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Flavor preference: Effects of ingestion-contingent intravenous saline or glucose
PhyMology and Behavior. 'CoL 6, pp. 341-~143. Pcriguaon ~
19"/1. Printed in Great Britain
Flavor Preference: Effects of IngestionContingent Intravenous Saline or Glucose' S. H. R E V U S K Y I, M. H. S M I T H , JR. A N D D. V. C H A L M E R S *
Department of Psychology, University of Washington, Seattle, Washington, U.S.A. (Received 5 October 1969) REW~KY, S. H., M. H. SMrrn, JR. AND D. V. ~ . Flavorpreference: Effectsof ingestion-contingent intravenous saline or glucose. PrlYstoL.BEHAV.6 (4) 341-343, 1971.--In Experiment 1,hungry rats with intravenous eannulae were fed a non-nutritive sweetened coffee solution on 5 occasions. Shortly after each such drinking period, one group was infused with 5-10 ml of 10~o glucose solution; a second group was similarly infused with 0.9 ~0 saline solution and a third group was not infused at all. In a subsequent preference test between sweetened coffee and saccharin solution, the saline group had a reduced preference for coffee relative to the other two groups, which did not differ significantly between themselves. Experiment 2 was a replication in which a number of procedural details were changed. The results were similar to those of Experiment I. Glucose
Food aversions
Feeding
Drinking
Homeostasis
A~m~o_.s often show an increased preference for foods which contain a needed commodity. In some of these cases, a feedback system based on the post-ingestional increase in the availability of the commodity may be at least partly responsible. However, the increased preference is not by itself evidence for such a feedback system. Conceivably, a need state might produce an increased preference for all substances containing the needed commodity without involvement of any feedback at all. If, however, an animal is fed a distinctively flavored substance which does not contain the needed commodity and later receives the commodity by means of an injection, any resultant increase in preference for the flavored substance can be validly attributed to the feedback. Two types of post-ingestional injections have been demonstrated to produce an increase in the preference for nutritionally inert, flavored substances. The first is parenteral injection of thiamine into thiamine deficient rats [1, 6, 13] and the second is intragastric injection of liquid food into hungry rats [9]. The purpose of the experiments reported here was to find out if intravenous infusion of glucose solution into hungry rats can produce a similar result. The experimental strategy was to feed the rats a non-nutritive solution of sweetened coffee and then to infuse them intravenously with glucose solution. Some days later, their preferences for the coffee solution were to be compared with those of two types of controls: rats which had previously been infused with isotonic saline solution after drinking the sweetened coffee and rats which were not infused at all after drinking the sweetened coffee. Infusion of glucose was selected as the means of producing
Intravenous feeding
the post-ingestional consequence because there are experimental reports that intravenous injection of 10% glucose dissolved in saline is an effective reward for motor operants in hungry rats [3] and in hungry rabbits [2]. Since Dietz and Capretta [5] and Garcia and Koelling [7] have found that the preference for a flavor is more readily changed by physiological aftereffects than is the probability of a motor operant, it seemed worthwhile to try to change the preference for a flavor by contingent infusion of glucose solution. METHODS
Two similar experiments were performed. Experiment 2 was a replication of Experiment 1 with certain changes based in part on the results of Experiment 1.
Experiment 1 The method of Davis [4] was used to implant permanent cannulae through the jugular veins into the hearts of male Sprague--Dawley rats weighing 200-300 g a d lib; anesthesia was administered by the method of Rye and Elder [11]. After surgery, all rats were placed on a feeding schedule in which they received 6 hr of free access to ground rat chow every second day. Water was continuously available except when test fluids were presented. A series of 5 training trials spaced two days apart usually began 2 days after the cannula was implanted. The deprivation level during training varied from 16 to 24 hr but was held constant for individual rats. The coffee solution consisted of 2.0% instant decaffeinated coffee and 0.1% sodium
XExperiment 1 was conducted at the US Army Medical Research Laboratory. We thank R. E. Hill, K. Kloet and N. Long for technical assistance, R. S. Dedrich and A. J. Neves for veterinary services and particularly J. D. Davis for advising us about surgical techniques. Experiment 2 was supported by USPHS grant MH-11148 and the writing of this paper was partly supported by USPHS grant MH-16423. sPresent address: Department of Psychology, Northern Illinois University, DeKalb, Illinois 60115. 8Present address: Department of Psychology, Pennsylvania State University, University Park, Pennsylvania 16802. 341
342 saccharin (w/v) and was available in the home cages until 30 rain after drinking began. The infusion through a motor-driven syringe began 10-20 min after the coffee was removed and lasted 53 min; the rats were restrained in an acrylic cage during infusion. The amount infused was 5 ml if the rat had consumed less than 2.5 g of coffee solution on its first trial and 10 ml otherwise. The substance infused into the experimental (glucose) rats was a 10 ~ solution of dextrose in pure water available commercially for medical purposes. The saline control rats were similarly infused with commercial isotonic (0.9 ~ w/v) saline. The noninfused control rats were simply left undisturbed in their home cages after consuming the coffee. On the day following the fifth and last coffee conditioning trial, a 0.1 ~ saccharin solution was substituted for water during the time the rats consumed their normal 6 hr meal. On the following day, the rats were allowed 3 hr of free access to both sweetened coffee and the saccharin solution. Preference for coffee was defined as the weight of coffee solution consumed divided by the total weight of fluid consumed and was calculated separately for each hour of the test. Fewer than 30 per cent of the rats that begin the experiment yielded data. Many died during training and others were discarded because their cannulae did not function properly. It was critical that the cannulae of the glucose rats not leak fluid outside of the vein because glucose solution in the neck cavity produces a number of effects which might be expected to be aversive. For this reason, the cannulae were checked periodically in various ways throughout the experiment. In a few tests, including a final test after all data were collected, from 2 to 5 mg of sodium pentobarbital were injected into the cannula. This dose will nearly immobilize a rat if injected directly into the vein, but will not produce an overt effect if it does not enter the blood stream directly. When a postexperimental test with pentobarbital indicated a poor cannula, the data were discarded if the rat was in the glucose or saline infusion group. Noninfused rats with defective cannulae were not discarded because their data did not seem different from the data of animals with functional cannulae and there was no theoretical reason to expect a difference. Usable data were obtained from 9 rats in the glucose group (4 had received 10 ml infusions and 5 had received 5 ml), 4 rats in the saline group (3 had received 10 ml and one had received 5 ml), and 11 rats in the noninfusion control group (3 of these had functional cannulae at the end of the experiment). That there were fewer rats in the saline group than in the glucose group does not indicate that a higher attrition rate was produced by the saline procedure; more rats were subjected to the glucose procedure because it seemed desirable for statistical reasons to have a larger number in that group. Although attrition was high, all rats who finished the experiment appeared in good health; there were no deaths during 7 post-experimental days of free access to food and water.
Experiment 2 This experiment differed from Experiment 1 in the following details. The male rats used were older and weighed over 350 g ad lib. They were habituated to the deprivation schedule prior to surgery so that initiation of deprivation did not immediately follow the trauma of surgery. The deprivation schedule was made less severe by increasing the duration of each meal of ground chow to 8 hr. Familiarization with 0.1 ~o saccharin solution occurred prior to surgery instead of after the training trials; 0.1 ~ saccharin solution was substituted for water for
REVUSKY, SMITH AND CHALMERS one 8 hr period and for one 16 hr period. This saccharin familiarization was temporally separated from feeding so that nutrients from the chow would not become associated with the saccharin flavor [12]. During surgery, halothane anesthesia [10] was used instead of the method of Rye and Elder [11 ]. The intravenous infusions were administered while the rats were unrestrained and noninfused rats were also placed in the infusion apparatus. The infused solutions were heated nearly to body temperature and the amount infused was 10 ml in 55 rain for all rats. The glucose solution was 5 % (w/v) instead of 10~o and there were only 3 training trials, instead of 5. The second and third hr of the preference test were combined. The techniques used in Experiment 2 must have been better than those used in Experiment I because the attrition rate was below 20 per cent. Data were obtained from 10 rats in the glucose group, 11 in the saline group, and 10 noninfused controls. RESULTS AND DISCUSSION
Tables 1 and 2 show that the two experiments led to identical conclusions about the relative effects of the three treatments on the preference for sweetened coffee solution when TABLE 1 MEAN PREFERENCES FOR COFFEE SOLUTION RELATIVE TO SACCHARIN SOLUTION IN EXPERIMENT 1
Hours of Test
Groups Saline
Noninfusion
Glucose
1 2 3
0.190 0.033 0.013
0.327 0.380 0.573
0.343 0.540 0.705
TABLE 2 MEAN PREFERENCES FOR COFFEE SOLUTION RELATIVE TO SACCHARIN SOLUTION IN EXPERIMENT 2
Hours of Test
Groups Saline
Noninfusion
Glucose
1 2, 3
0.088 0.044
0.231 0.245
0.350 0.258
the alternative was saccharin solution. In each experiment, the saline group exhibited a lower preference for coffee than either the noninfused group or the glucose group (each of 4 t-tests yielded p<0.05). In neither experiment was there a statistically significant difference in preference between the noninfused group and the glucose group. Thus infusion of glucose solution had no discernible effect on preference while infusion of saline solution acted as a punishment in the sense that its occurrence after the consumption of coffee reduced the preference for coffee. Another measure of the preference for coffee solution might be the amount of coffee solution consumed on the final training trial. In Experiment I, there were no reliable differences among groups on this measure; but in Experiment 2, differences among groups were detected (Table 3). On the
INTRAVENOUS INFUSIONS AND FLAVOR PREFERENCE TABLE 3 CONSUMPTION OF COFFEE SOLUTIONIN GRAMS DURING THE TRAINING TRIALS OF EXPERIMENT 2
Training Sessions
Groups Saline
Noninfusion
Glucose 0.95 1.99 3.27
1
1.35
1.43
2 3
1.59 1.75
3.47 7.38
third trial, the coffee consumption of the saline group was significantly lower (p<0.01, t-test) than the consumption of the noninfused group, confirming the earlier conclusion that infusion of saline solution was a punishment. There is also a suggestion in Table 3 that glucose infusion as well might have been a punishment. Coffee consumption on the third trial was lower in the glucose group than in the noninfused group (0.04
343 results of the two-bottle preference tests for Experiment 2 (Table 2) do not confirm such a conclusion, although twobottle test are generally more sensitive than one-bottle tests [8]. (b) On Trial I, prior to any infusions, the noninfused rats consumed more than the rats later infused with glucose, suggesting that uncontrolled individual differences might have been involved. (c) In Experiment 1, there was no indication of a reduced coffee consumption in the glucose group. The aversion to the coffee flavor exhibited by the saline group and the similarity in the coffee preferences of the glucose and noninfused groups are explicable by either of two hypotheses: The first is that infusion of isotonic saline solution produces an aversion because the NaCI in itself is aversive. An alternative is that the rise in blood glucose produced by the glucose infusion was a positive reinforcement but did not produce an increased preference for coffee because it was counteracted by a presumed aversive aftereffect concommitant with the intravenous infusion of any substance whatsoever. By this view, the saline infusion produced an aversion to the coffee flavor only because there was nothing positively reinforcing in the saline to counteract the aversive effects of the infusion itself. At present, experimentation to select between these possibilities is in progress.
REFERENCES I. Campbell, C. S. The development of specific preferences in 7. Garcia, J. and R. A. Koelling. Relation of cue to consequence thiamine deficient rats: evidence against mediation by afterin avoidance learning. Psychonom. Sci. 4: 123-124, 1966. tastes. Unpublished master's thesis, University of Illinois at 8, Grote, F. W. and R. T. Brown. Preference tests are more Chicago Circle, 1969. sensitive than daily intake measures. Sixteenth Annual Meeting 2. Chambers, R. M. Effects of intravenous glucose injections on of the Southeastern Psychological Association, Louisville, 1970. learning, general activity, and hunger drive. J. comp. physiol. 9. Holman, G. L. The intragastric reinforcement effect. J. comp. Psychol. 49: 558-564, 1956. physiol. Psychol. 69: 432--441, 1969. 3. Coppock, H. W. and R. M. Chambers. Reinforcement of 10, Luschei, E. S. and J. J. Mehaffey. Small animal anesthesia with position preference by automatic intravenous injections of halothane. J. appl. Physio122: 595-597, 1967. glucose, J. comp. physiol. Psyehol. 47: 355-358, 1954. 11. Rye, M. M. and S. T. Elder. A suggestion concerning the 4. Davis, J. D. A method for chronic intravenous infusion in anesthetization of the rat. J. exp. Analysis Behav. 9: 243-244, freely moving rats. J. exp. Analysis Behav. 9: 385-387, 1966. 1966. 5. Dietz, M. N. and P. J. Capretta. Modification of sugar and 12. Smith, M. P. and P. J. Capretta. Effects of drive level and sugar-saccharin preference in rats as a function of electrical experience on the reward value of saccharine solution. J. comp. shock to the mouth. Proceedings of the 75th Annual Convention physiol. Psychol. 49: 553-557, 1956. of the American Psychological Association. Washington: 13. Zahorik, D. M. and S. F. Maier. Appetitive conditioning with American Psychological Association, 1967. pp. 161-162. recovery from thiamine deficiency as the unconditionedstimulus 6. Garcia, J., F. R. Ervin, C. H. Yorke and R. A. Koelling. Psychonom. Sci. 17: 309-310, 1969. Conditioning with delayed vitamin injections. Science 155: 716-718, 1967.
19"/1. Printed in Great Britain
Flavor Preference: Effects of IngestionContingent Intravenous Saline or Glucose' S. H. R E V U S K Y I, M. H. S M I T H , JR. A N D D. V. C H A L M E R S *
Department of Psychology, University of Washington, Seattle, Washington, U.S.A. (Received 5 October 1969) REW~KY, S. H., M. H. SMrrn, JR. AND D. V. ~ . Flavorpreference: Effectsof ingestion-contingent intravenous saline or glucose. PrlYstoL.BEHAV.6 (4) 341-343, 1971.--In Experiment 1,hungry rats with intravenous eannulae were fed a non-nutritive sweetened coffee solution on 5 occasions. Shortly after each such drinking period, one group was infused with 5-10 ml of 10~o glucose solution; a second group was similarly infused with 0.9 ~0 saline solution and a third group was not infused at all. In a subsequent preference test between sweetened coffee and saccharin solution, the saline group had a reduced preference for coffee relative to the other two groups, which did not differ significantly between themselves. Experiment 2 was a replication in which a number of procedural details were changed. The results were similar to those of Experiment I. Glucose
Food aversions
Feeding
Drinking
Homeostasis
A~m~o_.s often show an increased preference for foods which contain a needed commodity. In some of these cases, a feedback system based on the post-ingestional increase in the availability of the commodity may be at least partly responsible. However, the increased preference is not by itself evidence for such a feedback system. Conceivably, a need state might produce an increased preference for all substances containing the needed commodity without involvement of any feedback at all. If, however, an animal is fed a distinctively flavored substance which does not contain the needed commodity and later receives the commodity by means of an injection, any resultant increase in preference for the flavored substance can be validly attributed to the feedback. Two types of post-ingestional injections have been demonstrated to produce an increase in the preference for nutritionally inert, flavored substances. The first is parenteral injection of thiamine into thiamine deficient rats [1, 6, 13] and the second is intragastric injection of liquid food into hungry rats [9]. The purpose of the experiments reported here was to find out if intravenous infusion of glucose solution into hungry rats can produce a similar result. The experimental strategy was to feed the rats a non-nutritive solution of sweetened coffee and then to infuse them intravenously with glucose solution. Some days later, their preferences for the coffee solution were to be compared with those of two types of controls: rats which had previously been infused with isotonic saline solution after drinking the sweetened coffee and rats which were not infused at all after drinking the sweetened coffee. Infusion of glucose was selected as the means of producing
Intravenous feeding
the post-ingestional consequence because there are experimental reports that intravenous injection of 10% glucose dissolved in saline is an effective reward for motor operants in hungry rats [3] and in hungry rabbits [2]. Since Dietz and Capretta [5] and Garcia and Koelling [7] have found that the preference for a flavor is more readily changed by physiological aftereffects than is the probability of a motor operant, it seemed worthwhile to try to change the preference for a flavor by contingent infusion of glucose solution. METHODS
Two similar experiments were performed. Experiment 2 was a replication of Experiment 1 with certain changes based in part on the results of Experiment 1.
Experiment 1 The method of Davis [4] was used to implant permanent cannulae through the jugular veins into the hearts of male Sprague--Dawley rats weighing 200-300 g a d lib; anesthesia was administered by the method of Rye and Elder [11]. After surgery, all rats were placed on a feeding schedule in which they received 6 hr of free access to ground rat chow every second day. Water was continuously available except when test fluids were presented. A series of 5 training trials spaced two days apart usually began 2 days after the cannula was implanted. The deprivation level during training varied from 16 to 24 hr but was held constant for individual rats. The coffee solution consisted of 2.0% instant decaffeinated coffee and 0.1% sodium
XExperiment 1 was conducted at the US Army Medical Research Laboratory. We thank R. E. Hill, K. Kloet and N. Long for technical assistance, R. S. Dedrich and A. J. Neves for veterinary services and particularly J. D. Davis for advising us about surgical techniques. Experiment 2 was supported by USPHS grant MH-11148 and the writing of this paper was partly supported by USPHS grant MH-16423. sPresent address: Department of Psychology, Northern Illinois University, DeKalb, Illinois 60115. 8Present address: Department of Psychology, Pennsylvania State University, University Park, Pennsylvania 16802. 341
342 saccharin (w/v) and was available in the home cages until 30 rain after drinking began. The infusion through a motor-driven syringe began 10-20 min after the coffee was removed and lasted 53 min; the rats were restrained in an acrylic cage during infusion. The amount infused was 5 ml if the rat had consumed less than 2.5 g of coffee solution on its first trial and 10 ml otherwise. The substance infused into the experimental (glucose) rats was a 10 ~ solution of dextrose in pure water available commercially for medical purposes. The saline control rats were similarly infused with commercial isotonic (0.9 ~ w/v) saline. The noninfused control rats were simply left undisturbed in their home cages after consuming the coffee. On the day following the fifth and last coffee conditioning trial, a 0.1 ~ saccharin solution was substituted for water during the time the rats consumed their normal 6 hr meal. On the following day, the rats were allowed 3 hr of free access to both sweetened coffee and the saccharin solution. Preference for coffee was defined as the weight of coffee solution consumed divided by the total weight of fluid consumed and was calculated separately for each hour of the test. Fewer than 30 per cent of the rats that begin the experiment yielded data. Many died during training and others were discarded because their cannulae did not function properly. It was critical that the cannulae of the glucose rats not leak fluid outside of the vein because glucose solution in the neck cavity produces a number of effects which might be expected to be aversive. For this reason, the cannulae were checked periodically in various ways throughout the experiment. In a few tests, including a final test after all data were collected, from 2 to 5 mg of sodium pentobarbital were injected into the cannula. This dose will nearly immobilize a rat if injected directly into the vein, but will not produce an overt effect if it does not enter the blood stream directly. When a postexperimental test with pentobarbital indicated a poor cannula, the data were discarded if the rat was in the glucose or saline infusion group. Noninfused rats with defective cannulae were not discarded because their data did not seem different from the data of animals with functional cannulae and there was no theoretical reason to expect a difference. Usable data were obtained from 9 rats in the glucose group (4 had received 10 ml infusions and 5 had received 5 ml), 4 rats in the saline group (3 had received 10 ml and one had received 5 ml), and 11 rats in the noninfusion control group (3 of these had functional cannulae at the end of the experiment). That there were fewer rats in the saline group than in the glucose group does not indicate that a higher attrition rate was produced by the saline procedure; more rats were subjected to the glucose procedure because it seemed desirable for statistical reasons to have a larger number in that group. Although attrition was high, all rats who finished the experiment appeared in good health; there were no deaths during 7 post-experimental days of free access to food and water.
Experiment 2 This experiment differed from Experiment 1 in the following details. The male rats used were older and weighed over 350 g ad lib. They were habituated to the deprivation schedule prior to surgery so that initiation of deprivation did not immediately follow the trauma of surgery. The deprivation schedule was made less severe by increasing the duration of each meal of ground chow to 8 hr. Familiarization with 0.1 ~o saccharin solution occurred prior to surgery instead of after the training trials; 0.1 ~ saccharin solution was substituted for water for
REVUSKY, SMITH AND CHALMERS one 8 hr period and for one 16 hr period. This saccharin familiarization was temporally separated from feeding so that nutrients from the chow would not become associated with the saccharin flavor [12]. During surgery, halothane anesthesia [10] was used instead of the method of Rye and Elder [11 ]. The intravenous infusions were administered while the rats were unrestrained and noninfused rats were also placed in the infusion apparatus. The infused solutions were heated nearly to body temperature and the amount infused was 10 ml in 55 rain for all rats. The glucose solution was 5 % (w/v) instead of 10~o and there were only 3 training trials, instead of 5. The second and third hr of the preference test were combined. The techniques used in Experiment 2 must have been better than those used in Experiment I because the attrition rate was below 20 per cent. Data were obtained from 10 rats in the glucose group, 11 in the saline group, and 10 noninfused controls. RESULTS AND DISCUSSION
Tables 1 and 2 show that the two experiments led to identical conclusions about the relative effects of the three treatments on the preference for sweetened coffee solution when TABLE 1 MEAN PREFERENCES FOR COFFEE SOLUTION RELATIVE TO SACCHARIN SOLUTION IN EXPERIMENT 1
Hours of Test
Groups Saline
Noninfusion
Glucose
1 2 3
0.190 0.033 0.013
0.327 0.380 0.573
0.343 0.540 0.705
TABLE 2 MEAN PREFERENCES FOR COFFEE SOLUTION RELATIVE TO SACCHARIN SOLUTION IN EXPERIMENT 2
Hours of Test
Groups Saline
Noninfusion
Glucose
1 2, 3
0.088 0.044
0.231 0.245
0.350 0.258
the alternative was saccharin solution. In each experiment, the saline group exhibited a lower preference for coffee than either the noninfused group or the glucose group (each of 4 t-tests yielded p<0.05). In neither experiment was there a statistically significant difference in preference between the noninfused group and the glucose group. Thus infusion of glucose solution had no discernible effect on preference while infusion of saline solution acted as a punishment in the sense that its occurrence after the consumption of coffee reduced the preference for coffee. Another measure of the preference for coffee solution might be the amount of coffee solution consumed on the final training trial. In Experiment I, there were no reliable differences among groups on this measure; but in Experiment 2, differences among groups were detected (Table 3). On the
INTRAVENOUS INFUSIONS AND FLAVOR PREFERENCE TABLE 3 CONSUMPTION OF COFFEE SOLUTIONIN GRAMS DURING THE TRAINING TRIALS OF EXPERIMENT 2
Training Sessions
Groups Saline
Noninfusion
Glucose 0.95 1.99 3.27
1
1.35
1.43
2 3
1.59 1.75
3.47 7.38
third trial, the coffee consumption of the saline group was significantly lower (p<0.01, t-test) than the consumption of the noninfused group, confirming the earlier conclusion that infusion of saline solution was a punishment. There is also a suggestion in Table 3 that glucose infusion as well might have been a punishment. Coffee consumption on the third trial was lower in the glucose group than in the noninfused group (0.04
343 results of the two-bottle preference tests for Experiment 2 (Table 2) do not confirm such a conclusion, although twobottle test are generally more sensitive than one-bottle tests [8]. (b) On Trial I, prior to any infusions, the noninfused rats consumed more than the rats later infused with glucose, suggesting that uncontrolled individual differences might have been involved. (c) In Experiment 1, there was no indication of a reduced coffee consumption in the glucose group. The aversion to the coffee flavor exhibited by the saline group and the similarity in the coffee preferences of the glucose and noninfused groups are explicable by either of two hypotheses: The first is that infusion of isotonic saline solution produces an aversion because the NaCI in itself is aversive. An alternative is that the rise in blood glucose produced by the glucose infusion was a positive reinforcement but did not produce an increased preference for coffee because it was counteracted by a presumed aversive aftereffect concommitant with the intravenous infusion of any substance whatsoever. By this view, the saline infusion produced an aversion to the coffee flavor only because there was nothing positively reinforcing in the saline to counteract the aversive effects of the infusion itself. At present, experimentation to select between these possibilities is in progress.
REFERENCES I. Campbell, C. S. The development of specific preferences in 7. Garcia, J. and R. A. Koelling. Relation of cue to consequence thiamine deficient rats: evidence against mediation by afterin avoidance learning. Psychonom. Sci. 4: 123-124, 1966. tastes. Unpublished master's thesis, University of Illinois at 8, Grote, F. W. and R. T. Brown. Preference tests are more Chicago Circle, 1969. sensitive than daily intake measures. Sixteenth Annual Meeting 2. Chambers, R. M. Effects of intravenous glucose injections on of the Southeastern Psychological Association, Louisville, 1970. learning, general activity, and hunger drive. J. comp. physiol. 9. Holman, G. L. The intragastric reinforcement effect. J. comp. Psychol. 49: 558-564, 1956. physiol. Psychol. 69: 432--441, 1969. 3. Coppock, H. W. and R. M. Chambers. Reinforcement of 10, Luschei, E. S. and J. J. Mehaffey. Small animal anesthesia with position preference by automatic intravenous injections of halothane. J. appl. Physio122: 595-597, 1967. glucose, J. comp. physiol. Psyehol. 47: 355-358, 1954. 11. Rye, M. M. and S. T. Elder. A suggestion concerning the 4. Davis, J. D. A method for chronic intravenous infusion in anesthetization of the rat. J. exp. Analysis Behav. 9: 243-244, freely moving rats. J. exp. Analysis Behav. 9: 385-387, 1966. 1966. 5. Dietz, M. N. and P. J. Capretta. Modification of sugar and 12. Smith, M. P. and P. J. Capretta. Effects of drive level and sugar-saccharin preference in rats as a function of electrical experience on the reward value of saccharine solution. J. comp. shock to the mouth. Proceedings of the 75th Annual Convention physiol. Psychol. 49: 553-557, 1956. of the American Psychological Association. Washington: 13. Zahorik, D. M. and S. F. Maier. Appetitive conditioning with American Psychological Association, 1967. pp. 161-162. recovery from thiamine deficiency as the unconditionedstimulus 6. Garcia, J., F. R. Ervin, C. H. Yorke and R. A. Koelling. Psychonom. Sci. 17: 309-310, 1969. Conditioning with delayed vitamin injections. Science 155: 716-718, 1967.