Tonic pain perception in the mouse: differential modulation by three receptor-selective opioid agonists

Area of Science:

• Pharmacology
• Neuroscience
• Pain Research

Background:

• Tonic nociception models may better represent clinical pain than acute models.
• Endogenous opioid systems play a role in pain modulation.
• A modified mouse paw formalin test was developed to study tonic pain.

Purpose of the Study:

• To investigate the modulatory roles and sites of action of endogenous opioid systems in tonic pain.
• To evaluate the antinociceptive activity of specific opioid receptor agonists (mu, kappa, delta) in a modified formalin test.

Main Methods:

• Utilized receptor-preferring agonists: sufentanil (mu), U-50,488H (kappa), and [D-Pen2,5]enkephalin (DPDPE, delta).
• Administered agonists systemically, spinally, and supraspinally in a modified mouse paw formalin test.
• Determined antinociceptive activity by measuring the reduction in paw licking, calculating A50 values for effective doses.

Main Results:

• Kappa and mu opioid receptor agonists (U-50,488H and sufentanil) demonstrated antinociceptive effects at spinal and supraspinal levels.
• DPDPE (delta opioid receptor agonist) was found to be inactive in this model within behaviorally acceptable dose ranges.
• Effective doses varied by administration route, with central administration generally requiring lower doses for kappa and mu agonists.

Conclusions:

• Kappa and mu opioid receptors, but not delta receptors, modulate tonic pain perception in mice at both spinal and supraspinal sites.
• The modified formalin test is a suitable model for preclinical evaluation of kappa agonists.
• Endogenous opioid systems, specifically kappa and mu pathways, are key targets for managing tonic pain.