Adenosinergic psychopharmaceuticals?

GABA and l-theanine mixture decreases sleep latency and improves NREM sleep

CONTEXT

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter and it is well established that activation of GABA_A receptors favours sleep. l-Theanine, a naturally occurring amino acid first discovered in green tea, is a well-known anti-anxiety supplement with proven relaxation benefits.

OBJECTIVE

This study investigated the potential synergistic sleep enhancement effect of GABA/l-theanine mixture.

MATERIALS AND METHODS

Pentobarbital-induced sleep test was applied to find proper concentration for sleep-promoting effect in ICR mice. Electroencephalogram (EEG) analysis was performed to investigate total sleeping time and sleep quality in normal SD rats and caffeine-induced awareness model. Real-time polymerase chain reaction (RT-PCR) was applied to investigate whether the sleep-promoting mechanism of GABA/l-theanine mixture involved transcriptional processes.

RESULTS

GABA/l-theanine mixture (100/20 mg/kg) showed a decrease in sleep latency (20.7 and 14.9%) and an increase in sleep duration (87.3 and 26.8%) compared to GABA or theanine alone. GABA/l-theanine mixture led to a significant increase in rapid eye movement (REM) (99.6%) and non-REM (NREM) (20.6%) compared to controls. The use of GABA/l-theanine mixture rather than GABA or l-theanine alone restored to normal levels sleep time and quality in the arousal animal model. The administration of GABA/l-theanine led to increased expression of GABA and the glutamate GluN1 receptor subunit.

CONCLUSIONS

GABA/l-theanine mixture has a positive synergistic effect on sleep quality and duration as compared to the GABA or l-theanine alone. The increase in GABA receptor and GluN1 expression is attributed to the potential neuromodulatory properties of GABA/l-theanine combination, which seems to affect sleep behaviour.

Antidepressant-Like Activity of Typical Antidepressant Drugs in the Forced Swim Test and Tail Suspension Test in Mice Is Augmented by DMPX, an Adenosine A2A Receptor Antagonist

Unsatisfactory therapeutic effects of currently used antidepressants force to search for new pharmacological treatment strategies. Recent research points to the relationship between depressive disorders and the adenosinergic system. Therefore, the main goal of our studies was to evaluate the effects of DMPX (3 mg/kg, i.p.), which possesses selectivity for adenosine A_2A receptors versus A₁ receptors, on the activity of imipramine (15 mg/kg, i.p.), escitalopram (2.5 mg/kg, i.p.), and reboxetine (2 mg/kg, i.p.) given in subtherapeutic doses. The studies carried out using the forced swim and tail suspension tests in mice showed that DMPX at a dose of 6 and 12 mg/kg exerts antidepressant-like effect and does not affect the locomotor activity. Co-administration of DMPX at a dose of 3 mg/kg with the studied antidepressant drugs caused the reduction of immobility time in both behavioral tests. The observed effect was not associated with an increase in the locomotor activity. To evaluate whether the observed effects were due to a pharmacokinetic/pharmacodynamic interaction, the levels of the antidepressants in blood and brain were measured using high-performance liquid chromatography. It can be assumed that the interaction between DMPX and imipramine was exclusively pharmacodynamic in nature, whereas an increased antidepressant activity of escitalopram and reboxetine was at least partly related to its pharmacokinetic interaction with DMPX.

L-theanine partially counteracts caffeine-induced sleep disturbances in rats

L-theanine has been reported to inhibit the excitatory effects of caffeine. The present study examined the effects of L-theanine on caffeine-induced sleep disturbances in rats. Rats received the following drug pairings: saline and saline (Control), 7.5 mg/kg caffeine and saline, or 7.5 mg/kg of caffeine followed by various doses of L-theanine (22.5, 37.5, 75, or 150 mg/kg). Vigilance states were divided into: wakefulness (W), transition to slow-wave sleep (tSWS), slow-wave sleep (SWS), and rapid-eye-movement sleep (REMS). Caffeine significantly increased the duration of W and decreased the duration of SWS and REMS compared to the Control. Although L-theanine failed to reverse the caffeine-induced W increase, at 22.5 and 37.5 mg/kg (but not at 75 and 150 mg/kg), it significantly reversed caffeine-induced decreases in SWS. In conclusion, low doses of L-theanine can partially reverse caffeine-induced reductions in SWS; however, effects of L-theanine on caffeine-induced insomnia do not appear to increase dose-dependently.

Adenosine receptors and brain diseases: neuroprotection and neurodegeneration

Adenosine acts in parallel as a neuromodulator and as a homeostatic modulator in the central nervous system. Its neuromodulatory role relies on a balanced activation of inhibitory A(1) receptors (A1R) and facilitatory A(2A) receptors (A2AR), mostly controlling excitatory glutamatergic synapses: A1R impose a tonic brake on excitatory transmission, whereas A2AR are selectively engaged to promote synaptic plasticity phenomena. This neuromodulatory role of adenosine is strikingly similar to the role of adenosine in the control of brain disorders; thus, A1R mostly act as a hurdle that needs to be overcame to begin neurodegeneration and, accordingly, A1R only effectively control neurodegeneration if activated in the temporal vicinity of brain insults; in contrast, the blockade of A2AR alleviates the long-term burden of brain disorders in different neurodegenerative conditions such as ischemia, epilepsy, Parkinson's or Alzheimer's disease and also seem to afford benefits in some psychiatric conditions. In spite of this qualitative agreement between neuromodulation and neuroprotection by A1R and A2AR, it is still unclear if the role of A1R and A2AR in the control of neuroprotection is mostly due to the control of glutamatergic transmission, or if it is instead due to the different homeostatic roles of these receptors related with the control of metabolism, of neuron-glia communication, of neuroinflammation, of neurogenesis or of the control of action of growth factors. In spite of this current mechanistic uncertainty, it seems evident that targeting adenosine receptors might indeed constitute a novel strategy to control the demise of different neurological and psychiatric disorders.

Adenosine A(2A) receptors in psychopharmacology: modulators of behavior, mood and cognition

The adenosine A(2A) receptor (A(2A)R) is in the center of a neuromodulatory network affecting a wide range of neuropsychiatric functions by interacting with and integrating several neurotransmitter systems, especially dopaminergic and glutamatergic neurotransmission. These interactions and integrations occur at multiple levels, including (1) direct receptor- receptor cross-talk at the cell membrane, (2) intracellular second messenger systems, (3) trans-synaptic actions via striatal collaterals or interneurons in the striatum, (4) and interactions at the network level of the basal ganglia. Consequently, A(2A)Rs constitute a novel target to modulate various psychiatric conditions. In the present review we will first summarize the molecular interaction of adenosine receptors with other neurotransmitter systems and then discuss the potential applications of A(2A)R agonists and antagonists in physiological and pathophysiological conditions, such as psychostimulant action, drug addiction, anxiety, depression, schizophrenia and learning and memory.

Potential therapeutic interest of adenosine A2A receptors in psychiatric disorders

The interest on targeting adenosine A(2A) receptors in the realm of psychiatric diseases first arose based on their tight physical and functional interaction with dopamine D(2) receptors. However, the role of central A(2A) receptors is now viewed as much broader than just controlling D(2) receptor function. Thus, there is currently a major interest in the ability of A(2A) receptors to control synaptic plasticity at glutamatergic synapses. This is due to a combined ability of A(2A) receptors to facilitate the release of glutamate and the activation of NMDA receptors. Therefore, A(2A) receptors are now conceived as a normalizing device promoting adequate adaptive responses in neuronal circuits, a role similar to that fulfilled, in essence, by dopamine. This makes A(2A) receptors particularly attractive targets to manage psychiatric disorders since adenosine may act as go-between glutamate and dopamine, two of the key players in mood processing. Furthermore, A(2A) receptors also control glia function and brain metabolic adaptation, two other emerging mechanisms to understand abnormal processing of mood, and A(2A) receptors are important players in controlling the demise of neurodegeneration, considered an amplificatory loop in psychiatric disorders. Current data only provide an indirect confirmation of this putative role of A(2A) receptors, based on the effects of caffeine (an antagonist of both A(1) and A(2A) receptors) in psychiatric disorders. However, the introduction of A(2A) receptors antagonists in clinics as anti-parkinsonian agents is hoped to bolster our knowledge on the role of A(2A) receptors in mood disorders in the near future.

Cannabis-associated psychosis: current status of research

Cannabis has been used for recreational, medicinal and religious purposes in different cultures since ancient times. There have been various reports of adverse effects due to or associated with cannabis consumption, including psychotic episodes. Historically, our understanding of these clinical observations has been significantly hindered by a lack of knowledge regarding their underlying neurobiological and pharmacological processes. However, the discovery of the endogenous cannabinoid system has allowed a greater understanding of these adverse effects to develop. From a clinical perspective, toxic or transient psychotic reactions to the administration of herbal cannabis preparations or specific cannabinoid compounds have to be differentiated from longer-lasting, persistent schizophrenia-like disorders associated with the use of cannabis/cannabinoids. The latter are most likely to be associated with a predisposition or vulnerability to schizophrenia. Interestingly, the recently suggested role of the endogenous cannabinoid system in schizophrenia not related to previous cannabinoid consumption introduces an additional perspective on the mechanism underlying cannabis-associated schizophrenia-like disorders, as well as on the effects of cannabis consumption in schizophrenia. At present, acute psychopharmacological treatment options for cannabis-associated transient and persistent schizophrenia-like psychotic episodes are similar and are based on the use of benzodiazepines and antipsychotics. However, new pharmacological strategies using the endogenous cannabinoid system as a primary target are under development. Long-term psychotherapeutic treatment options involve case management strategies and are mainly based on specialised psychotherapeutic programmes to encourage cannabis users to stop their use of the drug.

Effects of adenosine agonists and an antagonist on excitatory transmitter release from the ischemic rabbit hippocampus

The purpose of this study was to determine the effects of adenosine agonists and an antagonist on ischemia-induced extracellular glutamate concentrations in an animal model of transient cerebral ischemia using in vivo cerebral microdialysis. Fifty New Zealand white rabbits were randomly assigned to one of five groups (normothermia, hypothermia, cyclopentyladenosine (CPA), theophylline, or propentofylline). Microdialysis probes were stereotactically placed in the dorsal hippocampus. Twenty minutes before the onset of ischemia, either 1 mg/kg CPA, 5 mg/kg propentofylline, or 20 mg/kg theophylline were administered intravenously. Esophageal temperature was maintained at 38 degrees C, except in the hypothermic animals, which were cooled to 30 degrees C throughout the entire experiment. Two 12-min periods of cerebral ischemia, separated by a 105-min interval of reperfusion, were produced by inflating a neck tourniquet. High-performance liquid chromatography was used to determine the glutamate concentration in the microdialysate. There were no significant increases in glutamate concentrations during the first ischemic period in any of the five groups. During the second ischemic episode, glutamate concentrations in the normothermic group peaked at levels approximately three times higher than the initial values. A similar pattern of changes in glutamate concentrations was observed in the CPA, propentofylline, and theophylline groups. In the hypothermic group, the concentrations of glutamate remained at baseline levels during the entire experiment. Contrary to expectations, neither the adenosine agonists (CPA, propentofylline) nor the antagonist (theophylline) had any effect on extracellular glutamate concentrations in the peri-ischemic period. Although adenosine and its analogs may be cerebroprotective agents, their mechanism of action is not fully understood. The data derived from this study indicates that the acute administration of such agents had no effect on ischemia-induced glutamate release within the hippocampus under these experimental conditions. Based on these results, further work is needed to compare in vivo versus in vitro experimental results in acute and long-term treatment studies with adenosine receptor agonists and antagonists.

Physiological and pharmacological properties of adenosine: therapeutic implications

Adenosine is a nucleoside which has been shown to participate in the regulation of physiological activity in a variety of mammalian tissues, and has been recognized as a homeostatic neuromodulator. It exerts its actions via membrane-bound receptors which have been characterized using biochemical, electrophysiological and radioligand binding techniques. Adenosine has been implicated in the pharmacological actions of several classes of drugs. A number of studies strongly suggest that the nucleoside may regulate cellular activity in many pathological disorders and, in that respect, adenosine derivatives appear as promising candidates for the development of new therapeutic compounds, such as anticonvulsant, anti-ischemic, analgesic and neuroprotective agents.