- Email: [email protected]
Atropine methyl nitrate inhibits sham feeding in the rat
Pharmacology Biochemistry & Behavior, Vol. 8, pp. 405-407. Printed in the U.S.A.
Atropine Methyl Nitrate Inhibits Sham Feeding in the Rat D. L O R E N Z , 1 P. N A R D I AND G. P. SMITH
Department o f Psychiatry, Cornell University Medical College and the Edward W. Bourne Behavioral Research Laboratory, N e w York Hospital Westchester Division, White Plains, N Y 10605
(Received 30 November) LORENZ, D., P. NARDI AND G. P. SMITH. Atropine methyl nitrate inhibits sham feeding in the rat. PHARMAC. BIOCHEM. BEHAV. 8(4) 405-407, 1978.- Atropine methyl nitrate inhibited sham feeding of a liquid diet in a dose-related manner. Identical doses of atropine methyl nitrate had no effect on sham drinking of water. This differential effect is evidence that the inhibition of sham feeding was not due to peripheral anticholinergic disruption of licking, salivating or swallowing. The results suggest that peripheral blockade of cholinergic receptors of the muscarinic type is a mechanism for eliciting satiety for food in the rat. Atropine methyl nitrate
Anticholinergic
Muscarinicreceptor
ATROPINE methyl nitrate inhibits food intake [3,4]. The proposed mechanism of the anorexic action is thought to be indirect: peripheral, anticholinergic blockade of salivary secretion prevents normal mastication and swallowing of dry food [3,4]. This dry mouth hypothesis is supported by the report that atropine methyl nitrate (AMN) did not inhibit the intake of wet mash [2]. We report here, however, that AMN inhibits sham feeding of liquid diet, but not sham drinking of water, in a dose-related manner. This is critical evidence against the dry mouth hypothesis and suggests that peripheral anticholinergic blockade activates an inhibitory mechanism specific for feeding behavior. EXPERIMENT 1
Method Animals and procedure. Adult male albino rats (Sprague-Dawley, Hormone Assay Co., Chicago, IL), weighing between 3 5 0 - 4 5 0 g, were housed and tested in individual wire cages in a room maintained at about 24°C and lit from 0700 to 1900 hr. Under Chloropent anesthesia ( 2 - 3 ml/kg, IP, Fort Dodge, IA) each rat had a gastric cannula implanted along the greater curvature in the rumen of the stomach, as described by Antin et al. [ 1 ]. Seven to 10 days after surgery, rats were placed on a 17-hr food deprivation schedule. All tests were performed during the light phase (1000 to 1200 hr). Fifteen min before the start of a test, the screw occluding the gastric cannula was removed, gastric contents were flushed out with tap water, and an extension drainage tube was attached to the cannula of each rat. Rats were then returned to their cages ready to sham feed.
Satiety
S h a mfeeding
S h a mdrinking
Liquid diet (25% v/v, No. 116 EC, GIBCO, Grand Island, NY) was offered for 77 min. Sham intake was recorded every 5 min throughout the test. Behavior was sampled for 10 sec every 5 rain and the occurrence of resting behavior was noted. Resting behavior was defined as a relaxed, immobile posture with or without the head and trunk of the body positioned on the cage floor. The eyes could be open or closed. The occurrence of resting behavior was considered the terminal item in the behavioral sequence of satiety [ I ]. Since resting never occurs in sham feeding rats injected with saline [ 1 ], if AMN inhibited sham feeding and elicited resting behavior, it would suggest that AMN had produced satiety and not just disrupted feeding. Atropine methyl nitrate (AMN 2, 10, 50, 250, 1250 ~g/kg, No. A-0382, Sigma, St. Louis, MO) was dissolved in 1 ml of saline (0.9%) and administered intraperitoneally 17 rain after sham feeding began. All rats were tested twice with each dose. Injections of AMN alternated with 1 ml injections of saline (0.9%) in a crossover design. Gastric drainage was collected in plastic containers placed beneath the cages and measured after each test. If the volume of gastric drainage was less than 100% of the volume sham fed, the test was discarded. At the end of the test gastric cannulas were closed and rats were given the 25% liquid diet to eat for 1 hr. Purina pellets were then provided until 1700 hr when food was removed in preparation for the next day's test.
Results AMN inhibited sham feeding in a dose related manner (Fig. 1). The threshold dose was 10 ug/kg. The largest dose
Send reprint requests to D. Lorenz, Leidy Laboratory, Department of Biology, University of Pennsylvania, Philadelphia, PA 19104.
C o p y r i g h t © 1978 A N K H O
I n t e r n a t i o n a l Inc.--0091-3057/78/0804-0405500.60
406
L O R E N Z , N A R D I A N D SMITH
IO0 - -
Atropine methyl nitrate 1
Coolrolu
80-
Sham intake of liquid diet 60(mllhr)
iiiiii!i ii!iii!
iii~ ii!ii!i
iiiiiiii ii!!iii! 40-
!i!i i ~
20
10
50
250
~ilil) 1,250
Dose (lagI kg) FIG. 1. Sham intakes of 11 rats after intraperitoneal administration of saline or 5 doses of atropine methyl nitrate. *Denotes significant difference (p<0.025, matched pairs t-test). Each rat was tested twice with all doses of atropine methyl nitrate.
( 1 2 5 0 p g / k g ) i n h i b i t e d s h a m feeding 77%. T h e i n h i b i t i o n o f s h a m f e e d i n g was a c c o m p a n i e d b y a n increased i n c i d e n c e o f resting b e h a v i o r ( T a b l e 1). Since resting b e h a v i o r is t h e t e r m i n a l i t e m in the b e h a v i o r a l s e q u e n c e o f satiety, this result suggests t h a t A M N n o t o n l y i n h i b i t s s h a m feeding b u t also elicits satiety. A m o r e c o m p l e t e b e h a v i o r a l analysis, h o w e v e r , will be r e q u i r e d to e s t a b l i s h this p o i n t . It is possible t h a t A M N i n h i b i t s s h a m feeding b y d i s r u p t i n g t h e n o r m a l m o t o r i n t e g r a t i o n necessary for licking o r swallowing liquid f o o d . If this were so, A M N s h o u l d also i n h i b i t s h a m d r i n k i n g o f water. This was t e s t e d in t h e n e x t experiment.
TABLE 1 EFFECT OF ATROPINE METHYL NITRATE ON RESTING BEHAVIOR Atropine (/~g/kg) Number of rats resting
2 0
10 2
50 3
250 7
1250 11
were placed o n a 2 2 - h r w a t e r d e p r i v a t i o n s c h e d u l e w i t h P u r i n a rat pellets available at all t i m e s e x c e p t d u r i n g t h e test period. All tests were p e r f o r m e d d u r i n g t h e light p h a s e ( 1 0 0 0 - 1 2 0 0 hr). F i f t e e n m i n u t e s b e f o r e t h e start o f t h e test, t h e screw o c c l u d i n g the gastric c a n n u l a was r e m o v e d , t h e gastric c o n t e n t s were flushed o u t w i t h saline (0.9%), a n d a n e x t e n s i o n drainage t u b e was a t t a c h e d to t h e c a n n u l a of e a c h rat. Rats were t h e n r e t u r n e d to t h e i r cages r e a d y to s h a m drink. D e i o n i z e d w a t e r was o f f e r e d for 77 m i n . T h e rats were given a n i n t r a p e r i t o n e a l i n j e c t i o n o f e i t h e r AMN (2, 10, 50, 250, or 1250 ug/kg) dissolved in 1 m l o f saline (0.9%) or saline a l o n e 17 m i n a f t e r s h a m d r i n k i n g began. T h e doses o f AMN were a d m i n i s t e r e d in r a n d o m s e q u e n c e every o t h e r day in a crossover design. S h a m w a t e r i n t a k e s a n d a 10-sec sample o f b e h a v i o r were r e c o r d e d every 5 m i n in an i d e n t i c a l m a n n e r as described for t h e sham feeding tests. F o r an u n k n o w n r e a s o n ingested w a t e r did n o t d r a i n o u t t h e gastric fistula b y gravity as well as liquid diet did. To o v e r c o m e this p r o b l e m , the gastric e x t e n s i o n t u b e s were a s p i r a t e d w i t h a v a c u u m p u m p at regular intervals. This did n o t d i s r u p t s h a m d r i n k i n g b e h a v i o r a n d t h e r e c o v e r y of gastric c o n t e n t s averaged 98% o f t h e v o l u m e o f w a t e r ingested. T h e small v o l u m e s o f ingested w a t e r t h a t did n o t d r a i n o u t t h e gastric fistula were n o t sufficient to i n h i b i t s h a m drinking. Rats were r e q u i r e d to sham d r i n k t h r o u g h o u t a 7 7 - m i n p e r i o d w i t h o u t s t o p p i n g for m o r e t h a n o n e 5-min interval b e f o r e b e i n g t e s t e d w i t h AMN. T w o rats were e l i m i n a t e d o n t h e basis o f this criterion.
Results A M N did n o t i n h i b i t s h a m d r i n k i n g ( T a b l e 2). AMN also did n o t elicit resting b e h a v i o r in a dose-related m a n n e r ( r e s t i n g o c c u r r e d o n c e in 1 rat a f t e r 10 p g / k g a n d once in a n o t h e r rat a f t e r 20 ug/kg).
TABLE 2 EFFECT OF ATROPINE METHYL NITRATE ON SHAM DRINKING Dose of Atropine (/xg/kg)
Intake after Atropine (ml/hr)
Intake after Saline (ml/hr)
2 10 50 250 1250
70_+10.5 65_+11.3 87_+ 6.3 82_+ 10.1 69_+ 8.6
84-+i5.8 83_+14.9 70_+10.0 82_+ 12.3 84_+13.8
Number of rats resting means the number of rats that rested in at least I out of the 2 tests after a given dose of atropine methyl nitrate. No rat rested after saline injections. All rats rested after 1250 tzg/kg.
Mean sham intakes (_+SE) of water (ml/hr) after IP administration of atropine methyl nitrate or equivolumetric saline in 4 or 5 rats. No dose of atropine had a significant effect on sham drinking (matched pairs t-test, 2-tailed).
EXPERIMENT 2
DISCUSSION
Method Animals. Seven a d u l t male a l b i n o rats, weighing b e t w e e n 3 0 0 - 4 3 5 g were e q u i p p e d w i t h c h r o n i c gastric cannulas. Procedure A f t e r b o d y weights r e t u r n e d to p r e o p e r a t i v e levels, rats
I d e n t i c a l doses o f AMN i n h i b i t s h a m feeding, b u t n o t sham drinking. This d i f f e r e n t i a l effect o f AMN e x t e n d s earlier results b e c a u s e it was o b t a i n e d u n d e r c o n d i t i o n s in w h i c h a p p a r e n t l y i d e n t i c a l m o t o r acts were r e q u i r e d for ingesting liquid f o o d or water. It is possible t h a t this d i f f e r e n t i a l effect o n s h a m feeding a n d d r i n k i n g is due to t h e longer d e p r i v a t i o n p r i o r to sham d r i n k i n g t h a n sham
ATROPINE METHYL NITRATE INHIBITS SHAM FEEDING feeding (22 vs 17 hr), but we doubt it. We interpret this differential effect as evidence that the inhibition of sham feeding was not due to peripheral anticholinergic disruption of licking or swallowing. It was also not the result of anticholinergic blockade of salivation because it is hard to imagine how a dry mouth could inhibit liquid food intake. Sham feeding was very sensitive to AMN: 10 tag/kg produced a 16% inhibition under these test conditions. This dose is much smaller than doses used to inhibit normal feeding [3,4]. The reason(s) why the sham feeding rat is so much more sensitive to inhibition by AMN is not known, but the sensitivity of the sham feeding rat makes it a good preparation to analyze the inhibitory effect of AMN.
407 AMN not only inhibited sham feeding, it also elicited resting behavior in a dose related manner over the same dose range. This combination of inhibition of sham feeding and elicitation of resting behavior suggests, but does not prove, that peripheral blockade of cholinergic receptors of the muscarinic type is a mechanism for eliciting satiety for food in the rat. ACKNOWLEDGEMENTS We thank Alan Epstein, James Gibbs and F. Scott Kraly for their critiques of this manuscript. The research was supported by the Lineberry Fund, NIH Research Grants AM17240 and MH15455 and Career Development Award MH00149.
REFERENCES 1. Antin, J., J. Gibbs, J. Holt, R. C. Young and G. P. Smith. Cholecystokinin elicits the complete behavioral sequence of satiety in rats. J. comp. physiol. Psychol. 89: 784-790, 1975. 2. Burks, C. D. and A. E. Fisher. Anticholinergic blockade of schedule-induced polydipsia. Physiol. Behav. 5: 635-640, 1970.
3. Pradhan, S. N. and J. Roth. Comparative behavioral effects of several anticholinergic agents in rats. Psychopharmacologia 12: 358-366, 1968. 4. Stein, L. Anticholinergic drugs and the central control of thirst. Science 139: 46-48, 1963.
Atropine Methyl Nitrate Inhibits Sham Feeding in the Rat D. L O R E N Z , 1 P. N A R D I AND G. P. SMITH
Department o f Psychiatry, Cornell University Medical College and the Edward W. Bourne Behavioral Research Laboratory, N e w York Hospital Westchester Division, White Plains, N Y 10605
(Received 30 November) LORENZ, D., P. NARDI AND G. P. SMITH. Atropine methyl nitrate inhibits sham feeding in the rat. PHARMAC. BIOCHEM. BEHAV. 8(4) 405-407, 1978.- Atropine methyl nitrate inhibited sham feeding of a liquid diet in a dose-related manner. Identical doses of atropine methyl nitrate had no effect on sham drinking of water. This differential effect is evidence that the inhibition of sham feeding was not due to peripheral anticholinergic disruption of licking, salivating or swallowing. The results suggest that peripheral blockade of cholinergic receptors of the muscarinic type is a mechanism for eliciting satiety for food in the rat. Atropine methyl nitrate
Anticholinergic
Muscarinicreceptor
ATROPINE methyl nitrate inhibits food intake [3,4]. The proposed mechanism of the anorexic action is thought to be indirect: peripheral, anticholinergic blockade of salivary secretion prevents normal mastication and swallowing of dry food [3,4]. This dry mouth hypothesis is supported by the report that atropine methyl nitrate (AMN) did not inhibit the intake of wet mash [2]. We report here, however, that AMN inhibits sham feeding of liquid diet, but not sham drinking of water, in a dose-related manner. This is critical evidence against the dry mouth hypothesis and suggests that peripheral anticholinergic blockade activates an inhibitory mechanism specific for feeding behavior. EXPERIMENT 1
Method Animals and procedure. Adult male albino rats (Sprague-Dawley, Hormone Assay Co., Chicago, IL), weighing between 3 5 0 - 4 5 0 g, were housed and tested in individual wire cages in a room maintained at about 24°C and lit from 0700 to 1900 hr. Under Chloropent anesthesia ( 2 - 3 ml/kg, IP, Fort Dodge, IA) each rat had a gastric cannula implanted along the greater curvature in the rumen of the stomach, as described by Antin et al. [ 1 ]. Seven to 10 days after surgery, rats were placed on a 17-hr food deprivation schedule. All tests were performed during the light phase (1000 to 1200 hr). Fifteen min before the start of a test, the screw occluding the gastric cannula was removed, gastric contents were flushed out with tap water, and an extension drainage tube was attached to the cannula of each rat. Rats were then returned to their cages ready to sham feed.
Satiety
S h a mfeeding
S h a mdrinking
Liquid diet (25% v/v, No. 116 EC, GIBCO, Grand Island, NY) was offered for 77 min. Sham intake was recorded every 5 min throughout the test. Behavior was sampled for 10 sec every 5 rain and the occurrence of resting behavior was noted. Resting behavior was defined as a relaxed, immobile posture with or without the head and trunk of the body positioned on the cage floor. The eyes could be open or closed. The occurrence of resting behavior was considered the terminal item in the behavioral sequence of satiety [ I ]. Since resting never occurs in sham feeding rats injected with saline [ 1 ], if AMN inhibited sham feeding and elicited resting behavior, it would suggest that AMN had produced satiety and not just disrupted feeding. Atropine methyl nitrate (AMN 2, 10, 50, 250, 1250 ~g/kg, No. A-0382, Sigma, St. Louis, MO) was dissolved in 1 ml of saline (0.9%) and administered intraperitoneally 17 rain after sham feeding began. All rats were tested twice with each dose. Injections of AMN alternated with 1 ml injections of saline (0.9%) in a crossover design. Gastric drainage was collected in plastic containers placed beneath the cages and measured after each test. If the volume of gastric drainage was less than 100% of the volume sham fed, the test was discarded. At the end of the test gastric cannulas were closed and rats were given the 25% liquid diet to eat for 1 hr. Purina pellets were then provided until 1700 hr when food was removed in preparation for the next day's test.
Results AMN inhibited sham feeding in a dose related manner (Fig. 1). The threshold dose was 10 ug/kg. The largest dose
Send reprint requests to D. Lorenz, Leidy Laboratory, Department of Biology, University of Pennsylvania, Philadelphia, PA 19104.
C o p y r i g h t © 1978 A N K H O
I n t e r n a t i o n a l Inc.--0091-3057/78/0804-0405500.60
406
L O R E N Z , N A R D I A N D SMITH
IO0 - -
Atropine methyl nitrate 1
Coolrolu
80-
Sham intake of liquid diet 60(mllhr)
iiiiii!i ii!iii!
iii~ ii!ii!i
iiiiiiii ii!!iii! 40-
!i!i i ~
20
10
50
250
~ilil) 1,250
Dose (lagI kg) FIG. 1. Sham intakes of 11 rats after intraperitoneal administration of saline or 5 doses of atropine methyl nitrate. *Denotes significant difference (p<0.025, matched pairs t-test). Each rat was tested twice with all doses of atropine methyl nitrate.
( 1 2 5 0 p g / k g ) i n h i b i t e d s h a m feeding 77%. T h e i n h i b i t i o n o f s h a m f e e d i n g was a c c o m p a n i e d b y a n increased i n c i d e n c e o f resting b e h a v i o r ( T a b l e 1). Since resting b e h a v i o r is t h e t e r m i n a l i t e m in the b e h a v i o r a l s e q u e n c e o f satiety, this result suggests t h a t A M N n o t o n l y i n h i b i t s s h a m feeding b u t also elicits satiety. A m o r e c o m p l e t e b e h a v i o r a l analysis, h o w e v e r , will be r e q u i r e d to e s t a b l i s h this p o i n t . It is possible t h a t A M N i n h i b i t s s h a m feeding b y d i s r u p t i n g t h e n o r m a l m o t o r i n t e g r a t i o n necessary for licking o r swallowing liquid f o o d . If this were so, A M N s h o u l d also i n h i b i t s h a m d r i n k i n g o f water. This was t e s t e d in t h e n e x t experiment.
TABLE 1 EFFECT OF ATROPINE METHYL NITRATE ON RESTING BEHAVIOR Atropine (/~g/kg) Number of rats resting
2 0
10 2
50 3
250 7
1250 11
were placed o n a 2 2 - h r w a t e r d e p r i v a t i o n s c h e d u l e w i t h P u r i n a rat pellets available at all t i m e s e x c e p t d u r i n g t h e test period. All tests were p e r f o r m e d d u r i n g t h e light p h a s e ( 1 0 0 0 - 1 2 0 0 hr). F i f t e e n m i n u t e s b e f o r e t h e start o f t h e test, t h e screw o c c l u d i n g the gastric c a n n u l a was r e m o v e d , t h e gastric c o n t e n t s were flushed o u t w i t h saline (0.9%), a n d a n e x t e n s i o n drainage t u b e was a t t a c h e d to t h e c a n n u l a of e a c h rat. Rats were t h e n r e t u r n e d to t h e i r cages r e a d y to s h a m drink. D e i o n i z e d w a t e r was o f f e r e d for 77 m i n . T h e rats were given a n i n t r a p e r i t o n e a l i n j e c t i o n o f e i t h e r AMN (2, 10, 50, 250, or 1250 ug/kg) dissolved in 1 m l o f saline (0.9%) or saline a l o n e 17 m i n a f t e r s h a m d r i n k i n g began. T h e doses o f AMN were a d m i n i s t e r e d in r a n d o m s e q u e n c e every o t h e r day in a crossover design. S h a m w a t e r i n t a k e s a n d a 10-sec sample o f b e h a v i o r were r e c o r d e d every 5 m i n in an i d e n t i c a l m a n n e r as described for t h e sham feeding tests. F o r an u n k n o w n r e a s o n ingested w a t e r did n o t d r a i n o u t t h e gastric fistula b y gravity as well as liquid diet did. To o v e r c o m e this p r o b l e m , the gastric e x t e n s i o n t u b e s were a s p i r a t e d w i t h a v a c u u m p u m p at regular intervals. This did n o t d i s r u p t s h a m d r i n k i n g b e h a v i o r a n d t h e r e c o v e r y of gastric c o n t e n t s averaged 98% o f t h e v o l u m e o f w a t e r ingested. T h e small v o l u m e s o f ingested w a t e r t h a t did n o t d r a i n o u t t h e gastric fistula were n o t sufficient to i n h i b i t s h a m drinking. Rats were r e q u i r e d to sham d r i n k t h r o u g h o u t a 7 7 - m i n p e r i o d w i t h o u t s t o p p i n g for m o r e t h a n o n e 5-min interval b e f o r e b e i n g t e s t e d w i t h AMN. T w o rats were e l i m i n a t e d o n t h e basis o f this criterion.
Results A M N did n o t i n h i b i t s h a m d r i n k i n g ( T a b l e 2). AMN also did n o t elicit resting b e h a v i o r in a dose-related m a n n e r ( r e s t i n g o c c u r r e d o n c e in 1 rat a f t e r 10 p g / k g a n d once in a n o t h e r rat a f t e r 20 ug/kg).
TABLE 2 EFFECT OF ATROPINE METHYL NITRATE ON SHAM DRINKING Dose of Atropine (/xg/kg)
Intake after Atropine (ml/hr)
Intake after Saline (ml/hr)
2 10 50 250 1250
70_+10.5 65_+11.3 87_+ 6.3 82_+ 10.1 69_+ 8.6
84-+i5.8 83_+14.9 70_+10.0 82_+ 12.3 84_+13.8
Number of rats resting means the number of rats that rested in at least I out of the 2 tests after a given dose of atropine methyl nitrate. No rat rested after saline injections. All rats rested after 1250 tzg/kg.
Mean sham intakes (_+SE) of water (ml/hr) after IP administration of atropine methyl nitrate or equivolumetric saline in 4 or 5 rats. No dose of atropine had a significant effect on sham drinking (matched pairs t-test, 2-tailed).
EXPERIMENT 2
DISCUSSION
Method Animals. Seven a d u l t male a l b i n o rats, weighing b e t w e e n 3 0 0 - 4 3 5 g were e q u i p p e d w i t h c h r o n i c gastric cannulas. Procedure A f t e r b o d y weights r e t u r n e d to p r e o p e r a t i v e levels, rats
I d e n t i c a l doses o f AMN i n h i b i t s h a m feeding, b u t n o t sham drinking. This d i f f e r e n t i a l effect o f AMN e x t e n d s earlier results b e c a u s e it was o b t a i n e d u n d e r c o n d i t i o n s in w h i c h a p p a r e n t l y i d e n t i c a l m o t o r acts were r e q u i r e d for ingesting liquid f o o d or water. It is possible t h a t this d i f f e r e n t i a l effect o n s h a m feeding a n d d r i n k i n g is due to t h e longer d e p r i v a t i o n p r i o r to sham d r i n k i n g t h a n sham
ATROPINE METHYL NITRATE INHIBITS SHAM FEEDING feeding (22 vs 17 hr), but we doubt it. We interpret this differential effect as evidence that the inhibition of sham feeding was not due to peripheral anticholinergic disruption of licking or swallowing. It was also not the result of anticholinergic blockade of salivation because it is hard to imagine how a dry mouth could inhibit liquid food intake. Sham feeding was very sensitive to AMN: 10 tag/kg produced a 16% inhibition under these test conditions. This dose is much smaller than doses used to inhibit normal feeding [3,4]. The reason(s) why the sham feeding rat is so much more sensitive to inhibition by AMN is not known, but the sensitivity of the sham feeding rat makes it a good preparation to analyze the inhibitory effect of AMN.
407 AMN not only inhibited sham feeding, it also elicited resting behavior in a dose related manner over the same dose range. This combination of inhibition of sham feeding and elicitation of resting behavior suggests, but does not prove, that peripheral blockade of cholinergic receptors of the muscarinic type is a mechanism for eliciting satiety for food in the rat. ACKNOWLEDGEMENTS We thank Alan Epstein, James Gibbs and F. Scott Kraly for their critiques of this manuscript. The research was supported by the Lineberry Fund, NIH Research Grants AM17240 and MH15455 and Career Development Award MH00149.
REFERENCES 1. Antin, J., J. Gibbs, J. Holt, R. C. Young and G. P. Smith. Cholecystokinin elicits the complete behavioral sequence of satiety in rats. J. comp. physiol. Psychol. 89: 784-790, 1975. 2. Burks, C. D. and A. E. Fisher. Anticholinergic blockade of schedule-induced polydipsia. Physiol. Behav. 5: 635-640, 1970.
3. Pradhan, S. N. and J. Roth. Comparative behavioral effects of several anticholinergic agents in rats. Psychopharmacologia 12: 358-366, 1968. 4. Stein, L. Anticholinergic drugs and the central control of thirst. Science 139: 46-48, 1963.