Interactions of anticholinesterases with Achatina fulica acetylcholine responses and electrogenic sodium pump

Action potential bursts in central snail neurons elicited by paeonol: roles of ionic currents

AIM

to investigate the effects of 2'-hydroxy-4'-methoxyacetophenone (paeonol) on the electrophysiological behavior of a central neuron (right parietal 4; RP4) of the giant African snail (Achatina fulica Ferussac).

METHODS

intracellular recordings and the two-electrode voltage clamp method were used to study the effects of paeonol on the RP4 neuron.

RESULTS

the RP4 neuron generated spontaneous action potentials. Bath application of paeonol at a concentration of ≥ 500 micromol/L reversibly elicited action potential bursts in a concentration-dependent manner. Immersing the neurons in Co(2+)-substituted Ca(2+)-free solution did not block paeonol-elicited bursting. Pretreatment with the protein kinase A (PKA) inhibitor KT-5720 or the protein kinase C (PKC) inhibitor Ro 31-8220 did not affect the action potential bursts. Voltage-clamp studies revealed that paeonol at a concentration of 500 micromol/L had no remarkable effects on the total inward currents, whereas paeonol decreased the delayed rectifying K(+) current (I(KD)) and the fast-inactivating K(+) current (I(A)). Application of 4-aminopyridine (4-AP 5 mmol/L), an inhibitor of I(A), or charybdotoxin 250 nmol/L, an inhibitor of the Ca(2+)-activated K(+) current (I(K(Ca))), failed to elicit action potential bursts, whereas tetraethylammonium chloride (TEA 50 mmol/L), an I(KD) blocker, successfully elicited action potential bursts. At a lower concentration of 5 mmol/L, TEA facilitated the induction of action potential bursts elicited by paeonol.

CONCLUSION

paeonol elicited a bursting firing pattern of action potentials in the RP4 neuron and this activity relates closely to the inhibitory effects of paeonol on the I(KD).

(±)3,4-Methylenedioxyamphetamine elicits action potential bursts in a central snail neuron

The effects of (+/-)3,4-methylenedioxyamphetamine (MDA) were studied in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac, using the two-electrode voltage-clamp method. The RP4 neuron generated spontaneous action potentials. Extracellular or intracellular application of MDA elicited action potential bursts of the central RP4 neuron. The action potential bursts elicited by MDA were not blocked when neurons were immersed in high-Mg2+ solution, Ca2+-free solution, nor after continuous perfusion with atropine, d-tubocurarine, propranolol, prazosin, haloperidol, sulpiride or methiothepin. Notably, the induction of action potential bursts was blocked by pretreatment with protein kinase C (PKC) inhibitors, chelerythrine and Ro 31-8220, but not by protein kinase A (PKA) inhibitors, KT-5720 and H89, nor by the phospholipase C (PLC) inhibitor, U73122. PKC activators, i.e., phorbol 12,13-dibutyrate (PDBu) and 1-oleoyl-2-acety-sn-glycerol (OAG; a membrane-permeant DAG analog), facilitate the induction of action potential bursts elicited by MDA. Voltage-clamp studies revealed that MDA decreased the delayed rectifying K+ current (I(KD)) of the RP4 neuron. Further, although Ro 31-8220 did not affect the I(KD), Ro 31-8220 decreased the inhibitory effect of MDA on the I(KD). These results suggest that the generation of action potential bursts elicited by MDA was not due to (1) the synaptic effects of neurotransmitters, (2) the cholinergic, adrenergic, dopaminergic or serotoninergic receptors of the excitable membrane. Instead, the MDA-elicited action potential bursts are closely related to PKC activity and the inhibitory effects on the I(KD).

Cocaine elicits action potential bursts in a central snail neuron: The role of delayed rectifying K+ current

The effects of cocaine were studied in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac, using the two-electrode voltage-clamp method. The RP4 neuron generated spontaneous action potentials and bath application of cocaine (0.3-1 mM) reversibly elicited action potential bursts of the central RP4 neuron in a concentration-dependent manner. The action potential bursts were not blocked when neurons were immersed in high-Mg(2+)solution, Ca(2+)-free solution, nor after continuous perfusion with atropine, d-tubocurarine, propranolol, prazosin, haloperidol, or sulpiride. Similarly, the action potential bursts were not abolished by pretreatment with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, (9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester or anisomycin. Injection of hyperpolarizing current at an intensity of greater than 2 nA effectively suppressed the cocaine-elicited action potential bursts and no postsynaptic potentials were observed under these conditions. These results suggest that the generation of action potential bursts elicited by cocaine was not due to (1) the synaptic effects of neurotransmitters, (2) the cholinergic, adrenergic or dopaminergic receptors of the excitable membrane, or (3) the cAMP second messengers and new protein synthesis of the RP4 neuron. Notably, the induction of action potential bursts was blocked by pretreatment with 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione. Voltage-clamp studies conducted on the RP4 neuron revealed that cocaine at 0.3 mM decreased (1) the Ca(2+) current, (2) the delayed rectifying K(+) current, (3) the fast-inactivating K(+) current and (4) the Ca(2+)-activated K(+) current, but had no remarkable effects on the Na(+) current. Perfusion with Ca(2+)-free solution, which may abolish the Ca(2+) current and Ca(2+)-activated K(+) current, did not cause any bursts of action potentials in control RP4 neurons. Application of 4-aminopyridine, an inhibitor of fast-inactivating K(+) current, and paxilline, an inhibitor of Ca(2+)-activated K(+) current, failed to elicit action potential bursts, whereas tetraethylammonium chloride, a blocker of Ca(2+)-activated K(+) current and delayed rectifying K(+) current, and tacrine, an inhibitor of delayed rectifying K(+) current, successfully elicited action potential bursts. Further, while 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione did not affect the delayed rectifying K(+) current of the RP4 neuron, 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione decreased the inhibitory effect of cocaine on the delayed rectifying K(+) current. It is concluded that cocaine elicits action potential bursts in the central snail RP4 neuron and that the effect is closely related to the inhibitory effects on the delayed rectifying K(+) current.

Production of polyclonal antibodies against territrem B and detection of territrem B in the conidia of Aspergillus terreus 23-1 by immunoelectron microscopy

Territrem B, a fungal metabolite isolated from Aspergillums terreus 23-1, is a tremorgenic mycotoxin. Immunoelectron microscopy using anti-territrem B polyclonal antibody was used to detect territrem B in the fungal body of A. terreus 23-1 at different times of culture without shaking on potato dextrose (PD) agar medium. The anti-territrem B serum was produced by immunization of rabbits with 4beta-hydroxymethyl-4beta-demethylterritrem B-sccinate bound by a linker to keyhole limpet hemocyanin. This antiserum recognized territrems and immunoelectron microscopy using this antiserum, and colloidal gold-conjugated anti-rabbit IgG antibodies showed that territrem B was localized to the fungal body of A. terreus 23-1. Territrem B was first seen in the cytoplasm of the conidia after 4 days' culture on PD agar medium. Maximal territrem B production in the conidia was seen on the 14th day of culture; however, territrem B was not formed in the hyphae at any stage of culture. These results are consistent with the previous finding that the formation of territrems is related to fungal sporulation.