Scopolamine impairs learning performance of rats in a 14-unit T-maze

To assess involvement of muscarinic cholinergic systems in performance of a shock-motivated 14-unit T-maze task, 3-month old Fischer-344 rats were given an IP injection of scopolamine (0.1, 0.3, 1.0 or 3.0 mg/kg), methylscopolamine (1.0 mg/kg), or saline 30 min prior to maze training on 2 consecutive days. Scopolamine, but not methylscopolamine, impaired all components of acquisition performance. Measures of error performance, run time, shock duration, and number of shocks received were significantly increased but only at the 1.0 and 3.0 mg/kg scopolamine doses. The cognitive component of the task, measured by error performance, appeared most affected. Cognitive performance deficits observed following scopolamine administration in the present study resembled age-related impairments in rats and mice previously observed in this task. The cholinergic hypothesis of geriatric memory dysfunction appears to be implicated by these findings; however, the degree to which memory systems are involved remains unclear. Other performance variables such as discriminative control of stimuli or mechanisms of attention are implicated and discussed.     

Scopolamine does not affect footshock sensitivity in the rat

To test the generality of the finding of Feigley, et al. [8] that scopolamine increases sensitivity/reactivity to footshock, rats were tested under either scopolamine or saline conditions for sensitivity to footshock in an automated version of the flinch-jump paradigm. There was no significant trend toward increased sensitivity following scopolamine injection at any of the response magnitudes assessed. Since the previous study included an operant response in the measure of sensitivity, it was suggested that apparent effects of scopolamine on reactivity to footshock are dependent on the inclusion of an operant response in the measure of reactivity, and are not due to changes in sensory thresholds.     

Scopolamine produces environment-specific conditioned activity that is not blocked by pimozide in rats

A classical conditioning paradigm was employed to determine if the stimulant effects of the anticholinergic scopolamine could show conditioning. In experiment 1 rats had 12 60-min pairings of scopolamine (1.0 or 8.0 mg/kg, IP) with a distinctive environment that monitored horizontal and vertical activity. Experimental (paired) groups received the drug 30 min prior to each session, whereas control (unpaired) groups received saline. Following each session the Paired groups were injected with saline, and the Unpaired groups received the same doses of scopolamine. After every fourth pairing a test session assessed conditioning by comparing activity of paired und unpaired groups in response to saline. Scopolamine enhanced horizontal activity, although conditioning was seen only with 8.0 mg/kg. The low dose increased vertical activity, whereas an initial decrease was observed with 8.0 mg/kg. However, conditioned vertical activity was seen with both doses. Experiment 2 assessed the possible role of dopamine in conditioning with 8.0 mg/kg scopolamine. Rats treated as in experiment 1 were additionally given 0.4 mg/kg pimozide 4 h prior to each pairing session. Pimozide did not block scopolamine's stimulant effect. Conditioned horizontal and vertical activity were also observed, suggesting that this effect may be mediated by direct changes within cholinergic systems.     

Dose-specific effects of scopolamine on canine cognition: Impairment of visuospatial memory,but not visuospatial discrimination

Rationale The cholinergic system is linked extensively to memory, but its exact role remains controversial. In particular, scopolamine-induced impairment in rodents is not task specific, which may be due to difficulty in developing rodent protocols to assess deficits in recent memory, in which the remembered event is brief and distinct, and/or to non-specific behavioral impairment.Objectives The present study sought to determine whether scopolamine-induced deficits in recent memory, using a working memory task, could be dose-specifically dissociated from deficits in associative memory in dogs.Methods A Latin-square design was used to determine the effect of scopolamine (5, 10 and 15 g/kg; SC) on a variable delayed-non-matching-to-position (DNMP) task, which assesses visuospatial working memory. Subsequently, the minimal effective dose (15 g/kg; SC) was administered prior to testing on a landmark discrimination task, which provides a measure of allocentric spatial ability, a black-white discrimination task, an oddity discrimination task and tests of exploratory behavior. We also investigated the effects of a 30 g/kg dose (SC) on tests of oddity discrimination and behavioral activity.Results A 15 g/kg dose produced significant impairment on the DNMP task, but did not affect performance of any discrimination task and did not alter behavior on tests of open field or curiosity. A 30 g/kg dose caused disruption on discrimination performance and on open field measures.Conclusions Working memory performance is most sensitive to scopolamine-induced impairment and can be dissociated from scopolamine-induced deficits in discrimination performance and non-cognitive behaviors. The present results indicate that scopolamine-induced impairments of working memory in the dog can serve as a model of age-related cholinergic dysfunction.     

Aged dogs demonstrate both increased sensitivity to scopolamine impairment and decreased muscarinic receptor density

Memory deficits associated with aging and Alzheimer's disease have been linked to cholinergic dysfunction. The present study investigated this hypothesis by comparing the effects of the muscarinic cholinergic receptor antagonist scopolamine on recent memory performance and by examining muscarinic receptor density in aged and young dogs. Scopolamine (15 μg/kg; SC) was administered prior to testing young (M = 2.8 years) and aged (M = 13.0 years) dogs on a delayed-non-matching-to-position task (DNMP). Scopolamine significantly impaired performance of aged, but not young dogs. Muscarinic receptor density was assessed autoradiographically using the non-selective radioligand [³H]quinuclidinylbenzilate. Aged dogs (M = 14.1 years) showed significantly decreased density of muscarinic receptors in all brain regions examined except the cerebellum compared to young dogs (M = 3.7 years). The results are consistent with those seen in aged humans and Alzheimer's patients and support the hypothesis of age-dependent cholinergic dysfunction in the dog, although this was not directly determined in the current study. These findings demonstrate that markers of cholinergic hypofunction, in addition to the natural cognitive decline and amyloid pathology previously noted, are seen in canine aging. Collectively, this supports the use of the aged dog as a model for examining early pathological events in the development of Alzheimer's disease.     

Memory retention: effect of prolonged cholinergic stimulation in mice

The fundamental hypothesis that drugs may affect memory processing by prolonging transmitter action was tested by extending the time of drug action, using repeated administrations of the cholinergic agonist, arecoline hydrobromide (ARE). The ARE was injected intracerebroventricularly into mice immediately after training (T-maze footshock avoidance) and at 90-min intervals thereafter, for a total of 1, 2, or 3 injections. The results indicate that 1 injection had no effect whereas 3 successive injections significantly improved memory retention test performance. The results confirm the hypothesis being tested; six control groups ruled out other plausible interpretations of the results.     

Repeated exposure to diisopropylfluorophosphate (DFP) produces increased sensitivity to cholinergic antagonists in discrimination retention and reversal

This experiment examined the effects of repeated exposure to diisopropylfluorophosphate (DFP), an organophosphate anticholinesterase, on the retention and reversal of a visual discrimination and on the number of muscarinic receptors in the brain. Rats were trained in a serial reversal procedure. After achieving stable performance, the rats were divided into two groups. One group received repeated injections of DFP, the other group received saline injections. To determine whether DFP-treated rats would be more sensitive than normal rats to stresses on the cholinergic system, each rat was injected with saline or one of three doses of scopolamine, a muscarinic receptor blocker, prior to testing on every 6th day. DFP alone caused no impairment in performance. Scopolamine produced a greater impairment in DFP-treated rats than in control rats. Similar results were obtained in a second behavioral task, match-to-sample in a water maze, using the same DFP treatment protocol and only one dose of scopolamine. The number of muscarinic receptors and acetylcholinesterase activity levels were reduced on the 2nd and 15th day after the end of DFP treatment. These results demonstrate that although repeated exposure to organophosphate anticholinesterases may not alter discrimination behavior directly, it may compromise the central nervous system so that it cannot react normally when challenged.     

Phenserine: a physostigmine derivative that is a long-acting inhibitor of cholinesterase and demonstrates a wide dose range for attenuating a scopolamine-induced learning impairment of rats in a 14-unit T-maze

Phenserine ((–)-N-phenylcarbamoyl eseroline), a carbamate analog of physostigmine (Phy), is a long-acting inhibitor of cholinesterase. We have assessed the potential clinical value of phenserine for cholinomimetic therapy of cognitive impairments associated with aging and Alzheimer's disease by evaluating its duration of in vivo activity against rat plasma acetylcholinesterase (AChE) and its effect on attenuating a scopolamine-induced impairment in learning performance of young rats in a shock-motivated 14-unit T-maze. Phenserine achieved maximum AChE inhibition of 73.5% at 5 min and maintained a high and relatively constant inhibition for more than 8 h. For analysis of effects on learning performance, 69, 3-month-old male Fischer-344 rats were pretrained in a straight runway to avoid electric footshock. On the following day, each animal received 15 trials in the 14-unit T-maze. Sixty minutes prior to the maze training, each rat received the first IP injection of either vehicle (Tween 80, ethanol and 0.9% NaCl) or phenserine at 1.5, 3.0, 4.0, 5.0, 7.5, or 10.0 mg/kg. Then 30 min prior to the training, each animal received a second IP injection of either 0.9% NaCl or scopolamine hydrochloride (0.75 mg/kg; SCOP). Compared to the vehicle-SCOP group, all but the 7.5 mg/kg dose of phenserine significantly ameliorated error performance, runtime, shock frequency and shock duration in SCOP-treated rats at the final block of three trials. Appearing to have a long effect and a wide therapeutic window, phenserine deserves further study as a cognitive enhancer.     

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.     

Intrahippocampal infusion of ebselen impairs retention of an inhibitory avoidance task in rats

Ebselen is a seleno-compound used in the treatment of neurological disorders involving the glutamatergic system. Although ebselen is currently used in clinical trials, the physiological effects of this seleno-compound are poorly known. In this study, we investigated the effects of intrahippocampal infusion of ebselen (0.1-3 nmol) in rats submitted to an inhibitory avoidance task. Ebselen (1-3 nmol) infused after the training session impaired retention of inhibitory avoidance, tested 90 min or 24 h after the training session. Moreover, ebselen also impaired the retention when infused 30 min prior to training or 10 min prior to test sessions. In summary, ebselen impaired memory consolidation, acquisition and retrieval. This amnesic effect of ebselen could be related to oxidant activity at N-methyl-D-aspartate (NMDA) receptors. Our results indicate that more studies must be performed to investigate the mechanisms of this amnesic effect and whether ebselen has a cognition-impairing effect when administered chronically.     

Memory and postsynaptic cholinergic receptors in aging mice

Significant retention deficits were observed on passive avoidance tasks (step-down and step-through) in 15-, 20-, and 25-month-old male C57BL/6 mice compared with 4- and 8-month-old mice. In contrast, cholinergic muscarinic receptor binding ( [3H]quinuclidinyl benzilate) in cerebral cortex, striatum, hippocampus, and cerebellum in these same animals revealed no difference in this 4- to 25-month age range. In a separate comparison of 4- and 29-month-old female mice, [3H]QNB binding was significantly decreased in the older group in cerebral cortex, hippocampus, and striatum. Environmental enrichment, compared with an impoverished environment, significantly improved retention in mice on 24-hr step-down performance but affected QNB binding only minimally (6-7% decrease of QNB binding in cerebral cortex and hippocampus). Benzodiazepine ( [3H]flunitrazepam) receptor binding was significantly (12-15%) decreased in 29-month-old mice compared with 4-month-old mice in the cerebral cortex, hippocampus, cerebellum, and brain stem.     

Effects of disrupting the cholinergic system on short-term spatial memory in rats

The effects of disrupting the muscarinic or nicotinic systems on short-term spatial memory were investigated using a delayed matching to position (DMTP) procedure. Rats were trained on the DMTP until stability and then divided into two groups: one group was implanted with an indwelling cannula aimed at the lateral ventricle. The cannulated group received injections of selective muscarinic antagonists (pirenzepine, M1; AFDX 116, M2; UH-AH 37, M1/M3) or hemicholinium-3 (a choline uptake inhibitor). The remaining animals were treated with conventional muscarinic antagonists (scopolamine, methyl scopolamine) or nicotinic channel blockers (mecamylamine, hexamethonium). Scopolamine, methyl scopolamine and UH-AH 37 disrupted all performance parameters in a non-specific but dose related manner. Pirenzepine disrupted accuracy in a delay, but not dose dependent manner, and exerted no other negative effects on performance. Hemicholinium-3-induced performance deficits showed some elements of effects seen following pirenzepine and scopolamine (delay dependent effects on accuracy, some negative effects on other motoric aspects of performance). AFDX 116 and hexamethonium had no significant effects on performance with respect to control. Mecamylamine reduced accuracy and increased response latencies at the highest dose tested. These data indicate that muscarinic antagonists are more effective at disrupting mnemonic performance than nicotinic blockers, and moreover, that distinct muscarinic receptors may have differential effects on cognitive performance.     

Scopolamine attenuates the motor disruptions but not the attentional disturbances induced by haloperidol in a sustained attention task in the rat

Rats were trained to perform a sustained attention task that required the subject to insert its head into a cylindrical “observation tunnel” and wait for the presentation of one of three spatially separated visual stimuli located on the upper portion of the tunnel circumference. Detection of a briefly (0.125 s) presented “correct” stimulus, followed by the rats' forward nose poke, resulted in access to a reinforcement dipper lifted through the orifice in the floor of the tunnel. Nose pokes to the two incorrect stimuli resulted in a 5-s time-out period. The task maximized attention and minimized movement requirements. Performance was characterized in terms of accuracy (i.e. errors of omission, and errors of commission), time on task, and latency to respond to the stimuli (i.e., reaction time). Haloperidol (0.04, 0.08, and 0.16 mg/kg) increased errors of omission and reaction time. However, lack of significant correlations between these two measures suggested that attentional accuracy may be independent of motor slowing produced by this neuroleptic. Scopolamine (0.2 mg/kg) alone increased both errors of omission and commission, but did not affect reaction time to correct stimuli. The sustained attention task as implemented here may be useful in the simultaneous study of classical neuroleptics desirable and undesirable CNS effects. This research was supported by NIMH grant MH43429     

Mianserin as a discriminative stimulus in rats: asymmetrical cross-generalization with scopolamine

The present study was conducted to determine if the tetracyclic antidepressant mianserin could be established as a discrminative stimulus in rats. One group of rats was trained to discriminate mianserin (4.0 mg/kg, IP) from saline in a two-lever drug discrimination procedure, and a second group of rats was trained to discriminate the muscarinic cholinergic antagonist scopolamine (0.25 mg/kg, IP) from saline. Generalization testing with the training drugs yielded an ED₅₀ of 0.502 mg/kg for the mianserin-trained rats and an ED₅₀ of 0.048 mg/kg for the scopolamine-trained rats. Asymmetrical cross-generalization between mianserin and scopolamine was observed, because scopolamine produced mianserin-appropriate responding, but mianserin did not produce scopolamine-appropriate responding. This study is the first demonstration that rats can be trained to discriminate mianserin from saline and that antagonism of muscarinic cholinergic receptors is sufficient to produce mianserin-appropriate responding.     

Comparative in vivo effects of parathion on striatal acetylcholine accumulation in adult and aged rats

Aged rats are more sensitive to the acute toxicity of the prototype organophosphate insecticide, parathion. We compared the acute effects of parathion on diaphragm and brain regional cholinesterase activity, muscarinic receptor binding and striatal acetylcholine levels in 3- and 18-month-old male Sprague-Dawley rats. Adult and aged rats were surgically implanted with a microdialysis cannula into the right striatum 5-7 days prior to parathion treatment. Rats were given either vehicle (peanut oil, 2 ml/kg) or one of a range of dosages of parathion (adult: 1.8, 3.4, 6.0, 9.0, 18 and 27 mg/kg, s.c.; aged: 1.8, 3.4, 6 and 9 mg/kg, s.c.) and body weight, functional signs of toxicity, and nocturnal motor activity were recorded for seven days. Three and seven days after parathion treatment, microdialysis samples were collected and rats were subsequently sacrificed for biochemical measurements. Higher dosages of parathion led to significant time-dependent reductions in body weight in both age groups. Rats in both age groups treated with lower dosages showed few overt signs of cholinergic toxicity while equitoxic high dosages (adult, 27 mg/kg; aged, 9 mg/kg) elicited marked signs of cholinergic toxicity (involuntary movements and SLUD [i.e., acronym for Salivation, Lacrimation, Urination and Defecation] signs) with peak effects being noted 3-4 days after treatment. Nocturnal activity (ambulation and rearing) was reduced in both age groups following parathion dosing, with more prominent effects in adults and rearing being more consistently affected. Dose- and time-dependent inhibition of cholinesterase activity was noted in both diaphragm and striatum. Total muscarinic receptor ([(3)H]quinuclidinyl benzilate, QNB) binding was significantly lower in aged rats, and both total binding and muscarinic agonist ([(3)H]oxotremorine methiodide] binding was significantly reduced in both age-groups treated with the highest dosages of parathion (adult, 27 mg/kg; aged, 9 mg/kg). In contrast to relatively similar levels of cholinesterase inhibition, striatal extracellular acetylcholine levels were significantly lower (2.2- to 2.9-fold) in aged rats at both 3 and 7 day time-points compared to adult rats treated with equitoxic dosages (i.e., 9 and 27 mg/kg, respectively). No age-related differences in in vitro striatal acetylcholine synthesis or in vivo acetylcholine accumulation following direct infusion of the cholinesterase inhibitor neostigmine (1 microM) were noted. While aged rats are more sensitive than adults to the acute toxicity of parathion, lesser acetylcholine accumulation was noted in the striatum of aged rats exhibiting similar levels of cholinesterase inhibition. These findings suggest that lesser acetylcholine accumulation may be required to elicit cholinergic signs in the aged rat, possibly based on aging-associated changes in muscarinic receptor density.     

Combined stimulation of the glycine and polyamine sites of the NMDA receptor attenuates NMDA blockade-induced learning deficits of rats in a 14-unit T-maze

 The present study examined the effects of multi-site activation of the glycine and polyamine sites of the NMDA receptor on memory formation in rats learning a 14-unit T-maze task. The competitive NMDA receptor antagonist, (±)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP, 9 mg/kg), was used to impair learning. The objectives were two-fold: (1) to investigate the effects of independent stimulation of the strychnine-insensitive glycine site or the polyamine site; (2) to investigate the effects of simultaneous activation of these two sites. Male, Fischer-344 rats were pretrained to a criterion of 13 out of 15 shock avoidances in a straight runway, and 24 h later were trained in a 14-unit T-maze that also required shock avoidance. Prior to maze training, rats received intraperitoneal (IP) injections of saline, saline plus CPP, CPP plus the glycine agonist, D-cycloserine (DCS, 30 or 40 mg/kg), CPP plus the polyamine agonist, spermine (SPM, 2.5 or 5 mg/kg), or CPP plus a combination of DCS (7.5 mg/kg) and SPM (0.625 mg/kg). Individual administration of either DCS or SPM attenuated the CPP-induced maze learning impairment in a dose-dependent manner. However, the combined treatment with both DCS and SPM completely reversed the learning deficit at doses five-fold less than either drug given alone. These findings provide additional evidence that the glycine and polyamine modulatory sites of the NMDA receptor are involved in memory formation. Furthermore, the potent synergistic effect resulting from combined activation of the glycine and polyamine sites would suggest a stronger interaction between these two sites than previously considered, and might provide new therapeutic approaches for enhancing glutamatergic function. Received: 9 May 1997/Final version: 13 August 1997     

Effects of post-training d-amphetamine on acquisition of an appetitive autoshaped lever press response in rats

This experiment examined the effect of post-training d-amphetamine on retention in an appetitive autoshaping conditioning situation. Harlan Sprague-Dawley rats were first given ten autoshaping trials, followed by either three or four additional sessions of 50 trials (70 s intertrial interval) on which the conditioned stimulus (the extension of an illuminated Plexiglas lever for 10 s) and unconditioned stimulus (a 45 mg food pellet), were paired. d-Amphetamine (1 or 2 mg/kg) or saline was administered IP either immediately or 2 h following training. Rats injected with 1 mg/kg d-amphetamine immediately after the first training session made significantly more responses during the conditioned stimulus presentation on the following daily session of 50 trials. Thus, the amphetamine-treated rats acquired the lever press response faster than those given only saline. The amphetamine effects were time dependent: no significant effects were found if the injection was delayed until 2 h following training. These results agree with the findings of other instrumental aversive facilitation studies and suggest that d-amphetamine may enhance retention of the classically conditioned components of autoshaping.     

Age-related impairment in learning but not memory in SAMP8 female mice

Four-month and 12-month-old SAM (Senescence Accelerated Mouse)-P8 and -R1 female mice, either intact or ovariectomized, were trained to avoid foot shock in a T-maze. Mice were trained until they made their first avoidance. Memory retention of this task was then tested 1 week later. The results indicated that P8, but not R1, female mice whether intact or ovariectomized, showed an age-related learning impairment. This impairment was more apparent in ovariectomized mice, as ovariectomy was associated with a significant reduction in the mean trials to make an avoidance in all groups except 12-month P8 females. Of greatest interest was the absence of any age-related impairment of retention in P8 females. In several previous studies, SAM-P8, but not R1, males showed an age-related impairment of learning and memory. These results indicate that age-related impairment of memory in P8 mice may be inherited in a sex-related manner, and suggests that the mechanisms involved in the development of impaired learning and memory are different.     

Effects of intra-hippocampal scopolamine injections in a repeated spatial acquisition task in the rat

The involvement of hippocampal cholinergic synapses in spatial discrimination learning was evaluated by locally administering scopolamine into the hippocampus. Sixteen 16-month-old male Lewis rats received bilaterally implanted cannulae aimed at the dorsal part of the hippocampus. The rats were trained on a repeated acquisition test in the Morris water-escape task. In this procedure the invisible platform is randomly moved from day to day to one of four possible locations. Thus, the rat has to learn to localize the platform from day to day. On each day the rats received four pairs of trials. Scopolamine injections (35 µg in 1 l per hippocampus) were given to one group (n=8) on days 5 and 7. On days 6 and 8 all rats received saline injections. Place learning was retarded in the scopolamine-treated rats during the first swims of pairs of trials. During second swims the scopolamine-treated rats showed a general performance deficit, indicating that first and second swims were differentially affected. The data support the hypothesis that cholinergic neurotransmission in the dorsal hippocampus is involved in spatial learning processes.     

Effect of cobra neurotoxin on retention of a brightness discrimination in rats.

In view of some recent evidence that blockade of nicotinic acetylcholine receptors might interfere with memory recall, the possibility that intracranial injections of purified corbra neurotoxin, an irreversible nicotinic receptor blocker, would produce long-lasting amnesia was explored with a brightness discrimination habit in rats. The results indicate that even the highest tolerable dose of neurotoxin had no detectable effect on memory recall.