Analgesic effects of fibroblast growth factor in the rat

Attenuation of pain perception after transposition of the greater omentum to the cauda equina region of rats--a preliminary observation

BACKGROUND

This paper addresses a specific experimental design to suggest the possible role of the greater omentum in the modulation of pain in rats.

METHODS

Fifteen male Sprague-Dawley rats weighing between 275 and 325 g were selected. The animals were randomized and then anesthetized with pentobarbital (35 mg/kg) and divided into three groups: (1) sham: laparotomy followed by laminectomy with exposure of the spinal epidural space (n=5); (2) transposition of pedicled omentum (n=5) to the cauda equina epidural space; and (3) transposition of pedicled omentum (n=5) to the cauda equina intradural space. The animals were operated upon and once more randomized by an independent investigator, so that the groups were thought to be similar during post-operative testing. The latency of paw withdrawal to noxious heat stimulation was tested and the values (seconds) plotted for 1, 3, 6, 11, 14 and 30 days after surgery. Randomization codes were open after the animals were euthanized. The analysis of variance (ANOVA) without replication was applied for each of the dataset and comparisons established among the different study groups involved. The omenta were removed and standard immunohistochemistry was performed for gamma-amino-butyric acid (GABA), serotonin, calcitonin-gene related protein (CGRP), vascular intestinal peptide (VIP) and Met-enkephalin.

RESULTS

The response to high heating rates of stimulation favored intradural versus sham and epidural omental transpositions. High and low noxious heat stimulation suggested an increased threshold to noxious stimulation after the 3 and 30 days of omental transposition. In the low heat stimulation series, responses were comparatively higher than in the sham animals.

CONCLUSIONS

The suggested increased threshold of response to noxious stimulation after transposition of the greater omentum onto the spinal cord of rats suggested a novel role of the omentum and a potential future application in the clinical arena.

Fibroblast growth factor-2 acutely influences the impulse activity of rat dorsal horn neurones

The neurotrophic and neuroprotective actions of fibroblast growth factor-2 (FGF-2) are well-established. The signal cascade mediating these effects includes steps that are likely to influence also the electrical properties of neurones. However, the possibility that FGF-2 may acutely affect the processing of neuronal impulse activity is largely unexplored. In the present study the impulse activity of single dorsal horn neurones was recorded in the rat during ionophoretical administration of FGF-2 close to the neurones. Before and during FGF-2 ionophoresis the receptive field of each cell was tested with defined mechanical stimuli. At a concentration of 10 nM in the ionophoresis pipette, FGF-2 reduced the responses of the cells to mechanical stimulation. There was no preferential action of FGF-2 on a particular functional type of dorsal horn neurone; both non-nociceptive and nociceptive cells exhibited a reduced mechanical responsiveness. The background (ongoing) activity was also depressed in most neurones. The results of the study show that in addition to neurotrophic and neuroprotective actions FGF-2 has an acute inhibitory influence on the impulse activity of spinal sensory neurones. This depression of neuronal activity could add to the neuroprotective action of FGF-2 by counteracting glutamate excitotoxicity following a central nervous trauma.

Role of nitric oxide in the physiopathology of pain

Many painful disorders, including joint dysfunctions such as rheumatoid arthritis (RA) or temporomandibular joint disorders (TMD), are associated with hyperthermia of the overlying skin. The same is true of certain intractable chronic pain conditions, such as chronic orofacial pain, which may be associated with TMD. We suggest that this skin hyperthermia, caused by regional vasodilation, is induced by extravascular nitric oxide (NO). Extravascular NO can be produced in the affected joint by osteoblasts, chondrocytes, and macrophages, by mechanical stimulation of endothelial cells, or by stimulated neurons. In view of a strong correlation between pain and skin hyperthermia in these disorders, and the evidence that NO enhances the sensitivity of peripheral nociceptors, we also suggest that at least this kind of pain is associated with excessive local level of NO. This hypothesis can be verified by dynamic area telethermometry, assessing the effect of NO on the sympathetic nervous function. This mechanism, which is in line with the general role of NO as a mediator between different organ systems, also may be relevant to any pain associated with enhanced immune response. Clinical implications of the proposed mechanism are discussed.