Characterization and synaptic connectivity of melanopsin-containing ganglion cells in the primate retina Patricia R. Jusuf, 1,2 Sammy C. S. Lee, 1,2 Jens Hannibal 3,4 and Ulrike Gru ¨ nert 1,2 1 National Vision Research Institute of Australia, Cnr Keppel & Cardigan Streets, Carlton, Victoria 3053, Australia 2 Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, Victoria, Australia 3 Clinical Biochemistry, Rigshopitalet, Copenhagen, Denmark 4 Department of Clinical Biochemistry, Bispebjerg Hospital, Copenhagen, Denmark Keywords: amacrine input, bipolar input, Callithrix jacchus, intrinsically photosensitive ganglion cells, Macaca fascicularis, marmoset Abstract Melanopsin is a photopigment expressed in retinal ganglion cells, which are intrinsically photosensitive and are also involved in retinal circuits arising from rod and cone photoreceptors. This circuitry, however, is poorly understood. Here, we studied the morphology, distribution and synaptic input to melanopsin-containing ganglion cells in a New World monkey, the common marmoset (Callithrix jacchus). The dendrites of melanopsin-containing cells in marmoset stratify either close to the inner nuclear layer (outer stratifying), or close to the ganglion cell layer (inner stratifying). The dendritic fields of outer-stratifying cells tile the retina, with little overlap. However, the dendritic fields of outer-stratifying cells largely overlap with the dendritic fields of inner-stratifying cells. Thus, inner- stratifying and outer-stratifying cells may form functionally independent populations. The synaptic input to melanopsin-containing cells was determined using synaptic markers (antibodies to C-terminal binding protein 2, CtBP2, for presumed bipolar synapses, and antibodies to gephyrin for presumed amacrine synapses). Both outer-stratifying and inner-stratifying cells show colocalized immunoreactive puncta across their entire dendritic tree for both markers. The density of CtBP2 puncta on inner dendrites was about 50% higher than that on outer dendrites. The density of gephyrin puncta was comparable for outer and inner dendrites but higher than the density of CtBP2 puncta. The inner-stratifying cells may receive their input from a type of diffuse bipolar cell (DB6). Our results are consistent with the idea that both outer and inner melanopsin cells receive bipolar and amacrine input across their dendritic tree. Introduction Different aspects of the visual world are processed in parallel pathways. These pathways originate in the retina and reach higher brain regions via different types of ganglion cells. In primate retina, at least 15 ganglion cell types have been found, but only a few of them are well characterized (Kolb et al., 1992; Dacey, 2004; Yamada et al., 2005). Midget (parvocellular) ganglion cells convey signals used for red–green color vision and high spatial acuity vision [reviewed in Martin (1998)]. Parasol (magnocellular) ganglion cells relay achro- matic visual information used for motion perception (Merigan & Maunsell, 1993; Calkins, 1999). Small bistratified ganglion cells are thought to convey blue-ON ⁄ yellow-OFF signals to the koniocellular layers of the lateral geniculate nucleus (Dacey & Lee, 1994; Martin et al., 1997). They belong to the group of so-called ‘wide-field’ ganglion cells, which are characterized by large dendritic fields (Dacey, 2004). The present study concerns another type of wide-field cell, the melanopsin-containing ganglion cell, which was first described in rat retina [reviewed by Berson (2003)]. In primate retina, this cell has previously been termed G20, PT-sparse, giant very sparse, large very sparse, or giant sparse (Kolb et al., 1992; Rodieck & Watanabe, 1993; Peterson & Dacey, 1999; Dacey et al., 2003; Dacey, 2004; Yamada et al., 2005). Melanopsin ganglion cells are intrinsically photosensitive, i.e. they respond to light, even if synaptic transmission is blocked (Berson, 2003). They contribute to a monosynaptic retinohypothalamic tract leading to the suprachiasmatic nucleus, and play an important role in photoentrainment of the circadian system (Hannibal et al., 2002, 2004; Hattar et al., 2002; Sollars et al., 2003). Another major target is the olivary pretectal nucleus, where melanopsin cells are involved in the pupillary reflex [reviewed by Fu et al. (2005)]. Melanopsin ganglion cells also project to the thalamic visual relay nucleus, the lateral geniculate nucleus, suggesting that they may also play a role in the image-forming part of the visual system (Dacey et al., 2003, 2005; Hattar et al., 2006). Recently, some studies provided evidence that intrinsically photo- sensitive ganglion cells also receive light signals from rods and ⁄ or cones via bipolar and amacrine cells. First, an electron microscopic study of mouse retina identified synapses from both amacrine and bipolar cells onto the dendrites of immunolabeled melanopsin cells (Belenky et al., 2003). Second, electrophysiological recordings from Correspondence: Dr U. Gru ¨nert, 1 National Vision Research Institute of Australia, as above. E-mail: uhg@unimelb.edu.au Received 9 August 2007, revised 25 September 2007, accepted 1 October 2007 European Journal of Neuroscience, Vol. 26, pp. 2906–2921, 2007 doi:10.1111/j.1460-9568.2007.05924.x ª The Authors (2007). 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