0.74 +/- 0.23 in serotonin when quantal size appeared unchanged. 5. The 5-HT3 agonist 2-methyl-serotonin (52 microM, n = 4) partially mimicked the effect of serotonin, increasing the inhibitory noise without affecting the pyramidal neurone conductance. The serotonin-induced facilitation of unitary IPSPs was blocked by the 5-HT3 antagonists ICS 205-930 (1-90 nM, n = 3) and metoclopramide (30 microM, n = 1). 6. These results suggest that serotonin directly excites GABAergic interneurones acting on a 5-HT3 receptor and consequently increasing the frequency of inhibitory synaptic events recorded in CA1 pyramidal cells.

5-Hydroxytryptamine depresses reticulospinal excitatory postsynaptic potentials in motoneurons of the lamprey

Application of 5-hydroxytryptamine (5-HT) to the lamprey spinal cord in vitro reversibly depressed the chemical component of excitatory post-synaptic potentials recorded intracellularly in motoneurons and evoked by stimulation of single reticulospinal Müller cells. The depression could be produced either by local application of small volumes of 10 mM 5-HT to the surface of the spinal cord or by bath-application of 1 or 10 microM 5-HT. No effect on the input resistance of the postsynaptic cells or their sensitivity to glutamate, the suspected transmitter at this synapse, could be detected, suggesting the possibility of a presynaptic action of 5-HT at this synapse in the lamprey.

Differential distribution of serotoninergic inputs on the goldfish Mauthner cell

The morphology and distribution of the serotoninergic (5-HT) input to the Mauthner cell (M cell) of a teleost, Carassius auratus, were analyzed at the light microscopic level. Immunohistochemical methods revealed that 1) most fibers innervating the M cell originate from the ventral and lateroventral regions of the rhombencephalon; 2) two groups of fibers contribute to this innervation, thick ones (type I, 0.4-0.7 microns in diameter) with terminal endings and thin ones (type II, less than 0.2 microns) that issue numerous beaded varicosities 4-10 microns from the target cell and only occasional side endings contacting it; 3) the density of immunoreactive profiles is uneven over the whole cell and predominates on the ventral dendrite; and 4) the two sets of axons, although overlapping, do not have the same distribution. Specifically, both classes are present on the ventral dendrite, whereas type II fibers are the only ones observed on the soma, in the region of the initial segment of the axon, and in the vicinity of the lateral dendrite. Functionally identified inputs on the M cell also have a regionalized distribution, depending, for example, on whether they belong to excitatory or inhibitory networks. Thus we propose that 5-HT inputs have specific influences that are a function of their respective localization.