PerspectiveNeuroscience

Caffeine Boosts Bees' Memories

See allHide authors and affiliations

Science  08 Mar 2013:
Vol. 339, Issue 6124, pp. 1157-1159
DOI: 10.1126/science.1234411

Pollination systems are biological markets, where flower visitors choose between flower species on the basis of their quality, such as the sweetness and amount of nectar per flower. Plants in turn compete for pollinators and advertise their product through colorful visual displays and scents. A key challenge in floral advertising is that signals must be not only attractive but also memorable (1): The more distinct a flower signal, the more likely a pollinator is to remember it, increasing the probability that pollinators will visit more flowers of this species while ignoring competing flower species. On page 1202 of this issue, Wright et al. (2) report that some plant species appear to gain an unfair advantage in this competitive market by manipulating the memory of bees with psychoactive drugs.

Caffeine junkies of the wild?

(A) Honeybees often consume caffeinated drinks from discarded cans. Wright et al. show that some plants manipulate the memory of bees by adding caffeine to their nectar. (B) Various antennal odorant receptor types, each responsive to specific chemicals, send information to the mushroom bodies via the antennal lobes (12). A ventral unpaired neuron (VUMmx1) mediating the sucrose reward signal also connects to mushroom bodies (9). [Adapted from (6)] (C) In the odor-learning circuitry in the bee brain, projection neurons connect to Kenyon cells in the mushroom bodies (12). As Wright et al. show (2), simultaneous input to Kenyon cells from olfactory and reward pathways might strengthen synaptic connections between Kenyon cells and output neurons and between projection neurons and Kenyon cells. Caffeine increases transmission at the synapses between projection neurons and Kenyon cells and also enhances Kenyon cell excitability, facilitating the formation of longterm memories for floral scents (2).

PHOTO CREDITS: (PANEL A) AZIZ ALMUTAIRI/UNIVERSITY OF MASSACHUSETTS; MICROGRAPH (PANEL B), AXEL BROCKMANN/NCBS BANGALORE

Many plants contain alkaloids such as caffeine and nicotine. Their bitter taste deters herbivores; at high concentrations, they are toxic. The nectars of some flowers, in addition to various sugars, also contain such secondary compounds (3). This presents a puzzle, because pollinators typically reject bitter nectar (4). However, at low concentrations, bees appear to prefer caffeine-containing nectar (see the figure, panel A) (3). Wright et al. show that caffeine concentrations in the nectar of various Coffea and Citrus flower species never exceed levels at which they might deter bees. Hence, flowers are careful not to leak too much bitterness into the sweetness of their nectar. But why is there caffeine in nectar at all?

In mammals, caffeine is a cognitive enhancer (5). Wright et al. show that caffeine also has a dramatic effect on the long-term memory of bees. The authors trained bees to associate a floral scent with a sucrose reward. If, during training, the reward droplet contained caffeine, twice as many bees remembered the scent 3 days later.

The olfactory receptors of insects are distributed along their antennae (see the figure, panel B). Receptor cell axons extend to the primary olfactory centers, the antennal lobes (6). From the antennal lobes, projection neurons connect to the mushroom bodies, which mediate sensory integration and learning in insect brains (7). The mushroom bodies of honeybees contain about 370,000 so-called Kenyon cells. The projection neurons appear to release acetylcholine as the primary neurotransmitter, which binds to acetylcholine receptors in Kenyon cell dendrites (8). The mushroom bodies also receive information about sugary rewards from the bee's mouthparts by way of the VUMmx1 neuron, the single neuron that constitutes the reward pathway in bee olfactory learning (9).

Wright et al. found that caffeine increases the excitability of Kenyon cells. Blocking the acetylcholine receptors of Kenyon cells reverses the caffeine effect. This makes it likely that the effects of caffeine are due to increased activity of projection neurons from the antennal lobes.

The authors propose that the increased activation of Kenyon cells could be due to the interaction of caffeine and adenosine receptors in projection neurons. In mammals, adenosine acts as an inhibitory neuromodulator, and caffeine is a known antagonist of adenosine (5). The application of DPCPX (an antagonist of adenosine receptors) to bee brains produces a similar excitation of Kenyon cells as caffeine, indicating that the observed effects of caffeine on long-term memory could indeed be via blocking of adenosine receptors (2).

It is thus conceivable that caffeine facilitates strengthening of synaptic connections between Kenyon cells and olfactory projection neurons activated by a particular floral scent, especially under the modulatory influence of signals from the reward pathway (see the figure, panel C) (7). Connections between Kenyon cells and other neurons further downstream could also be affected (2).

The discovery of the cognitive effects of psychoactive drugs in floral nectar opens new perspectives in the competitive race between plant species to lure pollinators. Because these substances occur in many plant tissues (as deterrents to herbivores), they could be added to nectar at little extra cost to the plant, with profound effects on pollinator behavior. If as a result of caffeine ingestion, bees remember the traits of the flowers better, they might be more likely to stay faithful to these flowers and disregard others. Further research may show whether drugs in nectar not only influence pollinator preference via enhanced memory for floral traits but also might have addictive effects. These would be recognizable by drug seeking despite known adverse effects [such as predation risk at flowers (10)], relapses after periods of abstinence, or withdrawal symptoms (11).

References

View Abstract

Stay Connected to Science

Navigate This Article