Anatomical pathways connecting lip sensory structures and central nervous system in hirudinid leeches visualized by carbocyanine dyes and laser scanning confocal microscopy.

Chemoreception in Hirudo medicinalis is thought to be mediated by ciliated cells grouped in sensory structures, the sensilla, arranged in bands on the animal's dorsal lip (Elliott, 1986; Zipser et al., 1994). Furthermore, chemical and/or thermal stimulation of the dorsal lip in reduced preparations evokes changes in the electrical activity of the cephalic nerves that connect the head with the central nervous system. However, the complete trajectory by which the sensory afferents reach the cerebral ganglia has not been demonstrated anatomically. In this study, we traced these pathways following retrograde and/or anterograde transport of carbocyanine dyes (DiI, DiA and DiD) in the cephalic nerves of Hirudo medicinalis and a closely related species, Macrobdella decora. While information regarding Macrobdella's chemoreception is scarce, the two species show some differences with regard to their chemical preferences. Dyes were applied to the sensillar structures along the dorsal lip, or to the cut ends of individual cephalic nerves in fixed preparations that included the lip and attached nerves with or without the head ganglia. After a two week incubation, specimens were mounted and imaged using a confocal microscope. The results show that the axons of the sensory neurons in the sensilla project through the four pairs of cephalic nerves. The sensillar projections are however more numerous in the dorsal nerves than they are in the ventral ones. In addition, the organization of the sensillar bands, the morphology of the pathways and the sensory structures themselves appear to be identical for Hirudo and Macrobdella and therefore the behavioral differences in response to appetitive stimuli cannot be readily explained by differences in morphology.