Peripheral synapses at identifiable mechanosensory neurons in the spider Cupiennius salei: synapsin-like immunoreactivity

Indirect immunocytochemical tests were used at the light- and electron-micr oscopic levels to investigate peripheral chemical synapses in identified se nsory neurons of two types of cuticular mechanosensors in the spider Cupien nius salei Keys.: (1) in the lyriform slit-sense organ VS-3 (comprising 7-8 cuticular slits, each innervated by 2 bipolar sensory neurons) and (2) in tactile hair sensilla (each supplied with 3 bipolar sensory cells). All the se neurons are mechanosensitive. Application of a monoclonal antibody again st Drosophila synapsin revealed clear punctate immunofluorescence in whole- mount preparations of both mechanoreceptor types. The size and overall dist ribution of immunoreactive puncta suggested that these were labeled presyna ptic sites. Immunofluorescent puncta were 0.5-6.8 mu m long and located 0.5 -6.6 mu m apart from each other. They were concentrated at the initial axon segments of the sensory neurons, while the somata and the dendritic region s showed fewer puncta. Western blot analysis with the same synapsin antibod y against samples of spider sensory hypodermis and against samples from the central nervous system revealed a characteristic doubler band at 72 kDa an d 75 kDa, corresponding to the apparent molecular mass of synapsin in Droso phila and in mammals. Conventional transmissionelectron-microscopic stainin g demonstrated that numerous chemical synapses (with at least 2 vesicle typ es) were present at these mechanosensory neurons and their surrounding glia l sheath. The distribution of these synapses corresponded to our immunofluo rescence results. Ultrastructural examination of anti-synapsin-stained neur ons confirmed that reaction product was associated with synaptic vesicles. We assume that the peripheral synaptic contacts originate from efferents th at could exert a complex modulatory influence on mechanosensory activity.