Songbirds, passerine, have species-specific and individually unique songs and sing for courtship behavior, territorial defense, and individual identification. Like human infants acquire language-specific vocalization by listening to parents’ speech, juveniles of songbirds develop species-specific songs by matching their immature vocalizations to songs memorized in early development gradually with vocal motor learning. Expected to shed light on infants’ vocal learning, neural mechanisms of song learning have been investigated intensively, but little is known how neural circuit balances competing criteria, individual uniqueness and species-specificity. A songbird, zebra finches, are social breeder. Their juveniles listen to multiple conspecific songs and develop individually unique own songs. I review development of their individually unique and species-universal songs and neural activities in auditory area supporting the song acquisition in early development with discussion of open questions to be answered in this field.

Humans receive multiple environmental stimuli by sensory neurons and transmit their information to brain. The nervous system in brain integrates and discriminates their information, and sends appropriate instructions to various tissues in the body. However, molecular physiological mechanisms underlying integration and discrimination of multiple signals in nervous system remain poorly understood. Here, we review that cold acclimation in C. elegans can be useful experimental model for studying the neuronal circuit integrating two different environmental factors, temperature and oxygen. C. elegans can appropriately adapt to environmental change by integrating and distinguishing multiple sensory information in neuronal circuit consisting of only 302 neurons. Recently, we have found KQT-type potassium channel (KQT-2) is involved in cold acclimation. Interestingly, kqt-2 mutant showed stronger abnormal cold acclimation when they cultivated at higher oxygen concentration. The temperature-sensing neuron expressing KQT-2 potassium channel has connection from an oxygen-sensing neuron. Ca²⁺ imaging analysis suggested that the neuronal activity of the temperature-sensing neuron is modulated by oxygen signaling. We have proposed that molecular mechanisms and the simple neural circuit integrating two different sensory information.

Escape behaviors are crucial to survive predator encounters or aversive stimuli. The neural circuits mediating escape reactions of different animals have a common framework to trigger extremely fast and robust movement with minimum delay. Thus, the escape networks possibly represent functional architectures to perform most efficient sensory-motor processing in the brain. Here we review escape behaviors and underlying circuits of squid, crayfish, fruit fly, zebrafish and rodent. The escape circuits of these animals involve giant neurons, or also called as giant fibers or giant axons, to initiate fast escape. Without activation of the giant neurons, the animals can do escape or similar behaviors typically in response to less precious threads, but they are delayed and much slower than fast escape initiated by the giant neurons. Therefore, fast and slow escape circuits are built, probably in parallel, in the brain and the giant neurons play a key role to induce fast escape to avoid imminent danger. We also discuss why the giant neurons are built in the fast escape circuits by introducing their advantage to collect sensory information and to send ballistic motor output as fast as possible.

Animals are required to process enormous and complex olfactory stimuli for their survival and reproductive success. Information processing along parallel pathways is a common feature of insect and mammal olfactory systems, suggesting that parallel subsystems are an effective way of fast and reliable olfactory processing. In this review, I give an overview of insect olfactory systems and introduce the structure of parallel olfactory processing streams of the American cockroach Periplaneta americana. In the cockroach, parallel nature of olfactory pathways has been known from the periphery to the higher-order center, the mushroom body. In addition, recently, simultaneous intracellular recordings/stainings from mushroom body output neurons and giant inhibitory neurons suggested parallel feedback pathways in the mushroom body. Odor-evoked responses of secondary olfactory neurons suggest that one of the parallel pathways processes odor-specific aspects while the other processes odor timing and concentration. The parallel feedback pathways may provide different forms of negative feedback for different classes of mushroom body intrinsic neurons, which process and represent different parameters of olfactory information.

Molecular and cellular basis of sexual dimorphism in the insect brain has been extensively studied in the fruit fly Drosophila melanogaster, an excellent model insect with powerful genetics. Studies in Drosophila have revealed that sexual dimorphism in the Drosophila brain is regulated under the control of the sex-specific splicing cascade and sex-specific gene products of two transcription factor genes, fruitless and doublesex. In the Drosophila brain, the neurons expressing either (or both) of the genes form sexually dimorphic circuits that regulate sex-specific behaviors. Recent studies on fruitless and doublesex homologs in basal direct-developing insects support the idea that the molecular mechanism underlying the formation of sexually dimorphic circuits in Drosophila is not ubiquitous in insects. In this review, I summarize the molecular basis of neural sex determination in Drosophila, and recent findings in the evolution of sex-determination genes in non-Drosophila insects.