The role of neuropeptides in spinal nociceptive mechanisms with special emphasis on galanin, neuropeptide Y and orphanin FQ/nociceptin

Abstract

Several peptides present in sensory neurons and in the spinal cord have been recognized to have inhibitory action in nociception. In this thesis, we have used the flexor reflex preparation in rats to study the role of several proposed inhibitory neuropeptides, galanin, galanin message-associated peptide (GMAP), neuropeptide Y (NPY), orphanin FQ and nocistatin in spinal nociceptive processing under normal conditions, after inflammation or peripheral nerve injury.

GMAP is the C-terminal flanking peptide in the galanin precursor protein. Previous studies have shown that intrathecal (i.t.) GMAP moderately facilitated the flexor reflex and blocked the reflex facilitation following a train of conditioning stimuli (CS) applied to C-afferents. In study 1, we showed that the naturally formed N- and C-terminal fragments of GMAP are pharmacologically active and produce effects which are similar to the full peptide. Thus, the in vivo effect of GMAP may be mediated by these fragments.

It has been well established that exogenous and endogenous galanin has an inhibitory role in spinal nociception. In study II, we showed that carrageenan-induced inflammation is associated with a complex functional plasticity in the role of endogenous galanin in mediating spinal excitability. There appears to be an enhanced endogenous inhibitory control by galanin on C-afferent input during the peak of inflammation whereas during the recovery phase there may be a temporary reduction in galanin receptors, which may impair the action of exo- and endogenous galanin. In study III, we evaluated the effects of exogenous and endogenous galanin on neuropathic pain-like behaviors after partial sciatic nerve injury. I.t. galanin allviated mechanical and cold allodynia in nerve injured rats. However, endogenous galanin was not tonically active as the galanin receptor antagonist M35 has no effect. In agreement with the above, the upregulation of galanin in sensory neurons was more limited after partial injury than after complete nerve injury where endogenous galanin plays an inhibitory role.

In studies IV and V, the role of NPY in mediating the flexor reflex in normal, inflamed and axotornized rats was studied. In normal rats, i.t. NPY or a selective Y1 receptor agonist, (Leu31, Pro34)- NPY, produced a dose-dependent biphasic effect on the flexor reflex with facilitation at low doses and depression at high doses. In contrast, i.t. NPY(13-36), a selective agonist of the Y2 receptor, only facilitated the flexor reflex. One to three weeks after axotomy, i.t. NPY(13-36) produced dosedependent reflex depression. These results suggest that the Y1 receptor may mediate the depressive effect of i.t. NPY in rats with intact sciatic nerve, while both Y1 and Y2 receptors may be involved in mediating the depressive effect of NPY after axotomy. On the other hand, peripheral inflammation appears to increase the excitatory effect of i.t. NPY while NPY-mediated inhibition is unchanged.

In the final part of the thesis, the antinociceptive role of orphanin FQ, the endogenous ligand of the orphan opioid receptor-like receptors (ORL1), was studied. I.t. orphanin FQ primarily depressed the flexor reflex in normal, inflamed and axotomized rats. In behaving rats, i.t. orphanin FQ was antinociceptive and reduced abnormal pain-related behaviors after inflammation, peripheral nerve injury or spinal cord injury. Phe1 *(Ch2-NH)Gly2)-nociceptin-(1-13)NH2, a proposed antagonist of the ORL1 receptors, was a potent and stable agonist in rat spinal cord whereas nocistatin, another peptide derived from orphanin FQ precursor protein, interacted with orphanin FQ in a complex fashion with no antinociceptive property.

In conclusion studies performed in this thesis showed that in addition to the extensively studied excitatory peptides, several peptides can exert inhibitory action on spinal nociception. The inhibition can be on the basal nociceptive reflex as for NPY or orphanin FQ or on the hyperexcitable state as for galanin and GMAP. Peripheral nerve injury and inflammation induce plasticity in these peptidergic systems, leading to altered functions of these inhibitory peptides. Spinal administration of some of these peptides produces analgesia following inflammation and nerve injury. Thus, these peptides may present leads in developing novel analgesics in treating chronic pain.