Effects of high‐dose fenfluramine treatment on monoamine uptake sites in rat brain: Assessment using quantitative autoradiography

Abstract

Fenfluramine is an amphetamine derivative that in humans is used primarily as an anorectic agent in the treatment of obesity. In rats, subchronic high-dose d,l-fenfluramine treatment (24 mg/kg subcutaneously, twice daily for 4 days) causes long-lasting decreases in brain serotonin (5HT), its metabolite 5-hydroxyindoleacetic acid, and high-affinity 5HT uptake sites. Moreover, this high-dose treatment regimen causes both selective long-lasting decreases in fine-caliber 5HT-immunoreactive axons and appearance of other 5HT-immunoreactive axons with morphology characteristic of degenerating axons. Determination of the potential neurotoxic effects of fenfluramine treatment using immunohistochemistry is limited from the perspectives that staining is difficult to quantify and that it relies on presence of the antigen (in this case 5HT), and the 5HT-depleting effects of fenfluramine are well known. In the present study, we used quantitative in vitro autoradiography to assess, in detail, the density and regional distribution of [3H]paroxetine-labeled 5HT and [3H]mazindol-labeled catecholamine uptake sites in response to the high-dose fenfluramine treatment described above. Because monoamine uptake sites are concentrated on monoamine-containing nerve terminals, decreases in uptake site density would provide a quantitative assessment of potential neurotoxicity resulting from this fenfluramine treatment regimen. Marked decreases in densities of [3H]paroxetine-labeled 5HT uptake sites occurred in brain regions in which fenfluramine treatment decreased the density of 5HT-like immunostaining when compared to saline-treated control rats. These included cerebral cortex, caudate putamen, hippocampus, thalamus, and medial hypothalamus. Smaller, but nonetheless significant, decreases in density of [3H]paroxetine-labeled 5HT uptake sites were noted in brain regions in which partial sparing of 5HT-like immunoreactive fibers had been reported following fenfluramine treatment, specifically septum, lateral hypothalamus, and amygdala. In contrast, [3H]mazindol autoradiography revealed that total catecholamine (i.e., dopamine and norepinephrine) uptake sites in cerebral cortex, caudate putamen, and locus coeruleus, areas in which [3H]paroxetine-labeled 5HT uptake sites were significantly decreased, were unaffected by this fenfluramine treatment. These data support the hypothesis that subchronic, high-dose fenfluramine treatment causes selective degeneration of 5HT axons in rat brain. Since pharmacokinetic studies show that the dosing regimen used in this study exposes rat brain to concentrations of fenfluramine that are approximately 600 times greater than those resulting from the therapeutic oral dose, caution must be exercised in extrapolating these data to humans.

Loss of Serotonin Transporter Protein after MDMA and Other Ring-Substituted Amphetamines

“…In particular, after certain dosing conditions, MDMA (Green et al, 2003), PCA (Sanders-Bush and Steranka, 1978;Fuller, 1992), and FEN (Schuster et al, 1986;Appel et al, 1989Appel et al, , 1990McCann et al, 1997) produce long-term reductions in the concentrations of 5-HT and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), the activity of 5-HT's rate limiting synthetic enzyme, tryptophan hydroxylase (TPH), and the abundance of membrane and vesicular 5-HT transporters, as measured by [ 3 H]paroxetine and [ 3 H]DTBZ binding, respectively. In addition, tract-tracing studies after MDMA show reduced anterograde [ 3 H]proline transport along ascending 5-HT axonal projections (Callahan et al, 2001).…”

Combined phentermine/fenfluramine administration enhances depletion of serotonin from central terminal fields

“…Combined administration of Phen (5 mg/kg) with Fen (3.125 mg/kg) yielded significantly greater reductions than those observed with either drug alone in all regions except for the hippocampus and hypothalamus. *Significantly different from saline (P , 0.05);However, very little is known with regard to combined administration of Phen/Fen with respect to their longterm, possibly toxic effects, although repeated administration of Phen (Kleven et al, 1991) or Fen alone (Appel et al, 1989(Appel et al, , 1990McMaster, 1975, 1977;Kleven and Seiden, 1989;Molliver and Molliver, 1990;Schuster et al, 1986) are known to cause loss of DA and/or 5-HT in several animal models. In the present study, combined administration of Phen and Fen in rats caused a reduction in 5-HT levels and a loss of 5-HT transporters significantly greater than those induced by comparable doses of either drug administered alone.…”

MDMA (Ecstasy) Inhibition of MAO Type A and Type B: Comparisons with Fenfluramine and Fluoxetine (Prozac)

“…In contrast, MDMA ap pears to be slightly more potent than FEN at inhibition of reuptake (0.42 IlmollL, Steele et al 1987 and0.876 IlmollL, Borroni et al 1983, respectively). Finally, both drugs are toxic to serotonergic neurons (Appel et al 1990;Azmitia et al 1990;Battaglia et al 1987Battaglia et al , 1988 (Gu 1993) and in vivo (Sotelo and Zamora 1978). The fact that MDMA has greater toxicity is not due to its ability to induce release or bind to the serotonin transporter.…”