Adipokinetic hormone (AKH), energy budget and their effect on feeding and gustatory processes of foraging honey bees

Response to comment on "Food Wanting is Mediated by Transient Activation of Dopaminergic Signaling in the Honeybee Brain"

In a technical comment, Barron et al. (1) criticized the work of Huang et al. (2) putting the accent on the quantification of dopamine levels via high-performance liquid chromatography (HPLC), yet also including data interpretation through alternative hypotheses aimed at invalidating the original ones proposed by Huang et al. We thank the authors of this technical comment, which allows us to clarify technical aspects of our work that may have been unclear, and for promoting discussion around the conclusions of our work. Below we provide answers to the points raised in their comment.

Systemic glucose levels are modulated by specific wavelengths in the solar light spectrum that shift mitochondrial metabolism

Systemic glucose levels can be modulated with specific solar wavelengths that influence mitochondrial metabolism. Mitochondrial respiration can be modulated using light that shifts ATP production with exceptional conservation of effect across species, from insects to humans. Known wavelengths have opposing effects of photobiomodulation, with longer wavelengths (660–900 nm red/infrared) increasing ATP production, and 420 nm (blue) light suppressing metabolism. Increasing mitochondrial respiration should result in a greater demand for glucose, and a decrease should result in a reduced demand for glucose. Here we have tested the hypothesis that these wavelengths alter circulating glucose concentration. We first established an oral glucose tolerance test curve in a bumblebee model, which showed sustained increase in systemic glucose beyond that seen in mammals, with a gradual normalisation over eight hours. This extended period of increased systemic glucose provided a stable model for glucose manipulation. Bees were starved overnight and given a glucose load in the morning. In the first group glucose levels were examined at hourly intervals. In the second group, bees were additionally exposed to either 670 nm or 420 nm light and their blood glucose examined. Increasing mitochondrial activity with 670 nm light at the peak of circulating glucose, resulted in a significant 50% reduction in concentration measured. Exposure to 420nm light that retards mitochondrial respiration elevated systemic glucose levels by over 50%. The impact of 670 nm and 420 nm on mitochondria is highly conserved. Hence, different wavelengths of visible light may be used to modulate systemic metabolism bidirectionally and may prove an effective agent in mammals.

Food wanting is mediated by transient activation of dopaminergic signaling in the honey bee brain

The biological bases of wanting have been characterized in mammals, but whether an equivalent wanting system exists in insects remains unknown. In this study, we focused on honey bees, which perform intensive foraging activities to satisfy colony needs, and sought to determine whether foragers leave the hive driven by specific expectations about reward and whether they recollect these expectations during their waggle dances. We monitored foraging and dance behavior and simultaneously quantified and interfered with biogenic amine signaling in the bee brain. We show that a dopamine-dependent wanting system is activated transiently in the bee brain by increased appetite and individual recollection of profitable food sources, both en route to the goal and during waggle dances. Our results show that insects share with mammals common neural mechanisms for encoding wanting of stimuli with positive hedonic value.

The short neuropeptide F (sNPF) promotes the formation of appetitive visual memories in honey bees

Motivation can critically influence learning and memory. Multiple neural mechanisms regulate motivational states, among which signalling via specific neuropeptides, such as NPY in vertebrates and NPF and its short variant sNPF in invertebrates, plays an essential role. The honey bee (Apis mellifera) is a privileged model for the study of appetitive learning and memory. Bees learn and memorize sensory cues associated with nectar reward while foraging, and their learning is affected by their feeding state. However, the neural underpinnings of their motivational states remain poorly known. Here we focused on the short neuropeptide F (sNPF) and studied if it modulates the acquisition and formation of colour memories. Artificially increasing sNPF levels in partially fed foragers with a reduced motivation to learn colours resulted in significant colour learning and memory above the levels exhibited by starved foragers. Our results thus identify sNPF as a critical component of motivational processes involved in foraging and in the cognitive processes associated with this activity in honey bees.

The short neuropeptide F regulates appetitive but not aversive responsiveness in a social insect

The neuropeptide F (NPF) and its short version (sNPF) mediate food- and stress-related responses in solitary insects. In the honeybee, a social insect where food collection and defensive responses are socially regulated, only sNPF has an identified receptor. Here we increased artificially sNPF levels in honeybee foragers and studied the consequences of this manipulation in various forms of appetitive and aversive responsiveness. Increasing sNPF in partially fed bees turned them into the equivalent of starved animals, enhancing both their food consumption and responsiveness to appetitive gustatory and olfactory stimuli. Neural activity in the olfactory circuits of fed animals was reduced and could be rescued by sNPF treatment to the level of starved bees. In contrast, sNPF had no effect on responsiveness to nociceptive stimuli. Our results thus identify sNPF as a key modulator of hunger and food-related responses in bees, which are at the core of their foraging activities.

Bee year: Basic physiological strategies to cope with seasonality

Worker honey bees are subject to biochemical and physiological changes throughout the year. This study aimed to provide the reasons behind these fluctuations. The markers analysed included lipid, carbohydrate, and protein levels in the haemolymph; the activity of digestive enzymes in the midgut; the levels of adipokinetic hormone (AKH) in the bee central nervous system; the levels of vitellogenins in the bee venom and haemolymph; and the levels of melittin in the venom. The levels of all the main nutrients in the haemolymph peaked mostly within the period of maximal bee activity, whereas the activity of digestive enzymes mostly showed a two-peak course. Furthermore, the levels of AKHs fluctuated throughout the year, with modest but significant variations. These data suggest that the role of AKHs in bee energy metabolism is somewhat limited, and that bees rely more on available food and less on body deposits. Interestingly, the non-metabolic characteristics also fluctuated over the year. The vitellogenin peak reached its maximum in the haemolymph in winter, which is probably associated with the immunoprotection of long-lived winter bees. The analysis of bee venom showed the maximal levels of vitellogenin in autumn; however, it is not entirely clear why this is the case. Finally, melittin levels showed strong fluctuations, suggesting that seasonal control was unlikely.