Amphetamine and Other Weak Bases Act to Promote Reverse Transport of Dopamine in Ventral Midbrain Neurons

Abstract

Amphetamine-like psychostimulants are thought to produce rewarding effects by increasing dopamine levels at mesolimbic synapses. Paradoxically, dopamine uptake blockers, which generally increase extracellular dopamine, inhibit amphetamine-induced dopamine overflow. This effect could be due to either inhibition o f amphetamine uptake or inhibition of dopamine efflux through the transporter (reverse transport). We used weak bases and dopamine uptake blockers in ventral midbrain neuron cultures to separate the effects on blockade of amphetamine uptake from reverse transport ofdopamine. Amphetamine, ammonium chloride, tributylamine, and monensin, at concentrations that produce similar reductions in acidic pH gradients, increased dopamine release. This effect was inhibited by uptake blockers. Although in the case of amphetamine the inhibition of release could have been due to blockade of amphetamine uptake, inhibition also occurred with weak bases that are not transporter substrates. This suggests that reduction of vesicular pH gradients increases cytoplasmic dopamine which in turn promotes reverse transport. Consistent with this model, extracellular 3,4-dihydroxyphenylacetic acid was increased by ammonium chloride and monensin, as would be expected with elevated cytoplasmic dopamine levels. These findings extend the weak base mechanism of amphetamine action, in which amphetamine reduces vesicular pH gradients resulting in increased cytoplasmic dopamine that promotes reverse transport.

Differential action of (+)‐amphetamine on electrically evoked dopamine overflow in rat brain slices containing corpus striatum and nucleus accumbens

“…As dopamine is ionized at physiological pH, it cannot readily pass through membranes by diffusion in the absence of specific carrier molecules. Therefore, dopamine leaves the nerve terminal either by calcium-dependent exocytosis from vesicular stores, or by exchange diffusion, induced by displacement of dopamine from stores by drugs such as (+)-amphetamine (Sulzer et al, 1993). For exchange diffusion to operate, the dopamine uptake system must be functional, as inhibitors of the dopamine uptake system prevent dopamine displacement by (+)-amphetamine (Raiteri & Levi, 1978;Fisher & Cho, 1979;Parker & Cubeddu, 1988).…”

“…The hypothesis states that (+)-amphetamine acts as a substrate for the carrier molecule and accelerates the inward movement of the carrier, making it available for the outward transport of cytoplasmic free dopamine. The increase in the cytoplasmic concentration of (+ )-amphetamine results in an elevation of cytoplasmic free dopamine due to both the displacement of dopamine from intraneuronal stores (Moore, 1978;McMillan, 1983;Sulzer et al, 1993) and the monoamine oxidase inhibitory action of (+)-amphetamine (Miller et al, 1980). As a substrate for the dopamine carrier system, (+)-amphetamine also acts as a dopamine neuronal uptake inhibitor (Horn et al, 1970;Heikilla et al, 1975;Hyttel, 1978;Parker & Cubeddu, 1988).…”

Effects of Cocaine on c-Fos and NGFI-B mRNA Expression in Transgenic Mice Underexpressing Glucocorticoid Receptors

“…Whereas amphetamine elevates extracellular DA levels by acting both on its reuptake and its release (Sulzer and Rayport, 1990;Sulzer et al, 1993), COC rather acts solely by the former mechanism (Ritz et al, 1990a, b). Since c-fos induction after COC administration mostly depends on D 1 receptor stimulation (Drago et al, 1996;Das et al, 1997), DAT decrease levels in TG mice should then result in a decreased COC effect on D 1 receptor.…”

Action potentials and amphetamine release antipsychotic drug from dopamine neuron synaptic VMAT vesicles

“…We then examined the VMAT-dependent mechanism mediating APD release by the lower concentration of pCA. Amphetamines are VMAT substrates and can induce DA release from DA-containing vesicles either by acting as weak bases to reduce the vesicular pH gradient or by inducing VMAT-mediated exchange with luminal DA (14)(15)(16)(17)(18)(19). VMAT-mediated exchange can only occur with two VMAT substrate molecules (20).…”