ACETYLCHOLINE AND HISTAMINE ARE TRANSMITTER CANDIDATES IN IDENTIFIABLE MECHANOSENSITIVE NEURONS OF THE SPIDER CUPIENNIUS-SALEI - AN IMMUNOCYTOCHEMICAL STUDY

Histochemical and indirect immunocytochemical techniques were used to search for neuroactive substances and transmitter candidates in identi fied sensory neurons of two types of cuticular mechanoreceptors in the spider Cupiennius salei Keys.: (1) in lyriform slit-sense organ VS-3 (comprising 7-8 cuticular slits each innervated by 2 bipolar neurons), and (2) in tactile hairs (each supplied by 3 bipolar sensory cells). All neurons are mechanosensitive. A polyclonal antibody against cholin e acetyltransferase (ChAT) strongly labeled all cell bodies and affere nt fibers of both mechanoreceptor types. Western blot analysis using t he same antibody against samples of spider sensory hypodermis and agai nst samples from the central nervous system demonstrated a clear band at 65 kDa, corresponding to the molecular mass of ChAT in insects. Mor eover, staining for acetylcholine esterase (AChE) revealed AChE activi ty in one neuron of each mechanoreceptor type. Incubation with a polyc lonal antibody against histamine clearly labeled one neuron in each se t of sensilla, whereas activity in the remaining one or two cells was near background. All mechanoreceptor preparations treated with a poly clonal antiserum against serotonin tested negative, whereas sections t hrough the central nervous system of the same spiders were clearly lab eled for serotonin. The presence of ChAT-like immunoreactivity and ACh E implicates acetylcholine as a transmitter candidate in the two mecha noreceptive organs. We assume that histamine serves as a mechanosensor y co-transmitter in the central nervous system and may also act at per ipheral synapses that exist in these sensilla.