Activities of cAMP-dependent protein kinase and protein kinase C are modulated by desensitized nicotinic receptors in the rat brain

Targeting Phosphodiesterases in Pharmacotherapy for Substance Dependence

Substance dependence is a chronic relapsing brain disorder associated with adaptational changes in synaptic plasticity and neuronal functions. The high levels of substance consumption and relapse rate suggest more reliable medications are in need to better address the underlying causes of this disease. It has been well established that the intracellular second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) and their signaling systems play an important role in the molecular mechanisms of substance taking behaviors. On this basis, the phosphodiesterase (PDE) superfamily, which crucially controls cyclic nucleotide levels by catalyzing their hydrolysis, has been proposed as a novel class of therapeutic targets for substance use disorders. This chapter reviews the expression patterns of PDEs in the brain with regard to neural structures underlying the dependent process and highlights available evidence for a modulatory role of PDEs in substance dependence.

Transcriptional regulation of L-type calcium channel subtypes Cav1.2 and Cav1.3 by nicotine and their potential role in nicotine sensitization

INTRODUCTION

L-type calcium channel (LTCC) activity in the brain is mediated by 2 subtypes, Ca(v)1.2 and Ca(v)1.3. The individual contributions of these LTCC subtypes to the long-term pharmacological and behavioral effects of nicotine are unknown.

METHODS

Using quantitative in situ hybridization, we examined expression levels of Ca(v)1.2 and Ca(v)1.3 in forebrain regions of mice treated with nicotine (0.175 mg/kg) or saline for 1 or 14 days and sacrificed 24 hr or 7 days following the last injection. Additionally, we treated mice with nicotine for 14 days and then administered the nonspecific LTCC antagonist nifedipine twice daily during a 7-day abstinence period prior to testing for nicotine sensitization to determine the effect of LTCC blockade on sensitization.

RESULTS

Ca(v)1.2 mRNA was unaffected 24 hr following a single nicotine exposure, whereas Ca(v)1.3 mRNA was upregulated in several brain regions. Following 14 days of nicotine treatment and 24 hr of abstinence, Ca(v)1.2 mRNA was downregulated throughout the areas examined, whereas Ca(v)1.3 mRNA had mostly returned to control values. Following 7 days of abstinence, a strong upregulation of Ca(v)1.2 transcripts was observed, whereas Ca(v)1.3 mRNA was largely unaffected. In our sensitization study, nifedipine administered during nicotine abstinence impaired subsequent nicotine sensitization.

CONCLUSIONS

Our data suggest a differential involvement of Ca(v)1.2 and Ca(v)1.3 in nicotine-related processes. Ca(v)1.3 seems to be involved primarily during early exposure to nicotine. Ca(v)1.2 appears to play a role in the long-term molecular and behavioral changes that occur following chronic nicotine and abstinence. Nifedipine may counteract those nicotine-induced alterations in LTCC activity to impair nicotine sensitization.

Differential expression and function of nicotinic acetylcholine receptors in the urinary bladder epithelium of the rat

It has been previously determined that the epithelial lining of the urinary bladder, or urothelium, expresses two subtypes of nicotinic acetylcholine receptors (nAChRs) that mediate distinct physiological effects in vivo. These effects include inhibition of bladder reflexes through α7 receptors and an excitation of bladder reflexes through α3-containing (α3*) receptors. It is believed that urothelial receptors mediate their effects through modulating the release of neurotransmitters such as ATP that subsequently influence bladder afferent nerve excitability. Therefore, we examined the distribution of nAChRs in the urothelium, as well as their ability to influence the release of the neurotransmitter ATP. Immunofluorescent staining of both whole bladder tissue and primary urothelial cultures from the rat demonstrated that the urothelium contains both α3* and α7 receptors. In primary urothelial cultures, α7 stimulation with choline (10 μM to 1 mM) caused a decrease in basal ATP release while α3* stimulation with cytisine (1–100 μM) caused a concentration-dependent, biphasic response, with low concentrations (1–10 μM) inhibiting release and higher concentrations (50–100 μM) increasing release. These responses were mirrored in an in vitro, whole bladder preparation. In vivo, excitation of bladder reflexes in response to intravesical cytisine (100 μM) is blocked by systemic administration of the purinergic antagonist PPADS (1 or 3 μg kg(−1)). We also examined how each receptor subtype influenced intracellular Ca2+ levels in cultured urothelial cells. nAChR stimulation increased [Ca2+]i through distinct mechanisms: α7 through a ryanodine-sensitive intracellular mechanism and α3* through extracellular influx. In addition, our findings suggest interactions between nAChR subtypes whereby activation of α7 receptors inhibited the response to a subsequent activation of α3* receptors, preventing the increase in [Ca2+]i previously observed. This inhibitory effect appears to be mediated through protein kinase A- or protein kinase C-mediated pathways.

Protein kinases and addiction

Although drugs of abuse have different chemical structures and interact with different protein targets, all appear to usurp common neuronal systems that regulate reward and motivation. Addiction is a complex disease that is thought to involve drug-induced changes in synaptic plasticity due to alterations in cell signaling, gene transcription, and protein synthesis. Recent evidence suggests that drugs of abuse interact with and change a common network of signaling pathways that include a subset of specific protein kinases. The best studied of these kinases are reviewed here and include extracellular signal-regulated kinase, cAMP-dependent protein kinase, cyclin-dependent protein kinase 5, protein kinase C, calcium/calmodulin-dependent protein kinase II, and Fyn tyrosine kinase. These kinases have been implicated in various aspects of drug addiction including acute drug effects, drug self-administration, withdrawal, reinforcement, sensitization, and tolerance. Identifying protein kinase substrates and signaling pathways that contribute to the addicted state may provide novel approaches for new pharmacotherapies to treat drug addiction.

Nicotine stimulates human lung cancer cell growth by inducing fibronectin expression

The mechanisms by which tobacco promotes lung cancer remain incompletely understood. Herein, we report that nicotine, a major component of tobacco, promotes the proliferation of cultured non-small cell lung carcinoma (NSCLC) cells; this effect was most noticeable at 5 days. However, nicotine had no effect on apoptosis of NSCLC cells. In experiments designed to unveil the mechanisms for this effect, we found that nicotine also stimulated mRNA and protein expression of fibronectin. Fibronectin is a matrix glycoprotein that regulates important cellular processes (e.g., adhesion, proliferation, and differentiation) and is highly expressed in tobacco-related lung disorders. Of note, reagents against the integrin alpha5beta1 (antibodies, RGD peptides, alpha5 shRNA) blocked the mitogenic effects of nicotine. Thus, nicotine stimulated NSCLC cell proliferation indirectly via fibronectin induction. We then focused on the mechanisms responsible for nicotine-induced fibronectin expression in NSCLC cells and found that nicotine stimulated the surface expression of alpha7 nicotinic acetylcholine receptor (alpha7 nAChR), and that alpha-bungarotoxin, an inhibitor of alpha7 nAChR, abolished the nicotine-induced fibronectin response. The fibronectin-inducing effects of nicotine were associated with activation of extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3-K)/mammalian target of rapamycin (mTOR) signaling pathways, and were abrogated by inhibitors of ERK (PD98059), PI3-K (LY294002), and mTOR (rapamycin), but not by inhibitors of protein kinase (PK)C (calphostin C) and PKA (H89). These observations suggest that nicotine stimulates NSCLC proliferation through induction of fibronectin, and that these events are mediated through nAChR-mediated signals that include ERK and PI3-K/mTOR pathways. This work highlights the role of fibronectin and alpha5beta1 integrins as potential targets for anti-lung cancer therapies.

Effects of desensitized nicotinic receptors on rotational behavior in a 6-hydroxydopamine model of Parkinson's disease

The purpose of this study was to investigate the effects of desensitized nicotinic acetylcholine receptors (nAChRs) on rotational behavior in the unilateral 6-hydroxydopamine (6-OHDA) model of Parkinson's disease (PD). When rats were treated with different doses of nicotine, nAChRs were observed in activated, subacute desensitized, acute desensitized, and chronic desensitized states. The rotational behavior of the hemiparkinsonian rats was determined when nAChRs were in the activated or different desensitized states. The results showed that hemiparkinsonian rats exhibited no significant changes in apomorphine-induced rotation when brain nAChRs were in an activated state. However, hemiparkinsonian rats displayed a significant reduction in apomorphine-induced rotational behavior when brain nAChRs were in subacute, acute, or chronic desensitized states induced by repeated administration of nicotine. When nAChRs were blocked by the nAChR antagonist mecamylamine, the behavior of the hemiparkinsonian rats worsened. These results suggest that desensitized nAChRs can lead to behavioral improvement in the 6-OHDA rat model of PD.

Nicotine causes age-dependent changes in gene expression in the adolescent female rat brain

Humans often start smoking during adolescence. Recent results suggest that rodents may also be particularly vulnerable to nicotine dependence during adolescence. We examined the effect of chronic nicotine exposure on gene expression profiles during adolescence in female rats, who were dosed with nicotine (and control animals were dosed with saline) via subcutaneously implanted osmotic minipumps. Brain samples were collected at four ages: before puberty (postnatal day 25), at about the time of puberty in females (postnatal day 35), and after puberty (postnatal days 45 and 55). The expression of 7931 genes in three brain areas was measured using DNA microarrays. Quantitative RT-PCR was also employed to confirm the expression patterns of selected genes. We used a novel clustering technique (principal cluster analysis) to classify 162 nicotine-regulated genes into five clusters, of which only one (cluster A) showed similar patterns of gene expression across all three brain areas (ventral striatum, prefrontal cortex, and hippocampus). Three clusters of genes (A, B, and C) showed dramatic peaks in their nicotine responses at the same age (p35). The other two clusters (D1 and D2) showed smaller peaks and/or valleys in their nicotine responses at p35 and p45. Thus, the age of maximal gene expression response to nicotine in female rats corresponds approximately to the age of maximal behavioral response and the age of puberty.

Nicotine affects mineralized nodule formation by the human osteosarcoma cell line Saos-2

Several in vitro and in vivo studies have indicated that tobacco smoking may be an important risk factor for the development and severity of inflammatory periodontal disease. In the present study, we examined the effect of nicotine on cell proliferation, alkaline phosphatase (ALPase) activity, mineralized nodule formation, and the expression of extracellular matrix proteins in the human osteosarcoma cell line Saos-2. The cells were cultured with Dulbecco's modified Eagle medium containing 10% fetal bovine serum with 0, 10(-4) M, and 10(-3) M nicotine for up to 14 days. Mineralized nodule formation was examined by alizarin red staining, and the calcium content in mineralized nodules was determined using a calcium E-test kit. The expression of extracellular matrix proteins was estimated by determining the levels of their mRNAs using the real-time polymerase chain reaction. Mineralized nodule formation and calcium content in mineralized nodules were remarkably suppressed by nicotine on days 10 and 14 of culture, respectively. ALPase activity as well as type I collagen and osteopontin expression also decreased in the presence of nicotine after 5, 10, and 14 days of culture, respectively. By contrast, the amount of bone sialoprotein increased during 14 days of culture with nicotine. These results suggest that nicotine suppresses osteogenesis through a decrease in ALPase and type I collagen production by osteoblasts.

Mice lacking the alpha4 nicotinic receptor subunit fail to modulate dopaminergic neuronal arbors and possess impaired dopamine transporter function

Neuronal nicotinic acetylcholine receptors (nAChRs) at presynaptic sites can modulate dopaminergic synaptic transmission by regulating dopamine (DA) release and uptake. Dopaminergic transmission in nigrostriatal and mesolimbic pathways is vital for the coordination of movement and is associated with learning and behavioral reinforcement. We reported recently that the D2 DA receptor plays a central role in regulating the arbor size of substantia nigra dopaminergic neurons. Given the known effects of nAChRs on dopaminergic neurotransmission, we assessed the ability of the alpha4 nAChR subunit to regulate arbor size of dopaminergic neurons by comparing responses of wild-type and alpha4 nAChR subunit knockout [alpha4(-/-)] mice to long-term exposure to cocaine, amphetamine, nicotine, and haloperidol, and after substantia nigra neurotoxic lesioning. We found that dopaminergic neurons in adult drug-naive alpha4(-/-) mice had significantly larger terminal arbors, and despite normal short-term behavioral responses to drugs acting on pre- and postsynaptic D2 DA receptors, they were unable to modulate their terminal arbor in response to pharmacological manipulation or after lesioning. In addition, although synaptosome DA uptake studies showed that the interaction of the D2 DA receptor and the dopamine transporter (DAT) was preserved in alpha4(-/-) mice, DAT function was found to be impaired. These findings suggest that the alpha4 subunit of the nAChR is an independent regulator of terminal arbor size of nigrostriatal dopaminergic neurons and that reduced functionality of presynaptic DAT may contribute to this effect by impairing DA uptake.