Spectral classification

Plant Chemicals Affect the Human Brain?

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of the evolution of “primary metabolism.” This collective term encompasses the path-ways that modify and synthesize proteins, fats, carbohydrates, and nucleic acids to provide the specific compounds required for the organism’s physical construction and the production of energy. These processes are essentially the same across all life forms, with only a few minor variations. As an example, processes such as glycolysis and the citric acid (“tricarboxylic acid” or “Krebs”) cycle comprise complex series of enzymatic reactions that underpin the metabolic pathways that generate useable energy from carbohydrates, fats, and proteins and that also produce a host of substrates for other enzymatic processes. These pathways, which take place in the mitochondria of eukary-otes, are common to all living cells.¹⁹⁵

We can also directly trace the origins of the animal nervous system back 600 million years¹⁸⁹ to the single-celled common ancestor of the human and the chronoflagellate.

This organism already possessed the “genetic repertoire” and key molecular compo-nents that form the basis of neuronal function.¹⁹⁶ The many ancestral components common to contemporary vertebrates and chronoflagellates include the same calcium and sodium channels¹⁹⁶ that underpin neuronal electrical signals, and the proteins that control the release of neuronal neurotransmitters in animals.¹⁹⁷ Of course the common ancestor of animals and chronoflagellates didn’t arrive fully formed, and if we step farther back in time we can likewise discern many of the genetic origins of the animal nervous system in the last common ancestor of humans and plants. By a process of both divergent and convergent evolution these commonalities have, in turn, given rise to some surprising similarities between plants and humans in terms of the parameters that we tend to think of, rather egotistically, as being solely related to the functioning of the mammalian brain.

It is also possible to start to answer the question “Why do plant chemicals affect the human brain?” by considering some of these similarities; firstly those between plants and humans, and secondly those between humans and invertebrates, and, most par-ticularly, the plants’ most intimate neighbors, the insects.

■ T H E S I M I L A R I T I E S B E T W E E N P L A N T S A N D A N I M A L S

In very recent years there has been a growing appreciation of the complexity of the mechanisms and processes that dictate a plant’s perception of its surroundings, and how it responds to environmental stimuli. Indeed a whole new field of research, “plant neurobiology,” which considers the molecular, chemical, electrical, and hydraulic properties underlying plant perception and behavior, has been gradually emerging as a discipline.¹⁹⁸ An increased focus on these processes has thrown up some fascinating similarities between the “neurobiology” of the plant and the human. It seems apt to summarize some of the many similarities between plants and animals that may under-pin, in part, the central nervous system effects of phytochemicals.

Receptors and Signaling Cascades

All cells, whether they exist in the form of a single-celled organism or as a single component of a more complex multicellular organism, have to be able to receive infor-mation from their environment. For a single-celled organism this process might involve detecting a change in a chemical concentration gradient followed by an appropriate

movement; whereasfor the most complex organisms this process might involve signal-ing between many millions of cells, with a resultant change in physiological parameters or behavior. For instance, the average human brain contains something in the region of 23 billion neurons, each of which is interconnected to other neurons by some 5,000 to 10,000 synapses. On top of this there are also a slightly larger number of the glial cells that support neuronal functioning, and these again are interconnected with other glial cells and neurons (although this is poorly understood to date).¹⁹⁹ All of these cells com-municate via a bewildering array of neurotransmitters, neuromodulators, hormones, and signaling molecules, all of which function as part of multiple signal transduction systems. Typically, irrespective of the complexity of the organism, at a cellular level the signal transduction process comprises some form of extracellular signaling molecule binding to and activating one or more proteinaceous cellular receptors on the external surface of the cell’s membrane (in the case of signaling molecules that can penetrate the cell membrane, the receptors may be in the cell’s cytoplasm or nucleus) with enough specificity to differentiate the signal from other background signals. This is followed by transmission of the signal across the membrane and into the cell, where it is amplified and disseminated within the cell, resulting in a physical response, often dictated by changed gene expression in the cell nucleus.²⁰⁰

Having arisen in a common single-celled ancestor, much of the basic machinery and many of the processes of cellular signal transduction are conserved across phylogenetic kingdoms. The animal nervous system functions courtesy of two main types of mem-brane receptors for neurochemicals. The most abundant are “metabotropic” receptors, such as the “G-protein-coupled” receptor that winds in and out of the membrane. The external portion of this receptor is bound to by the signaling molecule, and the signal is then transmitted across the membrane to the internal portion of the receptor. It is then propagated within the cell via “second messenger” signal transduction pathways.

In contrast, “ionotropic” receptors, such as the “ligand-gated ion channel,” open or close a membrane channel in response to binding by the signaling molecule, allowing ions (sodium, calcium, potassium, or chloride) to flow directly into the cell. These ions either change the electrical properties of the cell or trigger second- messenger signal transduction pathways. As we’ll see below, both of these receptor types are conserved in humans and plants^201–204; in some cases the plant receptors are activated by the same chemicals that we typically think of as “neurotransmitters.” Other receptors commonly expressed in both plants and animals include the leucine-rich repeat receptors²⁰⁰ and a range of protein kinase receptors. This latter, ubiquitous group of receptors include the “receptor tyrosine kinases,” which are found in cells throughout the animal body, including the brain. In plants the conserved group of receptors are termed “receptor-like kinases.” In both cases these receptors are involved in transducing stress-related signals, which are delivered to the cell by hormones and other stress-related ligands.²⁰⁵

The similarity in signal transduction pathways extends to the inside of the cell, with the conserved receptors propagating their signal via the same signaling cascades.²⁰⁰ For instance, in the case of protein kinase receptors, the ubiquitous mitogen-activated protein kinase (MAPK) cascade, in which a series of protein kinases activate (phos-phorylate) each other in a chain reaction that amplifies and transmits the signal within the cell, is conserved in plants and humans.^206,207 Indeed, plants, due to duplication of the original common ancestral genes, have a particularly rich complement of more than 1,000 protein kinase genes; this is approximately double the number of kinase genes seen in the human genome.²⁰⁸ Intriguingly, while the downstream products of

MAPK activity in animal cells include the synthesis of a number of inflammatory fac-tors, including prostaglandins,^209,210 plants also enjoy an orthologous, genetically con-served response, but in their case the modified pathway leads to the synthesis of the jasmonate hormones, rather than prostaglandins, and ultimately the synthesis of a raft of defensive secondary chemicals^211–213 (see “Immune and Stress Responses” below, and Chapter 8).

Mammalian Neurotransmitters, Neurochemicals, and Receptors?

We immediately associate a number of chemicals solely with the functioning of the human or mammalian brain. However, many of the neurotransmitters, neuromodula-tors, and hormones that are integral to the functioning of our own central nervous system are in fact the chemical products of metabolic processes that existed before the differentiation of plants and animals over a billion years ago. Many of these “neuro-chemicals” were synthesized and served cellular functions in our common unicellular ancestors,203,214,215 and they have gone on to occupy key roles in the lives of plants, often sharing functional similarities with the roles they play as signaling molecules in mammals. The following describes the comparative plant and mammalian roles of a number of chemicals that are typically thought of as “neurochemicals.”

Acetylcholine

As a key mammalian neurotransmitter acetylcholine plays a part in the functioning of muscles and the parasympathetic nervous system and contributes to the majority of brain processes, including long-term potentiation and neuronal plasticity, sensory per-ception, arousal, and attention. It functions in animals by binding to ionotropic ‘nico-tinic’ receptors and G-protein-coupled ‘muscarinic’ receptors. However, more recently, the role of acetylcholine both in non-neuronal physiological processes in animals and in its ubiquitous role as a genetically conserved signaling molecule across all forms of life, including plants, has attracted increasing attention.²¹⁶ In plants acetylcholine contributes to the regulation of growth, germination, flowering, water homeostasis, and photosynthesis.203,216,217 It has been shown to carry out some of these functions by binding to receptors analogous to the nicotinic²¹⁸ and muscarinic receptors^219,220 found in animals. The latter receptors have been shown to exert their effects on, for instance, stomatal opening via the same signal transduction mechanisms that contribute to the effects of the analogous receptor in animal cells (i.e., an increase in cytosolic calcium).

Glutamate and Gamma-Aminobutyric Acid (GABA)

These amino acids (non-proteinogenic in the case of GABA) are the most abundant mammalian neurotransmitters and play approximately opposite roles in the verte-brate nervous systems. Glutamate is the most prevalent excitatory neurotransmitter, with profound roles in cognitive function, and in particular memory, due to its role in synaptic plasticity. GABA, on the other hand, is the major inhibitory neurotransmit-ter²²¹ with receptors on most, if not all, neurons in the central nervous system.²²² Both glutamate and GABA bind directly to their own ionotropic receptors and modulate the flow of ions (chloride in the case of GABA, and sodium and calcium in the case

of glutamate) across neuronal membranes, thereby modulating neuronal excitability.

They both also influence internal cellular processes by binding to G-protein-coupled membrane receptors.²¹⁵

In plants both glutamate and GABA also serve signaling functions. Glutamate plays a central role in many aspects of amino acid and nitrogen metabolism and acts as a precursor to chlorophyll in developing leaves. It also acts as a signaling molecule and plays multifarious roles in germination, growth, light-mediated development, carbon/nitrogen sensing, and responses to stressors. These roles include a contribu-tion to the jasmonic acid hormonal pathways.²²³ Many of these functions are as a con-sequence of direct binding by glutamate to phylogenetically conserved receptors in plant membranes. Evidence suggests that these receptors exert the same ionotropic effects as the animal homologues^215,223 and result in action potentials broadly similar to those generated in mammalian cells.²²⁴ GABA is synthesized from glutamate by the same enzymatic pathway in plants and animals.²²⁵ In plants GABA plays a central role in general metabolism and is rapidly produced in response to a variety of stress-ors.^226,227 Its secondary roles include the attenuation of oxidative stress and defense against insects and nematodes.^225,228 It also functions as an intracellular signaling mol-ecule, particularly during plant development. While unidentified as yet, the possibility of specific plant GABA receptors remains open, although it is also possible that GABA may also bind to a domain of the plant ionotropic glutamate receptor and modulate its function allosterically.²²⁷ GABA also features in emissions designed to communicate with other plants and organisms²²⁹ and may function as an induced defense compound in its own right.²³⁰

The Indoleamines—Serotonin and Melatonin

The ubiquitous amino acid tryptophan played a key role in the evolution of life due to its ability to absorb light to generate biological energy. This property, which is courtesy of its indole ring structure, made tryptophan and its derivatives key components of all light-capturing proteins, such as chlorophyll and rhodopsin. The same property underlies many of its functions in plant and animal physiology.²¹⁴

Across vertebrates the L-tryptophan–derived neurotransmitter serotonin (5- hydroxytryptamine [5-HT]) plays a role in most basic biological processes, includ-ing movement, breathinclud-ing, sexual reproduction, and temperature regulation. In higher animals it also modulates more complex behaviors such as sleeping, eating, memory and learning, attention, sexual activity, and, in humans, mood.²¹⁴ The closely related neurohormone melatonin (N-acetyl-5-methoxytryptamine) is synthesized by the pineal gland in the mammalian brain during darkness and plays diverse roles related to its key function in regulating circadian rhythms throughout the brain and body, including the modulation of sleep, mood, sexual behavior, and aspects of seasonal reproduction.^231,232

Plants also synthesize serotonin, melatonin, and a number of structurally related indoleamines, including the ubiquitous plant hormone auxin (indole-3-acetic acid).²³³ Although the role of these indoleamines in plants has received comparatively little attention, serotonin, often in interaction with auxin,²³⁴ has been shown to help regu-late root system growth and architecture, the maturation and ripening of fruit, and senescence. It is also induced in response to microbial attack and as a defense against herbivores.^235–238

Melatonin has been isolated in a diverse range of angiosperms, with quantities varying as a consequence of light levels as in animals. In plants its functions include an analogous role in regulating circadian rhythms (see below) and other photoperiod responses, and it has related effects on the flowering, germination, and growth of the plant above and below ground.^232,239

The Catecholamines—Dopamine, Adrenaline, and Noradrenaline

In both animals and plants dopamine and its derivatives adrenaline and noradrena-line (also known as epinephrine and norepinephrine) are synthesized from the amino acid L-phenylalanine via a conserved synthetic pathway. These three catecholamines are most closely associated with their role as animal neurochemicals. Dopamine has wide-ranging mammalian functions as a key neurotransmitter, with roles including the modulation of diverse aspects of cognitive function and behavior, motivation and reward, sexual gratification, and sleep. Adrenaline and noradrenaline are both neu-rotransmitters and neurohormones, with roles encompassing modulation of aspects of brain function, heart rate, gluco-regulation, the “fight-or-flight” response dictated by the sympathetic nervous system, and stress-related responses in most tissues of the body.

In plants dopamine is a key intermediary in the synthesis of a wide range of alkaloid secondary metabolites, including morphine and mescaline. The other catecholamines have also been isolated at varying concentrations in a wide range of plants, includ-ing many fruits and green food plants.²⁰² To date, their endogenous roles in the life of plants are poorly delineated. However, it is notable that dopamine is a potent an-tioxidant and that the catecholamines as a group may function as intermediaries in photosynthesis, and in the responses to stress and infection. They may also interact with auxin to modulate growth and influence flowering. Adrenaline and noradrena-line have also been shown to modulate sugar metabolism in plant tissue via similar mechanisms as seen in mammalian cells (i.e., inactivation of glycogen synthase and activation of phosphorylation). A number of indirect lines of evidence also suggest that the catecholamines may function by binding directly to as-yet-unidentified recep-tors in plant tissue.²⁰²

Purines

The purinergic signaling system employs purines such as adenosine, ATP, and py-rimidines. These molecules are released from the cell into the extracellular matrix to function as the most basic extracellular signaling molecules in both animals and plants. In animals, purines modulate the activity of neurons by acting as co- transmitters within all of the neurotransmitter systems, throughout both the central and peripheral nervous systems. They therefore play a plethora of roles in diverse tissues and organs. They do this via three discrete receptor subtypes. The most fa-miliar of these, and the earliest to be characterized, are the G-protein-coupled “P1”

adenosine receptors, but these have now been joined by two subtypes of “P2” purino-receptors, the ionotropic P2X and G-protein-coupled P2Y receptors that are bound by ATP and other purinergic signaling nucleotides.²⁴⁰ In plants, purines and py-rimidines function in a conserved capacity, triggering similar cellular transduction

pathways involving nitric oxide, calcium, and reactive oxygen species. Common roles across the taxa include modulation of growth, cell death, responses within the respective immune systems, and responses to biotic and abiotic stressors.²⁴¹ However, only indirect evidence points to the existence of membrane-bound purine receptors in plant cells to date.²⁴²

Steroid Hormones

Mammals express five types of steroidal hormones: mineralocorticoids and gluco-corticoids, and the three groups of female and male sex hormones, the progesto-gens, estrogens and androgens. These hormones contribute to a raft of physiologi-cal processes, including physiphysiologi-cal homeostasis, metabolism, development of sexual characteristics, brain function, and stress responses to illness and injury, including inflammatory and immune function responses. They do this via two distinct signal transduction mechanisms: they diffuse into cells and bind to cytoplasm or nucleus receptors, which generate slow cellular responses, and they also bind directly to mem-brane receptors, which generate a faster response.^243,244 The most abundant proges-togen is progesterone, which when synthesized in the ovaries, placenta, and adrenal glands is primarily associated with multiple aspects of female reproduction and lacta-tion. However, progesterone is also synthesized throughout the central and peripheral nervous system in both males and females; acting as a “neurosteroid,” playing multi-ple roles, including in myelination and neuroprotection.^245,246 Progesterone receptors are also expressed in diverse brain regions, including the hypothalamus, pituitary, and cortex, and progesterone modulates sexual behavior, parental behavior and aggres-sion, and mood.^246,247

Among the many steroids that plants synthesize, only the brassinosteroids were thought to exert hormonal effects until recently.²⁴⁴ However, a number of “mamma-lian” hormones, including progesterone, androgens, and estrogens, have been identi-fied in a wide range of tissues taken from a variety of plants.243,248–250 These steroids may function as precursors for other steroidal secondary metabolites.²⁴⁸ However, it is also notable that progesterone, in particular, exhibits many of the characteristics of an endogenous plant hormone.²⁴⁴ In this respect it is notable that in plants both cytoplasm/nucleus and membrane receptors have been identified for brassinosteroids, and these include several receptors that bind endogenously circulating progesterone (and other “mammalian” sex steroids) in plants.²⁴⁴ These include two membrane re-ceptors, “membrane steroid binding protein 1” (MSBP1) and “steroid binding pro-tein” (SBP), that are widely distributed throughout plant tissues. These receptors represent partially conserved orthologues of the mammalian “progesterone receptor membrane component 1” (PGRMC1) progesterone receptor.²⁴³ Similarly, circulating estrogens and specific estrogen-binding sites similar to the mammalian nuclear “es-trogen receptor α” have been identified in various tissues from two members of the Solanaceae.²⁵¹ The enzymatic pathways that result in the sex steroids in plants are also partially conserved.²⁴⁹ Unsurprisingly, the “mammalian” sex hormones exert a wide range of effects on plant growth and development. For instance, progesterone influ-ences root elongation, germination and pollen tube growth, flowering, seedling and plant growth, and antioxidative stress responses,²⁴⁴ in some cases showing strongly biphasic effects (e.g., increasing growth parameters at low doses and impairing them at higher doses).²⁴³

Nitric Oxide

Nitric oxide (NO) is a gaseous molecule whose small size and neutral charge make

Nitric oxide (NO) is a gaseous molecule whose small size and neutral charge make