Jing-Xia Hao

Chronic pain-related syndrome in rats after ischemic spinal cord lesion: a possible animal model for pain in patients with spinal cord injury

&NA; We examined a pain‐related syndrome, which includes mechanical allodynia and autotomy, in rats after ischemic spinal cord injury photochemically induced by laser irradiation for 5–20 min. This procedure results in an acute allodynia‐like phenomenon which lasts for several days and is possibly related to dysfunction of the GABAB system in the spinal cord. In some animals this is followed by a chronic allodynia‐like symptom with an onset varying between 1 week and 1.5 months after injury, expressed as a clearly painful reaction to light pressure applied to a skin area at or near the dermatome of the injured spinal segments. In the majority of rats the allodynia persists over several months, in some cases accompanied by autotomy of the hind paws. Pharmacological studies indicated that the allodynia in the majority of rats could be relieved by systemic tocainide (75 mg/kg). Morphine was only effective at a sedative dose (5 mg/kg). The allodynia was not relieved by baclofen, muscimol, clonidine or carbamazepine. Low‐dose systemic pentobarbital (5 mg/kg) had a slight beneficial effect. Guanethidine (20 mg/kg s.c.) did not abolish the allodynia in most of the rats. Histological examination revealed massive damage in the spinal cord. The dorsal roots of the irradiated segments were also injured. No morphological abnormalities were seen in the dorsal root ganglia. The mechanism that may account for this chronic pain‐related syndrome in spinally injured rats probably involves abnormalities in the central nervous system. The allodynia seen in chronic spinally injured rats was similar to some painful symptoms in patients after spinal cord injury or stroke. It is suggested that the chronic allodynia‐like phenomenon may represent an animal model for studying the mechanisms of chronic central pain.

Reduced antinociception and plasma extravasation in mice lacking a neuropeptide Y receptor

Neuropeptide Y (NPY) is believed to exert antinociceptive actions by inhibiting the release of substance P and other ‘pain neurotransmitters’ in the spinal cord dorsal horn. However, the physiological significance and potential therapeutic value of NPY remain obscure. It is also unclear which receptor subtype(s) are involved. To identify a possible physiological role for the NPY Y1 receptor in pain transmission, we generated NPY Y1 receptor null mutant (Y1-/-) mice by homologous recombination techniques. Here we show that Y1-/- mice develop hyperalgesia to acute thermal, cutaneous and visceral chemical pain, and exhibit mechanical hypersensitivity. Neuropathic pain is increased, and the mice show a complete absence of the pharmacological analgesic effects of NPY. In the periphery, Y1 receptor activation is sufficient and required for substance P release and the subsequent development of neurogenic inflammation and plasma leakage. We conclude that the Y1 receptor is required for central physiological and pharmacological NPY-induced analgesia and that its activation is both sufficient and required for the release of substance P and initiation of neurogenic inflammation.

Allodynia-like effects in rat after ischaemic spinal cord injury photochemically induced by laser irradiation

&NA; We report behaviours suggesting the presence of allodynia elicited by non‐noxious brushing and mechanical pressure following photochemically induced ischaemic spinal cord injury in the rat. Female rats were intravenously injected with Erythrosin B and the T10 vertebra was irradiated with a laser beam for 1, 5 or 10 min. These procedures initiated an intravascular photochemical reaction, resulting in ischaemic spinal cord injury. After irradiation a clear allodynia was observed in most rats, The animals vocalized intensely to light touch during gentle handling and were clearly agitated to light brushing of the flanks. The vocalization threshold in response to the mechanical pressure measured with von Frey hairs was markedly decreased during this period. In some animals the existence of spontaneous pain was suggested by spontaneous vocalization. The duration of the allodynia varied among animals from several hours to several days. The severity and duration of allodynia seemed not to be related to the duration of irradiation. In sham‐operated rats a slight, transient allodynia was also noted around the wound within a few hours after surgery, which was effectively relieved by systemic morphine (2 mg/kg, i.p.). Morphine (2 mg/kg, i.p.) also partially relieved the allodynia in spinally injured rats 4 h after irradiation. However, morphine, even at a higher dose (5 mg/kg, i.p.), failed to alleviate the allodynia in spinally injured rats 24–48 h after the injury. Systemic injection of the GABAB agonist baclofen (0.01‐0.1 mg/kg, i.p.), but not the GABAA agonist muscimol (1 mg/kg, i.p.), effectively relieved allodynia during this period. Pretreatment with guanethidine 24 h and just prior to the irradiation (20 mg/kg, s.c.) did not prevent the occurrence of allodynia in spinal cord injured rats, The present observation is the first to show that ischaemic spinal cord injury could result in cutaneous mechanical allodynia. This phenomenon is resistant to morphine and may not involve the sympathetic system. Histological examination of allodynic animals 3 days after spinal cord injury revealed considerable morphological damage in the dorsal spinal cord of a rat irradiated for 5 min. The related dorsal roots were also slightly affected in this animal, while the dorsal root ganglia were normal. However, in rats irradiated for 1 min, despite the existence of strong allodynia, no damage could be found at this time in the spinal cord, dorsal roots or dorsal root ganglia. It is suggested that functional deficits in the GABAB system in the spinal cord may be related to this allodynia‐like phenomenon. Allodynia following laser‐induced spinal cord injury may be a useful pain model for testing the efficacy of analgesic drugs against central pain of ischaemic origin.

Treatment of a chronic allodynia-like response in spinally injured rats: effects of systemically administered nitric oxide synthase inhibitors

&NA; We have previously reported that we have observed chronic pain‐like response to light mechanical stimuli (allodynia) in rats after severe spinal cord ischemia, which resembles some painful conditions in chronic spinally injured patients and is not relieved by a number of conventional analgesics used for treating chronic neuropathic pain. In the present study, we tested the effects of the non‐selective nitric oxide synthase (NOS) inhibitor NG‐nitro‐L‐arginine methyl ester (L‐NAME) and the selective neuronal NOS inhibitor 7‐nitro indazole (7‐NI) and 6‐nitro indazole (6‐NI) on the chronic allodynia‐like behavior. Systemic L‐NAME dose‐dependently relieved mechanical allodynia‐like response in a stereo‐specific and l‐arginine‐reversible manner without causing sedation or motor deficits. However, L‐NAME significantly elevated systemic blood pressure. Systemic 7‐NI relieved chronic allodynia in a l‐arginine reversible manner, did not increase blood pressure or induce sedation, but caused motor deficits at a high dose, which was not reversed by l‐arginine. Systemic 6‐NI also relieved the chronic allodynia, which was however associated with severe sedation. In order to exclude the possibility that the effect of L‐NAME on blood pressure was involved in the analgesic effect observed, the effect of systemically applied adrenaline was examined. Adrenaline increased the systemic blood pressure to a similar extent as L‐NAME, but did not relieve allodynia. It is suggested that blockade of NOS by L‐NAME relieved the chronic allodynia‐like behavior in spinally injured rats. This effect was likely to be mediated by a blockade of neuronal isoforms of NOS, as 7‐NI relieved the allodynia in a l‐arginine‐reversible manner. Consequently, generation of NO by neuronal NOS may be critically involved in the maintenance of this abnormal pain‐related sensation. The possibility of using NOS inhibitors as potential novel analgesics is discussed.

Anti-hyperalgesic and anti-allodynic effects of intrathecal nociceptin/orphanin FQ in rats after spinal cord injury, peripheral nerve injury and inflammation

&NA; We examined the effects of intrathecal nociceptin, the endogenous ligand for the orphan opioid receptor‐like receptor, on abnormal pain‐related behaviors in rats after carrageenan‐induced inflammation and photochemically‐induced peripheral nerve or spinal cord ischemic injury. Intrathecal nociceptin dose‐dependently alleviated mechanical and cold allodynia‐like behavior in the two models of neuropathic pain. The heat hyperalgesia associated with peripheral inflammation was also significantly reduced, although the efficacy of the anti‐hyperalgesic effect of nociceptin in the inflammation model was decreased. Intrathecal nociceptin also induced significant antinociceptin on the tail‐flick test in all three groups of rats. However, the antinociceptive effect of nociceptin was significantly reduced in rats with peripheral nerve injury. These results indicated that spinally administered nociceptin has anti‐allodynic and anti‐hyperalgesic effects in animal models of tonic or chronic pain of different origins. Peripheral inflammation and nerve injury may induce spinal plasticity which leads to altered potency and efficacy of nociceptin.

Decreased GABA immunoreactivity in spinal cord dorsal horn neurons after transient spinal cord ischemia in the rat

The number of GABA-like immunoreactive (LI) cells in lamina I-III of the rat spinal cord was significantly decreased bilaterally 48-72 h after photochemical induction of transient spinal cord ischemia compared to sham-operated controls. No significant changes in the number of GABA-LI cells were observed at cervical level. The number of GABA-LI cells was restored 2 weeks after ischemia. These data, together with recent behavioral and electrophysiological findings, suggest that decreased intraneuronal GABA levels after spinal cord ischemia may underlie the development of the temporary pain-like response to innocuous mechanical stimuli (allodynia) in rats after transient spinal cord ischemia.

Hyperalgesia and increased neuropathic pain-like response in mice lacking galanin receptor 1 receptors.

The neuropeptide galanin may have a role in modulation of nociception, particularly after peripheral nerve injury. The effect of galanin is mediated by at least three subtypes of receptors. In the present study, we assessed the nociceptive sensitivity in mice lacking the galanin receptor 1 gene (Galr1) and the development of neuropathic pain-like behaviours after photochemically induced partial sciatic nerve ischaemic injury. Under basal condition, Galr1 knock-out (Galr1(-/-)) mice had shortened response latency on the hot plate, but not tail flick and paw radiant heat, tests. The mechanical sensitivity was not different between Galr1(-/-) and wild type (Galr1(+/+)) mice, whereas the cold response was moderately enhanced in Galr1(-/-) mice. Both Galr1(-/-) mice and Galr1(+/+) controls developed mechanical and heat hypersensitivity after partial sciatic nerve injury. The duration of such pain-like behaviours was significantly increased in Galr1(-/-). The Galr1(-/-) mice and Galr1(+/+) mice did not differ in their recovery from deficits in toe-spread after sciatic nerve crush. The results provide some evidence for an inhibitory function for the neuropeptide galanin acting on galanin receptor 1 (GALR1) in nociception and neuropathic pain after peripheral nerve injury in mice.

Interferon-γ receptors in nociceptive pathways: Role in neuropathic pain-related behaviour

INTERFERON-γ receptor (IFN-γR) immunoreactivity was observed in the superficial dorsal horn and lateral spinal nucleus in rat and mouse spinal cord. Dorsal rhizotomies did not reduce immunoreactivity in the rat. IFN-γ distribution overlapped with nitric oxide synthase-1 immunoreactivity. In wild-type mice, intrathecal injections of mouse IFN-γ evoked biting behaviour, whereas mice with disruption of the functional gene for IFN-γR did not respond. Both types of mice had similar withdrawal thresholds to mechanical stimulation and reacted similarly to foot-pad carrageenan injections. In contrast to wild-type mice, IFN-γR knock-out mice did not show autotomy after sciatic nerve section. This study demonstrates a functional IFN-γR in spinal nociceptive pathways related to neuropathic pain.

Lack of cross-tolerance between the antinociceptive effect of intrathecal orphanin FQ and morphine in the rat

We assessed whether tolerance develops to the antinociceptive effect of intrathecal (i.t.) orphanin FQ, the endogenous ligand of the orphan opioid-like receptor, and whether there is a cross-tolerance between the antinociceptive effect of i.t. orphanin FQ and the mu opioid receptor agonist morphine. Orphanin FQ administered i.t. at 10 micrograms induced strong thermal antinociception as assessed with the tail flick test without inducing motor impairment or sedation. Tolerance developed in 4 days to the antinociceptive effect of orphanin FQ upon twice daily injections. However, rats rendered tolerant to orphanin FQ did not exhibit cross-tolerance to morphine. Moreover, in rats made tolerant to morphine, orphanin FQ induced similar antinociceptive effect as in morphine naive rats. The present results are the first to show that there is no cross-tolerance between orphanin FQ and morphine in eliciting spinal antinociception, supporting the notion that orphanin FQ produced spinal antinociception through a site which is different from classical opioid receptors.

Spantide II, a novel tachykinin antagonist, and galanin inhibit plasma extravasation induced by antidromic C-fiber stimulation in rat hindpaw

The effect of intradermal injection of Spantide II, a novel tachykinin antagonist, and the neuropeptide galanin on neurogenic plasma extravasation induced by antidromic stimulation of C-fibers in the sciatic nerve was examined in the hindpaws of rats. Activation of C-fibers by antidromic sciatic nerve stimulation (2 Hz, 5 min) consistently evoked a localized plasma extravasation of Evans Blue in the skin area of the hindpaw innervated by the sciatic nerve. Intradermal injection of 3 nmol Spantide II significantly inhibited this response. The plasma extravasation was nearly totally abolished when the concentration of Spantide II was increased to 9 nmol. Intradermal injection of 1.5 and 15 nmol galanin also inhibited plasma extravasation. Intradermal injection of 9 nmol Spantide II effectively blocked the plasma extravasion in the hindpaw induced by 8 nmol intravenous substance P. Plasma extravasation induced by intravenous substance P was also inhibited by the higher, but not by the lower, dose of galanin injected intradermally. The present results indicate that Spantide II, a potent non-toxic tachykinin antagonist, effectively blocks the neurogenic plasma extravasation induced by antidromic C-fiber stimulation, thus supporting the view that tachykinins play an important role in this neurogenic inflammatory process. It is further shown that galanin, a naturally occurring neuropeptide present in primary afferents, also inhibits C-fiber activation-evoked plasma extravasation, indicating an interaction between galanin and tachykinins in the peripheral terminals of primary afferents, possibly through both pre- and postsynaptic mechanisms.