The effects of scopolamine and clonidine upon the performance and learning of a motor skill

Muscarinic regulation of self-grooming behavior and ultrasonic vocalizations in the context of open-field habituation in rats

Laboratory rats repeatedly exposed to an open field (OF) apparatus display increasingly high levels of grooming -especially that characterized by long and complex sequences- which has been taken as an additional index of novelty habituation. We hypothesized that disrupting such a learning process by administering an amnesic drug as the antimuscarinic scopolamine (SCP) could delay the appearance of more complex grooming subtypes. Thus, rats were pretreated either with SCP (15 mg/kg or 30 mg/kg) or vehicle (VEH) upon four one-day apart OF (OF1-4). On a fifth assessment, all rats received VEH to analyze the likely carry-over effect of SCP. Finally, we measured 50-kHz and 22-kHz ultrasonic vocalizations (USVs) as reliable markers of positive and negative emotionality, respectively. We found that SCP increased locomotion during OF1 and reduced rearing on OF1-OF4, causing no disruption in habituation over tests. SCP prevented the increase of total grooming time by inhibiting complex grooming subtypes and promoting short cephalic sequences. Despite the SCP-induced alterations on grooming agreed with our hypotheses, those changes may have resulted from a motor impairment that could have also affected rearing behavior. Additionally, SCP suppressed 50-kHz USVs while marginally increased 22-kHz calls. Once SCP was withdrawn, rearing, grooming, and some 50-kHz USVs subtypes returned to VEH levels, suggesting that novelty habituation occurred despite the SCP administration. Altogether, that mixed profile of SCP-induced behavioral changes may derive from the complex interplay between the contrasting action of SCP on different brain regions and the doses here used.

Centrally acting antihypertensives change the psychogenic cardiovascular reactivity

Clonidine (CL) and Rilmenidine (RI) are among the most frequently prescribed centrally acting antihypertensives. Here, we compared CL and RI effects on psychogenic cardiovascular reactivity to sonant, luminous, motosensory, and vibrotactile stimuli during neurogenic hypertension. The femoral artery and vein of Wistar (WT - normotensive) and spontaneously hypertensive rats (SHR) were catheterized before (24 h interval) i.p. injection of vehicle (NaCl 0.9%, control - CT group), CL (10 µg/kg), or RI (10 µg/kg) and acute exposure to luminous (5000 lm), sonant (75 dB sudden tap), motor (180° cage twist), and air-jet (10 L/min - restraint and vibrotactile). Findings showed that: (i) CL or RI reduced the arterial pressure of SHR, without affecting basal heart rate in WT and SHR; (ii) different stimuli evoked pressor and tachycardic responses; (iii) CL and RI reduced pressor response to sound; (iv) CL or RI reduced pressor responses to luminous stimulus without a change in peak tachycardia in SHR; (v) cage twist increased blood pressure in SHR, which was attenuated by CL or RI; (vi) air-jet increased pressure and heart rate; (vii) CL or RI attenuated the pressor responses to air-jet in SHR while RI reduced the chronotropic reactivity in both strains. Altogether, both antihypertensives relieved the psychogenic cardiovascular responses to different stimuli. The RI elicited higher cardioprotective effects through a reduction in air-jet-induced tachycardia.

The neuropharmacology of implicit learning

Two decades of pharmacologic research on the human capacity to implicitly acquire knowledge as well as cognitive skills and procedures have yielded surprisingly few conclusive insights. We review the empirical literature of the neuropharmacology of implicit learning. We evaluate the findings in the context of relevant computational models related to neurotransmittors such as dopamine, serotonin, acetylcholine and noradrenalin. These include models for reinforcement learning, sequence production, and categorization. We conclude, based on the reviewed literature, that one can predict improved implicit acquisition by moderately elevated dopamine levels and impaired implicit acquisition by moderately decreased dopamine levels. These effects are most prominent in the dorsal striatum. This is supported by a range of behavioral tasks in the empirical literature. Similar predictions can be made for serotonin, although there is yet a lack of support in the literature for serotonin involvement in classical implicit learning tasks. There is currently a lack of evidence for a role of the noradrenergic and cholinergic systems in implicit and related forms of learning. GABA modulators, including benzodiazepines, seem to affect implicit learning in a complex manner and further research is needed. Finally, we identify allosteric AMPA receptors modulators as a potentially interesting target for future investigation of the neuropharmacology of procedural and implicit learning.

Selective effects of clonidine and temazepam on attention and memory

The present study compared the effects of clonidine and temazepam on performance on a range of tasks aiming to assess the role of central noradrenergic mechanisms in cognitive function. Fifteen healthy volunteers (seven male, eight female), aged 18-25 years, took part in a five-period crossover study in which they received placebo, temazepam (15 and 30 mg) and clonidine (150 and 300 microg) by mouth in counterbalanced order in sessions at least 4 days apart. A test battery was administered before treatment and at 45, 90 and 135 min after the dose. Performance on most tests was significantly impaired in a dose-related fashion, and subjective sedation was recorded for both drugs. The greatest impairments with clonidine were on attention in the presence of distractors. Clonidine did not affect the formation of new long-term memories, in contrast to temazepam, but did impair measures of working memory. Subjective effects, especially feelings of drunkenness and abnormality, were particularly marked with clonidine. These results support the suggestion that central noradrenergic function may be involved in preventing distraction, but do not confirm other reports suggesting that some aspects of performance are improved with clonidine.

Safety pharmacology--a progressive approach

Although deaths and life-threatening adverse drug reactions (ADRs) in Phase I clinical trials are extremely rare, less severe ADRs occur with an incidence of over 13%. Of the candidate drugs (CDs) that fail prior to marketing, it is generally acknowledged that about 1 in 5 do so because of ADRs in the clinic. Once new chemical entities (NCEs) are on the market, ADRs are estimated to be the fourth leading cause of death in the USA. These various statistics indicate that there is room for improvement in preclinical safety assessment, and a smarter approach to safety pharmacology (SP) can contribute to this. Rather than 'bundling' the SP studies together just prior to Phase I trials, a step-wise, streamlined approach can be adopted throughout the drug discovery process. In this way, the SP information can contribute to making informed judgements at each milestone throughout the preclinical drug discovery process: (i) to assist in series and compound selection; (ii) to assess potential risk of failure in the clinic due to ADRs; (iii) to predict potential ADRs that the clinical pharmacologists can focus on; (iv) to define a therapeutic window for acute dosing in humans. To achieve these objectives, the SP tests need to be carefully selected, adequately validated in-house, and be robust and reliable. To achieve (ii) above, outcome criteria have to be set which, for each test (in vitro and in vivo), take into account acceptable safety margins for the particular therapeutic target. Thus, highly sensitive and predictive SP tests positioned strategically and as early as possible should contribute to reducing attrition during clinical development and ultimately to marketing safer medicines more rapidly.

Dopamine-antagonistic, anticholinergic, and GABAergic effects on declarative and procedural memory functions

Declarative and procedural memory functions are related to dissociable neuroanatomic substrates. In the present study differential effects of pharmacologically induced changes in dopaminergic, GABAergic, and cholinergic activity in the brain on declarative (object and face recognition, immediate and delayed word recall) and procedural memory processes (compensatory tracking) were investigated. In a double-blind design, either 3 mg of haloperidol, 11 mg of midazolam, 1 mg of scopolamine, or placebo were administered to 80 healthy volunteers randomly assigned to one of the four drug conditions. Although all three drugs produced a detrimental effect on immediate and delayed word recall, recall performance was substantially more impaired by the benzodiazepine midazolam than by either haloperidol or scopolamine. While recognition of faces was affected by neither of the drugs, performance on object recognition was significantly decreased by midazolam as compared to placebo. Procedural learning was markedly impaired by all drugs but, again, the observed effect was most pronounced with midazolam. Additional analyses of measures of subjective activation, cortical arousal, and psychomotor performance argued against the assumption that the observed memory-impairing effects were secondary to drug-induced sedation. The overall pattern of results revealed that memory processes are much more susceptible to changes in GABAergic than in dopaminergic or cholinergic neurotransmitter activity. Furthermore, the present findings point to the conclusion that the modulating effects of dopaminergic, GABAergic, and cholinergic neurotransmitter systems on declarative and procedural memory functions are less specific than suggested by neuropsychological studies in patients.

Procedural learning in Parkinson's disease and cerebellar degeneration

We compared procedural learning, translation of procedural knowledge into declarative knowledge, and use of declarative knowledge in age-matched normal volunteers (n = 30), patients with Parkinson's disease (n = 20), and patients with cerebellar degeneration (n = 15) by using a serial reaction time task. Patients with Parkinson's disease achieved procedural knowledge and used declarative knowledge of the task to improve performance, but they required a larger number of repetitions of the task to translate procedural knowledge into declarative knowledge. Patients with cerebellar degeneration did not show performance improvement due to procedural learning, failed to achieve declarative knowledge, and showed limited use of declarative knowledge of the task to improve their performance. Both basal ganglia and cerebellum are involved in procedural learning, but their roles are different. The normal influence of the basal ganglia on the prefrontal cortex may be required for timely access of information to and from the working memory buffer, while the cerebellum may index and order events in the time domain and be therefore essential for any cognitive functions involving sequences.

Korsakoff's syndrome, cognition and clonidine

Eighteen patients suffering from Alcoholic Korsakoff's Syndrome participated in a placebo-controlled double-blind cross-over trial of clonidine 0.3 mg b.d. for two weeks versus matched placebo for two weeks. A detailed neuropsychological assessment was carried out at the end of each treatment phase and staff ratings of behaviour were also obtained. Clonidine treatment resulted in no significant improvement over placebo on any of the cognitive measures employed. The results contradict previous smaller studies which had suggested that chronic treatment with clonidine had a memory-enhancing effect in Korsakoff's syndrome.

Increased cognitive sensitivity to scopolamine with age and a perspective on the scopolamine model

18 older normal volunteers (mean age = 66.5 +/- 7.9 years) and 46 younger volunteers (mean age = 27.0 +/- 6.1 years) were administered the anticholinergic drug scopolamine (0.5 mg i.v.) followed by a battery of cognitive tests evaluating attention, learning and memory. The older subjects were significantly more impaired than the younger by scopolamine on some tests of learning and memory. This increased sensitivity of the older group to scopolamine is consistent with studies in animals and humans showing decreased cholinergic system function with age. The findings also indicate that age is an important variable to consider in using the scopolamine model of memory impairment. The cognitive impairment caused by scopolamine in younger subjects in this and prior studies is similar to some, but not all aspects of the impairment which occurs in normal aging. Scopolamine also caused impairments on digit span and word fluency tasks, which are not consistent with normal aging changes. In the older group of subjects, scopolamine produced aspects of the cognitive impairment which occurs in AD on tests of episodic memory and learning, vigilance-attention, category retrieval, digit span, and number of intrusions. Other areas of cognition that are of relevance to aging and AD such as psychomotor speed, praxis, concept formation and remote memory were not evaluated in this study. Some of these are being evaluated in ongoing studies, along with additional and more specific tests of retrieval from knowledge memory, implicit memory and attention. The scopolamine model has provided a fruitful pharmacologic starting point for the study of a number of cognitive operations. The idea of dissecting apart aspects of memory systems pharmacologically depends on the availability of neurochemically specific drugs and on the specificity and sensitivity of neuropsychological tests for distinct cognitive operations or domains. Further studies using such tools will aid not only in the understanding of the impairments which occur in aging and in AD, but also of the conceptualization of memory and other cognitive operations and ultimately the physiological mechanisms involved in memory and learning.

Cholinergic activity and constraints on information processing

In humans, close relationships are found between cholinergic activity and constraints placed on information processing operations. This is true for all operations where the effects of cholinergic activity have been studied. Studies of vigilance, memory, problem solving, stimulus processing and response processing are cited as illustrations. These studies suggest the hypothesis that cholinergic activity controls constraints in all information processing operations. Alternative hypotheses are proposed and experimental tests are suggested.

Memory for what it is and memory for what it means: a single case of Korsakoff's amnesia

A single case of Korsakoff's syndrome, showing classic circumscribed amnesia on traditional neuropsychological tests, performed a number of learning tasks previously used in primates to demonstrate the different anatomical basis of memory systems used to learn about the properties of objects or to learn about rules of responding. The patient (CJ) performed in a manner very similar to lesioned monkeys in that he could learn evaluative tasks (e.g. a red object is nice), but not rule based tasks (e.g. a red object means go left). In consequence CJ learned an object discrimination task in a manner qualitatively different from a group of non-amnesic controls. Although he could not learn rule based tasks, CJ could perform them once he had been given the rule. Detailed analysis of learning showed that CJ could choose the object he had chosen previously, but could not say where it had been or whether he had been rewarded. Furthermore he could recognise a word as familiar, while not remembering its source. We propose that he has a specific impairment in a memory system concerned with the representation of the signification of objects in particular contexts, while representation of the properties of objects remains intact.

Effect of dexmedetomidine and midazolam on human performance and mood

The effects of dexmedetomidine (DXM), a novel alpha 2-adrenoceptor agonist, on human performance and mood were studied in a double-blind randomized crossover study in 12 healthy student volunteers. Single IM doses of dexmedetomidine, 0.6 microgram/kg (DXM1) and 1.2 micrograms/kg (DXM2), 80 micrograms/kg midazolam (MID) and saline placebo, were given at one-week intervals. Performance was measured objectively and mood was assessed subjectively with visual analogue scales (VAS) at baseline and 40 min, 2 h, 4 h and 6 h after each injection. Blood pressure and heart rate in the sitting position were measured and venous blood was sampled during each testing round. DXM1 did not significantly impair cognitive (digit symbol substitution), coordinative (tracking) or reactive skills, and at 6 h it shortened reaction time and reduced errors in complex tracking. It produced exophoria, increased body sway with the eyes open and impaired subjective performance on VAS. The higher dose dexmedetomidine also caused impaired cognitive, coordinative and reactive skills (although a trend towards improved coordination was found at 6 h), reduced attention, produced exophoria, increased body sway with the eyes open and caused drowsiness, clumsiness, passiveness and mental slowness on the appropriate VAS scales. MID resembled DXM2 in impaired objective and subjective measures of skilled performance, although the objective effects of MID were of speedier onset and shorter duration. In contrast to MID, DXM caused a significant and dose-dependent decrease in systolic and diastolic blood pressure still detectable 6 h after injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Clonidine and scopolamine: differences and similarities in how they change human information processing

1. Humans have been tested on a choice reaction time task designed to disclose interactions between stimulus complexity and drug effect. 2. Tests were carried out using oral scopolamine (1.2 mg) and clonidine (0.2 mg). 3. Reaction times and event related potentials were measured. 4. Both drugs slowed reaction time and the N1 component of the ERP. 5. SCOP slows RTs to easy-to-discriminate stimuli more than RTs to harder stimuli. Its effect on P3 is the same for both types of stimuli. 6. CLON tends to slow P3 latencies to easy stimuli more than P3 latencies to harder stimuli, while the RT slowing is almost identical for both types of stimuli.