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    • ketorolac toradol The top panel of Fig depicts the

    2021-12-02

    The top panel of Fig. 3 depicts the average number of planaria visible during each observation for all cohorts in the group testing following light exposure. Although there was considerably more variability than in the light-exposure conditions, a t-test showed no significant difference in count between control versus ketorolac toradol exposure, t (118) = .4773, p =  .6340. The bottom panel of Fig. 3 depicts cumulative counts for each group. The cumulative count for histamine-exposed planaria was 95 and the cumulative count for the control cohorts was 88. The results of motility testing are depicted in Fig. 4. There was no difference in motility counts between the control and histamine exposure, t (34) = .2936, p =  .7708.
    Discussion Pairing histamine with a darkened petri dish increased the amount of time planaria spent in the light side of the petri dish when subsequently given a choice between the dark and light halves. This occurred whether we tested the planaria individually or in groups. We found no effect of histamine on motility, and pairing histamine with the light side of petri dish did not appear to affect behavior. To the best of our knowledge, this is both the first demonstration of CPA in planaria and the first examination of histamine on planarian phototaxis. The results add to a growing body of research showing that orderly results can be obtained when using planaria as an animal model for the study of psychoactive drugs [19,20]. Furthermore, the data expand similarities previously shown between planaria and vertebrates with hedonistic properties of drugs by showing parallels with the aversive properties of drugs as well. One limitation of our study is that the mechanism of aversion development is unknown. Although it is possible that CPA developed due to histamine affecting receptors in the planarian central nervous system, histamine could have had peripheral effects on sensory organs such as chemoreceptors or photoreceptors. Using an immunocytochemical assay, Panula, Erikson, Gustafsson, and Reuter [21] identified histamine receptors in the photoreceptors in D. tigrina. Interference with photoreception could have had lasting effects past the 30 min of exposure to histamine which may have affected phototaxis. In future studies, it may be prudent to include a washout period between drug exposure and testing to eliminate the possibility that behavioral effects are due to body burden of drug at the time of testing. Whether the changes in behavior were due to activation of histamine receptors and not other mechanisms may be testable. In vertebrates, histamine antagonists have been shown to eliminate or diminish the efficacy of histamine as a punisher. Goldberg [11] found that administration of the H1-receptor antagonist diphenhydramine prevented the punishing effects of histamine in squirrel monkeys responding for food. Podlesnik et al. [8] demonstrated that the histamine H1-receptor antagonists pyrilamine and ketotifen both limited the punishing effects of histamine on food-maintained responding in rats. Treatment of planaria with histamine antagonists prior to conditioning procedures is an avenue for further study. Effects of drug combinations in planaria have been successfully carried out before [22], so investigating the combined effects of two or more substances on phototaxic procedures in planaria may be feasible. Although planaria exposed to histamine in the dark spent significantly more time in the light than those exposed to fresh water only, they still allocated a majority of the choice sessions to the dark side of the petri dish. This may have been due to some of the methodological parameters. We used a light intensity that may have been too bright to allow for gross disruption of negative phototaxis. Greater light intensity has been shown to increase planaria’s speed away from a light source [23]. Experimentally determining a light intensity that produces indifference or a slight preference for a dark environment may allow for more sensitive measurement of conditioning. As stated by Bardo and Bevins [1, p. 39], “Although reducing an aversion to a context may be a mechanism by which drugs have a rewarding effect, the best approach for conducting CPP experiments is to construct an apparatus that minimizes any strong bias in preference for either context.” In addition, we did not have a precedent for the concentration of histamine, and had to make rough extrapolations from research done with intraveneous injections in rodents. We found no behavioral effects during pilot studies with 0.5 mM histamine. The punishing effect of histamine in vertebrates is dose dependent [8,9,14], and it may be of interest to evaluate the potential of various doses of histamine to produce CPA in planaria. Finally, we arranged a single pairing of histamine with the dark or light side of the petri dish. Although single pairings have been shown to produce CPP in invertebrates [24,25], it is possible that multiple pairings of histamine and the dark side of the petri dish would enhance CPA.