Opioids: Bridging Pain Relief, Social Behavior, Emotion Regulation, and Traditional Healing
While the opioid system’s primary function in non-mammalian animals appears to be restricted to analgesia, in humans it is pivotal in pain relief and impacts a myriad of physiological and behavioral processes. The roles we’ll outline here include, but are not limited to, the following areas [1] [2]. Let’s break down its influence:
- Reproductive and Developmental Processes:
- Sexual activity and hormones
- Pregnancy
- Development and endocrinology
- Mental and Emotional Health:
- Mental illness
- Mood states
- Physiological Functions:
- Gastrointestinal, renal, and hepatic functions
- Cardiovascular responses
- Respiration and thermoregulation
- Neurological and Motor Activities:
- Seizures and neurologic disorders
- General activity and locomotion
- Behavioral Responses:
- Substance abuse
- Eating and drinking
- Tolerance and dependence
- Stress and social status
- Other Functions
- Immunological responses
Moreover, the μ-opioid receptor (MOR) system has been shown to interact with the dopamine system in brain regions involved in reward processing [3] [4] [5], as well as with oxytocin and dopamine in the context of social bonding and social reward processing [6].
As the protective effects of the opioid system seem to play a crucial role in promoting psychological and psychosomatic resilience[24], some researchers even argue that the role of endogenous opioids in pain modulation is of secondary importance to their function in behavioral fine-tuning [25].
Opioids and Animal Social Behavior
The opioid system has been shown to play a significant role in the social behaviors of animals. For example, studies have revealed that exogenous opioid agonists can alleviate separation distress in puppies [7], while opioid antagonists can lead to an increase in distress vocalization in chicks [17]. This suggests that opioids are involved in the comfort provided by social environments. Furthermore, the activation of μ receptors has been found to be essential for bonding in monogamous adult voles [8]. Additionally, the opioid system has been identified as one of the dominant systems responsible for the modulation of social play in rodents [9]. Overall, these findings highlight the importance of opioids in social behavior and suggest that targeting the opioid system may have potential therapeutic applications in promoting social bonding and alleviating social distress.
Opioids and Human Emotion Regulation
In humans, the involvement of the endogenous opioid system in affective processes and its role in regulating emotions is well-established through fMRI and PET studies. Notably, μ receptors are expressed abundantly in the brain circuits supporting emotional processing. Studies conclude that there is a significant convergence between the opioidergic system and emotion circuits [24].
Opioids in Clinical Anxiety and PTSD
Furthermore, in human, opioid agonists have been shown to alleviate clinical anxiety, while opioid antagonism may lead to reductions in feelings of social connection [13]. Studies on PTSD patients, including soldiers after combat injury in Iraq and children with burns suggest that opioid administration following acute trauma could decrease the likelihood of PTSD [10] [11] [12]. This may be achieved through inhibition of fear conditioning, down-regulation of sensory processing, or generalized sedation. Overall, these findings highlight the therapeutic potential of opioid agonists in treating anxiety-related disorders.
Brain Opioid Theory of Social Attachment (BOTSA)
The Brain Opioid Theory of Social Attachment (BOTSA) [13] [14] suggests that endogenous opioids are critical for primate and human bonding, including parental attachment, romantic attachment, and social affiliation. Social isolation is believed to reduce the levels of natural opioids in our body, driving us to seek social interactions. When we engage in social contact, our body releases these opioids, leading to feelings of happiness and satisfaction. Notably, these natural opioids, like β-endorphin which is linked to social interactions, break down quickly in the body and don’t cause tolerance. This means we constantly need to engage socially to maintain these positive feelings .
Extensive data from animal studies [15] [16] [17] [18] [19] [20] [21] have provided support for the Brain Opioid Theory of Social Attachment, showing that manipulations of the endogenous opioid system can affect social bonding behavior. Emerging human studies have also provided evidence to support this theory [22] [23].
Adaptive Responses in the Opioid System and Immune Impacts
Studies, using radioligand binding data, have suggested that when there is an alteration in the levels of presynaptically released endogenous opioids, the postsynaptic receptors may attempt to compensate by increasing the number of opioid receptors. This adaptive response helps maintain the overall balance of the endogenous opioid system in the body.
Historically deemed immunosuppressive, recent studies reveal a multifaceted influence as opioids might suppress, stimulate, or exert a dual effect on the immune system [33]. The complex relationship between opioids and immune function is crucial to understand, especially considering the immune system’s role in processes like inflammation, tumor progression, and drug abuse.
Acupuncture, Electroacupuncture and Endorphine response
One of acupuncture’s well-established mechanisms is its ability to stimulate the hypothalamus and pituitary gland, leading to the release of β-endorphins, the body’s natural painkillers. This fact has been affirmed by numerous clinical trials conducted on both humans and animals.
One of the earliest studies providing concrete evidence of this mechanism was conducted in 1978 by Peng et al. (PMID: 710079). In their research on rabbits, they found that extracts from the brains and serum of electro-acupunctured subjects, when injected into other rabbits, resulted in a significant increase in pain thresholds. This analgesic effect, intriguingly, was reversed by naloxone, an opiate antagonist, highlighting the pivotal role of endorphins in acupuncture’s pain-relieving properties.
Electro-acupuncture [28], laser acupuncture [29] and acupressure [30] can generate similar effects. Some animal studies have also demonstrated that electro-acupuncture (EA) not only promotes the release of endorphines but also upregulates endogenous endorphine receptors [31] [32].
Yet, the repeated application of EA has been found to elicit a significant desensitization of the μ-opioid receptors (MORs), a phenomenon also observed with prolonged use of morphine [26].
Moreover, the therapeutic effects of electroacupuncture (EA) are frequency-dependent, as different frequencies can activate different brain regions and release different neuropeptides [27]. For example, 2 Hz EA stimulates the release of enkephalin, β-endorphin, and endomorphin, whereas 100 Hz EA selectively increases the release of dynorphin. Combining the two frequencies may promote a simultaneous release of all four opioid peptides, that may result in a maximal therapeutic effect.
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