Blockers for excitatory effects of achatin-I, a tetrapeptide having a D-phenylalanine residue, on a snail neurone

Emerging knowledge of regulatory roles of D-amino acids in bacteria

The d-enantiomers of amino acids have been thought to have relatively minor functions in biological processes. While l-amino acids clearly predominate in nature, d-amino acids are sometimes found in proteins that are not synthesized by ribosomes, and d-Ala and d-Glu are routinely found in the peptidoglycan cell wall of bacteria. Here, we review recent findings showing that d-amino acids have previously unappreciated regulatory roles in the bacterial kingdom. Many diverse bacterial phyla synthesize and release d-amino acids, including d-Met and d-Leu, which were not previously known to be made. These noncanonical d-amino acids regulate cell wall remodeling in stationary phase and cause biofilm dispersal in aging bacterial communities. Elucidating the mechanisms by which d-amino acids govern cell wall remodeling and biofilm disassembly will undoubtedly reveal new paradigms for understanding how extracytoplasmic processes are regulated as well as lead to development of novel therapeutics.

Ouabain-sensitive K(+)-dependent outward current caused by threo-beta-hydroxy-L-glutamic acid on a snail neuron

1. An analog of L-glutamic acid, threo-beta-hydroxy-L-glutamic acid (threo-L-BHGA), was applied locally to the giant neuron of an Achatina snail by pneumatic brief pressure ejection and induced an outward current (Iout) on the ventral-left cerebral distinct neurone (v-LCDN). The present study aimed to elucidate the ionic mechanisms of the Iout caused by threo-L-BHGA (ItL-BHGA) of v-LCDN and the effects of ouabain on this current under voltage clamp. 2. The reversal potentials of ItL-BHGA (EtL-BHGA) of v-LCDN in varied K+o were fitted to the Nernst equation as ItL-BHGA = IK (K+ current) and were almost unchanged in Cl-o-free and Na+o-reduced (20% of normal) states. The ItL-BHGA is due to the increase in permeability of the neuromembrane to K+(K(+)-dependent) and is neither Na(+)- nor Cl-(-)-dependent. K(+)-channel blockers, a mixture of tetraethyl-ammonium (TEA) and 4-amino-pyridine (4-AP), blocked ItL-BHGA mainly in a noncompetitive and partly in an uncompetitive manner. 3. Unexpectedly, ItL-BHGA of v-LCDN was almost abolished in the Na+o-free state and significantly reduced in the Cl-o-free state. However, an Na(+)-channel blocker, tetrodotoxin, showed a tendency to enhance ItL-BHGA. On the other hand, ItL-BHGA was enhanced in K+o-free state. 4. Ouabain markedly inhibited ItL-BHGA in both noncompetitive and uncompetitive manners. Benzamil, an inhibitor of the Na(+)-Ca2+ exchange applied simultaneously with ouabain could not prevent ouabain inhibition on ItL-BHGA. The currents induced by other putative neurotransmitters, including a K(+)-dependent Iout caused by dopamine on v-LCDN, were not affected by ouabain. 5. According to our previous study, the threo-L-BHGA receptors are not linked with protein kinases or calmodulin. Then, ItL-BHGA could be produced by the receptor K+ channel complex or the receptor-G-protein-K+ channel combination. The present results indicate that the ATPase activity inhibited by ouabain and the presence of extracellular Na+ and Cl- are needed for threo-L-BHGA to activate the K(+)-dependent structure. Furthermore, the K+o-free state, which inactivates the Na(+)-K+ pump, and tetrodotoxin, which suppresses the Na+ channel at least partly, did not affect the structure to be activated.

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.

Pharmacologic characteristics of excitatory gamma-amino-butyric acid (GABA) receptors in a snail neuron

1. The pharmacologic characteristics of excitatory gamma-aminobutyric acid (GABA) receptors, termed muscimol II type GABA receptors, found in a giant neuron type, v-LCDN (ventral-left cerebral distinct neuron), of an African giant snail (Achatina fulica Férussac), were studied using the mammalian GABA receptor agonists, antagonists and synergists and GABA uptake inhibitor using the voltage clamp technique. 2. GABA and its agonists, ejected by brief pressure, produced an inward current (Iin) of the following order of potency: trans-t-aminocrotonic acid (TACA) > GABA > muscimol > isoguvacine > 5-aminopentanoic acid and cis-4-aminocrotonic acid (CACA). (+/-)-Baclofen and 3-aminopropylphosphonic acid (APPA) were ineffective. The Iin values produced by GABA, TACA, isoguvacine and CACA were stable for at least 60 min, whereas the Iin induced by muscimol was not. 3. According to the dose-response curves of GABA, TACA, isoguvacine and CACA, measured by the varied pressure duration method, the ED50 value of CACA was larger than those of the other compounds, and Emax of TACA was larger than that of GABA, whereas Emax values of isoguvacine and CACA were smaller. 4. The perfusion of beta-alanine, pentobarbital and 5-aminopentanoic acid inhibited the Iin induced by GABA, whereas (-)-bicuculline, pitrazepin, diazepam and 2-hydroxysaclofen had no effect. 5. From the effects of beta-alanine on the dose-response curves of GABA, measured by the varied pressure duration method, beta-alanine competitively inhibited the Iin caused by GABA. According to the effects of pentobarbital on the dose-response curves of GABA, this drug noncompetitively inhibited the Iin using the varied pressure duration method, and partly competitively and partly noncompetitively using the Y-tube method. The effects of 5-aminopentanoic acid on the dose-response curves of GABA indicated that this drug noncompetitively inhibited the Iin using the varied pressure duration method, and partly noncompetitively and partly uncompetitively using the Y-tube method. 6. The pharmacologic features of the Achatina muscimol II type GABA receptors were similar to those of mammalian GABAC (GABAp1) receptors, except for the effects of pentobarbital.

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.

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

The effects of intracellular signal transduction system inhibitors on the inward current (Iin) caused by achatin-I (Gly-D-Phe-Ala-Asp), an Achatina endogenous tetrapeptide having a D-phenylalanine residue, applied locally onto the neurone tested, were examined under voltage clamp using two identifiable Achatina giant neurone types, v-RCDN (ventral-right cerebral distinct neurone) and PON (periodically oscillating neurone). H-89 (N-[2-(p-bromocinnamylamino)-ethyl]-5-isoquinolinesulfonamide) (adenosine-3',5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase inhibitor) markedly suppressed the achatin-I-induced Iin on PON, whereas this drug was ineffective on the Iin of v-RCDN. Dose (pressure duration)-response study of achatin-I on PON in a physiological solution and in the presence of H-89, and Lineweaver-Burk plot of these data, indicated that H-89 inhibited the Iin in a noncompetitive manner. KT5823 (N-methyl-(8R*,9S*,11S*)-(-)-9-methoxy-9-methoxycarbonyl-8-methyl-2,3,9, 10-tetrahydro-8,11-epoxy-1H,8H,11H-2, 7b,11a-triazadibenzo[a,g]cycloocta[c,d,e]-trinden-1-on e) (guanosine-3',5'-cyclic monophosphate (cyclic GMP)-dependent protein kinase inhibitor) suppressed the achatin-I-induced Iin of v-RCDN in mainly noncompetitive and partly uncompetitive manners, but this drug had no effect on the Iin of PON. W-7 (N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide) (calmodulin inhibitor) suppressed noncompetitively the Iin of PON, but this drug had no effect on the Iin of v-RCDN. IBMX (3-isobutyl-1-methylxanthine) (cyclic nucleotide phosphodiesterase inhibitor) enhanced the achatin-I-induced Iin of v-RCDN, but this drug was ineffective on the Iin of PON. However, IBMX might have effects on the achatin-I receptor sites on v-RCDN. These findings suggest multiple intracellular signal transduction pathways mediating the achatin-I-induced Iin: the Iin of PON is via cyclic AMP-dependent and probably Ca2+/calmodulin-dependent protein kinases, and that of v-RCDN via cyclic GMP-dependent protein kinase. Other signal transduction system inhibitors including calphostin C (2-[12-[2-(benzyloxy)-propyl]-3, 10-dihydro-4,9-dihydroxy-2,6,7,11-tetramethoxy-3,10-dioxo-1-per yleny]-1 -methylethyl carbonic acid 4-hydroxyphenyl ester) (protein kinase C inhibitor) did not significantly affect the Iin of both v-RCDN and PON.

Identifiable Achatina giant neurones: their localizations in ganglia, axonal pathways and pharmacological features

1. An African giant snail (Achatina fulica Férussac), originally from East Africa, is now found abundantly in tropical and subtropical regions of Asia, including Okinawa in Japan. This is one of the largest land snail species in the world. The Achatina central nervous system is composed of the buccal, cerebral and suboesophageal ganglia. The 37 giant neurones were identified in these ganglia by the series of studies conducted over about 20 years. The identifications were made by the localization of these neurones in the ganglia, their axonal pathways and their pharmacological features. 2. In the left buccal ganglion, the four giant neurones, d-LBAN, d-LBMB, d-LBCN and d-LBPN, were identified. In the left and right cerebral ganglia, d-LCDN, d-RCDN, v-LCDN and v-RCDN were identified. The suboesophageal ganglia are further composed of the left and right parietal, the visceral, the left and right pleural, and the left and right pedal ganglia. In the right parietal ganglion, PON, TAN, TAN-2, TAN-3, RAPN, d-RPLN, BAPN, LPPN, LBPN, LAPN and v-RPLN were identified. In the visceral ganglion, VIN, FAN, INN, d-VLN, v-VLN, v-VAN, LVMN, RVMN and v-VNAN were identified. In the left parietal ganglion, v-LPSN was identified. In the left and right pedal ganglia, LPeNLN, RPeNLN, d-LPeLN, d-LPeCN, d-RPeAN, d-LPeDN, d-LPeMN and d-LPeEN were identified. 3. Of the small molecule compounds tested, dopamine, 5-hydroxytryptamine, GABA, L-glutamic acid, threo- or erythro-beta-hydroxy-L-glutamic acid were effective on the Achatina giant neurones. We suppose that these compounds act as the neurotransmitters for these neurones. 4. Of the neuroactive peptides, achatin-I(Gly-D-Phe-Ala-Asp). APGW-amide(Ala-Pro-Gly-Trp-NH2) and Achatina cardioexcitatory peptide (ACEP-1)(Ser-Gly-Gln-Ser-Trp-Arg-Pro-Gln-Gly-Arg-Phe-NH2) were proposed as neurotransmitters, because these were effective on the Achatina giant neurones and their presence was demonstrated in the Achatina ganglia. Further, myomodulin (Pro-Met-Ser-Met-Leu-Arg-Leu-NH2), buccalin (Gly-Met-Asp-Ser-Leu-Ala-Phe-Ser-Gly-Gly-Leu-NH2), FMRFamide (Phe-Met-Arg-Phe-NH2). [Ser2]-Mytilus inhibitory peptide ([Ser2]-MIP) (Gly-Ser-Pro-Met-Phe-Val-NH2), catch-relaxing peptide (CARP) (Ala-Met-Pro-Met-Leu-Arg-Leu-NH2), oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2) and small cardioactive peptideB (SCPB) (Met-Asn-Tyr-Leu-Ala-Phe-Pro-Arg-Met-NH2) could also be neurotransmitters because these peptides were also effective on the Achatina giant neurones, though their presence in the ganglia of this animal has not yet been demonstrated. 5. Calcium current (ICa) was recorded from Achatina giant neurones in the Na(+)-free solution containing K(+)-channel blockers under voltage clamp. The Ca2+ antagonistic effects of brovincamine, verapamil, eperisone, diltiazem, monatepil, etc., were compared using the ICa of the Achatina neurones. 6. Almost all of the mammalian small molecule neurotransmitters were effective on the Achatina giant neurones, suggesting that these compounds are acting on the neurones of a wide variety of animal species. However, the pharmacological features of the Achatina neurone receptors to these compounds were not fully comparable to those of the mammalian receptors. For example, we proposed that beta-hydroxy-L-glutamic acid (either threo- or erythro-) could be an inhibitory neurotransmitter for an Achatina neurone. 7. In contrast, the Achatina giant neurones appear to have no receptor for the mammalian neuroactive peptides, except for oxytocin and Arg-vasotocin. On the other hand, many neuroactive peptides were isolated from invertebrate nervous tissues, including achatin-I, a neuroexcitatory tetrapeptide having a D-phenylalanine residue.