Effects of histamine on polyphosphoinositide metabolism in NG108-15 cells

The mRNA expression of serotonin 2C subtype receptors uncoupled with inositol hydrolysis in NG108-15 cells

Cell culture systems seem to be useful for clarifying the cellular physiological mechanisms of serotonin 2C subtype receptors (5-HT2CR) and related drug action mechanisms. However, there are still few reports about cells that contain intrinsic 5-HT2CR. This report demonstrates by using RT/PCR that 5-HT2CR mRNA exists in splicing variant forms in NGI08-15 cells. The PCR results using a pair of primers that recognized sequences near the third intracellular loop site showed two neighboring bands at about 500 bp upon electrophoresis in acrylamide gels. The sequence analysis demonstrated that one band was the rat 5-HT2CR sequence and the other one was that of the mouse. Serotonin, however, did not enhance the inositol phosphates formation in NG108-15 cells. It has been reported that post-translational modifications of RNA, splicing and editing, occur at the site of the second intracellular loop domain in 5-HT2CR mRNA. Accordingly, a pair of primers that recognized this site were designed. The molecular size of the PCR product was shorter than that expected based on the sequence of the native 5-HT2CR. The fragment lacked the 95 nucleotides of native 5-HT2CR mRNA. This seems to be the reason why serotonin did not enhance inositol phosphates formation in NG108-15 cells.

Distinct induction of c-fos mRNA in NG108-15 cells transfected with muscarinic m1 and m3 receptors

The differences of intracellular signalling mechanisms between muscarinic acetylcholine m1 and m3 receptors, which are coupled with polyphosphoinositide turnover, were examined by using m1- and m3-transfected NG108-15 cells. The c-fos mRNA was induced by 1 mM acetylcholine peak at 60 min in both m1 and m3 cells. The c-fos induction in m1 cells was inhibited by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM) and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7), but was not inhibited by prolonged treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA), suggesting that intracellular Ca2+ and calmodulin are involved in the induction. The c-fos induction in m3 cells was inhibited by BAPTA-AM and prolonged treatment with TPA, but was not influenced by W-7, suggesting that protein kinase C is mainly involved in m3-induced c-fos expression. Acetylcholine induced an increase in inositol phosphates and a transient increase in the intracellular concentration of Ca2+ in both m1 and m3 cells. Sustained stimulation of acetylcholine strongly increased the inositol monophosphate content in m3 cells, but that of inositol trisphosphate and inositol diphosphate in m1 cells. These results suggest that the difference between m1- and m3-induced c-fos mRNA induction mechanisms is due to the difference in respective properties in polyphosphoinositide turnover.

Arachidonic acid and its acyclic derivatives regulate short-term plasticity of the cholinoreceptors of neurons of the edible snail

The influence of eicosanoids on the dynamics of the extinction of the inward current, induced by repeated iontophoretic applications of acetylcholine to the soma has been demonstrated in identified RPa3 and LPa3 neurons of the edible snail by means of bielectrode recording of the membrane potential. The extracellular action of arachidonic acid (50-100 mumole/l) intensifies the extinction. Quinacrine (100-600 mumole/l), an inhibitor of phospholipase A2, which decreases the content of arachidonic acid in the cells, acts ambiguously. Nordihydroguaiaretic acid (3-10 mumole/l), which is an inhibitor of the lipoxygenase oxidation of arachidonic acid, attenuates the extinction. Indomethacin (10-50 mumole/l), a blocker of cyclooxygenase oxidation of arachidonic acid, does not influence extinction. All of the compounds investigated decrease the amplitude of the inward current induced by acetylcholine. The results obtained make it possible to hypothesize that arachidonic acid and its acyclic metabolites, which are formed as a result of lipoxygenase oxidation, regulate short-term plasticity of the cholinoreceptors of neurons of the edible snail. The cyclic eicosanoids which are formed during the cyclooxygenase oxidation of arachidonic acid do not exert a regulatory effect on the plasticity of the cholinoreceptors.

Histamine H1-receptors on astrocytes in primary cultures: a possible target for histaminergic neurones

The characteristics of histamine H1-receptors expressed on astrocytes from the cerebral cortex of new born rats were analysed by the [3H]-mepyramine binding assay. The apparent dissociation constant (Kd) was 10.4 nM and the binding capacity (Bmax) of 262 fmol/mg protein. H1-antagonists inhibited the [3H]mepyramine bindings and the isomers of chlorpheniramine showed a stereoselectivity for the inhibition of the bindings. Two distinct populations of cultured astrocytes, type-1 and type-2 astrocytes, were enriched and histamine-induced accumulations of inositol phosphates (IP) and cyclic AMP and histamine-evoked Ca++ signals were examined. Histamine stimulated the accumulation of IP in type-2 astrocytes, but not in type-1 astrocytes. The accumulation of cyclic AMP induced by histamine was observed in type-1 astrocytes, although not in type-2 astrocytes. Histamine-induced Ca++ signals were observed in 17.2% of type-1 astrocytes and in 72.9% of type-2 astrocytes. Histamine-induced Ca++ signals in type-2 astrocytes were antagonized by H1-antagonists, but not by H2- antagonists. Histamine-induced Ca++ signals were classified into 4 patterns, ie. transient, oscillatory, sustained and biphasic. When extracellular Ca++ was omitted or La was added to the extracellular medium, sustained phase of Ca++ signal disappeared and transient and oscillatory patterns were only observed. Phorbol ester inhibited histamine-induced Ca++ signals but pertussis toxin (IAP) and organic voltage dependent Ca++ channel blockers had no effect. Histamine-induced Ca++ elevation appeared initially in processes and then Ca++ wave propagated to the cell soma. Ca++ elevation was observed only in the processes in some cells.