Physiology & Behavior, Vol. 21, pp. 363-368. Pergamon Press and Brain Research Publ., 1978. Printed in the U.S.A.
Conditioned Taste Preferences in the Rat Induced by Self-stimulation AARON ETTENBERG
AND NORMAN
WHITE
McGill University, 1205 McGregor Avenue, Montreal, Quebec, Canada H3A 1BI ( R e c e i v e d 16 M a r c h 1978) E T r E N B E R G , A. AND N. WHITE. Conditioned taste preferences in the rat induced by self-stimulation. PHYSIOL. BEHAV. 21(3) 363-368, 1978.--Water-deprived rats with lateral hypothalamic electrodes were allowed to drink a novel tasting solution for 10 rain, followed immediately by 15 rain of intracranial self-stimulation. During a subsequent free-choice test with water, these rats exhibited a strong preference for the novel-tasting solution. Rats presented with either selfstimulation alone or the novel solution alone did not exhibit any such preference. The preference for the novel solution by the rats that experienced the pairing of this solution with self-stimulation was interpreted as a conditioned taste preference.
Conditioned taste preferences
Self-stimulation
Reinforcement
IT IS well established that making rats ill after they have ingested a novel tasting substance causes them to reject that substance for some time thereafter [8, 18, 19]. It is generally assumed that these conditioned taste aversions (CTA) are the result of associations that are formed when the novel taste and the negative affective consequences of the illness are paired [17]. An obvious parallel of the CTA paradigm is the conditioned taste preference (CTP) in which a novel taste becomes associated with positive affect [21,22]. Since intracranial self-stimulation appears to have positive affecrive consequences [11, 13, 14, 20], it seemed a reasonable hypothesis that rats would form a CTP for a novel tasting substance when the presentation of that substance was paired with a session of self-stimulation. The first experiment tested this hypothesis. EXPERIMENT 1 METHOD
Animals Thirty-two male albino rats weighing 250--300 g at the start of the experiment were used in this experiment. Each animal was individually housed and provided with ad lib access to food. Water was provided in two 100 ml graduated Richter tubes located on the front of each cage.
Surgery Bipolar stimulating electrodes (Plastic Products Co.) were aimed at the lateral hypothalamus of 16 rats under 50 mg/kg sodium pentobarbital anaesthesia. With the tooth bar of the stereotaxic instrument set at 3.2 mm above the interaural line the electrode coordinates were: 0.8 mm posterior to Bregma; 1.5 mm lateral to midline; 8.5 mm ventral to the skull surface. The remaining 16 rats were anaesthetized but did not undergo surgery.
Apparatus
The self-stimulation cage was a wooden box with a Plexiglas front (20 x 30 x 46 era). A Plexigias lever (10 × 15 cm) was mounted in the middle of one side wall of the cage 3.0 cm from the floor. Each press of the lever produced a 0.5 sec train of 60 H z sine-wave stimulation.
Procedure The animals were allowed 10 days to recover from surgery after which those with electrodes were trained to lever-press for intracranial stimulation. The current intensity for each animal was adjusted to a value that produced a steady rate of responding for 15 rain (current range: 15-35 /~A). The animals were given no further experience with the intracranial self-stimulation until the test day. Starting one week after the self-stimulation training the amount of water drunk from each of the Richter tubes on the front of each cage was recorded every 24 hr for six consecufive days. These amounts were used to determine the natural side preferences of each rat. On the sixth day both tubes were removed from all cages. After 48 hr of water deprivation each rat was individually placed into the self-stimulation cage which contained a Richter tube in place of the lever, The rats with electrodes were connected to the stimulator. All rats drank for 10 rain: half of the rats with electrodes and half of the unimplanted rats drank a solution o f 2 mg/ml instant decaffeinated coffee in tap water, the remaining rats drank plain tap water. At the end of the drinking period the Richter tube was removed and the rats were allowed access to the lever for 15 rain. The rats in the self-stimulation groups were given a few trains of " p r i m i n g " stimulation to ensure that they began responding immediately. Immediately after this procedure each animal was returned to its home cage. No liquids were available for one hr. T w o Richter tubes, one containing coffee solution and the
C o p y r i g h t © 1978 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/78/090363-06502.00/0
364
ETTENBERG AND WHITE
C-SS
W-SS
FIG. 1. Location of electrodes in self-stimulating rats. Numbers represent mm posterior to bregma. C-SS: coffee/sellstimulation group, W-SS: water/self-stimulation group. Sections are from Peilegfino and Cushman, 1967, 80. other containing water, were then placed on the front of each cage. The water tubes were placed on each rat's preferred side, and the coffee tubes were placed on each rat's nonpreferred side. The amounts of coffee and water drunk during this free-choice situation were recorded after 4 hr, 12 hr and 24 hr. Upon completion of the experiment the rats with electrodes were killed and perfused with physiological saline followed by a 10% Formalin solution. The brains were removed and fixed in 10% Formalin. The locations of the electrode tips were subsequently determined from 40/z thionin-stained frozen sections. RESULTS The electrode placements for the implanted animals are shown in Fig. 1. The electrodes were located in the area of the lateral hypothalamus dorsolateral to the fornix. The coffee consumption (expressed as a percent of total liquid intake) of the rats in each group during the first 4 hr, the next 8 hr and the subsequent 12 hr of free-choice are shown in Fig. 2. Both prior self-stimulation and prior exposure to coffee increased the consumption of coffee during the first 4 hr of the preference test. During the next 8 hr coffee consumption remained elevated in the coffee/self-stimulation (C-SS) group only, and during the final 12 hr of the preference test the initial preferences had disappeared completely. A separate 2 × 2 analysis of variance was computed on the arcsine transformed data for each of the three time intervals. During the first 4 hr of the preference test there was a significant main effect of prior exposure to coffee, F(1,28)=11.34, p < 0 . 0 1 , and a significant effect of prior self-stimulation, F(1,28) =9.27, p 0.05, Scheff6 post hoc comparisons revealed that prior self-stimulation alone did not significantly increase coffee consumption during the preference test (water/self-stimulation group (W-SS) vs. water/no selfstimulation group (W-NSS), F(1,28)=1.51, p>0.05). Prior exposure to coffee alone also had no significant effect on coffee consumption (coffee/no self-stimulation group (CNSS) vs. water/no self-stimulation group (W-NSS),
70. -~ "~ -~ ff~6
m
60.
• self-stimulation no self-stimulation
Z O 30. 23 O9 Z 0 (..,) ~J
20. 10.
0 (3 c
w 0-4
C
W 4 12 TIME IN HOURS
C W 12 24
FIG. 2. Mean coffee consumption (-+ SEM) of each group during three time intervals of a free-choice situation with water. Data are expressed as percent of total liquid intake. C=prior exposure to coffee; W=prior exposure to water. F( 1,28) = 2.12, p >0.05. On the other hand, prior exposure to coffee paired with self-stimulation resulted in a significantly greater consumption of coffee during the first 4 hr of the preference test than either prior coffee alone, C-SS vs. C-NSS, F(1,28)=9.48, p
Conditioned Taste Preferences in the Rat Induced by Self-stimulation AARON ETTENBERG
AND NORMAN
WHITE
McGill University, 1205 McGregor Avenue, Montreal, Quebec, Canada H3A 1BI ( R e c e i v e d 16 M a r c h 1978) E T r E N B E R G , A. AND N. WHITE. Conditioned taste preferences in the rat induced by self-stimulation. PHYSIOL. BEHAV. 21(3) 363-368, 1978.--Water-deprived rats with lateral hypothalamic electrodes were allowed to drink a novel tasting solution for 10 rain, followed immediately by 15 rain of intracranial self-stimulation. During a subsequent free-choice test with water, these rats exhibited a strong preference for the novel-tasting solution. Rats presented with either selfstimulation alone or the novel solution alone did not exhibit any such preference. The preference for the novel solution by the rats that experienced the pairing of this solution with self-stimulation was interpreted as a conditioned taste preference.
Conditioned taste preferences
Self-stimulation
Reinforcement
IT IS well established that making rats ill after they have ingested a novel tasting substance causes them to reject that substance for some time thereafter [8, 18, 19]. It is generally assumed that these conditioned taste aversions (CTA) are the result of associations that are formed when the novel taste and the negative affective consequences of the illness are paired [17]. An obvious parallel of the CTA paradigm is the conditioned taste preference (CTP) in which a novel taste becomes associated with positive affect [21,22]. Since intracranial self-stimulation appears to have positive affecrive consequences [11, 13, 14, 20], it seemed a reasonable hypothesis that rats would form a CTP for a novel tasting substance when the presentation of that substance was paired with a session of self-stimulation. The first experiment tested this hypothesis. EXPERIMENT 1 METHOD
Animals Thirty-two male albino rats weighing 250--300 g at the start of the experiment were used in this experiment. Each animal was individually housed and provided with ad lib access to food. Water was provided in two 100 ml graduated Richter tubes located on the front of each cage.
Surgery Bipolar stimulating electrodes (Plastic Products Co.) were aimed at the lateral hypothalamus of 16 rats under 50 mg/kg sodium pentobarbital anaesthesia. With the tooth bar of the stereotaxic instrument set at 3.2 mm above the interaural line the electrode coordinates were: 0.8 mm posterior to Bregma; 1.5 mm lateral to midline; 8.5 mm ventral to the skull surface. The remaining 16 rats were anaesthetized but did not undergo surgery.
Apparatus
The self-stimulation cage was a wooden box with a Plexiglas front (20 x 30 x 46 era). A Plexigias lever (10 × 15 cm) was mounted in the middle of one side wall of the cage 3.0 cm from the floor. Each press of the lever produced a 0.5 sec train of 60 H z sine-wave stimulation.
Procedure The animals were allowed 10 days to recover from surgery after which those with electrodes were trained to lever-press for intracranial stimulation. The current intensity for each animal was adjusted to a value that produced a steady rate of responding for 15 rain (current range: 15-35 /~A). The animals were given no further experience with the intracranial self-stimulation until the test day. Starting one week after the self-stimulation training the amount of water drunk from each of the Richter tubes on the front of each cage was recorded every 24 hr for six consecufive days. These amounts were used to determine the natural side preferences of each rat. On the sixth day both tubes were removed from all cages. After 48 hr of water deprivation each rat was individually placed into the self-stimulation cage which contained a Richter tube in place of the lever, The rats with electrodes were connected to the stimulator. All rats drank for 10 rain: half of the rats with electrodes and half of the unimplanted rats drank a solution o f 2 mg/ml instant decaffeinated coffee in tap water, the remaining rats drank plain tap water. At the end of the drinking period the Richter tube was removed and the rats were allowed access to the lever for 15 rain. The rats in the self-stimulation groups were given a few trains of " p r i m i n g " stimulation to ensure that they began responding immediately. Immediately after this procedure each animal was returned to its home cage. No liquids were available for one hr. T w o Richter tubes, one containing coffee solution and the
C o p y r i g h t © 1978 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/78/090363-06502.00/0
364
ETTENBERG AND WHITE
C-SS
W-SS
FIG. 1. Location of electrodes in self-stimulating rats. Numbers represent mm posterior to bregma. C-SS: coffee/sellstimulation group, W-SS: water/self-stimulation group. Sections are from Peilegfino and Cushman, 1967, 80. other containing water, were then placed on the front of each cage. The water tubes were placed on each rat's preferred side, and the coffee tubes were placed on each rat's nonpreferred side. The amounts of coffee and water drunk during this free-choice situation were recorded after 4 hr, 12 hr and 24 hr. Upon completion of the experiment the rats with electrodes were killed and perfused with physiological saline followed by a 10% Formalin solution. The brains were removed and fixed in 10% Formalin. The locations of the electrode tips were subsequently determined from 40/z thionin-stained frozen sections. RESULTS The electrode placements for the implanted animals are shown in Fig. 1. The electrodes were located in the area of the lateral hypothalamus dorsolateral to the fornix. The coffee consumption (expressed as a percent of total liquid intake) of the rats in each group during the first 4 hr, the next 8 hr and the subsequent 12 hr of free-choice are shown in Fig. 2. Both prior self-stimulation and prior exposure to coffee increased the consumption of coffee during the first 4 hr of the preference test. During the next 8 hr coffee consumption remained elevated in the coffee/self-stimulation (C-SS) group only, and during the final 12 hr of the preference test the initial preferences had disappeared completely. A separate 2 × 2 analysis of variance was computed on the arcsine transformed data for each of the three time intervals. During the first 4 hr of the preference test there was a significant main effect of prior exposure to coffee, F(1,28)=11.34, p < 0 . 0 1 , and a significant effect of prior self-stimulation, F(1,28) =9.27, p 0.05, Scheff6 post hoc comparisons revealed that prior self-stimulation alone did not significantly increase coffee consumption during the preference test (water/self-stimulation group (W-SS) vs. water/no selfstimulation group (W-NSS), F(1,28)=1.51, p>0.05). Prior exposure to coffee alone also had no significant effect on coffee consumption (coffee/no self-stimulation group (CNSS) vs. water/no self-stimulation group (W-NSS),
70. -~ "~ -~ ff~6
m
60.
• self-stimulation no self-stimulation
Z O 30. 23 O9 Z 0 (..,) ~J
20. 10.
0 (3 c
w 0-4
C
W 4 12 TIME IN HOURS
C W 12 24
FIG. 2. Mean coffee consumption (-+ SEM) of each group during three time intervals of a free-choice situation with water. Data are expressed as percent of total liquid intake. C=prior exposure to coffee; W=prior exposure to water. F( 1,28) = 2.12, p >0.05. On the other hand, prior exposure to coffee paired with self-stimulation resulted in a significantly greater consumption of coffee during the first 4 hr of the preference test than either prior coffee alone, C-SS vs. C-NSS, F(1,28)=9.48, p
