Therapeutic and adverse actions of serotonin transporter substrates

Abstract

A variety of drugs release serotonin (5-HT, 5-hydroxytryptamine) from neurons by acting as substrates for 5-HT transporter (SERT) proteins. This review summarizes the neurochemical, therapeutic, and adverse actions of substrate-type 5-HT-releasing agents. The appetite suppressant (±)-fenfluramine is composed of (+) and (−) isomers, which are N-de-ethylated in the liver to yield the metabolites (+)- and (−)-norfenfluramine. Fenfluramines and norfenfluramines are potent 5-HT releasers. (±)-3,4-Methylenedioxymethamphetamine ((±)-MDMA, “ecstasy”) and m-chlorophenylpiperazine (mCPP) are substrate-type 5-HT releasers. Fenfluramines, (±)-MDMA, and mCPP release neuronal 5-HT by a common non-exocytotic diffusion-exchange mechanism involving SERTs. (+)-Norfenfluramine is a potent 5-HT_2B and 5-HT_2C receptor agonist. The former activity may increase the risk of valvular heart disease, whereas the latter activity is implicated in the anorexic effect of systemic fenfluramine. Appetite suppressants that increase the risk for developing primary pulmonary hypertension (PPH) are all SERT substrates, but these drugs vary considerably in their propensity to increase this risk. For example, fenfluramine and aminorex are clearly linked to the occurrence of PPH, whereas other anorectics are not. Similarly, some SERT substrates deplete brain tissue 5-HT in animals (e.g., fenfluramine), while others do not (e.g., mCPP). In addition to the established indication of obesity, 5-HT releasers may help treat psychiatric disorders, such as drug and alcohol dependence, depression, and premenstrual syndrome. Viewed collectively, we believe new medications can be developed that selectively release 5-HT without increasing the risk for adverse effects of valvular heart disease, PPH, and neurotoxicity. Such agents may be useful for treating a variety of psychiatric disorders.

Introduction

Neurons that synthesize, store, and release monoamine transmitters [norepinephrine (NE), dopamine (DA), and serotonin (5-HT, 5-hydroxytryptamine)] are widely distributed in the mammalian CNS. These neurons possess specialized plasma membrane proteins that function to transport previously released transmitter molecules from the extracellular space back into the cytoplasm Amara & Kuhar, 1993, Masson et al., 1999. It is now well established that there are distinct transporter proteins expressed by NE neurons [i.e., NE transporters (NETs)], DA neurons (i.e., DA transporters), and 5-HT neurons [i.e., 5-HT transporters (SERTs)]. These proteins belong to a superfamily of Na⁺/Cl⁻-dependent transporters that share genetic, structural, and functional homologies Blakely et al., 1994, Uhl & Johnson, 1994. Under normal circumstances, the transporter-mediated uptake of monoamine transmitters is the principal mechanism for inactivation of monoaminergic signaling in the brain. Accordingly, a variety of therapeutic and abused drugs interact with monoamine transporter sites (Amara & Sonders, 1998).

Drugs that target transporter proteins can be divided into two basic classes: reuptake inhibitors and substrate-type releasers. Reuptake inhibitors bind to transporter proteins, but are not themselves transported. These drugs elevate extracellular transmitter concentrations by blocking transporter-mediated recapture of transmitter molecules from the synapse. Substrate-type releasers bind to transporter proteins, and these drugs are subsequently transported into the cytoplasm of nerve terminals. Releasers elevate extracellular transmitter concentrations by a two-pronged mechanism: (1) they promote efflux of transmitter by a process of transporter-mediated exchange and (2) they increase cytoplasmic levels of transmitter by disrupting storage of transmitters in vesicles Rudnick, 1997, Rudnick & Clark, 1993. This latter action increases the pool of neurotransmitter available for release by transporter-mediated exchange. Because substrate-type releasing agents must be transported into nerve terminals to promote transmitter release, reuptake inhibitors block the effects of releasers. Fig. 1 shows a schematic diagram depicting the mechanism of action of the prototypical 5-HT releaser fenfluramine. Fenfluramine acts as a substrate for SERT. SERTs, in turn, mediate the translocation of drug molecules into the cytoplasm in exchange for 5-HT molecules that flow out into the extracellular space (i.e., synapse). Note that fenfluramine is also a substrate for the vesicular monoamine transporter (VMAT) present on intracellular vesicle membranes Rudnick, 1997, Schuldiner et al., 1993. By disrupting compartmentalization of 5-HT into vesicles, fenfluramine increases the pool of cytoplasmic transmitter available for release.

Although both reuptake inhibitors and releasers elevate synaptic concentrations of transmitter via transporter-dependent processes, there are important differences in their precise modes of action. In particular, the activity of reuptake inhibitors requires ongoing release of transmitters via exocytosis—a process that is dependent upon electrical depolarization and extracellular Ca²⁺. Thus, the capability of reuptake inhibitors to increase synaptic transmitter levels is said to be impulse- and Ca²⁺-dependent. Releasing agents, on the other hand, increase synaptic transmitter levels by a process that is largely independent of ongoing cell firing and exocytotic transmitter release. Cell membrane autoreceptors mediate negative feedback mechanisms that serve to dampen the ability of reuptake inhibitors to elevate synaptic transmitters (reviewed in Pineyro & Blier, 1999). Such negative feedback effects exist for 5-HT Adell & Artigas, 1991, Rutter et al., 1995, Smith & Lakoski, 1997, DA (Hinerth et al., 2000), and NE (Mateo et al., 1998) neuron systems. While autoreceptor activation can completely abolish the ability of reuptake inhibitors to elevate synaptic transmitter levels, autoreceptor mechanisms have little or no effect on substrate-induced neurotransmitter release Florin et al., 1994, Gardier et al., 1994, Gundlah et al., 1997, Kamal et al., 1981, Kuczenski et al., 1990. Because of autoreceptor-mediated feedback inhibition, reuptake inhibitors tend to produce small increases in extracellular neurotransmitters, whereas releasers tend to produce more robust increases Gundlah et al., 1997, Scorza et al., 1999. The in vivo microdialysis data in Fig. 2 illustrate the modest and sustained elevation of extracellular 5-HT evoked by the 5-HT reuptake inhibitor fluoxetine compared with the much larger and transient effect of the 5-HT releaser (+)-fenfluramine Berger et al., 1992, Crespi et al., 1997, Gundlah et al., 1997.

A variety of 5-HT-selective reuptake inhibitors (SSRIs), such as fluoxetine, sertraline, and citalopram, are medications used for the treatment of psychiatric disorders, including depression, panic disorder, and obsessive-compulsive disorder (for reviews, see Gorman & Kent, 1999, Zohar & Westenberg, 2000). In contrast, there are far fewer 5-HT releasing agents that have been used in human patients. Because of the withdrawal of the appetite suppressants fenfluramine and (+)-fenfluramine from the market in September 1997 (Connolly & McGoon, 1999), there are currently no clinically available 5-HT releasing agents. A main goal of this review is to summarize the potential therapeutic uses and reported adverse effects of substrate-type 5-HT releasing agents, including the illicit drug 3,4-methylenedioxymethamphetamine ((±)-MDMA or “ecstasy”) Fitzgerald & Reid, 1990, Gudelsky & Nash, 1996, Johnson et al., 1991a, Rothman et al., 2001. Furthermore, we hope this review will foster continued interest in the development of novel and selective 5-HT releasers that might be used as effective medications.