Animals initiate behavior not only reflexively in response to external stimuli but also voluntarily depending on their internal state. What are the neuronal mechanisms that subserve voluntary initiation of behavior? I have identified brain neurons involved in voluntary walking of crayfish, Procambarus clarkii. I found brain neurons whose spike activities increased>1 sec before the spontaneous behavioral onset. This activity is indicative of readiness or preparatory activities in the vertebrate brain that precede the onset of voluntary actions. Therefore, I termed this activity as ‘readiness discharge’. Readiness discharge neurons were found to be silent when walking was initiated by external stimuli, but other neurons were specifically activated for the stimulus-evoked walking. Readiness discharge was not associated with any specific direction of walking. Other walking direction-specific neurons were activated after the readiness discharge units were activated, suggesting hierarchical control for the spontaneous initiation of walking from its general aspects to more specifics. Furthermore, I found brain neurons whose activities increased during walking and others that were activated at the termination of walking. Thus, it was revealed that voluntary walking is controlled by descending interneurons that are organized for behavioral initiation, continuation and termination in a hierarchical way. Further intracellular studies revealed that readiness discharge are shaped by sequential excitatory and inhibitory synaptic inputs rather than by endogenous excitability changes. The synaptic activation is most likely to take place in the medial protocerebrum in the crayfish brain. I propose that recurrent neural circuits subserve the activation.

View full abstract

Sea hares are slow moving animals that have soft body with vestigial shell. Since it is slow and soft, it potentially has high predation risk. However, not many predators attack sea hares and consume them in the field and laboratory environments. How sea hare can defend themselves? “Diet derived chemical defense” has been a hypothetical answer. Actually, the sea hares prefer chemically rich algae. Many researchers have tried to support this “Diet derived chemical defense” hypothesis with well designed experiments; however, the results have been ambiguous. The other hypothetical defense was “defense by inking”. Sea hares release purple secretion when they were attacked by predators. This inking might work as smoke screen by providing a visual obstacle or as deterrent to provide chemical defense. This hypothesis has been anecdotal for many years. However, recently, multiple mechanisms of the chemical defense by inking have been revealed that inking involves two types of secretions, purple ink and white opaline. The ink and opaline contain deterrent molecules which involves seaweed derived secondary metabolites and an enzyme-substrate mixture. The ink and opaline also contains appetitive molecules. These defensive molecules work on chemosensory systems of predators. The ink and opaline works not only against predators but also to conspecifics as alarm cue. This articles reviews recent studies on diet derived chemical defense in their body and ink of sea hares.

View full abstract

We have developed an experimental system for simultaneously recording the locomotor behavior and the neural activities from freely walking insects. The system consists of a motion-tracking subsystem, a motor-driven rotating connector (an active slip ring subsystem), and conventional extracellular recording devices. A pair of linear motion guides is orthogonally arranged to form a 2-D coordinate system above the experimental arena. The head stage of the 2-D guide is equipped with a CCD camera and the active slip ring subsystem. When an animal is detected in the arena by the CCD camera, a programmable logic controller (PLC) sends a feedback signal to the servomotors for the head stage so that the insect is always captured at the center of the image. Thus, the CCD camera continuously follows the animal, and then, the system measures two behavioral parameters, namely, the 2-D coordinates of the animal’s position within the arena and the orientation of the body axis. Electrical signals from neurons are fed to a conventional bio-electrical amplifier through the active slip ring subsystem. The tangling of electrode wires, which is necessarily caused when the animal turns, is automatically compensated for by the active slip ring subsystem. The new system successfully recorded the locomotor and brain neuron activities of field crickets within 12 h.

View full abstract

[in Japanese]

View full abstract