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Schedule-induced polydipsia experience decreases locomotor response to amphetamine
Brain Research, 445 (1988) 211-215
211
Elsevier BRE 13410
Schedule-induced polydipsia experience decreases locomotor response to amphetamine A. Tazi, R. Dantzer and M. Le Moal Laboratoire de Psychobiologie des Comportements Adaptatifs, L N.S.E.R.M. Unit~259, Universit~ de Bordeaux 11, Domaine de Carreire, Bordeaux (France)
(Accepted 22 September 1987) Key words: Schedule-induced polydipsia; Coping; Central dopaminergic system; Amphetamine
To investigate the influence of schedule-induced polydipsia (SIP) on central dopaminergic systems, rats trained in a SIP procedure were challenged with the psychostimulant and dopaminergic agonist, D-amphetamine. In a first experiment, rats that had access to water and developed SIP (SIP-positive) displayed a lower response to amphetamine than rats that had access to water but did not develop SIP (SIP-negative) and rats that had no access to water. There was no difference in the spontaneous activity of these different groups of animals. In a second experiment, SIP-positive rats displayed the same reduced response to amphetamine following only 10 min of SIP drinking. In addition, SIP-positive rats that were tested without access to water during the SIP test displayed an increased locomotor activity both after saline and amphetamine treatments. These results suggest that SIP has stress-reducing properties. INTRODUCTION Schedule-induced polydipsia (SIP) is a form of 'adjunctive' or displacement behavior that occurs when food-deprived animals are submitted to an intermittent schedule of food delivery with free access to water 7. Many explanations have been proposed to account for the development of SIP and related activities. In line with the initial proposition that adjunctive behaviors function as stabilizing influences on activities disrupted by conflict or the impossibility to reach a goal object 7'8, recent theories focus on adaptive functions that these activities might serve. The observation that plasma corticosterone levels were decreased in animals that developed SIP, while they remained elevated in animals that did not have the opportunity to perfornl this behavior, led Brett and Levine 3 to propose that SIP l~as de-arousal properties and that it represents an efficient way of coping with aversive situations. These results have been replicated 25 and extended to chain chewing in pigs 6. In ad-
dition, the ability to engage in adjunctive activities has been found to block the activation of endogenous pain inhibitory systems that is otherwise triggered by intermittent distribution of food to deprived animals 24. Although the neuroanatomical substrates of schedule-induced behavior have not yet been fully elucidated, there is evidence that central dopaminergic systems are involved in the development of this behavior. More specifically, selective lesions of the dopaminergic terminals within the nucleus accumbens block the development of schedule-induced drinking 2°'27, whereas selective lesions of the dopaminergic terminals within the lateral septum have the opposite effect 22. From these data and the observation that repeated injections of D-amphetamine facilitate the development of SIP 17, it may be speculated that the propensity to develop SIP is related te enhanced activity in brain dopaminergic systems, Central dopaminergic neurons have also been implicated in another form of compulsive behavior in response to a mild stress, i.e. tail-pinch-induced eating ~. More-
Correspondence: R. Dantzer, Laboratoire de Psychobiologie des Comportements Adaptatifs, I.N,S.E.R.M. Unit~ 259, Universit~ de Bordeaux II, Domaine de Carreire, rue Camille Saint-Sa~ns, 33077 Bordeaux Cedex, France.
0006-8993/88/$03.50 © 1988Elsevier Science Publishers B.V. (Biomedical Division)
212 over, repeated tail-pinc h leads to sensitization to the behavioral effects of amphetamine and this procedure is fully interchangeable with repeated injections of amphetamine. Since the ability to cope is an important variable in the interaction of stress and the behavioral effect of amphetamine 16, we postulated that the stress-reducing properties of SIP should lead to a reduced responsiveness to amphetamine. In the present experiments, we show that polydipsic rats display a lower behavioral activation in response to a low dose of amphetamine than do non-polydipsic rats. MATERIALS AND METHODS
Subjects The subjects were male Sprague-Dawley rats (Janvier, France) weighing about 250 g at the start of the experiment. They were housed 4-5 per cage. The vivarium was temperature-regulated and lights were maintained on a 12:12 h dark-light cycle. Experiments were run during the light part of the cycle.
Apparatus Four identical operant chambers measuring 29 x 29 x 39 cm and housed in sound-attenuating cabinets were used. A recessed food-tray (Campden Instruments, Ltd) was mounted at the floor level on one wall of the chamber. Food pellets (Noyes, 45 mg) were delivered to the tray by means of an automatic dispenser and could be obtained by the rat by pushing open a hinged perspex flap covering the front of the tray. The hinged flap was connected to a microswitch so that 'tray-entries' by the rat could be recorded automatically. A plastic bottle was mounted on the outside of the wall containing the food tray. A stainless steel spout protruded into the chamber at a height of 4 cm and at a distance of 6 cm from the food-tray. The experiment was controlled by a minicomputer which also recorded the occurrence of responses. Locomotor activity was measured in circular corridors, 12 cm wide and 170 cm long, equipped with photocell beams located 3 cm above the floor and counts were recorded automatically at 10-min intervals from a printout counter located outside the testing room.
Procedure After 48 h food deprivation, rats were placed indi-
vidually into the test chamber and they were allowed to eat 31 pellets from the food tray, with the flap open. On the next day the same procedure was repeated, except that the flap was lowered, so that the animals had to push it open to earn the 31 pellets distributed every 20-30 s. During the following days, the rats were exposed daily to a 30-min session during which they received 31 pellets according to a fixedtime 60-s schedule of food distribution (FT 60). Water was freely available throughout the session, except for some animals that did not have access to a water bottle. Licking of the water spout, tray entries and water intake were recorded. Animals received a supplementary amount of food following every session to maintain their body weight at 80-85% of their free-feeding body weight. After stabilization of water consumption that required 9-14 sessk, ns ,~f FT 60, rats were tested for locomotor activity. On the first test session, they were injected with saline immediately after the end of the FT 60 session and they were placed individually into the circular corridors for 60 min. On the next day, the same procedure was repeated, except that animals were injected intraperitoneally with D-amphetamine sulphate (1 mg/kg) before the activity test. In the first experiment, 35 rats were tested, 7 had no access to water (No-water) and 28 had the opportunity to drink during the session (Water). Locomotor activity was measured following sessions 10 and 11. In the second experiment, 32 rats were tested, all of whom had free access to water during the polydipsia experiment. Locomotor activity was measured following sessions 15 and 16 that lasted only 10 min to minimize the amount of water ingested. Half the animals that had developed drinking were tested in the absence of water for those sessions. Statistical analysis. Results expressed as total number of photocell counts per 10 min were submitted to a 2-way analysis of variance with repeated measurements on the time factor. Post-hoc comparisons of group means were carried out with the NewmanKeuls test. RESULTS
Experiment I: Effects of schedule-induced polydipsia on reactivity to amphetamine On the ninth session, 14 rats from the Water gror, p
213 displayed SIP drinking, as e v i d e n c e d by a m e a n water c o n s u m p t i o n of 14.6 + 1.28 ml, whereas the 14 o t h e r rats d r a n k only small a m o u n t s of water (1.63 + 0.13 ml). Rats that d r a n k in response to the SIP test were designated SIP-pos; those that did not drink were designated SIP-neg. T h e s e two groups were studied separately for their l o c o m o t o r activity following SIP sessions 10 and 11. A f t e r saline injection, l o c o m o t o r activity decreased significantly over time (time effect: F.~.~ = 117.5, P < 0.001), but there was no difference between groups (group x time interaction: Ft0,160 = 0°93) (Fig. 1). In contrast, the increase in l o c o m o t o r activity to D-amphetamine was higher in SIP-neg and in N o - w a t e r rats than in SIP-pos rats (group effect: ~.32 = 3.12, P < 0.05; group x time "~ateraction: Fi0.t60 = 2.15, P < 0.05).
Experiment 2: Effects o f a minimized SIP test on reactivity to amphetamine Since the high a m o u n t of water intake could have resulted in a decreased bioavailability of o - a m p h e t a mine in SlP-pos rats, Expt. 2 c o m p a r e d the response to D-amphetamine of SIP-pos and SIP-neg rats after only 10 min of SIP experience. In a d d i t i o n , to confirm the role of drinking in the decreased response to a m p h e t a m i n e , some of the SIP-pos rats were tested for their l o c o m o t o r activity following a session without access to water. O n SIP session 14, SlP-pos rats (n = 24) drank an average a m o u n t of 12.22 + 1.16 mi of water, while SIP-neg rats (n = 8) d r a n k only 1.33 + 0.15 ml. D u r i n g SIP sessions 15 and 16, that lasted only 10 rain, the SIP-pos animals that h a d access to water d r a n k 5.29 + 0.59 ml and 6.35 + 0.52 ml, respectively.
= ~ SIP-positive (n=15) =.--.., SIP-negative (n = 8 )L'~ SIP-positive, no.water (n=g) r - j
: e SIP-positive (n=14) -" -" SIP- negative (n = 14) -- = No-water (n=7)
I
I I
TOTAL ACTIVITY
160 mini
TOTAL ACTIVITY 160 mini 4 0 0 1 - " 2ooo~.
~
400 2oooI
e ~il
350 ,ooof tO 3 0 0
0
SAL
300
AMPH
--I .,J UJ
0 250
o,-
SAL
AMPH
250
g "in
200
d-AMPH
Z
(lmg/kg)
-
d-AMPH (lrng/kg) (D 16)
2O0
(Oll)
LU
~; 150
--
150 100 -
lOO 5o
o - 1o :;o :~o ,=b ~o 6~ TIIVlE (rain)
Io :;o :;o ,~o s'o 6b TIME (min)
Fig. 1. Influence of SIP on the locomotor response to D-amphetamine. SIP-positive refers to rats that displayed excessive drinking in response to intermittent food distribution; SIP-negative to rats that did not develop SIP, and No..water to rats that had no access to water during the SIP sessions. Each point represents the mean number of photocell counts per 10-min intervals. Values in the inset represent mean (_+ S.E.M.) of the •otal counts for the duration of the session. * Significantly different from SIP-positive rats, P < 11.(15.Newman-Keuls test.
o
io ~o :;o ,~o ,;o 6b TIME (mini
;o ~o ~;o ;o ~o 6'o TIME (mini
Fig. 2. Influence of minimized SIP on the locomotor response to D-amphetamine. SIP-positive and SIP-negative rats were characterized as described before. 'SIP-positive, no-water" refers to SIP-positive rats t.-sted without access to water on sessions 15 and 16. which lasted only Ill min. Each point represents the mean number of photocell counts per 10-rain inter~ vals. Values in the inset represent mean (_+ S.E.M.) of the total counts for the duration of the session. * Significantly different from SIP-positive rats, P < 11.{15,Newman-Keuls test.
214 After saline injection, locomotor activity decreased significantly over time (time effect, F5.~45 = 95.6, P < 0.001) and there was a significant difference between groups in this effect (group x time iateraction: F10a45 = 2.55, P < 0.01) (Fig. 2). SIP-pos rats tested without access to water displayed an enhanced locomotor activity during the first 10-min interval compared to other groups (P < 0.05) and the second 10-min interval compared to SIP-neg rats (P < 0.05). In contrast, the increase in locomotor activity to D-amphetamine was higher in SIP-pos rats tested without water and in SIP-neg rats than in SIPpos rats tested with access to water (group effect: F2.29 = 7.18, P < 0.01). DISCUSSION The present results demonstrate that rats that display excessive drinking in response to an intermittent schedule of food distribution show less of an increase in locomotor activity in response to a dose of D-amphetamine acting preferentially on the mesolimbic dopamine system 5a3 than non-polydipsic rats or rats tested without access to water. It could be argued that SIP-pos rats are less sensitive to amphetamine because they are actually exposed to a lower dose of D-amphetamine than rats that do not drink. This could be due to a decreased bioavailability of amphetamine, via the excessive amount of water ingested. There is, however, no evidence of hemodilution in polydipsic animals even after 4 h of a SIP test resulting in the intake of 80-100 ml water 14. This is due to the fact that SIP-pos rats do not retain water in the body fluid compartments, but rather excrete copious volume of extremely dilute urine. In the present experiments, the critical factor for the decreased responsiveness to amphetamine is the ability to drink rather than the amount of water ingested, since a short-duration SIP session that only allows ingestion of minimum amounts of water is as effective as a 30-min session to decrease locomotor activity in response to D-amphetamine. Plasma corticosterone levels have also been observed to decrease in SIP-pos animals in a manner independent of water intake. This decrease takes place in a few minutes and contrasts with the increased plasma corticosterone levels displayed by rats that have no access to water 3'25. The decreased responsiveness to ampheta-
mine displayed by SIP-pos rats is therefore another feature that adds up to the growing list of physiological consequences of the SIP experience, such as decreased pituitary-adrenal axis activation 3 and reduced tail-flick latencies 24. Dopaminergic neurons are very sensitive to stress; however, these variations are restricted to specific brain regions, such as the nucleus accumbens, the mesolimbic frontal cortex and the arcuate nucleus of the hypothalamus 2"Ha5"26. Food deprivation is also able to enhance the metabolism of brain dopamine in the hypothalamus 12 and in the prefontal cortex 4. In rats chronically adapted to a regimen of limited access to food, increased dopamine turnover during feeding was observed in the hypothalamus, the nucleus accumbens and the amygdala 9. Changes in hypothalamic dopamine metabolism, however, did not correlate with duration of access to food or with amount of food eaten l°. Since similar increases were observed in rats that were not provided with food but were exposed to stimuli associated with feeding 21, it may be postulated that dopaminergic neurons mediate the effects, of external stimuli on physiological and behavioral responses involved in motivated activities Is. The possibility that dopaminergic neurons are involved in behavioral responsiveness to stimuli associated with reward is supported by the finding of an enhanced response of rats to a tone or a light paired with food following amphetamine infusion into the nucleus accumbens 23 and the blockade of this effect following 6-hydroxydopamine lesions to the nucleus accumbens ~9. Since the same lesion blocks both the acquisition of SIP 2° and the enhanced corticosterone levels displayed by rats submitted to an intermittent food delivery schedule 27, mesolimbic dopamine may play a critical role in the cumulative activation effect of food delivery. In addition to its possible role in the motivational arousal produced by intermittent distribution of food, the mesolimbic dopamine system is well known to be implicated in the mediation of the locomotor response to amphetamine 5'13. Ou :~ data therefore suggest that the ability to engage in adjunctive activities reduces the activation of this system and that, within this context, schedule-induced drinking has, at least on a short-term basis, functions very similar to those of other coping behaviors 16.
215 REFERENCES 1 Antelman, S.M., Eichler, A.J., Black, C.A. and Kocan, D., Interchangeability of stress and amphetamine in sensitization, Science, 207 (1980) 329-331. 2 Blanc, G., Herv6, D., Simon, H., Lisoprawski, A., (31owinski, J. and Tassin, A., Response to stress of mesocortico-frontal dopaminergic neurons in rats after longterm isolation, Nature (Lond.), 284 (1980) 265-267. 3 Brett, L. and Levine, S., The pituitary-adrenal response to 'minimized' schedule-induced drinking, Physiol. Behav., 26 (1981) 153-158. 4 Carlson, J.N., Herrick, K.F., Baird, J.L. and Glick, S.D., Selective enhancement of dopamine utilization in the rat prefrontal cortex by food deprivation, Brain Research, 400 (1987) 200-203. 5 Creese, I. and Iversen, S.D., The pharmacological and anatomical substrates of the amphetamine response in the rat, Brain Research, 83 (1975) 419-436. 6 Dantzer, R. and Mormede, P., Pituitary-adrenal correlates of adjunctive activities in pigs, Horm. Behav., 16 (1981) 386-395. 7 Falk, J.L., The nature and determinants of adjunctive behavior, Physiol. Behav., 6 (1971) 577-588. 8 Faik, J.L., The origin and function of adjunctive behavior, Anita. Learn. Behav., 5 (1977) 325-335. 9 Heffner, T.G., Hartman, J.A. and Seiden, L.S., Feeding increases dopamine metabolism in the rat brain, Science, 208 (1980) 1168-1170. 10 Heffner, T.(3., Vosmer, (3. and Seiden, L.S., Time-dependent changes in hypothalamic dopamine metabolism during feeding in the rat, Pharmacol. Biochem. Behav., 20 (1984) 947-949. 11 Herman, J.P., Guilloneau, D., Dantzer, R., Scatton, B., Semerdjian-Rouquier, L. and Le Moal, M., Differential effects of inescapable footshocks and stimuli previously paired with inescapable footshocks on dopamine turnover in cortical and limbic areas of the rat, Life Sci., 30 (1982) 2207-2214. 12 Janwar-Uniyal, M., Darwish, M., Levin, B.E. and Leibowitz, S.F., Alterations in catecholamine levels and turnover in discrete brain areas after food deprivation, Pharmacol. Biochem. Behav., 26 (1987) 271-275. 13 Kelley, P.H. and Iversen, S.D., Selective 6-OHDA induced destruction of mesolimbic dopamine neurons: abolition of psychostimulant-induced locomotor activity in rats, Eur. J. Pharmacol., 40 (1976) 45-56. 14 Kenny, R.C., Wright, J.W. and Reynolds, T.J., Scheduleinduced polydipsia: the role of oral and plasma factors, Physiol. Behav., 17 (1976) 939-945.
15 Kobayashi, R.M., Palkovits, M., Kizer, J.S., Jacobowitz, D.M. and Kopin, I.J., Selective alterations of catecholamines and tyrosine hydroxylase activity in the hypothalamus following acute and chronic stress. In E. Usdin, R. Kvetnansky and I.J. Kopin (Eds.), Catecholamin¢- and Stress, Pergamon, Oxford, 1976, pp. 29-38. 16 MacLennan, A.J. and Maier, S.F., Coping and the stressinduced potentiation of stimulant stereotypy in the rat, Science, 219 (1983) 1091-1093. 17 Mittleman, (3. and Valenstein, E,S., Individual differences iE non-regulatory ingestive behavior and catecholamine sy::;tems, Brain Research, 348 (1985) 112-117. 18 t'anksepp, J., The anatomy of emotions. In. R. Plutchik an-d H. Kellerman (Eds.), Emotion: Theory, Research, and E,rr'z~:zce Vol. 3, Academic, New York, 1986, pp. 91-124: 19 Robbins, T.W. and Everitt, B.J., Functional studies of the central catecholamines. In J.R. Smythies and R.J. Bradley (Eds.), International Review of Neurology. Vol. 123, Academic, New York, 1982. 20 Robbins, T.W, and Koob, (3.F., Selective disruption of displacement behaviour by lesions of the mesolimbic dopamine system, Nature (Lond.), 285 (1980) 409-412. 21 Simansky, K.J., Bourbonais, K.A. and Smith, (3.P., Foodrelated stimuli increase the ratio of 3,4-dihydroxyphenylacetic acid to dopamine in the hypothalamus, Pharmacol. Biochem. Behav., 23 (1985) 253-258. 22 Taghzouti, K., Simon, H., Tazi, A., Dantzer, R. and Le Moal, M., The effect of 6-OHDA lesions of the lateral septurn on schedule-induced polydipsia, Behav. Brain Res., 15 (1985) 1-8. 23 Taylor, J.R. and Robbins, T.W., Enhanced behavioral control by conditioned reinforcers following microinjection of D-amphetamine into the nucleus accumbens, Psychopharmacology, 84 (1984) 405-412. 24 Tazi, A., Dantzer, R. and Le Moal, M., Prediction and control of food rewards modulate endogenous pain inhibitory systems, Behav. Brain Res., 23 (1987) 197-20a 25 Tazi, A., Dantzer, R., Mormede, P. and Le Moal, M., Pituitary adrenal correlates of schedule-induced polydipsia and wheel running in rats, Behav. Brain Res., 19 (1986) 249-256. 26 Thierry, A.M., Tassin, J.P., Blanc, G. and Glowinsky, J., Selective activation of the mesocorticai DA system by stress, Nature (Lond.), 263 (1976) 242-244. 27 Wallace, M., Singer, G., Finlay, J. and Gibson, S., The effect of 6-OHDA lesions of the nucleus accumbens septum on schedule-induced drinking, wheelrunning and plasma corticosterone levels in the rat, Pharmacol. Biochem. Be. hay., 18 (1983) 129-136.
211
Elsevier BRE 13410
Schedule-induced polydipsia experience decreases locomotor response to amphetamine A. Tazi, R. Dantzer and M. Le Moal Laboratoire de Psychobiologie des Comportements Adaptatifs, L N.S.E.R.M. Unit~259, Universit~ de Bordeaux 11, Domaine de Carreire, Bordeaux (France)
(Accepted 22 September 1987) Key words: Schedule-induced polydipsia; Coping; Central dopaminergic system; Amphetamine
To investigate the influence of schedule-induced polydipsia (SIP) on central dopaminergic systems, rats trained in a SIP procedure were challenged with the psychostimulant and dopaminergic agonist, D-amphetamine. In a first experiment, rats that had access to water and developed SIP (SIP-positive) displayed a lower response to amphetamine than rats that had access to water but did not develop SIP (SIP-negative) and rats that had no access to water. There was no difference in the spontaneous activity of these different groups of animals. In a second experiment, SIP-positive rats displayed the same reduced response to amphetamine following only 10 min of SIP drinking. In addition, SIP-positive rats that were tested without access to water during the SIP test displayed an increased locomotor activity both after saline and amphetamine treatments. These results suggest that SIP has stress-reducing properties. INTRODUCTION Schedule-induced polydipsia (SIP) is a form of 'adjunctive' or displacement behavior that occurs when food-deprived animals are submitted to an intermittent schedule of food delivery with free access to water 7. Many explanations have been proposed to account for the development of SIP and related activities. In line with the initial proposition that adjunctive behaviors function as stabilizing influences on activities disrupted by conflict or the impossibility to reach a goal object 7'8, recent theories focus on adaptive functions that these activities might serve. The observation that plasma corticosterone levels were decreased in animals that developed SIP, while they remained elevated in animals that did not have the opportunity to perfornl this behavior, led Brett and Levine 3 to propose that SIP l~as de-arousal properties and that it represents an efficient way of coping with aversive situations. These results have been replicated 25 and extended to chain chewing in pigs 6. In ad-
dition, the ability to engage in adjunctive activities has been found to block the activation of endogenous pain inhibitory systems that is otherwise triggered by intermittent distribution of food to deprived animals 24. Although the neuroanatomical substrates of schedule-induced behavior have not yet been fully elucidated, there is evidence that central dopaminergic systems are involved in the development of this behavior. More specifically, selective lesions of the dopaminergic terminals within the nucleus accumbens block the development of schedule-induced drinking 2°'27, whereas selective lesions of the dopaminergic terminals within the lateral septum have the opposite effect 22. From these data and the observation that repeated injections of D-amphetamine facilitate the development of SIP 17, it may be speculated that the propensity to develop SIP is related te enhanced activity in brain dopaminergic systems, Central dopaminergic neurons have also been implicated in another form of compulsive behavior in response to a mild stress, i.e. tail-pinch-induced eating ~. More-
Correspondence: R. Dantzer, Laboratoire de Psychobiologie des Comportements Adaptatifs, I.N,S.E.R.M. Unit~ 259, Universit~ de Bordeaux II, Domaine de Carreire, rue Camille Saint-Sa~ns, 33077 Bordeaux Cedex, France.
0006-8993/88/$03.50 © 1988Elsevier Science Publishers B.V. (Biomedical Division)
212 over, repeated tail-pinc h leads to sensitization to the behavioral effects of amphetamine and this procedure is fully interchangeable with repeated injections of amphetamine. Since the ability to cope is an important variable in the interaction of stress and the behavioral effect of amphetamine 16, we postulated that the stress-reducing properties of SIP should lead to a reduced responsiveness to amphetamine. In the present experiments, we show that polydipsic rats display a lower behavioral activation in response to a low dose of amphetamine than do non-polydipsic rats. MATERIALS AND METHODS
Subjects The subjects were male Sprague-Dawley rats (Janvier, France) weighing about 250 g at the start of the experiment. They were housed 4-5 per cage. The vivarium was temperature-regulated and lights were maintained on a 12:12 h dark-light cycle. Experiments were run during the light part of the cycle.
Apparatus Four identical operant chambers measuring 29 x 29 x 39 cm and housed in sound-attenuating cabinets were used. A recessed food-tray (Campden Instruments, Ltd) was mounted at the floor level on one wall of the chamber. Food pellets (Noyes, 45 mg) were delivered to the tray by means of an automatic dispenser and could be obtained by the rat by pushing open a hinged perspex flap covering the front of the tray. The hinged flap was connected to a microswitch so that 'tray-entries' by the rat could be recorded automatically. A plastic bottle was mounted on the outside of the wall containing the food tray. A stainless steel spout protruded into the chamber at a height of 4 cm and at a distance of 6 cm from the food-tray. The experiment was controlled by a minicomputer which also recorded the occurrence of responses. Locomotor activity was measured in circular corridors, 12 cm wide and 170 cm long, equipped with photocell beams located 3 cm above the floor and counts were recorded automatically at 10-min intervals from a printout counter located outside the testing room.
Procedure After 48 h food deprivation, rats were placed indi-
vidually into the test chamber and they were allowed to eat 31 pellets from the food tray, with the flap open. On the next day the same procedure was repeated, except that the flap was lowered, so that the animals had to push it open to earn the 31 pellets distributed every 20-30 s. During the following days, the rats were exposed daily to a 30-min session during which they received 31 pellets according to a fixedtime 60-s schedule of food distribution (FT 60). Water was freely available throughout the session, except for some animals that did not have access to a water bottle. Licking of the water spout, tray entries and water intake were recorded. Animals received a supplementary amount of food following every session to maintain their body weight at 80-85% of their free-feeding body weight. After stabilization of water consumption that required 9-14 sessk, ns ,~f FT 60, rats were tested for locomotor activity. On the first test session, they were injected with saline immediately after the end of the FT 60 session and they were placed individually into the circular corridors for 60 min. On the next day, the same procedure was repeated, except that animals were injected intraperitoneally with D-amphetamine sulphate (1 mg/kg) before the activity test. In the first experiment, 35 rats were tested, 7 had no access to water (No-water) and 28 had the opportunity to drink during the session (Water). Locomotor activity was measured following sessions 10 and 11. In the second experiment, 32 rats were tested, all of whom had free access to water during the polydipsia experiment. Locomotor activity was measured following sessions 15 and 16 that lasted only 10 min to minimize the amount of water ingested. Half the animals that had developed drinking were tested in the absence of water for those sessions. Statistical analysis. Results expressed as total number of photocell counts per 10 min were submitted to a 2-way analysis of variance with repeated measurements on the time factor. Post-hoc comparisons of group means were carried out with the NewmanKeuls test. RESULTS
Experiment I: Effects of schedule-induced polydipsia on reactivity to amphetamine On the ninth session, 14 rats from the Water gror, p
213 displayed SIP drinking, as e v i d e n c e d by a m e a n water c o n s u m p t i o n of 14.6 + 1.28 ml, whereas the 14 o t h e r rats d r a n k only small a m o u n t s of water (1.63 + 0.13 ml). Rats that d r a n k in response to the SIP test were designated SIP-pos; those that did not drink were designated SIP-neg. T h e s e two groups were studied separately for their l o c o m o t o r activity following SIP sessions 10 and 11. A f t e r saline injection, l o c o m o t o r activity decreased significantly over time (time effect: F.~.~ = 117.5, P < 0.001), but there was no difference between groups (group x time interaction: Ft0,160 = 0°93) (Fig. 1). In contrast, the increase in l o c o m o t o r activity to D-amphetamine was higher in SIP-neg and in N o - w a t e r rats than in SIP-pos rats (group effect: ~.32 = 3.12, P < 0.05; group x time "~ateraction: Fi0.t60 = 2.15, P < 0.05).
Experiment 2: Effects o f a minimized SIP test on reactivity to amphetamine Since the high a m o u n t of water intake could have resulted in a decreased bioavailability of o - a m p h e t a mine in SlP-pos rats, Expt. 2 c o m p a r e d the response to D-amphetamine of SIP-pos and SIP-neg rats after only 10 min of SIP experience. In a d d i t i o n , to confirm the role of drinking in the decreased response to a m p h e t a m i n e , some of the SIP-pos rats were tested for their l o c o m o t o r activity following a session without access to water. O n SIP session 14, SlP-pos rats (n = 24) drank an average a m o u n t of 12.22 + 1.16 mi of water, while SIP-neg rats (n = 8) d r a n k only 1.33 + 0.15 ml. D u r i n g SIP sessions 15 and 16, that lasted only 10 rain, the SIP-pos animals that h a d access to water d r a n k 5.29 + 0.59 ml and 6.35 + 0.52 ml, respectively.
= ~ SIP-positive (n=15) =.--.., SIP-negative (n = 8 )L'~ SIP-positive, no.water (n=g) r - j
: e SIP-positive (n=14) -" -" SIP- negative (n = 14) -- = No-water (n=7)
I
I I
TOTAL ACTIVITY
160 mini
TOTAL ACTIVITY 160 mini 4 0 0 1 - " 2ooo~.
~
400 2oooI
e ~il
350 ,ooof tO 3 0 0
0
SAL
300
AMPH
--I .,J UJ
0 250
o,-
SAL
AMPH
250
g "in
200
d-AMPH
Z
(lmg/kg)
-
d-AMPH (lrng/kg) (D 16)
2O0
(Oll)
LU
~; 150
--
150 100 -
lOO 5o
o - 1o :;o :~o ,=b ~o 6~ TIIVlE (rain)
Io :;o :;o ,~o s'o 6b TIME (min)
Fig. 1. Influence of SIP on the locomotor response to D-amphetamine. SIP-positive refers to rats that displayed excessive drinking in response to intermittent food distribution; SIP-negative to rats that did not develop SIP, and No..water to rats that had no access to water during the SIP sessions. Each point represents the mean number of photocell counts per 10-min intervals. Values in the inset represent mean (_+ S.E.M.) of the •otal counts for the duration of the session. * Significantly different from SIP-positive rats, P < 11.(15.Newman-Keuls test.
o
io ~o :;o ,~o ,;o 6b TIME (mini
;o ~o ~;o ;o ~o 6'o TIME (mini
Fig. 2. Influence of minimized SIP on the locomotor response to D-amphetamine. SIP-positive and SIP-negative rats were characterized as described before. 'SIP-positive, no-water" refers to SIP-positive rats t.-sted without access to water on sessions 15 and 16. which lasted only Ill min. Each point represents the mean number of photocell counts per 10-rain inter~ vals. Values in the inset represent mean (_+ S.E.M.) of the total counts for the duration of the session. * Significantly different from SIP-positive rats, P < 11.{15,Newman-Keuls test.
214 After saline injection, locomotor activity decreased significantly over time (time effect, F5.~45 = 95.6, P < 0.001) and there was a significant difference between groups in this effect (group x time iateraction: F10a45 = 2.55, P < 0.01) (Fig. 2). SIP-pos rats tested without access to water displayed an enhanced locomotor activity during the first 10-min interval compared to other groups (P < 0.05) and the second 10-min interval compared to SIP-neg rats (P < 0.05). In contrast, the increase in locomotor activity to D-amphetamine was higher in SIP-pos rats tested without water and in SIP-neg rats than in SIPpos rats tested with access to water (group effect: F2.29 = 7.18, P < 0.01). DISCUSSION The present results demonstrate that rats that display excessive drinking in response to an intermittent schedule of food distribution show less of an increase in locomotor activity in response to a dose of D-amphetamine acting preferentially on the mesolimbic dopamine system 5a3 than non-polydipsic rats or rats tested without access to water. It could be argued that SIP-pos rats are less sensitive to amphetamine because they are actually exposed to a lower dose of D-amphetamine than rats that do not drink. This could be due to a decreased bioavailability of amphetamine, via the excessive amount of water ingested. There is, however, no evidence of hemodilution in polydipsic animals even after 4 h of a SIP test resulting in the intake of 80-100 ml water 14. This is due to the fact that SIP-pos rats do not retain water in the body fluid compartments, but rather excrete copious volume of extremely dilute urine. In the present experiments, the critical factor for the decreased responsiveness to amphetamine is the ability to drink rather than the amount of water ingested, since a short-duration SIP session that only allows ingestion of minimum amounts of water is as effective as a 30-min session to decrease locomotor activity in response to D-amphetamine. Plasma corticosterone levels have also been observed to decrease in SIP-pos animals in a manner independent of water intake. This decrease takes place in a few minutes and contrasts with the increased plasma corticosterone levels displayed by rats that have no access to water 3'25. The decreased responsiveness to ampheta-
mine displayed by SIP-pos rats is therefore another feature that adds up to the growing list of physiological consequences of the SIP experience, such as decreased pituitary-adrenal axis activation 3 and reduced tail-flick latencies 24. Dopaminergic neurons are very sensitive to stress; however, these variations are restricted to specific brain regions, such as the nucleus accumbens, the mesolimbic frontal cortex and the arcuate nucleus of the hypothalamus 2"Ha5"26. Food deprivation is also able to enhance the metabolism of brain dopamine in the hypothalamus 12 and in the prefontal cortex 4. In rats chronically adapted to a regimen of limited access to food, increased dopamine turnover during feeding was observed in the hypothalamus, the nucleus accumbens and the amygdala 9. Changes in hypothalamic dopamine metabolism, however, did not correlate with duration of access to food or with amount of food eaten l°. Since similar increases were observed in rats that were not provided with food but were exposed to stimuli associated with feeding 21, it may be postulated that dopaminergic neurons mediate the effects, of external stimuli on physiological and behavioral responses involved in motivated activities Is. The possibility that dopaminergic neurons are involved in behavioral responsiveness to stimuli associated with reward is supported by the finding of an enhanced response of rats to a tone or a light paired with food following amphetamine infusion into the nucleus accumbens 23 and the blockade of this effect following 6-hydroxydopamine lesions to the nucleus accumbens ~9. Since the same lesion blocks both the acquisition of SIP 2° and the enhanced corticosterone levels displayed by rats submitted to an intermittent food delivery schedule 27, mesolimbic dopamine may play a critical role in the cumulative activation effect of food delivery. In addition to its possible role in the motivational arousal produced by intermittent distribution of food, the mesolimbic dopamine system is well known to be implicated in the mediation of the locomotor response to amphetamine 5'13. Ou :~ data therefore suggest that the ability to engage in adjunctive activities reduces the activation of this system and that, within this context, schedule-induced drinking has, at least on a short-term basis, functions very similar to those of other coping behaviors 16.
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