As cGMP is hydrolysed during the activation phase of phototransduction, its intracellular concentration decreases leading to the closure of CNG channel (the closed status is designated as CNG channel) (Dhallan et al. 1992, Chen et al. 1993). Channel closure reduces the inward flux of sodium and calcium (also known as the 'dark current') resulting in cell hyperpolarisation. Cooperative binding of cGMP to the CNG channel confers additional amplification to the phototransduction cascade (Pugh & Lamb 1993, Yau & Baylor 1989).