Abstract Intraocular co-grafts of rat fetal spinal cord and dorsal root ganglia were used to examine the en- hanced survival, growth, and differentiation of sensory neurons by nerve growth factor. E14 lumbar spinal seg- ments were implanted into the anterior eye chamber of capsaicin-pretreated rats. Two weeks later, an E14 dorsal root ganglion was implanted beside the spinal cord graft. Nerve growth factor or vehicle was injected weekly for 4 weeks into the anterior eye chamber. Co-grafts were examined weekly and, at 6 weeks, processed for calcito- nin gene-related peptide (CGRP) immunofluorescence. No differences in overall size were determined for the grafts. Co-grafts treated with nerve growth factor con- tained many more CGRP neurons (19.4 cells/20 μm) that were significantly larger (mean 764 μm 2 ) than neurons from control co-grafts (8.6 cells/20 μm; mean 373 μm 2 ). In co-grafts treated with nerve growth factor, CGRP-im- munoreactive fibers were extensive in the dorsal root ganglion, adjacent iris, and spinal cord compared to con- trol co-grafts. A few CGRP-positive motoneurons were observed in the spinal cord, but no differences in number or size of motoneurons were found. The current report demonstrates that spinal cord and dorsal root ganglia can be co-grafted in oculo for long periods of time. Many dorsal root ganglion neurons survive and send peripheral processes into the iris and central processes into the spi- nal cord under the influence of exogenous nerve growth factor. The intraocular graft paradigm can be of use to further examine the role of neurotrophic factors in regu- lating or modulating dorsal root ganglion and spinal cord neurons. Key words Primary afferents · Dorsal horn · Motoneurons · Rat (Sprague Dawley) Introduction Primary sensory neurons derived from neural crest cells are sensitive to neurotrophic factors during and after de- velopment. One neurotrophin, nerve growth factor (NGF), is important for the development and survival of small, peptide-containing, dorsal root ganglion (DRG) neurons and the differentiation of these neurons into no- ciceptors (Mendell 1994). During embryonic and postna- tal growth, DRG neurons bearing high-affinity NGF re- ceptors, i.e., trkA receptors, are influenced to grow pe- ripheral terminals into the epidermis by NGF-secreting epidermal cells (Davies et al. 1987; Davies 1992; Buchman and Davies 1993; Davis et al. 1993; Snider and McMahon 1998) and differentiate into A-δ high-thresh- old mechanoreceptors (HTMRs) and C-mechanoheat re- ceptors (MHRs; Ritter et al. 1991; Lewin and Mendell 1994). Postnatal administration of NGF antibodies caus- es many A-δ HTMRs to switch functional phenotype and become D-Hair neurons (low-threshold cutaneous mech- anoreceptors; Lewin et al. 1992). Genetic alterations of NGF and trkA receptor in humans and knockout mice or NGF antibody administration produce hypoalgesia with decreases in the number of small, peptide-containing DRG neurons, unmyelinated peripheral axons, and cuta- neous calcitonin gene-related peptide (CGRP) and sub- stance P (SP) immunoreactivities (Johnson et al. 1980; This study was supported by aSwedish MRC grant 14X-06555 (Å.S.), The Miami Project Foundation, The Marianne and Marcus Wallenberg Foundation, The Spinalis Foundations and The Daniel Heumann Fund for Spinal Cord Research (Å.S.). Thanks to Scan- dinavia/T.H. Greenberg fellowship (E.Å.), G.D. Searle, Monsanto Co., NIH Grant NS27213 (K.E.M.) K.E. Miller ( ✉ ) Department of Cell Biology, University of Oklahoma Health Sciences Center, POB 26901, Oklahoma City, OK 73190, USA e-mail: kenneth-miller@ouhsc.edu Tel: +1 405 271 2335, ext. 217 Fax: +1 405 271 3548 E. Åkesson · Å. Seiger Department of Clinical Neuroscience and Family Medicine, Division of Geriatric Medicine, Huddinge University Hospital, Karolinska Institute, S-141 86 Huddinge, Sweden K.E. Miller Central Nervous System Disease Research, G.D. Searle R&D/Monsanto Co., Chesterfield, MO 63198, USA Cell Tissue Res (1999) 298:243–253 © Springer-Verlag 1999 Digital Object Identifier (DOI) 10.1007/s004419900097 REGULAR ARTICLE Kenneth E. Miller · Elisabet Åkesson · Åke Seiger Nerve growth factor-induced stimulation of dorsal root ganglion/spinal cord co-grafts in oculo: enhanced survival and growth of CGRP-immunoreactive sensory neurons Received: 31 August 1998 / Accepted: 15 July 1999 / Published online: 21 September 1999