Nitric Oxide-Directed Synaptic Remodeling in the Adult Mammal CNS

Abstract

In adult mammals, learning, memory, and restoration of sensorimotor lost functions imply synaptic reorganization that requires diffusible messengers-mediated communication between presynaptic and postsynaptic structures. A candidate molecule to accomplish this function is the gaseous intercellular messenger nitric oxide (NO), which is involved in synaptogenesis and projection refinement during development; however, the role of NO in synaptic reorganization processes in adulthood remains to be established. In this work, we tested the hypothesis that this free radical is a mediator in the adult mammal CNS synaptic remodeling processes using a model of hypoglossal axonal injury recently developed by us. Axonal injury-induced disconnection of motoneurons from myocytes produces withdrawal of synaptic inputs to motoneurons and concomitant upregulation of the neuronal isoform of NO synthase (NOS-I). After recovery of the neuromuscular function, synaptic coverage is reestablished and NOS-I is downregulated. We also report, by using functional and morphological approaches, that chronic inhibition of the NO/cGMP pathway prevents synaptic withdrawal evoked by axon injury, despite the persistent muscle disconnection. After successful withdrawal of synaptic boutons, inhibition of NO synthesis, but not of cGMP, accelerated the recovery of synaptic coverage, although neuromuscular disconnection was maintained. Furthermore, protein S-nitrosylation was upregulated after nerve injury, and this effect was reversed by NOS-I inhibition. Our results suggest that during synaptic remodeling in the adult CNS, NO acts as a signal for synaptic detachment and inhibits synapse formation by cGMP-dependent and probably S-nitrosylation-mediated mechanisms, respectively. We also suggest a feasible role of NO in neurological disorders coursing with NOS-I upregulation.

Nitric Oxide Induces Pathological Synapse Loss by a Protein Kinase G-, Rho Kinase-Dependent Mechanism Preceded by Myosin Light Chain Phosphorylation

“…Axotomized HMNs upregulate nNOS and undergo excitatory synaptic stripping, which likely accounts for the strong reduction in their firing activities (Sunico et al, 2005). Systemic administration of inhibitors of the neuronal NO/cGMP pathway prevented synapse loss.…”

“…XIIth nerve crushing induced both de novo expression of nNOS and a NO/cGMP-dependent synaptic loss on adult HMNs (Sunico et al, 2005). One week after crushing, animals treated daily with the inactive stereoisomer D-NAME had a reduction in the frequency of VGLUT2-(3.4 Ϯ 0.29 puncta/100 m), but not in VGAT-ir (15.2 Ϯ 0.60 puncta/100 m) puncta compared with the intact side (VGLUT2, 6.5 Ϯ 0.45 puncta/100 m; VGAT, 14.5 Ϯ 0.72 puncta/100 m) (Fig.…”

“…6G,H). We selected this postinjury time window because nNOS was already upregulated (Sunico et al, 2005) and syn-ir reduction had not yet occurred (intact side, 16.9 Ϯ 0.55; injured side, 17.2 Ϯ 0.7 puncta/100 m). These results suggest that, after motor nerve injury in adult rats, NO, through sGC, mediates the loss of most ROCK-containing excitatory inputs, likely by a mechanism involving previous p-MLC.…”

“…Synaptic stripping from the motoneuron surface also occurs in the progression of several motoneuron pathologies, such as amyotrophic lateral sclerosis (ALS), progressive muscular atrophy, and traumatically damaged adult motor axons (Sumner, 1975;Ikemoto et al, 1994;Sasaki and Maruyama, 1994;Ince et al, 1995;Sunico et al, 2005). Understanding the molecular basis underlying this neurodegenerative event is of central interest for the development of new therapeutic tools.…”

“…iNOS (inducible NOS) is also upregulated in glial cells in these diseases as well as in MS and HIV dementia (Lee et al, 2003). Additionally, NO participates in projection refinement during development (Wu et al, 1994) and in synapse loss suffered by motoneurons after motor nerve injury (Sunico et al, 2005;Moreno-Ló pez and González-Forero, 2006). However, determining whether NO synthesis is not only necessary but also sufficient to induce synapse loss remains elusive.…”