Honeybee

A winged social insect of the family Apidea, subfamily Apinae, genus Apis, characterized by special organs of pollen and nectar collection and of honey production.

Humankind has displayed keen interest in bees since the dawn of history. There is no evidence that the opposite was ever true. Cave paintings dating 10000years ago already depict bold honey harvesters driving away the stinging bees with smoke (Menzel and Mercer 1987). A few millennia later, when the Almighty led the Israelites from slavery to freedom, he promised 'a good and large land, a land flowing with milk and honey' (Exodus 3:8). To this day, cultivation of the common honeybee, Apis mellifera L. (Rutner 1988), is a significant source of income to some, and a hobby to others; a cult encyclopaedia detailing whatever-you-ever-wanted-to-know-about-bees-and-never-dared-to-ask-them is claimed to have sold over the years more than half a million copies (Root 1972).

Bees are endowed with a remarkable behavioural repertoire that is manifested in both solitary adventures and social life. It is subserved by acute visual, odour, taste and tactile perceptions and discriminations, a magnetic sense, expert long-range navigation, skilful flight, dance 'language', and more (e.g. von Frisch 1967; Menzel and Mercer 1987; Getz and Page 1991; Seeley 1995; Menzel et al. 1996; Giurfa and Menzel 1997; Sirinivasan et al. 2000; Esch et al. 2001). There is a long and occasionally heated debate in the bee literature on how much of bee behaviour is innate and how much acquired throughout life (e.g. Lindauer 1967; Gould 1984), but clearly, under certain circumstances, bees are proficient learners (Bitterman et al. 1983; Lehrer 1993; Hammer and Menzel 1995; Brown et al. 1998; Erber et al. 1998). They manage to do all their tricks with a tiny brain (1 mm3, not strikingly larger than a single giant neuron in "Aplysia), that contains less than a million nerve cells. Despite the brain's compactness, compartmentalized sensory and associative centres, connecting pathways and certain individual neurons can be identified and manipulated. Bees are hence amenable not only to ethological and behavioural analysis, but also to neuroanatomical, neuropharmaco-logical, and cellular investigation.

Yet those are not the cognitive or neuronal virtues of the bee that have carried it to these pages. Clearly, many species do outperform the bee in behavioural complexity and brain power. The bee is of interest here because it is the subject of a systematic, multilevel, top-down research programme, that has successfully managed to link the phenomenology of ecological behaviour to the mechanistics of circuits and molecules. Furthermore, this programme attempts to explain concretely "real-life learning in terms of alterations in identifiable "internal representations. This critical step in bridging behaviour and brain is still a rather uncommon enterprise in the neurobiology of learning and memory.

Bees can be tested for memory as freely-behaving populations, freely-flying individuals, or restrained individuals. The latter situation offers advantages for mechanistic studies. The most popular paradigm is olfactory "classical conditioning (Bitterman et al. 1983; Hammer and Menzel 1995). A bee extends its proboscis (the insect's version of a tongue) when chemoreceptors on the proboscis or antennae are stimulated by food, e.g. a sucrose solution. This is called the 'proboscis extension reflex' (PER). In the context of classical conditioning, the sucrose is the unconditioned stimulus (US) and the PER the unconditioned response (UR). The response can be conditioned by pairing an odour, which initially is practically neutral with respect to the PER, with sucrose. Pursuing the terminology of classical conditioning, the odour is the conditioned stimulus (CS), and with conditioning comes to evoke the PER (the conditioned response, CR). Such learning is used in foraging, hence is couched in a language familiar to the bee. This is probably the reason why this learning is fast and robust.

Both the CS and the US pathways of the modifiable reflex have been mapped in the bee's brain. The CS pathway starts with the olfactory chemoreceptors, which project to the antennal lobes, the functional analogue of the mammalian olfactory bulb. In the lobes, the information is processed by approximately 5000 interneurons and 1000 projection neurons. The projection neurons reach other parts of the brain, including the 'mushroom bodies', a central processing area shown to play a part in learning in other insects as well ("Drosophila). The US pathway begins with the chemoreceptors that sense the sucrose. They send information to central motor neurons that control the proboscis, and to interneurons that innervate a number of brain areas and subserve the modulatory function of the US. Activity of one of these modulatory neurons, named VUMmx1, was shown to correlate with the US and, furthermore, to be capable of substituting for it (Hammer 1993; see 'mimicry' under "criterion, "method). The reward function offood can be substituted by microinjection of the "neurotransmitter octopamine into the mushroom bodies or the antennal lobes (Hammer and Menzel 1998). Specific temporal patterns of activation of cAMP-dependent "protein kinase sustain the associative long-term PER memory in the lobes (Muller 2000). The presence of extensive projections of US-related interneurons in the brain indicates that there are multiple sites ofconvergence of the CS and US in this system ("coincidence detection). The multiplicity of association loci is in accord with data from cellular and circuit analysis ofclassical conditioning in other organisms (e.g. "Aplysia). It is plausible to assume that the multiple sites of association in PER conditioning in the bee's brain are not functionally equivalent.

The analysis of conditioning in the honeybee has recently proceeded to target experience-dependent changes in the coherent activity of neuronal populations that are expected to encode internal representations ofodours and their hedonic valence. "Functional neuroimaging of neuronal "calcium currents unveiled specific odour-induced spatiotemporal activity "maps in the antennal lobes (Joerges et al. 1997), that were specifically modified in associative learning (Faber et al. 1999). Further research is needed to determine the causal relevance of these alterations in circuit activity to representational change (this caveat applies as well to the use-dependent morphological alterations that were detected in the olfactory glomeruli; Sigg et al. 1997). But already at this stage, the tiny brain of the bee is one of the first places in which the discussion of learning mechanisms addresses not only anatomical pathways, "synapses, and molecules, but also putative population-encoded internal representations—a real must for understanding the neurobiology of memory. The bee thus appears to navigate us in the right direction.

Selected associations: Associative learning, Classical conditioning, Simple system

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