Development and reversal of sensitization to amphetamine-induced hypophagia: role of temporal,pharmacological, and behavioral variables

This study shows that sensitization can develop to amphetamine-induced hypophagia and examines the stability of this effect following subsequent pharmacological and behavioral experience. Rats given 36 injections of either amphetamine (2.5 mg/kg; Group A) or saline (Group S) at 3-day intervals developed sensitization of hypophagia, as assessed by a shift to the left in the dose-response (DR) function. Group A also displayed sensitization of stereotypy, whereas Group S showed little change except at the highest dose. Subgroups from each group were then given daily injections of amphetamine (2 mg/kg) either before or after access to milk for 4 weeks. Other subgroups were given injections of saline as a control. On a final DR determination, these control groups showed no further changes in milk intake. In contrast, groups given chronic injections of amphetamineafter milk showed a loss of sensitization (DR3=DR1), whereas groups given the drugbefore milk developed tolerance that was limited to the chronic dose. These results demonstrate that (1) sensitization of amphetamine-induced hypophagia and stereotypy can develop independently; (2) sensitization of hypophagia can be reversed, without inducing tolerance, by subsequent daily exposure to the drug; and (3) prior sensitization of hypophagia does not preclude the subsequent development of tolerance if the drug is later given in the context of feeding.     

Loss of tolerance to amphetamine-induced hypophagia in rats: homeostatic readjustment vs. instrumental learning.

According to the homeostatic model, the loss of tolerance to amphetamine-induced hypophagia requires a period of unrestricted feeding in the drug-free state, which transforms the compensatory response mediating tolerance ("hyperhunger") into a functional disturbance to homeostasis. In the absence of such a disturbance, tolerance should be retained. To test this prediction, rats tolerant to amphetamine's hypophagic effect were given a 4-week tolerance retention period during which milk intakes were restricted and deprivation levels held relatively constant. During this period the rats were assigned to one of the following drug treatment conditions: 1) saline injections both before and after daily milk tests (saline group); 2) saline injections before, and amphetamine injections after, daily milk tests (after group); 3) no injections and no milk tests (no-treatment group); or 4) amphetamine injections before, and saline injections after, milk tests (before group). Despite the restricted feeding regimen, both the saline and after groups lost tolerance. These results do not support the homeostatic model, but are consistent with the instrumental learning model, which views drinking milk in the undrugged state as analogous to receiving noncontingent reinforcement.     

Role of behavioral and pharmacological variables in the loss of tolerance to amphetamine hypophagia

 Tolerance to amphetamine-induced hypophagia is lost when drug injections are withdrawn for 4 weeks while milk tests are continued (Wolgin and Hughes 1996). The purpose of this study was to determine whether the loss of tolerance is a function of drug withdrawal per se. Rats made tolerant to amphetamine (2 mg/kg, IP) were assigned to one of three groups. During the next 4 weeks (phase), one group continued to receive amphetamine injections prior to daily milk tests (Before group), one group received drug injections after the milk tests (After group), and one group received injections of saline prior to the milk tests (Saline group). Dose-response tests revealed that the Before group retained tolerance, whereas the After and Saline groups lost tolerance. When retested with chronic injections of amphetamine prior to milk, the After and Saline groups reacquired tolerance more rapidly, and to a greater extent, than non-tolerant controls. These results demonstrate that the loss of tolerance is not due to drug withdrawal per se, but may be due to the unlearning of behavioral strategies previously acquired under the drug. Received: 17 September 1996 / Final version: 10 March 1997     

Changes in behavioural contingencies produce a loss of tolerance to amphetamine hypophagia in rats despite continued feeding tests while drugged

Rats administered amphetamine prior to access to milk in bottles develop tolerance to the hypophagic effect of the drug by learning to suppress stereotyped behaviours that interfere with feeding. When tolerant rats are later allowed to drink milk from a bottle in an unintoxicated state, tolerance is lost, even when drug exposure is held constant by administration of the drug after the test. In the present experiment, we show that tolerance can also be lost in the face of continued administration of amphetamine prior to milk tests, as a result of changes in the contingencies of reinforcement that govern the suppression of stereotypy. Rats were injected with 2 mg/kg amphetamine and given access to milk in bottles for 16 trials. Tolerance to the hypophagic effect was confirmed by dose-response tests in which milk was available in bottles. The rats were then injected with 2 mg/kg amphetamine prior to intraoral milk infusions for 21 trials. This method of feeding did not require the suppression of stereotypy to obtain milk. Subsequent dose-response tests in which milk was again presented in bottles revealed that tolerance was lost, even though intoxicated feedings were never interrupted. These results demonstrate that the contingencies of reinforcement governing the suppression of stereotypy determine whether tolerance is retained or lost.     

Experiential constraints on the development of tolerance to amphetamine hypophagia following sensitization of stereotypy: instrumental contingencies regulate the expression of sensitization

 In previous research, sensitization of stereotypy induced by injections of 2.5 mg/kg amphetamine did not interfere with subsequent tolerance development to the hypophagic effect of 2 mg/kg. This study examined the effect of a higher sensitizing dose. Rats given intermittent injections of 5 mg/kg amphetamine and then challenged with various doses of amphetamine showed focused head scanning at 2 mg/kg and oral stereotypy at 4 mg/kg. In contrast, saline controls showed diffuse sniffing and head scanning at 2 and 4 mg/kg. Subgroups from each condition were then given daily injections of either amphetamine (2 mg/kg) or saline and access to milk for 30 min. Dose-response tests revealed that both drugged groups learned to suppress stereotypy in order to feed at 2 mg/kg, but only the non-sensitized group could do so at 4 mg/kg. These results demonstrate that (1) rats learn to suppress only those stereotyped movements that they experience in the context of feeding and (2) instrumental contingencies can influence the expression of behavioral sensitization. Received: 18 December 1997 / Final version: 3 June 1998     

Sensitization to haloperidol-induced suppression of milk intake: effect of interdose interval

The purpose of this study was to determine the effect of manipulating the interdose interval (IDI) on the suppression of milk intake induced by haloperidol (HAL). Groups of rats were given chronic injections of either HAL (0.625 mg/kg) or saline at IDIs of 1, 2, 7, or 14 days. Dose-response curves were determined at the conclusion of the chronic phase. The results indicated that injections of HAL given at IDIs of 1 or 2 days produced neither tolerance nor sensitization, whereas injections given at intervals of 7 or 14 days produced sensitization. Sensitization was also observed in the control groups, perhaps as a result of the intermittent schedule of HAL injections given during the dose-response tests. Sensitization to HAL was not accompanied by changes in sensitivity to amphetamine. The results of this experiment are consistent with those of other studies in showing that the behavioral effects of neuroleptics are strongly influenced by the schedule of injections. In addition, evidence is presented that sensitization to HAL-induced hypophagia is contingent on behavioral experience under the drug.     

The effects of dose and repeated administration on the longer-term hypophagia produced by amphetamine in rats

Rats are hypophagic approximately 1-3 and 13-27 h after receiving amphetamine (2.0 mg/kg). This study examined how these short- and longer-term phases of hypophagia were affected by repeated administration of different amphetamine doses. Throughout eight five-day tests, the rats could lever press for food pellets for 1-hour periods beginning every three hours. On test day 1, the rats were treated with saline, and on test day 3, they were treated with a dose of amphetamine. Across tests, for one group, treatment on day 3 alternated between 0.0 (saline) and 0.5 mg/kg amphetamine; for a second, group treatment on day 3 alternated between 1.0 and 2.0 mg/kg amphetamine; and for a third group, treatment on day 3 was always 1.0 mg/kg amphetamine. The patterns of food intake following day 1 saline and day 3 treatment were compared. Short-term food intake was abolished by 0.5, 1.0, and 2.0 mg/kg amphetamine, and no tolerance was observed to this effect. Longer-term hypophagia was produced by 1.0 and 2.0 but not by 0.5 mg/kg. Tolerance to longer-term hypophagia was seen when 1.0 mg/kg alone was used as the day 3 treatment, but not when 1.0 and 2.0 mg/kg were alternated across tests as the day 3 treatment. Short- and longer-term hypophagia were dissociated by threshold doses for elicitation and by differential tolerance. Occasional receipt of a higher amphetamine dose may sometimes increase the longer-term hypophagia produced by a lower dose.     

Sensitization of haloperidol-induced hypophagia is contingent on behavioral experience with food

Groups of rats were given injections of haloperiodol (0.31mg/kg) at weekly intervals either before or after access to sweetened milk. Control groups were given injections of saline. At the end of the chronic regimen, all groups received a single injection of haloperidol (0.15mg/kg) prior to milk access. Rats injected with the drug before milk during the chronic phase showed a progressive decrease in milk intake. When subsequently challenged with a lower dose, this group ingested less milk than any of the other groups, which did not differ from one other. These results demonstrate that sensitization of haloperidol-induced hypophagia is contingent on experience with milk while in the drugged state.     

Tolerance to the discriminative stimulus properties of d-amphetamine

Male rats (F-344) responding for milk on a VI 20 sec schedule of reinforcement were trained to discriminate which of two levers to press on the basis of whether they had been injected with d-amphetamine (0.50, 1.00 or 1.50 mg/kg) or saline 15 min prior to daily training sessions. Dose-response functions determined for each of the three (n = 6) training-dose groups indicated that ED₅₀ values were directly correlated with training dose. Two days following chronic amphetamine injections (a total of 78 mg/kg over 4 days) rats were tested for tolerance at a dose which normally produced about 80% drug-lever responding. Rats in all three groups showed tolerance to the cue properties of amphetamine. In the 0.50 and 1.00 mg/kg groups, complete tolerance was shown as evidenced by the fact that the drug lever responding did not differ from that which was appropriate following saline injections.     

Effects of acute and chronic cocaine on milk intake, body weight, and activity in bottle- and cannula-fed rats

The effects of cocaine on the milk intake, body weight and activity of bottle- and cannula-fed rats was compared under both acute and chronic dosing conditions. Bottle-fed rats were initially more hypophagic than cannula-fed rats when given acute injections of cocaine (4-40mg/kg). Following chronic injections of the drug (16mg/kg), bottle-fed rats developed tolerance, as shown by a rightward shift in the dose-response function for milk intake. Such tolerance was accompanied by a decrease in drug-induced motor activity. In contrast, cannula-fed rats showed marked sensitization of stereotyped movements. Bottle -fed rats showed marked sensitization of stereotyped movements. However, weight loss per se was not a determining factor in tolerance development, because cannula-fed rats given chronic injections of 32mg/kg cocaine lost even more weight, but did not become tolerant. These results suggest that, at moderate doses, cocaine suppresses feeding primarily by inducing behaviors that are incompatible with the appetitive phase of feeding, and that tolerance involves learning to inhibit such responses in order to feed.     

The process of learning in rats undergoing prolonged treatment with psychotropic agents

The effect of amphetamine and chlorpromazine on the process of learning and the change in performance of acquired skills in rats was studied. Maze tests were camed out for 23 days and conditioned reflex tests for 37 days. Animals received 0.5 mg/kg/day of amphetamine or 0.5 mg/kg/day of chlorpromazine added to the drinking water. Further groups received daily intraperitoneal injections of 0.25 mg/kg of amphetamine or 4 mg/kg of chlorpromazine.The learning index deteriorated initially by oral administration of amphetamine, but subsequently improved, whereas intraperitoneal treatment caused a slight deterioration.Both agents interfered with previously acquired conditioned responses.The frequency of pseudopositive responses was considerably higher following administration of amphetamine, but was unalterated by chlorpromazine.     

Haloperidol-induced catalepsy is modulated by the development of tolerance to amphetamine-induced anorexia

The effect of the development of tolerance to amphetamine-anorexia on both amphetamine-induced and haloperidol-induced motor effects was investigated. The animals in experiment 1 showed an acute anorexic reaction to 3 mg/kg amphetamine, whereas the rats in experiment 2 failed to meet the criterion level of acute anorexia. During initial training rats received 13 injections of saline or 3 mg/kg amphetamine intraperitoneally (i.p.) every other day. In both experiments, for one group each amphetamine injection was followed 20 min later by 30 min access to milk (CONT groups). In a second group, each amphetamine injection was followed 24 h later by 30 min access to milk (NONCON groups) and a third group received only saline injections and milk (controls). As expected, in experiment 1 originally anorexic animals in the CONT group developed tolerance, whereas the NONCON and control groups displayed no tolerance to amphetamine-anorexia. The NONCON group showed sig nificantly more stereotypy than either the CONT or control group. Furthermore, following 1.25 mg/kg haloperidol the CONT animals were less cataleptic than the NONCON and control groups which did not differ. In experiment 2, at the end of training the rats in all groups displayed no anorexia following amphetamine injection; they consumed an amount of milk equivalent to that normally consumed under no-drug conditions. Neither was there a difference in the amount of catalepsy between groups following injection of 1.25 mg/kg haloperidol.     

Procedures affecting maintenance and loss of tolerance to amphetamine-induced anorexia and cross-tolerance to haloperidol-induced catalepsy

Procedures affecting the maintenance and loss of tolerance to amphetamine anorexia were investigated. Following a period in which rats acquired tolerance to amphetamine-induced anorexia via the contingent- tolerance procedure, a variety of manipulations were investigated to determine which effected a loss of tolerance. The procedures included: continued milk availability or its absence, saline injections or no injections and a switch to non-contingent amphetamine injections. These procedures were each investigated while animals continued to be food-deprived or were maintained at their ad libitum weights. The animals maintained at 100% ad lib. lost tolerance whereas tolerance was maintained by the 85% groups, regardless of what other manipulations were given. Subsequently all animals were tested for catalepsy induced by 1.25 mg/kg haloperidol. Animals that had maintained tolerance to amphetamine anorexia (85% groups) were significantly less cataleptic than the animals that had lost tolerance to amphetamine anorexia (100% groups). The finding that tolerance to amphetamine anorexia is lost in animals that have ad lib. access to food indicates that a homeostatic imbalance caused by food-deprivation is perhaps the most important factor in the maintenance of tolerance to amphetamine anorexia.     

Tolerance to the behavioural effects of the selective 5-HT(1A) agent, ipsapirone

Effects of ipsaprione on rats were studied in procedures in which tolerance was assessed with operant responding and in feeding tests. Initially, the suppressant effect of ipsapirone on Fixed Ratio 20 behaviour was studied. During 33 days treatment (7.5mg/kg daily) no tolerance developed. Subsequently, the same rats received ipsapirone (7.5mg/kg) three times daily, at intervals of 2.5h, the first injection preceding operant sessions. Over 10 further days of treatment, there was still no evidence of tolerance. In a second study, ipsapirone was again administered at 7.5mg/kg before operant sessions, followed by 2 daily injections of 20mg/kg, at at 2.5h intervals after operant sessions. Under these conditions, some tolerance developed although it was incomplete and rapidly lost. The effects of ipsapirone on operant responding were found to be short-lasting. The suppressant action of 7.5mg/kg of ipsapirone (to 50% of baseline) was abolished if the drug was injected 4h before sessions. In other studies, we confirmed that tolerance develops very rapidly to hyperphagic actions of ipsapirone (see Kennett et al., 1987). We conclude that: 1) tolerance develops at differing rates to various effects of ipsapirone; 2) only those effects of ipsapirone (and related agents) which involve activation of presynaptic autoreceptors show rapid tolerance; 3) tolerance develops to effects on operant responding only if high doses are administered frequently. Since ipsapirone is short-acting, tolerance to such effects develops only when drug is continuously present in body tissue. The tolerance observed (to effects on operant behaviour) was probably of a pharmacodynamic nature and did not involve learning processes.     

Time dependent changes in anterograde scopolamine-induced amnesia in rats

These experiments studied the effect of scopolamine on memory formation and subsequent memory recall. Different groups of rats were trained on a Y-maze brightness discrimination task 20 min after IP injection of 2 mg/kg scopolamine HBr, an anticholinergic. Retention tests were then conducted 1 day or 2, 4, or 6 weeks after training. Deficits in retention performance were observed at 1 day and 2 weeks after training but not at the longer intervals. In addition, other rats were trained in the same manner and after the same dose of scopolamine but were then retention tested 20 min after 0.5 mg/kg physostigmine salicylate, a cholinesterase inhibitor. These subjects also showed deficits at 1 day and 2 weeks but were not different from controls at the longer intervals. Amnesia was not, however, produced after treatment with scopolamine methyl nitrate or by injections of scopolamine HBr administered immediately after training. These results suggest that scopolamine, present in the central nervous system during training or within the first few moments thereafter, modifies the formation of the memory trace in such a way that memory is not available for recall for a period of weeks.     

Learned suppression of stereotypy in amphetamine-treated rats: implications for understanding tolerance to amphetamine 'anorexia'

The purpose of this study was to determine whether amphetamine-treated rats can learn to suppress stereotyped movements in order to feed. Rats implanted with cannulae were reinforced with intraoral infusions of milk for holding their heads stationary within a narrow area of space defined by intersecting photobeams. Four of six rats given chronic injections of amphetamine (2mg/kg) learned the response. The amount of milk ingested as a result of the infusions increased over trials at a rate that was comparable to that of rats given milk in bottles. Despite the development of such 'tolerance', analysis of the temporal distribution of photobeam interruptions revealed residual effects of the drug. Specifically, amphetamine-treated rats had longer latencies to initiate infusions and displayed a more fragmented pattern of responding than did saline controls. These results demonstrate that rats can learn to inhibit amphetamine-induced sterotypy and support the view that tolerance to amphetamine 'anorexia' involves learning to suppress stereotyped movements that interfere with feeding. Parallels to the suppression of involuntary movements in humans are noted.     

Development and retention of tolerance to the sedative effects of chlordiazepoxide: Role of apparatus cues

Groups of rats were pretreated for 5 days with chlordiazepoxide (5 to 50 mg/kg) or with control water injections. On the sixth day the rats were given a test dose of chlordiazepoxide (10 mg/kg), or water. The rats that had received 5 days of pretreatment with chlordiazepoxide were significantly less sedated by the test dose than were those given chlordiazepoxide for the first time, i.e. they had developed tolerance. There were no significant differences between the two pretreatment groups in the extent of tolerance. A second experiment examined the effects of associating drug injections with apparatus cues. This had no effect on the development of tolerance, but had a significant effect on its retention: rats pretreated and replaced in their home cages showed complete recovery from tolerance after two drug-free days, whereas those placed in the apparatus after each day's injection retained some tolerance even after two drug-free weeks.     

Possible role of brain dopaminergic systems in the memory effects of central stimulants

In experiments on male Wistar rats trained and tested for memory retention on an active avoidance task in a maze, it was found that haloperidol at doses of 0.2 and 2 mg/kg injected intraperitoneally one hour prior to or immediately after training impaired learning and/or memory. The central stimulants caffeine (20 mg/kg), strychinine (1 mg/kg) and amphetamine (1 mg/kg) injected intraperitoneally immediately after training improved long-term retention; the central stimulants administered at the same doses but in combination with haloperidol (2 mg/kg) after training did not manifest their retention-facilitating effect. It is assumed that dopaminergic mechanisms underlie learning and memory processes in active avoidance conditioning and that an optimum activity of these mechanisms is necessary for the memory-facilitating effect of the central stimulants to appear.     

Effects of naltrexone on amphetamine-induced locomotion and rearing: acute and repeated injections

 The present experiment investigated the ability of the opiate receptor antagonist naltrexone to block the increased locomotion and rearing produced acutely by amphetamine as well as the sensitization of these responses produced when this drug is administered repeatedly. Rats in different groups received an injection of amphetamine (1.5 mg/kg, IP) or saline preceded 30 min earlier by an injection of naltrexone (0, 0.5, 1.0, 5.0 or 10.0 mg/kg, IP). Naltrexone dose-dependently reduced the rearing but had no effect on the locomotion produced by this dose of amphetamine. The locomotion and rearing observed following saline were not affected. This pattern of results was observed following each of six additional pairs of injections, one pair of injections given every third day. Once, soon (2–4 days) and once, long (9–12 days) after the last injection, all animals were injected with amphetamine (0.75 mg/kg, IP) in the absence of naltrexone (tests for sensitization). Animals having been pre-exposed to amphetamine preceded by naltrexone showed no evidence of sensitized rearing on either test, indicating that naltrexone blocked sensitization of this response to amphetamine. These animals, however, exhibited sensitized locomotion on both tests. These results suggest an important but complex role for dopamine-opioid interactions not only in the production of acute locomotor responding to amphetamine but also in the sensitization of locomotor responding when this drug is administered repeatedly. The present findings also suggest that amphetamine-induced rearing is more dependent than locomotion on neuronal mechanisms involving dopamine-opioid interactions. Received: 12 March 1996 / Final version: 2 January 1997     

Tolerance,withdrawal, and supersensitivity to dopamine mediated cues in a drug-drug discrimination

Rats were trained to discriminate between 0.25 mg/kg amphetamine (AMPH) and 0.03 mg/kg haloperidol (HAL) in a two-lever drug discrimination task. In order to test for a drug-induced withdrawal state, animals were assigned to one of three chronic treatment groups and given injections of AMPH, HAL, or distilled water (DW) for 10 consecutive days. Subjects from each treatment condition were then tested at 24, 48, or 72 h after the final injection. At the 24 h retest interval, subjects injected with AMPH responded as though administered an acute dose of HAL (0.028 mg/kg) and subjects injected with chronic HAL responded as though administered an acute dose of AMPH (0.15 mg/kg). By 72 h choice behavior had returned to pretreatment values. To determine whether the rebound observed after 10 days of drug treatment was present after a single injection, independent groups of subjects were injected with single doses of either 10 mg/kg AMPH or 1.0 mg/kg HAL and then retested from 4 h to 48 h later. Single doses of both AMPH and HAL produced significant rebounds that peaked between 20 h (AMPH) and 24 h (HAL) following administration. In a third experiment, animals were tested with or without acute doses of drug following pretreatment with either HAL or AMPH. Receptor supersensitivity accounts for the tolerance observed to HAL 24 h after treatment with 1.0 mg/kg HAL, whereas receptor subsensitivity accounts for the tolerance observed 20 h after treatment with 10 mg/kg AMPH.Some of the data presented here was presented at the meeting of the Society for Neurosciences, New Orleans, 1991.