• Spiradoline has a high affinity for the kappa opioid receptor, and is selective for the kappa opioid receptor unless extremely high doses are used.
• Spiradoline has been extensively used in animals and was shown to be safe in humans in limited clinical trials.
• Spiradoline is structurally different from the natural peptide agonists of the kappa opioid receptor.
• Spiradoline is non-addicting (unlike mu and delta opioid agonists)
• Spiradoline is commercially available

In humans, it had disappointing results since it did not provide the analgesic effect that they had hoped. In fact, it seems that the investigators kept pushing the doses of spiradoline higher and higher in hopes of getting analgesia, and several patients reported dysphoria and hallucinations at the higher doses. Otherwise, the drug appeared quite safe at the lower doses. We have used Spiradoline to regulate heart rate in mice expressing Ro1 in the heart. To measure heart rate and waveform, we implanted cardiac telemetry units into mice expressing Ro1 and administered the k opioid receptor agonist spiradoline to activate Ro1. Intraperitoneal (i.p.) administration of spiradoline at doses of 1x10^-7 to 1x10^-5 mol/kg (0.05 mg/kg to 5 mg/kg) caused a dose-dependent decrease in heartrate (Fig. 4a in the single mouse depicted in Redfern et al.). In other unpublished experiments we found that approximately 50% of the mice died (perhaps of respiratory failure) when we gave 1x10^-4 mol/kg, while all the mice survived when we used 1x10^-5 mol/kg (which is why this was our maximal dose. We have not tested these high doses in k-receptor KO mice, but we anticipate that it will not have these side effects.