Receptor-specific compounds were applied by retrograde microdialysis to the ventral tegmental area (VTA) of the rat brain. The effect of the intrategmental infusions on extracellular dopamine in the ipsilateral nucleus accumbens were recorded with a second microdialysis probe. Intrategmental infusion of muscimol (10-40 microM) or baclofen (50 microM) decreased extracellular dopamine in the nucleus accumbens. Intrategmental infusion of NMDA (1 mM, 15 min) or kainate (50 microM, 15 min) increased extracellular dopamine in the nucleus accumbens. The effects of the excitatory amino acids were suppressed by co-infusion of MK-801 (1 MM), (+)-3-amino-1-hydroxy-2-pyrrolidone [(+)-HA966; 1 mM], (+/-)-3(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 100 microM), and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX;300 microM). Intrategmental infusion of of carbachol (50 microM) increased extracellular dopamine in the nucleus accumbens. These results provide evidence for localization of GABAA, GABAB NMDA, non-NMDA, and cholinergic receptors on dopamine neurons in the VTA. Infusions of CPP, (+)-MK-801, (+)-HA966, CNQX, mecamylamine, atropine, or 3-[[(3,4-dichlorophenyl)methyl]propyl](diethoxymethyl) phosphonic acid (CGP 52432) into the VTA did not modify extracellular dopamine in the nucleus accumbens. Infusion of bicuculline (50 microM) and (-)-sulpiride (50 microM) was followed by an increase in extracellular dopamine in the nucleus accumbens. These data suggest that dopamine neurons in the VTA are tonically inhibited by GABA and dopamine by acting on GABAA, and D2 receptors, respectively. A tonic stimulation by glutamatergic or cholinergic neurons was not detected. Finally, results on A10 neurons are compared with earlier data on A9 neurons. A striking difference was found in that GABAA-dopamine interactions are indirect in the substantia nigra and direct in the VTA.

Microdialysis has become a frequently used method to study extracellular levels of GABA and glutamate in the central nervous system. However, the fact that the major part of GABA and glutamate as measured by microdialysis does not fulfill the classical criteria for exocytotic release questions the vesicular origin of the amino acids in dialysates. Glial metabolism or reversal of the (re)uptake sites has been suggested to be responsible for the pool of nonexocytotically released amino-acid transmitters that seem to predominate over the neuronal exocytotic pool. The origin of extracellular GABA and glutamate levels and, as a consequence, the implications of changes in these levels upon manipulations are therefore obscure. This review critically analyzes what microdialysis data signify, i.e., whether amino-acid neurotransmitters sampled by microdialysis represent synaptic release, carrier-mediated release, or glial metabolism. The basal levels of GABA and glutamate are virtually tetrodotoxin- and calcium-independent. Given the fact that evidence for nonexocytotic release mediated by reversal of the uptake sites as a release mechanism relevant for normal neurotransmission is so far limited to conditions of "excessive stimulation," basal levels most likely reflect a nonneuronal pool of amino acids. Extracellular GABA and glutamate concentrations can be enhanced by a wide variety of pharmacological and physiological manipulations. However, it is presently impossible to ascertain that the stimulated GABA and glutamate in dialysates are of neuronal origin. On the other hand, under certain stimulatory conditions, increases in amino-acid transmitters can be obtained in the presence of tetrodotoxin, again suggesting that aspecific factors not directly related to neurotransmission underlie these changes in extracellular levels. It is concluded that synaptic transmission of GABA and glutamate is strictly compartmentalized and as a result, these amino acids can hardly leak out of the synaptic cleft and reach the extracellular space where the dialysis probe samples.

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A;lthough glycosphingolipids are ubiquitously expressed and essential for multicellular organisms, surprisingly little is known about their intracellular functions. To explore the role of glycosphingolipids in membrane transport, we used the glycosphingolipid-deficient GM95 mouse melanoma cell line. We found that GM95 cells do not make melanin pigment because tyrosinase, the first and rate-limiting enzyme in melanin synthesis, was not targeted to melanosomes but accumulated in the Golgi complex. However, tyrosinase-related protein 1 still reached melanosomal structures via the plasma membrane instead of the direct pathway from the Golgi. Delivery of lysosomal enzymes from the Golgi complex to endosomes was normal, suggesting that this pathway is not affected by the absence of glycosphingolipids. Loss of pigmentation was due to tyrosinase mislocalization, since transfection of tyrosinase with an extended transmembrane domain, which bypassed the transport block, restored pigmentation. Transfection of ceramide glucosyltransferase or addition of glucosylsphingosine restored tyrosinase transport and pigmentation. We conclude that protein transport from Golgi to melanosomes via the direct pathway requires glycosphingolipids.

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