Imipramine stimulates phospholipase C activity in rat brain

The Cooperative Relationship between STAT5 and Reactive Oxygen Species in Leukemia: Mechanism and Therapeutic Potential

Reactive oxygen species (ROS) are now recognized as important second messengers with roles in many aspects of signaling during leukemogenesis. They serve as critical cell signaling molecules that regulate the activity of various enzymes including tyrosine phosphatases. ROS can induce inactivation of tyrosine phosphatases, which counteract the effects of tyrosine kinases. ROS increase phosphorylation of many proteins including signal transducer and activator of transcription-5 (STAT5) via Janus kinases (JAKs). STAT5 is aberrantly activated through phosphorylation in many types of cancer and this constitutive activation is associated with cell survival, proliferation, and self-renewal. Such leukemic activation of STAT5 is rarely caused by mutation of the STAT5 gene itself but instead by overactive mutant receptors with tyrosine kinase activity as well as JAK, SRC family protein tyrosine kinases (SFKs), and Abelson murine leukemia viral oncogene homolog (ABL) kinases. Interestingly, STAT5 suppresses transcription of several genes encoding antioxidant enzymes while simultaneously enhancing transcription of NADPH oxidase. By doing so, STAT5 activation promotes an overall elevation of ROS level, which acts as a feed-forward loop, especially in high risk Fms-related tyrosine kinase 3 (FLT3) mutant leukemia. Therefore, efforts have been made recently to target ROS in cancer cells. Drugs that are able to either quench ROS production or inversely augment ROS-related signaling pathways both have potential as cancer therapies and may afford some selectivity by activating feedback inhibition of the ROS-STAT5 kinome. This review summarizes the cooperative relationship between ROS and STAT5 and explores the pros and cons of emerging ROS-targeting therapies that are selective for leukemia characterized by persistent STAT5 phosphorylation.

The Chemical Sensitivity and Electrical Activity of Individual Olfactory Sensory Neurons to a Range of Sex Pheromones and Food Odors in the Goldfish

Although it is well established that the olfactory epithelium of teleost fish detects at least 6 classes of biologically relevant odorants using 5 types of olfactory sensory neurons (OSNs), little is understood about the specificity of individual OSNs and thus how they encode identity of natural odors. In this study, we used in vivo extracellular single-unit recording to examine the odor responsiveness and physiological characteristics of 109 individual OSNs in mature male goldfish to a broad range of biological odorants including feeding stimuli (amino acids, polyamines, nucleotides), sex pheromones (sex steroids, prostaglandins [PGs]), and a putative social cue (bile acids). Sixty-one OSNs were chemosensitive, with over half of these (36) responding to amino acids, 7 to polyamines, 7 to nucleotides, 5 to bile acids, 9 to PGs, and 7 to sex steroids. Approximately a quarter of the amino acid-sensitive units also responded to polyamines or nucleotides. Three of 6 amino acid-sensitive units responded to more than 1 amino acid compound, and 5 sex pheromone-sensitive units detected just 1 sex pheromone. While pheromone-sensitive OSNs also responded to the adenylyl cyclase activator, forskolin, amino acid-sensitive OSNs responded to either forskolin or a phospholipase C activator, imipramine. Most OSNs responded to odorants and activators with excitation. Our results suggest that pheromone information is encoded by OSNs specifically tuned to single sex pheromones and employ adenylyl cyclase, suggestive of a labeled-line organization, while food information is encoded by a combination of OSNs that use both adenylyl cyclase and phospholipase C and are often less specifically tuned.

Common and rare variant analysis in early-onset bipolar disorder vulnerability

Bipolar disorder is one of the most common and devastating psychiatric disorders whose mechanisms remain largely unknown. Despite a strong genetic contribution demonstrated by twin and adoption studies, a polygenic background influences this multifactorial and heterogeneous psychiatric disorder. To identify susceptibility genes on a severe and more familial sub-form of the disease, we conducted a genome-wide association study focused on 211 patients of French origin with an early age at onset and 1,719 controls, and then replicated our data on a German sample of 159 patients with early-onset bipolar disorder and 998 controls. Replication study and subsequent meta-analysis revealed two genes encoding proteins involved in phosphoinositide signalling pathway (PLEKHA5 and PLCXD3). We performed additional replication studies in two datasets from the WTCCC (764 patients and 2,938 controls) and the GAIN-TGen cohorts (1,524 patients and 1,436 controls) and found nominal P-values both in the PLCXD3 and PLEKHA5 loci with the WTCCC sample. In addition, we identified in the French cohort one affected individual with a deletion at the PLCXD3 locus and another one carrying a missense variation in PLCXD3 (p.R93H), both supporting a role of the phosphatidylinositol pathway in early-onset bipolar disorder vulnerability. Although the current nominally significant findings should be interpreted with caution and need replication in independent cohorts, this study supports the strategy to combine genetic approaches to determine the molecular mechanisms underlying bipolar disorder.

The tricyclic antidepressants amitriptyline, nortriptyline and imipramine are weak antagonists of human and rat alpha1B-adrenoceptors

Although it is long known that the tricyclic antidepressants amitriptyline, nortriptyline and imipramine inhibit the noradrenaline transporter and alpha(1)-adrenoceptors with similar affinities, which may lead to self-cancelling actions, the selectivity of these drugs for alpha(1)-adrenoceptor subtypes is unknown. The present study investigates the selectivity of amitriptyline, nortriptyline and imipramine for human recombinant and rat native alpha(1)-adrenoceptor subtypes. The selectivity of amitriptyline, nortriptyline and imipramine was investigated in HEK-293 cells expressing each of the human alpha(1)-subtypes and in rat native receptors from the vas deferens (alpha(1A)), spleen (alpha(1B)) and aorta (alpha(1D)) through [(3)H]prazosin binding, and noradrenaline-induced intracellular Ca(2+) increases and contraction assays. Amitriptyline, nortriptyline and imipramine showed considerably higher affinities for alpha(1A)- (approximately 25- to 80-fold) and alpha(1D)-adrenoceptors (approximately 10- to 25-fold) than for alpha(1B)-adrenoceptors in both contraction and [(3)H]prazosin binding assays with rat native and human receptors, respectively. In addition, amitriptyline, nortriptyline and imipramine were substantially more potent in the inhibition of noradrenaline-induced intracellular Ca(2+) increases in HEK-293 cells expressing alpha(1A)- or a truncated version of alpha(1D)-adrenoceptors which traffics more efficiently towards the cell membrane than in cells expressing alpha(1B)-adrenoceptors. Amitriptyline, nortriptyline and imipramine are much weaker antagonists of rat and human alpha(1B)-adrenoceptors than of alpha(1A)- and alpha(1D)-adrenoceptors. The differential affinities for these receptors indicate that the alpha(1)-adrenoceptor subtype which activation is most increased by the augmented noradrenaline availability resultant from the blockade of neuronal reuptake is the alpha(1B)-adrenoceptor. This may be important for the behavioural effects of these drugs.

Glucocorticoid receptor interaction with TrkB promotes BDNF-triggered PLC-gamma signaling for glutamate release via a glutamate transporter

An increase in glucocorticoid levels and down-regulation of BDNF (brain-derived neurotrophic factor) are supposed to be involved in the pathophysiology of depressive disorders. However, possible crosstalk between glucocorticoid- and BDNF-mediated neuronal functions in the CNS has not been elucidated. Here, we examined whether chronic glucocorticoid exposure influences BDNF-triggered intracellular signaling for glutamate release via a glutamate transporter. We found that chronic exposure to dexamethasone (DEX, a synthetic glucocorticoid) suppressed BDNF-induced glutamate release via weakening the activation of the PLC-gamma (phospholipase C-gamma)/Ca(2+) system in cultured cortical neurons. We demonstrated that the GR (glucocorticoid receptor) interacts with receptor tyrosine kinase for BDNF (TrkB). Following DEX treatment, TrkB-GR interaction was reduced due to the decline in GR expression. Corticosterone, a natural glucocorticoid, also reduced TrkB-GR interaction, BDNF-stimulated PLC-gamma, and BDNF-triggered glutamate release. Interestingly, BDNF-dependent binding of PLC-gamma to TrkB was diminished by DEX. SiRNA transfection to induce a decrease in endogenous GR mimicked the inhibitory action of DEX. Conversely, DEX-inhibited BDNF-activated PLC-gamma signaling for glutamate release was recovered by GR overexpression. We propose that TrkB-GR interaction plays a critical role in the BDNF-stimulated PLC-gamma pathway, which is required for glutamate release, and the decrease in TrkB-GR interaction caused by chronic exposure to glucocorticoids results in the suppression of BDNF-mediated neurotransmitter release via a glutamate transporter.

Diverse antidepressants increase CDP-diacylglycerol production and phosphatidylinositide resynthesis in depression-relevant regions of the rat brain

Background

Major depression is a serious mood disorder affecting millions of adults and children worldwide. While the etiopathology of depression remains obscure, antidepressant medications increase synaptic levels of monoamine neurotransmitters in brain regions associated with the disease. Monoamine transmitters activate multiple signaling cascades some of which have been investigated as potential mediators of depression or antidepressant drug action. However, the diacylglycerol arm of phosphoinositide signaling cascades has not been systematically investigated, even though downstream targets of this cascade have been implicated in depression. With the ultimate goal of uncovering the primary postsynaptic actions that may initiate cellular antidepressive signaling, we have examined the antidepressant-induced production of CDP-diacylglycerol which is both a product of diacylglycerol phosphorylation and a precursor for the synthesis of physiologically critical glycerophospholipids such as the phosphatidylinositides. For this, drug effects on [³H]cytidine-labeled CDP-diacylglycerol and [³H]inositol-labeled phosphatidylinositides were measured in response to the tricyclics desipramine and imipramine, the selective serotonin reuptake inhibitors fluoxetine and paroxetine, the atypical antidepressants maprotiline and nomifensine, and several monoamine oxidase inhibitors.

Results

Multiple compounds from each antidepressant category significantly stimulated [³H]CDP-diacylglycerol accumulation in cerebrocortical, hippocampal, and striatal tissues, and also enhanced the resynthesis of inositol phospholipids. Conversely, various antipsychotics, anxiolytics, and non-antidepressant psychotropic agents failed to significantly induce CDP-diacylglycerol or phosphoinositide synthesis. Drug-induced CDP-diacylglycerol accumulation was independent of lithium and only partially dependent on phosphoinositide hydrolysis, thus indicating that antidepressants can mobilize CDP-diacylglycerol from additional pools lying outside of the inositol cycle. Further, unlike direct serotonergic, muscarinic, or α-adrenergic agonists that elicited comparable or lower effects on CDP-diacylglycerol versus inositol phosphates, the antidepressants dose-dependently induced significantly greater accumulations of CDP-diacylglycerol.

Conclusion

Chemically divergent antidepressant agents commonly and significantly enhanced the accumulation of CDP-diacylglycerol. The latter is not only a derived product of phosphoinositide hydrolysis but is also a crucial intermediate in the biosynthesis of several signaling substrates. Hence, altered CDP-diacylglycerol signaling might be implicated in the pathophysiology of depression or the mechanism of action of diverse antidepressant medications.

Regulation of neuronal PLCgamma by chronic morphine

Alterations in neurotrophic signaling pathways may contribute to the changes in the mesolimbic dopamine system induced by chronic morphine exposure. In a rat model of morphine dependence, we previously identified increased levels of phospholipase C gamma-1 (PLCgamma1) immunoreactivity specifically within the ventral tegmental area (VTA) following chronic morphine treatment. Using an antibody specific for the tyrosine-phosphorylated, activated form of PLCgamma1, we now show that chronic morphine also significantly upregulates PLCgamma1 activity in the VTA, as well as in the nucleus accumbens and hippocampus, regions which are also implicated in the reinforcing properties of morphine. In contrast, no increase in PLCgamma1 activity was found in the substantia nigra or dorsal striatum. HSV-mediated overexpression of PLCgamma1 in PC12 cells induced ERK activation via a mechanism dependent, in part, on both MAP-ERK kinase (MEK) and protein kinase C. PLCgamma1 overexpression in the VTA similarly induced ERK activation in the VTA in vivo. As chronic morphine treatment has been shown to increase ERK activity within the VTA, the current results suggest that increased PLCgamma1 activity may be an upstream mediator of this effect.

Chronic antidepressants potentiate via sigma-1 receptors the brain-derived neurotrophic factor-induced signaling for glutamate release

Up-regulation of BDNF (brain-derived neurotrophic factor) has been suggested to contribute to the action of antidepressants. However, it is unclear whether chronic treatment with antidepressants may influence acute BDNF signaling in central nervous system neurons. Because BDNF has been shown by us to reinforce excitatory glutamatergic transmission in cultured cortical neurons via the phospholipase-gamma (PLC-gamma)/inositol 1,4,5-trisphosphate (IP3)/Ca2+ pathway (Numakawa, T., Yamagishi, S., Adachi, N., Matsumoto, T., Yokomaku, D., Yamada, M., and Hatanaka, H. (2002) J. Biol. Chem. 277, 6520-6529), we examined in this study the possible effects of pretreatment with antidepressants on the BDNF signaling through the PLC-gamma)/IP3/Ca2+ pathway. Furthermore, because the PLC-gamma/IP3/Ca2+ pathway is regulated by sigma-1 receptors (Hayashi, T., and Su, T. P. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 491-496), we examined whether the BDNF signaling is modulated by sigma-1 receptors (Sig-1R). We found that the BDNF-stimulated PLC-gamma activation and the ensued increase in intracellular Ca2+ ([Ca2+]i) were potentiated by pretreatment with imipramine or fluvoxamine, so was the BDNF-induced glutamate release. Furthermore, enhancement of the interaction between PLC-gamma and TrkB (receptor for BDNF) after imipramine pretreatment was observed. Interestingly, BD1047, a potent Sig-1R antagonist, blocked the imipramine-dependent potentiation on the BDNF-induced PLC-gamma activation and glutamate release. In contrast, overexpression of Sig-1R per se, without antidepressant pretreatment, enhances BDNF-induced PLC-gamma activation and glutamate release. These results suggest that antidepressant pretreatment selectively enhance the BDNF signaling on the PLC-gamma/IP3/Ca2+ pathway via Sig-1R, and that Sig-1R plays an important role in BDNF signaling leading to glutamate release.

Flagellin of Pseudomonas aeruginosa inhibits Na+ transport in airway epithelia

Pseudomonas aeruginosa causes severe life-threatening airway infections that are a frequent cause for hospitalization of cystic fibrosis (CF) patients. These Gram-negative pathogens possess flagella that contain the protein flagellin as a major structural component. Flagellin binds to the host cell glycolipid asialoGM1 (ASGM1), which appears enriched in luminal membranes of respiratory epithelial cells. We demonstrate that in mouse airways, luminal exposure to flagellin leads to inhibition of Na+ absorption by the epithelial Na+ channel ENaC, but does not directly induce a secretory response. Inhibition of ENaC was observed in tracheas of wild-type mice and was attenuated in mice homozygous for the frequent cystic fibrosis conductance regulator (CFTR) mutation G551D. Similar to flagellin, anti-ASGM1 antibody also inhibited ENaC. The inhibitory effects of flagellin on ENaC were attenuated by blockers of the purinergic signaling pathway, although an increase in the intracellular Ca2+ concentration by recombinant or purified flagellin or whole flagella was not observed. Because an inhibitor of the mitogen-activated protein kinase (MAPK) pathway also attenuated the effects of flagellin on Na+ absorption, we conclude that flagellin exclusively inhibits ENaC, probably due to release of ATP and activation of purinergic receptors of the P2Y subtype. Stimulation of these receptors activates the MAPK pathway, thereby leading to inhibition of ENaC. Thus, P. aeruginosa reduces Na+ absorption, which could enhance local mucociliary clearance, a mechanism that seem to be attenuated in CF.

Caveolin Scaffolding Peptide-1 Interferes With Norepinephrine-Induced PLC-β Activation in Cultured Rat Vascular Smooth Muscle Cells

Caveolins are a family of integral membrane proteins implicated in various cell functions, including the organization and inactivation of signaling molecules of G protein-coupled receptors. We tested the ability of human caveolin scaffolding peptide-1 (CSP-1) to regulate norepinephrine- (NE) or histamine (HIS)-induced increases on intracellular calcium concentrations ([Ca(2+)]i). In cultured rat vascular smooth muscle cells (VSMC), CSP-1 inhibited in a concentration-dependent manner NE- and HIS-induced increases in [Ca(2+)]i. This effect can be explained by the fact that CSP-1 inhibited a common signaling pathway. We tested the ability of this peptide to decrease the activation of PLC-beta3 and MAPK. CSP-1 inhibited the expression of the activated form of both enzymes, suggesting a direct effect of the peptide on the signaling cascade. CSP-1 readily enters VSMC in culture, as observed when FITC-conjugated CPS-1 is added to cell culture media. Taken together, these data suggest that CSP-1 blocks the effects of NE and HIS on [Ca(2+)]i of VSMC by inhibiting the activation of PLC-beta3 and MAPK.

The antidepressant imipramine inhibits M current by activating a phosphatidylinositol 4,5-bisphosphate (PIP2)-dependent pathway in rat sympathetic neurones

Little is known about the intracellular actions of imipramine (IMI) in the regulation of ion channels. We tested the action of IMI on the intracellular cascade that regulates M current (I(M)) in superior cervical ganglion neurones (SCGs). Dialysis of the cells with GDPbetaS, a G protein signaling blocker, did not disrupt the inhibition of I(M). When we incubated the cells with the phospholipase C (PLC) inhibitor U73122, it prevented the I(M) inhibition by IMI. Also, when we dialyzed the cells with an intracellular Ca2+ chelator, it did not disrupt I(M) inhibition by IMI, as occurs in the M1 cascade. When we incubated the cells with the generic kinase inhibitor wortmannin, it prevented the recovery of I(M) from the inhibition by IMI. Also, when we applied phosphatidylinositol 4,5-bisphosphate (PIP2) intracellularly, it diminished the inhibition of I(M) by IMI. Our findings suggest that PLC is the target for IMI, that recovery of I(M) needs lipid phosphorylation for PIP2 resynthesis, and that IMI inhibits I(M) by activating a PLC-dependent pathway, likely by decreasing the concentration of PIP2.

Amitriptyline enhances extracellular tissue levels of adenosine in the rat hindpaw and inhibits adenosine uptake

Local administration of amitriptyline into the rat hindpaw produces peripheral antinociception; this is reduced by adenosine receptor antagonists and appears to involve endogenous adenosine. The present study used peripheral microdialysis: (a) to determine whether amitriptyline could enhance extracellular tissue levels of endogenous adenosine in the rat hindpaw and (b) to examine mechanisms by which such an increase could occur. Local injection of amitriptyline into the plantar hindpaw, at doses that produce peripheral antinociception (100-300 nmol), produced an increase in local extracellular levels of adenosine. When injected in combination with formalin, which also enhances such levels of adenosine, an additive increase was observed. This adenosine originated partly as nucleotide, as inhibition of ecto-5'-nucleotidase reduced the amount of adenosine detected in the probe following administration of amitriptyline. When administered in combination with exogenous adenosine, amitriptyline augmented recovery of adenosine in the probe. Pretreatment of rats with capsaicin augmented the ability of amitriptyline to increase adenosine levels detected in the dialysis probe; it also enhanced tissue recovery of exogenously administered adenosine. In uptake studies using cultured rat C6 glioma cells, amitriptyline inhibited adenosine uptake by an adenosine transporter (IC50 0.37 +/- 0.12 mM). In enzyme assays, amitriptyline had no effect on adenosine kinase or adenosine deaminase activity. These results demonstrate that amitriptyline: (a) enhances extracellular tissue levels of adenosine in the rat hindpaw following local administration in vivo and (b) inhibits adenosine uptake but has no effect on metabolism in vitro. Therefore, increased extracellular adenosine levels in vivo appear to result partially from extracellular conversion of nucleotide and partially from inhibition of uptake.

Differential effect of imipramine and related compounds on Mg2+ efflux from rat erythrocytes

The effect of imipramine on Mg2+ efflux in NaCl medium (Na+/Mg2+ antiport), on Mg2+ efflux in choline.Cl medium (choline/Mg2+ antiport) and on Mg2+ efflux in sucrose medium (Cl- -coupled Mg2+ efflux) was investigated in rat erythrocytes. In non-Mg2+-loaded rat erythrocytes, imipramine stimulated Na+/Mg2+ antiport but inhibited choline/Mg2+ antiport and Cl- -coupled Mg2+ efflux. The same effect could be obtained by several other compounds structurally related to imipramine. These drugs contain a cyclic hydrophobic ring structure to which a four-membered secondary or tertiary amine side chain is attached. At a physiological pH, the amine side chain expresses a cationic choline-like structure. The inhibitory effect on choline/Mg2+ antiport is lost when the amine side chain is modified or abandoned, pointing to competition of the choline-like side chain with choline or another cation at the unspecific choline antiporter or at the Cl- -coupled Mg2+ efflux. Other related drugs either stimulated Na+/Mg2+ antiport and choline/Mg2+ antiport, or they were ineffective. For stimulation of Na+/Mg2+ antiport and choline/Mg2+ antiport, there is no specific common structural motif of the drugs tested. The effects of imipramine on Na+/Mg2+ antiport and choline/Mg2+ antiport are not mediated by PKCalpha but are caused by a direct reaction of imipramine with these transporters. By increasing the intracellular Mg2+ concentration, the stimulation of Na+/Mg2+ antiport at a physiological intracellular Mg2+ concentration changed to an inhibition of Na+/Mg2+ antiport. This effect can be explained by the hypothesis that Mg2+ loading induced an allosteric transition of the Mg2+/Mg2+ exchanger with low Na+/Mg2+ antiport capacity to the Na+/Mg2+ antiporter with high Na+/Mg2+ antiport capacity. Both forms of the Mg2+ exchanger may be differently affected by imipramine.

Adenophostin A and imipramine are two activators of the olfactory inositol 1,4,5-trisphosphate-gated channel in fish olfatory cilia

Binding of an odorant to its receptor activates the cAMP-dependent pathway, and also leads to inositol 1,4,5-trisphosphate (InsP(3)) production. This induces opening of a plasma membrane channel in olfactory receptor cells (ORCs). We investigated single-channel properties of this channel in the presence of a phospholipase C (PLC) activator (imipramine) and of a potent activator of the InsP(3)/Ca(2+) release channel (adenophostin A) by reconstituting carp olfactory cilia into planar lipid bilayers. In the presence of 53 mM barium as a charge carrier, the addition of 50 microM imipramine induced a current of 1.53+/-0.05 pA at 0 mV. There were two different mean open times (6.0+/-0.6 ms and 49.6+/-6.4 ms). The I/ V curve displayed a slope conductance of 50+/-2 pS. Channel activity was transient and was blocked by neomycin (50 microM). These observations suggest that imipramine may activate the olfactory InsP(3)-gated channel through PLC. Using the same ionic conditions, the application of 0.5 microM adenophostin A triggered a current of 1.47+/-0.04 pA at 0 mV. The I/ V curve displayed a slope conductance of 60+/-2 pS. This channel showed only a single mean open time (15.0+/-0.3 ms) and was strongly inhibited by ruthenium red (30 microM) and heparin (10 microg/mL). These results indicate that adenophostin A and imipramine may act on the ciliary InsP(3)-gated channel and are potentially valuable pharmacological tools for studying olfactory transduction mechanisms.

Microtubule-dependent redistribution of the type-1 inositol 1,4,5-trisphosphate receptor in A7r5 smooth muscle cells

In A7r5 vascular smooth muscle cells, the two expressed inositol 1,4,5-trisphosphate receptor (IP(3)R) isoforms were differentially localized. IP(3)R1 was predominantly localized in the perinuclear region, whereas IP(3)R3 was homogeneously distributed over the cytoplasm. Prolonged stimulation (1-5 hours) of cells with 3 microM arginine-vasopressin induced a redistribution of IP(3)R1 from the perinuclear region to the entire cytoplasm, whereas the localization of IP(3)R3 appeared to be unaffected. The redistribution process occurred independently of IP(3)R downregulation. No structural changes of the endoplasmic reticulum were observed, but SERCA-type Ca(2+) pumps redistributed similarly to IP(3)R1. The change in IP(3)R1 localization induced by arginine-vasopressin could be blocked by the simultaneous addition of nocodazole or taxol and depended on Ca(2+) release from intracellular stores since Ca(2+)-mobilizing agents such as thapsigargin and cyclopiazonic acid could induce the redistribution. Furthermore, various protein kinase C inhibitors could inhibit the redistribution of IP(3)R1, whereas the protein kinase C activator 1-oleoyl-2-acetyl-sn-glycerol induced the redistribution. Activation of protein kinase C also induced an outgrowth of the microtubules from the perinuclear region into the cytoplasm, similar to what was seen for the redistribution of IP(3)R1. Finally, blocking vesicular transport at the level of the intermediate compartment inhibited the redistribution. Taken together, these findings suggest a role for protein kinase C and microtubuli in the redistribution of IP(3)R1, which probably occurs via a mechanism of vesicular trafficking.

The antidepressant imipramine inhibits the M-type K+ current in rat sympathetic neurons

This study aimed to assess the effects of the antidepressant drug imipramine (IMI) on the neuronal M-type K+ current (IK(M)). We show that IMI reversibly reduces IK(M) with an IC50 of 7 microM. The V0.5 and slope factor of the steady state activation curve remained unchanged after IMI, indicating a mode of action that is voltage insensitive for blocking the M-channel. Patch pipette application of IMI elicits same inhibitory response suggesting a binding site on the M-channel accessible from both sides of the cell membrane. Accordingly, the inhibitory effect of IMI is larger by rising external pH near to the pKa of the drug. Therefore, we propose that a neutral form of IMI binds more efficiently to M-channels to exert its inhibitory action by a voltage-independent mechanism.

Modulation of spontaneous quantal release of neurotransmitters in the hippocampus

Presynaptic action potentials trigger the exocytosis of neurotransmitters. However, even in the absence of depolarisation-dependent Ca2+ entry nearby release sites, spontaneous vesicular release still occurs. Even though this happens at low rate, such spontaneous release may play a trophic role in maintaining the shape of dendritic structures. Like evoked responses, action potential-independent release is subject to modulation. This review describes some of the regulatory factors that rapidly and presynaptically regulate the ongoing Ca2+-independent release of neurotransmitters in the hippocampus. For instance, the electrical activity of the nerve ending, neurotransmitters, hypertonic solutions, neurotoxins, polycations, neurotrophic factors, immunoglobulins, cyclothiazide and psychotropic drugs can all modify the rate of spontaneous release. This can be achieved through various mechanisms that can be Ca2+-dependent or Ca2+-independent, protein kinase-dependent or independent. Since action potential-independent release contributes to the maintenance of dendritic structures, neuromodulators are likely to influence the density and/or length of dendritic spines, which in turn may modulate information processing in the central nervous system (CNS).

Up-regulation of nucleolin mRNA and protein in peripheral blood mononuclear cells by extracellular-regulated kinase

The signal transduction pathways regulating nucleolin mRNA and protein production have yet to be elucidated. Peripheral blood mononuclear cells treated with phorbol 12-myristate 13-acetate showed steady state levels of nucleolin mRNA that were 2-2.5-fold greater than untreated control cells. The up-regulation of nucleolin mRNA was substantially repressed by U0126, a specific inhibitor that blocks phosphorylation of extracellular-regulated kinase (ERK). Calcium ionophores and ionomycin also activated ERK and substantially elevated nucleolin mRNA levels, demonstrating phorbol 12-myristate 13-acetate and calcium signaling converge on ERK. Drugs that affected protein kinase C, protein kinase A, and phospholipase C signal transduction pathways did not alter nucleolin mRNA levels significantly. The half-life of nucleolin mRNA increased from 1.8 h in resting cells to 3.2 h with phorbol ester activation, suggesting ERK-mediated posttranscriptional regulation. Concomitantly, full-length nucleolin protein was increased. The higher levels of nucleolin protein were accompanied by increased binding of a 70-kDa nucleolin fragment to the 29-base instability element in the 3'-untranslated region of amyloid precursor protein (APP) mRNA in gel mobility shift assays. Supplementation of rabbit reticulocyte lysate with nucleolin decreased APP mRNA stability and protein production. These data suggest ERK up-regulates nucleolin posttranscriptionally thereby controlling APP production.

Regulation of GIRK channel deactivation by Galpha(q) and Galpha(i/o) pathways

G protein regulated inward rectifying potassium channels (GIRKs) are activated by G protein coupled receptors (GPCRs) via the G protein betagamma subunits. However, little is known about the effects of different GPCRs on the deactivation kinetics of transmitter-mediated GIRK currents. In the present study we investigated the influence of different GPCRs in the presence and absence of RGS proteins on the deactivation kinetics of GIRK channels by coexpressing the recombinant protein subunits in Xenopus oocytes. The stimulation of both G(i/o)- and G(q)-coupled pathways accelerated GIRK deactivation. GIRK currents deactivated faster upon stimulation of G(i/o)- and G(q)-coupled pathways by P(2)Y(2) receptors (P(2)Y(2)Rs) than upon activation of the G(i/o)-coupled pathway alone via muscarinic acetylcholine receptor M2 (M(2) mAChRs). This acceleration was found to be dependent on phospholipase C (PLC) and protein kinase C (PKC) activities and intracellular calcium. With the assumption that RGS2 has a higher affinity for Galpha(q) than Galpha(i/o), we demonstrated that the deactivation kinetics of GIRK channels can be differentially regulated by the relative amount of RGS proteins. These data indicate that transmitter-mediated deactivation of GIRK currents is modulated by crosstalk between G(i/o)- and G(q)-coupled pathways.

Modulation of odor-induced increases in [Ca(2+)](i) by inhibitors of protein kinases A and C in rat and human olfactory receptor neurons

Protein kinases A and C have been postulated to exert multiple effects on different elements of signal transduction pathways in olfactory receptor neurons. However, little is known about the modulation of olfactory responses by protein kinases in intact olfactory receptor neurons. To further elucidate the details of the modulation of odorant responsiveness by these protein kinases, we investigated the action of two protein kinase inhibitors: H89, an inhibitor of protein kinase A, and N-myristoylated EGF receptor, an inhibitor of protein kinase C, on odorant responsiveness in intact olfactory neurons. We isolated individual olfactory neurons from the adult human and rat olfactory epithelium and measured responses of the isolated cells to odorants or biochemical activators that have been shown to initiate cyclic AMP or inositol 1,4,5-trisphospate production in biochemical preparations. We employed calcium imaging techniques to measure odor-elicited changes in intracellular calcium that occur over several seconds. In human olfactory receptor neurons, the protein kinase A and C inhibitors affected the responses to different sets of odorants. In rats, however, the protein kinase C inhibitor affected responses to all odorants, while the protein kinase A inhibitor had no effect. In both species, the effect of inhibition of protein kinases was to enhance the elevation and block termination of intracellular calcium levels elicited by odorants. Our results show that protein kinases A and C may modulate odorant responses of olfactory neurons by regulating calcium fluxes that occur several seconds after odorant stimulation. The effects of protein kinase C inhibition are different in rat and human olfactory neurons, indicating that species differences are an important consideration when applying data from animal studies to apply to humans.

Imipramine inhibits Cl(-) secretion by desensitization of beta-adrenergic receptors in calu-3 human airway cells

Recent investigations have found that tetracyclic antidepressants like imipramine (IMP) have high-affinity sites not only in brain but also in mammalian lung. In the present study, we examined the effects of IMP on the Cl(-) secretion produced by isoproterenol (ISP), a beta-adrenergic receptor (beta-AR) agonist, in Calu-3 human airway cells. ISP applied in the basolateral solution generated a sustained short-circuit current that was abolished by diphenylamine-2-carboxylate, a Cl(-) channel blocker. IMP (0.01-1 mM) applied in the apical or basolateral solution for 30 min significantly inhibited the ISP-induced responses in a concentration-dependent manner, and the inhibitory effects of this drug were remarkable when applied from the apical rather than the basolateral side. ISP-induced responses were mimicked by forskolin- and 8-bromo-cyclic AMP-induced ones, but which were insensitive to IMP. These results indicate that IMP desensitizes the beta-AR on the basolateral membrane from the cytosolic side in Calu-3 cells.

Relaxant effects of antidepressants on human isolated mesenteric arteries

AIMS

The therapeutic action of tricyclic agents may be accompanied by unwanted effects on the cardiovascular system. The evidence for the effects on vascular and nonvascular smooth muscle comes from animal studies. Whether these studies can be extrapolated to human vessels remains to be determined. Therefore, the present study was designed to investigate the influence of amitriptyline, nortriptyline and sertraline on the contractile responses of human isolated mesenteric arteries to electrical field stimulation, noradrenaline and potassium chloride.

METHODS

Arterial segments (lumen diameter 0.8-1.2 mm) were obtained from portions of the human omentum during the course of 41 abdominal operations (22 men and 19 women), and rings 3 mm long were mounted in organ baths for isometric recording of tension. In some artery rings the endothelium was removed mechanically.

RESULTS

In precontracted artery rings amitriptyline, nortriptyline and sertraline (3x10(-7)-10(-4) m ) produced concentration-dependent relaxation that was independent of the presence or absence of vascular endothelium. Incubation with indomethacin (3x10(-6) m ) reduced the pD2 values thus indicating the participation of dilating prostanoid substances in this response. Amitriptyline and nortriptyline inhibited both the neurogenic-and noradrenaline-induced contractions. In contrast, only the highest concentration of sertraline reduced the adrenergic responses. Amitriptyline, nortriptyline and sertraline inhibited contractions elicited by KCl and produced rightward shifts of the concentration-response curve to CaCl2 following incubation in calcium-free solution.

CONCLUSIONS

These results indicate that amitriptyline and nortriptyline could act as adrenoceptor antagonists and direct inhibitors of smooth muscle contraction of human mesenteric arteries, whereas sertraline might principally exert its action only as direct inhibitor of smooth muscle contraction. This relaxant mechanism involves an interference with the entry of calcium.

Acute application of the tricyclic antidepressant desipramine presynaptically stimulates the exocytosis of glutamate in the hippocampus

Tricyclic antidepressants (e.g., imipramine, desipramine) are currently used in the treatment of mood disorders such as depression. At the cellular level they inhibit the re-uptake of the exocytosed monoamines serotonin and noradrenaline. However, they also stimulate phospholipase C activity and the production of the second messenger inositol 1,4,5-trisphosphate. Since phospholipase C activation can also lead to the production of the protein kinase C activator diacylglycerol, we have undertaken experiments to see whether acutely applied desipramine could change the synaptic strength of neurons in a protein kinase C-dependent manner. Experiments performed with cultured hippocampal neurons dissociated from neonatal rats revealed that desipramine rapidly enhanced the spontaneous vesicular release of glutamate. This was observed by measuring the frequency of tetrodotoxin-resistant spontaneous excitatory postsynaptic currents. Analysis of amplitude distribution histograms indicated a presynaptic site of action. The protein kinase inhibitor staurosporine and down-regulation of protein kinase C activity greatly reduced the desipramine-dependent enhancement of the frequency of tetrodotoxin-resistant spontaneous excitatory postsynaptic currents. This presynaptic modulation requires SNARE proteins because cleavage of SNAP-25 with the botulinum neurotoxin A strongly reduced the desipramine-induced glutamate release. Thus, acute applications of desipramine stimulated the ongoing neurotransmitter release pathway, probably by activating protein kinase C. Our data indicate that tricyclic antidepressant drugs not only act on serotoninergic and/or noradrenergic cells but can also modify the activity of glutamatergic neurons.

Bulbectomy-induced loss of raphe neurons is counteracted by antidepressant treatment

1. Bilateral olfactory bulb ablation was performed in C57B1/6j mice (C57). Separate groups of bulbectomized mice were treated with either antidepressants (trazodone, 20 mg/kg i.p., or amitriptyline, 20 mg/kg i.p.) or saline daily for 14 consecutive days starting 14 days after surgery. 2. Celloidine-imbedded 10 microns-thick brain sections containing the nucleus raphe dorsalis (NRD) or locus coeruleus (LC) were stained for Nissl, and the number of functional and pyknotic cells was counted out of 500 total cell count for each animal in every experimental group: sham-operated, bulbectomized treated with saline or one of the two antidepressants. 3. Bulbectomy produced a significant 4 times increase in the proportion of pyknotic cells in NRD as compared to sham-operated control. Both antidepressants reversed the effect bringing the number of pyknotic cells to control level. The proportion of pyknotic cells in LC was also slightly increased (61%) in the bulbectomized mice, but only amitriptyline was able to reverse the effect. 4. Widespread degeneration of the neurons in NRD caused by bulbectomy may be involved in the serotonergic component of the bulbectomy syndrome. The ability of antidepressants to diminish bulbectomy-induced loss of NRD neurons may underlie their restorative effect on the behavior and neurochemical characteristics of bulbectomized animals.