Background
Nitric oxide (NO) may function as a signaling molecule in controlling neuronal activity and plays an important role in governing sensory inputs during migraine [1]. Endogenous NO is produced by the constitutive isoforms of NO synthase, endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS). Asymmetric dimethylarginine (ADMA), a major endogenous inhibitor of NOS, inhibits NO production in vivo and in vitro [2, 3]. Besides ADMA, two other forms of methylated arginine — which can be considered arginine analogues — have been identified in eukaryotes: NG-monomethyl-l-arginine (l-NMMA), and ω-NG,N′G-symmetric dimethylarginine (SDMA) [4]. All three methylated arginines (ADMA, l-NMMA and SDMA) are inhibitors of arginine transport at superphysiological concentrations, while the physiological relevance of this inhibition remains unclear [5, 6]. Circulating ADMA is present at higher concentrations than l-NMMA and is often considered to be the principal inhibitor of NOS activity [2]. Most of ADMA is degraded by dimethylarginine dimethylaminohydrolase (DDAH), which hydrolyzes ADMA to L-citrulline and dimethylamine [7]. Therefore, this enzymatic pathway is a potential endogenous mechanism for the regulation of NO production by competitive inhibition. ADMA has been associated to cardiovascular risk [7, 8] as it seems involved in the development and progression of cardiovascular disease, via the inhibition of eNOS activity and increased production of superoxides [9]. However, high levels of ADMA and increased DDAH-1 expression have been detected in the brain, and spinal cord, thus suggesting a possible role for the ADMA-DDAH pathway in the modulation of neuronal activity [10–12]. This hypothesis seems even more compelling when considering that DDAH-1 co-localizes with nNOS [11]. Increased ADMA levels seem to induce endothelial dysfunction and oxidative stress [9, 12], two potential factors involved in migraine pathogenesis [13, 14]. Available data on ADMA plasma levels and migraine have yielded inconclusive findings so far [15–17] and there is no information on ADMA/DDAH pathway in animal models of migraine.
Exogenous NO, released by nitroglycerin (or glyceryl trinitrate, GTN), induces migraine-like headache in predisposed subjects and it has been used as a human [18, 19] and animal model for the study of migraine [20–22]. GTN also activates the NO synthetic pathway in humans and rats [23, 24].
In order to gain new insights in ADMA-DDAH-NO axis in migraine pain, in this study we investigated changes in brain and serum ADMA levels, together with nNOS and eNOS expression and DDHA-1 expression in discrete areas of the rat brain following GTN administration.