Effect of peripheral axotomy on dorsal root ganglion neuron phenotype and autonomy behaviour in neuropeptide Y-deficient mice.

The lumbar 5 (L5) dorsal root ganglia (DRGs) were studied in neuropeptide tyrosine (NPY)-deficient (-/-) and wild type (+/+) mice after unilateral sciatic nerve transection using in situ hybridization and immunohistochemistry. NPY, galanin and two NPY receptors (Y-Rs) were analyzed as well as self-mutilation behaviour (autotomy) and nociceptive thresholds. No difference between wild type and NPY-deficient mice was seen in the tail-flick or hot plate test. However, -/- mice showed a much stronger autotomy behaviour than wild type mice. NPY was not found in L5 DRGs in -/- mice, not even after axotomy. Galanin was upregulated to the same extent after axotomy in NPY-deficient and wild type mice. Y1- and Y2-R mRNAs were found mainly in small DRG neuron profiles. Both receptor mRNAs were downregulated after axotomy, to about the same extent in NPY-deficient as in wild type mice. In control and contralateral ganglia the mRNA levels of both receptors were lower in NPY-deficient mice than in wild type mice. The contralateral Y2-R mRNA levels did not reach control values in the NPY-deficient mice, as they did in the wild type mice. In both strains the Y1-R protein was decorating the somatic plasmalemma. The present results suggest that lack of NPY may cause exaggerated autotomy, a self-mutilation behaviour possibly related to pain sensation, in agreement with the described analgesic effect of NPY. Although significant differences in levels of Y1- and especially Y2-R mRNAs were observed between wild type and NPY-deficient mice, they were only moderate. These findings suggest that expression, regulation, localization and possible function of Y1- and Y2-Rs are not dependent on presence of the endogenous ligand. Also, deletion of NPY does not seem to influence the expression of the partly coexisting peptide galanin.