Self-intravenous drinking and regulation of hydromineral balance in rats recovering from lesions of the lateral hypothalamus

Physiology and Behavior, Vol. 14, pp. 457-464. Brain Research Publications Inc., 1975. Printed in the U.S.A.

Self-Intravenous Drinking and Regulation of Hydromineral Balance in Rats Recovering from Lesions of the Lateral Hypothalamus I NEIL ROWLAND 2 AND STYLIANOS NICOLAIDIS

Lab. de Neurophysiologie, Collbge de France, 75231 Paris Cedex 05

(Received 4 September 1974) ROWLAND, N. AND S. NICOLAIDIS. Self intravenous drinking and regulation of hydromineral balance in rats recovering from lesions of the lateral hypothalamus. PHYSIOL. BEHAV. 14(4) 457-464, 1975. - Rats recovering after lesions of the lateral hypothalamic area (LHA) were tested for their ability to regulate hydromineral balance by the control of intakes. In the first experiment these animals learned to self inject water intravenously, and maintained themselves in the prolonged absence of oral water. It was also found that these rats preferred to take water~V rather than orally. In a second experiment LHA rats were provided with all of their fluid requirements by a continuous IV infusion. Under these conditions, oral drinking was very much suppressed, while food intake remained normal. It would thus appear that given the chance not to ingest oral water, the LHA rat does not. It also follows that the LHA rat must be sensitive to its hydrational state, and can appropriately modify drinking behavior. The LHA rat may find the taste of water in some way unpleasant, which could account for these and other results. Lateral hypothalamus Hydromineral balance Systemic metering Oral factors Self-intravenous drinking IV infusion Desalivation Prandial drinking

THE lateral hypothalamic syndrome describes the complex of behavioral disturbances following large bilateral electrolytic lesions of the lateral hypothalamic area (LHA) [3]. After such lesions, rats are typically akinetic, aphagic and adipsic and require tube feeding. With passage of time the lost behaviors reappear in a well-known sequence [ 14,28]. Eventually the animals reach "Stage 4" of recovery [28] at which time they are able to maintain themselves on dry food and plain water. However this is only a state of partial recovery, since there are long-lasting residual abnormalities in regulatory behaviors. In particular, the Stage 4 rat responds very poorly to acute challenges to its hydromineral equilibrium induced, for example, by injections of hypertonic NaC1, or polyethylene glycol [ 3,26]. These findings have been ascribed to the irreversible disruption of neural systems involved in the control, of oral drinking behavior in response to body fluid deficits [3,13]. Support for this viewpoint was invoked from the observed prandial drinking pattern in Stage 4 LHA rats i.e. minute drafts of water taken after every few mouthfuls of dry food. In addition, these rats do not drink in the absence of food, nor to fluid deprivation [3 ]. It has been proposed by Kissileff and Epstein that the only stimulus to drinking in these animals is that of a "dry m o u t h " which may be due

Taste

to an impaired neural control of salivary secretion during the ingestion of dry food. Neurologically intact (NI) rats which are surgically desalivate show prandial drinking; desalivation of the Stage 4 LHA rat leads to an exaggeration of the prandial drinking pattern [ 13]. However this is indirect evidence, and before the residual drinking deficits in Stage 4 LHA rats are ascribed to the absence of systemic metering of hydromineral balance and/ or a loss of integration and control centers involved in thirst, it is necessary to exclude the possibility that peripheral components of the ingestive act are impaired. There is a paucity of data to this end, in particular relevant to the role of taste factors and oral metering in general. The present experiments are an attempt to evaluate the contribution of oral factors to drinking in the Stage 4 LHA rat. In Experiment 1 oral factors are essentially eliminated using a technique whereby self-intravenous (IV) drinking is substituted for oral drinking [ 17 ]. Under these conditions, rats recovering from LHA lesions were able to maintain hydromineral balance for long periods. In Experiment 2, continuous IV water infusions almost completely abolished oral drinking. These findings suggest that the LHA rat is capable of appropriately modulating oral drinking with changes of systemic hydration state.

Work carried out in the laboratory of J. LeMagnen during the tenure of a French government scholarship by N. R. This work forms part of a Ph. D. thesis submitted (by N. R.) to the University of London, January 1974. 2N. R. is now at the Psychobiology Program, University of Pittsburgh, Psychology Dept., Pittsburgh, Pennsylvania 15260, U.S.A. Reprints may be obtained from either author. 457

458

ROWLAND AND NICOLAIDIS GENERAL METHOD

Animals and Surgery Male and female wistar rats ( 2 3 0 - 3 5 0 g) were bilaterally lesioned in the LHA using stereotaxic procedures. Insect pin electrodes were insulated (Insl-X) except for a 0.3 mm tip, and lowered to coordinates AO 5 . 5 - 6 . 0 , L +2.0, D 7 . 0 - 7 . 5 down from dura (ear and mouthbar in horizontal plane [ 1] ). The two electrodes were clamped 4 mm apart in a holder and lowered simultaneously; in some animals these were implanted chronically without lesioning (rats with LC prefixes). Either at this time, or postlesion, cardiac catheters were implanted and secured to the skull with screws and dental cement [16]. Lesioning currents were typically 2 mA for 15 sec.

Housing and care Operated animals were housed individually in a room lighted from 0 8 0 0 - 2 0 0 0 hr, and ambient temperature 2 2 - 2 8 °. Lesioned rats were tube fed a liquid diet during the period of aphagia, and gradually weaned to dry cookies, wet cookies offered in shallow dishes and eventually onto regular lab chow (Extralabo M25 pellets or powder, or T Amidon, a semisynthetic diet) and oral fluids [27]. Occasionally, carrots and lettuce were given, since some of the animals will eat these foods before accepting other wet foods, and thereby ensure hydration. In a few rats, continuous IV infusions of glucose plus amino acid solutions were given instead of tube feeding. For up to 10 days this proved an effective way to maintain aphagic animals without the problems of severe diarrhea and dehydration typically seen during stomach feeding. However, it proved difficult to totally feed animals for longer periods without infections and general deterioration, and so this technique was not used as a routine recovery method.

Histology At the end of the experiment, or when the animals died, brains were Formalin fixed and cut frozen at 50 u. The sections were mounted unstained for inspection. EXPERIMENT

1:

SELF-INTRAVENOUS DRINKING IN LHA LESIONED RATS

This experiment is a direct test of whether the rat with LHA lesions can regulate its hydromineral balance through control of extra-oral intake. A demonstration of this ability would not be expected if the only stimulus to drinking in these rats is that of a dry mouth contingent upon dry food ingestion (since the presumed reinforcement would be alleviation of that dryness) [11, 12, 13]. Under the standard self-IV drinking conditions oral water is not available [17]. In this experiment, the LHA lesioned rats were also given a choice between oral and IV water.

Procedure Four female and 5 male LHA lesioned rats which had shown aphagia and adipsia were used. These rats were in a stage of accepting dry chow; 4 were still orally adipsic for all fluids, while five would ingest tapwater at the start of the experiment.

The animals were permanently connected to an infusion system via catheter and watertight swivel, and so were essentially unrestrained [17]. A lever was accessible at all times through the cage wall, and its depression led to the delivery of 0.8 ml of water in 120 sec. Presses during an injection had no effect. Dry chow (usually powder) was available at all times; in some cases meal patterns were recorded using either a strain gauge balance or a device in which the pressing aside of a light lever was necessary to obtain access to the food. In this latter case, the entries to the food cup and leverpresses for IV water were recorded on adjacent channels of a chart recorder, allowing temporal analysis of ingestive behaviors. After a few days, or when stable IV water intakes were observed, various tests were administered. In 3 rats, a 24 hr food deprivation was imposed, with IV water still available. In 7 rats the oral: IV preference was tested; the stage of experiment of this test, and oral fluid available, varied across individuals as detailed in the results.

Results All of the rats including those which were orally adipsic, learned to press the bar for IV water at least as well as NI rats. Usually, spontaneous pressing occurred within the first 24 hr, and no special training was necessary; in some cases the lever had to be pushed a little way inside the cage for one or two days, after which time it could be withdrawn (precluding accidental presses). The IV water intakes stabilized at about 15 ml/24 hr (Table 1). This level was attained after about 3 days, during which time a drop in bodyweight of about 18 g occurred. This is very similar to our observations in NI rats, where the 18 g loss was attributed to fluid loss [ 17]. Food intake remained at preexperimental levels for the duration of the experiment (range 6-29 days), leaky catheters and infections accounting for the LHA rats in the end. The IV water injections into the atrium caused no haemolysis and no apparent discomfort to the animals. Between 60 and 90 percent of lever pressing by the LHA rats occurred nocturnally, in close temporal relation with feeding (Fig. 1). This corresponds to the oral drinking pattern of these same animals, thus corroborating other reports [8,10]. While NI rats showed "periprandial" pressing for IV water [24], it is difficult to speak in terms of meals in LHA rats since they typically exhibit nibbling bouts of several hours duration (see also [10]). For this reason, LHA rats pressed during meals, and rather than present the obvious temporal relation seen from the chart records it is more constructive to describe some observations. Each night was spent in between 2 and 4 periods of prolonged (up to 4 hr) intense activity Rat L29 in Fig. 1 is representative. A sequence is typically made up of a minute or less of feeding (sometimes with spillage), followed by grooming, running around the cage, sniffing, and sometimes a leverpress by deliberately reaching through the window. Occasionally the rat would press the lever several times in rapid succession, but only the first press was effective in triggering injection. This whole sequence of consummatory behaviors is repeated many times during one of the prolonged activity periods, and hence during a meal several effective IV self injections occurred. A similar pattern with oral drinking substituting for the leverpress was observed in some rats after the IV experiment (and the

WATER R E G U L A T I O N IN RECOVERED LATERALS

459 TABLE1

RESULTS OF SELF-IV DRINKING (SID) EXPERIMENT IN 9 LHA LESIONED RATS. THE PRELESION WEIGHT, WEIGHT AT THE START OF THE SID EXPERIMENT ARE INDICATED, AS WELL AS IF THE RAT WAS ORALLY ADIPSIC (FOR PLAIN WATER) AT THE START. THE DAILY IV WATER INTAKE IS SHOWN, AND THE BODY WEIGHT AT THE END OF 8 DAYS (OR LESS, IF APPLICABLE) INDICATED. Bodyweight (g)

Intake (ml) on Consecutive Days of SID

Rat

Oral Adipsia

Prelesion

Start/SID

Day 8

1

2

L21

no

325

325

300

16

10

L29

no

250

265

242

9

L33

yes

235

216

220

19

3

5

6

7

1

17

4

6

13

4

2

30

32

45

21

27

29

6

4

8

10

7

16

18

10

7

13

8

~1

yes

250

230

210

1

14

6

19

7

7

LC2

yes

278

280

262

12

23

3

5

11

10

8

Days on SID

4

7

6

LC5

no

290

288

240

5

5

9

4

0

25

8

2

8

K3

no

292

292

267

0

4

6

4

9

6

4

3

18

K21

yes

275

270

265

52

44

12

39

11

35

27

21

K26

no

315

306

3~

3

2

46

16

20

22

20

22

25

13

12

10

16

12

17

13

16

14.7

Means

\ J \ J -3.6

-18.0

0*

*Indicates forced deprivation of IV water (not included in mean). For Rats L21, L29, LC2 oral water was available on Day 1 with intakes 10, 10, 0 ml respectively. Rat L33 had oral water Days 5 - 7 with intakes oral 10, 2, 5 ml. These oral intakes are not included in the totals.

chart records, of course, showed prandial drinking). It is also evident that the end of the period of oral adipsia (Stage 3) does not correspond with the return of the ability to show the self-IV behavior, since both orally adipsic LHA rats and the Stage 4 animals performed identically. The fragile nature of the LH preparations precluded the administration of a systematic battery of tests for regulation under the self-IV drinking conditions. However, when the IV intakes had stabilized and animals were in good health, the following tests were given: Oral w a t e r versus I V water. In two rats (L21 and L29) oral water was left available during the first 24 hr in the IV drinking paradigm. Both reduced oral intake from 25 to 10ml, while taking the rest (9 and 16 ml) IV. Thus the presence of oral water did not stop responding for IV water, and oral intake was appropriately reduced. Rats LC2 and K21 were orally adipsic when put in the experiment. In both cases oral fluid (sucrose for K21, water for LC2) was left available, but was untouched while responding for IV water was established adequately as for other rats (Table 1). Three rats were given a choice of oral and IV water after IV responding had been established. Rat L33 took 6 ml/day orally and I1 ml IV (4 day means), with the relative amount taken IV increasing with days. Rats L29 showed a 9:1 preference for IV during 4 days. The oral solution was at that time changed to 5% sucrose, and the IV intake dropped to 9 ml/day while the oral intake rose to 45 ml. The increased total intake was accompanied by BW gain. F o o d deprivation. In 2 of 3 rats studied, removal o f food for 24 hr led to an increase in the IV water intake (Fig. 1, Day 6). These animals in fact emptied their syringes (over

50 ml) during the night of deprivation. This can also be seen towards the end of the night of Day 8 in Fig. 1, when the animal had spilled all of its powdered food and was functionally deprived. The third animal showed essentially no change in the amount taken IV in the absence of food than when food was present. Thus the deprivation o f food in no way attenuates self-IV drinking in LHA rats as was also found in NI rats [24]. This result is in sharp contrast to the oral drinking behaviour of LHA rats during 24 ha" food deprivation. Five rats so studied drank a mean 6.0 ml (range 4 - 8 ) in the absence o f food. This is lower than the oral water intake of NI rats deprived of food for 24 hr (range 9--40 in a control experiment). It should be noted that this intake is not exactly zero, and corresponds with that found by some other workers [5,14]. Discussion

Rats recovering from LHA lesions learned to selfadminister IV water and thereby assure hydromineral balance over long periods. Their performance was very similar to that observed in NI rats in the same paradigm [ 17]. This suggests that the mechanisms responsible for the extra-oral dipsic behaviour are functional in the LHA rat (or that these functions have been replicated in other brain regions [27] ). Also, because LHA rats are clearly able to sense and act upon the consequences of a leverpress (i.e. IV water injection), it implies that systemic osmo and/or voloreceptors are functioning in an informative manner. The role of systemic receptors alone in the control o f fluid intake has been discussed previously, and by subtraction the role of orogastric factors evaluated [18]. One possible

460

ROWLAND AND NICOLAIDIS

I V

intake

L29

day

(~

10 m l ,f

-

8

,7 ,j

---

•

S o°

J

J m

j

, ~ - -SJ

g~

-

-

¢

J

o.

/ ~ ,

D

1~

ji

m . i

>

J

~

.o . _

!

_z ~

D

J

4 D

.,

E

m

p

~

.3 2

# - - '

(J

m

1 9

21

9h

FIG. 1. Cumulative self-IV drinking record for LHA lesioned Rat L29, for the first 8 experimental days (indicated at right). Each leverpress delivered 0.8 ml of water IV. The broken bars above the cumulative traces are ad lib meals of dry powdered chow. On Day 6, D = 24 hr food deprivation. Note the increased responding in the absence of food. This is partly replicated toward the end of the night on Day 8 when the rat had spilled all the food (hence no more meals). interpretation of the present experimental results would be that the orogastric (normally facilitatory) factors are unavailable or ineffective in the LHA rats. Trigeminal deafferentation is reported to lead to a lateral-like syndrome in the pigeon, with selective disruption of feeding [321. The impairment in effect of oral and/or gastric factors in LHA rats is indicated by the apparent preference for IV over oral water. This is in marked contrast to NI rats which show a total oral preference. Indeed, these results lend support to an earlier observation that LHA rats in some way found water in the mouth "unpleasant" [30]. This same argument could be invoked to explain why LHA rats do not drink orally in the absence of food; normally food might be ingested to mask the taste of water. Thus in the self-IV drinking paradigm, water intake (untasted) does continue during food deprivation. However, it is unwise to make a firm statement to this end, since the observed increases of responding in 2 rats might reflect frustrative behavior, and it is impossible to say how much of the IV intake may be due to such an emotional component. I n t h e I V - o n l y drinking conditions, food intake remained at normal levels. Hence, wetting of the m o u t h is in no way necessary or permissive to the ingestion of dry

food in LHA rats. The only way in which dry m o u t h factors could possibly intervene is via the elaboration of saliva contingent upon the IV water injection. This does not appear to be a significant factor in NI rats in the self-IV drinking paradigm [17], and it is most unlikely to be the case in LHA rats, especially if they indeed have impaired neural control of salivation [3]. EXPERIMENT 2: EFFECT OF CONTINUOUS IV WATER INFUSIONS ON ORAL WATER INTAKE IN LHA RATS The results of the first experiment suggested that LHA lesioned rats are capable of monitoring systemic water inputs at least as well as intact rats. If this was indeed the case then it appears that some factor of unpleasantness associated with oral water might be responsible for some of the other abnormal drinking traits of rats recovered from LHA lesions. A prediction arising from such a theory is that the LHA rat should not insest oral water (i.e. avoid the unpleasant) in conditions where hydrational requirements are m e t b y p a r e n t e r a l administration. The present experiment examines the effect on oral drinking of a continuous IV infusion of water or isotonic saline.

WATER REGULATION IN RECOVERED LATERALS

461

Procedure Five LHA lesioned rats were studied, four of them had recovered to Stage 4 of drinking tap water, and one was drinking 0.5% saccharin orally. Continuous IV water or isotonic NaCI infusions of 2 0 - 4 8 ml/24 hr were given as detailed in Table 2. Ad libitum food and fluid intakes were recorded daily. As a control, 2 NI rats were surgically desalivated [10] and fitted with cardiac catheters. After recovery these rats were also tested in the continuous infusion paradigm. Results The IV infusion of water in the partially recovered LHA rats led to a large decrease in oral fluid intake (Table 2), including the rat ingesting saccharin. Isotonic saline and water appeared to have similar effects on oral drinking. In all cases food intake was unchanged from preinfusion levels, and so nonspecific depressant effects cannot be invoked to account for the suppression of oral drinking. These reductions in oral drinking are larger than those observed in NI rats under similar conditions [18,23]. One additional rat, not reported in the Table 2, was infused with isotonic saline during the later stages of recovery, as shown in Fig. 2. Both oral water and saccharin were refused during infusion, but sucrose solution was ingested (Days 32 and 33) in the presence and absence of infusion, respectively. This sucrose ingestion yielded only some 8 kcal per day, so it could hardly be claimed that the the rat was eating the sucrose; instead it seems that palatability of the available fluid determines intake. In the case of the rats in Table 2, it is evident that even though water is ingested, it is very easy to suppress this intake.

In contrast the surgically desalivate rats showed some attenuation of both feeding and drinking during IV water infusion (Table 2), but the water to food ratio remained constant. Discussion The results of this experiment, as those of Experiment 1, indicate that the ingestion of dry food in LHA rats is not necessarily accompanied by the ingestion of water. The desalivate rat which needs to drink in order to swallow showed no change in the amount taken with a meal during parenteral supplement. The large reductions of drinking in LHA rats clearly demonstrate that a dry mouth interpretation is inadequate in these animals - the ingestion of dry food' is not necessarily accompanied by the ingestion of water. Indeed, the apparent avoidance of water shown by the LHA rat suggests an alternative explanation of the prandial drinking phenomenon, other than a dry mouth. It might reflect a strategy of eating to wash away the bad taste of water, which would also account for low intakes in the absence of food. This received indirect support from an experiment in which NI rats drinking quinine adulterated water were shown to engage in prandial drinking of the classical lateral style (unpublished observations). The reductions in oral drinking in LHA rats appear to contradict earlier results of Kissileff [ 11,12]. He gave intragastric water infusions via a nasopharyngeal catheter; the infusions were meal paired - every time the rat took a pellet a shot of water was delivered. In contrast to NI rats which showed large reductions of oral drinking in these conditions, LHA rats showed essentially no reduction in

TABLE 2 EFFECTS OF CONTINUOUS IV INFUSIONS OF WATER ON ORAL DRINKING IN RECOVERED LATERAL RATS AND IN DESALIVATE RATS

Rat

No Infusion Oral Intake R (ml) (ml/g)

Days

Infusion ml/24 hr

Infused Oral Intake ml/24 hr

R (ml/g)

Days

L21

24.8

1.36

6

35

7.1

0.37

12

LC3

20

0.99

5

20 30

10 4.4

0.54 0.26

3 5

LC5

11

0.79

1

22

3.2

0.19

6

K31

14

0.68

4

15"

1.8

0.15

4

K39

16t

1.00

1

15"

0.5t

0.04

4

DS21

92

4.77

3

40

67

4.50

3

DS23

60

2.55

6

30

33

2.37

3

Results are presented as means for periods of infusion or no infusion for each individual lateral (top) or neurologically intact desalivates (DS - lower part).R = water to food ratio. Mean infusion of water, or *0.9% saline/24 hr is tabulated, with the oral water intake. tRat K39 was in Stage 3 and drinking saccharin sweetened water at all times.

462

ROWLAND AND NICOLAll)IS ~teedlng

.10 m n bodywt ~5 g

~¢irafts

tM Sal

Oral fluid

ml

ml

25

';8

300 33

K26d -lateral-

32

Su

2 5 SU

f

31

J 2q

18

0 sac

18

0

H~O

18

13

su

28 cl,~y p()st

L

/ f

B

R

I

G

H

T 20

8 t3

FIG. 2. Protocol of Rat K26 recovering from LHA lesions. The continuous traces are cumulative food intakes of dry chow. The day postlesion is indicated on the far right, along with a bar graph of bodyweight. Of note are the slow, principally nocturnal meals. On the first 4 days shown the rat received a continuous IV infusion of 0.9% saline. The oral fluid intake on those days is indicated at the left; SU = 5% sucrose, SAC = 0.05% saccharin. When the orally available fluid was accepted (Days 32 and 33) the food intake was relatively high. The oral drinking showed a typical prandial pattern, as shown in the details at the top of the figure. Feeding is indicated by shaded blocks and drinking by the downward marks. About 2 ml of fluid was ingested with each meal shown. drinking. There are at least two possibilities for the discrepancies, both of which deserve further investigation. The first is that LHA rats, unlike NI rats, are unable to sense IG repletion in the short term (i.e. are not sensitive to short term factors which normally modulate ongoing ingestive behaviors). However, it is unclear why the overhydration (IG plus oral intakes) at the first meal should not be sufficient to reduce drinking at subsequent meals. Although Kissileff's infusion experiments were only 24 hr each, it is unlikely that these effects on drinking would take this long to become apparent, since in the present experim e n t s reductions in drinking were observed almost immediately. The second possibility concerns the sensitivity to certain short term feedback signals. The NI rat given IG nasopharyngeal infusions during spontaneous meals reduces oral drinking by some 0.9 ml per ml infused [1 1,12,18]. This impressive reduction was not observed when the same IG meal paired injections were given through a catheter which passes into the stomach transmurally and via a subcutaneous route [18]. Thus, in the normal rat it would appear that the nasopharyngeal sensations accompanying the gastric repletion are a key factor in short term satiety. It therefore remains a possibility that this nasopharyngeal cue is ineffective or unavailable to the LHA lesioned rat.

Histology The lesions of the present study were typically large, bilaterally symmetrical in the far lateral LHA at the level of

the ventromedial nuclei. Most of the LHA lateral to the fornix, as well as parts of the internal capsule, overlying zona incerta and subthalamus were invaded. The lesions appeared identical with others reported necessary to produce the lateral hypothalamic syndrome [ 28,31 ]. GENERAL DISCUSSION The results of the present experiments suggest that the rat partially recovered after LHA lesions is able to meter its hydromineral state. It is able to learn to self-IV inject water, apparently via the sensing of the injected water, and to appropriately adjust the leverpressing behavior to assure hydromineral balance over long periods. When systemic repletion is given by a programmed continuous infusion the LHA rats essentially stop drinking orally. This suggests that the LHA rats may find oral water mildly aversive, and under the conditions of (sensed) systemic repletion, the oral dipsic behavior is reduced. It has been proposed that NI self-IV drinking rats which show low daily fluid intakes compared to oral drinkers, are regulating body fluid balance about a lowered set point. In this mode of regulation (in the absence of orogastric metering of inputs or taste factors), it appeared that the kidney concentrates urine to near maximal values. Thus equilibrium is attained with lowered intakes and economy of fluid loss. These rats also failed to show operant responding for IV water after acute dipsogenic challenges such as NaC1, polyethylene glycol, and angiotensin [17]. This same lack of response to acute hypertonic NaC1 injec-

WATER REGULATION IN RECOVERED LATERALS tion has been observed in self IG drinkers [ 12], where only the stimulus has been eliminated from the act of water intake. Thus while these IV or IG rats survive adequately on a day to day basis they are unable to execute adaptive responses to extreme or sudden environmental challenges. Similarly, orally drinking LHA rats do not drink to acute challenges [3], although there is evidence that during prolonged [25] or nocturnal [22] testing they do in fact ingest considerable amounts of water. The removal of oropharyngeal sensations from the drinking act is not the only way in which such primary drinking [7] can be abolished. Rats which were chronically habituated to drinking quinine adulterated water were found to be severely impaired in responses to acute thirst stimuli [19]. The ad lib intakes of rats chronically exposed to quinine water were found to be low, as in self-IV drinkers, yet these animals do have their entire range of sensors available for metering of intake. The mechanisms underlying the observed lowered intakes in the two cases could be different - one resulting from the absence of peripheral metering, and the other from the negative stimulus properties of quinine. Alternatively, both of these examples have the same functional disruption of input to a mechanism controlling the ingestive act. That is, oral signals which are normally neutral or facilitatory to drinking are now either absent or inhibitory. The fact that either of these conceptualisations could account for the observed behavioral deficits, which strikingly resemble those seen in the orally drinking Stage 4 rat after LHA lesions, point to cautions in the interpretation of so called regulatory deficits [3], and also lead to questions concerning the semantics of regulation [ 19,29 ]. Further, it has been proposed that recovered laterals regulate BW about a lowered set point [21]. However this same argument could be used to describe rats eating [9] or drinking [19] unpalatable substances. The possibility that LHA lesioned rats remain at chronically lowered BW's might be secondary to them limiting oral water intake. Indeed, the fact that many reports of low water to food ratios in LHA lesioned rats (e.g. [5, 8, 23] ) of the order of 1.0 ml/g, as seen in quinine drinkers [19] support this suggestion. In fact, in unpublished studies on LHA rats, we have found that IV infusions of large amounts of water (50 ml/24 hr) for long periods lead to increased food intakes and substantial BW gain. At a descriptive level, therefore, the reaction of the LHA Stage 4 rat to water appears in many respects to resemble the reaction of the NI rat to quinine adulterated water. One of the cell types destroyed by LHA lesions could be an integrator of information from the oral and systemic

463 environments. Such neurons, which may be independently activated by both oral and IV water (or, in the opposite sense by NaC1) stimuli have been identified [2,15]. These cells could be involved in the metering of ingested water, in the capacity of anticipating the systemic repletion after absorption has occurred. In the self-IV drinking rat the oral input will be largely absent; in the quinine drinking rat the oral input might be modified by taste factors. In the positive direction, positive taste incentive alone is sufficient to elicit ingestion [4]. The fact that these units are activated by IV injections of NaC1 implies an input dependent upon present osmotic state. If these units were abolished by LHA lesions, the integrative or anticipatory functions would be lost although the independent activating signals, which presumably emanate from other regions, would still be available [2]. At any rate, there are a number of possibilities for the multiple cue control for the activation of a neural substrate for fluid seeking and ingestive behaviors [2, 6, 20]. The observation in the present experiments that the dehydration of the LHA lesioned rat by IV infusion led to the reduction of water-seeking behavior, while food seeking and ingestion was essentially unchanged, demonstrates that the neural substrates for feeding and for drinking may be independently activated in these animals. This conflicts with the interpretation that the recovering LHA lesioned rat engages in drinking only when it feeds, due to dry mouth factors [3]. In the absence of food, the LHA rat does not drink appreciable amounts of water, which has also been taken as supporting this viewpoint. However, preliminary observations (NR - April 1974) show that the food deprived LHA lesioned rat does initiate drinking bouts, but that these bouts are not sustained for more than a few licks. The 24h fluid intake is therefore negligible. Thus it appears that the water seeking substrate is activated in the absence of food, but fluid intakes are low because of an impairment in sustaining ingestive bouts. This model of independent activation of neural programmers for eating and drinking has been proposed in the intact rat [20]. In summary, to describe the LHA lesioned rat which has recovered to Stage 4 as having lost the ability of all regulatory drinking, and that this reflects loss of critical neural tissue [3], should be considered with some reservations. Given appropriate testing procedures (present work, see also [25]) it seems that these rats can respond to changes of the internal environment with behavior which tends to preserve hydromineral homeostasis. Clearly, the LHA rat is far from normal in these respects, but the peripheral sensory aspects seem to play a major role in the so-called permanent dipsic deficits.

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