Endocrine interactions between plants and animals: Implications of exogenous hormone sources for the evolution of hormone signaling

Shared regulatory function of non-genomic thyroid hormone signaling in echinoderm skeletogenesis

Thyroid hormones are crucial regulators of metamorphosis and development in bilaterians, particularly in chordate deuterostomes. Recent evidence suggests a role for thyroid hormone signaling, principally via 3,5,3',5'-Tetraiodo-L-thyronine (T4), in the regulation of metamorphosis, programmed cell death and skeletogenesis in echinoids (sea urchins and sand dollars) and sea stars. Here, we test whether TH signaling in skeletogenesis is a shared trait of Echinozoa (Echinoida and Holothouroida) and Asterozoa (Ophiourida and Asteroida). We demonstrate dramatic acceleration of skeletogenesis after TH treatment in three classes of echinoderms: sea urchins, sea stars, and brittle stars (echinoids, asteroids, and ophiuroids). Fluorescently labeled thyroid hormone analogues reveal thyroid hormone binding to cells proximal to regions of skeletogenesis in the gut and juvenile rudiment. We also identify, for the first time, a potential source of thyroxine during gastrulation in sea urchin embryos. Thyroxine-positive cells are present in tip of the archenteron. In addition, we detect thyroid hormone binding to the cell membrane and nucleus during metamorphic development in echinoderms. Immunohistochemistry of phosphorylated MAPK in the presence and absence of TH-binding inhibitors suggests that THs may act via phosphorylation of MAPK (ERK1/2) to accelerate initiation of skeletogenesis in the three echinoderm groups. Together, these results indicate that TH regulation of mesenchyme cell activity via integrin-mediated MAPK signaling may be a conserved mechanism for the regulation of skeletogenesis in echinoderm development. In addition, TH action via a nuclear thyroid hormone receptor may regulate metamorphic development. Our findings shed light on potentially ancient pathways of thyroid hormone activity in echinoids, ophiuroids, and asteroids, or on a signaling system that has been repeatedly co-opted to coordinate metamorphic development in bilaterians.

Thyroid hormone-induced cell death in sea urchin metamorphic development

Thyroid hormones (THs) are important regulators of development, metabolism and homeostasis in metazoans. Specifically, they have been shown to regulate the metamorphic transitions of vertebrates and invertebrates alike. Indirectly developing sea urchin larvae accelerate the formation of juvenile structures in response to thyroxine (T4) treatment, while reducing their larval arm length. The mechanisms underlying larval arm reduction are unknown and we hypothesized that programmed cell death (PCD) is linked to this process. To test this hypothesis, we measured larval arm retraction in response to different THs (T4, T3, rT3, Tetrac) and assessed cell death in larvae using three different methods (TUNEL, YO-PRO-1 and caspase-3 activity) in the sea urchin Strongylocentrotus purpuratus. We also compared the extent of PCD in response to TH treatment before and after the invagination of the larval ectoderm, which marks the initiation of juvenile development in larval sea urchin species. We found that T4 treatment results in the strongest reduction of larval arms but detected a significant increase of PCD in response to T4, T3 and Tetrac in post-ingression but not pre-ingression larvae. As post-ingression larvae have initiated metamorphic development and therefore allocate resources to both larval and the juvenile structures, these results provide evidence that THs regulate larval development differentially via PCD. PCD in combination with cell proliferation likely has a key function in sea urchin development.

Hormonal Regulation of Programmed Cell Death in Sea Urchin Metamorphosis

Programmed cell death (PCD) has been identified as a key process in the metamorphic transition of indirectly developing organisms such as frogs and insects. Many marine invertebrate species with indirect development and biphasic life cycles face the challenge of completing the metamorphic transition of the larval body into a juvenile when they settle into the benthic habitat. Some key characteristics stand out during this transition in comparison to frogs and insects: (1) the transition is often remarkably fast and (2) the larval body is largely abandoned and few structures transition into the juvenile stage. In sea urchins, a group with a drastic and fast metamorphosis, development and destruction of the larval body is regulated by endocrine signals. Here we provide a brief review of the basic regulatory mechanisms of PCD in animals. We then narrow our discussion to metamorphosis with a specific emphasis on sea urchins with indirect life histories and discuss the function of thyroid hormones and histamine in larval development, metamorphosis and settlement of the sea urchin Strongylocentrotus purpuratus. We were able to annotate the large majority of PCD related genes in the sea urchin S. purpuratus and ongoing studies on sea urchin metamorphosis will shed light on the regulatory architecture underlying this dramatic life history transition. While we find overwhelming evidence for hormonal regulation of PCD in animals, especially in the context of metamorphosis, the mechanisms in many marine invertebrate groups with indirect life histories requires more work. Hence, we propose that studies of PCD in animals requires functional studies in whole organisms rather than isolated cells. We predict that future work, targeting a broader array of organisms will not only help to reveal important new functions of PCD but provide a fundamentally new perspective on its use in a diversity of taxonomic, developmental, and ecological contexts.

Effect of different extracts and fractions of Senecio biafrae (Oliv. &Hiern) J. Moore on in vivo and in vitro parameters of folliculogenesis in experimental animals

BACKGROUND

Senecio biafrae is a medicinal plant widely used in traditional medicine to cure female infertility. Some effects have been pharmacologically demonstrated on immature female rats but in vivo and in vitro investigations are still necessary for determining its mechanism of action. The aim of the present study was to evaluate the estrogenic and FSH-like effects of the plant extracts and fractions on some fertility parameters in immature female rats and on in vitro survival and growth of swine preantral follicles.

METHODS

21-23 days old female Wistar rats orally received extracts and fractions of S. biafrae at 0, 8 and 64 mg/kg doses over 20 days. The LH, FSH, estradiol and progesterone serum levels were evaluated as well as the ovarian cholesterol, uterus and ovaries masses and proteins. The numbers of follicles at different developmental stages were recorded in ovarian cortexes after histology. Slices of swine ovarian cortexes were cultured along 1 or 7 days in alpha-minimum essential medium (α-MEM) and fixed for morphological analysis of preantral follicles. The fresh control, cultured control (CIV control) and different Senecio biafrae-treated ovarian fragments were analyzed for preantral follicles development. Treatments that showed the best follicle growth in culture were submitted to AgNOR test. The aqueous and MeOH/CH₂Cl₂ extracts as well as the ethyl acetate and hexane fractions of S. biafrae were submitted to the HPLC for analysis of polyphenolic secondary metabolites.

RESULTS

Ovarian and uterine proteins were significantly high (p < 0.01) in animals treated with the two dosages of ethyl acetate and n-butanol fractions. The same result was recorded with uterine proteins in animals treated with the hexane fraction. The FSH level significantly dropped with all ethanolic extract doses and with the 64 mg/kg dosage of the methanol/methylene chloride (MeOH/CH₂Cl₂) extract while LH was reduced (p < 0.01) in almost all the treated groups. Estradiol level was significantly increased (p < 0.001) in the three groups receiving the extracts, but reduced (p < 0.001) in the three groups receiving the fractions of the plant. The progesterone level increased with almost all the treated groups. Primary and secondary follicles augmented (p < 0.01) in MeOH/CH₂Cl₂ extract and n-butanol fraction while tertiary follicles increased with the same extract and the ethyl acetate fraction (p < 0.05). Treatments with aqueous and ethanolic extracts as well as ethyl acetate fraction led to a significant increase (p < 0.05) in the number of morphologically normal follicles after 7 days of culture as compared to the CIV control. The number of AgNOR dots per follicle was significantly low (p < 0.05) in all cultured groups as compared to the fresh control, except the ethyl acetate 2.8 ng/ml dosage. The same observation was done with AgNOR dots per cell in the 2.8 ng/ml dosage aqueous extract-treated fragments. The phenolic compounds mainly encountered in the plant, independently of the extract or fraction are apigenin, eugenol and rutin.

CONCLUSION

Extracts and fractions of S. biafrae have an important FSH-like effect which induces follicular survival and growth.

The evolutionary road to invertebrate thyroid hormone signaling: Perspectives for endocrine disruption processes

Thyroid hormones (THs) are the only iodine-containing hormones that play fundamental roles in chordates and non-chordates. The chemical nature, mode of action and the synthesis of THs are well established in mammals and other vertebrates. Although thyroid-like hormones have been detected in protostomes and non-chordate deuterostomes, TH signaling is poorly understood as compared to vertebrates, particularly in protostomes. Therefore, the central objective of this article is to review TH system components and TH-induced effects in non-vertebrate chordates, non-chordate deuterostomes and protostomes based on available genomes and functional information. To accomplish this task, we integrate here the available knowledge on the THs signaling across non-vertebrate chordates, non-chordate deuterostomes and protostomes by considering studies encompassing TH system components and physiological actions of THs. We also address the possible interactions of thyroid disrupting chemicals and their effects in protostomes and non-chordate deuterostomes. Finally, the perspectives on current and future challenges are discussed.

The relationship between ingested thyroid hormones, thyroid homeostasis and iodine metabolism in humans and teleost fish

Oral l-thyroxine (T4) therapy is used to treat human hypothyroidism but T4 fed to teleost fish does not raise plasma thyroid hormone (TH) levels nor induce growth, even though oral 3,5,3'-triiodo-l-thyronine (T3) is effective. This suggests a major difference in TH metabolism between teleosts and humans, often used as a starting thyroid model for lower vertebrates. To gain further insight on the proximate (mechanistic) and ultimate (survival value) factors underlying this difference, the several steps in TH homeostasis from intestinal TH uptake to hypothalamic-hypophyseal regulation were compared between humans and teleosts, and following dietary TH challenges. A major proximate factor limiting trout T4 uptake is a potent constitutive thiol-inhibited intestinal complete T4 deiodination that is ineffective for T3. At the hepatic level, T4 deiodination, conjugation and extensive biliary excretion with negligible T4 enterohepatic recycling can further block teleost T4 uptake to plasma. Such protection of plasma T4 from dietary T4 may be particularly critical for piscivorous fish consuming thyroid tissue, rich in T4 but not T3. It would prevent disruption by unregulated ingested T4 of the characteristic acute and transient changes in teleost plasma T4 due to diel rhythms, food intake and stress-related factors. These marked natural short-term fluctuations in teleost plasma T4 levels are enabled by the relatively small and rapidly-cleared plasma T4 pool, stemming largely from properties of the plasma T4-binding proteins. Humans, however, due mainly to plasma T4-binding globulin, have a relatively massive circulating pool of T4 and an extremely well-buffered free T4 level, consistent with the major TH role in regulating basal metabolic rate. Furthermore, this large well-buffered and slowly-cleared plasma T4 pool, in conjuction with enterohepatic recycling and relaxation of hypothalamic-hypophyseal negative feedback, allows humans to temporarily 'store' ingested T4 in plasma, thereby sparing endogenous TH secretion and conserving thyroidal iodine reserves. Indeed, iodine conservation is likely the key ultimate factor determining the divergent evolution of the human and teleost systems. For humans, ingested iodine in the form of I^-, or TH and their derivatives, is the sole iodine source and may be limiting in many environments. However, most freshwater teleosts, in addition to their ability to assimilate dietary I^-, can derive sufficient I^- from their copious gill irrigation, with no selective advantage in absorbing dietary T4 which would disrupt their natural acute and transient changes in plasma T4. Thus T4 may act also as a vitamin (vitamone) in humans but not in teleosts; in contrast, T3, naturally ingested at much lower levels, may act as a vitamone in both humans and teleosts.

Histone deacetylase activity mediates thermal plasticity in zebrafish (Danio rerio)

Regulatory mechanisms underlying thermal plasticity determine its evolution and potential to confer resilience to climate change. Here we show that class I and II histone deacetylases (HDAC) mediated thermal plasticity globally by shifting metabolomic profiles of cold acclimated zebrafish (Danio rerio) away from warm acclimated animals. HDAC activity promoted swimming performance, but reduced slow and fast myosin heavy chain content in cardiac and skeletal muscle. HDAC increased sarco-endoplasmic reticulum ATPase activity in cold-acclimated fish but not in warm-acclimated animals, and it promoted cardiac function (heart rate and relative stroke volume) in cold but not in warm-acclimated animals. HDAC are an evolutionarily ancient group of proteins, and our data show that they mediate the capacity for thermal plasticity, although the actual manifestation of plasticity is likely to be determined by interactions with other regulators such as AMP-activated protein kinase and thyroid hormone.

Hormonally active phytochemicals from macroalgae: A largely untapped source of ligands to deorphanize nuclear receptors in emerging marine animal models

Hormonally active phytochemicals (HAPs) are signaling molecules produced by plants that alter hormonal signaling in animals, due to consumption or environmental exposure. To date, HAPs have been investigated mainly in terrestrial ecosystems. To gain a full understanding of the origin and evolution of plant-animal interactions, it is necessary also to study these interactions in the marine environment, where the major photosynthetic lineages are very distant from the terrestrial plants. Here we focus on chemicals from red and brown macroalgae and point out their potential role as modulators of the endocrine system of aquatic animals through nuclear hormone receptors. We show that, regarding steroids and oxylipins, there are already some candidates available for further functional investigations of ligand-receptor interactions. Furthermore, several carotenoids, produced by cyanobacteria provide candidates that could be investigated with respect to their presence in macroalgae. Finally, regarding halogenated compounds, it is not clear yet which molecules could bridge the gap to explain the transition from lipid sensing to thyroid hormone high affinity binding among nuclear receptors.

Sea Urchin Larvae as a Model for Postembryonic Development

Larvae are a diverse set of postembryonic life forms distinct from juveniles or adults that have evolved in many animal phyla. Echinoids (sea urchins and sand dollars) generate rapidly developing, morphologically simple, and optically transparent larvae and are a well-established model system supported by a broad array of genomic resources, experimental approaches, and imaging techniques. As such, they provide a unique opportunity to study postembryonic processes such as endocrine signaling, immunity, host-microbe interactions, and regeneration. Here we review a broad array of literature focusing on these important processes in sea urchin larvae, providing support for the claim that they represent excellent experimental study systems. Specifically, there is strong evidence emerging that endocrine signaling, immunity, and host-microbe interactions play major roles in larval development and physiology. Future research should take advantage of sea urchin larvae as a model to study these processes in more detail.

Evolution of ligands, receptors and metabolizing enzymes of thyroid signaling

Thyroid hormones (THs) play important roles in vertebrates such as the control of the metabolism, development and seasonality. Given the pleiotropic effects of thyroid disorders (developmental delay, mood disorder, tachycardia, etc), THs signaling is highly investigated, specially using mammalian models. In addition, the critical role of TH in controlling frog metamorphosis has led to the use of Xenopus as another prominent model to study THs action. Nevertheless, animals regarded as non-model species can also improve our understanding of THs signaling. For instance, studies in amphioxus highlighted the role of Triac as a bona fide thyroid hormone receptor (TR) ligand. In this review, we discuss our current understanding of the THs signaling in the different taxa forming the metazoans (multicellular animals) group. We mainly focus on three actors of the THs signaling: the ligand, the receptor and the deiodinases, enzymes playing a critical role in THs metabolism. By doing so, we also pinpoint many key questions that remain unanswered. How can THs accelerate metamorphosis in tunicates and echinoderms while their TRs have not been yet demonstrated as functional THs receptors in these species? Do THs have a biological effect in insects and cnidarians even though they do not have any TR? What is the basic function of THs in invertebrate protostomia? These questions can appear disconnected from pharmacological issues and human applications, but the investigation of THs signaling at the metazoans scale can greatly improve our understanding of this major endocrinological pathway.

Nutrition and dopamine: An intake of tyrosine in royal jelly can affect the brain levels of dopamine in male honeybees (Apis mellifera L.)

Precursors of neuroactive substances can be obtained from dietary sources, which can affect the resulting production of such substances in the brain. In social species, an intake of the precursor in food could be controlled by social interactions. To test the effects of dietary tyrosine on the brain dopamine levels in social insect colonies, male and worker honeybees were fed tyrosine or royal jelly under experimental conditions and the brain levels of dopamine and its metabolite were then measured. The results showed that the levels of dopamine and its metabolite in the brains of 4- and 8-day-old workers and 8-day-old males were significantly higher in tyrosine-fed bees than in control bees, but the levels in 4-day-old males were not. The brain levels of dopamine and its metabolite in 4- and 8-day-old males and workers were significantly higher in royal jelly-fed bees than in control bees, except for one group of 4-day-old workers. Food exchanges with workers were observed in males during 1-3 days, but self-feedings were also during 5-7 days. These results suggest that the brain levels of dopamine in males can be controlled by an intake of tyrosine in food via exchanging food with nestmates and by self-feeding.

Effect of the aqueous extract of Senecio biafrae (Oliv. & Hiern) J. Moore on some fertility parameters in immature female rat

ETHNOPHARMACOLOGICAL RELEVANCE

Senecio biafrae is a plant from the huge family of Asteraceae used in the African pharmacopoeia for the treatment of many ailments among which is infertility.

MATERIAL AND METHODS

The aqueous extract, which was primarily subjected to polyphenol analysis, has been administered to immature female rats for 20 days at 8, 32, 64 and 128 mg/kg of body weight. The day following the treatment, the animals were sacrificed; their serum, ovary and uterus were retained respectively for reproductive hormones, ovarian and uterine proteins, and ovarian cholesterol assays.

RESULTS

Light body weight gain variation of treated animals was observed during the experimental period. A significant increase (p ˂ 0.05) in serum estradiol and proteins as well as in uterine weight (p ˂ 0.01) of all Senecio biafrae treated animals was noted. No significant variation was noticed in the ovarian weight and follicle numbers.

CONCLUSION

The various biochemical and physiological parameters of fertility were significantly improved with the aqueous extract of Senecio biafrae, thus attesting some of its traditional usage.

The evolution of endothermy is explained by thyroid hormone-mediated responses to cold in early vertebrates

The evolution of endothermy is one of the most intriguing and consistently debated topics in vertebrate biology, but the proximate mechanisms that mediated its evolution are unknown. Here, we suggest that the function of thyroid hormone in regulating physiological processes in response to cold is key to understanding the evolution of endothermy. We argue that the capacity of early chordates to produce thyroid hormone internally was the first step in this evolutionary process. Selection could then act on the capacity of thyroid hormone to regulate metabolism, muscle force production and cardiac performance to maintain their function against the negative thermodynamic effects of decreasing temperature. Thyroid-mediated cold acclimation would have been the principal selective advantage. The actions of thyroid hormone during cold acclimation in zebrafish are very similar to its role during endothermic thermogenesis. The thyroid-mediated increases in metabolism and locomotor performance in ectotherms eventually resulted in sufficient heat production to affect body temperature. From this point onwards, increased body temperature per se could be of selective advantage and reinforce thyroid-induced increases in physiological rates. Selection for increased body temperature would promote those mechanisms that maximise heat production, such as increased Na(+)/K(+)-ATPase activity, futile cycling by SERCA, and mitochondrial uncoupling, all of which are regulated by thyroid hormone. The specific end point of this broader evolutionary process would be endothermic thermoregulation. However, considering the evolution of endothermy in isolation is misleading because the selective advantages that drove the evolutionary process were independent from endothermy. In other words, without the selective advantages of thyroid-mediated cold acclimation in fish, there would be no endotherms.

The Na+/I- symporter (NIS): mechanism and medical impact

The Na(+)/I(-) symporter (NIS) is the plasma membrane glycoprotein that mediates active I(-) transport in the thyroid and other tissues, such as salivary glands, stomach, lactating breast, and small intestine. In the thyroid, NIS-mediated I(-) uptake plays a key role as the first step in the biosynthesis of the thyroid hormones, of which iodine is an essential constituent. These hormones are crucial for the development of the central nervous system and the lungs in the fetus and the newborn and for intermediary metabolism at all ages. Since the cloning of NIS in 1996, NIS research has become a major field of inquiry, with considerable impact on many basic and translational areas. In this article, we review the most recent findings on NIS, I(-) homeostasis, and related topics and place them in historical context. Among many other issues, we discuss the current outlook on iodide deficiency disorders, the present stage of understanding of the structure/function properties of NIS, information gleaned from the characterization of I(-) transport deficiency-causing NIS mutations, insights derived from the newly reported crystal structures of prokaryotic transporters and 3-dimensional homology modeling, and the novel discovery that NIS transports different substrates with different stoichiometries. A review of NIS regulatory mechanisms is provided, including a newly discovered one involving a K(+) channel that is required for NIS function in the thyroid. We also cover current and potential clinical applications of NIS, such as its central role in the treatment of thyroid cancer, its promising use as a reporter gene in imaging and diagnostic procedures, and the latest studies on NIS gene transfer aimed at extending radioiodide treatment to extrathyroidal cancers, including those involving specially engineered NIS molecules.

Thyroid hormone actions are temperature-specific and regulate thermal acclimation in zebrafish (Danio rerio)

BACKGROUND

Thyroid hormone (TH) is best known for its role in development in animals, and for its control of metabolic heat production (thermogenesis) during cold acclimation in mammals. It is unknown whether the regulatory role of TH in thermogenesis is derived in mammals, or whether TH also mediates thermal responses in earlier vertebrates. Ectothermic vertebrates show complex responses to temperature variation, but the mechanisms mediating these are poorly understood. The molecular mechanisms underpinning TH action are very similar across vertebrates, suggesting that TH may also regulate thermal responses in ectotherms. We therefore aimed to determine whether TH regulates thermal acclimation in the zebrafish (Danio rerio). We induced hypothyroidism, followed by supplementation with 3,5-diiodothyronine (T2) or 3,5,3'-triiodothyronine (T3) in zebrafish exposed to different chronic temperatures. We measured whole-animal responses (swimming performance and metabolic rates), tissue-specific regulatory enzyme activities, gene expression, and free levels of T2 and T3.

RESULTS

We found that both T3 and the lesser-known T2, regulate thermal acclimation in an ectotherm. To our knowledge, this is the first such study to show this. Hypothyroid treatment impaired performance measures in cold-acclimated but not warm-acclimated individuals, whereas supplementation with both TH metabolites restored performance. TH could either induce or repress responses, depending on the actual temperature and thermal history of the animal.

CONCLUSIONS

The low sensitivity to TH at warm temperatures could mean that increasing temperatures (that is, global warming) will reduce the capacity of animals to regulate their physiologies to match demands. We suggest that the properties that underlie the role of TH in thermal acclimation (temperature sensitivity and metabolic control) may have predisposed this hormone for a regulatory role in the evolution of endothermy.

Iodine accumulation in sea urchin larvae is dependent on peroxide

Iodine has many important biological functions and its concentrations vary with the environment. Recent research has provided novel insights into iodine uptake mechanisms in marine bacteria and kelp through hydrogen peroxide-dependent diffusion (PDD). This mechanism is distinct from sodium-dependent mechanisms known from vertebrates. In vertebrates, iodine accumulates in the thyroid gland by the action of the apical iodide transporter (AIT) and the sodium/iodide symporter (NIS). Neither of these proteins has, thus far, been identified outside of the chordates, and PDD (as an iodine uptake mechanism) has never been studied in animals. Using (125)I as a marker for total iodine influx, we tested iodine uptake via sodium-dependent transport versus PDD in embryos and larvae of the sea urchin Strongylocentrotus purpuratus. We found that iodine uptake in S. purpuratus is largely independent of NIS/AIT. Instead, we found that uptake is dependent on the presence and production of hydrogen peroxide, indicating that sea urchin larvae use PDD as a mechanism for iodine acquisition. Our data, for the first time, provide conclusive evidence for this mechanism in an animal. Furthermore, our data provide preliminary evidence that sodium-dependent iodine uptake via active transporter proteins is a synapomorphy of vertebrates.

Minireview: The neural regulation of the hypothalamic-pituitary-thyroid axis

Thyroid hormone (TH) signaling plays an important role in development and adult life. Many organisms may have evolved under selective pressure of exogenous TH, suggesting that thyroid hormone signaling is phylogenetically older than the systems that regulate their synthesis. Therefore, the negative feedback system by TH itself was probably the first mechanism of regulation of circulating TH levels. In humans and other vertebrates, it is well known that TH negatively regulates its own production through central actions that modulate the hypothalamic-pituitary-thyroid (HPT) axis. Indeed, primary hypothyroidism leads to the up-regulation of the genes encoding many key players in the HPT axis, such as TRH, type 2 deiodinase (dio2), pyroglutamyl peptidase II (PPII), TRH receptor 1 (TRHR1), and the TSH α- and β-subunits. However, in many physiological circumstances, the activity of the HPT axis is not always a function of circulating TH concentrations. Indeed, circadian changes in the HPT axis activity are not a consequence of oscillation in circulating TH levels. Similarly, during reduced food availability, several components of the HPT axis are down-regulated even in the presence of lower circulating TH levels, suggesting the presence of a regulatory pathway hierarchically higher than the feedback system. This minireview discusses the neural regulation of the HPT axis, focusing on both TH-dependent and -independent pathways and their potential integration.

Herbal extracts and phytochemicals: plant secondary metabolites and the enhancement of human brain function

Humans consume a wide range of foods, drugs, and dietary supplements that are derived from plants and which modify the functioning of the central nervous sytem (CNS). The psychoactive properties of these substances are attributable to the presence of plant secondary metabolites, chemicals that are not required for the immediate survival of the plant but which are synthesized to increase the fitness of the plant to survive by allowing it to interact with its environment, including pathogens and herbivorous and symbiotic insects. In many cases, the effects of these phytochemicals on the human CNS might be linked either to their ecological roles in the life of the plant or to molecular and biochemical similarities in the biology of plants and higher animals. This review assesses the current evidence for the efficacy of a range of readily available plant-based extracts and chemicals that may improve brain function and which have attracted sufficient research in this regard to reach a conclusion as to their potential effectiveness as nootropics. Many of these candidate phytochemicals/extracts can be grouped by the chemical nature of their potentially active secondary metabolite constituents into alkaloids (caffeine, nicotine), terpenes (ginkgo, ginseng, valerian, Melissa officinalis, sage), and phenolic compounds (curcumin, resveratrol, epigallocatechin-3-gallate, Hypericum perforatum, soy isoflavones). They are discussed in terms of how an increased understanding of the relationship between their ecological roles and CNS effects might further the field of natural, phytochemical drug discovery.