Representation of auditory signals in the M-cell: role of electrical synapses

Audiovisual integration in the Mauthner cell enhances escape probability and reduces response latency

Fast and accurate threat detection is critical for animal survival. Reducing perceptual ambiguity by integrating multiple sources of sensory information can enhance perception and reduce response latency. However, studies addressing the link between behavioral correlates of multisensory integration and its underlying neural basis are rare. Fish that detect an urgent threat escape with an explosive behavior known as C-start. The C-start is driven by an identified neural circuit centered on the Mauthner cell, an identified neuron capable of triggering escapes in response to visual and auditory stimuli. Here we demonstrate that goldfish can integrate visual looms and brief auditory stimuli to increase C-start probability. This multisensory enhancement is inversely correlated to the salience of the stimuli, with weaker auditory cues producing a proportionally stronger multisensory effect. We also show that multisensory stimuli reduced C-start response latency, with most escapes locked to the presentation of the auditory cue. We make a direct link between behavioral data and its underlying neural mechanism by reproducing the behavioral data with an integrate-and-fire computational model of the Mauthner cell. This model of the Mauthner cell circuit suggests that excitatory inputs integrated at the soma are key elements in multisensory decision making during fast C-start escapes. This provides a simple but powerful mechanism to enhance threat detection and survival.

Subcellular Dissection of a Simple Neural Circuit: Functional Domains of the Mauthner-Cell During Habituation

Sensorimotor integration is a pivotal feature of the nervous system for ensuring a coordinated motor response to external stimuli. In essence, such neural circuits can optimize behavioral performance based on the saliency of environmental cues. In zebrafish, habituation of the acoustic startle response (ASR) is a simple behavior integrated into the startle command neurons, called the Mauthner cells. Whereas the essential neuronal components that regulate the startle response have been identified, the principles of how this regulation is integrated at the subcellular regions of the Mauthner cell, which in turn modulate the performance of the behavior, is still not well understood. Here, we reveal mechanistically distinct dynamics of excitatory inputs converging onto the lateral dendrite (LD) and axon initial segment (AIS) of the Mauthner cell by in vivo imaging glutamate release using iGluSnFR, an ultrafast glutamate sensing fluorescent reporter. We find that modulation of glutamate release is dependent on NMDA receptor activity exclusively at the AIS, which is responsible for setting the sensitivity of the startle reflex and inducing a depression of synaptic activity during habituation. In contrast, glutamate-release at the LD is not regulated by NMDA receptors and serves as a baseline component of Mauthner cell activation. Finally, using in vivo calcium imaging at the feed-forward interneuron population component of the startle circuit, we reveal that these cells indeed play pivotal roles in both setting the startle threshold and habituation by modulating the AIS of the Mauthner cell. These results indicate that a command neuron may have several functionally distinct regions to regulate complex aspects of behavior.

Acoustic sound-induced stress response of Nile tilapia (Oreochromis niloticus) to long-term underwater sound transmissions of urban and shipping noises

Human-made impacts on the acoustic environment from marine industries is becoming a more significant issue with increasing public concern of environmental consequences. Even though there are several reports with scientific evidences on harmful influences of anthropogenic underwater sounds on the aquatic ecosystem, most of the studies so far dealt with trigger effects of short term noise impacts on aquatic animals. In the present study, however, long-term experimentation was conducted with Nile tilapia (Oreochromis niloticus) in order to figure out how fish may respond to long-term exposure of underwater sounds and if the level of response may change (increase or decline) over time. A startle reflex as a sign of stress was seen immediately at the start of the playbacks of ship noise or urban sounds in this study. Peaks of elevated respiratory movements of ventilation (opercula beats and pectoral wing rates) retained high over the following 30 days of sound initiation and underwent a declining trend over the following 90 days of exposure. At the end of the 120-day study period, the lowered response of fish after long-term sound exposure is likely due to the increased tolerance of fish to human-generated underwater sounds of urban and shipping noises. Different than short-term noise impacts, information on long-term exposure of anthropogenic underwater sounds is important for environmental management and setting new regulations for the sustainable use of water resources in the world.

Hypoxia Has a Lasting Effect on Fast-Startle Behavior of the Tropical Fish Haemulon plumieri

Anthropogenic activities and climate change have resulted in an increase of hypoxic conditions in nearshore ecosystems worldwide. Depending on the persistence of a hypoxic event, the survival of aquatic animals can be compromised. Temperate fish exposed to hypoxia display a reduction in the probability of eliciting startle responses thought to be important for escape from predation. Here we examine the effect of hypoxia on the probability of eliciting fast-startle responses (fast-starts) of a tropical fish, the white grunt (Haemulon plumieri), and whether hypoxia has a prolonged impact on behavior once the fish are returned to normoxic conditions. White grunts collected from the San Juan Bay Estuary in Puerto Rico were exposed to an oxygen concentration of 2.5 mg L^-1 (40% dissolved oxygen). We found a significant reduction in auditory-evoked fast-starts that lasted for at least 24 hours after fish were returned to normoxic conditions. Accessibility to the neuronal networks that underlie startle responses was an important motivator for this study. Mauthner cells are identifiable neurons found in most fish and amphibians, and these cells are known to initiate fast-starts in teleost fishes. The assumption that most of the short-latency responses in this study are Mauthner cell initiated provided the impetus to characterize the white grunt Mauthner cell. The identification of the cell provides a first step in understanding how low oxygen levels may impact a single cell and its circuit and the behavior it initiates.

Influence of Stimulus Intensity on Multimodal Integration in the Startle Escape System of Goldfish

Processing of multimodal information is essential for an organism to respond to environmental events. However, how multimodal integration in neurons translates into behavior is far from clear. Here, we investigate integration of biologically relevant visual and auditory information in the goldfish startle escape system in which paired Mauthner-cells (M-cells) initiate the behavior. Sound pips and visual looms as well as multimodal combinations of these stimuli were tested for their effectiveness of evoking the startle response. Results showed that adding a low intensity sound early during a visual loom (low visual effectiveness) produced a supralinear increase in startle responsiveness as compared to an increase expected from a linear summation of the two unimodal stimuli. In contrast, adding a sound pip late during the loom (high visual effectiveness) increased responsiveness consistent with a linear multimodal integration of the two stimuli. Together the results confirm the Inverse Effectiveness Principle (IEP) of multimodal integration proposed in other species. Given the well-established role of the M-cell as a multimodal integrator, these results suggest that IEP is computed in individual neurons that initiate vital behavioral decisions.

Differential processing in modality-specific Mauthner cell dendrites

KEY POINTS

The present study examines dendritic integrative processes that occur in many central neurons but have been challenging to study in vivo in the vertebrate brain. The Mauthner cell of goldfish receives auditory and visual information via two separate dendrites, providing a privileged scenario for in vivo examination of dendritic integration. The results show differential attenuation properties in the Mauthner cell dendrites arising at least partly from differences in cable properties and the nonlinear behaviour of the respective dendritic membranes. In addition to distinct modality-dependent membrane specialization in neighbouring dendrites of the Mauthner cell, we report cross-modal dendritic interactions via backpropagating postsynaptic potentials. Broadly, the results of the present study provide an exceptional example for the processing power of single neurons.

ABSTRACT

Animals process multimodal information for adaptive behavioural decisions. In fish, evasion of a diving bird that breaks the water surface depends on integrating visual and auditory stimuli with very different characteristics. How do neurons process such differential sensory inputs at the dendritic level? For that, we studied the Mauthner cells (M-cells) in the goldfish startle circuit, which receive visual and auditory inputs via two separate dendrites, both accessible for in vivo recordings. We investigated whether electrophysiological membrane properties and dendrite morphology, studied in vivo, play a role in selective sensory processing in the M-cell. The results obtained show that anatomical and electrophysiological differences between the dendrites combine to produce stronger attenuation of visually evoked postsynaptic potentials (PSPs) than to auditory evoked PSPs. Interestingly, our recordings showed also cross-modal dendritic interaction because auditory evoked PSPs invade the ventral dendrite (VD), as well as the opposite where visual PSPs invade the lateral dendrite (LD). However, these interactions were asymmetrical, with auditory PSPs being more prominent in the VD than visual PSPs in the LD. Modelling experiments imply that this asymmetry is caused by active conductances expressed in the proximal segments of the VD. The results obtained in the present study suggest modality-dependent membrane specialization in M-cell dendrites suited for processing stimuli of different time domains and, more broadly, provide a compelling example of information processing in single neurons.

Cellular Mechanisms of Cortisol-Induced Changes in Mauthner-Cell Excitability in the Startle Circuit of Goldfish

Predator pressure and olfactory cues (alarm substance) have been shown to modulate Mauthner cell (M-cell) initiated startle escape responses (C-starts) in teleost fish. The regulation of such adaptive responses to potential threats is thought to involve the release of steroid hormones such as cortisol. However, the mechanism by which cortisol may regulate M-cell excitability is not known. Here, we used intrasomatic, in vivo recordings to elucidate the acute effects of cortisol on M-cell membrane properties and sound evoked post-synaptic potentials (PSPs). Cortisol tonically decreased threshold current in the M-cell within 10 min before trending towards baseline excitability over an hour later, which may indicate the involvement of non-genomic mechanisms. Consistently, current ramp injection experiments showed that cortisol increased M-cell input resistance in the depolarizing membrane, i.e., by a voltage-dependent postsynaptic mechanism. Cortisol also increases the magnitude of sound-evoked M-cell PSPs by reducing the efficacy of local feedforward inhibition (FFI). Interestingly, another pre-synaptic inhibitory network mediating prepulse inhibition (PPI) remained unaffected. Together, our results suggest that cortisol rapidly increases M-cell excitability via a post-synaptic effector mechanism, likely a chloride conductance, which, in combination with its dampening effect on FFI, will modulate information processing to reach threshold. Given the central role of the M-cell in initiating startle, these results are consistent with a role of cortisol in mediating the expression of a vital behavior.

Anthropogenic noise compromises the anti-predator behaviour of the European seabass, Dicentrarchus labrax (L.)

Anthropogenic noise is a significant pollutant of the world's oceans, affecting behavioural and physiological traits in a range of species, including anti-predator behaviours. Using the open field test, we investigated the effects of recordings of piling and drilling noise on the anti-predator behaviour of captive juvenile European seabass in response to a visual stimulus (a predatory mimic). The impulsive nature of piling noise triggered a reflexive startle response, which contrasted the behaviour elicited by the continuous drilling noise. When presented with the predatory mimic, fish exposed to both piling and drilling noise explored the experimental arena more extensively than control fish exposed to ambient noise. Fish under drilling and piling conditions also exhibited reduced predator inspection behaviour. Piling and drilling noise induced stress as measured by ventilation rate. This study provides further evidence that the behaviour and physiology of European seabass is significantly affected by exposure to elevated noise levels.

Social Status-Dependent Shift in Neural Circuit Activation Affects Decision Making

In a social group, animals make behavioral decisions that fit their social ranks. These behavioral choices are dependent on the various social cues experienced during social interactions. In vertebrates, little is known of how social status affects the underlying neural mechanisms regulating decision-making circuits that drive competing behaviors. Here, we demonstrate that social status in zebrafish (Danio rerio) influences behavioral decisions by shifting the balance in neural circuit activation between two competing networks (escape and swim). We show that socially dominant animals enhance activation of the swim circuit. Conversely, social subordinates display a decreased activation of the swim circuit, but an enhanced activation of the escape circuit. In an effort to understand how social status mediates these effects, we constructed a neurocomputational model of the escape and swim circuits. The model replicates our findings and suggests that social status-related shift in circuit dynamics could be mediated by changes in the relative excitability of the escape and swim networks. Together, our results reveal that changes in the excitabilities of the Mauthner command neuron for escape and the inhibitory interneurons that regulate swimming provide a cellular mechanism for the nervous system to adapt to changes in social conditions by permitting the animal to select a socially appropriate behavioral response.SIGNIFICANCE STATEMENT Understanding how social factors influence nervous system function is of great importance. Using zebrafish as a model system, we demonstrate how social experience affects decision making to enable animals to produce socially appropriate behavior. Based on experimental evidence and computational modeling, we show that behavioral decisions reflect the interplay between competing neural circuits whose activation thresholds shift in accordance with social status. We demonstrate this through analysis of the behavior and neural circuit responses that drive escape and swim behaviors in fish. We show that socially subordinate animals favor escape over swimming, while socially dominants favor swimming over escape. We propose that these differences are mediated by shifts in relative circuit excitability.

Intensity-dependent timing and precision of startle response latency in larval zebrafish

KEY POINTS

Using high-speed videos time-locked with whole-animal electrical recordings, simultaneous measurement of behavioural kinematics and field potential parameters of C-start startle responses allowed for discrimination between short-latency and long-latency C-starts (SLCs vs. LLCs) in larval zebrafish. Apart from their latencies, SLC kinematics and SLC field potential parameters were intensity independent. Increasing stimulus intensity increased the probability of evoking an SLC and decreased mean SLC latencies while increasing their precision; subtraction of field potential latencies from SLC latencies revealed a fixed time delay between the two measurements that was intensity independent. The latency and the precision in the latency of the SLC field potentials were linearly correlated to the latencies and precision of the first evoked action potentials (spikes) in hair-cell afferent neurons of the lateral line. Together, these findings indicate that first spike latency (FSL) is a fast encoding mechanism that can serve to precisely initiate startle responses when speed is critical for survival.

ABSTRACT

Vertebrates rely on fast sensory encoding for rapid and precise initiation of startle responses. In afferent sensory neurons, trains of action potentials (spikes) encode stimulus intensity within the onset time of the first evoked spike (first spike latency; FSL) and the number of evoked spikes. For speed of initiation of startle responses, FSL would be the more advantageous mechanism to encode the intensity of a threat. However, the intensity dependence of FSL and spike number and whether either determines the precision of startle response initiation is not known. Here, we examined short-latency startle responses (SLCs) in larval zebrafish and tested the hypothesis that first spike latencies and their precision (jitter) determine the onset time and precision of SLCs. We evoked startle responses via activation of Channelrhodopsin (ChR2) expressed in ear and lateral line hair cells and acquired high-speed videos of head-fixed larvae while simultaneously recording underlying field potentials. This method allowed for discrimination between primary SLCs and less frequent, long-latency startle responses (LLCs). Quantification of SLC kinematics and field potential parameters revealed that, apart from their latencies, they were intensity independent. We found that increasing stimulus intensity decreased SLC latencies while increasing their precision, which was significantly correlated with corresponding changes in field potential latencies and their precision. Single afferent neuron recordings from the lateral line revealed a similar intensity-dependent decrease in first spike latencies and their jitter, which could account for the intensity-dependent changes in timing and precision of startle response latencies.

A convergent and essential interneuron pathway for Mauthner-cell-mediated escapes

The Mauthner cell (M-cell) is a command-like neuron in teleost fish whose firing in response to aversive stimuli is correlated with short-latency escapes [1-3]. M-cells have been proposed as evolutionary ancestors of startle response neurons of the mammalian reticular formation [4], and studies of this circuit have uncovered important principles in neurobiology that generalize to more complex vertebrate models [3]. The main excitatory input was thought to originate from multisensory afferents synapsing directly onto the M-cell dendrites [3]. Here, we describe an additional, convergent pathway that is essential for the M-cell-mediated startle behavior in larval zebrafish. It is composed of excitatory interneurons called spiral fiber neurons, which project to the M-cell axon hillock. By in vivo calcium imaging, we found that spiral fiber neurons are active in response to aversive stimuli capable of eliciting escapes. Like M-cell ablations, bilateral ablations of spiral fiber neurons largely eliminate short-latency escapes. Unilateral spiral fiber neuron ablations shift the directionality of escapes and indicate that spiral fiber neurons excite the M-cell in a lateralized manner. Their optogenetic activation increases the probability of short-latency escapes, supporting the notion that spiral fiber neurons help activate M-cell-mediated startle behavior. These results reveal that spiral fiber neurons are essential for the function of the M-cell in response to sensory cues and suggest that convergent excitatory inputs that differ in their input location and timing ensure reliable activation of the M-cell, a feedforward excitatory motif that may extend to other neural circuits.

Glycine and GABAA receptors mediate tonic and phasic inhibitory processes that contribute to prepulse inhibition in the goldfish startle network

Prepulse inhibition (PPI) is understood as a sensorimotor gating process that attenuates sensory flow to the startle pathway during early stages (20–1000 ms) of information processing. Here, we applied in vivo electrophysiology and pharmacology to determine if PPI is mediated by glycine receptors (GlyRs) and/or GABA_A receptors (GABA_ARs) in the goldfish auditory startle circuit. Specifically, we used selective antagonists to dissect the contributions of target receptors on sound-evoked postsynaptic potentials (PSPs) recorded in the neurons that initiate startle, the Mauthner-cells (M-cell). We found that strychnine, a GlyR antagonist, disrupted a fast-activated (5 ms) and rapidly (<50 ms) decaying (feed-forward) inhibitory process that contributes to PPI at 20 ms prepulse/pulse inter-stimulus intervals (ISI). Additionally we observed increases of the evoked postsynaptic potential (PSP) peak amplitude (+87.43 ± 21.53%, N = 9) and duration (+204 ± 48.91%, N = 9). In contrast, treatment with bicuculline, a GABA_AR antagonist, caused a general reduction in PPI across all tested interstimulus intervals (ISIs) (20–500 ms). Bicuculline also increased PSP peak amplitude (+133.8 ± 10.3%, N = 5) and PSP duration (+284.95 ± 65.64%, N = 5). Treatment with either antagonist also tonically increased post-synaptic excitability in the M-cells, reflected by an increase in the magnitude of antidromically-evoked action potentials (APs) by 15.07 ± 3.21%, N = 7 and 16.23 ± 7.08%, N = 5 for strychnine and bicuculline, respectively. These results suggest that GABA_ARs and GlyRs are functionally segregated to short- and longer-lasting sound-evoked (phasic) inhibitory processes that contribute to PPI, with the mediation of tonic inhibition by both receptor systems being critical for gain control within the M-cell startle circuit.

The Mauthner-cell circuit of fish as a model system for startle plasticity

The Mauthner-cell (M-cell) system of teleost fish has a long history as an experimental model for addressing a wide range of neurobiological questions. Principles derived from studies on this system have contributed significantly to our understanding at multiple levels, from mechanisms of synaptic transmission and synaptic plasticity to the concepts of a decision neuron that initiates key aspects of the startle behavior. Here we will review recent work that focuses on the neurophysiological and neuropharmacological basis for modifications in the M-cell circuit. After summarizing the main excitatory and inhibitory inputs to the M-cell, we review experiments showing startle response modulation by temperature, social status, and sensory filtering. Although very different in nature, actions of these three sources of modulation converge in the M-cell network. Mechanisms of modulation include altering the excitability of the M-cell itself as well as changes in excitatory and inhibitor drive, highlighting the role of balanced excitation and inhibition for escape decisions. One of the most extensively studied forms of startle plasticity in vertebrates is prepulse inhibition (PPI), a sensorimotor gating phenomenon, which is impaired in several information processing disorders. Finally, we review recent work in the M-cell system which focuses on the cellular mechanisms of PPI and its modulation by serotonin and dopamine.

The 5-HT5A receptor regulates excitability in the auditory startle circuit: functional implications for sensorimotor gating

Here we applied behavioral testing, pharmacology, and in vivo electrophysiology to determine the function of the serotonin 5-HT5A receptor in goldfish startle plasticity and sensorimotor gating. In an initial series of behavioral experiments, we characterized the effects of a selective 5-HT5A antagonist, SB-699551 (3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4'-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride), on prepulse inhibition of the acoustic startle response. Those experiments showed a dose-dependent decline in startle rates in prepulse conditions. Subsequent behavioral experiments showed that SB-699551 also reduced baseline startle rates (i.e., without prepulse). To determine the cellular mechanisms underlying these behaviors, we tested the effects of two distinct selective 5-HT5A antagonists, SB-699551 and A-843277 (N-(2,6-dimethoxybenzyl)-N'[4-(4-fluorophenyl)thiazol-2-yl]guanidine), on the intrinsic membrane properties and synaptic sound response of the Mauthner cell (M-cell), the decision-making neuron of the startle circuit. Auditory-evoked postsynaptic potentials recorded in the M-cell were similarly attenuated after treatment with either 5-HT5A antagonist (SB-699551, 26.41 ± 3.98% reduction; A-843277, 17.52 ± 6.24% reduction). This attenuation was produced by a tonic (intrinsic) reduction in M-cell input resistance, likely mediated by a Cl(-) conductance, that added to the extrinsic inhibition produced by an auditory prepulse. Interestingly, the effector mechanisms underlying neural prepulse inhibition itself were unaffected by antagonist treatment. In summary, these results provide an in vivo electrophysiological characterization of the 5-HT5A receptor and its behavioral relevance and provide a new perspective on the interaction of intrinsic and extrinsic modulatory mechanisms in startle plasticity and sensorimotor gating.

The physiology of fish behaviour: a selective review of developments over the past 40 years(§)

During the past 40 years many new techniques have emerged that have been pivotal in furthering understanding of the physiology of fish behaviour. Behavioural studies have been enhanced by video recording systems and software for computerized event recording analysis, fine scale anatomical studies by fluorescence confocal microscopy, neurophysiological studies by visualisation and neuroendocrinology with techniques for identifying, localizing and quantifying many neurochemicals within the central nervous system. This array of approaches has been complemented by developments in molecular biology that include the ability to monitor expression profiles for known genes in specific neural structures and within the whole transcriptome. This article explores how the deployment of new techniques during the last four decades has advanced the understanding of two extensively studied systems. The first of these is the fast-start escape response, concentrating on work on goldfish Carassius auratus and zebrafish Danio rerio. The second is the link between social experience and neuroendocrinology and how this relates to life-history traits in the cichlid Burton's mouthbrooder Astatotilapia burtoni. These two case studies are then used to explore the extent to which the behaviour of animals can be explained in terms of underlying physiological mechanisms.

Behavioral genetics in larval zebrafish: learning from the young

Deciphering the genetic code that determines how the vertebrate nervous system assembles into neural circuits that ultimately control behavior is a fascinating and challenging question in modern neurobiology. Because of the complexity of this problem, successful strategies require a simple yet focused experimental approach without limiting the scope of the discovery. Unbiased, large-scale forward genetic screens in invertebrate organisms have yielded great insight into the genetic regulation of neural circuit assembly and function. For many reasons, this highly successful approach has been difficult to recapitulate in the behavioral neuroscience field's classic vertebrate model organisms-rodents. Here, we discuss how larval zebrafish provide a promising model system to which we can apply the design of invertebrate behavior-based screens to reveal the genetic mechanisms critical for neural circuit assembly and function in vertebrates.

Characteristics of the anterior lateral line nerve input to the Mauthner cell

The goldfish Mauthner (M-) cells, a bilateral pair of reticulospinal neurons, initiate the auditory evoked escape behavior of teleosts. In an open field the fish reliably turns away from the sound source. This implies that the M-cells are capable of a decision-making process that requires the two cells to receive differential directional inputs. Recent studies have indicated that the lateral line (LL) system is necessary in the initial directionality of the escape. This information is thought to be transmitted to the M-cell by the anterior branch of the lateral line nerve (aLLn), which has a shorter conduction time then the posterior branch. This study is the first attempt to characterize the inputs from the aLLn to the M-cell. M-cell intracellular responses to aLLn stimulation indicate a fast monosynaptic input (0.80±0.03 ms) that has a small amplitude averaging 5.85±0.42 mV. This input is bilateral and has a significantly longer latency and smaller amplitude in the contralateral M-cell. Superimposed on the evoked excitatory postsynaptic potential (EPSP) is a shunting inhibition with a delay of 1 ms, which is characteristic of other sensory inputs to the M-cell. Pharmacological manipulation and 50 Hz stimulation reveal a component of the evoked EPSP that is electrotonic, a property favoring speed of transmission. In addition, this input is localized to the lateral dendrite proximal to the inputs from the inner ear. The short latency of these inputs and their proximity to the posterior eighth nerve afferents indicate a crucial role for the aLLn in influencing the excitability and directionality of the M-cell.

Dopaminergic-induced changes in Mauthner cell excitability disrupt prepulse inhibition in the startle circuit of goldfish

Prepulse inhibition (PPI) is a widespread sensorimotor gating phenomenon characterized by a decrease in startle magnitude if a nonstartling stimulus is presented 20-1,000 ms before a startling stimulus. Dopaminergic agonists disrupt behavioral PPI in various animal models. This provides an important neuropharmacological link to schizophrenia patients that typically show PPI deficits at distinct (60 ms) prepulse-pulse intervals. Here, we study time-dependent effects of dopaminergic modulation in the goldfish Mauthner cell (M-cell) startle network, which shows PPI-like behavioral and physiological startle attenuations. The unique experimental accessibility of the M-cell system allows investigating the underlying cellular mechanism with physiological stimuli in vivo. Our results show that the dopaminergic agonist apomorphine (2 mg/kg body wt) reduced synaptic M-cell PPI by 23.6% (n = 18; P = 0.009) for prepulse-pulse intervals of 50 ms, whereas other intervals showed no reduction. Consistently, application of the dopamine antagonist haloperidol (0.4 mg/kg body wt) restored PPI to control level. Current ramp injections while recording M-cell membrane potential revealed that apomorphine acts through a postsynaptic, time-dependent mechanism by deinactivating a M-cell membrane nonlinearity, effectively increasing input resistance close to threshold. This increase is most pronounced for prepulse-pulse intervals of 50 ms (47.9%, n = 8; P < 0.05) providing a time-dependent, cellular mechanism for dopaminergic disruption of PPI. These results provide, for the first time, direct evidence of dopaminergic modulation of PPI in the elementary startle circuit of vertebrates and reemphasize the potential of characterizing temporal aspects of PPI at the physiological level to understand its underlying mechanisms.

Atypical properties of release and short-term depression at a specialized nicotinic synapse in the Mauthner cell network

Many synapses exhibit temporally complex forms of activity-dependent short-term synaptic plasticity. The diversity of these phenomena reflects the evolutionary specialization of synapses within networks. We examined the properties of transmission and plasticity, in vivo, at an identified, specialized axo-axonic nicotinic synapse between the goldfish Mauthner cell and one of its targets, the cranial relay neuron (CRN), using intracellular paired recordings and low frequency (0.33-2 Hz) train stimulations. Depression of successive excitatory postsynaptic potentials (EPSPs), which dominates short-term plasticity, had two components. A fast component reduced the amplitude of EPSP(2), to less than 50% of EPSP(1). A slow component produced an additional 10-30% of amplitude reduction and developed with a time constant of tens of seconds. The latencies of the later depressed responses were ∼0.1 ms longer than that of EPSP(1), suggesting a reduced release probability. The Ca(2+) chelators EGTA and BAPTA, injected presynaptically, reduced all EPSPs and slowed development of the second component of depression. Interestingly, spike broadening, produced by injecting K(+) channel blockers, reduced release, but accelerated the kinetics of the slow component. Finally, Ba(2+) in the external medium enhanced release, and reduced the first component and slowed the development of the second component of depression. Taken together, these last two results, which are in contrast to observations at other synapses, and the two-component depression suggest atypical release properties at the output synapses of the Mauthner cell, which triggers an escape behavior. We suggest that the second component of depression provides an additional safety factor to prevent repetitive firing of the CRN.

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.

Fast synaptic transmission in the goldfish CNS mediated by multiple nicotinic receptors

Usually nicotinic receptors in the central nervous system only influence the strength of a signal between neurons. At a few critical connections, for instance some of those involved in the flight response, nicotinic receptors not only modulate the signal, they actually determine whether a signal is conveyed or not. We show at one of the few such connections accessible for study, up to three different nicotinic receptor subtypes mediate the signal. The subtypes appear to be clustered in separate locations. Depending on the number and combination of the subtypes present the signal can range from short to long duration and from low to high amplitude. This provides a critical connection with a built-in plasticity and may enable it to adapt to a changing environment.

Functional specializations of primary auditory afferents on the Mauthner cells: interactions between membrane and synaptic properties

Primary auditory afferents are usually perceived as passive, timing-preserving, lines of communication. Contrasting this view, a special class of auditory afferents to teleost Mauthner cells, a command neuron that organizes tail-flip escape responses, undergoes potentiation of their mixed (electrical and chemical) synapses in response to high frequency cellular activity. This property is likely to represent a mechanism of input sensitization as these neurons provide the Mauthner cell with essential information for the initiation of an escape response. We review here the anatomical and physiological specializations of these identifiable auditory afferents. In particular, we discuss how their membrane and synaptic properties act in concert to more efficaciously activate the Mauthner cells. The striking functional specializations of these neurons suggest that primary auditory afferents might be capable of more sophisticated contributions to auditory processing than has been generally recognized.

Phase encoding in the Mauthner system: implications in left-right sound source discrimination

The paired teleost Mauthner (M)-cells and their associated network serve as an excellent system to study the biophysical basis of decision making. In teleosts, an abrupt sound evokes an M-spike, triggering a C-start escape that is usually directed away from a sound source. The response latency is minimized by electrical synapses between auditory afferents and the M-cell lateral dendrite. Here, we demonstrate that the electrical synapses also mediate phase encoding. Ramped sound pressure waves (150-250 Hz) evoked electrotonic postsynaptic potentials in the M-cell locked to two diametrically opposed phase angles that were frequency dependent but intensity independent. Phase encoding was also evident at the behavioral level underwater, because the stimuli evoked directional C-starts with an onset that was phase locked to the sound wave. In interneurons inhibitory to the M-cell, these same stimuli also evoked phase-locked electrotonic postsynaptic potentials and action potentials. The resulting chemical and electrical (i.e., field effect) inhibitions functioned tonically and phasically, respectively. Phase encoding could be important in underwater sound source localization, which is thought to require a neural computation involving a phase comparison between the pressure and the directional particle motion components of sound. This computation may be implemented by an interplay between phase-dependent afferent excitation and feedforward inhibition that activates the appropriate M-cell and directs the C-start away from the sound source.

Auditory input to CNS is acquired coincidentally with development of inner ear after formation of functional afferent pathway in zebrafish

Auditory perception in vertebrates depends on transduction of sound into neural signals in the inner ear hair cells (HCs) and on transmission of these signals to the brain through auditory (VIIIth) nerve afferents. To investigate the developmental acquisition of auditory inputs by the CNS, we have electrophysiologically and morphologically examined the process of acquisition of auditory responsiveness by zebrafish macular HCs and the Mauthner cells (M-cells) in vivo. The M-cells are a paired large reticulospinal neurons in the hindbrain; they receive direct inputs from the VIIIth nerve afferents and initiate an acoustic startle response. Whole-cell recordings from the M-cells showed that sound-evoked postsynaptic currents were first observed around 40 h postfertilization (hpf); during subsequent development, onset latency decreased and amplitude increased. The appearance and development of microphonic potentials in the inner ear coincided with those of the acoustic responses of the M-cell, whereas the functional auditory circuits from the macular HCs to the M-cell were already formed at 27 hpf. These results suggest that the functional maturation of inner ear after formation of the auditory pathway is a critical process in the acquisition of auditory inputs by CNS neurons.

A role of electrical inhibition in sensorimotor integration

Although it is accepted that extracellular fields generated by neuronal activity can influence the excitability of neighboring cells, whether this form of neurotransmission has a functional role remains open. In vivo field effects occur in the teleost Mauthner (M)-cell system, where a combination of structural features support the concept of inhibitory electrical synapses. A single spike in one M-cell evoked within as little as 2.2 ms of the onset of an abrupt sound, simulating a predatory strike, initiates a startle-escape behavior [Zottoli SJ (1977) J Exp Biol 66:243-254]. We show that such sounds produce synchronized action potentials in as many as 20 or more interneurons that mediate feed-forward electrical inhibition of the M-cell. The resulting action currents produce an electrical inhibition that coincides with the electrotonic excitatory drive to the M-cell; the amplitude of the peak of the inhibition is approximately 40% of that of the excitation. When electrical inhibition is neutralized with an extracellular cathodal current pulse, subthreshold auditory stimuli are converted into ones that produce an M-spike. Because the timing of electrical inhibition is often the same as the latency of M-cell firing in freely swimming fish, we conclude that electrical inhibition participates in regulating the threshold of the acoustic startle-escape behavior. Therefore, a field effect is likely to be essential to the normal functioning of the neural network.

Otolith endorgan input to the Mauthner neuron in the goldfish

The Mauthner (M-) cell of the goldfish, Carassius auratus, triggers the rapid escape response of the fish in response to various stimuli, including visual and auditory. The large size and accessibility of the M-cell make it an ideal model system for the study of synaptic transmission, membrane properties, and sensory-motor gating. Although physiological recordings have suggested that afferents from all three of the inner ear endorgans (the saccule, lagena, and utricle) synapse directly on the ipsilateral M-cell, the specific contacts and anatomical distributions of these inputs along the M-cell lateral dendrite remain unknown. We traced specific branches of the auditory (VIIIth) nerve from the three otolith organs of the fish inner ear to the M-cell. The goldfish sacculus gives rise to the vast majority of inputs that contact a large portion of the M-cell lateral dendrite, and these inputs vary greatly in size. In contrast to the ubiquitous distribution of saccular inputs, those from the lagena are segregated to distal regions of the M-cell and synapse on the distal dorsal branch of the lateral dendrite. Similarly, inputs from the utricle are also segregated to distal regions, synapsing on the ventral branch of the lateral dendrite. These results demonstrate that nerves from all three endorgans contact the M-cell, with input-specific segregation of synapses along the M-cell lateral dendrite.

Behavioral and physiological characterization of sensorimotor gating in the goldfish startle response

Prepulse inhibition (PPI) is typically associated with an attenuation of auditory startle behavior in mammals and is presumably mediated within the brainstem startle circuit. However, the inhibitory mechanisms underlying PPI are not yet clear. We addressed this question with complementary behavioral and in vivo electrophysiological experiments in the startle escape circuit of goldfish, the Mauthner cell (M-cell) system. In the behavioral experiments we observed a 77.5% attenuation (PPI) of startle escape probability following auditory prepulse-pulse stimulation. The PPI effect was observed for prepulse-pulse interstimulus intervals (ISIs) ranging from 20 to 600 ms and its magnitude depended linearly on prepulse intensity over a range of 14 dB. Electrophysiological recordings of synaptic responses to a sound pulse in the M-cell, which is the sensorimotor neuron initiating startle escapes, showed a 21% reduction in amplitude of the dendritic postsynaptic potential (PSP) and a 23% reduction of the somatic PSP following a prepulse. In addition, a prepulse evoked a long-lasting (500 ms) decrease in M-cell excitability indicated by 1) an increased threshold current, 2) an inhibitory shunt of the action potential (AP), and 3) by a linearized M-cell membrane, which effectively impedes M-cell AP generation. Comparing the magnitude and kinetics of inhibitory shunts evoked by a prepulse in the M-cell dendrite and soma revealed a disproportionately larger and longer-lasting inhibition in the dendrite. These results suggest that the observed PPI-type attenuation of startle behavior can be correlated to distinct postsynaptic mechanisms mediated primarily at the M-cell lateral dendrite.

Correlation of C-start behaviors with neural activity recorded from the hindbrain in free-swimming goldfish (Carassius auratus)

Startle behaviors in teleost fishes are well suited for investigations of mechanisms of sensorimotor integration because the behavior is quantifiable and much of the underlying circuitry has been identified. The teleost C-start is triggered by an action potential in one of the two Mauthner (M) cells. To correlate C-start behavior with electrophysiology, extracellular recordings were obtained from the surface of the medulla oblongata in the hindbrain,close to the M-axons, in freely swimming goldfish monitored using high-speed video. The recordings included action potentials generated by the two M-axons,as well as neighboring axons in the dorsal medial longitudinal fasciculus. Axonal backfills indicated that the latter originate from identifiable reticulospinal somata in rhombomeres 2-8 and local interneurons. Diverse auditory and visual stimuli evoked behaviors with kinematics characteristic of the C-start, and the amplitude of the first component of the hindbrain field potential correlated with the C-start direction. The onset of the field potential preceded that of the simultaneously recorded trunk EMG and movement initiation by 1.08±0.04 and 8.13±0.17 ms, respectively. A subsequent longer latency field potential was predictive of a counterturn. These results indicate that characteristic features of the C-start can be extracted from the neural activity of the M-cell and a population of other reticulospinal neurons in free-swimming goldfish.