Early development of intrinsic photosensitivity of inner retinal neurones in the murine retina.

Abstract

Abstract: : Purpose:In the developing murine retina, the rod and cone opsins are expressed between post–natal day (P) 1 to P7 (Tarttelin et al., 2003) and the earliest visual responses emerge between P8 and P10. Melanopsin, the candidate photopigment for non–image forming responses to light is expressed earlier in development in the inner retina, by embryonic day 10.5 (Tarttelin et al., 2003). However, the functional significance of this is unclear. Methods:Using the Ca²⁺ sensitive fluorescent indicator FURA–2AM in the ganglion cell layer (GCL) of the isolated mouse retina (Sekaran et al, 2003), we investigated the development of light sensitivity. All experiments were performed in the presence of glutamate receptor antagonists to exclude any input from the photoreceptor cell layer. Results:Light stimulation at 470nm for 1 min induced increases in intracellular Ca²⁺ in a subset of the cells of the GCL in animals aged from P3 to P6. Both transient and sustained responses were observed. The light responses were intensity dependent. As the intensity of the light source was increased (range: 4.7 x 10¹³ to 2.9 x 10¹⁵ photons/sec/cm), the amplitude of the response increased and the latency decreased (range: ∼60 secs to 2 secs). Light responses could be elicited at lower levels of illumination (4.9 x 10¹² photons/cm/s) by increasing the duration of the stimulus, suggesting that the light responsive unit had a long integrating time constant. Conclusions:The results suggest that in the murine retina, the earliest light dependent activity in the GCL is associated with the inner retinal photoreceptors.

Keywords: retinal development • ganglion cells • calcium