What is a "low dose" of d-amphetamine for inducing behavioral effects in laboratory rats?

BACKGROUND: Because of the amphetamines' abuse potential and capability of exacerbating or inducing mood and psychotic disturbances, investigations of the behavioral effects of amphetamines commonly involve non-human animals, with the laboratory rat being by far the most common species used. Although investigators of the behavioral effects of amphetamine in rats sometimes refer to doses used as being "low", "moderate", "high", etc., it is not clear in what sense these terms apply. OBJECTIVES: To develop an operational definition of a low dose of amphetamine in rats, we reviewed studies that assessed the behavioral effects of dextroamphetamine (d-AMP) in rats in which some subset of doses, administered SC, IM or IP, was described as being "low". We then used the results of these studies to establish what the lowest effective dose ranges were across a variety of behavioral domains and compared these doses and their effects with those obtained with normal, healthy adult humans. RESULTS: While the range of the lowest doses used in the studies with rats was quite broad (0.025-2.0 mg/kg), the median lowest effective doses observed (in the studies using doses of 0.125 mg/kg or less) were between 0.125 and 0.165 mg/kg across the behavioral domains of consummatory behavior, unconditioned or spontaneous behavior, learned behavior, and drug discriminative control. This range of doses was also found to be comparable to the lowest behaviorally effective doses of d-AMP (SC or PO) in normal human adults, which suggests that the sensitivity to the behavioral effects of amphetamine in these two species is fairly comparable. CONCLUSIONS: Because of their ability to alter a wide variety of behaviors in rats, we conclude that low doses of d-AMP are in the 0.1-0.4 mg/kg range. Doses within this range typically: 1) constitute the ED50 in most drug discrimination/generalization procedures; 2) increase a variety of consummatory behaviors; 3) increase a variety of unconditioned or spontaneous motor activities; 4) increase low rate schedule-controlled behavior while exerting variable effects on high rate schedule controlled behavior; and 5) improve performance on some choice tasks, particularly those requiring sustained attention. Our analyses also indicate that, with respect to behavior, investigators do not always agree on what constitutes a low dose of amphetamine in rats and that doses assumed to be low for this species often are relatively high.

A verification of psychostimulant-induced improvement in sustained attention in rats: effects of d-amphetamine, nicotine, and pemoline.

Previous studies from this laboratory have demonstrated that a variety of psychostimulant drugs can improve the performance of rats trained in a 2-choice stimulus detection task in which the correct responses are indicated by a briefly illuminated light. To enhance the construct validity of the task for assessing sustained attention, the procedure was modified so that the precue interval across trials varied unpredictably between 3, 7, and 11 s. After training rats (N = 17) so that their baseline accuracy levels stabilized between 75% and 88% correct, their performance was assessed after administration of d-amphetamine (0.125-0.75 mg/kg sc), nicotine (0.25-0.75 mg/kg sc), and pemoline (5.0-30.0 po). At certain doses all 3 drugs induced performance improvements in mean choice accuracy and choice response time. Because the precue intervals varied unpredictably and the cue durations used to maintain the rats' baseline accuracy levels were typically short (range = 70-500 ms), the task conforms to most conditions typically required for assessing sustained attention. Results verify the proposal that psychostimulant drugs can enhance the attentiveness of animals in a fashion similar to that documented in humans.

Nicotine enhances stimulus detection performance of middle- and old-aged rats: a longitudinal study.

The effects of nicotine on sustained attention were tested in F344xBN male rats when they were chronologically middle and old aged. The rats (n = 11) were trained in a two-choice, stimulus detection task in which a press of one of two levers was reinforced with food, with the correct lever indicated by the position of a briefly illuminated light. They were tested when they were 24-25 and 34-35 months of age (i.e., at 60-68% and 85-95%, respectively of their expected median life span) after saline or 0.1-0.5 mg/kg doses of nicotine (SC). A significant dose-related improvement in percent correct choices and decrease in choice response times was found at both ages, and there was no significant main effect of age or an age by dose interaction. These results support the position that nicotine can enhance attentional processes in rats throughout their life span. Nicotine and other nicotinic agonists may have efficacy in the treatment of disorders such as Alzheimer's disease.

Facilitation of stimulus detection performance of rats with d-amphetamine: a function of dose and level of training.

Conditions under which amphetamine may facilitate stimulus detection task choice performance in rats were investigated. Rats (n=15) were trained in a two-choice, light-detection task to three successively more stringent criterion levels of task training (minimal, intermediate, and extended) and then tested after administration of saline, 0.25, 0.50, and 0.75 mg/kg d-amphetamine (AMP). For each training level, baseline levels of choice accuracy were maintained at approximately 82% by manipulating the animals' cue duration. No aspect of performance was enhanced by any dose of AMP after minimal criteria training, and there was a dose-dependent decrease in the number of trials completed. After the intermediate level of training, the 0.25 mg/kg dose of AMP reliably increased choice accuracy, there was no reliable change in choice reaction time, and there was a dose-dependent decrease in the number of trials completed. After the extended training, the 0.25 mg/kg dose of AMP reliably increased choice response accuracy, the 0.25 and 0.50 mg/kg doses of AMP reliably decreased choice reaction time, and there was no reliable change in the number of trials completed at any dose of AMP. These results support the contention that psychostimulants can facilitate the choice performance of rats in stimulus detection tasks if an appropriately low dose is used and the animal's behavior is strongly controlled by the stimulus-reinforcement contingencies of the task.

Fluoxetine alters the effects of cocaine on vigilance task performance of rats.

The potential modulating influence of fluoxetine (FLU) on the effects of cocaine (COC) on vigilance task performance was explored. Rats were trained in a discrete-trial task in which pressing one of two levers was reinforced by food, with the correct lever indicated by the position of a briefly illuminated light. Tests with FLU (2.5, 5.0, and 10.0 mg/kg) revealed no significant effects of FLU on mean choice accuracy, log choice reaction time, or number of omission trials; however, 10.0 mg/kg FLU did produce a small but reliable increase in mean log food retrieval latency. The effects of COC (1.25, 2.5 and 10.0 mg/kg) were then assessed following pretreatment with saline or FLU (2.5 and 10.0 mg/kg). COC increased mean choice accuracy, with maximal performance enhancement occurring with 2.5 mg/kg COC; mean log choice reaction time was decreased by all three doses of COC, although there was considerable within-subject variability at the 10.0 mg/kg dose of COC. Pretreatment with FLU reversed both of these performance-enhancing effects of COC in a dose-dependent fashion. Log food retrieval latency was not significantly affected by the lower doses of COC and was significantly increased by the 10.0 mg/kg dose of COC. FLU and COC appeared to interact in an additive fashion with this measure. The number of omission trials per session was significantly increased by 10.0 mg/kg COC--an effect that was not significantly altered by FLU pretreatment. These results support previous studies indicating that acute FLU exposure can reduce many of COC's properties that may contribute to COC's efficacy as a positive reinforcer.

Differential effects of d-amphetamine on vigilance in younger and older male rats.

After training to comparable levels of performance on a two-choice, discrete-trial vigilance task, younger (9 mo) and older (26 mo) male F344xBN rats were tested after SC injections of d-amphetamine (0.125, 0.25, 0.50, and 1.0 mg/kg). Relative to their saline treatment performance levels, both groups exhibited decreases in choice latencies under the lower doses of amphetamine and an increase in food retrieval latencies after 1.0 mg/kg amphetamine. The percentage of correct responses in the older animals was lower than in the younger animals at all doses of amphetamine, and the groups differed significantly at the 0.25 and 0.50 mg/kg doses. There were no significant differences between the groups in either of the latency measures at any of the doses of amphetamine. These results suggest, as has been demonstrated with cocaine, that the alertness-altering properties of amphetamine are qualitatively different in older and younger adult organisms.

Aging changes the effects of cocaine on vigilance task performance of rats.

To investigate age-related behavioral changes with the administration of cocaine, young (6-10 mo), middle-aged (12-18 mo), young-old (21-24 mo), and old (25-36 mo) rats that had been trained on a 2-choice, discrete trial vigilance task were tested under different doses of cocaine. The young and middle-aged rats exhibited significantly increased accuracy and decreased choice latencies following 2.5 mg/kg cocaine. A 15.0 mg/kg dose increased variability in these measures and increased food retrieval latencies. The young-old and old rats exhibited no significant changes in accuracy or choice latency to 1.25, 2.5, and 5.0 mg/kg cocaine; however, a dose of 15.0 mg/kg significantly reduced accuracy in both these groups and increased choice latencies in the old animals. Also, doses of 5.0 and 15.0 mg/kg tended to increase food retrieval latencies more in the two older groups than in the two younger groups. These results indicate that, rather than a simple increase or decrease in sensitivity, there is a qualitative change in cocaine's behavioral effects as rats age. These findings may reconcile long-standing discrepancies in the literature regarding age-related changes in the behavioral effects of amphetamine, with actions and effects very similar to those of cocaine.

Cocaine and vigilance task performance of rats: effects of delay of reinforcement.

In two experiments rats were food-reinforced for pressing one of two levers in an operant chamber, with the correct lever being indicated by the position of a briefly illuminated light. In Experiment 1 the levers were always in the chamber, whereas in Experiment 2 the levers were inserted into the chamber immediately after cue light termination and withdrawn immediately after a choice response. The rats were tested under four conditions: after an injection (SC) of saline or 2.5 mg/kg cocaine and with delay of reinforcement (DOR) of either 0 or 8 s. In both experiments, cocaine enhanced accuracy under the 0-s DOR condition. However, in neither experiment was there evidence of facilitation with cocaine under 8-s DOR, which by itself increased choice latencies and decreased accuracy when choice latencies exceeded 0.5 s. These results indicate that cocaine may only enhance performance in vigilance tasks under constrained conditions, e.g., those that require minimal levels of information processing.

Cocaine and level of arousal: effects on vigilance task performance of rats.

Rats were food-reinforced for pressing one of two levers in an operant chamber, with the correct lever being indicated by the position of a briefly illuminated light. After stable accuracy levels were achieved, the rats were tested after an injection of either saline or cocaine (2.5 mg/kg) under two conditions. In the "low arousal" condition, animals were tested during the light phase of a 12-hr light-dark cycle and were fed approximately 5 hr prior to testing. In the "high arousal" condition, animals were tested during the dark phase after approximately 28-hr food deprivation. As expected, accuracy was higher and median choice and food retrieval latencies were shorter under the high arousal condition. Contrary to predictions, cocaine enhanced accuracy under both conditions. These results indicate that cocaine-enhanced performance in some tasks is not necessarily dependent on the animal performing at suboptimal arousal levels.

Effects of cocaine and d-amphetamine on sustained and selective attention in rats.

The effects of cocaine and d-amphetamine were compared in two attention-loading tasks. Cued by the position of a light, rats were food-reinforced for pressing one of two levers in a 2-choice, discrete-trial procedure. In the "sustained attention" task, the cue light was illuminated for a brief period (1.8 sec or less) at the beginning of each trial. In the "selective attention" task, the cue light remained on until a level press, while a blinking light over the incorrect lever served as a distractor. In the sustained attention task, low doses of d-amphetamine (0.25 mg/kg SC) and cocaine (2.5 mg/kg SC) enhanced accuracy; some doses of d-amphetamine (0.75 mg/kg SC) and cocaine (1.25 and 2.5 mg/kg SC) also reduced choice latencies. In the selective attention task, the lower doses of these drugs had no effect on accuracy, the highest dose of d-amphetamine (1.25 mg/kg SC) disrupted accuracy, and all doses of the drugs reduced choice latencies. The time to retrieve food was increased in a dose-dependent fashion by both drugs in both tasks. These results indicate that, other than differences in potency, cocaine and d-amphetamine induce similar behavioral effects in attention-loading tasks, with improvement or interference with performance dependent on the dose and the type of attention demanded of the task.

Effects of naltrexone, and d-amphetamine, and their interaction on the stimulus control of choice behavior of rats.

The hypothesis that endogenous opioid peptides modulate attentional processes was tested. The effects of the opioid antagonist naltrexone (NALT), d-amphetamine (AMP), and their interaction were investigated in rats trained in a two-choice task in which the position of a short-duration light served as a cue for food-reinforced responses. NALT (0.25, 1.0, 5.0, and 10.0 mg/kg) produced no significant changes in performance (accuracy, choice latency, and food retrieval time). As predicted, AMP induced dose-dependent biphasic effects. Low doses of AMP (0.25 and 0.5 mg/kg) significantly enhanced accuracy, decreased choice latency, and lengthened food retrieval time; 1.25 mg/kg AMP disrupted accuracy, increased choice latency, and further lengthened food retrieval time. The combination of NALT (0.25, 1.0, and 10.0 mg/kg) and subthreshold doses of AMP (0.07 and 0.1 mg/kg) had no effect on performance except for an increase in food retrieval time with 10.0 mg/kg NALT, whereas the combination of NALT and moderate doses of AMP (0.5 and 1.0 mg/kg) disrupted accuracy, increased choice latencies, and lengthened food retrieval time. These results do not support the hypothesis that endogenous opioid peptides play a vital role in attentional processes or that opioid antagonists may be useful in the treatment of attentional deficit disorders.

Comparison of the effects of morphine and immobilization stress on discrimination performance of rats.

The effects of morphine and those of immobilization stress (IMS) on performance of rats in discrete-trial, two-choice discrimination tasks were compared. In Experiments 1 and 2, two shocks of different intensities were discriminative stimuli (SD) for choices in a T-maze. In Experiments 3 and 4, responses were right and left lever presses in an operant chamber, where correct responses were signaled by either shocks of different intensities (shock discrimination task) or by the position of a brief light flash (light discrimination task). In Experiment 1, both 70-min IMS and 5 mg/kg morphine induced greater disruptions in trials in which the higher shock was the SD, and there were no significant differences between treatments. However, in Experiment 2, only the effects of morphine were blocked by 10 mg/kg naltrexone. In Experiment 3, the effects of IMS and morphine were not selective with respect to SD shock level in the shock discrimination task, nor were they task (shock vs. light) selective. Also, the effect of morphine was significantly greater than the effect of IMS in both tasks. The results of Experiment 4 indicated that effects induced by actual alterations in the intensities of the SD shocks did not mimic those induced by either morphine or IMS but that abrupt decreases in the duration of the SD lights induced effects similar to those of morphine. These studies indicate that different mechanisms mediate the effects of IMS and morphine and much of their behavioral effects in shock discrimination tasks are due to dissociative processes rather than alterations in perceived shock intensity.

Acute morphine dependence: effects observed in shock and light discrimination tasks.

Alterations in shock discrimination accuracy in the rat, indicative of hyperalgesia, have been noted 1-3 days following a single injection of morphine. To establish the extent to which these "withdrawal-like" effects were specific to the shock discrimination paradigm, rats were trained in two separate discrimination tasks. The discriminative stimuli (SD,s) for correct lever presses were mild electric shocks of different intensities in one task and were short duration lights over the levers in the other. After achieving comparable accuracy levels in the two tasks, the animals were injected SC with 30 mg/kg morphine sulfate and performance levels assessed 1, 2, 3, and 7 days later. Shock discrimination accuracy was significantly enhanced on post-morphine day 2, while accuracy in the light position task was not significantly affected on any of the post-morphine test days. The results indicated that increased pain sensitivity, as well as other signs of dependence, can occur following acute exposure to morphine.

How do tranquilizing agents selectively inhibit conditioned avoidance responding?

A number of tranquilizing agents have been shown to inhibit conditioned avoidance responses (CAR) at doses that do not interfere with escape responses (ER). To test the hypothesis that this selective action may be due to differential response strengths of the two responses, rats were trained to press retractable levers in an operant chamber either to avoid a 0.5 mA shock during a 5.0-s warning period or to escape from a low-intensity shock within 5.0 s. The intensity of the latter shock was adjusted for each animal so that CAR and ER were comparable in terms of probability of occurrence and latency. While doses of chlorpromazine, clonidine, diazepam, and morphine that reduced CAR by 30%-50% did not significantly affect high-shock ER, i.e., ER on CAR trials where no CAR occurred, they interfered with low-shock ER to the same degree as CAR. These and other results suggest that the selective blockade of the CAR by these drugs in the CAR paradigm is primarily due to differential strengths of the CAR and ER. They also support studies concluding that tranquilizing drugs reduce avoidance because of a deficit in the ability to initiate motor responses, rather than interfering with associative processes or reducing situation-induced emotional reactions. However, the finding of a small differential effect, at least with chlorpromazine, on CAR and low-shock ER across trials within sessions indicated that different mechanisms may be involved in the suppression of these two responses.

Effects of immobilization stress on shock discrimination performance in rats.

It has been suggested that acute stress induces analgesia in rats and mice. However, stress can have a myriad of debilitating effects on organisms, and it is not clear whether stress decreases the perceived intensity of the painful stimulus (i.e., a change in sensitivity) or decreases the willingness or ability of the organism to carry out the appropriate bodily movement (i.e., a change in responsivity). Since traditional animal analgesia tests confound sensitivity with responsivity, we assessed the effects of immobilization stress on rats trained on a two-choice, two-shock discrimination task, in which the dependent measures are presumably less reflective of alterations in pain responsivity. Immobilization stress significantly disrupted shock discrimination performance, with the disruption occurring primarily in choice performance signalled by the higher intensity shock discriminative stimulus. These effects completely dissipated with successive daily exposures to stress. The data suggest that immobilization stress has a specific effect on pain sensitivity which is separate from its possible effects on pain responsivity.

Morphine dependence in rats assessed in a shock discrimination task.

The effects of chronic morphine sulfate (5 mg/kg/day for 14 days) on shock discrimination performance of rats was assessed in Experiment 1. Significant tolerance developed to the disruptive effects of morphine on performance. A significant increase in discrimination performance was found 48 h after the last injection in tests conducted without the injection environment cues present. Ten days after the chronic drug regimen, tests conducted 30 min after exposure to the injection-environment cues revealed no differences between animals previously administered morphine and control animals administered saline. In Experiment II, shock discrimination performance was assessed in a separate group of rats after exposure to a single injection of morphine sulfate (30 mg/kg), which eliminated associative processes, e.g., Pavlovian or instrumental conditioning, as factors in the subsequent behavioral tests for hyperalgesia. Significant changes in discrimination performance (primarily enhanced percent correct to the high shock stimulus) indicative of increased pain sensitivity were obtained in tests conducted 1, 2, and 3, but not 9 days after the injection. These experiments indicate that increased pain sensitivity, as opposed to hyperresponsivity operationally measured in traditional analgesia tests (e.g., hot plate, jump-flinch, and tail flick), is a component of morphine withdrawal of a nonassociative origin.

Effects of morphine, d-amphetamine, and pentobarbital on shock and light discrimination performance in rats.

The effects of 2 and 4 mg/kg morphine sulfate, 0.5 and 1 mg/kg d-amphetamine sulfate, and 6 and 12 mg/kg pentobaribital sodium were tested in rats in two different discrete-trial two-choice discrimination tasks. The discriminative stimuli for one task were high and low intensity shocks. In the other, correct choices were signaled by the position of a brief light flash. Morphine (4 mg/kg) significantly disrupted performance of both tasks, with more reliable disturbance occurring in the shock discrimination animals. Pentobarbital (12 mg/kg), while exerting noticeable effects on gross motor behavior, had little effect on discrimination performance; d-amphetamine (1 mg/kg) was disruptive of discrimination performance in only some animals. The results indicate that much of the effect of relatively low doses of morphine on the shock discrimination performance of rats may be due not to its putative specific antinociceptive properties, but to alterations in conceptual-judgmental processes or decreases in motivation (e.g., hunger) unrelated to pain.