Arne Mørk

Effects of GTP on hormone-stimulated adenylate cyclase activity in cerebral cortex, striatum, and hippocampus from rats treated chronically with lithium

The effects of lithium on guanosine triphosphate (GTP) stimulated adenylate cyclase activity and hormone-induced GTP activation of the enzyme have been studied in three regions of the rat brain. Chronic treatment with lithium, giving a serum lithium level of 0.71 +/- 24 mmol/L, reduced isoprenaline-induced GTP stimulation of adenylate cyclase activity in cortical membranes at concentrations of GTP up to 2 microM. No effect of lithium was observed at higher concentrations of GTP. The enzyme activity stimulated by GTP alone was unaltered by lithium ex vivo. In striatal membranes, lithium ex vivo decreased both dopamine-induced GTP activation of adenylate cyclase and GTP-stimulated adenylate cyclase activity at concentrations of GTP below 2 microM. No effects of lithium ex vivo were found in striatum at 2 microM GTP and above. In hippocampal membranes, lithium ex vivo did not influence either serotonin-induced GTP stimulation of the adenylate cyclase or GTP-stimulated enzyme activity at low levels of GTP. However, at 50 microM GTP, lithium ex vivo enhanced serotonin-stimulated enzyme activity. The present results suggest that lithium ex vivo decreases neurotransmitter activation of the cortical beta-adrenergic adenylate cyclase by influencing the mechanisms by which receptor agonists enhance the GTP stimulation of the adenylate cyclase. Furthermore, lithium ex vivo exerts a region-specific action on the brain adenylate cyclases, but in the brain regions studied, an effect of lithium on N-protein level might be of significance for the action of lithium ex vivo on neurotransmitter activation.

The effects of lithium in vitro and ex vivo on adenylate cyclase in brain are exerted by distinct mechanisms.

The effects of lithium on basal and forskolin-stimulated activity of adenylate cyclase in membrane preparations from cerebral cortex of the rat have been studied. Chronic treatment with lithium, yielding a level of lithium in serum of 0.71 +/- 0.18 mmol/l, reduced forskolin-stimulated activity in total homogenates but exerted no effect on the basal activity. Lithium in vitro, at 2 and 10 mM, did not influence the basal enzyme activity in membranes from either control or lithium-treated animals. The sensitivity of forskolin-stimulated adenylate cyclase to lithium in vitro was unaltered after chronic treatment and the in vitro and ex vivo effects of lithium on this parameter were additive. The inhibitory ex vivo effect of lithium was not antagonized by increasing concentrations of magnesium and the inhibitory effect of lithium ex vivo was still persistent after washing of the membranes. The present results indicate that lithium exerts its ex vivo effect on the activated cyclase, independently of the in vitro effect. Both effects may, however, contribute to the in vivo effect of lithium during chronic treatment.

Altered protein phosphorylation in the rat brain following chronic lithium and carbamazepine treatments.

Lithium and carbamazepine (CBZ) alter levels of specific kinase-activating second messengers generated by adenylate cyclases and the phosphoinositide system. Thus, lithium and CBZ may change endogenous protein phosphorylation mediated by cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC). The present study aimed at comparing the chronic effects of lithium and CBZ on protein phosphorylation in the rat brain by using quantitative autoradiography. Long-term treatments yielded plasma levels within the therapeutic range. In the particulate hippocampal fraction PKA-mediated phosphorylation of a 42 kDa protein and PKC-mediated phosphorylation of a 88 kDa protein were decreased after lithium treatment. In the cortical particulate fraction approximately 30% reduction in the PKA-mediated protein phosphorylation of several proteins was observed after lithium and CBZ treatments. In the same fraction, CBZ treatment significantly reduced PKC-mediated phosphorylation of several substrates by 30-40%. PKA activity was significantly reduced in cortex, but not in the hippocampus. Thus, both drugs exhibited fraction and region specificities.

Effects of lithium ex vivo on the GTP-mediated inhibition of calcium-stimulated adenylate cyclase activity in rat brain

The aim of this study was to investigate the effects of chronic lithium treatment on calcium (Ca2+)-stimulated adenylate cyclase activity in rat striatum and hippocampus, and to elucidate the effect of lithium treatment on the neurotransmitter/GTP-mediated inhibition of Ca2+-stimulated enzyme activity in the two brain areas. Lithium treatment, which gave a serum-lithium concentration of 0.9 +/- 0.16 mmol/l, enhanced Ca2+-stimulated enzyme activity in the hippocampus but reduced this activity in the striatum. Serotonin (5-HT) dose dependently reduced Ca2+-stimulated adenylate cyclase activity in the hippocampus, and chronic lithium administration reduced the ability of 1 microM 5-HT to inhibit Ca2+-stimulated enzyme activity. Furthermore, the 5-HT-induced GTP-mediated inhibition of Ca2+-stimulated adenylate cyclase activity in the hippocampus was markedly decreased by lithium. Increasing concentrations of dopamine in the striatum did not, however, affect Ca2+-stimulated adenylate cyclase activity and the inhibition of enzyme activity observed with increasing concentrations of GTP was not influenced by chronic lithium treatment. These results demonstrate that lithium ex vivo exerts dual and region-specific effects on Ca2+-stimulated adenylate cyclase in the brain. Furthermore, long-term administration of lithium could reduce the inhibitory effect of 5-HT on adenylate cyclase in the hippocampus, by influencing the inhibitory GTP-binding protein. The effects of lithium on serotonergic and dopaminergic neurotransmission could be involved in the therapeutic actions of lithium in manic-depressive illness.

Effects of Chronic Lithium Treatment on Agonist‐Enhanced Extracellular Concentrations of Cyclic AMP in the Dorsal Hippocampus of Freely Moving Rats

Abstract: Studies on brain slices and homogenates suggest that chronic lithium treatment affects the activity of adenylate cyclases in the brain. To investigate whether chronic lithium administration influences the cyclic AMP (cAMP) synthesis in vivo, we have used microdialysis to assess lithium‐induced alterations in extracellular concentrations of cAMP in the dorsal hippocampus of freely moving rats. Local infusion of noradrenaline or forskolin through the microdialysis probes produced rapid increases in the extracellular concentrations of cAMP in the dorsal hippocampus. Lithium administration for 4 weeks (serum lithium concentration of 0.8 ± 0.11 mmol/L) did not affect the baseline levels of cAMP. However, in rats fed a lithium‐supplemented diet, noradrenaline‐ and forskolin‐induced enhancement of cAMP levels was decreased in the dorsal hippocampus. The rats were videotaped 18 min before and 27 min after initiating the introduction of noradrenaline and forskolin into the dorsal hippocampus. The infusion of agonists induced a moderate behavioral excitation. Rats treated with lithium were less active compared with the control rats. Taken together, these data confirm that chronic lithium administration affects the cAMP signaling system in the brain of living animals, presumably by interfering with a site beyond the receptor level.

5-Hydroxytryptamine receptor agonists influence calcium-stimulated adenylate cyclase activity in the cerebral cortex and hippocampus of the rat

The effects of 5-hydroxytryptamine (5-HT) receptor agonists on calcium (Ca2+)-stimulated adenylate cyclase activity in the hippocampus and cerebral cortex of the rat were studied. In the presence of Ca2+ (1.5 microM), 5-HT dose dependently inhibited adenylate cyclase activity (EC50 = 10 +/- 2 nM). The inhibitory effect of 5-HT on Ca2(+)-stimulated adenylate cyclase was antagonized by spiperone (KB = 2 +/- 0.8 nM). The rank order of potency of 5-HT agonists to inhibit Ca2(+)-stimulated adenylate cyclase in the hippocampus was: 5-carboxamidotryptamine (5-CT) greater than 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) greater than 5-hydroxytryptamine (5-HT) = 5-methoxytryptamine (5-OCH3-T) greater than trifluoromethylphenylpiperazine (TFMPP) greater than m-chlorophenylpiperazine (mCPP). 2-Methyl-5-hydroxytryptamine (2-CH3-5-HT) did not exert an effect on Ca2(+)-stimulated enzyme activity. In the cerebral cortex 5-HT exerted a biphasic stimulatory effect on adenylate cyclase activity in the absence of Ca2+ (EC50 = 0.2 +/- 0.04 nM and 10 +/- 3 microM), whereas 8-OH-DPAT, 5-CT and 2-CH3-5-HT exerted a monophasic effect. In the presence of Ca2+ (1.5 microM), low concentrations of 5-HT, 8-OH-DPAT, 5-CT and 2-CH3-5-HT potentiated adenylate cyclase activity, whereas higher concentrations, except 2-CH3-5-HT, inhibited the enzyme activity. We propose that the 5-HT receptor mediating inhibition of Ca2(+)-stimulated adenylate cyclase in the rat hippocampus corresponds to the 5-HT1A subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of lithium treatment on extracellular serotonin levels in the dorsal hippocampus and wet-dog shakes in the rat

In the present study wet-dog shakes in rats were induced by local potassium (K+) depolarization in the dorsal hippocampus. Concurrently, changes in extracellular concentrations of cyclic AMP (cAMP) and serotonin (5-HT) were assessed by microdialysis. It has been well-established that lithium influences the synthesis of cAMP in the brain via effects on adenylate cyclases. In this study, the effect of chronic lithium treatment on the number of wet-dog shakes and the release of 5-HT was investigated. Wet-dog shakes, formation of cAMP and release of 5-HT were induced by perfusing a Ringer solution containing 60 mM K+ through the microdialysis probe for 20 min. Under some conditions, this high K+ solution also contained 20 microM forskolin. The number of wet-dog shakes and the formation of cAMP induced by K+ depolarization were enhanced by forskolin, while the K+ -stimulated release of 5-HT was unaffected by forskolin. Chronic lithium treatment, yielding a plasma lithium level of 0.78+/-0.09 mmol/l, decreased the number of wet-dog shakes but did not affect the extracellular level of 5-HT in the dorsal hippocampus. Chronic lithium treatment may affect the serotonergic wet-dog shake syndrome in the rat partly via the cAMP signalling system but does not seem to influence this syndrome by changing the release of 5-HT from nerve terminals in the dorsal hippocampus.

Chronic treatment with citalopram induces noradrenaline receptor hypoactivity. A microdialysis study

To investigate whether chronic citalopram administration influences the cyclic AMP (cAMP) synthesis in vivo, microdialysis was used to assess citalopram-induced alterations in extracellular concentrations of cAMP in the dorsal hippocampus of freely moving rats. Citalopram administration for 4 weeks (40 mg/kg p.o. daily) did not affect the baseline levels of cAMP but decreased the noradrenaline-induced enhancement of cAMP levels. No change in forskolin-induced enhancement of cAMP levels was observed. Citalopram in situ did not exert any effect on the cAMP levels. These data support the hypothesis that chronic administration of antidepressants alters the function of noradrenergic receptors.

The effect of acute and chronic lithium on forskolin-induced reduction of rat activity

SummaryForskolin is a diterpene derivative that activates adenylate cyclase and raises cAMP levels in the cell. Both ip and icv forskolin cause behavioral hypoactivity. Lithium has been reported for many years to block cAMP accumulation, but the behavioral relevance of this biochemical effect is not clear. We studied the effect of acute and chronic lithium on icv forskolininduced hypoactivity. Acute lithium had no effect, but chronic lithium significantly blocked forskolin-induced hypoactivity. The effect of chronic lithium occurred with both forskolin in DMSO and with a water-soluble forskolin derivative. These results suggest that this behavioral model can be used to investigate whether new inhibitors of adenylate cyclase possess lithium-like effects.

A comparative study on the effects of tetracyclines and lithium on the cyclic AMP second messenger system in rat brain

1. This study was aimed at investigating the effects of demeclocycline (DMC), minocycline (MC), and lithium (Li) in vitro on cyclic AMP (cAMP) accumulation in rat cerebral cortex stimulated by noradrenaline, forskolin, and ouabain. 2. DMC, MC, and Li dose-dependently reduced noradrenaline-stimulated cAMP formation in cortical slices, but only Li inhibited the cAMP formation induced by forskolin. 3. In contrast to Li, DMC and MC did not affect noradrenaline-stimulated adenylate cyclase activity in cortical membranes. 4. In cortical slices, ouabain stimulated the cAMP production (required the presence of extracellular Ca2+ and was blocked by verapamil). Ouabain-stimulated cAMP accumulation in cortical slices was inhibited by DMC, MC, and Li. 5. DMC and MC do not seem to interact directly with the adenylate cyclase as reported for Li. It is concluded that the tetracyclines, DMC and MC, affect the cAMP signaling system in rat brain by mechanisms that differ from that of Li. The decreased receptor agonist-stimulated cAMP production in cortical slices in the presence of DMC and MC may be due to the Ca(2+)-chelating ability of these tetracyclines.