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During many behaviors in vertebrates, the CNS generates asymmetric activities between the left and right sides to produce asymmetric body movements. For asymmetrical activations of the CNS, reciprocal inhibition between the left and right sides is believed to play a key role. However, the complexity of the CNS makes it difficult to identify the reciprocal inhibition circuits at the level of individual cells and the contribution of each neuron to the asymmetric activity. Using larval zebrafish, we examined this issue by investigating reciprocal inhibition circuits between a pair of Mauthner (M) cells, giant reticulospinal neurons that trigger fast escapes. Previous studies have shown that a class of excitatory neurons, called cranial relay neurons, is involved in the reciprocal inhibition pathway between the M cells. Using transgenic fish, in which two of the cranial relay neurons (Ta1 and Ta2) expressed GFP, we showed that Ta1 and Ta2 constitute major parts of the pathway. In larvae in which Ta1/Ta2 were laser-ablated, the amplitude of the reciprocal IPSPs dropped to less than one-third. Calcium imaging and electrophysiological recording showed that the occurrence probability of bilateral M-cell activation upon sound/vibration stimuli was greatly increased in the Ta1/Ta2-ablated larvae. Behavioral experiments revealed that the Ta1/Ta2 ablation resulted in shallower body bends during sound/vibration-evoked escapes, which is consistent with the observation that increased occurrence of bilateral M-cell activation impaired escape performance. Our study revealed major components of the reciprocal inhibition circuits in the M cell system and the behavioral importance of the circuits. [ABSTRACT FROM AUTHOR]

Segmental organization along the neuraxis is a prominent feature of the CNS in vertebrates. In a wide range of fishes, hindbrain segments contain orderly arranged reticulospinal neurons (RSNs). Individual RSNs in goldfish and zebrafish hindbrain are morphologically identified. RSNs sharing similar morphological features are called segmental homologs and repeated in adjacent segments. However, little is known about functional relationships among segmental homologs. Here we investigated the electrophysiological connectivity between the Mauthner cell (M-cell), a pair of giant RSNs in segment 4 (r4) that are known to trigger fast escape behavior, and different series of homologous RSNs in r4 -r6. Paired intracellular recordings in adult goldfish revealed unidirectional connections from the M-cell to RSNs. The connectivity was similar in morphological homologs. A single M-cell spike produced IPSPs in dorsally located RSNs (MiD cells) on the ipsilateral side and excitatory postsynaptic depolarization on the contralateral side, except for MiD2cm cells. The inhibitory or excitatory potentials effectively suppressed or enhanced target RSNs spiking, respectively. In contrast to the lateralized effects on MiD cells, single M-cell spiking elicited equally strong depolarizations on bilateral RSNs located ventrally (MiV cells), and the depolarization was high enough for MiV cells to burst. Therefore, the morphological homology of repeated RSNs in r4 -r6 and their functional M-cell connectivity were closely correlated, suggesting that each functional connection works as a functional motif during the M-cellinitiated escape. [ABSTRACT FROM AUTHOR]

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 Ca2+ 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. [ABSTRACT FROM AUTHOR]

Although behavior is ultimately guided by decision-making neurons and their associated networks, the mechanisms underlying neural decision-making in a behaviorally relevant context remain mostly elusive. To address this question, we analyzed goldfish escapes in response to distinct visual looming stimuli with high-speed video and compared them with electrophysiological responses of the Mauthner cell (M-cell), the threshold detector that initiates such behaviors. These looming stimuli evoke powerful and fast body-bend (C-start) escapes with response probabilities between 0.7 and 0.91 and mean latencies ranging from 142 to 716 ms. Chronic recordings showed that these C-starts are correlated with M-cell activity. Analysis of response latency as a function of the different optical parameters characterizing the stimuli suggests response threshold is closely correlated to a dynamically scaled function of angular retinal image size, θ(t), specifically κ(t) = θ(t = δ x e -βθ(t - δ), where the exponential term progressively reduces the weight of θ(t). Intracellular recordings show that looming stimuli typically evoked bursts of graded EPSPs with peak amplitudes up to 9 mV in the M-cell. The proposed scaling function κ(t) predicts the slope of the depolarizing envelope of these EPSPs and the timing of the largest peak. An analysis of the firing rate of presynaptic inhibitory interneurons suggests the timing of the EPSP peak is shaped by an interaction of excitatory and inhibitory inputs to the M-cell and corresponds to the temporal window in which the probabilistic decision of whether or not to escape is reached. [ABSTRACT FROM AUTHOR]

In the hindbrain of zebrafish and goldfish, reticulospinal (RS) neurons are arranged in seven segments, with segmental homologs in adjacent segments. The Mauthner cell (M-cell) in the fourth segment (r4) is known to trigger fast escape behavior. Its serial homologs, MiD2cm in r5 and MiD3cm in r6, are predicted to contribute to this behavior, which can be evoked by head-tap stimuli. However, little is known about their input- output properties. Therefore, we studied afferent projections from the auditory posterior eighth nerve (pVIIIn) and firing properties of MiD2cm and MiD3cm for comparison with the M-cell in adult goldfish. Labeling of RS neurons and the pVIIIn afferents with fluorescent tracers showed that the pVIIIn projected to r4 -r6. Tone burst and electrical stimulation of the pVIIIn evoked EPSPs in the M-cell, MiD2cm, and MiD3cm. Stepwise depolarization typically elicited a single spike at the onset in the M-cell but repetitive spiking in MiD2cm and MiD3cm. This atypical property of the M-cell was mediated by dendrotoxin-I (DTX-I)-sensitive voltage-gated potassium channels together with recurrent inhibition, because combined application of DTX-I, strychnine, and bicuculline led to continuous repetitive firing in M-cells. The M-cell but not MiD2cm or MiD3cm expressed Kv1.2, a DTX-I-sensitive potassium channel subunit. Thus, the M-cell and its segmental homologs may sense common auditory information but send different outputs to the spinal circuits to control adaptive escape behavior. [ABSTRACT FROM AUTHOR]

Hox genes are critical for establishing the segmental pattern of the vertebrate hindbrain. Changes in their expression can alter neural organization of hindbrain segments and may be a mechanism for brain evolution. To test the hypothesis that neurons induced through changes in Hox gene expression can integrate into functional neural circuits, we examined the roles of ectopic Mauthner cells (M-cells) in the escape response of larval zebrafish. The activity of the paired Mauthner cells in rhombomere 4 (r4) has been shown to be critical for generating a high-performance startle behavior in response to stimulation of the tail (Liu and Fetcho,1999). Previous studies have found that misexpression of particular Hox genes causes ectopic M-cells to be generated in r2 in addition to the r4 cells (Alexandra et al., 1996; McClintock et al., 2001). With calcium imaging, we found that the homeotically transformed neurons respond to startle stimuli. To determine the roles of ectopic and endogenous M-cells in the behavior, we lesioned the r2, r4, or both M-cells with cell-specific laser lesion and examined the effect on startle performance. Lesion of the normal M-cells did not decrease escape performance when the ectopic cells were present. These results indicate that the homeotically transformed Mauthner cells are fully functional in the escape circuit and are functionally redundant with normal M-cells. We suggest that such functional redundancy between neurons may provide a substrate for evolution of neural circuits. [ABSTRACT FROM AUTHOR]