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Schedule-induced polydipsia: Another look at water-intake volume regulation
Physiology & Behavior, Vol. 35, pp. 221-227. Copyright©Pergamon Press Ltd., 1985. Printed in the U.S.A.
0031-9384/85 $3,00 + .00
Schedule-Induced Polydipsia: Another Look at Water-Intake Volume Regulation J O S E P H H. P O R T E R
Department o f Psychology, Virginia Commonwealth University, 810 West Franklin St. Richmond, VA 23284 R e c e i v e d 23 N o v e m b e r 1983 PORTER, J. H. Schedule-induced polydipsia: Another look at water-intake volume regulation. PHYSIOL BEHAV 35(2) 221-227, 1985.--Experiment 1 found that rats regulated the number of drinks per session during schedule-induced polydipsia, rather than volume intake when water was obtained from different sized water dipper cups. However, when water availability was manipulated during polydipsia sessions by changing the aperture size of the water bottle tube (Experiment 2), the rats regulated total water-intake volume per session. Experiment 3 demonstrated that this volume regulation was more precise during schedule-induced drinking than during water deprivation induced drinking. Also, it was shown that the different sized apertures were effective in manipulating the rates of water availability and ingestion.These experiments demonstrated that volume regulation during schedule-induced polydipsia occurs only when water is freely available via drinking tubes. Adjunctive behavior Volume regulation Drink regulation Schedule-induced polydipsia Deprivation-induced drinking Water availability Oropharyngeal mechanisms Nonhomeostatic drinking Rats
A L T H O U G H schedule-induced polydipsia is a well-studied phenomenon (see [8,9]), there are many aspects about it which are not resolved. Falk [7] demonstrated that fooddeprived rats would drink an average of 92.5 ml of water in a 3.17 hour session when food pellets were delivered on a variable-interval one-minute reinforcement schedule. Schedule-induced polydipsia is characterized by a postpellet pattern of drinking (see [8,9]) in which almost every pellet is followed by a small draught of water (typically around 0.5 ml). Although most of the studies on scheduleinduced polydipsia have attempted to explain why the drinking develops initially, one line of research has attempted to determine why animals quit drinking after consuming each small draught of water. In other words, does the animal stop drinking because it has consumed a certain quantity (volume) of water or because it has spent a certain amount of time drinking (or taken a fixed number of drinks)? Freed and Mendelson [10] and Freed, Mendelson, and Bramble [11] reported that rats regulate volume intake or water during schedule-induced polydipsia, rather than the amount of time spent drinking. The rate of water ingestion was manipulated by using either a large (2.6 mm) or a small (1.0 mm) aperture in the drinking tube. The rats spent more time drinking when the small aperture (which had a slower rate of water flow) was present in order to maintain a relatively constant volume of water ingested during each session. They also found that water-intake volume regulation was more precise during schedule-induced polydipsia than
during water-deprivation induced drinking. Allen and Kenshalo [1] have reported volume intake regulation in Java Macaque monkeys when draught size per lick at a drinking spout was manipulated. Also, Magyar, Wardbillig, and Meyer [14] have reported that rats regulate volume intake during schedule-induced polydipsia during responsecontingent food schedules (the Freed and Mendelson studies [10,11] had used response-independent food schedules). Thus, these studies seem to offer strong support for the notion that animals regulate volume intake during scheduleinduced polydipsia test sessions. However, recent studies have challenged this conclusion. Wetherington and Ware [23] reported that volume intake regulation did not occur during schedule-induced polydipsia when the mode of water availability was manipulated by changing water dipper size (0.01 ml vs. 0.04 ml dippers). In a subsequent study, Wetherington, Lawler, and Blanco [22] parametrically varied dipper size to further examine this discrepant finding. They found that increasing the dipper size increased session water intake during schedule-induced polydipsia. These studies by Wetherington and her colleagues question the notion of a volume constancy mechanism in the regulation of schedule-induced polydipsia, since changing the mode of water availability (dipper vs. water spout) produced different results. The purpose of the present series of experiments was to re-examine the question of water-intake volume regulation of schedule-induced polydipsia. Besides the different modes of
1This research was supported in part by the Grant-in-Aid Program for Faculty of Virginia Commonwealth University.
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FIG. I. Mean water intake (ml) for each half of each session in Experiment 1 with the 0.06 ml dipper and the 0.10 ml dipper. The overall mean for the 15 sessions was 6.3 ml (SEM=0.29) for the 0.10 ml dipper and 3.9 ml (SEM=0.41) for the 0.06 ml dipper. It should be noted that the water intake values were derived from the number of dipper operations.
FIG. 2. Mean number of drinks (dipper operations) for each half of each session in Experiment 1 with the 0.06 ml dipper and the 0.10 ml dipper. The overall mean for the 15 sessions with 62.9 (SEM=4.1) for the 1.0 ml dipper and 65.4 (SEM=4.8) for the 0.06 ml dipper. It should be noted that the number of drinks actually represents the number of dipper operations.
water presentation and manipulation (dipper size vs. water spout aperture size) in the Wetherington et al. studies [22,23], another procedural difference may possibly account for these discrepant findings. In the Freed and Mendelson studies [10,11] the water tube sizes were alternated within sessions (i.e., intrasession); whereas, Wetherington et al. [22,23] manipulated dipper size across test sessions (i.e., intersession). In order to determine whether or not this procedural difference was responsible for these discrepant findings, intrasession manipulation of dipper size was examined in Experiment 1. This allowed a more direct comparison of the findings in the Wetherington et al. studies with the findings in the Freed and Mendelson studies.
Procedure
EXPERIMENT 1 METHOD Animals One adult naive male albino rat and four adult hooded rats (1 female, 3 males) were used. The hooded rats had previous experience with fixed-time (FT) food reinforcement schedules and schedule-induced polydipsia. They were housed individually in an animal colony room (7 a.m. light/6 p.m. dark cycle). Room temperature was maintained at approximately 72°F (22.2°C). Apparatus Test sessions were conducted in a BRS/LVE operantconditioning chamber (Moducage) which was enclosed in a sound-attenuated cubicle. The food magazine was located on the left side of the intelligence panel, the response lever in the center, and the water dipper on the right side. Two dipper cup sizes of 0.10 ml and 0.06 ml were used. A 28 v/dc houselight and white noise were present during each session. Reinforcers were 45 mg standard formula Noyes pellets. Standard solid state and electromechanical programming and recording equipment were located in an adjacent room.
Initially, the rats were placed on a 23-hr water deprivation schedule in order to train them to press the lever which operated the water dipper (0.10 ml dipper cup). After the bar-pressing response had been shaped, each rat obtained 100 water reinforcers according to a fixed-ratio (FR) 1 schedule. Then, the rats were returned to an ad lib food and water schedule for 14 days in their home cages. New freefeeding body weights (BW) were established (mean of the last two days), and the rats were then reduced to 80% BW and maintained at that level by restricting their daily ration of Purina rat chow. Daily test sessions lasted 50-min, and food pellets were delivered according to a non-contingent fixedtime one-minute (FT 1-min) schedule. For the first 20 sessions each bar press activated the water dipper (FR 1) and delivered 0.10 ml of water in order to establish scheduleinduced drinking. There was a one second delay between dipper activations so that a burst of responses would not be counted as several drinks. The dipper was normally in the up position so that it remained freely available to the rats. Also, the number of drinks was obtained from the number of dipper operations, not from the number of barpresses. The volume of water intake was obtained by multiplying the number of drinks during a session by the volume of the dipper which was present (either 0.10 ml or 0.6 ml). For the next 15 sessions, the 0.10 ml dipper cup and 0.06 ml dipper cup were alternated during each session. One size was available during the first half of the session (25-min), and the other during the second half. The order was counterbalanced across sessions. Changing the dipper cup halfway through the session took about one minute.
RESULTS AND DISCUSSION
The rats regulated the number of drinks during each half of the session, but did not regulate the volume of water consumed. It should be noted that the number of drinks actually
VOLUME REGULATION DURING SCHEDULE-INDUCED POLYDIPSIA TABLE 1 INDIVIDUAL SUBJECT DATA FROM FIG. 2 (EXPERIMENT 1) AND FIG. 4 (EXPERIMENT 2)
Mean Number of Drinks for Each Subject From Fig. 2 Subjects
0.10 ml Dipper Cup
0.06 mi Dipper Cup
R-I R-2 R-3 R-4 R-5 Mean
55.6 78.1 46.7 70.6 64.4 63.1
56.9 73.2 49.9 81.2 65.4 65.3
Mean Water Intakes for Each Subject From Fig. 4 Subjects
2.6 mm Hole
4.8 mm Hole
I-1 I-2 I-3 I-4 Mean
12.5 20.0 11.8 16.9 15.3
12.6 20.1 11.4 14.9 14.8
represents the number of dipper operations, and that the water intake values (ml) were derived from the number of dipper operations. Figure 1 shows the mean water intake (ml) for each half of each session. Except for session number 6, the volume of water intake was always substantially higher when the 0. I0 ml dipper cup was present. The average intake over all 15 sessions for the 0.10 ml dipper cup (mean=6.3 ml, SEM=0.29) was significantly greater, t (4) = 8.705, p <0.01, than intake when the 0.06 ml dipper cup (mean=3.9 ml, SEM=0.41) was present. Figure 2 shows that the mean number of drinks (i.e., the number of dipper operations) were similar for both halves of each session. Across all 15 sessions the mean number of drinks was 65.3 (SEM=5.0) for the 0.06 ml dipper cup and 63.1 (SEM=4.9) for the 0.10 ml dipper cup. The overall means were not significantly different, t(4)=0.90, p>0.05. Table 1 shows the mean number of drinks for each subject from Fig. 2. Instead of regulating the total volume of water intake, as Freed and Mendelson [10,11] reported, the rats in the present study regulated the number of drinks taken within each session (see Fig. 2) These results do, however, agree with Wetherington et al. 's [22,23] findings than when the mode of water availability to rats is via a water dipper, the volume of water consumed depends on the size of the water dipper. More water was consumed during a session with the large dipper (0.10 ml) than with the smaller dipper (0.6 ml). However, the rats in the present study demonstrated very precise regulation of the number of drinks during the session with both dipper cup sizes. In the Wetherington and Ware study [23] the rats not only consumed a larger volume of water with the large dipper cup (0.04 ml) than with the smaller dipper cup (0.01 ml), but also had a greater number of drinks (i.e., dipper operations) with the larger dipper cup than with the small dipper cup. In their subsequent study, however, Wetherington et al. [22] found that the number of drinks did not vary with dipper size which was varied systematically from 0.01 ml to 0.10 ml. One obvious difference between the present study and
223
Wetherington et a l . ' s studies [22,23] and the Freed and Mendelson's studies [10,11] was the method by which the water was obtained. In the present study rats had to press a lever to obtain water via a dipper cup whose size was varied. One might suspect that this mode of water availability places unusual constraints on the development of normal scheduleinduced polydipsia. However, Falk [8] has previously shown that rats can be trained to press a lever in order to obtain water during schedule-induced polydipsia. Also, Allen and Porter [2] have investigated schedule-induced polydipsia by requiring the rats to press a lever in order to obtain water from a dipper, rather than having free access to a water bottle. Both the pattern of drinking and the amounts of water consumed when dippers were used were comparable to normal schedule-induced polydipsia. In the present experiment the mean water intake for the total session (0.10 ml and 0.06 ml dipper data combined) across the 15 days was 10.2 ml (0.2 ml per pellet). Although this value is somewhat low, it is well within ranges of water intake which have been reported in polydipsia studies and observations in this laboratory in which access to water was via a water bottle tube. Figure 3 shows the relative frequency of drinks (dipper operations) as a function of fifteen-second bins during the one-minute interpellet intervals for the last three test sessions. As can be seen, all rats (with the exception of R-3 with the 0.10 ml dipper cup) displayed the greatest number of drinks during the first fifteen seconds of the interpellet intervals with both the 0.10 ml and 0.06 ml dipper cups. The number of drinks then decreased in a monotonic linear function (with the exception of R-3 again) throughout the rest of the one-minute interval. While polydipsia drinking with water dippers was extended somewhat further into the one-minute interval than is typically seen when water tubes are used, the majority of the drinking did occur during the first thirty seconds of the interpellet interval. Thus, the use of dipper cups in Experiment 1 did not appear to greatly alter the characteristics (i.e., volume ingested, pattern drinking) typically seen with schedule-induced polydipsia [8,9]. Experiment 1 demonstrated that the lack of volumeintake regulation in the Wetherington et al. studies [22,23] cannot be attributed to the procedural differences of intrasession vs. intersession manipulation of dipper cup size. Since Experiment 1 failed to replicate Freed and Mendelson's [10, l 1] findings, I felt a more systematic replication of their studies was necessary. In Experiment 2, the rate of water ingestion was varied by using two drinking tubes with different size apertures, which Freed and Mendelson have shown to be an effective way to change ingestion rate. Freed and Mendelson [10,11] and Magyar et al. [14] used 1.0 and 2.6 mm tube openings. In Experiment 2, 2.6 and 4.8 mm apertures were used in order to determine if the volume relation shown in those studies would be evident with a larger aperture. EXPERIMENT 2 METHOD Animals
Four adult naive female Sprague-Dawley rats were used. Housing conditions were the same as in Experiment 1. The rats were maintained at 80% BW by restricting their daily ration of Purina rat chow.
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Apparatus Test sessions were conducted in two Scientific Prototype operant-conditioning chambers (Model A-100) which were housed in sound-attenuated cubicles. A water bottle was mounted behind the intelligence panel on the right-hand side with the metal drinking tube recessed slightly behind a hole 20 mm in diameter. The food magazine was mounted on the left side of the panel and no lever was present. Standard drinking tubes with 2.6---0.10 mm apertures were used as small-holed tubes, and drinking tubes with 4.8±0.10 mm apertures were used as large-holed tubes. A 28 v/dc houselight and white noise were present during each session. Reinforcers were 45 mg standard formula Noyes pellets. Standard electromechanical programming and recording equipment were located in an adjacent room. Procedure One-hour test sessions were conducted daily. Food pel-
lets were delivered according to a FT 1-min schedule. The rats were given 20 1-hr sessions with the small-hole drinking tube (2.6 mm) in order to establish schedule-induced polydipsia. Then 18 sessions were conducted in which the large-hole and small-hole drinking t u b e s were alternated within each session. One size hole was available for the first half of the session (20-min), and the other size was available for the second half of the session. It took about one minute to weigh the water bottle and to change the drinking tube during the middle of the session. The order in which the two different sized drinking tubes were available during the session was counterbalanced across sessions. RESULTS
AND DISCUSSION
The rats regulated the volume of water-intake during the SIP sessions, so that similar amounts of water were consumed when either the large-hole or small-hole drinking tube was present. These data confirm Freed and Mendelson's [10,11] and Magyar et al. 's [14] findings of water-intake volume regulation during schedule-induced polydipsia when the rate of water ingestion is manipulated by changing the size of the drinking-tube aperture. Figure 4 presents the mean water intake for each half of each session for the 2.6 mm hole drinking tube and the 4.8 mm hole drinking tube. The amounts of water consumed during each half of the session were not significantly different, t(3)=l.0, p>0.05, with a mean intake across all 18 sessions of 15.3 ml (SEM=0.41) with the 2.6 mm hole and 14.8 ml (SEM=0.59) with the 4.8 mm hole. If the data from session number 1 are omitted, the mean intake for both the large and small aperture drinking tubes is 15.1 ml. Table 1 shows the mean water intakes for each subject from Fig. 4. In Experiment 1 with dipper cups, the rats regulated the number of drinks per session rather than total volume intake. In Experiment 2 with drinking tubes, the rats regulated total volume intake per session; however, no measure of drinking time was obtained with the different sized apertures. In order to determine whether or not the 2.6 mm and 4.8 mm holes produced different rates of water ingestion, Experiment 3 was conducted. In addition to measuring volume intake, the amount of time spent drinking was also measured. The pro-
VOLUME REGULATION DURING SCHEDULE-INDUCED POLYDIPSIA cedure used in Experiment 3 was similar to that used by Freed and Mendelson [ 10,1 l] for measuring drinking induced by water deprivation.
TABLE 2 MEAN DATAFROMWATER-DEPRIVATIONINDUCED DRINKINGIN EXPERIMENT 3 7Cater Intake (ml)
EXPERIMENT 3 METHOD
225
Animal
Time Spent Drinking (sec)
Drinking Rate (ml per sec)
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4.8mm
2.6ram
4.8ram
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4.8ram
12.8 12.1 10.1 14.6 12.4
13.6 14.9 13.4 15.6
60.2 127.0 149.2 50.4
31.9 124.3 84.1 24.5
0.213 0.095 0.068 0.290
0.426 0.120 0.159 0.637
14.4
96.7
66.2
0.167
0.336
A n i m a l s and A p p a r a t u s
The animals were the same as in Experiment 2. Testing was conducted in only one operant conditioning chamber. A running time meter was used to measure the amount of time spent drinking to the nearest tenth of a second.
I-1 1-2 1-3 I-4 Mean
Procedure
The rats were first placed on ad lib food and water until they recaptured their former free-feeding body weights. They were placed on a 23.5 hr schedule of water deprivation. F o r 2 days, the daily 30-min period of access to water was in the home cage (with 2.6 mm aperture). F o r the next 8 days, the rats received their 30-rain of access to water in the operant chamber. On half of the days the drinking tube with 2.6 mm hole was present, on the other half the 4.8 mm hole was present. The order in which the large and small holes were presented was counterbalanced across days. F o o d was continuously available in the home cage, but was not present in the operant chamber during the 30-rain period of access to water. Total water intake and time spent drinking were recorded during each 30-min period.
There were four half-hour sessions with the 2.6 mm hole and with the 4.8 mm hole.
iment do not support this finding, since no food was present during deprivation-induced drinking and significantly more water was consumed with the 4.8 mm hole drinking tube than with the 2.6 mm hole drinking tube. The reason for this discrepancy is unclear. GENERAL DISCUSSION
RESULTS AND DISCUSSION Table 2 presents the mean data for water-deprivation induced from four half-hour periods with the 2.6 mm hole and four half-hour periods with the 4.8 mm hole. All four rats drank significantly more water from the 4.8 mm hole than from the 2.6 mm hole, t(3)=3.130, p<0.05, one-tailed test. However, they spent significantly more time drinking from the 2.6 mm hole than the 4.8 mm hole, t(3)=2.362, p<0.05, one-tailed test. This resulted in a mean ingestion rate twice as large for the 4.8 mm hole as for the 2.6 mm hole (0.336 ml/sec and 0.167 ml/sec, respectively). These mean ingestion rates were significantly different, t(3)=2.362, p<0.05, onetailed test. These data parallel findings by Freed and Mendelson [10,11] with water- deprivation induced drinking, and indicate that the different sized holes in the drinking tubes used in Experiment 2 were effective in producing different rates of water ingestion. The results of Experiments 2 and 3 replicate Freed et al. 's fmdings [ 11] that the volume regulation of water was more precise during schedule-induced polydipsia than during water-deprivation induced drinking. During polydipsia testing there was no significant difference in the volume intake for the two different sized apertures (2.6 mm hole yielded a mean intake of 15.2 ml and the 4.8 mm hole yielded a mean intake of 14.8 ml); whereas, during deprivationinduced testing there was significantly more water consumed with the 4.8 mm hole than with the 2.6 mm hole (see Table 2). Also, it should be noted that there was no evidence of greater water spillage or leakage from the 4.8 mm aperature in either Experiments 2 or 3. However, in their second study, Freed and Mendelson [ 10] found that volume intake regulation during deprivation-induced drinking was as precise as during schedule-induced polydipsia when food was not present in the test chamber, but was not as precise when food was present in the test chamber. The results of the present exper-
If the volume constancy hypothesis proposed by Freed and Mendelson [10,11] is true, then it might be expected that volume intake should be regulated during polydipsia sessions regardless of the mode of access to water. However, Wetherington et al. [22,23] reported that rats did not display water-intake volume regulation when the rats had to press a lever to obtain water via different sized dipper cups. Experiment 1 in the present study also failed to demonstrate volume regulation when water availability was manipulated by alternating large (0.10 ml) and small (0.06 ml) dipper cups within polydipsia sessions (rather than between sessions as Wetherington et al. has done). Thus, the procedural difference o f i n t r a s e s s i o n (as in the Freed and Mendelson studies [10,11] and in Experiment 1 of the present study) vs. intersession (as in the Wetherington et al. studies [22,23]) manipulation of water availability was not important in the discrepant findings. I also have failed to find volume regulation using dipper cups when the sizes were manipulated across sessions rather than within sessions (unpublished observations). When water availability was manipulated with different sized holes in drinking tubes (Experiment 2), the rats regulated volume intake during schedule-induced polydipsia in the same manner as reported by Freed and Mendelson [10,11] and Magyar et al. [14]. This experiment also extended the previous findings of volume regulation to a larger size aperture (i.e., 4.8 mm). Experiment 3 verified that the 4.8 mm and 2.6 mm drinking tube holes were effective in producing different rates of water ingestion, and also demonstrated that water-intake volume regulation was more precise during schedule-induced polydipsia than during deprivation-induced drinking. While it is unclear as to why the mode of access to water (i.e., water tube vs. dipper cup) produces these different
226 results in volume regulation, several factors can be ruled out. As stated above, intrasession vs. intersession manipulations do not appear to be important. Another possibility might be that the dipper cup sizes used in the present study (0.10 ml and 0.06 ml) and in the Wetherington and Ware study (0.40 ml and 0.01 ml) prevented volume regulation during polydipsia sessions. However, Wetherington et al. [22] parametrically varied dipper cup sizes from 0.01 ml to 0. l0 ml and still failed to find any evidence for volume regulation during schedule-induced polydipsia. A third possibility is that the dipper cup mode of access to water may place unusual constraints on the development and maintenance of normal schedule-induced polydipsia. However, the rats in the present study and in previous studies displayed polydipsic levels of water intake (although they do tend to be lower than water intakes seen with drinking tubes). Also, the present study (see Fig. 3) demonstrated that the pattern of polydipsic drinking with dipper cups was similar to the post-pellet pattern typically seen when water is available via drinking tubes (see [8]); although it should be noted that Wetherington et al. [22] reported that peak drinking occurred in the middle of the interpellet interval when dipper cups were used. Magyar et al. [14] suggested that "volume regulation during schedule-induced polydipsia is achieved through reductions in competing responses, and changes in both the discriminative control of drinking and topography of licking" (p. 763), and that "intake regulation also depends upon the limitations and constraints imposed by the experimental contingencies" (p. 767). The results of the present study and the studies by Wetherington et al. [22,23] clearly support the last half of this suggestion. Changing the mode of access to water during schedule-induced polydipsia (i.e., using dipper cups) either prevented, or at least reduced, the ability of the rats to regulate session volume intake. However, the rats in the present study (Experiment 1) did regulate the number of drinks within each session. Thus, rats drinking in scheduleinduced polydipsia sessions regulate volume-intake when water is available via conventional drinking tubes, but appear to regulate the number of drinks when water is obtained by barpressing for water via dipper cups. Allen and Kenshalo [1] found volume regulation in monkeys drinking from a water spout in which the draught size could be manipulated. Thus, there are a great deal of data which support a volume constancy hypothesis, but there are clearly exceptions as the present study (Experiment 1) and the studies by Wetherington et al. [22,23] show. I have previously argued [19] that a two-factor theory can best explain schedule-induced polydipsia. The initial development of schedule-induced polydipsia is probably a result of a general increase in motor excitability mediated via the lateral hypothalamus (see [21]) or other neural mechanisms. This idea has received additional support from research by Robbins and Koob [20] who found that mesolimbic dopamine (DA) lesions disrupted the acquisition of schedule-induced polydipsia, but had no effect on drinking induced by water deprivation. Similarly, Porter, Goldsmith, McDonough, Heath and Johnson [17] have reported that DA blockers, pimozide and spiperone, prevented the acquisition of schedule-induced polydipsia. Also, in that study, deprivation-induced drinking and ad lib drinking were unaffected by the pimozide and spiperone injections. The second factor concerns the regulation of established schedule-induced polydipsia via some oropharyngeal metering mechanism. Although not a primary mechanism,
PORTER oropharyngeal metering does exert some control over normal water intake [3]. Also, Kissileff [13] has demonstrated that small amounts of water injected directly into the mouth through a cheek fistula will abolish prandial drinking in rats, but the same quantity of water has no effect on prandial drinking when injected into the stomach. However, normal food-associated drinking is suppressed by both intragastric and intraoral injections of water. Kissileff has defined prandial drinking as a type of nonhomeostatic drinking which is controlled by an oropharyngeal mechanism; whereas, foodassociated drinking is homeostatic drinking which is controlled by factors related to water balance. Schedule-induced polydipsia has also been classified as a form of nonhomeostatic drinking [13, 15, 16, 17, 19], and several studies have shown that schedule-induced polydipsia appears to be controlled by oropharyngeal factors rather than by hydration controls. Both Chapman ]4] and Kenney, Wright, and Reynolds [12] have shown that intragastric preloads of water do not suppress schedule-induced polydipsia; whereas, intraoral preloads of water (via implanted fistulas) abolish or depress schedule-induced polydipsia. Other studies [5, 8, 19] have shown that gastric or intraperitoneal preloads of water or isotonic saline typically have little effect on e s t a b l i s h e d polydipsia, although Corfield-Sumner and Bond [6] were able to suppress established polydipsia with very large (selfingested) preloads of a glucose/saccharin solution. Also, Porter et al. [18] have recently shown that 10 ml intraperitoneal preloads of water, but not isotonic saline, completely suppressed the a c q u i s i t i o n of schedule-induced polydipsia in four of six rats, but produced only a slight suppression of e s t a b l i s h e d polydipsia. Although it is somewhat speculative, if volume regulation during established schedule-induced polydipsia is normally controlled by oropharyngeal factors when rats are allowed to freely drink from water tubes, it is possible that this volume regulation may be disrupted when the mode of access to water is manipulated. When drinking from a water bottle tube, the flow of water is not interrupted until the rat terminates the drinking bout; however, by requiring the rats to barpress in order to obtain water, the normal sequence of licking and time spent drinking is obviously disrupted. One possible consequence of this disruption of normal drinking bouts is that the oropharyngeal metering of water intake may also be disrupted, and thus, the regulation of volume intake during schedule-induced polydipsia may be interfered with. Also, as Wetherington et al. [22] have pointed out, travel time and travel distance to and from the water source are greater when barpressing and drinking from a water dipper (which would require alternating between the lever and dipper cut) than when drinking from a water tube (which would require only a single trip). While the present study and previous studies [22,23] have not systematically explored the effects of "travel time" on the amount of schedule-induced drinking when water dippers are used, this factor should be examined in future studies in order to determine its influence on possible oropharyngeal mechanisms. In summary, the present study supports previous findings [1, 10, 11, 14] of water intake volume regulation during schedule-induced polydipsia, and found that this volume regulation is more precise during schedule-induced drinking, than during deprivation-induced drinking. However, this volume regulation occurs only when water is freely available via drinking tubes and is probably monitored by an oropharyngeal "metering" mechanism. When the mode of access to water during polydipsia is changed (i.e., barpres-
VOLUME
REGULATION
DURING SCHEDULE-INDUCED
sing for p r e s e n t a t i o n o f a d i p p e r cup), this v o l u m e r e g u l a t i o n is d i s r u p t e d . A l t h o u g h s p e c u l a t i v e , it is s u g g e s t e d t h a t this d i s r u p t i o n m a y b e d u e to a n i n t e r f e r e n c e w i t h o r o p h a r y n g e a l m e c h a n i s m s w h i c h n o r m a l l y c o n t r o l e s t a b l i s h e d polydipsic drinking.
POLYDIPSIA
227 ACKNOWLEDGEMENTS
I would like to thank Nazan N. Sozer and Judith E. Platko for their assistance in data collection, and Richard Young for drawing the figures.
REFERENCES 1. Allen, J. D. and D. R. Kenshalo, Jr. Schedule-induced drinking as a function of interpellet interval and draught size in the Java Macque. J Exp Anal Behav 30: 139-151, 1978. 2. Allen, J. D. and J. H. Porter. Demonstration of behavioral contrast with adjunctive drinking. Physiol Behav 15:511-515, 1975. 3. Blass, E. M., R. Jabaris and W. G. Hall. Oropharyngeal control of drinking in rats. J Comp Physiol Psychol 90: 909-916, 1976. 4. Chapman, H. W. Oropharyngeal determinants of nonregulatory drinking in the rat. Unpublished Dissertation. University of Pennsylvania, 1969. 5. Cope, C. L., D. J. Sanger and D. E. Blackman. Intragastric water and the acquisition of schedule-induced drinking. Behav Biol 17: 267-270, 1976. 6. Corfield-Sumner, P. K. and N. W. Bond. Effects of preloading on the acquisition and maintenance of schedule-induced polydipsia. Behav Biol 23: 238-242, 1978. 7. Falk, J. L. Production of polydipsia in normal rats by an intermittent food schedule. Science 133: 195-196, 1961. 8. Falk, J. L. Conditions producing psychogenic polydipsia in animals. Ann N Y Acad Sci 157: 569-589, 1969. 9. Falk, J. L. The nature and determinants of adjunctive behavior. Physiol Behav 6: 577-588, 1971. 10. Freed, W. J. and J. Mendelson. Water-intake volume regulation in the rat: Schedule-induced drinking compared with waterdeprivation-drinking. J Cornp Physiol Psychol 91: 564-573, 1977. 11. Freed, W. J., J. Mendelson and J. M. Bramble. Intake-volume regulation during schedule-induced polydipsia in rats. Behav Biol 16: 245-250, 1976. 12. Kenny, J. R., J. W. Wright and T. J. Reynolds. Scheduleinduced polydipsia: The role of oral and plasma factors. Physio! Behav 17: 939-945, 1976. 13. Kissileff, H. R. Nonhomeostatic controls of drinking. In: The Neuropsychology of Thirst: New Findings and Advances in Concepts, edited by A. N. Epstein, H. R. Kissileff and E. Stellar. New York: John Wiley and Sons. 1973.
14. Magyar, R. L., R. J. Waldbillig and M. E. Meyer. The development of water-intake regulation during schedule-induced polydipsia. Physiol Behav 25: 763-767, 1980. 15. McDonough, J. J. and J. H. Porter. Schedule-induced and water-deprivation-induced drinking in rats: Effects of hypertonic saline challenges to homeostatic thirst mechanism. Bull Psychon Soc 21: 403-406, 1983. 16. Porter, J. H. Schedule-induced polydipsia: Homeostatic or non-homeostatic regulation of water intake? Paper presented at the meeting of the Sixth International conference on the Physiology of Food and Fluid Intake, Jouyen-Josas, France, 1977. 17. Porter, J. H., P. A. Goldsmith, J. J. McDonough, G. F. Heath and D. N. Johnson. Differential effects of dopamine blockers on the acquisition of schedule-induced drinking and deprivationinduced drinking. Physiol Psychol 12: 302-306, 1984. 18. Porter, J. H., J. J. McDonough and R. Young. Intraperitoneal preloads of water, but not isotonic saline, suppress scheduleinduced polysipsia in rats. Physiol Behav 29: 795-801, 1982. 19. Porter, J. H., R. Young and T. P. Moeschl. Effects of water and saline preloads on schedule-induced polydipsia in the rat. Physiol Behav 21: 333-338, 1978. 20. Robbins, T. W. and G. F. Koob. Selective disruption of displacement behavior by lesions of the mesolimbic dopamine system. Nature 285: 409-412, 1980. 21. Wayner, M. J. The lateral hypothalamus and adjunctive drinking. In: Progress in Brain Research: Vol. 41: Integrative Hypothalamic Activity, edited by D. F. Swaab and J. P. Schade. Amsterdam: Elsevier, 1974. 22. Wetherington, C. L., C. P. Lawler and I. Bianco. Scheduleinduced polydipsia and size of water dipper. Physiol Behav 30: 669-673, 1983. 23. Wetherington, C. L. and R. W. Ware. Schedule-induced polydipsia and mode of access to water. Behav Anal Lett 1: 187-198, 1981.
0031-9384/85 $3,00 + .00
Schedule-Induced Polydipsia: Another Look at Water-Intake Volume Regulation J O S E P H H. P O R T E R
Department o f Psychology, Virginia Commonwealth University, 810 West Franklin St. Richmond, VA 23284 R e c e i v e d 23 N o v e m b e r 1983 PORTER, J. H. Schedule-induced polydipsia: Another look at water-intake volume regulation. PHYSIOL BEHAV 35(2) 221-227, 1985.--Experiment 1 found that rats regulated the number of drinks per session during schedule-induced polydipsia, rather than volume intake when water was obtained from different sized water dipper cups. However, when water availability was manipulated during polydipsia sessions by changing the aperture size of the water bottle tube (Experiment 2), the rats regulated total water-intake volume per session. Experiment 3 demonstrated that this volume regulation was more precise during schedule-induced drinking than during water deprivation induced drinking. Also, it was shown that the different sized apertures were effective in manipulating the rates of water availability and ingestion.These experiments demonstrated that volume regulation during schedule-induced polydipsia occurs only when water is freely available via drinking tubes. Adjunctive behavior Volume regulation Drink regulation Schedule-induced polydipsia Deprivation-induced drinking Water availability Oropharyngeal mechanisms Nonhomeostatic drinking Rats
A L T H O U G H schedule-induced polydipsia is a well-studied phenomenon (see [8,9]), there are many aspects about it which are not resolved. Falk [7] demonstrated that fooddeprived rats would drink an average of 92.5 ml of water in a 3.17 hour session when food pellets were delivered on a variable-interval one-minute reinforcement schedule. Schedule-induced polydipsia is characterized by a postpellet pattern of drinking (see [8,9]) in which almost every pellet is followed by a small draught of water (typically around 0.5 ml). Although most of the studies on scheduleinduced polydipsia have attempted to explain why the drinking develops initially, one line of research has attempted to determine why animals quit drinking after consuming each small draught of water. In other words, does the animal stop drinking because it has consumed a certain quantity (volume) of water or because it has spent a certain amount of time drinking (or taken a fixed number of drinks)? Freed and Mendelson [10] and Freed, Mendelson, and Bramble [11] reported that rats regulate volume intake or water during schedule-induced polydipsia, rather than the amount of time spent drinking. The rate of water ingestion was manipulated by using either a large (2.6 mm) or a small (1.0 mm) aperture in the drinking tube. The rats spent more time drinking when the small aperture (which had a slower rate of water flow) was present in order to maintain a relatively constant volume of water ingested during each session. They also found that water-intake volume regulation was more precise during schedule-induced polydipsia than
during water-deprivation induced drinking. Allen and Kenshalo [1] have reported volume intake regulation in Java Macaque monkeys when draught size per lick at a drinking spout was manipulated. Also, Magyar, Wardbillig, and Meyer [14] have reported that rats regulate volume intake during schedule-induced polydipsia during responsecontingent food schedules (the Freed and Mendelson studies [10,11] had used response-independent food schedules). Thus, these studies seem to offer strong support for the notion that animals regulate volume intake during scheduleinduced polydipsia test sessions. However, recent studies have challenged this conclusion. Wetherington and Ware [23] reported that volume intake regulation did not occur during schedule-induced polydipsia when the mode of water availability was manipulated by changing water dipper size (0.01 ml vs. 0.04 ml dippers). In a subsequent study, Wetherington, Lawler, and Blanco [22] parametrically varied dipper size to further examine this discrepant finding. They found that increasing the dipper size increased session water intake during schedule-induced polydipsia. These studies by Wetherington and her colleagues question the notion of a volume constancy mechanism in the regulation of schedule-induced polydipsia, since changing the mode of water availability (dipper vs. water spout) produced different results. The purpose of the present series of experiments was to re-examine the question of water-intake volume regulation of schedule-induced polydipsia. Besides the different modes of
1This research was supported in part by the Grant-in-Aid Program for Faculty of Virginia Commonwealth University.
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FIG. I. Mean water intake (ml) for each half of each session in Experiment 1 with the 0.06 ml dipper and the 0.10 ml dipper. The overall mean for the 15 sessions was 6.3 ml (SEM=0.29) for the 0.10 ml dipper and 3.9 ml (SEM=0.41) for the 0.06 ml dipper. It should be noted that the water intake values were derived from the number of dipper operations.
FIG. 2. Mean number of drinks (dipper operations) for each half of each session in Experiment 1 with the 0.06 ml dipper and the 0.10 ml dipper. The overall mean for the 15 sessions with 62.9 (SEM=4.1) for the 1.0 ml dipper and 65.4 (SEM=4.8) for the 0.06 ml dipper. It should be noted that the number of drinks actually represents the number of dipper operations.
water presentation and manipulation (dipper size vs. water spout aperture size) in the Wetherington et al. studies [22,23], another procedural difference may possibly account for these discrepant findings. In the Freed and Mendelson studies [10,11] the water tube sizes were alternated within sessions (i.e., intrasession); whereas, Wetherington et al. [22,23] manipulated dipper size across test sessions (i.e., intersession). In order to determine whether or not this procedural difference was responsible for these discrepant findings, intrasession manipulation of dipper size was examined in Experiment 1. This allowed a more direct comparison of the findings in the Wetherington et al. studies with the findings in the Freed and Mendelson studies.
Procedure
EXPERIMENT 1 METHOD Animals One adult naive male albino rat and four adult hooded rats (1 female, 3 males) were used. The hooded rats had previous experience with fixed-time (FT) food reinforcement schedules and schedule-induced polydipsia. They were housed individually in an animal colony room (7 a.m. light/6 p.m. dark cycle). Room temperature was maintained at approximately 72°F (22.2°C). Apparatus Test sessions were conducted in a BRS/LVE operantconditioning chamber (Moducage) which was enclosed in a sound-attenuated cubicle. The food magazine was located on the left side of the intelligence panel, the response lever in the center, and the water dipper on the right side. Two dipper cup sizes of 0.10 ml and 0.06 ml were used. A 28 v/dc houselight and white noise were present during each session. Reinforcers were 45 mg standard formula Noyes pellets. Standard solid state and electromechanical programming and recording equipment were located in an adjacent room.
Initially, the rats were placed on a 23-hr water deprivation schedule in order to train them to press the lever which operated the water dipper (0.10 ml dipper cup). After the bar-pressing response had been shaped, each rat obtained 100 water reinforcers according to a fixed-ratio (FR) 1 schedule. Then, the rats were returned to an ad lib food and water schedule for 14 days in their home cages. New freefeeding body weights (BW) were established (mean of the last two days), and the rats were then reduced to 80% BW and maintained at that level by restricting their daily ration of Purina rat chow. Daily test sessions lasted 50-min, and food pellets were delivered according to a non-contingent fixedtime one-minute (FT 1-min) schedule. For the first 20 sessions each bar press activated the water dipper (FR 1) and delivered 0.10 ml of water in order to establish scheduleinduced drinking. There was a one second delay between dipper activations so that a burst of responses would not be counted as several drinks. The dipper was normally in the up position so that it remained freely available to the rats. Also, the number of drinks was obtained from the number of dipper operations, not from the number of barpresses. The volume of water intake was obtained by multiplying the number of drinks during a session by the volume of the dipper which was present (either 0.10 ml or 0.6 ml). For the next 15 sessions, the 0.10 ml dipper cup and 0.06 ml dipper cup were alternated during each session. One size was available during the first half of the session (25-min), and the other during the second half. The order was counterbalanced across sessions. Changing the dipper cup halfway through the session took about one minute.
RESULTS AND DISCUSSION
The rats regulated the number of drinks during each half of the session, but did not regulate the volume of water consumed. It should be noted that the number of drinks actually
VOLUME REGULATION DURING SCHEDULE-INDUCED POLYDIPSIA TABLE 1 INDIVIDUAL SUBJECT DATA FROM FIG. 2 (EXPERIMENT 1) AND FIG. 4 (EXPERIMENT 2)
Mean Number of Drinks for Each Subject From Fig. 2 Subjects
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Mean Water Intakes for Each Subject From Fig. 4 Subjects
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represents the number of dipper operations, and that the water intake values (ml) were derived from the number of dipper operations. Figure 1 shows the mean water intake (ml) for each half of each session. Except for session number 6, the volume of water intake was always substantially higher when the 0. I0 ml dipper cup was present. The average intake over all 15 sessions for the 0.10 ml dipper cup (mean=6.3 ml, SEM=0.29) was significantly greater, t (4) = 8.705, p <0.01, than intake when the 0.06 ml dipper cup (mean=3.9 ml, SEM=0.41) was present. Figure 2 shows that the mean number of drinks (i.e., the number of dipper operations) were similar for both halves of each session. Across all 15 sessions the mean number of drinks was 65.3 (SEM=5.0) for the 0.06 ml dipper cup and 63.1 (SEM=4.9) for the 0.10 ml dipper cup. The overall means were not significantly different, t(4)=0.90, p>0.05. Table 1 shows the mean number of drinks for each subject from Fig. 2. Instead of regulating the total volume of water intake, as Freed and Mendelson [10,11] reported, the rats in the present study regulated the number of drinks taken within each session (see Fig. 2) These results do, however, agree with Wetherington et al. 's [22,23] findings than when the mode of water availability to rats is via a water dipper, the volume of water consumed depends on the size of the water dipper. More water was consumed during a session with the large dipper (0.10 ml) than with the smaller dipper (0.6 ml). However, the rats in the present study demonstrated very precise regulation of the number of drinks during the session with both dipper cup sizes. In the Wetherington and Ware study [23] the rats not only consumed a larger volume of water with the large dipper cup (0.04 ml) than with the smaller dipper cup (0.01 ml), but also had a greater number of drinks (i.e., dipper operations) with the larger dipper cup than with the small dipper cup. In their subsequent study, however, Wetherington et al. [22] found that the number of drinks did not vary with dipper size which was varied systematically from 0.01 ml to 0.10 ml. One obvious difference between the present study and
223
Wetherington et a l . ' s studies [22,23] and the Freed and Mendelson's studies [10,11] was the method by which the water was obtained. In the present study rats had to press a lever to obtain water via a dipper cup whose size was varied. One might suspect that this mode of water availability places unusual constraints on the development of normal scheduleinduced polydipsia. However, Falk [8] has previously shown that rats can be trained to press a lever in order to obtain water during schedule-induced polydipsia. Also, Allen and Porter [2] have investigated schedule-induced polydipsia by requiring the rats to press a lever in order to obtain water from a dipper, rather than having free access to a water bottle. Both the pattern of drinking and the amounts of water consumed when dippers were used were comparable to normal schedule-induced polydipsia. In the present experiment the mean water intake for the total session (0.10 ml and 0.06 ml dipper data combined) across the 15 days was 10.2 ml (0.2 ml per pellet). Although this value is somewhat low, it is well within ranges of water intake which have been reported in polydipsia studies and observations in this laboratory in which access to water was via a water bottle tube. Figure 3 shows the relative frequency of drinks (dipper operations) as a function of fifteen-second bins during the one-minute interpellet intervals for the last three test sessions. As can be seen, all rats (with the exception of R-3 with the 0.10 ml dipper cup) displayed the greatest number of drinks during the first fifteen seconds of the interpellet intervals with both the 0.10 ml and 0.06 ml dipper cups. The number of drinks then decreased in a monotonic linear function (with the exception of R-3 again) throughout the rest of the one-minute interval. While polydipsia drinking with water dippers was extended somewhat further into the one-minute interval than is typically seen when water tubes are used, the majority of the drinking did occur during the first thirty seconds of the interpellet interval. Thus, the use of dipper cups in Experiment 1 did not appear to greatly alter the characteristics (i.e., volume ingested, pattern drinking) typically seen with schedule-induced polydipsia [8,9]. Experiment 1 demonstrated that the lack of volumeintake regulation in the Wetherington et al. studies [22,23] cannot be attributed to the procedural differences of intrasession vs. intersession manipulation of dipper cup size. Since Experiment 1 failed to replicate Freed and Mendelson's [10, l 1] findings, I felt a more systematic replication of their studies was necessary. In Experiment 2, the rate of water ingestion was varied by using two drinking tubes with different size apertures, which Freed and Mendelson have shown to be an effective way to change ingestion rate. Freed and Mendelson [10,11] and Magyar et al. [14] used 1.0 and 2.6 mm tube openings. In Experiment 2, 2.6 and 4.8 mm apertures were used in order to determine if the volume relation shown in those studies would be evident with a larger aperture. EXPERIMENT 2 METHOD Animals
Four adult naive female Sprague-Dawley rats were used. Housing conditions were the same as in Experiment 1. The rats were maintained at 80% BW by restricting their daily ration of Purina rat chow.
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FIG. 3. Relative frequency of drinks as a function of fifteen-second bins during the one-minute interpellet intervals for the last three sessions of Experiment 1 for each rat for the 0.10 ml dipper cup and the 0.06 ml dipper cup. It should be noted that the number of drinks actually represents the number of dipper operations.
Apparatus Test sessions were conducted in two Scientific Prototype operant-conditioning chambers (Model A-100) which were housed in sound-attenuated cubicles. A water bottle was mounted behind the intelligence panel on the right-hand side with the metal drinking tube recessed slightly behind a hole 20 mm in diameter. The food magazine was mounted on the left side of the panel and no lever was present. Standard drinking tubes with 2.6---0.10 mm apertures were used as small-holed tubes, and drinking tubes with 4.8±0.10 mm apertures were used as large-holed tubes. A 28 v/dc houselight and white noise were present during each session. Reinforcers were 45 mg standard formula Noyes pellets. Standard electromechanical programming and recording equipment were located in an adjacent room. Procedure One-hour test sessions were conducted daily. Food pel-
lets were delivered according to a FT 1-min schedule. The rats were given 20 1-hr sessions with the small-hole drinking tube (2.6 mm) in order to establish schedule-induced polydipsia. Then 18 sessions were conducted in which the large-hole and small-hole drinking t u b e s were alternated within each session. One size hole was available for the first half of the session (20-min), and the other size was available for the second half of the session. It took about one minute to weigh the water bottle and to change the drinking tube during the middle of the session. The order in which the two different sized drinking tubes were available during the session was counterbalanced across sessions. RESULTS
AND DISCUSSION
The rats regulated the volume of water-intake during the SIP sessions, so that similar amounts of water were consumed when either the large-hole or small-hole drinking tube was present. These data confirm Freed and Mendelson's [10,11] and Magyar et al. 's [14] findings of water-intake volume regulation during schedule-induced polydipsia when the rate of water ingestion is manipulated by changing the size of the drinking-tube aperture. Figure 4 presents the mean water intake for each half of each session for the 2.6 mm hole drinking tube and the 4.8 mm hole drinking tube. The amounts of water consumed during each half of the session were not significantly different, t(3)=l.0, p>0.05, with a mean intake across all 18 sessions of 15.3 ml (SEM=0.41) with the 2.6 mm hole and 14.8 ml (SEM=0.59) with the 4.8 mm hole. If the data from session number 1 are omitted, the mean intake for both the large and small aperture drinking tubes is 15.1 ml. Table 1 shows the mean water intakes for each subject from Fig. 4. In Experiment 1 with dipper cups, the rats regulated the number of drinks per session rather than total volume intake. In Experiment 2 with drinking tubes, the rats regulated total volume intake per session; however, no measure of drinking time was obtained with the different sized apertures. In order to determine whether or not the 2.6 mm and 4.8 mm holes produced different rates of water ingestion, Experiment 3 was conducted. In addition to measuring volume intake, the amount of time spent drinking was also measured. The pro-
VOLUME REGULATION DURING SCHEDULE-INDUCED POLYDIPSIA cedure used in Experiment 3 was similar to that used by Freed and Mendelson [ 10,1 l] for measuring drinking induced by water deprivation.
TABLE 2 MEAN DATAFROMWATER-DEPRIVATIONINDUCED DRINKINGIN EXPERIMENT 3 7Cater Intake (ml)
EXPERIMENT 3 METHOD
225
Animal
Time Spent Drinking (sec)
Drinking Rate (ml per sec)
2.6mm
4.8mm
2.6ram
4.8ram
2.6mm
4.8ram
12.8 12.1 10.1 14.6 12.4
13.6 14.9 13.4 15.6
60.2 127.0 149.2 50.4
31.9 124.3 84.1 24.5
0.213 0.095 0.068 0.290
0.426 0.120 0.159 0.637
14.4
96.7
66.2
0.167
0.336
A n i m a l s and A p p a r a t u s
The animals were the same as in Experiment 2. Testing was conducted in only one operant conditioning chamber. A running time meter was used to measure the amount of time spent drinking to the nearest tenth of a second.
I-1 1-2 1-3 I-4 Mean
Procedure
The rats were first placed on ad lib food and water until they recaptured their former free-feeding body weights. They were placed on a 23.5 hr schedule of water deprivation. F o r 2 days, the daily 30-min period of access to water was in the home cage (with 2.6 mm aperture). F o r the next 8 days, the rats received their 30-rain of access to water in the operant chamber. On half of the days the drinking tube with 2.6 mm hole was present, on the other half the 4.8 mm hole was present. The order in which the large and small holes were presented was counterbalanced across days. F o o d was continuously available in the home cage, but was not present in the operant chamber during the 30-rain period of access to water. Total water intake and time spent drinking were recorded during each 30-min period.
There were four half-hour sessions with the 2.6 mm hole and with the 4.8 mm hole.
iment do not support this finding, since no food was present during deprivation-induced drinking and significantly more water was consumed with the 4.8 mm hole drinking tube than with the 2.6 mm hole drinking tube. The reason for this discrepancy is unclear. GENERAL DISCUSSION
RESULTS AND DISCUSSION Table 2 presents the mean data for water-deprivation induced from four half-hour periods with the 2.6 mm hole and four half-hour periods with the 4.8 mm hole. All four rats drank significantly more water from the 4.8 mm hole than from the 2.6 mm hole, t(3)=3.130, p<0.05, one-tailed test. However, they spent significantly more time drinking from the 2.6 mm hole than the 4.8 mm hole, t(3)=2.362, p<0.05, one-tailed test. This resulted in a mean ingestion rate twice as large for the 4.8 mm hole as for the 2.6 mm hole (0.336 ml/sec and 0.167 ml/sec, respectively). These mean ingestion rates were significantly different, t(3)=2.362, p<0.05, onetailed test. These data parallel findings by Freed and Mendelson [10,11] with water- deprivation induced drinking, and indicate that the different sized holes in the drinking tubes used in Experiment 2 were effective in producing different rates of water ingestion. The results of Experiments 2 and 3 replicate Freed et al. 's fmdings [ 11] that the volume regulation of water was more precise during schedule-induced polydipsia than during water-deprivation induced drinking. During polydipsia testing there was no significant difference in the volume intake for the two different sized apertures (2.6 mm hole yielded a mean intake of 15.2 ml and the 4.8 mm hole yielded a mean intake of 14.8 ml); whereas, during deprivationinduced testing there was significantly more water consumed with the 4.8 mm hole than with the 2.6 mm hole (see Table 2). Also, it should be noted that there was no evidence of greater water spillage or leakage from the 4.8 mm aperature in either Experiments 2 or 3. However, in their second study, Freed and Mendelson [ 10] found that volume intake regulation during deprivation-induced drinking was as precise as during schedule-induced polydipsia when food was not present in the test chamber, but was not as precise when food was present in the test chamber. The results of the present exper-
If the volume constancy hypothesis proposed by Freed and Mendelson [10,11] is true, then it might be expected that volume intake should be regulated during polydipsia sessions regardless of the mode of access to water. However, Wetherington et al. [22,23] reported that rats did not display water-intake volume regulation when the rats had to press a lever to obtain water via different sized dipper cups. Experiment 1 in the present study also failed to demonstrate volume regulation when water availability was manipulated by alternating large (0.10 ml) and small (0.06 ml) dipper cups within polydipsia sessions (rather than between sessions as Wetherington et al. has done). Thus, the procedural difference o f i n t r a s e s s i o n (as in the Freed and Mendelson studies [10,11] and in Experiment 1 of the present study) vs. intersession (as in the Wetherington et al. studies [22,23]) manipulation of water availability was not important in the discrepant findings. I also have failed to find volume regulation using dipper cups when the sizes were manipulated across sessions rather than within sessions (unpublished observations). When water availability was manipulated with different sized holes in drinking tubes (Experiment 2), the rats regulated volume intake during schedule-induced polydipsia in the same manner as reported by Freed and Mendelson [10,11] and Magyar et al. [14]. This experiment also extended the previous findings of volume regulation to a larger size aperture (i.e., 4.8 mm). Experiment 3 verified that the 4.8 mm and 2.6 mm drinking tube holes were effective in producing different rates of water ingestion, and also demonstrated that water-intake volume regulation was more precise during schedule-induced polydipsia than during deprivation-induced drinking. While it is unclear as to why the mode of access to water (i.e., water tube vs. dipper cup) produces these different
226 results in volume regulation, several factors can be ruled out. As stated above, intrasession vs. intersession manipulations do not appear to be important. Another possibility might be that the dipper cup sizes used in the present study (0.10 ml and 0.06 ml) and in the Wetherington and Ware study (0.40 ml and 0.01 ml) prevented volume regulation during polydipsia sessions. However, Wetherington et al. [22] parametrically varied dipper cup sizes from 0.01 ml to 0. l0 ml and still failed to find any evidence for volume regulation during schedule-induced polydipsia. A third possibility is that the dipper cup mode of access to water may place unusual constraints on the development and maintenance of normal schedule-induced polydipsia. However, the rats in the present study and in previous studies displayed polydipsic levels of water intake (although they do tend to be lower than water intakes seen with drinking tubes). Also, the present study (see Fig. 3) demonstrated that the pattern of polydipsic drinking with dipper cups was similar to the post-pellet pattern typically seen when water is available via drinking tubes (see [8]); although it should be noted that Wetherington et al. [22] reported that peak drinking occurred in the middle of the interpellet interval when dipper cups were used. Magyar et al. [14] suggested that "volume regulation during schedule-induced polydipsia is achieved through reductions in competing responses, and changes in both the discriminative control of drinking and topography of licking" (p. 763), and that "intake regulation also depends upon the limitations and constraints imposed by the experimental contingencies" (p. 767). The results of the present study and the studies by Wetherington et al. [22,23] clearly support the last half of this suggestion. Changing the mode of access to water during schedule-induced polydipsia (i.e., using dipper cups) either prevented, or at least reduced, the ability of the rats to regulate session volume intake. However, the rats in the present study (Experiment 1) did regulate the number of drinks within each session. Thus, rats drinking in scheduleinduced polydipsia sessions regulate volume-intake when water is available via conventional drinking tubes, but appear to regulate the number of drinks when water is obtained by barpressing for water via dipper cups. Allen and Kenshalo [1] found volume regulation in monkeys drinking from a water spout in which the draught size could be manipulated. Thus, there are a great deal of data which support a volume constancy hypothesis, but there are clearly exceptions as the present study (Experiment 1) and the studies by Wetherington et al. [22,23] show. I have previously argued [19] that a two-factor theory can best explain schedule-induced polydipsia. The initial development of schedule-induced polydipsia is probably a result of a general increase in motor excitability mediated via the lateral hypothalamus (see [21]) or other neural mechanisms. This idea has received additional support from research by Robbins and Koob [20] who found that mesolimbic dopamine (DA) lesions disrupted the acquisition of schedule-induced polydipsia, but had no effect on drinking induced by water deprivation. Similarly, Porter, Goldsmith, McDonough, Heath and Johnson [17] have reported that DA blockers, pimozide and spiperone, prevented the acquisition of schedule-induced polydipsia. Also, in that study, deprivation-induced drinking and ad lib drinking were unaffected by the pimozide and spiperone injections. The second factor concerns the regulation of established schedule-induced polydipsia via some oropharyngeal metering mechanism. Although not a primary mechanism,
PORTER oropharyngeal metering does exert some control over normal water intake [3]. Also, Kissileff [13] has demonstrated that small amounts of water injected directly into the mouth through a cheek fistula will abolish prandial drinking in rats, but the same quantity of water has no effect on prandial drinking when injected into the stomach. However, normal food-associated drinking is suppressed by both intragastric and intraoral injections of water. Kissileff has defined prandial drinking as a type of nonhomeostatic drinking which is controlled by an oropharyngeal mechanism; whereas, foodassociated drinking is homeostatic drinking which is controlled by factors related to water balance. Schedule-induced polydipsia has also been classified as a form of nonhomeostatic drinking [13, 15, 16, 17, 19], and several studies have shown that schedule-induced polydipsia appears to be controlled by oropharyngeal factors rather than by hydration controls. Both Chapman ]4] and Kenney, Wright, and Reynolds [12] have shown that intragastric preloads of water do not suppress schedule-induced polydipsia; whereas, intraoral preloads of water (via implanted fistulas) abolish or depress schedule-induced polydipsia. Other studies [5, 8, 19] have shown that gastric or intraperitoneal preloads of water or isotonic saline typically have little effect on e s t a b l i s h e d polydipsia, although Corfield-Sumner and Bond [6] were able to suppress established polydipsia with very large (selfingested) preloads of a glucose/saccharin solution. Also, Porter et al. [18] have recently shown that 10 ml intraperitoneal preloads of water, but not isotonic saline, completely suppressed the a c q u i s i t i o n of schedule-induced polydipsia in four of six rats, but produced only a slight suppression of e s t a b l i s h e d polydipsia. Although it is somewhat speculative, if volume regulation during established schedule-induced polydipsia is normally controlled by oropharyngeal factors when rats are allowed to freely drink from water tubes, it is possible that this volume regulation may be disrupted when the mode of access to water is manipulated. When drinking from a water bottle tube, the flow of water is not interrupted until the rat terminates the drinking bout; however, by requiring the rats to barpress in order to obtain water, the normal sequence of licking and time spent drinking is obviously disrupted. One possible consequence of this disruption of normal drinking bouts is that the oropharyngeal metering of water intake may also be disrupted, and thus, the regulation of volume intake during schedule-induced polydipsia may be interfered with. Also, as Wetherington et al. [22] have pointed out, travel time and travel distance to and from the water source are greater when barpressing and drinking from a water dipper (which would require alternating between the lever and dipper cut) than when drinking from a water tube (which would require only a single trip). While the present study and previous studies [22,23] have not systematically explored the effects of "travel time" on the amount of schedule-induced drinking when water dippers are used, this factor should be examined in future studies in order to determine its influence on possible oropharyngeal mechanisms. In summary, the present study supports previous findings [1, 10, 11, 14] of water intake volume regulation during schedule-induced polydipsia, and found that this volume regulation is more precise during schedule-induced drinking, than during deprivation-induced drinking. However, this volume regulation occurs only when water is freely available via drinking tubes and is probably monitored by an oropharyngeal "metering" mechanism. When the mode of access to water during polydipsia is changed (i.e., barpres-
VOLUME
REGULATION
DURING SCHEDULE-INDUCED
sing for p r e s e n t a t i o n o f a d i p p e r cup), this v o l u m e r e g u l a t i o n is d i s r u p t e d . A l t h o u g h s p e c u l a t i v e , it is s u g g e s t e d t h a t this d i s r u p t i o n m a y b e d u e to a n i n t e r f e r e n c e w i t h o r o p h a r y n g e a l m e c h a n i s m s w h i c h n o r m a l l y c o n t r o l e s t a b l i s h e d polydipsic drinking.
POLYDIPSIA
227 ACKNOWLEDGEMENTS
I would like to thank Nazan N. Sozer and Judith E. Platko for their assistance in data collection, and Richard Young for drawing the figures.
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14. Magyar, R. L., R. J. Waldbillig and M. E. Meyer. The development of water-intake regulation during schedule-induced polydipsia. Physiol Behav 25: 763-767, 1980. 15. McDonough, J. J. and J. H. Porter. Schedule-induced and water-deprivation-induced drinking in rats: Effects of hypertonic saline challenges to homeostatic thirst mechanism. Bull Psychon Soc 21: 403-406, 1983. 16. Porter, J. H. Schedule-induced polydipsia: Homeostatic or non-homeostatic regulation of water intake? Paper presented at the meeting of the Sixth International conference on the Physiology of Food and Fluid Intake, Jouyen-Josas, France, 1977. 17. Porter, J. H., P. A. Goldsmith, J. J. McDonough, G. F. Heath and D. N. Johnson. Differential effects of dopamine blockers on the acquisition of schedule-induced drinking and deprivationinduced drinking. Physiol Psychol 12: 302-306, 1984. 18. Porter, J. H., J. J. McDonough and R. Young. Intraperitoneal preloads of water, but not isotonic saline, suppress scheduleinduced polysipsia in rats. Physiol Behav 29: 795-801, 1982. 19. Porter, J. H., R. Young and T. P. Moeschl. Effects of water and saline preloads on schedule-induced polydipsia in the rat. Physiol Behav 21: 333-338, 1978. 20. Robbins, T. W. and G. F. Koob. Selective disruption of displacement behavior by lesions of the mesolimbic dopamine system. Nature 285: 409-412, 1980. 21. Wayner, M. J. The lateral hypothalamus and adjunctive drinking. In: Progress in Brain Research: Vol. 41: Integrative Hypothalamic Activity, edited by D. F. Swaab and J. P. Schade. Amsterdam: Elsevier, 1974. 22. Wetherington, C. L., C. P. Lawler and I. Bianco. Scheduleinduced polydipsia and size of water dipper. Physiol Behav 30: 669-673, 1983. 23. Wetherington, C. L. and R. W. Ware. Schedule-induced polydipsia and mode of access to water. Behav Anal Lett 1: 187-198, 1981.