Combined effect of Neurotropin® and methylcobalamin on postherpetic neuralgia in mice infected with herpes simplex virus type-1.

BACKGROUND: Postherpetic pain (PHP) is difficult to control. Although Neurotropin® (NTP) and methylcobalamin (MCB) are often prescribed to treat the pain, the efficacy of combined treatment for PHP remains imcompletely understood. OBJECTIVE: In this study, we investigate the combined effects of NTP and MCB on PHP in mice. METHODS: NTP and MCB were administered from day 10-29 after herpes simplex virus type-1 (HSV-1) infection. The pain-related responses were evaluated using a paint brush. The expression of neuropathy-related factor (ATF3) and nerve repair factors (GAP-43 and SPRR1A) in the dorsal root ganglion (DRG) and neurons in the skin were evaluated by immunohistochemical staining. Nerve growth factor (NGF) and neurotrophin-3 (NT3) mRNA expression levels were evaluated using real-time PCR. RESULTS: Repeated treatment with NTP and MCB after the acute phase inhibited PHP. Combined treatment with these drugs inhibited PHP at an earlier stage than either treatment alone. In the DRG of HSV-1-infected mice, MCB, but not NTP, decreased the number of cells expressing ATF3 and increased the number of cells expressing GAP-43- and SPRR1A. In addition, MCB, but not NTP, also increased and recovered non-myelinated neurons decreased in the lesional skin. NTP increased the mRNA levels of NTF3 in keratinocytes, while MCB increased that of NGF in Schwann cells. CONCLUSION: These results suggest that combined treatment with NTP and MCB is useful for the treatment of PHP. The combined effect may be attributed to the different analgesic mechanisms of these drugs.

Excitatory effect of Neurotropin(®) on noradrenergic neurons in rat locus coeruleus.

AIMS: Although the clinical use of Neurotropin® as an analgesic for chronic pain has been firmly established, its analgesic mechanism is still unclear. In this study, we investigate the direct effects of Neurotropin using an electrophysiological method. MAIN METHODS: Blind patch-clamp recordings were made from rat locus coeruleus (LC) and periaqueductal gray (PAG) neurons in brainstem slices of normal rats. The effects of intracerebroventricular (icv) injection of Neurotropin on nociceptive transmission were recorded from spinal substantia gelatinosa (SG) neurons in fifth lumbar spinal nerve-ligated (L5-SNL) rats using an in vivo patch-clamp method. KEY FINDINGS: Neurotropin (0.2­1.0 NU/mL) dose-dependently increased the firing rate in noradrenergic LC neurons of normal rats. Under the voltage-clamp condition, Neurotropin induced an inward current in 90% of LC neurons thatwas not affected by tetrodotoxin or an injection of GDP-ß-S (G protein inhibitor) through recording pipettes. In contrast, Neurotropin had no effects on all PAG neurons tested. Using in vivo patch-clamp recordings, the icv injection of Neurotropin inhibited both frequency and amplitude of pinch-evoked excitatory postsynaptic currents of SG neurons in L5-SNL rats. These results suggest that Neurotropin directly excites the descending noradrenergic LC neurons and inhibits nociceptive transmission in the spinal dorsal horn. SIGNIFICANCE: This study is the first direct demonstration that Neurotropin activates the noradrenergic descending pain inhibitory systems, and this would reinforce the usefulness of Neurotropin in the treatment of human neuropathic pain.

Combined antiallodynic effect of Neurotropin® and pregabalin in rats with L5-spinal nerve ligation.

AIMS: In this study, we investigated the combined effect of Neurotropin® and pregabalin for L5-spinal nerve ligation (L5-SNL) model in rats and thiopental-induced sleep in mice. MAIN METHODS: The left fifth lumbar nerve of rats was tightly ligated with silk sutures under pentobarbital anesthesia. The hindpaw withdrawal threshold was measured by application of von Frey filaments. Thiopental sodium was intravenously administered in mice and sleeping time was measured. In L5-SNL rats, an isobolographic analysis was performed to clarify the combined antiallodynic effect of Neurotropin and pregabalin 14 days after ligation in rats. In isobolographic analysis and thiopental-induced sleep test, Neurotropin and pregabalin were orally administered to coincide with the timing of the peak effect of each drug. KEY FINDINGS: Neurotropin (50-200 NU/kg) and pregabalin (2.5-10mg/kg) showed a dose-dependent antiallodynic action in L5-SNL rats. The antiallodynic effect of pregabalin was reversed by intrathecal injection of yohimbine or ondansetron. Isobolographic analysis suggested that the combined antiallodynic effect of Neurotropin and pregabalin in L5-SNL rats may have been more than a mere additive effect. Neurotropin (50-400 NU/kg) had no effect on thiopental-induced sleeping time whereas pregabalin (30-100mg/kg) significantly prolonged it. When the dose of pregabalin was 30 mg/kg, Neurotropin (50-400 NU/kg) did not further exacerbate the prolongation effect of pregabalin on thiopental-induced sleep. SIGNIFICANCE: It was suggested that when Neurotropin was administered in combination with pregabalin, it might provide more effective pain relief than that obtained with each agent alone in neuropathic pain without aggravating adverse effects of pregabalin.

The antiallodynic effect of Neurotropin is mediated via activation of descending pain inhibitory systems in rats with spinal nerve ligation.

BACKGROUND: Neurotropin, a nonprotein extract isolated from inflamed skin of rabbits inoculated with vaccinia virus, is widely used in Japan to treat chronic pain such as neuropathic pain. Although some studies have been conducted on the mechanism of the antiallodynic action of Neurotropin, this mechanism has yet to be adequately clarified. METHODS: The left fifth lumbar nerve of rats was tightly ligated with silk sutures under pentobarbital anesthesia. Mechanical allodynia was confirmed by measuring the hindpaw withdrawal threshold in response to application of von Frey filaments. Behavioral tests were performed at 28 days after nerve ligation. Neurotropin was administered IV, intrathecally or intracerebroventricularly in L5 spinal nerve ligation (L5-SNL) rats. We examined the effects of noradrenergic, serotonergic and gamma-aminobutyric acid (GABA)ergic antagonists on the antiallodynic action of Neurotropin in L5-SNL rats. Yohimbine hydrochloride (yohimbine) was used as an alpha(2) adrenoceptor antagonist, ketanserin tartrate (ketanserin) as a 5-HT(2A) receptor antagonist, MDL72,222 as a 5-HT(3) receptor antagonist, (-)-bicuculline methobromide (bicuculline) as a GABA(A) receptor antagonist, and CGP35,348 as a GABA(B) receptor antagonist, and intrathecally injected. RESULTS: IV (50-100 NU/kg) doses of Neurotropin elicited an antiallodynic action in L5-SNL rats. Moreover, intracerebroventricular (400 mNU/rat), but not intrathecal, injection of Neurotropin inhibited allodynia. The antiallodynic action of Neurotropin (100 NU/kg, IV) was antagonized by intrathecal injections of yohimbine (10 nmol/rat), ketanserin (30 nmol/rat), MDL72,222(30 nmol/rat), bicuculline (0.6 nmol/rat) and CGP35348 (30 nmol/rat). On the other hand, the antiallodynic action of intrathecally injected m-CPBG (5-HT(3) receptor agonist) was reversed by intrathecal injection of bicuculline and CGP35348, suggesting interaction of 5-HT(3) receptors and spinal inhibitory (GABAergic) interneurons. CONCLUSIONS: These results suggest that the antiallodynic effect of Neurotropin is mediated via activation of descending pain inhibitory systems, such as the noradrenergic and serotonergic systems, which project from supraspinal sites to the spinal dorsal horn. In addition, activation of inhibitory GABAergic interneurons via 5-HT(3) receptors by serotonin released in the spinal dorsal horn may also be involved in the antiallodynic action of Neurotropin.

Mechanisms of analgesic action of neurotropin on chronic pain in adjuvant-induced arthritic rat: roles of descending noradrenergic and serotonergic systems.

Neurotropin((R)), a non-protein extract from the inflamed skin of rabbits inoculated with vaccinia virus, has been clinically used as an analgesic drug for treatment of chronic pain. In this study, we investigated the analgesic mechanisms of Neurotropin in the adjuvant-induced arthritic rat, a chronic pain model with inflammation. Neurotropin caused dose-dependent inhibition of hyperalgesia in the adjuvant-induced arthritic rat after single intravenous (10 - 100 NU/kg) and oral (30 - 200 NU/kg) administration. The analgesic effect of Neurotropin (intravenous 100 NU/kg and oral 200 NU/kg) was significantly inhibited by intrathecal injections of the alpha(2)-adrenoceptor antagonist yohimbine (30 nmol/animal) and the selective 5-HT(3) serotonin receptor antagonist MDL72222 (30 nmol/animal), and slightly inhibited by the non-selective serotonin receptor antagonist methysergide (100 nmol/animal). The results suggest that the analgesic action of Neurotropin is at least in part due to the enhancement of noradrenergic and serotonergic descending pain inhibitory pathways. Neurotropin may be useful for the clinical management of chronic pain diseases such as a rheumatoid arthritis and osteoarthritis.