Peripheral synapses at identified mechanosensory neurons in spiders: Three-dimensional reconstruction and GABA immunocytochemistry

The mechanosensory organs of arachnids receive diverse peripheral inputs. L ittle is known about the origin, distribution, and function of these chemic al synapses, which we examined in lyriform slit sense organ VS-3 of the spi der Cupiennius salei. The cuticular slits of this organ are each associated with two large bipolar mechanosensory neurons with different adaptation ra tes. With intracellular recording, we have now been able to correlate direc tly the staining intensity of a neuron for acetylcholinesterase with its ad aptation rate, thus allowing us simply to stain a neuron to identify its fu nctional type. All rapidly adapting neurons stain more heavily than slowly adapting neurons. Immunostaining of whole-mount preparations reveals GABA-l ike immunoreactive fibers forming numerous varicosities at the surface of a ll sensory neurons in VS-3; peripheral GABA-like immunoreactive somata are lacking. Sectioning the leg nerve procures rapid degeneration of most fiber profiles, confirming that the fibers are efferent. Punctate synapsin-like immunoreactivity colocalizes to these varicosities, although some synapsin- like immunoreactive puncta are GABA-immunonegative, Fibers with similar imm unoreactivities are also associated with trichobothria, tactile hairs, inte rnal joint receptors, i.e. other types of spider mechanosensory organs. In organ VS-3, immunoreactivity is most dense across the initial axon segment. The exact distribution of peripheral synapses was reconstructed from a 10- mu m-long electron micrograph series of the dendritic, somatic, and initial axon regions of acetylcholinesterase-stained VS-3 neurons. These reveal a pattern similar to that of the synapsin-like immunoreactivity. Two differen t types of synapse were distinguished on the basis of their presynaptic ves icle populations. Many peripheral synapses thus appear to derive from effer ent GABA-like immunoreactive fibers and probably provide centrifugal inhibi tory control of primary mechanosensory activities.