Lumbar intrathecal (i.t.) injection of Dermorphin-saporin (500 ng) in rats has been shown to selectively destroy spinal cord dorsal horn neurons expressing the mu-opioid receptor (MOR) with resulting attenuation of antinociceptive effects of systemic and i.t. morphine to low intensity (44 C) but not high (52 C) heat and increased responding in the formalin test. The current study examined the effects of lumbar i.t. Derm-sap on morphine antinociception during capsaicin-induced thermal hyperalgesia and in the formalin test. Effects of Derm-sap on primary afferent thermal nociceptors were assessed in baseline hotplate and tail flick testing and after systemic loperamide, a peripherally restricted mu opioid. Sixteen male rats and twenty female rats received a single lumbar intrathecal injection of 10 ul of PBS or Derm-sap (500 ng). Sensitivity to mu opioids was evaluated on the 44 C hotplate test under baseline conditions in the female rats and three hours after topical plantar capsaicin cream (0.94%) in the male rats. Effects of morphine (10 mg/kg, s.c.) on formalin behavior (25 ul of 5%) was evaluated in female rats for 90 min after plantar formalin injection. Immunohistochemical staining for GIRK2, a postsynaptic potassium channel essential for morphine analgesia, was evaluated in lumbar dorsal horns of PBS and Derm-sap rats. Derm-sap produced: 1) no change in baseline tail flick responses, 2) reduced antinociceptive effects of systemic morphine in the 44 C hotplate test after plantar capsaicin; 3) reduced antinociceptive effects of systemic morphine, but not loperamide, in the 44 C hotplate test; 4) reduced antinociceptive effects of morphine in the formalin test and 4) decreased GIRK2 staining in the dorsal horn. These observations demonstrate an important role for dorsal horn MOR-expressing neurons in analgesic and anti-hyperalgesic actions of morphine.