Involvement of the Opioid Pathway in the Antinociceptive Activity of the Aqueous Stem Extract from Doyerea emetocathartica

Background: Doyerea emetocathartica (Cucurbitaceae) is endemic to the Brazilian Caatinga, commonly known as “cabeça-de-negro”, and is traditionally used to treat various conditions, including pain. Previous studies have demonstrated that the aqueous stem extract exhibits anti-inflammatory activity, which may be associated with the presence of cayaponoside D2, a major component of this extract. Given this, the aim of this study was to characterize the aqueous extract of D. emetocatharthica stems in terms of its Cayaponoside D2 content and to evaluate its antinociceptive activity using in vivo models, as well as to assess the involvement of the opioid pathway in the observed effect. Methods: For this purpose, the extract was analyzed by HPLC to quantify cayaponoside D2, and formalin- and acetic acid-induced nociception models were used to evaluate antinociceptive activity; the rats were divided into six groups (n = 6). The test groups received the aqueous extract of the D. emetocathartica stem (DEAQ) at doses of 25, 50, and 100 mg/kg; the control group received 0.9% saline; and morphine and indomethacin were used as positive controls. To investigate the mechanism of action involved in the antinociceptive effect, an evaluation was conducted of the role of the opioid pathway and the involvement of opioid receptors in the antinociceptive effect in the presence of specific antagonists: naloxone, CTOP, naltrindole, and Nor-BNI. Results: The analgesic activity of DEAQ was demonstrated in acetic acid-induced abdominal writhing and formalin-induced nociception models, with the most pronounced effects observed at doses of 50 and 100 mg kg−1. In the formalin test, DEAQ showed a predominant effect during the inflammatory phase. Mechanistic investigations revealed that the antinociceptive activity involves the opioid system, mainly through μ-opioid receptors, with additional participation of δ-opioid receptors, as evidenced by the partial reversal of the effect by selective antagonists. These findings suggest that DEAQ may exert a multimodal analgesic effect involving both opioid and non-opioid pathways. This activity can be attributed to the presence of cayaponoside D2, which was quantified in the extract at a concentration of 108 mg g−1 of dry extract. Conclusions: These findings demonstrate that the aqueous stem extract from D. emetocathartica exhibits promising analgesic activity mediated by the opioid pathway, reinforcing its pharmacological potential and supporting its traditional use in pain management.