Anatomical organization of antennal lobe projection neurons in the moth Heliothis virescens

Abstract

A prerequisite for understanding how odor information is coded in the central nervous system is to know the morphology and spatial relationship of the principal neurons forming the olfactory pathways. The present account provides an anatomical description of the morphology of the neuronal connections between the antennal lobe and the protocerebrum in the moth Heliothis virescens, a species used for studies of olfactory processing and learning. Intracellular labeling and antennal lobe focal injections with dextran fluorescent markers were combined with neuropil immunostaining and three-dimensional reconstructions. The experiments revealed four antennocerebral tracts, the inner, middle, outer, and dorsomedial, and eight morphological types of projection neurons in addition to a neuron with an unpaired median soma in the subesophageal ganglion. Multiglomerular projection neurons, present in all but the dorsomedial antennocerebral tract, project in several olfactory foci of the protocerebral neuropil. With few exceptions, these neurons do not innervate the calyces of the mushroom body. Uniglomerular projection neurons appear most numerous in the inner antennocerebral tract but are also present in the outer and dorsomedial tracts. These neurons always ramify in the calyces of the mushroom body and in the lateral horn. The projection areas of the neurons following different tracts are largely separated in the secondary olfactory centers. This is most evident in the lateral horn, whereas, in the calyces, the axonal ramifications are more intermingled. The mushroom body architecture, revealed by neuropil immunolabeling, showed striking similarities to that of other lepidopteran species as well as insects of other taxa.

γ-Aminobutyric acid immunostaining in the antennal lobe of the moth Heliothis virescens and its colocalization with neuropeptides

“…The logic of this projection pattern is demonstrated in the noctuid moth Heliothis virescens in which four male-specific receptor neuron types carry olfactory information from the external world to specific addresses in the brain, i.e., the four subunits of the Berg et al 1995;Christensen et al 1995;Hansson et al 1995;Vickers et al 1998;Vickers and Christensen 2003). The ordinary glomeruli, which are targets of plant odor sensory neurons, are present in both males and females (Stranden et al 2003;Rø et al 2006). In H. virescens, the total number of antennal-lobe glomeruli, including dimorphic and isomorphic units, comprises 64 in the male and 62 in the female (Berg et al 2002).…”

Effects of two bitter substances on olfactory conditioning in the moth Heliothis virescens

“…The olfactory pathways involved in olfactory conditioning have been extensively studied and are well described in several species, including A. mellifera and the moth Heliothis virescens. The odorants are detected by olfactory receptor neurons located on the antennae, and olfactory information is transmitted via synapses within the glomeruli of the antennal lobes to local interneurons that carry out local computation, and to projection neurons (Menzel and Giurfa, 2001;Mustaparta and Stranden, 2005;Rø et al, 2007). Projection neurons further convey odour information via the antennocerebral tracts to the calyces of the mushroom bodies and to the lateral horn, a premotor area.…”

The anatomical pathways for antennal sensory information in the central nervous system of the cricket, Gryllus bimaculatus

“…LH). Previously the projection patterns of AL-PNs have been investigated in a wide variety of insects (fly: Stocker et al, 1990;cockroach: Malun et al, 1993;moth: Homberg et al, 1988, Rø et al, 2007; honeybee: Kirschner et al, 2006;ant: Zube et al, 2008). As in the other species studied we found in the cricket that the IACT was more frequently and more intensely stained.…”

“…In the cockroach, AL-PNs project via ACT2 and there they also exhibit uniglomerular arborization (Malun et al, 1993). However, in the honeybee and moth the AL-PNs project via l-ACT (synonymous with OACT) but also exhibit uniglomerular arborization (Abel et al, 2001;Müller et al, 2002;Rø et al, 2007).…”

“…In a number of different species, the chemical information having been processed by local interneurons in the AL, is sent up to 'higher centers' (the calyx of the mushroom body, and the lateral protocerebrum) by projection neurons (PNs) via several neuronal tracts, known as the antenno-cerebral tracts (ACTs) (moth: Homberg et al, 1988;Rø et al, 2007; cockroach: Malun et al, 1993;honeybee: Kirchner et al, 2006;ant: Zube et al, 2008). In G. bimaculatus, however, only a part of projection routes and terminating regions of the PNs are known .…”