The effects of cAMP, Ca2+, and phorbol esters on ouabain-induced depression of acetylcholine responses inHelix neurons

Abstract

1. Using internal perfusion and concentration-clamp procedures applied to Helix neurons, the effects of cAMP, Ca2+, and phorbol esters on ouabain-induced depression of acetylcholine Cl-dependent responses were determined. 2. Intracellular cAMP (10(-4) M) depressed those acetylcholine responses which were blocked by ouabain but had no effect on ouabain-insensitive acetylcholine responses. In the presence of elevated intracellular cAMP, ouabain had no further depressant effect on these acetylcholine responses. Both elevated cAMP and ouabain reduced the acetylcholine response without altering the current-voltage curves. 3. An increase in intracellular Ca2+ concentration depressed the amplitude of current induced by application of acetylcholine in neurons with ouabain-sensitive responses and shifted the dose-response relationship to the right. However, elevated Ca2+ did not reduce the maximal response induced by acetylcholine, nor did it prevent the reduction of that response by ouabain. 4. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent stimulator of protein kinase C activity, caused depression of both the ouabain-sensitive and the ouabain-insensitive acetylcholine responses. The inhibitory effect of TPA was markedly enhanced after addition of ATP to the intracellular medium and was greatly reduced by cooling to 5 degrees C. The blocking effect of ouabain, however, reexamined in the presence of TPA. 5. These observations are consistent with the hypothesis that the depression of acetylcholine induced Cl--responses in Helix neurons is a result of an increase in intracellular cAMP concentration but is unrelated to activation of protein kinase C or increases in intracellular Ca2+.

Na+/K+-pump and neurotransmitter membrane receptors

“…In a later paper, Arvanov et al (1992b) showed that ouabain selectively suppressed Helix A-type neuron responses to ACh, which were due to the selective increase in membrane permeability to chloride. This effect of ouabain is mediated by an increase in cAMP levels.…”

“…An increase in the concentration of free Ca 2+ in the dialysed neurons of the pond snail ( Lymnaea stagnalis ) and grape snail ( H. pomatia ) reduces the amplitude of the inward Cl − current caused by acetylcholine (ACh) (Chemeris et al 1982; Arvanov et al 1992b). An investigation of the inhibitory effect of ouabain on the somatic cholinergic receptors of the neurons of the grape snail has shown that the amplitude of the ACh current depends on the concentration of free Ca 2+ in the cytoplasm (Pivovarov and Boguslavskii 2001).…”

“…On the other hand, inactivation of the Na + /K + -pump results in an increase in the level of ATP and cAMP (Arvanov et al 1992b; Saghian et al 1996), reduction in the level of cGMP and stimulation of the phosphorylation of the neuron membrane of Helix (Ayrapetyan 1990). These authors concluded there was a close correlation between Na + /K + -ATPase activity, the intracellular level of cyclic nucleotides and the level of membrane phosphorylation.…”

Involvement of Na,K-pump in SEPYLRFamide-mediated reduction of cholinosensitivity in Helix neurons

“…Elevation of free intracellular Ca 2+ reduces ACh-induced inward Cl − currents in Lymnaea stagnalis [12,52] and Helix pomatia [3] dialysed neurons. This means that an artificial increase of intracellular free Ca 2+ in the cytoplasm has a similar effect to that of FaRPs on somatic acetylcholine receptors.…”

“…One possible regulator is the Na,K-pump. It is known from the literature that inhibition of the Na,K-pump by ouabain evokes a decrease of ACh-induced currents of H. pomatia dialysed neurons [3]. Ouabain can elevate intracellular calcium via the activation of the reverse mode of Na/Ca-exchange (Na + -efflux, Ca 2+ -influx) [1,11,13,15,20,31,45,46] and stimulate Ca 2+ release from internal stores [4,32,46,47].…”