Intrathecal administration of PNA targeting galanin receptor reduces galanin-mediated inhibitory effect in the rat spinal cord.

Peptide nucleic acids (PNA) are nucleic acid analogues containing neutral amide backbone, forming stable and tight complexes with complementary DNA/RNA. However, it is unclear whether unmodified PNA can efficiently penetrate neuronal tissue in order to act as antisense reagent. Here we show that intrathecal (i.t.) injection of an unmodified antisense PNA complementary to the rat galanin receptor type 1 (GalR1) mRNA is able to block the inhibitory effect of i.t. administered galanin on spinal nociceptive transmission. Autoradiographic ligand binding studies using [125I]galanin show that the unmodified PNA is able to reduce the density of galanin binding sites in the dorsal horn. Thus, unmodified PNA applied i.t. appears to function as an effective antisense reagent in rat spinal cord in vivo.

Nociceptin/orphanin FQ in spinal nociceptive mechanisms under normal and pathological conditions.

Nociceptin and its receptor are present in dorsal spinal cord, indicating a possible role for this peptide in pain transmission. The majority of functional studies using behavioral and electrophysiological studies have shown that nociceptin applied at spinal level produces antinociception through pre- and post-synaptic mechanisms. The spinal inhibitory effect of nociceptin is not sensitive to antagonists of opioid receptors such as naloxone. Thus, nociceptin-induced antinociception is mediated by a novel mechanism independent of activation of classic opioid receptors. This has raised the possibility that agonists of the nociceptin receptor may represent a novel class of analgesics. Supporting this hypothesis, several groups have shown that intrathecal nociceptin alleviated hyperalgesic and allodynic responses in rats after inflammation or partial peripheral nerve injury. Electrophysiological studies have also indicated that the antinociceptive potency of spinal nociceptin is maintained or enhanced after nerve injury. It is concluded that the predominant action of nociceptin in the spinal cord appears to be inhibitory. The physiological role of nociceptin in spinal nociceptive mechanisms remains to be defined. Moreover, further evaluation of nociceptin as a new analgesic calls the development of non-peptide brain penetrating agents.

Effects of intrathecal morphine, clonidine and baclofen on allodynia after partial sciatic nerve injury in the rat.

BACKGROUND: Increased response to mechanical or cold stimulation of hind paws was observed in rats with partial sciatic nerve injury as a result of photochemically induced ischemia. The present study examined the effects of intrathecal morphine, clonidine and baclofen on the allodynia-like responses. METHODS: The left sciatic nerves of rats were irradiated for 2 min with an argon ion laser under chloral hydrate anesthesia. The threshold of paw withdrawal to mechanical stimulation was determined with a series of monofilaments (von Frey hairs). The response to cold stimulation was tested by spraying ethyl chloride on the plantar surface of the paw. When rats were exhibiting stable mechanical and cold allodynia-like behaviors after nerve injury, the effects of i.t. morphine (1, 2, 7 microg), clonidine (1, 2, 7 microg) and baclofen (0.1, 0.2, 0.7, 9 microg) in a cumulative dose regime were investigated. RESULTS: Intrathecal morphine dose-dependently alleviated the mechanical and cold allodynia without inducing motor impairment or sedation. Intrathecal clonidine did not alter the response of hind paws to mechanical stimulation, but reduced the cold allodynia. Intrathecal baclofen reduced the responses of rats to mechanical stimulation only at doses that also induced profound motor deficits. CONCLUSIONS: The present data suggest that intrathecal morphine, and to some extent clonidine, but not baclofen, alleviated the abnormal pain-related behaviors in this new rat model of partial peripheral nerve injury. Differences in the pharmacological profile between the present model and other models of peripheral nerve injury are discussed.

The effect of intrathecal selective agonists of Y1 and Y2 neuropeptide Y receptors on the flexor reflex in normal and axotomized rats.

We have examined the effects of intrathecal (i.t.) administration of [Leu31,Pro34]-neuropeptide Y (NPY) or NPY-(13-36), selective agonists of NPY Y1 or Y2 receptors, respectively, on the excitability of the flexor reflex in normal rats and after unilateral transection of the sciatic nerve. In rats with intact and sectioned sciatic nerves, i.t. [Leu31,Pro34]-NPY induced a similar biphasic effect on the flexor reflex with facilitation at low doses and facilitation followed by depression at high doses. In contrast, i.t. NPY-(13-36) only facilitated the flexor reflex in normal rats, and at high dose it caused ongoing discharges in the electromyogram. NPY-(13-36) caused dose-dependent depression of the flexor reflex in rats after sciatic nerve transection, in addition to its facilitatory effect. Topical application of [Leu31,Pro34]-NPY or NPY-(13-36) caused a moderate and brief reduction in spinal cord blood flow. No difference was noted between the vasoconstrictive effect of [Leu31,Pro34]-NPY and NPY-(13-36). It is suggested that activation of Y1 receptors may be primarily responsible for the reflex depressive effect of i.t. neuropeptide Y in rats with intact sciatic nerves, whereas both Y1 and Y2 receptors may be involved in mediating the depressive effect of NPY after axotomy.

Effects of intrathecal orphanin FQ on a flexor reflex in the rat after inflammation or peripheral nerve section.

We examined the effects of intrathecal orphanin FQ, the endogenous ligand for the orphan opioid-like receptor, on the hamstring nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats after carrageenan-induced inflammation or unilateral sciatic nerve transection. As described previously [Xu, X.-J., Hao, J.-X., Wiesenfeld-Hallin, Z., 1996. Orphanin FQ or antiorphanin FQ: potent spinal antinociceptive effect of orphanin FQ/orphanin FQ in the rat. NeuroReport 7, 2092-2094.], intrathecal orphanin FQ induced a dose-dependent depression of the flexor reflex with a ED50 of 965 ng. Initial reflex facilitation was noted in some experiments at lower doses (10 or 100 ng). A similar bi-phasic response pattern to intrathecal orphanin FQ was observed in experiments conducted in inflamed or axotomized rats. However, the magnitude of the initial reflex facilitation was significantly increased in inflamed rats compared to normals whereas the duration of reflex depression was significantly shortened. The ED50 for reflex depression was 2.4 jig for inflamed rats. In contrast, axotomy did not significantly alter the facilitatory and depressive effect of orphanin FQ with ED50 for reflex depression being 374 ng. These results confirmed an inhibitory action of orphanin FQ on spinal nociception in rats. It is suggested that the effect of orphanin FQ may be modulated by inflammation and nerve injury. In particular, unlike morphine, there seems to be no reduction in the effect of spinal orphanin FQ in inducing antinociception after peripheral nerve axotomy.

Intrathecal galanin alleviates allodynia-like behaviour in rats after partial peripheral nerve injury.

We have previously suggested that the neuropeptides galanin and galanin message-associated peptide (GMAP) may have an inhibitory role in spinal nociception. The present study examined the effects of intrathecal (i.t.) administration of these two peptides on allodynia-like behaviours in response to mechanical and cold stimulation in rats after photochemically induced ischaemic peripheral nerve injury. I.t. galanin significantly alleviated the mechanical- and cold-allodynia-like behaviours in nerve injured rats, and was not associated with motor impairment or sedation. I.t. GMAP relieved mechanical allodynia much less than galanin. I.t. M-35, a high-affinity galanin receptor antagonist, did not significantly alter the response of the rats to mechanical or cold stimulation. At 1 or 2 weeks postinjury, around 15% of dorsal root ganglion (DRG) neuron profiles showed galanin-like immunoreactivity. These profiles were mostly small sized. Although the number of galanin positive cells was thus increased in the DRG in the present model, the increase was substantially less than after complete sciatic nerve section, as previously shown. The present results showed that spinal administration of galanin inhibited some abnormal pain-like behaviours in rats after partial peripheral nerve injury. These results further support an inhibitory function for galanin in nociception. However, endogenous galanin may not play a significant role in suppressing nociceptive input after partial ischaemic peripheral nerve injury, as the upregulation of galanin is moderate.

Effects of intrathecal nocistatin on the flexor reflex and its interaction with orphanin FQ nociceptin.

We studied the effects of intrathecal (i.t.) nocistatin, a peptide identified from the precursor of orphanin FQ/nociceptin (OFQ) on the spinal nociceptive flexor reflex in decerebrate, spinalized, unanesthetized rats and its interaction with i.t. OFQ. Nocistatin induced a moderate, non-dose-dependent facilitation of the flexor reflex without producing reflex depression whereas i.t. OFQ induced a biphasic dose-dependent facilitatory and inhibitory effect. The facilitatory effect of low dose (0.55 pmol) OFQ was significantly increased by nocistatin. On the other hand, the duration, but not magnitude, of reflex depression induced by a high (550 pmol) dose of OFQ was significantly shortened by 5.5 nmol nocistatin. Thus, nocistatin interacts with OFQ in a complex fashion, increasing excitation and reducing inhibition. No evidence was obtained for an antinociceptive effect of nocistatin in rat spinal cord.

The effect of intrathecal neuropeptide Y on the flexor reflex in rats after carrageenan-induced inflammation.

We examined the effects of intrathecal (i.t.) administration of neuropeptide Y (NPY) on the excitability of the flexor reflex in normal rats and 24 h after inflammation induced by subcutaneous carrageenan. In normal rats, i.t. NPY at low doses (10 and 100 ng) caused a brief facilitation of the flexor reflex with no subsequent depression. At higher doses (1 and 10 microg), the effect of NPY was mainly inhibitory, causing substantial and usually prolonged depression of the flexor reflex. At 24 h after the injection of carrageenan, when inflammation was at its peak, the magnitude of the reflex was increased and discharge duration became prolonged. I.t. NPY produced similar pattern of dose-dependent facilitatory and depressive effects on the flexor reflex. The facilitatory effect of i.t. NPY, particularly for the higher doses, was significantly enhanced in inflamed rats compared to normals. In contrast, the depressive effect of high doses of i.t. NPY was unchanged. These data suggest that the changes in levels of NPY and NPY receptors in the spinal cord known to occur after inflammation, are associated with an increased excitatory effect of this peptide.

[Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2, a proposed antagonist of the nociceptin receptor, is a potent and stable agonist in the rat spinal cord.

[Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 is an nociceptin analogue which has been shown to be a selective antagonist of the nociceptin receptor in peripheral tissues. We now report that intrathecal [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 produced a dose-dependent depression of the nociceptive flexor reflex in rats, an effect that is similar to nociceptin. The duration of depression produced by [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 was significantly more prolonged than by nociceptin. The reflex depressive effect of nociceptin was not blocked by [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2. The results indicated that the proposed nociceptin receptor antagonist [Phe1psi(CH2-NH)Gly2]-nociceptin-(1-13)NH2 is a potent agonist in rat spinal cord and more resistant to enzymatic degradation compared to nociceptin.

On the role of galanin in mediating spinal flexor reflex excitability in inflammation.

The effects of exogenous and endogenous galanin on spinal flexor reflex excitability was evaluated in rats one to eight days after the induction of inflammation by subcutaneous injection of carrageenan into the sural nerve innervation area. In normal rats, electrical stimulation of C-fibres in the sural nerve elicited a brisk reflex discharge. Conditioning stimulation of C-fibres (1/s) generated a gradual increase in reflex magnitude (wind-up), which was followed by a period of reflex hyperexcitability. Intrathecal galanin dose-dependently blocked reflex hyperexcitability induced by C-fibre conditioning stimulation whereas i.t. M-35, a high-affinity galanin receptor antagonist, moderately potentiated this effect. At one to three days after the injection of carrageenen, when inflammation was at its peak, the magnitude of the reflex was significantly increased and discharge duration became prolonged. However, wind-up and reflex hyperexcitability were significantly reduced. Furthermore, reduced reflex excitability during conditioning stimulation ("wind-down") and depression of the reflex were sometimes present, which are rarely observed in normal rats. Intrathecal galanin reduced hyperexcitability during inflammation, although its potency was weaker than in normals. However, the galanin receptor antagonist M-35 strongly enhanced wind-up and reflex hyperexcitability, similarly as in normal rats. The baseline flexor reflex, wind-up and C-fibre conditioning stimulation-induced facilitation were normalized four to eight days after carrageenan injection when signs of inflammation were diminishing. Interestingly, intrathecal galanin and M-35 failed to influence spinal excitability. The results suggest a complex functional plasticity in the role of endogenous galanin in mediating spinal excitability during inflammation. There appears to be an enhanced endogenous inhibitory control by galanin on C-afferent input during the peak of inflammation, which may explain the relative ineffectiveness of exogenous galanin. During the recovery phase there may be a reduction in galanin receptors, which may impair the action of endogenous and exogenous galanin. These results further support the notion that galanin is an endogenous inhibitory peptide in nociception.

The effect of naturally occurring fragments of galanin message-associated peptide on spinal cord excitability in rats.

Galanin message-associated peptide (GMAP), a 60 amino acid fragment of galanin precursor protein, is present in dorsal root ganglion cells and upon intrathecal (i.t.) administration influences the spinal nociceptive flexor reflex in rat in a complex manner. The present study assessed the effects on spinal cord excitability of N-terminal fragment GMAP (1-30) and C-terminal fragments GMAP (34-60) or GMAP (35-60), which were formed from GMAP following enzymatic degradation. The effect of the fragments was compared with the effects of the complete peptide sequence. The GMAP fragments slightly facilitated the flexor reflex and dose-dependently blocked hyperexcitability following C-fiber conditioning stimulation. The potency of the blocking effect of GMAP (1-30) was comparable to GMAP (1-60) and was one order of magnitude higher than the potency of the C-terminal fragments. The results indicated that both naturally formed N- and C-terminal fragments of GMAP are pharmacologically active and produce effects which are similar to the full peptide sequence.