Effects of MK-801 stereoisomers on schedule-controlled behavior in rats.

Behavioral effects of (+)MK-801 (0.03-0.32 mg/kg) and (-)MK-801 (0.32-3.20 mg/kg) were evaluated in rats using a multiple fixed-ratio, fixed-interval (FR20, FI2) schedule of food presentation. Both enantiomers produced dose-dependent decreases in response rate under the FR20 and in this respect (+)MK-801 was approximately ten times as potent as (-)MK-801. Under the FI2 schedule component, the (+) enantiomer produced substantial increases as well as decreases in response rate whereas the (-) enantiomer produced only decreases. When 0.178 mg/kg (+)MK-801 and 1.78 mg/kg (-)MK-801 were administered for 11 consecutive days, tolerance developed to the decrease in response rate under the FR20 schedule component. Tolerance to the effects of the (+) enantiomer under the FI2 schedule component was indicated by progressively larger increases in response rate than those observed during acute administration. These results support potential therapeutic applications of MK-801.

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.

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

The influence of environmental variables on the development of tolerance to physostigmine's effects in rats was examined using multiple fixed-ratio, extinction schedules of food presentation. Initial administration of physostigmine (0.4 mg/kg) produced nearly maximal decreases in the number of food pellets delivered, running response rate, and overall response rate, under multiple FR 10, EXT and multiple FR 50, EXT schedules. With repeated administration, tolerance to physostigmine's effects was observed when 10 responses were required to produce reinforcement but was not observed when 50 responses were required to produce reinforcement. Tolerance under the multiple FR 10, EXT schedule of reinforcement was also observed when physostigmine was administered post-session. When tolerance was acquired, it was retained for up to 25 drug-free days. These results suggest that tolerance to physostigmine's effects on schedule-controlled behavior is strongly influenced by response requirement, independently of physostigmine-induced reinforcement loss. Additionally, tolerance is not dependent on experience with the schedule while under the effects of physostigmine, and is retained for a substantial period of time in the absence of continued physostigmine administration.

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.

Evaluation of neuroprotection and behavioral recovery by the kappa-opioid, PD117302 following transient forebrain ischemia.

The effects of the selective kappa-opioid, PD117302 ((+/-)-trans-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]benzo[b]thiophene-4-acetamide), on transient (15 min) global forebrain ischemia, induced by four-vessel occlusion, was evaluated using a multiple fixed-ratio, fixed-interval schedule of food presentation in rats. The schedule produced distinctive patterns of responding in the fixed-ratio and fixed-interval components. Ischemia produced CA1 hippocampal necrosis and prolonged suppression of responding under both schedule components. When responding resumed, the pattern of responding rapidly returned. Response disruption and CA1 hippocampal necrosis were minimal or nonexistent in sham-occluded rats. Behavioral recovery time under both components of the schedule of reinforcement correlated with CA1 necrosis. On average, CA1 necrosis was less, and behavioral recovery time was shorter, in rats treated with PD117302 postocclusion as compared with vehicle-treated rats. The difference, however, did not reach statistical significance. These results demonstrate the utility of schedule-controlled responding for evaluating potentially therapeutic compounds for the treatment of ischemic injury. These results also further characterize the neuroprotective actions of kappa opioids.

Dose-dependent brainstem neuropathology following repeated arteether administration in rats.

Histopathological effects of the artemisinin antimalarial, beta-arteether, were evaluated in rats. Arteether (3.125-12.5 mg/kg/day, IM, in sesame oil) was administered for 7 consecutive days. Seven days following the last injection, histological evaluation of the brainstem was performed. Rats treated with 12.5 mg/kg showed significant neuropathology, including chromatolysis, in the nucleus trapezoideus and nucleus superior olive. To a lesser extent, neuropathology was present in the nucleus ruber. Mild neuropathology was also detected in other brainstem regions examined. Although no statistically significant neuropathology was found for the groups treated with 6.25 mg/kg/day and 3.125 mg/kg/day, substantial neuropathology was observed in a single rat in each of these treatment conditions. These results confirm and extend previous studies demonstrating brainstem neurotoxicity from artemisinin antimalarials. Furthermore, these results suggest that, in rats, brainstem auditory pathways may be particularly vulnerable. Early detection of arteether neuropathology may, therefore, require examination of auditory functions.

Tifluadom's effects under electric shock titration and tail-immersion procedures in squirrel monkeys.

The analgesic efficacy of the kappa-opioid benzodiazepine, tifluadom, was examined in squirrel monkeys using electric shock titration and tail-immersion procedures. Tifluadom produced dose-dependent increases in the shock intensity that maintained responding under the shock titration schedule without substantially decreasing response rates. Tifluadom also increased the latency of tail-withdrawal from 55 degrees C water. Naloxone attenuated tifluadom's effects under both procedures. These results extend previous reports of tifluadom's analgesic characteristics and suggest that the tail-immersion procedure is a useful analgesic assay in squirrel monkeys.

Tolerance to oxotremorine's effects on schedule-controlled behavior in physostigmine-tolerant rats.

Tolerance to the effects of physostigmine and oxotremorine in rats was evaluated using a multiple fixed-ratio 10, extinction schedule of food presentation. Physostigmine was administered either once daily or three times daily for 18 consecutive days. Tolerance to physostigmine's response decreasing effects was observed under both administration regimens. Cumulative dose-effect functions for oxotremorine (0.0056-0.562 mg/kg) were determined before and after chronic physostigmine administration. Oxotremorine's potency to produce response rate suppression decreased in rats receiving physostigmine three times daily but did not substantially change in rats receiving single daily injections. These results demonstrate that the dose or duration of action of physostigmine can determine whether tolerance to physostigmine's effects is accompanied by cross-tolerance to oxotremorine's effects.

Acute high dose arteether toxicity in rats.

Acute high dose administration of the artemisinin antimalarial, beta-arteether (AE), was evaluated in rats using an auditory discrimination task (ADT) and histology. After rats were trained on the ADT, AE (25, 75, 125 mg/kg, i.m.) or vehicle (sesame oil) was administered and behavioral performance was evaluated for 11 consecutive days. Histological evaluation of the brains was performed using thionine and cupric-silver staining. Damaged cells were counted in specific brainstem nuclei of all rats and a qualitative analysis of the rostral-caudal extent of selected brains was performed. Behavioral performance was not significantly affected by any treatment although some evidence of disruption was observed, particularly after the largest dose. At 125 mg/kg, AE produced statistically significant neuropathology, including chromatolysis, in the nucleus trapezoideus and nucleus superior olive. AE at 75 mg/kg, produced significant neuropathology in the nucleus trapezoideus. Neither AE at 25 mg/kg, nor vehicle produced damage. Qualitative analysis revealed a pattern of neuropathology focused in the brainstem. The results show that, in rats, a single dose of AE can produce a pattern of brainstem neuropathology and that specific brainstem nuclei, including auditory nuclei, are particularly vulnerable. These results are consistent with, and extend, previous studies demonstrating brainstem neurotoxicity from repeated AE administration. Moreover, early detection of AE-induced neuropathology is problematic and may require selective examination of brainstem functions.

Perceptual bisection in rats: the effects of physostigmine, scopolamine and pirenzepine.

The effects of cholinergic drugs on three different perceptual bisection tasks were studied in rats. Physostigmine (0.056-0.56 mg/kg), a reversible anticholinesterase, produced dose-dependent decrements in discriminability (A'), but did not affect the bisection point (BP) in visual duration, auditory duration, and auditory intensity bisection tasks. This finding is consistent with results previously obtained in an auditory duration bisection task with an irreversible anticholinesterase, diisopropyl phosphofluoridate. Scopolamine (0.075-0.422 mg/kg), a muscarinic cholinergic-receptor antagonist, produced dose-dependent decrements in both A' and BP in visual and auditory duration bisection tasks. The behavioral antagonism between physostigmine (0.56 mg/kg) and scopolamine (0.075-0.237 mg/kg) was studied in the visual and auditory duration bisection tasks. The BP was not affected by physostigmine alone or in combination with scopolamine, except at the largest dose of scopolamine, which produced a reliable decrement in the BP. A', however, was equally decreased by physostigmine alone and all combinations of physostigmine and scopolamine. Pirenzepine (1, 3 and 10 mg/kg), a selective high-affinity M1 muscarinic antagonist, had no effect on A' or the BP in the duration bisection tasks, suggesting changes in perception produced by muscarinic antagonists do not involve the M1 receptor subtype. The similar drug effects in different sensory modalities (visual and auditory) and perceptual systems (subjective duration and loudness) suggest that cholinergic drugs may affect perceptual mechanisms responsible for sensory coding, such as the output of a neural generator.

Evaluation of transient forebrain ischemia induced by four vessel occlusion using schedule-controlled behavior.

Transient (5-min) global forebrain ischemia, induced by four- vessel occlusion, was assessed using a multiple fixed-ratio, fixed-interval schedule of food presentation in five rats. Under control conditions, the schedule produced distinctive response rates in each schedule component. Initially, ischemia disrupted responding under both schedule components, and to approximately the same degree. In general, total session responses returned to pre-occlusion levels during the course of 45 post-occlusion days, however, response rates under the fixed-interval component showed slightly less recovery than those under the fixed-ratio component. Histological assessment revealed considerable variability in hippocampal damage between rats. Severe damage in the CA1, CA2, and CA3 formations was observed in a single rat, and that rat also showed the greatest degree of response disruption. These results suggest that schedule-controlled responding may be a valuable method for assessing the effects of ischemic injury, and thus, putative neuroprotective compounds, on complex behavior.

Effects of arteether on an auditory radial-arm maze task in rats.

We evaluated the behavioral and neural toxicity of the artemisinin antimalarial compound, arteether (AE), using a novel radial-arm maze procedure. We have previously shown that AE can produce a distinctive pattern of neurotoxicity in the brainstem and that auditory nuclei are particularly vulnerable. Thus, we assessed performance which depended upon auditory processing. We trained rats to choose one of eight arms of a radial maze, depending upon which arm served as the source of a white noise stimulus. Correct responses produced food reinforcement while incorrect choices had no programmed consequences. When the task was acquired, AE (25 mg/kg/day; n=7) or oil vehicle (n=7) was administered (intramuscularly) for seven consecutive days. Behavioral sessions were conducted during the days of drug administrations and for 7 days following drug administrations. Subsequently, histopathology was conducted and a quantitative assessment of the nucleus trapezoideus was made. AE produced a progressive deficit in performance on the maze task. That is, accuracy decreased, choice latency increased, and the number of trials completed decreased. Moreover, the greatest deficits were observed during the period following drug administrations. AE-treated rats revealed marked damage in the nucleus trapezoideus. The damage included chromatolysis, necrosis, and gliosis. Vehicle-treated rats did not show performance deficits or neuropathology. These results extend earlier studies and show that AE can produce damage in the n. trapezoideus of rats, which is associated with performance deficits on a complex auditory task. Thus, the auditory radial-arm maze task is a useful tool for assessing AE-induced toxicity.

Effects of atropine and azaprophen on matching and detection in rhesus monkeys.

The effects of the anticholinergic atropine and azaprophen, a novel, conformationally restricted analog of atropine, were examined in rhesus monkeys using delayed match-to-sample and detection tasks. Both compounds (0.01-0.32 mg/kg) produced dose-dependent decreases in the rate of responding under both tasks. Drug effects on the match-to-sample task correlated with drug effects on the detection task. Both compounds produced decreases in the percentage of correct responses on the match-to-sample task when choice trials occurred 4 or 16 sec, but not 0.01 sec, following sample presentation. Doses of atropine and azaprophen decreasing accuracy on the match-to-sample task also decreased the number of responses on the task. In general, atropine was slightly more potent than azaprophen on both tasks. These results further characterize azaprophen's anticholinergic effects.

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.

Behavioral and pharmacological assessment of butyrylcholinesterase in rats.

Advances in the treatment of organophosphorus (OP) toxicity have focussed on the use of exogenous cholinesterases to act as scavengers for the OP agent. To further investigate the feasibility of the scavenger approach, we evaluated the effects of highly purified horse serum butyrylcholinesterase (HS-BChE) on performance in rats. HS-BChE (5000 U, IP) produced substantial increases in blood enzyme activity for up to 72 h after injection. HS-BChE (5000 U, IP) had no effect on acquisition or retention of a passive avoidance task. In contrast, atropine sulfate (10 mg/kg) impaired retention when tested 168 h after administration. When examined for 10 days following administration, HS-BChE (7500 U, IP) had no effect on either total daily motor activity or circadian pattern of activity. HS-BChE (5000 U, IM) also had no acute or prolonged effects on the rate of lever pressing maintained by a VI56 s schedule of food reinforcement. HS-BChE (7500 U, IM) was observed to confer significant, but partial, protection against response rate decreases produced by the OP, MEPQ, under the VI56 s schedule of reinforcement. These results suggest that, in rats, HS-BChE, at doses that attenuate OP toxicity, may be devoid of cognitive or motor effects.

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.

Behavioral and neural toxicity of the artemisinin antimalarial, arteether, but not artesunate and artelinate, in rats.

Three artemisinin antimalarials, arteether (AE), artesunate (AS), and artelinate (AL) were evaluated in rats using an auditory discrimination task (ADT) and neurohistology. After rats were trained on the ADT, equimolar doses of AE (25 mg/kg, in sesame oil, n=6), AS (31 mg/kg, in sodium carbonate, n=6), and AL (36 mg/kg, in saline, n=6), or vehicle (sodium carbonate, n=6) were administered (IM) for 7 consecutive days. Behavioral performance was evaluated, during daily sessions, before, during, and after administration. Histological evaluation of the brains was performed using thionine staining, and damaged cells were counted in specific brainstem nuclei of all rats. Behavioral performance was not significantly affected in any rats treated with AS, AL, or vehicle. Furthermore, histological examination of the brains of rats treated with AS, AL, and vehicle did not show damage. In stark contrast, all rats treated with AE showed a progressive and severe decline in performance on the ADT. The deficit was characterized by decreases in accuracy, increases in response time and, eventually, response suppression. When performance on the ADT was suppressed, rats also showed gross behavioral signs of toxicity that included tremor, gait disturbances, and lethargy. Subsequent histological assessment of AE-treated rats revealed marked damage in the brainstem nuclei, ruber, superior olive, trapezoideus, and inferior vestibular. The damage included chromatolysis, necrosis, and gliosis. These results demonstrate distinct differences in the ability of artemisinins to produce neurotoxicity. Further research is needed to uncover pharmacokinetic and metabolic differences in artemisinins that may predict neurotoxic potential.

Behavioral effects of some diphenyl-substituted antimuscarinics: comparison with cocaine and atropine.

To more fully characterize the behavioral excitatory effects observed with certain diphenyl-substituted antimuscarinics, various behavioral effects of benactyzine, a prototype excitatory antimuscarinic, was evaluated in rats. These effects were compared to those of cocaine, atropine, and azaprophen, a muscarinic antagonist that contains both the diphenyl substituents of benactyzine and a ring isomeric with the tropane ring of atropine. Under a fixed-interval 5-min schedule of food presentation, cocaine and benactyzine increased response rates. Atropine and azaprophen only decreased responding. The muscarinic agonist oxotremorine attenuated the rate-increasing effects but did not alter the disruptions in the temporal patterning produced by benactyzine or shift the dose-effect function to the right. In rats discriminating 10 mg/kg cocaine from saline, benactyzine partially substituted for cocaine, producing a maximum of 50% cocaine-appropriate responses. Benactyzine fully substituted for scopolamine in rats discriminating 0.056 mg/kg scopolamine from saline. All antimuscarinics increased locomotor activity when activity levels were low in control animals, but the increases were less than those produced by cocaine. Cocaine increased both locomotor activity and fixed-interval responding at comparable doses, whereas 10-fold higher doses of benactyzine were required to increase locomotor activity. These results support the following conclusions: 1) In addition to its classical antimuscarinic behavioral profile, benactyzine has behavioral excitatory actions similar in some respects to those of cocaine; 2) the behavioral excitatory effects of benactyzine do not appear to be due solely to antagonism of muscarinic receptors; and 3) the alkyl-ester may be an important structural feature of diphenyl-substituted antimuscarinics for the induction of behavioral stimulation.

Behavioral and neural toxicity of arteether in rats.

Repeated administration of the artemisinin antimalarial compound, 3-arteether (AE) (25 mg/kg, i.m.) was evaluated in rats using a two-choice, discrete trial, auditory discrimination task and subsequent neurohistology. Rats were trained to choose one of two response levers following presentation of white noise or a tone + white noise. Increasing and decreasing the intensity of the tone increased and decreased discriminability, respectively, and differential reinforcement density produced systematic changes in response bias. AE (n = 5) or vehicle (n = 5) was injected daily (9-12 days). Initial injections of AE did not affect behavioral performance. Continuing daily injections produced significant decreases in choice accuracy and significant increases in choice reaction time. When overt signs of severe toxicity were observed, rats were sacrificed and significant neural pathology was observed in the nucleus trapezoideus of AE-treated rats. In a subsequent experiment, AE was injected for 3 (n = 5), 5 (n = 5), or 7 (n = 5), consecutive days and performance was examined for an additional 7 days. Behavioral disruption was only observed in rats receiving AE for 7 days and the greatest degree of disruption occurred after AE injections were completed. Histopathological examination showed significant neural pathology in the nuclei trapezoideus, superior olive, and ruber of rats receiving 7- and 5-day AE regimens, and in the nucleus trapezoideus of rats receiving the 3-day regimen. Thus, behavioral disruption reflected, but did not predict, neuropathology. These results confirm and extend earlier results demonstrating neurotoxicity of AE in rats. Further, these results demonstrate that the auditory discrimination task provides an objective behavioral measure of AE neurotoxicity, and thus, can serve as a valuable tool for the safety development of AE and other artemisinin antimalarial compounds.

Effects of morphine, clonidine, and intensity change on electric-shock discrimination.

The effects of morphine, clonidine, and changes in stimulus intensity were examined in squirrel monkeys responding on one of two levers following brief presentations of one of two electric-shock intensities (0.1 and 0.5 mA). Responses were designated as correct or incorrect depending on which shock intensity had been presented and which lever was pressed. Morphine (0.42 to 1.80 mg/kg) and clonidine (0.075 to 0.18 mg/kg) decreased percentage correct responding. Morphine and clonidine also increased response latency and the number of shock presentations that were not followed by responses. Changes in shock intensity also decreased percentage correct responding but had no effect on response latency or on the number of shock presentations not followed by responses.