Themushroom bodies orcorpora pedunculata are a pair of structures in thebrain ofarthropods, includinginsects andcrustaceans,[2] and someannelids (notably theragwormPlatynereis dumerilii).[3] They are known to play a role inolfactory learning and memory. In most insects, the mushroom bodies and thelateral horn are the two higher brain regions that receive olfactory information from theantennal lobe via projection neurons.[4] They were first identified and described by French biologistFélix Dujardin in 1850.[5][6]
Mushroom bodies are usually described asneuropils, i.e., as dense networks ofneuronal processes (dendrite andaxon terminals) andglia. They get their name from their roughly hemisphericalcalyx, a protuberance that is joined to the rest of the brain by a central nerve tract orpeduncle.
Most of our current knowledge of mushroom bodies comes from studies of a few species of insect, especially thecockroachPeriplaneta americana, thehoney beeApis mellifera,[7] thelocust and the fruit flyDrosophila melanogaster. Studies of fruit fly mushroom bodies have been particularly important for understanding the genetic basis of mushroom body functioning, since theirgenome has been sequenced and a vast number of tools to manipulate their gene expression exist.
In theinsect brain, the peduncles of the mushroom bodies extend through themidbrain. They are mainly composed of the long, densely packed nerve fibres of theKenyon cells, the intrinsic neurons of the mushroom bodies. These cells have been found in the mushroom bodies of all species that have been investigated, though their number varies. Fruit flies, for example, have around 2,500, whereas cockroaches have about 200,000.
A locust brain dissection to expose the central brain and carry out electro-physiology recordings can be seen here.[8]
Historically, it was believed that only insects had mushroom bodies, because they were not present in crabs and lobsters. However, their discovery in themantis shrimp in 2017 lead to the later conclusion[2] that the mushroom body is theancestral state of allarthropods, and that this feature was later lost in crabs and lobsters.[2]
Mushroom bodies are best known for their role inolfactory associative learning. These olfactory signals are received fromdopaminergic,octopaminergic,cholinergic,serotonergic, andGABAergic neurons outside the MB.[9] They are largest in theHymenoptera, which are known to have particularly elaborate control over olfactory behaviours. However, since mushroom bodies are also found inanosmic primitive insects, their role is likely to extend beyond olfactory processing. Anatomical studies suggest a role in the processing ofvisual andmechanosensory input in some species.[10] InHymenoptera in particular, subregions of the mushroom body neuropil are specialized to receive olfactory, visual, or both types of sensory input.[11] In Hymenoptera, olfactory input is layered in the calyx. In ants, several layers can be discriminated, corresponding to different clusters of glomeruli in theantennal lobes, perhaps for processing different classes of odors.[4][12] There are two main groups ofprojection neurons dividing the antennal lobe into two main regions, anterior and posterior. Projection neuron groups are segregated, innervating glomerular groups separately and sending axons by separate routes, either through the medial-antenno protocerebral tract (m-APT) or through the lateral-antenno protocerebral tract (l-APT), and connecting with two layers in the calyx of the mushroom bodies. In these layers the organization of the two efferent regions of the antennal lobe is represented topographically, establishing a coarseodotopic map of the antennal lobe in the region of thelip of the mushroom bodies.[4][12]
Mushroom bodies are known to be involved inlearning andmemory, particularly forsmell, and thus are the subject of current intense research. In larger insects, studies suggest that mushroom bodies have other learning and memory functions, likeassociative memory, sensory filtering,motor control, and place memory. Research implies that mushroom bodies generally act as a sort ofcoincidence detector, integrating multi-modal inputs[4] and creating novel associations, thus suggesting their role in learning and memory.[13] Recent work also shows evidence for the involvement of the mushroom body in innate olfactory behaviors through interactions with the lateral horn,[14][15] possibly making use of the partially stereotyped sensory responses of the mushroom body output neurons (MBONs) across individuals.[16] Although the connections between the projection neurons and the Kenyon cells are random (i.e., not stereotyped across individuals),[17] the stereotypy in MBON responses is made possible by the dense convergence of many Kenyon cells onto a few MBONs along with other network properties.[16]
Information about odors may be encoded in the mushroom body by the identities of the responsive neurons as well as the timing of their spikes.[18] Experiments in locusts have shown thatKenyon cells have their activity synchronized to 20-Hzneural oscillations and are particularly responsive to projection neuron spikes at specific phases of the oscillatory cycle.[19]
The neurons which receive signals fromserotonergic andGABAergic neurons outside the MB produce wakefulness, and experimentally stimulating these serotonergic upstream neurons forces sleep. The target neurons in the MB are inhibited byserotonin,GABA, and the combination of both. On the other handoctopamine does not seem to affect the MB's sleep function.[9]
We know that mushroom body structures are important forolfactorylearning andmemory inDrosophila because theirablation destroys this function.[21] The mushroom body is also able to combine information from the internal state of the body and the olfactory input to determine innate behavior.[22] The exact roles of the specific neurons making up the mushroom bodies are still unclear. However, these structures are studied extensively because much is known about theirgenetic make-up. There are three specific classes of neurons that make up the mushroom body lobes: α/β, α’/β’, and γ neurons, which all have distinct gene expression. A topic of current research is which of these substructures in the mushroom body are involved in each phase and process of learning and memory.[23]Drosophila mushroom bodies are also often used to study learning and memory and are manipulated due to their relatively discrete nature. Typically, olfactory learning assays consist of exposing flies to two odors separately; one is paired with electric shock pulses (theconditioned stimulus, or CS+), and the second is not (unconditioned stimulus, or US). After this training period, flies are placed in aT-maze with the two odors placed individually on either end of the horizontal ‘T’ arms. The percent of flies that avoid the CS+ is calculated, with high avoidance considered evidence of learning and memory.[24]
Recent studies combining odor conditioning and cellular imaging have identified six memory traces that coincide withmolecular changes in theDrosophilaolfactory system. Three of these traces are associated with early forming behavioral memory. One such trace was visualized in theantennal lobe (AL) bysynapto-pHluorin reporter molecules. Immediately after conditioning, an additional set ofprojection neurons in a set of eightglomeruli in the AL becomes synaptically activated by the conditioned odor, and lasts for only 7 minutes.[25] A second trace is detectable byGCaMP expression, and thus an increase in Ca2+ influx, in the α’/β’ axons of the mushroom body neurons.[26] This is a longer-lasting trace, present for up to one hour following conditioning. The third memory trace is the reduction of activity of the anterior-paired lateral neuron, which acts as a memory formation suppressor through one of its inhibitoryGABAergic receptors. Decrease incalcium response of APL neurons and subsequent decrease inGABA release onto the mushroom bodies persisted up to 5 minutes after odor conditioning.[27]
The intermediate term memory trace is dependent on expression of theamn gene located in dorsal paired medial neurons. An increase incalcium influx and synaptic release that innervates the mushroom bodies becomes detectable approximately 30 minutes after pairing of electric shock with an odor, and persists for at least an hour.[28] Both long-term memory traces that have been mapped depend on activity and protein synthesis ofCREB andCaMKII, and only exist after spaced conditioning. The first trace is detected in α/β neurons between 9 and 24 hours after conditioning, and is characterized by an increase incalcium influx in response to the conditioned odor.[29] The second long-term memory trace forms in the γ mushroom bodies and is detected by increasecalcium influx between 18 and 24 hours after conditioning[30]
Cyclic adenosine monophosphate (cAMP or cyclic AMP) is a second messenger that has been implicated in facilitating mushroom bodycalcium influx inDrosophila melanogaster mushroom body neurons. cAMP elevation induces presynaptic plasticity in Drosophila. cAMP levels are affected by bothneurotransmitters, such asdopamine andoctopamine, and odors themselves. Dopamine and octopamine are released by mushroom bodyinterneurons, while odors directly activate neurons in the olfactory pathway, causing calcium influx throughvoltage-gated calcium channels.[31]
In aclassical conditioning paradigm, pairing neuronaldepolarization (viaacetylcholine application to represent theodor orCS) with subsequent dopamine application (to represent the shock orUS), results in a synergistic increase incAMP in the mushroom body lobes.[31] These results suggest that the mushroom body lobes are a critical site of CS/US integration via the action of cAMP. This synergistic effect was originally observed inAplysia, where pairing calcium influx with activation ofG protein signaling byserotonin generates a similar synergistic increase in cAMP.[32]
Additionally, this synergistic increase in cAMP is mediated by and dependent onrutabaga adenylyl cyclase (rut AC), which is sensitive to both calcium (which results from voltage-gated calcium channel opening by odors) andG protein stimulation (caused by dopamine).[31] While a forward pairing of neuronaldepolarization and dopamine, (acetylcholine followed by dopamine) results in a synergistic increase in cAMP, a forward pairing of neuronaldepolarization and octopamine produces a sub-additive effect on cAMP.[31] More specifically, this means that this pairing produces significantly less cAMP than the sum of each stimulus individually in the lobes. Therefore, rut AC in mushroom body neurons works as acoincidence detector with dopamine and octopamine functioning bidirectionally to affect cAMP levels.[31]
Protein kinase A (PKA) has been found to play an important role in learning and memory inDrosophila.[33] Whencalcium enters a cell and binds withcalmodulin, it stimulatesadenylate cyclase (AC), which is encoded by therutabaga gene (rut).[34] This AC activation increases the concentration ofcAMP, which activates PKA.[34] Whendopamine, an aversive olfactory stimulant, is applied it activates PKA specifically in the vertical mushroom body lobes.[34] This spatial specificity is regulated by the dunce (dnc) PDE, a cAMP-specificphosphodiesterase. If the dunce gene is abolished, as found in thednc mutant, the spatial specificity is not maintained. In contrast, an appetitive stimulation created by anoctopamine application increases PKA in all lobes.[34] In therut mutant, a genotype in which the rutabaga is abolished, the responses to both dopamine and octopamine were greatly reduced and close to experimental noise.
Acetylcholine, which represents theconditioned stimulus, leads to a strong increase in PKA activation compared to stimulation with dopamine or octopamine alone.[34] This reaction is abolished inrut mutants, which demonstrates that PKA is essential for sensory integration.[34] The specificity of activation of the alpha lobe in the presence of dopamine is maintained when dopamine is in combination with acetylcholine.[34] Essentially, during a conditioning paradigm when a conditioned stimulus is paired with an unconditioned stimulus, PKA exhibits heightened activation. This shows that PKA is required for conditioned learning inDrosophila melanogaster.
Stimulus → output responses are the product of pairs ofexcitation andinhibition. This is the same pattern of organisation as withmammals' brains. These patterns may, as with mammals, precede action. As of 2021[update] this is an area only recently elucidated by Zwaka et al 2018, Duer et al 2015, and Paffhausen et al 2020.[7]