Modulation of vigilance and behavioral activation by alpha-1 adrenoceptors in the rat

Theory of visual attention (TVA) applied to rats performing the 5-choice serial reaction time task: differential effects of dopaminergic and noradrenergic manipulations

RATIONALE

Attention is compromised in many psychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD). While dopamine and noradrenaline systems have been implicated in ADHD, their exact role in attentional processing is yet unknown.

OBJECTIVES

We applied the theory of visual attention (TVA) model, adapted from human research, to the rat 5-choice serial reaction time task (5CSRTT) to investigate catecholaminergic modulation of visual attentional processing in healthy subjects of high- and low-attention phenotypes.

METHODS

Rats trained on the standard 5CSRTT and tested with variable stimulus durations were treated systemically with noradrenergic and/or dopaminergic agents (atomoxetine, methylphenidate, amphetamine, phenylephrine and atipamezole). TVA modelling was applied to estimate visual processing speed for correct and incorrect visual perceptual categorisations, independent of motor reaction times, as measures of attentional capacity.

RESULTS

Atomoxetine and phenylephrine decreased response frequencies, including premature responses, increased omissions and slowed responding. In contrast, methylphenidate, amphetamine and atipamezole sped up responding and increased premature responses. Visual processing speed was also affected differentially. Atomoxetine and phenylephrine slowed, whereas methylphenidate and atipamezole sped up, visual processing, both for correct and incorrect categorisations. Amphetamine selectively improved visual processing for correct, though not incorrect, responses in high-attention rats only, possibly reflecting improved attention.

CONCLUSIONS

These data indicate that the application of TVA to the 5CSRTT provides an enhanced sensitivity to capturing attentional effects. Unexpectedly, we found overall slowing effects, including impaired visual processing, following drugs either increasing extracellular noradrenaline (atomoxetine) or activating the α1-adrenoceptor (phenylephrine), while also ameliorating premature responses (impulsivity). In contrast, amphetamine had potential pro-attentional effects by enhancing visual processing, probably due to central dopamine upregulation.

The role of anterior insula-brainstem projections and alpha-1 noradrenergic receptors for compulsion-like and alcohol-only drinking

Compulsion-like alcohol drinking (CLAD), where consumption continues despite negative consequences, is a major obstacle to treating alcohol use disorder. The locus coeruleus area in the brainstem and norepinephrine receptor (NER) signaling in forebrain cortical regions have been implicated in adaptive responding under stress, which is conceptually similar to compulsion-like responding (adaptive responding despite the presence of stress or conflict). Thus, we examined whether anterior insula (aINS)-to-brainstem connections and alpha-1 NERs regulated compulsion-like intake and alcohol-only drinking (AOD). Halorhodopsin inhibition of aINS-brainstem significantly reduced CLAD, with no effect on alcohol-only or saccharin intake, suggesting a specific aINS-brainstem role in aversion-resistant drinking. In contrast, prazosin inhibition of alpha-1 NERs systemically reduced both CLAD and AOD. Similar to systemic inhibition, intra-aINS alpha-1-NER antagonism reduced both CLAD and AOD. Global aINS inhibition with GABAR agonists also strongly reduced both CLAD and AOD, without impacting saccharin intake or locomotion, while aINS inhibition of calcium-permeable AMPARs (with NASPM) reduced CLAD without impacting AOD. Finally, prazosin inhibition of CLAD and AOD was not correlated with each other, systemically or within aINS, suggesting the possibility that different aINS pathways regulate CLAD versus AOD, which will require further study to definitively address. Together, our results provide important new information showing that some aINS pathways (aINS-brainstem and NASPM-sensitive) specifically regulate compulsion-like alcohol consumption, while aINS more generally may contain parallel pathways promoting CLAD versus AOD. These findings also support the importance of the adaptive stress response system for multiple forms of alcohol drinking.

Sex differences in noradrenergic modulation of attention and impulsivity in rats

RATIONALE

Noradrenaline (NE) is closely related to attentive performance and impulsive control. However, the potential sex differences regarding attention and impulsivity under the noradrenergic modulation have been largely neglected. Therefore, our study aimed to investigate whether male and female rats exhibit differential responses to NE-related drugs during the five-choice serial reaction time task (5CSRT).

METHODS

Male and female rats were trained in 5CSRT and administered with different NE drugs after obtaining stable baseline performance: atipamezole, a highly selective α2 receptor antagonist; prazosin, an α1 receptor antagonist; and atomoxetine, a selective NE reuptake inhibitor. Later, prazosin was selected to co-administration with atomoxetine.

RESULTS

Male and female rats exhibited equal learning speed, and no significant baseline differences were found as measured by the 5CSRT. Atomoxetine decreased premature responses in both sexes, but the extent of this reduction was different, with the reduction greater in males. Besides, atomoxetine (1.8 mg/kg) increased the error of omissions in females. The high dose of prazosin (0.5 mg/kg) decreased the accuracy only in male rats, but this was ameliorated by the co-administration with atomoxetine.

CONCLUSIONS

Atomoxetine showed significant improvement in impulsivity, but atomoxetine had less beneficial effects on impulsive control in females than in males, and it even impaired attentional performance in female rats. The α1 receptors were mainly responsible for NE drug-related sex differences in attention rather than impulsivity. The results obtained in this study indicate that the sex differences exist in both attention and impulsivity by the modulation of noradrenaline and raise the concern to improve sex-specific treatments.

Association study of Catechol-o-methyltransferase and Alpha-1-adrenergic receptor gene polymorphisms with multiple phenotypes of heroin use disorder

Heroin use disorder is a chronic relapsing brain disease containing multiple phenotypes. These phenotypes vary among heroin users and might be influenced by genetic factors. Single-nucleotide polymorphisms (SNPs) of catechol-O-methyltransferase (COMT) and alpha-1-adrenergic receptor (ADRA1A) genes are associated with heroin use disorder. However, it has not been clarified which phenotypes of heroin use disorder are related to these genes. To address this question, we recruited 801 unrelated heroin users and divided them into different subgroups according to four important phenotypes of heroin use disorder. Then 7 SNPs in the functional region of these genes were systematically screened and genotyped using a SNaPshot assay. We found that the A allele of ADRA1A rs1048101 was associated with a shorter duration of transition from first use to addiction. Subjects with the C allele of ADRA1A rs3808585 were more susceptible to memory impairment after heroin use disorder. Subjects with the G allele of COMT rs769224 were more likely to take a higher dose of heroin every day. Our study confirmed the association between polymorphisms of COMT and ADRA1A with those specific phenotypes of heroin use disorder, which will be instructive for the precise treatment of the disease.

Noradrenergic α1-Adrenoceptor Actions in the Primate Dorsolateral Prefrontal Cortex

Noradrenergic (NE) α1-adrenoceptors (α1-ARs) contribute to arousal mechanisms and play an important role in therapeutic medications such as those for the treatment of posttraumatic stress disorder (PTSD). However, little is known about how α1-AR stimulation influences neuronal firing in the dorsolateral prefrontal cortex (dlPFC), a newly evolved region that is dysfunctional in PTSD and other mental illnesses. The current study examined the effects of α1-AR manipulation on neuronal firing in dlPFC of rhesus monkeys performing a visuospatial working memory task, focusing on the "delay cells" that maintain spatially tuned information across the delay period. Iontophoresis of the α1-AR antagonist HEAT (2-{[β-(4-hydroxyphenyl)ethyl]aminomethyl}-1-tetralone) had mixed effects, reducing firing in a majority of neurons but having nonsignificant excitatory effects or no effect in remaining delay cells. These data suggest that endogenous NE has excitatory effects in some delay cells under basal conditions. In contrast, the α1-AR agonists phenylephrine and cirazoline suppressed delay cell firing and this was blocked by coadministration of HEAT. These results indicate an inverted-U dose response for α1-AR actions, with mixed excitatory actions under basal conditions and suppressed firing with high levels of α1-AR stimulation such as with stress exposure. Immunoelectron microscopy revealed α1-AR expression presynaptically in axons and axon terminals and postsynaptically in spines, dendrites, and astrocytes. It is possible that α1-AR excitatory effects arise from presynaptic excitation of glutamate release, whereas postsynaptic actions suppress firing through calcium-protein kinase C opening of potassium channels on spines. The latter may predominate under stressful conditions, leading to loss of dlPFC regulation during uncontrollable stress.SIGNIFICANCE STATEMENT Noradrenergic stimulation of α1-adrenoceptors (α1-ARs) is implicated in posttraumatic stress disorder (PTSD) and other mental disorders that involve dysfunction of the prefrontal cortex, a brain region that provides top-down control. However, the location and contribution of α1-ARs to prefrontal cortical physiology in primates has received little attention. This study found that α1-ARs are located near prefrontal synapses and that α1-AR stimulation has mixed effects under basal conditions. However, high levels of α1-AR stimulation, as occur with stress, suppress neuronal firing. These findings help to explain why we lose top-down control under conditions of uncontrollable stress when there are high levels of noradrenergic release in brain and why blocking α1-AR, such as with prazosin, may be helpful in the treatment of PTSD.

Selective activation of Dopamine D3 receptors and norepinephrine transporter blockade enhances sustained attention

Catecholamine transmitters dopamine (DA) and norepinephrine (NE) regulate prefrontal cortical (PFC) circuit activity and PFC-mediated executive functions. Accordingly, pharmacological agents that influence catecholamine neurotransmission exert prominent effects on cognition. Many such agents are used clinically to treat attention disorders. For example, methylphenidate blocks DA and NE reuptake and is the leading choice for attention deficit hyperactivity disorder (ADHD) treatment. Recently, we have designed SK609 - a selective small molecule agonist of the DA D3 receptor (D3R). In this study, we further characterized SK609's ability to selectively inhibit the reuptake of NE by NE transporters (NET). Our results indicate SK609 selectively inhibits NET with a K_i value of ∼500 nM and behaves as a NET substrate. Systemic dosing of SK609 (4 mg/kg; i.p.) in naïve rats produced a 300% and 160% increase in NE and DA, respectively, in the PFC as measured by microdialysis. Based on these neurochemical results, SK609 was tested in a PFC-dependent, visually-guided sustained attention task in rats. SK609 improved performance in a dose-dependent manner with a classical inverted-U dose response function with a peak effect at 4 mg/kg. SK609's peak effect was blocked by a pre-treatment with either the D2/D3R antagonist raclopride (0.05 mg/kg; i.p) or the alpha-1 adrenergic receptor antagonist prazosin (0.25 mg/kg; i.p), confirming a role for both DA and NE in promoting sustained attention. Additionally, SK609 improved sustained attention more prominently among low-performing animals. Doses of SK609 (2, 4, and 8 mg/kg) associated with cognitive enhancement did not produce an increase in spontaneous locomotor activity, suggesting a lack of side effects mediated by DA transporter (DAT) activity. These results demonstrate that the novel catecholaminergic modulator SK609 has the potential to treat sustained attention deficits without affecting DAT activity, distinguishing it from amphetamines and methylphenidate.

Drug-free and context-dependent locomotor hyperactivity in DBA/2 J mice previously treated with repeated cocaine: Relationship with behavioral sensitization and role of noradrenergic receptors

Drug-associated contexts and discrete cues can trigger motivational states responsible for drug-seeking behavior and relapse. In preclinical research, drug-free conditioned hyperactivity has been used to investigate the expression of memories associated with psychostimulant drug effects. Addictive drugs can produce long-lasting sensitization to their psychomotor actions, a phenomenon known as behavioral sensitization. The neuroplasticity underlying behavioral sensitization appears to be involved in pathological drug pursuit and abuse. In the present study we evaluated drug-free, context-dependent hyperactivity in DBA/2 J mice previously treated with cocaine and we explored whether this conditioned effect was related to behavioral sensitization. Given the role of noradrenergic (NA) neurotransmission in memory retrieval, consolidation and reconsolidation processes, we also investigated whether conditioned hyperactivity in a drug-free state was mediated by NA receptors. Animals underwent a sensitization protocol with six cocaine injections (0, 5, 10 or 20 mg/kg) paired to a particular floor cue. Three days after this sensitization phase, all animals were exposed to the same familiar floor environment without drug treatment. A second test with an unfamiliar floor was conducted 24 h later. Conditioned hyperactivity was also explored after one or three cocaine pairings and was evaluated for its duration (with repeated familiar vs. unfamiliar floor tests). In a series of pharmacological experiments, we evaluated the effects propranolol (a non-selective antagonist of β1- and β2-receptors) and prazosin (α1-receptor antagonist) on conditioned hyperactivity. Cocaine treatment produced both robust sensitization and drug-free conditioned hyperactivity, an effect that lasted up to 17 days (with cocaine 20 mg/kg). A significant correlation between the magnitude of cocaine sensitization and the level of conditioned hyperactivity was found. Propranolol, but not prazosin, blocked context-dependent hyperlocomotion in a drug-free state. Our data, together with a vast body of literature, indicate that the NA system plays a key role in the retrieval and behavioral expression of drug-associated memories.

The α-1 adrenoceptor (ADRA1A) genotype moderates the magnitude of acute cocaine-induced subjective effects in cocaine-dependent individuals

OBJECTIVES

We examined whether a functional variant of the ADRA1A gene moderated cocaine-induced subjective effects in a group of cocaine-dependent individuals.

METHODS

This study was a within-participant, double-blind, placebo-controlled inpatient human laboratory evaluation of 65 nontreatment-seeking, cocaine-dependent [Diagnostic and Statistical Manual of Mental Disorders, 4th ed. (DSM-IV)] individuals aged 18-55 years. Participants received both placebo (saline, IV) and cocaine (40 mg, IV), and subjective responses were assessed 15 min before receiving an infusion and at 5 min intervals for the subsequent 20 min. The rs1048101 variant of the α1A-adrenoceptor (ADRA1A) gene was genotyped and it was evaluated whether the Cys to Arg substitution at codon 347 in exon 2 (Cys347Arg) moderated the magnitude of the subjective effects produced by cocaine.

RESULTS

Thirty (46%) participants were found to have the major allele CC genotype and 35 (44%) carried at least one minor T-allele of rs1048101 (TT or TC genotype). Individuals with the CC genotype showed greater responses for 'desire' (P<0.0001), 'high' (P<0.0001), 'any drug effect' (P<0.0001), 'like cocaine' (P<0.0001), and 'likely to use cocaine if given access' (P<0.05) with experiment-wise significance.

CONCLUSION

This study indicates that the ADRA1A genotype could be used to identify individuals for whom acute cocaine exposure may be more rewarding and by inference may result in greater difficulty in establishing and/or maintaining abstinence from cocaine.

Rodent Test of Attention and Impulsivity: The 5-Choice Serial Reaction Time Task

The 5-choice serial reaction time task (5-CSRTT) is employed extensively to measure attention in rodents. The assay involves animals trained to respond to a brief, unpredictable visual stimulus presented in one of five locations. The effects of experimental manipulations on response speed and choice accuracy are measured, and each related to attentional performance. The 5-CSRTT is also used to measure motor impulsivity. Adapted from a human task, the 5-CSRTT can be employed with rodents or primates, highlighting its translational value. Another strength of this procedure is its adaptability to task modification. An example is the 5-choice continuous performance task, which has both target and non-target trial types. Overall, the 5-CSRTT has proven to be valuable for drug discovery efforts aimed at identifying new agents for the treatment of central nervous system disorders and for further understanding the neurobiological processes of attention and impulsivity. Its flexibility offers considerable scope to the experimenter, and in this respect the task continues to evolve. © 2017 by John Wiley & Sons, Inc.

Auditory p300 event-related potentials in children with Sydenham's chorea

P300 event-related potentials (ERPs), objective measures related to cognitive processing, have not been studied in Sydenham's chorea (SC) patients.

PURPOSE

To assess cognitive impairment with P300 ERPs.

METHOD

Seventeen patients with SC and 20 unaffected healthy children were included. Stanford-Binet test was used for psychometric assessment, and odd-ball paradigm was used for auditory ERPs.

RESULTS

There was no significant difference in P300 latencies between the SC-pretreatment group, SC-posttreatment group and control group (p>0.05). Mean interpeak latencies in SC-pretreatment group and SC-posttreatment group showed significant prolongation compared with the control group (p<0.05). Mean interpeak latencies in SC-posttreatment group were significantly decreased compared with SC-pretreatment group (p<0.05). Compared to controls, patients did not show significant difference in Stanford-Binet intelligence examination.

CONCLUSION

This report suggests that interpeak latencies and amplitudes of P300 ERPs could be useful for detecting and monitoring cognitive impairment in SC patients.

Attention

The ability to focus one's attention on important environmental stimuli while ignoring irrelevant stimuli is fundamental to human cognition and intellectual function. Attention is inextricably linked to perception, learning and memory, and executive function; however, it is often impaired in a variety of neuropsychiatric disorders, including Alzheimer's disease, schizophrenia, depression, and attention deficit hyperactivity disorder (ADHD). Accordingly, attention is considered as an important therapeutic target in these disorders. The purpose of this chapter is to provide an overview of the most common behavioral paradigms of attention that have been used in animals (particularly rodents) and to review the literature where these tasks have been employed to elucidate neurobiological substrates of attention as well as to evaluate novel pharmacological agents for their potential as treatments for disorders of attention. These paradigms include two tasks of sustained attention that were developed as rodent analogues of the human Continuous Performance Task (CPT), the Five-Choice Serial Reaction Time Task (5-CSRTT) and the more recently introduced Five-Choice Continuous Performance Task (5C-CPT), and the Signal Detection Task (SDT) which was designed to emphasize temporal components of attention.

Assessment of attention and inhibitory control in rodent developmental neurotoxicity studies

In designing screens to assess potential neurotoxicants, the paramount goal is that the selected assessment tools detect dysfunction if it exists. This goal is particularly challenging in the case of cognitive assessments. Cognition is not a unitary phenomenon, and indeed there is growing evidence that different aspects of cognitive functioning are subserved by distinct neural systems. As a result, if a particular neurotoxicant selectively damages certain neural systems but not others, it can impair some cognitive, sensory, or affective functions, but leave many others intact. Accordingly, studies with human subjects use batteries of cognitive tests, cognizant of the fact that no one test is capable of detecting all forms of cognitive dysfunction. In contrast, assessment of cognitive functioning in non-human animal developmental neurotoxicity (DNT) studies typically consists of a single, presumably representative, "learning and memory" task that is expected to detect all potential effects on cognitive functioning. Streamlining the cognitive assessment in these studies saves time and money, but these shortcuts can have serious consequences if the aspect of cognitive functioning that is impaired is not tapped by the single selected task. In particular, executive functioning - a constellation of cognitive functions which enables the organism to focus on multiple streams of information simultaneously, and revise plans as necessary - is poorly assessed in most animal DNT studies. The failure to adequately assess these functions - which include attention, working memory, inhibitory control, and planning - is particularly worrisome in light of evidence that the neural systems that subserve these functions may be uniquely vulnerable to early developmental insults. We illustrate the importance of tapping these areas of functioning in DNT studies by describing the pattern of effects produced by early developmental Pb exposure. Rats exposed to lead (Pb) early in development were tested on a series of automated attention tasks, as well as on a radial arm maze task. The lead-exposed rats were not impaired in this demanding radial arm maze task, despite conditions which tapped the limits of both working and long-term memory. In contrast, the automated tests designed to assess rodent executive functioning revealed selective and functionally important deficits in attention and regulation of emotion or negative affect (produced by committing an error or not receiving an expected reward). This example underscores the importance of including tasks to specifically tap executive functioning in DNT batteries. Such tasks are not only sensitive but can also shed light on the specific nature of the dysfunction, and they can implicate dysfunction of specific neural systems, information which can be used to design therapeutic interventions. Although the use of such tasks increases the time and effort needed to complete the battery, the benefits outweigh the cost, in light of the greater sensitivity of the battery and the more complete characterization of effects.

Pay attention to impulsivity: modelling low attentive and high impulsive subtypes of adult ADHD in the 5-choice continuous performance task (5C-CPT) in female rats

Varying levels of attention and impulsivity deficits are core features of the three subtypes of adult attention deficit-hyperactivity disorder (ADHD). To date, little is known about the neurobiological correlates of these subtypes. Development of a translational animal model is essential to improve our understanding and improve therapeutic strategies. The 5-choice continuous performance task (5C-CPT) in rats can be used to examine different forms of attention and impulsivity. Adult rats were trained to pre-set 5C-CPT criterion and subsequently separated into subgroups according to baseline levels of sustained attention, vigilance, premature responding and response disinhibition in the 5C-CPT. The behavioural subgroups were selected to represent the different subtypes of adult ADHD. Consequently, effects of the clinically used pharmacotherapies (methylphenidate and atomoxetine) were assessed in the different subgroups. Four subgroups were identified: low-attentive (LA), high-attentive (HA), high-impulsive (HI) and low-impulsive (LI). Methylphenidate and atomoxetine produced differential effects in the subgroups. Methylphenidate increased sustained attention and vigilance in LA animals, and reduced premature responding in HI animals. Atomoxetine also improved sustained attention and vigilance in LA animals, and reduced response disinhibition and premature responding in HI animals. This is the first study using adult rats to demonstrate the translational value of the 5C-CPT to select subgroups of rats, which may be used to model the subtypes observed in adult ADHD. Our findings suggest that this as an important paradigm to increase our understanding of the neurobiological underpinnings of adult ADHD-subtypes and their response to pharmacotherapy.

CNTRICS final animal model task selection: control of attention

Schizophrenia is associated with impaired attention. The top-down control of attention, defined as the ability to guide and refocus attention in accordance with internal goals and representations, was identified by the Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia (CNTRICS) initiative as an important construct for task development and research. A recent CNTRICS meeting identified three tasks commonly used with rodent models as having high construct validity and promise for further development: The 5-choice serial reaction time task, the 5-choice continuous performance task, and the distractor condition sustained attention task. Here we describe their current status, including data on their neural substrates, evidence for sensitivity to neuropharmacological manipulations and genetic influences, and data from animal models of the cognitive deficits of schizophrenia. A common strength is the development of parallel human tasks to facilitate connections to the neural circuitry and drug development research done in these animal models. We conclude with recommendations for the steps needed to improve testing so that it better represents the complex biological and behavioral picture presented by schizophrenia.

Pharmacogenetic randomized trial for cocaine abuse: disulfiram and α1A-adrenoceptor gene variation

Disulfiram is a cocaine addiction pharmacotherapy that inhibits dopamine β-hydroxylase (DβH) and reduces norepinephrine production. We examined whether a functional variant of the ADRA1A gene (Cys to Arg at codon 347 in exon 2, Cys347Arg) may enhance treatment response through decreased stimulation of this α1A-adrenoceptor, since antagonists of this receptor show promise in reducing cocaine use. Sixty-nine cocaine and opioid co-dependent (DSM-IV) subjects were stabilized on methadone for two weeks and subsequently randomized into disulfiram (250 mg/day, N=32) and placebo groups (N=37) for 10 weeks. We genotyped the ADRA1A gene polymorphism (rs1048101) and evaluated its role for increasing cocaine free urines in those subjects treated with disulfiram using repeated measures analysis of variance, corrected for population structure. The 47 patients who carried at least one T allele of rs1048101 (TT or TC genotype) reduced their cocaine positive urines from 84% to 56% on disulfiram (p=0.0001), while the 22 patients with the major allele CC genotype showed no disulfiram effect. This study indicates that a patient's ADRA1A genotype could be used to identify a subset of individuals for which disulfiram and, perhaps, other α1-adrenoceptor blockers may be an effective pharmacotherapy for cocaine dependence.

Effect of Block of α1-adrenoceptors on Overall Motor Activity but not on Spatial Cognition in the Object-Position Recognition Task

Prazosin, an α1-adrenoceptor antagonist, is well known for its depressant effect on motivation and motor activity, while it has no effect on retention of spatial behavior in several tasks, e.g. in the Morris water maze and radial arm maze. The role of α1-adrenoceptors in operant tasks with stimulus-controlled behavior has not yet been tested. The present study investigated the effect of prazosin on the modulation of overall motor activity and on cognitive performance in a spatial operant task called object-position recognition task, where operant behavior (lever pressing) was controlled by spatial stimuli displayed on a computer screen. This task has been previously showed to be hippocampal-dependent. Pre-test injection of prazosin at the dose of 3 mg/kg decreased the responding rate, while it did not affect the recognition of object’s position. In conclusion, we validated the new cognitive test with a drug with known pharmacological effects on behavior and confirmed the depressant effect of prazosin on motor activity and no effect on retrieval of spatial memory in the hippocampal-dependent operant task.

Noradrenergic modulation of cognition: therapeutic implications

The noradrenaline (norepinephrine) system exerts profound influences on cognition via ascending projections to the forebrain, mostly originating from the locus coeruleus. This paper provides an overview of available infrahuman and healthy human studies, exploring the effects of specific noradrenergic manipulations on dissociable cognitive functions, including attention, working memory, cognitive flexibility, response inhibition and emotional memory. Remarkable parallels across species have been reported which may account for the mechanisms by which noradrenergic medications exert their beneficial effects in disorders such as depression and attention-deficit hyperactivity disorder (ADHD). The literature is discussed in relation to prevailing models of noradrenergic influences over cognition and novel therapeutic directions, including in relation to investigating the effects of noradrenergic manipulations on other disorders characterized by impulsivity, and dementias. Unanswered questions are also highlighted, along with key avenues for future research, both proof-of-concept and clinical.

Noradrenergic versus dopaminergic modulation of impulsivity, attention and monitoring behaviour in rats performing the stop-signal task: possible relevance to ADHD

RATIONALE

Deficient response inhibition is a prominent feature of many pathological conditions characterised by impulsive and compulsive behaviour. Clinically effective doses of catecholamine reuptake inhibitors are able to improve such inhibitory deficits as measured by the stop-signal task (SST) in humans and other animals. However, the precise therapeutic mode of action of these compounds in terms of their relative effects on dopamine (DA) and noradrenaline (NA) systems in prefrontal cortical and striatal regions mediating attention and cognitive control remains unclear.

OBJECTIVES

We sought to fractionate the effects of global catecholaminergic manipulations on SST performance by using receptor-specific compounds for NA or DA. The results are described in terms of the effects of modulating specific receptor subtypes on various behavioural measures such as response inhibition, perseveration, sustained attention, error monitoring and motivation.

RESULTS

Blockade of α2-adrenoceptors improved sustained attention and response inhibition, whereas α1 and β1/2 adrenergic receptor antagonists disrupted go performance and sustained attention, respectively. No relevant effects were obtained after targeting DA D1, D2 or D4 receptors, while both a D3 receptor agonist and antagonist improved post-error slowing and compulsive nose-poke behaviour, though generally impairing other task measures.

CONCLUSIONS

Our results suggest that the use of specific pharmacological agents targeting α2 and β noradrenergic receptors may improve existing treatments for attentional deficits and impulsivity, whereas DA D3 receptors may modulate error monitoring and perseverative behaviour.

α-Adrenergic receptors in auditory cue detection: α2 receptor blockade suppresses false alarm responding in the rat

Numerous studies have suggested a facilitatory role of the noradrenergic system in attention. Cognitive functions relating to attentive states--arousal, motivation, behavioral flexibility, and working memory--are enhanced by norepinephrine release throughout the brain. The present study addresses the role of the adrenergic system on stimulus validity and sustained attention within the auditory system. We examined the effects of adrenoceptor stimulation via systemic injection of α1 and α2-adrenoceptor antagonist and agonist drugs, prazosin (1 mg/kg), phenylephrine (0.1 mg/kg), yohimbine (1 mg/kg), and clonidine (0.0375 mg/kg), respectively. Our results indicate that α1-adrenergic stimulation is ineffective in modulating the biological assessment of auditory signal validity in the non-stressed rat, while α2-adrenoceptor antagonist and agonist drugs were effective in modulating both accuracy and response latencies in the habituated animal. Remarkably, blockade of α2-adrenoceptors significantly improved the animal's ability to correctly reject non-signal events. These findings indicate not only a state dependent noradrenergic component of auditory attentional processing, but a potential therapeutic use for drugs targeting norepinephrine release in neurological disorders ranging from Alzheimer's disease to schizophrenia.

On the improvement of inhibitory response control and visuospatial attention by indirect and direct adrenoceptor agonists

RATIONALE

The clinical efficacy of the monoamine and noradrenaline transporter inhibitors methylphenidate and atomoxetine in attention deficit/hyperactivity disorder implicates noradrenergic neurotransmission in modulating inhibitory response control processes. Nonetheless, it is unclear which adrenoceptor subtypes are involved in these effects.

OBJECTIVES

The present study aimed at investigating the effects of adrenoceptor agonists on inhibitory response control as assessed in the rodent 5-choice serial reaction time task, a widely used translational model to measure this executive cognitive function.

RESULTS

Consistent with the previous reported effects of atomoxetine, the noradrenaline transporter inhibitor desipramine improved inhibitory response control, albeit the effect size was smaller compared to that of atomoxetine. Methylphenidate exerted a bimodal effect on inhibitory response control. Interestingly, the preferential β2-adrenoceptor agonist clenbuterol improved inhibitory response control. Moreover, clenbuterol improved visuospatial attention in the task, an effect that was also observed with the preferential β1-adrenoceptor agonist dobutamine. By contrast, although the preferential α1-adrenoceptor and α2-adrenoceptor agonists (phenylephrine and clonidine, respectively) and the non-selective β-adrenoceptor agonist (isoprenaline) were found to alter inhibitory response control, this was probably secondary to the simultaneous increments in response latencies and omissions observed at effective doses.

CONCLUSIONS

Taken together, these findings further strengthen the notion of noradrenergic modulation of inhibitory response control and attentional processes and particularly reveal the involvement of β2-adrenoceptors therein.

Pharmacological enhancement of memory and executive functioning in laboratory animals

Investigating how different pharmacological compounds may enhance learning, memory, and higher-order cognitive functions in laboratory animals is the first critical step toward the development of cognitive enhancers that may be used to ameliorate impairments in these functions in patients suffering from neuropsychiatric disorders. Rather than focus on one aspect of cognition, or class of drug, in this review we provide a broad overview of how distinct classes of pharmacological compounds may enhance different types of memory and executive functioning, particularly those mediated by the prefrontal cortex. These include recognition memory, attention, working memory, and different components of behavioral flexibility. A key emphasis is placed on comparing and contrasting the effects of certain drugs on different cognitive and mnemonic functions, highlighting methodological issues associated with this type of research, tasks used to investigate these functions, and avenues for future research. Viewed collectively, studies of the neuropharmacological basis of cognition in rodents and non-human primates have identified targets that will hopefully open new avenues for the treatment of cognitive disabilities in persons affected by mental disorders.

Comparative anatomy of the locus coeruleus in humans and nonhuman primates

The locus coeruleus (LC) is a dense cluster of neurons that projects axons throughout the neuroaxis and is located in the rostral pontine tegmentum extending from the level of the inferior colliculus to the motor nucleus of the trigeminal nerve. LC neurons are lost in the course of several neurodegenerative disorders, including Alzheimer's and Parkinson's diseases. In this study we used Nissl staining and tyrosine hydroxylase (TH) immunoreactivity to compare the human LC with that of closely related primate species, including great and lesser apes, and macaque monkeys. TH catalyzes the initial and rate-limiting step in catecholamine biosynthesis. The number of TH-immunoreactive (TH-ir) neurons was estimated in each species using stereologic methods. In the LC of humans the mean total number of TH-ir neurons was significantly higher compared to the other primates. Because the total number of TH-ir neurons in the LC was highly correlated with the species mean volume of the medulla oblongata, cerebellum, and neocortical gray matter, we conclude that much of the observed phylogenetic variation can be explained by anatomical scaling. Notably, the total number of LC neurons in humans was most closely predicted by the nonhuman allometric scaling relationship relative to medulla size, whereas the number of LC neurons in humans was considerably lower than predicted according to neocortex and cerebellum volume.

Alpha adrenergic modulation on effects of norepinephrine transporter inhibitor reboxetine in five-choice serial reaction time task

The study examined the effects of a norepinephrine transporter (NET) inhibitor reboxetine (RBX) on an attentional performance test. Adult SD rats trained with five-choice serial reaction time task (5-CSRTT) were administered with RBX (0, 3.0 and 10 mg/kg) in the testing day. Alpha-1 adrenergic receptor antagonist PRA and alpha-2 adrenergic receptor antagonist RX821002 were used to clarify the RBX effect. Results revealed that rat received RBX at 10 mg/kg had an increase in the percentage of the correct response and decreases in the numbers of premature response. Alpha-1 adrenergic receptor antagonist Prazosin (PRA) at 0.1 mg/kg reversed the RBX augmented correct responding rate. However, alpha-2 adrenergic receptor antagonist RX821002 at 0.05 and 0.1 mg/kg dose dependently reversed the RBX reduced impulsive responding. Our results suggested that RBX as a norepinephrine transporter inhibitor can be beneficial in both attentional accuracy and response control and alpha-1 and alpha-2 adrenergic receptors might be involved differently.

Candidate gene analysis in an on-going genome-wide association study of attention-deficit hyperactivity disorder: suggestive association signals in ADRA1A

OBJECTIVES

Attention-deficit hyperactivity disorder (ADHD) is a highly heritable, common developmental disorder. Although a few confirmed associations have emerged from candidate gene studies, these have shown the same limitations that have become evident in the study of other complex diseases, often with inconsistent and nonreplicated results across different studies.

METHODS

In this report, 27 ADHD candidate genes were explored in greater depth using high-density tag single nucleotide polymorphism (SNP) genotyping. Association with 557 SNPs was tested using the transmission disequilibrium test in 270 nuclear pedigrees selected from an ongoing ADHD genetic study that includes all disease subtypes.

RESULTS

SNPs in seven genes including SLC1A3, SLC6A3, HTR4, ADRA1A, HTR2A, SNAP25, and COMT showed a nominal level of association with ADHD (P values <0.05), but none remained significant after a stringent correction for the total number of tests performed.

CONCLUSION

The strongest signal emerged from SNPs in the promoter region (rs3808585) and in an intron (rs17426222, rs4732682, rs573514) of ADRA1A, all located within the same haplotype block. Some of the SNPs in HTR2A and COMT have already been reported by others, whereas other SNPs will need confirmation in independent samples.

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

Epinephrine-induced enhancement of memory retrieval for inhibitory avoidance conditioning in preweanling Sprague-Dawley rats

Developmental research on memory is commonly conducted using preweanling rats, but the extent to which these animals are susceptible to hormone-induced memory retrieval is unclear. This study examined the effects of epinephrine (.001, .01, or .1 mg/kg) on retrieval of IA conditioning in 17-day-old infants. Animals tested 24 hr following training performed significantly worse than infants tested 5 min after training and adults tested 24 hr later, indicating that infantile amnesia had occurred. Epinephrine attenuated this deficit in a dose-dependent manner, with a significant improvement in performance at doses of .01 mg/kg for latency and at .01 and .1 mg/kg for safe side-dependent measures. The role of epinephrine as a memory modulator is discussed in terms of its neurobiological and internal contextual effects.

Modafinil improves attention, inhibitory control, and reaction time in healthy, middle-aged rats

This study examined the effect of the novel psychostimulant modafinil (Provigil) on a variety of cognitive and behavioral measures including associative learning, sustained attention, inhibitory control, and reaction time. Middle-aged female rats (18-20 months old) were administered oral doses of modafinil (0, 8, 32, and 64 mg/kg) and tested in a 3-choice visual discrimination and sustained attention task. Modafinil produced a dose-dependent pattern of improved response accuracy and impulse control (fewer premature responses) and shorter response latencies, without affecting omission errors, motivation or motor control. Although the biochemical mechanism of modafinil is unknown, these results suggest a profile differing from typical psychostimulants (e.g., amphetamine). The implications of these findings for treatment of narcolepsy, ADHD, and various arousal-related disorders are considered. Further research is needed to examine the relative safety, effectiveness, and addictive potential of modafinil, as well as, its effects in comparison with other performance-enhancing drugs (e.g., caffeine, nicotine, and amphetamines).

Modulation of nicotine-induced attentional enhancement in rats by adrenoceptor antagonists

RATIONALE

Understanding the neuropharmacological mechanisms mediating attentional enhancement by nicotine would help a targeted search for nicotinic compounds with retained therapeutic but reduced unwanted side-effects. Previous studies suggested that the dopamine-releasing effects of nicotine may not be of primary importance for its attention-enhancing properties.

OBJECTIVES

The present study examined the role of noradrenergic neurotransmission for the effects of nicotine on different response indices of an attentional paradigm.

METHODS

The effects of systemic injections of the alpha(1)-adrenoceptor antagonist prazosin that also displays significant affinity at alpha(2B) and alpha(2C)-adrenoceptors and the beta-adrenoceptor antagonist propranolol were tested in both the presence and absence of nicotine in rats trained in a version of the five-choice serial reaction time task.

RESULTS

Nicotine generally enhanced the accuracy of signal detection, reduced omission errors and shortened response latencies. At the largest doses tested, both prazosin (1 mg/kg) and propranolol (10 mg/kg) impaired performance. For propranolol, these effects depended on the rate of target signal presentation. The two compounds differentially modulated the effects of nicotine. Propranolol (6 mg/kg and 10 mg/kg) but not prazosin reduced its effects on omission errors and accuracy. By contrast, prazosin (0.5 mg/kg) reversed the nicotine-induced reductions in response latency.

CONCLUSIONS

The data provide the first evidence that beta-adrenoceptors are involved in mediating the effects of nicotine on signal detection, while activation of alpha-adrenoceptors may contribute to its effects on response speed. This is a further indication that, from among nicotine's wide range of neuropharmacological effects, specific facets can be dissociated that are responsible for its attention-enhancing properties.

Psychopharmacological approaches to modulating attention in the five-choice serial reaction time task: implications for schizophrenia

RATIONALE

In schizophrenia, attentional disturbance is a core feature which may not only accompany the disorder, but may precede the onset of psychiatric symptoms.

OBJECTIVES

The five-choice serial reaction time task (5CSRTT) is a test of visuo-spatial attention that has been used extensively in rats for measuring the effects of systemic and central neurochemical manipulations on various aspects of attentional performance, including selective attention, vigilance and executive control. These findings are relevant to our understanding of the neural systems that may be compromised in patients with schizophrenia.

METHODS

The 5CSRTT is conducted in an operant chamber that has multiple response locations, in which brief visual stimuli can be presented randomly. Performance is maintained using food reinforcers to criterion levels of accuracy. Various aspects of performance are measured, including attentional accuracy and premature responding, especially under different attentional challenges.

RESULTS

The effects of systemic and intra-cerebral infusions of selective dopamine, serotonin and cholinergic receptor agents on the 5CSRTT are reviewed with a view to identifying attention-enhancing effects that may be relevant to the treatment of cognitive deficits in schizophrenia. In addition, some novel agents such as modafinil and histamine receptor agents are also considered. Examining the effects of selective neurochemical lesions helped define the neural locus of attentional effects. Similarly, findings from microdialysis studies helped identify the extracellular changes in neurotransmitters and their metabolites in freely moving rats during performance of the 5CSRTT.

CONCLUSIONS

The monoaminergic and cholinergic systems have independent but complementary roles in attentional function, as measured by the 5CSRTT. These functions are predominantly under the control of the prefrontal cortex and striatum. These conclusions are considered in the context of their application towards therapeutic approaches for attentional disturbances that are typically observed in schizophrenic patients.

alpha 1d Adrenoceptor signaling is required for stimulus induced locomotor activity

alpha 1 Adrenergic receptors mediate a variety of physiological responses and have been well studied in the cardiovascular and peripheral nervous system. However, their role in the central nervous system remains ill defined because of the lack of highly specific ligands to the alpha1 receptor subtypes. Here, we have employed gene targeting to elucidate the role of alpha 1d receptors in vivo. In addition to disrupting function, the insertion of the lacZ gene into the alpha 1d receptor locus enabled the specific identification of cells expressing the alpha 1d gene. These cells are localized in the cortex, hippocampus, olfactory bulb, dorsal geniculate and ventral posterolateral nuclei of the thalamus. Behaviorally, the alpha 1d(-/-) mice show normal locomotor activity during the subjective day, or resting phase of their cycle. However, during subjective night, or active phase, wheel-running activity is significantly reduced in mutant mice. Furthermore, these mice show a reduction in exploratory rearing behavior in a novel cage environment. Lastly, alpha 1d(-/-) mice show reduced hyperlocomotion after acute amphetamine administration. Together, these data reveal the functional importance of alpha 1d adrenoceptors in mediating a variety of stimulus-induced changes in locomotor behaviors. While the sensitivity of noradrenergic neurons to environmental stimuli has been well documented, our data demonstrate that at least some of these post-synaptic responses are mediated by alpha 1d adrenergic receptors.

Alpha-adrenoceptor-mediated modulation of 5-HT2 receptor agonist induced impulsive responding in a 5-choice serial reaction time task

The activation of 5-HT2A receptors has been shown to enhance the probability of premature responding, regarded as a form of motor impulsive behaviour. At the behavioural level, the interaction of alpha-adrenoceptors and 5-HT2 receptors has been linked to head twitch behaviour, regarded as an experimental model of compulsive behaviour. The aim was to determine whether the probability of premature responding induced by an excess activation of 5-HT2A receptors can be modulated by the blockade of alpha1- or alpha2- adrenoceptors. In the experiments, the 5-choice serial reaction time task was used to measure attention and response control of the rats. The experiments assessed the effects of (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) 0.1-0.2 mg/kg subcutaneously, a 5-HT2A/2C agonist, and prazosin, an alpha1-adrenoceptor antagonist, alone or in combination, on the performance of rats. In an additional experiment to examine the possible role of the alpha2-adrenoceptors, a potent, selective and specific alpha2-adrenoceptor antagonist, atipamezole, was given alone or in combination with DOI. Results showed that DOI increased the probability of premature responses, but it did not affect the choice accuracy. Prazosin (0.1 or 0.3 mg/kg, subcutaneously), given on its own had no effects on probability of responding prematurely, but prazosin (0.3 mg/kg.) was able to attenuate the DOI-induced responding. Atipamezole (0.1 mg/kg, s.c.) did not attenuate the effect of DOI on probability of premature responding. When given at lower doses, DOI (0.03 mg/kg) and atipamezole (0.03 mg/kg) synergistically increased the probability of premature responding, whereas a higher dose of atipamezole (0.3 mg/kg) on its own increased the probability of responding prematurely, but this effect was not additive to that of 0.1 mg/kg DOI. These data indicate that 5-HT2 receptor activation enhances impulsive responding and this effect can be diminished by the blockade of alpha1-adrenoceptors. Atipamezole, an alpha2-antagonist, enhances the probability of premature responding and shares the mechanism of action with the 5-HT2 agonist in this respect. These results provide evidence for an interaction between the serotonergic 5-HT2 receptors and alpha-adrenoceptors in the modulation of response control to the motor impulsivity type of behaviour (premature responding) in addition to that of compulsory behaviour (head shakes) found previously.

Animal models of attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) involves clinically heterogeneous dysfunctions of sustained attention, with behavioral overactivity and impulsivity, of juvenile onset. Experimental models, in addition to mimicking syndromal features, should resemble the clinical condition in pathophysiology, and predict potential new treatments. One of the most extensively evaluated animal models of ADHD is the spontaneously hypertensive rat. Other models include additional genetic variants (dopamine transporter gene knock-out mouse, coloboma mouse, Naples hyperexcitable rat, acallosal mouse, hyposexual rat, and population-extreme rodents), neonatal lesioning of dopamine neurons with 6-hydroxydopamine, and exposure to other neurotoxins or hippocampal irradiation. None is fully comparable to clinical ADHD. The pathophysiology involved varies, including both deficient and excessive dopaminergic functioning, and probable involvement of other monoamine neurotransmitters. Improved models as well as further testing of their ability to predict treatment responses are required.

An in vivo binding assay to determine central alpha(1)-adrenoceptor occupancy using [(3)H]prazosin

An alpha(1) adrenoceptor (alpha(1)-AdR) assay using [(3)H]prazosin binding in mouse brain is described which allows in vivo determination of central alpha(1)-AdR occupancy for ligands with alpha(1)-AdR affinity. Binding of [3H]prazosin in rat and mouse brain membranes in vitro was used to characterise the pharmacological profile of alpha(1)-AdRs in order to determine any potential species variations. Saturation and displacement studies yielded comparable affinity and pharmacological profile for [(3)H]prazosin binding in mouse and rat brain homogenates. These studies confirmed the absence of species variation for ligands in central alpha(1)-AdR pharmacology which is in good agreement with previous studies in rat brain. Subsequently, in vivo binding of [(3)H]prazosin in mouse whole brain was used to measure the occupancy of a number of AdR ligands. Timecourse studies revealed that a [3H]prazosin (5 mu Ci/mouse) pretreatment time of at least 20 min following intravenous (i.v.) administration was required for optimal specific binding. Ligands were administered systemically 40 min prior to i.v. administration of radiolabel. The alpha(1)-adrenoceptor ligands prazosin (ED(50)=0.15 mg/kg i.p.), benoxathian (0.52 mg/kg i.p.) and phentolamine (51 mg/kg i.p.) were all able to block in vivo [(3)H]prazosin binding from mouse brain. In addition, receptor occupancy values for a number of compounds including haloperidol (ED(50)=0.83 mg/kg s.c.), clozapine (2.2 mg/kg s.c.) and MDL-100907 [R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol], (10 mg/kg s.c.)], which possess high to moderate affinity at alpha(1)-adrenoceptors, were also determined. These results suggest that in the mouse, [(3)H]prazosin binding can be used to measure in vivo receptor occupancy of ligands with affinity at central alpha(1)-adrenoceptors.

A behavioral study of alpha-1b adrenergic receptor knockout mice: increased reaction to novelty and selectively reduced learning capacities

Knockout mice lacking the alpha-1b adrenergic receptor were tested in behavioral experiments. Reaction to novelty was first assessed in a simple test in which the time taken by the knockout mice and their littermate controls to enter a second compartment was compared. Then the mice were tested in an open field to which unknown objects were subsequently added. Special novelty was introduced by moving one of the familiar objects to another location in the open field. Spatial behavior and memory were further studied in a homing board test, and in the water maze. The alpha-1b knockout mice showed an enhanced reactivity to new situations. They were faster to enter the new environment, covered longer paths in the open field, and spent more time exploring the new objects. They reacted like controls to modification inducing spatial novelty. In the homing board test, both the knockout mice and the control mice seemed to use a combination of distant visual and proximal olfactory cues, showing place preference only if the two types of cues were redundant. In the water maze the alpha-1b knockout mice were unable to learn the task, which was confirmed in a probe trial without platform. They were perfectly able, however, to escape in a visible platform procedure. These results confirm previous findings showing that the noradrenergic pathway is important for the modulation of behaviors such as reaction to novelty and exploration, and suggest that this is mediated, at least partly, through the alpha-1b adrenergic receptors. The lack of alpha-1b adrenergic receptors in spatial orientation does not seem important in cue-rich tasks but may interfere with orientation in situations providing distant cues only.

Decreased exploratory activity and impaired passive avoidance behaviour in mice deficient for the alpha(1b)-adrenoceptor

There is growing evidence that a dysfunction of central noradrenergic neurotransmission is involved in age-related impairments of cognitive performance and the pathophysiology of Alzheimer's disease (AD). A reduction of density of central alpha(1)-adrenergic receptors (alpha(1)-AR) has been shown in aging and AD brains. Three alpha(1)-AR subtypes (alpha(1a), alpha(1b) and alpha(1d)) have been identified by molecular cloning. However, very little is known about the functional role of distinct alpha(1)-AR subtypes in the brain. This problem was specifically addressed using a model of knockout mouse deficient in alpha(1b)-AR (alpha(1B)-/-) because these animals show a 40% reduction of alpha(1)-AR density in the brain as already reported. In comparison to the wild-type mice (alpha(1B)+/+), alpha(1B)-/- mice showed significantly reduced square entries and a reduced rearing behaviour was observed over all sessions in the open field. In passive avoidance procedures, alpha(1B)-/- mice showed a tendency towards decreased short-term-latency and a significant decline in long-term-latency. The present results indicate that mutation of a single member of the alpha(1)-AR gene family creates a distinct phenotype and provide evidence that alpha(1B)-AR is possibly involved in modulation of memory consolidation and fear-motivated exploratory activity. Furthermore, this model of knockout mice may be useful in elucidating the role of alpha(1B)-AR in dementias involving deficits of the noradrenergic system.

Correlation between serum concentrations following continuous intravenous infusion of dexmedetomidine or medetomidine in cats and their sedative and analgesic effects

Dexmedetomidine (DEX) may have some therapeutic advantages over the racemate medetomidine (MED). Here we have examined how serum concentrations of DEX correlate with some of its anaesthetic effects. Cats (n = 6) were administered with a continuous stepwise intravenous (i.v.) infusion of DEX or MED on different occasions in a cross-over design. Maintenance infusion rates (mg/kg/min) used were: DEX = 0.25 (MED = 0.50); DEX = 1 (MED = 2) and DEX = 4 (MED = 8) for infusion steps 1, 2 and 3, respectively. Each maintenance infusion lasted at least 50 min and was preceded with a loading dose. There was no significant difference between serum DEX and 0.5 serum MED concentrations at any dose level nor was there a significant difference between serum DEX and the (entire) serum MED concentrations. There was no significant difference between DEX and MED for sedation, analgesia, muscular relaxation and heart and respiratory rates. For both DEX and MED, serum drug concentration and analgesia were dose-dependent and sedation increased until the end of infusion step 2 (dose level 2) and decreased at the end of step 3 (dose level 3). Muscular relaxation was not dose-dependent. We conclude that increasing the blood concentration of DEX or MED beyond a certain level decreases the level of sedation instead of increasing it even though analgesia increases. The rate at which DEX and MED are metabolized in cats may not be the same.

Effects of D-cycloserine, a positive modulator of N-methyl-D-aspartate receptors, and ST 587, a putative alpha-1 adrenergic agonist, individually and in combination, on the non-delayed and delayed foraging behaviour of rats assessed in the radial arm maze

The present study investigated whether alpha-1 adrenergic and glutamatergic N-methyl-D-aspartate (NMDA) receptor-mediated mechanisms interact in memory processes, by examining the effects of individual and combined systemic administration of ST 587, a putative alpha-1 agonist, and D-cycloserine (DCS), a partial agonist at the glycine-B binding site of the NMDA receptor, on the performance of rats in non-delayed and delayed (4-6 h) foraging behaviour in the radial arm maze task, using the delayed non-matching to sample (DNMTS) version. The results indicated that DCS (5.0 mg/kg) decreased working memory errors, i.e. the number of re-entries into the previously visited arms during the sampling phase. In addition, both ST 587 (100 microg/kg) and DCS (10 mg/kg), when administered alone 30 min before a sampling phase, improved retention of this task as reflected by the increased number of correct choices before the first error during the retention phase. The combined administration of ST 587 and DCS, however, did not lead to better retention in the DNMTS task compared with the administration of each of the drugs alone. Combinations of sub-threshold doses of ST 587 (50 or 75 microg/kg) and DCS (5.0 or 7.5 mg/kg) also did not improve retention in this task. DCS (5.0 or 7.5 mg/kg) increased activity as indicated by the increased number of arm entries in a given time during the sampling phase. These findings suggest that the systemic administration of a positive modulator of the NMDA receptor facilitates hippocampal-dependent memory functions, but that these effects are not enhanced by combined administration with an alpha-1 agonist, even though the alpha-1 agonist is effective when given alone. The results support the idea that NMDA receptors modulate both mnemonic and non-mnemonic functions in the brain.

Central alpha1-adrenoceptors: their role in the modulation of attention and memory formation

Adrenoceptors presently are classified into three main subclasses: alpha1-, alpha2-, and beta-receptors, each with three (perhaps more) subtypes. All three alpha1-adrenoceptor subtypes are present in rat brain. The purpose of this review is to assess the role of alpha1-adrenoceptors in the modulation of synaptic transmission and plasticity, as well as their ability to modulate higher cerebral functions, such as attentional and memory processes. However, since there are no truly subtype-specific agonists or antagonists available at present, it is virtually impossible to allocate a particular central effect to one or other of the subtypes. The activation of alpha1-adrenoceptors reduces the firing probability and glutamate release in the cornu ammonis of the hippocampus. Alpha1-Adrenoceptors may flexibly modulate weak and strong activation of the pyramidal neurones in the neocortex. Alpha1-Adrenoceptors play only a minor role in the modulation of long-term potentiation in the hippocampus, and may influence many brain functions also via non-neuronal mechanisms. since glial cells can express alpha1-adrenoceptors. At the behavioural level, the activation of alpha1-adrenoceptors promotes vigilance and influences working memory and behavioural activation, while having only a minor role in the modulation of long-term memory.