Maryka Quik

Guidelines on nicotine dose selection for in vivo research

RationaleThis review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.ObjectivesThis review capitalizes on the authors’ collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.ResultsSeven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.ConclusionsThe selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose–response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Effect of vasoactive intestinal peptide (VIP) and other peptides on cAMP accumulation in rat brain.

Abstract The effects of a number of peptides which have recently been demonstrated to be present in brain, were determined on adenylate cyclase activity in homogenates and on cAMP and cGMP levels in rat brain slices. Substance P, luteinizing hormone releasing factor, thyrotropin releasing factor, somatostatin, glucagon, and neurotensin were without effect in any of these tests. When slices from a number of regions of rat brain were incubated in the presence of 0.5 μM vasoactive intestinal peptide (VIP), a significant increase in the accumulation of cAMP over basal values was observed. There were no changes in cGMP levels. VIP also caused an increase in cell-free adenylate cyclase activity of striatal, cortical and hippocampal homogenates, and this response was considerably increased in the presence of guanylyl-imidodiphosphate (GMP-PNP). The phosphodiesterase inhibitor isobutylmethylxanthine caused a 3 to 6-fold increase in basal levels of cAMP in brain slices, but VIP was still able to elicit a further increase, indicating that its effects on cAMP accumulation were probably due to activation of adenylate cyclase. The increase in cAMP in cortical and hypothalamic but not striatal slices was affected by alterations in the calcium concentration of the incubation medium. When tissue slices were incubated in the presence of VIP and a variety of antagonist drugs (propranolol, phenoxybenzamine, α-flupenthixol, naloxone), no alteration in the VIP induced increase in cAMP was observed. Furthermore, when VIP was incubated in the presence of agonists (dopamine, noradrenaline, isoproterenol, prostaglandin E 1 , adenosine), the induced increase in cAMP levels was additive to that caused by VIP. These results support a role for VIP as a neuromodulatory or neuro-transmitter compound in the central nervous system, (CNS), mediating its action through the adenylate cyclase/cAMP system.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease.

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinsons disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Localization of nicotinic receptor subunit mRNAs in monkey brain by in situ hybridization

Nicotinic receptors are implicated in memory, learning, locomotor activity, and addiction. Identification of the specific receptor subtypes that mediate these behaviors is essential for understanding their role in central nervous system (CNS) function. Although expression of nicotinic receptor transcript has been studied in rodent brain, their localization in the monkey CNS, which may be a better model for the human brain, is not yet known. We therefore investigated the distribution of α4, α6, α7, β2, β3, and β4 receptors subunit mRNAs in the monkey brain by using in situ hybridization. α4 and α7 mRNAs were very widely expressed, with a substantial degree of overlap in their distribution, except for the reticular nucleus of the thalamus in which α7 mRNA was much more prominently expressed. β2 and β4 mRNA were also widely distributed, although β4 was more prominently localized in thalamic nuclei than β2. The distribution of α6 and β3 mRNA was very distinct from that of the other transcripts, being restricted to catecholaminergic nuclei, the cerebellum, and a few other areas. Although there were similarities in distribution of the nicotinic receptor subunit mRNAs in monkey and rodent brain, there were prominent differences in areas such as the caudate, putamen, locus coeruleus, medial habenula, and cerebellum. In fact, the distribution of α4 and α7 mRNAs in the monkey caudate and putamen was more similar to that reported in the human than rodent brain. These findings have implications for the development of drug therapies for neurological disorders, such as Alzheimers and Parkinsons disease, in which nicotinic receptors are decreased. J. Comp. Neurol. 425:58–69, 2000.

Relationship among nigrostriatal denervation, parkinsonism, and dyskinesias in the MPTP primate model

Presynaptic denervation is likely to play an important role in the pathophysiology of dyskinesias that develop after levodopa administration to patients with Parkinsons disease. In this study, the thresholds of nigrostriatal damage necessary for the occurrence of parkinsonism and levodopa‐induced involuntary movements were compared in squirrel monkeys lesioned with 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP). Animals treated with a regimen of MPTP that caused parkinsonism displayed ≥95% striatal dopamine depletion, 90% reduction of striatal dopamine uptake sites, and 70% nigral neuronal loss. Levodopa administration ameliorated the parkinsonian signs of these monkeys but also induced dyskinesias. A separate group of animals was treated with a milder MPTP regimen that caused 60%–70% striatal dopamine depletion, a 50% decrease in dopamine transporter, and 40% loss of dopaminergic nigral neurons. While these monkeys displayed no behavioral signs of parkinsonism, they all became dyskinetic after levodopa administration. The priming effect of levodopa, that is, the recurrence of dyskinesias in animals previously exposed to the drug, was compared in severely versus mildly lesioned monkeys. When severely injured parkinsonian animals underwent a second cycle of levodopa treatment, they immediately and consistently developed involuntary movements. In contrast, the recurrence of dyskinesias in primed monkeys with a partial nigrostriatal lesion required several levodopa administrations and remained relatively sporadic. The data indicate that moderate nigrostriatal damage which does not induce clinical parkinsonism predisposes to levodopa‐induced dyskinesias. Once dyskinesias have been induced, the severity of denervation may enhance the sensitivity to subsequent levodopa exposures.

Nicotine reduces levodopa-induced dyskinesias in lesioned monkeys

Levodopa, the gold standard for Parkinsons disease treatment, is associated with debilitating abnormal involuntary movements or dyskinesias, for which few treatments are currently available. Studies have implicated numerous neurotransmitters in the development of levodopa‐induced dyskinesias. However, the cholinergic system has received little attention despite an extensive overlap between dopaminergic terminals and cholinergic interneurons in the striatum and the well‐known ability of nicotine to stimulate striatal dopamine release. Our objective, therefore, was to determine whether nicotine treatment reduced levodopa‐induced dyskinesias.

Nicotine as a potential neuroprotective agent for Parkinson’s disease

Converging research efforts suggest that nicotine and other drugs that act at nicotinic acetylcholine receptors (nAChRs) may be beneficial in the management of Parkinsons disease. This idea initially stemmed from the results of epidemiological studies that demonstrated that smoking is associated with a decreased incidence of Parkinsons disease. The subsequent finding that nicotine administration protected against nigrostriatal damage in parkinsonian animal models led to the idea that nicotine in tobacco products may contribute to this apparent protective action. Nicotine most likely exerts its effects by interacting at nAChRs. Accumulating research indicates that multiple subtypes containing nAChRs, including α4β2, α6β2, and/or α7, may be involved. Stimulation of nAChRs initially activates various intracellular transduction pathways primarily via alterations in calcium signaling. Consequent adaptations in immune responsiveness and trophic factors may ultimately mediate nicotines ability to reduce/halt the neuronal damage that arises in Parkinsons disease. In addition to a potential neuroprotective action, nicotine also has antidepressant properties and improves attention/cognition. Altogether, these findings suggest that nicotine and nAChR drugs represent promising therapeutic agents for the management of Parkinsons disease.

Nicotine and Nicotinic Receptors; Relevance to Parkinson’s Disease

The development of nicotinic agonists for therapy in neurodegenerative disorders such as Parkinsons disease is an area currently receiving considerable attention. The rationale for such work stems from findings that reveal a loss of nicotinic receptors in Parkinsons disease brains. These results, coupled with reports that nicotine treatment relieves some of the symptoms of this disorder, provides support for the contention that nicotine and/or nicotinic agonists may be beneficial for acute symptomatic treatment. Moreover, the observation that there is a decreased incidence of Parkinsons disease with tobacco use, possibly due to the nicotine in tobacco products, may imply that such drugs are useful for long-term neuroprotection. However, there are multiple nicotinic receptor populations in the brain with different functional properties. Identification of the subtypes involved in nigrostriatal dopaminergic activity is therefore critical for the rational use of selective therapeutic agents for symptomatic treatment and/or neuroprotection. Accumulating evidence, both in rodents and nonhuman primates now indicate that alpha6* nicotinic receptors are present on nigrostriatal dopaminergic neurons, and furthermore, that receptors containing this subunit may be most vulnerable to nigrostriatal damage, at least in nonhuman primates. These data suggest that nicotinic receptor ligands directed to alpha6* nicotinic receptors might be particularly relevant for Parkinsons disease therapeutics.

α6β2* and α4β2* Nicotinic Acetylcholine Receptors As Drug Targets for Parkinson's Disease

Parkinsons disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinsons disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the “gold standard” for Parkinsons disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinsons disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinsons disease.

Continuous and Intermittent Nicotine Treatment Reduces l-3,4-Dihydroxyphenylalanine (l-DOPA)-Induced Dyskinesias in a Rat Model of Parkinson's Disease

The development of abnormal involuntary movements (AIMs) or dyskinesias is a serious complication of l-DOPA [l-3,4-dihydroxyphenylalanine] therapy for Parkinsons disease. Our previous work had shown that intermittent nicotine dosing reduced l-DOPA-induced dyskinetic-like movements in nonhuman primates. A readily available nicotine formulation is the nicotine patch, which provides a constant source of nicotine. However, constant nicotine administration more readily desensitizes nicotinic receptors, to possibly yield alternate behavioral outcomes. Therefore, we investigated whether constant nicotine administration reduced l-DOPA-induced AIMs in a rat parkinsonian model, with results compared with those with intermittent nicotine dosing. Rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion were exposed to either intermittent (drinking water) or constant (minipump) nicotine for ≥2 weeks at doses that yielded plasma levels of the nicotine metabolite cotinine similar to those in smokers. The rats were next treated with l-DOPA/benserazide (8 or 12 mg/kg/15 mg/kg) for ≥3 weeks to allow for the development of AIMs, with nicotine treatment continued. Both modes of nicotine administration resulted in ≥50% decline in l-DOPA-induced AIMs. Nicotine treatment also significantly reduced AIMs in l-DOPA-primed rats using either dosing regimen, whereas nicotine removal led to an increase in AIMs. There was no effect of nicotine on various measures of motor performance in 6-OHDA-lesioned rats. In summary, nicotine provided either via the drinking water or minipump reduced l-DOPA-induced AIMs in a rat model of Parkinsons disease. These results suggest that either intermittent or constant nicotine treatment may be useful in the treatment of l-DOPA-induced dyskinesias in patients with Parkinsons disease.