The extraction and assay of substance P in the rat sciatic nerve

Clenbuterol stimulates neurotrophic support in streptozotocin-diabetic rats

BACKGROUND

The diabetic neuropathies are characterized by axonal degeneration of peripheral nerves. The commonest variant of this condition is distal symmetrical polyneuropathy, in which the principal presenting features can be explained by impaired neurotrophic support of sensory neurones.

AIMS

The aims of these experiments were to determine whether deficits in nerve growth factor (NGF) content and the products of its target genes (substance P and calcitonin-gene related peptide; CGRP) in the sciatic nerve of streptozotocin-induced rats could be prevented using clenbuterol, which has been demonstrated to induce NGF synthesis in vitro.

RESULTS

Diabetes of 8 weeks duration produced a reduction in measurable sciatic nerve NGF-like immunoreactivity (NGF-LI) content of 53% (p < 0.01). This was accompanied by depletion of substance P (of 43%; p < 0.01) and CGRP (of 45%; p < 0.01). Treatment with clenbuterol (30 microg/kg, 100 microg/kg and 300 microg/kg; daily subcutaneous injection) significantly increased the NGF-LI and neuropeptide content of the sciatic nerve, with 30 microg/kg of clenbuterol normalising the levels in diabetic animals. The alterations in steady state sciatic nerve NGF-LI were proportionally related to the amount of NGF-LI delivered to the primary afferent neurones via retrograde transport. Thus, there was a significant reduction in accumulation of NGF-LI distal to a 12-h ligature in diabetic untreated rats (p < 0.01), which was normalized by clenbuterol (30 microg/kg; p < 0.05). Treatment of control or diabetic rats with clenbuterol did not affect the local production of NGF-LI in the sciatic nerve adjacent to and triggered by an occluding ligature. The change in NGF-LI content of neuronal tissue in the clenbuterol treated rats was a reflection of the capture of NGF derived from the peripheral targets of the sciatic nerve. The soleus and EDL muscles showed reductions in NGF-LI in untreated diabetic animals. Clenbuterol significantly (p < 0.05) increased NGF-LI in the soleus muscle.

CONCLUSION

Clenbuterol could be a useful tool in alleviating the deficits in neurotrophin support in diabetes.

Detection of increased tissue concentrations of nerve growth factor with an improved extraction procedure

Nerve growth factor (NGF) is a protein essential for the survival and normal function of sympathetic neurons. Two-site immunoassays have been developed over the past decade in several laboratories and used to estimate its endogenous concentrations in a variety of effector tissues. However, levels appear restricted to a narrow range, display only a poor correlation with innervation density, and show obvious inter- and intralaboratory variations, the origins of which are unclear. This led us to examine alternative extraction procedures for NGF before quantification. In particular, we have found treatment of tissue extracts with high and low pH in the presence of detergent results in the detection of higher NGF concentrations in immunoassays using either polyclonal or commercially available monoclonal antibodies. These increases were tissue-specific (sciatic nerve, mesenteric arteries, and thoracic aorta > heart and brain > sympathetic ganglia > abdominal aorta) and as much as 10 times greater than the amounts detected by traditional procedures. The method should also prove useful for the assay of other members of the neurotrophin family when appropriate antibodies become available.