Chronic nicotine administration increases NGF-like immunoreactivity in frontoparietal cerebral cortex

Insights into Pharmacological Activities of Nicotine and 6-Hydroxy-L-nicotine, a Bacterial Nicotine Derivative: A Systematic Review

The purported cognitive benefits associated with nicotine and its metabolites in the brain are a matter of debate. In this review, the impact of the pharmacologically active metabolite of a nicotine derivative produced by bacteria named 6-hydroxy-L-nicotine (6HLN) on memory, oxidative stress, and the activity of the cholinergic system in the brain was examined. A search in the PubMed, Science Direct, Web of Science, and Google Scholar databases, limiting entries to those published between 1992 and 2023, was conducted. The search focused specifically on articles about nicotine metabolites, memory, oxidative stress, and cholinergic system activity, as well as enzymes or pathways related to nicotine degradation in bacteria. The preliminary search resulted in 696 articles, and following the application of exclusion criteria, 212 articles were deemed eligible for inclusion. This review focuses on experimental studies supporting nicotine catabolism in bacteria, and the chemical and pharmacological activities of nicotine and its metabolite 6HLN.

Potential of Glial Cell Modulators in the Management of Substance Use Disorders

The pervasive and devastating nature of substance use disorders underlies the need for the continued development of novel pharmacotherapies. We now know that glia play a much greater role in neuronal processes than once believed.  The various types of glial cells (e.g., astrocytes, microglial, oligodendrocytes) participate in numerous functions that are crucial to healthy central nervous system function. Drugs of abuse have been shown to interact with glia in ways that directly contribute to the pharmacodynamic effects responsible for their abuse potential. Through their effect upon glia, drugs of abuse also alter brain function resulting in behavioral changes associated with substance use disorders. Therefore, drug-induced changes in glia and inflammation within the central nervous system (neuroinflammation) have been investigated to treat various aspects of drug abuse and dependence. This article presents a brief overview of the effects of each of the major classes of addictive drugs on glia. Next, the paper reviews the pre-clinical and clinical studies assessing the effects that glial modulators have on abuse-related behavioral effects, such as pleasure, withdrawal, and motivation. There is a strong body of pre-clinical literature demonstrating the general effectiveness of several glia-modulating drugs in models of reward and relapse. Clinical studies have also yielded promising results, though not as robust. There is still much to disentangle regarding the integration between addictive drugs and glial cells. Improved understanding of the relationship between glia and the pathophysiology of drug abuse should allow for more precise exploration in the development and testing of glial-directed treatments for substance use disorders.

Revisiting nicotine’s role in the ageing brain and cognitive impairment

Brain ageing is a complex process which in its pathologic form is associated with learning and memory dysfunction or cognitive impairment. During ageing, changes in cholinergic innervations and reduced acetylcholinergic tonus may trigger a series of molecular pathways participating in oxidative stress, excitotoxicity, amyloid-β toxicity, apoptosis, neuroinflammation, and perturb neurotrophic factors in the brain. Nicotine is an exogenous agonist of nicotinic acetylcholine receptors (nAChRs) and acts as a pharmacological chaperone in the regulation of nAChR expression, potentially intervening in age-related changes in diverse molecular pathways leading to pathology. Although nicotine has therapeutic potential, paradoxical effects have been reported, possibly due to its inverted U-shape dose-response effects or pharmacokinetic factors. Additionally, nicotine administration should result in optimum therapeutic effects without imparting abuse potential or toxicity. Overall, this review aims to compile the previous and most recent data on nicotine and its effects on cognition-related mechanisms and age-related cognitive impairment.

The missing link between long-term stimulation of nicotinic receptors and the increases of acetylcholine release and vasodilation in the cerebral cortex of aged rats

In adult rats (4-9 months), chronic nicotine infusion increases the basal level of acetylcholine (ACh) release in the cerebral cortex and enhances responses of cortical ACh release and cortical vasodilation elicited by nucleus basalis of Meynert (NBM) stimulation. In the present study, we examined whether these effects of nicotine are detected in aged rats. Aged rats (27-30 months) received sustained subcutaneous nicotine (100 μg/kg/h) or saline for 14 days. Under urethane anesthesia, ACh release and regional blood flow in the parietal cortex were measured. The basal level of ACh release in the cerebral cortex was not changed by chronic nicotine. In addition, the magnitudes of ACh release and vasodilation by NBM stimulation were similar between the saline-treated and nicotine-treated groups. The lack of an effect of chronic nicotine in aged rats may be due to a decrease in nicotinic receptors in the cerebral cortex during aging (Nordberg et al., J Neurosci Res 31:103-111, 1992).

Neurobiology of nAChRs and cognition: a mini review of Dr. Jerry J. Buccafusco's contributions over a 25 year career

This review highlights some of the many contributions of the late Dr. Jerry J. Buccafusco to the neurobiology of nicotinic acetylcholine receptors (nAChRs) and cognition over a 25 year period. The article is written by two of Dr. Buccafusco's professional colleagues, one from academia and one from the pharmaceutical industry. While Dr. Buccafusco's expertise in the cholinergic field was extensive, his insights into the practical relevance of his work (with a long-term goal of formulating new drug development strategies) were unique, and a great asset to both the basic science community and pharmaceutical companies. In 1988, Dr. Buccafusco's laboratory was the first to report the cognitive enhancing action of low doses of nicotine in non-human primates. Since that time he studied a large number of novel pro-cognitive agents from several pharmacological classes in rodents as well as monkeys. Based on years of observing paradoxical effects of nicotinic ligands in vitro and in vivo, Dr. Buccafusco made the provocative argument that it might be possible to develop new chemical entities (with pro-cognitive actions) that have the ability to desensitize nAChRs without producing an antecedent agonist action. Some of his more recent work focused on development of single molecular entities that act on multiple CNS targets (including nAChRs) to enhance cognition, provide neuroprotection, and/or provide additional therapeutic actions (e.g., antipsychotic effects). Dr. Buccafusco's influence will live on in the work of the numerous graduate students, postdoctoral fellows, and junior faculty that he mentored over the years who now serve in prestigious positions throughout the world.

Revisiting the cholinergic hypothesis in the development of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia affecting the elderly population today; however, there is currently no accurate description of the etiology of this devastating disorder. No single factor has been demonstrated as being causative; however, an alternative co-factors theory suggests that the interaction of multiple risk factors is responsible for AD. We have used this model, in combination with the original cholinergic hypothesis of AD to propose a "new" cholinergic hypothesis that we present in this review. This new version takes into account recent findings from the literature and our reports of removal of medial septum cholinergic projections to the hippocampus reduces both behavioural and anatomical plasticity, resulting in greater cognitive impairment in response to secondary insults (stress, injury, disease, etc.). We will first summarize the experimental results and discuss some potential mechanisms that could explain our results. We will then present our 'new' version of the cholinergic hypothesis and how it relates to the field of AD research today. Finally we will discuss some of the implications for treatment that arise from this model and present directions for future study.

Sustained subcutaneous infusion of nicotine enhances cholinergic vasodilation in the cerebral cortex induced by stimulation of the nucleus basalis of Meynert in rats

The present study examined the effects of sustained nicotine exposure on the cholinergic vasodilative system originating in the nucleus basalis of Meynert (NBM) and projecting to the cerebral cortex in rats. Rats received sustained subcutaneous nicotine (100μg/kg/h) for 14 days. Under urethane anesthesia, the vasodilation response and acetylcholine release in the parietal cortex induced by electrical stimulation of the NBM (10-200μA) were measured. The basal level of acetylcholine release was significantly higher in nicotine-treated rats than in saline-treated control rats. In the control rats, both the acetylcholine release and blood flow were increased by NBM stimulation in a stimulus intensity-dependent manner, and a threshold of 50μA. In nicotine-treated rats, the threshold intensity of NBM stimulation producing increases in acetylcholine release and blood flow was reduced to 20μA. The stimulus intensity-dependent acetylcholine release and vasodilation by NBM stimulation were significantly larger in nicotine-treated rats than in control rats. We conclude that sustained subcutaneous infusion of nicotine enhances cholinergic vasodilative system in the cerebral cortex originating in the NBM.

Nicotine sensitization and analysis of brain-derived neurotrophic factor in adolescent beta-arrestin-2 knockout mice

Nicotine sensitization and levels of brain-derived neurotrophic factor (BDNF) were analyzed in adolescent beta-arrestin-2 knockout (betaA-2 KO) and wild type (WT) mice. The beta-arrestin-2 protein has been shown to be important in G-protein hydrolysis and receptor internalization. Four- to five-week-old adolescent betaA-2 KO and WT C57/Bl6 mice were administered either nicotine (0.5 mg/kg free base) or saline 10 min before being placed into a locomotor arena on each of 7 (Experiment 1) or 14 (Experiment 2) consecutive days. A nicotine challenge was given 7 days after sensitization was complete. In Experiment 1, betaA-2 KO mice administered nicotine or saline and WT mice administered nicotine demonstrated significant hypoactivity during early in testing, and neither WT nor betaA-2 KO mice administered nicotine demonstrated sensitization. On the nicotine challenge, WT mice administered nicotine demonstrated significantly higher activity levels compared to all groups, and this same group demonstrated significantly higher levels of accumbal BDNF compared to all groups. In Experiment 2, betaA-2 KO mice were again hypoactive compared to WT mice, whereas WT mice administered nicotine demonstrated significant hypoactivity during initial testing and significantly higher levels of activity compared to all other groups late in testing. On the nicotine challenge, WT mice that received nicotine demonstrated a significant increase in activity compared to all groups, and showed increased accumbal BDNF compared to all groups. These results show that the beta-arrestin-2 protein is important in induction and expression of nicotine sensitization as well as nicotine's effects on accumbal BDNF.

Immunohistochemical increase in cyclooxygenase-2 without apoptosis in different brain areas of subchronic nicotine- and D-amphetamine-treated rats

Cyclooxygenase-2 (COX-2) upregulation has been related to both neurodegeneration and physiological processes. To clarify whether nicotine-induced upregulation of COX-2 occurs, and to analyse its significance, a comparative immunohistochemical and Western blot study was performed on the frontoparietal cortex, hippocampus and cerebellar cortex of rats treated (14 days) with nicotine, D(+)amphetamine (0.35 and 1.16 mg free base/kg/day, respectively), or both drugs simultaneously. None of these treatments promoted neuronal apoptosis. Lipid peroxidation increased in the hippocampus of the nicotine-treated rats and in all the brain regions examined in the D(+)amphetamine rats, but not in the double-treated animals. Both molecules increased the COX-2 content (as determined by the number of immunopositive neurons and the intensity of their immunodeposits) in an area-, layer- and neuron type-dependent manner, in all brain regions in which a large number of COX-2 immunopositive neurons were observed in controls (the somatosensory cortical areas, CA-1, CA-3, the gyrus dentatus, the ectorhinal/perirhinal areas, and the gyrus cingularis). No increase was seen in the motor cortical areas, while a reduction was recorded in the cerebellar cortex; these regions had only a few immunopositive neurons in controls. Western blot analysis revealed a 50-80% increase in COX-2 in the brain cortex and hippocampus of nicotine-treated rats, and similar increases (150-200%) in the cortex of the D(+)amphetamine- and nicotine + D(+)amphetamine-treated rats. Nicotine-induced upregulation of COX-2 seems to be related to neuronal plasticity rather than neurodegeneration. Nicotine agonists might be useful in the treatment of cognitive disorders.

Adolescent nicotine treatment changes the response of acetylcholine systems to subsequent nicotine administration in adulthood

Nicotine alters the developmental trajectory of acetylcholine (ACh) systems in the immature brain, with vulnerability extending from fetal stages through adolescence. We administered nicotine to adolescent rats (postnatal days PN30-47) and then examined the subsequent response to nicotine given in adulthood (PN90-107), simulating plasma levels in smokers, and performing evaluations during nicotine treatment (PN105) and withdrawal (PN110, PN120 and PN130), as well as assessing persistent changes at 6 months of age (PN180). We measured nicotinic acetylcholine receptor (nAChR) binding, choline acetyltransferase (ChAT) activity, a marker for ACh terminals, and hemicholinium-3 (HC3) binding to the choline transporter, an index of ACh presynaptic activity. By itself, adolescent nicotine exposure evoked sex-selective deficits in cerebrocortical HC3 binding while elevating ChAT in young adulthood in striatum and midbrain. Nicotine given in adulthood produced profound nAChR upregulation lasting 2 weeks after discontinuing treatment, and decrements in cerebrocortical and striatal HC3 binding emerged during withdrawal, indicative of reduced ACh synaptic activity. For all three parameters, adolescent nicotine altered the responses to nicotine given in adulthood, producing both sensitization and desensitization that depended on sex and brain region, effects that parallel the disparate behavioral outcomes reported for these treatments. The interaction seen here for the impact of adolescent nicotine exposure on adult nicotine responses was substantially greater than that found previously for the effects of prenatal nicotine exposure on adult responses. Our findings thus reinforce the importance of adolescence as a critical period in which the future responsiveness to nicotine is programmed.

Chronic low-dose administration of nicotine facilitates recovery and synaptic change after focal ischemia in rats

The current study examines the effects of chronic administration of nicotine on motor behavior after focal stroke in rats. Animals were trained in a tray-reaching task for 2weeks and then they were divided into: (1) control+saline (2) control+nicotine (3) stroke+saline, and (4) stroke+nicotine groups. Lesions were produced by devascularization of the surface blood vessels of the sensorimotor cortex contralateral to the forepaw used for reaching. Forty-eight hours after the lesions, and for a total of 12days, animals received two daily injections of either nicotine (0.3mg/kg) or saline (0.9%). Animals were tested in a motor battery 1week after the lesions and every other week for a total of 7weeks. Pyramidal cells in forelimb and cingulate areas were then examined for dendritic length and branching using a Golgi-Cox procedure. Behavioral results demonstrated that by the end of the testing stroke+nicotine animals showed significant behavioral improvement relative to stroke+saline animals. Stroke+nicotine animals showed an increase in dendritic length and branching in pyramidal cells of the forelimb and cingulate areas. The results suggest that the behavioral enhancement in the stroke+nicotine group might be attributable to the enhanced dendritic growth in residual cortical motor regions.

Repeated nicotine exposure in rats: effects on memory function, cholinergic markers and nerve growth factor

A decrease in the number of nicotinic-acetylcholine receptors (nAChRs) in the brain is thought to contribute to the cognitive dysfunction associated with diseases as diverse as Alzheimer's disease and schizophrenia. Interestingly, nicotine and similar compounds have been shown to enhance memory function and increase the expression of nAChRs and therefore, could have a therapeutic role in the aforementioned diseases. Nicotine has also been shown to exert positive effects on certain neurotrophins such as nerve growth factor (NGF), and therefore could play a role beyond mere symptomatic therapy. However, to date, comprehensive studies of nicotine's effects on the expression of specific acetylcholine (ACh) receptor subtypes, key cholinergic proteins (that are regulated by NGF) such as choline acetyltransferase (ChAT) and the vesicular ACh transporter (VAChT) are lacking. Studies to further investigate the effects of nicotine on NGF especially its high- and low-affinity receptors are also needed. In the present study, male Wistar rats exposed a relatively low dosage of nicotine (0.35 mg/kg every 12 h) for 14 days demonstrated improved memory performance (assessed in two separate water maze testing methods) when compared with controls. Autoradiographic experiments indicated that nicotine increased [3H]-epibatidine, [125I]-alpha-bungarotoxin and [3H]-AFDX384, but not [3H]-pirenzepine binding sites in several learning- and memory-related brain areas. The expression of ChAT, VAChT, as well as tropomyosin-receptor kinase A (TrkA) NGF receptors and phospho-TrK receptors was increased by nicotine in the hippocampus. No changes were observed in the levels of the NGF peptide or low affinity p75 neurotrophin receptors (p75NTR), however. These results suggest that repeated exposure to nicotine results in positive effects on central cholinergic markers and memory function, which may be mediated via effects on high-affinity NGF receptors.

Prevention of diminished parasympathetic control of the heart in experimental heart failure

Decreased synaptic transmission in parasympathetic ganglia contributes to abnormal parasympathetic function in heart failure (HF). Because nicotinic ACh receptors (nAChR) mediate synaptic transmission at the ganglion and upregulate in response to chronic exposure to agonist in vitro, we tested the hypothesis that repeated exposures of ganglionic neurons to a nAChR agonist can prevent a loss of parasympathetic control in HF. Two sets of experiments were performed. In set 1, unpaced control dogs and dogs undergoing pacing-induced HF were treated with a repeated intravenous nicotinic agonist during the development of HF. Under conditions of sympathetic blockade, R-R responses to a bolus injection of 200 μg 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP; nicotinic agonist) were found to be increased five times over the untreated group after 6 wk. In experimental set 2, dogs treated with weekly DMPP injections and in HF were anesthetized and underwent electrical stimulation of the right vagus nerve, which showed sinus cycle length responses >10 times that of controls ( P < 0.05). Complete ganglionic blockade with hexamethonium abolished all responses, confirming that synaptic transmission was mediated entirely by nAChRs in both controls and HF. Despite decreased ganglionic function leading to reduced parasympathetic control of the heart in HF, repeated exposure with a nicotinic agonist during the development of HF results in not only preserved but also supranormal effects of parasympathetic stimulation on the sinus node.