Biomedical subjects

R S Jope

Publications and source records attributed to R S Jope.

At least 55 records · Page 3Linked to original sources

PubMed1997

Lithium attenuates nerve growth factor-induced activation of AP-1 DNA binding activity in PC12 cells.

This investigation tested if lithium, the primary therapeutic agent for bipolar mood disorder, modulated activation of the AP-1 transcription factor in PC12 cells treated with nerve growth factor (NGF), which induces robust responses in these cells. NGF induced large, time-dependent increases in AP-1 DNA binding activity. Pretreatment with 5 mmol/L lithium for 24 h reduced AP-1 induction by NGF by 42%; shorter treatments and lower concentrations of lithium had smaller inhibitory effects on AP-1. This effect of lithium was not limited to AP-1, as it also inhibited NGF-induced cyclic AMP responsive element (CRE) DNA binding activity. In contrast, activation of AP-1 and CRE by forskolin was unaffected by lithium. AP-1 constituent proteins were differentially susceptible to lithium, as cJun was reduced by 55%, cFos was unaffected by lithium, and an intermediate effect was observed with Jun B. These results reveal that lithium modulates the activation of transcription factors in a neuronal cell model, indicating that selective regulation of gene expression may contribute to the long term in vivo effect of lithium.

Activating Transcription Factor 2↗

PubMed1997

Comparison of [3H]phosphatidylinositol and [3H]phosphatidylinositol 4,5-bisphosphate hydrolysis in postmortem human brain membranes and characterization of stimulation by dopamine D1 receptors.

Assessing the function of the phosphoinositide signal transduction system in membranes prepared from postmortem human brain by measuring the hydrolysis of exogenous labeled phosphoinositides has been applied to studies of a variety of CNS disorders in recent years. Two issues concerning such studies were addressed in the current investigation: how do [3H]phosphatidylinositol and [3H]phosphatidylinositol 4,5-bisphosphate compare as substrates, and how do dopamine D1 receptors influence phosphoinositide signaling? Comparisons of [3H]phosphatidylinositol and [3H] phosphatidylinositol 4,5-bisphosphate hydrolysis stimulated by guanosine-5'-O-(3-thiotriphosphate)-activated G proteins and by several receptor agonists demonstrated that in most cases each substrate gave similar relative results in membranes prepared from prefrontal cortices of six individuals. However, using optimal assay conditions, [3H]phosphatidylinositol produced a greater signal-to-noise ratio compared with [3H] phosphatidylinositol 4,5-bisphosphate. Dopamine D1 receptors were demonstrated to be directly coupled to phosphoinositide hydrolysis in human brain membranes, and this response was shown to be mediated by the G(q/11) G protein subtype and by the beta-subtype of phospholipase C. Therefore, these results demonstrate that [3H]phosphatidylinositol is a suitable substrate to measure phosphoinositide hydrolysis in human brain membranes and that dopamine D1 receptors directly stimulate this signaling system.

2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-ben↗

PubMed1996

Cholinergic stimulation of AP-1 and NF kappa B transcription factors is differentially sensitive to oxidative stress in SH-SY5Y neuroblastoma: relationship to phosphoinositide hydrolysis.

Oxidative stress appears to contribute to neuronal dysfunction in a number of neurodegenerative conditions, notably including Alzheimer's disease, in which cholinergic receptor-linked signal transduction activity is severely impaired. To test whether oxidative stress could contribute to deficits in cholinergic signaling, responses to carbachol were measured in human neuroblastoma SH-SY5Y cells exposed to H2O2. DNA binding activities of two transcription factors that are respondent to oxidative conditions, AP-1 and NF kappa B, were measured in nuclear extracts. H2O2 and carbachol individually induced dose- and time-dependent increases in AP-1 and NF kappa B. In contrast, when given together, H2O2 concentration dependently (30-300 microM) inhibited the increase after carbachol in AP-1. Carbachol's stimulation of NF kappa B was not inhibited except with a high concentration (300 microM) of H2O2, which was associated with impaired activation of protein kinase C. Lower concentrations of H2O2 (30-300 microM) inhibited carbachol-induced [3H]phosphoinositide hydrolysis, and this inhibition correlated (r = 0.95) with the inhibition of carbachol-induced AP-1. Activation [3H]phosphoinositide hydrolysis by the calcium ionophore ionomycin was unaffected by H2O2, indicating that phospholipase C and phosphoinositides were impervious to this treatment. In contrast, activation with NaF of G-proteins coupled to phospholipase C was concentration dependently inhibited by H2O2, indicating impaired G-protein function. These effects of H2O2 are similar to signaling impairments reported in Alzheimer's disease brain, which involve deficits in receptor- and G-protein-stimulated phosphoinositide hydrolysis, but not phospholipase C activity. Thus, these findings indicate that oxidative stress may contribute to impaired phosphoinositide signaling in neurological disorders in which oxidative stress occurs, and that oxidative stress can differentially influence transcription factors activated by cholinergic stimulation.

Carbachol↗

PubMed1996

Alterations in phosphoinositide signaling and G-protein levels in depressed suicide brain.

The function of the phosphoinositide signal transduction system and the levels of heterotrimeric G-protein alpha-subunits were examined in postmortem prefrontal cortex regions (8/9) and region (10) from suicide victims with major depression and matched control subjects without psychiatric illness. The hydrolysis of [3H]phosphatidylinositol (PI) stimulated by phospholipase C, GTP-gamma-S, NaF, and neurotransmitter receptor agonists was measured in membrane preparations from both groups. Phospholipase C-beta activity was similar in depressed suicide and control subjects in the two regions of prefrontal cortex. In prefrontal cortex (10), but not in (8/9), the GTP-gamma-S concentration-dependent stimulation of [3H]PI hydrolysis was significantly lower (30%) in the depressed suicide group compared to the control group. Receptor-coupled, G-protein-mediated [3H]PI hydrolysis induced with carbachol, histamine, trans-1-aminocyclopentyl-1, 3-dicarboxylic acid (ACPD, a glutamatergic metabotropic receptor agonist), serotonin, or 2-methylthio-adenosine triphosphate (2mATP, a purinergic receptor agonist) in the presence of GTP-gamma-S stimulated equivalent responses in the two groups of subjects in each brain region. In prefrontal cortex (10) there was a 68% increase in the level of the 45 kDa subtype of G alpha s and in prefrontal cortex (8/9) there was a significant decrease (21%) in the level of G alpha i2 in the depressed suicide group compared to the control group. Levels of other heterotrimeric G-protein alpha-subunits (G alpha q/11, G alpha i1, and G alpha o) were not different in depressed suicide and control subjects in either brain region. Moreover, there were no differences in the levels of phospholipase C-beta or protein kinase C-alpha in the two groups of subjects in either brain region examined. These results demonstrate that in the prefrontal cortex of suicide victims with major depression compared to normal control subjects there is a region-specific alteration of G-protein-induced activation of the phosphoinositide signal transduction system and in the levels of G-protein alpha-subunits involved in cyclic AMP synthesis. These findings provide direct evidence in human brain that these two important signal transduction systems are altered in suicide subjects with major depression.

Adult↗

PubMed1996

Phosphoinositide signaling in human brain.

The phosphoinositide signal transduction system constitutes one of the primary means for intercellular communication in the central nervous system, but only recently has this system been studied in human brain. Although some investigations have studied phosphoinositide signaling in slices from biopsied human brain, due to the limited access to such material a greater number of studies have utilized membranes prepared from postmortem human brain. With membranes exposed to exogenous labeled phosphoinositides, activation of phospholipase C with calcium, with G-proteins stimulated by GTP gamma S or NaF, or with several receptor agonists, have demonstrated that all of the components of the phosphoinositide system are retained in human brain membranes and are responsive to appropriate stimuli. Investigators have begun to examine the effects of neurological (Alzheimer's disease, epilepsy, Parkinson's disease) and psychiatric (schizophrenia, major depression, bipolar affective disorder) diseases on the activity of the phosphoinositide system. Alzheimer's disease has been studied to the greatest extent and a severe deficit in phosphoinositide signaling has been identified in most studies. In addition, brain regionally selective deficits in G-protein function associated with phosphoinositide signaling have been reported in subjects with major depression or with bipolar affective disorder, and in the latter an ameliorative effect of the therapeutic drug lithium was identified. Although significant progress has been achieved in studying the phosphoinositide system in human brain, many issues remaining to be addressed are discussed in this review. With carefully controlled studies, it appears that much will be learned in the near future about the phosphoinositide signal transduction system in human brain and the effects of a variety of disorders on its function.

Alzheimer Disease↗

PubMed1996

The phosphoinositide signal transduction system is impaired in bipolar affective disorder brain.

The function of the phosphoinositide second messenger system was assessed in occipital, temporal, and frontal cortex obtained postmortem from subjects with bipolar affective disorder and matched controls by measuring the hydrolysis of [3H]phosphatidylinositol ([3H]PI) incubated with membrane preparations and several different stimulatory agents. Phospholipase C activity, measured in the presence of 0.1 mM Ca2+ to stimulate the enzyme, was not different in bipolar and control samples. G proteins coupled to phospholipase C were concentration-dependently activated by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and by NaF. GTP gamma S-stimulated [3H]Pl hydrolysis was markedly lower (50%) at all tested concentrations (0.3-10 microM GTP gamma S) in occipital cortical membranes from bipolar compared with control subjects. Responses to GTP gamma S in temporal and frontal cortical membranes were similar in bipolars and controls, as were responses to NaF in all three regions. Brain lithium concentrations correlated directly with GTP gamma S-stimulated [3H]Pl hydrolysis in bipolar occipital, but not temporal or frontal, cortex. Carbachol, histamine, trans-1-aminocyclopentyl-1,3-dicarboxylic acid, serotonin, and ATP each activated [3H]Pl hydrolysis above that obtained with GTP gamma S alone, and these responses were similar in bipolars and controls except for deficits in the responses to carbachol and serotonin in the occipital cortex, which were equivalent to the deficit detected with GTP gamma S alone. Thus, among the three cortical regions examined there was a selective impairment in G protein-stimulated [3H]Pl hydrolysis in occipital cortical membranes from bipolar compared with control subjects. These results directly demonstrate decreased activity of the phosphoinositide signal transduction system in specific brain regions in bipolar affective disorder.

Adenosine Triphosphate↗

PubMed1995

Inhibition of NFkB DNA binding activity by glucocorticoids in rat brain.

The influence of glucocorticoids on the transcription factor NFkB was investigated by using the gel mobility shift assay with nuclear extracts prepared from rat cerebral cortex and hippocampus after a variety of in vivo treatments. Following stimulation with each of three treatments, kainate, pilocarpine, or lithium plus pilocarpine-induced seizures, NFkB DNA binding activity was significantly greater in the cortex and hippocampus from adrenalectomized than from adrenal-intact rats. These results indicate that in rat brain glucocorticoids inhibit NFkB activity in addition to the previously reported inhibition of the transcription factor AP-1 (activator protein 1). Impairment of stimulus-induced transcription factor activity may contribute to the deleterious effects of prolonged elevations of glucocorticoids on neuronal function.

Adrenalectomy↗

PubMed1995

Modulation by inositol of cholinergic- and serotonergic-induced seizures in lithium-treated rats.

Hippocampal and cortical EEG recordings in rats were used to monitor the in vivo modulation by lithium of responses to agonists for 5HT2/5HT1c serotonergic (DOI) and cholinergic (pilocarpine) receptors and the influence of inositol administration. Administration of DOI (8 mg/kg) or pilocarpine (30 mg/kg) to rats pretreated with lithium acutely (3 mmol/kg) or chronically (dietary, 4 weeks) resulted in seizures, whereas these doses did not cause seizures without lithium pretreatment. This indicated that lithium most likely affects a signal transduction process common to both systems, which is the phosphoinositide second messenger system. To examine the potential influence of altered inositol levels on these responses, we tested the effects of infusions (10 mg, i.c.v.) of myo-inositol, a precursor of phosphoinositide synthesis, and of epi-inositol, an isomer not used for phosphoinositide synthesis. Administration of myo-inositol (10 mg) slightly reduced the incidence of seizures induced by acute lithium plus DOI but almost completely blocked seizures induced by acute lithium plus pilocarpine. This was surprising since seizures induced by acute lithium plus DOI were less severe than those after acute lithium plus pilocarpine, but myo-inositol was more effective in blocking the latter. Epi-inositol also blocked seizures under both conditions but it was less effective than myo-inositol after treatment with acute lithium plus pilocarpine. The latencies to seizures and/or severity of seizures were potentiated more by chronic than acute lithium pretreatment with both DOI and pilocarpine, but attenuation by myo-inositol was less with each agonist after chronic lithium compared with acute lithium treatment. Peripheral administration of a high dose of myo-inositol blocked seizures induced by acute lithium plus pilocarpine, but the inositol treatment itself was toxic and caused seizures prior to pilocarpine administration, so the mechanism of action cannot simply be attributed to increased brain inositol levels. These results demonstrate that lithium modulates the in vivo responses to DOI and pilocarpine, most probably through an effect on the phosphoinositide signal transduction system. They also show that centrally administered myo-inositol modifies responses to these agents, but the effectiveness of epi-inositol and other results leave unclear the mechanistic basis of its actions.

Amphetamines↗

PubMed1995

Circadian variation in rat brain AP-1 DNA binding activity after cholinergic stimulation: modulation by lithium.

The potential influence of a circadian rhythm and its modulation by lithium on the stimulation of AP-1 DNA binding activity by the cholinergic agonist pilocarpine was investigated in rat cerebral cortex. Stimulation of AP-1 binding after pilocarpine (30 mg/kg) was evident within 1 h and was maximally stimulated by 200% at 2 h. Pilocarpine-stimulated AP-1 binding exhibited a circadian rhythm in AP-1 binding measured at 0800, 1200, and 1600 hours, 2 h after pilocarpine. Pilocarpine-stimulated AP-1 binding at 0800 hours was approximately twice the level measured at 1600 hours. After acute lithium treatment, pilocarpine administration induced generalized seizures after about 20 min and stimulated AP-1 binding which increased continuously for 4.5 h, at which time the stimulation was 900% above control. A circadian variation was apparent in AP-1 binding stimulated by acute lithium plus pilocarpine, with stimulation at 0800 hours being 1.5 times that at 1600 hours. After chronic lithium and pilocarpine, which also produced seizures, there was no circadian variation in pilocarpine-stimulated AP-1 binding. Thus pilocarpine-induced AP-1 binding in rat cerebral cortex was influenced by a circadian rhythm, but this was abolished by chronic lithium administration.

Animals↗

PubMed1995

Diurnal variation in kainate-induced AP-1 activation in rat brain: influence of glucocorticoids.

The large diurnal rhythm of circulating glucocorticoid levels was used to determine if physiological fluctuations of glucocorticoids were capable of modulating kainate-induced immediate early gene (IEG) activation, measured as AP-1 DNA binding activity, in rat brain since administered dexamethasone previously had been shown to be inhibitory. AP-1 activity in the cerebral cortex 1.5 h after kainate treatment measured at 08.00 h (4.9-fold control) was more than twice the stimulation obtained at 16.00 h (1.8-fold). These times of day are associated with reported low and high levels of circulating glucocorticoids at 08.00 and 16.00 h, respectively. To test if there was a causal relationship, kainate-induced AP-1 activity was measured at both times in adrenalectomized rats. Adrenalectomy abolished the attenuation of the response to kainate found in intact rats at 16.00 h, indicating that the diurnal fluctuations in circulating glucocorticoids contribute to modulation of IEG responses to kainate. Neither AP-1 activity in the hippocampus nor cyclic AMP response element activation in either brain region measured after kainate treatment was influenced by the time of day or by adrenalectomy. Immunoprecipitation of glucocorticoid receptors from cortical nuclear extracts co-precipitated c-Jun, indicating that the mechanism accounting for the suppression of AP-1 activity by glucocorticoids may involve direct interactions between activated glucocorticoid receptors and AP-1 constituent proteins. These results extend previous reports that administered glucocorticoids inhibit AP-1 activity by demonstrating that this occurs with endogenous glucocorticoids as a consequence of the circadian rhythm of circulating glucocorticoids and demonstrate that responses to kainate vary dependent upon the time of day.

Animals↗

PubMed1995

Phosphoinositide hydrolysis, G alpha q, phospholipase C, and protein kinase C in post mortem human brain: effects of post mortem interval, subject age, and Alzheimer's disease.

Influences of post mortem time interval, subject age and Alzheimer's disease were investigated on several components of the phosphoinositide second messenger system, including stimulation of [3H]phosphatidylinositol hydrolysis by GTP[S] and several receptor agonists and the levels of Galphaq, beta, delta and gamma subtypes of phospholipase C, and five protein kinase C isoforms, in membranes prepared from post mortem human prefrontal cortex. Most of these components were stable with post mortem delays in the range of 5-21 h, but decreases of Galphaq and the alpha and xi protein kinase C subtypes were detected. Within the subject age range of 19-100 years, G-protein- and agonist-induced [3H]phosphatidylinositol hydrolysis decreased, as did levels of Galphaq, but the levels of phospholipase C and protein kinase C subtypes were generally unchanged. In Alzheimer's disease, compared with age- and post mortem interval-matched controls, there was a decrease in [3H]phosphatidylinositol hydrolysis stimulated by G-proteins and by several receptor agonists, but the levels of Galphaq and most of the phospholipase C and protein kinase C isoforms were unaffected. The greatest deficits, which were >50%, occurred with GTP[S]- and carbachol-induced [3H]phosphatidylinositol hydrolysis, indicating that this G-protein function and the response to cholinergic stimulation are significantly impaired in Alzheimer's disease. In summary a comprehensive assessment of several components of the phosphoinositide second messenger system was made in post mortem human brain. Most elements were stable within the post mortem interval range of 5-21 h, lending validity to measurements using these tissues. Significant age-related reductions in several components were identified, indicating loss of responses with increasing age. Most importantly, severe reductions in responses to several stimuli were found in Alzheimer's disease brain, deficits in signal transduction which may contribute to impaired cognition and to the limited therapeutic responses to drugs, such as those used to activate cholinergic receptors coupled with the phosphoinositide system.

Adult↗

PubMed1995

Defective expression of hematopoietic cell protein tyrosine phosphatase (HCP) in lymphoid cells blocks Fas-mediated apoptosis.

Protein tyrosine dephosphorylation after Fas cross-linking occurred in Fas apoptosis-sensitive CEM-6 cells but not in Fas apoptosis-resistant MOLT-4 cells, and apoptosis in the CEM-6 cells could be inhibited by the protein tyrosine phosphatase inhibitor, pervanadate. The time course and level of dephosphorylation were correlated with increased hematopoietic cell protein tyrosine phosphatase (HCP) activity, but not with the activity of two other tyrosine phosphatases. The level of expression of HCP was correlated with Fas apoptosis function in eleven human and murine Fas-positive lymphoid cell lines. Expression of recombinant HCP in the MOLT-4 cell line converted this Fas apoptosis-resistant cell line to Fas apoptosis sensitive. HCP-mutant mev/mev mice exhibited increased expression of Fas but decreased Fas-mediated apoptosis function in lymphoid organs after anti-mouse Fas antibody treatment in vivo. Thus, HCP-mediated protein dephosphorylation is involved in the delivery of the Fas apoptosis signal in lymphoid cells.

Animals↗

PubMed1995

Pertussis toxin-sensitive G protein alpha-subunits: production of monoclonal antibodies and detection of differential increases on differentiation of PC12 and LA-N-5 cells.

Monoclonal antibodies were produced that are specific for the three major pertussis toxin-sensitive G protein alpha-subunits present in mammalian brain--alpha o, alpha i1, and alpha i2--using purified bovine brain G proteins, purified rat brain G proteins, and purified recombinant alpha i2, respectively. These monoclonal antibodies were used to monitor changes in the concentrations of the three G protein alpha-subunits during differentiation of PC12 cells and human neuroblastoma LA-N-5 cells. In PC12 cells, levels of alpha i1 but not alpha i2 increased during nerve growth factor-induced differentiation. In contrast, alpha i2 but not alpha i1 increased when LA-N-5 cells were differentiated with retinoic acid. The concentration of alpha o increased in both cell lines during differentiation. Electrophoretic resolution of alpha o subtypes revealed that although alpha o2 was the major subtype in undifferentiated cells, only the concentration of alpha o1 increased during differentiation of both PC12 and LA-N-5 cells. The level of 43-kDa growth-associated protein, a protein known to associate with alpha o, increased similarly to that of alpha o1. ADP-ribosylation of alpha o, alpha i1, and alpha i2 with pertussis toxin did not alter the reactivities of the monoclonal antibodies, but toxin treatment of cells reduced the concentrations of each protein after 24 h. There was no change in the concentration of alpha q, which is not ADP-ribosylated by pertussis toxin. Thus, these new monoclonal antibodies enabled the detection of differential increases in subtypes of alpha i and alpha o associated with neuronal differentiation.

Adenosine Diphosphate Ribose↗

PubMed1995

Selective inhibition of the expression of signal transduction proteins by lithium in nerve growth factor-differentiated PC12 cells.

This investigation examined if lithium, the primary therapeutic treatment for bipolar affective disorder, modulated the levels of selected signal transduction proteins in PC12 cells. Nerve growth factor (NGF) induced differentiation of PC12 cells, and after 12 days of NGF treatment there were large increases in the levels of the heterotrimeric G protein subunits alpha o1, alpha i1, beta, and alpha s, small increases in those of alpha i2 and alpha q, and a slight decrease in that of alpha o2. Lithium (1 mM, equivalent to the therapeutic concentration) selectively reduced NGF-induced increases in levels of G protein subunits, generally having the greatest inhibition on those that were increased the most by NGF. Lithium at 5 mM had greater inhibitory effects than 1 mM lithium on NGF-induced increases in levels of G proteins, but neither concentration of lithium affected the induction of the cytoskeletal protein beta-tubulin. Examination of other proteins involved in signal transduction revealed that 12 days of NGF treatment increased the level of protein kinase C-alpha, but not those of the beta, epsilon, or zeta subtypes, and did not alter the levels of beta, gamma, or delta phospholipase C. Pretreatment with lithium inhibited the increase in content of protein kinase C-alpha induced by NGF but had little effect on the proteins not responsive to NGF except for decreasing the levels of protein kinase C-epsilon. The inhibitory effect of lithium was found not to be due to inhibition of NGF-induced tyrosine phosphorylation, which was unaffected by 5 mM lithium, or to inositol depletion. In summary, use of the dynamic system of NGF-induced PC12 cell differentiation provided a sensitive model in which to identify signal transduction proteins that were influenced by lithium treatment. The large changes caused by a therapeutically equivalent concentration of lithium lend support to the proposal that the selective inhibitory effects of lithium on subtypes of G proteins and protein kinase C may be important therapeutic targets.

Animals↗

PubMed1994

[3H]PtdIns hydrolysis in postmortem human brain membranes is mediated by the G-proteins Gq/11 and phospholipase C-beta.

A method utilizing exogenously added [3H]PtdIns incubated with membranes prepared from postmoretem human brain has been shown to provide a means of measuring agonist-induced, guanosine 5'-O-(thiotriphosphate) (GTP[S])-dependent hydrolysis of [3H]PtdIns, thus allowing investigations of the activity of the phosphoinositide second-messenger system in accessible human brain tissue. Agonists inducing [3H]PtdIns hydrolysis include carbachol, trans-1-aminocyclopentyl-1,3-dicarboxylate (ACPD; a glutamatergic metabotropic receptor agonist), serotonin and ATP, with the latter two agonists producing the largest responses. In addition to ATP, [3H]PtdIns hydrolysis was induced by ADP and by 2-methylthio-ATP, indicating that P2-purinergic receptors mediate this process. Subtype-selective antibodies we used to identify Gq/11 and phospholipase C-beta as the G-protein and phospholipase C subtypes that mediated GTP[S]-induced and agonist-induced [3H]PtdIns hydrolysis. These results demonstrate that this method reveals that agonist-induced, GTP[S]-dependent [3H]PtdIns hydrolysis is retained in postmortem human brain membranes with properties similar to rat brain. This method should allow studies of the modulation of phosphoinositide hydrolysis in human brain and investigations of potential alterations in postmortem brain from subjects with neurological and psychiatric diseases.

Adenosine Triphosphate↗

PubMed1994

G-proteins modulate amiloride-sensitive sodium channels.

We examined the regulation of an amiloride-sensitive sodium conductance expressed in human B lymphoid cells. This conductance was activated by two independent pathways, one involving cyclic adenylyl monophosphate (cAMP)-dependent protein kinase and the other involving a pertussis toxin-sensitive G-protein. Cholera toxin, presumably by increasing cellular cAMP, and pertussis toxin, which ADP-ribosylates certain GTP-binding proteins, both independently increased the amiloride-sensitive sodium conductance. Simultaneous treatment with both toxins, however, failed to increase the sodium conductance, implying that a single set of sodium channels was being affected by both toxins. In cells preactivated with pertussis toxin, 8-chlorophenylthio-cAMP inhibited the activated sodium conductance back to the basal level. Thus, cyclic AMP-dependent pathways can either activate or inhibit amiloride-sensitive sodium channels, depending upon the activation state of a pertussis toxin-sensitive GTP-binding protein. These findings support a hypothesis for the regulation of amiloride-sensitive sodium channels which incorporates the independent effects of cholera and pertussis toxins, and in which cyclic AMP can play a dual role in the regulation of channel activity.

Amiloride↗

PubMed1994

Lithium potentiates phosphoinositide-linked 5-HT receptor stimulation in vivo.

The therapeutic effect of lithium in manic-depressive illness may involve alterations in the activity of the phosphoinositide second messenger system. Lithium administration to rats potentiates responses to cholinergic agonists, as evidenced by the production of seizures in lithium-treated rats after normally nonconvulsant doses of cholinergic agonists. We now report that lithium also potentiates the response to a serotonin (5-HT) agonist, DOI, that activates 5-HT2/5-HT1C receptors coupled to phosphoinositide hydrolysis. EEG recordings showed that administration of DOI (8 mg kg-1) to lithium-treated, but not to lithium-naive, rats caused seizures which were blocked by ritanserin pretreatment. These results demonstrate that lithium pretreatment causes a normally subconvulsive dose of serotonergic, in addition to cholinergic, agonists to induce seizures. Since DOI, like cholinergic agonists, activates receptors coupled with phosphoinositide hydrolysis and lithium potentiates responses to each, this second messenger system is likely to be involved in this effect of lithium.

Amphetamines↗

PubMed1994

Brain G-protein proteolysis by calpain: enhancement by lithium.

Heterotrimeric G-proteins mediate many receptor-coupled signal transduction processes and the cellular concentrations of G-proteins are modulated by several factors, including development, activity, and drugs. The mechanisms causing changes in G-protein concentrations are mostly unknown. The purpose of this study was to determine if G-proteins could be proteolyzed by calpain, a calcium-activated neutral protease that has been linked with neuronal plasticity. In membranes prepared from rat cerebral cortex, calpain rapidly cleaved the alpha-subunit of Go but did not hydrolyze beta-subunits. Comparisons of the proteolysis of different alpha-subunits revealed that they were differentially susceptible to calpain-induced proteolysis in the order of alpha s > alpha o > alpha q > alpha i. Preincubation of cortical membranes with GTP gamma S, which binds to G alpha and causes its dissociation from the beta gamma dimer, reduced calpain-mediated proteolysis of alpha o. Lithium, the primary treatment for mania, enhanced the calpain-mediated proteolysis of alpha o in the heterotrimeric state but did not affect proteolysis of dissociated, GTP gamma S-bound alpha o. These results demonstrate that proteolysis by calpain is a potential mechanism by which cellular G-protein concentrations can be regulated.

Animals↗