Abstract
Abstract: :
Purpose: To understand the contribution of intracellular calcium sources and their modulation, to neurotransmitter release in the inner plexiform layer of the salamander retina. Methods: Whole–cell patch clamp techniques were used in combination with intracellular calcium release modulators while recording light responses from third–order neurons in dark–adapted salamander retinal slices. Electrical stimulation and hyper–osmotic sucrose puffs were also used to stimulate the inner retina while recording from third–order neurons in order to isolate the bipolar to ganglion cell and amacrine to ganglion cell synapses. Results: Intracellular calcium levels are known to play a key role in eliciting and regulating neurotransmitter release in neurons. Though the overall process of calcium influx through voltage–gated channels leading to intracellular calcium induced calcium release has been documented, little is known about how different internal calcium release mechanisms interact and contribute to light–evoked excitatory and inhibitory synaptic transmission in the retina. We investigated the contribution of two sources of intracellular calcium release, the Ryanodine receptor (RyR) and the IP3 receptor (IP3–R) pathways, to synaptic transmission in the inner retina. 10mM Caffeine, a RyR calcium release modulator, rapidly blocked light–evoked inhibitory responses. Concomitant with the loss of IPSPs, light–evoked EPSPs were still observed at both light onset and offset in most cells. Caffeine does not effectively block light–evoked excitatory responses in the time scale during which it blocks inhibition in these third–order neurons. We often observe a transient enhancement of EPSPs during application of caffeine. When the influence of IP3 receptors was tested using 100µM 2–APB, an IP3 receptor blocker, it induced an increase of spontaneous synaptic activity in contrast to caffeine, which reduced spontaneous activity. The 2–APB induced spontaneous activity was inhibitory, since it could be abolished by a cocktail of 50µM picrotoxin and 10µM strychnine. Conclusions:We find that caffeine and 2–APB, working through RyR and IP3–R pathways respectively, differentially affect excitatory and inhibitory responses in third order neurons.
Keywords: calcium • ganglion cells • synapse