Effects of fenfluramine on the adrenergic system

Regional cerebral cortical activation in monoamine oxidase A-deficient mice: differential effects of chronic versus acute elevations in serotonin and norepinephrine

Mice deficient in monoamine oxidase A have previously been shown to demonstrate a chronic elevation of serotonin and norepinephrine in the brain. Using the autoradiographic [14C]iodo-antipyrine method, we examined cerebral cortical blood flow in conscious, restrained four- to five-month-old knock-out and wild-type animals following the intraperitoneal administration of either saline or D-fenfluramine. Knock-out animals administered saline, compared to their wild-type counterparts, demonstrated a significantly higher regional cortical blood flow in somatosensory and barrel field neocortex, an area which previous histological studies have shown to be characterized by abnormal serotonergic projection fibers and absent barrel formation. Regional cortical blood flow was significantly lower in knock-out than in wild-type mice in the entorhinal and midline motor cortex, with non-significant decreases noted in the olfactory, piriform and retrosplenial cortices and the amygdala. We compared the above findings to those obtained in response to D-fenfluramine which, in conjunction with its metabolite D-norfenfluramine, results in acute elevations of brain levels of serotonin and norepinephrine. Administration of D-fenfluramine (21. 2mg/kg) resulted in changes in regional cortical perfusion in most brain regions of both knock-out and wild-type mice that were opposite to the genotypic differences seen in perfusion in response to saline. Fenfluramine significantly increased regional cortical blood flow in the allocortex (olfactory, piriform, entorhinal) and the amygdala, and significantly decreased regional cortical blood flow in the somatosensory, barrel field, midline motor and retrosplenial cortices. Changes in regional perfusion in response to fenfluramine were topographically equivalent in knock-out and wild-type mice, although in knock-out mice such changes were of greater magnitude. Our study suggests that the effects on regional cortical blood flow of a lifelong absence of monoamine oxidase A, and the consequent chronic increase in serotonin and norepinephrine, differ from those attributable to acute increases in these neurotransmitters following fenfluramine administration. Such a differential response may reflect neurodevelopmental abnormalities and/or effects of a chronic physiological adaptation on the regulation of cortical activation.

Fenfluramine and N-ethyl amphetamine: comparison of the reinforcing and rate-decreasing actions in the rhesus monkey

N-ethyl amphetamine HCl (NEA) and fenfluramine HCl (meta-trifluoromethyl N-ethyl amphetamine) were evaluated as reinforcers in rhesus monkeys that had been previously trained to press a lever using food presentations and cocaine HCl injections as reinforcers. Each daily session consisted of episodic opportunities to obtain reinforcers under a fixed-ratio schedule of 30. A drug period was interpolated between two periods in which lever-press responding was maintained by food presentations. Compared to saline, none of the drugs altered the rate of responding in the food periods which preceded the drug sessions, indicating the absence of residual response-disrupting drug actions from previous sessions. However, NEA and fenfluramine self-injection resulted in dose-related decreases in response rates during the food periods which immediately followed the drug sessions. Cocaine HCl (30 mug/kg/injection) maintained high response rates at over one response/second during the drug periods, as did the same dose of NEA. Doses of 10 and 100 mug/kg/injection of NEA as well as all doses of fenfluramine HCl (10 through 300 mug/kg/injection) maintained rates that were not different from those associated with saline injections. These results substantiate and extend earlier findings with fenfluramine and indicate that its failure to act as a reinforcer is attributable to its meta-trifluoromethyl group.