Anxiolytic Activity of Several Herbs

Anxiety disorders represent one of the most widespread mental health challenges, affecting millions of individuals globally and significantly impairing quality of life. According to the World Health Organization, nearly 301 million people worldwide were living with an anxiety disorder in 2019, including around 58 million children and adolescents, making them the most common mental health disorders across populations [1]. Beyond individual suffering, anxiety disorders are associated with increased risk of comorbid depression, substance abuse, cardiovascular disease, and reduced workplace productivity, contributing to a substantial socioeconomic burden [2]. Currently available pharmacological treatments, including benzodiazepines, selective serotonin reuptake inhibitors (SSRIs), and serotonin–norepinephrine reuptake inhibitors (SNRIs), provide effective symptom relief but are not without limitations. Benzodiazepines are widely prescribed due to their rapid anxiolytic effect; however, they are associated with tolerance, dependence, cognitive impairment, and withdrawal syndromes, making them unsuitable for long-term management. On the other hand, SSRIs and SNRIs are better suited for chronic use but present challenges such as delayed onset of action, gastrointestinal side effects, insomnia, and sexual dysfunction, which frequently reduce patient adherence [3]. These limitations underscore the need for safer and more tolerable alternatives, especially for patients requiring long-term therapy. In recent decades, herbal medicines have attracted increasing attention as complementary or alternative approaches for managing anxiety. Many botanicals have been used in Ayurveda, Traditional Chinese Medicine, and Western herbal practices for centuries to reduce stress, improve sleep, and stabilize mood. Modern pharmacological investigations reveal that herbal drugs may exert anxiolytic effects by enhancing GABAergic activity, modulating serotonergic and dopaminergic signaling, regulating the hypothalamic–pituitary–adrenal (HPA) axis, and reducing oxidative stress [4]. These multimodal mechanisms may offer therapeutic benefits with fewer adverse effects compared to conventional anxiolytics. Several herbal drugs have shown promise in both preclinical and clinical settings. For example, Withania somnifera (Ashwagandha) has demonstrated significant reductions in stress and anxiety scores in randomized controlled trials, likely due to its adaptogenic effects and cortisol-lowering properties. Matricaria chamomilla (Chamomile) has been studied in generalized anxiety disorder with encouraging results, while Passiflora incarnata (Passionflower) and Valeriana officinalis (Valerian) exhibit GABA-mediated anxiolytic activity. Conversely, Piper methysticum (Kava) shows proven efficacy but its clinical use is controversial due to reports of hepatotoxicity [5]. Despite promising evidence, challenges such as lack of standardization, variability in phytochemical composition, herb–drug interactions, and limited large-scale clinical trials hinder the integration of herbal drugs into mainstream therapy. This review aims to explore the anxiolytic potential of herbal medicines, summarizing their mechanisms of action, preclinical and clinical evidence, safety profile, and future research directions.

2. Pathophysiology of Anxiety Disorders

The pathophysiology of anxiety disorders is multifactorial, involving complex interactions among neurotransmitter systems, stress response pathways, and oxidative mechanisms. Understanding these neurobiological substrates is critical for identifying therapeutic targets of herbal anxiolytics.

1. Neurotransmitter Dysregulation

• GABAergic system: Reduced gamma-aminobutyric acid (GABA) activity contributes to neuronal hyperexcitability and heightened anxiety. Several herbal drugs such as Valeriana officinalis and Passiflora incarnata are reported to enhance GABAergic transmission [6].
• Serotonergic system: Altered serotonin (5-HT) signaling, particularly at 5-HT1A receptors, has been strongly linked to anxiety disorders. Bacopa monnieri and Hypericum perforatum are suggested to influence serotonergic modulation [6].
• Noradrenergic and dopaminergic systems: Dysregulated noradrenaline from the locus coeruleus and altered dopamine pathways contribute to hypervigilance and fear responses. Adaptogenic herbs like Withania somnifera may restore this balance [6].

2. Hypothalamic–Pituitary–Adrenal (HPA) Axis Dysfunction

• Chronic stress leads to hyperactivity of the HPA axis, causing elevated cortisol secretion and impaired feedback inhibition, thereby perpetuating anxiety symptoms.
• Dysregulated cortisol signaling is a hallmark of generalized anxiety and related disorders [7].
• Herbal adaptogens such as Withania somnifera demonstrate cortisol-lowering effects, highlighting their therapeutic potential.

3. Neuroinflammation and Oxidative Stress

• Anxiety disorders are associated with increased oxidative stress and elevated pro-inflammatory cytokines, which impair neuronal signaling.
• Oxidative damage affects hippocampal and amygdala function, brain regions central to fear and anxiety processing [8].
• Herbal agents like Bacopa monnieri and Nardostachys jatamansi, rich in antioxidants, may counteract these effects.

4. Structural and Functional Brain Abnormalities

• Neuroimaging studies indicate hyperactivity in the amygdala and reduced top-down regulation by the prefrontal cortex, contributing to excessive worry and fear responses [6].
• Altered connectivity within limbic circuits sustains chronic anxiety symptoms. Herbal drugs with neurotrophic and neuroprotective effects may indirectly modulate these networks.

3. Mechanisms of Action of Herbal Anxiolytics

3.1 GABAergic Modulation (9 – 12)

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system. Reduced GABA activity is strongly linked with anxiety disorders, as low inhibition leads to excessive neuronal excitability and heightened stress responses.

• Several herbs (e.g., Valerian, Passionflower, Kava) contain phytochemicals that enhance GABAergic transmission by binding to GABAA_AA​ receptors, increasing chloride ion influx, and producing calming effects similar to benzodiazepines.
• Unlike synthetic benzodiazepines, these herbal agents often exert gentler effects and may have fewer issues with dependence and tolerance.

3.2 Serotonergic Modulation

The serotonin (5-HT) system is another major regulator of mood and anxiety. Dysregulation of serotonin pathways, particularly 5-HT1A_1A1​A and 5-HT2A_2A2​A receptors, has been implicated in generalized anxiety disorder, social anxiety, and panic disorder.

• Herbal medicines like Ashwagandha and Chamomile have been shown in preclinical and clinical studies to modulate serotonergic transmission, either by acting as partial agonists on serotonin receptors or enhancing serotonin availability.
• This mechanism is somewhat similar to SSRIs but tends to involve multiple receptor sites simultaneously, providing a more balanced effect.

3.3 Dopaminergic and Adrenergic Pathways

Dopamine and norepinephrine are excitatory neurotransmitters that regulate arousal, motivation, and stress responses. Overactivation of these pathways is linked to restlessness, hypervigilance, and anxiety.

• Herbal extracts (e.g., Brahmi and Jatamansi) can modulate dopaminergic and adrenergic signaling, helping to restore equilibrium between excitatory and inhibitory neurotransmitters.
• By reducing excessive catecholamine activity, these herbs promote relaxation, reduce palpitations, and alleviate anxiety-related physical symptoms.

3.4 Cortisol Suppression and HPA Axis Regulation

The hypothalamic–pituitary–adrenal (HPA) axis is central to the body’s stress response. Chronic stress leads to excessive cortisol release, which is associated with anxiety, sleep disturbances, and impaired cognition.

• Adaptogenic herbs like Ashwagandha and Rhodiola reduce cortisol levels and normalize HPA axis activity.
• This neuroendocrine regulation helps the body adapt to stress more effectively, preventing the vicious cycle of chronic anxiety.

3.5 Antioxidant and Neuroprotective Activity

Oxidative stress and neuroinflammation are increasingly recognized as important contributors to anxiety disorders. Reactive oxygen species (ROS) can damage neurons, impair neurotransmitter function, and disrupt neural circuits involved in emotional regulation.

• Herbs such as Brahmi, Chamomile, and Jatamansi contain flavonoids, saponins, and other phytochemicals with strong antioxidant and anti-inflammatory properties.
• These compounds help protect neurons from oxidative damage, support mitochondrial function, and promote neurogenesis, which may explain the long-term benefits of herbal treatments in anxiety.

4. Major Herbal Drugs with Anxiolytic Potential

4.1 Ashwagandha (Withania somnifera)

Ashwagandha, commonly known as Indian ginseng or winter cherry, is a well-known adaptogenic herb in Ayurveda. It helps the body cope with stress and anxiety by reducing cortisol levels and normalizing hypothalamic–pituitary–adrenal (HPA) axis function. Preclinical studies show that Ashwagandha enhances GABAergic signaling and also influences serotonin pathways, leading to calming effects. Clinical trials suggest it can reduce stress, improve sleep, and lower anxiety scores in generalized anxiety disorder patients (13,14).

4.2 Brahmi (Bacopa monnieri)

Brahmi is a traditional Ayurvedic herb used for cognitive enhancement and anxiety. Its active compounds, bacosides, possess antioxidant and neuroprotective properties. Experimental studies reveal that Brahmi modulates serotonin and dopamine levels, reduces oxidative stress, and helps normalize stress-induced changes in neurotransmitters (15). Human studies suggest improved memory, attention, and reduced anxiety symptoms, making it both a nootropic and anxiolytic herb.

4.3 Jatamansi (Nardostachys jatamansi)

Jatamansi, a Himalayan herb, has been traditionally used as a tranquilizer and sleep aid. Animal studies show that it enhances GABA and serotonin activity, while also exerting strong antioxidant effects that protect neurons from stress-related damage. Its anxiolytic effect is believed to be similar to mild benzodiazepines, but with fewer side effects. It has also shown potential in reducing neuroinflammation linked to anxiety disorders (16).

4.4 Valerian (Valeriana officinalis)

Valerian root is widely used in Western herbal medicine for insomnia and anxiety. Its phytochemicals, such as valerenic acid, act on GABAA_AA​ receptors, increasing inhibitory neurotransmission. Valerian is often compared to benzodiazepines in mechanism but produces milder sedation and has less dependence risk. Clinical studies show improvements in sleep quality and anxiety reduction, particularly in generalized anxiety and premenstrual syndrome (17).

4.5 Chamomile (Matricaria chamomilla)

Chamomile is one of the most commonly used medicinal herbs for relaxation. Its flavonoid apigenin binds to benzodiazepine receptors in the brain, producing a mild sedative effect. Chamomile also has anti-inflammatory and antioxidant properties, which may contribute to its anxiolytic action. Clinical evidence suggests that chamomile extracts can reduce symptoms in patients with generalized anxiety disorder and improve sleep quality (14,17).

4.6 Passionflower (Passiflora incarnata)

Passionflower is traditionally used in Native American and European medicine as a calming agent. Its alkaloids and flavonoids enhance GABA activity, leading to anxiolytic and sedative effects. Studies suggest passionflower may reduce restlessness, improve sleep, and decrease physiological symptoms of anxiety such as palpitations. Some trials have even compared passionflower favorably with oxazepam in generalized anxiety disorder patients (13,18).

4.7 Kava (Piper methysticum)

Kava, native to the South Pacific, is one of the most well-studied herbal anxiolytics. Its active compounds, kavalactones, modulate GABAergic and dopaminergic pathways, producing relaxation without significant cognitive impairment. Clinical trials demonstrate significant reductions in anxiety symptoms, making it one of the best-supported herbal remedies. However, long-term or excessive use has been linked to hepatotoxicity, which limits its widespread clinical acceptance (14,18).

5. Experimental Methods to Assess Anxiolytic Activity in Mice

Animal models are widely used to evaluate the anxiolytic potential of herbal and synthetic drugs. These methods allow researchers to assess behavioral, physiological, and biochemical responses that mimic human anxiety. Below are the most common tests applied in preclinical studies.

5.1 Elevated Plus Maze (EPM) Test

The EPM is one of the most widely used models for anxiety. It consists of two open arms and two closed arms elevated above the floor. Normally, mice avoid open arms due to fear of open spaces. Anxiolytic drugs increase the time spent and number of entries into the open arms, whereas anxiogenic agents reduce these measures (19). The test is highly sensitive to benzodiazepines and GABAergic compounds, making it useful for herbal studies such as Ashwagandha and Valerian.

5.2 Light/Dark Box Test

This test is based on rodents’ natural aversion to brightly lit areas. The apparatus contains two compartments: one brightly lit and one dark. Anxiolytic agents increase the time spent in the light box and the number of transitions between compartments, suggesting reduced anxiety (20). Herbal extracts like chamomile and passionflower have been tested using this model.

5.3 Open Field Test (OFT)

The OFT measures both locomotor activity and anxiety. Mice are placed in a large open arena, and their movements are recorded. Increased time in the central zone indicates reduced anxiety, as mice typically prefer staying near walls (thigmotaxis). In addition, locomotion and grooming behavior provide insights into drug-induced sedation versus anxiolysis (21).

5.4 Hole Board Test

This test assesses exploratory behavior by counting the number of times a mouse dips its head into holes in a floor board. Increased head-dipping is interpreted as an anxiolytic effect, while reduced activity may indicate sedation or high anxiety. The hole board is especially useful for distinguishing between sedative and anxiolytic actions of herbal drugs (22).

5.5 Vogel Conflict Test

This is a punished drinking test where thirsty mice are given water but punished with mild shocks for licks. Anxiolytic drugs, such as benzodiazepines, increase the number of punished responses, showing reduced anxiety. It is considered a more pharmacologically specific model, though technically demanding compared to maze-based tests (23).

5.6 Social Interaction Test

This test measures the amount of time two unfamiliar mice spend interacting in a neutral arena. Normally, stress and anxiety reduce social behavior. Anxiolytic drugs increase the duration of social interaction, whereas anxiogenic drugs reduce it. This test is particularly relevant for herbal drugs that modulate serotonin and dopamine pathways, like Brahmi and Kava (20).

5.7 Biochemical Markers (Neurotransmitters and Stress Hormones)

In addition to behavioral models, biochemical assays are used to confirm anxiolytic mechanisms. Common markers include:

• GABA and glutamate levels → reflect excitatory/inhibitory balance.
• Serotonin (5-HT) and dopamine levels → key in mood regulation.
• Cortisol (corticosterone in rodents) → indicates stress response via the HPA axis.
• Oxidative stress markers (lipid peroxidation, antioxidant enzymes) → show neuroprotective effects of herbal drugs (21,22).

These biomarkers strengthen the link between observed behavior and neurochemical mechanisms.