Release of dopamine in vivo from cat substantia nigra

Abstract

THE central aminergic neurones present some original characteristics. By means of quantitative autoradiographic studies, it has been demonstrated that serotoninergic terminals projecting into the cerebral cortex and the locus coeruleus exhibit very few classical synaptic contacts^1,2. This has also been seen with cortical noradrenergic terminals¹. It has been suggested that the amines could be released not only in synapses, but also from varicosities devoid of synaptic junctions: they could thus act as neuromodulators at some distance from the release sites. It has also been proposed that dopamine could be released from dendrites of nigrostriatal dopaminergic neurones³. Indeed, a release of endogenous dopamine (Cuello, personal communication) or of labelled dopamine^4,5 previously taken up in tissues, has been demonstrated using slices of the rat substantia nigra exposed to a high concentration of potassium (30 mM). This process was calcium dependent. An indication of possible dendritic release was also obtained indirectly in vivo by measuring dopamine metabolites in the rat substantia nigra⁶. Dihydroxyphenylacetic acid and homovanillic acid levels were increased after stimulation of the medial forebrain bundle, which contains axons of various ascending aminergic pathways including the nigrostriatal dopaminergic system. Therefore, dopamine could be released not only from nerve terminals in the striatum, but also from dendrites in the substantia nigra. If the latter mechanism was occuring in physiological states, it could have important implications for the understanding of the regulatory processes involved in the control of the activity of the dopaminergic neurones. It thus seemed necessary to demonstrate directly an in vivo release of dopamine in the substantia nigra.