Neuroprotective and Neurodestructive Functions of Nitric Oxide after Spinal Cord Hemisection Mei Xu, Yee-Kong Ng, 1 and Seng-Kee Leong Department of Anatomy, Faculty of Medicine, The National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received June 7, 1999; accepted September 27, 1999 Nitric oxide (NO) may subserve different functions in different central neurons subjected to axotomy. The difference may depend on whether the neurons basally express neuronal nitric oxide synthase (nNOS), a bio- synthetic enzyme of NO. This is supported by our previous finding that suggests the differential role of NO in neurons of nucleus dorsalis (ND) and red nucleus (RN) which have different basal expression of nNOS. This study aimed to establish firmly the functions of NO, as revealed by nNOS immunoreactivity and nico- tinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry, by the administration of endogenous NO donor, L-arginine (L-arg), and NOS inhibitor, L-N G -nitroarginine methyl ester (L-NAME). To relate the role of NO to glutamate receptors (GluR), the distributions of a-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid receptor (AMPAR) and N- methyl-D-aspartate receptor (NMDAR) in the two nuclei were revealed by immunohistochemical techniques. nNOS immunoreactivity was void in ND neurons, but expressed weakly in the RN normally. It was induced in ipsilateral ND neurons and upregulated on both sides of RN after spinal cord hemisection. Neuronal loss in the ipsilateral ND was augmented by L-arg, but reduced by L-NAME. In the contralateral RN, L-arg attenuated neuronal loss. NMDAR1 was present in most neurons in ND. After axotomy, some NMDAR1 immunoreactive neurons of the ipsilateral ND were induced to express NOS, whereas RN neurons showed strong staining for NMDAR1 and all the AMPA sub- units. Most of the NOS-positive neurons in the RN were coexistent with GluR2 in normal rats and those sub- jected to axotomy. The present data demonstrated that NO exerted neurodestructive function in the non-NOS- containing ND neurons characterized by NMDAR as the predominant glutamate receptor. NO might be beneficial to the NOS-containing RN neurons. This could be attributed to the presence of GluR2. Possible diverse synthesizing pathways of NO in two different central nuclei were suggested from the observation that NOS was colocalized with NADPH-d in ND neu- rons, but not in RN neurons. r 2000 Academic Press Key Words: NO; nucleus dorsalis; red nucleus; lower spinal cord hemisection; L-arg; L-NAME; AMPA recep- tor; NMDA receptor. INTRODUCTION Nitric oxide (NO) has been shown to be an important neurotransmitter in the central nervous system (CNS) (6). It also exerts a neuromodulatory effect on the release of certain neurotransmitters (27). An increase in NO production under some neuropathological condi- tions is presumed by the elevated expression of nitric oxide synthase (NOS). NO may act as a neurotoxic agent in brain ischemia and hypoxia (1, 7). However, some other reports suggest that NO may also possess neuroprotective properties. This is evidenced by the observation that NOS-containing neurons are particu- larly resistant to neurodegeneration in Huntington’s disease and excitatory amino acid neurotoxicity (15, 18). Glutamate receptors constitute the most abundant class of neurotransmitter receptors in the vertebrate CNS. Although important to normal CNS function, it is also implicated in excitotoxicity (3). N-Methyl-D- aspartate (NMDA) receptor presumably accounts for most neurotoxicity associated with synaptically re- leased glutamate in various pathologic conditions (9, 22). This type of toxicity is exquisitely dependent on Ca 21 (24). a-Amino-3-hydroxy-5-methyl-4-isoxazolepro- pionic acid (AMPA) subunits are involved in the re- duced NMDA receptor-mediated toxicity in NOS- positive neurons (21). Despite the extensive studies on the functions of NO and glutamate receptors and their possible roles in neurodestruction or neuroprotection, the issues are still clouded with controversies. Nucleus dorsalis (ND) is a bilateral spinal cord nucleus contain- ing neurons whose axons ascend ipsilaterally to the cerebellum. The red nucleus (RN) is situated bilaterally in the midbrain, and the majority of its axons descend contralaterally to the spinal cord. Spinal cord hemisec- tion simultaneously axotomizes neurons in the ipsilat- 1 To whom correspondence should be addressed. Fax: 65-778 7643 E-mail: antngyk@nus.edu.sg. Experimental Neurology 161, 472–480 (2000) doi:10.1006/exnr.1999.7278, available online at http://www.idealibrary.com on 472 0014-4886/00 $35.00 Copyright r 2000 by Academic Press All rights of reproduction in any form reserved.