Effects of caffeine and various xanthines on hornets and bees

Invasive ant learning is not affected by seven potential neuroactive chemicals

Argentine ants Linepithema humile are one of the most damaging invasive alien species worldwide. Enhancing or disrupting cognitive abilities, such as learning, has the potential to improve management efforts, for example by increasing preference for a bait, or improving ants' ability to learn its characteristics or location. Nectar-feeding insects are often the victims of psychoactive manipulation, with plants lacing their nectar with secondary metabolites such as alkaloids and non-protein amino acids which often alter learning, foraging, or recruitment. However, the effect of neuroactive chemicals has seldomly been explored in ants. Here, we test the effects of seven potential neuroactive chemicals-two alkaloids: caffeine and nicotine; two biogenic amines: dopamine and octopamine, and three nonprotein amino acids: β-alanine, GABA and taurine-on the cognitive abilities of invasive L. humile using bifurcation mazes. Our results confirm that these ants are strong associative learners, requiring as little as one experience to develop an association. However, we show no short-term effect of any of the chemicals tested on spatial learning, and in addition no effect of caffeine on short-term olfactory learning. This lack of effect is surprising, given the extensive reports of the tested chemicals affecting learning and foraging in bees. This mismatch could be due to the heavy bias towards bees in the literature, a positive result publication bias, or differences in methodology.

Bumble bees show an induced preference for flowers when primed with caffeinated nectar and a target floral odor

Caffeine is a widely occurring plant defense chemical^1,2 that occurs in the nectar of some plants, e.g., Coffea or Citrus spp., where it may influence pollinator behavior to enhance pollination.^3,4 Honey bees fed caffeine form longer lasting olfactory memory associations,⁵ which could give plants with caffeinated nectar an adaptive advantage by inducing more visits to flowers. Caffeinated free-flying bees show enhanced learning performance⁶ and are more likely to revisit a caffeinated target feeder or artificial flower,^7-9 although it is not clear whether improved memory of the target cues or the perception of caffeine as a reward is the cause. Here, we show that inexperienced bumble bees (Bombus terrestris) locate new food sources emitting a learned floral odor more consistently if they have been fed caffeine. In laboratory arena tests, we fed bees a caffeinated food alongside a floral odor blend (priming) and then used robotic experimental flowers¹⁰ to disentangle the effects of caffeine improving memory for learned food-associated cues versus caffeine as a reward. Inexperienced bees primed with caffeine made more initial visits to target robotic flowers emitting the target odor compared to control bees or those primed with odor alone. Caffeine-primed bees tended to improve their floral handling time faster. Although the effects of caffeine were short lived, we show that food-locating behaviors in free-flying bumble bees can be enhanced by caffeine provided in the nest. Consequently, there is potential to redesign commercial colonies to enhance bees' forage focus or even bias bees to forage on a specific crop.

Identification of Immune Regulatory Genes in Apis mellifera through Caffeine Treatment

Plants and pollinators are mutually beneficial: plants provide nectar as a food source and in return their pollen is disseminated by pollinators such as honeybees. Some plants secrete chemicals to deter herbivores as a protective measure, among which is caffeine, a naturally occurring, bitter tasting, and pharmacologically active secondary compound. It can be found in low concentrations in the nectars of some plants and as such, when pollinators consume nectar, they also take in small amounts of caffeine. Whilst caffeine has been indicated as an antioxidant in both mammals and insects, the effect on insect immunity is unclear. In the present study, honeybees were treated with caffeine and the expression profiles of genes involved in immune responses were measured to evaluate the influence of caffeine on immunity. In addition, honeybees were infected with deformed wing virus (DWV) to study how caffeine affects their response against pathogens. Our results showed that caffeine can increase the expression of genes involved in immunity and reduce virus copy numbers, indicating that it has the potential to help honeybees fight against viral infection. The present study provides a valuable insight into the mechanism by which honeybees react to biotic stress and how caffeine can serve as a positive contributor, thus having a potential application in beekeeping.

Impact of alkaloids in food consumption, metabolism and survival in a blood-sucking insect

The sense of taste provides information about the “good” or “bad” quality of a food source, which may be potentially nutritious or toxic. Most alkaloids taste bitter to humans, and because bitter taste is synonymous of noxious food, they are generally rejected. This response may be due to an innate low palatability or due to a malaise that occurs after food ingestion, which could even lead to death. We investigated in the kissing bug Rhodnius prolixus, whether alkaloids such as quinine, caffeine and theophylline, are merely distasteful, or if anti-appetitive responses are caused by a post-ingestion physiological effect, or both of these options. Although anti-appetitive responses were observed for the three alkaloids, only caffeine and theophylline affect metabolic and respiratory parameters that reflected an underlying physiological stress following their ingestion. Furthermore, caffeine caused the highest mortality. In contrast, quinine appears to be a merely unpalatable compound. The sense of taste helps insects to avoid making wrong feeding decisions, such as the intake of bitter/toxic foods, and thus avoid potentially harmful effects on health, a mechanism preserved in obligate hematophagous insects.

Exploring cocoa properties: is theobromine a cognitive modulator?

Nutritional qualities of cocoa have been acknowledged by several authors; a particular focus has been placed on its high content of flavanols, known for their excellent antioxidant properties and subsequent protective effect on cardio- and cerebrovascular systems as well as for neuromodulatory and neuroprotective actions. Other active components of cocoa are methylxanthines (caffeine and theobromine). Whereas the effects of caffeine are extensively researched, the same is not the case for theobromine; this review summarizes evidence on the effect of theobromine on cognitive functions. Considering animal studies, it can be asserted that acute exposition to theobromine has a reduced and delayed nootropic effect with respect to caffeine, whereas both animal and human studies suggested a potential neuroprotective action of long-term assumption of theobromine through a reduction of Aβ amyloid pathology, which is commonly observed in Alzheimer's disease patients' brains. Hence, the conceivable action of theobromine alone and associated with caffeine or other cocoa constituents on cognitive modulation is yet underexplored and future studies are needed to shed light on this promising molecule.

Inhibitory effects of caffeine on gustatory plasticity in the nematode Caenorhabditis elegans

The effects of caffeine on salt chemotaxis learning were investigated using the nematode Caenorhabditis elegans. To estimate the degree of salt chemotaxis learning, nematodes were placed in a mixed solution of NaCl and caffeine, and then the chemotaxis index of NaCl was obtained from the nematodes placed on agar medium after pre-exposure to caffeine concentrations of 0.01, 0.1, 0.3, and 1.0%. Locomotor activity and preference behavior for caffeine were also estimated under these caffeine conditions. Nematodes pre-exposed to 0.3% caffeine showed inhibition of salt chemotaxis learning. Additional experiments indicated that nematodes showed a preference response to the middle concentration of caffeine (0.1%), with preference behavior declining in the 0.3% caffeine condition. Stable locomotor activity was observed under 0.01–0.3% caffeine conditions. These results suggest that salt chemotaxis learning with 0.3% caffeine is useful for investigating the effects of caffeine on learning in nematodes.

The buzz on caffeine in invertebrates: effects on behavior and molecular mechanisms

A number of recent studies from as diverse fields as plant-pollinator interactions, analyses of caffeine as an environmental pollutant, and the ability of caffeine to provide protection against neurodegenerative diseases have generated interest in understanding the actions of caffeine in invertebrates. This review summarizes what is currently known about the effects of caffeine on behavior and its molecular mechanisms in invertebrates. Caffeine appears to have similar effects on locomotion and sleep in both invertebrates and mammals. Furthermore, as in mammals, caffeine appears to have complex effects on learning and memory. However, the underlying mechanisms for these effects may differ between invertebrates and vertebrates. While caffeine's ability to cause release of intracellular calcium stores via ryanodine receptors and its actions as a phosphodiesterase inhibitor have been clearly established in invertebrates, its ability to interact with invertebrate adenosine receptors remains an important open question. Initial studies in insects and mollusks suggest an interaction between caffeine and the dopamine signaling pathway; more work needs to be done to understand the mechanisms by which caffeine influences signaling via biogenic amines. As of yet, little is known about whether other actions of caffeine in vertebrates, such as its effects on GABAA and glycine receptors, are conserved. Furthermore, the pharmacokinetics of caffeine remains to be elucidated. Overall behavioral responses to caffeine appear to be conserved amongst organisms; however, we are just beginning to understand the mechanisms underlying its effects across animal phyla.

Associations between purine metabolites and clinical symptoms in schizophrenia

BACKGROUND

The antioxidant defense system, which is known to be dysregulated in schizophrenia, is closely linked to the dynamics of purine pathway. Thus, alterations in the homeostatic balance in the purine pathway may be involved in the pathophysiology of schizophrenia.

METHODOLOGY/PRINCIPAL FINDINGS

Breakdown products in purine pathway were measured using high-pressure liquid chromatography coupled with a coulometric multi-electrode array system for 25 first-episode neuroleptic-naïve patients with schizophrenia at baseline and at 4-weeks following initiation of treatment with antipsychotic medication. Associations between these metabolites and clinical and neurological symptoms were examined at both time points. The ratio of uric acid and guanine measured at baseline predicted clinical improvement following four weeks of treatment with antipsychotic medication. Baseline levels of purine metabolites also predicted clinical and neurological symtpoms recorded at baseline; level of guanosine was associated with degree of clinical thought disturbance, and the ratio of xanthosine to guanosine at baseline predicted degree of impairment in the repetition and sequencing of actions.

CONCLUSIONS/SIGNIFICANCE

Findings suggest an association between optimal levels of purine byproducts and dynamics in clinical symptoms and adjustment, as well as in the integrity of sensory and motor processing. Taken together, alterations in purine catabolism may have clinical relevance in schizophrenia pathology.

Theobromine and the pharmacology of cocoa

The effects of theobromine in man are underresearched, possibly owing to the assumption that it is behaviourally inert. Toxicology research in animals may appear to provide alarming results, but these cannot be extrapolated to humans for a number of reasons. Domestic animals and animals used for racing competitions need to be guarded from chocolate and cocoa-containing foods, including foods containing cocoa husks. Research ought to include caffeine as a comparative agent, and underlying mechanisms need to be further explored. Of all constituents proposed to play a role in our liking for chocolate, caffeine is the most convincing, though a role for theobromine cannot be ruled out. Most other substances are unlikely to exude a psychopharmacological effect owing to extremely low concentrations or the inability to reach the blood-brain barrier, whilst chocolate craving and addiction need to be explained by means of a culturally determined ambivalence towards chocolate.

Some thoughts on the physiology of caffeine in coffee: and a glimpse of metabolite profiling

Human beings enjoy the flavor and stimulating activity of a cup of coffee without knowing that by doing so, they are part of a 'food web' and receive signals coffee plants build to improve their struggle for life. This review is centered in the first part on the purine alkaloid caffeine and its physiological role in the coffee plant's life cycle. Many of the thoughts and ideas presented here are plain speculation, because the real research revealing the secrets of plant physiology such as e.g. the formation of the coffee bean with all its ingredients, has just started. The recent achievements in molecular biology made it possible to tackle and answer new questions regarding the regulation of secondary metabolism in the coffee plant organs at selected stages of their development. Brazilian research groups have much contributed to the recent progress in molecular biology and physiology of coffee. Among them was Maro R. Söndahl, in commemoration of whom this article has been written. Thus, the second part reports on the very first steps Maro and I made together into a very new field of coffee, that is metabolite profiling. The outcome was amazing and gives an idea of the great potential of this technique to map in future the complex network of the coffee metabolom.

Effects of caffeine on olfactory and visual learning in the honey bee (Apis mellifera)

Although caffeine is known to improve alertness and arousal in humans and other mammals, its impacts on specific behaviours, including complex cognitive processes, remain controversial. We reasoned that the availability of an easily manipulable, but behaviourally complex invertebrate organism with a simpler nervous system would be beneficial to this field of research. We used a popular behavioural model, the honeybee, to evaluate the effects of caffeine on (1) the development of olfactory learning and (2) the performance in complex learning paradigms, including a 'delayed-match-to-sample' task and visual associative learning. To evaluate the efficacy of caffeine treatment, a variety of doses (0.4-400 ng/1 mg of body mass) were applied topically to tethered individuals. Behavioural testing was performed with either tethered or free-flying adult honeybees. We show that caffeine has marked cognitive effects in this species. In young honeybees, it reduces the age at which restrained individuals are able to learn an olfactory associative task, whereas in older, free-flying bees, caffeine improves both motivation and cognitive performance in complex learning tasks. Our results suggest that the honeybee model may be useful in explaining caffeine-related behavioural changes not only in this species, but also in mammalian systems.

Discriminative stimulus and subjective effects of theobromine and caffeine in humans

Theobromine versus placebo discrimination and caffeine versus placebo discrimination were studied in two consecutive experiments in seven volunteers who abstained from methylxanthines. Daily sessions involved PO double-blind ingestion of two sets of capsules sequentially, one of which contained drug and the other placebo. Subjects attempted to identify, and were later informed, which set of capsules contained the drug. In each experiment subjects were exposed to progressively lower doses. Five subjects acquired the theobromine discrimination; the lowest dose discriminated ranged from 100 to 560 mg. All seven subjects acquired the caffeine discrimination; the lowest dose discriminated ranged from 1.8 to 178 mg. A final experiment evaluated subjective effect ratings following 560 mg theobromine, 178 mg caffeine and placebo, which were administered double-blind in capsules once daily, five times each in mixed sequence. Caffeine produced changes in both group and individual ratings (e.g. increased well-being, energy, social disposition and alert). Theobromine did not produce changes in group ratings but changed ratings in some subjects. Across subjects, sensitivity to caffeine discriminative effects in the discrimination experiment correlated significantly with the number and magnitude of caffeine subjective effects in the final experiment. This study documents modest discriminative effects of theobromine in humans, but the basis of the discrimination is unclear. This study suggests that commonly consumed cocoa products contain behaviorally active doses of caffeine and possibly theobromine.

Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects

Caffeine is the most widely consumed central-nervous-system stimulant. Three main mechanisms of action of caffeine on the central nervous system have been described. Mobilization of intracellular calcium and inhibition of specific phosphodiesterases only occur at high non-physiological concentrations of caffeine. The only likely mechanism of action of the methylxanthine is the antagonism at the level of adenosine receptors. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons. The methylxanthine induces dose-response increases in locomotor activity in animals. Its psychostimulant action on man is, however, often subtle and not very easy to detect. The effects of caffeine on learning, memory, performance and coordination are rather related to the methylxanthine action on arousal, vigilance and fatigue. Caffeine exerts obvious effects on anxiety and sleep which vary according to individual sensitivity to the methylxanthine. However, children in general do not appear more sensitive to methylxanthine effects than adults. The central nervous system does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported.