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Given the evolutionary importance of social ties for survival, humans are thought to have evolved psychobiological mechanisms to monitor and safeguard the status of their social bonds. At the psychological level, self-esteem is proposed to function as a gauge—‘sociometer’—reflecting one’s social belongingness status. At the biological level, endogenous opioids appear to be an important substrate for the hedonic signalling needed to regulate social behaviour. We investigated whether endogenous opioids may serve as the biological correlate of the sociometer. We administered 50 mg naltrexone (an opioid receptor antagonist) and placebo in a counterbalanced order to 26 male and female participants on two occasions ∼1 week apart. Participants reported lower levels of self-esteem—particularly self-liking—on the naltrexone (vs placebo) day. We also explored a potential behavioural consequence of naltrexone administration: attentional bias to accepting (smiling) faces—an early-stage perceptual process thought to maximize opportunities to restore social connection. Participants exhibited heightened attentional bias towards accepting faces on the naltrexone (vs placebo) day, which we interpret as an indicator of heightened social need under opioid receptor blockade. We discuss implications of these findings for understanding the neurobiological underpinnings of sociality as well as the relationship between adverse social conditions, low self-esteem and psychopathology.

Music’s universality and its ability to deeply affect emotions suggest an evolutionary origin. Previous investigators have found that naltrexone (NTX), a μ-opioid antagonist, may induce reversible anhedonia, attenuating both positive and negative emotions. The neurochemical basis of musical experience is not well-understood, and the NTX-induced anhedonia hypothesis has not been tested with music. Accordingly, we administered NTX or placebo on two different days in a double-blind crossover study, and assessed participants’ responses to music using both psychophysiological (objective) and behavioral (subjective) measures. We found that both positive and negative emotions were attenuated. We conclude that endogenous opioids are critical to experiencing both positive and negative emotions in music, and that music uses the same reward pathways as food, drug and sexual pleasure. Our findings add to the growing body of evidence for the evolutionary biological substrates of music.

Empathy is thought to be unique to higher primates, possibly to humans alone. We report the modulation of pain sensitivity in mice produced solely by exposure to their cagemates, but not to strangers, in pain. Mice tested in dyads and given an identical noxious stimulus displayed increased pain behaviors with statistically greater co-occurrence, effects dependent on visual observation. When familiar mice were given noxious stimuli of different intensities, their pain behavior was influenced by their neighbor's status bidirectionally. Finally, observation of a cagemate in pain altered pain sensitivity of an entirely different modality, suggesting that nociceptive mechanisms in general are sensitized.

Amiloride is a nonspecific blocker of acid-sensing ion channels (ASICs) that have been recently implicated in the mediation of mechanical and chemical/inflammatory nociception. Preliminary data using a transgenic model are suggestive of sex differences in the role of ASICs. We report here that systemic administration of amiloride (10–70 mg/kg ip) produces a robust, dose-dependent blockade of late/tonic phase nociceptive behavior on the mouse formalin test (5%; 20 μl) in female but not male mice, completely abolishing the known sex difference in formalin test response. Adult gonadectomy produced a “switching” of sex differences in amiloride efficacy, with castrated males displaying an amiloride blockade and ovariectomized females rendered less sensitive to amiloride. Gonadectomized mice could be switched back to their intact status using chronic estrogen benzoate or testosterone propionate replacement via osmotic minipump (6 μg/day or 250 μg/day, respectively). It is unclear whether this striking sex difference is due to sex-specific involvement of ASICs in pain processing, but the present data represent one of the first demonstrations of pain-related sex differences with no obvious opioid involvement.