Testing and comparison of non-opioid analgesics in amphibians

Abstract

Because of the lack of information about effective analgesics in non-mammalian vertebrates, the potency of various non-opioid agents were tested in a model of analgesia by using Northern grass frogs (Rana pipiens). This alternative model has been used widely for investigating opioid analgesic action. Potential non-opioid analgesics tested included antipsychotic, benzodiazepine, barbiturate, antihistamine, non-steroidal anti-inflammatory (NSAID), and partial opioid agents. Northern grass frogs were acclimated to lab conditions in individual cages. Drugs were administered systemically through the dorsal lymph sac, and analgesic effects were estimated by using the acetic acid test (AAT). The AAT is done by placing logarithmic dilutions of acid dropwise on the dorsum of the animal's thigh until a wiping response is obtained. At various doses, chlorpromazine and haloperidol (antipsychotics), chlordiazepoxide (a benzodiazepine), buprenorphine (a partial opioid agonist), and diphenhydramine (a histamine antagonist) produced moderate to strong analgesic effects. Indomethacin and ketorolac (NSAIDs), butorphanol (a partial opioid agonist), and pentobarbital (a barbiturate) produced weaker but noticeable analgesic effects. Our results are the first to document the effectiveness of a wide array of pharmacologically active agents in a novel amphibian model for analgesia. These findings provide needed data regarding the use of alternative, non-opioid agents for the treatment of pain in amphibians and other poikilothermic species.

Figure 1

Time course of the analgesic effects of morphine, chlorpromazine, chlordiazepoxide, and buprenorphine after systemic administration in amphibians (n = 4 to 6 per group).○, morphine (300 nmol/g); ▽, chlorpromazine (100 nmol/g);■, chlordiazepoxide (300 nmol/g); ⋇, buprenorphine (30 nmol/g). [“Key” deleted from illustration] Saline-injected control group did not give a significant effect and is omitted for clarity.

Figure 2

Time course of the analgesic effects of morphine, chlorpromazine, diphenhydramine, and buprenorphine following systemic administration in amphibians (n = 4 to 6 animals per group). ●, morphine (300 nmol/g); ▽, chlorpromazine (30 nmol/g); ■, diphenhydramine (200 nmol/g); ⋄, haloperidol (30 nmol/g). [“Key” deleted from illustration] Saline-injected control group did not give a significant effect and is omitted for clarity.

Figure 3

Mean peak analgesic effect [Is rewording accurate?] of morphine, chlorpromazine, chlordiazepoxide, and buprenorphine, diphenhydramine, and buprenorphine after systemic administration in amphibians (n = 4 to 6 per group). Saline-injected control group is included for comparison. *, P< 0.05 (Student’s t test) versus saline control group. [Please add “nmol/g” to doses given on s-axis of figure. Please “define” bars—1 standard deviation?]