Evaluation of muscarinic and nicotinic receptor antagonists on attention and working memory

Intermittent stimulation in the nucleus basalis of meynert improves sustained attention in rhesus monkeys

Sustained attention is essential in important behaviors in daily life. Many neuropsychiatric disorders are characterized by a compromised ability to sustain attention, making this cognitive domain an important therapeutic target. In this study, we tested a novel method of improving sustained attention. Monkeys were engaged in a continuous performance task (CPT) while the nucleus basalis of Meynert (NB), the main source of cholinergic innervation of the neocortex, was stimulated. Intermittent NB stimulation improved the animals' performance by increasing the hit rate and decreasing the false alarm rate. Administration of the cholinesterase inhibitor donepezil or the muscarinic antagonist scopolamine alone impaired performance, whereas the nicotinic antagonist mecamylamine alone improved performance. Applying NB stimulation while mecamylamine or donepezil were administered impaired CPT performance. Methylphenidate, a monoaminergic psychostimulant, was applied in conjunction with intermittent stimulation as a negative control, as it does not directly modulate cholinergic output. Methylphenidate also improved performance, and it produced further improvement when combined with NB stimulation. The additive effect of the combination suggested NB stimulation altered behavior independently from methylphenidate effects. We conclude that basal forebrain projections contribute to sustained attention, and that intermittent NB stimulation is an effective way of improving performance.

Task- and Treatment Length-Dependent Effects of Vortioxetine on Scopolamine-Induced Cognitive Dysfunction and Hippocampal Extracellular Acetylcholine in Rats

Major depressive disorder (MDD) is a common psychiatric disorder that often features impairments in cognitive function, and these cognitive symptoms can be important determinants of functional ability. Vortioxetine is a multimodal antidepressant that may improve some aspects of cognitive function in patients with MDD, including attention, processing speed, executive function, and memory. However, the cause of these effects is unclear, and there are several competing theories on the underlying mechanism, notably including regionally-selective downstream enhancement of glutamate neurotransmission and increased acetylcholine (ACh) neurotransmission. The current work sought to evaluate the ACh hypothesis by examining vortioxetine’s ability to reverse scopolamine-induced impairments in rodent tests of memory and attention. Additionally, vortioxetine’s effects on hippocampal extracellular ACh levels were examined alongside studies of vortioxetine’s pharmacokinetic profile. We found that acute vortioxetine reversed scopolamine-induced impairments in social and object recognition memory, but did not alter scopolamine-induced impairments in attention. Acute vortioxetine also induced a modest and short-lived increase in hippocampal ACh levels. However, this short-term effect is at variance with vortioxetine’s moderately long brain half life (5.1 hours). Interestingly, subchronic vortioxetine treatment failed to reverse scopolamine-induced social recognition memory deficits and had no effects on basal hippocampal ACh levels. These data suggest that vortioxetine has some effects on memory that could be mediated through cholinergic neurotransmission, however these effects are modest and only seen under acute dosing conditions. These limitations may argue against cholinergic mechanisms being the primary mediator of vortioxetine′s cognitive effects, which are observed under chronic dosing conditions in patients with MDD.

Roles of cholinergic receptors during attentional modulation of cue detection

Basal forebrain corticopetal cholinergic neurons are known to be necessary for normal attentional processing. Alterations of cholinergic system functioning have been associated with several neuropsychiatric diseases, such as Alzheimer’s disease and schizophrenia, in which attentional dysfunction is thought to be a key contributing factor. Loss of cortical cholinergic inputs impairs performance in attention-demanding tasks. Moreover, measures of acetylcholine with microdialysis and, more recently, of choline with enzyme-coated microelectrodes have begun to elucidate the precise cognitive demands that activate the cholinergic system on distinct time scales. However, the receptor actions following acetylcholine release under attentionally-challenging conditions are only beginning to be understood. The present review is designed to summarize the evidence regarding the actions of acetylcholine at muscarinic and nicotinic receptors under cognitively challenging conditions in order to evaluate the functions mediated by these two different cholinergic receptor classes. Moreover, evidence that supports beneficial effects of muscarinic muscarinic-1 receptor agonists and selective nicotinic receptor subtype agonists for cognitive processing will be discussed. Finally, some challenges and limitations of targeting the cholinergic system for treating cognitive deficits along with future research directions will be mentioned. In conclusion, multiple aspects of cholinergic neurotransmission must be considered when attempting to restore function of this neuromodulatory system.

Chronic scopolamine-injection-induced cognitive deficit on reward-directed instrumental learning in rat is associated with CREB signaling activity in the cerebral cortex and dorsal hippocampus

RATIONALE

Scopolamine, a nonselective muscarinic receptor antagonist, has been used in experimental animal models of dementia. It has been demonstrated to disrupt performances in a battery of behavioral tests. However, no attempt has been made to determine how scopolamine-treated animals would respond to a series of reward-directed instrumental learning (RDIL) tasks.

OBJECTIVES

The present study was designed to investigate the effects of chronic intraperitoneal injection of scopolamine in Wistar rats on RDIL, as well as on the expression of memory-related molecules in the dorsal hippocampus (DH) and cerebral cortex (CCx).

METHODS

The effects of the pretraining injection of scopolamine on the acquisition of instrumental response (experiment 1) were first investigated. Then, the effects of post-training manipulation on the maintenance of instrumental response and the responses to changes in contingency degradation and signal discrimination were assessed (experiment 2). Finally, the expression of cyclic AMP response element-binding protein (CREB), phosphorylated CREB, and brain-derived neurotrophic factor in the DH and CCx were examined using Western blotting and enzyme-linked immunosorbent assay.

RESULTS

The acquisition of instrumental conditioning is more vulnerable than its maintenance. The 3.0-mg/kg dose of scopolamine rendered rats unable to make adaptive changes in facing contingency degradation and correct responses in signal discrimination tasks. Furthermore, CREB signaling was inactivated by pretraining scopolamine treatment in both the DH and CCx. Nevertheless, this pathway was selectively suppressed by post-training treatment only in the CCx during memory reconsolidation.

CONCLUSIONS

The results suggest that scopolamine-induced cognitive deficits on RDIL are related to the distinguishing alteration of CREB signaling in the DH and CCx.

Effects of the neurotensin NTS₁ receptor agonist PD149163 on visual signal detection in rats

Antipsychotic drugs provide limited efficacy for cognitive impairment in schizophrenia. Recent studies have found that the neurotensin NTS1 receptor agonist and putative atypical antipsychotic drug PD149163 reverses deficits in sensory-gating and novel object recognition, suggesting that this compound may have the potential to improve cognitive functioning in schizophrenia. The present study sought to extend these investigations by evaluating the effects of PD149163 on sustained attention using a visual signal detection operant task in rats. PD149163, the atypical antipsychotic drug clozapine, and the dopamine D2/3 receptor antagonist raclopride all significantly decreased percent "hit" accuracy, while none of these compounds altered "correct rejections" (compared to vehicle control). Clozapine and raclopride significantly increased response latency, while high doses of PD149163 and raclopride significantly increased trial omissions. Nicotine, which was tested as a positive control, significantly improved overall performance in this task and did not affect response latency or trial omissions. The present findings suggest that neurotensin NTS1 receptor agonists, like antipsychotic drugs, may inhibit sustained attention in this task despite having different pharmacological mechanisms of action.

Task demands dissociate the effects of muscarinic M1 receptor blockade and protein kinase C inhibition on attentional performance in rats

The cholinergic system is known to be necessary for normal attentional processing. However, the receptors and mechanisms mediating the effects of acetylcholine on attention remain unclear. Previous work in our laboratory suggested that cholinergic muscarinic receptors are critical for maintaining performance in an attention-demanding task in rats. We examined the role of the muscarinic M(1) receptor and protein kinase C (PKC), which is activated by the M(1) receptor, in attention task performance. Rats were trained in an attention-demanding task requiring discrimination of brief (500, 100, 25 ms) visual signals from trials with no signal presentation. The effects of muscarinic M(1) receptor blockade were assessed by administering dicyclomine (0-5.0 mg/kg). The effects of PKC inhibition were assessed by administering chelerythrine chloride (0-2.0 mg/kg). Dicyclomine decreased the accuracy of detecting longer signals in this attention task, including when attentional demands were increased by flashing a houselight throughout the session. Chelerythrine chloride decreased the accuracy of signal detection in the standard version of the task but not when the houselight was flashed throughout the session. The present findings indicate that muscarinic M(1) receptors are critical for maintaining performance when attentional demands are increased, and that PKC activity may contribute to some aspects of attentional performance.

Low-dose nicotine facilitates spatial memory in ApoE-knockout mice in the radial arm maze

Here, we investigated the effects of nicotine on spatial memory in ApoE-knockout (ApoE-KO) and wild-type (WT) mice in a radial arm maze. Training occurred on three consecutive days and the test was performed on day 4, with one trial per day. Then on day 4, animals were administered nicotine (0.1, 0.25, 0.5, and 1.0 mg/kg) or the antagonist of nicotinic receptors (nAChRs) mecamylamine (MEC 2 mg/kg) alone or together with 0.1 mg/kg nicotine. The number of errors in the first eight choices was recorded. The results were that 0.1 mg/kg nicotine decreased errors in ApoE-KO mice, while 0.1 and 0.25 mg/kg nicotine reduced errors in WT mice, indicating that lower doses of nicotine elicit a memory improvement. In contrast, 1.0 mg/kg nicotine increased errors in WT mice, but not in ApoE-KO mice. MEC alone had no noticeable effect on errors in either strain of mice. However, co-administration of 0.1 mg/kg nicotine and MEC increased errors and reduced the effects of nicotine in WT mice, but not in ApoE-KO mice. Our study found a biphasic effect of nicotine in WT mice: it improves spatial memory at lower doses and impairs it at a higher dose. In ApoE-KO mice, nicotine improves memory at a low dose and has no effect at a higher dose, suggesting that the ApoE deficiency may influence the efficacy of nicotine. Moreover, a reversal of nicotinic effects with MEC was seen in WT mice, indicating the likelihood of the involvement of nAChRs in the spatial-memory response to nicotine.

Cholinergic receptor subtypes and their role in cognition, emotion, and vigilance control: an overview of preclinical and clinical findings

RATIONALE

The cholinergic system has long been linked to cognitive processes. Two main classes of acetylcholine (ACh) receptors exist in the human brain, namely muscarinic and nicotinic receptors, of which several subtypes occur.

OBJECTIVES

This review seeks to provide an overview of previous findings on the influence of cholinergic receptor manipulations on cognition in animals and humans, with particular emphasis on the role of selected cholinergic receptor subtypes. Furthermore, the involvement of these receptor subtypes in the regulation of emotion and brain electrical activity as measured by electroencephalography (EEG) shall be addressed since these domains are considered to be important modulators of cognitive functioning.

RESULTS

In regard to cognition, the muscarinic receptor subtypes have been implicated mainly in memory functions, but have also been linked to attentional processes. The nicotinic α7 receptor subtype is involved in working memory, whereas the α4β2* subtype has been linked to tests of attention. Both muscarinic and nicotinic cholinergic mechanisms play a role in modulating brain electrical activity. Nicotinic receptors have been strongly associated with the modulation of depression and anxiety.

CONCLUSIONS

Cholinergic receptor manipulations have an effect on cognition, emotion, and brain electrical activity as measured by EEG. Changes in cognition can result from direct cholinergic receptor manipulation or from cholinergically induced changes in vigilance or affective state.

Subtype-selective nicotinic agonists enhance olfactory working memory in normal rats: a novel use of the odour span task

Nicotinic agonists have been shown to enhance performance in cognitive tasks based on attention and memory. The aim of this study was to use a test of olfactory working memory; the odour span task (OST) in rodents, to investigate the effects of subtype-specific nicotinic agonists on working memory in normal rats. Rats were trained in a non-matching to sample (NMTS) rule and then the full OST, which involved identifying a novel odour from an increasing number of presented odours. Male hooded Lister rats were treated with nicotine, selective nicotinic agonists or vehicle (saline). In order to validate the task, muscarinic and nicotinic receptor antagonists were also examined. Nicotine at both 0.05 and 0.1mg/kg significantly increased mean span length in the OST. The selective alpha 4 beta 2 nicotinic receptor agonist metanicotine (0.1mg/kg s.c.) and the selective alpha 7 nicotinic receptor agonist (R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide) (compound A, 10mg/kg i.p.) also improved performance. In contrast, mecamylamine and scopolamine significantly decreased mean span length. These findings suggest a role for the activation of both alpha 4 beta 2 and alpha 7 subtypes of neuronal nicotinic receptor in mediating enhancements of olfactory working memory capacity in normal, non-compromised rats. These nicotinic receptor subtypes may therefore prove to be useful targets for the development of novel treatments for neuropsychiatric disorders that involve cognitive dysfunction.

Cognitive effects of psychotomimetic drugs in rats discriminating number cues

RATIONALE

Deficits in memory and attention are broadly acknowledged during psychosis; however, experiments on modeled psychosis often test working memory without systematic manipulation of attentional demands.

OBJECTIVES

The major research goal was discovering which neurobehavioral processes, attention, or memory contributed more to drug-provoked performance deficits.

MATERIALS AND METHODS

Rats were trained to perform operant ratio discrimination (RD) tasks wherein the number of presses at a rear-wall lever was discriminated using one of two front-wall levers. Effects from four psychotomimetic drugs, the serotonin agonist 2,5-dimethoxy-4-iodoamphetamine, the noncompetitive NMDA-glutamate receptor antagonist phencyclidine (PCP), and two CB1-selective cannabinoid agonists, WIN 55,512-2 and AM 411, were assessed using a signal detection analytical overlay to dissociate cognitive from noncognitive motor and motivational disruptions. Further methods allowed dissociation of attention compromises from mnemonic deficits.

RESULTS

For each test compound, at least one dose elicited decreased RD accuracy without affecting response rates, and task difficulty was shown to be a crucial dictator of accuracy effect specificities. Effects from both PCP and WIN 55,512-2 biased animals to select the response lever conditioned for denser reinforcement. The same two drugs rendered peculiar response patterns in distracter light session components, considering light blinks were included to divert subjects' attention away from task-relevant information. The response patterns determined during distracter components of PCP/WIN testing sessions, counterintuitively, suggest performance enhancement.

CONCLUSION

Comprehensive viewing of RD performance patterns after drug administration indicates that sustained attention and transient information management are significantly impaired during the drug-induced psychosis state, while selective attention is less affected.

Systemic and intrabasalis administration of the orexin-1 receptor antagonist, SB-334867, disrupts attentional performance in rats

RATIONALE

Orexin neurons project to a number of brain regions, including onto basal forebrain cholinergic neurons. Basal forebrain corticopetal cholinergic neurons are known to be necessary for normal attentional performance. Thus, the orexin system may contribute to attentional processing.

OBJECTIVES

We tested whether blockade of orexin-1 receptors would disrupt attentional performance.

METHODS

Rats were trained in a two-lever sustained attention task that required discrimination of a visual signal (500, 100, 25 ms) from trials with no signal presentation. Rats received systemic or intrabasalis administration of the orexin-1 receptor antagonist, SB-334867, prior to task performance.

RESULTS

Systemic administration of the orexin-1 receptor antagonist, SB-334867 (5.0 mg/kg), decreased detection of the longest duration signal. Intrabasalis SB-334867 (0.60 microg) decreased overall accuracy on trials with longer signal durations.

CONCLUSIONS

These findings suggest that orexins contribute to attentional processing, although neural circuits outside of basal forebrain corticopetal cholinergic neurons may mediate some of these effects.

Participation of cholinergic system in memory deficits induced by blockade of hippocampal mGlu1 receptors

We investigated the role of hippocampal group I metabotropic glutamate receptors (mGlu(1) receptors) in the retrieval process of spatial memory using 8-arm radial maze performance with 4 arms baited. In addition, the participation of the cholinergic system in memory deficits induced by mGlu(1) receptors antagonist was studied. Intrahippocampal injection of (R,S)-1-Aminoindan-1,5-dicarboxylic acid (AIDA), a mGlu(1) receptor antagonist, significantly increased total error, reference memory error and working memory error at a dose of 20 nmol/side. Donepezil (0.5 and 1.0 mg/kg, p.o.) showed an ameliorative effect on AIDA-induced memory deficits. Improvement by donepezil of AIDA-induced memory deficits was antagonized by scopolamine (5 nmol/side) but not by mecamylamine (200 nmol/side) at a dose that did not affect performance. These findings clearly indicate that hippocampal mGlu(1) receptors play an important role in the retrieval of spatial memory. Furthermore, we found that hippocampal mGlu(1) receptors were closely associated with muscarinic receptors in memory performance.