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. 2013 Feb;23(1):76–83. doi: 10.1016/j.conb.2012.09.002

Figure 1.

Figure 1

Proposed model for the perception and integration of sensory information in the male CNS. (a) Olfaction: Olfactory receptor neurons (ORNs) in the antenna detect odors and send axons to glomeruli of the antennal lobe of the brain. DA1 and VA1lm glomeruli receive pheromonal information from Or67d-expressing neurons (which respond to the male pheromone 11-cis-vaccenyl acetate, cVA) [24,31] and Or47b-expressing neurons (which respond to unidentified female and male-derived odors) [58,59]. While Or67d ORNs express fru, Or47b ORNs do not. Olfactory information is propagated through projection neurons to higher brain centers, such as the mushroom body and lateral horn. OR67d/DA1 neurons target fru-expressing second order neurons in a specific region of the lateral horn associated with pheromone processing. Ultimately these second order neurons form downstream fru-expressing connections (via third and fourth order neurons) that terminate in the abdominal ganglia [24,25•]. VL2a glomerulus receives information from Ir84a-expressing neurons, which respond to odors derived from host food/oviposition substrates [34]. VL2a projection neurons are segregated from projection neurons responding to general food odor pathways but they are anatomically interconnected with the VA1lm/DA1 pheromone pathways and target a specific area in the lateral horn involved in pheromone processing [34]. Note that only half of the male CNS is shown in the schematic. Adapted from [60]. (b) Gustation: In males, non-volatile pheromones are primarily detected by gustatory receptor neurons (GRNs) of the foreleg tarsi. Gr33a and Gr32a are thought to detect female-specific pheromones (acting as aphrodisiacs) and Gr39a is thought to detect male-specific pheromones (acting as aversive stimuli) [38,40,41]. GRNs expressing these Grs do not express fruitless. However, projections of Gr32a-expressing terminate in the SOG are likely to form connections with fru-expressing neurons of the ventrolateral protocerebrum that relay this signal to the higher-order lateral protocerebrum [42]. Projections of Gr33a-expressing and Gr39a-expressing GRNs remain unknown. GRNs expressing proprioceptive receptors pickpocket-23 (ppk23), ppk29 or ppk25 co-express fruitless and have also been implicated in courtship [19•,44,45,47,48]. All three send sexually dimorphic projections to the first thoracic ganglia, of which ppk23-expressing and ppk29-expressing GRNs ascend to terminate in the SOG [19•,44,45,48]. Although downstream connections have not been described, studies have shown that gustatory information is relayed from the SOG to a distinct region of the lateral protocerebrum [16•,19•]. One population of ppk23-expressing GRNs respond to male-specific pheromones (to promote male–male repulsion) [44], while a second population responds to female-specific pheromones (to promote male–female courtship) [44,45]. Most aforementioned receptors are also expressed in the mouthparts; however, their specific role in these tissues has not been determined. Note that only half of the male CNS is shown in the schematic.