Microiontophoresis of biogenic amines on cortical neurons: amounts of NA, DA and 5-HT ejected, compared with tissue content

The present series of studies were carried out to quantify the amounts of dopamine (DA), noradrenaline (NA) and serotonin (5-HT) ejected from iontophoresis micropipettes and that produce inhibitory and modulatory effects on cortical neurons, in the frontoparietal cortex of the rat and in the occipital cortex of the cat. Using radioactive isotopes of the biogenic amines the effective transport number (n) was found to be 0.08 for DA; 0.13 for NA, and 0.22 for 5-HT. In addition, similar determinations were made, for comparison purposes, of the transport numbers of the neurotransmitters acetylcholine (ACh; n = 0.44), gamma-amino-n-butyrate (GABA, n = 13), and glutamate (GLU; n = 0.27). The quantities ejected under in vivo conditions were then estimated using Faraday's formula and fell in the pmol range (10(-12) mol). The effects of DA, NA and 5-HT on cortical units were studied; the amounts ejected were compared with the endogenous tissue content of these amines, determined by means of specific and sensitive radioenzymatic assays in the regions where the microiontophoretic experiments were performed. These results are discussed in the light of the anatomical, biochemical and electrophysiological data suggesting a modulatory role for the biogenic amines in the cerebral neocortex.