April 2010
Volume 51, Issue 13
Free
ARVO Annual Meeting Abstract  |   April 2010
Pentobarbital Inhibits Response of Direction Selective Ganglion Cell Presynaptically
Author Affiliations & Notes
  • M. Y. Lipin
    Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania
  • R. G. Smith
    Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania
  • Footnotes
    Commercial Relationships  M.Y. Lipin, None; R.G. Smith, None.
  • Footnotes
    Support  NIH Grant EY016607
Investigative Ophthalmology & Visual Science April 2010, Vol.51, 1866. doi:
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      M. Y. Lipin, R. G. Smith; Pentobarbital Inhibits Response of Direction Selective Ganglion Cell Presynaptically. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1866.

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Abstract

Purpose: : The retinal direction selective ganglion cell (DSGC) responds preferentially to motion in one direction. The crucial role in directional selectivity belongs to starburst amacrine cells, which release GABA to inhibit bipolar cell terminals and DSGCs, and acetylcholine to excite DSGCs. Pentobarbital is known to potentiate GABA-gated currents and to antagonize nicotinic receptors, and suppresses responses of ganglion cells, thus raising the specific question of whether and how it might suppress DSGC’s response. Possible mechanisms are postsynaptic inhibition of the DSGC, postsynaptic suppression of cholinergic excitation of DSGCs, and presynaptic inhibition of bipolar cell terminals.

Methods: : To study the mechanism of directional selectivity, we used an in vitro whole-mount retina (35 ºC) from a guinea pig anesthetized with pentobarbital. Although pentobarbital is widely used for this purpose, it suppresses the light response in vivo, and we wanted to determine its effect on the in vitro preparation. We recorded DSGC responses to a moving dark bar (400 µm/s) in whole-cell patch mode, and applied conductance analysis to distinguish the excitatory and inhibitory inputs.

Results: : Pentobarbital at a concentration equivalent to anesthesia (100 µM in vitro), completely suppressed the extracellular response of DSGCs (n=6) to a moving dark bar. It increased the tonic DSGC conductance by 1.25 nS (by ~20%, n=8), and decreased light-evoked excitatory and inhibitory conductances by 75% and 94%, respectively. A lower concentration (25µM) increased the DSGC conductance by 0.18 nS (by ~3%, n=8), and decreased light-evoked excitatory and inhibitory conductances by 58% and 29%, respectively. The light-evoked conductances were about the same order of magnitude as the tonic cell conductance. Consistent with previous reports, gabazine, an antagonist of GABAA receptors, when applied at low concentration (3 µM) abolished DS, however, when applied in the presence of 100 µM pentobarbital, the DS light response was preserved. Gabazine at high concentration (100 µM) abolished the increase in tonic DSGC conductance and the pentobarbital-induced suppression of glutamatergic and cholinergic input.

Conclusions: : The results suggest that pentobarbital suppresses the light response of the DSGC predominantly by enhancing presynaptic inhibition of bipolar cell terminals rather than by enhancing shunting or suppression of excitation postsynaptically.

Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • drug toxicity/drug effects • inhibitory receptors 
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