Functions of Neurotransmitters on Different Evolutionary Steps

The function of compounds named neurotransmitters originates from simple chemot-axis and chemosignaling of microbial cells and leads to intercellular communication (Fig. 2.1). The so-called neurotransmitters may regulate (as hormones) growth and development of other unicellular organisms, and be attractants or repellents for them. In higher concentrations the same substances also play a defense role (for saving or

Fig. 2.1 The scheme of the evolution in the neurotransmitter (biomediator) function

aggression) or, in some cases, serve as an origin of cultural food. The following step in evolution includes the development of colonial relations (parasitar or symbiotic) and then the formation of multicellular organisms that forms more specialized function of biomediators in the irritation transfer along the multicellular system. This evolution way leads us to the concept of neurology, not only for animals, but also for plants (Baluska et al. 2005; Brenner et al. 2006; Murch 2006).

In Table 2.3, we compare the main functions of the neurotransmitters in all kingdoms. The realization of the irritation impulse transfer into a cell from the surface or between compartments of the cell occurs with the participation of neu-rotransmitters, and on cellular level the compounds may induce different reactions. Neurotransmitters are stored in secretory vesicles, and then can be liberated within cell or out. Primary reaction to acetylcholine is often a change in membrane permeability for ions, while other reactions for both the neurotransmitter and biogenic amines are connected with the systems of secondary messengers - cyclic nucleotides, Ca2+, inositol-3-phosphate, etc.

First of all, the neurotransmitter functioning as chemosignal (the neurotransmitter is released from one cell and percepted by another) occurs in certain structural forms - the chink forming between cells or between organelles within cell. We can usually see the chink between plasmic membranes of any contacted cells such as

Table 2.3 The established functions of neurotransmitters in living organisms

Neurotransmitter

Microorganisms

Plants

Animals

Acetylcholine

Regulation of motility

Dopamine

Stimulation of gram negative and gram positive bacterial growth and virulence

Norepinephrine (adrenaline)

Bacterial growth stimulation

Epinephrine (adrenaline) Bacterial growth stimulation

Serotonin

Regulation of membrane permeability and other cellular reactions up to growth and development in many plant species

Regulation of many cellular processes from growth and development to defense reactions

Stimulation of growth of culture and cellular aggregation bacteria Streptococcus faecalis, yeast Candida guillermondii, E. coli K-12 and Rhodospirillum rubrum. Regulation of membrane potential

Regulation of many cellular processes from growth and development to defense reactions

Regulation of many cellular processes from ion permeability, growth and development to defense reactions Regulation of growth and development of many plant cells

Regulation of cell proliferation, growth and morphogenesis. The carriage of nerve impulses across the synaptic chink, from one neuron to another of impulses across the "motor plate," from a neuron to a muscle cell, where it generates muscle contractions Decreases peripheral vascular resistance, increases pulse pressure and mean arterial pressure. The positive chronotropic effect produces a small increase in heart rate as well. Important for forming memories. In embryos of Vertebrata and lower animals may regulate development Increases peripheral vascular resistance, pulse pressure and mean arterial pressure as well as stimulates of the thrombocytes' aggregation Induced vasodilation (mainly in skeletal muscle) and vasoconstriction (especially skin and viscera) Control of appetite, sleep, memory and learning, temperature regulation, mood, behavior (including sexual and hallucinogenic), vascular function, muscle contraction, endocrine regulation, and depression. In embryos of Vertebrata and lower animals may regulate development

Histamine Stimulation of cultural growth and Regulation of the growth and Involves in many allergic reactions and cellular aggregation of E. coli K-12 development at stress increases permeability of capillaries, arterial pressure is decreased, but increases intracranial pressure that causes headache, smooth musculature of lungs is reduced, causing suffocation, causes the expansion of vessels and the reddening of the skin, the swelling of clothStimulation of the secretion of gastric juice, saliva (digestive hormone)

Sources: (Anuchin et al. 2007, 2008; Buznikov 1967, 1987, 1990 Faust and Doetsch 1971; Burton et al. 2002; Freestone et al. 2007; Lyte and Ernst 1992, 1993;

Lyte et al. 1997; Oleskin et al. 1998a, b; Oleskin 2007; Roshchina 1991, 2001a; Strakhovskaya et al. 1993)

Chink as a base for contacts between organelles within cell between cells

Fig. 2.2 Structure where the neurotransmitters action is possible the membranes of unicellular contacting organisms and synaptic membranes of cells in organisms with a nervous system (Roshchina 1991, 2001a, b; Buznikov et al. 1996; Shmukler et al. 2007). As seen in the scheme presented in Fig. 2.2, at any membrane contacts, either time-changed or constant, chinks may be formed. There are chinks between endoplasmic reticulum and organelles within cells, or between different cells. Today, constant or temporary chinks between cells or within the cell are considered a necessary structural form for the chemosignal transfer.

As can be seen from Table 2.3, common cellular effects of neurotransmitters in any type of living kingdom cell are the changes in membrane permeability (short-time effects) and the regulation of growth and development (long-time effects). Regulatory function of the neurotransmitters appears to be an ancient function, relating processes occurring both within a cell and the environmental unicellular populations. The secretion that contains neurotransmitters is released out of any cell and may contact with other cells at chemotaxis. The cell, which receives similar chemosignals, responds primarily by the changes in ion permeability and the formation of action potential, and then with various metabolic and growth reactions. Some other aspects and details will be described below.

2.3.1.1 Functions in Microorganisms

The first report, showing acetylcholine production in bacterium strains, was from L. plantarum and. L. odontolyticus (Stephenson and Rowatt 1947). Approximately 5 jmg acetylcholine/mg dry wt. cells/h was formed if the bacteria were grown both in vegetable juice and washed cells. Acetylcholine can be also used as substrate for microorganisms, and regulates their development in special conditions (Imshenetskii et al. 1974). Its regulation of motility peculiar to photosynthesing bacteria Rhodospirillum rubrum and Thiospirillum jenenese has also been shown (Faust and Doetsch 1971).

Catecholamines can regulate the growth of Gram-negative bacteria, including E. coli (where concentration dependent specificity was observed with response to norepinephrine ยป epinephrine > dopamine), Y. enterocolitica and P.s aeruginosa (Lyte and Ernst 1992; Freestone et al. 1999). Dopamine also stimulates the cultural growth of E. coli, Y. enterocolica, S. enterica, S. epidermidis, etc., and the cellular aggregation and formation of colonies of E. coli and S. epidermidis (Lyte and Ernst

1993; Neal et al. 2001; Freestone et al. 2007; Anuchin et al. 2007, 2008). Similar effects on Gram-negative bacteria E. coli, S. enterica and Y. enterocolitica were observed for norepinephrine (Lyte and Ernst 1992, 1993; Lyte et al. 1997; Freestone et al. 1999, 2007; Burton et al. 2002) and on E .coli for epinephrine (Anuchin et al. 2007; Freestone et al. 2007). Serotonin stimulated cultural growth and cellular aggregation of bacterial species, including Streptococcus faecalis, the yeast Candida guillermondii, (Strakhovskaya et al. 1993), E. coli K-12 and Rhodospirillum rubrum at concentrations of 2 x 10-7-2 x 10-5 M (Oleskin et al. 1998a, b; Anuchin et al. 2007, 2008). Moreover, histamine showed similar effects on E. coli (Anuchin et al. 2007, 2008). Serotonin at 10-6-10-5 M concentrations also inhibited light-dependent membrane potential generation in Rsp. rubrum, but in the myxobacterium Polyangium sp. serotonin stimulates cell aggregation and myxospore formation (Oleskin et al. 1998a, b). At concentrations near 20 |M, serotonin inhibits cell aggregation and microbial culture growth and photo-dependent membrane potential of the bacterium Rsp. rubrum. At micromolar amounts, the effects presumably result from the specific action of serotonin as an intercellular communication agent accelerating and possibly synchronizing the development of the microbial cell population. According to Oleskin et al. (1998a, b), the growth stimulation of microorganisms by serotonin over a millimolar to micromolar range has been demonstrated in prokaryotes, both Gram-positive including Streptococcus faecalis (Strakhovskaya et al. 1993) and Gram-negative bacteria including E. coli and Rsp. rubrum (Oleskin et al. 1998a, b). In some cases, such as that for Bacillus brevis, the degree of growth stimulation achieves 100% of control. Freestone et al. (2008a, b) showed that cat-echolamine stress hormones can significantly increase the growth of a wide range of gram negative and gram positive bacteria. Using a novel two-fluorophore chemotaxis assay, it was found that E. coli is attracted to epinephrine and norepi-nephrine (and also increased the bacterial motility and biofilm formation), while it is repelled by indole (Bansal et al. 2007). Moreover, epinephrine/norepineph-rine upregulated the expression of genes involved in surface colonization and virulence, while exposure to indole decreased their expression (Bansal et al. 2007). Histamine synthesis by respiratory tract microorganisms: was also observed, and its possible role in pathogenicity considered (Devalia et al. 1989).

Much attention has also been given to the role of colonial organization and intercellular communication in parasite/commensal/symbiont-multicellular host organism systems. Data from the literature on the ability of microorganisms to form plant hormones (biogenic amines) have been reviewed by Tsavkelova et al. (2006), who discuss the Rhodospirillum rubrum pathways whereby the biogenic amines are metabolized, and their effects on the development and activity (physiological and biochemical) of the microorganisms are considered. The role as hormones and hormone-like substances is in the formation of association-type (microorganismhost) interactions. The review by Oleskin et al. (2000) suggested that the integrity and coherence of microbial populations (colonies, biofilms, etc.) be viewed as peculiar so-called "super-organisms," which are thought to have become multicel-lular organisms during the course of evolution. This included such relevant phenomena as apoptosis, bacterial altruism, quorum effects, collective differentiation of microbial cells, and the formation of population-level structures such as an extracellular matrix. Emphasis can also be placed on the channels in colonies and agents of intercellular communication in microbial populations. The involvement of a large number of evolutionarily conserved communicational facilities and patterns of intercellular interactions can therefore be underscored. Moreover, an interesting fact is the 5-hydroxytryptophan conversion to serotonin under UV-irradiation (Fraikin et al. 1989). This neurotransmitter may serve as a protector for microorganisms in similar unfavorable conditions. For example, dinoflagellates (a large group of flagellate protists contained in marine plankton) and green algae Gonyaulax polyedra synthesize the protector melatonin, using serotonin as a precursor (Balzer et al. 1993). Circadian rhythms of indoleamines in the dinoflagellate Gonyaulax polyedra and persistence of melatonin rhythm in constant darkness, have a relationship to 5-methoxytryptamine.

2.3.1.2 Function in Plants

In plants, neurotransmitters demonstrate a high biological activity, playing a role as chemosignals, regulators of membrane permeability, growth and development regulators, etc. (Roshchina (1991, 2001a). Some examples will be considered below.

A signaling role of acetylcholine is well seen as the participation in plant root-shoot signal transduction (Wang et al. 2003b; Baluska et al. 2004, 2005; Brenner et al. 2006). Acetylcholine causes rooting in leaf explants of in vitro raised tomato (Lycopersicon esculentum Miller) seedlings (Bamel et al. 2007). Contractile effects of acetylcholine connected with membrane ion permeability were also observed in the regulation of the stomata function - the opening and closing movement in plants such as Vicia faba and Pisum sativum (Wang et al. 1998, 1999a, 2000). It was established that muscarinic and nicotinic acetylcholine receptors are involved in the event. A regulatory role for acetylcholine and its antagonists in inward rectified K+ channels from guard cells protoplasts from leaf stomata of Vicia faba was found (Leng et al. 2000). Ca2+ and Ca-related systems were found to participate in acetyl-choline-regulated signal transduction during stomata opening and closing (Wang et al. 2003a; Meng et al. 2004). A chloride channel in the tonoplast (vacuolar membrane) of Chara corallina also responds to acetylcholine (Gong and Bisson 2002). Electric processes participate in the electrical signaling, memory and rapid closure of the carnivorous plant Dionaea muscipula Ellis (Venus flytrap), and acetylcholine is thought to include in the phenomenon (Volkov et al. 2009).

Acetylcholine and cholinergic system play essential roles in plant fertilization and breeding. For example, lower activities of acetylcholinesterase and choline acetyltrans-ferase in pistils (Tezuka et al. 2007) or in pollen (Kovaleva and Roshchina 1997) were associated with self-incompatibility. A role for acetylcholine can be proposed as dealt with phytochrome and photoreceptor in the growth regulation as well. Wisniewska and Tretyn 2003). There is a connection between some fungal infections (in particular for the Fusarium fungi) and the accumulation of plant growth regulators, gibberellic acid, and auxins. Acetylcholine and antibody against acetylcholinesterase may inhibit biosynthesis of gibberellic acid, one of the main growth hormones (Beri and Gupta 2007). The enzyme may also be included in choline-auxin relations that affected plant growth processes. Direct evidence for the hydrolysis of choline-auxin or indole acetylcholine conjugates by pea cholinesterase has been demonstrated by some authors (Ballal et al. 1993; Bozso et al. 1995; Fluck et al. 2000).

A defense function for catecholamines in the plant cell has also been considered in the literature (Roshchina 1991, 2001a; Szopa et al. 2001; Kulma and Szopa 2007). Increased dopamine content in some algae, in particular Ulvaria obscura, has led to the consideration of the neurotransmitter as a feeding deterrent (van Alstyne et al. 2006). This is a novel ecological role for a catecholamine. The confirmation of dopamine production acting as defense mechanism against grazers was done from experiments with isopods, snails, and sea urchin eating the agar-based foods contained exogenous dopamine. Damaged algae were also found to release a water-soluble reddish-black substance (dopachrome) that inhibits the development of brown algal embryos, reduced the rates of macroalgal and epiphyte growth and caused increase mortality in oyster larvae (Nelson et al. 2003). Further, serotonin itself (Roshchina 2001a) and its derivatives, such as melatonin (Posmyk and Janas 2009), may also play a protectory role as antioxidants in various plants.

2.3.1.3 Functions in Animals

Currently, we have information about cellular functions for all animal organisms, including those which lack a nervous system and specialized functions peculiar to multicellular organisms with nervous system. First are related to the growth (similar with microbial and plant systems) and morphogenetic reactions. According to modern concepts, acetylcholine and serotonin may play a morphogenetic role in animals - from lower to higher ones (Buznikov 1990; Buznikov et al. 1996; Lauder and Schambra 1999).

The specialized function of neurochemical compounds concerned with the transmission of signals from one neuron to the next across synapses has been considered almost exclusively for neuronal systems as described in classical animal physiology. Neurotransmitters are also found at the axon endings of motor neurons, where they stimulate the muscle fibers to contract. The first of the neurotransmitters to be studied, acetylcholine, transfers nerve impulses from one neuron to another, where it propagates nerve impulses in the receiving neuron, or from a neuron to a muscle cell, where it generates muscle contractions. Moreover, genetic defects of acetylcholine signaling promote protein degradation in muscle cells (Szewczyk et al. 2000). It is obviously important to have proper nervous system and muscle functioning. In the adult nervous system, neurotransmitters mediate cellular communication within neuronal circuits. In developing tissues and primitive organisms, neurotransmitters subserve growth regulatory and morphogenetic functions as regulators of embryogenesis (Buznikov 2007). They regulate growth, differentiation, and plasticity of developing central nervous system neurons. Cellular effects of acetylcholine in animals may also be related to pathogenesis of diseases such as acute and chronic inflammation, local and systemic infection, dementia, atherosclerosis, and finally cancer (Wessler et al. 2001).

2.3.1.4 Possible Evolution of Neurotransmitter Reception

Since neurotransmitters are found in all living organisms - from unicellular to multicellular ones, Christophersen (1991) has described their possible evolution in terms of the molecular structure of neurotransmitters and adaptive variance in their metabolism, like that known for hormone receptors (Csaba 1980). The similarity of domains in signal receptors (Berman et al. 1991) was seen to compare with the physicochemical properties of signal receptor domains as the basis for sequence comparison. Christophersen (1991) advanced the hypothesis that all metabolites, even minor ones, are expressed as a result of stimuli and are directed against or support actions of receptor-based systems that reflect the evolution of receptors. For example, there is a similarity in some domains of rhodopsin, bacteriorhodop-sin, and neurotransmitter receptors (Pertseva 1989, 1990a, b; Fryxell and Meyerowitz 1991). Recently, transgenic technique has permitted the expression of the human dopamine receptor in the potato Solanum tuberosum (Skirycz et al. 2005). A blockade of catecholamine-induced growth by adrenergic and dopamin-ergic receptor antagonists has been also observed for E. coli O157:H7, S. enterica and Y. enterocolitica (Freestone et al. 2007) The similarity and universality of basic endocrine mechanisms of the living world are shown in the examples of the development of receptor-based mechanisms of protozoa and invertebrates (Csaba and Muller 1996). First of all, there are conservative parts or domains in modern cholino- or aminoreceptor, which are also found in prokaryotes and had no changed in the evolution (Pertseva 1989, 1990a, b). Homology of some bacterial proteins (from Mycobacterium smegmatis, Corynebacterium glutamicum, and Halobacterium salinarum) to mammalian neurotransmitter transporters (for example vesicular monoamine transporter) was observed as well (Vardy et al. 2005). Today, molecular evolution of the nicotinic acetylcholine receptor has also been confirmed by the multigene family in excitable cells of highly organized animals (Le Novere and Changeux 1995).

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