The kidneys play pivotal roles in acid-base homeostasis, and the acid-secreting (␣-type) and bicarbonate-secreting (␤-type) intercalated cells in the collecting ducts are major sites for the final modulation of urinary acid secretion. Since the H ؉ -ATPase and anion exchanger activities in these two types of intercalated cells exhibit opposite polarities, it has been suggested that the ␣-and ␤-intercalated cells are interchangeable via a cell polarity change. Immunohistological studies, however, have failed to confirm that the apical anion exchanger of ␤-intercalated cells is the band 3 protein localized to the basolateral membrane of ␣-intercalated cells. In the present study, we show the evidence that a novel member of the anion exchanger and sodium bicarbonate cotransporter superfamily is an apical anion exchanger of ␤-intercalated cells. Cloned cDNA from the ␤-intercalated cells shows about 30% homology with anion exchanger types 1-3, and functional expression of this protein in COS-7 cells and Xenopus oocytes showed sodium-independent and 4,4-diisothiocyanostilbene-2,2-disulfonic acid-insensitive anion exchanger activity. Furthermore, immunohistological studies revealed that this novel anion exchanger is present on the apical membrane of ␤-intercalated cells, although some ␤-intercalated cells were negative for AE4 staining. We conclude that our newly cloned transporter is an apical anion exchanger of the ␤-intercalated cells, whereas our data do not exclude the possibility that there may be another form of anion exchanger in these cells.

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Alpha-methyl-p-tyrosine (AMPT) is known to inhibit the formation of dopamine (DA) in the cytosol of dopaminergic neurons and is therefore used to study the role of the cytosolic DA pools. AMPT is usually administered systemically. In the present study, however, the effects of locally infused AMPT on the efflux of DA from the nucleus accumbens and dorsal striatum were analyzed, using in vivo brain microdialysis in unanesthetized rats. The administration of AMPT (100 microM, 4 h) into the nucleus accumbens reduced accumbal DA output to 30% of its baseline level. When it was infused into the dorsal striatum, however, it reduced striatal DA output to 60% of its baseline level. At first sight, these data suggest that the amount of DA available from the AMPT-sensitive pool is larger in the nucleus accumbens than in the striatum. However, this cannot be the case, as the decrease in accumbal and striatal DA efflux induced by systemic administration of AMPT (250 mg/kg given intra-peritoneally) was identical. These results show that local infusion of AMPT is a valuable tool for analyzing the role of AMPT-sensitive pools within a particular brain area, but it cannot be used to compare effects across different brain structures because a fixed dose of AMPT differentially affected the nucleus accumbens and the dorsal striatum.

Microdialysis technique was used to study the effects of the locally applied alpha adrenoceptor agonist phenylephrine and antagonist phentolamine on the basal noradrenaline efflux as well as on the noradrenaline uptake inhibitor desipramine-elicited noradrenaline efflux in the nucleus accumbens (NAc) of freely moving rats. Tetrodotoxin reduced basal noradrenaline efflux by 72%, whereas desipramine increased it by 204%. Phenylephrine reduced the basal noradrenaline efflux by 32% and phentolamine blocked this effect. Phentolamine elevated the basal noradrenaline efflux by 150% and phenylephrine counteracted this effect. The desipramine-elicited noradrenaline efflux was not affected by phenylephrine, but enhanced by phentolamine. Desipramine counteracted the effects of phenylephrine and potentiated those of phentolamine. These results indicate that the accumbal noradrenaline efflux is under inhibitory control of alpha adrenoceptors that are suggested to be presynaptically located on adrenergic nerve terminals in the NAc. Furthermore, this study suggests that the conformational state of alpha adrenoceptors varies across the available amount of noradrenaline. The clinical impact of these data is discussed.

Activation of mu-opioid receptors in the nucleus accumbens (NAc) is known to increase accumbal dopamine efflux in rats. Endomorphin-2 (Tyr-Pro-Phe-Phe-NH 2 ; EM-2) and endomorphin-1 (Tyr-Pro-Trp-Phe-NH 2 ; EM-1) are suggested to be the endogenous ligands for the mu-opioid receptor. As the ability of EM-2 and EM-1 to alter the accumbal extracellular dopamine level has not yet been studied in freely moving rats, the present study was performed, using a microdialysis technique that allows on-line monitoring of the extracellular dopamine with a temporal resolution of 5 min. A 25 min infusion of either EM-2 or EM-1 into the NAc (5, 25, and 50 nmol) produced a dose-dependent increase of the accumbal dopamine level. The EM-2 (50 nmol)-and EM-1 (25 and 50 nmol)-induced dopamine efflux were abolished by intra-accumbal perfusion of tetrodotoxin (2 mM). Intra-accumbal perfusion of the mu-opioid receptor antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH 2 ; 3 nmol) failed to affect the EM-2 (50 nmol)-induced dopamine release, whereas it significantly inhibited the EM-1 (25 and 50 nmol)-induced dopamine release. The EM-1 (50 nmol)-induced accumbal dopamine efflux was significantly reduced by the systemic administration of the putative mu1-opioid receptor antagonist naloxonazine (15 mg/kg, intraperitoneally (i.p.), given 24 h before starting the perfusion). Systemic administration of the aspecific opioid receptor antagonist naloxone (1 mg/kg, i.p., given 10 or 20 min before starting the perfusion) also failed to affect the EM-2 (50 nmol)-induced dopamine efflux, whereas it significantly inhibited the EM-1 (25 and 50 nmol)-induced dopamine efflux. The present study shows that the intra-accumbal infusion of EM-2 and EM-1 increases accumbal dopamine efflux by mechanisms that fully differ. It is concluded that the effects of EM-2 are not mediated via opioid receptors in contrast to the effects of EM-1 that are mediated via mu1-opioid receptors in the NAc.