Tazu Aoki

The habenula is crucial for experience-dependent modification of fear responses in zebrafish

The zebrafish dorsal habenula (dHb) shows conspicuous asymmetry in its connection with the interpeduncular nucleus (IPN) and is equivalent to the mammalian medial habenula. Genetic inactivation of the lateral subnucleus of dHb (dHbL) biased fish towards freezing rather than the normal flight response to a conditioned fear stimulus, suggesting that the dHbL-IPN pathway is important for controlling experience-dependent modification of fear responses.

Identification of the Zebrafish Ventral Habenula As a Homolog of the Mammalian Lateral Habenula

The mammalian habenula consists of the medial and lateral habenulae. Recent behavioral and electrophysiological studies suggested that the lateral habenula plays a pivotal role in controlling motor and cognitive behaviors by influencing the activity of dopaminergic and serotonergic neurons. Despite the functional significance, manipulating neural activity in this pathway remains difficult because of the absence of a genetically accessible animal model such as zebrafish. To address the level of lateral habenula conservation in zebrafish, we applied the tract-tracing technique to GFP (green fluorescent protein)-expressing transgenic zebrafish to identify habenular neurons that project to the raphe nuclei, a major target of the mammalian lateral habenula. Axonal tracing in live and fixed fish showed projection of zebrafish ventral habenula axons to the ventral part of the median raphe, but not to the interpeduncular nucleus where the dorsal habenula projected. The ventral habenula expressed protocadherin 10a, a specific marker of the rat lateral habenula, whereas the dorsal habenula showed no such expression. Gene expression analyses revealed that the ventromedially positioned ventral habenula in the adult originated from the region of primordium lateral to the dorsal habenula during development. This suggested that zebrafish habenulae emerge during development with mediolateral orientation similar to that of the mammalian medial and lateral habenulae. These findings indicated that the lateral habenular pathways are evolutionarily conserved pathways and might control adaptive behaviors in vertebrates through the regulation of monoaminergic activities.

The Habenulo-Raphe Serotonergic Circuit Encodes an Aversive Expectation Value Essential for Adaptive Active Avoidance of Danger

Anticipation of danger at first elicits panic in animals, but later it helps them to avoid the real threat adaptively. In zebrafish, as fish experience more and more danger, neurons in the ventral habenula (vHb) showed tonic increase in the activity to the presented cue and activated serotonergic neurons in the median raphe (MR). This neuronal activity could represent the expectation of a dangerous outcome and be used for comparison with a real outcome when the fish is learning how to escape from a dangerous to a safer environment. Indeed, inhibiting synaptic transmission from vHb to MR impaired adaptive avoidance learning, while panic behavior induced by classical fear conditioning remained intact. Furthermore, artificially triggering this negative outcome expectation signal by optogenetic stimulation of vHb neurons evoked place avoidance behavior. Thus, vHb-MR circuit is essential for representing the level of expected danger and behavioral programming to adaptively avoid potential hazard.

Social conflict resolution regulated by two dorsal habenular subregions in zebrafish

How to win a fish fight When to cease aggression and escape is an important decision that fighting animals must make. Chou et al. characterized the role of two nuclei in a brain area of the zebrafish called the dorsal habenula (dHb) during social aggression (see the Perspective by Desban and Wyart). Silencing the lateral dHb reduced the likelihood of winning a fight, whereas silencing the medial dHb increased the likelihood of winning. Thus, these two nuclei antagonistically control the threshold for surrender. Science, this issue p. 87; see also p. 42 The neuronal basis for keeping the aggression of fighting fish in check is elucidated. [Also see Perspective by Desban and Wyart] When animals encounter conflict they initiate and escalate aggression to establish and maintain a social hierarchy. The neural mechanisms by which animals resolve fighting behaviors to determine such social hierarchies remain unknown. We identified two subregions of the dorsal habenula (dHb) in zebrafish that antagonistically regulate the outcome of conflict. The losing experience reduced neural transmission in the lateral subregion of dHb (dHbL)–dorsal/intermediate interpeduncular nucleus (d/iIPN) circuit. Silencing of the dHbL or medial subregion of dHb (dHbM) caused a stronger predisposition to lose or win a fight, respectively. These results demonstrate that the dHbL and dHbM comprise a dual control system for conflict resolution of social aggression.

Imaging of Neural Ensemble for the Retrieval of a Learned Behavioral Program

The encoding of long-term associative memories for learned behaviors is a fundamental brain function. Yet, how behavior is stably consolidated and retrieved in the vertebrate cortex is poorly understood. We trained zebrafish in aversive reinforcement learning and measured calcium signals across their entire brain during retrieval of the learned response. A discrete area of dorsal telencephalon that was inactive immediately after training became active the next day. Analysis of the identified area indicated that it was specific and essential for long-term memory retrieval and contained electrophysiological responses entrained to the learning stimulus. When the behavioral rule changed, a rapid spatial shift in the functional map across the telencephalon was observed. These results demonstrate that the retrieval of long-term memories for learned behaviors can be studied at the whole-brain scale in behaving zebrafish in vivo. Moreover, the findings indicate that consolidated memory traces can be rapidly modified during reinforcement learning.

Visualization of zebrafish striatum direct and indirect pathway projection neurons

The prefrontal cortex is credited with contributing to relational reasoning, or the ability to integrate multiple acquired associations to generate new relationships. We have recorded single-unit activity from the lateral prefrontal cortex (LPFC) and the striatum while the monkeys performed a sequential paired-association task with asymmetric reward schedule. In the task, the monkeys learned two sequences of associated stimuli: A1-B1-C1 and A2-B2C2. The asymmetric reward rule was instructed by pairing C1 (or C2) with large (or small) reward block by block. The monkey also learned associations between new stimuli (e.g. N1, N2) and B1 and B2. The new stimuli were presented as the first cue in sequential paired-association trials instead of the old stimuli (A1 and A2). The findings from single-unit activity suggest that the LPFC can use an internal model of category to transfer reward information associated with one stimulus to other stimuli, even to new stimuli that had never been paired with different amount of reward. The striatum only uses direct experience between conditioned stimuli and reward to predict reward. One prediction from this hypothesis is that if the LPFC is inactivated, the monkey still correctly predicts reward for old stimuli through the striatal pathway, but has deficits in predicting reward for new stimuli. We injected muscimol to locally inactivate the LPFC, and also saline into the LPFC as control. In saline sessions, the monkey had significantly higher choice accuracy for new stimuli in large than in small reward trials, but this difference disappeared in muscimol session, consistent with the prediction. Together with single-unit activity data, our results suggest that the LPFC play a critical role in category-based reward inference.

Activation of distinct neural ensemble in zebrafish telencephalon following the go/no-go rule change in the goal directed active avoidance learning

-amyloid precursor protein (APP) in enlarged early endosomes. However, it remains unclear how endocytic dysfunction is induced in an age-dependent manner. We have previously shown that the interaction between dyneindynactin complexes is clearly attenuated in aged monkey brains, suggesting that dynein-mediated transport dysfunction exists in aged brains. Thus, in the present study, we assessed our hypothesis that the dysfunction of dyneinmediated transport would be responsible for endocytic dysfunction leading to AD pathology. First, we examined immunohistochemistry and westernblot by using young and aged monkey brains to investigate age-related endocytic pathology. Immunohistochemical and westernblot analyses revealed that age-dependent endocytic pathology was accompanied by an increase in Rab GTPases in aged monkey brains. We also examined RNAi studies to assess whether dynein dysfunction can reproduce endocytic pathology as seen in aged monkey brains. Evidently, we demonstrated that siRNA-induced dynein dysfunction reproduced the endocytic pathology accompanied by increased Rab GTPases seen in aged monkey brains. Moreover, it also resulted in endosomal APP accumulation characterized by increased -site cleavage. These findings suggest that dynein dysfunction may underlie age-dependent endocytic dysfunction via the upregulation of Rab GTPases, leading to AD pathology.

Optical imaging analysis of neural activity of zebrafish telencephalon in the goal directed behavior

O3-G2-5 Genetic inactivation of the habenulo-interpeduncular projection enhances the conditioned fear response in zebrafish Masakazu Agetsuma1, Hidenori Aizawa1, Tazu Aoki1, Mikako Takahoko1, Ryoko Nakayama1, Toshiyuki Shiraki1, Midori Goto1, Koichi Kawakami2, Shin-ichi Higashijima3, Hitoshi Okamoto1 1 RIKEN BSI, Japan; 2 National Institute of Genetics, Japan; 3 Okazaki Institute for Integrative Bioscience, Japan

Genetic inactivation of the habenulo-interpeduncular projection enhances the conditioned fear response in zebrafish

O3-G2-5 Genetic inactivation of the habenulo-interpeduncular projection enhances the conditioned fear response in zebrafish Masakazu Agetsuma1, Hidenori Aizawa1, Tazu Aoki1, Mikako Takahoko1, Ryoko Nakayama1, Toshiyuki Shiraki1, Midori Goto1, Koichi Kawakami2, Shin-ichi Higashijima3, Hitoshi Okamoto1 1 RIKEN BSI, Japan; 2 National Institute of Genetics, Japan; 3 Okazaki Institute for Integrative Bioscience, Japan