Characterization of the retinal ganglion cells projecting to the SCN

In mammals, light entrainment of the circadian clock requires input from the retina, which communicates with the SCN via retinohypothalamic tract. This neuronal projection constitutes the sole source of retinal input to the circadian system, and it is formed by the axons of a small subset of retinal ganglion cells (RGCs). Although retinal input is essential for photoentrainment, rod and cone photoreceptors are not required. Instead, recent findings suggest that RGCs that project to the SCN may themselves function as circadian photoreceptors. In striking contrast to their visual counterparts, RGCs projecting to the SCN express a novel pigment, melanopsin, and exhibit intrinsic light sensitivity. In order to understand the process of circadian entrainment, we must first understand how RGCs of the RHT generate and shape the retinal input to the circadian system. Our working model is that RGCs projecting to the circadian system are intrinsically sensitive to light due to a signaling cascade triggered by photoexcited melanopsin that activates a non-selective cation channel that depolarizes the plasma membrane. This depolarization, in turn, activates a suite of voltage-gated conductances that confers unique firing properties on these neurons.

In collaboration with Dr. David Robinson (CROET) and Dr. Lane Brown (NSI), we have investigated the morphological and electrophysiological properties of this unique class of RGCs. Although SCN-projecting RGCs resemble Type III cells in form, they display strikingly different physiological properties from these neurons (Warren et al., Eur J Neurosci, 2003). First, in response to the injection of a sustained depolarizing current, SCN-projecting cells fired in a transient fashion, in contrast to most RGCs, which fired robust trains of action potentials. Second, in response to light, SCN-projecting RGCs exhibited an intensity-dependent transient depolarization in the absence of rod and cone input. In response to varying light intensities, SCN-projecting RGCs exhibited a graded transient inward current that peaked within 5 seconds and decayed to a plateau. The voltage dependence of the light-activated current was obtained by subtracting currents elicited by a voltage ramp before and during illumination. The light-activated current displayed both inward and outward rectification and was unaffected by substitution of extracellular Na+ with choline. In both respects, the intrinsic light-activated current observed in SCN-projecting RGCs resembles currents carried by ion channels of the transient receptor potential (trp) family, which are known to mediate the light response of invertebrate photoreceptors (Warren et al., Eur J Neurosci, 2003).

Our current work is:

  1. Characterizing the signaling properties of hetrologously-expressed melanopsin
  2. Identifying the intracellular signaling pathway that generates the intrinsic light response
  3. Determining the molecular identity of ion channels giving rise to the light-activated current.