Abstract
The olfactory pathway in the insect brain is anatomically well described from the antennal lobe (AL) to the mushroom bodies and the lateral protocerebrum (LP) in several species. Less is known about the further connections of the olfactory network in protocerebrum and how information about relevant plant odorants and mixtures are represented in this network, resulting in output information mediated by descending neurons. In the present study we have recorded intracellularly followed by dye injections from neurons in the LP and superior protocerebrum (SP) of the moth, Heliothis virescens. As relevant stimuli, we have used selected primary plant odorants and mixtures of them. The results provide the morphology and physiological responses of neurons involved in a putative circuit connecting the mushroom body lobes, the SP, and the LP, as well as input to SP and LP by one multiglomerular AL neuron and output from the LP by one descending neuron. All neurons responded to a particular mixture of ten primary plant odorants, some of them also to single odorants of the mixture. Altogether, the physiological data indicate integration in protocerebral neurons of information from several of the receptor neuron types functionally described in this species.
Highlights
The intimate relationship between insects and plants relies to a large extent on plant produced chemical cues and sophisticated olfactory and gustatory systems evolved in insects
Based on intracellular recordings from superior protocerebrum (SP) and lateral protocerebrum (LP), we have described neurons involved in a putative circuit processing plant odor information in the brain of H. virescens females, 28 neurons presented in this study
We have stimulated with primary odorants, each identified as the best odorant for a particular olfactory receptor neurons (ORNs)
Summary
The intimate relationship between insects and plants relies to a large extent on plant produced chemical cues and sophisticated olfactory and gustatory systems evolved in insects The importance of these senses is reflected in the numerous sensory organs, sensilla, on the various appendages and the large areas of the brain devoted to chemosensory coding and learning. As shown in the fruit fly (Drosophila melanogaster) and other species, the olfactory receptor neurons (ORNs) consist of subtypes, each subtype expressing one and the same type of receptor proteins and sending their primary axons to one specific glomerulus, exceptionally two, in the AL (Vosshall et al, 1999; Vosshall and Stocker, 2007) This implies that each glomerulus is considered to receive odor information from one type of ORNs. The number of glomeruli is species specific, like 43 in the fruit fly, 60–67 in moth species, including Heliothis virescens (66 in females), and 160 in the honeybee (Apis mellifera; Flanagan and Mercer, 1989; Stocker et al, 1990; Berg et al, 2002; Løfaldli et al, 2010). The processed information is further mediated to the protocerebral areas, mainly via three parallel antenno-protocerebral tracts (APTs), the medial ( called the inner antenno-cerebral tract, ACT), the lateral ( called the outer ACT), and the medio-lateral
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