Opioids and Motivation

Abstract

Emotions, experienced subjectively, appear enigmatic and inaccessible to scientific analysis, but affect many concrete psychological phenomena, such as pain sensitivity and reactions to every-day experiences. Recent evidence suggests that opioid peptides, in particular β-endorphin, may influence motivational processes by affecting emotions. Opioid peptides are found in the nervous system and peripheral tissues and produce effects similar to those of opioid drugs. β-Endorphin, whose action has a long duration, may function as a hormone and as a neuromodulator, rather than as a classical neurotransmitter. It is released by stress and causes hyperthermia and an increase in threshold pain. Endogenous opioids also stimulate food and water intake. Biochemical and behavioural experiments indicate that endogenous opioids, in particular β-endorphin, are released in purely rewarding situations. β-Endorphin may stimulate intake by supporting positive emotional reactions; in addition, it reduces negative reactions to aversive stimuli. This is consistent with the fact that β-endorphin produces positive emotional changes and that the aversiveness of antagonists, which reflects opioid blockade, is eliminated by destruction of the β-endorphin system in the brain. Such an emotional effect would also explain behavioural changes, for example, a decreased sensitivity to noxious stimulation and an increase in exploration tendencies. In nature, such changes might aid in foraging and in the formation of positive associations between environment and reward. Opioids lower the threshold for rewarding electrical stimulation to the brain. They also alter the response to sensory input and modify the initiation of motor activity. Naturally released opioids may, through effects on sensory processing, enable sensory input to excite more easily reward systems. Consistent with such a general effect are reports of opioid influences on several forms of motivated behaviour other than food and water intake. Some behaviours which are stimulated by aversive stimuli, such as shock-induced fighting and the need for closeness in social animals, are inhibited by opioids, whereas inter-male aggression is potentiated. Current results suggest that opioids suppress sexuality, perhaps through an interaction with hormones. Opioid effects on learning and memory can probably be explained in terms of their emotional actions.