Blackwell Publishing Journal Backfiles 1879-2005
Primary cultures of gerbil mesencephalon were used for studying the modulation exerted by tachykinin NK3 receptor activation on the activity of dopamine (DA) neurons. Dopamine neurons were identified by their ability to take up [3H]DA in a nomifensine-dependent manner. Moreover, tyrosine hydroxylase immunohistochemistry revealed that these neurons accounted for 5–7% of the total cell population. The NK3 receptor agonists, senktide (EC50= 0.58 nM) and [MePhe7]neurokinin B (EC50= 3 nM), increased spontaneous [3H]DA release in a concentration-dependent manner. In contrast, tested at a supramaximal concentration (10-7 M), neither septide nor substance P were found to affect [3H]DA release. The senktide-evoked [3H]DA release was not observed when extracellular Ca2+ was chelated, but was unaffected by nomifensine. This indicates that this increase in [3H]DA outflow resulted more from an exocytotic process than from reversal of carrier-mediated DA uptake. Moreover, the senktide effect was unaffected by the Na+ channel blocker tetrodotoxin, a result suggesting a direct action of senktide on DA neurons. The non-peptide NK3 receptor antagonist, SR 142801, shifted or blocked (ICs0 = 0.89 nM) the senktide-evoked [3H]DA release, while its (-)-antipode, SR 142806, was 80-fold less potent, in agreement with binding data. Selective antagonists for NK, (SR 140333) or NK2 (SR 48968) receptors failed to reduce the senktide effect. Light scanning microscopic analysis of mesencephalic cells loaded with the Ca2+ sensitive dye, fluo-3, showed that senktide induced a rise in cytosolic Ca2+ in 8-10% of the cell population. The senktide-induced elevation in intracellular Ca2+ was rapid in onset and transient (at lo4 M) or more sustained with no further increase in fluorescence intensity (at 10-7 M). The proportion of senktide-responsive cells was not significantly modified when extracellular Ca2+ was chelated, but was reduced by 87% in the presence of SR 142801 and by 75% in cultures that were pre-treated with the DA neurotoxin l-methyl-4-phenylpyridinium. The present study shows that enhancement of spontaneous [3H]DA release and intracellular Ca2+ mobilization may be observed after NK3 receptor stimulation and that both biochemical events are likely to occur in DA neurons.
Type of Medium: