Environmental influences on the development of tolerance to the effects of physostigmine on schedule-controlled behavior

Extra (non-contingent) food does not affect behavioral tolerance to d-amphetamine's rate-decreasing effects

Tolerance refers to the diminished effect of a drug following its repeated administration such that a larger dose is needed to obtain the initial effect. Tolerance to a drug's effects on operant behavior is more likely to develop when initial drug administration results in a loss of reinforcement. It remains unknown how offsetting loss of reinforcement influences the development of tolerance. Providing extra (non-contingent) food pellets was hypothesized to impede the development of tolerance to effects of a repeatedly administered dose of d-amphetamine that reduced the number of food pellets earned by rats. A multiple schedule with two variable-interval (VI) 60-s components resulting in food delivery was used to maintain lever pressing. Extra food pellets were provided in one of those components according to a variable-time (VT) 120-s or a VT 30-s schedule for separate groups of rats (n=6 for each group). Effects of d-amphetamine (0.1-3.0mg/kg) were tested before (acute) and during (chronic) injections (45days) of an individually selected, repeatedly administered dose that reduced the number of food pellets earned by at least 50% compared to when saline was tested acutely. There was a dose-dependent decrease in lever-press rates and food pellets earned. The development of tolerance was quantified by dividing the area-under-the-curve (AUC) of the chronic dose-effect function by the AUC of the acute dose-effect function. Tolerance developed to a similar extent in components with and without extra food pellets for rats in both groups. These results indicate that offsetting reinforcement loss, at least as studied here, did not differentially affect tolerance development.

Evaluation of miosis, behavior and cholinesterase inhibition from low-level, whole-body vapor exposure to soman in African green monkeys (Chlorocebus sabeus)

BACKGROUND

Relatively little is known about the effects of very low-level exposures to nerve agents where few signs or symptoms are present.

METHODS

African green monkeys (Chlorocebus sabeus) (n = 8) were exposed for 10 min, whole-body, to a single concentration of soman (0.028-0.891 mg/m³).

RESULTS

EC₅₀ values for miosis were determined to be 0.055 mg/m³ and 0.132 mg/m³ when defined as a 50 percent reduction in pupil area and diameter, respectively. In general, performance on a serial probe recognition task remained unchanged at lower concentrations, but responding was suppressed at the largest concentration tested. Soman produced concentration-dependent inhibition of acetylcholinesterase activity and, to a lesser extent, butyrylcholinesterase activity.

CONCLUSIONS

These results characterize threshold soman exposure concentrations that produce miosis in the absence of other overt signs of toxicity and extend previous studies indicating that miosis is a valuable early indicator for the detection of soman vapor exposure.

Repeated post- or presession cocaine administration: roles of dose and fixed-ratio schedule

Effects of repeated administration of cocaine to animals behaving under operant contingencies have depended on when the drug is given. Moderate doses given presession have generally led to a decrease in the drug's effect, an outcome usually referred to as tolerance. When these same doses have been given after sessions, the usual result has been no change or an increase in the drug's effects, with the latter usually referred to as sensitization. In the present study, repeated postsession administration of a relatively small dose of cocaine (3.0 or 5.6 mg/kg) to pigeons responding under a multiple fixed-ratio 5, fixed-ratio 100 schedule of food presentation generally resulted in tolerance to the rate-decreasing effects of the drug. When the same dose was given before sessions, little additional tolerance was observed, although some subjects showed further tolerance in the small-ratio component. A regimen of repeated postsession injection of larger (10.0-23.0 mg/kg) doses suppressed key pecking during the session; responding resumed following discontinuation of postsession administrations. Effects of postsession administration of cocaine, therefore, depended on the dose, with smaller doses leading to tolerance and larger ones to suppression of behavior during the session. Effects of postsession drug administration of either small or large doses were not related to whether effects of postsession drug were experienced mainly in the operant test chamber or in the pigeon's home cage. The results with large postsession doses are compatible with a view that the drug acted as a Pavlovian unconditional stimulus, with the session-related stimuli acting as a long-duration Pavlovian conditional stimulus. Tolerance following postsession administration of the smaller doses challenges the view that it depended on experiencing the drug's effects while the arranged reinforcement contingencies were in effect.

The combined effects of pyridostigmine and chronic stress on brain cortical and blood acetylcholinesterase, corticosterone, prolactin and alternation performance in rats

Thousands of soldiers who served in the Gulf War have symptoms that have been collectively termed Gulf War Illness (GWI). It has been suggested that a combination of operational stress and pyridostigmine, a drug given as a pretreatment to protect soldiers against the effects of exposure to nerve agents, might have had unexpected adverse health effects causing these symptoms. Our laboratory has previously modeled operational stress in rats using a paradigm of around-the-clock intermittent signalled footshock. In the present studies, this model was used to investigate the potential synergistic effects of chronic stress and pyridostigmine on physiology and behavior. Seventy-two rats were trained to perform an alternation lever pressing task to earn their entire daily food intake. The rats were then implanted with osmotic minipumps containing vehicle, pyridostigmine (25 mg/ml pyridostigmine bromide) or physostigmine (20 mg/ml eserine hemisulfate). The pumps delivered 1 microl/h, which resulted in a cumulative dosing of approximately 1.5 mg/kg/day of pyridostigmine or 1.2 mg/kg/day of physostigmine, equimolar doses of the two drugs. The rats were then returned to their home cages where performance continued to be measured 24 h/day. After 4 days, 24 of the 72 rats were trained to escape signalled footshock (avoidance-escape group) and 24 other rats (yoked-stressed group) were each paired to a rat in the avoidance-escape group. The remaining 24 rats were not subjected to footshock (unstressed group). Shock trials were intermittently presented in the home cage 24 h/day for 3 days, while alternation performance continued to be measured. Since only 12 test cages were available, each condition was repeated to achieve a final n of six rats per group. Pyridostigmine and physostigmine each decreased blood acetylcholinesterase levels by approximately 50%. Physostigmine also decreased brain cortical acetylcholinesterase levels by approximately 50%, while pyridostigmine had no effect on cortical acetylcholinesterase activity. Alternation performance was impaired on the first day of stress and then recovered. Neither pyridostigmine nor physostigmine affected performance in the absence of stress or increased the effects of stress alone. Corticosterone was significantly increased in the yoked stress group compared to unstressed controls. These data suggest that pyridostigmine does not exacerbate the effects of stress on performance or levels of stress hormones. Furthermore, these data do not suggest that stress enables pyridostigmine to cross the blood brain barrier.

Effects of cocaine and morphine under mixed-ratio schedules of food delivery: support for a behavioral momentum analysis

Previous studies have shown that ratio size influenced the development of tolerance under simple and multiple schedules, but not under progressive-ratio (PR) schedules. PR schedules share certain features with mixed-ratio (MR) schedules, and pilot data suggested that ratio size fails to modulate tolerance to cocaine or morphine under MR schedules. The present study examined more comprehensively the pre- and postchronic effects of cocaine and (in separate birds) morphine under MR schedules with fixed-ratio (FR) 5 and FR 95, FR 25 and FR 75, and FR 50 and FR 50 components. Acute doses of cocaine and morphine initially were given in an ascending series (beginning with 0.56 mg/kg) until responding was reduced to near-zero levels. Chronic (daily) dosing with a dose that reduced, but did not eliminate, responding then occurred until response rates stabilized. Finally, postchronic dose-response determinations were conducted. Both cocaine and morphine reduced response rates at all FR values. Tolerance was consistently observed to the effects of morphine, but not to those of cocaine. With both drugs the degree of tolerance observed did not vary as a function of FR value. These findings, like those obtained under PR schedules, indicate that ratio size does not always modulate drug tolerance. A behavioral momentum analysis of drug action appears to account for whether or not ratio size modulates tolerance, and such an analysis is provided.

Specificity of effects of chronically administered diazepam on the responding of rats under two different spaced-responding schedules

Lever pressing of rats was maintained in different chambers during two different sessions each day. At 0900 h, responding was maintained under a two-component multiple schedule in which responses initiated an interval that had to elapse before delivery of food (time delay of 20 s and 40 s). In this schedule, a 'response-pause' sequence preceded reinforcers, and acutely administered diazepam only decreased responding. At 1400 h, responding by the same subjects was maintained under a different two-component multiple schedule, in which individual responses initiated an interval that had to be terminated by another response before delivery of food (DRL 20 s and 40 s). In this second schedule, a 'response-pause-response' sequence preceded reinforcers, and acutely administered diazepam increased responding. After studying the acute behavioral effects of diazepam during each separate 'timing' schedule, animals systematically received 1.7 mg/kg per day diazepam 2-5 min prior to their different schedule components, in order to study the influence of reinforcement contingency on the chronic effects of this drug. Diminution of the initial effects of diazepam during daily drug administration prior to DRL 20 s responding did not extend to DRL 40 s responding or to time-delay responding, and tolerance did not develop at all for time-delay responding. When diazepam was again administered after all the daily schedules for approximately 1 month, and then given before the individual DRL schedules, DRL responding was increased again as it had been prior to chronic drug administration. These results suggest that the behavioral effects of acutely administered diazepam are influenced by different 'timing' requirements, and that the behavioral effects of chronically administered diazepam are influenced by 'timing' requirements and by drug- and chamber-related stimuli.

Behavioral comparison of the oximes TMB-4, 2-PAM, and HI-6 in rats using operant conditioning

It has recently been shown that oximes can amplify the ability of cholinesterases to scavenge organophosphorus (OP) agents. Since both OP agents and oximes can disrupt performance, behavioral evaluation of bioscavenger therapies using oximes can be hindered. Therefore, we investigated the ability of three oximes, administered alone, to disrupt performance. The effects of trimedoxime bromide (TMB-4) (3.16-56.2 mg/kg), pralidoxime chloride (2-PAM) (10.0-237.1 mg/kg), and, 1-([[4-amincarbonyl)pyridino]-methoxy]-methyl)-2, 4-bis[(hydroxyimino)methyl] pyridinium dichloride monohydrate (HI-6) (10.0-237.1 mg/kg) were evaluated in rats using a variable-interval 56 (VI 56) s schedule of food reinforcement. Under control conditions, the VI 56 s schedule produced a constant rate of responding (i.e., lever-pressing). All three oximes produced dose-dependent decreases in responding, and the largest doses of TMB-4 and 2-PAM produced complete or nearly complete suppression of responding in all rats. Only the largest dose of HI-6 suppressed responding. Analysis of the dose-effect functions demonstrated that TMB-4 was substantially more potent than 2-PAM, which was slightly more potent than HI-6, for producing response suppression. These results establish doses of each oxime that will not contribute to disruption of responding, and thus, facilitate future evaluation of bioscavenger therapies against OP toxicity.

Differential effects of clozapine and pimozide on fixed-ratio responding during repeated dosing

Previous research has shown that the differential development of tolerance to the disruption of operant responding produced by repeated dosing with pimozide (PMZ) or clozapine (CLZ) can distinguish these two drugs. In the present study, the effects of PMZ (1 mg/kg) and CLZ (10 mg/kg) on response rate and response duration in rats lever pressing for food reward under a fixed-ratio 30 (FR-30) operant schedule were examined. PMZ suppressed response rates across all 10 days of drug dosing; CLZ produced an initial response rate decrease, with partial recovery (50%) occurring within the 10 day period. Similarly, PMZ produced an increase in response duration that persisted into the postdrug vehicle-injection period, while CLZ did not significantly change response duration. The prolonged suppression of FR responding produced by PMZ is similar to the lack of tolerance to this drug in other types of operant schedules. In contrast, CLZ's effects on response rate are schedule dependent. These results suggest that changes in response duration with repeated dosing may more reliably differentiate typical and atypical neuroleptics than do changes in response rate under FR schedules.

Effects of reinforcer limitations on fixed-ratio responding during repeated administration of chlorpromazine

Key-pecking was maintained under a 30-response fixed-ratio schedule of food presentation and pigeons received 30 mg/kg/day chlorpromazine immediately prior to experimental sessions. After responding stabilized during chronic drug and animals were receiving all available reinforcers, the time available for receiving reinforcers was systematically varied. When that time was decreased, animals initially received fewer reinforcers but responding subsequently increased to an extent that animals once more received all available reinforcers. When that time was again increased, responding once again decreased but only to an extent that animals continued receiving all available reinforcers. During subsequent reversals of the time for reinforcer availability, responding increased or decreased to an extent always resulting in the maximum number of reinforcers. When drug was discontinued, responding returned to predrug levels. These results demonstrate that effects of chronically administered chlorpromazine were influenced by both drug activity at its site of action and by the behavioral process of reinforcement.

Effects of physostigmine and scopolamine on rats' performances in object-recognition and radial-maze tests

The effects of physostigmine and scopolamine were evaluated on working memory of rats in object recognition and radial-maze tests. Three doses of physostigmine hemi-sulfate (Phys: 0.05, 0.10 and 0.20 mg/kg), five doses of scopolamine hydrobromide (Scop: 0.125, 0.25, 0.5, 1.0 and 2.0 mg/kg), and one dose of scopolamine methylbromide (Mscop: 2.0 mg/kg) were used. In object recognition test, rats were submitted to three or four intertrial delay conditions (1-min, 15-min and either 60-min or 24-h). The higher doses of Scop (1.0 and 2.0 mg/kg) in 1-min and 15-min delay and of Phys (0.20 mg/kg) in 1-min delay impaired discrimination between new and familiar objects. Mscop impaired discrimination between objects in 60-min but not in 1-min and 15-min delay. This effect may be state dependent. Radial-maze learning was impaired by the lower doses of scopolamine (0.25 and 0.50 mg/kg) which had no effect in object recognition test. These results show that in our conditions, object recognition is less sensitive than radial-maze test to cholinergic drugs.

Situational specificity of tolerance to effects of phencyclidine on responding of rats under fixed-ratio and spaced-responding schedules

Responding of rats (n = 5) was maintained under DRL (lever) and Time-Delay (nose-key) schedules of food presentation in different experimental chambers during two separate daily sessions. Tolerance that developed to rate-decreasing effects of phencyclidine for nose-key pressing under the Time-Delay schedule did not extend to effects of phencyclidine on lever pressing under the DRL schedule. In a second experiment, both lever and nose-key pressing of rats were maintained under individual and multiple fixed-ratio schedules. One group of animals (n = 5) experienced both the individual and the multiple schedules in the same experimental chamber and another group (n = 5) experienced the individual and the multiple schedules in different experimental chambers. Tolerance that developed to behavioral effects of phencyclidine during the individual schedule did not extend to responding on even the same manipulandum under the multiple schedule in a different experimental chamber. In contrast, tolerance that developed to behavioral effects of phencyclidine during the individual schedule did extend to responding on even the different manipulandum under the multiple schedule in the same experimental chamber. Thus, tolerance that developed in the environment that was coincident with the pharmacologic actions of phencyclidine did not extend to similar operants in a different environmental condition, but did extend even to a different operant and schedule context in the same environmental condition.

Effects of shock intensity on observed tolerance to decreased avoidance responding by clonidine

Interruption of a photobeam by rats was maintained under a Sidman avoidance schedule, and moderate response rates were maintained at low frequencies of electrical stimulation. After acute injections of clonidine, responding decreased, and frequency of electric stimulation increased, in a dose-dependent manner. At a lower intensity of electric stimulation, response-suppressive effects of clonidine did not diminish for up to 40 sessions with daily administration of clonidine. At a higher stimulus intensity, however, response-suppressive effects of clonidine diminished within 15 sessions, and stimulus frequency was at control level after 40 sessions with daily administration of clonidine. Behavioral consequences altered the effects of chronic clonidine so that tolerance was observed at a higher, but not a lower, intensity of electric stimulation.

Relationship of the behavioral effects of aprophen, atropine and scopolamine to antagonism of the behavioral effects of physostigmine

Behavioral effects of aprophen, atropine and scopolamine, in rats, were examined under a multiple schedule of food presentation and at different injection-test times. The effects of the varied treatments were compared to the ability of the drugs, under identical conditions, to prevent the behavioral effects of the anticholinesterase, physostigmine. Potencies of the antagonists to decrease response rates varied across three log units. All three antagonists produced dose-related attenuation of the response suppressant effects of physostigmine. In general, aprophen was a better antagonist than scopolamine or atropine. It blocked behavioral effects of physostigmine across a wider range of doses than the other compounds, and did so with less behavioral disruption. Although substantial differences between the three antagonists were observed, the behavioral effects of all three antagonists (when administered alone) were positively correlated with their efficacy as antagonists of the response suppressant effects of physostigmine.

Effects of fixed-ratio requirement on observed tolerance to decreased responding by clonidine

Responding of rats was maintained under either a 10- or a 40-response fixed-ratio schedule, and "local" rates of responding were 0.29-0.37 responses per sec for both schedules. Clonidine decreased responding for both schedules in a similar and dose-dependent manner, and the largest dose tested (0.3 mg/kg) completely suppressed behavior. When 0.1 mg/kg was administered immediately prior to 30 daily experimental sessions, FR10 responding recovered to control levels within 15 sessions, whereas FR40 responding recovered only to approximately 60% of control level at asymptote. These results continue to identify boundary conditions for the influence of reinforcer loss on tolerance development, and they emphasize the overriding influence of behavioral processes on observed tolerance to the behavioral effects of drugs.

Effects of azaprophen, scopolamine and trihexyphenidyl on schedule-controlled behavior, before and after chronic physostigmine

The effects of the muscarinic acetylcholine receptor antagonists, azaprophen (0.3-10.0 mg/kg), scopolamine (0.01-3.0 mg/kg) and trihexyphenidyl (0.3-10.0 mg/kg) were examined in rats using a VI 18 s schedule of food reinforcement, before and after chronic physostigmine administration. All three compounds produced dose-dependent decreases [corrected] in the rate of responding. Scopolamine was more potent than trihexyphenidyl which was equipotent to azaprophen. All three compounds antagonized the response rate-decreasing effects of physostigmine in a dose-dependent fashion. Following 43 consecutive daily administrations of physostigmine (0.4 mg/kg), partial tolerance developed to its response rate-decreasing effects. When the three antagonists were again examined (alone and in combination with physostigmine), their effects were generally unchanged. These results further characterize the behavioral effects of azaprophen, scopolamine and trihexyphenidyl. These results also suggest that tolerance to physostigmine's effects can be mediated through behavioral rather than pharmacological mechanisms.