Effect of pyrethroids on pentobarbital-induced sleeping time in relation to the chemical structure

Neurobehavioral toxicology of pyrethroid insecticides in adult animals: a critical review

Pyrethroids are pesticides with high selectivity for insects. In order to identify strengths and gaps in the database for pyrethroid neurobehavioral toxicology, we have critically analyzed the data from peer-reviewed literature. This review includes dose-response data that have been recently generated demonstrating consistent findings for low-dose, acute, oral exposure to pyrethroids in small rodents. All pyrethroids tested (i.e., about twenty compounds), regardless of structure, produce a decrease in motor activity in a variety of test protocols. The range of relative potencies varies more than two orders of magnitude, and thresholds for motor activity were found well below doses that produce overt signs of poisoning. Six compounds (allethrin, permethrin, cis-permethrin, deltamethrin, cypermethrin, and fenvalerate) impair schedule-controlled operant responding, seven compounds (pyrethrum, bifenthrin, S-bioallethrin, permethrin, beta-cyfluthrin, cypermethrin, and deltamethrin) decrease grip strength, and two compounds (deltamethrin and alpha-cypermethrin) produce incoordination using the rotarod. In addition, while compounds lacking an alpha-cyano group (e.g., cismethrin, permethrin, bifenthrin) induce an increase in acoustic-evoked startle response amplitude, cyano compounds (e.g., deltamethrin, cypermethrin, cyfluthrin) produce the opposite outcome. Other endpoints (e.g., tremor intensity, sensory response) have been only occasionally explored. A synthesis of the neurobehavioral evidence relating to the action of pyrethroids indicates that some differences in the experimental findings across compounds are also present in the low-effective dose range. For risk assessment purposes, a strategy that takes into account data from an array of neurobehavioral endpoints is needed to capture the heterogeneity of pyrethroid-induced adverse effects and accurately inform policy decisions.

Species difference in the inhibition of pentobarbital metabolism by empenthrin

Empenthrin, synthetic pyrethroid, prolonged the pentobarbital-induced sleeping time in mice, but not in rats, guinea pigs or hamsters. Empenthrin did not delay the clearance of pentobarbital from serum in dogs. In addition, empenthrin dose-dependently inhibited in vitro metabolism of pentobarbital in mice, but not in rats, guinea pigs, hamsters or rabbits. Lineweaver-Burk plots indicated that the inhibition was competitive in mice. Microsomal fractions of recombinant yeast expressing human cytochrome P-450 (CYP)s were used to determine the inhibitory effect of empenthrin on pentobarbital metabolism in humans. CYP2B6 and CYP2D6 were responsible for biotransformation of pentobarbital to a pentobarbital alcohol identified as 5-ethyl-5-(1'-methyl-3'-hydroxybutyl) barbituric acid. The structure of pentobarbital fit the criteria for a CYP2D6 substrate on computational analysis. Empenthrin did not inhibit the pentobarbital metabolism catalyzed by these two CYPs. These findings suggest that the inhibition of pentobarbital metabolism by empenthrin in mice does not occur in other species including humans.

Mechanism of prolongation of pentobarbital-induced sleeping time by empenthrin in mice

The effect of empenthrin, a synthetic pyrethroid, on pentobarbital (PTB)-induced sleeping time was examined in mice and rats. In mice, pretreatment with empenthrin prolonged PTB-induced sleeping time in a dose-dependent manner. The maximum effect on PTB-sleeping time was noted when mice were pretreated orally with empenthrin 2-4 h before PTB injection. However, empenthrin did not change the sleeping time induced by diethyl ether which is hardly metabolized in liver. Empenthrin inhibited the clearance of serum PTB in mic, but did not change the PTB concentration in serum at which animals recovered from sleeping. To examine the effect of PTB on metabolic enzymes in mouse liver, PTB was incubated aerobically with a hepatic microsomal fraction in the presence of NADPH at 37 degrees C. Empenthrin inhibited the vitro metabolism of PTB dose-dependently. In rats, empenthrin neither changed the PTB sleeping time, nor inhibited the clearance of serum PTB. No inhibitory effect of empenthrin was observed on the in vitro metabolism of PTB using rat hepatic microsomal fraction. These findings indicate that empenthrin prolongs PTB-sleeping time in mice through an inhibition of the PTB-metabolizing enzyme(s) in the liver , an effect that does not occur in rats. Also, there is a clear species-specificity in the inhibitory effect of empenthrin on the PTB-metabolizing enzyme(s).