Multimodal efferent and recurrent neurons in the medial lobes of cockroachmushroom bodies

Previous electrophysiological studies of cockroach mushroom bodies demonstr ated the sensitivity of efferent neurons to multimodal stimuli. The present account describes the morphology and physiology of several types of effere nt neurons with dendrites in the medial lobes. In general, efferent neurons respond to a variety of modalities in a context-specific manner, respondin g to specific combinations or specific sequences of multimodal stimuli. Eff erent neurons that show endogenous activity have dendritic specializations that extend to laminae of Kenyon cell axons equipped with many synaptic ves icles, termed "dark" laminae. Efferent neurons that are active only during stimulation have dendritic specializations that branch mainly among Kenyon cell axons having few vesicles and forming the "pale" laminae. A new catego ry of "recurrent" efferent neuron has been identified that provides feedbac k or feedforward connections between different parts of the mushroom body. Some of these neurons are immunopositive to antibodies raised against the i nhibitory transmitter gamma-aminobutyric acid. Feedback pathways to the cal yces arise from satellite neuropils adjacent to the medial lobes, which rec eive axon collaterals of efferent neurons. Efferent neurons are uniquely id entifiable. Each morphological type occurs at the same location in the mush room bodies of different individuals. Medial lobe efferent neurons terminat e in the lateral protocerebrum among the endings of antennal lobe projectio n neurons. It is suggested that information about the sensory context of ol factory (or other) stimuli is relayed by efferent neurons to the lateral pr otocerebrum where it is integrated with information about odors relayed by antennal lobe projection neurons. (C) 1999 Wiley-Liss, Inc.