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Journal of Neuroscience, Vol 10, 383-393, Copyright © 1990 by Society for Neuroscience
The physiology of somatostatin in the rabbit retina
RA Zalutsky and RF Miller
Washington University School of Medicine, Department of Ophthalmology, St. Louis, Missouri 63110.
The neuropeptide somatostatin (SS) has been localized to neurons of the
rabbit retina by immunochemical and biochemical methods (Sagar et al.,
1982, 1986; Marshak and Yamada 1984). We examined the effects of bath-
applied SS on neurons of the rabbit retina, using intra- and extracellular
electrophysiological techniques in an in vitro retina eyecup preparation.
All commonly encountered ganglion cell receptive field types were affected
by SS, and the effects were of 3 kinds: The first was a general excitation,
occurring with a threshold concentration of about 100 nM; the onset of the
excitation was too slow (seconds) for SS to participate in any rapid
light-evoked responses. The second SS effect was an increase in the
"signal-to-noise ratio," defined here as the ratio of light-evoked to
spontaneous spiking, which resulted from a decrease in spontaneous activity
and, usually, a concomitant increase in light-evoked spiking. The third
effect was a shift in center-surround balance towards a more dominant
center. The signal-to-noise and center-surround effects were evident at
concentrations as low as 0.5 nM; both were slow onset (tens of seconds) and
long lasting (tens of minutes). SS acted at multiple levels within the
retinal circuitry to produce the observed changes in ganglion cell output.
These effects included direct actions on ganglion and amacrine cells, and a
decrease in the efficiency with which horizontal cells could drive the
retinal network. At least part of these SS actions on third-order neurons
resulted from a decrease in conductance to ions with an equilibrium
potential more positive than dark membrane potential. The
degradation-resistant SS agonist SMS201-995 had effects qualitatively and
quantitatively similar to those of SS, suggesting that SS may be degraded
slowly enough to act at a distance from its sites of release. While no
adequate SS antagonist is available, the greater sensitivity to exogenous
SS, in retinas depleted of their SS content (with cysteamine), suggests a
role for endogenous SS. The potency of SS also reinforces this view. The
results of this study suggest that SS may be a neuromodulator in the rabbit
retina, producing long-lasting changes in the "signal-to-noise" discharge
pattern and center-surround balance of ganglion cells.
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