Comparative effects of alpha-2 receptor agents and THA on the performance of adult and aged rats in the delayed non-matching to position task

Preventive strategies and potential therapeutic interventions for delirium in sepsis

Delirium is the most frequent and severe clinical presentation of brain dysfunction in critically ill septic patients with an incidence ranging from 9% to 71%. Delirium represents a significant burden for patients and relatives, as well as to the health care system, resulting in higher costs, long-term cognitive impairment and significant risk of death after 6 months. Current interventions for the prevention of delirium typically involve early recognition and amelioration of modifiable risk factors and treatment of underlying conditions that predisposes the individual to delirium. Several pharmacological interventions to prevent and treat delirium have been tested, although their effectiveness remains uncertain, especially in larger and more homogeneous subgroups of ICU patients, like in patients with sepsis. To date, there is inconsistent and conflicting data regarding the efficacy of any particular pharmacological agent, thus substantial attention has been paid to non-pharmacological interventions and preventive strategies should be applied to every patient admitted in the ICU. Future trials should be designed to evaluate the impact of these pharmacologic interventions on the prevention and treatment of delirium on clinically relevant outcomes such as length of stay, hospital mortality and long-term cognitive function. The role of specific medications like statins in delirium prevention is also yet to be evaluated.

Facilitation of spatial working memory performance following intra-prefrontal cortical administration of the adrenergic alpha1 agonist phenylephrine

RATIONALE

Spatial working memory is dependent on the appropriate functioning of the prefrontal cortex (PFC). PFC activity can be modulated by noradrenaline (NA) released by afferent projections from the locus coeruleus. The coreuleo-cortical NA system could therefore be a target for cognitive enhancers of spatial working memory. Of the three classes of NA receptor potentially involved, the α2 and α1 classes seem most significant, though agents targeting these receptors have yielded mixed results. This may be partially due to the use of behavioural assays that do not translate effectively from the laboratory to the clinical setting. Use of a paradigm with improved translational potential may be essential to resolve these discrepancies.

OBJECTIVES

The objective of this study was to assess the effects of PFC-infused α2 and α1 adrenergic receptor agonists on spatial working memory performance in the touchscreen continuous trial-unique non-matching to location (cTUNL) task in rats.

METHODS

Young male rats were trained in the cTUNL paradigm. Cannulation of the mPFC allowed direct administration of GABA agonists for task validation, and phenylephrine and guanfacine to determine the effects of adrenergic agonists on task performance.

RESULTS

Infusion of muscimol and baclofen resulted in a delay-dependent impairment. Administration of the α2 agonist guanfacine had no effect, whilst infusion of the α1 agonist phenylephrine significantly improved working memory performance.

CONCLUSIONS

Spatial working memory as measured in the rat cTUNL task is dependent on the mPFC. Enhancement of noradrenergic signalling enhanced performance in this paradigm, suggesting a significant role for the α1 receptor in this facilitation.

Pharmacological properties, central nervous system effects, and potential therapeutic applications of atipamezole, a selective alpha2-adrenoceptor antagonist

Atipamezole is an alpha2-adrenoceptor antagonist with an imidazole structure. Receptor binding studies indicate that its affinity for alpha2-adrenoceptors and its alpha2/alpha1 selectivity ratio are considerably higher than those of yohimbine, the prototype alpha2-adrenoceptor antagonist. Atipamezole is not selective for subtypes of alpha2-adrenoceptors. Unlike many other alpha2-adrenoceptor antagonists, it has negligible affinity for 5-HT1A and I2 binding sites. Atipamezole is rapidly absorbed and distributed from the periphery to the central nervous system. In humans, atipamezole at doses up to 30 mg/subject produced no cardiovascular or subjective side effects, while at a high dose (100 mg/subject) it produced subjective symptoms, such as motor restlessness, and an increase in blood pressure. Atipamezole rapidly reverses sedation/anesthesia induced by alpha2-adrenoceptor agonists. Due to this property, atipamezole is commonly used by veterinarians to awaken animals from sedation/anesthesia induced by alpha2-adrenoceptor agonists alone or in combination with various anesthetics. Atipamezole increased sexual activity in rats and monkeys. In animals with sustained nociception, atipamezole increased pain-related responses by blocking the noradrenergic feedback inhibition of pain. In tests assessing cognitive functions, atipamezole at low doses has beneficial effects on alertness, selective attention, planning, learning, and recall in experimental animals, but not necessarily on short-term working memory. At higher doses atipamezole impaired performance in tests of cognitive functions, probably due to noradrenergic overactivity. Recent experimental animal studies suggest that atipamezole might have beneficial effects in the recovery from brain damage and might potentiate the anti-Parkinsonian effects of dopaminergic drugs. In phase I studies atipamezole has been well tolerated by human subjects.

Delayed non-matching to position performance in aged hybrid Fischer 344 x brown Norway rats: a longitudinal study

In this study, the effects of aging on the performance in a delayed non-matching to position (DNMTP) task were investigated longitudinally in hybrid Fischer 344 x Brown Norway rats. The rats were first trained to perform the task. Subsequently, their performance was assessed monthly from 28 to 34 months of age. The measures of responding on the DNMTP schedule did not decrease in the course of the study. After the last DNMTP test, choline acetyltransferase (ChAT) activity and glial fibrillary acidic protein (GFAP) content were measured in frontal cortex and hippocampus. We found that higher levels of GFAP in the frontal cortex, but not hippocampus, were associated with a poorer performance in the DNMTP task. Our findings support the notion that repeated testing prevents the age-related decline in cognitive functions that has been reported in cross-sectional studies. Pathology of the frontal cortex seems to predict a faster rate of forgetting in aging rats.

Systemic administration of atipamezole, a selective antagonist of alpha-2 adrenoceptors, facilitates behavioural activity but does not influence short-term or long-term memory in trimethyltin-intoxicated and control rats

The present study used trimethyltin (TMT)-intoxicated rats as a model for the behavioural syndrome seen after neuronal damage to the limbic system. Behavioural assessments indicated increased locomotor activity and reduced number of groomings in an open-arena task in TMT-intoxicated (6.6 mg/kg as a free base) rats, as has been found previously. A novel finding was the severe deficit in swimming to a visible platform in the water maze task, with reduced swimming speed at the beginning of the training period. During the reacquisition phase of a radial arm maze task, TMT-intoxicated rats made more short-term and long-term memory errors, and their behavioural activity was increased in comparison with controls. The administration of atipamezole (300 micrograms/kg), a selective antagonist of alpha 2-adrenoceptors, enhanced locomotor activity compared to saline-treated rats, but these effects did not differ between the TMT group and their controls. Atipamezole did not enhance short-term or long-term memory in either TMT or control groups. Taken together, the present data indicate that TMT intoxication is a model for global dementia rather than for a specific loss of relational memory. Previous studies on the neurochemical effects of TMT and the alleviation or prevention of neurotoxicity of TMT are reviewed.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Stimulation and blockade of alpha1 adrenoceptors affect behavioural activity, but not spatial working memory assessed by delayed non-matching to position task in rats

The present study investigated the role of alpha1 adrenergic receptors in the modulation of working memory and behavioural activity by assessing the effects of alpha1 adrenergic receptor stimulation or blockade on the performance of rats in a delayed non-matching to position task. St-587 (a putative agonist of alpha1 adrenoceptors) at a dose of 100 microg/kg slightly increased choice accuracy (per cent correct responses) of rats, but the effect was delay-independent which is interpreted as an improvement in choice accuracy and non-mnemonic (non-working memory) in character. Neither St-587 (300 or 1000 microg/kg) nor prazosin (a prototype antagonist of alpha1 adrenoceptors) (100 or 300 microg/kg) significantly affected the choice accuracy of rats in this task. Prazosin 300 microg/kg lengthened the latency for correct responses in the working memory task but did not affect food collection latencies. This combination of effects may reflect decreased motor output. St-587 300 and 1000 microg/kg, but not prazosin, increased food collection latencies in the working memory task. Thus, the present results suggest that alpha1 adrenergic receptors do not play any important role in spatial working memory as assessed using the delayed non-matching to position task, but that modulation of alpha1 adrenoceptors may affect motor activity and motivation in rats.

Metrifonate improves spatial navigation and avoidance behavior in scopolamine-treated, medial septum-lesioned and aged rats

We investigated the effects of acute p.o. pretraining treatment with an indirect acetylcholinesterase inhibitor, metrifonate, on water maze spatial navigation and passive avoidance behavior. Metrifonate (10-100 mg/kg, orally, p.o.) did not improve the water maze or passive avoidance performance of young intact rats. However, in young rats metrifonate over a broad dosage range (10-100 mg/kg, p.o.) was able to alleviate the adverse effects of scopolamine (a muscarinic acetylcholine receptor antagonist; 0.4 and 2.0 mg/kg in water maze and passive avoidance study, respectively) and medial septum-lesioning on spatial reference and working memory and passive avoidance performance. In old (23-month-old) rats, a defect of water maze and passive avoidance behavior was observed. In old rats, metrifonate improved spatial reference memory function in the water maze and also passive avoidance at 10-30 mg/kg, but the 3 mg/kg dose was ineffective. Very old (27-month-old) rats had a more severe impairment of water maze performance than old rats, and metrifonate 3-30 mg/kg did not improve their spatial navigation. These results show that metrifonate may over a wide range of doses stimulate cognitive functioning, but during advanced aging neurobiological defects develop that may mask some of the therapeutic effects of metrifonate in rats.

Alpha 2-adrenoceptor agonist, dexmedetomidine, protects against kainic acid-induced convulsions and neuronal damage

Kainic acid (KA)-induced convulsions are accompanied by histopathological changes that are most prominent in the temporal lobe structures. In the present study, we investigated whether a selective alpha2-adrenoceptor agonist, dexmedetomidine could attenuate KA-induced epileptic convulsions and subsequent neuronal damage in the rat hippocampus. Rats were pretreated 30 min before KA injection (9 mg/kg, i.p.) with dexmedetomidine (3 micrograms/kg, s.c.). The behavior of animals was observed for at least 3 h. Dexmedetomidine suppressed the development (p < 0.001), generalization (p < 0.05) and severity (p < 0.01) of convulsions. In addition, histological analysis revealed that dexmedetomidine-treated animals without convulsions or with only partial convulsions had no neuronal damage in the principal cell layers of the hippocampus. A selective alpha2-antagonist, atipamezole (1 mg/kg, s.c.) potentiated KA-induced convulsions and increased the mortality in status epilepticus. In conclusion, the present study demonstrated that dexmedetomidine, in addition to possessing anticonvulsant properties, has a neuroprotective effect in the KA model of status epilepticus.

The effects of d-cycloserine, a partial agonist at the glycine binding site, on spatial learning and working memory in scopolamine-treated rats

The present study investigated the effect of d-cycloserine, a partial agonist at the glycine binding site on NMDA receptor complex, on the performance of scopolamine-treated adult rats in a water maze task assessing spatial learning and in a delayed non-matching to position task assessing working memory in a spatial context. In the spatial learning task, scopolamine (0.4 mg/kg, i.p.) impaired acquisition (increased escape latency and distance) and increased swimming speed of rats. D-cycloserine (1.0 mg/kg, i.p.) reversed the deficits in acquisition performance but not the increases in behavioral activity. In the working memory task, scopolamine (0.2 mg/kg, i.p.) produced deficits on nonmnemonic rather than on mnemonic performance factors; scopolamine delay-independently decreased the percent correct responses and reduced behavioral activity of rats. D-cycloserine (1.0, 3.0 and 10 mg/kg, i.p.) did not reverse these performance deficits. When administered alone, the moderate to higher doses of d-cycloserine had no effects on working memory but the lower dose produced slight deficits in mnemonic performance factors; the 1.0 mg/kg dose delay-dependently decreased the percent correct responses without affecting behavioral activity of rats. In the water maze task, d-cycloserine had no effects on acquisition performance or behavioral activity of rats. These results suggest that acute, systemic administration of d-cycloserine does not improve spatial learning or working memory. However, at appropriate doses this agent may be efficacious in disease states of central cholinergic hypofunction since 1.0 mg/kg d-cycloserine was able to reverse the scopolamine-induced deficits in acquisition.

Effect of sex and age on brain monoamines and spatial learning in rats

The concentrations of noradrenaline (NA), dopamine (DA), serotonin (5-HT), and their metabolites were measured in the prefrontal cortex, caudate-putamen, and hippocampus in young (3 months) and aged (27-31 months) Wistar rats of both sexes. Age-related changes were found in prefrontal NA and HVA/DA ratio, striatal DA and DOPAC/DA ratio, and striatal and hippocampal 5-HT and 5-HIAA/5-HT ratio. Age and sex dependent changes were found in striatal DA and DOPAC/DA ratio, and hippocampal MHPG-SO4/NA ratio. The aged rats were tested in spatial discrimination and reversal tasks in a T maze. The effects of alpha 2-agonist medetomidine (3 micrograms/kg) on the task performance were assessed in relation to individual variation in monoamine metabolism. Medetomidine impaired spatial discrimination learning of the aged rats by interacting with the hippocampal 5-HT turnover. Medetomidine improved reversal learning through an interaction with the striatal DA turnover and reduced the number of perseverative errors after reversal, mainly due to its interaction with the prefrontal NA turnover. It is concluded that the memory enhancing effect of drugs acting through the brain monoamine systems is highly dependent on the stage of degeneration of these systems that show considerable individual variation in aged animals.

Tacrine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy in Alzheimer's disease

Tacrine is a centrally acting cholinesterase inhibitor with additional pharmacological activity on monoamine levels and ion channels. It has been postulated that some or all of these additional properties may also be relevant to the mode of action of the drug. There are wide interindividual variations in pharmacological and clinical response to tacrine, possibly related to interindividual variation in bioavailability. Tacrine appears to improve cognitive function and behavioural deficits in a proportion of patients with Alzheimer's disease, at dosages of 80 to 160 mg/day. In the best designed trials, 30 to 51% of evaluable patients showed an improvement of at least 4 points on the cognitive subscale of the Alzheimer's Disease Assessment Scale, versus 16 to 25% of placebo recipients. A similar proportion of tacrine recipients were judged to have improved when global assessment scales were used. There was a significant dose-response relationship up to 160 mg/day. However, large numbers of patients were withdrawn during the trials, many because of tacrine-associated increases in transaminase levels. Elevated liver enzyme levels occurred in about 50% of tacrine recipients (reaching clinical significance in about 25%). Cholinergic symptoms also occurred more often in tacrine recipients than in those receiving placebo. A gradual increase in tacrine dosage, at 6-week intervals, is recommended when initiating therapy, and weekly serum transaminase monitoring is required for 6 weeks after each dosage increase. Despite the limitations implied by the low proportion of responders and high incidence of hepatic adverse effects associated with therapy, tacrine appears to make a measurable difference in both cognitive and behavioural function in a proportion of patients with Alzheimer's disease--a welcome advance in an area previously devoid of acceptable treatment options.

The effects of selective alpha-2 adrenergic agents on the performance of rats in a 5-choice serial reaction time task

The present study investigated how the systemic administration of alpha-2 adrenergic agents that modulate the function of the noradrenergic system in brain, affect rousal and sustained attention. Food-deprivated rats were trained to detect and respond to brief flashes of light presented randomly in one of five spatially diverse locations. The effects of single-dose administrations of dexmedetomidine, an alpha-2 agonist, and atipamezole, an alpha-2 antagonists, on the choice accuracy, errors of omissions, speed of responding, and collection of the reward could be assessed in this task. Dexmedetomidine increased the amount of omissions, speed of response, and decreased the number of premature responses, although it did not markedly lengthen response latencies and food collection latency. Atipamezole increased the number of premature responses. Neither dexmedetomidine nor atipamezole had any effect on choice accuracy of rats in this task. The results suggest that dexmedetomidine decreased behavioral activity and arousal of rats, whereas atipamezole had mild stimulant effect on behavior.

The effects of tacrine and zacopride on the performance of adult rats in the working memory task

1. The present study investigated the effects of tacrine (an inhibitor of acetylcholinesterase) and zacopride (the antagonist of 5-HT3 receptors) on the performance of adult rats in a continuous operant delayed non-matching to position task assessing spatial working memory. 2. Adult rats had a decline in the percent correct responses at the longest delays (16 and 30 sec) in this task. Tacrine (1.0 mg/kg) or zacopride (0.0025, 0.05, 1.0 mg/kg) did not increase the percent correct responses at any time delays. The higher dose of tacrine reduced behavioural activity (e.g. the decreased number of trials completed and increased sample press latency) of rats during memory testing, and it slightly increased choice accuracy across all the delays. 3. The combination of zacopride (1.0 mg/kg) and tacrine (1.0 mg/kg) increased the percent correct responses at the shortest delays, but not at the longest delays. 4. These results indicate a non-mnemonic improvement in the accuracy performance of rats, and they suggest that the effects of acute, systemic administrations of zacopride (which is thought to increase the release of acetylcholine) or/and tacrine (which inhibits the breakdown of acetylcholine) do not improve spatial working/short-term memory in rats.