Relationship between radioprotective and neuromotor effects of S-2(3-aminopropyl-amino)ethylphosphorothioate (WR-2721) in mice

Comparison of behavioral and radioprotective effects of WR-2721 and WR-3689

The behavioral effects of the radioprotectant agents ethiofos, S-2-(3-aminopropylamino)ethylphosphorothioic acid (WR-2721) and S-2-(3-methylaminopropyl)aminoethylphosphorothioic acid (WR-3689) were evaluated in rats trained to respond under a multiple fixed-interval 120-s, fixed-ratio 50-response (mult FI FR) schedule of milk reinforcement. Each compound produced dose-dependent reductions in responding under both schedules over the same dose range (100-180 mg/kg, IP); ED50s indicated that WR-3689 was slightly more potent than WR-2721. On several performance measures, WR-3689 produced greater decrements during a second dose-effect determination, whereas WR-2721 had more pronounced effects during the initial one. In a second series of studies, low (56 mg/kg) and high (180 mg/kg) doses of both drugs were tested for radioprotective effects in rats responding under an FR-50 schedule of milk reinforcement and exposed to a nonlethal (5 gray, Gy) or lethal (10 Gy) dose of ionizing radiation (60Co gamma rays). Neither dose of radiation altered FR response rates on the day of exposure (day 1). Five Gy of gamma radiation produced a 25-40% reduction in response rates on days 2-5 (24-72 h) after exposure. Neither dose of WR-2721 or WR-3689 provided significant protection against these performance decrements. All groups exposed to 10 Gy experienced a progressive decline in FR responding on days 2-5 after exposure. Performance of groups that received pretreatment with the 180-mg/kg dose of either drug or the 56-mg/kg dose of WR-3689 was maintained at significantly higher levels than saline-treated controls on days 4-5 after exposure to 10 Gy; however, even at these higher levels of performance response rates remained below 50% of preirradiation control levels. Subsequently, 56 and 180 mg/kg WR-3689 and 180 mg/kg WR-2721 were found to provide protection against the lethal consequences of the 10-Gy exposure. Thus, neither WR-2721 nor WR-3689 afforded any significant short-term protection against radiation-induced performance decrements when these drugs were administered at either behaviorally ineffective or behaviorally disruptive doses. Rather, the beneficial effects of these drugs paralleled their ability to antagonize radiation-induced lethality.

Behavioral toxicity of selected radioprotectors

Effective radioprotection with minimal behavioral disruption is essential for the selection of protective agents to be used in manned spaceflight. This overview summarizes the studies on the behavioral toxicity of selected radioprotectors classified as phosphorothioates (WR-2721, WR-3689), bioactive lipids (16, 16 dimethylprostaglandin E2(DiPGE2), platelet activating factor (PAF), leukotriene C4), and immunomodulators (glucan, synthetic trehalose dicorynomycolate, and interleukin-1). Behavioral toxicity was examined in laboratory mice using a locomotor activity test. For all compounds tested, there was a dose-dependent decrease in locomotor behavior that paralleled the dose-dependent increase in radioprotection. While combinations of radioprotective compounds (DiPGE2 plus WR-2721) increased radioprotection, they also decreased locomotor activity. The central nervous system stimulant, caffeine, was able to mitigate the locomotor decrement produced by WR-3689 or PAF.

Effect of the radioprotector WR-2721 on operant behavior in the rat

The effect of WR-2721 on performance maintained by a fixed-ratio 20 (FR-20) schedule for water reinforcement was studied in male Sprague-Dawley rats. Graded doses of WR-2721 (range 25-100 mg/kg) were administered IP immediately prior to a 60 min test session. WR-2721 had a dose dependent monotonic disruptive effect on FR responding, with significant effects at doses of 50, 75 and 100 mg/kg. WR-2721 also decreased postsession water consumption, but only one significant effect at the highest dose (100 mg/kg). Both slopes of the dose-response regression line are parallel in effect. These data indicate that WR-2721 may affect drinking motivation, which could disrupt operant performance, and WR-2721 affects motor behavior at lower doses than those that depress "motivation" to drink. The log dose-probit analysis on the all-or-none disruptive pattern of pause of responding observed from cumulative records disclosed that the slope of this regression line (s = 1.11) was also almost identical to that of reinforcer decrement analyzed from graded dose-response relationship (s = 1.14) and shared the same estimated ED50's (58.5 and 55.6 mg/kg, respectively). A preliminary study using a variety of pharmacological interventions was also carried out to ascertain if the general functional gastrointestinal disorders produced by WR-2721 may subserve the behavioral deficits. Subcutaneous pretreatments with various selective, peripherally active, gastroprotective drugs [cimetidine (30 and 60 mg/kg), pirenzepine (5 and 10 mg/kg) and domperidone (1, 5 and 10 mg/kg)] 30 min prior to challenge with WR-2721 at dose of 100 mg/kg, demonstrated that these drugs did not yield any apparent significant attenuative effects.(ABSTRACT TRUNCATED AT 250 WORDS)