Neuroscience Research using Small Animals on a Chip: From Nematodes to Zebrafish Larvae

Optogenetic activation of hypothalamic AgRP neurons in transgenic zebrafish larvae increased food intake

Agouti Related Peptide (AgRP) neurons are located in the hypothalamus, and upon stimulation, these neurons regulate hunger and hunger-mediated behaviors, especially food-seeking and compulsive eating. AgRP neurons are naturally activated by ghrelin binding onto the ghrelin receptors on the neuron surface during starvation or fasting state to evoke the aforementioned behaviors. In this study, we used channelrhodopsin (ChR2), an optogenetic actuator, to control AgRP neuronal activity. For the first time, we observed food-intake behavior in zebrafish larvae by optogenetically triggering AgRP1 neurons. We created a transgenic line, Tg(AgRP1:ChR2-Kaede), where ChR2-Kaede is expressed in AgRP1 neurons. Transgenic zebrafish Tg(AgRP1:ChR2-Kaede) larvae at 6 days post fertilization and wild-type (ABWT) larvae were used to compare the suction behavior. We found that AgRP1 neuron activation in transgenic larvae led to a significantly higher food-consumption behavior than wildtype larvae when analyzed using Particle Image Velocimetry (PIV) to calculate the food particle velocity initiated by larval suction behavior. These findings in this novel transgenic zebrafish model would be useful in studying various hunger-related behaviors, their underlying neural circuits, and substrates subjected to different chemical stimuli, including drugs of abuse.

Brain stimulation-on-a-chip: a neuromodulation platform for brain slices

Electrical stimulation of ex vivo brain tissue slices has been a method used to understand mechanisms imparted by transcranial direct current stimulation (tDCS), but there are significant direct current electric field (dcEF) dosage and electrochemical by-product concerns in conventional experimental setups that may impact translational findings. Therefore, we developed an on-chip platform with fluidic, electrochemical, and magnetically-induced spatial control. Fluidically, the chamber geometrically confines precise dcEF delivery to the enclosed brain slice and allows for tissue recovery in order to monitor post-stimulation effects. Electrochemically, conducting hydrogel electrodes mitigate stimulation-induced faradaic reactions typical of commonly-used metal electrodes. Magnetically, we applied ferromagnetic substrates beneath the tissue and used an external permanent magnet to enable in situ rotational control in relation to the dcEF. By combining the microfluidic chamber with live-cell calcium imaging and electrophysiological recordings, we showcased the potential to study the acute and lasting effects of dcEFs with the potential of providing multi-session stimulation. This on-chip bioelectronic platform presents a modernized yet simple solution to electrically stimulate explanted tissue by offering more environmental control to users, which unlocks new opportunities to conduct thorough brain stimulation mechanistic investigations.

Generation of glial cell-derived neurotrophic factor (gdnf) morphants in zebrafish larvae by cerebroventricular microinjection of vivo morpholino

Dopaminergic neurons in the brain are an important source of dopamine, which is a crucial neurotransmitter for wellbeing, memory, reward, and motor control. Deficiency of dopamine due to advanced age and accumulative dopaminergic neuron defects can lead to movement disorders such as Parkinson's disease. Glial cell-derived neurotrophic factor (GDNF) is one of many factors involved in dopaminergic neuron development and/or survival. However, other endogenous GDNF functions in the brain await further investigation. Zebrafish is a well-established genetic model for neurodevelopment and neurodegeneration studies. Importantly, zebrafish shares approximately 70% functional orthologs with human genes including GDNF. To gain a better understanding on the precise functional role of gdnf in dopaminergic neurons, our laboratory devised a targeted knockdown of gdnf in the zebrafish larval brain using vivo morpholino. Here, detailed protocols on the generation of gdnf morphants using vivo morpholino are outlined. This method can be applied for targeting of genes in the brain to determine specific spatiotemporal gene function in situ.

Single-Shot Light-Field Microscopy: An Emerging Tool for 3D Biomedical Imaging

3D microscopy is a useful tool to visualize the detailed structures and mechanisms of biomedical specimens. In particular, biophysical phenomena such as neural activity require fast 3D volumetric imaging because fluorescence signals degrade quickly. A light-field microscope (LFM) has recently attracted attention as a high-speed volumetric imaging technique by recording 3D information in a single-snapshot. This review highlighted recent progress in LFM techniques for 3D biomedical applications. In detail, various image reconstruction algorithms according to LFM configurations are explained, and several biomedical applications such as neuron activity localization, live-cell imaging, locomotion analysis, and single-molecule visualization are introduced. We also discuss deep learning-based LFMs to enhance image resolution and reduce reconstruction artifacts.