Effects of oxytocin-family peptides and substance P on locomotor activity and filial preferences in visually naïve chicks

Nonapeptides from the vasopressin/oxytocin family have been implicated in a wide variety of social behaviours across vertebrates. Experimental manipulations that alter nonapeptide levels or receptor function in the brain have provided evidence for understanding how nonapeptides influence responses to social stimuli in adults. While behaviours in adults have been extensively studied, much less in known about roles of nonapeptides in early life and the development of affiliative social behaviours. We examined an experience-independent preference (social predisposition) that is present at hatching and is characterized by the tendency of visually naïve chicks (Gallus gallus) to prefer to approach a stuffed hen stimulus over a control stimulus in a choice test. Among chicks that show the social predisposition preference, bilateral intracranial mesotocin injections resulted in higher mean hen preference scores compared with saline-injected controls. Equimolar doses of mesotocin and vasotocin injections had different effects on locomotor activity: vasotocin, but not mesotocin, resulted in hypoactivity. We also tested whether intraperitoneal substance P had an effect on hen preference scores because previous research has proposed that vasotocin effects on social approach are mediated by peripheral release of substance P, but found no significant effect. All together, our data suggest that mesotocin signalling may be important for social predispositions and can potentially enhance the perceived salience of social stimuli soon after hatching. Specifically, mesotocin release and signalling in the brain may regulate the ability to recognize naturalistic stimuli and/or to act on the motivation to approach naturalistic stimuli.