Methodologic aspects of sodium appetite: an addendum

Irrelevant Incentive Learning during Instrumental Conditioning: The Role of the Drive-Reinforcer and Response-Reinforcer Relationships

Four experiments investigated the processes by which a motivationally-induced change in the value of the training reinforcer affects instrumental performance. Initially, thirsty rats were trained to lever press for either a sodium or non-sodium solution. In Experiment I sodium-trained rats responded faster in extinction following the induction of a sodium appetite, but not following either food or water deprivation. Thus, enhanced extinction performance depends upon the relevance of the training reinforcer to the test drive state. The remaining experiments examined the role of the instrumental contingency. Animals received response-contingent presentations of one solution alternated either within (Experiments II and III) or between sessions (Experiment IV) with non-contingent presentations of another solution. Neither procedure yielded convincing evidence that contingent sodium presentations generated more responding in extinction under a sodium appetite than did non-contingent sodium presentations. On the basis of these results, we argue that the instrumental contingency itself does not play a major role in this irrelevant incentive effect.

Irrelevant Incentive Learning during Training on Ratio and Interval Schedules

Two experiments investigated the effect of a motivationally-induced change in the value of the training reinforcer on instrumental performance. Initially, thirsty rats were trained to lever press for either a sodium or a potassium solution. Responding in an extinction test was then measured following the induction of sodium appetite. In Experiment I sodium-trained rats responded faster in a test given one day following the end of instrumental training. Furthermore, the relative size of this irrelevant incentive effect did not depend upon whether a ratio or interval schedule was employed during training. Delaying the test for eight days following the end of training abolished the difference between the test performance of sodium- and potassium-trained animals. Experiment II provided a further study of the effect of the training schedule when the introduction of the sodium reinforcer was delayed until responding was well established. Again the relative size of the difference between the performance of sodium- and potassium-trained animals was comparable following training on ratio and interval schedules. The insensitivity of this irrelevant incentive effect to the training contingency is in marked contrast to previous failures to detect an effect of reinforcer revaluation brought about by aversion conditioning following training on an interval schedule (Dickinson, Nicholas and Adams, 1983).

A novel method for induction of salt appetite in rats

A number of procedures exist for the experimental induction of sodium appetite. With the exception of a low sodium diet, nearly all of these methods are invasive, requiring injections, surgery, or both. In addition to stimulating intake of concentrated salt, some of them produce substantial side-effects like reduced food intake and weight gain. The present experiment was designed to evaluate the efficacy of a novel non-invasive method to induce a salt appetite. We investigated the effects of ingesting 100 microM amiloride, a diuretic and natriuretic compound, on urine quantity, electrolyte balance, 0.5 1M sodium chloride (NaCl) intake, water intake, and Na(+)-free chow intake, and weight gain in rats. A water ingestion only group served as control. Consumption of amiloride in mixture with water produced greater loss of urinary Na(+) and intake of 0.51 M NaCl compared with controls. This treatment was without effect on food intake and only modestly influenced weight gain. These results demonstrate a rapid and non-invasive method for the induction of salt appetite free of unwanted side-effects.

Changes in stimulus salience as a result of stimulus preexposure: evidence from aversive and appetitive testing procedures

In two experiments, rats received preexposure to three compound flavor stimuli, AX, BX, and CX, where X represents a saline solution. AX and BX were presented in alternation; CX, on a separate block of trials. The value of X was then modified, being devalued by aversive conditioning in Experiment 1, and rendered valuable by the induction of a state of salt need in Experiment 2. When given a choice between BX and CX, the rats consumed more of BX than of CX in Experiment 1, and more of CX than of BX in Experiment 2, suggesting that B and C differed in their ability to modulate the response governed by the X element. It was suggested that blocked preexposure to CX reduces the salience of the C stimulus but that the salience of B is maintained by preexposure in which BX is alternated with AX. The implications of this result for the phenomenon of perceptual learning are discussed.

Furosemide, sodium appetite, and ingestive behavior

Sodium appetite is often produced experimentally by using the diuretic furosemide (Furo) to induce a rapid loss of urinary sodium. The present experiments were designed to investigate the dose-dependent relationship between renal and behavioral responses to Furo. We compared the effects of five different Furo doses (0.5, 1, 2, 6, and 10 mg) on 3% NaCl intake, water intake, Na(+)-free chow intake, urine quantity, electrolyte balance, and weight gain in rats. The Na(+) loss produced by Furo injection was dose dependent from 0.5 to 10 mg and did not change across repeated depletions. There was only a weak correspondence, however, between these dose-dependent changes in renal function and subsequent sodium appetite. This suggests that net Na(+) loss is not the only determinant of sodium intake. Moreover, at the two higher doses of Furo, both food intake and weight dropped significantly, but these did not change following the three lower ones. Given these substantial side effects, the preferred dose of Furo for inducing a salt appetite should not exceed 2.0 mg.

Sodium deficiency enhances the behavioral responses to centrally administered vasopressin in rats

The ability of sodium deficiency to stimulate vasopressin (VP) release was examined by determining if sodium deficiency sensitizes the animal to the behavioral disruption caused by intraventricular injections of VP. In sodium-replete rats, intraventricular injections of 50 ng VP on Day 1 had no effect on behavior, but this dose elicited abnormal behaviors (barrel rolls, hind-limb extensions) when administered on Day 2, indicating a sensitization phenomenon. In separate experiments, the first intraventricular injection of 50 ng VP in sodium-deficient but not in sodium-replete rats also elicited barrel rotations followed by hind-limb extension. Intraventricular injection of VP also disrupted motor behavior in sodium-replete rats that had multiple prior experiences with sodium deficiency but not in naive rats. These results show that sodium deficiency can mimic the effect of central injections of VP in sensitizing the brain to the behavioral effects of exogenous VP. This suggests that sodium deficiency induces the central release of VP.

Brain vasopressin and sodium appetite

Intraventricular injections of vasopressin (VP) and antagonists with varying degrees of specificity for the VP receptors were used to identify the action of endogenous brain VP on 0.3 M NaCl intake by sodium-deficient rats. Lateral ventricular injections of 100 ng and 1 microg VP caused barrel rotations and a dramatic decrease in NaCl intake by sodium-deficient rats and suppressed sucrose intake. Intraventricular injection of the V(1)/V(2) receptor antagonist [d(CH(2))(5)(1),O-Et-Tyr(2),Val(4), Arg(8)]VP and the V(1) receptor antagonist [d(CH(2))(5)(1),O-Me-Tyr(2),Arg(8)]VP (MeT-AVP) significantly suppressed NaCl intake by sodium-deficient rats without causing motor disturbances. MeT-AVP had no effect on sucrose intake (0.1 M). In contrast, the selective V(2) receptor antagonist had no significant effect on NaCl intake. Last, injections of 100 ng MeT-AVP decreased mean arterial blood pressure (MAP), whereas 100 ng VP elevated MAP and pretreatment with MeT-AVP blocked the pressor effect of VP. These results indicate that the effects produced by 100 ng MeT-AVP represent receptor antagonistic activity. These findings suggest that the effect of exogenous VP on salt intake is secondary to motor disruptions and that endogenous brain VP neurotransmission acting at V(1) receptors plays a role in the arousal of salt appetite.

Learned preferences for the flavor of salted food

We examined whether rats can associate the flavor of their food with its salt content, and whether this association is influenced by sodium status during training and testing. During two pairs of 2-h training trials, rats ate flavored food containing 1.75% NaCl or an alternatively flavored unsalted food. The motivational state of the rats was manipulated prior to each trial by combined 48-h dietary sodium deprivation and furosemide treatment (severe sodium depletion), 48-h dietary sodium deprivation (mild sodium deprivation), or continued maintenance on stock diet containing 1% NaCl (sodium replete). When later given a choice between the two flavors, all rats preferred food containing the salt-paired flavor. The strength of this preference was unaffected by motivational state during training or by the salt content of the test foods, but was modulated by the motivational state of the rats during the preference test. Preference for the salt-paired flavor was strongest when rats were tested after severe sodium depletion, less strong after mild sodium deprivation, and absent when sodium replete. These results indicate that deprivation state during training has little effect on learned preferences for the flavor of salted food but deprivation state during testing affects the expression of this learning.

Brief preoperative exposure to saline protects rats against behavioral impairments in salt appetite following central gustatory damage

Adult rats who receive large electrolytic lesions at the level of the thalamic taste relay display impairments in taste related behavior, specifically the appetite for salt that results from changes in sodium balance. However, if the rats are given preoperative experience of ingesting a salty taste they are protected against these impairments in salt appetite that result from the lesion. In the present study we found that a brief (30 s) preoperative exposure to saline is sufficient to protect adult rats against expected deficits in salt appetite that normally result from central gustatory damage at the level of the thalamic taste relay. It is becoming increasingly clear that preoperative events can lessen the severity of the effects of brain damage. The results are discussed in the context of the appetite for salt, and preoperative immunization from behavioral deficits that result from brain damage.

Circadian wheel-running activity of rats under schedules of limited daily access to salt

Schedules of limited daily access to food result in 'anticipatory' activity preceding each daily feeding. It is well-established that such food-anticipatory activity depends on a food-entrainable circadian timing mechanism. In the present study, we sought to extend the generality of these results by maintaining rats in running wheels under schedules which provided access to salt solutions or to salty food for 2 hr each day. The animals were subjected to dietary, pharmacological and surgical treatments that promote salt appetite. However, we found no evidence for daily salt-anticipatory wheel running activity in any condition.

The effects of disconnecting the subfornical organ on behavioral and physiological responses to alterations of body sodium

Rats with knife cuts transecting the subfornical organ efferent projections were examined for behavioral and renal responses to experimental alterations in body sodium. Lack of subfornical organ efferent projections did not impair behavioral and renal compensatory responses either in the latency to respond or in magnitude of the response.

Effect of total amygdalectomy upon regulation of salt intake in rats

Sodium appetite was studied in rats with lesions destroying the entire amygdaloid nuclear complex. The rats were totally aphagic and adipsic for several days following lesioning but regained nearly normal levels of food and water intake about 2 to 3 weeks postoperatively. Intake of 3% saline was observed after induction of sodium appetite by treatment with a mineralocorticoid and a natriuretic agent. Rats with amygdaloid lesions generally manifested severe but not total loss of sodium appetite. Regulation of water intake was also moderately to severely impaired. Suggestive evidence was obtained that recovery of sodium appetite in amygdalectomized rats can be enhanced by postoperative experience with sodium appetite and saline reinforcement.