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Am J Physiol Regul Integr Comp Physiol 276: R1339-R1345, 1999;
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Vol. 276, Issue 5, R1339-R1345, May 1999

Sodium depletion and aldosterone decrease dopamine transporter activity in nucleus accumbens but not striatum

Mitchell F. Roitman1, Terrell A. Patterson2, Randall R. Sakai3, Ilene L. Bernstein1,4, and Dianne P. Figlewicz1,4,5

1 Program in Neurobiology and Behavior and 4 Department of Psychology, University of Washington, Seattle 98195; 5 Metabolism/Endocrinology (151), Veterans Affairs Puget Sound Health Care System, Seattle, Washington 98108; 2 Pfizer Central Research, Groton, Connecticut 06340; and 3 Department of Animal Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Motivated behaviors, including sodium (Na) appetite, are correlated with increased dopamine (DA) transmission in the nucleus accumbens (NAc). DA transporter (DAT) modulation affects DA transmission and may play a role in motivated behaviors. In vivo Na depletion, which reliably induces Na appetite, was correlated with robust decreases in DA uptake via the DAT in the rat NAc with rotating disk electrode voltammetry [1,277 ± 162 vs. 575 ± 89 pmol · s-1 · g-1; Vmax of transport for control vs. Na-depleted tissue]. Plasma aldosterone (Aldo) levels increase after in vivo Na depletion and contribute to Na appetite. Decreased DAT activity in the NAc was observed after in vitro Aldo treatment (428 ± 28 vs. 300 ± 25 pmol · s-1 · g-1). Neither treatment affected DAT activity in the striatum. These results suggest that a direct action of Aldo is one possible mechanism by which Na depletion induces a reduction in DAT activity in the NAc. Reduced DAT activity may play a role in generating increased NAc DA transmission during Na appetite, which may underlie the motivating properties of Na for the Na-depleted rat.

sodium appetite; motivation; reward; rat


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

DOPAMINE (DA) transmission is increased in the nucleus accumbens (NAc) during a variety of motivated behaviors (e.g., intracranial stimulation, drug taking, sex, and feeding) (13, 18, 23, 38-40). The role of NAc DA in ingestive behavior has received considerable attention, and DA concentration measurements coupled with lesion and antagonist treatment studies have implicated NAc DA in mediating the reinforcement of palatable foods and motivated ingestive behaviors (for review, see Refs. 34, 36). Ingestion of concentrated Na-containing substances is different from ingestion of palatable foods in that, under normal circumstances, concentrated Na-containing substances are avoided or rejected. However, when rats become depleted of Na, they display Na appetite: a highly motivated behavior characterized by the avid ingestion of concentrated Na-containing substances (10). Na-depleted rats, in contrast to rats that are Na replete, appear to find the taste of hypertonic Na solutions palatable and rewarding (6, 9). There is a small but growing body of literature implicating NAc DA transmission in the expression of Na appetite. Selective DA antagonists suppress sham drinking of NaCl solutions in Na-depleted rats (29). Substantial increases in NAc DA levels and DA metabolism in Na-depleted rats ingesting NaCl have been reported (14, 18). These studies provide both indirect and direct evidence of increased DA transmission during Na appetite. However, the mechanisms by which DA transmission is increased during Na appetite expression remain unclear.

One primary regulator of extracellular DA concentration during DA transmission is the DA reuptake transporter (DAT), which clears synaptically released DA with high affinity and specificity (3, 33). It has been shown recently that reducing DAT protein levels in the striatum, although it does not affect basal extracellular DA concentration, can lead to dramatic differences in DA overflow after stimulated DA release (33). Studies from our laboratory and others have shown that the rate of DA uptake via the DAT is sensitive to physiological state and hormonal regulation. Reduced DAT uptake rates in the striatum were demonstrated in food-deprived rats (26). In addition, effects of the steroid hormones corticosterone (Cort) and estrogen on DAT uptake rates have been observed (17, 37, and T. A. Patterson and D. Figlewicz Lattemann, unpublished observations). In the present study, rotating disk electrode voltammetry (RDEV) was used to test the hypothesis that Na depletion, which reliably gives rise to the robust consumption of hypertonic Na solutions, decreases DAT function. Furthermore, the steroid hormone aldosterone (Aldo), important for the expression of Na appetite (31), was tested to see if it could directly decrease DAT function. DAT activity after in vivo Na depletion or in vitro Aldo treatment was compared with control treatments in two DA terminal sites: the NAc and the striatum. If Na depletion was correlated with reduced reuptake rates, then there could be greater DA concentrations in DA target areas during expression of Na appetite. Increased DA transmission could contribute to mediating the reinforcing properties of hypertonic Na solutions for the Na-depleted rat.


    MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Animals. Male Sprague-Dawley rats, 200-250 g (Simonsen, Gilroy, CA), were used in all experiments. Rats were maintained on a 12:12-h light-dark cycle (lights on 0700, lights off 1900), with food and water available ad libitum unless noted. All procedures were approved by the Institutional Animal Care and Use Committees of the Veterans Affairs Puget Sound Health Care System and the University of Washington. Rat euthanasia was performed by guillotine. Immediately after euthanasia, brains were rapidly removed, and striatum or NAc was dissected on an iced (4°C) glass plate.

Experimental design. To determine the effect of acute in vivo Na depletion on DA uptake via the DAT, rats were treated 24 h before euthanasia with the diuretic furosemide (10 mg/kg sc in 2 equal injections spaced 1 h apart; Astra USA, Westborough, MA) or vehicle (0.9% NaCl sc) as a control. Just before the first injection, food and water were removed, and rats were weighed. Injections were given, and, at 1 h after the second injection, rats were weighed again to confirm that furosemide-treated animals experienced diuresis. Water bottles were returned to the cages along with food. Rats treated with furosemide received an Na-deficient diet (ICN Biochemicals, Costa Mesa, CA). This procedure reliably induces an Na appetite (41). Rats were weighed 24 h posttreatment to ensure that furosemide-treated animals regained any loss in body weight as a result of the furosemide-induced diuresis. Rats were then euthanized, and trunk blood was collected to determine circulating levels of hormones (n = 7 and 8 for Na-depleted and control rats, respectively). Dissected NAc or striatum was finely minced, washed eight times in Krebs bicarbonate buffer, which was equilibrated with 95% CO2-5% O2 and warmed to 37°C, and DA uptake was assessed (see In vitro measurement of DA uptake via DAT).

To determine the effect of the mineralocorticoid Aldo on DA uptake via the DAT, naive rats with ad libitum access to normal lab chow and water were used. After euthanasia, dissection, and washing, one NAc or one striatum from each rat was treated with Aldo (aldosterone hemisuccinate; Steraloids, Wilton, NH), the contralateral tissue was treated with vehicle, and treatment with respect to side and order of voltammetric measurement was counterbalanced. Tissue was incubated with vehicle (0.0008, 0.008, or 0.04% ethanol in distilled, deionized H2O) or Aldo in 0.0008, 0.008, or 0.04% ethanol for final concentrations of 20, 200, or 1,000 nM, respectively. The number of NAc samples used was 3, 3, and 4 for the low-, medium-, and high-control solutions, respectively, and 3, 4, and 4 for the low-, medium-, and high-Aldo concentrations, respectively. The number of striatal samples used was 4 for the high-control solution and 4 for the high-Aldo concentration. RDEV measurement of DA uptake was then performed as described below, but only one concentration of exogenous DA was used for this study (4,000 nM).

In vitro measurement of DA uptake via DAT. RDEV, which has been described in detail previously (22), was used to determine the effect of Na depletion on the kinetics of DA uptake by the DAT or the effect of Aldo treatment on DA uptake by the DAT with a single concentration of DA. This technique has been shown to be a rapid method for the determination of the kinetics of active uptake by the DA transporter (22). Briefly, striatum and NAc suspensions were prepared as described above, and voltammetric measurements were made. Tissue and treatment order were counterbalanced. Suspensions were washed eight times (1/3 volume wash each) in 95% O2-5% CO2 saturated physiological buffer (in mM: 124 NaCl, 3.0 KCl, 1.24 KH2PO4, 1.3 MgSO4, 2.5 CaCl2, 26 NaHCO3, and 10 glucose), resulting in a pH of 7.4 and a final volume of 500 µl. The solution was transferred to the electrochemical cell and stirred with a glassy carbon electrode (rotation speed 2,000 rpm controlled with a Pine Instruments AFMSR high-performance rotator; Grove City, PA). The applied potential was +450 mV vs. Ag-AgCl. The electrochemical signal was monitored on a strip-chart recorder (LKB-rec 102; Pharmacia, Stockholm, Sweden). After a steady baseline was established, a single addition of DA was made (final concentration 500, 1,000, 2,000 or 4,000 nM), and the uptake profile was recorded. The number of NAc samples used was 3, 6, 4, and 4 from control rats and 3, 5, 5, and 4 for Na-depleted rats at the 500, 1,000, 2,000, and 4,000 nM DA addition, respectively. The number of striatal samples used was 3, 3, 3, and 5 from control rats and 3, 3, 3, and 4 from Na-depleted rats at the 500, 1,000, 2,000, and 4,000 nM DA addition, respectively. Initial zero-order velocity of transport was obtained from the slope of the first 30 s of the uptake profile. Maximal velocity (Vmax) and Michaelis-Menten constant (Km) of transport were determined by using the initial velocity of uptake as a function of DA concentration added, and nonlinear analysis for adherence to the Michaelis-Menten equation was tested (analysis performed using Prism; GraphPad Software, San Diego, CA). Significant differences between the parameters resulting from regression analysis were determined using a z-test.

[125I]RTI-121 binding assay. Competition for binding of the high-affinity, high-specificity cocaine analog RTI-121 by Aldo was measured to test for a possible interaction of Aldo directly with the DAT. Striatum (n = 3) or NAc (n = 3) was dissected and immediately frozen at -80°C. The binding assay methodology was similar to that of Patterson and Figlewicz Lattemann (27). Briefly, on the day of the binding assay, striatum or NAc was placed in binding assay buffer (in mM: 10 Na2HPO2, 1.8 KH2PO4, 136 NaCl, and 2.8 KCl, pH 7.4) at 4°C, at a concentration of ~1 mg tissue wet wt/ml. Brain regions were homogenized with an electric homogenizer (Tissue Tearor; BiospecProducts, Bartlesville, OK) for 10 s and then centrifuged at 50,000 g for 10 min. Pellets were washed, and the homogenization and centrifugation procedure was repeated. The final pellet was resuspended in the original volume of assay buffer. Protein concentration was measured with a bicinchoninic acid protein assay reagent kit and microplate spectrophotometer (model MR 500; Dynatech, Chantilly, VA). The binding reaction mixture consisted of 100 µl of membrane suspension (~0.1 mg), 50 pM [125I]RTI-121, and increasing concentrations of Aldo (0-1,000 nM). Displacement of [125I]RTI-121 binding with 1 µM unlabeled RTI-121 or 5.0 µM mazindol, a nonspecific inhibitor of the DAT, served as positive controls. After incubation at either 25°C for 1 h [previously established conditions in this laboratory (Patterson and Figlewicz Lattemann, unpublished observations)] or 37°C for 30 min (simulating the RDEV assay conditions), the assays were terminated by rapid filtration of samples onto Whatman GF/B glass fiber filters (presoaked in 0.05% polyethyleneimine) with a Brandel cell harvester (Gaithersburg, MD). Filters were washed three times with ice-cold assay buffer, and radioactivity was measured on a gamma counter (APEX Automatic Gamma Counter; Micromedic Systems). Binding studies were performed in triplicate.

RIA for plasma Aldo and Cort. Plasma Aldo was determined using a commercially available RIA kit (Diagnostic Products) for Aldo. The kit uses 125I-labeled Aldo and requires no extraction or chromatography for analysis. Plasma Cort was also determined by RIA using [3H]Cort and a commercially available antibody (no. B21-42; Endocrine Sciences, Tarzana, CA). The assay methodology was that used by Spencer et al. (35).


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Adrenal hormone concentrations in Na-depleted vs. -replete rats. Na depletion with the diuretic furosemide leads to high circulating levels of plasma Aldo (30). Analysis of Aldo and the glucocorticoid Cort from the plasma of rats in which we measured DA uptake was measured by RIA, and the results are shown in Fig. 1. Plasma Aldo concentration in rats depleted of Na was significantly increased compared with saline-treated controls, consistent with previous reports (30) [t(13) = 4.39, P < 0.01; Fig. 1, top]. In contrast, there was no difference in plasma Cort concentrations between groups (Fig. 1, bottom).


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  		Fig. 1.   Plasma concentration of steroid hormones determined by RIA. Data are means ± SE. Top: mineralocorticoid aldosterone (Aldo); n = 7 and 8 Na-depleted and control samples, respectively. # P < 0.01 vs. control. Bottom: glucocorticoid corticosterone (Cort); n = 7 and 5 Na-depleted and control samples, respectively.

Effect of Na depletion on DA uptake rates in NAc and striatum. The rate of DA uptake via the DAT in either the NAc or striatum was assessed by RDEV in rats depleted of Na or rats that were in positive Na balance. We observed a decrease in DA uptake at several concentrations of DA in the NAc but no difference at any concentration of DA in the striatum for Na-depleted vs. -replete rats (Figs. 2 and 3). Statistical significance of these results was explored with a three-way ANOVA, which revealed main effects of physiological state [control vs. Na depleted; F(1,60) = 12.36, P < 0.01], type of tissue [NAc vs. striatum; F (1,60) = 14.09, P < 0.01], and concentration of DA [F(3,60) = 102.2, P < 0.01]. In addition, there were significant interactions between type of tissue and physiological state [F(1,60) = 8.41, P < 0.01] and concentration of DA and physiological state [F(3,60) = 3.69, P < 0.05]. Because of the significant main effect of tissue type, kinetic analysis of the concentration effect was performed separately for NAc and striatum. For each type of tissue, uptake rate vs. concentration curves for Na-depleted and -replete rats fit Michaelis-Menten kinetics and can be seen in Figs. 2 and 3. With respect to NAc tissue, we found a significant decrease in the Vmax for DA (1,277 ± 162 vs. 575 ± 89 pmol · s-1 · g wet wt-1, control vs. Na depleted; z = 3.78, P < 0.01). In addition, there was a significant decrease in the Km of DA transport (3.65 ± 0.81 vs. 1.59 ± 0.57 µM, control vs. Na depleted; z = 2.09, P < 0.05).


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  		Fig. 2.   Initial velocity of uptake of dopamine (DA) as a function of concentration of DA in nucleus accumbens (NAc) suspensions. Data for each concentration are means ± SE of several independent measurements. , Uptake in NAc suspensions from control-treated rats (n = 3, 6, 4, and 4 for 500, 1,000, 2,000, and 4,000 nM DA, respectively); , uptake in NAc suspensions from Na-depleted rats (n = 3, 5, 5, and 4 for 500, 1,000, 2,000, and 4,000 nM DA, respectively). Nonlinear regression analysis demonstrates adherence to Michaelis-Menten equation (r2 <=  0.99) as shown by lines of fit to data points.



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  		Fig. 3.   Initial velocity of uptake of DA as a function of concentration of DA in striatal suspensions. Data for each concentration are means ± SE of several independent measurements. , Uptake in striatal suspensions from control-treated rats (n = 3, 3, 3, and 5 for 500, 1,000, 2,000, and 4,000 nM DA, respectively); , uptake in striatal suspensions from Na-depleted rats (n = 3, 3, 3, and 4 for 500, 1,000, 2,000, and 4,000 nM DA, respectively). Nonlinear regression analysis demonstrates adherence to Michaelis-Menten equation (r2 <=  0.99) as shown by lines of fit to data points.

The significant interaction between type of tissue and physiological state suggested that, of the two tissue types investigated here, Na depletion affected DA uptake in only the NAc. This was confirmed when kinetic analysis of the DA uptake by concentration curves in the striatum was performed. There was no effect of Na depletion on either the Vmax (935 ± 75 vs. 873 ± 98 pmol · s-1 · g wet wt-1, control vs. Na depleted) or Km (1.95 ± 0.32 vs. 1.84 ± 0.57 µM, control vs. Na depleted) in the striatum.

Effects of in vitro Aldo on DA uptake rate in the NAc and striatum. Because Na depletion correlates with decreased DA uptake via the DAT, and because plasma Aldo is elevated in Na-depleted rats, we investigated whether direct Aldo treatment could lead to decreased DAT activity. Although there were separate vehicle-treated groups for each concentration of Aldo, rate measurements across vehicle groups did not statistically differ and thus data from vehicle groups were combined. After 30 min of incubation, the reduction in uptake of 4,000 nM DA after Aldo in the NAc was significant at the highest concentration (1,000 nM) of Aldo used [t(12) = 2.68, P < 0.05; Fig. 4]. There was no effect of 1,000 nM Aldo on DAT activity in the striatum compared with vehicle-treated striatum tissue (Fig. 5). Uptake rates in control tissue after the addition of 4,000 nM DA were lower in this study compared with rates obtained above. This can be attributed to an effect of the vehicle in this study, consistent with other studies from our laboratory (A. Zavosh and D. Latteman, unpublished data).


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  		Fig. 4.   Effect of in vitro Aldo on DA uptake in NAc. After 30-min incubation with either vehicle (n = 10) or Aldo (20 nM, n = 3; 200 nM, n = 4; 1,000 nM, n = 4). DA (final conc. 4,000 nM) uptake was measured by rotary disk electrode voltammetry (RDEV). Data are means ± SE. * P < 0.05 vs. vehicle.



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  		Fig. 5.   Lack of effect of in vitro Aldo (1,000 nM) on striatal DA uptake. Data are means ± SE; n = 4 for each group.

Competition of Aldo with [125I]RTI-121 binding in NAc and striatum. To determine if the effects observed with Aldo treatment were a result of a direct interaction of Aldo with the cocaine binding site of the DAT, we measured binding of the specific, high-affinity cocaine analog RTI-121 in the presence of a range of concentrations of Aldo (10-1,000 nM). We used semipurified plasma membranes from the NAc or striatum. Identical results were obtained when binding assays were run under two different conditions (see [125I]RTI-121 binding assay; data from 30-min, 37°C assay condition shown in Fig. 6). As a positive control, displacement of [125I]RTI-121 binding by unlabeled RTI-121 or the less-specific pharmacological agent mazindol was observed. However, at no concentration did Aldo decrease [125I]RTI-121 binding in either the NAc or the striatum (Fig. 6, top and bottom, respectively).


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  		Fig. 6.   Lack of effect of several concentrations of Aldo on displacement of [125I]RTI-121 binding. Measurement was made on semipurified membranes (n = 3 preparations). Addition of unlabeled RTI (1 µM) or mazindol (Maz, 5 µM) served as controls, in which displacement of [125I]RTI-121 binding was observed. Top: NAc membranes. Bottom: striatal membranes.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

The present studies addressed whether DAT activity could be reduced by a treatment that induces the drive underlying the motivated behavior of Na appetite. DAT activity was evaluated in an in vitro preparation. The major findings were significant reductions of DA uptake rate in the NAc after in vivo Na depletion and in vitro tissue treatment with the hormone Aldo. In the first of these studies, tissue was obtained at a time when Na was depleted in vivo (consistent with the elevation in plasma Aldo observed) and a strong Na appetite would be displayed (29). Two brain regions rich in DA terminals and containing high levels of DAT were examined: the striatum and the NAc (8, 15). Twenty-four hours after Na depletion with diuretic treatment, there was a significant decrease in DA uptake rate in the NAc but not in the striatum. Kinetic analysis of uptake suggested that the effect may be accounted for by reduction in DAT protein in NAc synaptic membranes (significant reduction in Vmax for tissue taken from Na-depleted rats).

The temporary loss in body weight engendered by the diuresis is unlikely to be responsible for the change in DA uptake in the NAc. Twenty-four-hour food deprivation, which causes a significant and sustained drop in body weight, has no effect on NAc DA uptake (26). Furthermore, it is also unlikely that in vivo Na depletion by furosemide reduced Na at the level of the basal ganglia, which could account for the decrease in DA uptake. Although Na depletion leads to Na appetite, it does not appear that brain levels of Na per se trigger Na appetite. For instance, experimentally lower cerebrospinal fluid (CSF) Na concentration in rats fails to induce an Na appetite (12). Importantly, it has been shown here that DA uptake rates in the striatum were unchanged by in vivo Na depletion. Thus it is likely that some agent other than reduced CSF or central nervous system (CNS) Na is responsible for the reduction in DAT activity.

One possible mechanism by which Na depletion could result in decreased DAT function in the NAc was investigated in the second part of these studies. In this and other studies, robust increases in plasma levels of the mineralocorticoid hormone Aldo after Na depletion have been demonstrated (30). Together with another hormone, angiotensin II, Aldo is thought to play an important role in triggering Na appetite (11, 31, 42). In tissue taken from naive rats, a concentration-dependent decrease in DA uptake rate after 30 min incubation with Aldo in vitro was observed. Similar to the effects of Na depletion, the effect of Aldo incubation was observed in the NAc but not in the striatum. Analogous to the in vivo effect of Na depletion, the failure of Aldo to alter DA uptake in the striatum argues against nonspecific effects of Aldo on the DAT. In addition, Aldo failed to compete for binding to at least one extracellular domain of the DAT with radioactively labeled RTI-121 in both DA terminal regions examined. These results are consistent with a role for Aldo in mediating the decrease in DA uptake observed after Na depletion. Furthermore, it is unlikely that the actions of Aldo were a result of a direct interaction with the DAT itself.

Steroid hormones are classically thought to act by diffusing through the plasma membrane, binding to intracellular receptors, and having direct genomic effects. The observed effects of Aldo in these studies were relatively rapid (observed at 30 min) and, taken together with the fact that our preparation isolated DA terminals from their cell bodies, suggest a nongenomic route of action. It is now established that steroid hormone actions may engage nongenomic as well as genomic pathways. Estrogen has been shown to alter calcium conductance in striatal neurons even when the hormone was modified such that it was unable to diffuse through the cell membrane (24). Estrogen has also been shown to rapidly decrease DA uptake rate in the NAc, presumably through nongenomic mechanisms (37). Recently, Cort has been shown to modulate the DAT in a protein kinase C (PKC)-sensitive manner (Patterson and Figlewicz Lattemann, unpublished observations). Finally, Aldo has been shown to rapidly activate a proton conductance in cultured kidney cells even when genomic pathways are blocked (16). The authors provide evidence that this action of Aldo is mediated by G protein-dependent stimulation of PKC. Because Cort and Aldo can bind to one common intracellular receptor (1, 4), perhaps Aldo can also activate the same PKC-mediated mechanism as can Cort to modulate DAT activity. Cort can be ruled out as a possible mechanism by which the DAT is modulated in Na-depleted rats in this study, because plasma Cort levels did not differ between groups. Regional specificity for both PKC and adenylate cyclase isoforms in the CNS has been reported (7, 25). A quantitative or qualitative difference between NAc and striatum expression of a postreceptor signaling protein coupled to a receptor for Aldo and Cort may underlie the regional specificity of these hormonal effects on DAT activity.

In the present studies, Na depletion and Aldo treatment similarly and selectively affected NAc DAT. The localization of the effect to the NAc is striking given the difference in the types of behaviors in which NAc and striatum are thought to participate. Although controversy remains (32), the NAc is thought to be involved primarily in incentive motivation and mechanisms of reinforcement and reward (5, 21, 28). In particular, manipulations of NAc activity have been shown to have consequences for ingestive behavior (20). The striatum is thought mainly to facilitate motor behavior and the sequencing of motor action (2, 28).

A motivated state implies that a stimulus that is neutral or moderately reinforcing becomes more reinforcing with the change in state. This is true for the motivated state of Na depletion. Under normal circumstances, rats avoid or reject concentrated Na solutions. However, when Na depleted, rats will avidly ingest the very same solutions (10). Ingestion of concentrated Na solutions when rats are Na depleted leads to robust increases in extracellular DA in the NAc (18). Our findings argue for a role of reduced uptake via the DAT in the increase of extracellular NAc DA levels reported during the expression of Na appetite.

Perspectives

Reuptake by the presynaptically localized DAT is the chief mechanism by which DA is cleared from the synapse and is therefore critical in the regulation of extracellular DA concentration during DA transmission (3, 19, 33). The physiological importance of DAT activity during DA transmission was illustrated strikingly by Silvia et al. (33) in rats infused with antisense oligonucleotides to reduce DAT protein unilaterally. These researchers, measuring DA overflow in the striatum, showed that although basal DA overflow was unchanged by DAT protein reduction, stimulated DA release resulted in a dramatically larger DA overflow in the treated side of the brain. Thus modulation of the DAT can be thought of as setting the gain of DA transmission. Based on the work of Silvia et al. (33), it is unlikely that the change in DAT activity detected in the present study would, by itself, lead to differences in basal NAc DA concentrations between Na-depleted and control rats. However, the increased gain of DA transmission set by the effects of Na depletion on NAc DAT likely contributes to the robust increase in NAc DA during the expression of Na appetite as observed by Hoebel et al. (18). This increase in DA signaling may be partly responsible for the dramatic shift in behavior toward concentrated Na solutions after Na depletion, from avoidance to avid ingestion and from aversion to reward. Thus our study represents one example whereby a physiological modulation of synaptic DA function may contribute to an adaptive change in behavior.


    ACKNOWLEDGEMENTS

The authors acknowledge the excellent technical assistance of Aryana Zavosh.


    FOOTNOTES

This work was supported by National Institutes of Health Grants DK-40963, DK-48061, DC-00248, and T32HD-07391 and by Dept. of Veterans Affairs Merit Review.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests and other correspondence: M. F. Roitman, Box 351525 Guthrie Hall, Univ. of Washington, Seattle, WA 98195 (E-mail: roitman{at}u.washington.edu).

Received 4 November 1998; accepted in final form 19 January 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

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