Multiple intracellular signal transduction pathways mediating inward current produced by the neuropeptide, achatin-I

Minocycline inhibits D-amphetamine-elicited action potential bursts in a central snail neuron

Minocycline is a second-generation tetracycline that has been reported to have powerful neuroprotective properties. In our previous studies, we found that d-amphetamine (AMPH) elicited action potential bursts in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac. This study sought to determine the effects of minocycline on the AMPH-elicited action potential pattern changes in the central snail neuron, using the two-electrode voltage clamping method. Extracellular application of AMPH at 300 μM elicited action potential bursts in the RP4 neuron. Minocycline dose-dependently (300-900 μM) inhibited the action potential bursts elicited by AMPH. The inhibitory effects of minocycline on AMPH-elicited action potential bursts were restored by forskolin (50 μM), an adenylate cyclase activator, and by dibutyryl cAMP (N(6),2'-O-Dibutyryladenosine 3',5'-cyclic monophosphate; 1mM), a membrane-permeable cAMP analog. Co-administration of forskolin (50 μM) plus tetraethylammonium chloride (TEA; 5mM) or co-administration of TEA (5mM) plus dibutyryl cAMP (1mM) also elicited action potential bursts, which were prevented and inhibited by minocycline. In addition, minocycline prevented and inhibited forskolin (100 μM)-elicited action potential bursts. Notably, TEA (50mM)-elicited action potential bursts in the RP4 neuron were not affected by minocycline. Minocycline did not affect steady-state outward currents of the RP4 neuron. However, minocycline did decrease the AMPH-elicited steady-state current changes. Similarly, minocycline decreased the effects of forskolin-elicited steady-state current changes. Pretreatment with H89 (N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride; 10 μM), a protein kinase A inhibitor, inhibited AMPH-elicited action potential bursts and decreased AMPH-elicited steady-state current changes. These results suggest that the cAMP-protein kinase A signaling pathway and the steady-state current are involved in the inhibitory effects of minocycline upon AMPH-elicited action potential bursts.

NMR investigation of the electrostatic effect in binding of a neuropeptide, achatin-I, to phosphatidylcholine bilayers

Achatin-I (Gly1-d-Phe2-Ala3-Asp4), known as a neuropeptide containing a d-amino acid, binds to the surface of a zwitterionic phosphatidylcholine (PC) membrane only when the peptide N-terminal amino group is in the ionized state, NH3+ (Kimura, T.; Okamura, E.; Matubayasi, N.; Asami, K.; Nakahara, M. Biophys. J. 2004, 87, 375-385). To gain mechanistic insights into how the binding equilibrium is delicately controlled by the ionization state of the N-terminal amino group, peptide-lipid binding interactions are investigated by selectively enriched 15N (at the N-terminus) and natural-abundance 13C NMR spectroscopy. Upon binding to the PC membrane, the 15N NMR of the N-terminal NH3(+) shifts upfield. This observation supports a mechanism that the role of the N-terminal NH3(+) in stabilizing the binding state is through electrostatic attraction with a headgroup negative charge, i.e., PO4(-). Interestingly, when the side chain beta-carboxyl group in Asp4 is deionized at acidic pH, the 15N signal of the N-terminal NH3(+) exhibits no significant chemical-shift change upon membrane binding of achatin-I. The Asp4 side chain thus regulates efficiency of the electrostatic binding between the peptide N-terminal NH3(+) and the lipid headgroup PO4(-). 13C chemical shifts in the hydrophobic D-Phe2 residue are largely perturbed upon membrane binding, in the case where the side chain beta-CO2(-) in Asp4 is deionized; the deionization of Asp4 beta-CO2(-) increases the net hydrophobicity of achatin-I with a reduction of both the electrostatic hydration and the electrostatic attraction with the headgroup N(CH3)3(+) in the most superficial region of the PC membrane, resulting in deeper anchoring of the phenyl ring. Hence, the electrostatic effect of the side chain beta-CO2(-) in Asp4 floats achatin-I on the PC membrane surface, and the binding equilibrium is sensitively controlled by the ionization state of the N-terminal NH3(+).

NMR study on the binding of neuropeptide achatin-I to phospholipid bilayer: the equilibrium, location, and peptide conformation

Molecular mechanism of the binding of neuropeptide achatin-I (Gly-D-Phe-Ala-Asp) to large unilamellar vesicles of zwitterionic egg-yolk phosphatidylcholine (EPC) was investigated by means of natural-abundance (13)C and high-resolution (of 0.01 Hz order) (1)H NMR spectroscopy. The binding equilibrium was found to be sensitive to the ionization state of the N-terminal NH(3)(+) group in achatin-I; the de-ionization of NH(3)(+) decreases the bound fraction of the peptide from approximately 15% to nearly none. The electrostatic attraction between the N-terminal positive NH(3)(+) group and the negative PO(4)(-) group in the EPC headgroup plays an important role in controlling the equilibrium. Analysis of the (13)C chemical shifts (delta) of EPC showed that the binding location of the peptide within the bilayer is the polar region between the glycerol and ester groups. The binding caused upfield changes Delta delta of the (13)C resonance for almost all the carbon sites in achatin-I. The changes Delta delta for the ionic Asp at the C-terminus are more than five times as large as those for the other residues. The drastic changes for Asp result from the dehydration of the ionic CO(2)(-) groups, which are strongly hydrated by electrostatic interactions in bulk water. The side-chain conformational equilibria of the aromatic d-Phe and ionic Asp residues were both affected by the binding, and the induced changes in the equilibria appear to reflect the peptide-lipid hydrophobic interactions.

Effects of inhibitors for intracellular signal transduction systems on the inward current produced by GABA in a snail neuron

1. An inward current (I[in]) was produced by gamma-aminobutyric acid (GABA) and muscimol, but not by baclofen, in an identifiable giant neuron type, v-LCDN (ventral-left cerebral distinct neuron), of an African giant snail (Achatina fulica Ferussac) under voltage clamp. 2. The pharmacological features of the excitatory GABA receptors in this Achatina neuron type, termed the Achatina muscimol II type GABA receptors, were mainly comparable to those of the mammalian GABA(C) receptors. 3. It was demonstrated in the present study that the following inhibitors for intracellular signal transduction systems showed no significant effect on the I(in) produced by GABA in this Achatina neuron type: H-7 [1-(5-isoquinolinyl sulfonyl)-2-methylpiperazine], an inhibitor of cyclic AMP-dependent protein kinase (PKA), cyclic GMP-dependent protein kinase (PKG) and protein kinase C (PKC); H-8 (N-[2-(methylamino)-ethyl]-5-isoquinolinesulfonamide), a PKA and PKG inhibitor; H-9 [N-(2-aminoethyl)-5-isoquinolinesulfonamide], a PKA inhibitor; staurosporine ((9alpha,10beta,11beta,13alpha)-(+)-2,3,10,11,12 ,13-hexahydro-10-methoxy-9-methyl-11-(methylamino)-9,13-epoxy-1H,9H-d iindolo[1,2,3-gh: 3',2',1'-1m]pyrrolo[3,4-j] [1,7]benzodiazonin-1-one), a PKA and PKC inhibitor; KT5823 ((8R,9S, 11S)-9-methoxy-9-methoxycarbonyl-2N,8-dimethyl-2,3,9,10-tetrahydro-8,11- epoxy-1H,8H,11H-2,7b,11a-triazadibenzo[a,g]cycloocta[c,d,e]- trinden-1-one), a PKG inhibitor; W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], a calmodulin inhibitor; ML-9 [1-(5-chloronaphthalene-1-sulfonyl-1H-hexahydro-1,4-diazepine hydrochloride], a myosin light-chain kinase inhibitor; genistein [5,7-dihydroxy-3-(4-hydroxyphenyl)-4H-1-benzopyran-4-one], a tyrosine protein kinase inhibitor; IBMX (3-isobutyl-1-methylxanthine), a cyclic nucleotide phosphodiesterase (PDE) inhibitor; fluphenazine nitrogen-mustard (2-chloroethyl)-4[3-(2-trifluoromethyl-10-phenothiazinyl)-propyl]p iperazine dihydrochloride), a calmodulin-dependent PDE inhibitor; calyculin A, a type 1 protein phosphatase inhibitor; and okadaic acid (9,10-deepithio-9,10-didehydroacanthifolicin), a type 1, 2A and 2B protein phosphatase inhibitor. 4. With these results, it was proposed that the excitatory Achatina muscimol II type GABA receptors in v-LCDN are not metabotropic but ionotropic.

Synthesis of achatin-I (Gly-D-Phe-L-Ala-L-Asp) analogs having dehydrophenylalanine or aminoisobutyric acid residue at position 2, and their effects on Achatina giant neurons

1. Achatin-I (Gly-D-Phe-L-Ala-L-Asp), a neuroactive tetrapeptide having a D-phenylalanine residue, has been proposed to be an excitatory neurotransmitter of Achatina giant neurons. It was revealed that the D-Phe2 residue is essential for bioactivity of achatin-I, which seems to adopt beta-turn conformation. In the present study, in order to investigate the structure-activity relationships of achatin-I and its derivatives, the two highly constrained analogs of achatin-I, [delta ZPhe2]achatin-I (Gly-delta ZPhe-L-Ala-L-Asp) (delta ZPhe: (Z)-alpha,beta-dehydrophenylalanine) and [Aib2]achatin-I (Gly-Aib-L-Ala-L-Asp) (Aib: alpha-aminoisobutyric acid), were synthesized, and their effects on the two identifiable Achatina giant neuron types, PON (periodically oscillating neuron) and v-RCDN (ventral-right cerebral distinct neuron), were examined in comparison with those of achatin-I under voltage clamp. 2. Achatin-I (n = 6), ejected onto the neurone by brief pneumatic pressure (2 kg/cm2, 400 ms, 10(-3) M, at 10-min intervals), produced an inward current (Im) on PON. The Iin value (mean +/- SEM) was 0.44 +/- 0.03 nA. The interval between the achatin-I ejection and the Iin peak was 14.74 +/- 3.15 s (n = 6). [delta ZPhe2]achatin-I (n = 6) and [Aib2]achatin-I (n = 6) had no effect on this neuron type. 3. On the other hand, achatin-I (n = 10) and [delta ZPhe2]-achatin-I (n = 10), ejected by brief pressure, produced an Iin on v-RCDN. The Iin values were 0.85 +/- 0.07 nA for achatin-I and 0.48 +/- 0.05 nA (p < 0.01, compared with that of achatin-I by Student's t-test for paired data) for [delta ZPhe2]achatin-I. The intervals between the compound ejection and the Iin peak were 5.95 +/- 0.33 s for achatin-I and 8.70 +/- 0.81 s (p < 0.05, compared with that of achatin-I) for [delta ZPhe2]achatin-I. [Aib2]achatin-I (n = 10) had no effect on this neuron type.

Blocking effects of promethazine, triprolidine and their analogues on the excitation caused by the peptide, achatin-I

An Achatina endogenous tetrapeptide, achatin-I (Gly-D-Phe-Ala-Asp), applied by brief pressure, produced an inward current (Iin) on an Achatina giant neurone type, PON (periodically oscillating neurone). Promethazine, triprolidine and their analogues tested, applied by perfusion, showed a tendency to inhibit the Iin, suggesting that the effective structures vary to a wide extent. With respect to promethazine and its analogues, the presence of 2-bromo, 5-oxo, 3-dimethylsulfamido and 2-methoxy weakened the effects. 10-(2-methylamino-2-methylethyl) instead of 10-(2-dimethylamino-2-methylethyl) of promethazine and the azepine ring instead of phenothiazine ring potentiated the effects. From the dose (pressure duration)-response study of achatin-I, the two promethazine analogues, RP 6497 and RP 6549 (the structures are shown in Fig. 1), inhibited the Iin in partly competitive and partly noncompetitive manners. Regarding triprolidine and its analogues, the compounds in Z-configuration seemed to be more effective than those in E-configuration. The presence of 4-methyl in 1-phenyl, and 1-(4-pyridyl) instead of 1-(2-pyridyl) potentiated the effects. 3-Dimethylamino instead of 3-pyrrolidino weakened the effects. The two triprolidine analogues, Trip Der 3 and Trip Der 6 (the structures in Fig. 2), inhibited the Iin in an uncompetitive manner.