Identification and characterization of the opiate receptor in the ciliated protozoan, Tetrahymena

Effects of Gonadotropin-Releasing Hormone (GnRH) and Its Analogues on the Physiological Behaviors and Hormone Content of Tetrahymena pyriformis

The unicellular Tetrahymena distinguishes structure-related vertebrate hormones by its chemosensory reactions. In the present work, the selectivity of hormone receptors was evaluated by analyzing the effects of various gonadotropin-releasing hormone (GnRH) analogs (GnRH-I, GnRH-III) as well as truncated (Ac-SHDWKPG-NH₂) and dimer derivatives ([GnRH-III(C)]₂ and [GnRH-III(CGFLG)]₂) of GnRH-III on (i) locomotory behaviors, (ii) cell proliferation, and (iii) intracellular hormone contents of Tetrahymena pyriformis. The migration, intracellular hormone content, and proliferation of Tetrahymena were investigated by microscope-assisted tracking analysis, flow cytometry, and a CASY TT cell counter, respectively. Depending on the length of linker sequence between the two GnRH-III monomers, the GnRH-III dimers had the opposite effect on Tetrahymena migration. [GnRH-III(CGFLG)]₂ dimer had a slow, serpentine-like movement, while [GnRH-III(C)]₂ dimer had a rather linear swimming pattern. All GnRH-III derivatives significantly induced cell growth after 6 h incubation. Endogenous histamine content was uniformly enhanced by Ac-SHDWKPG-NH₂ and GnRH-III dimers, while some differences between the hormonal activities of GnRHs were manifested in their effects on intracellular levels of serotonin and endorphin. The GnRH peptides could directly affect Tetrahymena migration and proliferation in a structure-dependent manner, and they could indirectly regulate these reactions by paracrine/autocrine mechanisms. Present results support the theory that recognition ability and selectivity of mammalian hormone receptors can be deduced from a phylogenetically ancient level like the unicellular Tetrahymena.

Interaction of nicotine with morphine potency in Paramecium caudatum

Aims

Many studies have been carried out about the interaction between nicotine and morphine in higher animals. Although previous behavioral and pharmacological evidence indicated the presence of opioid system in protozoa, there is no available data about nicotine effect on the potency of morphine in eukaryotic unicellular organisms such as protozoa. Hence, this work aims to investigate the interaction of nicotine with morphine in the protozoan Paramecium caudatum.

Main methods

According to our innovative model, the movement behavior of P. caudatum was investigated with a numerical scale using the Sedgewick-Rafter counting chamber at the field of view of 4X light microscope objective lens, such that the difference in number of Paramecia cells at definite moments after injection of drugs/substances was considered as a criterion for the behavioral response of P. caudatum.

Key findings

Results indicated the variations of morphine potency at the dose of 2 μg morphine accompanied by different doses of nicotine in P. caudatum so that the highest aggregation of Paramecia cells occurred at the dose of 2 μg morphine + 4 μg nicotine.

Significance

This confirmed that in eukaryotic unicellular organisms such as P. caudatum, nicotine can reinforce the morphine potency in a dose-dependent manner.

The hormonal system of the unicellular Tetrahymena: a review with evolutionary aspects

The unicellular ciliate, Tetrahymena has receptors for hormones of the higher ranked animals, these hormones (e.g. insulin, triiodothyronine, ACTH, histamine, etc.) are also produced by it and it has signal pathways and second messengers for signal transmission. These components are chemically and functionally very similar to that of mammalian ones. The exogenously given hormones regulate different functions, as movement, phagocytosis, chemotaxis, cell growth, secretion, excretion and the cells' own hormone production. The receptors are extremely sensitive, certain hormones are sensed (and response is provoked) at 10-21 M concentration, which makes likely that the function could work by the effect of hormones produced by the Tetrahymena itself. The signal reception is selective, it can differentiate between closely related hormones. The review is listing the hormones produced by the Tetrahymena, the receptors which can receive signals and the signal pathways and second messengers as well, as the known effects of mammalian hormones to the life functions of Tetrahymena. The possible and justified role of hormonal system in the Tetrahymena as a single cell and inside the Tetrahymena population, as a community is discussed. The unicellular hormonal system and mammalian endocrine system are compared and evolutionary conclusions are drawn.

Thyrotropin (TSH) regulates triiodothyronine (T3) production in the unicellular Tetrahymena

The aim of the experiments was to study the regulation of triiodothyronine (T3) production in the unicellular Tetrahymena. Untreated and troph-hormone treated specimen were prepared and in different timepoints T3 content was measured and compared by immunocytochemical flow cytometry. 0.1 or 0.001 IU TSH in tryptone-yeast medium stimulated T3 synthesis at 10, 20, 30 min, but does not stimulate after 1 h. The overlapping gonadotropic hormone (GTH) also did it, however only at 10 min. In Losina salt solution (physiological for Tetrahymena) the effect was weaker, however outer amino acid source was not absolutely needed for the production of the hormone. The results show that the TSH regulation of thyroid hormone synthesis (storage, secretion) and troph-hormone overlap can be deduced to a unicellular level. This may allow the hypothesis that the endocrine mechanisms proved at a low level of phylogeny are preserved for the higher ranked organisms.

Effect of different concentrations of serotonin, histamine and insulin on the hormone (serotonin and ACTH) production of Tetrahymena in nutrient-free physiological milieu

Cell populations of Tetrahymena pyriformisGL were kept in nutrient-free (Losina) milieu and treated with different (10(-6)-10(-21)M) concentrations of serotonin, histamine or insulin for 30 min. Following that the hormone (serotonin and adrenocorticotropin (ACTH) content of the cells were measured by immunocytochemical flow cytometric method. Serotonin reduced histamine when applied in 10(-12) and 10(-15)M concentrations, while elevated ACTH levels when applied in 10(-6), 10(-9) and 10(-21)M concentrations. Histamine reduced serotonin concentration at 10(-9)-10(-21)M concentrations and increased ACTH in 10(-6)M. Insulin elevated both hormones' content in each concentration except at 10(-12)M. The results demonstrate that (1) in nutrient-free conditions the hormonal effects differ from that of nutrient-rich (tryptone+yeast) condition; (2) there is an optimal hormone concentration, which causes the strongest effect and this is different for each hormones; (3) the hormone receptors of Tetrahymena are very sensitive; as they react to zeptomolar concentrations. Such small concentration is even more effective than higher ones. Since hormones must become highly diluted in the natural environment of Tetrahymena, it seems that such low concentrations are the actual physiological concentrations.

Hormonal effects on Tetrahymena: change in case of combined treatment

In order to approach their natural conditions, populations of Tetrahymena were kept in Losina-Losinky's salt solution for 1 h, than in the tryptone+yeast medium. During this time they were treated with histamine, serotonin or insulin, or with the combinations of these hormones. Effect of the combined treatments on the production of serotonin (5HT), or adrenocorticotropic hormone (ACTH) or triiodothyronine (T₃) by the cells was compared to the effect of single-hormone treatments. Significant differences were seen between the results obtained following the single or combined treatments. There was no summation of the effects, however an elevation or diminution of the hormone production was observed after the combined treatment, as compared with the untreated controls or with the use of one of the hormones in the samples. The experiments demonstrate that there is a hormonal regulation between the Tetrahymena cells and the hormones influence each other's effect.

Effect of femtomolar concentrations of hormones on insulin binding by Tetrahymena, as a function of time

The unicellular ciliate Tetrahymena, contains and binds hormones, characteristic of vertebrates. Earlier experiments demonstrated the effect of extremely low concentrations of hormones. In the present experiments, the effect of various hormones (endorphin, serotonin, histamine, insulin and epidermal growth factor [EGF]) in 10(-15) M, or oxytocin, gonadotropin at 0.001 IU concentrations) on the binding of FITC-insulin was studied by using flow cytometry and confocal microscopy, after 1, 5, 15, 30 and 60 min. Six of the seven hormones promptly decreased the cells' hormone binding capacity, the exception being EGF, and in four cases (endorphin, serotonin, insulin and oxytocin) the reduction was enormous. The decreased binding was durable. However, in the case of endorphin and oxytocin after 30 min, and in the case of serotonin after 60 min the binding returned to the control level. In the case of oxytocin after 60 min, binding significantly surpassed the control level. Histamine returned to the control level after 15 min, but after that the binding became even lower. EGF provoked special behaviour: it increased hormone binding after 30 and 60 min. The results call attention to the extreme sensitivity of Tetrahymena receptors to hormonal inductions and to its quick response ability.

Hormonal imprinting: phylogeny, ontogeny, diseases and possible role in present-day human evolution

Hormonal (chemical) imprinting which was first observed (and named) by us in the seventies of the last century, is a general biological phenomenon which takes place when the developing receptor meets its target hormone for the first time. Under the effect of imprinting, receptors mature and reach their maximal binding capacity. It also influences the cells' hormone production and different functions depending on receptors and hormones. Hormonal imprinting is present already at the unicellular level causing the development of specific receptors and helping the easier recognition of useful or harmful surrounding molecules. The phenomenon is an important factor in the survival of the species, as the effect of imprinting is transmitted to the progeny cell generations. At the same time it possibly helps the selection of molecules which are suitable for acting as hormones in higher ranked animals. In mammals, hormonal imprinting takes place perinatally and determines the function of receptor-signal-transduction systems as well as hormone production for life. However, there are other critical imprinting periods for continuously developing cells. Excess of the target hormones or presence of foreign molecules which are able to bind to the receptors, provoke faulty imprinting in the critical periods with life-long morphological, biochemical, functional or behavioural consequences. As many receptor-bound foreign molecules are used as medical treatments and many such molecules are present around us and inside us as environmental pollutants, they--causing faulty imprinting--are able to predispose the (human) organism to cardiovascular, endocrine, metabolic and cancerous diseases. It seems likely that this effect is connected with disturbance of DNA methylation process in the critical periods of life. There are some signs of the transgenerational effect of faulty imprinting and this could be manifested in the evolution of humans by an epigenetic route.

Dynamin-association with agonist-mediated sequestration of beta-adrenergic receptor in single-cell eukaryote Paramecium

Evidence that dynamin is associated with the sequestration of the Paramecium beta(2)-adrenergic receptor (betaAR) immunoanalogue is presented. We previously reported a dramatic change in the distribution of betaAR analogue in the subcellular fractions upon isoproterenol treatment: it is redistributed from the membraneous to the cytosolic fraction, as revealed by quantitative image analysis of western blots. Here we confirm and extend this observation by laser scanning confocal and immunogold electron microscopy. In the presence of isoproterenol (10 micro mol l(-1)) betaAR translocated from the cell surface into dynamin-positive vesicles in the cytoplasmic compartment, as observed by dual fluorochrome immunolabeling in a series of the confocal optical sections. Colocalization of betaAR and dynamin in the tiny endocytic vesicles was detected by further electron microscopic studies. Generally receptor sequestration follows its desensitization, which is initiated by receptor phosphorylation by G-protein-coupled receptor kinase. We cloned and sequenced the gene fragment of 407 nucleotides homologous to the beta-adrenergic receptor kinase (betaARK): its deduced amino acid sequence shows 51.6% homology in 126 amino acids that overlap with the human betaARK2 (GRK3), and may participate in Paramecium betaAR desensitization. These results suggest that the molecular machinery for the desensitization/sequestration of the receptor immunorelated to vertebrate betaAR exists in unicellular PARAMECIUM:

Opioid Activity of beta-Endorphin-like Proteins from Tetrahymena

Morphine and other opioids have been reported to modulate phagocytosis in the ciliate Tetrahymena. However, the endogenous signaling molecule responsible for these effects remains uncharacterized. In this work we present evidence for the presence of beta-endorphin-like protein(s) in Tetrahymena thermophila. Subcellular extracts and cell-free culture supernatants were fractionated by hydrophobic chromatography on Sep Pack C18 columns and by affinity chromatography on polyclonal anti-beta-endorphin columns. Both preparations exhibited opioid-like effects in two different systems: 1) they inhibited phagocytosis in murine peritoneal macrophages, and 2) they blocked the response to mechanical stimuli in the ciliate Stentor. Both of these effects were reversed by naloxone, consistent with an opioid receptor-mediated mechanism. Chromatographic (HPLC) fractionation of the subcellular extracts resolved a component with beta-endorphin-like immunoreactivity, whose retention time was similar to that of the human beta-endorphin standard. Fractions were also analyzed by immunoblots using a monoclonal antibody that recognizes the N-terminus of human beta-endorphin. This antibody detected two antigenic components (corresponding to Mr 9,000 and Mr 12,000 polypeptides) in subcellular extracts, but only a single antigen (corresponding to a Mr 7,000 polypeptide) in culture supernatants. These results indicate that Tetrahymena produces one or more proteins that share some properties with beta-endorphin and that these may form part of an opioid mechanism that originated early in evolution.

Chemotactic activity of oligopeptides containing an EWS motif on Tetrahymena pyriformis: the effect of amidation of the C-terminal residue

Chemotactic properties of 3-7-mer peptides containing an EWS motive and their peptide amides synthesized and characterized by us were investigated in Tetrahymena pyriformis GL model. Analysis of the peptide acids shows that SEWS possesses exceptionally strong (660%+/-21; 430%+/-18) chemoattractant ability at 10(-12) and 10(-11) m respectively. The shorter peptide (EWS) possesses chemorepellent activity, while longer peptides display neutral (WSEWS) or moderate chemoattractant (EWSEWS and GEWSEWS) chemotactic ability. Amidation of the C-terminus can significantly modify the character of peptides: it points to the conclusion that a free alpha-COOH group at this position is required for the high efficiency of SEWS, while in the shorter (EWS) and longer peptides (WSEWS and EWSEWS) amidation can result in chemoattractant ligands. Evaluation of the structure-function relationship of these compounds establishes the significance of Glu (E) with its high surface-exposed area and negatively-charged side chain. The high discriminative ability and good chemotactic responsiveness of Tetrahymena support the theory that a chemotactic signalling mechanism working in higher levels of phylogeny is a well conserved and inducible one even in protozoa.

Partial cloning of putative G-proteins modulating mechanotransduction in the ciliate stentor

Signal transduction systems known to utilize G-proteins in higher eukaryotes undoubtedly evolved prior to the development of metazoa. Pharmacological evidence indicates that the ciliates Paramecium, Stentor, and Tetrahymena all utilize signaling systems similar to those found in mammals. However, there has been relatively little direct evidence for the existence of G-proteins in ciliates. Since highly conserved heterotrimeric G-proteins form the basis of receptor-coupled signal transduction systems in a wide variety of metazoa, it is of interest to know if these important signaling molecules were early to evolve and are present and functionally important in a wide variety of unicellular organisms. We have previously shown that mechanotransduction in Stentor is modulated by opiates in a manner that may involve pertussis toxin-sensitive G-proteins. Here we utilize drugs known to interact with G-proteins to further test for the involvement of these important signaling molecules in Stentor mechanotransduction. We present behavioral and electrophysiological data demonstrating that putative G-proteins in Stentor decrease mechanical sensitivity by modulating the mechanotransduction process. In addition, we report the partial cloning of 4 G-protein alpha-subunits from Stentor. We confirm that these clones are of Stentor origin and are transcribed. Furthermore, we employ antisense oligodeoxynucleotide-mediated knockout to demonstrate that these ciliate G-proteins exert a modulatory influence on Stentor behavior, and that a G1/G0-like clone mediates the inhibitory action of beta-endorphin on mechanotransduction.

Insulin uptake, localization and production in previously insulin treated and untreated Tetrahymena. Data on the mechanism of hormonal imprinting

Confocal microscopic experiments demonstrate the presence of insulin in Tetrahymena, observed also in earlier experiments. However, there is a broad spectrum of insulin-containing cells from the immunocytochemically insulin-free, to the strongly antibody-reactive ones. During 1 h of insulin treatment (imprinting) the cells gradually bind and take up insulin, and the process is slow. One minute after the start of treatment there is not difference in the number of insulin antibody-reactive cells and amount of insulin. After 5 or 10 min the cells bind and contain more insulin and after 1 h most of the cells are densely packed with the insulin antibody-reactive material. Insulin imprinting accelerates binding and uptake alike: 48 h after imprinting and 1 min after the start of the second treatment, more insulin is present on the surface and inside the cells, than after 10 min in the first-time treated cells. Theoretically, this effect of hormonal imprinting helps to maintain the species by facilitating molecular recognition and binding as well as uptake of useful molecules. The experiments also support previous observations on the parallel receptor-evoking (strengthening) and hormone-producing effect of hormonal imprinting.

Chemotactic selection of Tetrahymena pyriformis GL induced with histamine, di-iodotyrosine or insulin

It has been hypothesized that in phylogeny the encounter between potential signalling molecules and the continously changing cell membrane could result in the formation of a ligand specific receptor. This chemical (hormonal) imprinting is then transmitted to the progeny generations. It is, however, very difficult to know whether the selection of cells with receptor-like patterns or amplification of complete receptor-like patterns led to the formation of the receptor-hormone complex. The new technique of 'chemotactic selection provides a physiological response-guided selection of cells. It also enables the testing of subpopulations with the characteristic selector ligand. We show here that of three chemotactic ligands (histamine, di-iodotyrosine (T2) and human insulin), insulin and T2 selected subpopulations express a significantly high chemotactic response. Since the control medium has a selector capacity itself, we introduced a chemotactic selection coefficient (Chsel) which facilitates the comparison of all groups. Using this factor we found that insulin (Chsel = 1.57), functions as a strong selector and T2 (Chsel = 0.98), was a weak selector. Morphometric evaluation of the cells showed a good correlation between chemotactic responsiveness and morphometric characteristics of subpopulations selected with insulin and histamine. T2 data suggest that the long lasting responsiveness is not general, but might be subpopulation specific.

Prolonged elevation of insulin content in the unicellular tetrahymena after insulin treatment: induction of insulin production or storage?

In the unicellular organism, Tetrahymena, the first encounter with an exogeneously given hormone results in hormonal imprinting. This causes an increase of the binding capacity of receptors and the production of the appropriate hormone in the progeny generations of the treated cell. In the present experiments the quantity (using radioimmunoassay) and localization (using confocal laser scanning microscopy) of the immunologically insulin-like material (hereafter insulin) were studied for 10 days after 4 h or 24 h 10(-6) M insulin treatment (hormonal imprinting). Forty-eight hours after both insulin treatments a high quantity of insulin was present in the cells. This value was also significantly increased after 96 h. After 8 days the difference to the control was significant only in the 24 h treated group. Confocal microscopy (using antibody to pig insulin) localized insulin in the cell body. The oral field contained extremely high quantities of the endogeneous hormone. Insulin treatment (after 48 and 96 h) caused an elevation of insulin content in general, and specific accumulation in the posterior sections of the cell, around the nucleus and in the periphery were observed. Ten days after both treatments only the peripheral region of the cell body and the ciliary row contained more insulin than the control. This means that after insulin treatment the quantity of insulin increases for a lengthy time period which is followed by the expression of insulin in the peripheral region. Insulin contained by Tetrahymena 48 h after imprinting stimulated glucose uptake of rat diaphragm.

Enzymes related to catecholamine biosynthesis in Tetrahymena pyriformis. Presence of GTP cyclohydrolase I

We first identified GTP cyclohydrolase I activity (EC 3.5.4.16) in the ciliated protozoa, Tetrahymena pyriformis. The Vmax value of the enzyme in the cellular extract of T. pyriformis was 255 pmol mg-1 protein h-1. Michaelis-Menten kinetics indicated a positive cooperative binding of GTP to the enzyme. The GTP concentration producing half-maximal velocity was 0.8 mM. By high-performance liquid chromatography (HPLC) with fluorescence detection, a major peak corresponding to D-monapterin (2-amino-4-hydroxy-6-[(1'R,2'R)-1',2',3'-trihydroxypropyl]pteridin e, D-threo-neopterin) and minor peaks of D-erythro-neopterin and L-erythro-biopterin were found to be present in the cellular extract of Tetrahymena. Thus, it is strongly suggested that Tetrahymena converts GTP into unconjugated pteridine derivatives. In this study, dopamine was detected as the major catecholamine, while neither epinephrine nor norepinephrine was identified. Indeed, this protozoa was shown to possess the activity of a dopamine synthesizing enzyme, aromatic L-amino acid decarboxylase. On the other hand, activities of tyrosine hydroxylase or tyrosinase which converts tyrosine into dopa, the substrate of aromatic L-amino acid decarboxylase, could not be detected in this protozoa. Furthermore, neither dopamine beta-hydroxylase activity nor phenylethanolamine N-methyltransferase activity could be identified by the HPLC methods.

Thermoregulatory changes by hypoxia: lessons from the paramecium

1. In organisms ranging from paramecia to mammals, hypoxia elicits a regulated decrease in body temperature (Tb). A decrease in Tb is an important adaptation to hypoxia primarily because it lowers metabolic rate when oxygen supply is limited, thus facilitating survival. 2. Although this beneficial response is extremely widespread among taxa, little is known of the cellular mechanisms that mediate hypoxia-induced decreases in Tb. This is due, in large part, to the extreme complexity of vertebrate thermoregulatory systems. 3. The thermoregulatory system of the unicellular paramecium is much simpler than that of vertebrates, yet it responds similarly to hypoxia. Research has explored the functional importance of hypoxia-induced decreases in Tb. In addition, a number of possible mediators and signalling pathways in hypoxia-induced reductions in Tb have been assessed. 4. In Paramecium caudatum, hypoxia appears to exert its thermoregulatory effects by inhibiting oxidative phosphorylation. Decreases in intracellular [ATP] and pH may be important intermediate signals. In addition, an endogenous opioid system appears to help mediate hypoxia-induced changes in thermoregulatory behaviour.

Signaling in unicellular eukaryotes

Aspects of intercellular and intracellular signaling systems in cell survival, proliferation, differentiation, chemosensory behavior, and programmed cell death in free-living unicellular eukaryotes have been reviewed. Comparisons have been made with both bacteria and metazoa. The central organisms were flagellates (Trypanosoma, Leishmania, and Crithidia), slime molds (Dictyostelium), yeast cells (Saccharomyces cerevisiae), and ciliates (Paramecium, Euplotes, and Tetrahymena). There are two novel aspects in this review. First, cellular responses are viewed in an evolutionary perspective, rather than from the more prevailing one, in which the unicellular eukaryotes are seen by the mammalian organisms. Second, results obtained with cell cultures in minimal, chemically defined nutrient media at low cell densities where intercellular signaling is strongly reduced are discussed. These results shed light on control mechanisms and their cooperation inside the living cell. Intracellular systems have many common features in unicellular and multicellular organisms.

Opioid receptor types for endogenous enkephalin in the thoracic ganglion of the crab, Carcinus maenas

In crustaceans, the endogenous opioid peptides, enkephalins, are known to be concentrated in the thoracic ganglion, although they have been demonstrated in all parts of the nervous system. Bmax and Kd measurements have been obtained for the binding of ligands used to characterize delta- and kappa-type opioid receptors in vertebrates. High- and low affinity binding of [3H] [2-D-Pen5-D Pen] enkephalin ([3H]DPDPE) has been measured with a Kd = 9.2 +/- 2.4 nM, Bmax = 153 fmol/mg, and Kd = 243 +/- 27 nM, Bmax = 1.785 pmol/mg, respectively. In addition a kappa-type receptor with Kd 85.5 +/- 12.6 nM and Bmax = 21.138 pmol/mg protein has been recorded. Binding characteristics of several ligands were monitored. Electrophoretic studies of affinity chromatographically purified receptor fractions revealed a molecular mass of 60 kDa. Isoelectric focusing showed a specific binding of [3H]DPDPE to thoracic ganglion membranes at a pl of 5.5.

A novel opioid mechanism seems to modulate phagocytosis in Tetrahymena

We have previously reported that a beta-endorphin-like substance inhibits phagocytosis in Tetrahymena perhaps by a mu-like opioid receptor. We now report a further characterization of the elements involved in the signal transduction mechanism of this opioid. Affinity chromatography followed by immunoblots of both intracellular extracts and extracellular medium reveal the presence of two main proteins of 64 and 75 kDa. These molecular weights are much higher than that of any known opioid peptide or precursor protein and suggest that we may be dealing with either a novel opioid or with proteins that by chance cross-react with anti-beta-endorphin antibody. Nevertheless, when the biological activity of these proteins was tested it was found that they had an effect similar to that of mammalian beta-endorphin, namely inhibition of phagocytosis by a naloxone-reversible mechanism. We have probed a size-selected Tetrahymena library with a pro-opiomelanocortin probe and have obtained several positive clones; the sequencing of their inserts should establish whether we are dealing with a bona fide member of the opioid family. Another aspect we have been studying is the G-proteins which appear to be involved in the modulation of phagocytosis. We have found, by means of Western blotting (using an antibody against the conserved GTP-binding region of the alpha-subunit), two bands of 51 and 59 kDa; no alpha-subunit of 59 kDa had been reported previously and may represent a novel G-protein. In spite of these differences, the opioid signal transduction mechanism appears to remarkably resemble that present in more complex organisms.

Method for determination of chemoattraction in Tetrahymena pyriformis

Earlier works presented a variety of techniques for detection of chemoattractants in ciliates. The present agar layer model and the used double-P cutter give an easy, reproducible way of quantitative evaluation of positive chemotaxis. Beside the two-channel chamber, the multi-channel one seems to be also useful in characterization of different taxons or different chemoattractants.

Immunocytochemical verification of the insulin receptor's specificity in the nuclear envelope of Tetrahymena. Comparison with receptors of the plasma membrane

The unicellular Tetrahymena possess hormone receptors in the nuclear envelope similarly to higher rank animals. These receptors bind insulin and their specificity is detectable by monoclonal antibodies developed to insulin. The hormonal (insulin) pretreatment (imprinting) of the cell did not alter the binding capacity of the nuclear membrane, demonstrated by antibody-technique. The specific binding characteristics of the plasma membrane was demonstrated and this was significantly increased following imprinting. In the nucleus of Tetrahymena presence of insulin was not detected by immunocytochemical method.

The opiate system in invertebrates

The presence in diverse species of a similar mode of communication, that of a soluble messenger binding to a receptor, raises the question as to whether the specific components of this system are equally widespread. Do invertebrates use the same hormones and receptors as vertebrates do? Invertebrates ranging from unicellular organisms to insects have been shown to contain opiate-like peptides and binding sites, and they exhibit biological responses to opiates. However, critical genetic data are lacking. It is not known how signal systems arise phylogenetically, but it is conceivable that signal molecules that are already present cause the formation of their own receptors from membrane proteins.

Pharmacological characterization of an opioid receptor in the ciliate Tetrahymena

A pharmacological characterization has been performed of the opioid receptor involved in modulation of phagocytosis in the protozoan ciliate Tetrahymena. Studies on inhibition of phagocytosis by mammalian prototypic opioid agonists revealed that morphine and beta-endorphin have the highest intrinsic activity, whereas all the other opioids tested can only be considered partial agonists. However, morphine (a mu-receptor agonist) is twice as potent as beta-endorphin (a delta-receptor agonist). Furthermore, the sensitivity for the opioid antagonist naloxone, determined in the presence of morphine and beta-endorphin, is very similar to the sensitivity exhibited by mammalian tissues rich in mu-opioid receptors. We suggest that the opioid receptor coupled to phagocytosis in Tetrahymena is mu-like in some of its pharmacological characteristics and may serve as a model system for studies on opioid receptor function and evolution.

The E. coli envY gene encodes a high affinity opioid binding site

In an attempt to isolate a cDNA encoding an opioid receptor, a cDNA library was constructed in the lambda ZAP vector using NG108-15 mRNA as template. Using an in vitro transcription-translation assay and a sib selection strategy, a single phage was isolated. An RNA transcribed from this cDNA was able to direct in vitro translation of opioid binding sites. The insert was sequenced and comparison with data banks showed a 100% homology with the E. coli envY gene. We assume that the presence of the envY sequence in the NG108-15 cDNA library was due to a contamination of the lambda ZAP vector with E. coli DNA. A search for opioid binding sites on E. coli strains showed that envY+ strains, but not envY- mutants were able to bind opiates. On envY+ cells, the sites are stereospecific, saturable and of high affinity for the opiate ligands. These sites bind opiate agonists and antagonists but neither mu nor delta opioid peptides. In contrast, rabbit reticulocyte lysate primed with RNA transcribed in vitro from the envY sequence elicited the synthesis of an opioid binding site with mixed mu and delta properties. In addition, transfection of the envY sequence into mammalian cells resulted in the expression of opioid binding sites. Depending on the type of cells transfected, these sites were selective for either the mu or delta ligands.

The binding of diazepam in the mitochondria of Tetrahymena pyriformis as detected by quantitative high resolution autoradiography

Tritiated diazepam accumulates mainly in the mitochondria of the unicellular Tetrahymena. This is the case in both a single (the first encounter) and a repeated (one day or a week after the first) administration of the drug. When imprinting of Tetrahymena by diazepam (the first encounter) is followed a week later by the administration of the labelled drug, the membranes of the vesicles, too, show the appearance of label. Regarding the studies presented here, the unicellular Tetrahymena also contain diazepam receptors in the mitochondria as suggested for cells of higher rank animals.

Cortical ultrastructure and chemoreception in ciliated protists (Ciliophora)

The ciliated protists (ciliates) offer a unique opportunity to explore the relationship between chemoreception and cell structure. Ciliates resemble chemosensory neurons in their responses to stimuli and presence of cilia. Ciliates have highly patterned surfaces that should permit precise localization of chemoreceptors in relation to effector organelles. Furthermore, ciliates are easy to grow and to manipulate genetically; they can also be readily studied biochemically and by electrophysiological techniques. This review contains a comparative description of the ultrastructural features of the ciliate cell surface relevant to chemoreception, examines the structural features of putative chemoreceptive cilia, and provides a summary of the electron microscopic information available so far bearing on chemoreceptive aspects of swimming, feeding, excretion, endocytosis, and sexual responses of ciliates. The electron microscopic identification and localization of specific chemoreceptive macromolecules and organelles at the molecular level have not yet been achieved in ciliates. These await the development of specific probes for chemoreceptor and transduction macromolecules. Nevertheless, the electron microscope has provided a wealth of information about the surface features of ciliates where chemoreception is believed to take place. Such morphological information will prove essential to a complete understanding of reception and transduction at the molecular level. In the ciliates, major questions to be answered relate to the apportionment of chemoreceptive functions between the cilia and cell soma, the global distribution of receptors in relation to the anterior-posterior, dorsal-ventral, and left-right axes of the cell, and the relationship of receptors to ultrastructural components of the cell coat, cell membrane, and cytoskeleton.

An opioid growth factor regulates the replication of microorganisms

An opioid growth factor (OGF), [Met5]-enkephalin, interacts with the zeta (zeta) opioid receptor to modulate development of eukaryotes. We have found that [Met5]-enkephalin, an endogenous opioid peptide serves to inhibit the growth of S. aureus. This effect on growth involves cell proliferative events and is under tonic control, since potent opioid antagonists accelerate cell replication. Both the OGF and zeta opioid receptor were associated with these microorganisms. Other opioid receptors (mu, delta and kappa) were not detected. OGF also controlled the growth of other bacteria: P. aeruginosa and S. marcesans. These results indicate that OGF and its receptor, known to be important in the regulation of mammalian development, also function in the growth of simple unicellular organisms. We suggest that the endogenous opioid system related to growth originated billions of years ago.

Direct membrane effects of morphine and endorphins on Amoeba proteus

Morphine, leu-enkephalinamide, met-enkephalin, alpha-neoendorphin and its Arg8 1-8 fragment increase contractile vacuole output in the freshwater Amoeba proteus at 18 microM. Significant effects of leu-enkephalin and naloxone are obtained at 180 microM. All compounds have reached their maximal activity at 720 microM. Alpha-neoendorphin and leu-enkephalin are inactive in the presence of isotonic, non-penetration sucrose, hence these compounds increase plasma membrane permeability to water. Results from molecular modeling show a clear correlation of activity with amphiphilicity, charge distribution and general flexibility of molecules. We conclude that, like previously-studied vasopressin analogues and non-hormonal amphiphilic peptides, active opioids embed themselves into the Amoeba plasma membrane, disrupting the lipid bilayer and increasing its permeability. In our Amoeba system, naloxone, a general morphine-like inhibitor, blocks active opioids as well as a vasopressin analogue. Naloxone, being less active than other tested amphiphiles, acts as a membrane stabilizer, protecting the lipid bilayer against the disruption action of more active compounds.

Chemosensory behaviour of Tetrahymena

Free swimming cells of the ciliated protozoan Tetrahymena are attracted to certain chemicals by chemokinesis. However, a special type of chemotaxis in response to a chemical gradient is found in cells gliding very slowly in semisolid media. In contrast to classical chemotaxis by leukocytes, which is solely positive towards chemo-attractants, the oriented chemokinesis by gliding Tetrahymena involves both positive and negative elements. The major chemo-attractants are peptides and/or proteins, and they may be compounds which signal the presence of food in the natural environment of this freshwater phagotroph.

Effect of chronic opioid treatment on phagocytosis in Tetrahymena

Opioid inhibition of phagocytosis in the protozoan ciliate Tetrahymena is antagonized by naloxone and this antagonism can be surmounted by increasing agonist concentration, which suggests a receptor-mediated mechanism. Desensitization of the opioid effect is time dependent in addition to concentration dependent. Chronic exposure to opioids results in the development of tolerance to the inhibitory effect of the agonists, and withdrawal of the latter results in a decrease in phagocytic capacity, which suggests that a state akin to dependence has been developed in these cells. Naloxone appears to behave as a partial agonist in tolerant cells, and there seems to exist cross-tolerance to mu and delta agonists.

Day-night rhythms in the inhibitory effects of 60 Hz magnetic fields on opiate-mediated 'analgesic' behaviors of the land snail, Cepaea nemoralis

There is accumulating evidence that magnetic fields can affect a variety of opioid-mediated behavioral and physiological functions. The present experiments were designed to examine the effects of various durations of day- and night-time exposures to low intensity (1.0 gauss rms) 60 Hz magnetic fields on light (L) and dark (D) period opioid-mediated aversive thermal ('nociceptive') responses and morphine-induced 'analgesia' in the nocturnally-crepuscularly active land snail, Cepaea nemoralis. The snails displayed a LD rhythm in the latency of their aversive (40 degrees C) thermal (nociceptive) responses, showing a significantly greater response latency at night than during the day. Administration of morphine (10 mg/kg/2.0 microliters) elicited significant increases in the thermal response latencies indicative of the induction of analgesia. The snails displayed a significantly greater analgesic response at night than during the day. Exposure (0.50, 2, 12, 48 or 120 h in L or D) to the 60 Hz fields reduced morphine-induced analgesia in both the L and D periods, with the magnetic stimuli having significantly greater inhibitory effects in the D period. The magnetic fields also significantly attenuated the level of the dark period basal nociceptive response latencies, while not affecting the light period responses. In both the L and D periods the degree of attenuation of the analgesic and nociceptive response latencies was related to the duration of exposure to the 60 Hz magnetic fields.(ABSTRACT TRUNCATED AT 250 WORDS)

The opiate receptor: a single 110 kDa recognition molecule appears to be conserved in Tetrahymena, leech, and rat

We compared the molecular nature of the rat brain opiate receptor with that of the invertebrate leech, Haemopis marmorata, and the protozoan, Tetrahymena, in order to examine the issue of apparent receptor heterogeneity with respect to biochemical structure. A binding study with rat brain membrane verified that [125I]beta-endorphin [( 125I]beta E), a broad specificity ligand, is displaced by the antagonist (-)-naloxone, but not the inactive stereoisomer (+)-naloxone; agonists considered prototypes for mu, delta, and kappa opiate receptors all displayed stereospecific binding displacement. For SDS-PAGE analysis of the opiate receptor [125I]beta-endorphin was covalently affixed to its recognition molecule with the cross-linking reagent DSS. Primary reaction products occur at 110, 58/55, and 29 kDa. Cross-linking products of all 3 molecular weights are effectively reversed by opiate ligands, regardless of their mu, delta, or kappa specificities. Peptide mapping studies in SDS gels, using limited proteolysis, showed that the 110 kDa band can be digested into 58 and 29 kDa fragments and the 58 kDa band into a 29 kDa fragment. Additional smaller molecular weight fragments were generated from the 110, 58/55, and 29 kDa bands which shared their molecular weights. Two possible explanations for the extensive sequence homology between the three major cross-linking products are: (1) the 110 kDa species is the opiate receptor, and the 58 and 29 kDa species are proteolytic fragments; and (2) one of the lower molecular weight species is the opiate receptor, and adjacent receptors are aggregated into the 110 kDa complex through cross-linking.(ABSTRACT TRUNCATED AT 250 WORDS)