Inducible galanin and GalR2 receptor system in motor neuron injury and regeneration

Galanin has been ascribed several physiological roles that are thought to be mediated via multiple galanin receptors. Recently, two galanin receptors--galanin receptor-1 (GalR1) and galanin receptor-2 (GalR2)--have been cloned and characterized and shown to have differences in amino acid sequence, pharmacology, and second messenger signaling systems. Previous studies have demonstrated an up-regulation of galanin expression in damaged neurons of several different types. Using in situ hybridization histochemistry this study investigated whether adult cranial motor neurons express mRNAs encoding GalR1 and/or GalR2 and explored possible time-dependent changes in these transcripts following facial nerve injury. GalR2 mRNA levels were increased in the ipsilateral facial nucleus 3 (approximately 1.8-fold) and 7 days (approximately 3.7-fold) after unilateral facial nerve crush and had returned to levels equivalent to those in contralateral controls by 14-21 days. GalR1 mRNA was not detected in facial nuclei of naive, sham-operated, or operated rats but was present in adjacent reticular nuclei. Galanin mRNA levels were also increased eight- to 10-fold in the ipsilateral facial nucleus following nerve injury. These experiments confirm the putative importance of galanin signaling systems after nerve injury by demonstrating a differential response of galanin receptor subtypes and suggest an important "autoreceptor" role for the GalR2 receptor in these processes.