Abstract
Purpose: :
Previous studies have demonstrated that muscarinic acetylcholine receptors (mAChRs) modulate retinal ganglion cell response properties and that the reported effects vary by ganglion cell type. There are more than 10 types of ganglion cells identified in the mammalian retina. The diversity of ganglion cells and mAChRs makes it difficult to interpret the various effects of specific mAChR modulation. Thus, the purpose of this study was to identify specific ganglion cell types whose response properties were affected by muscarinic activation. This portion of the study focused on the modulation of morphologically and physiologically identified ganglion cell subtypes by m1-m4 mAChR activation.
Methods: :
Rabbit eyecups were used for whole cell patch clamp recordings to test the effects of mAChR agents on sustained ganglion cell light responses. Cell attached recording was used to determine the subtypes of brisk and sluggish sustained ON and OFF cells. Light responses were recorded before, during, and after the application of the specific muscarinic antagonists: non-selective Atropine, Pirenzepine (m1), Himbacine (m2 and m4), and 4-diphenylacetoxy-N-methylpiperidine methiodide (m3). The non-selective mAChR agonist methacholine was puff applied to activate mAChRs before and after blockade of mAChR subtypes. Cell morphology was confirmed by injecting Lucifer Yellow.
Results: :
Both ON and OFF sustained ganglion cell light responses were modulated by mAChR activation. Methacholine depolarized ON sustained cells, while it did not show obvious effect on OFF sustained cells’ membrane potentials. The specific antagonists of all muscarinic subtypes (m1-m4) suppressed sustained ON responses of ON sustained cells. In OFF sustained cells, the m2 and m4 mAChR antagonist himbacine increased OFF sustained responses. Application of the m3 antagonist 4-DAMP suppressed OFF responses, while blockade of m1 receptors had no significant effect on the OFF component.
Conclusions: :
The light responses of concentric sustained ganglion cells were affected by specific mAChR blockade. However, muscarinic blockade affected only the center responses. Whether the effects on light responses were excitatory or inhibitory, mAChR antagonists affected only the ON responses of ON cells and only the OFF responses of OFF cells. They did not affect the antagonistic surround of either ON or OFF cells. In ON center sustained cells, specific mAChR antagonists consistently suppressed the center type responses, while they either activated or suppressed the center responses of OFF sustained GCs. These results suggest that the center light responses were mediated by different mAChR subtypes expressed by GCs themselves as well as through intermediate neurons.
Keywords: acetylcholine • ganglion cells • electrophysiology: non-clinical