Central Nervous System Agents in Medicinal Chemistry - Volume 10, Issue 3, 2010

Nicotine/nicotine agonists or allosteric modulators of nicotine receptors have been suggested as the most important therapeutic agents in the prevention and clinical control of cognitive impairment which characteryze neuropsychiatric and neurodegenerative disorders such as schizophrenia, attention deficit/hyperactivity disorder and Alzheimer's disease. Both clinical studies and animal experiments support the important role of the nicotinic systems in learning, different kind of memory and cognition. For development of nicotinic treatments we have a well characterized lead compound, nicotine. However, the neural nicotinic mechanisms underlying cognitive functions are not well known because the side effects of nicotine overdose have hindered the development of this therapeutical line. The new development of non-toxic, brain specific nicotine drugs need a full knolewdge of these mechamism and a reevaluation of the nicotine effects. This review aims to analize the diferent kind of effects of nicotine on the Central Nervous System (CNS), especially on the cortex and hippocampus. Nicotine effects are, theorically and/or practically, of variable character depending on dayly dose and time of treatment; on the subtype and density of the different nicotinic receptors existing in the distinct brain regions; on the processes of desensitization and tolerance of nicotinic receptors and on other neuronal factors. Nicotine produces the above mentioned activation of the cognitive functions acting directly or indirectly on cortical neurons. In some experiments, high doses of nicotine can impair memory. This substance induces increases in the glycolytic pahtway and Krebs cycle of neurons, as well as brain blood flow. Nicotine also produces an increase in NGF immunoreactivity in frontoparietal cortex. All these neuronal changes may cause different positive effects such as neuroprotection, neuroplasticity and better preformance of synaptic circuits. The benefit of other neuronal changes can be matter of discussion such as some modifications in synaptic transmission, the COX-2 increase in frontoparietal cortex and hippocampus or the changes in the antioxidant systems. Finally, other neuronal changes can be of negative effect such as the induction of apoptosis and oxidative stress (DNA damage, ROS and lipid peroxide increase). All these described effects explain both the beneficial and neurotoxic consequeces of the activation of the nicotinic receptors. The diversity and variability of the nicotinic effects should take into account when nicotine agonists will be used as a possible cognitive treatment.

Antidepressants are widely used to treat several anxiety disorders, among which generalized anxiety disorder (GAD) and panic disorder (PD). Serotonin (5-HT) is believed to play a key role in the mode of action of these agents, a major question being which pathways and receptor subtypes are involved in each type of anxiety disorder. The dual role of 5-HT in defense hypothesis assumes that 5-HT facilitates defensive responses to potential threat, like inhibitory avoidance, related to anxiety, whereas it inhibits defensive responses to proximal danger, like one-way escape, related to panic. The former action would be exerted at the forebrain, chiefly the amygdala and medial prefrontal cortex (PFC), while the latter would be exerted at the dorsal periaqueductal gray (DPAG) matter of the midbrain. The present review is focused on studies designed to test this hypothesis, performed in animal models of anxiety and panic, as well as in human experimental anxiety tests. The reviewed results suggest that chronic, but not acute, administration of antidepressants suppress panic attacks by increasing the release of 5-HT and enhancing the responsivity of post-synaptic 5-HT1A and 5-HT2A receptors in the DPAG. The attenuation of generalized anxiety, also caused by the same drug treatment, would be due to the desensitization of 5-HT2C receptors and, less certainly, to increased stimulation of 5-HT1A receptors in forebrain structures. This action would result in less activation of the amygdala, medial PFC and insula by warning signals, as shown by the reviewed results obtained with functional neuroimaging in healthy volunteers and patients with anxiety disorders.

Ethnopharmacological research investigates the plants and other medicinal and toxic substances utilized by different traditional populations. One approach in this field is a literature search of the available publications on medicinal plants. The purpose of the current study was to select plants with psychoactive effects described in a Brazilian literary work written by Pio Correa in 1926. Those mentioned plants were classified in accordance with their indications for use as stimulants and depressors of the central nervous system. For the phytochemical study herein, we researched these species via a database search, and all the obtained information was compiled into a new database to analyze possible correlations between the chemical compounds and the psychoactive categories. Of the 813 plants searched in the literary work, 104 presented chemical data in the scientific periodicals consulted. Seventy-five of them belong to the stimulant category, while 31 are depressors and two of them belong to both categories. Phenols and flavonoids were the main compounds observed in plants of both categories, though at different frequencies. Monoterpenes (29.9%) and sesquiterpenes (28.6%) were also observed in plants from the stimulant category, while 25.8% of plants from the depressor category were comprised of carotenoids and 22.6% of steroids. The main specific compounds were identified as ferulic acid, α-pinene, limonene, α-humulene and kaempferol among the stimulant plants. Otherwise, in depressor plants were characterized caffeic acid, kaempferol, quercetin, β-carotene, physalins and withanolides as specific compounds. The association between ethnopharmacological and chemotaxonomic data, as presented in this study, could support plant selection in further investigations by research groups whose studies focus on psychoactive plants as potential therapeutics.

Ayurveda is a Sanskrit word, which means “the scripture for longevity”. It represents an ancient system of traditional medicine prevalent in India and in several other south Asian countries. It is based on a holistic view of treatment which is believed to cure human diseases through establishment of equilibrium in the different elements of human life, the body, the mind, the intellect and the soul [1]. Ayurveda dates back to the period of the Indus Valley civilization (about 3000 B.C) and has been passed on through generations of oral tradition, like the other four sacred texts (Rigveda, Yajurveda, Samaveda and Atharvanaveda) which were composed between 12th and 7th century B.C [2, 3]. References to the herbal medicines of Ayurveda are found in all of the other four Vedas, suggesting that Ayurveda predates the other Vedas by at least several centuries. It was already in full practice at the time of Buddha (6th century B.C) and had produced two of the greatest physicians of ancient India, Charaka and Shushrutha who composed the basic texts of their trade, the Samhitas. By this time, ayurveda had already developed eight different subspecialties of medical treatment, named Ashtanga, which included surgery, internal medicine, pediatrics, toxicology, health and longevity, and spiritual healing [4]. Ayurvedic medicine was mainly composed of herbal preparations which were occasionally combined with different levels of other compounds, as supplements [5]. In the Ayurvedic system, the herbs used for medicinal purposes are classed as brain tonics or rejuvenators. Among the plants most often used in Ayurveda are, in the descending order of importance: (a) Ashwagandha, (b) Brahmi, (c) Jatamansi, (d) Jyotishmati, (e) Mandukparni, (f) Shankhapushpi, and (g) Vacha. The general appearance of these seven plants is shown in Fig. (1) Their corresponding Latin names, as employed in current scientific literature, the botanical families that each of them belongs to, their normal habitats in different areas of the world, as well as the common synonyms by which they are known, are shown in the Table 1. The scientific investigations concerning the best known and most scientifically investigated of these herbs, Ashwagandha will be discussed in detail in this review. Ashwagandha (Withania somnifera, WS), also commonly known, in different parts of the world, as Indian ginseng, Winter cherry, Ajagandha, Kanaje Hindi and Samm Al Ferakh, is a plant belonging to the Solanaceae family. It is also known in different linguistic areas in India by its local vernacular names [6]. It grows prolifically in dry regions of South Asia, Central Asia and Africa, particularly in India, Pakistan, Bangladesh, Sri Lanka, Afghanistan, South Africa, Egypt, Morocco, Congo and Jordon [7]. In India, it is cultivated, on a commercial scale, in the states of Madhya Pradesh, Uttar Pradesh, Punjab, Gujarat and Rajasthan [6]. In Sanskrit, ashwagandha, the Indian name for WS, means “odor of the horse”, probably originating from the odor of its root which resembles that of a sweaty horse. The name “somnifera” in Latin means “sleep-inducer” which probably refers to its extensive use as a remedy against stress from a variety of daily chores. Some herbalists refer to ashwagandha as Indian ginseng, since it is used in India, in a way similar to how ginseng is used in traditional Chinese medicine to treat a large variety of human diseases [8]. Ashwagandha is a shrub whose various parts (berries, leaves and roots) have been used by Ayurvedic practitioners as folk remedies, or as aphrodisiacs and diuretics. The fresh roots are sometimes boiled in milk, in order to leach out undesirable constituents. The berries are sometimes used as a substitute to coagulate milk in cheese making. In Ayurveda, the herbal preparation is referred to as a “rasayana”, an elixir that works, in a nonspecific, global fashion, to increase human health and longevity. It is also considered an adaptogen, a nontoxic medication that normalizes physiological functions, disturbed by chronic stress, through correction of imbalances in the neuroendocrine and immune systems [9, 10]. The scientific research that has been carried out on Ashwagandha and other ayurvedic herbal medicines may be classified into three major categories, taking into consideration the endogenous or exogenous phenomena that are known to cause physiological disequilibrium leading to the pathological state; (A) pharmacological and therapeutic effects of extracts, purified compounds or multi-herbal mixtures on specific non-neurological diseases; (B) pharmacological and therapeutic effects of extracts, purified compounds or multi-herbal mixtures on neurodegenerative disorders; and (C) biochemical, physiological and genetic studies on the herbal plants themselves, in order to distinguish between those originating from different habitats, or to improve the known medicinal quality of the indigenous plant. Some of the major points on its use in the treatment of neurodegenerative disorders are described below.

Attention Deficit Hyperactivity Disorder (ADHD) is a common behavioral disorder of children and is treated by psychostimulants. Psychostimulant exposure to children at the time of neuronal development can cause behavioral and physiological changes continuing during adulthood. Most of the studies on psychostimulants investigate the acute effects of the drug. The objective of this study was to investigate whether acute or chronic exposure to methylphenidate (MPD), the drug most often used to treat ADHD in children, will modulate the diurnal activity pattern of young rats. Maintaining the diurnal activity pattern is a physiological process that regulates the internal homeostasis. Dose response protocol was used to study the effect of acute and chronic MPD in four young post natal day 40 (P 40) rat groups, (each N=8), as follows: saline (control) group, and 0.6, 2.5, or 10.0 mg/kg i.p. MPD groups, respectively. The experiment was performed over 11 consecutive days of continuous locomotor activity recording using the open field assay. The data evaluation was divided into four phases as follows: acute, induction, washout and expression phases. There was a dose-dependent increase in the average locomotor activity in the first few hours post-injection. Analysis of the diurnal rhythmic pattern of locomotion in the three dose groups compared to control demonstrated that only the 10.0 mg/kg MPD elicited significant changes in diurnal pattern activity in the washout and the expression phase. In addition, this study indicated that chronic MPD treatment elicits dose dependent anticipation and/or withdrawal and behavioral sensitization.