Dopamine modulates a variety of animal behaviors that range from sleep and learning to courtship and aggression. Besides its well-known phasic firing to natural reward, a substantial amount of mid-brain dopamine neurons (DANs) are known to exhibit ongoing intrinsic activity in the absence of an external stimulus. While accumulating evidence points at functional implications for these intrinsic “spontaneous activities” of DANs in cognitive processes such as learning, a causal link to behavior and its underlying mechanisms has yet to be elucidated. It has recently been shown that DANs of the model organism Drosophila melanogaster are also spontaneously active, and state-of-art physiological approaches have uncovered that this activity reflects the behavioral/internal states of the animal. Strikingly, genetic manipulation of basal DAN activity resulted in behavioral alterations in the animal, providing critical evidence that links spontaneous DAN activity to behavioral states. Furthermore, circuit level analyses have started to reveal cellular and molecular mechanisms that mediate or regulate spontaneous DAN activity. Through reviewing recent findings in different animals with the major focus on flies, we will discuss potential roles of this physiological phenomenon in directing animal behaviors.

Soil unicellular eukaryotes such as the ciliate Colpoda successively adapt to a terrestrial environment by promptly transforming into a resting cyst resistant to desiccation, freezing, high temperatures, UV light or acid, before the water puddles in which the vegetative cells proliferate dry out. In this review, we show the signaling pathways leading to the encystment of Colpoda cucullus Nag-1 which are triggerred by Ca²⁺, and discuss the roles of cAMP-dependent phosphorylated proteins and differentially expressed proteins during the cyst formation.

In an unpredictable environment with varied food sources, animals must appropriately regulate the amount and type of food they consume. Indeed, hunger, a motivational state representing the physiological need for food, is not a unitary phenomenon, but rather can exist individually for specific nutrients, such as salt or protein. The shifting of food preference driven by nutrient-specific hunger can be essential for survival1, yet little is known about the mechanisms underlying this process. Here, we identify a small subset of dopaminergic neurons that mediate protein-specific hunger in Drosophila. These neurons are both necessary and sufficient for the homeostatic drive to consume protein. The activity of these neurons is increased following protein deprivation. Activation of these neurons not only promote protein intake, but also restrict sugar consumption, and perform both functions by signaling to distinct downstream neurons via discrete terminal branches. Remarkably, protein deprivation triggers plastic changes in the protein-specific, but not sugar-specific, branch of these dopaminergic neurons, thus enabling selective and sustained consumption of protein.