MODULATION OF DOPAMINE AND NORADRENALINE RELEASE AND OF INTRACELLULAR CA2+ CONCENTRATION BY PRESYNAPTIC GLUTAMATE RECEPTORS IN HIPPOCAMPUS

1 We studied the release of [H-3]-dopamine and [H-3]-noradrenaline (NA) from hippocampal synaptosomes induced by glutamate receptors and the associated Ca2+ influx through Ca2+ channels. The release of tritiated neurotransmitters was studied by use of superfusion system and the intracellular free Ca2+ concentration ([Ca2+](i)) was determined by a fluorimetric assay with Indo-1 as a probe for Ca2+. 2 Presynaptic glutamate receptor activation induced Ca2+-dependent release of [H-3]-dopamine and [H-3]-NA from rat hippocampal synaptosomes. Thus, L-glutamate induced the release of both neurotransmitters in a dose-dependent manner (EC(50) = 5.62 mu M), and the effect of 100 mu M L-glutamate was inhibited by 83.8% in the presence of 10 mu M 6-cyano-7-nitroquinoxaline-2,3-dioxine (CNQX), but was not affected by 1 mu M (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801). 3 Other glutamate receptor agonists also stimulated the Ca2+-dependent release of [H-3]-dopamine and [H-3]-NA as follows: N-methyl-D-aspartate (NMDA), at 200 mu M, released 3.65 +/- 0.23% of the total H-3 catecholamines, and this effect was inhibited by 81.2% in the presence of 1 mu M MK-801; quisqualate (50 mu M), S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolopropionic acid (AMPA) (100 mu M) or kainate (100 mu M) released 1.57 +/- 0.26%, 1.93 +/- 0.17% and 2.09 +/- 0.22%, of the total H-3 catecholamines, respectively. 4 The ionotropic glutamate receptor agonist, AMPA, induced an increase in the [Ca2+](i) which was inhibited by 58.6% in the presence of 10 mu M CNQX. In contrast, the increase in [Ca2+](i) due to stimulation by glutamate was not sensitive to CNQX or MK-801. 5 Nitrendipine, at 1 mu M, did not inhibit the neurotransmitter release induced by AMPA, but, both 0.5 mu M omega-conotoxin GVIA (omega-CgTx) and 100 nM omega-Aga IVA reduced catecholamine release to 49.03 +/- 3.79% and 46.06 +/- 10.51% of the control, respectively. In the presence of both toxins the release was reduced to 12.58 +/- 4.64% of the control. 6 The results indicate that activation of presynaptic glutamate receptors of the NMDA and non-NMDA type induces the release of [H-3]-dopamine and [H-3]-NA from rat hippocampal synaptosomes and that the release induced by AMPA involves the activation of N- and P-type Ca2+ channels which allow the influx of Ca2+ that triggers the H-3 catecholamines release.