Journal of Neuroscience, Vol 9, 1872-1882, Copyright © 1989 by Society for Neuroscience
Cholinergic synapses and the organization of contrast detection in the crayfish optic lobe
C Pfeiffer and RM Glantz
Department of Biology, Rice University, Houston, Texas 77251.
The actions of acetylcholine (ACh) were examined on 4 classes of
multicolumnar interneurons whose dendrites lie in close proximity to the
putative cholinergic transmedullary neurons described in the companion
report. ACh-elicited responses in each cell type resemble visually elicited
synaptic events and persist following synaptic blockade with 20 mM CoCl2.
Tangential cells exhibit a hyperpolarizing response to ACh that resembles
the visual response in reversal potential and dependence on extracellular
chloride. The visual response is potentiated by the anticholinesterase,
neostigmine (0.1 mM). Visual and carbachol-elicited responses are blocked
by nicotinic ganglionic antagonists (e.g., 10(-6) M pempidine) that are
10-100 times more potent than D-tubocurarine. Medullary amacrine cells
exhibit depolarizing responses to ACh (10(-6) M) and light with similar
reversal potentials. The visual response is potentiated by neostigmine.
Dimming fibers respond to light and ACh with a hyperpolarization that
inhibits the maintained discharge. The sustaining fiber response to ACh
reflects both direct responses and indirectly elicited synaptic actions.
The direct action is a hyperpolarization possibly related to the visual
"off-response." It is associated with an increased conductance and a
reversal potential negative to the dark potential. The off-response is
abolished by curare and pempidine and potentiated by neostigmine. ACh
appears to orchestrate several aspects of the dual- channel contrast
detection system of the optic lobe. The actions of ACh on tangential cells,
amacrine cells, and dimming fibers are all consistent with the effects of a
spatially localized increment in light intensity and a corresponding local
release of ACh in the retinotopic columnar array.
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