Attenuation of stimulus sensitivity by scopolamine

The value of spontaneous alternation behavior (SAB) as a test of retention in pharmacological investigations of memory

Because of its reliance on memory, the tendency for rats, mice and other animals to alternate successive choices of T- or Y-maze arms has assumed considerable popularity in pharmacological studies of spatial memory as a quick and simple measure of retention that avoids the need for extensive training and the use of conventional reinforcers. Two forms of this tendency have been utilized, namely two-trial and continuous spontaneous alternation behavior (SAB). However, as the behavior can also reflect drug-related changes in sensory/attentional, motivational and performance processes, SAB should not be unquestionably accepted as a measure of memory alone. While assessments of post-acquisition drug effects on longer term memory may be possible through the appropriate timing of drug administration, this is more problematic if SAB is used as a measure of shorter term memory. Even though SAB can be a useful index of responsiveness to novelty, its value as a measure of retention is less certain. In this latter respect, a possible alternative to SAB testing might be the recently developed form of the related procedure, responsiveness to change.

Counteractive effects of a partial (sabcomeline) and a full (RS86) muscarinic receptor agonist on deficits in radial maze performance induced by S-AMPA lesions of the basal forebrain and medial septal area

After S-AMPA (8.0 mM) lesions to the nucleus basalis and medial septal regions, at the source of the cortical and hippocampal branches of the forebrain cholinergic projection system, rats displayed long-lasting and relatively stable impairment in long-term reference and short-term working memory in both spatial (place) and associative (cue) radial maze tasks. Treatment with four doses of the partial agonist at the M1 cholinergic muscarinic receptor, sabcomeline (formerly known as SB 202026: 0.01-0.156 mg/kg), substantially reduced working and reference memory errors in both tasks in lesioned rats, in a mainly dose-related manner. These effects were more consistent than those found with the direct muscarinic agonist RS86 (0.05-0.781 mg/kg). The performance of non-lesioned controls was largely unaffected by either treatment. These findings are consistent with previous evidence for cholinergic participation in the radial maze deficits induced by excitotoxic lesions to the forebrain cholinergic projection system. They show that with a relatively selective lesion, which respectively, reduced choline acetyltransferase activity to 36.5 and 22.5% of control level in frontal and dorsolateral cortex, and to 61.8 and 69.2% of control level in dorsal and ventral hippocampus, lesioned rats were responsive to pharmacological treatments aimed to enhance cholinergic function by full or partial agonist activity at M1 receptors. Findings that nicotine (0.1 mg/kg) also reduced radial maze errors in the lesioned animals supports the suggestion that lesion-induced deficits in radial maze performance were amenable to improvement by cholinergic receptor manipulation. However, given the potential adverse side effects of full receptor agonists, which nonselectively target cholinergic receptors throughout the organism, the functional efficacy of sabcomeline, which shows regional selectivity for the central M1 receptor subtypes, suggests that deleterious effects of cholinergic depletion on cognition can be counteracted without incurring the risk of unwanted side effects.

Continuous ICV infusion of scopolamine impairs sustained attention of rhesus monkeys

Systemic administration of anticholinergic agents impairs cognitive performance in animals and man. The anticholinergic, scopolamine, has profound effects on peripheral and central cholinergic function, making interpretation of its effects on cognitive performance difficult. To circumvent this problem, scopolamine was administered directly to the central nervous system of rhesus monkeys using a subcutaneously implanted infusion pump connected to a cannulae directed toward the right lateral ventricle. Intracerebroventricular (ICV) infusion of scopolamine (0.004, 0.012, 12.5, and 40.0 micrograms/kg/h) produced a dose-dependent decrease in the number of responses on a continuous performance task. Response decrements produced by scopolamine were seen mainly during the last half of the test session and at short stimulus durations. These data suggest that scopolamine produces a deficit in sustained attention or slowing of information processing that is mediated through direct central cholinergic blockade in the rhesus monkey.

Behavioral pharmacology and biochemistry of central cholinergic neurotransmission

Systemically administered cholinergic (muscarinic) receptor antagonists can impair the acquisition and post-acquisition performance of a variety of learned behaviors. acquisition performance of a variety of learned behaviors. At present, there is no consensus about the psychological mechanisms underlying these deficits. Behavioral inhibition, working (short-term) memory, reference (long-term) memory, attention, movement and strategy selection, and stimulus processing are among the constructs that have been proposed as underlying the effects of muscarinic receptor blockade. On the basis of neuroanatomical and neuropharmacological considerations it is contended that debates about the nature of the mediating events are pointless because they are on an anatomy that does not exist. Specifically, given that cholinergic neurons innervate almost the entire neuraxis and that muscarinic cholinergic receptors are distributed throughout the central nervous system, it is virtually certain that systemically applied antimuscarinic drugs will influence a broad spectrum of brain functions. In addition, the nature of the deficits produced by scopolamine and atropine, which are competitive antagonists, will depend on the regional endogenous rate of acetylcholine release, which may in turn be influenced by the particular environment and/or level of training imposed on the animal. As the literature seems to indicate, therefore, the effects of competitive antagonists will vary as a function of both the behavioral test and the level of training. Accordingly, attempts at unitary formulations of central cholinergic function are ill-conceived and illusory. Another approach to understanding central cholinergic function has been based on the use of local injections of excitotoxins into brain regions such as the basal forebrain that contain cholinergic neurons. Recent published reports indicate, that many of the behavioral deficits observed after ibotenic acid lesions of the basal forebrain are due primarily to the loss of non-cholinergic neurons. The inherent limitations of the excitotoxin lesion approach for unravelling the functions of central cholinergic systems are such that they cannot produce definitive information and might best, therefore, be abandoned. At present, a reliable selective toxin for cholinergic neurons is not available and urgently required. Until such a compound is identified, local intracerebral applications of antimuscarinic agents may be the preferred procedure for studying the behavioral correlates of regional blockade of cholinergic activity. Brain microdialysis in freely moving animals also holds considerable promise with respect to defining the circumstances under which acetylcholine is released in discrete regions of the central nervous system. At present, the function of central cholinergic systems and the possible role of each in learning and memory remain poorly understood.

Effects of cholinergic-rich neural grafts on radial maze performance of rats after excitotoxic lesions of the forebrain cholinergic projection system--II. Cholinergic drugs as probes to investigate lesion-induced deficits and transplant-induced functional recovery

The effects of two doses of muscarinic (arecoline and scopolamine) and nicotinic (nicotine and mecamylamine) cholinergic receptor agonists and antagonists on the radial maze errors of rats, performing poorly after ibotenate lesions to the nucleus basalis and medial septal brain regions, were assessed before and after transplantation of cholinergic-rich and -poor fetal grafts, using tasks which measured short- (working) and long-term (reference) spatial and associative memory. Lesioned rats showed improvement with the agonists, and impairment with the antagonists, at low doses which did not affect the performance of controls; these effects were more marked for working than reference memory, especially in the spatial task. The peripherally acting antagonists N-methylscopolamine and hexamethonium did not affect the performance of control or lesioned rats. Effects of the cholinergic probes were re-examined 16 weeks after grafting, in groups with cholinergic-rich grafts to cortex and/or hippocampus which showed functional recovery, and groups with cholinergic-rich grafts to basal forebrain, or cholinergic-poor grafts to basal forebrain, cortex, and hippocampus, which showed no improvement. All lesioned rats, regardless of site, type, or efficacy of transplant, continued to show marked impairment with the antagonists. Poorly performing grafted animals also showed improvement with the agonists. In rats with behaviourally effective cholinergic-rich grafts, arecoline had no effect, but nicotine substantially increased working and reference memory errors, particularly spatial working memory errors. Rats with grafts in both cortex and hippocampus showed the largest increases in errors after nicotine. These results show that lesioned rats were more sensitive to the bi-directional effects of cholinergic receptor ligands than controls, consistent with a role for acetylcholine in the lesion-induced deficits. The predominant effect of drugs on working memory may also be consistent with disruption of acquisition rather than of storage or retrieval processes in memory, and may be related to impairment of attention. The results further show that, despite behavioural recovery, supersensitive responses to cholinergic drugs were not normalized in rats with cholinergic-rich grafts, and that an additive interaction between graft and host may have occurred in response to nicotine.

Scopolamine impairs response-to-change based on distal cues in the rat

The effect of a scopolamine injection (1 mg/kg, IP) on response-to-change behavior was investigated in two experiments. After exploration of a T-maze with one arm black and the other white (Trial 1), rats were tested with both arms either black or white (Trial 2). Experiment 1 revealed that the opportunity to make body turns into the arms did not help scopolamine-injected rats to locate the changed arm after visual exploration of the arms during Trial 1. Saline-injected animals chose the changed arm. In Experiment 2, rats were allowed to move freely into the arms during Trial 1. During Trial 2, they were tested either from the same start as that used during Trial 1 or from a different start 180 degrees from the original start. While scopolamine-injected animals reacted appropriately to the change when tested from the same start, they were impaired when tested from the opposite start. In both conditions, saline-injected animals chose the changed arm. These results, together with others, support the notion that the cholinergic system plays a crucial role in the processing of distal information.

Signal detection analysis of ethanol effects on a complex conditional discrimination

The effects of ethanol on a conditional object identification task were investigated using an operant analog of Signal Detection Analysis. Water and three doses of ethanol (0.40, 0.75 and 1.5 g/kg) were orally administered on three separate occasions to three adult squirrel monkeys. Significant discrimination impairment as a function of increasing ethanol dose was observed. At the 1.5 g/kg dose, impairment extended to nonspecific effects, with subjects ceasing to respond early into the session. Subsequent signal detection analyses revealed that the reduction in performance resulted from losses in discriminability. Response bias was found to change unpredictably and independently of ethanol administration. Reaction time measures also showed no changes except a moderate, nonsignificant, facilitation in speed at the lowest (0.40 g/kg) dose. Taken together, these data suggest that ethanol acts to impair complex, or cognitive, performance by disrupting current sources of stimulus control within the range of doses tested.

Contrasting baseline-dependent effects of amphetamine, chlorpromazine and scopolamine on response switching in the pigeon

Both amphetamine and scopolamine increase low rates and reduce high rates of responding. It has been suggested that the dependence of the effects of these drugs on control rate may be due to their non-specific disruptive cue properties rather than to specific pharmacological actions. To examine whether this possible non-specific disruption also applied to response-choice, pigeons were trained under a schedule in which 30 key-peck responses were required. This fixed-ratio could be completed by responding in any order on either or both of two keys, and then the first switch between the two keys was reinforced by the presentation of food. Under control conditions, the probability of the birds switching between the keys increased as the ratio progressed, resulting in performance which could be analysed in a manner analogous to rate-dependent analyses of responding under fixed-interval (FI) schedules. The birds were then treated with amphetamine (0.4-3.2 mg/kg), chlorpromazine (1.0-30 mg/kg) and scopolamine (0.01-0.10 mg/kg). Amphetamine increased switching at all baselines, to probabilities greater than chance (i.e. P. switch greater than 0.5). Scopolamine resulted in response choice converging towards chance, whereas chlorpromazine reduced switching when the baseline probabilities were high. Thus: amphetamine increases switching in pigeons as it has previously been shown to do in rats, and this effect is not due to regression towards random choice, the opposite effect of a reduction in switching can occur after chlorpromazine treatment, and whereas the effects of amphetamine and scopolamine on response rate may be similar, the effects of the two drugs on response choice are dissociable, with only scopolamine resulting in a randomisation of performance.

Scopolamine impairs the response-to-change following observation of the environment but not after its exploration by the rat

The tendency to select the T-maze arm that has been changed in brightness between two successive trials (response-to-change) was investigated in rats injected with scopolamine (Sc) or saline (NaCl) 20 min before the test. In the "passive" version of the test, when in trial 1 rats could inspect the white-black arms through clear partitions blocking the entrance to the arms, a dose of when in trial 1 rats could inspect the white-black arms through clear partitions blocking the entrance to the arms, a dose of 1.0 mg/kg Sc decreased significantly the number of changed arm choices in trial 2, as compared to saline controls. A lower dose of Sc (0.5 mg/kg) was ineffective. In the "active" test version, when in trial 1 the rats were allowed to explore the white-black arms, doses of 1.0 and 2.0 mg/kg Sc did not affect the preference for the changed arm in trial 2. NaCl rats showed a significant preference for the changed arm choices in both tests. The scopolamine effects on response-to-change, i.e., impairment of performance in the passive but not in the active version, were essentially the same as those found by us previously in hippocampal rats.

The effects of morphine and scopolamine on auditory discrimination in squirrel monkeys

In squirrel monkeys trained to discriminate between two tones, morphine consistently increased the percent of response failures during tone periods; however, it had no consistent effect on the percent correct responses. In contrast, scopolamine decreased percent correct responses as well as increasing percent response failures in two monkeys not required to respond to produce the tone periods. In the two monkeys required to initiate tone periods by responding, high doses of scopolamine reduced the number of tones presented.

Effects of scopolamine and nicotine on human rapid information processing performance

In the first experiment, after a 10-min baseline test on a rapid information processing task, subjects received oral doses of either placebo, methscopolamine 1.2 mg, scopolamine 0.6 mg or scopolamine 1.2 mg, and 1 h later performed the task again for a 20-min period. Following scopolamine 1.2 mg, correct detections were significantly lower over the 20-min period, whereas no such decrement was observed in the other three conditions. In the second experiment a similar design was used to study the effects of nicotine 0.5 mg, 1.0 mg and 1.5 mg and placebo, except that post-drug testing was carried out 10 min after baseline due to the faster absorption of nicotine. Nicotine helped prevent both the decline in detections and the increase in reaction time which occurred over time in the placebo condition. These findings indicate that compounds with opposite effects on central cholinergic pathways produce opposite effects on the performance of a task involving rapid information processing, and are consistent with previous findings from this laboratory.

The separate and combined effects of scopolamine and nicotine on human information processing

Previous work in this and another laboratory has shown that nicotine tablets improve the performance of a rapid information processing task and reduce the Stroop effect, whereas scopolamine has the opposite effects. The purpose of this study was to extend these previous findings by determining whether, when administered together, these two drugs have mutually antagonistic effects on task performance. Two experiments are reported, both using within-subjects double-blind Latin Square designs. In the first, six subjects received single and combined doses of scopolamine 1.2 mg and nicotine 1.5 mg, and there was some evidence that the two drugs had mutually antagonistic effects on the rapid information processing task. In the second experiment 12 subjects received the same doses, but rapid information processing testing was carried out over a longer time period and Stroop testing was introduced at the end of the 2.5 h session. Nicotine was found to counteract the depression of performance produced by scopolamine on both the rapid information task and the Stroop test. These results provide further support for the theory that central cholinergic pathways play a major role in human information processing.

Scopolamine and acquisition of go-no go avoidance: a further analysis of the perseverative antimuscarinic deficit

Rats treated with scopolamine (0.5 mg/kg SC daily) during the acquisition of a discrimination task with symmetrical negative reinforcement (light-go, noise/light-no go) showed a learning impairment, with both active and passive avoidance deficits. In the initial stage of such training, however, fewer passive avoidance errors and more active avoidance errors were made by treated animals if active avoidance pretraining had occurred in the no-drug state. A similar experiment using the same stimulus arrangement with asymmetrical reinforcement (no punishment of intertrial, and no go signal, responses) showed a scopolamine effect consisting mainly of increased responding to extinction signals and during intertrial intervals, with little or no active avoidance deficit. Furthermore, interactions due to changes in treatment conditions in successive stages of training were minimized in the latter task, suggesting that the effects of the shift-no shift factor on distribution of errors in the early stages of active-passive avoidance learning were unlikely to have been due to a genuine drug dissociation. Overall, these results and others obtained previously in the same and related tasks tend to rule out some unidimensional explanations of antimuscarinic effects, e.g., response disinhibition (an exclusively motor deficit) or impairment of stimulus sensitivity (an exclusively sensory deficit). The data rather confirm the notion of a sensorimotor drug bias leading to a shift in response prepotencies depending jointly on stimuli, responses, and response consequences. Prior learning history and behavioural compensation for adverse treatment consequences at the reinforcement level may interact with the sensorimotor bias so as to produce "set perseveration" (perseveration of response tendencies).

Cholinergic drug effects on visual discriminations: a signal detection analysis

Signal detection analysis was used to examine the effects of scopolamine, amphetamine, and physostigmine on a brightness discrimination task. Four groups of rats were exposed to different reinforcement contingencies for correct responses in the presence of S+ stimuli and correct response failures in the presence of S- stimuli. Under non-drug conditions, orderly signal detection data were obtained with the group design. Contrary to a disinhibition hypothesis of cholinergic effects, scopolamine disrupted stimulus sensitivity and not response bias, but only when the discrimination was difficult. Drugs did not affect sensitivity of an earlier, simpler discrimination. The drug effects on response tendency for this simple discrimination task were difficult to interpret.

Bilateral skin conductance and the pupillary light-dark reflex: manipulation by chlorpromazine, haloperidol, scopolamine, and placebo

Cholinergic blocking with scopolamine produces skin conductance orientating response (SCOR) nonresponding in normal subjects. This may be one of a number of causes for nonresponding in schizophrenic subjects. Blockade of dopamine with haloperidol produces an increase in amplitude and shortening of recovery time in the SCOR of normal subjects. This result closely resembles that of Nielsen and Petersen (1976) who found a similar pattern of responding in normal subjects who scored high on a scale of schizophrenism. These results, along with those for chlorpromazine and the pupillographic effects of the three drugs are discussed in terms of biochemical working hypotheses of schizophrenic subclassification.

A reappraisal of scopolamine effects on inhibition

A series of related experiments was conducted to examine the effects of scopolamine on discrimination performance in the presence or absence of a stimulus signalling non-reinforcement. In Experiment 1, rats trained to respond on 1 of two levers in the presence of a 1000-Hz tone and on the other lever in the presence of a 3000-Hz tone were not reinforced when white noise was added to 1 of the tones. Pairing white noise with the other tone during an extinction session demonstrated that the white noise had become a conditioned inhibitory stimulus. In Experiment 2, scopolamine decreased responding and discrimination accuracy on the excitatory (reinforced) trials, and increased responding on the inhibitory (non-reinforced) trials. The magnitude of the drug's effect was similar on excitatory and inhibitory trials. Using combination of visual and auditory discriminative stimuli, Experiment 3 confirmed the results of Experiment 2. These experiments show that scopolamine disrupts animals' ability to discriminate, and that scopolamine-induced increases in non-rewarded responses cannot be attributed solely to a disinhibitory effect of the drug as Carlton (1969) and others have claimed.

Benzodiazepines and discrimination behaviour: dissociation of response and sensory factors

Ten London pigeons were trained on a schedule which allowed concurrent measurement of motor and exteroceptive sensory changes. The task involved a conditional colour discrimination contingent upon the completion of a fixed interval schedule of responding. As a preliminary pharmacological study the effects of the benzodiazepines, chlordiazepoxide and flurazepam were investigated. Both drugs enhanced perseverative responding (after completion of the FI), and to a variable degree responding during the FI, although there was no evidence for an increase in responding during the inter-trial-intervals. On the other hand, no marked changes in discrimination performance were observed. It is concluded that the most significant effects of these benzodiazepines are on motor mechanisms.

Effect of scopolamine on the reactivity of the albino rat to footshock

To determine if anticholergic drugs altered reactions to footshock, 9 female albino rats were tested for escape latencies following unsignaled presentations of footshock in a two-chambered shuttlebox. Different intensities of footshock (0, 0.04, 0.07, and 0.10 ma) were varied orthogonally with various doses of intraperitoneally injected scopolamine hydrobromide (0, 1.0, 4.0 and 16.0 mg/kg). Shock trials were randomly alternated with nonshock (pseudoshock) trials to estimate any drug-induced activity increase which might occur independently of any alteration in reactivity to aversive stimulation. Results indicated that scopolamine produced a significant increase in reactivity to footshock (i.e., shorter escape latencies) at near-threshold intensities as well as producing the expected increase in general activity.

Scopolamine induced changes in activity and reactions to novelty

The behavior of hooded rats was observed in an exploration box comprising novel and familiar halves following IP injections of 0.1, 0.25, 0.5, 0.75 or 1.00 mg/kg scopolamine or isotonic saline. Drug administration occurred after, rather than before, exposure to one of the alternative halves. All doses decreased reactions to the previously inaccessible novel half but decreases were greater for the 2 lowest doses. Rearing behavior was also suppressed by each dose whereas the number of apparatus cells entered (locomotion) was decreased by low doses but increased by high. The 3 behavioral measures showed declines in frequency during the course of each experimental session. However, low doses of the drug enhanced and high doses retarded these declines for rearing and cells entered. The study illustrated the difficulty in explaining data by unitary processes (such as attenuated habituation) when several behavioral indices and drug doses are employed within a single investigation.

The effects of scopolamine on a two-cue discrimination

Rats were trained with a tone, light or a tone—light combination as the discriminative stimulus. These groups were tested after doses of scopolamine and it was found that groups trained with a single cue were more sensitive to the drug than double-cue groups, although their pre-drug responding was similar. A similar pattern was found among individuals in the double-cue groups in which there was a significant correlation between dependency on a single cue, as shown in transfer tests, and drug sensitivity. These results were interpreted in terms of scopolamine-induced changes in stimulus sensitivity produced by a modification of the neural mechanisms controlling attention.