Toxin-induced conditioned disgust responses: Comparing emetine and scopolamine methyl nitrate (SMN) to lithium...

Chemotherapy patients often develop anticipatory nausea, a learned response produced by an environmental cue triggering nausea in the absence of the emetic stimulus. This can be modelled in rats by pairing the administration of an emetic treatment with exposure to a distinct context. Rats are a non-emetic species, but they are still capable of experiencing nausea, which is observable through the presence of the gaping response. The rat model of anticipatory nausea mainly utilizes lithium chloride (LiCl) to evoke the gaping response, yet various other putative emetic toxins and agents have remained unexplored. The current study determined if emetine, found in ipecac, and scopolamine methyl nitrate (SMN), a derivative of scopolamine, can evoke the conditioned gaping response. Adult male Long Evans rats (n = 40) were administered emetine (5.0 mg/kg, intraperitoneal [i.p.]), SMN (0.5 or 1.0 mg/kg, i.p.), LiCl (127 mg/kg, i.p.), or sodium chloride (NaCl, 0.9 %, i.p., control) before exposure to a distinct context for 30 min, over four conditioning trials, followed by four extinction (drug-free) trials. During the extinction trials, no treatments were administered, and rats were placed directly into the context. Rat behaviour on all of the trials was videotaped and scored for the frequency of the conditioned gaping response. Findings from this study suggest that, in comparison to LiCl, emetine is an effective conditioning stimulus, whereas SMN is not.