The Journal of Neuroscience, January 1, 1999, 19(1):298-310
Peripheral Synapses at Identified Mechanosensory Neurons in
Spiders: Three-Dimensional Reconstruction and GABA
Immunocytochemistry
Ruth
Fabian-Fine1, 2,
Ulli
Höger1,
Ernst-August
Seyfarth1, and
Ian A.
Meinertzhagen2
1 Zoologisches Institut, J. W. Goethe-Universität, D-60054 Frankfurt am Main, Germany, and
2 Neuroscience Institute, Dalhousie University, Halifax,
Nova Scotia, Canada B3H 4J1
The mechanosensory organs of arachnids receive diverse peripheral
inputs. Little is known about the origin, distribution, and function of
these chemical synapses, which we examined in lyriform slit sense organ
VS-3 of the spider Cupiennius salei. The cuticular slits
of this organ are each associated with two large bipolar
mechanosensory neurons with different adaptation rates. With
intracellular recording, we have now been able to correlate directly
the staining intensity of a neuron for acetylcholinesterase with its
adaptation rate, thus allowing us simply to stain a neuron to identify
its functional type. All rapidly adapting neurons stain more heavily
than slowly adapting neurons. Immunostaining of whole-mount
preparations reveals GABA-like immunoreactive fibers forming
numerous varicosities at the surface of all 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 immunoreactivities are also associated with trichobothria, tactile hairs, internal 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-µm-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
different types of synapse were distinguished on the basis of their
presynaptic vesicle populations. Many peripheral synapses thus appear
to derive from efferent GABA-like immunoreactive fibers and probably
provide centrifugal inhibitory control of primary mechanosensory activities.
Key words:
peripheral synapses; mechanoreceptors; synapsin; efferent
control; immunocytochemistry; three-dimensional reconstruction; ultrastructure; spider; acetylcholinesterase
Copyright © 1999 Society for Neuroscience 0270-6474/99/191298-13$05.00/0
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