Introduction
Early sea urchin embryos were among the first subjects of pre-nervous neurotransmitter function research (Buznikov et al., Reference Buznikov, Chudakova and Zvezdina1964; Buznikov, Reference Buznikov1989). During the next decades a wealth of embryo physiological data was obtained on the participation of the transmitters, in particular in the control of cleavage divisions (Buznikov, Reference Buznikov1989), state of the cytoskeleton (Buznikov & Grigoriev, Reference Buznikov and Grigoriev1990), blastomere interactions (Shmukler, Reference Shmukler1981, Reference Shmukler2010) and ciliary motility (Doran et al., Reference Doran, Doran, Koss, Tran, Christopher, Gallin and Goldberg2004). Maternal serotonin was also shown to participate in morphogenesis regulation (Yavarone et al., Reference Yavarone, Shuey, Tamir, Sadler and Lauder1993; Buznikov et al., Reference Buznikov, Shmukler and Lauder1996; Côté et al., Reference Côté, Fligny, Bayard, Launay, Gershon, Mallet and Vodjdani2007). The majority of embryo physiological data was accumulated quite a long period ago when neither a modern panel of ligands nor transmitter receptor classification existed (Hoyer et al., Reference Hoyer, Clarke, Fozard, Hartig, Martin, Mylecharane, Saxena and Humphrey1994). In many cases, questions arose concerning the receptor specificity of ligands used in earlier works on sea urchin embryos (Landau et al., Reference Landau, Buznikov, Kabankin, Kolbanov, Suvorov and Teplitz1977; Buznikov, Reference Buznikov1989), which were based on chemical structure only, and when sea urchin transmitter receptors were not characterized adequately. In the context of modern techniques available, a revision of previous data using contemporary approaches, in particular receptor classification elaborated in the last decades and newly synthesized and characterized ligands, is now required. Such an investigation, mainly focusing on the relatively late effects of serotonergic ligands, has been undertaken in embryos and the larvae of sea urchins (Buznikov et al., Reference Buznikov, Peterson, Nikitina, Bezuglov and Lauder2005). We carried out similar experiments on cleaving sea urchin Paracentrotus lividus and Arbacia lixula together with molecular biology research on the expression of the components of serotonergic system. Whereas a wide molecular biological investigation of the expression of the components of the serotonergic mechanism had been carried out in amphibian embryos (Nikishin et al., Reference Nikishin, Kremnyov, Konduktorova and Shmukler2012a), only sparse data on the serotonergic system (Nikishin et al., Reference Nikishin, Semenova and Shmukler2012b) and GABAergic system (Kaeser et al., Reference Kaeser, Rabe and Saha2011) of sea urchins existed. The present work is devoted to the characterization of the pre-nervous serotonergic system of the sea urchin P. lividus, and investigation of the expression and functional activity of the components taking part in the synthesis, vesicular transport and reception of serotonin.
Materials and methods
Experiments were performed in the embryos of sea urchins Paracentrotus lividus (Lamarck, 1816) and Arbacia lixula (Linnaeus, 1758) at the Institute of Marine Biology (Adriatic Sea, Kotor, Montenegro). Embryos were obtained using artificial fertilization according to a standard protocol (Buznikov & Podmarev, Reference Buznikov and Podmarev1975).
Sea urchin embryos and larvae were fixed in a 10-fold volume of RNALater (Ambion) for molecular genetic research, and the total RNA was isolated using TRIReagent (Sigma), according to the manufacturer's instructions, and treated with DNase I (Fermentas) to remove the genomic DNA; 1 mg of RNA, M-MLV reverse transcriptase (Evrogen) and random hexanucleotides (Silex) were used for cDNA synthesis. PCR was performed on an MJ Mini thermal cycler (BioRad) using colored Taq polymerase (Silex) and specific oligonucleotides (Lytech) using the parameters that were selected considering the sequence of the primers and the length of the product (Table 1). To exclude false-positive results, negative controls were included (PCR without reverse transcription and PCR without cDNA). The PCR products were analysed by 1.5% agarose gel electrophoresis with ethidium bromide (0.5 mg/ml). Primers were designed by Lasergene PrimerSelect (DNASTAR) using sequences from the NCBI GenBank database (Table 2). Prediction of transmembrane helices and topology of proteins were performed using open access online service HMMTOP (Tusnády & Simon, Reference Tusnády and Simon1998, Reference Tusnády and Simon2001). PCR products of serotonin transporter (SERT)-like, dopamine active transporter (DAT)-like and vesicular monoamine transporter (VMAT)-like genes were obtained from P. lividus embryos using specific oligonucleotides selected on the basis of mRNA sequences of S. purpuratus (Table 2). Products were isolated, cloned, sequenced (Evrogen) and deposited into GenBank with the following accession numbers: KC599202 (SERT-like) and KC599201 (VMAT-like).
Table 1 Specific oligonucleotides used in the present work
Table 2 GenBank sequences used in the work
Embryos were incubated for immunohistochemistry in artificial sea water, containing substances under investigation – serotonin hydrochloride (Tocris Bioscience #3547), 5-hydroxy-l-tryptophan (Sigma-Aldrich #H9772), 4-chloro-l-phenylalanine (Sigma-Aldrich #C8655), for 3 h at room temperature (20°C), and then fixed in 4% paraformaldehyde, transferred into methanol and stored at –20°C. Immunohistochemical staining was performed with primary polyclonal anti-serotonin rabbit antibodies (Chemicon #AB938) and anti-rabbit IgG-Atto 633 antibody produced in goat (Sigma-Aldrich #41176). Preparations were viewed under confocal microscopes either an Olympus FluoView FV10i (Confocal Microscopy Laboratory, Lomonosov Moscow State University) or a Leica TCS SP (Optical Research Group, Koltzov Institute of Developmental Biology) at equal laser intensity and detector sensitivity. The intensity of immunofluorescence was measured using ImageJ software (NIH), and statistical analysis was performed using STATISTICA software (StatSoft).
In embryo physiological experiments substances were added to the medium after elevation of the fertilization envelope and evaluation of the percentage of fertilization (more than 90%). Embryos were incubated at room temperature and the percentage of embryos that successfully passed first cleavage division was recorded. Substances used in the experiments are as follows: (S)-WAY 100135 dihydrochloride (Tocris Bioscience #1253), methiothepin maleate (Tocris Bioscience #0582), cyproheptadine hydrochloride sesquihydrate (Sigma-Aldrich #C6022), GR 55562 dihydrochloride (Tocris Bioscience #1054), SB 242084 (Tocris Bioscience #2901), spiperone (Sigma-Aldrich #S7395), 5-nonyl-oxytryptamine oxalate (Tocris Bioscience #0901), NAN-190 hydrobromide (Tocris Bioscience #0553), 3-tropanylindole-3-carboxylate methiodide (Sigma-Aldrich #T113), mianserin hydrochloride (Tocris Bioscience #0997), clozapine (Sigma-Aldrich #C6305), BW 723C86 hydrochloride (Tocris Bioscience #1059). Arachidonoyl serotonin and arachidonoyl-dopamine were synthesized in the Laboratory of Oxylipins, Shemyakin–Ovchinnikov Institute of Bioorganic Chemistry as described (Bisogno et al., Reference Bisogno, Melck, De Petrocellis, Bobrov, Gretskaya, Bezuglov, Sitachitta, Gerwick and Di Marzo1998; Bezuglov et al., Reference Bezuglov, Bobrov, Gretskaya, Gonchar, Zinchenko, Melck, Bisogno, Di Marzo, Kuklev, Rossi, Vidal and Durand2001).
Results
PCR analysis
To identify the components of transmitter systems possibly involved in the regulation of early development of sea urchins, the screening of P. lividus genes homologous to transmitter receptors and transporters was performed, also as a temporal analysis of their expression using reverse transcription PCR. There are several genes that encode G protein coupled receptors (GPCR), predicted in the GNOMON project as serotonin receptor homologues in the completely sequenced genome of the purple sea urchin Strongylocentrotus purpuratus. A BLAST search for homologues of these genes in the Expressed Sequence Tags (EST) database of P. lividus was performed. EST clones homologous to four HTR-like genes of S. purpuratus were identified (Table 2). PCR analysis of mRNA expression of these receptors during embryogenesis was performed and showed that all four genes are expressed in all stages of P. lividus development, including the earliest ones (Fig. 1). At the same time, the analysis of amino acid sequences of these receptors was performed using the online service HMMTOP, and revealed that the third transmembrane domain of all four receptors lacks the aspartate residue (Fig. 2 A) that is highly conservative in metabotropic serotonin receptors (Padayatti et al., Reference Padayatti, Wang, Gupta, Orban, Sun, Salom, Jordan, Palczewski and Chance2013).
Figure 1 PCR analysis of the mRNA expression of homologues of monoaminergic transmitter system components during P. lividus embryonic development. Cl, cleavage stage (16–32 blastomeres); eBl, early blastula; G, gastrula; mBl, mesenchyme blastula; Oo, oocyte; Pl, pluteus; Pr, prism.
Figure 2 Alignment of amino acid sequences of monoamine receptors (A), membrane transporters (B) and vesicular transporters (C) of human (Hs) and sea urchin S. purpuratus (Spur). Highly conserved amino acid residues are marked by asterisks. Amino acids mismatched with the reference human sequence are marked in black, and conserved amino acid substitutions are marked in grey. The boundaries of the transmembrane domains are marked under the alignments.
Another approach was therefore used to detect transmitter receptors expressed during early P. lividus embryogenesis. A BLAST search for sequences homologous to human serotonin receptor HTR1A and those obtained from early P. lividus embryos (EST clones tagged EGG, CLEB or MBSB) was undertaken in the EST database. Sequences found in this way were searched for using BLAST analysis for corresponding homologous S. purpuratus genes, whose amino acid sequences in turn were analysed using the HMMTOP service to check for the presence of an aspartate residue at the third transmembrane domain. Thus, the sequences homologous to S. purpuratus genes GNOMON predicted as TyrR2-like and D2-like were detected (Table 2). PCR analysis of mRNA expression of these receptors has shown that a D2-like receptor is expressed during whole embryonic development whereas the TyrR2-like receptor starts to be expressed from the gastrula stage. It needs to be taken into an account that these are monoaminergic receptors, whose specificity was determined formally and may be imprecise, as in the case of the abovementioned receptors erroneously attributed as serotonergic ones.
There are genes in the S. purpuratus genome that are annotated as homologous to monoamine transporters: membrane transporters of serotonin SERT-like, and norepinephrine NET-like1 and NET-like2, active dopamine transporter DAT-like, and vesicular monoamine transporter VMAT-like (Table 2). A search for homologous sequences in the P. lividus EST database was successful in cases of NET-like1 and NET-like2 only. Specific oligonucleotides for the investigation of the expression of the rest transporters were selected to the most conservative regions of S. purpuratus mRNA sequences of SERT-like, DAT-like and VMAT-like. Results of PCR analysis are presented in Fig. 1. The gene homologous to SERT-like was expressed in all of the developmental stages investigated. The most pronounced expression was found in the earliest stages such as oocyte and cleavage divisions, then decreased during early development gradually until zero at the gastrula stage and increased again at the neural stages (prism, pluteus). The expression of the gene homologous to DAT-like was detected at the pluteus stage only. At the same time, two homologues of the norepinephrine transporter NET-like1 and NET-like2 are expressed at all developmental stages. Computational analysis of amino acid sequences of transporters under investigation have shown that conservative residues characteristic for membrane monoamine transporters (Penado et al., Reference Penado, Rudnick and Stephan1998) are present in SERT-like and NET2-like (Fig. 2 B). A gene that is homologous to VMAT-like starts to be expressed at the prism stage only, and its amino acid sequence contains amino acid residues that are conservative for vesicular monoamine transporters (Schuldiner et al., Reference Schuldiner, Shirvan and Linial1995) (see Fig. 2 C).
Immunohistochemical experiments
Serotonin was detected immunohistochemically in all blastomeres of P. lividus (Fig. 3 A). Blastomere cytoplasm was stained uniformly as close grains. With uniform cytoplasm staining there were large grains distributed over the blastomere volume but they were concentrated at the surface membrane of the embryos. The negative control (without first antibodies) showed staining neither of the cytoplasm nor of large grains, and therefore the staining in the experimental samples was due to the anti-serotonin antibody immune reaction. To check the activity of serotonin reuptake, the incubation of sea urchin embryos at the cleavage division and blastula stages in the presence of serotonin (100 μM) was carried out. Subsequent immunostaining of incubated embryos have shown an increase in its intensity in the cytoplasm (Fig. 3 B) up to 188% as compared with the control at the cleavage stage (Fig. 3 E) and to 131% at the early blastula stage (Fig. 3 F). To check the activity of aromatic l-amino acid decarboxylase (AADC) embryos were incubated with serotonin precursor 5-hydroxytryptophan (HTP) (100 μM). Immunohistochemical staining elicited an increase in serotonin levels in the cytoplasm of embryos incubated in HTP (Fig. 3 C) to 297% as compared with the control at the cleavage stage (Fig. 3 E) and to 190% at the early blastula stage (Fig. 3 F). Immunohistochemical staining of the embryos incubated with tryptophan hydroxylase (TPH) inhibitor PCPA (100 μM) showed a decrease in cytoplasmic serotonin (Fig. 3 D) to 56% as compared with the control at the cleavage stage (Fig. 3 E) and to 22% at the early blastula stage (Fig. 3F). The number and staining intensity of large granules in the cytoplasm and at the surface of blastomeres were not changed with incubation in 5-hydroxytryptamine (5-HT), HTP or PCPA (Fig. 3 B–D).
Figure 3 Immunohistochemical detection of serotonin in P. lividus blastula stage embryos in the control (A) and after 3 h incubation in the 5-HT (B), HTP (C) and PCPA (D). Changes of serotonin immunoreactivity intensity after 3 h incubation of cleavage embryos (E) and blastula stage embryos (F) with 5-HT, HTP and PCPA. Bars show standard error. Significance by Mann–Whitney U-test: *P < 0.005.
Embryo physiological experiments
The sensitivity of embryos to serotonergic and dopaminergic ligands was studied by their ability to block the first cleavage division of P. lividus. The minimum concentrations of ligands that blocked first cleavage division are presented in Table 3 and show that the most effective were ligands characterized as 5-HT1-antagonists in mammals. The whole range of comparative ligand activity is as follows:
Table 3 Minimal concentrations of serotonergic ligands blocking cleavage divisions of P. lividus and A. lixula embryos
Note: No effect: has no cytostatic effect at concentrations below 100 μM.
methiothepine = 5-nonyl-oxytryptamine > inmecarb > cyproheptadine > (S)-WAY-100635 = SB 242084 = clozapin > MDL 72832 = BW 723C86 > GR 55562 > NAN-190
It needs to be noted that not only serotonin antagonists were able to block or inhibit first cleavage division but also some agonists such as BW 723C86. At the same time 5HT3-antagonists tropisetron and granisetron had no embryostatic effects.
A slightly different range of ligand activity was obtained in the parallel experiments with A. lixula embryos (Table 3):
(S)-WAY-100635 > 5-nonyl-oxytryptamine >> inmecarb > cyproheptadine = mianserin >> SB 242084 = clozapin = MDL 72832 = BW 723C86 = 8-OH-DPAT > GR 55562 = NAN-190 >>> methiothepine (ineffective)
The activity of 5HT1-receptor ligands was more pronounced compared with other types of ligands in this sea urchin species. In contrast with P. lividus, early A. lixula embryos were sensitive to mianserin and 8-OH-DPAT but not to methiothepine. A. lixula embryos also have far higher sensitivity to two substances as compared with P. lividus – 4 μM versus 50 μM for (S)-WAY-100635 and 7.5 μM versus 20 μM for 5-nonyl-oxytryptamine, correspondingly.
The specificity of the serotonergic mechanism of ligand action was investigated using the addition of transmitters or their conjugates with arachidonic acid together with antagonists. Serotonin has a relatively weak protective action against the effects of serotonin receptor antagonists in P. lividus embryos (Table 4). At the same time, arachidonoyl transmitter derivatives, firstly serotonin (АА-5-HT), had a far more pronounced protective effect. АА-5-HT effectively protected the development against inmecarb, (S)-WAY-100635, cyproheptadine and methiothepin. Arachidonoyl-5-methoxy-tryptamine (АА-5-МОТ) and arachidonoyl-dopamine (AA-DA) had less pronounced protective effects. Arachidonic acid itself never showed a protective action against serotonin antagonists. Together with arachidonic transmitter analogues, the activator of adenylyl cyclase forskolin (100 μM) and dibutyryl-cAMP (100 μM) also had a protective effect against cytostatic effect of methiothepin. Eticlopride and haloperidol, dopamine antagonists, effectively blocked the development of P. lividus embryos at concentrations of 50 and 20 μM, correspondingly. None of the substances used had a protective effect against eticlopride, whereas AA-DA effectively protected from the haloperidol effect.
Table 4 Effects of various protectors against cytostatic action of transmitter ligands in P. lividus embryos
Note: Effects are presented as the increase in a number of cleaving embryos after addition of protectors (%). Concentration of protectors in all experiments was 100 μM.
Discussion
The results of the present work allow some preliminary conclusions concerning the structure of the embryonic transmitter system. Serotonin is present in all species of early embryos studied (Buznikov, Reference Buznikov1989). In particular in sea urchins it is distributed over the cytoplasm (Buznikov et al., Reference Buznikov, Manukhin, Rakic, Kudriashova and Mndzhoian1979; Markova et al., Reference Markova, Buznikov, Kovaćević, Rakić, Salimova and Volina1985; Buznikov et al., Reference Buznikov, Nikitina, Voronezhskaya, Bezuglov, Willows and Nezlin2003; Amireault & Dubé, Reference Amireault and Dubé2005). The present immunohistochemical research showed the uniform localization of serotonin.
Immunohistochemical results of incubation of sea urchin embryos with serotonin precursor HTP and with tryptophan hydroxylase inhibitor PCPA (Fig. 3 C, D) have shown continuous activity of both synthetic enzymes – tryptophan hydroxylase and aromatic amino acid decarboxylase – during whole early development. Sea urchins are not exclusive in this respect as the activity of a serotonin synthesis system at the early stages of the development was shown in a number of species (Buznikov et al., Reference Buznikov, Nikitina, Voronezhskaya, Bezuglov, Willows and Nezlin2003). In some cases this system was detected in yolk granules, however serotonin is always detected in the cytoplasm only (Buznikov, Reference Buznikov1989).
Membrane and vesicular monoamine transporters are important components of monoaminergic systems. The present work has shown that several genes homologous to monoamine transporters are expressed in the early stages of embryonic development. Analysis of protein amino acid sequences encoded by these genes proved the presence of conservative residues in SERT-like and NET-like2. Both genes are expressed in the early P. lividus developmental stages, and a SERT-like homologue has interesting expression dynamics: it expresses most pronouncedly in the early developmental stages and then practically disappears at the gastrula stage which coincides with the dynamics of serotonin itself in sea urchin embryos (Renaud et al., Reference Renaud, Parisi, Capasso and De Prisco1983). Together with immunohistochemical data on the increase of intracellular serotonin levels after incubation with this transmitter this probably shows the functional role of serotonin transporters at the early stages of the development – during cleavage divisions and blastulation. These results also support earlier data on the activity of serotonin membrane transport in early sea urchin embryos (Buznikov, Reference Buznikov1984) and other animals (Amireault & Dubé, Reference Amireault and Dubé2005). The gene from S. purpuratus genome homologous to VMAT is annotated, whose amino acid sequence contains conservative residues that are characteristic for the vesicular monoamine transporter. PCR analysis has shown that the expression of this gene is absent in the early stages of development. Vesicular monoamine transporters are needed for the accumulation of monoaminergic transmitters, including serotonin, into vesicles for the realization of their intercellular signalling function in adult organisms. This mechanism is probably absent in early sea urchin embryos. It was earlier demonstrated that reserpine and other inhibitors of vesicular monoamine transport do not influence the level of monoamines inside cells at the pre-gastrulation stages, proving the absence of a system of vesicular transport in early embryos (Markova et al., Reference Markova, Buznikov, Kovaćević, Rakić, Salimova and Volina1985). Nevertheless, it is known that early sea urchin embryos transport the serotonin to the outer medium (Buznikov, Reference Buznikov1967; Renaud et al., Reference Renaud, Parisi, Capasso and De Prisco1983), and serotonin can be accumulated at the interblastomere spaces of early embryos (Markova et al., Reference Markova, Buznikov, Kovaćević, Rakić, Salimova and Volina1985). Mechanisms of such serotonin excretion remain unclear when taking into account the absence of vesicular transport. This is probably achieved through any membrane transporters able to undertake reverse activity (Richerson & Wu, Reference Richerson and Wu2003). The function of transmitter transport in early embryos may be coupled to some factors. First, in connection with the weakness of the enzymatic system of transmitter degradation, efflux of transmitter to the outer medium can be the main way of its inactivation in intracellular regulatory processes (Buznikov, Reference Buznikov1989). Moreover, transmitters transported to the external medium can take part in blastomere interaction (Shmukler, Reference Shmukler1993), and reuptake of serotonin into the blastomeres thus limits this process.
A key link of transmitter signalling is a receptor, and it is therefore important to know which receptors mediate transmitter effects in early embryonic development. Generally the results of embryo-physiological experiments show significant differences between neurotransmitter receptors of sea urchins and mammalian ones, in which the ligands used were characterized. In particular, some serotonin agonists in mammals (5-nonyl-oxytryptamine, BW 723C86, 8-OH-DPAT) may act as blockers of the development in sea urchin embryos. Besides, there are interspecies differences in the sensitivity of A. lixula and P. lividus embryos that are proved by variations of the ranges of ligand activities of these two species. This is probably the reason for the ineffectiveness of 5HT1А-receptor ligands, embryostatic in species used in the present work, in cleaving embryos of L. variegatus (Buznikov et al., Reference Buznikov, Peterson, Nikitina, Bezuglov and Lauder2005).
Nevertheless, it is clear that the cytostatic effects of transmitter ligands are mediated by metabotropic receptors. This opinion is supported in particular by a lack of embryostatic effects of channel receptor antagonists, as in previous study (Buznikov et al., Reference Buznikov, Peterson, Nikitina, Bezuglov and Lauder2005). Ligands, characterized as antagonists of 5HT1-receptors, were most effective that generally corresponds to previous results although obtained in an unusual experimental design – with the administration of ligands after the first cleavage division not after fertilization (Buznikov et al., Reference Buznikov, Peterson, Nikitina, Bezuglov and Lauder2005). However, taking into account our data on the expression of transmitter receptors during the early development of sea urchins, the data of the later publication have to be taken with special caution, because pharmacological characteristics obtained in mammals were applied there to sea urchins directly. At least partially, ligand effects are mediated by the adenylyl cyclase signal pathway, a finding that is supported by the protective action of forskolin and dibutyryl-cAMP and corresponds to earlier data obtained in another sea urchin species (Shmukler et al., Reference Shmukler, Grigoriev, Buznikov and Turpaev1986; Rostomyan et al., Reference Rostomyan, Abramyan, Buznikov and Gusareva1985). Similar data were obtained earlier (Carginale et al., Reference Carginale, Capasso, Madonna, Borrelli and Parisi1992) for dopaminergic ligands, including, however, metergoline, which is now recognized as a 5-HT1-, 5-HT2- and 5-HT7-antagonist. Thus, transmitter receptors taking part in serotonin signal function in sea urchin embryos are probably GPCR.
Our molecular genetic study of receptors homologous to transmitter GPCR, detected five genes that are expressed in early P. lividus development, probably playing a role in the serotonergic regulation of the cell cycle during cleavage. However, the amino acid sequence of D2-like only has an aspartate residue in the third transmembrane domain that is conservative among the metabotropic receptors of serotonin and other monoamines (van Rhee & Jacobson, Reference Van Rhee and Jacobson1996). The fact that the receptors studied are annotated as HTR-like but do not have this highly conservative amino acid residue means that prediction of GPCRs types using GNOMON methods may be imprecise (Nagy et al., Reference Nagy, Hegyi, Farkas, Tordai, Kozma, Bányai and Patthy2008). Taking into account pharmacological data obtained from P. lividus embryos, the D2-like receptor is the most probable link of pre-nervous monoamine functions in sea urchins. Further functional studies of this receptor may allow characterization of its real type and pharmacological properties also as elicit its role in the regulation of early sea urchin development.
Study of the specificity of early sea urchin embryo transmitter receptors using arachidonoyl derivatives of the transmitters brought some contradictions. Protective action against the cytostatic effects of methiothepin, inmecarb, cyproheptadine, and (S)-WAY-100635 had not only AA-5-HT but also AA-5-MOT and AA-DA. Furthermore, serotonin and АА-5-HT had no protective action against the cytostatic effect of clozapine which is active on the serotonin and dopamine receptors of mammals, whereas AA-DA had some protective action. At the same time, the cytostatic effect of spiperone, which is an antagonist of serotonin and dopamine receptors, was decreased by the addition of both serotonin and AA-DA. This again stresses a possible similarity of embryonic dopamine and serotonin receptors. These results suggest that either dopamine receptors are functionally active in early P. lividus development together with serotonin ones, or that embryonic receptors have combined sensitivity to both of these transmitters. Simultaneous expression of the receptors of different transmitters coupled to the same system of second messengers is characteristic for early development of some other animals (Dubé & Amireault, Reference Dubé and Amireault2007; Nikishin et al., Reference Nikishin, Kremnyov, Konduktorova and Shmukler2012a). Moreover, various types of GPCRs may interact and thus change their functional properties (Kamal & Jockers, Reference Kamal and Jockers2011). Such mechanisms might be present in the transmitter regulation of early sea urchin development.
Finally, one peculiarity of the embryonic transmitter system that needs to be noted that is indirectly supported by our experiments with arachidonoyl transmitter derivatives, having high lipophility and easily penetrating the embryonic cells is the intracellular localization of the receptors (Landau et al., Reference Landau, Buznikov, Kabankin, Kolbanov, Suvorov and Teplitz1977; Buznikov & Shmukler, Reference Buznikov and Shmukler1978; Buznikov, Reference Buznikov1989). This suggestion is supported by the significantly more pronounced protective action of arachidonoyl derivatives of the transmitters that easily penetrate the cell as compared with serotonin and dopamine per se, that cannot enter the cell without the participation of specific membrane transporters. It needs to be noted that in the analogous experiments of Buznikov and co-authors (Reference Buznikov, Peterson, Nikitina, Bezuglov and Lauder2005) lipophilic AA-5-HT and hydrophilic 5HTQ, which practically do not penetrate the cell of sea urchin embryos, show their protective effects in principally different ways. The specificity of protective action of the conjugates of transmitters with fatty acids is supported by the absence of any protective action of arachidonic acid.
Thus, previous understandings of the transmitter regulation of sea urchins’ early development have been confirmed by the present work in its main features. However, this is in need of some updates both concerning the specificity of pharmacological tools used and in the solution of the problem of key links in the process – embryonic receptors, which occur in a more complex way in sea urchins than in amphibians and mammals (Nikishin et al., Reference Nikishin, Kremnyov, Konduktorova and Shmukler2012a; Veselá et al., Reference Veselá, Rehák, Mihalik, Czikková, Pokorný and Koppel2003). The study of transmitter receptors in early sea invertebrates needs to be continued.
Acknowledgements
The authors are grateful to Dr A. Joksimović and Mr B. Lazarević for their help in the organization of experiments, Dr I. A. Kosevich for providing serotonin antibodies and Dr N. Gretskaya for the synthesis of arachidonoyl neurotransmitters.
Financial support
This work was supported by the Russian Fund for Basic Research grant 14-04-00110-а for D.N., V.B. and Y.S.
