Otolith endorgan input to the Mauthner neuron in the goldfish

Vestibular physiology and function in zebrafish

The vestibular system of the inner ear provides information about head motion and spatial orientation relative to gravity to ensure gaze stability, balance, and postural control. Zebrafish, like humans, have five sensory patches per ear that serve as peripheral vestibular organs, with the addition of the lagena and macula neglecta. The zebrafish inner ear can be easily studied due to its accessible location, the transparent tissue of larval fish, and the early development of vestibular behaviors. Thus, zebrafish are an excellent model for studying the development, physiology, and function of the vestibular system. Recent work has made great strides to elucidate vestibular neural circuitry in fish, tracing sensory transmission from receptors in the periphery to central computational circuits driving vestibular reflexes. Here we highlight recent work that illuminates the functional organization of vestibular sensory epithelia, innervating first-order afferent neurons, and second-order neuronal targets in the hindbrain. Using a combination of genetic, anatomical, electrophysiological, and optical techniques, these studies have probed the roles of vestibular sensory signals in fish gaze, postural, and swimming behaviors. We discuss remaining questions in vestibular development and organization that are tractable in the zebrafish model.

Serum calcium levels are associated with cognitive function in hypoparathyroidism: a neuropsychological and biochemical study in an Italian cohort of patients with chronic post-surgical hypoparathyroidism

PURPOSE

Hypoparathyroidism (HypoPT) is a rare endocrine disease and conventional therapy is based on calcium and vitamin D analogues. Conventional therapy does not restore calcium homeostasis and patients complain with neuropsychological symptoms, which have been evaluated with nonspecific self-administered questionnaires. This study aims to evaluate cognitive functions of patients with chronic post-surgical (PS)-HypoPT compared to a control population, using a standardized neuropsychological approach and evaluating the relationship with serum calcium (Alb-Ca).

METHODS

Observational, monocentric study on 33 patients with PS-HypoPT and 24 controls, in whom biochemical testing and a standardized neuropsychological assessment by a trained psychologist were performed.

RESULTS

In patients with PS-HypoPT, low Alb-Ca correlated with a worse performance on semantic memory abilities and executive function, as suggested by a significant inverse correlation between Alb-Ca and Trail Making Test A (TMT-A) scores (r = - 0.423; p = 0.014) and by a positive correlation with Semantic Fluency Test scores (SF)(r = 0.510; p = 0.002). PS-HypoPT patients with Alb-Ca ≤ 8.9 mg/dl had a significantly lower test performance compared with PS-HypoPT patients with Alb-Ca > 8.9 mg/dl, both at the TMT-A test (mean score: 34.53-18.55; p < 0.0001) and at SF test (mean score: 41.94-48.68; p = 0.01) and also a significantly lower test performance compared with control patients' group at TMT-A (mean score: 34.53-25.5; p = 0.0057).

CONCLUSIONS

Patients with chronic PS-HypoPT in conventional therapy do not show a severe cognitive impairment; however, cognitive functions namely visuo-spatial attention, executive function and semantic memory appear to be modulated by Alb-Ca and impaired by its low levels.

Multimodal mechanosensing enables treefrog embryos to escape egg-predators

Mechanosensory-cued hatching (MCH) is widespread, diverse and important for survival in many animals. From flatworms and insects to frogs and turtles, embryos use mechanosensory cues and signals to inform hatching timing, yet mechanisms mediating mechanosensing in ovo are largely unknown. The arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, hatch prematurely to escape predation, cued by physical disturbance in snake attacks. When otoconial organs in the developing vestibular system become functional, this response strengthens, but its earlier occurrence indicates another sensor must contribute. Post-hatching, tadpoles use lateral line neuromasts to detect water motion. We ablated neuromast function with gentamicin to assess their role in A. callidryas' hatching response to disturbance. Prior to vestibular function, this nearly eliminated the hatching response to a complex simulated attack cue, egg jiggling, revealing that neuromasts mediate early MCH. Vestibular function onset increased hatching, independent of neuromast function, indicating young embryos use multiple mechanosensory systems. MCH increased developmentally. All older embryos hatched in response to egg jiggling, but neuromast function reduced response latency. In contrast, neuromast ablation had no effect on the timing or level of hatching in motion-only vibration playbacks. It appears only a subset of egg-disturbance cues stimulate neuromasts; thus, embryos in attacked clutches may receive unimodal or multimodal stimuli. Agalychnis callidryas embryos have more neuromasts than described for any other species at hatching, suggesting precocious sensory development may facilitate MCH. Our findings provide insight into the behavioral roles of two mechanosensory systems in ovo and open possibilities for exploring sensory perception across taxa in early life stages.

Coordinated Expression of Two Types of Low-Threshold K+ Channels Establishes Unique Single Spiking of Mauthner Cells among Segmentally Homologous Neurons in the Zebrafish Hindbrain

Expression of different ion channels permits homologously-generated neurons to acquire different types of excitability and thus code various kinds of input information. Mauthner (M) series neurons in the teleost hindbrain consist of M cells and their morphological homologs, which are repeated in adjacent segments and share auditory inputs. When excited, M cells generate a single spike at the onset of abrupt stimuli, while their homologs encode input intensity with firing frequency. Our previous study in zebrafish showed that immature M cells burst phasically at 2 d postfertilization (dpf) and acquire single spiking at 4 dpf by specific expression of auxiliary Kvβ2 subunits in M cells in association with common expression of Kv1.1 channels in the M series. Here, we further reveal the ionic mechanisms underlying this functional differentiation. Pharmacological blocking of Kv7/KCNQ in addition to Kv1 altered mature M cells to fire tonically, similar to the homologs. In contrast, blocking either channel alone caused M cells to burst phasically. M cells at 2 dpf fired tonically after blocking Kv7. In situ hybridization revealed specific Kv7.4/KCNQ4 expression in M cells at 2 dpf. Kv7.4 and Kv1.1 channels expressed in Xenopus oocytes exhibited low-threshold outward currents with slow and fast rise times, while coexpression of Kvβ2 accelerated and increased Kv1.1 currents, respectively. Computational models, modified from a mouse cochlear neuron model, demonstrated that Kv7.4 channels suppress repetitive firing to produce spike-frequency adaptation, while Kvβ2-associated Kv1.1 channels increase firing threshold and decrease the onset latency of spiking. Altogether, coordinated expression of these low-threshold K⁺ channels with Kvβ2 functionally differentiates M cells among homologous neurons.

Mechanosensory Lateral Line Nerve Projections to Auditory Neurons in the Dorsal Descending Octaval Nucleus in the Goldfish, Carassius auratus

The nucleus medialis is the main first-order target of the mechanosensory lateral line (LL) system. This report definitively demonstrates that mechanosensory LL inputs also terminate in the ipsilateral dorsal portion of the descending octaval nucleus (dDO) in the goldfish. The dDO, which is the main first-order auditory nucleus in bony fishes, includes neurons that receive direct input from the otolithic end organs of the inner ear and project to the auditory midbrain. There are two groups of such auditory projection neurons: medial and lateral. The medial and the lateral groups in turn contain several neuronal populations, each of which includes one or more morphological cell types. In goldfish, the exclusively mechanosensory anterior and posterior LL nerves terminate only on specific cell types of auditory projection neurons in the lateral dDO group. Single neurons in the lateral dDO group may receive input from both anterior and posterior LL nerves. It is possible that some of the lateral dDO neurons that receive LL input also receive input from one or more of the otolithic end organs. These results are consistent with functional studies demonstrating low frequency acoustic sensitivity of the mechanosensory LL in teleosts, and they reveal that the anatomical substrate for sensory integration of otolithic and LL inputs is present at the origin of the central ascending auditory pathway in an otophysine fish.

Sensory hair cell death and regeneration in fishes

Sensory hair cells are specialized mechanotransductive receptors required for hearing and vestibular function. Loss of hair cells in humans and other mammals is permanent and causes reduced hearing and balance. In the early 1980’s, it was shown that hair cells continue to be added to the inner ear sensory epithelia in cartilaginous and bony fishes. Soon thereafter, hair cell regeneration was documented in the chick cochlea following acoustic trauma. Since then, research using chick and other avian models has led to great insights into hair cell death and regeneration. However, with the rise of the zebrafish as a model organism for studying disease and developmental processes, there has been an increased interest in studying sensory hair cell death and regeneration in its lateral line and inner ears. Advances derived from studies in zebrafish and other fish species include understanding the effect of ototoxins on hair cells and finding otoprotectants to mitigate ototoxin damage, the role of cellular proliferation vs. direct transdifferentiation during hair cell regeneration, and elucidating cellular pathways involved in the regeneration process. This review will summarize research on hair cell death and regeneration using fish models, indicate the potential strengths and weaknesses of these models, and discuss several emerging areas of future studies.

Electrical synaptic transmission in developing zebrafish: properties and molecular composition of gap junctions at a central auditory synapse

In contrast to the knowledge of chemical synapses, little is known regarding the properties of gap junction-mediated electrical synapses in developing zebrafish, which provide a valuable model to study neural function at the systems level. Identifiable "mixed" (electrical and chemical) auditory synaptic contacts known as "club endings" on Mauthner cells (2 large reticulospinal neurons involved in tail-flip escape responses) allow exploration of electrical transmission in fish. Here, we show that paralleling the development of auditory responses, electrical synapses at these contacts become anatomically identifiable at day 3 postfertilization, reaching a number of ∼6 between days 4 and 9. Furthermore, each terminal contains ∼18 gap junctions, representing between 2,000 and 3,000 connexon channels formed by the teleost homologs of mammalian connexin 36. Electrophysiological recordings revealed that gap junctions at each of these contacts are functional and that synaptic transmission has properties that are comparable with those of adult fish. Thus a surprisingly small number of mixed synapses are responsible for the acquisition of auditory responses by the Mauthner cells, and these are likely sufficient to support escape behaviors at early developmental stages.

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.

Coexpression of auxiliary Kvβ2 subunits with Kv1.1 channels is required for developmental acquisition of unique firing properties of zebrafish Mauthner cells

Each neuron possesses a unique firing property, which is largely attributed to heterogeneity in the composition of voltage-gated ion channel complexes. Zebrafish Mauthner (M) cells, which are bilaterally paired giant reticulospinal neurons (RSNs) in the hindbrain and induce rapid escape behavior, generate only a single spike at the onset of depolarization. This single spiking is in contrast with the repetitive firing of the M cell's morphologically homologous RSNs, MiD2cm and MiD3cm, which are also involved in escapes. However, how the unique firing property of M cells is established and the underlying molecular mechanisms remain unclear. In the present study, we first demonstrated that the single-spiking property of M cells was acquired at 4 days postfertilization (dpf), accompanied by an increase in dendrotoxin I (DTX)-sensitive low-threshold K(+) currents, prior to which the M cell repetitively fires as its homologs. Second, in situ hybridization showed that among DTX-sensitive Kv1 channel α-subunits, zKv1.1a was unexpectedly expressed even in the homologs and the bursting M cells at 2 dpf. In contrast, zKvβ2b, an auxiliary β-subunit of Kv1 channels, was expressed only in the single-spiking M cells. Third, zKv1.1a expressed in Xenopus oocytes functioned as a low-threshold K(+) channel, and its currents were enhanced by coexpression of zKvβ2b subunits. Finally, knockdown of zKvβ2b expression in zebrafish larvae resulted in repetitive firing of M cells at 4 dpf. Taken together, these results suggest that associative expression of Kvβ2 subunits with Kv1.1 channels is crucial for developmental acquisition of the unique firing properties of the M cells among homologous neurons.

Role of the lateral line mechanosensory system in directionality of goldfish auditory evoked escape response

Goldfish (Carassius auratus) escape responses to sudden auditory stimuli are mediated by a pair of reticulospinal neurons, the Mauthner (M-) cells, which integrate mechanosensory inputs from the inner ear and the lateral line (LL) to initiate a fast directional response away from the aversive stimulus. This behavior is context dependent; when near an obstruction the fish may rather turn towards the sound to avoid hitting the object. Mechanisms underlying this directionality remain unknown. Here we investigate the contribution of the LL system to auditory evoked escapes and provide behavioral evidence that it transmits stimulus - and environmental-dependent information that determines the initial response direction of the escape. We quantified escape latency, probability and directionality following abrupt sound stimuli before and after removal of the entire LL with 0.03 mmol l(-1) cobalt chloride (CoCl(2)), 0.002% gentamicin or selective posterior LL nerve (pLLn) transection. CoCl(2) significantly increased escape onset latency without affecting probability and reduced open field directionality from 77% to chance, 52%. This effect on directionality was also observed with gentamicin. Transection of the pLLn had no effect on directionality, indicating the anterior LL nerve (aLLn) afferents are more likely to transmit directional information to the M-cell. When the fish were near a wall, the error rate was quadrupled by both CoCl(2) and pLLn transection. Visual elimination had no influence on directionality unless combined with LL elimination.

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.

Activation of a multisensory, multifunctional nucleus in the zebrafish midbrain during diverse locomotor behaviors

Action potentials from the brain control the activity of spinal neural networks to produce, by as yet unknown mechanisms, a variety of motor behaviors. Particularly lacking are details on how identified descending neurons integrate diverse sensory inputs to generate specific locomotor patterns. We have examined the operations of the principal neurons in an intriguing midbrain nucleus, the nucleus of the medial longitudinal fasciculus (nMLF), in the larval zebrafish. The nMLF is the most rostral grouping of neurons that projects from the brain well into the spinal cord of teleost fishes, yet there is little direct physiological data available regarding its function. We report here that a distinct set of large, individually-identifiable neurons in nMLF (the MeL and MeM neurons) are activated by diverse sensory stimuli and contribute to distinct locomotor behaviors. Using in vivo confocal calcium imaging we observed that both photic and mechanical stimuli elicit calcium responses indicative of the firing of action potentials. Calcium responses were observed simultaneously with distinct swimming, turning and struggling movements of the larval trunk. While selectively contralateral responses were at times observed in response to a head-tap stimulus, these nMLF cells showed roughly similar numbers of bilateral responses. Calcium responses were observed at a range of latencies, suggesting involvement with both slow swimming patterns and the burst swimming component of the escape behavior. The MeL cells in particular were strongly activated during light-evoked slow swimming. The activation of MeL cells during the slow and burst forward swim gaits is consistent with their driving and/or coordinating the activity of slow and fast central pattern generators in spinal cord. As such, the MeL cells may help to shape a variety of larval behaviors including the optomotor response, escape swimming and prey capture.

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.

Initiation of Mauthner- or non-Mauthner-mediated fast escape evoked by different modes of sensory input

Brainstem reticulospinal neurons (RSNs) serve as the major descending system in vertebrate sensorimotor integration. One of the paired RSNs in zebrafish, the Mauthner (M) cell, is thought to initiate fast escape from sudden noxious stimuli. Two other paired RSNs, morphologically homologous to the M-cell, are also suggested to play key roles in controlling fast escape. However, the relationship among activities of the M-cell and its homologs during fast escape and the sensory inputs that elicit escape via their activation are unclear. We have monitored hindbrain RSN activity simultaneously with tail flip movement during fast escape in zebrafish. Confocal calcium imaging of RSNs was performed on larvae rostrally embedded in agar but with their tails allowed to move freely. Application of a pulsed waterjet to the otic vesicle (OV) to activate acousticovestibular input elicited contralateral fast tail flips with short latency and an apparent Ca(2+) increase, reflecting a single action potential, in the ipsilateral M-cell (M-escape). Application of waterjet to head skin for tactile stimulation elicited fast escapes, but onset was delayed and the M-cell did not fire (non-M-escape). After eliminating either the M-cell or OV, only non-M-escape was initiated. Simultaneous high-speed confocal imaging of the M-cell and one of its homologs, MiD3cm, revealed complementary activation during fast escape: MiD3cm activity was low during M-escape but high during non-M-escape. These results suggest that M-cell firing is necessary for fast escape with short latency elicited by acousticovestibular input and that MiD3cm is more involved in non-M-escape driven by head-tactile input.

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.