Opioid system and human emotions

Abstract

Emotions are states of vigilant readiness that guide human and animal behaviour during survival-salient situations. Categorical models of emotions posit neurally and physiologically distinct basic human emotions (anger, fear, disgust, happiness, sadness and surprise) that govern different survival functions. Opioid receptors are expressed abundantly in the mammalian emotion circuit, and the opioid system modulates a variety of functions related to arousal and motivation. Yet, its specific contribution to different basic emotions has remained poorly understood. Here, we review how the endogenous opioid system and particularly the μ receptor contribute to emotional processing in humans. Activation of the endogenous opioid system is consistently associated with both pleasant and unpleasant emotions. In general, exogenous opioid agonists facilitate approach-oriented emotions (anger, pleasure) and inhibit avoidance-oriented emotions (fear, sadness). Opioids also modulate social bonding and affiliative behaviour, and prolonged opioid abuse may render both social bonding and emotion recognition circuits dysfunctional. However, there is no clear evidence that the opioid system is able to affect the emotions associated with surprise and disgust. Taken together, the opioid systems contribute to a wide array of positive and negative emotions through their general ability to modulate the approach versus avoidance motivation associated with specific emotions. Because of the protective effects of opioid system-mediated prosociality and positive mood, the opioid system may constitute an important factor contributing to psychological and psychosomatic resilience.

Linked articles: This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Figure 1

Theoretical models of emotions. The most widely accepted view considers emotions as discrete functional systems, so that each of them is specialized for a different survival function (Ekman, 1992; Panksepp, 1982). Dimensional models of emotion in turn explain behavioural, physiological and subjective similarity across emotions by lower‐order dimensions or systems (typically one for pleasure‐displeasure and another for calmness‐arousal), whose relative activity may generate different patterns of emotionality (Russell, 1980). Both types of models assume, at least implicitly, that approach‐avoidance motivation is an important property of all emotional states, partially independent of pleasure and arousal (Elliot et al., 2013). Face stimuli are reprinted with permission from (Lundqvist et al., 1988).

Figure 2

Overlap between the human emotion circuit and the μ receptor system. (A) Distribution of μ receptors in the human brain. Mean non‐displaceable binding potential (BP _ND) image of 89 PET scans from healthy volunteers with the MOR‐selective agonist radioligand [¹¹C]‐carfentanil. (B) Brain regions responding consistently (forward inference) during emotional experience and perception in fMRI studies in NeuroSynth meta‐analysis (n _studies = 1609, P < 0.05 false discovery rate corrected) overlaid on the mean distribution of μ receptors as measured by [¹¹C]‐carfentanil PET.

Figure 3

Opioidergic responses to emotions measured with PET. ALE meta‐analysis map shows that limbic components of the emotion–motivation circuit show consistent opioidergic responses during emotional stimulation. The data are thresholded at P < 0.05, false discovery rate corrected.