Glutamate in dopamine neurons: synaptic versus diffuse transmission

There is solid electron microscopic data demonstrating the existence of dopamine (DA) axon terminals (varicosities) with or without synaptic membrane specializations (junctional complexes) in many parts of the CNS, and notably in neostriatum and nucleus accumbens. The dual morphological character of these DA innervations has led to the suggestion that the meso-telencephalic DA system operates by diffuse (or volume) as well as by classical synaptic transmission. In the last decade, electrophysiological and neurochemical evidence has also accumulated indicating that monoamine neurons in various parts of the CNS, and particularly the mesencephalic DA neurons, might release glutamate as a co-transmitter. Following the identification of the vesicular transporters for glutamate (VGluT), in situ hybridization and RT-PCR studies carried out on isolated neurons or standard tissue cultures, and more recently in vivo, have shown that VGluT2 mRNA may be expressed in a significant proportion of mesencephalic DA neurons, at least in the ventral tegmental area. A current study also suggests that the co-expression of tyrosine hydroxylase (TH) and VGluT2 by these neurons is regulated during embryonic development, and may be derepressed or reactivated postnatally following their partial destruction by neonatal administration of 6-hydroxydopamine (6-OHDA). In both 15 day-old and adult rats subjected or not to the neonatal 6-OHDA lesion, concurrent electron microscopic examination of the nucleus accumbens after dual immunocytochemical labeling for TH and VGluT2 reveals the co-existence of the two proteins in a significant proportion of these axon terminals. Moreover, all TH varicosities which co-localize VGluT2 are synaptic, as if there was a link between the potential of DA axon terminals to release glutamate and their establishment of synaptic junctions. Together with the RT-PCR and in situ hybridization data demonstrating the co-localization of TH and VGluT2 mRNA in mesencephalic neurons of the VTA, these observations raise a number of fundamental questions regarding the functioning of the meso-telencephalic DA system in healthy or diseased brain.