Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, PC38
The contribution of melanopsin-driven photoreception to light-evoked activity within the mouse olivary pretectal nucleus
A. E. Allen1, T. M. Brown1, J. Gigg1, R. J. Lucas1
1. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom.
In addition to rod and cone photoreceptors, the retina contains a subset of retinal ganglion cells that are rendered intrinsically photosensitive due to the expression of the photopigment melanopsin. These cells, termed mRGCs (melanopsin-expressing retinal ganglion cells), innervate several central targets, notably those associated with non-image forming responses to light. One such area is the olivary pretectal nucleus (OPN), a relay-station for the pupillary light reflex. We aimed to characterise the contribution of melanopsin-photoreception to light-evoked activity within the OPN in vivo. To achieve this, changes in spike-firing rate were assessed via multi-electrode recordings in the pretectum of mice anaesthetised with urethane (1.5g/kg). Responses to increasing intensities of blue (460nm) and red (640nm) light were measured, for both short (2s) and long (30s) duration stimuli. To isolate rod/cone vs. mRGC-driven activity, we compared responses in wildtype mice with those of rodless+coneless mice, and melanopsin knockout (Opn4-/-) mice. Light responses within the wildtype OPN were characterised by strong transient increases in firing rate at lights ON and OFF, and sustained elevations in firing rate for the duration of stimulus presentation. Responses showed irradiance-dependent increases in amplitude and response speed at intensities ranging from 9.8 to 15.8 log10 photons/cm2/sec. In rodless+coneless mice, transient increases in firing rate were entirely absent. Instead, neurons typically showed sluggish increases in firing rate that gradually increased to a peak around 10s after stimulus onset, and persisted for around 20s after offset. The rodless+coneless OPN also was much less sensitive, with a threshold for detectable responses around 13.8 log10 photons/cm2/sec. Conversely, Opn4-/- mice retained fast ON and OFF transient responses as seen in wildtypes, but showed relatively little sustained responses throughout light stimuli. The sensitivity of Opn4-/- mice was similar to that of wildtype mice, with irradiance-dependent responses occurring in the range of 9.8 to 15.8 log10 photons/cm2/sec. These data reveal that while rod and cone photoreceptors drive the fast transient responses to light onset and offset, melanopsin contributes to sustained elements of light-evoked activity within the OPN, especially at higher light intensities. They further suggest that melanopsin makes a unique contribution to the sensory capabilities of the mammalian visual system.
Where applicable, experiments conform with Society ethical requirements