Brain Research 973 (2003) 146–150 www.elsevier.com / locate / brainres Short communication Retinal ganglion cells projecting to the dorsal raphe and lateral geniculate complex in Mongolian gerbils * Katherine V. Fite , Melissa A. Birkett, Andrea Smith, Skirmantas Janusonis, Stephen McLaughlin Neuroscience and Behavior Program, Tobin Hall, University of Massachusetts, Amherst, MA 01003, USA Accepted 28 February 2003 Abstract Injections of rhodamine-B into the dorsal raphe nucleus (DRN) and Fluoro-Gold into the lateral geniculate nucleus (LGN) revealed double-labeled retinal ganglion cells (DL RGCs) projecting to both nuclei. The soma-size distribution of DL RGCs was compared with three other distributions: DRN-projecting RGCs, LGN-projecting RGCs, and a large sample of RGCs labeled via the optic nerve with DiI. DL RGC soma diameters fell primarily within the mid-to-upper size range of all three distributions. DL RGCs may provide information to both nuclei concerning comparable aspects of light and visual stimulation via collateralized axons.  2003 Elsevier Science B.V. All rights reserved. Theme: Sensory systems Topic: Subcortical visual pathways Keywords: Optic axon collateral; Double-labeled retinal ganglion cell; Serotonin system; Rodent visual system The large, serotonergic dorsal raphe nucleus (DRN) those of the ‘nonimage-forming’ subsystem of retinal receives direct retinal afferents in a number of species, afferents, which encodes primarily the photic and temporal including cats [10], rats [9,32], Chilean degus [7], tree characteristics of visual stimulation [2,8,22,23,26]. shrews [31] and Mongolian gerbils [9]. Gerbils are highly In order to determine whether the retinal-DRN pathway visual animals with a diurnal / crepuscular activity rhythm in gerbils may contain collaterals of optic axons that [12,15,30], and in this species, retinal-DRN terminals are innervate the large LGN complex, rhodamine-B was well placed to influence serotonergic neurons, particularly injected into the DRN and Fluoro-Gold was injected into those in the large, lateral subdivision which project to the the LGN complex. RGCs positive for both tracers were superior colliculus and lateral geniculate complex [9,17]. identified and analyzed in whole-mount retinas with regard The retinal-DRN pathway in gerbils originates from RGCs to soma sizes and retinal location. In addition, the soma- distributed in nonrandom fashion over the entire retina and size distribution of double-labeled (DL) RGCs was com- comprise about 1% of the total RGC population [9]. The pared with a previously described DRN–RGC distribution soma-diameter size range of DRN-RGCs is comparable to [9], with a population of LGN-projecting RGCs, and with that of the overall RGC population, suggesting that this a large, random sample of RGCs labeled via the transected pathway may include collaterals of optic afferents that optic nerve. innervate other retinorecipient nuclei. Extensive evidence Adult, male, Mongolian gerbils weighing 50–70 g for collateralized optic axons has been described in rodent ( Meriones unguiculatus) were used as subjects. All pro- visual systems [3–6,20,27]. The retinal-DRN pathway cedures were approved by the Institutional Animal Care shows several morphological features [7,9] comparable to and Use Committee of the University of Massachusetts in accordance with NIH and USDA guidelines. Animals were anesthetized with an intraperitoneal injection (mixture of *Corresponding author. Tel.: 11-413-545-0351; fax: 11-413 545- ketamine, 100 mg/kg and xylazine, 10 mg/kg). A total of 0996. E-mail address: kfite@psych.umass.edu (K.V. Fite). nine animals each received injections of two different 0006-8993 / 03 / $ – see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0006-8993(03)02549-6