A cell type–specific cortico-subcortical brain circuit for investigatory and novelty-seeking behavior

A brain circuit that drives and gates curiosity Curiosity is what drives organisms to investigate each other and their environment. It is considered by many to be as intrinsic as hunger and thirst, but the neurobiological mechanisms behind curiosity have remained elusive. In mice, Ahmadlou et al. found that a specific population of genetically identified γ-aminobutyric acid (GABA)—ergic neurons in a brain region called the zona incerta receive excitatory input in the form of novelty and/or arousal information from the prelimbic cortex, and these neurons send inhibitory projections to the periaqueductal gray region (see the Perspective by Farahbakhsh and Siciliano). This circuitry is necessary for the exploration of new objects and conspecifics. Science, this issue p. eabe9681; see also p. 684 A subpopulation of inhibitory neurons in the zona incerta drives mice to investigate objects and companions. INTRODUCTION Motivational drives are internal states that can be different even in similar interactions with external stimuli. Curiosity as the motivational drive for novelty-seeking and investigating the surrounding environment is for survival as essential and intrinsic as hunger. Curiosity, hunger, and appetitive aggression drive three different goal-directed behaviors—novelty seeking, food eating, and hunting—but these behaviors are composed of similar actions in animals. This similarity of actions has made it challenging to study novelty seeking and distinguish it from eating and hunting in nonarticulating animals. The brain mechanisms underlying this basic survival drive, curiosity, and novelty-seeking behavior have remained unclear. RATIONALE In spite of having well-developed techniques to study mouse brain circuits, there are many controversial and different results in the field of motivational behavior. This has left the functions of motivational brain regions such as the zona incerta (ZI) still uncertain. Not having a transparent, nonreinforced, and easily replicable paradigm is one of the main causes of this uncertainty. Therefore, we chose a simple solution to conduct our research: giving the mouse freedom to choose what it wants—double free-access choice. By examining mice in an experimental battery of object free-access double-choice (FADC) and social interaction tests—using optogenetics, chemogenetics, calcium fiber photometry, multichannel recording electrophysiology, and multicolor mRNA in situ hybridization—we uncovered a cell type–specific cortico-subcortical brain circuit of the curiosity and novelty-seeking behavior. RESULTS We analyzed the transitions within action sequences in object FADC and social interaction tests. Frequency and hidden Markov model analyses showed that mice choose different action sequences in interaction with novel objects and in early periods of interaction with novel conspecifics compared with interaction with familiar objects or later periods of interaction with conspecifics, which we categorized as deep and shallow investigation, respectively. This finding helped us to define a measure of depth of investigation that indicates how much a mouse prefers deep over shallow investigation and reflects the mouse’s motivational level to investigate, regardless of total duration of investigation. Optogenetic activation of inhibitory neurons in medial ZI (ZIm), ZImGAD2 neurons, showed a dramatic increase in positive arousal level, depth of investigation, and duration of interaction with conspecifics and novel objects compared with familiar objects, crickets, and food. Optogenetic or chemogenetic deactivation of these neurons decreased depth and duration of investigation. Moreover, we found that ZImGAD2 neurons are more active during deep investigation as compared with during shallow investigation. We found that activation of prelimbic cortex (PL) axons into ZIm increases arousal level, and chemogenetic deactivation of these axons decreases the duration and depth of investigation. Calcium fiber photometry of these axons showed no difference in activity between shallow and deep investigation, suggesting a nonspecific motivation. Optogenetic activation of ZImGAD2 axons into lateral periaqueductal gray (lPAG) increases the arousal level, whereas chemogenetic deactivation of these axons decreases duration and depth of investigation. Calcium fiber photometry of these axons showed high activity during deep investigation and no significant activity during shallow investigation, suggesting a thresholding mechanism. Last, we found a new subpopulation of inhibitory neurons in ZIm expressing tachykinin 1 (TAC1) that monosynaptically receive PL inputs and project to lPAG. Optogenetic activation and deactivation of these neurons, respectively, increased and decreased depth and duration of investigation. CONCLUSION Our experiments revealed different action sequences based on the motivational level of novelty seeking. Moreover, we uncovered a new brain circuit underlying curiosity and novelty-seeking behavior, connecting excitatory neurons of PL to lPAG through TAC1+ inhibitory neurons of ZIm. Brain mechanism of curiosity. (A) How we mapped motivational level to action sequences. (B) Experimental battery to distinguish novelty-seeking behavior from food eating and hunting in mice with photoactivation of ZImGAD2 neurons. (C) Schematic of calcium activity in PL→ZIm, ZIm, and ZIm→PAG during shallow and deep investigation. (D) TAC1+ neurons as a subpopulation of ZImGAD2 neurons receive input from PL and project to PAG. HMM, hidden Markov model. Exploring the physical and social environment is essential for understanding the surrounding world. We do not know how novelty-seeking motivation initiates the complex sequence of actions that make up investigatory behavior. We found in mice that inhibitory neurons in the medial zona incerta (ZIm), a subthalamic brain region, are essential for the decision to investigate an object or a conspecific. These neurons receive excitatory input from the prelimbic cortex to signal the initiation of exploration. This signal is modulated in the ZIm by the level of investigatory motivation. Increased activity in the ZIm instigates deep investigative action by inhibiting the periaqueductal gray region. A subpopulation of inhibitory ZIm neurons expressing tachykinin 1 (TAC1) modulates the investigatory behavior.