Is GABA involved in the development of caffeine tolerance?

Caffeine induces neurobehavioral effects through modulating neurotransmitters

Evidence demonstrates that chronic caffeine exposure, primarily through consumption of coffee or tea, leads to increased alertness and anxiety. Preclinical and clinical studies showed that caffeine induced beneficial effects on mood and cognition. Other studies using molecular techniques have reported that caffeine exhibited neuroprotective effects in animal models by protecting dopaminergic neurons. Moreover, caffeine interacts with dopaminergic system, which leads to improvements in neurobehavioral measures in animal models of depression or attention deficit hyperactivity disorder (ADHD). Glutamatergic receptors have been found to be involved on the neurobiological effects of caffeine. Additionally, caffeine has been found to suppress the inhibitory (GABAergic) activity and modulate GABA receptors. Studies have also found that modulating these neurotransmitters leads to neurobehavioral effects. The linkage between the modulatory role of caffeine on neurotransmitters and neurobehavioral effects has not been fully discussed. The purpose of this review is to discuss in detail the role of neurotransmitters in the effects of caffeine on neurobehavioral disorders.

Modulatory effects of caffeine on oxidative stress and anxiety-like behavior in ovariectomized rats

Menopause is accompanied by enhanced oxidative stress and behavioral changes, effects attenuated by antioxidants. The aim of this study was to evaluate the effects of caffeine on behavior and oxidative stress in an experimental model of menopause. Female rats were divided into the following groups: sham-operated (CON), sham-operated and caffeine-treated (CAF), ovariectomized (OVX), ovariectomized and caffeine-treated (OVX+CAF). Caffeine (6 mg/kg) and vehicle were administered for 21 days (subchronic) and 42 days (chronic), using 2 experimental subsets. Behavioral tests and oxidative stress parameters in the blood, whole brain, and hippocampus were assessed. The subchronic administration of caffeine decreased the lipid peroxidation and improved the antioxidant defense in the blood and brain. The GSH/GGSG ratio in the brain was improved by chronic administration, with reduced activities of antioxidant enzymes and enhanced nitric oxide and malondialdehyde levels. In particular, the lipid peroxidation in the hippocampus decreased in both experiments. The rats became hyperactive after 21 days of treatment, but no effect was observed after chronic administration. In both experimental subsets, caffeine had anxiolytic effects as tested in elevated plus maze. The administration of low doses of caffeine, for a short period of time, may be a new therapeutic approach to modulating the oxidative stress and anxiety in menopause.

Does caffeine reverse the EAC cell-induced immune suppression?

The aim of this study was to investigate the effect of long-term consumption of caffeine in the development of Ehrlich ascites carcinoma (EAC) cells in adult female mice, 25–30 g, in relation to immune response. Mice were treated with caffeine (20 mg kg−1 daily, p.o.) for 22–27 consecutive days or inoculated with EAC cells (5 times 106 cells/mL, i.p.), or both. Control mice, corresponding to experimental groups, were treated with corresponding vehicles under similar conditions. The lymphocyte viability, mitogen-induced proliferating activity, cytotoxicity and DNA fragmentation from blood, spleen and thymus of both control and experimental groups were measured as immune response parameters. An immune response index, corticosterone, was also measured in adrenals and plasma under similar conditions. Results showed that development of EAC cells caused immune suppression with a reduction of lymphocyte viability, cytotoxicity and proliferative activity and induction of DNA fragmentation in those tissues, as well as an increase in plasma corticosterone. Though long-term caffeine treatment (which resulted in tolerance to caffeine) alone did not alter significantly any of the immune response parameters studied, including corticosterone status (immune biomarker), the continuation of caffeine treatment during the development of EAC cells either restored or reduced the EAC cell-induced alteration in these parameters, including the HPA axis biomarker. These results suggest that long-term caffeine intake may inhibit or reverse the EAC cell-induced immune suppression.

Reversal of caffeine-induced anxiety by neurosteroid 3-alpha-hydroxy-5-alpha-pregnane-20-one in rats

Caffeine has been shown to increase brain and plasma content of neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP) that allosterically modulates GABA(A) receptors. The present study evaluated the role of neurosteroid 3alpha,5alpha-THP in the caffeine-induced anxiogenic-like effect using the elevated plus-maze (EPM) test in rats. Acute administration of caffeine (50 or 100mg/kg, i.p.) produced anxiogenic-like activity that was reversed by pretreatment with the neurosteroid 3alpha,5alpha-THP or progesterone, the GABA(A) agonist muscimol, or the benzodiazepine receptor agonist diazepam. On the contrary, caffeine produced higher anxiety in animals previously treated with the GABA(A) receptor antagonist, bicuculline or either of the various neurosteroid biosynthesis enzyme inhibitors viz. trilostane, finasteride or indomethacin. Furthermore, pretreatment with DHEAS, a neurosteroid that negatively modulates GABA(A) receptors also enhanced the caffeine-induced anxiety. Moreover, adrenalectomy potentiated the anxiogenic-like response of caffeine indicating the contributory role of peripheral steroidogenesis. Thus, it is speculated that neurosteroid 3alpha,5alpha-THP through positive modulation of GABA(A) receptor activity may serve as a counter-regulatory mechanism against caffeine-induced anxiety.

Effects of caffeine on locomotor activity of horses: determination of the no-effect threshold

Caffeine is the legal stimulant consumed most extensively by the human world population and may be found eventually in the urine and/or blood of race horses. The fact that caffeine is in foods led us to determine the highest no-effect dose (HNED) of caffeine on the spontaneous locomotor activity of horses and then to quantify this substance in urine until it disappeared. We built two behavioural stalls equipped with juxtaposed photoelectric sensors that emit infrared beams that divide the stall into nine sectors in a 'tic-tac-toe' fashion. Each time a beam was interrupted by a leg of the horse, a pulse was generated; the pulses were counted at 5-min intervals and stored by a microcomputer. Environmental effects were minimized by installing exhaust fans producing white noise that obscured outside sounds. One-way observation windows prevented the animals from seeing outside. The sensors were turned on 45 min before drug administration (saline control or caffeine). The animals were observed for up to 8 h after i.v. administration of 2.0, 2.5, 3.0 or 5.0 mg caffeine kg(-1). The HNED of caffeine for stimulation of the spontaneous locomotor activity of horses was 2.0 mg kg(-1). The quantification of caffeine in urine and plasma samples was done by gradient HPLC with UV detection. The no-effect threshold should not be greater than 2.0 microg caffeine ml(-1) plasma or 5.0 microg caffeine ml(-1) urine.

Lack of synergism between caffeine and SKF 38393 on rotational behavior in 6-hydroxydopamine-denervated rats

We have recently shown a synergistic effect between caffeine and the dopamine D(2) receptor agonist, bromocriptine, on contralateral rotational behavior in unilaterally 6-hydroxydopamine-denervated rats. In addition, we found that bromocriptine prevented caffeine-induced tolerance to this behavior following repeated treatment. In the present study, we investigated whether or not the dopamine D(1) receptor agonist, (+/-)-phenyl-2,3,4, 5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF 38393), presented similar characteristics. Different groups of rats received simultaneous injections of either vehicle plus vehicle, caffeine (40 mg/kg) plus vehicle, SKF 38393 (0.5, 1, 2, and 4 mg/kg) plus vehicle, or caffeine plus SKF 38393 (0.5, 1, 2, and 4 mg/kg) for 5 consecutive days, and both ipsilateral and contralateral rotational behavior was measured. Results showed that, on the first day of treatment, caffeine produced significantly more rotational behavior than did a low dose of SKF 38393 (0.5 mg/kg), and significantly less turning than at higher doses (2 and 4 mg/kg). Combined treatment with caffeine and a high dose of SKF 38393 (4 mg/kg) produced significantly more rotational behavior than did caffeine plus vehicle. With repeated administration, caffeine produced sustained tolerance to its effects on rotational behavior, whereas SKF 38393 did not. In the groups treated with low doses of SKF 38393 (0.5, and 1 mg/kg) plus caffeine, tolerance was observed while in the groups that received high doses of SKF 38393 (2 and 4 mg/kg) plus caffeine, no tolerance was observed to rotational behavior. These results suggest that maximal stimulation of dopamine D(1) receptors may be needed to prevent the tolerance effects of caffeine in this animal model. This finding may have clinical relevance to the therapeutic treatment of Parkinson's disease.

The best defense against hypoglycemia is to recognize it: is caffeine useful?

Caffeine, 1,3,7trimethylxanthine, is used by 80% of the adult population of the world in its various forms. Even the simple pleasure of consuming this socially acceptable drug has implications for the person with diabetes mellitus. Caffeine may increase an individual's sensitivity to hypoglycemia through the combined effects of reducing substrate delivery to the brain via constriction of the cerebral arteries, whilst simultaneously increasing brain glucose metabolism and augmenting catecholamine production. This article summarizes the evidence supporting the hypothesis that caffeine influences the perception of and physiological response to hypoglycemia. Under laboratory conditions, acute ingestion of caffeine markedly enhances the symptomatic and sympathoadrenal responses to hypoglycemia in both healthy volunteers and patients with type 1 diabetes. Recently a study of free-living people with type 1 diabetes showed that caffeine consumption increased the awareness of hypoglycemia. Caffeine has been associated with a number of negative effects and addiction. Most serious of these associations are ischemic heart disease and hypertension, the relationships have not been clearly established and the evidence to date is controversial. Thus we conclude that in modest doses, caffeine may be a useful adjuvant therapy for patients with hypoglycemia unawareness. For once here is a therapy which is inexpensive, safe, and remarkably popular with its consumers.