Neural pathways for the processing of alarm pheromone in the ant brain

Social insects like ants exhibit sophisticated communication by means of pheromones, one example of which is the use of alarm pheromones to alert nestmates for colony defense. In the ant Camponotus obscuripes, we have reported that information about formic acid and n-undecane, alarm pheromone components, is processed in a set of specific glomeruli in the antennal lobe (primary olfactory center). Alarm pheromone signals are then transmitted, mainly via uniglomerular projection neurons (uni-PNs), to the protocerebrum (PR), where sensory signals are integrated to form motor commands for behavioral responses. In this study, we physiologically and morphologically characterized 63 alarm pheromone-sensitive PR neurons in ants by using intracellular recording and staining techniques. Most of the pheromone-sensitive PR neurons had dendrites in the mushroom body (MB), the lateral horn, or the medial PR. Some neurons with dendrites in these areas responded specifically to formic acid or n-undecane and may participate in the control of specific behavioral responses to each pheromone component. Other neurons responded also to non-pheromonal odors, in contrast to uni-PNs, most of which responded specifically to alarm pheromones. Responses to non-pheromonal odors were most prominent in efferent neurons of the MB lobe, suggesting that they may participate in integration of pheromonal and non-pheromonal information. We found a class of PR neurons that receives input in all of these pheromone-processing areas and terminates in a variety of premotor areas. These neurons may participate in the control of pheromone-sensitized aggressive behavior, which is triggered by non-pheromonal sensory stimuli associated with a potential enemy.