Long-term memory shapes the primary olfactory center of an insect brain

Abstract

The storage of stable memories is generally considered to rely on changes in the functional properties and/or the synaptic connectivity of neural networks. However, these changes are not easily tractable given the complexity of the learning procedures and brain circuits studied. Such a search can be narrowed down by studying memories of specific stimuli in a given sensory modality and by working on networks with a modular and relatively simple organization. We have therefore focused on associative memories of individual odors and the possible related changes in the honeybee primary olfactory center, the antennal lobe (AL). As this brain structure is organized in well-identified morpho-functional units, the glomeruli, we looked for evidence of structural and functional plasticity in these units in relation with the bees' ability to store long-term memories (LTMs) of specific odors. Restrained bees were trained to form an odor-specific LTM in an appetitive Pavlovian conditioning protocol. The stability and specificity of this memory was tested behaviorally 3 d after conditioning. At that time, we performed both a structural and a functional analysis on a subset of 17 identified glomeruli by measuring glomerular volume under confocal microscopy, and odor-evoked activity, using in vivo calcium imaging. We show that long-term olfactory memory for a given odor is associated with volume increases in a subset of glomeruli. Independent of these structural changes, odor-evoked activity was not modified. Lastly, we show that structural glomerular plasticity can be predicted based on a putative model of interglomerular connections.[Supplemental material is available online at http: //www.learnmem.org.]In nature, animals' survival relies on their capacity to adapt to changes in the environment by constantly learning and memorizing novel information and modifying their behavior accordingly. This capacity relies on the long-term storage of learned information involving specific stable modifications of neural networks, both in their connectivity and in the strength of synaptic transmission (Matsuzaki 2007). For such long-term memory (LTM) to be specific for particular stimuli (for instance, an odor), they must rely on specific neural traces of these stimuli in the brain. A crucial question is whether such specific traces can be tracked down to individual neural units, given the complexity of the neuronal networks usually involved in the formation of LTM. Brain regions constituted of relatively few neurons and organized in clearly identified modules offer an excellent opportunity to answer this question. Among these is the insect antennal lobe (AL), an olfactory center that shares many similarities with the vertebrate olfactory bulb, but provides numerical simplicity. In the honeybee, the AL contains ;160 interconnected neuropil subunits, the glomeruli, which can be unambiguously identified across individuals (Galizia et al. 1999a). Experiments using in vivo calcium imaging have shown that glomeruli are functional units for odor...

Long-Term Memory Leads to Synaptic Reorganization in the Mushroom Bodies: A Memory Trace in the Insect Brain?

“…Indeed, plasticity of PN responses was observed both in Drosophila (Yu et al, 2004) and Apis (Fernandez et al, 2009). Supporting this view, LTM formation induces structural reorganizations in AL glomeruli, which could result in increased activity from a subset of the PNs (Hourcade et al, 2009). Even so, the observed changes would be widely distributed in the lip volume as the boutons of individual PNs are themselves widely scattered across the lip volume (Müller et al, 2002).…”

“…As a third control, bees from the paired ActD group received the same conditioning procedure as in the paired group, but were injected with 1 l of 1.5 mM Actinomycin D (Sigma-Aldrich) using a precision syringe (Hamilton) into their thorax, 3 h after learning. We previously showed that these conditions allowed to abolish LTM (Hourcade et al, 2009). To control for possible effects of the injection on memory, bees from all the other groups were injected with 1 l of PBS following the same procedure.…”

Neural substrate for higher-order learning in an insect: Mushroom bodies are necessary for configural discriminations

“…In the bee, a convergence between odor and sucrose pathways also exists at the level of the AL and the LH because VUMmx1, the instructive neuron that signals the presence of sucrose reward in the bee brain, contacts the olfactory circuit at these stages besides that of the MBs (50). Functional and structural changes have been found in the ALs of adult bees after elemental olfactory learning in the laboratory (66)(67)(68) or shortly after emergence in the hive (69,70). We thus suggest that, in the absence of functional MBs, elemental learning is still possible via the ALs.…”

Cyclic nucleotide–gated channels, calmodulin, adenylyl cyclase, and calcium/calmodulin-dependent protein kinase II are required for late, but not early, long-term memory formation in the honeybee

“…The antennal lobes (ALs), primary olfactory centers receiving olfactory information from olfactory receptor neurons on the antennae, and the mushroom bodies (MBs), multimodal structures receiving processed olfactory information from the ALs as well as visual, mechanosensory, and gustatory input, are the main candidates for hosting long-term memory traces. Both regions exhibit structural modifications after formation of an olfactory l-LTM (Hourcade et al 2009(Hourcade et al , 2010Arenas et al 2012).…”