Pain Modulation from the Locus Coeruleus in a Model of Hydrocephalus: Searching for Oxidative Stress-Induced Noradrenergic Neuroprotection

Abstract

Pain transmission at the spinal cord is modulated by noradrenaline (NA)-mediated actions that arise from supraspinal areas. We studied the locus coeruleus (LC) to evaluate the expression of the cathecolamine-synthetizing enzyme tyrosine hydroxylase (TH) and search for local oxidative stress and possible consequences in descending pain modulation in a model of hydrocephalus, a disease characterized by enlargement of the cerebral ventricular system usually due to the obstruction of cerebrospinal fluid flow. Four weeks after kaolin injection into the cisterna magna, immunodetection of the catecholamine-synthetizing enzymes TH and dopamine-β-hydroxylase (DBH) was performed in the LC and spinal cord. Colocalization of the oxidative stress marker 8-OHdG (8-hydroxyguanosine; 8-OHdG), with TH in the LC was performed. Formalin was injected in the hindpaw both for behavioral nociceptive evaluation and the immunodetection of Fos expression in the spinal cord. Hydrocephalic rats presented with a higher expression of TH at the LC, of TH and DBH at the spinal dorsal horn along with decreased nociceptive behavioral responses in the second (inflammatory) phase of the formalin test, and formalin-evoked Fos expression at the spinal dorsal horn. The expression of 8-OHdG was increased in the LC neurons, with higher co-localization in TH-immunoreactive neurons. Collectively, the results indicate increased noradrenergic expression at the LC during hydrocephalus. The strong oxidative stress damage at the LC neurons may lead to local neuroprotective-mediated increases in NA levels. The increased expression of catecholamine-synthetizing enzymes along with the decreased nociception-induced neuronal activation of dorsal horn neurons and behavioral pain signs may indicate that hydrocephalus is associated with alterations in descending pain modulation.

Noradrenergic Pathways Involved in Micturition in an Animal Model of Hydrocephalus—Implications for Urinary Dysfunction

“…Descending noradrenaline projections that innervate the lumbosacral segments of the spinal cord are required for the coordination of bladder contractions and EUS relaxation during voiding [15,38]. We previously showed that the PAG is strongly affected by the dilation of the cerebral ventricles in the hydrocephalus model used in the present study [21]. Here, the PAG was analyzed to evaluate the expression of the c-Fos protooncogene, as this is an indicator of neuronal activity [39].…”

“…Furthermore, these animals also showed increased levels of DBH, the enzyme involved in the synthesis of noradrenaline, at the Onuf's nucleus, where the motoneurons that regulate bladder contraction are located. We previously reported an increase in noradrenergic innervation in hydrocephalic animals at the LC [21]. The spinal noradrenergic system has a modulatory role in the control of the micturition reflex, and it is likely that an association between altered micturition and changes in noradrenergic innervation at the Onuf´s nucleus occurs.…”

“…Since knowledge about the mechanism behind urinary dysfunction during hydrocephalus is scarce and considering that two key areas involved in the neuronal control of micturition are affected in this condition, namely the PAG and the LC, our aim was to determine the micturition profile of hydrocephalic animals and evaluate the degree of neuronal activation at the PAG. We have recently demonstrated that the LC undergoes major alterations in hydrocephalic animals, with an increase in local levels of noradrenalinesynthetizing enzymes, which affect descending pain modulation through increases in noradrenergic innervation of the spinal dorsal horn [21]. We now continue the characterization of neurological dysfunctions in hydrocephalus.…”

“…The region of the head and neck was shaved, and under a binocular loupe, the cervical muscles were carefully separated to expose the atlantooccipital membrane. A sterile Kaolin (Sigma-Aldrich, St. Louis, MO, USA) 20% suspension in a volume of 0.04 mL was slowly injected into the cisterna magna at a slow rate with a 27 G needle, as previously described [21,25]. The Kaolin injection was administered after the CSF aspiration to confirm access to intracisternal space.…”

“…The immunohistochemical detection of Fos was elected because it is the most widely and reliably used functional anatomical marker of activated neurons in the central nervous system [34][35][36]. The induction of hydrocephaly was performed as above, but each animal received 0.05 mL of 20% Kaolin suspension (n = 8) or saline (n = 6), as described previously [21]. All the immunohistochemical procedures were similar to the DBH immunostaining but the blocking solution was 10% normal swine serum (NSS), the primary antibody was a polyclonal anti-Fos antibody raised in rabbit (Calbiochem, San Diego, CA, USA, Cat.…”