Axons of Alpha Ganglion Cells Regenerate Faster Than Other Types Into a Peripheral Nerve Graft in Adult Cats Hideki Maki, 1 Masami Watanabe, 2 * Yoshihito Tokita, 3 Kiyoshi Saito, 1 and Jun Yoshida 1 1 Department of Neurosurgery, Nagoya University School of Medicine, Nagoya, Aichi, Japan 2 Department of Physiology, Institute for Developmental Research, Kasugai, Aichi, Japan 3 Department of Perinatology, Institute for Developmental Research, Kasugai, Aichi, Japan We reported previously that alpha ganglion cells in the cat retina have the highest ability in axonal regeneration. To examine whether alpha cells regenerate axons faster, we estimated the rate of axonal regeneration of retinal ganglion cells (RGCs) with transplantation of a peripheral nerve (PN) segment. After 4, 6, and 8 weeks of survival, regenerated RGCs were double-labeled with two fluores- cent dyes injected separately at 10 mm and 20 mm from the connected site. From a scatter diagram of double- labeling ratios, we estimated that axons reached 20 mm by 3.2 weeks. Immunostaining suggested that first axon sprouts entered a PN segment on Day 4. These values enabled us to estimate average rates of axonal regener- ation as 1.1 mm/day for all the RGCs. Proportions of cell types of regenerated RGCs were obtained with Lucifer yellow injections, and those of alpha cells were higher than those in normal retinas in any periods. From analysis of scatter diagrams, we estimated axonal growth rate of alpha, beta, and non-alpha/beta cells as 1.4, 1.1, 1.0 mm/day, respectively. The higher regeneration rate of alpha cells may reflect greater regenerative ability com- pared to other cell types. The present system also pro- vides control values when a method to promote axonal regeneration is developed. © 2003 Wiley-Liss, Inc. Key words: retinal ganglion cells; axonal regeneration rate; peripheral nerve transplantation; adult cats Axons of retinal ganglion cells (RGCs) in adult mammals regenerate when a segment of peripheral nerve (PN) is transplanted to the transected end of the optic nerve (ON) (Aguayo, 1985; So and Aguayo, 1985). We have been studying ON regeneration in adult cats because cats have a developed visual system, as primates do, and are suitable for research on functional recovery of impaired visual pathways (Watanabe et al., 1991; Watanabe and Fukuda, 2002). We have revealed some aspects of cat RGCs with regenerated axons. Regenerated RGCs retain their morphological and physiological properties even for 60 days after axotomy (Watanabe et al., 1993; Miyoshi et al., 1999; Watanabe and Fukuda, 2002). Alpha cells have a greater ability for axonal regeneration than do other cell types (Watanabe et al., 1993; Miyoshi et al., 1999). Their higher regenerative ability can be attributed partly to higher resistance against axotomy; alpha cells survive axo- tomy better than beta cells for 14 days or longer than 60 days (Holla ¨nder et al., 1985; Silveira et al., 1994; Wa- tanabe et al., 1995, 2001). Although cat RGCs regenerate axotomized axons, fewer than 4% regenerate their axons into a PN graft (Watanabe et al., 1993), and the numbers are too small to expect functional recovery of impaired acute vision. It is reasonable to think that promotion of axotomized RGC survival results in more regenerated RGCs. In fact, when the survival of RGCs is raised with an injection of neu- rotrophic factors and forskolin, an accelerator of adenylate cyclase, the numbers of regenerated RGCs increase sig- nificantly (Watanabe et al., 2003); however, axonal regen- eration of surviving RGCs is not promoted. Moreover, we observed that most of these surviving RGCs without regeneration belonged to non-alpha/beta (NAB) cells. This observation suggested that a method that facilitates axon elongation should be developed, to promote both axonal regeneration and survival of axotomized RGCs. To develop a method facilitating axonal regeneration, actual regeneration rates of alpha, beta and NAB cells are required to assess the method. We examined regeneration rates of axotomized RGCs with PN transplantation in adult cats. We first estimated axonal regeneration rate as a whole. The esti- Contract grant sponsor: Ministry of Education, Culture, Sports, Science and Technology; Contract grant number: 1169427, 12358016, 12671733; Contract grant sponsor: Special Coordination Fund for Promoting Science and Technology. *Correspondence to: Masami Watanabe, Department of Physiology, Insti- tute for Developmental Research, Kasugai, Aichi 480-0392, Japan. E-mail: mwatan@inst-hsc.pref.aichi.jp Received 18 September 2002; Revised 25 November 2002; Accepted 5 December 2002 Journal of Neuroscience Research 72:218 –226 (2003) © 2003 Wiley-Liss, Inc.