In vivo neurotransmitter release in the locus coeruleus--effects of hyperforin, inescapable shock and fear

St. John's wort (Hypericum perforatum) and depression: what happens to the neurotransmitter systems?

St. John's wort (Hypericum perforatum) is a herbaceous plant containing many bioactive molecules including naphthodianthrones, phloroglucinol derivatives, flavonoids, bioflavonoids, proanthocyanidins, and chlorogenic acid. Evidence has shown the therapeutic effects of St. John's wort and especially its two major active components, hyperforin and hypericin, on different psychiatric and mood disorders such as posttraumatic stress disorder (PTSD), attention-deficit hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD), and anxiety disorders. St. John's wort also induces antidepressant effects. In this review study, we aimed to discuss the role of St. John's wort in modulating depression, with respect to the role of different neurotransmitter systems in the brain. We discussed changes in the neurotransmitter levels in depression, and following use of St. John's wort. It was concluded that changes in the function and level of neurotransmitters in depression are complex. Also, St. John's wort can induce inconsistent effects on neurotransmitter levels. We also found that glutamate and acetylcholine may be the most important neurotransmitters to study in future works, because the function of both neurotransmitters in depression is unclear. In addition, St. John's wort induces a dualistic modulation on the activity of cholinergic signaling, which can be an interesting topic for future studies.

Repeated fear-induced diurnal rhythm disruptions predict PTSD-like sensitized physiological acute stress responses in F344 rats

AIM

To identify objective factors that can predict future sensitized stress responses, thus allowing for effective intervention prior to developing sensitization and subsequent stress-related disorders, including post-traumatic stress disorder (PTSD).

METHODS

Adult male F344 rats implanted with biotelemetry devices were exposed to repeated conditioned fear or control conditions for 22 days followed by exposure to either no, mild or severe acute stress on day 23. Diurnal rhythms of locomotor activity (LA), heart rate (HR) and core body temperature (CBT) were biotelemetrically monitored throughout the study. In a subset of rat not implanted, corticosterone and indices of chronic stress were measured immediately following stress.

RESULTS

Rats exposed to repeated fear had fear-evoked increases in behavioural freezing and HR/CBT during exposure to the fear environment and displayed indices of chronic stress. Repeated fear produced flattening of diurnal rhythms in LA, HR and CBT. Repeated fear did not sensitize the corticosterone response to acute stress, but produced sensitized HR/CBT responses following acute stress, relative to the effect of acute stress in the absence of a history of repeated fear. Greater diurnal rhythm disruptions during repeated fear predicted sensitized acute stress-induced physiological responses. Rats exposed to repeated fear also displayed flattened diurnal LA and basal increases in HR.

CONCLUSIONS

Exposure to repeated fear produces outcomes consistent with those observed in PTSD. The results suggest that diurnal rhythm disruptions during chronic stressors may help predict sensitized physiological stress responses following traumatic events. Monitoring diurnal disruptions during repeated stress may thus help predict susceptibility to PTSD.

Hyperforin activates nonselective cation channels (NSCCs)

A large body of evidence supports the preclinical antidepressant profile of hyperforin including inhibition of the synaptosomal uptake of several neurotransmitters by hyperforin and studies in behavioural models. In contrast to other antidepressants, hyperforin does not directly inhibit neurotransmitter transporters, but instead uptake inhibition seems to be the consequence of an elevated intracellular sodium concentration ([Na+]i). The mechanism of hyperforin-induced elevation of [Na+]i was investigated using two different cell types: human platelets and rat pheochromocytoma cells (PC12 cells). In both cell systems, hyperforin increased both [Na+]i and free intracellular Ca2+ concentration ([Ca2+]i). One pathway for Na+ and Ca2+ entry is mediated by nonselective cation channels (NSCCs), which can be blocked by SK&F 96365 and LOE 908. LOE 908 is a blocker of both NSCC1 and NSCC2 subclasses, while SK&F 96365 blocks NSCC2 only. Both SK&F 96365 and LOE 908 completely inhibited the hyperforin-induced influx of Na+ and Ca2+ into platelets and PC12 cells. This indicates that hyperforin is mainly active upon NSCC2. The effect of hyperforin is inhibited by La3+ and Gd3+, indicating that there is a potential homology with canonical transient receptor potential protein channels (TRPC channels). Moreover, La3+ and Gd3+ attenuate the effect of hyperforin on serotonin uptake in human platelets. Additionally, hyperforin induces barium influx in PC12 cells and this influx can be inhibited by SK&F 96365, LOE 908, Gd3+ and La3+. In summary, these findings suggest that hyperforin represents a new principle for preclinical antidepressant activity, modulating brain neurotransmission by inhibition of neurotransmitter uptake via activation of NSCCs.British Journal of Pharmacology (2005) 145, 75-83. doi:10.1038/sj.bjp.0706155.

Role of hyperforin in the pharmacological activities of St. John's Wort

The phloroglucinol derivative hyperforin has been recently shown to be a major antidepressant component in the extract of Hypericum perforatum. Experimental studies clearly demonstrated its activity in different behavioral models of depression. Moreover clinical studies linked the therapeutic efficacy of Hypericum extracts to their hyperforin content, in a dose-dependent manner. The molecular mechanism of action of hyperforin is still under investigation. Hyperforin has been shown to inhibit, like conventional antidepressants, the neuronal uptake of serotonin, norepinephrine and dopamine. However, hyperforin inhibits also the uptake of gamma-aminobutyric acid (GABA) and L-glutamate. The uptake inhibition by hyperforin does not involve specific binding sites at the transporter molecules; its mechanism of action seems to be related to sodium conductive pathways, leading to an elevation in intracellular Na(+) concentration. Other additional mechanisms of action of hyperforin, involving ionic conductances as well synaptosomal and vesicular function, have been suggested. In addition to its antidepressant activity, hyperforin has many other pharmacological effects in vivo (anxiolytic-like, cognition-enhancing effects) and in vitro (antioxidant, anticyclooxygenase-1, and anticarcinogenic effects). These effects could be of clinical importance. On the other hand, the role of hyperforin in the pharmacological interactions occurring during Hypericum extract therapy must be fully investigated. Hyperforin seems to be responsible for the induction of liver cytochrome oxidase enzymes and intestinal P-glycoprotein. Several pharmacokinetic studies performed in rats and humans demonstrated oral bioavailability of hyperforin from Hypericum extract. Only recently a new chromatographic method for detection of hyperforin in the brain tissue has been developed and validated. Taking into account the chemical instability of hyperforin, current efforts are directed to the synthesis of new neuroactive derivatives.

Mechanism of action of St John's wort in depression : what is known?

Extracts of Hypericum perforatum L. (St John's wort) are now successfully competing for status as a standard antidepressant therapy. Because of this, great effort has been devoted to identifying the active antidepressant compounds in the extract. From a phytochemical point of view, St John's wort is one of the best-investigated medicinal plants. A series of bioactive compounds has been detected in the crude material, namely flavonol derivatives, biflavones, proanthocyanidines, xanthones, phloroglucinols and naphthodianthrones. Although St John's wort has been subjected to extensive scientific studies in the last decade, there are still many open questions about its pharmacology and mechanism of action. Initial biochemical studies reported that St John's wort is only a weak inhibitor of monoamine oxidase-A and -B activity but that it inhibits the synaptosomal uptake of serotonin, dopamine and noradrenaline (norepinephrine) with approximately equal affinity. However, other in vitro binding assays carried out using St John's wort extract demonstrated significant affinity for adenosine, GABA(A), GABA(B) and glutamate receptors. In vivo St John's wort extract leads to a downregulation of beta-adrenergic receptors and an upregulation of serotonin 5-HT(2) receptors in the rat frontal cortex and causes changes in neurotransmitter concentrations in brain areas that are implicated in depression. In studies using the rat forced swimming test, an animal model of depression, St John's wort extracts induced a significant reduction of immobility. In other experimental models of depression, including acute and chronic forms of escape deficit induced by stressors, St John's wort extract was shown to protect rats from the consequences of unavoidable stress. Recent neuroendocrine studies suggest that St John's wort is involved in the regulation of genes that control hypothalamic-pituitary-adrenal axis function. With regard to the antidepressant effects of St John's wort extract, many of the pharmacological activities appear to be attributable to the naphthodianthrone hypericin, the phloroglucinol derivative hyperforin and several flavonoids. This review integrates new findings of possible mechanisms that may underlie the antidepressant action of St John's wort and its active constituents with a large body of existing literature.