Selective α7 nicotinic receptor activation by AZD0328 enhances cortical dopamine release and improves learning and attentional processes

doi:10.1016/j.bcp.2009.07.005

Biochemical Pharmacology

Volume 78, Issue 7, 1 October 2009, Pages 880-888

https://doi.org/10.1016/j.bcp.2009.07.005 Get rights and content

Abstract

AZD0328, a novel spirofuropyridine neuronal nicotinic receptor partial agonist, was used to investigate the role of α7 neuronal nicotinic receptor (NNR) activation in the modulation of midbrain dopamine neuron function, cortical dopamine release and on two behavioral tasks known to be dependent on optimal levels of cortical dopamine. In vivo recordings from area 10 (ventral tegmental area) in rat brain showed an increased firing of putative dopamine neurons in response to low (0.00138 mg/kg) doses of AZD0328. Bursting patterns of dopamine neuron activity remained largely unchanged by application of AZD0328. In vivo microdialysis in awake rats showed an increase in extracellular prefrontal cortical dopamine in response to low doses of AZD0328. Compound-stimulated dopamine release showed an inverted dose effect relation that was maximal at the lowest dose tested (0.00178 mg/kg). Peak extracellular dopamine levels were reached 2 h after dosing with AZD0328. Acquisition of operant responding with delayed reinforcement in rats was dose dependently enhanced by AZD0328 with a plateau effect measured at 0.003 mg/kg. This effect was blocked by pre-treatment of animals with the selective α7 antagonist methyllycaconitine. AZD0328 improved novel object recognition in mice over a broad range of doses (0.00178–1.78 mg/kg) and the compound effect was found to be absent in homozygous α7 KO animals. Together, these data indicate that selective interaction with α7 NNRs by AZD0328 selectively enhances midbrain dopaminergic neuronal activity causing an enhancement of cortical dopamine levels; these neurochemical changes likely, underlie the positive behavioral responses observed in two different animal models. Our results suggest selective α7 NNR agonists may have significant therapeutic utility in neurologic and psychiatric indications where cognitive deficits and dopamine neuron dysfunction co-exist.

Graphical abstract

α7 Neuronal nicotinic receptor activation of midbrain dopamine neurons improves selective attention and acquisition of operant and object recognition behaviors.

Introduction

Nicotine and selective NNR agonists activate central neuronal nicotinic receptors and have been shown to enhance memory and attention. Activation of NNRs has been shown to modulate neurotransmitter release and function throughout the central nervous system, specifically this cognition enhancement is thought to be, at least in part, mediated through modulation of dopaminergic and glutamatergic transmission [1]. NNR subtypes composed of pentameric arrangements of α2, α4, α6, β2, and β4 subunits as well as α7 subunits show distinct neuro-anatomical distributions in mammalian brain consistent with involvement in the modulation of excitatory and inhibitory synaptic networks. Particularly, these subtypes of NNRs are expressed in the mesostriatal and mesolimbic dopamine pathways and have been shown to modulate the firing patterns of these dopaminergic neurons [2], [3], [4], [5]. Appropriate activation of midbrain dopaminergic neurons is crucial to many of the motivational aspects of goal directed behavior and decision-making. These aspects of behavior include the allocation of attentional resources and processes involved in reward evaluation, prediction, and outcome expectancy. In both human and animal models, the functional properties of the dopamine system influence an organism's ability to alter behavioral choices based upon recent experience and have been shown to correlate with improvements in learning and memory [3], [21]. While dopaminergic dysfunction is implicated in the motivational and cognitive abnormalities associated with a number of neurological and psychiatric disorders, neuronal mechanisms regulating dopaminergic function in vivo are not well established.

Despite an incomplete understanding of dopamine's regulation in vivo, converging molecular, pharmacological, and physiological evidence indicates a role for cholinergic inputs in the regulation of the midbrain dopaminergic system. Of particular importance in the control of dopamine release is a diverse set of nicotinic receptors located both directly on dopamine-producing cells within the ventral tegmental area (VTA) as well as on local inhibitory interneurons. Nicotine directly activates VTA neurons causing a number of time-dependent changes in dopamine release. Dopamine system activation is thought to account for the rewarding properties of nicotine and may also partially account for nicotine-associated improvements in attention and memory [3]. Nevertheless, the functional roles played by distinct subtypes of nicotinic receptors relative to the dopamine system remain to be fully elucidated.

Here we show that administration of low doses of a selective α7 NNR agonist, AZD0328, leads to a significant increase in the excitability of midbrain dopaminergic neurons with no independent change in the spike patterns produced by these neurons. Within the same dose range, AZD0328 led to a significant increase in cortical dopamine release in awake rodents and improved both conditioned response learning and memory retention in an object recognition task. The selective α7-receptor antagonist, methyllycaconitine (MLA), blocked the dopamine releasing and memory improving functions of AZD0328. Interestingly, during the training phase of the object recognition test, AZD0328 administration led to a robust increase in the targeted and selective exploration of objects without an overall increase in locomotor activity. To confirm the primary receptor requirement for AZD0328's mechanism of action, we tested the effect of AZD0328 on novel object recognition (NOR) in Chrna7 knockout mice and their non-transgenic littermates. This novel finding suggests a role for α7 NNRs in the selective allocation of attentional resources mediated, in part, by an enhancement in dopamine release. Together, these data provide mechanistic insight into how highly selective α7 agonists can be used to treat attentional and motivational abnormalities associated with a number of neurological and psychiatric conditions including: Alzheimer's disease, schizophrenia, bipolar disorder and attention deficit hyperactivity disorder (ADHD).

Section snippets

Test compounds

AZD0328 ((2′R)-spiro-[1-azabicyclo[2.2.2]octane-3,2′(3′H)-furo[2,3-b]pyridine] d-tartrate) (structure shown in Fig. 1) was prepared by AstraZeneca Wilmington CNS Discovery Chemistry [6]. For in vitro studies, AZD0328 was dissolved in distilled water or DMSO before use. For in vivo studies, AZD0328 was dissolved in 0.9% saline or phosphate buffered saline before use. Chloral hydrate, ketamine, methyllycaconitine, xylazine and all other reagents were purchased from Sigma–Aldrich (St. Louis, MO).

Receptor binding

Results

The in vitro binding characteristics of AZD0328 were determined on nicotinic receptor subtypes, and the structurally related 5HT₃ receptor, using a wide variety of radioligands and membrane preparations (Table 1). AZD0328 exhibited high affinity for the native α7 NNR of rat hippocampal membranes using the classical α7 antagonist radioligand [¹²⁵I]α-bungarotoxin. AZD0328 exhibited similar high affinity for the human α7 NNR stably expressed in HEK-293 cell membranes as assessed with [¹²⁵I]α-BTX

Discussion

Numerous α7 NNR agonists have been reported in the literature [19] and a few have now begun to yield clinical data in early trials with normal subjects, Alzheimer's and schizophrenia patients [20]. AZD0328 is unique in that its exceptional drug-like properties allow the attainment of efficacious brain exposure with low doses in animal models (Table 5). The consistency of these low dose effects across multiple preclinical models has permitted us to efficiently translate effects on dopamine

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Selective α7 nicotinic receptor activation by AZD0328 enhances cortical dopamine release and improves learning and attentional processes

Abstract

Graphical abstract

Introduction

Section snippets

Test compounds

Receptor binding

Results

Discussion

Neuron

Bioorg Med Chem Lett

Neuron

Behav Brain Res

Behav Process

Pharmacol Biochem Behav

Behav Brain Res

Prog Med Chem

Neuron

Brain Res

Curr Opin Neurobiol

Neuropsychologia

Behav Brain Res

Behav Brain Res

Pharm Sci

Nicotinic acetylcholine receptors and nicotinic cholinergic mechanisms of the central nervous system

Annu Rev Pharmacol Toxicol