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Lower efficacy MOR agonists like buprenorphine can produce analgesia with improved safety. We recently described a series of novel phenylmorphan-based MOR agonists with graded MOR efficacies, including several with sub-buprenorphine efficacies, and we described their acute effectiveness in mice to produce (a) antinociception in an assay of pain-related locomotor depression, and (b) improved safety on a range of side effects. Effects of repeated treatment with these compounds have not been examined.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Objective<\/jats:title>\n                    <jats:p>The present objective was to compare effects of repeated treatment with four opioids that ranged from high to low MOR efficacy (morphine&gt; buprenorphine\u2009&gt;\u2009JL-2-39\u2009&gt;\u2009DC-1-76.1).<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods<\/jats:title>\n                    <jats:p>Female and male ICR mice were treated once daily for 7 days with a selected dose of each drug\u2009\u00b1\u2009intraperitoneal lactic acid (IP acid) as a noxious stimulus and evaluated for vertical and horizontal locomotor activity on Days 1 and 7.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>IP acid alone produced sustained, concentration-dependent, and pain-related locomotor depression. When administered as a daily pretreatment to IP acid, all four opioids produced sustained antinociception expressed as significant alleviation of IP acid-induced behavioral depression on Days 1 and 7. However, when these opioids were administered alone, effects were efficacy dependent. Morphine produced locomotor stimulation and sensitization; buprenorphine and JL-2-39 produced locomotor stimulation without sensitization; DC-1-76.1 produced neither stimulation or sensitization.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusions<\/jats:title>\n                    <jats:p>These findings support continued consideration of low-efficacy MOR agonists as candidate analgesics that can produce sustained antinociception with reduced side effects during repeated treatment.<\/jats:p>\n                  <\/jats:sec>","DOI":"10.1007\/s00213-026-07117-5","type":"journal-article","created":{"date-parts":[[2026,7,2]],"date-time":"2026-07-02T10:29:50Z","timestamp":1782988190000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Effects of repeated treatment with opioids that vary in mu opioid receptor efficacy on pain-depressed locomotor behavior in mice"],"prefix":"10.1007","author":[{"given":"Jawaun","family":"Harris","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Dana R.","family":"Chambers","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Delmis E.","family":"Hernandez","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Arthur E.","family":"Jacobson","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Joshua A.","family":"Lutz","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kenner C.","family":"Rice","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Agnieszka","family":"Sulima","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"S. 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