Properties of cAMP-induced transmembrane current in mollusc neurons

Effects of serotonin and cAMP on calcium currents in different neurones of Helix pomatia

Effects of application of serotonin (5-HT) and intracellular administration of cyclic adenosine monophosphate (cAMP) on voltage-gated calcium current (ICa) were studied in isolated, intracellularly perfused Helix pomatia neurones. Two types of the effects of 5-HT (1-10 microM) were observed in different neurones: reversible inhibition (by about 20%) or reversible potentiation (up to 50%) of the current amplitude. Some cells did not respond to 5-HT application. In cells with the potentiating effect of 5-HT, ICa could also be increased by intracellular introduction of cAMP (100 microM), but not the guanosine analogue, cGMP (50-100 microM). These effects were not additive and could be potentiated by theophylline (5 mM) and 3-isobutyl-1-methylxanthine (IBMX, 100-500 microM); they could be mimicked by forskolin (10-50 microM) and abolished by tolbutamide (1-5 mM) or protein kinase inhibitor (500 micrograms/ml), indicating that cAMP-dependent phosphorylation mediates the potentiating action of 5-HT on ICa. In neurones showing inhibitory effect of 5-HT, neither cAMP nor forskolin increased ICa. Methiothepin (10-50 microM), a 5-HT1,2 receptor antagonist, irreversibly inhibited the potentiating effect of 5-HT, while antagonists of 5-HT2 receptors cyproheptadine (10-50 microM) or ketanserine (10-60 microM) and of 5-HT3 receptors ISC 205-930 (10-50 microM) or cocaine (5-25 microM) had no effect on ICa and its enhancement by 5-HT. It is suggested that in certain snail neurones the possibility of cAMP-dependent up-regulation of ICa correlates with the presence of 5-HT1-like receptors in the neuronal membrane.

Properties of cyclic AMP-dependent inward current in two identified neurons of the snail Lymnaea stagnalis

1. Intracellular iontophoresis of cyclic AMP induces a slow, inward sodium current in identified neurons B1 and RPD1 of Lymnaea stagnalis, which is TTX-insensitive and voltage independent. 2. High extracellular Ca2+ or Co2+ ions cause a reduction in the amplitude of the current, whereas low Ca2+ enhances it. 3. Agents known to increase intracellular cyclic AMP lead to depolarization and increased activity in B1, but RPD1 is unaffected. 4. The properties of cyclic AMP-stimulated inward current in neurons of L. stagnalis are compared with those of similar currents in other gastropod species.

Blocking effect of La3+ ions on transmembrane ionic current evoked by intracellular cyclic AMP injection in identified Helix pomatia neurons

Using the fluorescent probe fura-2 for measurements of the cytoplasmic concentration of free Ca2+ ions [( Ca]in) in combination with conventional current- or voltage-clamp methods, we studied the effects of La3+ ions on the cellular responses evoked by intracellular cAMP (adenosine 3',5'-cyclic monophosphate) injections into isolated Helix pomatia neurons. La3+ ions in submillimolar concentrations decreased cAMP-evoked [Ca]in transients. Transmembrane ionic currents evoked by cAMP injections were completely blocked by La3+ ions (1.0 mM) in all neurons under investigation. La(3+)-induced effects essentially differ from each other in RPa1 (right parietal 1) and LPa3 (left parietal 3) neurons. La3+ ions in milli- and submillimolar concentrations strongly affect different subcellular systems, especially, those which regulate an intracellular calcium concentration.

Free calcium transients and oscillations in nerve cells

Changes in the intracellular level of free calcium induced by different influences in neurones of the snail Helix pomatia have been measured by changes in Fura-2 fluorescence. Thymol in submillimolar concentrations induced the release of stored intracellular calcium. This effect was similar to xanthine-induced release. IP3 and Gpp[NH]p injections also released intracellular calcium. The response to cAMP injections was more complicated and included, probably, both the release of stored calcium and its influx through membrane channels. Oscillations of intracellular free calcium are described. It has been suggested that oscillations can occur only in cases where the mechanism of Ca-dependent calcium release is activated.

Oxytocin-induced sodium current is mediated by cAMP-dependent protein phosphorylation in an identified snail neuron

The intracellular biochemical process underlying oxytocin-induced change of membrane properties was analyzed in an identified neuron of Achatina fulica Férussac, using pressure injection technique and pharmacological tools. Oxytocin dose-dependently enhanced the negative slope resistance (NSR) region on the current-voltage relation. The oxytocin-induced current was attenuated by a reduction of extracellular Na+ and not influenced by the addition of 100 microM tetrodotoxin (TTX) to the medium, suggesting that this current is predominantly due to the activation of TTX-resistant Na+ channels. In the Ca2(+)-free state, substituted by an equivalent amount of Co2+, the amplitude of oxytocin-induced current was somewhat reduced at the NSR region but it was not influenced at less than -60 mV. Application of 100 microM isobutylmethylxanthine, a phosphodiesterase inhibitor, augmented the oxytocin-induced current. Pressure injection of 10 mM adenosine 3',5'-cyclic monophosphate (cAMP) elicited a Na(+)-dependent inward current similar to the oxytocin response. The further role of cAMP linked with the oxytocin-induced current was investigated using two kinds of cAMP-dependent protein kinase inhibitors, isoquinolinesulfonamide (H-8) and protein kinase inhibitor (PKI). Extracellular application of H-8 or pressure injection of PKI, prior to oxytocin application, both blocked the oxytocin-induced current. Based on these results, oxytocin-elicited inward currents may mediate cAMP-dependent protein phosphorylation mainly by activation of Na+ channels.

Diversity of calcium ion channels in cellular membranes

Successful introduction of techniques for separation of different ionic currents and recording of single channel activity has demonstrated the diversity of membrane structures responsible for generation of calcium signal during various forms of cellular activity. In excitable cells the electrically-operated calcium channels have been separated into two types functioning in different membrane potential ranges (low- and high-threshold ones). The low-threshold channels are ontogenetically primary and may play a role in regulation of cell development and differentiation. A similar function may also be characteristic of chemically-operated channels in some highly specialized cells (lymphocytes). The high-threshold channels in excitable cells generate an intracellular signal coupling membrane excitation and intracellular metabolic processes responsible for specific cellular reactions (among them retention of traces of previous activity in neurons--"learning"--being especially important). Chemically-operated N-methyl-D-aspartate-channels also participate in this function. The calcium signal can be potentiated by activation of calcium-operated channels in the membranes of intracellular structures, resulting in the liberation of calcium ions from the intracellular stores. Although different types of calcium channels have some common features in their structure which may indicate their genetic similarity, their specific properties make them well suited for participation in a wide range of cellular mechanisms.

Action of cyclic AMP on intracellular calcium concentration and bursting activity

In order to clarify its role in the provocation of seizure activity, the effects of cyclic AMP were examined on the intracellular calcium concentration of pentylenetetrazol (PTZ)-sensitive neurons as well as of PTZ-non-sensitive neurons of the Japanese land snail, Euhadra peliomphala. Extracellular application of isobutylmethylxanthine (IBMX) and dibutyryl cyclic AMP showed bursting activity-like firing in the PTZ-sensitive neurons. When 5'-guanylylimidodiphosphate (GNP-PNP) was injected into PTZ-sensitive neurons in the extracellular presence of IBMX and dibutyryl cyclic AMP, bursting activity followed by long-lasting hyperpolarization occurred. Intracellular injection of cyclic AMP into PTZ-sensitive neurons caused hyperpolarization coincident with an increase in intracellular calcium concentration. This increase in intracellular calcium concentration was the same under conditions in which the calcium influx was inhibited by the substitution of extracellular calcium chloride by cobalt chloride. In PTZ-non-sensitive neurons, cyclic AMP-induced bursting activity was not observed. These results suggest that an increase in cyclic AMP provoked bursting activity via an increase in intracellular calcium concentration.

Cyclic AMP-stimulated sodium current in identified feeding neurons of Lymnaea stagnalis

Iontophoretic injection of cAMP elicits a slow, transient inward current in identified buccal feeding motoneurons and in the giant cerebral interneuron of the snail, Lymnaea stagnalis. The current is voltage independent, and is abolished in the absence of extracellular Na+. Application of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) causes a marked increase in both amplitude and duration of cAMP-stimulated inward current. The amplitude of the current is reduced following prolonged application of depolarizing pulses to the cell. However, generation of high-frequency bursts of action potentials lasting up to 20 s has no significant effect on the amplitude of the cAMP-induced current measured subsequently. Bath application of the cAMP analogue 8-chlorophenylthio-cAMP or of IBMX leads to enhanced bursting activity in buccal motoneurons. It is suggested that cAMP sensitivity in feeding motoneurons provides a mechanism for adjusting the cells' responsiveness to rhythmic synaptic inputs during the generation of feeding motor output.