Action of fenfluramine, phenylpropanolamine, phentermine and diethylpropion on acoustic startle in rats.

Four commonly used anorectics which are amphetamine analogues were tested for their action on responsiveness in an acoustic startle test when rats were given daily IP injections adequate to produce a change in body weight. Drugs were given for 22 days. None of these drugs increased startle responsiveness as does the amphetamine parent compound. Instead, fenfluramine and phenylpropanolamine decreased startle responsiveness and phentermine and diethylpropion produced no change. There was no relationship between drug action and body weight. Partial tolerance was found for the fenfluramine action on startle and complete tolerance was found for its action on body weight gain. The fenfluramine action is compatible with the extensive literature on humans and animals indicating sedative properties.