In a project jointly led by the Kunes and Engert laboratories, co-first authors Caroline Wee, Erin Song, and Maxim Nikitchenko used the larval zebrafish to take a deep dive into how social cues are represented in the brain and how those cues modulate behavior. Their results appear in a paper published this week in Nature Communications (PDF).
Social interactions are an essential aspect of human experience, and the absence of social stimuli often leads to a sense of loneliness, pain, and anxiety. Similarly, in animals, social isolation or stimulation have profound influences on behavior. The larval zebrafish is no different. However, when larval zebrafish perceive visual or mechanosensory stimuli indicating the presence of other unrelated fish, they tend to swim away, avoiding the social stimuli (e.g. Harpaz et al, 2021). However, larval zebrafish have been previously shown to swim near olfactory cues signaling the presence of kin (Gerlach and Wullimann 2021).
To elucidate how larval zebrafish respond to social cues, the researchers first exploited a brain-wide activity mapping method (Randlett et al. 2015). They found that many zebrafish brain regions that are activated by 2 hours of social isolation were also activated by painful or aversive stimuli One of these regions was a population of oxytocin-expressing neurons in the preoptic area, which they had previously shown to be an essential for the animal’s pain responses (Wee et al. 2019). In mammals, oxytocin neurons are a multi-sensory and multi-functional population. Beyond a conserved involvement in nociception, oxytocin is also a potent appetite suppressant. These oxytocin neurons may play a similar role in other vertebrates, such as zebrafish.
If social isolation triggers activity in pain-related oxytocinergic neurons, can the presence of other larval zebrafish shut these neurons down? To figure this out, the authors distinguished which social cues (i.e. seeing, smelling, or feeling the presence of others) might reduce aversive responses in the fish. Previous work from the Engert laboratory (Harpaz et al, 2021) found that larvae swim away from visual stimuli mimicking other larvae. However, results from others (Gerlach et al. 2008) suggested larvae might swim toward water that had previously contained conspecifics. The authors hypothesized that an olfactory cue could be involved. Interestingly, water containing the scent of other larvae, particularly that of siblings, reduced the activity of this pain-sensing circuit, whereas visual cues did not ameliorate the activity, showing that the zebrafish-scented water did indeed activate the olfactory bulb.The authors posited that intermediary neurons in the zebrafish forebrain might transform these olfactory kin cues into an inhibitory signal to oxytocin neurons. Together with Engert Lab postdoc Kristian Herrera, they identified neurons in the subpallium, directly downstream of the olfactory bulb, that did exactly this; in fact, artificially activating these subpallial neurons alone was sufficient to suppress oxytocin neuron activity.
Since oxytocin neurons drive pain-induced behaviors, the authors hypothesized that kin cues should then ameliorate such responses to pain. Indeed, when the researchers paired a noxious stimulus with kin olfactory cues, both oxytocin neuron activity and defensive responses were suppressed relative to when a control (plain water) cue was presented. This phenomenon, also known as “social buffering,” is widespread throughout the animal kingdom.
The authors also observed that temporarily socially-isolated zebrafish larvae also consumed less food on average than fish in a group, but when the fish were exposed to kin odors, they ate as much as the control fish. Indeed, ablation or inhibition of the oxytocin circuit paralleled the appetite-enhancing effect of kin odors in isolated fish. Similarly, oxytocin neuron stimulation suppressed feeding in groups.
Overall, these findings enabled the team to identify an olfactory-subpallial-oxytocinergic circuit that integrates social odor cues to control both appetite and avoidance behaviors. The oxytocin neurons appear to be central hubs that receive multiple cues and control multiple behaviors, allowing the zebrafish to respond appropriately to its social context. Even at young larval stages, zebrafish are particularly sensitive to the cues of genetically-related kin, demonstrating an early capacity for social recognition and preference.