Abstracts

Rationale: Safety pharmacology studies often include experiments on seizure threshold in rodents for detection of proconvulsant drug activity. Therefore, reliable identification of proconvulsant drug activity by preclinical models is of particular importance. The timed intravenous (i.v.) pentylenetetrazole (PTZ) infusion seizure test is used as a standard model for preclinical assessment of proconvulsant drug activity. However, it has been revealed that substances, which show proconvulsant effects in the PTZ test, may have the opposite effect on electrical seizure thresholds. To address this point, we compared effects of several substances, including both anti- and proconvulsant drugs, on seizure thresholds in the timed i.v. PTZ infusion test with those elicited in the maximal electroshock seizure threshold (MEST) test. Methods: In freely moving adult male Wistar-Unilever rats PTZ was infused at a concentration of 0.8% at 1 ml/min via the tail vein. The threshold to induce a first myoclonic twitch or a clonic seizure in mg/kg PTZ infused was determined following vehicle treatment, and threshold determination was repeated 2-3 days later 30 minutes after intraperitoneal injection of one of the following drugs: phenobarbital, 20 mg/kg; d-amphetamine, 5 mg/kg; chlorpromazine, 3 mg/kg; theophylline, 30 and 50 mg/kg; caffeine, 60 and 80 mg/kg; or of tramadol, 2.5; 5, 10, and 20 mg/kg. Each rat served as its own control; 3-8 rats were used per experiment. For the MEST test, the same doses and pretreatment time were used to record the CC50, i.e. the convulsive current needed to induce either a forelimb or hindlimb tonus following current application via corneal electrodes in 50% of the animals, using 14-24 rats per MEST determination. In addition, latency to and duration of seizure endpoints as well as mortality were recorded. Results: Phenobarbital was shown to enhance seizure thresholds for recorded endpoints in both the PTZ test as well as the MEST test. In contrast, the known proconvulsant drug d-amphetamine significantly decreased PTZ seizure threshold whereas it increased the CC50 in the MEST test. Similarly, chlorpromazine exhibited a proconvulsant effect in the PTZ test, whereas we observed no influence on MEST seizure threshold. Interestingly, with caffeine and theophylline, we found no proconvulsant effects in the PTZ test, but theophylline even increased seizure threshold for myoclonic twitch. On the contrary, methylxanthines dose-dependently lowered seizure threshold in the MEST test. At 2.5 mg/kg, the opioid tramadol did not alter seizure threshold in both models. However, at higher doses, it significantly decreased seizure thresholds in the PTZ test, whereas it exerted anticonvulsant effects in the MEST test. Conclusions: Despite the fact that both seizure threshold models are suitable to detect proconvulsant drug activity, results are not always consistent. Thus, the sole use of one test to assess the proconvulsant potential of a substance during drug development is not sufficient as it may lead to false negative or positive conclusions.