Mu opioid receptors and analgesia at the site of a peripheral nerve injury

Antihyperalgesic effect of simultaneously released hydromorphone and bupivacaine from polymer fibers in the rat chronic constriction injury model

We aimed to evaluate the antihyperalgesic efficacy of a combination of hydromorphone (HM) and bupivacaine (BP) delivered via controlled release from a biodegradable cylindrical rod. In vivo studies were performed using a rat model of thermal hyperalgesia induced by chronic constriction injury (CCI) of the sciatic nerve with loose ligatures. Poly(lactic-co-glycolic acid) (PLGA) rods (10 mm length, 1 mm diameter) loaded with HM (5 mg per rod), BP (5 mg per rod) or no drug (placebo) were implanted subcutaneously, in single or dual pairs, adjacent to the constriction injury, immediately after nerve ligation. We evaluated the efficacy of two dose levels for each drug, alone or in combination, in attenuating thermal hyperesthesia over a period of 12 days according to a prevention protocol. Plasma levels of drugs released from the rods and also released in an in vitro simulation were evaluated. In vitro studies demonstrated that drug release is maintained for at least 10 days. HM (5 mg) alone and BP (5 mg) alone did not attenuate hyperalgesia. Their combination provided a significant increase in the paw withdrawal latency as compared to single agents or placebo. When the dose in each group was doubled, implanting four rods, significant attenuation of hyperalgesia was observed. Analyses of rods retrieved after termination of experiments (after 12 days) revealed 30% residual HM and 70% residual BP content. Prolonged delivery of HM and BP alone or in combination via locally applied PLGA rods may offer a feasible alternative to provide long-lasting analgesia.

Peripherally restricted opioid drugs: advances and retreats

PURPOSE OF REVIEW

The aim of this article is to familiarize anesthesiologists with recent research investigating peripheral opioid analgesia. The review focuses on studies of peripheral receptor distribution in humans and clinical data that support the hypothesis of peripheral analgesia that have been published over the past 2 years.

RECENT FINDINGS

Recent anatomical studies using human tissue have detected the presence of mu-opioid receptors in epidermal and dermal layers of normal skin, although expression was not altered in certain dermal clinical conditions. Nerve fibers of human dental pulp also contained detectable mu-opioid receptors. A survey of the clinical literature on peripherally mediated mu analgesia suggests that under conditions of inflammation, the onset of analgesia is typically delayed, but once initiated, is of long duration. A peripherally selective kappa agonist was efficacious in relieving the pain associated with chronic pancreatitis.

SUMMARY

Peripheral analgesia mediated through the mu-opioid receptor has been demonstrated in a variety of clinical settings, with the preponderance of data generated with arthroscopic procedures. Receptor localization studies using human tissue have corroborated the presence of the mu-opioid receptor in the periphery. Future research is needed to investigate the clinical utility of kappa and delta-opioid receptor agonists in the peripheral setting.

Attacking pain at its source: new perspectives on opioids

The treatment of severe pain with opioids has thus far been limited by their unwanted central side effects. Recent research promises new approaches, including opioid analgesics acting outside the central nervous system, targeting of opioid peptide-containing immune cells to peripheral damaged tissue, and gene transfer to enhance opioid production at sites of injury.