In Vivo Mechanisms Underlying Dopamine Release from Rat Nigrostriatal Terminals: I. Studies Using Veratrine and Ouabain

: The in vivo mechanisms underlying the dopamine (DA)‐releasing actions of veratrine and ouabain in the striatum of halothane‐anaesthetised rats have been investigated using brain microdialysis. Relevant catecholamines and indoleamines were separated and quantified using HPLC combined with an electrochemical detection system. Veratrine (10 μg/ml‐1 mg/ml) and ouabain (10 μM‐1 mM) were added to the medium perfusing the dialysis probes. Both compounds increased dialysate DA content in a dose‐related manner. Dialysate levels of the DA metabolites 3,4‐dihydroxyphenylacetic acid and homovanillic acid and the serotonin metabolite 5‐hydroxyindoleacetic acid were reduced by both veratrine and ouabain. Veratrine‐induced DA efflux was maximal in the first 20‐min sample collected after drug infusion began, whereas the maximal effect of ouabain was not observed until 20–40 min after administration began. Veratrine‐induced DA efflux was unaffected by systemic injection of the DA uptake inhibitor nomifensine but was inhibited by either coperfusion of tetrodotoxin (TTX) or removal of calcium from the perfusing buffer. These data suggest that veratrine induces release of DA via a carrier‐independent mechanism, perhaps involving an exocytotic release process. In contrast, ouabain‐induced DA release was reduced by nomifensine but was inhibited to a lesser degree by calcium depletion and TTX. Detailed analyses of these data suggest that although ouabain initially induces release of DA via a carrier‐dependent mechanism, an exocytotic process may also be involved. The finding that ouabain‐induced DA efflux exhibits a degree of TTX and calcium sensitivity suggests that membrane depolarisation caused by Na+,K+‐ATPase blockade opens voltage‐gated sodium channels and initiates an exocytotic release of DA. The intracellular pools of DA involved in the release of DA induced by veratrine and ouabain were also examined. Depletion of vesicular pools of DA by pretreatment with reserpine reduced the amount of DA release induced by both agents, although this effect was only significant in the case of veratrine. However, in reserpinised animals the residual amount of DA release induced by veratrine was inhibited by nomifensine, a result suggesting that DA may be released via a carrier‐dependent process in the absence of vesicular DA. Newly synthesised pools of DA were also depleted by pretreatment with the DA synthesis inhibitor α‐methyl‐p‐tyrosine. Under these conditions, both veratrine‐ a