Nicotinic acetylcholine receptors and the ascending dopamine pathways

α6ß2* and α4ß2* nicotinic receptors both regulate dopamine signaling with increased nigrostriatal damage: relevance to Parkinson's disease

Nicotinic receptors (nAChRs) are important modulators of dopaminergic transmission in striatum, a region critical to Parkinson's disease. The nAChRs mainly involved are the α6β2* and α4β2* subtypes. Lesion studies show that the α6β2* receptor is decreased to a much greater extent with nigrostriatal damage than the α4β2* subtype raising the question whether this latter nAChR population is more important with increased nigrostriatal damage. To address this, we investigated the effect of varying nigrostriatal damage on α6β2* and α4β2* receptor-modulated dopamine signaling using cyclic voltammetry. This approach offers the advantage that changes in dopamine release can be observed under different neuronal firing conditions. Total single-pulse-evoked dopamine release decreased in direct proportion to declines in the dopamine transporter and dopamine uptake. We next used α-conotoxinMII and mecamylamine to understand the role of the α4β2* and α6β2* subtypes in release. Single-pulse-stimulated α6β2* and α4β2* receptor dopamine release decreased to a similar extent with increasing nigrostriatal damage, indicating that both subtypes contribute to the control of dopaminergic transmission with lesioning. Total burst-stimulated dopamine release also decreased proportionately with nigrostriatal damage. However, the role of the α4β2* and α6β2* nAChRs varied with different degrees of lesioning, suggesting that the two subtypes play a unique function with burst firing, with a somewhat more prominent and possibly more selective role for the α6β2* subtype. These data have important therapeutic implications because they suggest that drugs directed to both α4β2* and α6β2* nAChRs may be useful in the treatment of neurological disorders such as Parkinson's disease.

Acetylcholine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area of the midbrain form the nigrostriatal and mesocorticolimbic dopaminergic pathways that, respectively, project to dorsal and ventral striatum (including prefrontal cortex). These midbrain dopaminergic nuclei and their respective forebrain and cortical target areas are well established as serving a critical role in mediating voluntary motor control, as evidenced in Parkinson's disease, and incentive-motivated behaviors and cognitive functions, as exhibited in drug addiction and schizophrenia, respectively. Although it cannot be disputed that excitatory and inhibitory amino acid-based neurotransmitters, such as glutamate and GABA, play a vital role in modulating activity of midbrain dopaminergic neurons, recent evidence suggests that acetylcholine may be as important in regulating dopaminergic transmission. Midbrain dopaminergic cell tonic and phasic activity is closely dependent upon projections from hindbrain pedunculopontine and the laterodorsal tegmental nuclei, which comprises the only known cholinergic inputs to these neurons. In close coordination with glutamatergic and GABAergic activity, these excitatory cholinergic projections activate nicotinic and muscarinic acetylcholine receptors within the substantia nigra and ventral tegmental area to modulate dopamine transmission in the dorsal/ventral striatum and prefrontal cortex. Additionally, acetylcholine-containing interneurons in the striatum also constitute an important neural substrate to provide further cholinergic modulation of forebrain striatal dopaminergic transmission. In this review, we examine neurological and psychopathological conditions associated with dysfunctions in the interaction of acetylcholine and dopamine and conventional and new pharmacological approaches to treat these disorders.

Effects of the alpha4beta2 partial agonist varenicline on brain activity and working memory in abstinent smokers

BACKGROUND

Cognitive alterations are a core symptom of nicotine withdrawal, contributing to smoking relapse. In rodents and humans, cognitive deficits can be reversed by treatment with the alpha4beta2 nicotinic receptor partial agonist varenicline. This neuroimaging study examined the neural mechanisms that underlie these effects.

METHODS

Twenty-two smokers completed 13 days of varenicline and placebo treatment in a double-blind crossover study with two functional magnetic resonance imaging sessions: after 3 days of abstinence while on varenicline and after 3 days of abstinence while on placebo (counterbalanced randomized order, 2-week washout). Blood oxygenation level-dependent (BOLD) data were acquired during performance of a visual N-back working memory task.

RESULTS

In a region of interest analysis, significant effects of treatment on mean percent signal change (varenicline > placebo) were observed in the dorsal anterior cingulate/medial frontal cortex, left dorsolateral prefrontal cortex, and right dorsolateral prefrontal cortex. In a cross-region model, there was a significant interaction of treatment by memory load, indicating significant increases in BOLD signal for varenicline versus placebo at the 2-back and 3-back levels but not the 1-back level. Varenicline improved performance (correct response time) in highly dependent smokers with no effect among less dependent smokers. In highly dependent smokers, faster correct response time was associated with increased BOLD signal.

CONCLUSIONS

This study provides novel evidence that the alpha4beta2 partial agonist varenicline increases working memory-related brain activity after 3 days of nicotine abstinence, particularly at high levels of task difficulty, with associated improvements in cognitive performance among highly dependent smokers.