Dissociable effects of histamine H1 antagonists on reaction-time performance in rats

Medications influencing central cholinergic pathways affect fixation stability, saccadic response time and associated eye movement dynamics during a temporally-cued visual reaction time task

RATIONALE

Anticholinergic medications largely exert their effects due to actions on the muscarinic receptor, which mediates the functions of acetylcholine in the peripheral and central nervous systems. In the central nervous system, acetylcholine plays an important role in the modulation of movement.

OBJECTIVE

This study investigated the effects of over-the-counter medications with varying degrees of central anticholinergic properties on fixation stability, saccadic response time and the dynamics associated with this eye movement during a temporally-cued visual reaction time task, in order to establish the significance of central cholinergic pathways in influencing eye movements during reaction time tasks.

METHODS

Twenty-two participants were recruited into the placebo-controlled, human double-blind, four-way crossover investigation. Eye tracking technology recorded eye movements while participants reacted to visual stimuli following temporally informative and uninformative cues. The task was performed pre-ingestion as well as 0.5 and 2 h post-ingestion of promethazine hydrochloride (strong centrally acting anticholinergic), hyoscine hydrobromide (moderate centrally acting anticholinergic), hyoscine butylbromide (anticholinergic devoid of central properties) and a placebo.

RESULTS

Promethazine decreased fixation stability during the reaction time task. In addition, promethazine was the only drug to increase saccadic response time during temporally informative and uninformative cued trials, whereby effects on response time were more pronounced following temporally informative cues. Promethazine also decreased saccadic amplitude and increased saccadic duration during the temporally-cued reaction time task.

CONCLUSION

Collectively, the results of the study highlight the significant role that central cholinergic pathways play in the control of eye movements during tasks that involve stimulus identification and motor responses following temporal cues.

Convergent pharmacological mechanisms in impulsivity and addiction: insights from rodent models

Research over the last two decades has widely demonstrated that impulsivity, in its various forms, is antecedent to the development of drug addiction and an important behavioural trait underlying the inability of addicts to refrain from continued drug use. Impulsivity describes a variety of rapidly and prematurely expressed behaviours that span several domains from impaired response inhibition to an intolerance of delayed rewards, and is a core symptom of attention deficit hyperactivity disorder (ADHD) and other brain disorders. Various theories have been advanced to explain how impulsivity interacts with addiction both causally and as a consequence of chronic drug abuse; these acknowledge the strong overlaps in neural circuitry and mechanisms between impulsivity and addiction and the seemingly paradoxical treatment of ADHD with stimulant drugs with high abuse potential. Recent years have witnessed unprecedented progress in the elucidation of pharmacological mechanisms underpinning impulsivity. Collectively, this work has significantly improved the prospect for new therapies in ADHD as well as our understanding of the neural mechanisms underlying the shift from recreational drug use to addiction. In this review, we consider the extent to which pharmacological interventions that target impulsive behaviour are also effective in animal models of addiction. We highlight several promising examples of convergence based on empirical findings in rodent-based studies.

Neurobehavioral effects of head-only gamma-radiation exposure in rats

The present report describes initial steps in the development of an animal model for assessing the effects of low levels of radiation encountered in the space environment on human cognitive function by examining the effects of radiation on a range of neurobehavioral functions in rodents that are similar to a number of basic human cognitive functions. The present report presents baseline data on the effects of gamma radiation on neurobehavioral functions in rodents (psychomotor speed, discrimination accuracy and inhibitory control) that are similar to those in humans. Two groups of eight Long-Evans rats were trained to perform a reaction-time task that required them to depress a lever for 1-3 s and to release the lever within 1.5 s of a release stimulus (correct trial) to receive a reward. Releasing the lever prior to the release stimulus (error) terminated the trial. One group was exposed to head-only gamma radiation (5 Gy at a dose rate of 1 Gy/min), while the second group was sham-irradiated using the same anesthesia protocol. The irradiated group showed significant deficits in both performance accuracy (percentage correct scores) and performance reliability (false alarm scores) from 1 to 4 months after irradiation, indicating clear performance impairments. The increase in false alarm scores is consistent with reduced inhibitory control and a shift toward increased anticipatory responses at the cost of decreased accuracy. The nonirradiated group showed no such changes over the same period.

Tolerability and safety of fluvoxamine and other antidepressants

Selective serotonin [5-hydroxytryptamine (5-HT)] reuptake inhibitors (SSRIs) and the 5-HT noradrenaline reuptake inhibitor, venlafaxine, are mainstays in treatment for depression. The highly specific actions of SSRIs of enhancing serotonergic neurotransmission appears to explain their benefit, while lack of direct actions on other neurotransmitter systems is responsible for their superior safety profile compared with tricyclic antidepressants. Although SSRIs (and venlafaxine) have similar adverse effects, certain differences are emerging. Fluvoxamine may have fewer effects on sexual dysfunction and sleep pattern. SSRIs have a cardiovascular safety profile superior to that of tricyclic antidepressants for patients with cardiovascular disease; fluvoxamine is safe in patients with cardiovascular disease and in the elderly. A discontinuation syndrome may develop upon abrupt SSRI cessation. SSRIs are more tolerable than tricyclic antidepressants in overdose, and there is no conclusive evidence to suggest that they are associated with an increased risk of suicide. Although the literature suggests that there are no clinically significant differences in efficacy amongst SSRIs, treatment decisions need to be based on considerations such as patient acceptability, response history and toxicity.

Acute and separate modulation of motor and cognitive performance in parkinsonian rats by bilateral stimulation of the subthalamic nucleus

The subthalamic nucleus (STN) is involved in motor and cognitive performance through its key role in the basal ganglia-thalamocortical circuits, but how these different modalities (motor and cognition) are controlled (similar vs. dissimilar) has not yet been elucidated. In the present study, the effects of bilateral STN deep brain stimulation (DBS) on motor and cognitive performance were investigated in a rat model of Parkinson disease (PD). After being trained in a choice reaction time (CRT) task, rats received bilateral injections of 6-hydroxydopamine (6-OHDA) into the striatum. One group of 6-OHDA animals was implanted bilaterally with stimulation electrodes at the level of the STN. Stimulations were performed at 130 Hz (frequency), 60 micros (pulse width), and varying amplitudes of 1, 3, 30, and 150 microA during the CRT task. Finally, rats were sacrificed and the brains processed for staining to determine the dopaminergic lesion (TH immunohistochemistry) and localization of the electrode tip (HE histochemistry). Bilateral 6-OHDA infusion significantly decreased (70%) the number of dopaminergic cells in the substantia nigra pars compacta (SNc) and increased motor time (MT), proportion of premature responding (PR), and reaction time (RT). Bilateral STN stimulation with an amplitude of 3 microA normalized 6-OHDA-induced deficits in PR and RT. Simulation with an amplitude of 30 microA reversed the lesion-induced deficits in MT and RT. Our data show for the first time that bilateral STN stimulation differentially affected the 6-OHDA-induced motor and cognitive deficits. This means that basal ganglia-thalamocortical motor and associative circuits responsible for specific motor and cognitive performance, which are processed through the STN, have unique physiological properties that can acutely and separately be modulated by specific electrical stimuli.

Resident cardiac mast cells and ischemia-reperfusion injury

BACKGROUND

The intense inflammatory reaction following reperfusion of ischemic myocardium has been implicated as a factor in the extension of myocardial injury. One of the therapeutic goals of modern cardiology is to design strategies to limit the infarct size following myocardial infarction. A sound understanding of the inflammatory cascade that involves the release of various proinflammatory mediators from cardiac cells is necessary before a specific intervention is pursued.

OBSERVATION

Summarized is the role of resident cardiac mast cells, which are noted to release inflammatory mediators, in ischemia-reperfusion-induced myocardial injury. Various pharmacologic interventions, such as disodium cromoglycate and ketotifen, that stabilize cardiac mast cells, or agents such as chlorpheniramine and cetirizine that prevent their degranulation during ischemia and reperfusion, may prove to be potential therapeutic agents to limit or salvage ischemia-reperfusion-induced injury.

CONCLUSION

On the basis of the effects of histamine H1 antagonists, adrenoceptor blockers, cellular calcium and nitric oxide modulators, as well as inhibitors of phosphodiesterase and mitogen-activated protein kinase on mast cells, cardiac resident mast cells may represent a novel target for the development of cardioprotective agents.