Olfactory Interference during Inhibitory Backward Pairing in Honey Bees

Background
Restrained worker honey bees are a valuable model for studying the behavioral and neural bases of olfactory plasticity. The proboscis extension response (PER; the proboscis is the mouthpart of honey bees) is released in response to sucrose stimulation. If sucrose stimulation is preceded one or a few times by an odor (forward pairing), the bee will form a memory for this association, and subsequent presentations of the odor alone are sufficient to elicit the PER. However, backward pairing between the two stimuli (sucrose, then odor) has not been studied to any great extent in bees, although the vertebrate literature indicates that it elicits a form of inhibitory plasticity.
Methodology/Principal Findings
If hungry bees are fed with sucrose, they will release a long lasting PER; however, this PER can be interrupted if an odor is presented 15 seconds (but not 7 or 30 seconds) after the sucrose (backward pairing). We refer to this previously unreported process as olfactory interference. Bees receiving this 15 second backward pairing show reduced performance after a subsequent single forward pairing (excitatory conditioning) trial. Analysis of the results supported a relationship between olfactory interference and a form of backward pairing-induced inhibitory learning/memory. Injecting the drug cimetidine into the deutocerebrum impaired olfactory interference.
Conclusions/Significance
Olfactory interference depends on the associative link between odor and PER, rather than between odor and sucrose. Furthermore, pairing an odor with sucrose can lead either to association of this odor to PER or to the inhibition of PER by this odor. Olfactory interference may provide insight into processes that gate how excitatory and inhibitory memories for odor-PER associations are formed.

背景（Background）
固定处理的工蜂是研究嗅觉可塑性行为与神经基础的理想模型。喙伸反应（proboscis extension response, PER；喙为蜜蜂的口器）可由蔗糖刺激诱发。若在蔗糖刺激前先施加一次或数次气味刺激（正向配对），蜜蜂会形成该关联记忆，后续仅单独呈现该气味即可诱发PER。不过，尽管脊椎动物相关研究表明，两刺激反向配对（先蔗糖后气味）可诱发一类抑制性可塑性，但此类操作在蜜蜂中的研究仍较为匮乏。

方法与主要发现（Methodology/Principal Findings）
若对饥饿蜜蜂饲喂蔗糖，其会产生持续时间较长的PER；但若在蔗糖刺激后15秒（而非7秒或30秒）施加气味刺激（反向配对），则该PER会被阻断。我们将这一此前未被报道的过程命名为嗅觉干扰（olfactory interference）。接受15秒反向配对处理的蜜蜂，在后续单次正向配对（兴奋性条件化）实验中的表现会出现下降。结果分析表明，嗅觉干扰与反向配对诱导的抑制性学习/记忆存在关联。向蜜蜂中脑（deutocerebrum）注射西咪替丁（cimetidine）会损害嗅觉干扰效应。

结论与意义（Conclusions/Significance）
嗅觉干扰依赖于气味与PER之间的关联联结，而非气味与蔗糖之间的关联。此外，气味与蔗糖的配对既可以促成该气味与PER的关联，也可能导致该气味对PER产生抑制作用。嗅觉干扰或可帮助我们解析调控气味-PER关联的兴奋性与抑制性记忆形成过程的机制。