Aphagia in the presence of drinking an isosmotic NaCl solution

Physiology and Behavior, Vol. 8, pp. 623-630, Brain Research Publications Inc., 1972. Printed in Great Britain.

Aphagia in the Presence of Drinking an Isosmotic NaC1 Solution' JAN W. KAKOLEWSKI

University of South Dakota, School of Medicine, Vermillion, South Dakota, 57069, U.S.A. AND EDWARD DEAUX ~

Antioch College, Yellow Springs, Ohio, 45387, U.S.A. (Received 7 S e p t e m b e r 1971) J. W. ANDE. DEAUX. Aphasia in thepresenceof drinking an isosmoticNaCIsolution. PHYSIOL.B~,AV. g (4) 623-630, 1972.--Rats with familial diabetes insipidus (D.I.) were exposed to liquids in which the solute was either excreted (NaCl), metabolized (glucose), or partially excreted-metabolized (milk). In the presence of an isosmotic solution where all the solvent was utilized for clearance of the solute (NaCI), rats continued to drink but displayed aphasia; decrease in the osmolality of NaCI solutions resulted in a proportional recovery of solid food ingestion. Exposure to milk resulted in hypophagia. Glucose solution did not interfere with the maintenance of eating, however, rats with D.I. did not increase their intake of glucose solution during solid food deprivation. Experimental evidence supports the view that the state of hydration as indicated by the body-fluid osmolality is essential for the maintenance of solid food ingestion. The evidence for a primary caloric regulation was doubtful. KAKOLEWSKI,

Familial diabetes insipidus Body-fluid osmolality Caloric regulation Brattleboro strain

Eating

Apahgia

Drinking

Osmotic regulation

(for example, glucose) would not induce aphagia; the glucose, following a fast removal from the extraceUular fluid, would free the solvent (water) and promote a decrease in BFO. The following experiments were designed to test these predictions.

THE STATE of body-fluid osmolality (BFO) serves as a cue in the initiation of ingestive behaviors [5, 6, 8, 15]. A decrease in BFO [5, 14, 15] is necessary for the initiation of solid food ingestion. If an animal is not capable of lowering its BFO, either as a result of drinking or freeing metabolic water, no eating behavior (aphagia) will occur. Thus, under certain conditions, aphasia could be induced even in the presence of drinking behavior per se, providing the ingested fluid would not decrease BFO. Rats with diabetes insipidus (D.I.) were used to test this prediction. Under optimal feeding conditions, rats with D.I. are not capable of concentrating their urine in excess of BFO [7], and if self-loaded with an isosmotic NaCI solution, the load is excreted [26, 27, 28] and the animals have practically no other source of water to fulfill their hydration needs. Therefore, exposure of rats with D.L to solutions of NaCI, where osmolality is either below or close to BFO, would decrease or eliminate solid food ingestion. Similarly, a decrease in solid food intake would be expected to result if these animals were fed a slowly metabolized liquid (such as milk), which had been adjusted to an osmolality approximately equal to BFO. On the other hand, use of quickly metabolized solids in the drinking water

GENERAL PROCEDURE AND METHODS Previous research employing rats with surgically induced D.I. has shown that animals are capable of ingesting large quantities of a NaC1 solution [26, 27, 28] and that they prefer NaCl solutions (within a range) over water [29]. Since considerable individual differences occur in rats with surgically induced D.I. [4, 7], rats of the Brattleboro strain with familial hypothalamic D.I. were used to secure a homogeneous sample. The isosmotic drinking solutions were adjusted to equal the rats' plasma osmolality according to Valtin and Schroeder's report [30]. Throughout all the experiments the same group of homozygous, female rats, obtained from the Carworth Company were used. At the onset of the experiments the rats were 95 days of age and the average body weight was 160 g. The animals were kept on a 12-hr light, 12-hr dark cycle in

XThis research was supported in part by National Institute of Mental Health Grant M-4529, Research Grant NGL-36-005 from the National Aeronautics and Space Administration, and a College Science Improvement Program Grant from the National Science Foundation while J. W. Kakolewski was at the Fels Research Institute in Yellow Springs, Ohio. 2We thank Dorothy Clarke for invaluable help in the preparation of the manuscript, and Gale Deaux for the graphs. 623

624

KAKOLEWSKI AND DEAUX

an air-conditioned room with a temperature of 72°F. F o o d in the form of ground Wayne Lab-Blox F-6 (Wayne Chow), administered in spillproof Wahmann tunnels, and distilled water were available, unless otherwise specified. Fluid spillage was collected in cylinders mounted under the drinking tubes. The consumption and body weight data were measured with the following accuracy: fluid, + 0 . 5 g , food, ±0.5 g, body weight, :~1 g. All experiments were separated by a period of time required for the baselines of water, food intake, and body weight, to return to normal values. The osmolality of the solutions and urine specimens collected were determined by means of an Advanced Instruments osmometer (Model 3L) in a 0.3 ml sample to the nearest 0.1 mOsm/kg. Urine samples were collected during the period of baseline measures (about 20-30 min) when no food or fluids were available, thus preventing possible contamination of the samples. The urine was captured on aluminum foil pans and analysed without delay. On the two occasions when the volume of the sample was insufficient for osmometric analysis, a refiactometric estimate of the total solids (TS~o) was made and the value converted into osmolality [Urine osmolality---- 147.13TS~o+33.7 (formula obtained in our laboratory)]. The term aphagla in this paper refers to a 24-hr food intake which did not exceed 1 g. This amount was smaller than the assumed single meal size for our animals (10 meals per 24 hr, 12-14 g food per animal).

solution; the smallest decrease occurred when the rats ingested the 74 mOsm/kg solution, and a 2.5 g gain followed 24-hi exposure to distilled water. Although there was apparent nonlinear interaction of food and solution ingestion as determinants of body-weight change, the greater influence of food intake was evident. However, since food intake was found to be a function of solution intake, body-weight change was considered to be primarily related to the effect of ingesting solutions of different osmolalities.

S00

450

40C

3so

30C

EXPERIMENT 1

Procedure Prior to this study, daily baseline measures of water, food intake, and body-weight changes were collected for 3 weeks. A total of 12 animals was used. The experiment involved exposure of each animal, every third day, to a solution of either 74, 156, 234 or 312 mOsm/kg of NaCI instead of distilled water. The sequence of exposure was conducted according to a Latin Square design and each of four orders of solution presentation was given to a set of three rats. Throughout the 14 days of treatment measures of the 24-hr food, fluid intake, and body-weight change were continued. At the end of each 24-hr period a urine sample was collected from each animal and the osmolality determined. Results The data were analyzed separately for each of the four variables measured; changes in consummatory behavior and urine concentration, which occurred during exposure to the NaCI solutions were compared with the baseline data collected earlier. The mean 24-hr consumption of water and of the NaCI solutions are presented in Fig. 1. The intake was an inverted V-shaped function of the concentration of the solutions, with rats consuming more of the 156 mOsm/ kg NaCI solution than any other. The amounts of distilled water and isosmotic, 312 mOsm/kg NaCI solution consumed were approximately the same. F o o d consumption was inversely related to the osmolality of the solutions (Fig. 2). When the animals were given the 312 m O s m / k g N a C l solution their mean 24-hr food intake was 0.42 g. F o o d intake in the presence of the weakest solution (74 mOsm/kg) was 12.17 g, which was approximately equal to that found with water (12.41 g). The body-weight changes were linearly related to the osmolality of the solutions (Fig, 3). The greatest 24-hr loss, 24.67g, occurred during exposure to the 312mOsm/kg

20C

15(

I

I

312

234

I 156

I 74

I 0

Osmolality of NaCI Solutions in mOsm/kg

FIG. 1. Mean 24 hr consumption of distilled water (O) and NaC! solutions in the presence of food.

The urine osmolality data are presented in Fig. 4. Again, a positive, linear relationship was found between the solution and urine osmolalities. The mean urine osmolality, during exposure to the higher NaCI concentrations of 234 and 312 mOsm/kg, exceeded the values of BFO, suggesting the emergence of a self-imposed osmotic diuresis. An analysis of variance was conducted on each of the four sets of data, excluding from each set the measures taken during exposure to distilled water, and the results are presented in Table 1. The main effect of Solutions was significant at the p < 0.001 level in all four variables, while in no case was the between-subject Sequence effect significant. The significance of the Order effect in the solution intake and body-weight change data indicated an interaction between the effect of exposure to the solution and the order in which it was presented. N o similar interaction was found in the food intake and urine osmolality data.

APHAGIA AND DRINKING

625

13.

8

12.

..

11.

lo.

-ij

i

~

5

*iI 18

20 22

*!f I

312

234

15G

74

28t

0

B

312

Osmolality of NaCl Solutions in mOsm/kg

I 234

I

I

I

156

74

0

Osmolality of NoCI Solutions in mO sm/kg

FIG. 2. Mean 24 hr food consumption in the presence of distilled water (O) or NaCI solutions.

FIG. 3. Body-weight changes during a 24 hr exposure to food and distilled water (O) or NaC1 solutions.

TABLE 1 ANALYSIS OF VARIANCE OF THE RESULTS OF EXPERIMENT 1

Variable Solution intake

Food intake

Body-weight change

Urine osmolality

Source

df

F

p

Sequence

3/8

2.34

>0.05

Order

3/24

7.45

<0.01

Solution

3/24

14.31

<0.001

Sequence

3/8

0.65

> 0.05

Order

3/24

2.86

>0.05

Solution

3/24

264.62

Sequence

3/8

0.89

> 0.05

Order

3/24

3.25

<0.05

Solution

3/24

174.99

Sequence

3/8

Order Solution

< 0.001

< 0.001

1.65

> 0.05

3/24

0.56

> 0.05

3/24

31.35

<0.001

626

KAKOLEWSKI AND DEAUX excess of BFO [7], which prevented any BFO decrease while an isosmotic NaCI solution was ingested. Treatment with A D H should result in freeing part of the osmotically-bound water present in the ingested isosmotic NaCI solution and provide the necessary hydration conditions for solid food ingestion. To test this prediction, rats with D.I. were given an isosmotic NaCI solution and solid food, and, in addition, were treated with A D H .

500

450

40(]

Procedure Four days following the completion of Experiment 1, five rats with D.I., randomly selected, were injected subcutaneously with A D H in the form of Pitressin Tannate (generously supplied by Dr. F. C. Armstrong and Dr. A. C. Bratton, Jr., Parke Davis and Company) in oil with a dosage of 0.5 U per rat. The rats were maintained on ground Wayne Chow and 3 1 2 m O s m / k g N a C l solution. Twentyfour hr food, solution intake, and body weight changes were measured. Urine osmolality was determined on a sample collected at the end of observation. Within-subject comparisons were made between each animal's consumption during the A D H treatment and their previous measures in Experiment 1.

0 E -~

30C

o 0 ...;

2so

20(

150

ol

I 312

I 234

i 156

I 74

I 0

Osmolality of NoCl Solutions in mOsm/kg

FIG. 4. Urine osmolality determined in a random sample collected during the exposure to food and distilled water (O) or NaCI solutions.

Discussion The results indicated that: (a) rats with D.I. consume more hypoosmotic NaCI solution than distilled water; (b) food intake was an inverse function of the osmolality of the solution ingested; and, (c) exposure to isosmotic NaCI solution resulted in cessation of eating. Thus, during exposure to isosmotic solution, aphagia occurred in spite of the ingestion of more than 200 g of solution. Excessive drinking was not sufficient for the initiation of solid food ingestion, since the consumption of large quantities of isosmotic NaCI solution was not accompanied by eating in rats with D.I. In addition, food intake was inversely related to the solutions' osmolalities or to the amount of water available for hydration. Only during exposure to the lowest NaCI concentration of 74 mOsm/kg was eating behavior not suppressed relative to the consumption found when animals were exposed to distilled water. However, in compensation, the amount of ingested 74 mOsm/kg NaCI solution was nearly twice the amount of water consumed in a 24-hr period, although, again, there was practically no difference in food intake.

Results The mean intake of 312 mOsm/kg NaCI solution following A D H administration decreased to l 1 5 g as compared to 171.6 g consumed by the same rats in Experiment 1. The difference was significant ( t = 3.43, d r = 4 , p <0.05). F o o d consumption increased and a mean of 8.4 g was ingested during the 24-hr following A D H injection, as compared to 0.8 g consumed by the same rats in Experiment 1 during the exposure to an isosmotic NaCI solution without A D H treatment. This difference was also significant (t = 11.21, dr= 4, p < 0.001). Thus the A D H treatment produced a significant increase in food intake and a significant decrease in solution intake. The effect of A D H on urine osmolality was minimal following the 24 hr which elapsed from the time of injection. The mean osmolality was 482.7 mOsm/kg, as compared to 451.1 mOsm/kg found in Experiment 1. The difference was not significant (t = 0.30, d f = 4). Discussion Although rats with D.I. displayed aphagia while drinking an isosmotic NaCI solution (Experiment 1), administration of A D H reinstated solid food ingestion. It is known that in the presence of A D H , rats with D.I. increase the load of solids in their urine [7, 30] and, as a consequence previously osmotically-bound water was freed from the isosmotic NaCI solution. The relatively small dosage of A D H did not restore the concentrating capacity of the kidneys throughout the entire period of observation (24 hr) as indicated by the lack of a significant increase in urine osmolality. However, independently, it was established that within a few hours following treatment with the same A D H dosage, urine concentration was significantly increased and urine volume and polydipsia decreased for approximately 16 hr.

EXPERIMENT 2

EXPERIMENT 3

The absence of an antidiuretic hormone (ADH) rendered rats with D.I. incapable of concentration of their urine in

In order to establish whether the aphagia in animals with D.I. occurs regardless of the osmotic or caloric properties

APHAGIA AND DRINKING

627

of the solid food, five rats with D.I. which had been exposed to isosmotic NaCI solution were tested. Regular food which contained NaCI and other electrolytes was replaced by an electrolyte-free diet. Since, under this condition, no hydration could take place, it was predicted that animals would ingest no more of a new diet than they did of the regular food (Experiment 1), despite the new diet's alleviating effect on the D.I. condition [7]. Introduction of an electrolyte-free diet provided an opportunity to test another prediction. If the quantity of food ingested is controlled by an osmotic feedback [6, 11, 15j, then, the amount of food consumed should have an inverse l elationship to the potential osmotic load it contains. If rats with D.I. were injected with A D H and then exposed to an isosmotic NaCI solution, it is reasonable to believe that more of the food with low osmotic potential (electrolytefree) would be ingested than of the normal diet (Wayne Chow).

Procedure Twenty days after the completion of Experiment 2 the same five rats with D.I. were exposed to 312 mOsm/kg NaCI solution and an electrolyte-free diet (Nutritional Biochemlcals Corporation's sodium-free diet containing 6% alphacel, 7% corn oil, 66% sucrose, 20% vitamin-free casein, and vitamin diet fortification mixture) for two consecutwe days. At the end of the first day consumption and body weight were measured and each rat injected with 0.5 U of Pitressin Tannate. The second day consumption and body-weight measures were again recorded. Within-subject comparisons were made between each animal's consumption during the two days and the intake found in Experiments 1 and 2.

diet containing electrolytes (t = 7.99, df= 4, p < 0.01). The solution intake decreased, and whereas the rats had consumed 115 g of 312 mOsm]kg NaCI solution following their previous A D H injection, when exposed to the normal diet, they drank 60.6 g in this experiment. The difference was not significant, however (t = 2.55, d f : 4, 0.10 > p > 0.05, two-tailed test).

Discussion In the presence of an isosmotic NaCI solution, rats with D.I. did not ingest an electrolyte-free diet. The small amount consumed (0.9 g) indicated that at least some of the rats sampled the food but the consumption was not maintained and the food remained neutral [17]. This ob~rvation does not support the hypothesis that the rat's sweet tooth may play a primary role in feeding regulation [13]. The difference in consumption of low and high osmotic diets, following A D H treatment, supported the assumption of the presence of an osmotic feedback but further investigations are necessary to establish how an osmotic factor may have participated in the control of the cessation of eating (satiation). EXPERIMENT 4

This experiment is given to show how solid food consumption changed when a milk-type solution was provided. Bruce and Kennedy [4] noted that rats with surgically induced D.I. ingested a limited but steady amount of milk. The water ingested in the milk may be sufficient for metabolism of the milk content but may not secure optimal hydration, and, as a consequence, a deficit in solid food ingestion will take place.

Results In the absence of A D H treatment, consumption of 312 mOsm/kg NaCl solution did not differ from that found in Experiment 1 (t----0.63, d f = 4), and food consumption remained negligible with no significant difference in the intake of the two diets (t = 0.20, dr= 4). Thus, exposure to isosmotic NaCl solution resulted in aphagia regardless of the composition of the diet. Following A D H treatment consumption of electrolyte-free diet increased to 12.7g, which was significantly higher than ingestion of the normal

Procedure Eleven Brattleboro rats with D.I. were used. After steady baselines in food, water intake, and body weight were established, the animals were exposed for 24 hr to the regular Wayne Chow. Instead of water, SMA (Wyeth Laboratories, Inc.) formula, dissolved in water to 322 mOsm/kg, was provided. The formula was sterilized in order to prevent spoilage. The caloric content of the formula amounted to approximately 6 calories per 100 g of liquid. Twenty-four

TABLE 2 COMPARISON OF THE INGESTIVEBEHAVIORIN ADH DEFICIENT AND ADH TREATED RATS

Experiment

Maintenance conditions

ADH* injection

Intake Food Solution

1

Wayne Chow, •312 mOsm/kg NaCI

none

0.85

171.0

2

Wayne Chow, 312 mOsm]kg NaCI

0.5 U per rat

8.40

115.0

3, Day one

Electrolyte-free, 312 mOsm/kg NaCI

none

0.90

188.4

3, Day two

Electrolyte-free, 312 mOsm/kg NaCI

0.5 U per rat

12.70

60.6

*A suspension of Vasopressin Tannatc in peanut oil.

628

KAKOLEWSKI AND DEAUX

hr after exposure to the formula, intakes and body weight were measured. Within-animal comparisons between each animal's consumption during exposure to SMA and their earlier baseline measures were made.

Results Food consumption decreased in all animals. The mean intake was 1.36 g and was significantly lower when compared to a mean of 12.91 g intake when water instead of SMA was present (t-----21.84, d f = 10, p < 0.001). The intake of SMA varied but the average of 51.2 ml was ingested (range from 32 to 90 g). The combined caloric value of substances ingested in Wayne Chow and SMA formula amounted to an average of 8 calories per animal which was substantially lower than their normal daily intake of approximately 40 calories. Body weight changes varied from animal to animal. Some gained and some lost and the overall decrease was 3 g. Thus, in spite of the decrease in solid food ingestion, the weight loss was smaller, compared to animals experiencing aphagia induced by a non-nutritive isosmotic solution (Experiment 1).

Discussion Rats with familial D.I. display a severe hypophagia when a milk-type, isosmotic solution is the only fluid available. A somewhat similar design led Bruce and Kennedy [4] to conclude that in rats with surgically induced D.L the caloric content was the determinant of the amount of milk ingested. Since their study gave no detailed information regarding the osmotic parameters, conflicting interpretations based on similar experimental designs cannot be reconciled presently. It is noteworthy, however, that when milk was supplemented with water, solid food ingestion was restored. In short, restoration of hydration corrected the hypophagia.

(t : 4.23, d f : 10, p < 0.01). The mean food consumption for the experimental and control animals was 4.88 and 4.02 respectively, and the difference was not significant (t : 1.26, d f = 10). Prior to exposure to the glucose solution the animals ingested 7.85 and 6.90g and this difference was also not significant (t-----1.64, d f = 10). Exposure to the glucose solution resulted in a significant decrease in food consumption in both groups; the mean decrease in the experimental group was 2.97 g (t : 9.23, d f : 5, p < 0.001) and in the control group 2.88 g (t = 4.89, d r : 5, p < 0.01). Each animal in the experimental group decreased the glucose consumption during food deprivation and the mean intake of 75.3 g was significantly lower as compared to the intake in the presence of food (t = 7.12, d f : 5, p < 0.001). In the control group all animals increased glucose intake and the mean consumption of 91.5 g was significantly higher compared to the intake during feeding ad lib (t----3.93, d f = 5, p < 0.02). Although the mean of glucose intake during food deprivation was different in these two groups, the difference was not significant (t ---- 1.53, d f = 10).

Discussion The decrease in food consumption in both groups suggested that glucose was monitored and included in the total energy balance. Preference for this solution (unpublished observation) and a decrease in food intake suggested a relatively precise (together with hedonic), controlled energy intake. However, during solid food deprivation, which, by itself, alleviates the D.I. condition [7], the animals with D.I. paradoxically decreased the intake of glucose solution, leaving in doubt the assumption of a precise energy control, even if under some conditions hydration needs appeared to be satisfied. G E N E R A L DISCUSSION

EXPERIMENT 5

This experiment is centered on the influence of isosmotic glucose consumption in the presence or absence of food. In the ingested isosmotic glucose solution the solid is removed quickly (10-20 min), the solvent is freed and utilized for hydration. Intact rats increase the intake of glucose solution during solid food deprivation--a phenomenon frequently utilized as evidence for caloric regulation [12]. The intake of an isosmotic glucose solution in rats with D.I. in the presence and absence of solid food was, therefore, tested.

Procedure A group of six female homozygeous and six control female heterozygous Brattleboro rats of the same age were subjects in this part of the experiment. The animals were exposed to 325mOsm/kg glucose solution for two consecutive days; the first day in the presence of Wayne Chow and the second day with the Chow removed. Food, solution consumption, and body weight were measured every 24 hr. Since the two groups differed in body weight and consumption, the intake was estimated relative to their body weight (g consumed per 100 g of body weight).

Results The mean 24-hr consumption of glucose solution in the presence of food was 99.6 g for the experimental group, and 59.8g for the controls. The difference was significant

The experiments indicate that solid food ingestion in rats with D.I. occurs only when feeding conditions secure a level of hydration allowing a BFO decrease. In all conditions where water was available either from hypoosmotic solutions, or other solutions where the solid was quickly metabolized (glucose solution), rats maintained their solid food ingestion. Exposure to liquids, from which substances had been slowly removed (i.e., milk) or all the solvent had been used for the clearance of the solid, as in the isosmotic NaCI solution [26, 27, 28], resulted in severe hypophagia or aphagia in spite of the presence of drinking. Treatment with A D H effectively abolished the aphagia. Previous reports reveal that initiation of solid food intake in normal animals was elicited by various degrees of rehydration [14] and abolished by prevention of a decrease in BFO [15]. With the orderly relationship between the water available from hypoosmotic NaCI solution and the quantity of solid food ingestion established in rats with D.I., the hypothesis that hydration (which allows a BFO decrease) is a primary condition for the initiation and maintenance of solid food ingestion is further substantiated. The consistency of results and the speed of elicitation of consummatory responses favors the osmotic mechanism as being principally involved in what is called a short term feeding regulation. The precision of osmotically determined adjustments (Experiments l, 2, and 3) is in significant contrast to a regulation which could be a result of cues derived from energy metabolism (Experiments 4 and 5).

APHAGIA AND DRINKING

629

There is ample evidence which indicates that intake of solid food depends upon the state of hydration [21, 23, 24]. The observation that water deprivation resulted in progressive limitation of solid food ingestion [2] has been routinely confirmed by many investigators. Hsiao [10, 11] reported detailed relationship between the concentration of NaCI in the drinking solution and the subsequent decrease in food consumption. In this context, the low survival rate of normal rats [9] or rats with D.I. [26, 27, 28], which were kept on a solution of NaC1, can be attributed to the emergence of hypophagia or aphagia which Swann [26, 27] and Heller [9] failed to observe. The importance of a decrease in BFO as a necessary condition for the regulation of solid food ingestion is indirectly evident in the work by Booth and Pitt [3] when they reported that insulin-induced food ingestion was preceded by a significant water intake. This observation was confirmed and extended in our laboratory; the injection with insuline or pretreatment with desoxyglucose (which, according to Smith and Epstein [22] resulted in an increased food ingestion) failed to elicit significant increases in food intake in dehydrated or partially rehydrated rats (unpublished observation). Novin [19] previously reported an increased water intake following insuline administration. An alternative to water deprivation is the addition of osmotically effective particles to the drinking water. Adolph [1] showed that food intake decreased in proportion to the degree of concentration of solids in sea water which he supplied as the only source for drinking. Since sea water was also consumed in decreasing amounts, the quantity of both components of ingestive behavior, i.e., drinking and eating, were diminished. A point in common with the present results is that rats with D.I., when exposed to isosmotic NaCI solution and Adolph's animals which were exposed to sea water, were left without a possibility to gain water other than that required for the elimination of the osmotic load. As a consequence, there was in Adolph's animals little margin for a possible hormonal manipulation (e.g., injection of ADH), since the absolute concentration of urine appeared to be close to the maximal concentration ability of the kidneys. The behavioral control of fluid intake in rats with D.I. does not differ from the regulation in normal rats. It is axiomatic that in diabetes insipidus an intact thirst mechanism is essential for the development of polydipsia [4, 20]. Moreover, rats with D.I. respond to hedonic and colligative properties in solutions. Teitlebaum et aL [29] reported that rats with surgically induced D.I. display an exacerbated acceptance (or preference in a short-term test situation) of hypotonic NaCI solutions. Starting with approximate

isosmotic solutions ingestion declined with increased concentrations, a pattern similar to that found in normal animals. Furthermore, normal but dehydrated animals ingest more NaCI solution than water in order to regain their fluid balance [18, 25]. Similarly, rats with D.I. display an increased intake of NaCI solutions which indicates that the mechanism governing the consumption of solutions, which contain an effective osmotic load, was intact and active despite the negligible effectiveness when the isosmotic NaCI solution was ingested. It appears that in Brattleboro D.I. rats the genetically transferred defect may be confined to the absence of A D H exclusively and the structures involved in the behavioral control of fluid ingestion do not differ from normal rats. One form of D.I. can be induced by surgically removing hypothalamic structures involved in the production and secretion of A D H (presumed to belong to osmoreceptors), and it is clear that this site cannot be involved in the regulation of behavioral adjustments to changes in the osmotic balance. Since animals with D.I. respond both to hyperosmolality (drinking) and hypoosmolality (food ingestion) other sites sensitive to BFO changes must be sought out. There is, however, no prospect for defining such structures at present. Removal of other sites involved in consumption (e.g., lateral or ventromedial hypothalamus) results in transient deviations and a subsequent recovery of ingestive regulation [16]. In any event, the incapability of A D H production is not essential for the maintenance of behavioral (consummatory) adjustments occurring in response to BFO changes. Conclusions from these experiments indicate that a precise prediction of the consummatory behavior can be made if the osmotic factor is used as a reference point, but only few generalizations were derived if energy adjustments are considered as the principle in (short-term) regulation of consummatory behavior. ADH-free animals present an unique opportunity to assess the animals' sensitivity to the colligative properties of the ingesta. In normal animals the presence of this hormone may slightly complicate the assessment of the behavioral correlates of osmoregulation because lndwidual differences in the quantity of A D H (stored and released) may result in some variability in the short-term consummatory regulation. If A D H is excluded as a variable then the occurrence of feeding behavior is even more consistent and highly predictable. The ingestive behavior of rats without A D H reflects the function of an almost perfect biological osmometer. On the other hand the pcrticular solutes involved in osmotic monitoring and the mechanisms of function of osmoreceptive structures must await further investigation.

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