The Analgesic Effect of Morphine on Peripheral Opioid Receptors: An Experimental Research

Opioids constitute a class of pharmacological compounds that play a crucial role in pain management. However, despite being among the most potent and versatile analgesics currently available, our understanding of pharmacokinetics and pharmacodynamic properties remains incomplete, with various aspects of their interaction with the human body requiring further research. Although highly effective in alleviating severe pain, the widespread use of opioids is limited by the high incidence of serious side effects including respiratory depression, dependency, tolerance, nausea, and constipation [1]. These negative consequences can be severe and pose significant risks to patient health. Additionally, long-term opioid use may negatively impact quality of life and daily functioning [2, 3].

In the late 1980s, studies began to highlight that the action of opioids is not limited solely to opioid receptors in the brain and spinal cord, but also affects peripheral sensory neurons [4]. Research has shown that intrinsic modulation of pain sensations can occur at the level of peripheral nerve endings as well [5, 6]. The peripheral analgesic effects of opioids have been more evident in pathophysiological conditions such as inflammation, tissue injuries, or neuropathy [7, 8]. During inflammation, the permeability of the blood-brain barrier increases, and the expression of opioid receptors on peripheral sensory neurons is upregulated. This facilitates the interaction of opioids with their receptors at the site of injury, enhancing their analgesic effects [9]. Tissue damage results in the release of inflammatory mediators and an increase in opioid receptors in the peripheral nervous system. This localized upregulation allows opioids to exert their pain-relieving effects more effectively in the injured area [10]. In neuropathic pain, changes in the nervous system, such as altered receptor expression and increased release of endogenous opioids, enhance the peripheral action of exogenous opioids. These changes make peripheral opioid receptors more responsive to analgesic treatment [11].

Considering the experiments conducted over time, we aimed to assess whether locally administered morphine generates a local analgesic effect and to determine the time point after the onset of inflammation at which this effect is most pronounced.

The current study protocol was approved by the Ethics Committee of “Carol Davila” University of Medicine and Pharmacy, Bucharest, Romania (7590/22065) for studies involving animals, in conformity with 43/2014 Law regarding animal protection used in scientific purposes, with further completions and 86/609/CEE Directive from 24 November 1986 regarding acts with power of law and administrative acts of member states for animal protection used in experimental purposes and other scientific purposes.

Initially, it was believed that opioids exerted analgesic effects exclusively through actions within the central nervous system [13]. However, in recent decades, evidence has emerged suggesting that opioid antinociception can be initiated by activating opioid receptors located outside the central nervous system [14]. One of the earliest findings supports the notion that morphine could elicit analgesic effects upon local application to painful peripheral areas. Since then, numerous clinical and experimental reports have been published confirming similar observations [15].

Peripheral opioid receptors are located outside the central nervous system that interact with opioid substances. They are classified into peripheral mu (MOR), delta (DOR), and kappa (KOR) receptors [16]. Their main role is to modulate sensations of pain and inflammation outside the brain and spinal cord. Activation of peripheral opioid receptors can produce analgesic effects, reducing the sensation of pain and inflammation in those areas [17]. Although initial attempts to demonstrate peripheral opioid analgesia in unaffected tissues yielded controversial results, subsequent studies conducted by Stein and his research team in pathological pain models were more successful and showed that the local injection of low, systemically inactive doses of agonists produced dose-dependent, stereospecific, and reversible analgesia through selective opioid antagonists [18]. Additionally, antinociception has been demonstrated in models of neuropathic, visceral, incisional, thermal, bone, and cancer pain. According to in vitro studies, the co-administration of agonists may act synergistically [19].

The peripheral analgesic effects of opioids are particularly evident in inflamed tissue. Under such conditions, the synthesis and expression of opioid receptors in dorsal root ganglia are increased [20, 21]. According to a review by researcher Stein, interleukin IL-4 and activator protein-1 are involved in stimulating the transcription of opioid receptors [22]. Furthermore, inflammation in peripheral tissues can amplify the synthesis and expression of these receptors, facilitating their axonal transport and increasing agonistic efficacy at nerve terminals. Inflammation may also increase the number of sensory nerve terminals and disrupt the perineural barrier, thereby promoting the access of opioid agonists to their receptors [23]. Clinical studies indicate that the application of opioid agonists to un-injured nerves is not reliably effective in producing analgesic effects, suggesting that inflammation promotes the accessibility and efficient coupling of opioid receptors in primary afferent neurons [24, 25]. Additionally, the secretion of endogenous opioid ligands in inflamed tissues may contribute to synergistic interactions at peripheral opioid receptors [26].

Our study showed that the limb withdrawal time was significantly higher in rats treated with intraplantar morphine compared to those in the control group. The study observed variations in the effectiveness of morphine depending on the timing of treatment post-carrageenan administration. We must consider that in the carrageenan-induced inflammation model, the peak level of inflammation is reached 3–5 hours after administration [27]. Carrageenan-induced local inflammation exhibits an acute phase (within the first 3 hours) dominated by the release of pro-inflammatory mediators such as histamine, serotonin, and bradykinin [28]. This is followed by an intermediate phase lasting up to 6 hours, where leukocyte infiltration and the release of large amounts of prostaglandins predominate. After 24 hours, the inflammation is characterized by a subacute phase with intense cellular infiltration [29].

These changes likely explain why the analgesic effect is most pronounced 3 hours after carrageenan administration, a time when the inflammation is actively progressing with the release of prostaglandins and other inflammatory cytokines. Additionally, this phase may also be marked by increased sensitivity of peripheral neurons due to the higher expression of opioid receptors at this stage of inflammation [30].

This highlights the importance of considering the temporal aspect of treatment in assessing analgesic efficacy. While the study focused on evaluating the analgesic effect of morphine, future research could delve into the underlying mechanisms responsible for its efficacy in inflammatory pain.

However, it is important to acknowledge certain limitations of the study. While the carrageenan-induced inflammation model is widely used and well-characterized, it may not fully replicate the complexity of clinical inflammatory pain conditions. Moreover, the study primarily focused on assessing the analgesic effects of morphine and did not delve into the underlying molecular mechanisms involved.

Locally administered morphine (intraplantar) demonstrates a significant analgesic effect in a carrageenan-induced inflammation model in rats, with its efficacy varying depending on the timing of administration following inflammation induction. The most pronounced analgesic effects are observed when morphine is administered immediately after carrageenan injection and at 3 hours post-administration, a period corresponding to the peak of inflammation characterized by the release of prostaglandins and other inflammatory cytokines. These findings suggest that locally administered morphine could be a promising option for pain relief in inflammatory conditions, with the timing of administration being crucial to maximizing its effectiveness. The enhanced sensitivity of peripheral neurons due to increased opioid receptor expression during this phase further supports the importance of precise timing in achieving optimal analgesic outcomes.