An Intraplantar Hypertonic Saline Assay in Mice for Rapid Screening of Analgesics

Development of new analgesics is limited by shortcomings of existing preclinical screening assays such as wide variations in response, suitability for a narrow range of analgesics, and propensity to induce tissue damage. Our aim was to determine the feasibility of a new in vivo animal assay as an analgesic screen based on nociceptive responses (licking and biting) after intraplantar (i.pl.) injection of hypertonic saline (HS) in mice.

With approval from the Institutional Animal Care Committee, we conducted a randomized, investigator-blinded in vivo study in adult CD-1 mice. We first studied the concentration-response relationship, time course, and sex difference of animals' nociceptive responses to HS. Subsequently, we assessed the screening ability of the HS assay to detect a range of established analgesics belonging to different classes. Finally, we performed histopathologic studies to assess potential tissue damage.

The response produced by i.pl. HS was greater and longer in female than in male mice. The responses to HS were concentration dependent with minimal variance. Ten percent HS evoked a maximal response within the first 5 minutes. Morphine dose-dependently attenuated animals' nociceptive responses (1-10 mg/kg intraperitoneally [i.p.]). The peripherally restricted µ-opioid receptor agonist, loperamide, reduced nociceptive responses when injected locally (30-100 µg/paw, i.pl.) but not systemically (1-10 mg/kg, i.p.). Acetylsalicylic acid (300 mg/kg, i.p.), naproxen (150 mg/kg, i.p), and acetaminophen (300 mg/kg, i.p.) all decreased nociceptive responses, as did i.pl. coinjections of lidocaine (0.003%-1%) with 10% HS. Histopathologic assessment revealed no tissue damage due to HS.

The i.pl. HS assay is easily performed, rapidly detects standard analgesics, and produces minimal animal suffering without tissue damage. We propose this assay as a useful addition to the armamentarium of existing preclinical analgesic screens.