Effect of Neurotropin on Hyperalgesia Induced by Prostaglandin E2, Naloxone, Melatonin and Dark Condition in Mice

Diurnal changes of tonic nociceptive responses in mice: evidence for a proalgesic role of melatonin

Diurnal variations in tonic pain reactions have been described in mice tested in Spring, but the underlying mechanisms are still unknown. We tested the potential role of melatonin, a key hormone in the control of neuro-endocrine circadian rhythms. The experiments were performed in male CBA/J mice housed under controlled temperature, humidity, and light (12/12 dark/light cycle) conditions, during the Light (7-10a.m.) or Dark (7-10p.m.) phases of the diurnal cycle. In a first group of experiments, animals were either pretreated with i.p. saline (controls) or with the melatonin receptor antagonist, luzindole (30 mg/kg), before the s.c. injection of a dilute formalin solution into a hindpaw. In control animals, pain-related behavioral reactions (licking and flinching) were higher in the evening (Dark) than in the morning (Light), both during the first (0-10 min) and the second (11-55 min) phase of the response to s.c. formalin. In animals pre-treated with luzindole, no diurnal changes occurred, pain reactions in the Dark being similar to those of the Light Control group. In a second group of experiments, artificial pinealectomy, obtained by exposing animals to continuous light for 48 h, also reduced pain reactions in the evening to levels comparable to those in the morning. Receptor autoradiography showed lower binding availability at spinal cord level in mice sacrificed during the Dark, as expected from the circadian pattern of melatonin secretion. A further significant decrease of melatonin receptor binding was induced by noxious stimulation. These results suggest a proalgesic role of endogenous melatonin in tonic pain.

Action of neurotropin on rat hypothalamic neurons in tissue slices

To clarify some of the actions of Neurotropin (NSP), intra- and extracellular measurements were made of 140 paraventricular (PVN) and 48 ventromedial nucleus (VMH) neurons in rat hypothalamic tissue slices in vitro while perfusing NSP, an extract from the inflamed skin of rabbits inoculated with vaccinia virus, through the recording chamber. NSP mostly decreased activity in PVN neurons (inhibition, 46; excitation, 27; excitation-inhibition, 5; no response, 62), and mostly increased it in VMH neurons (excitation, 12; inhibition, 3; no response, 33). Inhibition of PVN neurons by NSP was due to hyperpolarization with no change in membrane conductance. Since ouabain antagonized the NSP-induced inhibition of PVN neurons, the effect was probably due to activation of the sodium pump. Activity of some NSP-responsive PVN neurons was increased by increase of extracellular osmolarity, and activity of some NSP-responsive VMH neurons was increased by glucose application. The results suggest central modulation of autonomic or neuroendocrinological function by distinct NSP influence on PVN and VMH neural activity.

Mechanism of the analgesic effect of neurotropin

Neurotropin, an extract from the inflamed skin of vaccinia virus-inoculated rabbits, has been observed clinically to be effective for treating pain in patients with lumbago, SMON and other neuropathies. In the present study, we examined the mechanism of the antinociceptive effect of neurotropin in mice in relation to administration routes, opioids, and noradrenergic or GABAergic drugs, by the tail pressure method. The antinociceptive effects of neurotropin were large when administered by the i.p. and intracisternal (i.cist.) routes, but comparatively small in the case of the intrathecal (i.th.) route. Neurotropin may thus act at the supraspinal level rather than on the spinal cord. The antinociceptive effect of neurotropin was not blocked by naloxone, and no cross-tolerance developed between neurotropin and morphine. The effect of neurotropin was blocked by phentolamine and reserpine, but not by atropine. Its effect was enhanced by GABA, muscimol, aminooxyacetic acid and diaminobutyric acid, but not by baclofen, and blocked by bicuculline methiodide. From these results, the antinociceptive action of neurotropin appears to be non-opioid in nature, and may possible be mediated by the noradrenergic and GABAergic systems, but unrelated to the cholinergic system.