Neuronal nitric oxide synthase is upregulated in a subset of primary sensory afferents after nerve injury which are necessary for analgesia from α2-adrenoceptor stimulation

Abstract

Abstractα2-Adrenoceptor (AR) agonists increase in analgesic potency and efficacy after peripheral nerve injury, and their effects are blocked by neuronal nitric oxide synthase (nNOS) inhibitors and M4 muscarinic receptor antagonists only after injury. We tested whether nNOS and M4 muscarinic receptors are co-expressed in the spinal cord, and whether destruction of a subset of sensory afferents which are essential to α2-AR analgesia would also destroy nNOS and M4 receptor expression.Male Sprague Dawley rats underwent left L5 and L6 spinal nerve ligation. Lumbar spinal cord was removed and immunostained for M4 muscarinic receptors and nNOS alone and for co-expression. Others received intrathecal injection of saporin linked to an antibody to the neurotrophin receptor p75 NTR , which eliminates cells expressing this receptor and the analgesic effects of α2-AR agonists.nNOS staining of fibers in the superficial dorsal horn was dramatically increased after spinal nerve ligation, and this was abolished by saporin linked anti-p75 NTR treatment. In contrast, nNOS staining in dorsal horn neurons was unaltered by these manipulations. M4 receptors were present on neurons in the dorsal horn, some of which co-expressed nNOS, but their pattern of expression was not altered by these manipulations.Peripheral nerve injury increases nNOS expression in fibers in the superficial dorsal horn, some of which likely express p75 NTR , and α2-AR agonists may reduce injury-induced sensitization by activation of nNOS in these fibers In contrast, changes in nNOS and M4 receptor location on spinal cord neurons are not responsible for increased analgesic potency of α 2-AR agonists after nerve injury.

Localization of neurones expressing the gap junction protein Connexin45 within the adult spinal dorsal horn: a study using Cx45-eGFP reporter mice

“…This antibody specifically recognizes human, mouse and rat nNOS (Biomol data sheet). Antibody staining in mouse spinal cord reveals identical patterns to those previously documented (Maihofner et al 2000; Ma and Eisenach 2007). …”

α2‐Agonists as analgesic agents

“…24 It has been reported that the a 2 -agonists increase in analgesic potency and efficacy after nerve injury in animals 25 and patients with cancer pain. 26 In addition Ma and Eisenach 27 have reported that a 2 Agonists reduce injury-induced sensitization through the up-regulation of nitric oxide synthase.…”

Distribution of Nitrergic Neurons in the Dorsal Root Ganglia of the Bottlenose Dolphin (Tursiops truncatus)

“…In addition, immunohistochemical, electrophysiological, and pharmacological studies have suggested that NO acts as a neu-rotransmitter or neuromodulator in the processing of nociceptive stimuli. However, the influences of NO upon nociceptive transmission at different levels of the spinal cord are opposing and complex, and the exact sites and mechanisms of these actions are controversial (Meller and Gebhart, 1993;Cížkova et al, 2002;Ruscheweyh et al, 2006;Boettger et al, 2007;Ma and Eisenach, 2007;Kawano et al, 2009). In fact, both hyperalgesic and analgesic effects of NO were demonstrated at different levels of the neuraxis (Hoheisel et al, 2005).…”