Answer
Cyclic GMMP (cGMP) is the ligand that opens the sodium (Na+) ion channels in the photoreceptors ( rods and cones) causing dark current to flow.
Work Step by Step
In darkness, sodium ions flow through open ligand-gated ion channels in plasma membranes of photoreceptors(rods and cones).The ligand that holds the ion channels open is cyclic-GMP. This allows the flow of sodium ions, through the receptor plasma membrane, and into the receptor cytosol. The current associated with the flow of these sodium ions is appropriately called a "dark current."
Under these circumstances the photoreceptors are partially depolarized( -30 mV). At the synapse between photoreceptor and some bipolar cells, there is a constant flow of glutamate, an inhibitory neurotransmitter. The effect of glutamate is to hyperpolarize the bipolar cell, trigger an inhibitory post-synaptic potential (IPSP), and prevent the transmission of a signal to the ganglion cell. This inhibition is over come in light with the enzymatic hydrolysis of cGMP.
