1. We have used intracellular recording and staining, followed by reconstruction from serial sections, to characterize the responses and structure of projection neurons (PNs) that link the antennal lobe (AL) to other regions of the brain of the male sphinx moth Manduca sexta. 2. Dendritic arborizations of the AL PNs were usually restricted either to ordinary glomeruli or to the male-specific macroglomerular complex (MGC) within the AL neuropil. Dendritic fields in the MGC appeared to belong to distinct partitions within the MGC. PNs innervating the ordinary glomeruli had arborizations in a single glomerulus (uniglomerular) or in more than one ordinary glomerulus of one AL (multiglomerular) or in one case, in single glomeruli in both ALs (bilateral-uniglomerular). One PN innervated the MGC and many or all ordinary glomeruli of the AL. 3. PNs with dendritic arborizations in the ordinary glomeruli and PNs associated with the MGC typically projected both to the calyces of the ipsilateral mushroom body and to the lateral protocerebrum, but some differences in the patterns of termination in those regions have been noted for the two classes of PNs. One PN conspicuously lacked branches in the calyces but did project to the lateral protocerebrum. The PN innervating the MGC and many ordinary glomeruli projected to the calyces of the ipsilateral mushroom body and the superior protocerebrum. 4. Crude sex-pheromone extracts excited all neurons with arborizations in the MGC, although some were inhibited by other odors. One P(MGC) was excited by crude sex-pheromone extract and by a mimic of one component of the pheromone blend but was inhibited by another component of the blend. 5. PNs with dendritic arborizations in ordinary glomeruli were excited or inhibited by certain non-pheromonal odors. Some of these PNs also responded to mechanosensory stimulation of the antennae. 6. The PN with dendritic arborizations in the MGC and many ordinary glomeruli was excited by crude sex-pheromone extracts and non-pheromonal odors and also responded to mechanosensory stimulation of the antenna.

The macroglomerular complex (MGC) is the first-order center for synaptic processing of olfactory information about the female sex pheromone in the male moth brain. We have investigated the MGC subdivisions of the male silkmoth Bombyx mori by use of three-dimensional reconstruction of the MGC from sequential series of confocal slice images. The B. mori MGC consists of three subdivisions similar to those of Manduca sexta: the cumulus, toroid and horseshoe. Intracellular recording and staining revealed that responses of MGC projection neurons to pheromonal stimulation correlate with their dendritic arborizations in the subdivisions of the MGC (the cumulus, toroid and horseshoe) and each subdivision specific projection neuron transmits information to different regions in the calyces of the mushroom body and the inferior lateral protocerebrum. We revealed that major pheromone component information is transferred to the medial part of the inferior lateral protocerebrum through three different antennocerebral pathways. Although it is generally accepted that the calyces of the mushroom body and the inferior lateral protocerebrum are the target sites for pheromone information from the MGC in moths, our results suggest that the medial part of the inferior lateral protocerebrum may be a more important processing site for major pheromonal information in B. mori.

The antennal lobe (AL) of the male silkworm moth Bombyx mori contains 60 +/- 2 ventrally located antennal glomeruli and a dorsal macroglomerular complex (MGC) consisting of three subdivisions. The response patterns of MGC projection neurons (PNs) to pheromonal stimuli correlate with their dendritic arborization in the subdivisions of the MGC. However, the representation of this pheromonal information in the lateral protocerebrum (LPC), which is the target site of the AL PNs, is not well known. We performed nitric oxide (NO)-induced anti-cGMP immunohistochemistry and found that the PNs which respond to the major pheromone component (bombykol) express strong immunoreactivity. They project to a specific area, the delta area in the inferior lateral protocerebrum (DeltaILPC), which clearly represents the processing center for the major pheromone component. Furthermore, to examine the projection sites in the LPC from each subdivision of the MGC, we performed double-labeling of each type of MGC-PNs, combined with NO-induced anti-cGMP immunohistochemistry. We revealed that projections from each subdivision of the MGC overlapped or separated in specific regions of the DeltaILPC. These results suggest that integration and segregation of pheromone information may occur in the DeltaILPC.

No abstract

No abstract

The lateral accessory lobe (LAL) and the ventral protocerebrum (VPC) are a pair of symmetrical neural structures in the insect brain. The LAL-VPC is regarded as the major target of olfactory responding neurons as well as the control center for olfactory-evoked sequential zigzag turns. Previous studies of the silkworm moth Bombyx mori showed that these turns are controlled by long-lasting anti-phasic activities of the flip-flopping descending neurons with dendrites in the LAL-VPC. To elucidate the neural mechanisms underlying the generation of this alternating activity between the LAL-VPC units of both hemispheres, we first analyzed the detailed neural architecture of the LAL-VPC and identified five subregions. We then investigated the morphology and physiological responses of the LAL-VPC neurons by intracellular recording and staining and morphologically identified three types of bilateral neurons and three types of unilateral neurons. Bilateral neurons showed either brief or cyclic long-lasting responses. At least some neurons of the latter type produced gamma-aminobutyric acid (GABA). Unilateral neurons linking the LAL and VPC, in contrast, showed long-lasting or quick alternating activity. Timing analysis of the activity onset of each neural type suggests that quick reciprocal neural transmission between unilateral neurons would be responsible for the generation of long-lasting activity in one LAL-VPC unit, which lasts for up to a few seconds. Reciprocal inhibition and excitation by the bilateral neurons with long-lasting activities would mediate the alternating long-lasting activity between both LAL-VPC units, which might last for up to 20 seconds.

The lateral accessory lobe (LAL) mediates signals from the central complex to the thoracic motor centers. The results obtained from different insects suggest that the LAL is highly relevant to the locomotion. Perhaps due to its deep location and lack of clear anatomical boundaries, few studies have focused on this brain region. Systematic data of LAL interneurons are available in the silkmoth. We here review individual neurons constituting the LAL by comparing the silkmoth and other insects. The survey through the connectivity and intrinsic organization suggests potential homology in the organization of the LAL among insects.

In insects and other animals, intraspecific communication between individuals of the opposite sex is mediated in part by chemical signals called sex pheromones. In most moth species, male moths rely heavily on species-specific sex pheromones emitted by female moths to identify and orient towards an appropriate mating partner among a large number of sympatric insect species. The silkmoth, Bombyx mori, utilizes the simplest possible pheromone system, in which a single pheromone component, (E, Z)-10,12-hexadecadienol (bombykol), is sufficient to elicit full sexual behavior. We have previously shown that the sex pheromone receptor BmOR1 mediates specific detection of bombykol in the antennae of male silkmoths. However, it is unclear whether the sex pheromone receptor is the minimally sufficient determination factor that triggers initiation of orientation behavior towards a potential mate. Using transgenic silkmoths expressing the sex pheromone receptor PxOR1 of the diamondback moth Plutella xylostella in BmOR1-expressing neurons, we show that the selectivity of the sex pheromone receptor determines the chemical response specificity of sexual behavior in the silkmoth. Bombykol receptor neurons expressing PxOR1 responded to its specific ligand, (Z)-11-hexadecenal (Z11-16:Ald), in a dose-dependent manner. Male moths expressing PxOR1 exhibited typical pheromone orientation behavior and copulation attempts in response to Z11-16:Ald and to females of P. xylostella. Transformation of the bombykol receptor neurons had no effect on their projections in the antennal lobe. These results indicate that activation of bombykol receptor neurons alone is sufficient to trigger full sexual behavior. Thus, a single gene defines behavioral selectivity in sex pheromone communication in the silkmoth. Our findings show that a single molecular determinant can not only function as a modulator of behavior but also as an all-or-nothing initiator of a complex species-specific behavioral sequence.